Polymer Self-Assembly and Thin Film Deposition in
Supercritical Fluids
by
Nastaran Yousefi
M.Sc., University of Western Ontario, 2014
B.Sc., Islamic Azad University, 2009
Thesis Submitted in Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
in the
Department of Chemistry
Faculty of Science
© Nastaran Yousefi 2021
SIMON FRASER UNIVERSITY
Spring 2021
Copyright in this work rests with the author. Please ensure that any reproduction
or re-use is done in accordance with the relevant national copyright legislation.
Declaration of Committee
Name:
Nastaran Yousefi
Degree:
Doctor of Philosophy (Chemistry)
Thesis title:
Polymer Self-Assembly and Thin Film Deposition
in Supercritical Fluids
Committee:
Chair:
Corina Andreoiu
Associate Professor, Chemistry
Loren Kaake
Supervisor
Associate Professor, Chemistry
Paul Percival
Committee Member
Professor Emeritus, Chemistry
Vance Williams
Committee Member
Professor, Chemistry
Neil Branda
Examiner
Professor, Chemistry
James Watkins
External Examiner
Professor
Polymer Science and Engineering
University of Massachusetts Amherst
ii
Abstract
Patterning of flexible electronic devices using large-area printing techniques is the focus
of intense research due to their promise of producing low-cost, light-weight, and flexible devices.
The successful integration of advanced materials like semiconductor nanocrystals, carbon
nanotubes and polymer semiconductors into microscale electronic devices requires deposition
techniques that are robust, scalable, and enable fine patterning. To this end, we have established
a deposition technique that leverages the unique solubility properties of supercritical fluids. The
technique is the solution-phase analog of physical vapour deposition and allows thin films of a
semiconducting polymer to be grown without the need for in-situ chemical reactions. To
demonstrate the flexibility of the technique, we demonstrated precise control over the location of
material deposition using a combination of photolithography and resistive heating. The versatility
of the technique is demonstrated by creating a patterned film on the concave interior of a silicone
hemisphere, a substrate that cannot be patterned via any other technique. More generally, the
ability to control the deposition of solution processed materials with lithographic accuracy provides
the long sought-after bridge between top-down and bottom-up self-assembly.
In addition, we investigated the self-assembly of polymers in supercritical fluids by
depositing thin films and studying their morphology using polarized optical microscopy and
grazing incidence wide angle x-ray scattering. We summarized our observations with a two-step
model for film formation. The first step is pre-aggregation in solution whereby the local crystalline
order is established, and the solution turbulence can easily disrupt the solution-phase selfassembly. The second step to film formation is the longer length scale organization that is
influenced by the chain mobility on the surface. We identified pressure and solvent additive as
two powerful tools to facilitate the local crystalline order and longer length scale organization. The
work demonstrated key insights necessary to optimizing thin-film morphologies and principles for
understanding self-assembly in supercritical fluids that could be applied to self-assembly of
materials in other contexts. Finally, we developed a simple empirical model based on classical
thermodynamics that highlights the interplay of intermolecular interactions and solvent entropy
and describes both the temperature and pressure dependence of polymer solubility in
supercritical fluids.
iii
Keywords:
supercritical
fluids;
self-assembly;
semiconducting
semiconductors; thin film deposition; thin film morphology
iv
polymers;
organic
Dedication
To my brother, Pedram, for sacrificing his own happiness for mine and our family.
v
Acknowledgements
I would like to extend my sincerest appreciation to my supervisor, Dr. Loren Kaake, for
providing me with every opportunity to thrive, and more importantly, paved my way to become a
competent scientist. I will always appreciate his insatiable curiosity for science, his willingness to
be unconventional, and his boldness when it comes to scientific ideas and pushing barriers.
I would like to thank the members of my supervisory committee, Dr. Percival, and Dr.
Williams for their constant support and guidance throughout my PhD studies and providing me
with constructive feedbacks during annual committee meetings.
To my academic family at Simon Fraser University and particularly the Department of
Chemistry, you made my journey one full of wonderful memories and I will cherish those memories
forever. Thanks for supporting me through tough times with insurmountable amount of chocolate,
hugs, and words of encouragement. I consider myself lucky to be surrounded by such intelligent
and thoughtful people.
To my fiancé, Dr. Michael Meanwell, thank you for your love, constant encouragement,
and above all your genuine kindness. I am forever grateful for having you by my side during the
most challenging years of my life and I will not forget how you empowered me in different ways
every day. Indeed, there is no better feeling than having a person like you by my side to celebrate
my accomplishments with and hold me when I fail miserably.
To my family and friends, this thesis would have not been completed without your
unconditional love, collective kindness, and unwavering support. You all managed to amazingly
show up and support me in the most difficult moments of my life and lifting my spirits.
Lastly, to my brother Pedram, I am forever in your debt for always making me a priority in
your life, listening to my challenges patiently, imparting your wisdom gracefully whenever
possible, and above all always being there for me.
vi
Table of Contents
Declaration of Committee .................................................................................................... ii
Abstract ............................................................................................................................... iii
Dedication ............................................................................................................................v
Acknowledgements ............................................................................................................. vi
Table of Contents ............................................................................................................... vii
List of Figures.......................................................................................................................x
List of Acronyms and Symbols ......................................................................................... xvi
Chapter 1. Introduction .................................................................................................. 1
1.1. Past, Present, and Future of Organic Semiconductors ............................................ 1
1.2. Research Motivation .................................................................................................. 3
1.3. Thesis Overview ........................................................................................................ 6
Chapter 2. Background .................................................................................................. 8
2.1. Organic Semiconductors ........................................................................................... 8
2.1.1.
Doping of Semiconducting Polymers ..............................................................10
2.2. Charge Transport Mechanism in Semiconductors ..................................................13
2.2.1.
Band Theory in Inorganic Semiconductors .....................................................14
2.2.2.
Charge Transport Mechanisms in Organic Semiconductors ..........................17
Band-Like Transport ...................................................................................................18
Marcus Charge Transfer ............................................................................................19
Polaronic Transport ....................................................................................................20
2.2.3.
Disordered-Based Transport in Organic Semiconductors ..............................22
Nearest Neighbor Hopping .........................................................................................22
Variable Range Hopping (Mott and Efros–Shklovskii)...............................................22
Multiple Trap and Release (MTR) ..............................................................................23
2.2.4.
The Influence of Microstructure on Charge Transport in Semiconducting
Polymers .........................................................................................................................25
The Role of Regioregularity........................................................................................27
The Role of Molecular Weight ....................................................................................29
The Role of Chain Rigidity and Side-chain Unit.........................................................30
The Role of Processing Conditions ............................................................................31
2.3. Fluid Mechanics .......................................................................................................35
2.3.1.
Laminar and Turbulent Flow............................................................................36
2.3.2.
Heat Transfer via Convection or Conduction ..................................................37
Conduction (or diffusion) ............................................................................................37
Convection ..................................................................................................................38
Radiation .....................................................................................................................38
2.3.3.
Rayleigh Number .............................................................................................39
2.3.4.
Rayleigh-Bénard convection ...........................................................................40
2.4. Solubility and Thermodynamics...............................................................................41
2.4.1.
The Lattice Model of Solutions ........................................................................42
vii
2.4.2.
Flory-Huggins Theory ......................................................................................44
2.5. Supercritical Fluids ..................................................................................................48
2.5.1.
The van der Waals Equation of State .............................................................52
Mixtures Containing Polymer Components................................................................54
Mixtures Containing Supercritical Components .........................................................55
Chapter 3. Methods ......................................................................................................56
3.1. High-Pressure System Design ................................................................................56
3.2. In-situ Transmission UV-vis Spectroscopy Setup ...................................................59
3.3. Gravimetric Analysis of Saturated Solutions ...........................................................60
3.4. Substrate Preparation for Thin Film Deposition ......................................................61
3.5. Spectroscopic Techniques ......................................................................................62
3.5.1.
Transmission Ultraviolet-visible Spectroscopy ...............................................62
3.5.2.
Raman Spectroscopy ......................................................................................63
3.6. Grazing-Incidence Wide-Angle X-ray Scattering ....................................................64
3.7. Microscopy Techniques for Morphology Analysis of Deposited Thin Film .............67
3.7.1.
Optical Microscopy ..........................................................................................68
3.7.2.
Polarized Optical Microscopy ..........................................................................68
3.8. Optical Lithography for Pattern Development .........................................................71
Chapter 4. Physical Supercritical Fluid Deposition of Semiconducting Polymers
on Curved and Flexible Surfaces.........................................................................75
4.1. Introduction ..............................................................................................................75
4.2. Results and Discussion ...........................................................................................77
4.2.1.
Study of PBTTT-C14 Solubility in Supercritical n-pentane ..............................77
4.2.2.
PBTTT-C14 Thin Film Growth in Supercritical n-pentane................................81
4.2.3.
Patterned Deposition of PBTTT-C14 on Multifarious Substrates ....................85
4.3. Conclusion ...............................................................................................................89
4.4. Methods ...................................................................................................................90
4.4.1.
Transmission UV-vis Spectroscopic Measurements ......................................90
4.4.2.
Substrate Preparation for Thin Film Deposition ..............................................91
4.4.3.
Thin Film Deposition Conditions .....................................................................91
4.4.4.
Gravimetric Analysis ........................................................................................91
4.4.5.
Pattern Development .......................................................................................92
4.4.6.
Characterization of Deposited Thin film ..........................................................92
Chapter 5. Physical Supercritical Fluid Deposition of Aliphatic Polymer Films:
Controlling the Crystallinity with Pressure ........................................................94
5.1. Introduction ..............................................................................................................94
5.2. Results and Discussion ...........................................................................................97
5.2.1.
Isotactic Polypropylene Solubility in Supercritical n-pentane .........................97
5.2.2.
Empirical Model for Polymer Solubility at Elevated Pressures .......................98
5.2.3.
Characterization of Deposited Thin Film .......................................................101
5.3. Conclusion .............................................................................................................107
5.4. Methods .................................................................................................................108
viii
5.4.1.
5.4.2.
5.4.3.
Solubility Measurement via Gravimetric Analysis .........................................108
Substrate Preparation for Thin Film Deposition and Deposition Condition ..108
Characterization of Deposited Thin Film .......................................................109
Chapter 6. Physical Supercritical Fluid Deposition of Aliphatic Polymer Films:
Controlling the Crystallinity with Solvent Additive .........................................110
6.1. Introduction ............................................................................................................110
6.2. Results and Discussions........................................................................................112
6.2.1.
Solubility of Isotactic Polypropylene in a Binary Solvent System .................112
6.2.2.
Polymer Solubility Model for a Binary Solvent System .................................114
6.2.3.
Characterization of iPP Thin Film..................................................................115
6.3. Conclusion .............................................................................................................121
6.4. Methods .................................................................................................................121
6.4.1.
Solubility Measurement via Gravimetric Analysis .........................................121
6.4.2.
Substrate Preparation for Thin Film Deposition and Deposition Condition ..122
6.4.3.
Characterization of Deposited Thin Film .......................................................122
6.4.4.
High-temperature Size Exclusion Chromatography .....................................122
Chapter 7. Conclusions and Future Directions .......................................................123
7.1. Conclusions ...........................................................................................................123
7.2. Future Directions....................................................................................................125
References .......................................................................................................................129
ix
List of Figures
Figure 1.1. The advancement of semiconducting polymer mobility. State-of-the-art
polymer mobilities have improved by over six orders of magnitude in the last
30 years. The asterisk indicates materials aligned using a special technique.
Reprinted with permission from reference 20.................................................. 2
Figure 1.2. Several deposition methods for growing thin films of organic semiconductors.
I. Vacuum deposition, II. Inkjet printing, III. Gravure printing, IV. Aerosol jet
printing, V. Spin-coating, VI. Spray-coating, VII. Slot casting, VIII. Doctor
blade. ................................................................................................................ 4
Figure 2.1. Schematic of energy-level splitting in alkenes with increasing conjugation
length and the resultant decrease in bandgap. ............................................... 9
Figure 2.2. Chemical structures of some of the commonly used semiconducting small
molecules and polymers: (a) Copper phthalocyanine, (b) 6,13bis(triisopropylsilylethinyl) pentacene or TIPS-pentacene, (c) Rubrene, (d)
Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] or PBTTT-C14,
(e) Poly(triaryl amine), Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] or
PTAA, (f) Poly(9,9-dioctylfluorene-alt-benzothiadiazole) or F8BT. ............... 10
Figure 2.3. Doping mechanisms in semiconducting polymers and their application.
Reprinted with permission from reference 54................................................ 11
Figure 2.4. To achieve n‐type doping in organics the dopant has to donate electrons to the
LUMO states while p‐type dopants obtain electrons from the HOMO states
and create holes............................................................................................. 12
Figure 2.5. Schematic of band structures in metal, semiconductor, and insulator. ......... 15
Figure 2.6. The reduced E-K diagram for a semiconducting material illustrating valence
and conduction bands. ................................................................................... 16
Figure 2.7. Molecular packing motifs in (a) tetracene with herringbone structure packing
motif and (b) rubrene with slipped-cofacial packing of the π-conjugated
tetracene backbones. All the hydrogen atoms are omitted for clarity. Adapted
with permission from reference 76. ............................................................... 18
Figure 2.8. A schematic representing electron transfer in a biased double quantum
well. λ is the reorganization energy, εij is the difference in minimum of the
potential energy wells. The Marcus hopping path is indicated by the solid-red
arrow............................................................................................................... 20
Figure 2.9. Schematic of (a) polaronic band model and (b) small-polaron hopping model.
The hopping process to the neighbouring site is dependent on the
reorganization energy (Λ) corresponding to the vertical transition energy
between the two potential energy curves and the electronic coupling between
the localized molecular orbitals (τ). Adapted with permission from reference
94.................................................................................................................... 21
Figure 2.10. Schematic representation of main transport mechanisms in organic
semiconductors with the temperature dependence of the mobility calculated
by different transport models. Adapted with permission from reference 109.
........................................................................................................................ 25
x
Figure 2.11. Charge transport processes and disorder at different length scales in a twodimensional sheet of edge-on regioregular P3HT. Reprinted with permission
from reference 114. ........................................................................................ 26
Figure 2.12. The specific relative arrangement of the side chains in a chain defines
different configurations. (a) Structures of possible couplings in the dimers of
3-alkylthiophene rings (H:head, T:tail). (b) Structures of possible regioisomeric
triads (HH–TT, HT–HH, HT–HT, and TT–HT), of which HT–HT is the
regioregular isomer and displays better solid-state packing of the polymer. (C)
The solid-state packing efficiency of HT-coupled P3HT. .............................. 28
Figure 2.13. P3HT films morphology as evidenced by AFM. (a) Crystalline rod-like
morphology of Mn = 3.2 kD and (b) nodule structure for Mn = 31 kD. (c). Plot of
field-effect mobility versus the number average molecular weight. Group A and
B refer to the bromine-terminated polymers modified by different routes and
group C is the methythiophene-terminated polymers. Reprinted with
permission from reference 134. Schematics are transport models in low and
high-Mw films. Charge carriers are trapped on nanorods (highlighted in yellow)
in the low Mw case, while long chains in high-Mw films bridge the ordered
regions and soften the boundaries (marked with red arrow). Reprinted with
permission from reference 135. ..................................................................... 30
Figure 2.14. AFM phase images of thin RR-P3HT films of different molecular weight
achieved by three different casting techniques. Adapted with permission from
reference 155. ................................................................................................ 32
Figure 2.15. Thin films of P(NDI2OD-T2) deposited by spincoating from solutions CN:CF
(a, b, c), DCB (d, e, f), and toluene (g, h, i). Cross-polarized optical microscope
images (a, d, g) and AFM topography images (b, c, e, f, h, i). Reprinted with
permission from reference 175. ..................................................................... 33
Figure 2.16. Molecular orientation of P3HT in thin layers with respect to the substrate
surface. (a) edge-on, (b) face-on, (c) end-on. Reprinted with permission from
reference 178. ................................................................................................ 34
Figure 2.17. Molecular orientation of P3HT on different surfaces. Tapping mode scanning
force microscope images of a regioregular P3HT ML_NH 2 film [(a) topography
and (a’) phase], and of a P3HT ML_CH3 film [(b) topography and (b’) phase].
Schematic representation of the different conformations [(c) edge-on and (c’)
face-on] according to interfacial characteristics. Adapted with permission from
reference 180. ................................................................................................ 35
Figure 2.18. The three regimes of flow demonstrating the transition from laminar flow at
low Reynolds number to turbulent flow at high Reynolds number. ............... 37
Figure 2.19. A case showing fluid being held between two flat, parallel plates. (a) At low
temperature gradients, the fluid is stable. (b) As the temperature gradient
increases, natural convection sets in the form of regular convection cell. (c) At
high ΔT, the fluid reach the state of turbulent convection. Schematic of
convection cells: (d) two-dimentional rolls and (e) hexagonal convection
cells; l and g indicates different ciculation dirrection. .................................... 40
Figure 2.20. Rayleigh-Bénard convection. (a) Schematic of the cyclical motion creating
Rayleigh-Bénard convection cells (Left drawing). A snapshot taken from a
movie based on data from a Rayleigh-Bénard convection simulation is
provided on the right side where red and blue indicate hot and cold fluid. Photo
credit: Physics of Fluids Group, University of Twente, 2018. (b) Time-lapse
xi
photograph of hexagonal Rayleigh-Bénard convective cells. Flow lines are
manifested by aluminum fakes and 10-second exposure. Photo credit: M. Van
Dyke, 1982, An Album of Fluid Motion. ......................................................... 41
Figure 2.21. A two-dimensional square-lattice example of the lattice model of solutions.
The filled circles portray solute molecules and open circles represent solvent
molecules. ...................................................................................................... 42
Figure 2.22. Different type of polymer phase diagram and miscibility gaps. The two-phase
region is denoted by “2” on the phase diagram. ............................................ 47
Figure 2.23. (a) The phase diagram for a typical pure substance. The red and blue points
correspond to the gas–liquid–solid triple point and critical point repectively. (b)
The transition of CO2 into supercritical phase. 1) Below the critical point with
two distinct phases. 2) As the temperature of the system increases, the liquid
starts to expand. 3) With further temperature increase, the two phases start to
become less distinct. 4) A new supercritical phase forms. 5) As the system is
cooled down, the reverse process initiates. 6) With further temperature
decrease, the phase separation into liquid and vapor starts to take place. .. 49
Figure 2.24. Phase diagram. (a) Classical fluid state plane demonstrating different
supercritical states structure. The critical isobar and isotherm lines are shown
by blue and red dotted lines respectively. (b) Revised fluid state plane with
coexistence and pseudoboiling lines. Adapted with permission from reference
191.................................................................................................................. 51
Figure 2.25. (a) Temperature-pressure phase diagrams of n-pentane (Tr : 196.45 °C, Pr :
33.25 atm) and toluene (Tr : 318.64 °C, Pr : 40.72 atm). (b). Densitytemperature phase diagrams of n-pentane (ρr : 0.273 g.ml-1) and toluene (ρr :
0.291 g.ml-1). Data retrieved from NIST Chemistry WebBook on October,
2020................................................................................................................ 52
Figure 2.26. Isotherms of the van der Waals equation of state for 3 different temperatures.
(b) Maxwell construction for van der Walls isotherm. The isobaric line is
constructed such that equal areas are found for I.II.III and III.V.VI. (c) The
phase diagram produced based on the Maxwell construction for different
temperatures. ................................................................................................. 54
Figure 3.1. Drawing of the high-pressure chamber and some of its components. Bolts,
heating elements, and mounting elements are excluded for clarity. ............. 57
Figure 3.2. Schematic of supercritical tabletop setup (top view) and its components: (1):
N2 cylinder, (2): solvent bottle, (3): Omega benchtop PID controller, (4):
pressure display connected to the pressure transducer, (5): manual pressure
generator, (6): cartridge heaters x4, (7): inlet valve, (8): outlet valve, (9): cold
solvent trap, (10): Lexan safety box, (11): vacuum hose. ............................. 58
Figure 3.3. Schematic of a section of supercritical tabletop setup (side view) and different
components used for the in-situ transmission UV-vis spectroscopy............. 60
Figure 3.4. Schematic of a section of supercritical tabletop setup (side view) for gravimetric
measurements. .............................................................................................. 61
Figure 3.5. The sample holder (a) front view and (b) backside view. The ITO coated glass
substrate (labeled 1) is secured between two polyether ether ketone (PEEK)
legs (labeled 6) and two copper legs (labeled 4) facing each other. Kapton
sheet is used as an O-ring to ensure the proper sealing (labeled 5). Also,
Kapton wings were used to prevent short circuit when the chamber is fully
xii
closed (labeled 7). The temperature of the substrate surface is monitored
using thermocouples (labeled 3) and the voltage is applied to the substrate
through copper wires (labeled 2). .................................................................. 62
Figure 3.6. The scattering processes that can occur when IR light interacts with a molecule
(left) and electronic states diagram of a molecule (right) illustrating the origin
of Rayleigh, Stokes and Anti-Stokes Raman Scattering. .............................. 64
Figure 3.7. Grazing incidence x-ray scattering geometry, at small and large angles
(GISAXS and GIWAXS). ................................................................................ 64
Figure 3.8. Schematic of film crystallinity and corresponding 2D GIWAXS data. (a) vertical
lamellar stacking, (b) crystallites with both vertical and horizontal orientation,
(c) oriented domains around the horizontal direction and (d) full rotational
disorder of crystallites. ................................................................................... 66
Figure 3.9. Schematic illustration of the face-on and edge-on configurations of P3HT
chains. ............................................................................................................ 66
Figure 3.10. GIWAXS patterns of P3HT with predominantly (a) face-on and (b) edge-on
orientation. Reprinted with permission from Dr. Kevin G. Yager, Brookhaven
National Laboratory, 2020. ............................................................................ 67
Figure 3.11. Different types of microscopes and their resolving power range. ............... 68
Figure 3.12. Comparison between an isotropic material (left) with only one refractive index
for all propagation directions and a birefringent material (right) that has two
different refractive indices, allowing two different oscillation directions for the
light: a fast and a slow direction, hence a double image. Motic Incorporation
Limited Copyright, 2002-2016........................................................................ 69
Figure 3.13. (a) A representation of the refractive indices in different directions using the
optical indicatrix for a birefringent material. (b) A uniaxial indicatrix where the
one optical axis is along the c axis—ellipsoidal indicatrix. (c) A schematic
showing an atom feeling different spring strength in different crystalline
direction. ......................................................................................................... 70
Figure 3.14. The schematic of polarized optical microscope (left) and the corresponding
optical path and different light components (right). ....................................... 71
Figure 3.15. Photolithography steps and subtractive pattern transfer. ............................ 73
Figure 3.16. Different types of reaction mechanism during plasma etching.................... 74
Figure 4.1. UV-vis spectral measurements for the chamber and its contents (solution) for
several temperatures and a single pressure. Reprinted with permission from
reference 253. ................................................................................................ 78
Figure 4.2. (a) Integrated UV-vis absorbance as a function of temperature for several
pressures. (b) Estimated center of main absorbance peak as a function of
temperature for several pressures. Reprinted with permission from reference
253.................................................................................................................. 79
Figure 4.3. Total integrated absorbance (Left axis, a.u.) and concentration (Right axis, mg
mL-1) as a function of temperature at 7.0 MPa. Reprinted with permission from
reference 253. ................................................................................................ 80
Figure 4.4. Optical microscope images of PBTTT-C14 films grown onto ITO substrate with
Twall ≈ 130 °C and Tsub ≈ 160 °C for 90 minutes under several pressure
conditions (100x magnification). Adapted with permission from reference 253.
........................................................................................................................ 82
xiii
Figure 4.5. Ex situ UV-vis absorbance spectra of the films shown in Figure 4.4. Reprinted
with permission from reference 253. ............................................................. 83
Figure 4.6. Raman spectra of PBTTT-C14 as received from the supplier and PBTTT-C14
thin film deposited on ITO coated glass after 90 minutes deposition at 17.2
MPa. Reprinted with permission from reference 253. ................................... 84
Figure 4.7. In situ UV-vis absorbance at 625 nm collected as a function of time, monitoring
film growth (Twall ≈ 130 °C and Tsubstrate ≈ 160 °C). Reprinted with permission
from reference 253. ........................................................................................ 85
Figure 4.8. Deposition of PBTTT-C14 on ITO coated glass after a 3 hrs deposition at 7.0
MPa. The side-view cartoon of patterned substrate is provided at the top of
each microscope image (The height scale is ~ 200 nm while the width scale
is ~ 200 μm). The corresponding (a) top-view optical microscope image of
patterned ITO and (b) deposited PBTTT-C14 lines on patterned ITO substrate.
Reprinted with permission from reference 253.............................................. 86
Figure 4.9. Deposition onto PMMA (PMMA film thickness = 165 ± 15 nm). Side-view
cartoon is drawn on top of each microscopy image. (a) Top-view optical
microscope image of patterned substrate and (b) Deposited PBTTT-C14
patterns with deposition condition of: time = 3 hours, P = 7.0 MPa. Reprinted
with permission from reference 253. ............................................................. 87
Figure 4.10. Deposition on PDMS hemisphere. (a) Cartoon of top and side view of object.
(b) Optical microscope image of PDMS with embedded nichrome wire. The
image was taken from an angle looking into the bowl of the hemisphere. (c)
Optical microscope image of PDMS hemisphere after deposition and removal
of nichrome wire (deposition time = 4 h, P = 7.0 MPa). The image has been
taken from the top on the flat surface of the hemisphere. Reprinted with
permission from reference 253. ..................................................................... 89
Figure 5.1. Isobaric concentration of iPP in n-pentane as a function of temperature.
Reprinted with permission from reference 338.............................................. 97
Figure 5.2. Isotherm concentration of iPP as a function of pressure. Reprinted with
permission from reference 338. ................................................................... 101
Figure 5.3. Polarized optical microscopy images of iPP films grown in supercritical npentane (x10) at different pressures. Adapted with permission from reference
338................................................................................................................ 102
Figure 5.4. GIWAXS patterns of iPP films grown in pressurized n-pentane at different
pressures (log scale) and their Azimuthally-integrated GIWAXS patterns.
Adapted with permission from reference 338. ............................................. 104
Figure 5.5. (a) GIWAXS partial pole figures of the iPP films grown in n-pentane at different
pressures. (b) Proposed model for the two preferred orientations of iPP chains.
Reprinted with permission from reference 338............................................ 105
Figure 5.6. Cartoon representing the change in the fluid temperature and flow during the
deposition process. Black lines indicate iPP chains, white lines describe
turbulent flow. Reprinted with permission from reference 338. ................... 107
Figure 6.1. Isobaric concentration of iPP in n-pentane:acetone as a function of
temperature at P = 10.3MPa........................................................................ 113
Figure 6.2. Polarized optical microscopy images of iPP films grown in supercritical npentane and n-pentane:acetone (x10) at different pressures. .................... 116
xiv
Figure 6.3. GIWAXS patterns of iPP films grown in pressurized n-pentane:acetone
solutions at different pressures. Intensity is plotted on a log scale. ............ 117
Figure 6.4. Azimuthally-integrated linecuts of the GIWAXS data for iPP films grown at
different pressures. (a) pure n-pentane. (b) n-pentane + 1% acetone. (c) npentane + 10% acetone. .............................................................................. 118
Figure 6.5. Cartoon representing the deposition mechanism in n-pentane:acetone. Black
lines indicate polymer chains, blue ovals indicate acetone solvent shell, false
color gradient represents local density of n-pentane. ................................. 120
Figure 6.6. Polarized optical microscopy image (x10) of iPP film grown in supercritical npentane at 10.3 MPa in the presence of 10% acetone. .............................. 120
Figure 7.1. AFM height images of pure PBTTT films deposited using spin-coating (a), slowdrying (b), drop-casting (c), and physical supercritical fluid deposition at 3.5
MPa (d). None of the PBTTT films presented here were thermally annealed.
The scale bar is the same for all the images. Images a, b, and c were reprinted
with permission from reference 380. ........................................................... 125
Figure 7.2. AFM height images of PBTTT-C14 films deposited in (a) supercritical n-pentane
and (b) n-pentane + 0.5% mol toluene at 3.5 MPa. .................................... 127
Figure 7.3. Schematic of a proposed sample holder design with an incorporation of a
PEEK mesh in front of the ITO substrate to reduce the fluid turbulence near
the substrate. ............................................................................................... 128
xv
List of Acronyms and Symbols
∇T
Temperature Gradient
2D
Two Dimensional
4D LABS
4D Laboratories
A
Absorbance
a.u.
Arbitrary Units
AFM
Atomic Force Microscopy
AWG
American Wire Gauge
BeCu
Beryllium Copper
BT
Benzothiadiazole
CCD
Charge-Coupled Device
CDT
Cyclopentadithiophene
CF
Chloroform
CFD
Computational Fluid Dynamics
CN
Chloronaphthalene
CTRW
Continuous Time Random Walk
DNA
Deoxyribonucleic Acid
DPP
Diketopyrrolopyrrole
e
Electric Charge
E
Electric Field
EMI
Electromagnetic Interference
EoS
Equation of State
ES
Efros-Shklovskii
ET
Mean Energy of The Trap States
ɛ
Energy Associated with the Intermolecular Forces
ɛ
Molar Absorptivity
F8BT
Poly(9,9-dioctylfluorene-alt-Benzothiadiazole)
FET
Field-Effect Transistor
G
Gibbs Free Energy
xvi
Gex
Excess Gibbs Free Energy
GIWAXS
Grazing-Incidence Wide-Angle X-Ray Scattering
GIXS
Grazing Incidence X-Ray Scattering
Gr
Grashof Number
H
Enthalpy
HL
Halogen Light
HMDS
Hexamethyldisilazane
HOMO
Highest Occupied Molecular Orbital
HXMA
Hard X-Ray Micro-Analysis
I
Light Intensity
IDT
Indacenodithiophene
iPP
Isotactic Polypropylene
ITO
Indium Tin Oxide
J
Current Density
K
Solubility Coefficient
k
Wavevector
L
Length
LAC
Library and Archives Canada
LC
Counter Length
LCST
Lower Critical Solution Temperature
LED
Light Emitting Diode
LP
Persistence Length
LUMO
Lowest Unoccupied Molecular Orbital
m*
Effective Mass
me
Electron Mass
ML
Monolayer
Mn
Number Average Molecular Weight
MOF
Metal−Organic Framework
MTR
Multiple Trap and Release
xvii
MW
Weight Average Molecular Weight
MWD
Molecular Weight Distribution
n
Refractive Index
NDI
Naphtalenediimide
NEXAFS
Near Edge X-Ray Absorption Fine Structure
NIST
National Institute of Standards and Technology
NLO
Nonlinear Optical
Ø
Lattice Volume Fraction
OECT
Organic Electrochemical Transistor
OFET
Organic Field-Effect Transistor
OPVC
Organic Photovoltaic Cell
OSC
Organic Semiconductors
P
Pressure
P(NDI2OD-T2)
Poly(([N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-Alt-5,5'-(2,2'-bithiophene))
P3HT
Poly(3-hexylthiophene)
PBTTT
Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2b]thiophene]
PC-SAFT
Perturbed Chain-Statistical Associating Fluid Theory
PDI
Polydispersity Index
PDMS
Poly(dimethylsiloxane)
PDTTT
Poly(2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene)
PEDOT
Poly(3,4-ethylenedioxythiophene)
PID
Proportional–Integral–Derivative
PMMA
Poly(methyl methacrylate)
POM
Polarized Optical Microscopy
Pr
Prandtl Number
Pr
Critical Pressure
PSS
Polystyrene Sulfonate
xviii
PTAA
Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]
PTI
Industrial Pressure Transducers
q
Scattering Vector
R
Differential Retardation
Ra
Rayleigh Number
Re
Reynolds Number
RFID
Radio-Frequency Identification
rhop
Average Hopping Length
RIE
Reactive Ion Etching
Rij
Hopping Space Range
RR
Regioregular
RRa
Regiorandom
S
Entropy
SAFT
Statistical Associating Fluid Theory
SAM
Self-Assembled Monolayers
SC
Supercritical
sccm
Standard Cubic Centimeter Per Minute
ScCO2
Supercritical Carbon Dioxide
SCF
Supercritical Fluid
SEM
Scanning Electron Microscopy
SFU
Simon Fraser University
Si
Silicon
SEC
Size Exclusion Chromatography
T
Temperature
TCB
Trichlorobenzene
TCB
Trichlorobenzene
TER
Total External Reflection
TIPS
6,13-bis(triisopropylsilylethinyl)
Tr
Critical Temperature
xix
UCST
Upper Critical Solution Temperature
UV-vis
Ultraviolet–Visible
vdW
Van Der Waals
VLE
Vapor-Liquid Equilibrium
VRH
Variable Range Hopping
X
Mole Fraction
Z
Compressibility Factor
α
Alpha
αc
Total External Reflection Angle
β
Beta
γ
Gamma
ΔF
Change in Helmholtz Energy
ΔG
Change in Gibbs Free Energy
ΔH
Change in Enthalpy
Δn
Birefringence
ΔP
Change in Pressure
ΔSC
Change in Configurational Entropy
ΔSP
Change in Entropy of Pure Solvent
ΔT
Temperature Gradient (K)
ΔU
Change in Internal Energy
ΔV
Change in Volume
λ
Wavelength
Λ
Transition Energy
μ
Charge Carrier Mobility
ν
Average Velocity
νd
Drift Velocity
ξ0
Localization Length at Zero Magnetic Field
π
(Pi) Bonding Orbital(s)
π*
(Pi-Star) Antibonding Orbital(s)
xx
ρ
Density
Ρ
Momentum
σ
Electrical Conductivity
τ
Scattering Time
χ
Interaction Parameter
ω
Acentric Factor
ѵ
Fluid Kinematic Viscosity
xxi
Chapter 1.
Introduction
1.1. Past, Present, and Future of Organic Semiconductors
The beginnings of the modern-day organic semiconductor started over more than
a century ago. Key discoveries contributing to its development include the first studies of
photoconductivity of anthracene crystals (early 20 th century) and the discovery of
electroluminescence in the 1960s. These early investigations on organic small molecules
helped scientists understand the fundamental processes involved in optical excitation and
charge carrier transport.1,2 While most of the prospective advantages of organic
electronics were proposed decades ago, organic electroluminescent diodes are the only
organic electronic application to have successfully entered the commercial market, whose
display market in smartphones (Samsung, Apple) and televisions (LG) will grow above
US$300 billion by 2025.3,4
In 1977, Shirakawa, MacDiarmid and Heeger reported the first use of an organic
polymer, polyacetylene, as a conducting material. This landmark discovery soon led to
several applications of polyacetylene-based materials including uses as organic
photoconductors, conductive coatings, and photoreceptors in electrophotography.5,6 The
field has since evolved from these first generation polymers toward more complex
polymers such as soluble poly(alkylthiophenes) and poly(2,5-di(thiophen-2-yl)thieno[3,2b]thiophene) (PDTTT). More recently, there has been significant focus on donor-acceptor
co-polymers
which
are
comprised
of
monomeric
building
blocks
based
on
indacenodithiophene (IDT), diketopyrrolopyrrole (DPP), naphtalenediimide (NDI),
cyclopentadithiophene (CDT), benzothiadiazole (BT), thiophene or isoindigo.7
Commercialization efforts for organic semiconductors are driven by their many
unique properties which distinguish them from crystalline materials (i.e. silicon or gallium
arsenide) and allows for the tuning of their physical mechanics. The seminal work on
undoped small-molecule organic semiconductors goes back to 1980s. Notably, during this
time, Tang demonstrated the use of an organic heterojunction consisting of p- and nconducting materials in a photovoltaic cell.8 Furthermore, Tsumura, Burroughes, and
Horowitz independently showed the successful fabrication of thin film transistors from
conjugated polymers and oligomers.9-11 An important breakthrough came from the
1
demonstration of high-performance electroluminescent diodes from molecular films 12 and
conjugated polymers.13 Subsequently, as a result of these discoveries,
the first
commercial products were developed incorporating organic light-emitting device displays
and lighting. Other applications of organic semiconductors are in organic photovoltaic cells
(OPVCs) and organic field-effect transistors (OFETs). While not destined to replace
silicon-based technologies, organic semiconductors promise the advent of flexible solar
cells, low-cost printed integrated circuits, and large area, flexible light sources and
displays.
The electronic performance of devices based on organic semiconductors depends
largely on the rate with which charge carriers move within the π-conjugated materials. The
charge carrier mobility of semiconducting polymers has been dramatically increased over
the years and has achieved performances exceeding that of amorphous silicon (>1 cm 2
V-1 s-1),14-18 reaching mobilities comparable with commercial metal–oxide transistors (10–
30 cm2 V-1 s-1) (Figure 1.1).19 This steady improvement in polymer mobility is primarily due
to significant advances in molecular design and processing techniques.
Figure 1.1. The advancement of semiconducting polymer mobility. State-of-the-art
polymer mobilities have improved by over six orders of magnitude in the last 30
years. The asterisk indicates materials aligned using a special technique. Reprinted
with permission from reference 20.
2
The commercialization of semiconducting polymers is very promising owing to
advancements in their electronic performance, combined with their low-temperature
solution processing, easy deposition, and mechanical flexibility.
Nonetheless, the
continuous growth of the field will be dependent on the simultaneous effort to design new
molecules, understand the fundamentals governing the structure-property relationships,
and explore new areas for commercialization. The development of synthetic
methodologies for semiconducting polymers has mainly focused on developing synthetic
routes that allow for control over chain length, chain length distribution, and regioregularity
– features that greatly affect intra- or inter-chain packing and hence charge transport.
Moreover, the synthetic approaches must be scalable and show reliable solution
processability while avoiding toxic reagents and by-products.
Additional progress has come from studies on charge transport physics focusing
on how chain conformations, doping, and ion incorporation can impact charge transport
characteristics of semiconducting polymers. For instance, transport studies have shed
light on how different polymer film morphologies demonstrate different optoelectronic
performance, thus suggesting new processing design strategies are critical for further
performance improvement.7,21 In recent years, semiconducting polymers have received
much attention (owing to their flexibility/stretchability, biocompatibility, and some cases
biodegradability) in bioelectronic applications such as biosensors, 22,23 actuators for drug
release,24,25 and neural photostimulation.26-28 The basic unit for the majority of these
devices is an organic electrochemical transistor (OECT) consisting of p-type poly(3hexylthiophene) (P3HT) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
(PEDOT:PSS) polymers.29,30 The continued development of these bioelectronics holds
great promise for next-generation medical technology.
1.2. Research Motivation
As mentioned in the previous section, advances in processing strategies play a
crucial role in continuing to push the performance further for the organic optoelectronic
devices. For this reason, more attention has been paid to further performance
improvements based on processing techniques and achieving optimal morphology.
Additionally, research efforts have became more focused on establishing new strategies
that do not merely enable the control of morphology, but also facilitate process scale-up
for large scale production.
3
The crucial building blocks in any electronic devices consist of thin films (100-300
nm) deposited on a substrate. Based on the application of the device, this layer can be
composed of either a single or a blend of two or more materials. In general, it is essential
for the device performance that the deposition is homogeneous over a large area and that
the layer thickness is well-controlled. There are significant differences in deposition
methods for the two classes of organic semiconductors (e.g. small molecules and
polymers) with their advantages and disadvantages. Figure 1.2 demonstrates some of the
methods which have been employed to form thin organic films.
I.
VIII.
II.
VII.
III.
VI.
IV.
V.
Figure 1.2. Several deposition methods for growing thin films of organic
semiconductors. I. Vacuum deposition, II. Inkjet printing, III. Gravure printing, IV.
Aerosol jet printing, V. Spin-coating, VI. Spray-coating, VII. Slot casting, VIII. Doctor
blade.
Small molecules are processed mainly via vacuum thermal evaporation and are
entirely solvent-free. The vacuum deposition offers a comparatively easy path for the
preparation of a well-defined films (high resolution) and high purity films for multi-layer
systems. In contrast, semiconducting polymers are mainly processed from solution since
they would otherwise decompose into smaller molecules during this high-temperature
sublimation process. There are two deposition categories for solution-soluble
4
semiconducting polymers: deposition of a soluble precursor from a solution and
subsequent conversion to the final film, and direct deposition from solution. Spin-coating
is one of the most popular examples of direct solution deposition, often used for polymers
such as poly(3-hexylthiophene) (P3HT).
As mentioned, the properties of the thin film are not merely dictated by the nature
of the constituent materials as the film nanostructure strongly affects device performance.
For instance, in thin film transistors, the morphology of the thin film plays a major role in
switching, processing, and transmitting electronic information. Wang et al.31 used three
deposition methods (spin-coating, drop-casting, and dip-coating) for the fabrication of
regioregular P3HT film in organic thin film transistor structures and discovered dip-coating
ultrathin films show significantly higher mobility due to the improved structural order of the
semiconducting molecules at the interface. In the same manner, the control over the
mesoscopic order in polymers can be carried out via different complementary approaches
such as post-deposition thermal annealing,32,33 solvent additive processing,34,35 strainaligning,36,37 and modification of the substrate.38-40 These strategies have been shown to
improve mobility while also leading to a higher degree of crystalline order observed in the
polymer thin films. Thus, the development of organic electronic devices goes hand in hand
with nano-engineering of organic thin films via manipulation of the supramolecular
organization.
Although spin-coating is the commonly used approach to fabricate high efficiency
organic thin films, it is not suitable for the large-scale deposition of organic semiconductors
in electronics. The main criteria for a process to be deemed efficient for electronics
requires moderate temperature, high rate, continuity, and a small number of simple
process steps. As a result, printing techniques such as flexographic, gravure, screen and
ink-jet printing have attracted much interest as promising technologies for the successful
realization of organic electronics. Despite the remarkable performance reported in
literature for organic-based integrated circuits,41 RFID-tags,42,43 and displays,44,45 the
adaptation of mass-printing methods for the fabrication of these electronic devices is
complicated and requires significant modifications in processes and materials.
5
1.3. Thesis Overview
Although there are several deposition techniques for the fabrication of thin film
organic semiconductors, due to specific limitation of each techniques, new solution
processing methods are actively being investigated. The motivation behind the research
comprising this thesis is to develop a novel deposition technique using supercritical fluids.
Supercritical fluids (SCFs) are exceptional solvents for triggering the precipitation of
organic molecules. For instance, ScCO2 has been utilized as an antisolvent to control the
precipitation of C60 crystals from the solution. Additionally, manipulating temperature and
pressure of SCFs can alter solvent properties drastically, making SCFs a great tool in
controlling the morphology of organic semiconductor films. The process discussed in this
thesis is related to a method for depositing metal films from organometallic precursors,
called cold wall deposition.
•
Chapter 2: The purpose of this chapter is to provide a set of relevant
background concepts for the research presented thereafter in this thesis. This
chapter mainly focuses on explaining the basic physical and electronic
properties of organic semiconductors and the theories describing the charge
transport in them. Furthermore, the thermodynamic of polymer solutions using
different models, supercritical fluid properties, and fluid dynamics are
reviewed.
•
Chapter 3: This chapter describes the experimental setup for film deposition
and solubility measurements. It also includes the relevant details regarding the
custom-made supercritical fluid chamber, in-situ UV-vis spectroscopy set up,
materials and other technical equipment used during research.
•
Chapter 4: This chapter discusses our initial steps towards developing a novel
deposition
technique
including
the
solubility
study
of
a
selected
semiconducting polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2b]thiophene] (PBTTT-C14), in n-pentane at different temperatures and
pressures. Additionally, the technological relevance of this technique is
exhibited by depositing finely patterned features onto flat, curved, and flexible
substrates.
6
•
Chapter 5: In this chapter, the self-assembly of isotactic polypropylene films
from supercritical n-pentane is investigated to gain a deeper understanding of
the polymer self-assembly in SCFs. Additionally, a simple thermodynamic
model is proposed to predict both the temperature and pressure dependency
of the polymer solubility. Further, the morphology of the deposited films is
investigated to establish a relationship with pressure.
•
Chapter 6: This chapter examines the effect of solvent additive on solubility
and self-assembly of polymers in supercritical fluids using isotactic
polypropylene in n-pentane:acetone solution as the model system. The
morphology of the deposited films in different solution systems is compared
and a film formation model is proposed that can be applied to self-assembly
of materials in other contexts.
•
Chapter 7: This chapter provides a summary of the key results presented in
this thesis and how these findings are relevant to further performance
improvements of the organic optoelectronic devices. In addition, this chapter
provides some future directions for further improvement of supercritical fluid
deposition technique as a sustainable processing technique and achieving
optimal morphology.
7
Chapter 2.
Background
2.1. Organic Semiconductors
Organic semiconductors (OSCs) are a class of molecules and polymers with solidstate properties typically associated with semiconducting materials. For example, their
lowest energy electronic absorption is in the visible or near infrared frequency range, they
are insulating in their undoped state, and can be used in a variety of devices like light
emitting diodes, field effect transistors, and solar cells.
Organic semiconductors have sp2-hybridization, giving rise to π (bonding) or π*
(anti-bonding) orbitals. As a result, the molecule planarizes and allows the π/π* state to
be delocalized over several carbon atoms. This process is known as conjugation and the
delocalization of electron density between neighbouring carbon atoms stabilizes this
electronic configuration in the (π) configuration.46 As the conjugation length increases, the
energy difference between the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) decreases. The energy gap between LUMO and
HOMO is analogous to the bandgap in a crystalline material such as silicon.47 Therefore,
such conjugated systems can act as semiconductors. For instance, the polymerization of
ethylene can reduce the band gap energy from 6.7 eV for the monomer to almost 1.5 eV
for stretch-oriented polyacetylene as shown in Figure 2.1.
8
Figure 2.1. Schematic of energy-level splitting in alkenes with increasing
conjugation length and the resultant decrease in bandgap.
Organic semiconductors can be divided into two different classes — small
molecules (molecular crystals or thin films) and polymers. As illustrated in Figure 2.2,
semiconducting small molecules are usually derivatives of flat, large, aromatic molecules
such as polyarenes. In the case of semiconducting polymers, the polymer backbone is
formed by a carbon chain with alternating single and double bonds or (hetero)aromatic
rings.48 Early examples of conjugated polymers include polyaniline, polythiophene, and
polyphenylene vinylene. However, these polymers suffer from poor solubility, making them
difficult to use in sophisticated devices. This challenge was addressed by the addition of
side chains which improved polymer solubility in organic solvents. 49 However, it is now
known that they can also play a significant role in polymer packing during thin film
processing by altering electronic interactions.50-52 The classic example of side chain impact
on polymer semiconductor properties is the regiorandom versus regioregular poly(3hexylthiophene) as will be discussed in more details in section 2.2.4.
9
a.
b.
c.
d.
e.
f.
Figure 2.2. Chemical structures of some of the commonly used semiconducting
small molecules and polymers: (a) Copper phthalocyanine, (b) 6,13bis(triisopropylsilylethinyl) pentacene or TIPS-pentacene, (c) Rubrene, (d) Poly[2,5bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] or PBTTT-C14, (e) Poly(triaryl
amine), Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] or PTAA, (f) Poly(9,9dioctylfluorene-alt-benzothiadiazole) or F8BT.
2.1.1. Doping of Semiconducting Polymers
Semiconducting polymers lack intrinsic charge carriers (holes, or electrons); thus,
charge carriers must be introduced into their system via doping in order to possess
electronic functionality.53 Different methods of doping of semiconducting polymers are
summarized in Figure 2.3. The two primary methods of doping semiconducting polymers
are chemical or electrochemical doping.
10
“Chemical”
Electrical conductivity
Conductivity approaching that of copper
Chemical doping induces solubility
Transparent electrodes, antistatics
EMI shielding, conducting fibers
“Electrochemical”
Control of electrochemical potential
Electrochemical batteries
Electrochromism and "Smart Windows"
Light-emitting electrochemical cells
Doping of
Conjugated
Polymers
“Interfacial”
Charge injection without counterions
“Photochemical”
Organic FET circuits
Tunneling injection in LEDs
High-performance optical materials
1-d Nonlinear optical phenomena
Photoinduced electron transfer
Photovoltaic devices
Tunable NLO properties
Figure 2.3. Doping mechanisms in semiconducting polymers and their application.
Reprinted with permission from reference 54.
One of the methods of doping in organic semiconductors is analogous to inorganic
materials and is illustrated in Figure 2.4. The generation of extra mobile charge is achieved
by the addition of electron donors or acceptors to the material, a process known as
chemical doping. In chemical doping, a polymer is exposed either to an oxidant (p-type
doping) or reductant (n-type doping) where a direct charge transfer initiates the doping
process.55-57 Here is the example of chemical p-type doping where iodine is used as an
oxidant and creat holes
(𝑆𝐶 𝑝𝑜𝑙𝑦𝑚𝑒𝑟)𝑛 +
3
2
𝑛𝑦 (𝐼2 ) → [(𝑆𝐶 𝑝𝑜𝑙𝑦𝑚𝑒𝑟 ) +𝑦 (𝐼3− )𝑦 ]𝑛
11
(2.1)
Vacuum Level
Energy
LUMO
HOMO
Matrix
Dopant
Matrix
Matrix
p-type Doping
Dopant
Matrix
n-type Doping
Figure 2.4. To achieve n‐type doping in organics the dopant has to donate electrons
to the LUMO states while p‐type dopants obtain electrons from the HOMO states
and create holes.
In contrast, the electrochemical doping process can take place in an electrolyte
solution where the polymer-coated working electrode is submerged, and the doping level
can be easily tuned by the applied voltage between electrodes. As a result of the created
electrical potential difference, charges (electrons or holes) are delivered to the conducting
polymer, while counter ions from the electrolyte diffuse into (or out of) the polymer. An
example of an electrochemical p-type doping is illustrated here where lithium
tetrafluoroborate is served as an electrolyte
(𝑆𝐶 𝑝𝑜𝑙𝑦𝑚𝑒𝑟)𝑛 + [𝐿𝑖 + (𝐵𝐹4−) ]𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 → [(𝑆𝐶 𝑝𝑜𝑙𝑦𝑚𝑒𝑟)+𝑦 (𝐵𝐹4− )𝑦 ]𝑛 + 𝐿𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒
(2.2)
In Eq. 2.2 lithium is shown as being reduced at the counter electrode to maintain
charge neutrality of the expression. This is not always necessary. The counter electrode
itself can become charged in a manner similar to a capacitor to effect charge neutrality.
Additionally, charge carriers can be created in semiconducting polymers by photon
absorption and charge separation of the photoexcited species.
In contrast to chemical
and electrochemical doping, where the induced electrical conductivity is permanent, the
conductivity following photoexcitation and charge separation lasts only until the excitations
are either trapped or decay back to the ground state.54
12
In the case of interfacial doping, the polymer is in contact with a metallic surface
and charge injection happens from the metallic contacts, at the interface, by either adding
electrons to the π* band or removing an electron from π band of the semiconducting
polymer based on the voltage biased applied. Although the semiconducting polymer is
oxidized or reduced during the process of interfacial doping, the doping mechanism is in
contrast with chemical and electrochemical doping since it does not involve the
introduction of any redox agent in the process. By introducing charges via an applied
voltage, the semiconducting polymer can be used as the active layer in thin film diodes58
and field effect transistors (FETs).10,59,60 The development of polymer based light-emitting
diodes (LEDs) are the result of dual carrier (electrons and holes) injection in metalpolymer-metal structures.61
2.2. Charge Transport Mechanism in Semiconductors
The electrical conductivity of a material is a measure of the amount of electric
current it can carry under the influence of an applied electric potential and is denoted by
the symbol (𝜎). The electrical conductivity is dependent on both external parameters such
as electric field strength and temperature, as well as intrinsic material properties and the
number of charge carriers. The conductivity can be expressed as the product of charge
carrier concentrations (𝑛), their electric charge (𝑞), and charge carrier mobility (𝜇):
𝜎 = 𝑛𝑞𝜇
(2.3)
As discussed previously charge carrier concentration in conjugated polymers can
be changed by a variety of means. The relationship of mobility to temperature, electric
field strengths, charge carrier density, molecular properties, and material properties such
as thin film morphology can be quite complex. To develop a molecular-level description of
carrier motion, it is common to express the mobility in terms of drift velocity, defined as the
average velocity (𝑣), and electric field strength (𝐸):
𝑣 = 𝜇𝐸
(2.4)
The Drude model is a simple model describing the factors that contribute to the
conductivity of charges in materials by applying Boltzmann kinetic theory. Essentially,
Drude model treats the free electrons in a metal like an ideal gas that move against a
background of heavy fixed ions and is scattered randomly by nuclei. Though the Drude
13
model is limited, it can provide some insight into the types of processes that influence
electrical conductivity. Based on Ohm’s law, a constant electric force will result in a
constant electrical current. On the other hand, electrons experience a force under the
application of a constant electric field and accelerate according to Newton’s laws.
Therefore, it is essential to consider some kind of resistance to electron movement to be
able to bridge these two theories. The Drude theory makes assumptions that electrons
scatter with the probability of
𝑑𝑡
𝜏
at the 𝑑𝑡 time interval and that electrons stop upon
⃑⃑⃑′ = 0). The average time between collisions is
scattering and return to zero momentum (𝑝
the scattering time (𝜏) and the distance travelled between collisions is known as the mean
free path (𝑙). After a collision, the electron velocity is randomly directed and therefore the
average velocity is zero. However, electrons can still respond to external forces in between
scattering events. The applied force can be due to applied electric (or magnetic) field that
couples to the electron’s charge, similar to the Lorentz force. The electron mobility can be
quantified based on the electron velocity before the collision takes place by setting the
Lorentz force equation equal to Newton’s equation of motion and integrating from zero to
the scattering time (𝜏).62-64
𝜇=
𝑞𝜏
(2.5)
𝑚𝑒
where 𝑚𝑒 is the electron mass. In the free electron gas model, an electron has the mass
of 𝑚𝑒 . Upon the application of an electric field, the average velocity of all electrons would
reach their maximum value (known as drift velocity 𝑣𝑑 ). As a result, the current density for
𝑛 mobile electrons per unit volume can be achieved by
𝐽 = 𝜎𝐸 = −𝑛𝑞𝑣𝑑
(2.6)
where 𝑛 is the density of electrons and 𝑞 is the electron charge. In order to make drift
velocity (𝑣𝑑 ) independent of applied electric field (𝐸), the conductivity (𝜎) can be defined
as a product of 𝜇 , where 𝜇 is the mobility equal to
𝑣𝑑
𝐸
.
2.2.1. Band Theory in Inorganic Semiconductors
In a single atom, electrons reside in distinct energy states in orbits and when many
atoms are brought together to form a solid, the energy states mix to form a continuous set
14
of states (energy bands). There are 3 general forms of band structures in solids that is
categorized solids into metals, semiconductors, and insulators. As shown in Figure 2.5,
the valence band is consists of fully occupied energy states, while the conduction band is
mainly unoccupied energy states. In metals, there is an overlap between the conduction
and valence bands, resulting in the conduction energy bands being to some degree filled
by electrons at any temperature, while the semiconductors and the insulators have fully
filled valence bands separated by the band gap from the empty conduction bands at T =
0 K.63 Band gap is the energy difference between the valence band maximum and
conduction band minimum. As illustrated in Figure 2.5, the band gap is much narrower in
semiconductors compared to insulators, allowing electrons that are thermally excited to
contribute to conductivity by moving above the Fermi level. Fermi level in band theory
represents the maximum energy state that electrons can populate at T = 0 K.
Conduction Band
Energy
Conduction Band
Conduction Band
Fermi Level
Valence Band
Valence Band
Valence Band
Metal
Semiconductor
Insulator
Band Gap
Figure 2.5. Schematic of band structures in metal, semiconductor, and insulator.
Figure 2.5 is an essential schematic for conceptualizing the band theory, however,
it lacks meaningful information when it comes to describing the optical and electrical
properties of metals and semiconductors. As a result, the real band structure in three
dimensions is illustrated by 𝐸(𝐸𝑛𝑒𝑟𝑔𝑦) − 𝑘 (𝑤𝑎𝑣𝑒𝑣𝑒𝑐𝑡𝑜𝑟) diagram (i.e. band structure
diagram) to describe the relationship between the energy and wavevector of quantum
mechanical states for electrons in the material. As shown in Figure 2.6, the 𝐸 − 𝑘 diagram
has a parabolic shape, either concave up or down, which is like the dispersion relation for
free electrons quantum mechanically, where 𝐸 is directly proportional to 𝑘 2 .
15
E
Band Gap
Conduction Band
k
Valence Band
Figure 2.6. The reduced E-K diagram for a semiconducting material illustrating
valence and conduction bands.
The electron in a periodic potential is accelerated relative to the lattice in an applied
electric field such that its mass is equal to an effective mass (denoted 𝑚∗ ).
𝐸=
ħ2 𝑘 2
(2.7)
2𝑚∗
Based on Eq. 2.7, the 𝑚∗ is not a fundamental constant and its value is dependent
on the shape of the band. Specifically, the effective mass depends on the steepness of
the parabola around the conduction band minimum or the valence band maximum.
𝑚∗ =
ħ2
(2.8)
𝜕2 𝐸
𝜕𝑘2
Using equation (2.8), the effective mass for an electron in the conduction band or
a hole in the valence band can be calculated.
In a perfectly crystalline network of atoms, such as silicon crystal, the charge
carriers can move as plane waves (Bloch waves) with very high velocity. In other words,
Bloch electrons possess wavefunctions delocalized over the entire crystalline lattice. 63,65
However, scattering of electron wavefunctions from phonons and defects in the crystal
can reduce the mean free path of a charge carrier, lowering its charge carrier velocity. 66
Since the number of phonons increases at higher temperatures, the mean drift velocity
16
decreases with increasing temperature.67 This is the hallmark of conduction via a band
mechanism.
2.2.2. Charge Transport Mechanisms in Organic Semiconductors
The basic solid-state properties of organic semiconductors differ considerably from
their inorganic counterparts. In organic semiconductors, every molecule is attracted to the
neighboring molecule via the weak van der Waals interaction. As a result of weak
intermolecular interaction in organic semiconductors, the charge carrier (electron and
hole) mobility values are typically much lower than observed mobilities in their inorganic
counterparts (typically less than 10 cm2 V-1 s-1, except for rubrene).47,68
Additionally, small changes in molecular packing can result in drastically different
electronic properties and mobilities in organic semiconductors. A classic example
exhibiting the impact of molecular packing is the comparison between tetracene and
rubrene. Figure 2.7 displays the chemical structures of tetracene and rubrene
accompanied with their molecular packing motifs. Rubrene chemical structure consists of
a tetracene backbone with four phenyl rings attached to the two central fused rings (Figure
2.7.b) and has hole mobility as high as 40 cm2 V-1 s-1, while tetracene mobility reaches
values of 2.4 cm2 V-1 s-1. 69,70 The origin of this outstanding transport property for rubrene
is due to the presence of conduction band structure.71-73 In rubrene, the strong
intermolecular electronic couplings due to the slipped-cofacial packing of the π-conjugated
tetracene backbones74,75 results in electronic bands whose dispersion behavior associates
closely with the band-like transport mechanism. In contrast, tetracene exhibits a flat band
structure, indicating a very large effective mass, meaning that the charges are localized.
This is an example of how molecular structure changes can impact the molecular packing
and ultimately the charge transport. Hence, in addition to the chemical structure, other
elements such as the relative positions of the molecules in the crystals can also readily
influence molecular packing and result in drastic changes in electronic properties. Based
on these, it is challenging to establish a general charge transport mechanism in organic
semiconductors by applying the conventional charge transport descriptions.
17
a.
b.
Figure 2.7. Molecular packing motifs in (a) tetracene with herringbone structure
packing motif and (b) rubrene with slipped-cofacial packing of the π-conjugated
tetracene backbones. All the hydrogen atoms are omitted for clarity. Adapted with
permission from reference 76.
Thus far, there is not a universally accepted, general model describing charge
transport in organic semiconductors. Nevertheless, a great deal is understood about the
nature of charge transport in organic semiconductors with the most common models are
band transport, polaronic transport, and disorder-based transport. The applicability of a
model to a particular material (crystals as well as molecular and polymeric materials)
depends on number of factors including the strength of electronic coupling, and the thin
film structure.
Band-Like Transport
As discussed previously, the large values of charge carrier mobility in highly
crystalline molecules such as rubrene77 and TIPS-pentacene78,79 indicates a transport
mechanism similar to inorganic semiconductors where the charges move as highly
delocalized plane waves in a conduction band with a mean free path that is larger than
the nearest neighbor distance.2 However, in contrast to the classical description of band
transport which implies charge carriers are delocalized over a large distance compared to
lattice spacing, the delocalization of electrons in organic semiconductors is limited to few
molecules and in fact is not spatially fully extended to Bloch electrons. 80 Thus, the bandlike mechanism occurs mainly in crystals featuring delocalized charge carriers.81 However,
not all molecules exhibit this behavior, and other mechanisms must be used for different
18
classes of organic semiconductors.82,83 The band-like transport mechanism is mainly
realized in highly ordered organic materials, whereas the more disordered the materials
are, the more relevant hopping transport becomes.
Marcus Charge Transfer
In semiconducting polymers, Marcus theory is often invoked as a rational for
charge transport. The basic Marcus theory is the proper model for redox reactions in polar
environments, where low-frequency rotational vibrations and other solvent modes are
treated classically. According to the Marcus theory, electron transfer can occur without
breaking or making any bonds, through a sequence of transfers (jumps) over energy
barriers. In Figure 2.8, the Marcus electron transfer path is indicated by a solid red line,
where 𝜆 corresponds to the reorganization energy. Hence, the rate of electron transfer in
Marcus theory can be expressed by the free energy barrier (∆𝐺) using the below equation
𝑘CT = 𝐴 exp [−
(∆𝐺+𝜆)2
4𝜆𝑘B 𝑇
]
(2.9)
where 𝐴 is proportional to the electronic coupling between the initial and final state of the
charge transfer. Based on Eq. 2.9, maximum charge transfer rate is achieved under
conditions of −∆𝐺 = 𝜆, when the transfer is barrierless, meaning the acceptor parabola
crosses the donor state energy minimum. However, the more common scenario is that
∆𝐺 = 0, making the reorganization energy the primary barrier to transport.
19
Energy
Nuclear Rearrangement
Figure 2.8. A schematic representing electron transfer in a biased double quantum
well. λ is the reorganization energy, εij is the difference in minimum of the potential
energy wells. The Marcus hopping path is indicated by the solid-red arrow.
Marcus theory was first used by Brédas et al. to describe the charge transport in
organic semiconductors.84 Later on, Heeger presented the fundamental principles for
charge transport in organic and polymeric semiconductors using Marcus theory.54 In the
case of semiconducting polymers, charge transport can be described as an electron
transfer (hop) from a charged oligomer to an adjacent neutral oligomer. In the hopping
regime, the fast charge transfer processes entail large transfer integrals and a weak
coupling of the charges to the vibrations of the oligomer backbones. Therefore, Marcus
charge transfer model is appropriate when the intermolecular transfer integrals are much
smaller than the charge reorganization energy. However, the calculated mobility values
are often underestimated in Marcus theory, especially when charge delocalization is
present. For instance, a theoretical hole mobility calculated by Marcus theory for
pentacene resulted in values ranging from 6 to 15 cm2 V-1 s-1,85 while the experimental
values for single crystal field-effect transistor reached mobilities of 15 to 40 cm2 V-1 s-1.86
Polaronic Transport
The term “polaron” was first introduced by Lev Landau to describe an electron that
is moving in a dielectric crystal where the atoms shift from equilibrium to screen the
electron charge.87 During the early years of research in organic materials, the notion of
polaron was adapted to describe the localization of charge carriers in molecular crystals
20
due to the interaction of charge carriers with the surrounding electrons and nuclei in the
lattice attributed.1 Depending on the interplay between the intra and intermolecular
electron–lattice interactions, one can describe transport mechanism in different
categories. For instance, in single-crystal pentacene, when the relative strength of the
localization energy (E1) is comparable to the electronic intermolecular interaction (J), the
transport moves away from hopping to a polaronic transport. Although, the polaron
becomes unstable for intermolecular interaction strength above 120 meV and transport
becomes band-like (typically when J>> E1).88
In the polaronic band model the delocalization of carrier wavefunction happens
over many molecules and propagates with a small deformation of the lattice (Figure 2.9.a).
In organic materials, the polaron movement is usually treated in terms of the small-polaron
model introduced by Holstein and Friedman, where the carrier wavefunction is localized
on one molecular site, as illustrated in Figure 2.9.b, in contrast to polaronic band
model.89,90 Later, the mechanism of the small-polaron hopping was verified by L. B. Schein
et al. based on their observation in molecularly doped polymers.91 The polaron transport
was also reported in molecularly doped polymers by Parris et al. via combining the
experimental observation and the Marcus rate model.92 Recent experiments carried out
by Blom et al. indicate that polaron hopping is an important mechanism in the conduction
of charges in polymer films.93
b.
Potential energy
a.
𝚲
𝝉
Reaction Coordinate
Figure 2.9. Schematic of (a) polaronic band model and (b) small-polaron hopping
model. The hopping process to the neighbouring site is dependent on the
reorganization energy (𝚲) corresponding to the vertical transition energy between
the two potential energy curves and the electronic coupling between the localized
molecular orbitals (𝝉). Adapted with permission from reference 94.
21
2.2.3. Disordered-Based Transport in Organic Semiconductors
Because organic semiconductor thin films are disordered, charge carrier transport
is often described in terms of disordered semiconductor theory. Much of the original theory
was developed to describe the transport of charges in lightly doped semiconductors, which
was later extended to polycrystalline and amorphous inorganic semiconductors. This
section will review a few of the most important charge transport mechanisms in disordered
semiconductor theory.
Nearest Neighbor Hopping
In disordered semiconductors, charge carrier transport at sufficiently low
temperatures becomes dominated by hopping mechanism between localized state
relatively close to the Fermi level. The probability distribution function of hopping is
dependent on both the spatial and energetic separation of hoping sites. The hop to the
nearest neighboring site is often the most dominant hop due to the exponential character
of the hopping probabilities:
𝑃𝑖𝑗 ≈ exp (−2𝛼𝑅𝑖𝑗 −
𝐸𝑗 −𝐸𝑖
𝑘B 𝑇
) , 𝐸𝑗 > 𝐸𝑖
(2.10)
𝑃𝑖𝑗 ≈ exp(−2𝛼𝑅𝑖𝑗 ) , 𝐸𝑗 ≤ 𝐸𝑖
(2.11)
where 𝑃𝑖𝑗 is the probability of a carrier tunneling from a localized state 𝑖 (𝐸𝑖 ) to an empty
state 𝑗 (𝐸𝑗 ), 𝛼 reflects the potential landscape surrounding the hopping sites, and 𝑅𝑖𝑗 is the
hopping space range. Hopping conduction follows the simple Arrhenius-like law as
𝜎 = 𝜎́ exp[−
∆𝐸 ~
∆𝐸
𝑘B 𝑇
]
(2.12)
1
(2.13)
𝑁𝐸𝐹 𝑎 3
where 𝜎́ is a constant, ∆𝐸 is the activation energy, 𝑘B is the Boltzmann constant, 𝑇 is the
absolute temperature, and 𝑎 is the distance between nearest neighbours.
Variable Range Hopping (Mott and Efros–Shklovskii)
Variable range hopping (VRH) has been one of the widely used models to describe
the charge transport in crystalline and non-crystalline materials including amorphous
22
conducting polymers.95-97 Mott noted the hopping conductivity can be expressed in terms
of the tunneling between localized wave functions and thermal activation
𝜎 ∝ exp[−
2𝑟hop
𝜉0
−
∆𝐸
𝑘B 𝑇
]
(2.14)
where 𝑟hop is the temperature-dependent average hopping length, 𝜉0 is the localization
length at zero magnetic field, ∆𝐸 is the width of the energy interval near the Fermi level
where hopping happens, and 𝑘B𝑇 is the thermal energy. The transport is dominated by
thermally activated nearest neighbor hopping in the high-temperature limit resulting in
Arrhenius-like conductivity. On the other hand, at lower temperatures, the hopping is more
energetically favorable to occur over distances that on average are larger than the
nearest-neighboring distance.98 As a result, the conductivity can be expressed based on
the Mott’s law of VRH
𝜎(𝑇) ∝ exp[−( 𝑇0 / 𝑇) 𝑆 ]
(2.15)
where 𝑆 is defined based on the system dimensions and is equal to 1⁄(𝑑 + 1). Another
VRH-based conduction model is the Efros-Shklovskii (ES) which accounts for a vanishing
density of localized electron states near the Fermi level as a result of electron-electron
interactions.99 This leads to the variable range hopping conductivity which redefines the
exponent 𝑆 in Equation (2.15) to a constant value of 0.5 regardless of dimensions.99,100 At
the extreme low temperature regime, the conduction occurs by field-assisted hoping,
where the average hopping length follows an electric field dependence, resulting in the
field-driven, highly non-linear, and temperature-independent transport. 101-103
Multiple Trap and Release (MTR)
The multiple trap and release (MTR) model was introduced to depict the effect of
shallow traps on the conductivity. Defects in a semiconducting material create localized
electronic states in the band gap. Among all the mid-gap states, shallow traps are the
most important one in the MTR model. There are two types of traps categorized based on
their position to the mobility edge: shallow and deep traps. The mobility edge is defined
as the energy level that separates the extended states from the localized ones in
disordered materials. The shallow traps are located within a few 𝑘B 𝑇 of the mobility edge
and can influence the charge transport. Thus, if a charge is trapped in one of the shallow
23
traps, it can be thermally activated and released to the band after a fixed trapping time
(𝜏𝑡𝑟 ) where it travels as a Bloch wave. Based on the MTR model, transport happens
through a sequence of trapping events and thermal releases. 104,105 The MTR model has
been successfully used to describe the temperature dependent and gate voltage
dependent transport of OFETs using the below equation106-108
𝜎(𝑇) ∝ 𝜎0 𝛼 exp[−
𝐸𝑇
𝑘B 𝑇
]
(2.16)
where 𝜎0 is the intrinsic trap-free conductivity, 𝛼 reflects the ratio between the effective
density of states near the transport band and the density of traps, and 𝐸𝑇 is the mean
energy of the trap state.
Figure 2.10 summarizes the band-like transport and the most common transport
models discussed for disordered semiconductors, accompanied with the calculated
mobility and temperature relationships predicted from these models. In these calculations,
only the essential factors for the mobility–temperature relationship were varied, while
keeping other parameters constant. In the band-like model (Figure 2.10.a), α corresponds
to the scattering factor, indicating that as the temperature increases, the mobility drops
sharper at lower scattering factor values. In the case of VRH model (Figure 2.10.b), T1 is
proportional to the size of the localized states and as it increases, the mobility decreases.
However, at higher T1 values, mobility becomes more temperature dependent. In the MTR
model (Figure 2.10.c), ET corresponds to varying mean energy of the trap states. The MTR
model result highlights the impact of ET on mobility to be more significant at lower
temperatures. Overall, the MTR model predicts a decrease in mobility with an increase in
energy of the trap states.
24
Bandlike Transport
Multiple Trap and Release
Ec
ET,1
ET,2
ET,3
E
E
Ec
E
Variable Range Hopping
Distance
Distance
Distance
Log DOS
∝
∝
(−
)
/
∝
(−
)
Figure 2.10. Schematic representation of main transport mechanisms in organic
semiconductors with the temperature dependence of the mobility calculated by
different transport models. Adapted with permission from reference 109.
2.2.4. The Influence of Microstructure on Charge Transport in
Semiconducting Polymers
In the previous section, the charge transport physics of organic semiconductors
and different transport mechanisms were discussed in detail. In this section, the attention
is focused on the structural parameters influencing the charge transport in organic
semiconductors.
Understanding
the
structure-property
relationship
in
organic
semiconductors is essential for charge carrier mobility enhancement by improving the
ability to control the microstructures.
The macroscopic electronic properties of organic semiconductors are not only
influenced by the molecular structure and the chemical nature, but also the solid-state
assembly of the components (polymers or molecules).110-112 Because the length scales of
polymer semiconductor devices are typically larger than the chain length the effective
transport of charges in a semiconductor device requires that the charges be able to move
25
both rapidly along the chain and easily pass from chain to chain. 113 Thus, charge transport
in conjugated polymers averages local properties, developing at the nanometer scale,
over distances orders of magnitude larger. This highlights how vital it is to investigate
processes across multiple length scales when studying charge transport in conjugated
polymers.
There are many different morphologies that semiconducting polymers can adopt.
The earliest systems were largely amorphous and exhibited low charge carrier mobilities.
Later with the advent of solubilizing side chains, polymers which adopted more crystalline
structure were created, most notably, regioregular P3HT. This system possesses
considerably high charge carrier mobility and as such has become an important
benchmark for discussing the relationship between thin film morphology and charge
transport. Figure 2.11 shows some important considerations for describing the thin-film
morphology of regioregular P3HT.
Intra-chain Transfer
Intra-chain
Transfer
Inter-chain
Transfer
Ordered Region
Disordered Region
Ordered Region
Figure 2.11. Charge transport processes and disorder at different length scales in a
two-dimensional sheet of edge-on regioregular P3HT. Reprinted with permission
from reference 114.
Semiconducting polymers, with semicrystalline nature, form thin films with ordered
regions (crystallites) distributed throughout a disordered (amorphous) matrix. As
illustrated in Figure 2.11, the charge transport in ordered regions of polymer thin films is
governed by an intra-chain pathway along the π-conjugated backbone and an inter-chain
pathway along the π-π overlaps between the face-to-face π-π stacked backbones.115,116
26
However, the charge transport can also take place between the adjacent ordered regions.
For example, long polymer chains, known as polymer tie chains, link crystallites in
proximity to each other.117,118 As expected, the mobility on the same chain is orders of
magnitude larger than that measured over macroscopic distances. 119 For example, the
mobility of charges along isolated ladder-type polymer chains is reported to be as high as
600
cm2 V-1 s-1 whereas the intra-chain mobility is found to be 30 cm2 V-1 s-1.120 Thus,
charge transport is limited by the most difficult (longest time scale) transport processes
and is therefore dominated by the transport between crystallites. 113 As mentioned
previously, transport between crystallites is mainly through the polymer tie chains taking
either a direct tie line path between two crystallites, or a path requiring inter-chain hopping.
Based on Monte Carlo simulations, the slow interchain hopping is predicted to be the
dominant path at low fraction crystallinity, whereas at higher fraction crystallinity, the onchain mobility limits transport.118
Given the significant impact of microstructure on charge transport in polymer
semiconductors, the processing condition is as influential parameter as the chemical
structure in the charge transport processes.121 The local packing of polymer chains can
dramatically change conduction pathways on longer length scales and result in different
mobility values. The challenge is that the local packing is highly dependent on different
parameters such as the monomer structure (regioregularity), polymer molecular weight
(Mw) and molecular weight distribution (MWD),122 the degree of alignment of polymer
backbones (chain rigidity),123 side-chain length,124 and processing conditions, making it
difficult to predict the morphologies on such multiple length scales.
The Role of Regioregularity
It has been demonstrated that the regioregularity (the percentage of head-to-tail
linkages between monomer units in the polymer backbone) 125 affects the kinetics and
thermodynamics of polymer self-assembly and consequently film morphologies. As shown
in Figure 2.12.a, when alkyl chains are introduced in the thiophene dimers, there are three
possible configurations based on the arrangement of alkyl chains: head-to-head (HH),
head-to-tail (HT), and tail-to-tail (TT). Subsequently, there are four different regioisomeric
thiophene triads (HH–TT, HT–HH, HT–HT, and TT–HT, Figure 2.12.b). As the number of
monomer units increases in the polymer chain, the side chains can be arranged in a
repeating pattern of (HT-HT)n and form a regioregular isomer (Figure 2.12.c). On the other
27
hand, in the regiorandom isomer, there is no established pattern for the arrangements of
side chains and the placement of side chains is arbitrary ( …HT-TH-HT-HT-TT-TH…).
a.
HH
HT
TT
b.
HH-TH
HH-TT
HT-HT
TT-HT
c.
Figure 2.12. The specific relative arrangement of the side chains in a chain defines
different configurations. (a) Structures of possible couplings in the dimers of 3alkylthiophene rings (H:head, T:tail). (b) Structures of possible regioisomeric triads
(HH–TT, HT–HH, HT–HT, and TT–HT), of which HT–HT is the regioregular isomer
and displays better solid-state packing of the polymer. (C) The solid-state packing
efficiency of HT-coupled P3HT.
In the case of P3HT, the regiorandom version has a twisted chain conformation
with poor packing and low crystallinity.126 On the other hand, the polymers in the
regioregular fashion possess planar conformation, assembling into more crystalline
28
structures with lamella packing and better π-stacking.126-128 When the alkyl chains are
introduced in the thiophene units in regioregular fashion, head-to-tail (Figure 2.12), the
alkyl chains trigger their interdigitation which in turn forces the polymer chains to assemble
into lamellae. The regioregular frame has shown to increase the charge carrier mobility
several order of magnitude due to the delocalization of polarons over several
molecules.129,130 Additionally, a comparison study showed that regioregular P3HT (>91%
of HT attachment) formed lamellae with edge-on orientation with respect to the substrate,
whereas less regioregular P3HT (~80%) exhibited face-on orientation and mobilities
several orders of magnitude lower.38,113
The Role of Molecular Weight
Molecular weight of a semiconducting polymer can have a profound effect on its
thin film structure and consequently, its charge carrier mobility. It is typically observed that
low molecular weight polymers have higher crystallinity than their high molecular weight
counterparts, with crystallinity being correlated with higher charge carrier mobility in the
case of regioregular vs. regiorandom P3HT. However, it has been reported for several
conjugated polymers that despite the morphology analysis showing higher degree of
crystallinity in low-MW polymer films, higher charge carrier mobilities is usually achieved
by higher MWs.131-133 This behavior is due to an increase of polymer chains content that
are long enough to bridge between crystalline domains and enable electrically connective
pathways (Figure 2.13).134-136 For instance, the mobility in P3HT-FETs increases from
10-5 to 0.1 cm2 V-1 s-1 within a relatively narrow range of molecular weights (2–50 kg.mol-1)
and then saturates. Furthermore, higher charge carrier mobility in high-Mw P3HT has
been attributed to the reduction of hopping events and ensuring good electrical
connectivity. This is due to the longer chains allowing charge carriers to travel longer
distances on a chain rather than hopping to another chain.137,138
29
a.
c.
b.
Figure 2.13. P3HT films morphology as evidenced by AFM. (a) Crystalline rod-like
morphology of Mn = 3.2 kD and (b) nodule structure for Mn = 31 kD. (c). Plot of fieldeffect mobility versus the number average molecular weight. Group A and B refer
to the bromine-terminated polymers modified by different routes and group C is the
methythiophene-terminated polymers. Reprinted with permission from reference
134. Schematics are transport models in low and high-Mw films. Charge carriers are
trapped on nanorods (highlighted in yellow) in the low Mw case, while long chains
in high-Mw films bridge the ordered regions and soften the boundaries (marked with
red arrow). Reprinted with permission from reference 135.
The Role of Chain Rigidity and Side-chain Unit
The rigidity of polymer chain is computed by its persistence length (L P), the length
of chain over which the chain orientation remains similar. Persistence length defines the
regimes across which theoretical treatments of chain conformations vary.139 The polymer
behaves like a flexible chain if the contour length (LC, the length of a chain at its maximum
physical extension) is significantly larger than LP and is considered a rigid rod when LC of
a chain is considerably smaller than LP. In the intermediate regime when LC is comparable
to LP, the semiflexible chain is described by a worm-like chain.139,140
Most semiconducting polymers are classified as rod-polymers due to their stiff and
rigid backbones.141,142 In particular, the fairly rigid thienothiophene units planarize the
backbone which enhances the π–π stacking and promote intra- and intermolecular order
and efficient charge transport.143 FETs based on PBTTT can reach mobilities of up to 0.72
cm2 V-1 s-1 after annealing into the mesophase, while high performance donor-acceptor
polymers has demonstrated mobilities as high as 3.6 cm2 V-1 s-1 due to their higher
persistence length.115,144,145 The higher persistence length also facilitate the connectivity
between polymer chains, which critically control structural parameters, including the
fraction and size of ordered regions and the distance between them.117,146
30
Due to the challenges correlated with the solubility and processability, often the
alkyl functionalities are required for very rigid and high MW polymers. However, the chain
rigidity of conjugated polymers is very sensitive to the addition of alkyl chains and
ultimately alters the microstructure of the material in the solid state.147 The side chains
separate the conjugated backbones and as a result the intermolecular overlap
decreases.148 Thus, the challenge is to find a side-chain chemistry that achieves polymer
solubility without any detrimental effect on packing and mobilities. 149,150 For instance, in
cyclopentadithiophene-benzothiadiazole copolymers, the π-stacking distance decreases
from 3.8 to 3.5 Å by moving the branch point further away from the conjugated backbone
and as a result the mobility values span from 10−4 to 0.41 cm2 V-1 s-1,reaching their highest
value when the branch point is furthest from the polymer backbone. 151
The Role of Processing Conditions
The control of thin film morphology is not only dependent on the intrinsic molecular
parameters to achieve a self-assembly of the polymers into a certain degree of
macroscopic ordered regions. But also, extrinsic parameters such as device architecture
optimization and material processing conditions strongly affect the morphology. 110 Indeed,
different processing techniques offer control over macroscopic self-assembly to the level
that power conversion efficiency,152-154 carrier mobility,35,155 and conductivity in polymer
electronic devices vary several orders of magnitude based on the morphology. 156,157
Processing parameters acting as the primary handles for greatly influencing
morphology of semiconducting polymers in thin films are solvent, 158-160 solvent
additives,153,161,162 deposition methods,113,155,163-165 postdeposition treatments, 166-168 and
surface treatments of the substrates.169,170
The impact of processing condition on thin film morphology is illustrated in Figure
2.14, which shows AFM images of P3HT films cast by three different methods under
identical conditions. In the case of dip-coating and spin-coating, chloroform was used as
the solvent and drop-cast films were obtained from a solvent mixture of chloroform and
tetrahydrofuran (14:3, v/v). While there are rod-like structures in spin-coated films from
low molecular weight samples, the films are almost featureless at high Mn. In comparison,
the slow solvent evaporation by dip-coating results in well-defined nanostructures that are
consistent in the wide range of molecular weight.155 Furthermore, the studies of charge
transport in ultrathin dip-coated films (thickness of a few nanometers) proved the impact
31
of such slow solvent evaporation by exhibiting mobilities as high as 0.2 cm2 V-1 s-1.31 In
P3HT spin-coated layers, the microcrystalline order can be significantly improved by using
high boiling point solvents such as trichlorobenzene (TCB), as evidenced by X-ray
diffraction and AFM.159 This also causes a significant enhancement of the charge carrier
Spin-coating
Drop-casting
Dip-coating
mobility compared to layers cast from chloroform.
Mn = 1.9 kDa
Mn =5.6 kDa
Mn =10.8 kDa
Mn = 27 kDa
Figure 2.14. AFM phase images of thin RR-P3HT films of different molecular weight
achieved by three different casting techniques. Adapted with permission from
reference 155.
It is intriguing that even the level of molecular pre-aggregation in solution prior to
deposition can impact the deposited film morphology. 171-173 For instance, thin films of
P(NDI2OD-T2) deposited via spin-coating using acetonitrile as the solvent do not reveal
any significant features when imaged using a polarized light microscope and also the
topography does not reveal any evident texture. In contrast, the same deposition using
dichlorobenzene results in 10–20 nm wide fibrillar-like supramolecular structures visible in
film topography images (Figure 2.15). Despite the strong directional coherence within
bundles (in the order of hundreds of nanometers long),174 there is no order present
between bundles which explains the lack of any features when imaged via optical
polarized microscope. The highest degree of aggregation in solution is prompted by
32
solvents such as toluene or mesitylene, exhibiting marked birefringence, with continuous
features extending to millimeters. Regarding the film topography, fibrils are observed
similar to DCB case, however directionality is preserved on much wider scales which is
the basis of the observed birefringence (Figure 2.15).
Figure 2.15. Thin films of P(NDI2OD-T2) deposited by spincoating from solutions
CN:CF (a, b, c), DCB (d, e, f), and toluene (g, h, i). Cross-polarized optical
microscope images (a, d, g) and AFM topography images (b, c, e, f, h, i). Reprinted
with permission from reference 175.
The link between charge transport and crystalline texture (the out-of-plane
orientation of crystallites with respect to the substrate) was first demonstrated in a TFT
showing three orders of magnitude higher mobilities because of P3HT films containing
edge-on oriented crystallites (Figure 2.16.a) rather than face-on textured crystallites
(Figure 2.16.b) by altering the regioregularity of P3HT and the casting method. 113
However, it was later proven that P3HT regioregularity in the 90–97% range affects the
33
overall crystallinity of the film but has little impact on the texture. 176 Further studies with Xray absorption near edge structure (NEXAFS) in highly regioregular P3HT films prove that
plane-on orientation is preferred at fast spin speed due to the rapid drying of the film. 177
Thus, it is likely that texture is controlled by a combination of regioregularity and filmformation kinetics.
a.
b.
c.
Figure 2.16. Molecular orientation of P3HT in thin layers with respect to the
substrate surface. (a) edge-on, (b) face-on, (c) end-on. Reprinted with permission
from reference 178.
Charge transport is extremely sensitive to the nature of the surface on which the
molecules self-assemble as it can significantly influence the film texture. For instance,
substrate
functionalization
with
self-assembled
monolayers
(SAMs)
such
as
hexamethyldisilazane, octadecyltrichlorosilane, and γ-aminopropyltriethoxysilane directs
the preferential polymer backbone orientation relative to the substrate and results in an
edge-on texture.122, 179-181 On the other hand, substrates treated with octyltrichlorosilane
leads to the formation of a face-on oriented P3HT monolayer (ML).180,181 As illustrated in
Figure 2.17, depending on the surface properties obtained by SAMs of different end-group
functionalization, the P3HT chains in the monolayer films can adopt two different
conformations (edge-on and face-on).
34
a
a'
b
b'
C’
C
Figure 2.17. Molecular orientation of P3HT on different surfaces. Tapping mode
scanning force microscope images of a regioregular P3HT ML_NH 2 film [(a)
topography and (a’) phase], and of a P3HT ML_CH 3 film [(b) topography and (b’)
phase]. Schematic representation of the different conformations [(c) edge-on and
(c’) face-on] according to interfacial characteristics. Adapted with permission from
reference 180.
To summarize, to achieve the maximum charge carrier mobilities of organic
semiconductors, it is essential to ensure that charges are able to move from molecule to
molecule (chain to chain) without experiencing scattering or electronic trapping. A broad
investigation of the conditions that influence charge carrier mobilities have been carried
out, indicating many factors such as molecular weight, regioregularity, and processing
conditions. In particular, the plethora of parameters that can be tailored using different
processing conditions offers a unique opportunity to control the macroscopic electronic
properties of organic semiconductors.
2.3. Fluid Mechanics
The self-assembly process that controls polymer film morphology occurs in
solution during film formation. This statement also holds true for polymer self-assembly in
supercritical fluids. As such, a rudimentary understanding of important concepts in fluid
mechanics is important to understanding physical supercritical fluid deposition.
Fluid mechanics describes the effects of forces and energy exerted on fluids
(liquids, gases, and plasmas) and is divided into fluid statics (fluid at rest) and fluid
dynamics (fluid in motion). Fluid mechanics is a key part of chemical engineering and is
used to design reactors and processes for large scale chemical manufacturing. In the
context of organic semiconductors, fluid mechanics governs the process of spin coating
35
and is an important consideration when designing inks for printed electronics. As will be
demonstrated later, the fluid mechanics operating in a supercritical fluid vessel can exert
an important influence on the self-assembly of thin films.
The fluid movement is described by the known laws of physics for mass,
momentum,
and
energy
conservation
as
embodied
by
the
Navier–Stokes
equations. However, solutions of the Navier–Stokes equations are quite difficult, and only
a small number of analytical solutions have been obtained and are typically limited to
reduced dimensions and smooth flow. In general, chaos and turbulence have prevented
the development of analytical solutions, and researchers frequently employ computational
fluid dynamics to design everything from airplanes to underwater bridge supports.
2.3.1. Laminar and Turbulent Flow
Determining whether flow is laminar or turbulent is a vital first step to understanding
the fluid dynamics at play in each context. Streamlines describe the path followed by a
parcel of fluid as it traverses the region of interest with closely spaced streamlines
associated with faster flow. Laminar flow is described by smooth streamlines, describing
well-defined paths of fluid movement. In addition, a laminar flow can be distinguished as
either steady or unsteady. The flow is considered steady when its properties (e.g.,
temperature, pressure, velocity, and density) at different points are independent of time.
In contrast, the flow parameters are a function of time in an unsteady flow. It is important
to note that steady flow does not necessarily mean fluid is flowing at a constant
acceleration or velocity as their properties might be a function of space. For instance, the
steady flow in a curved pipe experience acceleration through moving to a different spatial
position.
On the other hand, turbulent flow is described by irregular fluctuations of
streamlines and unstable flow. Intuitively, one expects smooth flow at low speeds. As the
flow speed increases, so does the propensity for turbulence, and eventually, the
streamlines become complicated, the flow becomes turbulent and instabilities occur
(Figure 2.18). In addition, smooth flow is favored in viscous liquids which resist the
influence of rapidly changing forces. The transition from laminar to turbulent flow is
dependent on other parameters like density, pressure, and temperature. These influences
can be combined into a dimensionless number called the Reynolds number (𝑅𝑒)
36
𝑅𝑒 =
𝜐𝐿
(2.17)
𝜈
where 𝜈 is the fluid kinematic viscosity (m2 s-1), 𝜐 is the flow speed (m s-1), and 𝐿 is the
length of the sheer layer (m).
Laminar
Transitional
Turbulent
Figure 2.18. The three regimes of flow demonstrating the transition from laminar
flow at low Reynolds number to turbulent flow at high Reynolds number.
2.3.2. Heat Transfer via Convection or Conduction
The research described in this thesis deals with heat transfer in supercritical fluids.
It is therefore useful to describe a few general aspects of heat transfer not typically
discussed within the traditional chemistry curriculum. Heat transfer is classified into three
main mechanisms: conduction, convection, and radiation, which will be discussed briefly
in this section.
Conduction (or diffusion)
In the absence of any material flow, conductive heat flow involves the transfer of
heat from one location to another by transferring molecular agitation, often in the form of
acoustic phonons for electrically insulating materials. The principle of thermal conduction
(Fourier’s law) connects the rate of heat transfer to the temperature gradient and the area
through which the heat flows. The differential form of Fourier’s law of thermal conduction
37
is provided in Equation (2.18), where q
⃑ is the local heat flux density (W m-2), 𝑘 is the
materials conductivity (W m-1 K-1), and ⃑∇𝑇 is the temperature gradient (K m-1).
𝑞 = −𝑘𝛻⃑ 𝑇
(2.18)
It should be noted that a material’s thermal conductivity changes with temperature.
However, for many materials the variation can be ignored over a range of useful
temperatures.
Convection
Convection is a heat transfer mode via the motion of fluid (matter) due to the
temperature (density) gradient within the material. These fluid motions are called
convection currents, and the convective movement of fluid persists until there is no
temperature gradient present between regions. If the convective heat transfer is a result
of buoyancy forces, it is considered natural convection. The buoyancy forces result from
density differences due to variations of temperature in the fluid. On the other hand, forced
convection involves driving the fluid transfer from one location to another by use of external
mechanical means, such as a pump or fan.
Radiation
Radiative heat transfer occurs when matter emits thermal energy via photons as
electromagnetic waves. In the absence of a medium, thermal radiation can be propagated
through a vacuum and its release rate is proportional to its temperature to the fourth power
(∝ T4). Temperature also affects the frequency and the wavelength of the radiated waves
that constitute an emission spectrum.
Table 2.1. Fundamental modes of heat transfer and their differences.
Conduction
Convection
Radiation
Occurs between media by direct
contact
Can occur in static object
Fourier’s law of thermal diffusion
Occurs within the
medium
requires fluid motion
Navier Stokes equation
Occurs through electromagnetic
waves
can occur across a vacuum
Blackbody radiation
Table 2.1 summarizes different modes of heat transfer and their properties. In a
situation where both conduction and convection mechanisms are feasible for heat transfer,
the dominant mode is predicted by the Rayleigh number.
38
2.3.3. Rayleigh Number
The Rayleigh number (𝑅𝑎) is a parameter that measures the relative importance
between the effects of the buoyancy forces and the effects of the viscosity forces and
thermal conduction. The interplay between the gravitational force pulling the cooler fluid
down and the viscous damping force in the fluid is expressed by a non-dimensional factor,
Rayleigh number, specified as
𝑅𝑎 =
𝑔𝛽∆𝑇𝐿3
𝜈𝛼
= 𝐺𝑟 × 𝑃𝑟
(2.19)
where 𝑔 is acceleration due to gravity, 𝛽 is coefficient of thermal expansion of the fluid, ∆𝑇
is temperature difference, 𝐿 is length, ν is kinematic viscosity and 𝛼 is thermal diffusivity
of the fluid. The Grashof Number (𝐺𝑟) represents the ratio of the buoyancy force due to
the spatial variation in fluid density to viscous force acting on a fluid in the boundary layer.
The Prandtl Number (𝑃𝑟) describes the relationship between momentum diffusivity and
thermal diffusivity.
The magnitude of the Rayleigh number can be used for both characterizing the
fluid’s flow regime and predicting convectional instabilities. As shown in Figure 2.19, the
turbulent flow regime corresponds to the higher range for the Rayleigh number (Figure
2.19.c), while the lower range Rayleigh number is related to laminar flow (Figure 2.19.b).
Below a certain value of Rayleigh number, there is no motion in the fluid and heat transfer
is purely via conduction (Figure 2.19.a) and when it passes a critical value heat transfer
happens by natural convection. There is typically no convective motion at values lower
than the critical value of Rayleigh number, Ra < 1700.182
Additionally, the Rayleigh number can be used as a criterion to predict
convectional instabilities and convection cell appearance. In the case of spatially periodic
flow, commonly observed at lower values of Rayleigh number, the convection cells look
identical and closely pack. Most frequently, the idealised convection cells are categorized
into two-dimensional rolls (Figure 2.19.d), square cells (superpositions of two sets of
mutually perpendicular rolls), and hexagonal cells (Figure 2.19.e, superpositions of three
sets of rolls rotated by an angle of 2π/3 to one another). However, as the Rayleigh number
value increases and the turbulent convection becomes dominant, the thermal boundary
layer thickness decreases linearly and results in a sharp drop in temperature near the
boundary layers.
39
c.
b.
a.
Cold T = T0
No motion
Hot T = T0 + ΔT
(Low ΔT)
Bénard cells
d.
Turbulent convection
(High ΔT)
e.
Figure 2.19. A case showing fluid being held between two flat, parallel plates. (a) At
low temperature gradients, the fluid is stable. (b) As the temperature gradient
increases, natural convection sets in the form of regular convection cell. (c) At high
ΔT, the fluid reach the state of turbulent convection. Schematic of convection cells:
(d) two-dimentional rolls and (e) hexagonal convection cells; l and g indicates
different ciculation dirrection.
2.3.4. Rayleigh-Bénard convection
Rayleigh-Bénard convection is driven by a non-uniform temperature distribution
across the horizontally extended boundaries of a fluid's layer. This induces a turbulent
convective fluid motion at high temperature gradients (Ra > critical value), creating
convection in alternating patterns of upward and downward motion.183 If the temperature
gradient in the sealed container exceeds a threshold value, a pattern of convention cells
starts to appear. This is based on the buoyant forces pushing the less-dense fluid up
towards the cooler end of the container while the cooler and denser fluid at the top sinks
and displaces the warmer fluid (Figure 2.20.a). Due to the continuous density gradient,
the resultant cyclical motion creates a honeycomb pattern called Bénard convection cells
(Figure 2.19.b).184 Rayleigh-Bénard convection is considered a prototype for nonlinear
systems exhibiting pattern forming in chaotic dynamics and fully developed turbulence.185
However, when the Rayleigh number increases several times the critical value, these
patterns become unstable, oscillatory patterns arise, and convection becomes chaotic.
40
a.
b.
Figure 2.20. Rayleigh-Bénard convection. (a) Schematic of the cyclical motion
creating Rayleigh-Bénard convection cells (Left drawing). A snapshot taken from a
movie based on data from a Rayleigh-Bénard convection simulation is provided on
the right side where red and blue indicate hot and cold fluid. Photo credit: Physics
of Fluids Group, University of Twente, 2018. (b) Time-lapse photograph of
hexagonal Rayleigh-Bénard convective cells. Flow lines are manifested by
aluminum fakes and 10-second exposure. Photo credit: M. Van Dyke, 1982, An
Album of Fluid Motion.
2.4. Solubility and Thermodynamics
The method of depositing polymers from supercritical solvents relies on the
saturation concentration of polymer solutions. It is therefore vital to understand solvation
thermodynamics. The discussion will begin with a general statement of the process in
terms of the Gibbs free energy before proceeding to models of liquid-liquid mixtures based
on statistical thermodynamics. With the basic terminology outlined, a discussion of FloryHuggins theory of polymer solutions will then take place.
The spontaneity of a chemical process at constant pressure is governed by the
change in the Gibbs free energy (∆𝐺), with negative values of ∆𝐺 indicating a
41
thermodynamically favorable process. The free energy of mixing is thermodynamically
favored at constant temperature and pressure if the Gibbs free energy change is negative.
In Equation (2.20), entropy and enthalpy of mixing are denoted by ∆𝑆𝑚 and
∆𝐻𝑚 respectively.
∆𝐺𝑚 = ∆𝐻𝑚 − 𝑇∆𝑆𝑚
(2.20)
2.4.1. The Lattice Model of Solutions
Liquid-liquid solutions are a convenient starting place for describing mixing
thermodynamics, as liquids are well approximated as incompressible and possessing
short-range order. The lattice model of solutions assumes that both solvent and solute
occupy similar volumes with molecules being randomly packed together in a regular lattice
(Figure 2.21). While the molecules are presumed to be positioned in a box, they can still
move by swapping places. If 𝑁1 is the number of solvent molecules and 𝑁2 is the number
of solute molecules, for the simple square lattice shown in Figure 2.21, the lattice would
contain 𝑁0 = 𝑁1 + 𝑁2 lattice sites.
Figure 2.21. A two-dimensional square-lattice example of the lattice model of
solutions. The filled circles portray solute molecules and open circles represent
solvent molecules.
Since it is considered that the lattice does not change size upon solute introduction,
there can be neither a change in pressure nor a change in volume, meaning that pressurevolume work is zero, making enthalpy and internal energy equal. This has the additional
consequence of making the Gibbs free energy equal to the Helmholtz energy in this model.
42
The internal energy of mixing can be approximated by the van Laar equation which
parameterizes the intermolecular interactions between the two solution components. This
provides an accurate estimate even for solutions containing polymers where the size of
solvent and solute particles is not comparable.
∆𝑈𝑚 = 𝑧𝑁1 𝜙2 ∆𝜀
(2.21)
1
∆𝜀 ≈ 𝜀12 − (𝜀11 + 𝜀22 )
(2.22)
2
In Equation 2.21, 𝑧 is the coordination number of the lattice, for example the
number of nearest neighbor lattice sites, and 𝜙2 is the volume fraction of the solute. In
Equation 2.22, the parameter 𝜀11 is the energy associated with the intermolecular forces
between nearest neighbor solvent molecules, 𝜀22 is the energy associated with the forces
between nearest neighbor solute molecules, and 𝜀12 is the energy associated with the
forces between a solute-solvent pair which are nearest neighbors to each other. Typically,
𝜀11 and 𝜀22 represent a more stable interaction than 𝜀12 , making the internal energy almost
always a positive contribution to the Helmholtz energy of mixing. In other words, the
internal energy typically does not favor mixing.
The entropy of a solution can be readily calculated in terms of the number of
possible microstates using the Boltzmann equation. Assuming N0 site lattice containing
N1 molecules of solvent and N2 molecules of solute, the entropy can be written as
𝑆 = 𝑘B (ln 𝑁0 ! − ln 𝑁1 ! − ln 𝑁2!)
(2.23)
Before mixing, the system only possesses a single microstate, making the entropy
of the original segregated arrangement zero and therefore ∆𝑆𝑚 = 𝑆𝑐 , where 𝑆𝑐 is
configurational entropy. Using Stirling’s approximation for factorials and the relationship
of 𝑁0 = 𝑁1 + 𝑁2 , the solution entropy can be rewritten as
∆𝑆𝑚 = − 𝑘B (𝑁1 ln 𝑋1 + 𝑁2 ln 𝑋2)
(2.24)
In the above equation, 𝑋1 and 𝑋2 are the mole fractions of solvent and solute,
respectively. By rewriting the solution entropy based on their relative concentrations,
Equation 2.24 can be expressed as
43
∆𝑆𝑚
𝑁𝑘B
= −𝑥 ln 𝑥 − (1 − 𝑥) ln(1 − 𝑥)
(2.25)
where 𝑥 = 𝑁1 /𝑁 and 1 − 𝑥 = 𝑁2 /𝑁. In an ideal solution the ∆𝑆𝑚 is always positive and
∆𝑈𝑚 = 0 , as result the ∆𝐹𝑚 has a negative value and favors mixing. Therefore, ∆𝐹𝑚 for an
ideal solution is
∆𝐹𝑚
𝑁𝑘B𝑇
= −𝑥 ln 𝑥 − (1 − 𝑥) ln(1 − 𝑥)
In contrast to ideal solutions,
(2.26)
∆𝑈𝑚 = 0 does not hold anymore for regular
solutions, and the mixing involves the energies of solution. The model describing regular
solution was first introduced by Hildebrand, showing that beside the entropy of solution
driving the solvation, regular solutions are driven also by the energy of the mean-field
form. However, there are still systems that are even more complicated than regular
solution model, for instance polymer solutions where solute and solvent are not
comparable in size.
2.4.2. Flory-Huggins Theory
Flory-Huggins Theory applies the lattice model of liquid-liquid mixtures to polymer
solutions, providing a good estimate of the free energy of polymer solutions mixing and is
based on the lattice approach with a mean-field estimate. The system is defined similar to
a lattice consisting of 𝑁 sites with equal volume 𝑣0. In dealing with polymers, the degree
of polymerization (𝑥𝑖 ) should be considered when calculating the volume fraction.
However, the assumption is made that the monomer volume (𝑣𝑖 ) is equal to the volume of
a lattice site (𝑣0) and therefore the total volume for species 𝑖 (𝑉𝑖 ) is given as
𝑉𝑖 = 𝑛𝑖 𝑥𝑖 𝑣0
(2.27)
where 𝑛𝑖 is the number of molecules. Based on this, the lattice volume fraction (𝜙i )
occupied either by a solvent molecule or polymer can be defined as
𝜙i =
𝑛𝑖𝑥𝑖
𝑛1 𝑥1 + 𝑛2 𝑥2
=
𝑛𝑖 𝑥𝑖
(2.28)
𝑁
In the case of ideal polymer chains, the change in the conformational entropy of
polymer chains is insignificant when transitioning from free polymers in solid form to
44
polymers in solution. Hence, the entropy associated with mixing polymers should be
mainly from translational motion. The entire chain is considered a large molecule that
gains more translational entropy by having more available locations to place its center of
mass, just as in ideal solutions. Consequently, the entropy per molecule can be written as
𝑆𝑖 = 𝑘B ln 𝑉𝑖
(2.29)
Therefore, the change in the entropy of mixing can be written as
∆𝑆𝑚 = (𝑛1𝑆1 ) 𝑀 + (𝑛2𝑆2 )𝑀 − (𝑛1 𝑆1 )𝑃 − (𝑛2 𝑆2 )𝑃
(2.30)
where “𝑃” and “𝑀” denote pure and mixed states, respectively. The rearrangement of
Equation (2.30) results in a similar form of entropy discussed for the regular solution theory
in the previous section. The only difference is that mole fraction is replaced by volume
fraction (𝜙) and the degree of polymerization is in the pre-factor as shown below:
∆𝑆𝑚
𝑁
𝜙1
= −𝑘B [
𝑥1
ln 𝜙1 +
𝜙2
𝑥2
ln 𝜙2 ]
(2.31)
The enthalpy of mixing for polymers can either be positive (disfavoring mixing) or
negative, favoring mixing, the less common scenario. The energy of polymer mixing can
be derived the same way as in the regular solution theory, taking into account for all
interactions between solvent-solvent (𝜀11 ), monomer-monomer (𝜀22 ), and monomersolvent (𝜀12 ):
1
∆𝜀 = 𝜀12 − (𝜀11 + 𝜀22 )
(2.32)
2
Upon mixing, the probability of adjacent site being occupied by either monomer or
solvent can be used to calculate the energy of the mixed state:
∆𝑈𝑚 =
𝑁1 𝑁2
𝑁0
𝑧∆𝜀
(2.33)
where 𝑧 is the number of neighbours in the lattice site. The energy of mixing per lattice
site can be rewritten by substituting the mole fractions with volume fractions 𝜙1 and 𝜙2
Δ𝑈𝑚
𝑁𝑘B𝑇
= 𝜒𝜙1 𝜙2
(2.34)
45
The 𝜒 parameter is the interaction parameter derived by using a mean-field
approximation and is defined as
𝜒=
𝑧
𝑘B 𝑇
(𝜀12 −
𝜀11 +𝜀22
2
)
(2.35)
where the 𝜀 corresponds to the molecular interactions between monomers and solvent
molecules. The combination of Equations (2.31) and (2.34) results in the Helmholtz free
energy of mixing for polymer solutions
∆𝐹𝑚 = ∆𝑈𝑚 − 𝑇∆𝑆𝑚
Δ𝐹𝑚
𝑁𝑘B𝑇
= 𝜒𝜙1 𝜙2 +
𝜙1
𝑥1
(2.36)
ln 𝜙1 +
𝜙2
𝑥2
ln 𝜙2
(2.37)
In the case of solvent-polymer mixture (𝑥1 = 1 and 𝑥2 = large) and polymerpolymer mixture (𝑥1 = large and 𝑥2 = large), the asymmetric free energies are dictated
by the degrees of polymerization. Additionally, the favorable entropy of mixing decreases
as the degree of polymerization of either or both species increases.
The derived Flory-Huggins Theory for the Gibbs free energy of polymer solution
mixing has shown that 𝜒 is inversely proportional to temperature and thus the energy of
mixing will vary with temperature. This indicates that polymer solubility can change
significantly over small ranges of temperatures. To calculate the miscibility/phase
separation conditions for a polymer solution, a phase diagram can be constructed. To
construct the phase diagram, the Gibbs energy of the homogeneous solution with the
Gibbs energy of two coexisting phases should be compared. Rearranging the FloryHuggins Equation (2.37), Δ𝐹𝑚 can be rewritten as
Δ𝐹𝑚
𝑁𝑘B 𝑇
=
𝜙𝐴
𝑁A
ln 𝜙𝐴 +
𝜙𝐵
𝑁B
ln 𝜙𝐵 + 𝜒𝜙𝐴 𝜙𝐵
(2.38)
where the term 𝑁 denotes the degree of polymerization of the species and Δ𝐹𝑚 is the free
energy of mixing per site. The first two terms in Equation (2.38) represent the ideal or
combinatorial part of the entropy of mixing. Although the combinatorial entropy of mixing
is much smaller for polymers than for low molar mass compounds, the entropy still favors
mixing. However, the third term in Equation (2.38) is related to the non-zero enthalpy of
46
mixing and promotes phase separation. Based on Equation (2.35), the temperature
dependency of the 𝜒 parameter can be parameterized as
𝜒(𝑇) = α +
𝛽
(2.39)
𝑇
This approach implies that 𝜒 ∝
1
𝑇
, if the influence of α is considered insignificant,
and hence 𝜒 −1 can be considered as an analog of T. The temperature dependency of the
interaction parameter results in different type of polymer phase diagrams as illustrated in
Figure 2.22. The temperature-composition diagram displayed in Figure 2.22 show the
binodal curves (or coexistence curves) separating the one-phase region from the twophase region. In Figure 2.22.a, the upper critical solution temperature (UCST)
corresponds to a temperature above which the mixture components are miscible in all
proportions and below which it will phase separate. It is the interactions between the
components that causes an enthalpically driven (∆𝐻𝑚 > 0) phase separation. In contrast
to UCST, the lower critical solution temperature (LCST) behavior is observed when there
is an entropic driving force for phase separation. As shown in Figure 2.22.b, above the
lower critical solution temperature, the solution separates into two phases and below it
exists as one homogeneous phase. Additionally, it is possible for a polymer solution
system to have both UCST and LCST (Figure 2.22.c and d) or neither (Figure 2.22.e). For
instance, in Figure 2.22.c, the system is miscible in all proportion for temperatures
between the LCST and the UCST but becomes thermodynamically unstable at higher and
lower temperatures.
Temperature
a.
0
c.
b.
2
d.
2
UCST
2
LCST
2
Composition
e.
2
2
2
1
Figure 2.22. Different type of polymer phase diagram and miscibility gaps. The twophase region is denoted by “2” on the phase diagram.
47
2.5. Supercritical Fluids
The physical supercritical fluid deposition technique discussed for polymer films
relies on the distinctive properties of the supercritical fluids and the ability to tune their
solvation dynamics by changing solvent temperature and pressure. Given the controversy
over the terminology of “supercritical fluids”, this section starts with the common
description of supercritical fluid, following by the discussion regarding what should be
considered a supercritical fluid based on the phase diagram. Subsequently, the discussion
goes into whether the van der Waals equation of state is reliable in predicting the phase
behavior in mixtures containing polymer and supercritical components.
Traditionally, a supercritical fluid is characterized as a substance above its critical
temperature (Tr) and critical pressure (Pr). In the supercritical region, the matter does not
exhibit characteristics of either gas or liquid phase and becomes a fluid with liquid-like
densities and gas-like viscosities. This behavior was first observed in 1822 by French
engineer and physicist, Charles Cagniard de La Tour, in his well-known cannon barrel
experiment. He heated different substances, both liquid and vapour, in a sealed canon
and continuously shook the container and listened to the sound of a rolling flint ball until
at a certain temperature he observed that the sound stopped. Later, he was able to
observe this phenomenon in a glass apparatus and witness the single supercritical fluid
phase. Afterwards, Irish chemist Thomas Andrews came up with the name “supercritical
fluid” for this phenomenan.186
48
a.
Pressure
Liquid
Solid
Vapour
Temperature
b.
1
2
3
4
5
6
Figure 2.23. (a) The phase diagram for a typical pure substance. The red and blue
points correspond to the gas–liquid–solid triple point and critical point repectively.
(b) The transition of CO2 into supercritical phase. 1) Below the critical point with two
distinct phases. 2) As the temperature of the system increases, the liquid starts to
expand. 3) With further temperature increase, the two phases start to become less
distinct. 4) A new supercritical phase forms. 5) As the system is cooled down, the
reverse process initiates. 6) With further temperature decrease, the phase
separation into liquid and vapor starts to take place.
The state of a substance as a function of temperature and pressure can be
displayed using a phase diagram. A general phase diagram for a pure substance is
illustrated in Figure 2.23.a, and shows different thermodynamically stable regions,
separated by phase boundary lines. The phase boundary lines indicating the conditions
under which two phases of matter can coexist at equilibrium. The point on the phase
diagram where the phase boundary lines intersect is called the triple point, labeled by a
red point on Figure 2.23.a, where all three distinct phases of matter (solid, liquid, gas)
coexist. Another distinct point on the phase diagram is the critical point labelled blue on
the same figure. Figure 2.23.b displays a series of snapshots capturing the transition of
liquid CO2 into supercritical fluid when heated in a sealed container. If a liquid is heated in
a closed container, the vapour pressure increases, thus the vapour density increases as
temperature increases. At the same time, the density of the liquid drops due to thermal
expansion. This trend continues until the density of the liquid becomes equal to the density
49
of the vapour and the phase boundary between liquid and vapour phases disappear. The
point at which the phase boundary ceases to exist is called the critical point.
Conventionally, the phase diagram is divided into 4 regions separated by the
critical isobar and isotherm, shown by dotted blue and red lines respectively on Figure
2.24.a. There is a disagreement in the scientific community regarding the regions
(quadrants) considered to be supercritical fluid. For instance, the quadrants II, III, and IV
are deemed supercritical fluids, as in neither of them a phase equilibrium is possible.187
This is in agreement with Younglove’s view considering IL and IV as liquid and vapour, and
calls everything else fluid.188 On the other hand, Tucker believes only states above the
critical temperature, corresponding to quadrants II and III, are supercritical, while
Oefelein189 and Candel et al.190 refer to quadrant IV as transcritical, and quadrant III as
supercritical. Further analysis of this classical four quadrant state plane reveals that the
physical justification of these definitions is likely oversimplified. For this reason, a modified
diagram capturing the more subtle aspects of the phase behavior in the vicinity of the
critical point was proposed by Banuti & Hickey, 191 as shown in Figure 2.24.b. The
coexistence line separates the liquid and vapor, and the fluid is in a liquid state for
pressures above the coexistence line. At subcritical pressure, the coexistence line divides
liquid and vapor; at supercritical pressure, the liquid needs to pass through the
pseudoboiling-line before it transforms to a vapor state. At temperatures far above the
critical temperature, the vapor acts like an ideal gas. The transformations of the fluid above
supercritical pressure occur continuously, without a traditional phase change.
50
Density
Low
High
P
IV
IL
Ideal gas
Pcr
Vapour
Liquid
III
II
IV
Tcr
T
Figure 2.24. Phase diagram. (a) Classical fluid state plane demonstrating different
supercritical states structure. The critical isobar and isotherm lines are shown by
blue and red dotted lines respectively. (b) Revised fluid state plane with coexistence
and pseudoboiling lines. Adapted with permission from reference 191.
As a concrete example, Figure 2.25.a displays the phase diagrams of n-pentane
and toluene. As illustrated, the gas-liquid equilibrium curve is interrupted at the critical
point, providing a continuum of physicochemical properties. Beyond the critical point, the
molecules are not held together as strongly as in the liquid phase via intermolecular forces
as the number of them decreases. From the microscopic point of view, the molecules in
the liquid phase can be imagined as trapped in each others’ potential fields. As the
temperature rises, the average kinetic energy of molecules increases and when it reaches
a specific temperature, they have enough energy to break free from this potential well.
This analogy implies that the critical temperature is approximately proportional to the
potential well depth.192
The decreased intermolecular forces in supercritical fluids significantly impact the
fluid’s physical properties. For instance, the density of n-pentane and toluene decreases
as temperature increases and significantly drops near the critical region of the fluid (Figure
2.25.b). The solubility of different compounds in supercritical fluids depends on the fluid
density, in particular solvent density around the solute. In supercritical fluids, the local
solvent density around the solute is higher than the bulk density if the solvent-solute
interaction is favorable, leading to higher solvation. Consequently, what makes
supercritical fluid an interesting solvent is the ability to easily tailor its solubility power by
changing the temperature and pressure, and consequently the fluid density.
51
n-Pentane
Toluene
0.9
-1
Density (g.ml )
Pressure (atm)
n-Pentane
Toluene
40
20
0
0.6
0.3
0
100
200
Temperature (°C)
300
0
100
200
300
Temperature (°C)
Figure 2.25. (a) Temperature-pressure phase diagrams of n-pentane (Tr : 196.45 °C,
Pr : 33.25 atm) and toluene (T r : 318.64 °C, Pr : 40.72 atm). (b). Density-temperature
phase diagrams of n-pentane (ρr : 0.273 g.ml-1) and toluene (ρr : 0.291 g.ml-1). Data
retrieved from NIST Chemistry WebBook on October, 2020.
In general, the liquid-like densities of supercritical fluids combined with their gaslike diffusivities, enable the dissolution and fast transport of species rather easily
compared to their liquid state, and make supercritical fluid an excellent candidate as a
deposition medium. Moreover, supercritical fluids leave no solvent residue on the
substrate surface and have low surface tension. More importantly, supercritical fluids are
considered environmentally preferable solvents for not only in deposition and preparation
of supported metal nanostructures, but also in the area of polymer processing. 193,194
2.5.1. The van der Waals Equation of State
The van der Waals equation of state is a simple model that considers the
interaction between gas molecules and it is used to develop phase diagrams in addition
to understanding the behavior of gases. The van der Waals equation of state has the form
𝑃=
𝑅𝑇
𝑉−𝑏
−
𝑎
(2.40)
𝑉2
where 𝑃 is the external pressure and 𝑉 is the molar volume. In this equation, 𝑎 and 𝑏
parameters are normally estimated from the critical point and considered constants
(substance specific). However, these parameters can be functions of temperature and
52
some fluid properties such as acentric factor (𝜔), critical compressibility factor (𝑍𝑐 ), etc. In
fact, certain theoretical and empirical restrictions must be imposed for estimating these
parameters.195
To examine the extent to which the van der Waals equation of state is reliable in
predicting the phase behavior, one can start with the construction of isotherms. As shown
in Figure 2.26, the isotherms (line of constant temperature) take different shapes
depending on the temperature value. At temperatures that exceed the critical temperature,
decreasing the molar volume results in a monotonic increase in pressure, until it reaches
the point where molecules are closely packed, and the pressure diverges. The molar
volume cannot take values smaller than 𝑏, meaning the molecules reached their minimum
possible distance. In contrast, for temperatures below the critical temperature, as molar
volume is decreased the pressure goes through an initial rise, but then falls, and once
more rises (goes through a loop), before reaching molar volume values equal to 𝑏. This
implies that there is a region where compressing the fluid can cause its pressure to
decrease, corresponding to an unstable phase. This instability is interpreted as a liquidgas phase transition. Based on the isotherm for temperatures below the critical
temperature, there are two stable regions corresponding to relatively small and large molar
volumes separated with an unstable region.196 In the region of large molar volume (𝑣 ≫
𝑏), the increase in the pressure is monotonic as expected with a decrease in molar
volume. This stable region is the gas state. In the region of relatively small molar volumes
(𝑣 ~ 𝑏), molecules are very densely packed, thus small change in the volume requires a
great deal of pressure. This state defines a liquid phase. Therefore, the unstable region
corresponds to a volume region that liquid and gas are coexisting. To address this
anomaly in the isotherm, the Maxwell construction approach connects the high and low
molar volume regions by replacing the loop with one isobaric straight line such that the
areas cut off above and below this line are equal (Figure 2.26.b).197 Based on the new
constructed isotherm, pressure remains constant as the volume decreases, until all the
gas is converted into liquid and afterwards, pressure starts to increase as the liquid volume
decreases.198
53
a.
b.
c.
PC ,TC
P
P
P
T > Tc
T = Tc
Liquid
IV
I
T < Tc
b
III
Gas
V
II
V
V
T
Figure 2.26. Isotherms of the van der Waals equation of state for 3 different
temperatures. (b) Maxwell construction for van der Walls isotherm. The isobaric
line is constructed such that equal areas are found for I.II.III and III.V.VI. (c) The
phase diagram produced based on the Maxwell construction for different
temperatures.
The pressure at the phase transition can be established using the isotherms. The
phase diagram can be generated by repeating the Maxwell construction for different
temperatures and plotting the vapor pressure extracted versus the temperature (Figure
2.26.c). As the temperature approaches the critical temperature, the phase boundary
between liquid and gas disappears and they are not distinguishable from each other
anymore. There is no more phase transformation above this point and only a single-phase
fluid exists.
Mixtures Containing Polymer Components
Similar to the modeling of conventional phase equilibrium, there are several
polymer-specific equation of states which can be classified into two broad groups: 1)
excess Gibbs free energy (Gex) or activity coefficient models and 2) EoS models.199 While
the activity coefficient models are believed to be more suitable for complex phase
behavior, the EoS models have been more successful, especially for high pressure
systems. One of the examples of the cubic EoS applicable to large molecules and
polymers is Sako, Wu, and Prausnitz (SWP). 200 The SWP equation has been used, with
relative success, for mixtures using simple mixing rules. Other EoS and G ex models that
have been used with some success are Panagiotopoulos-Reid (PR)201 and SoaveRedlich-Kwong (SRK)202 using mainly Wong-Sandler (WS)203 and Zhong-Masuoka (ZM)204
mixing rules. Nevertheless, the most promising methods are usually based on the
combination of activity coefficient models and EoS for describing mixture phase behavior.
54
Mixtures Containing Supercritical Components
The virial EoS was the first model used to describe phase behavior of systems with
supercritical fluids. However, the most reliable models have been based on the cubic EoS
such as Soave-Redlich–Kwong (Eq. 2.41)202 and Peng-Robinson (Eq. 2.42)205 equations
of state.
𝑃=
𝑃=
𝑅𝑇
𝑉𝑚 −𝑏
𝑅𝑇
𝑉𝑚 −𝑏
−
−
𝑎
(2.41)
√𝑇𝑉𝑚 (𝑉𝑚 +𝑏)
𝑎.𝛼
(2.42)
2 +2𝑏𝑉 −𝑏 2
𝑉𝑚
𝑚
The SRK EoS is a modified version of the simple RK EoS with a function 𝛼 (𝑇, 𝜔)
involving the temperature and the acentric factor and can be applied to hydrocarbons. On
the other hand, the PR EoS model provides reasonable accuracy near the critical point
and is a superior model in calculations of the compressibility factors and liquid densities
of many materials. There are several combination models based on cubic EoS and mixing
rules presented in the literature but lack accurate representation of phase equilibrium in
systems containing a supercritical fluid.206 Nonetheless, it has been proven that the best
predictions are the result of using the Gibbs free energy models and nonquadratic mixing
rules.
55
Chapter 3.
Methods
This chapter provides a review of relevant concepts and experimental approaches
implemented throughout this thesis. The first section provides a detailed description of the
supercritical fluid chamber and the complete tabletop setup, following by the procedure
detailing the substrate preparation for deposition in the supercritical fluid chamber.
Thereafter, a summary of spectroscopic techniques used for establishing the solubility
behaviour of polymers and their chemical characterization are described. Subsequently,
relevant microscopy tools utilized for the deposited films texture and phase behavior
analyses are introduced. Finally, the basic principles and tools employed to successfully
develop patterns on the substrate for polymer deposition are outlined.
3.1. High-Pressure System Design
The custom pressure chamber was constructed at the SFU machine shop from
beryllium copper (BeCu) by drilling three perpendicular holes into a cube of raw material.
In total, the vessel has six ports, including an inlet positioned at the bottom of the chamber
(Figure 3.1, Y-Axis) for the introduction of fluids inside the chamber, and an outlet situated
at the top port (Figure 3.1, Y-Axis) for the exhaustion of fluids from the chamber.
Additionally, there are two sapphire windows positioned to face each other and allow for
in-situ transmission UV–vis spectroscopy (Figure 3.1, X-Axis). The port for the introduction
of a substrate for deposition is perpendicular to the sapphire windows (Figure 3.1, Z-Axis),
allowing to monitor the film growth on the substrate during deposition The chamber was
sealed by securing the ports with BeCu flanges using black oxide finished alloy steel bolts
(M5-0.8, class 12.9, manufactured by HOLO-KROME®), leaving approximately 27 mL of
internal volume. As a safety consideration, the volume of the system was kept low to
reduce the total energy of the system. The stored mechanical energy of a filled (27 mL)
and pressurized chamber (38 MPa), is equal to dropping a box weighing 5 kg from a 0.13
meters height, or the rupture of mountain bike tire when inflated to a typical pressure.
Furthermore, an O-ring made of Kapton® polyimide sheets (0.005" Thick), shown as black
O-ring in Figure 3.1, was placed in the groove of each port prior to the attachment of
flanges.
56
Figure 3.1. Drawing of the high-pressure chamber and some of its components.
Bolts, heating elements, and mounting elements are excluded for clarity.
The chamber was pressurized using a manual pressure generator purchased from
High Pressure Equipment Co. (HiP 62-6-10). The pressure generator is a manually
operated piston designed to compress liquid within a small volume to develop pressure.
The pressure was monitored with a transducer (Swagelok PTI series) and is read out with
a small LED display.
Figure 3.2 illustrates the complete assembly of supercritical fluid setup including
Swagelok fittings and stainless steel tubing. All the wetted parts (beside the sapphire
windows) are made of 316 or 17-4PH stainless steel. The commercially purchased system
components are rated to 69.0 MPa or greater and were regularly tested up to 24.0 MPa
to ensure the integrity of the system at elevated pressures. In addition, there was a 38.0
MPa rupture disc acting as a primary safety measure to prevent over pressurization. As
an additional safety consideration, the supercritical chamber was stationed inside a Lexan
safety box (labeled 10 in Figure 3.2). Furthermore, it is important to provide an active
ventilation of the vessel to rapidly remove any flammable solvent vapors inside the safety
box in the event of an unplanned depressurization (vacuum hose is labeled 11 in Figure
3.2).
Materials are introduced into the chamber by placing them into a crucible and
placing the crucible at the chamber bottom. The crucible containing the solute (polymer)
57
was made from glass with a cylindrical shape (diameter = 11.60 mm, height = 6.80 mm,
thickness = 0.85 mm). After placing the polymer sample inside the crucible, it was capped
with glass wool and wrapped with copper mesh to prevent the uncontrolled dispersion of
polymer inside the chamber. The prepared and filled crucible was then placed at the
bottom of the chamber prior to sealing the chamber. The scrupulous removal of oxygen
is an important safety measure that allows us to work safely with flammable solvents under
elevated pressures and temperatures. This was accomplished by purging the chamber
with nitrogen via an inlet valve (labeled 7 in Figure 3.2) for minimum 15 minutes to force
the air and moisture out of the chamber through the outlet valve (labeled 8 in Figure 3.2).
The solvent was also deoxygenated prior to addition to the chamber by nitrogen purging.
The purging was achieved via bubbling nitrogen through the solvent for approximately 20
minutes. Both the solvent and chamber were deoxygenated prior to increasing the
chamber’s temperature.
Figure 3.2. Schematic of supercritical tabletop setup (top view) and its components:
(1): N2 cylinder, (2): solvent bottle, (3): Omega benchtop PID controller, (4): pressure
display connected to the pressure transducer, (5): manual pressure generator, (6):
cartridge heaters x4, (7): inlet valve, (8): outlet valve, (9): cold solvent trap, (10):
Lexan safety box, (11): vacuum hose.
In order to set the temperature of the fluid inside the chamber and initiate the
polymer dissolution process, the exterior of the chamber was heated by an Omega
benchtop PID controller (CSi32 Series with 0.04 °C temperature stability) used to drive
58
four cartridge heaters connected in parallel and placed symmetrically about the edges of
the chamber (labelled 6 in Figure 3.2). A ground-fault circuit interrupter, or GFCI, was also
added between the wall power and the power control unit to shut off electrical power in
the event of a ground-fault. Concurrently, the chamber’s internal pressure was monitored
via pressure display (labeled 5 in Figure 3.2), and adjusted, if necessary, using the manual
pressure generator (labeled 5 in Figure 3.2).
3.2. In-situ Transmission UV-vis Spectroscopy Setup
The setup for in-situ transmission UV-vis spectroscopy is illustrated in Figure 3.3.
The light source and detector were positioned outside the Lexan safety box that houses
the supercritical fluid chamber. The optical fibers were fed through small holes that were
drilled through the Lexan safety box and securely held in position. Afterwards, they were
aligned facing the sapphire windows as portrayed in Figure 3.3. An Ocean Optics
USB4000 spectrometer that covers 200−1100 nm ranges with a Toshiba TCD1304AP
(3648-element linear silicon CCD array) detector was used. The halogen light source was
also purchased from Ocean Optics (HL-2000 with 360−2400 nm wavelength range) and
the path length was approximately 15 cm.
The reference spectrum used for the measurements was collected from the
solution inside the chamber at room temperature. It was also noted that transmission was
slightly higher at increased pressure, thus a different blank was collected for each
pressure studied. Additionally, the resolution of the spectra was deliberately decreased to
8 nm to increase the signal-to-noise ratio and thus ensure accurate readings at high
absorbance values.
It was imperative for the accuracy of the data collected that the UV-vis spectra
collection as a function of temperature was carried out in a successive manner. Thus,
UV−vis spectroscopic measurements were performed after the system was given
approximately 15 minutes to reach equilibrium and ensuring polymer is dissolved in the
supercritical fluid. After UV-vis spectra were collected and the appropriate blank spectrum
was subtracted, the data were fitted with two Gaussian peaks using IGOR Pro
(Wavemetrics) to establish the solubility behavior as a function of temperature.
59
At the end of each experiment using in-situ transmission UV-vis spectroscopy, the
chamber contents were exhausted through the chamber outlet into a polypropylene vessel
with a volume of 500 mL (labelled 9 in Figure 3.2). The vessel has an inlet port to accept
the chamber contents and a wide outlet port to prevent over-pressurizing the vessel.
Figure 3.3. Schematic of a section of supercritical tabletop setup (side view) and
different components used for the in-situ transmission UV-vis spectroscopy.
3.3. Gravimetric Analysis of Saturated Solutions
In case of the gravimetric analysis, the polymer solubility as a function of
temperature was established via direct polymer mass measurement from saturated
solutions. To measure the mass of polymer, the chamber contents (saturated solutions)
were exhausted through the chamber outlet into a polypropylene vessel with a volume of
500 mL. As a matter of safety, pressure should never be allowed to build inside the
collection vessel. It is also important to provide active ventilation of the vessel to rapidly
remove any flammable solvent vapors. The rapid expansion of the chamber contents
cooled the solution below its boiling point, allowing liquid solvent and polymer to be
collected. Afterwards, solvent was removed from the slurry under reduced pressure and
the dried polymer mass was weighed.
60
Figure 3.4. Schematic of a section of supercritical tabletop setup (side view) for
gravimetric measurements.
3.4. Substrate Preparation for Thin Film Deposition
The substrates used for film deposition were ITO-coated glass slides purchased
from Ossila Ltd. The thickness of ITO coating was in the range of 40 to 100 nm with
average resistance of 20-60 Ω/sq. ITO glass slides were chosen as substrates because
of their optical transparency, electrical conductivity, and ease of use. A thin layer of gold
(≈ 50 nm) was deposited near the edges of the substrates by using physical vapor
deposition to facilitate a uniform current through the ITO film. The ITO glass slides were
cleaned with acetone and isopropyl alcohol consecutively before mounting them on the
sample holder and placing the assembly into the chamber. The substrate was placed
parallel to the sapphire windows to facilitate the collection of UV-vis spectra during film
growth.
61
a.
4
7
b.
1
3
6
5
2
Figure 3.5. The sample holder (a) front view and (b) backside view. The ITO coated
glass substrate (labeled 1) is secured between two polyether ether ketone (PEEK)
legs (labeled 6) and two copper legs (labeled 4) facing each other. Kapton sheet is
used as an O-ring to ensure the proper sealing (labeled 5). Also, Kapton wings were
used to prevent short circuit when the chamber is fully closed (labeled 7). The
temperature of the substrate surface is monitored using thermocouples (labeled 3)
and the voltage is applied to the substrate through copper wires (labeled 2).
3.5. Spectroscopic Techniques
Light interacts with materials in several different ways. During these interactions,
light can be transmitted, reflected, or scattered depending on the material’s chemical
composition and the energy of the incident light. Spectroscopy techniques utilize lightmatter interactions to obtain information regarding the structure and properties of the
material.207 As mentioned previously, in-situ UV-vis spectroscopy was used to
characterize the concentration of compounds dissolved in supercritical fluids. In addition,
Raman spectroscopy was used to characterize polymer films chemical composition. Both
of these important techniques will be discussed below.
3.5.1. Transmission Ultraviolet-visible Spectroscopy
The transmission UV-vis spectroscopy was one of the main techniques that helped
us to investigate the solubility properties of pressurized polymer solutions at several
different temperatures and pressures, identifying the optimized deposition condition. As
described in greater detail above, one way this was done was to collect in-situ UV-vis
spectra of polymer solutions.
62
The UV-vis spectroscopy is based on the absorption of ultraviolet or visible light by
matter. As the light passes through the matter, electrons are promoted from the ground
state to an electronic excited state by absorbing a photon. As a result, the intensity of
incident light reduces after absorption, which is directly proportional to the molar
concentration of absorbing groups (called chromophores). The Beer-Lambert Law,
Equation 3.1, describes how absorption is commonly related to the optical path length (𝑙),
molar absorptivity (𝜀), and the molar concentration of the species (𝑐).208
𝐴 = 𝑙𝑜𝑔
𝐼0
𝐼
= 𝜀. 𝑙. 𝑐
(3.1)
UV-vis spectroscopy can be used for obtaining information about a compound in
solution and in solid state.
3.5.2. Raman Spectroscopy
Raman spectroscopy is a vibrational spectroscopic technique in which the sample
is irradiated with narrow band laser light. Interactions with molecular vibrations shift the
wavelength of light by an amount equal to the vibrational energy. The scattered light is
filtered to remove the main laser line which corresponds to Rayleigh scattering and allows
the shift from the main laser line to be more easily detected. This step is important because
Raman scattered light constitutes only a small portion of the total signal reaching the
detector. In Figure 3.6 different energy states that are involved in Raman spectra are
illustrated. Raman scattered light with the same wavelength as the incident light is called
Rayleigh scattering. Raman scattered light with shorter wavelength than Rayleigh
scattered light is called anti-Stokes, and that with longer wavelength is called Stokes. The
change in the energy of the incident photon is directly related to the specific vibrational
mode of a molecule and is typically reported in wavenumber (cm-1). Additionally, the
surrounding environment of the functional group can result in a change in the peak
position. For instance, the presence of hydrogen-bonding interactions can lead to
significant frequency shifts of the order of magnitude of hundreds of cm-1.209 In contrast to
IR spectroscopy that probes dipole moment changes, Raman looks at the molecular
bond’s polarizability providing a complementary probe of intramolecular vibrations. 210
63
Figure 3.6. The scattering processes that can occur when IR light interacts with a
molecule (left) and electronic states diagram of a molecule (right) illustrating the
origin of Rayleigh, Stokes and Anti-Stokes Raman Scattering.
3.6. Grazing-Incidence Wide-Angle X-ray Scattering
Grazing-incidence wide-angle x-ray scattering (GIWAXS) is a method developed
for studying nanostructures. Following the emergence of synchrotron radiation sources, a
collection of surface x-ray scattering and diffraction techniques have emerged to study
materials at the surface or interface with small volumes of matter.211 More importantly,
they act as a complementary technique to atomic force microscopy and scanning electron
microscopy which typically only measure the surface of the sample.
Figure 3.7. Grazing incidence x-ray scattering geometry, at small and large angles
(GISAXS and GIWAXS).
64
Scattering techniques are based on interactions of x-ray beams with sample and
analyzing the resulting x-ray scattering pattern. At high frequency regime such as x-rays,
the refractive index of all materials is less than one and x-rays with incident angles smaller
or equal to the critical angle undergo total external reflection (TER) at an interface and will
not penetrate the material’s bulk (< few nanometers).212 Hence, utilizing the technique of
x-ray diffraction at very low grazing incidence angles (typically tenths of a degree) with
respect to the sample surface helps to maximize the scattering contribution of the
investigated material relative to the substrate. Thus, minimizing the unwanted background
scattering (both elastic and inelastic) from the bulk.213 In Figure 3.7, a schematic sketch of
the grazing incidence x-ray scattering geometry at different angles is illustrated where the
incident angle αi is close to the angle of total external reflection αc . GIWAXS corresponds
to large values of the scattering angle (2θ) between 𝑘𝑖 and 𝑘𝑓 , and thus large values of
the in-plane (2θ𝑓 ) and out-off-plane (αf ) scattering angles. The direction of the scattered
wave (kf ) is detected at a direction defined by slits and makes an angle αf with respect to
the sample surface and an in-plane angle 2θ𝑓 with respect to the transmitted beam. The
wavevector transfer is defined as 𝑞 = 𝑘𝑓 − 𝑘𝑖 and is often broken down into two
components: 𝑞⊥ perpendicular to the surface and 𝑞∥ parallel to the surface.214 In the case
of wide-angle x-ray scattering, one can probe the atomic scale order by measuring the
scattering intensity and building the intensity distribution in reciprocal space.
Figure 3.8 summarizes the possible 2D GIWAXS patterns observed based on the
crystallinity and orientation of the crystals with respect to the substrate surface. As shown
in Figure 3.8.a, well-pronounced Bragg peaks are observed in the case of a highly
crystalline film with parallel orientation to the substrate surface. In the case of both parallel
and perpendicular orientation of crystallites present in the film, Bragg peaks appear along
both the vertical and horizontal directions (Figure 3.8.b). The more textured film with
randomly orientated domains results in the broadening of the Bragg peaks. For instance,
the Bragg peaks along the vertical direction will broaden when domains are oriented with
an angular distribution around the horizontal direction (Figure 3.8.c). Figure 3.8.d
illustrates the case of disordered film with large degree of crystallites orientation resulting
in Bragg peaks smearing out into Debye-Scherrer-like rings.215
65
Figure 3.8. Schematic of film crystallinity and corresponding 2D GIWAXS data. (a)
vertical lamellar stacking, (b) crystallites with both vertical and horizontal
orientation, (c) oriented domains around the horizontal direction and (d) full
rotational disorder of crystallites.
GIWAXS has been widely used in polymer science, as the scattering data profile
can provide valuable information regarding the polymer chain orientation on the substrate.
However, in the case of conjugated polymers, the high degree of paracrystallinity
broadens the Bragg peaks and additional analysis is required.216 For instance, poly(3hexylthiophene) (P3HT) when cast as a thin film, can crystalize into two major unit cells
with distinct chain orientations. As illustrated in Figure 3.9, the polymer chains can adopt
either the edge-on or face-on orientation with respect to the substrate. In the edge-on
orientation, the side-chains direction (lamellar stacking) is normal to the substrate. In
contrast, the face-on orientation involves the aromatic rings (π-π stacking) facing the
substrate.
Figure 3.9. Schematic illustration of the face-on and edge-on configurations of P3HT
chains.
66
Figure 3.10 is the corresponding GIWAXS scattering profiles for predominately
face-on (a) and edge-on (b) orientations of P3HT. In the predominate face-on orientation,
Figure 3.10.a, the lamellae peak appears in the in-plane direction (𝑞𝑥 axis) and the π-π
stacking peak in the out-of-plane orientation (q z axis). In Figure 3.10.b, the lamellar peaks
appear along the 𝑞𝑧 axis at ~ 0.4 Å−1 (in the out-of-plane orientation) with a repeating
distance of ~ 1.6 nm corresponding to the parallel lamellae, while the π-π stacking
orientation peak is at 𝑞𝑥 ~ 1.6 Å−1 in the in-plane direction with a spacing of ~ 0.39 nm.
Figure 3.10. GIWAXS patterns of P3HT with predominantly (a) face-on and (b) edgeon orientation. Reprinted with permission from Dr. Kevin G. Yager, Brookhaven
National Laboratory, 2020.
3.7. Microscopy Techniques for Morphology Analysis of
Deposited Thin Film
Microscopy produces images of samples that are too small to be seen with the
naked eye using a variety of techniques. There are three main types of microscopy:
optical, scanning probes, and charged particle (electron and ion) each with their set of
advantages and limitations. Different types of microscopes have different resolving
powers as indicated in Figure 3.11. The resolving power of a microscope can be defined
as the inverse of a distance between two points on a specimen that can still be
distinguished as distinct entities.
67
Figure 3.11. Different types of microscopes and their resolving power range.
3.7.1. Optical Microscopy
Optical microscopy refers to any type of microscopy in which visible light is being
used to visualize images. Since the visible light is used in this technique, the magnification
is limited by the resolving power achieved by the wavelength of the visible light. Abbe’s
equation can be used to calculate the approximate resolving power of an optical
microscope:
𝑅𝑒𝑠𝑜𝑙𝑣𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟 =
1
∆𝑑
=
2𝑛.𝑠𝑖𝑛 𝜃
(3.2)
𝜆
where 𝑛 is the refractive index of the medium separating object and aperture and 𝜆 is the
wavelength of the light illuminating the specimen. As a result, there are two strategies to
increase the resolving power: decreasing the wavelength (𝜆) or increasing the numerical
aperture (2𝑛. sin 𝜃) either through an increase of 𝑛 or the angle of light coming from the
object.
The bright-field microscope is one of the most common and simplest optical
microscopes. In bright-field microscopy, a specimen is illuminated from below and the light
is collected by an objective lens situated above the specimen. The objective magnifies the
light and transmits it to the oculars and/or camera, where the enlarged image of the
specimen is displayed. The observed contrast in the bright field image (dark sample on
bright background) is due to the light absorbance in dense areas of the specimen.
3.7.2. Polarized Optical Microscopy
The polarized optical microscopy (POM) is a useful microscopy technique that
facilitates the observation of birefringent specimens. Although, birefringence is mainly
associated with crystals at the microscopic level, it can also be observed at the
macroscopic level with polymers and fibers. The intrinsic birefringence is usually related
68
to the spatial arrangement of atomic groups and molecules, while the form birefringence
is based on the spatial arrangement of objects (rods or plates) in a medium of a different
refractive index.
Birefringence is the property of crystalline materials that have different indices of
refraction in the primary refractive index axes and are defined as optically anisotropic
materials.217
𝐵𝑖𝑟𝑒𝑓𝑟𝑖𝑛𝑔𝑒𝑛𝑐𝑒 = ∆𝑛 = 𝑛∥ − 𝑛⊥
(3.3)
where 𝑛∥ and 𝑛⊥ are corresponding to the refractive indices of the polarized light parallel
and perpendicular to the optical axes.218
Figure 3.12. Comparison between an isotropic material (left) with only one refractive
index for all propagation directions and a birefringent material (right) that has two
different refractive indices, allowing two different oscillation directions for the light:
a fast and a slow direction, hence a double image. Motic Incorporation Limited
Copyright, 2002-2016.
To provide a representation of the refractive indices in different directions, the
optical indicatrix is plotted in Figure 3.13.a. An optical indicatrix is an ellipsoid that
represents geometrically the variation of three primary refractive indices na, nb, and nc and
the overall refractive index of the material. Isotropic materials, such as cubic crystals, only
have one refractive index showing the same optical properties in all directions. In contrast,
anisotropic materials like tetragonal, hexagonal, or trigonal crystals, have two unique
primary refractive indices. The ne that is along the c axis is defined as the optical axis as
it is the axis for which light experiences no birefringence (parallel to the symmetry axis, so
perpendicular to the circular section), and the other primary refractive indices is no along
69
the crystallographic a and b axes (Figure 3.13.b). The rest of the crystal systems have two
optical axes and are named biaxial.
Figure 3.13. (a) A representation of the refractive indices in different directions
using the optical indicatrix for a birefringent material. (b) A uniaxial indicatrix where
the one optical axis is along the c axis—ellipsoidal indicatrix. (c) A schematic
showing an atom feeling different spring strength in different crystalline direction.
Most incoherent light sources generate unpolarized light. The generation of linearly
polarized light from such a source requires the use of polarizing optics typically made from
natural minerals or synthetic materials consisting of oriented crystallites. Polarized optical
microscopes use multiple polarizers to study the birefringence of a specimen. The first
polarizer in POM setup is located before the condenser that illuminates the specimen
(Figure 3.14), converting unpolarized light into plane-polarized light. The second polarizer
in POM, called analyzer, is placed above the objective with the axis perpendicular to the
first one (crossed polarizer configuration). In this configuration, the transmitted light will be
blocked by the analyzer and a dark background present in the eyepiece. However, if the
specimen is an anisotropic material, it changes the state of polarization of light propagating
through it and results in bright regions appearing in the dark background.
The birefringent specimen allows the polarized light along the optical axis (called
the extraordinary ray, ne) and the polarized light perpendicular to it (called the ordinary
ray, no) to travel at different speed. Consequently, these two light components that were
in phase when they entered the sample, are retarded at a different speed, and exit the
sample out of phase. Later, when passing through the analyzer, the two light components
recombine with constructive and destructive interference. The analyzer ensures the same
70
amplitude at the time of recombination for maximum contrast, while their differential
retardation (𝑅) can be measured using a polarized microscope:
𝑅 = ∆𝑛 × 𝑙
(3.4)
where ∆𝑛 is the product of the birefringence and 𝑙 is the path length through the material.
Birefringent materials are observed as bright objects on a dark background under
polarizing optical microscope. The brightness of the images is influenced by many factors,
such as intensity of birefringence, thickness of the specimen along the optical pathway,
and the alignment of the specimen parallel to one of the two polarizers. 219
Image Plane
Retardation
Recombined
Light Rays
After
Interference
Tube Lens
Analyzer
Ordinary and
Extraordinary
Light Rays
Objective
Birefringent
Specimen
Condenser
Plane Polarized
Light
Polarizer
Unpolarized Light
Figure 3.14. The schematic of polarized optical microscope (left) and the
corresponding optical path and different light components (right).
3.8.
Optical Lithography for Pattern Development
The word “Lithography” originates from a traditional planographic printing
technique dating back to the late 18th century which involves printing from a plane surface.
71
This technique relies on the physicochemical property that oil, and water do not mix.
Therefore, the printing elements are soaked with a greasy ink, whereas the nonprinting
elements are moistened with water and are ink repellent. The technology for
microelectronics fabrication shares similarities with this much older technique and uses
light in the process of pattern creation. Thus, it is commonly termed optical lithography or
photolithography. In photolithography, a mask is used to create a pattern into a
photosensitive emulsion (photoresist) coated onto a surface. All the steps involved in the
photolithographic process follow the same principle. A schematic presentation of the
photolithography process is shown in Figure 3.15. The procedure starts with chemically
cleaning the substrate to remove any traces of impurities. This is vital for promoting the
adhesion of a thin uniform layer of photoresist. The photoresist is deposited on the
substrate surface using spin coating. There are two types of photoresist:
positive
photoresist becomes more soluble after exposure to a light source and negative
photoresist which becomes less soluble after exposure. Regions of soluble and less
soluble material are created by selectively exposing the sample to a light source using a
photomask. During the exposure process the pattern on the mask is transferred to the
photoresist. Then, the substrate is immersed in a specific solution called “developer” which
dissolves away the exposed areas of the positive photoresist or the unexposed areas of
the negative photoresist. The substrate is then subjected to subtractive (etching) or
additive (deposition) techniques.220 In the etching process, material is removed from the
entire surface, resulting in the removal of material where the substrate is exposed, and
removal of layers of photoresist where the substrate is protected. In the deposition
process, material is deposited across the entire surface, causing material to be deposited
onto the substrate in places where the substrate is exposed.
Etching processes can be characterized as a liquid (wet) or plasma (dry) etching.
In microfabrication, there is a variety of wet etchants such as buffered HF used for silicon
dioxide (SiO2) or silicon nitride (Si3N4) etching. However, the modern microfabrication
technologies are moving away from the wet etching processes due to the dimension
control (isotropy), undercutting, and adhesion problems they encounter in these
processes. Hence, plasma etching currently plays a significant role in integrated circuit
manufacturing owing to its anisotropy, compatibility to automation and vacuum processing
technologies, and significantly reduced amount of liquid waste.221
72
Figure 3.15. Photolithography steps and subtractive pattern transfer.
In plasma etching the material is removed using plasma (a discharge gas) that
consists of reactive radicals, ions, electrons, and neutrons. The gas source for plasma
usually contains small molecules rich in chlorine or fluorine such as CCl4. As the active
species in plasma gets adsorbed on the surface, they react with the surface material and
form volatile products. Afterwards, the volatile products leave the surface and are pumped
out with the effluents of the plasma. Figure 3.16 demonstrates different mechanisms
during which the surface is etched by plasma active species. In principle, there are three
different mechanisms: sputter etching initiated by the bombardments of surface with high
energy ions, chemical etching caused by neutral species reacting with the surface
materials, and reactive ion etching (RIE) which is a combination of physical sputtering and
chemical activity of reactive species with the materials on the surface.222 The etching
73
profile in RIE can be tuned by adjusting pressure, gas flow, and the power of the applied
radio frequency. Additionally, this can produce an anisotropic etch profile.
Figure 3.16. Different types of reaction mechanism during plasma etching.
74
Chapter 4. Physical Supercritical Fluid Deposition
of Semiconducting Polymers on Curved and Flexible
Surfaces
The results presented in this chapter have been reported in part, see:
Yousefi, N.; Maala, J.J.; Louie, M.; Storback, J.; Kaake, L.G. ACS Appl. Mater.
Interfaces., 2020, 12, 17961-17968.
Several people contributed to the work presented in this chapter. We are grateful
to Simon Fraser University machine shop staff for machining the chamber components,
especially those made from beryllium copper. Undergraduate students, Mikayla Louie,
Janneus J. Maala and Jacob Storback who aided in assembly and troubleshooting of the
pressure cell and performed important preliminary experiments. Furthermore, Dr. Wen
Zhou helped in measuring the Raman spectrum of PBTTT-C14.
4.1. Introduction
Fine control over the deposition of solution-processed materials is a general
problem in the science of self-assembly. To fabricate nanoscale structures, two
approaches are
commonly used. The top-down approach is exemplified by
photolithography whereby larger objects are cut into smaller pieces. The bottom-up
approach
uses
intermolecular
self-assembly
to
create
larger
structures.223-225
Unfortunately, there is very little overlap between these two approaches, placing limits on
both. The molecular level complexity that is available, for example, to biological systems
cannot be achieved by using a top-down manufacturing technique. On the other hand, the
use of intermolecular forces alone in the placement of nanoscale structures encounters
75
severe challenges on account of the second law of thermodynamics. Bluntly, a
nanomachine is only useful insofar that the work it performs impacts macroscale objects
or improves health, and current manufacturing techniques have not yet bridged this gap.
Supercritical fluids provide a unique intermediate between the liquid and vapor
phase, and for this reason, we are exploring them as a means of controlling self-assembly.
Frequently, supercritical fluids are used in polymer synthesis, 226-231 and the reduction of
metal salts can be used to create nanoscale structures.232,233 In addition, supercritical
carbon dioxide (ScCO2) has been used in the processing of organic field effect transistors
(OFETs),234 organic photovoltaics (OPVs),235,236 metal−organic framework (MOF),237,238
and aerogels.239,240 We are interested in a process for material deposition that does not
require in situ chemical reactions, which we term physical supercritical fluid deposition to
differentiate it from chemical deposition techniques and suggest a comparison with
physical vapor deposition. A process that deposits material without in situ chemical
reactions is preferable because it allows the compounds of interest to be synthesized,
purified, and characterized before deposition. This allows for molecules of significantly
higher quality and greater complexity to be employed.
The field of organic semiconductors is one where controlling material selfassembly on many length scales is an important challenge. Most commercial technologies
using organic semiconductors employ physical vapor deposition to create thin films. The
technique is robust and scalable, and fine shadow masks can be used for micrometerscale resolution.241-244 However, the technique requires that the molecules of interest
sublime at a temperature lower than their decomposition temperature, limiting the
molecular weight of the materials that can be deposited.
Thin films of solution-processable organic semiconductors have excellent
properties.245-248 However, patterned deposition is more technically complicated than
using a shadow mask. Printing techniques are common, with each technique offering
trade-offs in terms of scalability, ease of ink formulation, and ultimate resolution. For
example, direct stamping techniques can deliver superior resolution relative to other
techniques but have severe challenges regarding layer-to-layer registry. On the contrary,
roll-to-roll printing offers the most scalable method of producing patterned polymer
semiconductor films, but ink formulation is difficult and the finest commonly achieved
resolution is ~ 100 μm.249,250 Aerosol jet printing produces the finest resolution at 20-30
76
μm251,252 and can deposit material onto convex surfaces, but it requires a print head,
making the scalability of the technique unclear. One of the key limitations of all the
presented techniques is their inability to work with substrates of negative curvature.
The importance of demonstrating a scalable, high-resolution deposition technique
in the field of polymer semiconductors stems from their ever-increasing performance. The
charge carrier mobility of polymers has steadily increased for over 30 years, with top
performers now on par with polysilicon.20 Moreover, material quality does not seem to
have encountered any fundamental limitations, which argues in favor of optimism
regarding further improvements. This seems to suggest that the terms “high performance”
and “polymer electronics” need not be at odds with each other. Indeed, the word “cheap”
(as representing lower quality than the conventional alternative) need not universally apply
to a whole class of materials that have the vast parameter space of organic synthesis at
their disposal. However, to begin leveraging this potential, an additive manufacturing
technique that delivers control over solution processed materials with an accuracy and
fidelity comparable to photolithography must be developed.
We provide the first demonstration that the unique properties of supercritical fluids
can be used not only to deposit films of a solution processed material but also to direct its
deposition with unprecedented control. The directed self-assembly process uses
photolithography, providing a heretofore unexplored connection between top-down and
bottom-up self-assembly. The fundamental science underlying self-assembly in
supercritical fluids is almost wholly unexplored, and this work provides a unique point of
entry to this line of inquiry.
4.2. Results and Discussion
4.2.1. Study of PBTTT-C14 Solubility in Supercritical n-pentane
A small glass crucible was filled with poly[2,5-bis(3-tetradecylthiophen-2yl)thieno[3,2-b]thiophene] (PBTTT-C14).143 The crucible was capped with glass wool and
placed at the bottom of the chamber. The chamber was then filled to the point of
overflowing with deoxygenated pentane. The temperature and pressure of the chamber
were set by using the manual pressure generator and a temperature control unit. UV-vis
spectra of the chamber and its contents were collected 15 min after the chamber had
77
come to a steady state temperature to ensure saturation of the solution. Spectra collected
at room temperature were used as the blank. Figure 4.1 shows absorbance spectra
collected at 7.0 MPa as a function of temperature. Because of the high absorbance values
involved, the resolution of the spectrometer was decreased, and the spectra were fitted
with two Gaussian peaks to capture the behavior as precisely as possible.
Absorbance
5
P = 7.0 MPa
60 °C
90 °C
120 °C
130 °C
140 °C
150 °C
160 °C
4
3
2
1
0
350
400
450
500
Wavelength (nm)
550
600
Figure 4.1. UV-vis spectral measurements for the chamber and its contents
(solution) for several temperatures and a single pressure. Reprinted with
permission from reference 253.
Absorbance spectra displayed in Figure 4.1 show an increase for temperatures up
to 130 °C. As the temperature is further increased, the absorbance decreases. To
visualize the trends in overall absorbance as a function of temperature and pressure,
Figure 4.2.a displays the total integrated intensity obtained from a fit of the data.
Absorbance increases with temperature, reaching a maximum value at ≈ 130 °C. Further
increases in temperature result in decreasing absorbance.
78
b.
17.2 MPa
7.0
3.5
1.7
Peak center (nm)
Absorbance (a.u.)
a.
440
420
17.2 MPa
7.0
3.5
1.7
400
380
60
100
140
Temperature (°C)
60
100
140
Temperature (°C)
Figure 4.2. (a) Integrated UV-vis absorbance as a function of temperature for several
pressures. (b) Estimated center of main absorbance peak as a function of
temperature for several pressures. Reprinted with permission from reference 253.
The absorption peak position is also a function of temperature, showing a red-shift
in the peak maximum for temperatures up to 130 °C. As the temperature is further
increased, a blue-shift is observed. To visualize the trends in peak position as a function
of temperature and pressure, Figure 4.2.b displays absorbance peak center as a function
of temperature (calculated by the weighted average of the two Gaussian peaks used to fit
the data). The peak shifts toward longer wavelengths with increasing temperature up to a
maximum, decreasing with further increases in temperature. Very little pressure
dependence is observed with respect to the peak position.
The results of Figure 4.1 and Figure 4.2 are best interpreted as demonstrating
changes in the amount and type of dissolved material. An alternative hypothesis is that
the UV-vis spectra of semiconducting polymers are simply reflecting a known response
from temperature and pressure. A monotonic blue-shift with increasing temperature was
observed previously,254 and a monotonic red-shift was observed in response to
pressure.255 Neither effect alters the overall absorbance, in line with spectroscopic sum
rules. Instead, the initial increase in absorbance we observe reflects an increase in the
amount of dissolved material, and the decrease with further temperature increases
indicates a decrease in the amount of dissolved material. This result was also confirmed
via gravimetric analysis (see Figure 4.3). The peak shift is reflective of fractionation
behavior. As temperature is increased, a red-shift of the spectrum is observed, indicating
an increase in the conjugation length of the material.256 This interpretation is consistent
79
with the solubility behavior of polymers in supercritical fluids, which favors the solvation of
low molecular weight and amorphous material.227,257-259 As the temperature is further
increased, the solubility of the polymer decreases with the longer conjugation length
0.20
7.0 MPa
Absorbance
Concentration
0.15
0.10
[PBTTT]
Absorbance (a.u.)
fraction leaving the solution first.
0.05
0.00
60
80
100 120 140
Tempearture (°C)
160
Figure 4.3. Total integrated absorbance (Left axis, a.u.) and concentration (Right
axis, mg mL-1) as a function of temperature at 7.0 MPa. Reprinted with permission
from reference 253.
The observed nonmonotonic solubility behavior runs counter to the properties of
typical polymer solutions at ambient pressure, which tend toward increasing solubility with
increasing temperature. However, at pressures above PC, the solvent transitions
continuously from liquid-like to gas-like behavior, and in the limit of pure gas-like behavior
the solubility of PBTTT-C14 is negligible. Connecting these two regimes with a continuous
line suggests strongly that a peak in the solubility will be observed. This behavior is wellknown in the supercritical fluid literature and is described as an hourglass-shaped region
of immiscibility in temperature-composition space.260
Because material becomes less soluble with increased temperature, the entropy
of solvation must be negative. This is typically explained by the increasing importance of
𝛥𝑉𝑠𝑜𝑙𝑣𝑎𝑡𝑖𝑜𝑛 at higher temperatures and lower fluid densities.194, 261 Microscopically, polymer
solvation is accompanied by the formation of a solvent shell. In a liquid nearing its critical
temperature, the solvent density of this shell exceeds that of the bulk. Breaking the solvent
shell increases the entropy of the solvent, causing the precipitation of the solute.262
Following McHugh,194 the problem can be approached quantitatively by using the
80
Sanchez-Lacombe equation of state, which treats the compressibility of the solvent.263 A
quantitative treatment will be discussed in more detail in Chapter 5.
4.2.2. PBTTT-C14 Thin Film Growth in Supercritical n-pentane
The observed maximum in material solubility can be leveraged to grow films on a
substrate without the need for chemical reactions. This is the key principle of physical
supercritical fluid deposition. To cause material deposition onto a substrate, we hold the
temperature of the cell wall at the solubility maximum (Twall ≈ 130 °C) and resistively heat
a substrate immersed in the fluid (Tsub ≈ 160 °C). Because the solubility of the material is
lower at the substrate surface, material will precipitate to form a thin film. Figure 4.4 shows
optical microscope images of a series of films grown on resistively heated indium tin oxide
(ITO) coated glass substrates. The exposure and saturation in these images have been
adjusted to better provide visual confirmation of film deposition and allow the film
uniformities to be compared.
81
10.4 MPa
7.0 MPa
17.2 MPa
Spin-coated
Figure 4.4. Optical microscope images of PBTTT-C14 films grown onto ITO substrate
with Twall ≈ 130 °C and Tsub ≈ 160 °C for 90 minutes under several pressure conditions
(100x magnification). Adapted with permission from reference 253.
UV-vis spectra of the films grown onto ITO substrates were collected ex situ and
are displayed in Figure 4.5. The spectra show a blue-shift in comparison to the spin-coated
film of the as-received PBTTT-C14. This is consistent with the observation in Figure 4.1,
which suggests polymer fractionation behavior.
82
Absorbance (norm)
spin coated
7.0 MPa
10.4 MPa
17.2 MPa
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
Figure 4.5. Ex situ UV-vis absorbance spectra of the films shown in Figure 4.4.
Reprinted with permission from reference 253.
Raman measurements between a deposited film and the as-received material
were collected. As illustrated in Figure 4.6, both collected Raman spectra share common
features associated with the diagnostic peaks in the range of 1300-1600 cm-1, indicating
that the deposited material is chemically unchanged. The thienothiophene core and
thiophene rings are known to possess three main modes. According to previous reports,
the C-C intra-ring stretching peak position is located at 1380 cm-1, the symmetric C=C
stretching peak position is at 1445 cm -1, and the C=C/C-C stretching is at ~ 1488 cm-1.
These modes are known to be sensitive to π-electron delocalization (conjugation length)
in polythiophene.264
For both materials, we observed three main peaks, which are consistent between
the two samples. The appearance of our spectra corresponds well with previous
reports.264,265 We observe a strong peak at 1395 cm-1 (labeled A), another at 1415 cm-1
(labeled B) and the highest energy mode is observed at 1490 cm-1 (labeled C). We
interpret the consistency between the peak positions of the two samples to demonstrate
that the material deposited under supercritical conditions remained chemically unchanged
during the deposition process. To better visualize the differences in the peak intensities,
the spectra were normalized to the strongest mode, 1390 cm -1. The main difference is
attributed to the intensity decrease of C=C/C-C stretching mode for PBTTT-C14 thin film at
~ 1488 cm-1. As a result, the height ratio of mode A to mode C (IA /IC) is observed to
increase, which has been previously related to the conjugation length.265,266
83
Norm. Raman Intensity (a.u.)
Thin film
As-received
1.0
AB
C
0.5
0.0
1200
1300
1400 -1 1500
Raman Shift (cm )
1600
Figure 4.6. Raman spectra of PBTTT-C14 as received from the supplier and PBTTTC14 thin film deposited on ITO coated glass after 90 minutes deposition at 17.2 MPa.
Reprinted with permission from reference 253.
Some differences in the microscale and nanoscale morphology are observed
between the films. For example, nanowires of PBTTT-C14 have been precipitated from
supercritical hexane,267 a morphology we observe in some (but not all) of our thin films.
Like any other processing technique, film morphology is sensitive to the conditions of film
deposition. Because charge carrier mobility is also strongly dependent on film morphology,
further studies are necessary to control and optimize charge carrier mobility.
Film deposition was monitored in situ by collecting UV-vis spectra of the ITO
coated glass substrates. Figure 4.7 shows the absorbance at 625 nm plotted as a function
of time during thin film deposition. This value was chosen because very little light was
transmitted at shorter wavelengths. We interpret the increase in absorbance as an
increase in film thickness and infer that film thickness increases with respect to time in an
approximately linear fashion. This indicates that control over film thickness is relatively
straightforward and the technique can be used to create films of arbitrary average
thickness.
84
4
4.0 MPa
7.0 MPa
10.0 MPa
17.2 MPa
Absorbance
3
2
1
0
0
20
40
60
Time (min)
Figure 4.7. In situ UV-vis absorbance at 625 nm collected as a function of time,
monitoring film growth (Twall ≈ 130 °C and Tsubstrate ≈ 160 °C). Reprinted with
permission from reference 253.
4.2.3. Patterned Deposition of PBTTT-C14 on Multifarious Substrates
To demonstrate the wider applicability of this technique, we developed a means of
forming patterns of PBTTT-C14 on an ITO coated glass substrate. A simple way to produce
a pattern in the deposited material is to heat the substrate unevenly, selectively depositing
material on the hottest regions of the substrate (Figure 4.8 for a schematic). Thin lines (5
μm) of ITO were created by conventional lithographic techniques, removing the ITO with
a reactive ion etcher. Optical microscope images of the results are shown in Figure 4.8.a
where the ITO lines can be faintly seen against the glass substrate.
85
a.
b.
50 μm
Figure 4.8. Deposition of PBTTT-C14 on ITO coated glass after a 3 hrs deposition at
7.0 MPa. The side-view cartoon of patterned substrate is provided at the top of each
microscope image (The height scale is ~ 200 nm while the width scale is ~ 200 μm).
The corresponding (a) top-view optical microscope image of patterned ITO and (b)
deposited PBTTT-C14 lines on patterned ITO substrate. Reprinted with permission
from reference 253.
The ITO lines were addressed electrically and heated resistively relative to the wall
of the supercritical fluid chamber (which was held at 130 °C). Figure 4.8.b displays optical
microscope images of the resulting films. Although some amount of overcoating should
be expected, the PBTTT-C14 lines possess a nearly identical line width relative to the
underlying ITO lines. With a line width of 5 μm, this is a 4-fold improvement relative to the
line width achievable via aerosol printing techniques. 252 Moreover, the sharpness of the
lines indicates that the line width of the technique can likely be improved by a factor of 10,
making this demonstration a key step toward using this technique to guide the selfassembly of nanostructured materials. The coffee ring effect,268 is also absent an
important advantage of physical supercritical fluid deposition relative to standard solution
processing methods like inkjet printing269 where drying must be carefully controlled.270
86
a.
b.
50 μm
Figure 4.9. Deposition onto PMMA (PMMA film thickness = 165 ± 15 nm). Side-view
cartoon is drawn on top of each microscopy image. (a) Top-view optical microscope
image of patterned substrate and (b) Deposited PBTTT-C14 patterns with deposition
condition of: time = 3 hours, P = 7.0 MPa. Reprinted with permission from reference
253.
The deposition of semiconducting polymer lines with a line width smaller than
conventional printing techniques is an important step toward using it as a manufacturing
technique. However, depositing material onto lithographically ITO coated glass does not
demonstrate the full potential of the technique. A more commercially applicable deposition
mode would be to use the lithographically coated ITO glass as a deposition master,
coating it with a substrate that can later be removed, allowing the deposition master to be
reused. To demonstrate this, we performed a patterned deposition onto a thin film of crosslinked poly(methyl methacrylate) (PMMA). By depositing a thin and removable layer onto
the ITO coated glass, we use the ITO glass as a pattern forming master, allowing us to
create patterns on the thin film substrate (PMMA). The thin-film substrate could then be
removed from the pattern-forming master and used in, for example, a flexible electronics
application. This process could be repeated without the need to replace the
pattern-forming master, which could be reused in a manner similar to a photo mask, a
shadow mask, or a printing plate. Of course, there is nothing special about ITO coated
glass in this context; any pattern-forming master capable of supporting sharp thermal
gradients would suffice.
As a first step to demonstrating this process, we coated a patterned ITO substrate
with PMMA. The film was crosslinked by heating to 220 °C for at least 30 minutes. 271
Figure 4.9 shows a schematic and optical microscope image of the patterned ITO films
87
with a PMMA overlayer. The deposition of PBTTT-C14 was performed exactly as described
in the context of Figure 4.8, with the results being shown in Figure 4.9. No obvious
increase in the line width was observed; the PMMA overlayer did not alter the resolution
of the finished pattern. In fact, the lower surface energy of PMMA relative to glass seems
to have prevented the deposition of small particles on portions of the substrate where
deposition was not desired.
As another demonstration of a unique power of this deposition technique, we
deposited PBTTT-C14 on the concave interior of a poly(dimethylsiloxane) (PDMS) shell.
PDMS is a popular silicone elastomer often employed in flexible and stretchable
electronics.272-276 The ability to address concave surfaces is unique to our technique;
surfaces of high negative curvature cannot be addressed by a print head, nor can they be
deposited on via vapor transport techniques. The ability to deposit material on such a
surface not only hints at potential applications in flexible electronics but also illustrates that
the technique can be used to deposit patterns on surfaces of any form.
A thin PDMS shell was cast in a die made of a polyimide base and a ball bearing.
Before the die was assembled and filled with PDMS, a nichrome wire (AWG = 40, d =
0.0799 mm) was embedded into the die. Figure 4.10 shows a schematic of the object and
a microscope image of the object before and after deposition. The object was placed into
the supercritical fluid chamber, and deposition was performed in a manner similar to the
previous experiments. The nichrome wire was gently removed from the PDMS shell after
the deposition, and the resulting object was imaged with a microscope. The results of the
deposition are shown in Figure 4.10 (c), demonstrating deposition of PBTTT-C14 onto a
flexible surface of large negative curvature.
88
a.
b.
2 mm
c.
Figure 4.10. Deposition on PDMS hemisphere. (a) Cartoon of top and side view of
object. (b) Optical microscope image of PDMS with embedded nichrome wire. The
image was taken from an angle looking into the bowl of the hemisphere. (c) Optical
microscope image of PDMS hemisphere after deposition and removal of nichrome
wire (deposition time = 4 h, P = 7.0 MPa). The image has been taken from the top on
the flat surface of the hemisphere. Reprinted with permission from reference 253.
The research performed in connection with Figure 4.10 brings forward two
challenges that must be overcome to create high-density circuitry on highly curved and
flexible surfaces. The first challenge is to create a deposition master to impart high-density
patterns on surfaces of nontrivial curvature (i.e., shapes that cannot be flattened). The
second challenge is the difficulty of imaging objects that have microscopic patterns but
macroscopic curvature. In other words, conventional microscopy with its limited depth of
focus is ill suited to imaging micrometer scale elements on a surface that extends several
millimeters vertical to the focal plane. The ability to both create and characterize patterns
of this kind is critical to leveraging the full potential of supercritical fluid deposition and
stretchable electronics as a whole.
4.3. Conclusion
We presented the initial demonstration of a method that utilizes supercritical fluids
and photolithography to direct the self-assembly of a material that can only be processed
from solution. The process relies on common properties of solvents above their critical
pressure, where solutes can exhibit a maximum in their solubility as a function of
temperature. This phenomenon allows films to be formed onto a heated substrate. ITO
glass substrates were patterned by using photolithography and, when resistively heated,
89
directed the formation of polymer lines 5 μm in width. This outperforms the line width of
printing techniques like aerosol jet by at least a factor of 4. The pattern forming substrate
can be used as a deposition master when overcoated with a flexible and potentially
removable thin film substrate. This was demonstrated with a cross-linked PMMA layer.
The deposition technique can work with substrates of virtually any shape, as demonstrated
by depositing films onto the concave interior of a PDMS hemispherical shell. The pattern
forming substrate was created with photolithography, and the patterned films have nearly
identical line width, suggesting that much higher resolution patterns can be created. The
coupling of photolithography with solution phase self-assembly is the key step to bridge
the gap between top-down nanofabrication and bottom-up self-assembly.
4.4. Methods
4.4.1. Transmission UV-vis Spectroscopic Measurements
A custom-built pressure vessel from a block of beryllium copper (BeCu) was
utilized for supercritical experiments. The specifications of the high-pressure vessel can
be found in section 3.1. As mentioned previously, the pressure vessel has six ports in
total, including two sapphire windows to allow in-situ transmission UV-vis spectroscopy.
Transmission UV-vis spectroscopic measurements were performed by placing PBTTT-C14
into a crucible at the bottom of the chamber before the vessel was sealed and filled with
solvent. The chamber exterior was heated by an Omega benchtop PID controller used to
drive four cartridge heaters. After the temperature stabilized, the solution was pressurized
to 1.7, 3.5, 7.0, and 17.2 MPa by using the manual pressure generator. The system was
given ∼15 minutes to reach equilibrium before solution absorbance measurements were
collected. The in-situ transmission UV-vis spectroscopic measurements details are
explained in section 3.2. The reference used for the transmission measurements was the
chamber content at room temperature. However, the blank spectra were collected for each
pressure as the transmission of the apparatus was slightly higher at increased pressure.
This could be attributed to the better refractive index matching between the sapphire
windows and the fluid. The resolution of the spectra was decreased to 8 nm to achieve
higher signal-to-noise ratio and obtain accurate readings at high absorbance values. The
fitting of the data was carried out with two Gaussian peaks using IGOR Pro (Wavemetrics).
90
4.4.2. Substrate Preparation for Thin Film Deposition
The substrates used for film deposition were ITO-coated glass slides with average
resistance of 20-60 Ω/sq. ITO glass slides were chosen as substrates because of their
optical transparency, electrical conductivity, and ease of use. A thin layer of gold (≈ 50
nm) was deposited near the edges of the substrates by using physical vapor deposition to
facilitate a uniform current through the ITO film. The ITO glass slides were cleaned with
acetone and isopropyl alcohol consecutively before mounting them on the sample holder
and placing the assembly into the chamber. The substrate was placed parallel to the
sapphire windows to facilitate the collection of UV-vis spectra during film growth.
4.4.3. Thin Film Deposition Conditions
The deposition of PBTTT-C14 in pressurized n-pentane was performed by first
increasing the temperature of the solution to 130 °C. When pressure and temperature had
stabilized, the temperature of the ITO glass slide was increased to 160 °C. Increasing the
substrate temperature had a subtle impact on the chamber pressure which was addressed
accordingly to maintain the desired pressure. In-situ UV-vis measurements were collected
during deposition beginning at the 10-minute mark of the deposition process, after the
temperature and pressure of the system had stabilized. Spectra were collected every 5
minutes thereafter to monitor the rate of deposition.
4.4.4. Gravimetric Analysis
Gravimetric analysis of PBTTT-C14 saturated solutions in n-pentane was
perfomred by exhausting the chamber contents through the chamber outlet into a
polypropylene vessel with a volume of 500 mL. In more detail, a crucible containing
PBTTT-C14 was placed at the bottom of the custom pressure cell. The cell was overfilled
with deoxygenated pentane before being sealed and pressurized. The vessel had an inlet
port to accept the chamber contents and a wide outlet port to avoid over-pressurizing the
vessel (the setup assembly can be found in Figure 3.2). The rapid expansion of the
chamber contents cooled the solution below its boiling point, allowing liquid n-pentane and
(precipitated) PBTTT-C14 to be collected. Solvent (n-pentane) was removed from the slurry
under reduced pressure, and the dried PBTTT-C14 material was weighed.
91
4.4.5. Pattern Development
The patterned ITO-coated substrates were prepared through photolithography
using the OAI Model 800 MBA Mask Aligner in the 4D LABS facility at Simon Fraser
University. The photomasks, fused quartz plates covered with patterned microstructures
of chromium, were also fabricated at 4D LABS. First, the substrates were placed in
Nanostrip2X solution for ~2 hours. Second, the substrates were plasma etched with
oxygen for 3 minutes by using a Technics PE-IIA Plasma Etch system. To ensure the
adhesion of photoresist to the substrate surface, hexamethyldisilazane (HMDS) vapor was
reacted with the surface prior to spin-coating of the photoresist. After the development of
the photoresist, the ITO patterns were generated by using a reactive ion etching plasma
system from SENTECH (SU 591) located in the 4D LABS user facility at Simon Fraser
University. The etch recipe was, 12 mTorr/ 400 W/ 2 sccm Cl2/ 40 sccm BCl3 / 40 sccm
Ar.277 Removal of the photoresist yielded ITO lines 5 μm in width with 50 μm of bare glass
between the traces.
Films of cross-linked PMMA were created by using a solution of PMMA (SigmaAldrich, average MW ≈ 996,000) in butyl acetate (20 mg.mL−1). The film was spin-cast at
1000 rotations per minute for 90 seconds and was crosslinked by heating to 220 °C for at
least 30 minutes.271 The resulting film thickness was 165 ± 15 nm as measured by a Bruker
Dektak XT profilometer. The PDMS substrate was fabricated by using premixed and
degassed PDMS (Sylgard 184 silicone elastomer kit, base and curing agent mixed at a
ratio of 10:1). A thin layer of PDMS was shaped into a plano-convex by using a piece of
concave DuPont Vespel Polyimide. A zigzagged shaped nichrome wire (AWG = 40, d =
0.0799 mm) was embedded into the PDMS layer before curing. Afterward, the curing was
performed in an oven at 100 °C for an hour.
4.4.6. Characterization of Deposited Thin film
Ex-situ UV-vis spectra of the deposited films, including those prepared by spincoating, were collected with an Agilent 8453 UV-vis spectrophotometer. Spin-coated films
were prepared by using PBTTT-C14 in 1,2-dichlorobenzene, (10 mg.mL−1), and the
resulting film thickness was ~ 30 nm as measured by a Bruker Dimension Icon atomic
force microscope.
92
Raman spectra were collected using a Reinshaw inVia Raman microscope (50×
objective) with an excitation wavelength of (785 nm/50 mW) and acquisition time of 10
seconds. The laser power was set to 1% of its maximum power for as-received PBTTTC14 and 50% for the PBTTT-C14 thin film grown in the supercritical fluid chamber at a
pressure of 17.2 MPa. All spectra were background-corrected, including the removal of
signals from ITO in the case of the film deposited from supercritical fluids. Spectra were
normalized to the peak intensity at 1390 cm-1.
Bright-field microscope images were collected with the Axioplan 2 imaging
universal microscope from Zeiss, which was coupled with a RETIGA EXi scientific camera
from QImaging.
93
Chapter 5. Physical Supercritical Fluid Deposition
of Aliphatic Polymer Films: Controlling the
Crystallinity with Pressure
The results presented in this chapter have been reported in part, see:
Yousefi, N.; Saeedi Saghez, B; Pettipas, R. D.; Kelly, T. L.; Kaake, L. G. Mater.
mg/ml
solution
substrate
Chem. Front., 2021, 5, 1428-1437.
T
P
Isobaric
iPP Thin Film
ITO Coated Glass
PC ,TC
𝑙
𝑔
T
The depositions of isotactic polypropylene in supercritical fluids presented in this
chapter were carried out by Nastaran Yousefi and undergraduate student Behrad Saeedi
Saghez assisted in performing the depositions. Dr. Timothy Kelly supervised the collection
and analysis of GIWAXS data and contributed insightful discussions to this work.
5.1. Introduction
The science of nanofabrication has achieved tremendous success in scaling
circuitry via photolithography.278,279 This top-down approach can be contrasted with the
bottom-up approach employed in biology, where larger, more complicated structures are
created from molecular building blocks. Synthetic developments along this line have
created an incredible diversity of structures, for example molecular machines, 280,281 DNA
nano-origami,282-284 and micellular structures for drug delivery,285-288 to name but a few.
Moreover, a plethora of inorganic structures can be grown from solution, including
nanowires,289-291 colloidal nanocrystals,292-294 and plasmonic nanostructures.295-297 To date,
94
the top-down and bottom-up approaches to self-assembly and nanofabrication exist in
largely non-intersecting regions of chemical parameter space. This limits the chemical
complexity of materials that can be used in a photolithographic process, and limits the
reproducibility, scalability, and control of bottom-up self-assembly.
To address this challenge and better exploit the possibilities afforded by
connecting top-down with bottom-up approaches to self-assembly, we introduced a
method of film formation that utilizes supercritical fluids (SCFs) in Chapter 4.253 The
technique works by exploiting a maximum in the isobaric solubility of a material to deposit
films onto a heated substrate. In the previous work, the substrate was heated resistively
by passing current through indium tin oxide (ITO) coated glass substrates. Film deposition
was controlled by lithographically patterning the ITO films, producing local resistive
heating. Material was found to deposit only where the ITO was present, even when the
substrate was coated with a thin film of poly(methylmethacrylate). This demonstration
provides a clear path towards controlling solution-phase self-assembly of high molecular
weight materials using an additive process that is in turn controlled via photolithography.
The additive nature of the pattern formation is critical to providing a clear path towards the
unification of top-down and bottom-up approaches to self-assembly and nanofabrication.
However, self-assembly in supercritical fluids is not well understood despite a long history
of materials-processing using supercritical fluids.
Supercritical fluids exhibit properties intermediate to liquids and gases; these
properties can be tuned by varying the temperature and pressure of the system. More
generally, a supercritical fluid can be described as a fluid that is pressurized to above or
near its critical pressure and held at a temperature near its critical temperature. While this
region of the phase diagram cannot be strictly defined in terms of a first-order phase
transition, it can be described by several distinct regions. In general, any material in the
vicinity of its critical temperature and pressure can be described as a supercritical fluid. 186
The ability of supercritical fluids to exhibit phase behavior intermediate to liquid and vapor
has provided many successful applications in areas of polymer synthesis, 226-231,298 and
creation of nanoscale structures via reduction of metal salts. 232,233 Moreover, supercritical
fluids have been employed in medicine for processing of tissue engineering scaffolds. 299303
95
In order to better understand materials self-assembly in supercritical fluids, we
chose to study isotactic polypropylene (iPP). It is one of the most widely used plastics
because of its thermoprocessability, stability, and good mechanical properties. 304-306 In
addition, it is one of the best-understood polymers, and its properties have been further
tailored by altering the chain architecture or by means of post-processing techniques.307311
These techniques are targeted to modify aspects of the structural hierarchy (crystallite,
lamella, and aggregates) that ultimately determine the macroscopic material properties.
Owing to its regioregularity, iPP is a highly crystalline polymer with at least 4
different reported structures. The α-form (monoclinic, the most frequent form), β-form
(hexagonal), γ-form (triclinic) and one smectic form.312-315 In addition, the material exhibits
diverse self-assembly properties at longer length scales. The most commonly observed
structure consists of stacked crystalline lamellae which further assemble to form
spherulites and cylindrites.316 Other observed motifs include dendrites, quadrites, and
hedrites. 308,317-321
The process of crystallization in thin polymer films is different than the polymer
melt, resulting in different spherulite morphologies. 322 In a polymer thin film, the spatial
restriction causes a preferential orientation of the lamellae that contrasts with the
continuous branching and epitaxial growth observed in polymer melts.323-325 In the case of
polymer thin films, pre-aggregation of the polymer happens in the solution state prior to
film deposition and solvent chemistry can alter the degree of pre-aggregation.326,327
There has been an interest in supercritical fluids or compressible dense fluids over
the last few decades due to their ability to tune their solvation power by changing solvent
temperature and pressure.262,328-330 In particular, polymer solubility in SCFs has brought
numerous opportunities for physicochemical changes in polymers. 331-334 SCFs have been
used to lower polymers’ glass transition and induce re-crystallization.333,335 In addition,
crystallization from polymer solutions in dense fluids promotes the formation of different
polymorphic states or development of different crystal morphologies such as zigzag or
helical chain conformations observed in polystyrene.336,337
96
5.2. Results and Discussion
5.2.1. Isotactic Polypropylene Solubility in Supercritical n-pentane
Before growing films of iPP, it is necessary to understand its solubility properties
at elevated temperature and pressure. More specifically, the technique relies on a
maximum in the isobaric solubility as a function of temperature. The measurement was
performed gravimetrically; the solution was exhausted from the chamber and the mass of
dissolved iPP was measured in the same manner described in section 4.4.4. Figure 5.1
shows the equilibrium concentration of iPP in n-pentane as a function of temperature at
different pressures. Qualitatively, the concentration increases with temperature up to 418
K, after which further increases in temperature result in a drop in saturation concentration
up to 433 K.338 The observed nonmonotonic solubility behavior for iPP agrees with our
previous study in which we investigated the solubility of PBTTT-C14 in the same solvent
(see Chapter 4).253 It should be mentioned that at higher temperatures, we observed that
the upper critical solution temperature (UCST) of iPP is reached and the compound
becomes miscible in all proportions. This condition was scrupulously avoided in
8
3.5 MPa
7.0 MPa
10.3 MPa
17.2 MPa
-1
Concentration (mg.ml )
subsequent experiments.
6
4
2
0
320
360
400
Temperature (K)
440
Figure 5.1. Isobaric concentration of iPP in n-pentane as a function of temperature.
Reprinted with permission from reference 338.
97
5.2.2. Empirical Model for Polymer Solubility at Elevated Pressures
The nonmonotonic solubility behavior of iPP is an important aspect of the film
deposition technique presented here and warrants further comment. At ambient
pressures, the free energy of solvation is driven by increasing polymer entropy. 194, 261 At
higher pressures, decreasing solubility with increasing temperature indicates a negative
entropy of solvation as the enthalpy of solvation in hydrocarbon solvents is not strongly
temperature dependent. This negative entropy contribution results from the solvent
entropy. Specifically, polymer solvation is accomplished by the creation of a solvent shell
surrounding the macromolecule. As the temperature approaches the critical temperature
from below, the solvent shell becomes higher in density than the surrounding solution.
Breaking this shell and releasing the solvent into the bulk increases the overall entropy of
the system, causing the polymer to precipitate.262
Polymer solution thermodynamics is typically approached from the Flory-Huggins
formalism,339,340 with corrections to the effective interaction parameter being commonly
used to describe deviations that lead to a lower critical solution temperature (LCST), for
example.341-343 However, the model is derived under the explicit assumption of an
incompressible solution and as such, performs poorly as it is taken further from this original
context. To extend the generality of the Flory-Huggins model, a statistical model based on
lattice fluids was put forward by Sanchez and Lacombe. 263 However, the price of this
generality is that the expression is quite involved and generating a fit to our data is not
feasible. Here, we propose an empirically motivated formulation that describes the change
in the equilibrium concentration of polymer at elevated pressures and temperatures, where
the solvent compressibility plays a dominant role.
We begin by using classical thermodynamics to describe the problem broadly. In
particular, the existence of a maximum in the isobaric solubility versus temperature implies
that the derivative of free energy with respect to the moles of dissolved polymer (𝑛2 ) and
temperature is zero.
(
𝜕2𝐺
) =0
(5.1)
𝜕𝑛2 𝜕𝑇 𝑃
Using the partial derivative relating Gibbs free energy and entropy
98
𝜕𝐺
( ) = −𝑆
(5.2)
𝜕𝑇 𝑃
It is then clear that at the maximum in solubility, the entropy of solvation is also at a
maximum.
(
𝜕𝑆
) =0
(5.3)
𝜕𝑛2 𝑃
Equation 5.3 can be interpreted to mean that the entropy of dissolving additional
polymeric material is counterbalanced by a decrease in solvent entropy. As described
above, this behavior results from the necessity of forming a solvent shell around the solute.
Under these conditions, releasing solvent from the solvent shell increases translational
entropy, resulting in a negative entropy of solvation, favoring the precipitation of polymeric
material.
We therefore write the Gibbs energy in terms of the enthalpy ∆𝐻, which contains
the familiar solubility parameters, the configurational entropy of the solvent and polymer
∆𝑆𝐶 , and the entropy of solvation related to the pure solvent ∆𝑆𝑃 .
∆𝐺 = ∆𝐻 − 𝑇∆𝑆𝐶 − 𝑇∆𝑆𝑃
(5.4)
Instead of taking derivatives of ∆𝐺, we use it to write the solubility coefficient 𝐾,
and assume that this coefficient is proportional to the mole fraction of dissolved polymer
to within a multiplicative and an additive constant.
∆𝐺 = −𝑅𝑇 𝑙𝑛 𝐾 = −𝑅𝑇 𝑙𝑛 [
𝜙2
𝑎
𝑏
− ]
(5.5)
𝑎
Upon rearranging Eq. 5.5 and using Eq. 5.4, we obtain the following:
𝜙2 = 𝑏 + 𝑎 𝑒𝑥𝑝 [−
∆𝐻−𝑇𝛥𝑆𝐶 −𝑇𝛥𝑆𝑝
𝑅𝑇
]
(5.6)
In the case of our system, the configurational entropy plays only a small role in
determining the solubility. This is appropriate given the low concentration regime we are
working at and the high molecular weight of the polymer. We will therefore assume this
term to be a small contribution that can be accounted for within 𝑎 and/or 𝑏, as it most
strongly influences solubility behavior at low temperatures, away from the region of the
phase diagram that we are most interested in.
99
The relationship between the solvent entropy and internal energy is of primary
importance in determining the solution properties. The US National Institute of Standards
and Technology (NIST) has an extensive database where data regarding the properties
of pure fluids is tabulated and accessible. Under the conditions of our experiment, the
entropy of pure n-pentane increases linearly as a function of temperature. We therefore
write the entropy of solvation as follows:
𝛥𝑆𝑝 = −𝛾𝑇
(5.7)
The sign and temperature dependence of the entropy allows Eq. 5.6 to be rewritten using
𝑇0 ≡ √∆𝐻 ⁄𝛾 as follows:
𝜙2 (𝑇) = 𝑏 + 𝑎 𝑒𝑥𝑝 [−
𝛾(𝑇−𝑇0 )2 +2𝛾𝑇 𝑇0
𝑅𝑇
]
(5.8)
Incorporating 2𝛾𝑇0 into 𝑎 yields the final expression which we use to fit our data.
𝜙2(𝑇) = 𝑏 + 𝑎 𝑒𝑥𝑝 [−
𝛾(𝑇−𝑇0 )2
𝑅𝑇
]
(5.9)
Figure 5.1 compares the solubility data obtained gravimetrically with a fit of Eq.
5.9. Qualitatively, the fit matches the observations with very little dissolved material at the
lowest temperature and a peak in the solubility (𝑇0 ) at approximately 417 K. However, to
fit the pressure dependent data in Figure 5.1, we describe the change in volume according
to the appropriate Maxwell relation,
𝜕∆𝐺
∆𝑉 ≡ (
𝜕𝑃
)
(5.10)
𝑇
and write the pressure dependence of the solubility as follows:
𝜙2 (𝑃) = 𝑏 + 𝑎 𝑒𝑥𝑝 [−
𝑃∆𝑉
𝑅𝑇
]
(5.11)
Figure 5.2 compares the solubility of iPP in n-pentane at different pressures at
constant temperatures as a function of pressure. As shown, the iPP solubility drops as a
function of the pressure. The decline in solubility seems to be more pronounced at lower
pressures and as pressure further increases the effect is of lesser importance. 338 The
results and the form of Eq. 5.11 are in good agreement with previous reports on the
solubility of iPP in n-pentane.344
100
Concentration (mg/ml)
403 k
423 K
8
418 K
433 K
7
6
5
3
6
9
12
15
18
Pressure (MPa)
Figure 5.2. Isotherm concentration of iPP as a function of pressure. Reprinted with
permission from reference 338.
In a similar manner to our previous study on PBTTT-C14, the decrease in the
isobaric solubility of iPP as a function of temperature can be used to form thin films on a
heated substrate. We held the temperature of the cell wall at the solubility maximum (Twall
≈ 418 K) and resistively heated an ITO coated glass substrate that was immersed in the
fluid (Tsub ≈ 433 K). This procedure causes material to precipitate onto the substrate
surface, forming a thin film.
5.2.3. Characterization of Deposited Thin Film
Figure 5.3 shows the deposited films with a series of polarized optical microscope
(POM) images. The bright regions in POM micrographs represent the crystalline domains
of the iPP film. The observation of decreasing brightness with increasing pressure during
deposition suggests that the crystallinity of thin film decreases with pressure. Furthermore,
samples which do exhibit crystallinity (i.e. films grown at 3.5 MPa) do not resemble the
commonly observed spherulitic morphology.345,346 Instead, we observe a growth process
similar to that of diffusion-limited aggregation. In this growth mode, a crystal or aggregate
would grow more randomly, which results in fractal-like forms.345
101
3.5 MPa
7.0 MPa
10.3 MPa
17.2 MPa
Figure 5.3. Polarized optical microscopy images of iPP films grown in supercritical
n-pentane (x10) at different pressures. Adapted with permission from reference 338.
To further demonstrate decreasing crystallinity with increasing pressure and to
determine the crystal structure of our films, we performed 2D grazing-incidence wideangle x-ray scattering experiments. Figure 5.4 displays the observed scattering pattern
and their corresponding azimuthal integration from films grown at several different
pressures. The GIWAXS patterns are consistent with the α-form of iPP. In particular, the
(110)α, (130)α, (040)α, (111)α, (131)α, and (041)α peaks, diagnostic of the α-form are
observed.347-349 Moreover, the azimuthally-integrated GIWAXS patterns in Figure 5.4 show
scattering intensity as a function of scattering vector (q). The observed scattering pattern
is consistent with the iPP α-phase,347,348 with Miller indices of the reflections identified in
102
red. In addition, a feature attributed to β(300) is observed at 1.09 Å -1, indicating a small
amount of the β-phase in iPP film grown at 3.5 MPa. This feature is not present at higher
pressures.
In addition, the intensity of the diagnostic peaks decreases for iPP films as
pressure increases. This is consistent with our initial interpretation of the POM images,
suggesting that the sample crystallinity decreases with increasing deposition pressure. 338
Previous studies of high-molecular-weight iPP demonstrate that it adopts the γ-form under
high-pressure conditions, with the highest percentage of γ crystallinity found at 200 MPa.
350-353
We have not observed any evidence of γ-phase in our films, consistent with the
lower pressure range of our experiment (<35 MPa).
103
α(13 )/(041)
α(200)
α(150)/(060)
α(111)
α(040)
α(130)
α(110)
β(300)
3.5 MPa
7.0 MPa
10.3 MPa
17.2 MPa
Figure 5.4. GIWAXS patterns of iPP films grown in pressurized n-pentane at
different pressures (log scale) and their Azimuthally-integrated GIWAXS patterns.
Adapted with permission from reference 338.
104
The GIWAXS patterns also provide information regarding the preferred orientation
of the iPP chains relative to the substrate. This can be more easily shown with partial pole
figures in Figure 5.5.a, constructed from the data in Figure 5.4. The data show that the
(110) reflection has two distinct preferred orientations, both out-of-plane (along qz) and inplane (along qr). The (040) reflection is preferentially oriented along qz, which is consistent
with the observed in-plane (110) scattering. This suggests two preferred orientations for
the iPP crystallites, shown in Figure 5.5.b, with either q(110) or q(040) normal to the substrate.
In both orientations, the iPP chains lie parallel to the substrate.338
a.
7.0 MPa
3.5 MPa
(
)
(
)+(
10.3 MPa
17.2 MPa
)
b.
Figure 5.5. (a) GIWAXS partial pole figures of the iPP films grown in n-pentane at
different pressures. (b) Proposed model for the two preferred orientations of iPP
chains. Reprinted with permission from reference 338.
As mentioned earlier, the non-spherulitic morphologies of the iPP films grown
under supercritical conditions suggests that diffusion-limited aggregation is the primary
growth mechanism. This rationalizes the more chaotic assembly of iPP than would be
inferred from large scale spherulitic morphologies. As a result of the diffusion-limited
aggregation mechanism, aggregation in solution (a.k.a pre-aggregation) is most likely
responsible for the pressure dependence of the crystallinity.
105
If polymer chains aggregate prior to deposition, fluid properties must be used to
explain the decrease in crystallinity at elevated pressures. Because the chamber is sealed
and does not contain a free surface, the Rayleigh number (𝑅𝑎) can be used to distinguish
between possible flow regimes.354 This quantity can be expressed in terms of the
isothermal compressibility (𝛽), the magnitude of the thermal gradient (𝛥𝑇), density (𝜌),
dynamic viscosity (𝜇), thermal diffusivity (𝛼), and the length scale relevant to the problem
(𝑥), presumably the distance between the chamber wall and the substrate in this
experiment, which is approximately 2 cm.
𝑅𝑎 =
𝛽𝛥𝑇𝜌𝑔𝑥3
(5.12)
𝜇𝛼
Three regimes of fluid flow can be distinguished. At low Rayleigh number, (𝑅𝑎 <
103 ) the fluid does not flow. In this case, mass and thermal transport are driven by
diffusion. At higher Rayleigh number, turbulence develops, and flow is driven by density
fluctuations arising from thermal gradients. In this flow regime, most of the temperature
drop occurs in a thin fluid layer near the boundary where flow can be thought to be
laminar.355 At very high Rayleigh number (𝑅𝑎 > 1013 ) the turbulence disrupts even the
boundary layer, entering what is called “ultimate” flow. 356 We visually observe light
scattering at the substrate surface during deposition which suggests that flow is in the
turbulent regime. This is confirmed by an estimate of the Rayleigh number (𝑅𝑎) on pure
n-pentane from data tabulated by NIST which places 𝑅𝑎 ≈ 1011 . For completeness, we
also estimate the Prandtl number (𝑃𝑟) to be approximately 𝑃𝑟 = 3.5.
The flow regime appropriate for our experimental conditions and our observations
suggest that during film deposition, the temperature drops rapidly in a thin boundary layer
near the ITO surface. This is depicted in Figure 5.6, a cartoon which describes our
proposed mechanism of film formation.
106
Figure 5.6. Cartoon representing the change in the fluid temperature and flow
during the deposition process. Black lines indicate iPP chains, white lines describe
turbulent flow. Reprinted with permission from reference 338.
The presence of polymer in solution affects solution viscosity, with higher
concentrations of polymer being associated with lower 𝑅𝑎 and less chaotic flows. We
therefore hypothesize that the lower concentration of polymer at higher pressures
increases 𝑅𝑎 making the flow more turbulent, inhibiting the self-assembly of crystallites.
Given the limited diffusion of polymers once they have impacted the surface, smaller and
less ordered material would result in more amorphous films. This explanation is in accord
with the pressure dependence of our POM and GIWAXS results.
5.3. Conclusion
We measured the solubility of isotactic polypropylene in supercritical n-pentane as
a function of temperature and pressure by means of gravimetric analysis. The isobaric
solubility exhibits a clear peak versus temperature, while the isothermal solubility exhibits
a decrease with pressure for the lowest pressures and is largely pressure independent
thereafter. We described our observations with a simple model based on classical
thermodynamics. Thin films were formed by resistively heating an ITO coated glass slide
immersed in a saturated solution. Films were grown at several pressures and their
morphology was studied using polarized optical microscopy and grazing incidence wide
angle x-ray scattering. Both the microscopy and the x-ray scattering were consistent with
an overall decrease in crystallinity. The α-phase of the material was the predominant
crystalline form with the dominant chain orientation parallel to the substrate.
During deposition, we observe light scattering near the substrate surface,
indicative of solution turbulence. In addition, pure n-pentane exhibits high Rayleigh
107
number under these conditions, also pointing towards a turbulent flow regime. We
presented a mechanistic description of film formation. First, material precipitates from
solution in a pre-aggregation step forming crystallites if not disrupted by solution
turbulence. Second, material is deposited onto the substrate where it has limited ability to
reorganize to form larger scale structures. These findings provide us with basic principles
governing the self-assembly in supercritical fluids and highlight the importance of
engineering low Rayleigh number flows to achieve more controlled self-assembly.
5.4. Methods
5.4.1. Solubility Measurement via Gravimetric Analysis
Solubility measurements were performed by placing isotactic polypropylene
(Sigma Aldrich, average MW ~ 250,000) into a crucible at the bottom of the chamber before
the vessel was sealed and filled with deoxygenated solvent. The chamber exterior was
heated using an Omega benchtop PID controller. After the temperature stabilized, the
solution was pressurized to 3.5, 7.0, 10.3, and 17.2 MPa using the manual pressure
generator and given approximately four hours to reach saturation. Afterward, gravimetric
analysis of saturated solutions of isotactic polypropylene in n-pentane were carried out by
exhausting the chamber contents through the chamber outlet into a polypropylene vessel.
The gravimetric analysis was carried out similar to PBTTT-C14 analysis discussed in
section 4.4.4.
5.4.2. Substrate Preparation for Thin Film Deposition and Deposition
Condition
The substrates used for deposition were the same ITO coated glass slides used
for the deposition of PBTTT-C14. The substrate specification and preparation steps can be
found in section 4.4.2. The deposition of isotactic polypropylene in pressurized n-pentane
was carried out by first increasing the temperature of the solution to 418 K. When pressure
and temperature had stabilized, the temperature of the ITO glass slide was increased to
433 K. Increasing the substrate temperature had a subtle effect on the chamber pressure
which was adjusted to maintain the desired pressure.
108
5.4.3. Characterization of Deposited Thin Film
Texture and phase behavior analyses were carried out using polarized optical
microscopy (POM) on an Olympus BS50 microscope equipped with cross polarizers.
GIWAXS measurements were performed at the Hard x-ray MicroAnalysis (HXMA)
beamline of the Canadian Light Source. An energy of 12.688 keV was selected using a
Si(111) monochromator. The beam size was defined by slits having a 0.2 mm vertical gap
and a 0.3 mm horizontal gap, and the angle of incidence was set to 0.1°. The diffraction
patterns were collected on a Rayonix SX165 CCD camera (80 μm pixel size; 16.3 cm
diameter) using an acquisition time of 30 s. The sample-to-detector distance (224 mm)
was calibrated using a silver behenate powder standard. The GIWAXS data were
processed using the GIXSGUI software package in MATLAB.357 The patterns were
calibrated, solid angle and polarization corrections applied, and the data was reshaped to
account for the missing wedge along q z.
109
Chapter 6. Physical Supercritical Fluid Deposition
of Aliphatic Polymer Films: Controlling the
Crystallinity with Solvent Additive
The results presented in this chapter have been reported in part, see:
Yousefi, N.; Saeedi Saghez, B; Pettipas, R. D.; Kelly, T. L.; Kaake, L. G. New J.
𝑆𝑜𝑙 𝑡𝑖𝑜𝑛
𝑆 𝑏𝑠𝑡𝑟𝑎𝑡𝑒
Chem., Submitted for publication, 2021.
mg/ml
Acetone
Acetone
Spherulite Growth
Pentane
T
P
Isobaric
𝑙
PC ,TC
𝑔
iPP Thin Film
ITO Coated Glass
T
The depositions of isotactic polypropylene in supercritical fluids presented in this
chapter were carried out by Nastaran Yousefi and undergraduate student, Behrad Saeedi
Saghez assisted in performing the depositions. Dr. Timothy Kelly supervised the collection
and analysis of GIWAXS data and contributed insightful discussions to this work.
6.1. Introduction
Photolithography, the prime example of top-down nanomanufacturing methods is
ubiquitous in the microelectronics industry.278,279,358 In contrast, bottom-up processes
create more complex structures from the self-assembly of molecular and atomic
components.359 The bottom-up approach has resulted in diverse set of novel structures
such as molecular machines,280,281 DNA nano-origami,282-284 and micellular structures for
drug delivery,285-288 amongst many others. Furthermore, a plethora of inorganic structures
can be created including nanowires,289-291 nanotubes,360 colloidal nanocrystals,292-294 and
plasmonically active nanostructures.295-297 The central challenge in nanotechnology is that
these approaches are still largely non-overlapping. This limits the chemical complexity of
materials that can be assembled using top-down lithography, and limits the reproducibility
110
and hence, the scalability of manufacturing processes that use bottom-up self-assembly
to position nanoscale structures on surfaces.
In an effort to address this challenge, we have introduced a film formation
technique using supercritical fluids (SCFs). 253 The technique relies on a maximum in the
isobaric solubility of a material to deposit films onto a heated substrate. Resistive heating
provides the most straightforward means of increasing substrate temperature. This
approach to self-assembly provides an entirely new path towards well-controlled selfassembly of high molecular weight materials in the solution phase using an additive
process that is in turn controlled via photolithography. The successful coupling of solution
phase self-assembly and lithography bridges the gap between bottom-up and top-down
self-assembly. However, the mechanism by which the self-assembly takes place in
supercritical fluids is not well understood despite a long history of materials processing
using supercritical fluids.
The SCF is a state intermediate between gas and liquid and its properties, for
example, solvation power can be easily tuned by changing temperature and/or pressure
of the system.262,328-330 The unique combination of liquid-like density, and gas-like
diffusivity and solvating properties of SCFs have resulted in a variety of successful
applications in the field of polymer science. For example, SCF’s can be used as a solvent
for synthesis,226-231,298 or imparting chemical and morphological changes.331-334 Other
studies have used supercritical fluids to lower polymers’ glass transition and induce recrystallization.333,335 Additionally, significant morphological changes are induced by the
crystallization from polymer solutions at high pressures such as decrease in the crystal
lamellae thickness in polycaprolactone332 and zigzag or helical chain conformations
observed in polystyrene.336,337
In order to gain a better insight into polymer self-assembly in supercritical fluids,
we selected isotactic polypropylene (iPP) since it is one of the most well-understood
crystalline polymers.304-306 This polymer has properties that can be further tailored by
altering the chain architecture or by means of post-processing techniques.307-311 These
techniques are designed to alter structural hierarchy (crystallite, lamellae, and aggregates)
that ultimately determine the macroscopic material properties.
111
The crystallization of iPP proceeds by the known two-step process of nucleation,
followed by crystal growth.361,362 The crystalline phase may adopt different structures and
morphologies based on the crystallization conditions.363,364 For instance, thin polymer films
and materials formed from the melt have different spherulite morphologies. 322 The
difference is mainly due to the spatial restriction that lamellae experience in polymer thin
film, inhibiting the primarily epitaxial growth seen in polymer melts. 323-325
To gain better control of self-assembly in supercritical fluids, we investigated
solvent additives and/or mixed solvents. The use of additives in polymer processing is
common, especially in the context of nucleation agents and plasticizers. 365,366 The use of
solvent additives, intended to evaporate during thin film formation, has been extensively
investigated in the organic electronics community, 34,35,157,367 and has been shown to
profoundly influence solar cell efficiencies by altering nanoscale self-assembly.153,154,368-370
In the case of polymer thin films, the polymer pre-aggregation in the solution state and the
solvent chemistry play pivotal roles in the formation of different morphologies in the
deposited films.326,327 The solvent additive approach has been demonstrated as a
promising method to alter polymer aggregations and obtain different thin film
morphologies.371-375
6.2. Results and Discussions
6.2.1. Solubility of Isotactic Polypropylene in a Binary Solvent System
In order to study the iPP solubility and its self-assembly in supercritical npentane:acetone, we
established the
equilibrium concentration
of
iPP
in n-
pentane:acetone solutions at different temperatures and pressures using gravimetric
analysis. Figure 6.1 displays the concentration of iPP as a function of temperature at a
pressure of 10.3 MPa for each solvent system. Initially, the concentration of iPP increases
with temperature before reaching a maximum and decreasing with further temperature
increases. This nonmonotonic behavior was observed previously with PBTTT-C14 (refer to
section 4.2.1). It should be pointed out that we observe an upper critical solution
temperature (UCST) of 448 K and have deliberately conducted our experiments below this
temperature. We note that the UCST we observe is higher than a previously reported
LCST for this system.376 We interpret the previous work as demonstrating nonmonotonic
solubility behavior rather than a true transition from miscibility to phase-separated
112
behavior.
Increasing the volume fraction of acetone causes the temperature of the
concentration maximum to shift towards higher temperatures. In addition, the peak
appears to sharpen.
Concentration (mg/ml)
3.0
2.5
2.0
P= 10.3 MPa
pentane
pentane + 1% acetone
pentane + 10% acetone
1.5
1.0
0.5
0.0
340
360
380
400
420
Concentration (mg/ml)
Figure 6.1. Isobaric concentration of iPP in n-pentane:acetone as a function of
temperature at P = 10.3MPa.
The observation of a solubility maximum at temperatures lower than a UCST is not
easily explained using the standard Flory-Huggins approach to solution thermodynamics.
In this approach, increasing polymer entropy promotes solvation while mismatched
solubility parameters inhibit solvation. The relative contribution of mismatched solubility
parameters to the free energy of mixing decreases monotonically with temperature,
eventually leading to a UCST, beyond which the polymer is miscible with the solvent.
Conversely, decreasing solubility with increasing temperature requires a negative entropy
of solvation in the absence of specific intermolecular forces which can be broken (e.g.
hydrogen bonds).
A negative entropy of solvation points towards the importance of the solvent
entropy in the solvation process. As the critical temperature of the solvent is approached,
the density of the solvent decreases substantially, and a chemical equilibrium can be
thought to exist between the solvent shell and the bulk solvent. Releasing solvent
molecules from the shell and precipitating the polymer increases the overall entropy,
leading to a negative entropy of solvation.
113
6.2.2. Polymer Solubility Model for a Binary Solvent System
A few quantitative models predicting polymer solubility in supercritical fluids exist
including those of Sanchez-Lacombe, Simha-Somcynsky, the Statistical Associating Fluid
Theory (SAFT) and the perturbed chain (PC)-SAFT.263, 335, 377, 378 However, these models
are too complex to fit our data set and, in their generality, lack intuitive transparency. To
address this, we developed a simple model based on classical thermodynamics to
describe polymer solubility for the more limited case in which we are primarily interested
(refer to section 5.2.2.)
We describe the free energy, ∆𝐺, of the system in terms of the enthalpy ∆𝐻, which
includes the intermolecular forces and is typically given in terms of solubility parameters.
Equation 6.1 also contains the configurational entropy of the solvent and polymer ∆𝑆𝐶 , and
the entropy of solvation related to the pure solvent ∆𝑆𝑃 .
∆𝐺 = ∆𝐻 − 𝑇∆𝑆𝐶 − 𝑇∆𝑆𝑃
(6.1)
Focusing only on the interplay between ∆𝐻 and ∆𝑆𝑃 , we can derive an expression
for the concentration of iPP versus temperature as follows:
𝜙2 = 𝑏 + 𝑎 𝑒𝑥𝑝 [−
𝛾(𝑇−𝑇0 )2
𝑅𝑇
]
(6.2)
Where 𝑇0 ≡ √∆𝐻 ⁄𝛾 with 𝛾 describing the slope of a linear increase in ∆𝑆P with
respect to temperature. This assumption is consistent with the behavior of solvent entropy
with respect to temperature.
Figure 6.1 shows the fit with respect to the collected data. The peak in the
concentration versus temperature can be fit with an approximately Gaussian lineshape. In
addition, Eq. 6.2 makes specific predictions about the position of the peak with respect to
changing solvent composition. We expect that acetone, an antisolvent for iPP, should
have a larger ∆𝐻 of solvation, and exhibit a higher temperature peak in the isobaric
solubility. This is consistent with our observations, demonstrating the validity of Eq. 6.2 in
qualitatively describing this system. The peak also appears to narrow, but noise in the
data prevents more specific conclusions being drawn. Gravimetric analysis is a robust
technique, but not exceptionally precise and has very poor throughput.
114
6.2.3. Characterization of iPP Thin Film
Using the solubility data in Figure 6.1, we devised the deposition conditions that
would produce thin films of iPP from supercritical n-pentane:acetone solutions. The
temperature of the chamber wall was set to 418 K (423 K in case of n-pentane:acetone).
And, an ITO coated glass substrate was resistively heated to 433 K to initiate the
deposition. These conditions were reproduced for several pressures, allowing us to
examine effect of pressure and changing solvent composition.
Figure 6.2 presents a series of polarized optical microscope images of the
deposited iPP films. Bright regions are typically interpreted to indicate the presence of
birefringent crystallites. We observe a decrease in brightness of POM micrographs with
an increase in pressure across all solvent systems, indicating reduced crystallinity of thin
films with pressure. In addition, the films grown in n-pentane with crystalline domains (i.e.
films grown at 3.5 MPa) do not show similar features to the commonly observed
spherulites in iPP films. The lack of order in crystalline regions resembles the diffusion
limited aggregation growth process in which the mechanism of growth is more random
and results in fractal structures.345
On the other hand, the addition of acetone to n-pentane not only enhances
spherulite formation in iPP films but also increases spherulite diameter. The influence of
solvent additive is more pronounced at low pressures especially at 3.5 MPa with the
largest spherulites being formed with the highest concentration of acetone. We postulate
that the acetone remains in the film for a short time following material deposition,
increasing chain mobility following deposition. As a side note, we observe dendritic ß-iPP
lamellae in case of 1% acetone addition at 7.0 MPa. The crystallization of ß-iPP in thin
films requires α-form crystals to act as nucleation sites for ß-iPP crystals switching the
direction of lamellar growth.
115
7.0 MPa
10.3 MPa
17.2 MPa
10% acetone
1% acetone
Pure pentane
3.5 MPa
Figure 6.2. Polarized optical microscopy images of iPP films grown in supercritical
n-pentane and n-pentane:acetone (x10) at different pressures.
The description emerging from the POM analysis is that crystallinity decreases as
pressure increases and increases as acetone concentration increases. To further study
the effect of pressure and solvent additive on the morphology of deposited films, we
performed 2D grazing-incidence wide-angle x-ray scattering experiments. Figure 6.3
displays the observed scattering pattern from films grown at several different pressures
from n-pentane:acetone solutions. The GIWAXS patterns are consistent with the α-form
of iPP for all samples. Specifically, the diagnostic peaks of the α-form, (110)α, (130)α,
(040)α, (111)α, (131)α, and (041)α are observed.347,348 Moreover, the intensity of the
diagnostic peaks decreases for all iPP films as the pressure is increased.
116
7.0 MPa
10.3 MPa
17.2 MPa
10% acetone
1% acetone
Pure pentane
3.5 MPa
Figure 6.3. GIWAXS patterns of iPP films grown in pressurized n-pentane:acetone
solutions at different pressures. Intensity is plotted on a log scale.
Figure 6.4 displays azimuthally integrated linecuts of the GIWAXS data for films
grown in pure n-pentane and n-pentane:acetone at several different pressures. The
azimuthal integrations show a scattering pattern similar to that of the α-iPP previously
reported.313,347,349 In addition, there is a decrease in the intensity of the peaks for iPP films
with increasing pressure. This is consistent with our analysis of POM images suggesting
that an increase in deposition pressure results in a reduced crystallinity in iPP films. Apart
from the α-iPP features, there is also a (300) β peak indicating the formation of a small
amount of ß-iPP at 3.5 MPa. However, this feature does not appear at pressures higher
than 3.5 MPa. Additionally, Ɣ-iPP is reported to be formed only at significantly elevated
pressures with the maximum fraction of Ɣ-iPP occurring at 200 MPa. .350-353 However, we
observed feature corresponding to Ɣ-iPP in the film deposited at 17.2 MPa in the presence
of 10% acetone, namely (111)Ɣ and (117)Ɣ reflections.
117
Ɣ- (111) Ɣ- (117)
Figure 6.4. Azimuthally-integrated linecuts of the GIWAXS data for iPP films grown
at different pressures. (a) pure n-pentane. (b) n-pentane + 1% acetone. (c) n-pentane
+ 10% acetone.
Furthermore, the number average molecular weight (Mn), weight average
molecular weight (Mw), and polydispersity index (PDI) of as-received iPP sample was
compared with iPP samples collected via gravimetric analysis from supercritical npentane:acetone solutions. This investigation was carried out as there is a possibility of
polymer fractionation in supercritical fluids with respect to molecular weight via
118
isothermally increasing pressure profile above the melting temperature of the polymer.379
The molecular weight measurements were performed via size exclusion chromatography
(SEC) and the results are provided in Table 6.1. Based on the results, there is no
consistent trend that can correlate the crystallinity degree observed in iPP films to the
polymer molecular weight. Hence, fluid properties must be primarily responsible for the
observed reduced crystallinity in iPP films as a function of deposition pressure.
Table 6.1. The number average molecular weight (Mn), weight average molecular
weight (Mw), and polydispersity index (PDI) of as-received iPP sample and iPP
samples collected via gravimetric analysis from supercritical n-pentane:acetone
solutions at 418 K (423 K in case of n-pentane:acetone) at different pressures via
size exclusion chromatography.
Sample
Mn
MW
PDI
iPP, as received
120,094
4,729,300
39.380
iPP in n-pentane, 3.5 MPa
129,828
6,407,432
49.353
iPP in n-pentane, 7.0 MPa
136,908
7,607,899
55.570
iPP in n-pentane, 10.3 MPa
131,275
7,001,019
53.331
iPP in n-pentane, 17.2 MPa
134,660
6,092,031
45.240
iPP in n-pentane + 1% acetone, 10.3 MPa
127,078
5,306,952
41.761
iPP in n-pentane + 10% acetone, 10.3 MPa
140,478
4,952,082
35.252
To synthesize our findings, a schematic drawing of the deposition mechanism is
provided in Figure 6.5. In this figure, the change in solvent density is represented by false
color gradient, with lower solvent density nearer the substrate. We propose that the drop
in solvent density in this region is correlated with the precipitation of material from npentane, where the entropy of the system favors disruption of the solvent shell.
119
Figure 6.5. Cartoon representing the deposition mechanism in n-pentane:acetone.
Black lines indicate polymer chains, blue ovals indicate acetone solvent shell, false
color gradient represents local density of n-pentane.
Additionally, it is anticipated that the flow regime is turbulent, exhibiting high
Rayleigh number. This was indeed confirmed by the observed light scattering near the
substrate surface during the deposition process. Moreover, ring-like structures are
observed in several POM images at high pressures (see Figure 6.6) which we interpret as
indicative of Rayleigh-Bénard convection cells.
Figure 6.6. Polarized optical microscopy image (x10) of iPP film grown in
supercritical n-pentane at 10.3 MPa in the presence of 10% acetone.
120
The presence of acetone has two complementary effects on the deposition. First,
it acts as a viscosity modifier, decreasing the Rayleigh number of the system. This leads
to a decrease in bulk solution turbulence and an increase in the thickness of the laminar
boundary layer at the heated surface. Both effects lead to a more controlled polymer
aggregation in the boundary layer. The second effect of acetone is that it remains with the
polymer as it is deposited onto the surface, allowing for greater chain mobility, further
increasing crystallinity.
6.3. Conclusion
We investigated the solubility of isotactic polypropylene in supercritical n-pentane
as a function of temperature via gravimetric analysis. The isobaric solubility displayed a
peak with temperature which can be described by an empirically motivated model.
Furthermore, the influence of a solvent additive was studied by adding acetone to npentane. A peak shift and a peak narrowing were observed, both of which are predicted
by lower overall solvent entropy. Based on the solubility behavior, deposition conditions
were established and iPP thin films were grown by resistively heating a substrate in the
saturated solution. The morphology of the thin films grown at different pressures were
studied using polarized optical microscopy and grazing incidence wide angle x-ray
scattering. The results confirmed the decrease in crystallinity with pressure due to an
increased turbulence near the substrate surface. The addition of acetone enhanced larger
scale self-assembly, evidenced by spherulite formation as seen in the POM images. Our
findings are consistent with a two-step film formation model in which the initial step
involves pre-aggregation in solution that establishes the local crystalline order. The
second step involves chain mobility on the surface, that increases in the presence of
acetone, with acetone presumably being slower to leave the film. In conclusion, the results
presented here demonstrate the key principles of self-assembly in supercritical fluids
which may be applicable to a wide range of materials and deposition conditions.
6.4. Methods
6.4.1. Solubility Measurement via Gravimetric Analysis
The solubility measurements were carried out for all the studied solvent systems
via gravimetric analysis discussed in detail in section 5.4.1.
121
6.4.2. Substrate Preparation for Thin Film Deposition and Deposition
Condition
ITO coated glass slides were used as the substrate for deposition of iPP (for
specifications refer to section 4.4.2). The deposition of isotactic polypropylene in
pressurized n-pentane was performed by first increasing the temperature of the solution
to 418 K (423 K in case of n-pentane:acetone). When pressure and temperature had
stabilized, the temperature of the ITO glass slide was increased to 433 K to initiate the
deposition.
6.4.3. Characterization of Deposited Thin Film
The iPP deposited films were analyzed using POM and GIWAXS. Detailed
information is available in section 5.4.3.
6.4.4. High-temperature Size Exclusion Chromatography
Number average molecular weight (Mn), weight average molecular weight (Mw),
and polydispersity index (PDI) were evaluated by high-temperature size exclusion
chromatography (SEC) using 1,2,4-trichlorobenzene and performed on a EcoSEC HLC8321GPC/HT (Tosoh Bioscience) equipped with a single TSKgel GPC column (GMHHRH; 300 mm × 7.8 mm) calibrated with monodisperse polystyrene standards. The samples
were prepared using 1 mg/mL of sample in trichlorobenzene (TCB), which were allowed
to stir at 80 °C for 12 h prior to injection. The analysis of the samples was performed at
180 °C with a flow rate of 1.0 mL/min with injection quantities of 300 µL. The data was
collected and integrated using EcoSEC 8321GPC HT software suite.
122
Chapter 7.
Conclusions and Future Directions
7.1. Conclusions
This thesis has outlined a novel solution-based deposition technique for growing
polymer thin films based on the distinctive properties of supercritical fluids. The primary
goal of the project is to establish a solution-phase analog of physical vapour deposition
method that does not require any in-situ chemical reactions, and more importantly offers
superior pattern resolution than printing techniques like aerosol jet. To achieve this goal,
the initial step involved investigating the solubility behaviour of a selected semiconducting
polymer, PBTTT-C14, in pentane at different temperatures and pressures. The deposition
in supercritical fluid was established based on a somewhat common property of a solutesolvent system, a maximum in their solubility as a function of temperature. This
phenomenon allows films to be formed onto a heated substrate. Taking advantage of this
unique property of supercritical fluids, ITO glass substrates were resistively heated and
thin films of PBTTT-C14 were grown in supercritical pentane. Subsequently, ITO glass
substrates were patterned by using photolithography and directed the formation of
polymer lines 5 μm in width, exceeding the line width of printing techniques like aerosol
jet by a least a factor of 4. Afterwards, we demonstrated the deposition of PBTTT-C14 on
finely patterned features onto a cross-linked PMMA layer and a PDMS hemispherical
shell. The patterned films had nearly identical line width, suggesting that much higher
resolution patterns can be created. The ability to deposit materials on flexible polymer
films and curved surfaces is particularly important as it highlights the potential use of this
deposition technique in the industrial manufacturing of flexible electronics. However, what
makes the physical supercritical fluid deposition noteworthy is that it provides an
unprecedented opportunity to couple photolithography with solution phase self-assembly.
This removes the barrier of the chemical complexity of materials that can be used in a
photolithographic process and the potential scalability of the technique for industrial
applications.
Despite a long history of using supercritical fluids as a processing media for
materials, the mechanism for self-assembly in supercritical fluids is still not fully
understood. Therefore, the natural next step was to study polymer self-assembly in
supercritical fluids. To achieve this goal, the self-assembly of isotactic polypropylene films
123
from supercritical n-pentane was investigated. The deposited thin film morphology was
studied using POM and GIWAXS, and the findings were summarized with a proposed twostep model describing the film formation in supercritical fluids. The initial step of selfassembly occurs in solution, enabling pre-aggregation in solution phase and the formation
of local crystallites. The second step involves the longer length scale organization on the
substrate surface. Given that pre-aggregation takes place in solution, the fluid mechanics
is an important consideration for controlling the polymer assembly. Indeed, our findings
provided evidence supporting that an increase in solution turbulence disrupts the solutionphase self-assembly and results in reduced crystallinity of deposited films. Apart from this,
the second step to film formation is strongly dependent on the chain mobility on the
surface, accommodating polymer chain reorganization to form larger scale structures. To
further explore what impacts the chain mobility on the surface, acetone was added as an
additive to n-pentane and isotactic polypropylene films were deposited. The presence of
acetone significantly changed the morphology of the deposited film. It was inferred based
on our findings that acetone primary acts as a viscosity modifier and decreases the bulk
fluid turbulence leading to a more controlled pre-aggregation in solution. Moreover,
acetone remains with the polymer as it deposits on the surface and facilitate higher
polymer chain mobility, which in turn allows for higher length order and increased
crystallinity. These findings highlight the significant impact solvent engineering can play in
controlling the self-assembly of polymers in supercritical fluids and achieving more
controlled self-assembly.
Additionally, we were able to develop a simple thermodynamic model to predict
both the temperature and pressure dependence of the polymer solubility in supercritical
fluids based on the interplay of intermolecular interactions and solvent entropy. To validate
the model, the equilibrium concentrations of isotactic polypropylene polymer predicted by
the model were compared with the experimental data collected at elevated pressures and
temperatures. The model results qualitatively match the experimental results, both
manifesting very little dissolved materials at the lowest temperature and the existence of
a maximum in the isobaric solubility versus temperature. Furthermore, the isothermal
solubility of iPP was investigated experimentally and compared with the proposed
thermodynamic model. The isothermal solubility exhibits a decrease with pressure for the
lowest pressures and is largely pressure independent thereafter. These results were in
124
good agreement with the proposed thermodynamic model predictions and with previous
reports on the solubility of iPP in n-pentane.
7.2. Future Directions
The development of a unique deposition technique that allows thin films of polymer
to be grown in supercritical fluids is very promising, primarily because there is no need for
in-situ chemical reactions, and it can be easily adapted for deposition of a variety of
materials. But more importantly, the morphology of the deposited films using supercritical
fluids can be altered by means of both pressure and solvent additive. Nonetheless, the
technique is still in its infancy and its further growth will be highly dependent on the efforts
to investigate the transport properties of organic semiconductor films grown from
supercritical fluids and subsequently compare the results to films deposited using other
available techniques. As discussed in Chapter 2, the charge transport in organic
semiconductors depends largely on the structural order at different levels and as a result
the processing techniques is of paramount importance. Based on our preliminary results,
the morphology of the PBTTT-C14 films deposited via physical supercritical fluid deposition
technique is significantly different than those deposited through other techniques. The
AFM images of the PBTTT films grown via spin-coating, slow-drying, drop-casting and
physical supercritical fluid deposition are presented in Figure 7.1 for comparison.
a.
b.
c.
d.
2 μm
Figure 7.1. AFM height images of pure PBTTT films deposited using spin-coating
(a), slow-drying (b), drop-casting (c), and physical supercritical fluid deposition at
3.5 MPa (d). None of the PBTTT films presented here were thermally annealed. The
scale bar is the same for all the images. Images a, b, and c were reprinted with
permission from reference 380.
There is a clear morphological difference among the four deposited films, which
points out that the polymer self-assembly can be varied significantly by changing the
processing technique. For instance, the spin-coated film looks unstructured compared to
125
slow-dried and drop-casted films where fibril-like aggregates are more distinctively visible
due to an increased film solidification time. On the other hand, the PBTTT film grown in
supercritical fluids reveals fibrillar aggregates that reach lengths in the order of
micrometers. This observation is in accordance with the deposition mechanism offered
previously in this thesis. The two-step polymer self-assembly in supercritical fluids,
accommodate the timeframe and the freedom necessary for the polymer chains to
assemble in a more ordered aggregates, hence enhancing the structural order to a higher
degree in the deposited films. Indeed, such morphology modification will impact the
transport properties, which demonstrates how crucial it is that future works involve the
study of transport properties of films grown from supercritical fluids.
In this thesis I reported how solvent additive can control the crystallinity of isotactic
polypropylene thin films and based on the observations a deposition mechanism was
proposed. I propose a separate set of experiments to assess the influence of solvent
additive on organic semiconductor films nanostructure and investigate the structureproperty relationship governing charge transport. Preliminary experiments demonstrate
the role that solvent additive plays in altering the morphologies of deposited thin films in
iPP, suggesting that similar strategies can be employed with PBTTT-C14. Figure 7.2
presents the AFM images of PBTTT-C14 thin films deposited in supercritical n-pentane
(Figure 7.2.a) and n-pentane + 0.5% mol toluene (Figure 7.2.b) at 3.5 MPa. The findings
highlighted that in the presence of toluene, the fibrillar aggregates observed in pure npentane start to disappear and the morphology shifts towards the formation of longer fibers
reaching lengths of 4-5 micrometers. Concurrently, it appears that the reduced number of
aggregates in the presence of toluene can result in reduced interconnectivity between
them. How this modification in morphology translates to the electronic properties of
polymer semiconductor films is the question that needs further investigation in the future.
126
a.
b.
Figure 7.2. AFM height images of PBTTT-C14 films deposited in (a) supercritical npentane and (b) n-pentane + 0.5% mol toluene at 3.5 MPa.
As discussed in Chapter 5, the degree of crystallinity in deposited thin films was
pressure dependent, which is due to the change of fluid turbulence near the substrate
surface as a function of pressure. Given the impact of fluid mechanics on the final
morphology of thin films, one of the approaches to regulate the fluid turbulence can be
achieved by incorporating some changes in the current substrate holder design to modify
the boundary layer thickness. This approach is based on the 𝑅𝑎 ∝ 𝑙3 relationship
between Rayleigh number (𝑅𝑎) and characteristic length of the fluid domain (𝑙). In order
to decrease the turbulence where self-assembly of polymer crystallites take place, the
thickness of the boundary layer must be decreased. One possible approach to achieve
this would be to add a layer of fine mesh in close distance from the ITO substrate, thus
creating an array of cells in the boundary layer where fluid can flow through mesh
openings. This proposed strategy can lead to the reduction of characteristic length to that
of the mesh opening sizes and as a result reducing the 𝑅𝑎 number.
127
Backside support
ITO substrate
PEEK mesh
Pogo pin
Figure 7.3. Schematic of a proposed sample holder design with an incorporation of
a PEEK mesh in front of the ITO substrate to reduce the fluid turbulence near the
substrate.
Another possible direction of future work could involve the expansion of the
physical supercritical fluid deposition technique to other classes of materials, extending its
application beyond semiconducting and aliphatic polymers. For instance, the physical
supercritical fluid deposition of organic small molecules, nanoparticles, metals, and metal
oxides. This avenue is particularly critical in justifying the physical supercritical fluid
deposition technique as a compelling processing technology that provides advantages
over the existing techniques based on its application diversity and economic, and technical
considerations.
128
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Stefano Bellucci
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