Project report on Growth of ZnO Nanowire and It's applications as Photodetector

Growth of ZnO Nanowire and
Its Application as UV Photodetector
A
PROJECT
Submitted in partial fulfilment
of
BACHELOR OF SCIENCE
In
PHYSICS
By
Jyotismat Raul
[ Roll no.:- 15PHY028]
GOVT COLLEGE (AUTO), ANGUL

ACKNOWLEDGEMENT
This project is by far the most significant accomplishment in my life and it
would be impossible without people (especially my family) who supported me and
believed in me .I would like to express my sincere gratitude to my guide Mr.
Chiranjib Sahu ,Dept . of Physics , Govt.(Autonomous) college , Angul for giving
me the opportunity to work with him and also providing excellent guidance
and continuous assistance throughout the project work . His constantadvice ,
assertions , appreciation were very virtual and irrevocable , giving us that boost
without which it wouldn’t have been possible for us to finish our project . I am
thankful to him for his encouragement throughout the project .
Jyotismat Raul
Roll no: 15PHY028

CERTIFICATE BY H.O.D.
This is certified that the term paper entitled “Growth of ZnO Nanowire and
Its Application as UV Photodetector" submitted by Jyotismat Raul bearing
Examination Roll No- 15PHY028 has been successfully completed
under guidance of Mr. Chiranjib Sahu and is being submitted to the
Department of Physics for evolution as part of 6th semester Exam 2018 of
govt. College(Auto) Angul.
Sign. Of H.O.D (Dept. of Physics)

CERTIFICATE BY THE GUIDE
This is certified that the term paper entitled “ Growth of ZnO Nanowire and
Its Application as UV Photodetector"submitted by Jyotismat Raul bearing
Examination Roll No- 15PHY028 and college Roll No- BS15-285 in
practical fulfilment of the requirement of requirements of 6th semester B.Sc.
examination (Physics) 2018 of Govt. College (Autonomous), Angul is the product
of his original research works and has been prepared under my guidance and
supervision.
Sign. of Guide (Dept. of Physics)

DECLARATION
I certify that the work contained in the project is original and has been
done by myself under the general supervision of my supervisor.The work
has not been submitted to any other institute for any degree . I have
followed the guidelines provided by the institute in preparing the project.
Whenever I have quoted written materials from other sources, I have put
them under quotation marks and given due credit to the sources by citing
them and giving required details in the references.
Jyotismat Raul

ABSTRACT
Zinc Oxide (ZnO) nanostructures with their significant properties have various
application in optoelectronics, sensor devices and energy conversion devices. This
report includes deposition of ZnO film by rf reactive sputtering on glass substrate. Its
analysis using XRD, FESEM and UV-VIS spectroscopy to study its morphology,
crystal structure, band gap and transmittance. Further, the deposited film is taken as
substrate for the growth of ZnO nanowires using Hydrothermal Method. Various
parameters affecting the growth conditions like growth temperature, growth time, and
precursor concentration have been studied and optimizations of reaction conditions
are done. Analysis of the samples fabricated under different conditions is carried out
using XRD and FESEM characterization techniques. Zinc oxide nanowires have their
application as UV photodetector due to their wide band gap and high surface to
volume ratio. To enhance the absorption ability, carrier concentration and
photoresponse, Zinc Oxide nanowires surface is functionalized using Poly(vinyl
alcohol) for the application of UV photodetector. Photoresponse and sensitivity is
studied for both coated and uncoated, using I-V characterization. To analyse optical
switching action under the illumination of UV light photoresponse of these nanowires
is plotted with respect to the time. Keywords: Zinc Oxide, Thin film, Nanowires,
Hydrothermal Method, Polymer Functionalization, UV Photodetector

CONTENTS
1. INTRODUCTION
2. PHYSICAL PROPERTIES OF ZNO
3. CRYSTAL STRUCTURE OF ZNO:
4. ZNO - THIN FILM AND NANOWIRES
5. CHARACTERISATION TECHNIQUES
6. SYNTHESIS OF ZNO NANOSTRUCTURES
7. RESULTS & DISCUSSION
• ZNO THIN FILM
• ZNO NANOWIRES
8. CONCLUSION
9. REFERENCES

INTRODUCTION
The significantly different physical properties of nanostructured materials in
comparison to their bulk counterpart made them important as widely used material in
the field of science and technology. The change in physical properties lies in their
characteristic structural features in between the isolated atoms and the bulk
macroscopic materials. On atomic level, there is a change in band structure due to
quantum confinement, which is due to the changes in the atomic structure resulting
from the direct influence of the ultra-small length scale on the energy band structure.
The significantly important electronic, mechanical, optical and magnetic properties
of the nanoscale materials can be attributed to the changes in the total energy and band
structure of the system resulting from the confinement in particular dimension.
Materials in nano dimension have their properties more influenced by surface
interaction than their bulk part; an increase in surface to volume ratio ensures more
effective surface phenomena. Nanowire is one of the nanostructured material which
has its length confined in one dimension. They are characterized by very high aspect
ratio, length lies in micrometre range and diameter in nanometer range. Nanowires
can be taken as model system to study influence of dimensional confinement on the
optical, magnetic, electrical and mechanical properties of a material. ZnO nanowires
have their application in optoelectronic and energy conversion devices. Zinc oxide
has been widely studied since 1935 [1].
It is an important II–VI compound semiconductor material having optical and
electrical properties which can be used in a number of applications, like high
transmittance conductive oxide coatings for solar cells, gas sensors, chemical sensor,
UV photodetectors, and bulk acoustic wave resonators. It has wide direct band gap
energy of 3.37 eV, making it transparent for visible light and operates in the UV to
blue wavelengths. The exciton binding energy is 60 meV for ZnO; the higher exciton
binding energy enhances the luminescence efficiency of light emission and make sure
that excitonic transition is less affected by thermal energy.
ZnO has exhibited better radiation resistance for possible devices used in space and
nuclear applications. ZnO is an amphoteric oxide having isoelectric point of 9.5 which
can be grown on inexpensive substrate, such as glass, at relatively low temperatures.
It is bio-safe and biocompatible. ZnO nanostructures, such as nanowires and
nanorods

are ideal for detection applications due to its large surface area to volume ratio. ZnO
nanowires and nanorods are attractive components for nanometre scale electronic and
photonic device applications because of their unique chemical and physical
properties.
A wide variety of nanodevices including ultraviolet detectors, sensors, field effect
transistors, Schottky diodes, intramolecular p– n junction diodes and light emitting
device arrays have been fabricated utilizing ZnO nanorods (nanowires) [2]. Owing to
its application synthesis of ZnO nanostructures is currently attracting intense
worldwide interest. ZnO have a piezoelectric tensor equal to or even greater than that
of GaN and AlN which means that ZnO is a suitable candidate for device applications
requiring a large electromechanical coupling. The thermal conductivity, ‘κ’ of a
semiconductor is an important property when considering high-power/high
temperature devices. It is a kinetic property affected by the vibrational, rotational and
electronic degrees of freedom, and is predominately limited by phonon-phonon
scattering in a pure crystal. ZnO, like most other semiconductors, contains a large
number of point defects, which have a significant effect on the thermal conductivity.
It has wide industrial application due to the development of growth technologies for
the synthesis of high quality single crystals and epitaxial layers which allows the
realization of ZnO-based electronic and optoelectronic devices. Improvements in
growth technology of ZnO nanostructures, epitaxial layers, single crystals, nanowires
and nanoparticles, ZnO devices have large potential to be increasingly functional in
the near future.
Physical Properties of ZnO
Molar Mass 81.409gm/mol
Appearance White solid
Odour Odourless

Density 5.606gm/cm
Melting point 19750 ºC
(Decomposes)
Boiling Point 23600ºC
Solubility in water 0.16mg/100ml(30ºC)
Band gap 3.37 eV (Direct)
Refractive Index 2.00
Crystal Structure of ZnO:
Most of group 12–16 binary compounds crystallize in the form
of cubic rock salt or cubic zinc blende structure or hexagonal wurtzite structure. The
rock salt structure is that of common table salt which may be obtained at very high
temperature and pressure. In normal ambient conditions the wurtzite symmetry is
thermodynamically favoured over the zinc blende [1]. In both the cases, each cation
is surrounded by four anion arranged in four corners of a regular tetrahedron with the
cation at the center. Similarly, four cations surround each anion. Zn-O bond possesses
good ionic character which is reflected in its band gap. This tetrahedral coordination
gives rise to polar symmetry. However what distinguishes the wurtzite structure from
the typical zinc blende structure is its rotation symmetry along <111 > direction, in
wurtzite structure the successive tetrahedrons are on top of each other and in zinc
blende structure they are not aligned. The wurtzite structure has hexagonal unit cell
structure with two lattice parameters a=3.2495 A and c=5.2069 A. The measure of

the amount by which each atom is displaced with respect to next along the caxis is
given by the parameter u, and experimentally, zinc oxide wurtzite structure is found
to have u= 0.38.
There is lot of work available on growth of one dimensional zinc oxide
nanostructures. A few applicable methods to grow nanomaterials include wet
chemical method, metal organic chemical vapour deposition, pulsed laser deposition,
molecular beam epitaxy and even top-down approaches by etching. Among all the
above method, wet chemical method is comparatively attractive for several reasons –
they are low cost, less hazardous, and capable of easy scaling up; the growth occurs
at comparatively low temperature, compatible with flexible organic substrate, metal
catalyst is not needed and there are number of parameter can be tuned to effectively
control the morphology and properties of final product [3]. Nanowires grown in my
work follows wet chemical synthesis- the hydrothermal method because it does not
require any specialized equipment and it is very cheap and easy to do. This method if
does not provide uniformity, is very much economic.
In present work zinc oxide nanowires are grown on glass substrates with seeded layer
of ZnO crystal as the study of optical properties could be best done on a substrate that
is transparent. The growth of nanowires on glass substrate has been studied by varying
reaction parameters like temperature, concentration and time. The main advantage of
zinc oxide nanowires over others in same fields is the wide band gap of zinc oxide
which can be utilized in various electronic devices. Functionalization of nanowires
surface is carried out using poly(vinyl alcohol) (PVA) for application of UV
photodetector.

3.ZnO - Thin Film and Nanowires
3.1 Thin Film
There are various types of nanostructures playing a vital role in the development of
science and technology. Thin films are of importance as they provide large surface
interaction, less consumption of material, less power consumption, are light weight
and portable, material property can be varied in wide range, can be easily doped and
patterned in micrometer range. A few methods to deposit thin films are Physical
Vapour Deposition, Chemical Vapour Deposition, Molecular Beam Epitaxy, and
Pulsed Laser Deposition. Sputtering which comes under Physical Vapour Deposition
have advantage compared to other methods as it gives good film quality, less wastage
of material, low cost methods for growth thin film.
3.1.1 Chemical Vapour Deposition (CVD)
Chemical Vapour Deposition (CVD) technology is interesting as it gives rise to high-
quality films and applicable to large-scale production. In the CVD method, precursors
are introduced inside the chamber by carrier gases or in gaseous phase, they made to
flow through a nozzle towards the substrate where they react to give a deposited film
on the substrate. This is comparatively a high temperature deposition method. Plasma
enhancing and light photons can be used to provide required activation energy for the
reaction and hence can occur at comparatively low temperature. ZnO deposition
occurs as a result of chemical reactions of vapour-phase precursors on the substrate,
which are introduced into the growth zone by the carrier gas. Metals can be injected
into the surface with their in the form of their organic compound like to introduce Zn,
Diethylzinc can be used which has a boiling point of 117ºC The reactions take place
in a reactor where a necessary temperature profile is created in the gas flow direction.

3.1.2 Pulsed Laser Deposition (PLD)
Pulsed laser deposition uses a pulsed laser of high power to irradiate a target surface.
On irradiating the target surface, target surface sublimes to eject its atoms. These
atoms acquire a directional flow towards the substrate surface and get condensed on
the substrate surface. Advantage of a PLD system is it maintains the stoichiometry of
the compound, gives good film quality, and gives a film of high purity. But it has a
demerit of low area deposition. The quality of the grown film depends on the substrate
temperature, ambient gas condition and intensity of the laser used.
3.1.3 Molecular Beam Epitaxy
This method is ultra-high vacuum based method which gives highly crystalline
epitaxial layer. In this method source is made to sublime inside the effusion cell and
then a directional flow is made towards the heated substrate in very controlled
manner. It has a slow deposition rate with frequent shuttering. Thin film grown by
this method have high purity so this also requires a highly pure source material. It has
an advantage of in-situ analysis for the modulation of each grown layer using RHEED
(Reflection High Energy Electron Diffraction) and RGA(Residual Gas Analysis) to
maintain cleanliness of the chamber.
3.1.4 Sputtering
Sputtering involves ionization of an inert gas under the effect of high potential applied
across the electrodes. At breakdown potential of sputtering gas, it ionizes into ions
and electron, ions under the effect of electric field move towards the cathode (target)
to collide with target surface causing removal of target atom from the surface of target

with other species like reflected neutrals, ions, secondary electrons etc. Due to
momentum transfer between target atom and sputtering ion, ejected target atom
attains a directional velocity towards the substrate and get deposited on the substrate
surface. Power supplied in sputtering can be of dc or rf type. DC power supply having
low cost have demerits of target poisoning and not applicability for the insulating
target, whereas, rf sputtering having relatively high cost can ensure better film quality
and applicable to all kind of targets (metal, semiconductor, insulator). In case of
reactive sputtering, where a reactive gas is made to flow inside the chamber, reacts
with the target material to form compound on the substrate surface, for which
substrate heating is provided.
3.2 Nanowires
There are two basic approaches to synthesize nanowires –
1. Top-Down
2. Bottom-Up
Top down approach reduces a large piece of a material to small pieces, by means of
lithography or electrophoresis to form nanowires. Bottom-Up approach synthesizes
nanowires by combining constituents adatoms.
3.2.1 Hydrothermal method
ZnO nanowire arrays can be made in aqueous solutions, using two-step process.
• Preparation of ZnO seeds on desired substrate
• Growth of nanowires on seeded substrate

ZnO seed layer can be prepared by any thin film deposition technique, namely,
sputtering, spin coating or dip coating. After preparing the thin film on the substrate,
it is placed in equimolar solution of Zinc Nitrate (Zn(NO3)) and
Hexamethylenetetraamine (HMT) such that the coated surface directly faces the
solution. Reaction is kept at 90ºC for around 4-8 hours. After some time solution turns
milky indicating the formation of white precipitate of Zinc Oxide. With the end of
growth time, substrate is taken out of the solution and rinsed slowly in De-ionized
water to remove any loosely adsorbed layer. Then it is kept for drying at room
temperature.
Reaction Mechanism –
(CH2)6N4 + 6 H2O 6 HCHO + 4 NH3
NH3 + H2O NH3·H2O
NH3·H2O N 4
+
+ OH-
Zn2+
+ 2 OH-
Zn(OH)2
Zn(OH)2 ZnO + H2O
Role of HMTA –
In the formation of nanowires HMTA acts as a weak base and plays most fundamental
role for the growth of ZnO nanowires. It slowly hydrolyze in water solution to
produce OH-
ion and this OH-
ion is important to precipitate out Zn2+
ion in high pH
environment. The solution medium having a pH 7-8 is supposed to be good for the
proper growth of nanowires.

When used as substrates for epitaxy, proper surface preparation is necessary to
evaluate the quality of hydrothermally grown ZnO. In order to reduce the strains and
dislocation density in epitaxial ZnO and related films, closely lattice matched
substrates are favoured for growth. Also with seeded ZnO layer the texturing of
nanowires results from a thermodynamically controlled nucleation and is independent
on its interaction with the substrate.
3.3 ZnO as UV Photodetector
ZnO nanowires due to their wide band gap of 3.37 eV, wide range of
photoluminescence emission band from blue to yellow, high quantum yield and
stability of its nanoparticles in aqueous solution for biological labelling can be used
as UV sensors.
Ideal properties of a UV sensors are as follows:
• Fast Response time
• Small reset time
• High selectivity
• High responsivity
• Good signal to noise ratio
UV sensing Mechanism
As ZnO is an n-type semiconductor, O2 molecules from environment get adsorbed on
nanowires surface forming a depletion layer with low conductivity near the nanowires
surface.
O2 (g) + e-
O2
-
(ad)

Upon exposure to the UV light there be production of e-
-hole pair, holes will migrate
to the surface of nanowires which has negatively charged oxygen molecule while
electrons can move through the bulk part with less chance of recombination hence
increasing the photo current.
Properties of ZnO nanowires as UV photodetector :
• Conductivity of ZnO nanowires is highly sensitive to the exposure of UV light.
• Photoresponse is linearly dependent on power of illumination.
• It has excellent wavelength selectivity.
• Large photo response can be detected at higher bias voltage.
• Response cut-off wavelength is found to be ~ 370 nm.
• Thinner nanowires have more sensitivity.
• Photoresponse dependence on the ambient gas conditions.
• ZnO nanowires have limited UV sensibility due to presence of point defects and
confined dimensionality.
Change in charge transport mechanism on functionalization:
Free electron inside the nanowire may get trapped by the positively charges
PDADMAC surface. On UV illumination these localized electrons can trap photo-
generated holes and reduces e-
- hole recombination rate and hence increases the
carrier life time. Due to this electron conduction takes place in the bulk of the material
while recombination takes place at the surface of the material hence increases photo-
conductance.
In case of PVA coating, PVA acts as O2 barrier and reduces the thickness of depletion
layer on the surface of nanowire hence reduces the trapping of carrier charges on the
surface of nanowire.

Releasing more available carrier for photoconduction and, therefore, gives better
photoresponse.
CHARACTERISATION TECHNIQUES
4.1 X-Ray Diffraction
This method gives the information about the crystalline nature of the sample which
includes miller indices, crystal structure, phase composition, nature of sample
(amorphous or crystalline). In this method X rays are made to fall on the sample if the
falling X rays satisfies the Bragg’s condition they gets diffracted from the sample.
Intensity of diffracted X ray is plotted with respect to scattering angle. Higher
intensity ensures more number of atoms lies in that particular plane. This technique
can also be used to study change in crystal structures in various experimental
conditions like thermal distortion.
Determination of crystal size:
The X-ray diffraction analysis in one of the most applicable method for the
estimation of crystallite size in nanomaterials. The broadening of the Bragg peaks
contains the development of the crystallite refinement and internal stain. To size
broadening and stain broadening, the full width at half maximum (FWHM) of the
Brag peaks as a function of the diffraction angle is analysed. Crystallite size of the
deposits is calculated by the X- ray diffraction (XRD) peak broadening. The
diffraction patterns are obtained using Cu Kα radiation. The grain size can be
estimated the Scherrer equation

D=0.9* λ/ β cos θ
where, λ : is wave length of X-ray, β is FWHM in radian, θ is peak angle.
4.2 Field Emission Scanning Electron Microscopy
It is one of the widely used methods for material characterization specially to study
their morphology and topological features. This method utilizes emission of electron
from a cathode when high potential is applied across its end. This electron beam
accelerated through a high potential and attains high momentum causing reduction in
its wavelength and according to the Rayleigh criteria provide high resolution image
of the sample under investigation. Electron beam is made to pass through a set of
electromagnetic lenses to focus properly on the specimen surface. Interaction of
electron with the specimen can result in secondary electrons, backscattered electrons,
Auger electrons etc to come out of the sample carrying various kind of information.
Detection of secondary electron is most common mode of detection, as they have low
energy and originated from a few nanometres of the surface. it is detected by
scintillator or photomultiplier tube. This gives information about the surface
morphology of the specimen. Backscattered electrons are high energy electrons
resulting from the reflection or backscattering from the volume of specimen. As they
comes from the comparatively more depth inside the specimen, they carry information
about the topology of the specimen, change in specimen property with the depth can
also be analyzed with detection of backscattered electrons.

Following information can be extracted from FESEM analysis :
• Use of field emission results in improved spatial resolution, less damage of the
specimen surface and minimized surface charging. It can be give following
information about the specimen surface
• It can also be used for the cross section analysis of the semiconductor device for
gate width, gate oxide, film thickness and construction details
• It can derive information about structure uniformity determination and advanced
coating thickness of elemental composition measurement and contamination
feature of the geometry.
4.3 UV-VISIBLE Spectroscopy
UV visible spectroscopy uses wavelength ranges from UV to visible range to study
transmittance, reflectance, and absorbance of the material under investigation. It can
be used to find information about the band gap of a deposited semiconductor film.
UV/Vis spectroscopy is often find its application in the quantitative determination of
solutions of transition metal ions and highly conjugated organic compounds because
of following :
• Transition metal ions shows color because they have vacant d orbitals in which an
electron can make transition by absorbing radiation in visible range and come back
to original state by emitting a radiation.
• Organic compounds having high degree of conjugation can also absorb light in UV-
VIS range in this case the solvent plays an important role e.g. ethanol shows weak
absorption for most of the wavelengths. Solvent polarity and pH is important while
taking absorption spectrum of the organic compounds.

Band Gap of a semiconductor can be measured using UV-Visible spectroscopy by
following relation –
(αhν)1/n
= B (hν - Eg)
α : Absorption Coefficient = -log(T)/(d)
T=Transmittance,
d=Thickness of the film
n = 2 Indirect band Gap, ½ Direct Band Gap
B = Constant related to transition probability
UV-Visible spectroscopy is most often used in a quantitative way to determine
concentrations of an absorbing species in solution, using the Beer-Lambert law:
A = - log10 (I/I0) = ε. c. L
Where, A = Measured Absorbance
I0 = Intensity of Incident Light
I = Transmitted Intensity
L=PathLength through the sample
c=Concentration of Absorbing species
I-V Measurement
I-V characteristic of a circuit shows the impedances of a circuit and the junction
property of a contact. It can be measured by making contact on the semiconductor
surface. In case of Ohmic contact, it has a linear relation but in case of Schottky
contact there is non-linear behaviour for IV curve. It is one of the basic instrument to
quantify electrical properties of an instrument and analyse the characteristics of
number of instruments which include semiconductors, MOSFETs and junction
properties like Metal-O-Semiconductor and Metal-Oxide-Metal etc

Project report on Growth of ZnO Nanowire and It's applications as Photodetector

SYNTHESIS OF ZnO NANOSTRUCTURES
5. FABRICATION
Fabrication of any thin film or nanostructure requires a substrate. In this work for the
deposition of thin film and growth of nanowires glass substrate is taken. Glass
substrate is used because of its low cost, stability upto a growth temperature of 400
ºC, favourable for large area deposition, easy availability, insulating in nature,
transparent and smooth surface. Before putting the glass substrate for deposition it
needed to be cleaned, to remove any foreign contaminants. Glass cleaning involves
following procedure:
1. First clean the glass slide using soap solution.
2. Sonicate it in Acetone for 10 minutes and then rinse with DI water.
3. Sonicate it in IPA (isopropyl alcohol) for 10 minutes and rinse with DI water.
4. Dry the cleaned glass substrate.
5.1 Growth of ZnO thin film by Sputtering
RF magnetron sputtering is employed to grow Zinc Oxide thin films. Thin film is
grown by reactive sputtering using Zn as target material and O2 as reactive gas. To
start with RF sputtering method, first pure (99.999 %) ZnO target is fixed at the
cathode and cleaned glass substrate is loaded on the anode electrode. Target and
substrate distance is kept fixed at 10 cm then chamber is closed. To remove unwanted
gases and moisture high vacuum of 8.5×10-7
mbar is obtained using Diffusion Pump
with Rotary Pump acting as forepump and backing pump. After running the set up for
around one hour desired vacuum is achieved. Then Ar and O2 gas is made to flow
inside the chamber maintain Ar to O2 the flow ratio of 2:3, with partially closed high

vacuum valve. Working pressure inside the chamber is kept constant at 8×10-3
mbar.
For the proper bonding between the Zn and O at the substrate surface is maintained
at 200ºC. RF power of 150 W is applied to the electrodes and film is deposited for 30
minutes. After 30 minutes chamber is left for cooling for around 30 minutes and then
substrate with deposited film is taken out of the chamber.
RF Magnetron Co-Sputtering Unit, Electronic Materials and Devices Laboratory,
Department of Physics NIT Rourkela

Deposition mechanism
High RF power of frequency 13.56 MHz applied inside the chamber, ionizes the gas
molecules, these ions moves inside the chamber under the effect of oscillating RF
electric field. These ions in their collision with target eject the target atoms due to
momentum transfer, target atom is ejected with other species like secondary ions,
reflected neutrals, ions etc. This ejected target atom due to its directional momentum
reaches to the substrate, loses its energy on substrate surface to get deposited.
Substrate heating is provided in case of reactive sputtering so to reach the activation
energy required to form Zn-O bond on the substrate surface and to achieve proper
orientation. Continuing this procedure for 30 minutes a uniform thin layer of film is
formed on substrate surface.
5.2 Synthesis of ZnO nanowires by Hydrothermal Method
In this method, equimolar concentration of Zinc Nitrate and
Hexamethylenetetraamine (HMT) is taken by dissolving each reagent in 100 ml of DI
water. This solution is kept inside a bottle and glass substrate with seeded ZnO layer
is kept inside the bottle in such a way so that the deposited side faces downward inside
the bottle. This solution is then heated at 90˚C for 4-8 hours. After some time
depending upon the concentration and temperature of the reaction solution inside the
bottle turns white showing the formation of zinc oxide. After the designated time
sample is taken out of the bottle and rinsed slowly with DI water 2-3 times to remove
any weakly adsorbed layer. Then it is kept for drying at room temperature.

Surface Functionalization
Surface Functionalization is done to enhance the surface property of a material for a
particular purpose. There are a number of ways in which surface functionalization
affects the surface interaction like it may increase the electron transport property and
absorption ability, can cause surface band bending and reduction of surface trapping
the charge carriers. On functionalization an extra energy state is introduced by the
polymer which lies in the band gap and conduction band of ZnO this extra energy
level acts as hopping state and increases the excitation probability of electron to the
conduction band [9]. Considering the application of ZnO nanowires for UV sensing
two materials are used for surface modification:
Surface Capping by Poly(vinyl alcohol)
• First as grown ZnO nanowires are taken.
• 0.2 gm of PVA dissolved in 50 ml of DI water at 80ºC for 2 hours.
• Then solution is kept at room temperature for one day to get uniform solution.
• As grown nanowires are dipped into the solution at 50ºC for one hour and then
dried.
For the measurement of I-V characteristic of the polymer coated and uncoated
nanowire sample contacts are made by masking the surface with mask having dot
diameter of 1 mm and is put into thermal evaporation set up to form conducting dots
of Aluminium film to measure the conductive property of the ZnO surface.

Picometer /Voltage Source , EMD Lab Dept. of Physics NIT- Rourkela

6. RESULTS & DISCUSSION
6.1. ZnO Thin Film
6.1.1. Structural Characterization (X-Ray Diffraction)
X- ray Diffraction date shown in this report is taken from Rigaku Ultima IV
diffractometer using Cu Kα-Radiation (λ=1.5418Å) ranging from 200
<θ<800
at
operating voltage of 40 kV and operating current of 40 mA.
Figure 6.1.1 XRD pattern of the ZnO thin film deposited on glass substrate
• It is evident from the XRD pattern that deposited film have high crystallinity.
• Maximum peak intensity is obtained at 2θ value of 34.779º with corresponding
interplaner distance d = 2.5773 Aº.
• Crystallite size is found to be around 14 nm
• A narrow peak is observed showing large crystallite size and good crystal quality.

6.1.2 Surface Morphology
FIELD EMISSION SCANNING ELECTRON MICROSCOPY (FESEM):
The morphology of the sample was observed in field emission scanning electron
microscopy (FESEM) using NOVA NanoSEM 450 scanning electron microscope.
Thickness of the film is found to be 796.04 nm. It can be seen that surface of film
have a uniform and smooth surface with large grain size.
Figure 6.1.2 a) lateral view of the ZnO thin film b) Surface morphology of ZnO Thin
Film
6.1.3. UV- Visible Spectroscopy
UV-Visible spectroscopy was performed to study optical properties of the thin film.
The room temperature UV-Visible transmittance spectra was taken in the range of
200-800 nm is shown in the figure.
a b

Band Gap Calculation : (αhν)1/n = B (hν - Eg)
α : absorption Coefficient
n = 2 Indirect band Gap, ½ Direct Band Gap
B = Constant related to transition probability
• Band Gap (Eg) is found to be 3.26 eV.
• Sudden rise in transmittance at = 385 nm and it has high transmission coefficient
for UVA to visible light.

Figure 6.1.3 a) Band gap calculation b) Transmittance Curve
6.2. ZnO NANOWIRES
In this section, ZnO nanowires grown by hydrothermal method are studied in detail
for their morphology and crystal structure. Further effect of precursor concentration,
growth temperature, growth time on morphology and crystallinity of ZnO nanowires
put under discussion through XRD and FESEM results.
a
b

6.2.1. X-Ray Diffraction:
The X-ray diffraction data were collected on a Rigaku Ultima IV diffractometer using
Cu
Kα-radiation (λ=1.5418Å) over a range 200
<θ<800
at operating voltage of 40 kV and
operating current of 40 mA. In this section XRD pattern of nanowires for various
concentration and growth temperature have been analyzed.
Varying Precursor Concentration
Maximum peak intensity is observed for 0.075 M solution, showing that at this molar
concentration well aligned nanowires are formed. New peaks are observed
corresponding to the plane (102) and (103) with increased intensity on increase in
temperature. From this observation it can be said that increment in precursor
concentration in the solution causes rapid nucleation of ZnO molecules and due to
rapid nucleation molecules doesn’t have time to find the most relaxed state and hence
other growth direction can also be seen.
Varying Growth Temperature
It is observed that on increasing the growth temperature peak intensity of (002) plane
increases. This can be attributed to the availability of required energy to the molecules
to align themselves at proper orientation. Appearance of new peaks is observed with
increase in temperature showing effect of thermal distortion on the crystalline
structure .

Figure 6.6.1 a) Variation with concentration b) Variation with temperature
a
b

6.2.2. Field Emission Scanning Electron Microscopy:
Varying Temperature
Temperature variation plays an important role in maintaining the aspect ratio. Aspect
ratio is determined by relative growth of polar and non-polar surfaces. It increases
with increase in temperature with maximum obtained at 90ºC. At low temperature
mobility and diffusion length of ions on the substrate is very low, which ceases the
movement of ions and hence large nuclei are formed reducing the density of
nanowires. At an increased temperature mobility and diffusion length is large enough
to reach the site of already grown nanowires resulting in lower density .
Figure 6.2.2 (I) Change in surface morphology of ZnO nanowires for with variation
in growth temperature a) T= 90ºC b) T= 80ºC c) T= 70ºC d) T= 60ºC
a b
c
d

Varying Precursor Concentration
When the precursor concentration in the solution increases, the nucleation of ZnO is
so rapid that many ZnO nuclei form in the initial stage. These nuclei may aggregate
together due to excess saturation. Also the chemical potential inside the solution body
increases with increase in zinc concentration. This increased zinc chemical potential
inside the solution can be balanced by generation of more nucleation sites on the
substrate surface, and therefore, the density of ZnO NWs will increase. If the zinc
concentration is further increased, the density of ZnO NWs remains approximately
steady with a slight tendency to decrease. After saturation density more arrived ions
will not contribute to the new nucleation sites but will dissolve into the solution, hence
density will be constant .

6.2.3 UV Detection Property of ZnO nanowires
UV sensing property of ZnO nanowires have been studied by analysing I-V curve for
both uncoated nanowires and Poly(vinyl alcohol) coated nanowires. It is observed
that current increases with increase in bias voltage for both coated and uncoated
nanowires that for coated ZnO nanowires but increment for coated nanowire is more
significant. Polymer coated nanowires have more sensitivity, faster response and
better recovery time than uncoated nanowires.
Figure 6.2.2 (II) Change in surface
morphology with change in precursor
concentration
a) Concentration = 0.025 M
b) Concentration = 0.075 M
c) Concentration = 0.125 M
a b
c

Idea for the PVA coating came from storage of food in food industries. Generally,
PVA is used to create a barrier for the oxygen ensuring proper storage. In UV
detection mechanism, it reduces the width of depletion layer of oxygen on the surface
of nanowire and hence reduces the surface carrier trapping of the carriers. Decay in
photocurrent due to photocurrent relaxation decreases. During the steady illumination
decrease in photocurrent for PVA coated nanowire is less than for as grown
nanowires. Because of the thin depletion layer there is a fast recovery of photocurrent
in coated nanowires .
Figure 6.2.3 (II) I-T Characteristics

a
• Figure 6.2.3 a) I-V Characteristics of coated ZnO nanowires -
b) I-V Characteristics of uncoated ZnO nanowires Increment in photocurrent for
coated surface in more significant

• Increment in photocurrent for coated surface in more significant -
• Increment in uncoated surface ~ 18 times
• Increment in coated surface ~ 50 times
• Decrement in photocurrent for long time UV exposure -
• Coated ~ 3.10 %
• Uncoated ~ 13.42 %
Conclusion
Thin Film
In this review, ZnO thin film grown by RF magnetron sputtering is studied for its
crystallinity, crystallite size, thickness, surface morphology and band gap. It is
observed that deposited thin film have high crystallinity with characteristic peak at
2θ value of 34.91 Degree. Inter-planer distance for the planes where most of the
ZnO atoms are arranged is (d) 2.6 Aº. It has crystallite size of around 14 nm.
Thickness of film is found to be 796.04 nm and band gap is 3.26 eV.
Transmittance curve shows transparency of the ZnO film for UV and visible light.
Nanowire
Zinc oxide nanowires have been grown on seeded crystalline Zinc Oxide layer
deposited on Glass Substrate by Hydrothermal Method using
Hexamethylenetetraamine (HMT) and Zinc Nitrate as precursor solution, resulting
in well aligned nanowires. It can also be concluded that seeded zinc oxide layer is
critical for the growth of nanowires. Nanowires morphology, length and density is

significantly affected by precursor concentration, growth temperature and growth
time. Due to wide band gap of ZnO which falls in UV range, ZnO nanowires can be
used as UV photodetector. To enhance the UV absorption ability and electron
transport property, surface functionalization is carried out using Poly(vinyl alcohol).
From I-V characteristics it is found that functionalized nanowires shows better
switching action and large deviation of photo current from dark current. This
behaviour of ZnO nanowires shows their suitability for commercial UV
photodetector which can be manufactured using Schottky contact for better response
References
1) Chennupati Jagadish and Stephen Pearton, “Zinc Oxide Bulk, Thin Films and
Nanostructures Processing, Properties and Applications,” Chapter 1, page 1-3
2) Anderson Janotti and Chris G Van de Walle Fundamentals of zinc oxide as a
Semiconductor, Rep. Prog. Phys. 72 (2009) 126501
3) Alison Goodsell, Zinc Oxide Nanowire Growth, Thayer School of Engineering,
4) Dartmouth College, Hanover, New Hampshire 03755
5) Milton Ohring, Materials Science of Thin Films, Chapter 6-Chemical Vapor
Deposition
6) Sheng Xu and Zhong Lin Wang, Nano Res. 2011, 4(11): 1013–1098
7) Yu et al. Nanoscale Research Letters 2012, 7:517
8) Jorge L. Gomez, Onur Tigli, J Mater Sci (2013) 48:612–624
9) Lori E. Greene, Matt Law, Dawud H. Tan, Max Montano, Josh Goldberger, Gabor
10) Somorjai, and Peidong Yang Nano Lett., Vol. 5, No. 7, 2005

11) Sachindra Nath Das, Kyeong-Ju Moon, Jyoti Prakash Kar, Ji-Hyuk Choi, Junjie
Xiong, Tae Il Lee, and Jae-Min Myoung, APPLIED PHYSICS LETTERS 97,
022103 2010
12) Lori E. Greene, Benjamin D. Yuhas, Matt Law, David Zitoun, and Peidong
Yang, Inorganic Chemistry, Vol. 45, No. 19, 2006 7535
13) Jia Grace Lu, Paichun Chang, Zhiyong Fan, Materials Science and Engineering
R 52 (2006) 49–91

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