"This accepted author manuscript is copyrighted and published by Elsevier.
It is posted here by agreement between Elsevier and MTA. The definitive
version of the text was subsequently published in [Applied Surface Science,
2017, 417, 218-223; DOI 10.1016/j.apsusc.2017.03.212]. Available under
license CC-BY-NC-ND."
- 1/20 -
Characterization of PLD grown WO3 thin films for gas sensing
Stefan I. Boyadjiev1,2*, Velichka Georgieva1, Nicolaie Stefan3, George E. Stan4, Natalia
Mihailescu3, Anita Visan3, Ion N. Mihailescu3, Cristina Besleaga4, and Imre M. Szilágyi2,5
1
Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko chaussee
blvd., 1784 Sofia, Bulgaria
2
MTA-BME Technical Analytical Chemistry Research Group, Szent Gellért tér 4, Budapest,
H-1111, Hungary
3
National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Street,
Magurele-Ilfov, RO-077125, Romania
4
National Institute of Materials Physics, 405A Atomistilor Street, Magurele-Ilfov,
RO-077125, Romania
5
Budapest University of Technology and Economics, Department of Inorganic and
Analytical Chemistry, H-1111 Budapest, Szent Gellért tér 4, Hungary
E-mail: boiajiev@gmail.com
Abstract: Tungsten trioxide (WO3) thin films were grown by pulsed laser deposition (PLD)
with the aim to be applied in gas sensors. The films were studied by atomic force microscopy
(AFM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and
profilometry. To study the gas sensing behavior of these WO3 films, they were deposited on
quartz resonators and the quartz crystal microbalance (QCM) method was applied to analyze
their gas sensitivity. Synthesis of tetragonal-WO3 films starting from a target with
predominantly monoclinic WO3 phase was observed. The films deposited at 300 °C
presented a surface topology favorable for the sorption properties, consisting of a film
matrix with protruding craters/cavities. QCM prototype sensors with such films were tested
for NO2 sensing. The PLD grown WO3 thin films show good sensitivity and fast reaction at
- 2/20 -
room temperature, even in as-deposited state. With the presented technology, the
manufacturing of QCM gas sensors is simple, fast and cost-effective, and it is also suitable
for energy-effective portable equipment for on-line monitoring of environmental changes.
Keywords: gas sensor, quartz crystal microbalance, pulsed laser deposition, thin film, WO3
1. Introduction
Tungsten trioxide (WO3) is one of the most widely studied semiconductor oxides, due to its
remarkable physical, optical and optoelectronic properties [1–2]. WO3 grown as thin film
can be used for various applications, such as electrochromic devices (in displays, smart
windows and optical switching coatings) [1–3], sensing numerous toxic and inflammable
gases [4–7], photocatalysis [8–9], solar energy conversion [2], water splitting [10], and
many others.
The quartz crystal microbalance (QCM) is a widely-used technique for detecting the mass of
thin films deposited on the crystal surface in the sub-nanogram range, but it could be also
applied for monitoring the adsorption of very small amounts of various toxic gases. When
the QCM method is used for gas sensing, the surface properties of the sensing material are of
higher importance, compared with its bulk. Very thin films are also applicable for gas
sensors [11–12], while their morphology is of high importance. Also, compared with other
methods, the advantages of QCM for sensing gases are the high sensitivity, simple
technological implementation and easy real-time monitoring, capability of operating at room
temperature, relative independence from electromagnetic fields and rapid temperature
changes, fast response even at low concentrations, durability, portability, low energy
consumption and cost [13–14].
Although WO3 shows good sensitivity to many gases [4–7], it is possible to further improve
its gas sensing properties by fostering its nanostructured morphology to increase the specific
surface area. Therefore, already various techniques have been adopted to grow thin films
from oxide materials with higher specific surface area.
Pulsed laser deposition (PLD), also called deposition by laser ablation, is a physical vapor
deposition (PVD) process based on the vaporization of condensed matter by means of
photons [15–17]. A highly intense short-pulsed (usually of nano- or pico-seconds)
- 3/20 -
high-power laser beam is focused under vacuum or a working gas atmosphere on a target of
the deposited material. If the laser fluence exceeds a specific threshold, the ablated material
is directed forward, in form of a plume, towards the deposition substrate, where film is
formed by re-condensation [15–16].
As the species arriving on the substrate are highly energetic, the synthesized films elicit high
adherence. Moreover, the plasma plume plays a notable piston-like role, pushing and
carrying the removed matter from the ablated target to the substrate [16].
PLD is a versatile PVD method which presents a distinct set of advantages, which currently
increases its appeal in the sensors field. The main ones are: i) congruent target-to-substrate
transfer of substances with intricate stoichiometries; ii) control over the growing films
morphology by adapting the deposition variables (possibility for growth of rough or
nanostructured surfaces); and iii) plasma direct action upon surface can result in a rather
large hardness, along with a high adherence of the growing film to the substrate.
i) The first feature is particularly important in the case of WO3, which could be
chemically modified during transfer. However, as shown in our previous studies [18], ns
excimer laser pulses can promote the growth of stoichiometric WO3 film on substrate after
laser transfer.
ii) The deposition of substance in form of nanoparticles covering the surface of film is a
characteristic of PLD, which can facilitate the natural growth of nanostructured surfaces
with an increased active area beneficial for gas sensing [19–21].
iii) The films obtained by PLD after the plasma transfer are usually very adherent to
substrate and also have a rather large hardness, which is of key importance for the long term
use as gas sensors in specialized devices [17, 22–23].
Therefore, in the present study PLD was chosen as method allowing the growth of highly
pure and adherent thin films with larger surface area, which is more beneficial morphology
for gas sensing, compared with other PVD methods.
The usual PLD drawbacks are the rather non-uniform thickness distribution, the possible
presence of bigger size (submicronic or even micronic) droplets and the rather limited
deposition area of usually only few square centimeters [15–16]. But when preparing QCM
gas sensors none of these limitations are significant for the fabricated device, since of high
importance are only the surface properties of the films, and the active sensing area of the
- 4/20 -
QCM sensor is rather small.
Reactive PLD is a strategy to enhance the reactivity between the background gas and the
ablated species in order to reconstitute on the substrate the stoichiometry of the likely lost
elements in the deposited films. For example, due to its expansion velocity, the lighter
oxygen species develop a larger angular distribution in the plume than the heavier elements,
such as tungsten, and additional sources to compensate the expected oxygen loss can be
necessary [15–17].
In this study, we prepared QCM gas sensors with PLD grown WO3 thin films, which were
characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier
transform infrared (FTIR) spectroscopy and profilometry. The gas sensing was investigated
by detecting various concentrations of NO2 in dry air at room temperature.
2. Experimental methods
2.1 Preparation of the gas sensors
Previously, prototype QCM gas sensors with several transition metal oxide films (e.g. TiO2,
MoO3, ZnO) were already fabricated by our team and tested for sensitivity to NO2 in a
specially designed experimental set-up [24]. These films were mostly prepared by reactive
magnetron sputtering [25–26] and atomic layer deposition (ALD) [11–12]. In the present
study, we focused on the sensing behavior of WO3 thin films grown by PLD. Such films
were deposited either on quartz resonators (AT-cut 16 MHz QCM with 4 mm gold electrodes
on both sides), which were used for preparing the QCM gas sensors [11–12], or on Si
substrates, which were used for further characterization of the films. Prior to deposition all
substrates were cleaned in an ultrasonic bath with acetone and ethanol for 10 minutes, rinsed
in deionized water and finally purged with synthetic air. All depositions were carried out in a
standard reactive PLD (RPLD) setup [16] consisting of a stainless-steel vacuum reaction
chamber equipped with a gas flow, heating and vacuum pump systems, MgF2 optics and
windows. Before deposition, the reactor chamber was evacuated down to a residual pressure
of ~5x10-5 Pa in order to avoid contamination. Then, a flux of pure O2 (99.999%) was
introduced to attain an oxygen pressure of 16 Pa. A commercial monoclinic WO3 powder
(Fluka, purity 99.99%), compacted at 2 MPa and sintered (1100 oC for 6 h), was used as
target to be ablated by a KrF (248 nm, 25 ns) pulsed laser source at a fluence of 1.2 J/cm2 and
- 5/20 -
repetition rate of 40 Hz. The samples were deposited on heated substrates (at 150 or 300 oC)
with a definite number of laser pulses (N), and oxygen pressure in the reactor chamber. The
target was rotated at a 0.5 Hz frequency, to avoid its piercing and to assure a smooth ablation
procedure. For all experiments a target-to-substrate distance of 40-mm was used. The
deposition parameters are summarized in Table 1.
2.2 Characterization techniques
a) The surface morphology of films was analyzed by Atomic Force Microscopy (AFM) in
non-contact mode using an NT-MDT NTEGRA Probe NanoLaboratory system (NT-MDT
NSG01 cantilever with tip radius of 10 nm).
b) Fourier transform infrared (FTIR) spectrometry studies were performed in transmission
mode with a Shimadzu 8400S instrument. The investigated range was chosen between
500–4000 cm−1, with a resolution of 4 cm−1.
c) The structure of the WO3 films samples was investigated by Grazing Incidence X-Ray
Diffraction (GIXRD) using a Bruker D8 Advance diffractometer, in parallel beam setting,
with Cu Kα (λ = 1.5418 Å) incident radiation. The incidence angle was set to 2°, and the
scattered intensity was scanned within the range (20–60)° (2θ), with a step size of 0.04°, and
6 s per step.
d) The initial parameters of the QCM sensors and their quality were evaluated by measuring
the main equivalent dynamic parameters: equivalent dynamic resistance Rq and static
capacitance C0 using a selective level meter. The other parameters, such as the quality factor
(Q), were calculated [27]. The QCM gas sensors with PLD grown WO3 films were installed
on a special holder inside the test chamber. The chamber was first evacuated, and then the
test gas, with a certain concentration, was released in a continuous flow. The velocities of
both the carrier (high purity synthetic air) and test (NO2) gases were measured and
controlled by mass flow controllers, their ratio being defined by the desired concentration. A
frequency counter (Hameg 8123) connected to the QCM, as well as to the computer for data
recording, registered the QCM frequency. Thus, the frequency change as a function of time
was recorded. The gas sensing measurements were based on the correlation between the
frequency shift and the additional sorbed mass loading of the resonator. The correlation
between the frequency shift (∆F) and the sorbed mass (∆m) for AT-cut quartz is given by
- 6/20 -
Sauerbrey equation [28]. This way the sorbed mass can be calculated and the gas sensing can
be quantitatively determined. The initial frequency value was considered the one measured
in the carrier gas flow, under saturation conditions. The temperature of the QCM was
measured by a Pt thermal sensor positioned near the QCM and was maintained constant
during the measurements, thus avoiding inaccuracies caused by temperature changes. In
order to eliminate selectivity error from sorbed water vapours, first the films were saturated
to H2O (equilibrium in the measured frequency was achieved), and then these values were
taken as initial for the NO2 measurements.
3. Results and Discussion
Our previous studies revealed that the roughness of the resonator electrodes influences the
gas sensing properties of the WO3 thin films [29]. It was concluded that that the optimal
conditions can be defined by a compromise between a surface with higher roughness, which
has better sorption but slower desorption and decreased quality of the QCM, and smoother
one, possessing a decreased sensing ability. Another approach to improve the sorption
properties is to design a surface morphology of the sensing film with higher roughness,
which is beneficial for the sorption but does not reduce the QCM quality (still, here the
desorption speed is also influenced, so optimization is also needed). The PLD method
generally offers possibilities for deposition of thin films with higher specific surface area,
compared with the previously studied WO3 films deposited by sputtering. Two different
deposition temperatures were used for synthetizing films with different degrees of
crystallinity and surface roughness.
The PLD method was found to be suitable for deposition of WO3 films on QCM for sensor
applications. The films were uniform and with good quality and adhesion to the QCM
electrodes. The thickness of the films of ~600 nm for the 301 sample and ~1500 nm for the
151 sample was rather high, which is not specifically needed for the sensing properties of the
films when using the QCM method (in our previous studies it was found out that much
thinner films also have rather good sensing properties when using the QCM method
[11–12]). But since the deposition rates while using PLD are rather high compared to many
other methods, growing a bit thicker films, which could be better for long term usage, can be
easily and cost-effectively facilitated.
- 7/20 -
Since the surface morphology and roughness is of high importance for the sorption of the gas
molecules, the films were first analyzed by AFM. The AFM images, collected at different
magnifications (Fig. 1), indicated in the case of WO3 films deposited at the lower
temperature (i.e. 150 °C) a typical PLD morphology composed of closely-packed
quasi-equiaxial spherical grains (Fig. 1-a,b). The majority of grains in the case of this type of
sample had diameters in the range ~112 – 120 nm, with only scarce cases of extremes, with
sizes of ~90 and ~200 nm. Instead, the sample deposited at the higher temperature (i.e.
300 °C) presented a radically different surface topology: a very smooth film matrix with
fairly well-distributed protruding craters/dents (Fig. 1-c,d). The topographies presented in
Fig. 1 were characteristic for the entire films surface, pointing to a good morphological
uniformity. The root mean square roughness (RRMS) of the WO3 films deposited at 150 °C
and 300 °C, determined on the basis of the 10x10 μm2 AFM images, were of ~17 nm and ~4
nm, respectively.
It is not clear yet why these dents have formed at the films deposited at 300 °C. It might be
due to a desorption process of species arriving at the substrate surface, not finding an energy
favorable spot, in this case of a higher temperature and fast film structuring process. Or some
substrate topological defects could act as energy barriers for some species, and as they do not
find the energy favorable site, they desorb. Thus, in such spots, the deposition rate is lower,
and this does not allow for the films to fill out the substrate voids. In any case, such
morphology is favorable for the sorption properties of the film, though it can be assumed that
the desorption times could be also increased.
The FTIR spectra of the source powder and sintered PLD target are presented comparatively
in Fig. 2a. All absorption bands characteristic to a monoclinic WO3-type phase were
identified: asymmetric stretching (νas) motions band within the equatorial plane (~646 cm–1),
ν(W–Ointra–W) stretching vibrations of the bridging oxygen (~788 cm–1), and ν(W–Ointer–W)
stretching vibrations of the bridging oxygens (~866 cm–1) [30–33]. The shallow peaks at
~1012 and ~1040 cm–1 are assigned to the stretching of the W=O bonds at the surface of the
grains [34–35]. The absorbance bands of the PLD target are both more conspicuous and
slightly shifted to higher wave numbers with respect to the raw starting powder, which
suggests the increased short-range order of this material. The IR spectra of PLD films (Fig.
2b) displayed a slightly different allure, with the W-O and W=O vibrational stretching bands
- 8/20 -
maxima somewhat shifted and less defined with respect to ones elicited by the bulk WO3
materials (Fig. 2a).
The XRD analyses have been carried out in symmetric (θ-θ) geometry on the source powder
material and the PLD sintered target (Fig. 3a). The sintering treatment induced an increased
crystallization of the WO3 material, as indicated by the sharper and more defined diffraction
peaks. In both powder and target cases, the monoclinic WO3 phase is the prominent one
(ICDD: 01-083-0950), with the triclinic-WO3 (ICDD: 01-083-0947) present only as a
supplemental minor phase. If in the case of the powder, the presence of triclinic-WO3 is only
hinted by some fainted shoulders, in the case of the target, the sintering treatment induces the
crystallinity increase of this minor phase, making it easier and safer to be ascertained. (Fig.
3a – inset).
The GIXRD analyses, performed on the PLD films (Fig. 3b), indicated the amorphous status
of the WO3 film deposited at 150°C, at the sensitivity limit of the apparatus. The film
deposited at 300 °C consisted of tetragonal WO3 crystalline phase (ICDD: 01-089-8052).
The high intensity of the 002 peak with respect to the reference ICDD file points toward a
c-axis preferential oriented growth of the crystallites. This is supported also by the modified
relative intensities (compared to the ICDD file) of the recorded peaks, as a function of the tilt
angle: the absence of reflections of planes perpendicular to c-axis (e.g., 110, 200), and the
presence of peaks appertaining to planes tilted with various angles with respect to the surface
(e.g., 102, 112, 202, 104, etc). The crystalline coherence length (“crystallite size”) was
estimated from the full-width at half maximum of the 002 diffraction line using the Scherrer
equation. The line width was corrected for instrumental broadening using a corundum
standard reference (NIST SRM 1976). A crystalline coherence length of ~51 nm has been
calculated. The zoomed region (the intensity scales of the graphical representations were
chosen so that to emphasize the low intensity humps), presented in Fig. 3b-inset, indicated
that the film deposited at 300 °C, has as well an amorphous minor component.
The crystal structure of WO3 is temperature dependent: α-WO3 – tetragonal (~740 – 900 °C),
β-WO3 – orthorhombic (~330 – 740 °C), γ-WO3 – monoclinic (~17 – 330 °C), δ-WO3 –
triclinic (–43 – 17 °C), and ε-WO3 – monoclinic (< –43 °C) [36–37]. However, in
non-equilibrium thermodynamic conditions, such as the ones typically met in the case of
physical vapor deposition methods, obtaining various WO3 phases outside the
- 9/20 -
above-mentioned temperature ranges, seems reasonable. The synthesis by PLD of
tetragonal-WO3 films starting from a target with different crystal structure is not
unprecedented. Mitsugi et al. [38] has obtained tetragonal-WO3 films from a triclinic-WO3
target by PLD in (fairly) comparable deposition conditions.
For a well-working QCM sensor of main importance are: i) the fast and stable response
(stable decrease in the frequency when the tested gas is introduced in the chamber); ii)
reaching saturation (equilibrium between the sorbed and desorbed molecules of the detected
gas at certain concentration) in a reasonable time; and iii) the sorption to be a physical
process during which the sensor can be easily recovered (and the initial values for the
frequency are reached after the tested gas is removed from the chamber). All these were
observed for the presently studied PLD grown WO3 films. Also, our prototype QCM sensors
with PLD deposited WO3 thin films showed good sensitivity to NO2. In Fig. 4 and Fig. 5 are
presented the frequency-time characteristics (FTCs) of such a QCM sensor measured at 500
and 1000 ppm NO2 concentrations. They were both typical for a well-working sensor.
At concentration of 500 ppm (Fig. 4) the frequency change until reaching saturation was
Δf≈50 Hz. The recovery was complete and the initial values of the resonator were achieved
after removing the NO2 gas from the chamber. The sorption was considered to be a fully
physical process, shown by the full recovery of the sensor in a comparably short time.
At higher concentrations the kinetics of the sorption and desorption processes was faster and
better expressed (Fig. 5). At a concentration of 1000 ppm, the change of frequency until
reaching saturation was Δf≈80 Hz. With increasing NO2 concentration, the sorption was
faster, while the time for reaching dynamic equilibrium was a bit longer, corresponding to
the higher adsorbed mass on the film surface. The sorption was still physical and the
resonator could be fully recovered, while the recovery times remained almost the same. This
shows that the dents present on the surface do not have a negative influence on the
desorption process, while most probably being beneficial for the sorption properties.
At both concentrations, the sorption process could be divided in two stages: faster at the
beginning and then slower until reaching saturation. The length of the stages was
proportional to the sorbed mass and was similar at the both concentration – around 2/3 of the
saturation time for the faster stage, and around 1/3 for the slower one.
Since the studied QCM sensors operate at room temperature, they generally do not have
- 10/20 -
problems with long term cycling as many conductometric sensors, which are working only
at high operational temperatures. Similar QCM gas sensors were tested from our team after
many years (some after even 7–8 years) and showed similar parameters, proving their ability
to detect for many cycles and long terms [25–26].
From our previous studies [11–12, 24–26, 29] and the results from the present one, could be
noted that the main advantages of the PLD method for preparing QCM gas sensors in
comparison with the others that our team used before are: i) the fast deposition times, which
together with the easier and less costly preparation of the PLD target make the method more
cost-effective, compared with both the reactive sputtering and ALD; ii) easily prepared
nanostructured surface, which was hardly (for longer deposition times and in a smaller
degree) achieved by reactive sputtering, something that is of high importance for the sensing
properties of the as-grown films.
4. Conclusions
Synthesis by PLD of tetragonal-WO3 films starting from a target with predominantly
monoclinic WO3 phase was observed. The PLD method was found to be suitable for fast and
cost-effective deposition of WO3 thin films for QCM gas sensors. The films deposited at
300 °C presented a surface topology favorable for the sorption properties, consisting a film
matrix with fairly well-distributed protruding craters/cavities. At the same time, it was found
that it does not have negative influence on the desorption process. These PLD grown WO3
thin films showed good sensitivity to NO2 at room temperature and capability to very well
register various concentrations. The sorption was fully reversible and the sensors could be
recovered in considerably short time.
Acknowledgments
S.I.B. acknowledges the Postdoctoral Fellowship programme of the Hungarian Academy of
Sciences (2013–2015) and the INERA REGPOT Project of Institute of Solid State Physics,
Bulgarian Academy of Sciences. N.S., N.M., A.V., F.M.M and I.N.M. acknowledge the Core
Program LAPLACE–IV 2016–2017. G.E.S. and C.B. thank the NIMP Core Programme PN
16 48–3/2016. I.M.S. thanks a János Bolyai Research Fellowship of the Hungarian
Academy of Sciences and an OTKA–PD–109129 grant.
- 11/20 -
References
[1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier,
Amsterdam, 1995.
[2] K.A. Gesheva, Thin film optical coatings for effective solar energy utilization,
Nova Science, New York, 2007.
[3] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism and
Electrochromic Devices, Cambridge university press, Cambridge, 2007.
[4] C. Balázsi, L. Wang, E.O. Zayim, I.M. Szilágyi, K. Sedlacková, J. Pfeifer, A.L. Tóth,
P.-I. Gouma, J. Eur. Ceram. Soc. 28 (2008) 913.
[5] M. Blo, M.C. Carotta, S. Galliera, S. Gherardi, A. Giberti, V. Guidi, C. Malagù, G.
Martinelli, M. Sacerdoti, B. Vendemiati, A. Zanni, Sens. Actuator B-Chem. 103 (2004)
213.
[6] L. Guatwei, T. Xian-ju, C. Choongyew, A. Gaiktin, A.B.M. Zailani, J. Mater. Sci. Eng.
5 (2011) 27.
[7] I.M. Szilágyi, S. Saukko, J. Mizseic, A.L. Tóth, J. Madarász, G. Pokol, Solid State Sci.
12 (2010) 1857.
[8] Y. Wicaksana, S. Liu, J. Scott, R. Amal, Molecules 19 (2014) 17747.
[9] I.M. Szilágyi, B. Forizs, O. Rosseler, A. Szegedi, P. Nemeth, P. Kiraly, G. Tarkanyi, B.
Vajna, K. Varga-Josepovits, K. Laszlo, A.L. Tóth, P. Baranyai, M. Leskelä, J. Catal. 294
(2012) 119D.
[10] B. Marsen, E.L. Miller, D. Paluselli, R.E. Rocheleau, Int. J. Hydrogen Energy 32
(2007) 3110.
[11] S. Boyadjiev, V. Georgieva, L. Vergov, Z. Baji, F. Gáber, I.M. Szilágyi, J. Phys.: Conf.
Ser. 559 (2014) 012013.
[12] S.I. Boyadjiev, V. Georgieva, R. Yordanov, Z. Raicheva, I.M. Szilágyi, Appl. Surf. Sci.
387 (2016) 1230–1235.
[13] A. Cunningham, Introduction to Bioanalytical Sensors, Wiley, New York, 1998.
[14] A. Mirmohseni, K. Rostamizadeh, Sensors 6 (2006) 324–334.
[15] C.W. Schneider and T. Lippert, “Laser ablation and thin film deposition”, Chapter 5
in: P. Schaaf (Ed.) “Laser Processing of materials Fundamentals, applications and
- 12/20 -
developments”, Springer series in Materials science 139, Springer, Berlin, 2010, pp.
89–112.
[16] I.N. Mihailescu, E. Gyorgy, “Pulsed Laser Deposition: An Overview”, in:
“International Trends in Optics and Photonics”, T. Asakura (Ed.), Springer, Heidelberg,
1999.
[17] R. Eason, Pulsed Laser Deposition of Thin Films – Applications-Led Growth of
Functional Materials, Wiley-Interscience, New York, 2007.
[18] E. György, G. Socol, I.N. Mihailescu, C. Ducu, S. Ciuca, J. Appl. Phys. 97 (2005)
093527.
[19] G. Socol, E. Axente, C. Ristoscu, F. Sima, A. Popescu, N. Stefan, L. Escoubas, J.
Ferreira, S. Bakalova, A. Szekeres, I.N. Mihailescu, J. Appl. Phys. 102 (2007) 083103.
[20] A. Klini, V. Zorba, E. Gyorgy, C. Ristoscu, V.S. Teodorescu, I.N. Mihailescu, C.
Fotakis, Laser Phys. 13 (2003) 1325–1329.
[21] C. Ghica, C. Ristoscu, G. Socol, D. Brodoceanu, L.C. Nistor, I.N. Mihailescu, A. Klini,
C. Fotakis, Appl. Surf. Sci. 252 (2006) 4672–4677.
[22] E. Gyorgy and I. N. Mihailescu, “Enzyme-Based Biosensors for Trace Detection”,
chapter 9 in “Life Cycle Analysis of Nanoparticles: Risk, Assessment, and Sustainability”,
A. Vaseashta (Ed.), DEStech Publications, 2015, pp. 231–248.
[23] C. Ristoscu, I.N. Mihailescu, D. Caiteanu, C.N. Mihailescu, T. Mazingue, L. Escoubas,
A. Perrone, H. Du, “Nanostructured thin optical sensors for detection of gas traces”,
chapter 2 in “Functionalized Nanoscale Materials, Devices, & Systems”, A. Vaseashta, and
I.N. Mihailescu (Eds.), Springer Science + Business Media B.V., 2008, pp. 27–50.
[24] V. Georgieva, P. Stefanov, L. Spassov, Z. Raicheva, M. Atanassov, T. Tincheva, E.
Manolov, L. Vergov, J. Optoelectr. Adv. Mater. 11 (2009) 1363–1366.
[25] R. Yordanov, S. Boyadjiev, V. Georgieva, Digest J. Nanomat. Biostruct. 9 (2014)
467–474.
[26] R. Yordanov, S. Boyadjiev, V. Georgieva, L. Vergov, J. Phys.: Conf. Ser. 514 (2014)
012040.
[27] S. Manolov, H. Tihchev, Generators, Tehnika, Sofia, 1982.
[28] G.Z. Sauerbrey, Physik 155 (1959) 206–212.
[29] V. Georgieva, Z. Raicheva, A. Grechnikov, V. Gadjanova, M. Atanassov, J. Lazarov, E.
- 13/20 -
Manolov, J. Phys.: Conf. Ser. 253 (2010) 012046.
[30] L.M. Bertus, C. Faure, A. Danine, C. Labrugere, G. Campet, A. Rougier, A. Duta,
Mater. Chem. Phys. 140 (2013) 49–59.
[31] N. Sharma, M. Deepa, P. Varshney, S.A. Agnihotry, Thin Solid Films 401 (2001)
45–51.
[32] J. Díaz-Reyes, V. Dorantes-García, A. Pérez-Benítez, J.A. Balderas-López,
Superficies y Vacío 21 (2008) 12–17.
[33] S.M. Kanan, Z. Lu, J.K. Cox, G. Bernhardt, C.P. Tripp, Langmuir 18 (2002)
1707–1712.
[34] H.I.S. Nogueira, A.M.V. Cavaleiro, J. Rocha, T. Trindade, J.D. Pedrosa de Jesus,
Mater. Res. Bull. 39 (2004) 683–693.
[35] U. Opara Krasovec, A. Surca Vuk, B. Orel, Electrochim. Acta 46 (2001) 1921–1929.
[36] H. Kalhori, S.B. Porter, A.S. Esmaeily, M. Coey, M. Ranjbar, H. Salamat, Appl. Surf.
Sci. 390 (2016) 43–49.
[37] M.C. Rao, Journal of Non-Oxide Glasses 5 (2013) 1.
[38] F. Mitsugi, E. Hiraiwa, T. Ikegami, K. Ebihara, R.K. Thareja, Jpn. J. Appl. Phys. 41
(2002) 5372–5375.
Table Caption
Table I. Experimental conditions for PLD deposited WO3 thin films.
Figure Captions
Fig. 1. Characteristic AFM images, collected at different scales (10×10 μm2 (a,c) and 2×2
μm2 (b,d)), for the 151 (a,b) and 301 (c,d) WO3 thin films fabricated by PLD on Si <100>
mirror polished substrates.
Fig. 2. Comparative FTIR spectra of (a) WO3 source powder and sintered PLD target, and
(b) WO3 films deposited by PLD.
Fig. 3. Comparative GIXRD patterns of the (a) source powder and PLD sintered target, and
(b) WO3 films fabricated by PLD at 150 °C and 300 °C. Insets: zoomed selected regions of
the GIXRD diagrams.
- 14/20 -
Fig. 4. FTC of PLD grown at 300 °C WO3 thin film towards 500 ppm NO2.
Fig. 5. FTC of PLD grown at 300 °C WO3 thin film towards 1000 ppm NO2.
- 15/20 -
Table I. Experimental conditions for PLD deposited WO3 thin films.
Sample
code
DT-S
(mm)
WO3-151
Fluence
(J/cm 2)
O2
pressure
(Pa)
40
16
1.2
40
WO3-301
Repetation
rate, f
(Hz)
1.2
- 16/20 -
T
(°C)
No. of
pulses
(K=103)
Deposition
time
(sec)
150
22 K
550
300
8K
200
Figures:
Fig. 1.
- 17/20 -
Fig. 2.
- 18/20 -
Fig. 3.
- 19/20 -
Fig. 4.
- 20/20 -
Fig. 5
- 21/20 -