Characterization of Tungsten Oxide Thin Films Produced by Spark

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Characterization of Tungsten Oxide Thin Films Produced by Spark Ablation for NO2 Gas Sensing Nishchay Isaac, Marco Valenti, Andreas Schmidt-Ott, and George Biskos ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11078 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 29, 2016

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ACS Applied Materials & Interfaces

Characterization of Tungsten Oxide Thin Films Produced by Spark Ablation for NO2 Gas Sensing

Nishchay A. Isaac1, Marco Valenti1, Andreas Schmidt-Ott1, George Biskos1,2,3*

1

2

Faculty of Applied Sciences, Delft University of Technology, Delft, 2628-BL, The Netherlands

Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft 2628-CN, The Netherlands 3

Energy Environment and Water Research Center, The Cyprus Institute, Nicosia 2121, Cyprus

*Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Tungsten oxides (WOx) thin films are currently being used in electro-chromic devices, solar-cells and gas sensors as a result of their versatile and unique characteristics. In this paper we produce nanoparticulate WOx films by spark ablation and focused inertial deposition, and demonstrate their application for NO2 sensing. The primary particles in the as-deposited film samples are amorphous with sizes ranging from 10 to 15 nm. To crystalize the samples, the as-deposited films are annealed at 500 °C in air. This also caused the primary particles to grow to 30 - 50 nm by sintering. The morphologies and crystal structures of the resulting materials are studied using Scanning and Transmission Electron Microscopy and X-Ray Diffraction, whereas information on composition and oxidation states are determined by X-Ray Photoemission Spectroscopy. The observed sensitivity of the resistance of the annealed films is ~100 when exposed to 1 ppm NO2 in air at 200 °C, which provides a considerable margin for employing them in gas sensors for measuring even lower concentrations. The films show a stable and repeatable response pattern. Considering the numerous advantages of spark ablation for fabricating nanoparticulate thin films, the results reported in this paper provide a promising first step towards the production of high sensitivity and high accuracy sensors.

Keywords: spark ablation, film morphology, tungsten oxide, annealing, NO2 sensing


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Introduction

Oxides of tungsten find applications in a wide number of novel products including smart windows1,2, dye synthesized solar cells3, photocatalysts4 and gas sensors5–7. By probing changes of their conductivity or optical properties, earlier works have demonstrated that WO3 thin films can be used to measure the concentration of gases such as H2, H2S, NH3, SO2, and NOx in the ppm range8,9. WO3 films offer good selectivity in particular towards NO2 due to the high reactivity and oxidizing properties if the later.10,11

To make tungsten oxide films sensitive to concentrations that are environmentally relevant (i.e., in the ppb range), however, their sensitivity has to be improved substantially. One way to achieve that is to increase their surface-to-volume ratio by reducing their thickness or by nanostructuring them. Controlling the size of their building blocks, nanoparticulate films can be tailored to have extremely high surface area thereby increasing the number of interaction sites with the analyte molecules. Apart from increasing sensitivity, changing the size of the constituent nanoparticles can also affect the response times of the films12.

Sensing properties of metal semiconductor films are mostly determined by the adsorbed oxygen species13,14. Being electronegative, the adsorbed oxygen ions take up the charge carriers (i.e., the electrons) from the surface of WOx, forming a charge depletion region. NO2, which has a higher electron affinity than oxygen, interacts with the adsorbed oxygen species and the metal oxide to trap more electrons. This increases the thickness of the depletion layer, thereby increasing the resistance of the films. Although the interaction mechanisms are not fully understood, recent density functional theory calculations15,16 indicate that the Fermi level of the


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energy band of tungsten oxide shifts towards higher energies, inducing changes in their resistance, upon adsorption of NO2 molecules16.

WOx nanoparticles and nanomaterials can be synthesized by a number of liquid-phase and gasphase methods17–20. Among those, synthesizing nanoparticle building blocks by spark ablation in the gas phase has many advantages21. First of all, the technique can produce nanoparticle building blocks and therefore nanomaterials of very high purity. In addition, it provides good control over particles size and composition, providing flexibility for the fabrication of materials for numerous applications including gas sensors. Last, but not least, it can offer the possibility of mixing two or more materials uniformly in the nanostructure, providing an additional feature for tuning the resulting nanomaterials. First introduced in 198822, spark ablation has been recently used for synthesizing Pd-based nanoparticles for hydrogen storage23 and sensing24, as well as for producing photocatalysts for water splitting25.

Here we produce WOx nanoparticulate thin films and demonstrate their capability for sensing NO2 in air. The morphology and crystal structure of the films are characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). The composition and oxidation state of the samples are determined by X-Ray photoelectron spectroscopy (XPS). Changes in the resistance of the resulting films are measured while exposing them to 1 ppm NO2 in air. The effect of changing film morphology on the sensing performance of the films is also investigated systematically.


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1. Materials and Methods Nanoparticulate Film Production

Thin films tested in this work are prepared using nanoparticles produced by a spark discharge generator (SDG). In brief, induced sparks between two electrodes held at a fixed distance next to each other vaporize the electrode material. The resulting vapor clouds are rapidly cooled by a high quenching gas flow that passes through the gap between the two electrodes, causing the formation of atomic clusters by nucleation26. These clusters further grow to form singlet nanoparticles27 and agglomerates by condensation and coagulation, respectively28. The resulting aerosol is passed through a series of aerodynamic focusing lenses29 and an inertial impactor30 where the nanoparticles are inertially deposited on a substrate (i.e., on the sensor chip in our case). The experimental setup used for the deposition is shown in Figure 1. Two tungsten electrodes (6 mm in diameter and 30 mm long), with one connected to a high-voltage power supply and the other to the ground, are placed next to each other inside a chamber. Ar gas (99.999% purity) flowing through the gap between the two electrodes at a rate of 1.5 lpm quenches the vapors produced by the sparks. The resulting aerosol is then passed through the series of critical orifices, which focuses the particles to a narrow beam before deposition while maintaining a constant flow through the impactor. One end of the exhaust line is at atmospheric pressure, as shown in Figure 1, ensuring that the spark chamber is also at atmospheric pressure.


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Gas lines SDG

1.5 Lpm

Electrical lines

HV Supply

MFC

Ground electrode

HEPA Filter To Exhaust By-Pass Valve Critical Orifice

Ar

Aerodynamic lenses & Inertial impactor

To Exhaust Vacuum Pump

Figure 1. Schematic diagram of the experimental setup used to produce the nanoparticulate thin films. Vapors produced by the sparks in the SDG are quenched by an Ar gas (99.999% purity) flow. The resulting aerosols downstream the SDG are passed through aerodynamic lenses to focus the particles in a narrow beam, and an inertial impactor to deposit them on the circuit substrates.

Particle and Film Characterization

A Scanning Mobility Particle Sizer31 (SMPS) was used to determine the size distribution of the particles produced by the SDG. The SMPS consisted of a custom-made Differential Mobility Analyzer32 (DMA) to select the sampled particles based on their electrical mobility, and a Condensation Particle Counter33 (CPC; GRIMM 5.400) for counting them. To determine the deposition rates we measured the thickness of the deposited films at specific time intervals using a stylus profiler (BRUKER Dektak 8). The deposited films were not entirely flat but exhibited concave-like profiles as shown in our previous work (cf. Figure 4 of Isaac et al.24).


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To characterize the overall structure of the resulting materials we used a Scanning Electron Microscope (SEM; JEOL JSM-6010LA) operated between 5 and 15 kV. Individual nanoparticles from the as-deposited films were placed on copper grids (Quantifoil® microgrid) and observed using a Transmission Electron Microscope (TEM; JEOL JEM 1400) operated at 120 kV. To place the particles from the annealed samples on the copper grids for observations we used the following procedure20: ethanol droplets (99.9% purity) were deposited onto the film to disperse it for a few seconds, and drops from the resulting solution were applied onto the TEM grid. TEM measurements were performed on the samples after the ethanol was allowed to evaporate at room temperature.

The crystal structure of the films was studied by X-ray Diffraction (XRD; Bruker AXS D8 Advance) with a Co Kα radiation source (λ = 1.78897 Å). XRD measurements were made in 2θ angles ranging from 20° to 50°. As-deposited films were found to be amorphous. In order to determine the oxidation state of the tungsten in the films, we employed an X-Ray photoemission spectrometer (Thermo Scientific k-Alpha) that employed an Al X-ray source, a 180° double focusing hemispherical analyser-128-channel detector, and a 4-keV ion-gun. All spectra were recorded using monochromatized Al-Kα (1486.68 eV) radiation. The X-ray source was operated at an acceleration voltage of 12 kV.

Sensing experiments

Following the characterization of the WOx films produced by spark ablation, we evaluated their capability to sense NO2. Thin films of tungsten oxide were deposited on commercial alumina substrates with inter-digitated Au electrodes (Electronics design center CWRU, Sensor # 102) and annealed at 500 °C in air. Figure 2 shows the setup used for testing the sensing


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performance of the resulting sensors. The films were fixed onto a substrate holder, which was then inserted into a tubular furnace. The gas flow over the sensor was kept constant at 150 ml min-1. The furnace temperature was controlled to 200 °C, while a thermocouple not belonging to the controller was used to continuously monitor the substrate temperature during the measurements.

Gas lines Electrical lines

Thermocouple #2

Temp. monitor

Temp. controller

Mass flow controllers Gas inlet

Thermocouple #1 Tubular furnace

Gas outlet

Substrate Holder 10 ppm NO2

Air

Electrometer Computer

Figure 2. Schematic layout of the experimental setup used for the sensing measurements. Test samples were fixed onto the substrate holder that was inserted into the tubular furnace. Changes in the resistance of the samples as a function of the NO2 concentration in the gas was measured by an electrometer (Keithley 6517A). The concentration of NO2 in air is adjusted by controlling the mixing of two flow streams: one with pure synthetic air and one containing 10 ppm NO2. The electrical resistance of the samples was measured with an electrometer (Keithley 6517A). The concentration of NO2 in the air flow through the tube furnace was controlled by mixing two flows adjusted independently by two mass flow controllers (Bronkhorst, F-201CV): the gas in the first flow was pure synthetic air while in the second it was synthetic air with 10 ppm NO2. Samples with NO2 concentrations down to 250 ppb in air could be achieved with this experimental system.


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2. Results and Discussion 3.1 Evaluation of agglomerate / particle sizes

Figure 3 shows the size distribution of the WOx particles measured by the SMPS just before deposition onto the substrate (cf. Figure 1). The mean size of the particles was ca. 100 nm. The WOx particles produced by the SDG were agglomerates28 consisting of primary particles of ca. 10 nm (cf. Figure 3 and discussion further on). We should point out, however, that the size of the agglomerates in the gas phase does not play any direct role in the sensing performance of the films, for which only the size of the primary particles resulted after annealing is important (see discussion further on). Although the size of the agglomerates could be modified by changing gas flow rate through the chamber or the particle concentration27, we did not attempt to do that in this work.

Figure 3. Particle size distribution of tungsten oxide agglomerates produced by the SDG.


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3.2 Thin film morphology

Figure 4 shows SEM images obtained for as-deposited (left column) and air-annealed tungsten oxide films (right column). The deposition time used to produce these samples ranged from 15 to 60 minutes.

As deposited

Annealed


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Figure 4. SEM micrographs of the nanoparticulate films produced by spark ablation and inertial deposition over 15 (a, b), 20 (c, d), 30 (e, f), and 60 (g, h) minutes. The left column shows the asdeposited films while the right column shows the same films after annealing at 500 °C in air for 1 hour. As shown in the micrographs, almost all the nanoparticulate tungsten oxide thin films produced by the method described above exhibited cracks. The cracks were observed to be larger, both in terms of length and depth, for the thicker films34,35. There are many mechanisms/forces that can initiate the cracking in the films. Bombarding the already formed nanoparticulate layer by the depositing nanoparticles during fabrication can cause mechanical stress. In addition, cohesive forces between nanoparticles become stronger during deposition, which result in closing the voids, deforming the crystallites and inducing tensile stresses in the film. The combined effect of these different stresses cause crack initiation and channeling34–37 in the as-deposited films.

A number of mechanisms enhancing the cracks also take place during annealing. Thermal stresses arise by the different degree of thermal expansion of the film and the substrate38,39. Increase in overall residual stresses due to thermal effects enhances crack channeling of the film. In addition, sintering of primary particles at the elevated temperatures eventually leads to shrinkage of the material, which in turn forms wider cracks. This is clearly observed by comparing the micrographs of the as-deposited with the annealed samples shown in Figure 4. Although these cracks can reduce the quality of the films by increasing dramatically their


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resistance on the one hand, on the other they can be advantageous for gas sensing by providing easy access of the analyte gas molecules to the interaction sites. The size and concentration of the agglomerates suspended in the gas depend on the operating conditions of the SDG. For the quenching flow rate (1.5 lpm) and spark repetition frequency (270 Hz) we used, the mean size of the agglomerates was ca. 100 nm while their concentration was in the order of 105 particles cm-3. Figure 5a shows TEM images of the as-deposited nanoparticles on copper TEM grids. Agglomerated structures consisting of primary particles having sizes between 10 and 15 nm are observed. Figure 5b shows TEM images of extracts of the material after the deposited film was annealed in air at 500 °C. Evidently, the primary particles have grown to sizes that range from 30 to 50 nm due to sintering induced by the high annealing temperatures40,41.

Figure 5. TEM image of the WOx agglomerates produced directly by the SDG (a) and after annealing at 500 °C in air (b).

3.3 Film Crystal Structure The as-deposited tungsten oxide films are amorphous in nature and therefore do not show any peaks in the X-Ray Diffraction (XRD) measurements (data not shown). The films crystalize after annealing in air at 500 °C for 1 hour as shown in the TEM observations (cf. Figure 5) and the XRD spectrum (cf. Figure 6). Close observation between 28° and 29° in the spectrum shows two


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distinguishable peaks. Each of these two peaks is indexed by different crystal structures of WOx, suggesting the co-existence of two different phases in the film (cf. XPS measurements further below). Co-existence of crystal structures for tungsten oxides have been reported earlier in the literature42–44.

Figure 6. XRD pattern of a WOx film annealed at 500 °C in air. The two distinguishable peaks between 2θ range of 28° and 29° indicate the co-existence of two different crystal structures in the film.

3.4 XPS Binding spectroscopy

Figure 7a shows X-Ray photoemission-binding spectra from the annealed films produced by spark ablation and inertial deposition in comparison with a spectrum obtained from a commercial WO3 powder (99.8% purity; Johnson Matthey GmbH). Both spectra show two peaks for orbital configurations of W4f7/2 and W4f5/2, indicating the existence of W6+ in our samples45,46. The good agreement in the peak position of the spectra also suggests that the sample produced by


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spark ablation is pure WO3. Small differences of the peak positions (i.e., the peaks of the our films and of the commercial powder are respectively at 35.9 and 35.8 eV for W4f7/2 and at 38.0 and 37.9 eV for W4f5) can be attributed to uniformity and grain morphology differences between the two materials, and are within experimental error. Because XPS is a surface chemical analysis technique, the estimated stoichiometry corresponds only to that of the surface.

Figure 7. XPS spectra from the WOx samples produced by spark ablation and inertial deposition, annealed at 500 °C in air, before (a) and after (b) etching 100 nm off the surface of the film. Both samples are compared with XPS spectra from a commercial WO3 powder. The peak positions of the spectra indicate a W6+ oxidation state for the surface sample and a shift from this oxidation state for the etched sample, indicating the presence of a sub-oxide.

To determine the oxidation state of the film in the bulk we performed a second XPS measurement (cf. Figure 7b). For this measurement the top 100 nm (approximately) layer of the deposition was etched away and an XPS measurement was performed on the surface that was revealed. The XPS peaks are again compared with those from the commercial WO3 powder. Additional peaks in the XPS spectrum are observed at lower binding energies, which is very similar to the XPS analysis provided by Xie et al.47, corresponding to tungsten oxidation states of Wx+ and W0. Since the nanomaterial fabrication takes place under a 99.999% pure Ar environment, potential sources of oxygen include impurities in the carrier gas, as well as the


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atmospheric oxygen when the sample is taken out of the deposition setup and during air annealing. The surface exposed to atmospheric oxygen consists of stoichiometric WO3. The underlying layers of the film (which are exposed to limited amounts of oxygen during fabrication), however, are not fully oxidized, with the extend of oxidation being dependent on depth. The co-existence of stoichiometric and non-stoichiometric tungsten oxides can also explain the two peaks observed in the XRD pattern as discussed in the previous section.

4. NO2 resistance based sensing

Figure 8 shows cyclic measurements of the resistance of a 2.5-µm-thick WOx film exposed to NO2 in air at 200 °C, which is the temperature that our films exhibit the highest sensitivity (cf. inset of Figure 8, and discussion further below). Each cycle consists of a 2-hour exposure to pure air followed by a 30-min exposure to air containing 1 ppm NO2. As expected, the electrical resistance of the films is increased when NO2 molecules are introduced in the surrounding air. As shown in Figure 8, the response of the sensor does not change significantly with cycling, indicating that the film does not change its properties over cycling and that most of the NO2 molecules chemisorbed onto their surface during exposure desorb when the NO2 is removed from the surrounding air. The sensitivity and recovery time of the sensors are used here to characterize their overall response. The sensitivity of the sensor is determined by ܵ = ܴேைమ / ܴ஺௜௥ , where ܴேைమ is the measured electrical resistance of the film when exposed to air containing traces of NO2, and ܴ஺௜௥ is the film resistance when exposed to pure air. The recovery time (߬ோ ) of the sensor is defined as the time the film required to decrease its resistance from ܴேைమ to nearly ܴ஺௜௥ (more specifically to ܴ஺௜௥ + (0.1×(ܴேைమ -ܴ஺௜௥ ))) when removing the NO2 traces from the surrounding air.


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The response of the films exhibited a sensitivity that increased logarithmically with increasing NO2 concentration in air from 0.5 to 10 ppm (data not shown). As reflected by the measurements shown in Figure 9, the WOx films exhibited a sensitivity of ~ 100 at 1 ppm of NO2, which is remarkably high compared to WOx porous films synthesized with other techniques (e.g., electrochemical anodization of W films; S ~ 40)48. Since the resistance change in metal oxide gas sensors is caused by surface oxidation reactions, the sensitivity of the sensor strongly depends on the active sites on the surface of the film49,50. The high sensitivity of the sensors can therefore be explained by the large surface area of the nanoparticulate thin films due to the small WOx grain size and their high porosity24, which is partly attributed to the fractal nature of the deposited agglomerates.

Figure 8. Change of the resistance of nanoparticulate WOx film deposited on interdigitized Au electrodes (deposition time of 30 min) when exposed alternatively to 1 ppm NO2 in air at 200 °C. Inset: normalized sensitivity of the WOx films as a function of temperature.

The sensing performance of films with different thickness (i.e., using different deposition times) when exposed to 10 ppm NO2 in air are shown in Figure 9, whereas the characteristics of


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the sensor are summarized in Table 1. The thickness of the films depends linearly on deposition time, with a slope of 0.165 µm/min as determined by profiliometry. The thicker films exhibit higher conductivity compared to their thinner counterparts because of the higher number of available electrical pathways between the inter-digitated Au electrodes. Interestingly, as the film thickness was increased by changing the deposition time from 10 to 30 minutes, the recovery time decreased progressively from 15 to 7 minutes. This can be explained by the fact that the thicker films have more micro-sized cracks (cf. Figure 4), providing shorter alternative pathways for NO2 molecules to escape from the bulk of the film after desorption. The remarkably high sensitivities of the films show that spark ablation coupled with inertial deposition is a promising technique for fabricating porous metal oxide nanostructured materials for gas sensing. This is especially true considering that important sensor film characteristics (e.g., particle size, composition, etc.) can be fine-tuned independently to further improve the response of the sensor due to the flexibility of the spark discharge as particle source. In many applications, including use of the films in sensors for environmental monitoring, their sensitivity will be affected by humidity and the presence of other oxidizing/reducing gases. Water vapor molecules in the gas sample will adsorb on the films and form hydroxyl ions, which in turn will introduce electrons that increase the conductivity of the films. When other oxidizing or reducing gases are present in the sample, they will compete with NO2 by respectively increasing or decreasing the resistance of the films. The high sensitivity of the films reported in this work, however, increases their possibility to exhibit selectivity towards NO2 without any further treatment of the samples. To fully investigate the applicability of these films for specific applications, further studies are required to investigate their performance and stability under specific conditions.


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Figure 9. Recovery of the resistance of nanoparticulate WOx films of different thickness (indicated by the different deposition times) during NO2 desorption.

Table 1 Response kinetics of the different thickness films when exposed to 10 ppm NO2 in air. Deposition Time (min)

Sensitivity

Response Time (min)

Recovery Time (min)

10

95

5.6

15.7

20

858

5.9

9.0

30

511

5.9

8.0

5. Conclusions

In this work we have characterized WOx nanoparticulate thin films produced by spark ablation and inertial deposition and have shown that they can be used in solid-state sensors for measuring the concentration of NO2 in air. Apart from their nanoparticulate structure, the films exhibit cracks resulting from film stresses developed during deposition and annealing. As-deposited


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films are amorphous in nature but form stoichiometric WO3 at the film surface upon annealing. Non-stoichiometric sub-oxidized tungsten oxides are present at the bulk material as indicated by XRD and XPS observations.

These films show a stable and repeatable resistance sensitivity of ~ 100 when exposed to 1 ppm NO2 in air at 200 °C, which can be attributed to their large surface area, their nanostructured morphology, and the high purity of the nanoparticle building blocks. The sensitivity of the films, which was found to be high enough even at NO2 concentrations as low as 0.5 ppm, increased with increasing the analyte concentration. Experiments with films of different thicknesses showed that the thicker the films are, the faster the recovery times they exhibit. This can be attributed to better accessibility of the target gas molecules to the active sites of the films through their micron-sized cracks provided their porosity is the same. Considering the flexibility of controlling and tuning a number of other parameters of the nanomaterials (e.g., grain size, composition, and porosity), the technique described in this work holds great promises for further advancements in sensor technology.

Acknowledgements

The authors would like to thank J.Feng and T. Pfeiffer for their help with the experimental setup, S. Sachdev for XPS analysis, Gaurav Deshpande & Prakhar Kiyawat for their valuable inputs in setup drawings and J. Middelkoop for performing TEM measurements. GB also acknowledges project 11SYN_5_1861, DE_SPARK_NANO_GEN for funding part of the research. This research project is implemented within the framework of the Action «Cooperation 2011 - Partnerships of Production and Research. Institutions in Focused Research and


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Technology» of the Operational Programme «Competitiveness and Entrepreneurship» (OPCE ΙΙ), (Action’s Beneficiary: General Secretariat for Research and Technology - MIA-RTDI), and is co-financed by the European Regional Development Fund (ERDF) and the Greek State.


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Characterization of Tungsten Oxide Thin Films Produced by Spark

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