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Article

Structural, Morphological, and Optical Properties of Iron Doped WO3 Thin Film Prepared by Pulsed Laser Deposition

by
Mariana Osiac
1,*,
Nicoleta Cioatera
2 and
Maria Jigau
1
1
Department of Physics, Faculty of Science, University of Craiova, 200585 Craiova, Romania
2
Department of Chemistry, Faculty of Science, University of Craiova, 200585 Craiova, Romania
*
Author to whom correspondence should be addressed.
Coatings 2020, 10(4), 412; https://doi.org/10.3390/coatings10040412
Submission received: 17 March 2020 / Revised: 14 April 2020 / Accepted: 16 April 2020 / Published: 21 April 2020
(This article belongs to the Section Surface Characterization, Deposition and Modification)
Browse Figures
Versions Notes

Abstract

:
The iron doped tungsten-oxide (Fe and WO3) thin film with different morphology and crystalline structures were obtained for different substrate temperatures at the oxygen pressure of 14.66 Pa. The Fe-doped WO3 films were deposited by pulsed laser deposition (PLD). The influence of the substrate temperature on the surface and on the crystalline phases of the films was studied. The XRD (X-ray diffraction) analysis indicates the changing in the crystalline phases from γ-monoclinic to a mixture of γ-monoclinic and hexagonal phases dependent on the temperature of annealing and as-grown films. Related to the as-grown and annealing films conditions, the SEM (scanning electron microscopy) shows a change in the image surface from nanoneedles, to nanoporous, and further to long nanowires and broad nanobands. Energy-dispersive X-ray spectroscopy (EDX) shows the elemental composition of the Fe-doped WO3 film as-grown and after annealing treatment. Raman spectroscopy presented the main vibration mode of the Fe-doped WO3 thin film. The optical energy bandgap of the films is decreasing as the substrate temperature increases.

1. Introduction

At present, since industrialization continues to advance, air pollution has become a topic of current interest for health, research, and detection of polluting gases. Detection of toxic gases from air is mainly done by gas sensors. Gas sensors should have some characteristics such as high sensitivity and selectivity, minimum concentration of detectable gases, fast response/recovery speed, reversibility, minimum energy consumption, and low fabrication cost [1]. Metal-oxide semiconductor thin films are important material because of their potential applications in photo-catalysis [2], solar cells, and gas sensors. Nanostructured metal oxide semiconductors thin films such as WO3, SnO2, ZnO and In2O3 are intensively studied, being an important subject in the research field of resistivity type gas sensors, since the interaction between the analysts and sensing materials thin film of the surface lead to significant variation in the electrical resistivity. The tungsten trioxide (WO3) is an n-type semiconductor with a wide band gap of 2.5 to 3 eV and is commonly used as sensing material because it reacts to a large number of gas molecules and has a strong variation in resistance [3]. Similar to other semiconducting oxide such as SnO2, ZnO and In2O3, its film surface is characterized by the oxygen vacancies. Among metal oxide semiconductors, tungsten oxide is inexpensive and chemically stable with relatively high electrical conductivity. Tungsten oxide has been used in various fields as a gas, humidity and temperature sensors, optical modulation devices and flat panel displays [3,4,5,6,7]. Recently, in gas sensing applications, WO3 has been tested against various concentrations of toxic gases, i.e., CO, NOx, SO2, H2S, NH3 and acetone [8,9,10,11,12,13,14,15,16,17,18]. Therefore, In-doped WO3 sensors [19] have been observed to have high response to acetone, while Fe-doped WO3 [20] had good sensing performance to ethanol at room temperature. An optimal response to NO2 gas were tests with Sb-doped WO3 sensors [21]. Moreover, the gas sensing properties of the prepared Fe–C-co-doped WO3 spheres were tested and the effects of working temperature and Fe content on the responses to acetone were optimized. It was proposed the synthesized hierarchical WO3 microspheres film with the combined effect of Fe–C co-doping in WO3 improved the sensitivity and selectivity of acetone [22]. To enhance the gas sensing performance, doping with various dopants such as Ag [23], Pd [14,18,24], Pt [13,16] and Fe [17,20] is an effective method to introduce oxygen vacancies in the structure of oxides. Moreover, to improve the gas-sensing performance of WO3 many attempts have been done to obtain films with different morphologies, i.e., nanoparticles [25], nanofibers [26], nanorods [6], nanoneedles [27], nanotubes [28], nanosheets [28,29], and core-shell microspheres [30]. It is known that certain diseases are associated with specific inorganic gases and volatile organic compounds (VOCs) in human breath. Particularly, diabetes, which is seriously harmful to human health, can be diagnosed by acetone detection from human breath. Detection and monitoring the illness by breath, in the early stages, drastically reduce the diagnostic medical costs and improve the quality of life of patients suffering from chronic illnesses. In addition, acetone is a type of toxic and explosive gas, and it is used in industries and labs, being harmful to health. Several metal oxides such a as Fe2O3 [5,30], In2O3 [31], WO3 [32] and a portable Si-doped WO3 nanostructured [31,32,33], have been used to distinguish the diabetic from the healthy by detecting the acetone concentration. Another organic compound such as formaldehyde (HCHO), which has a carcinogenic nature, is a gas that is hazardous for the environment, and its detection is always important in biomedical applications and food industries [34,35,36,37]. Ethanol (C2H5OH), an inflammable volatile organic compound, is most extensively used as alcohol, with many applications in the food and chemical industries, biomedical as well as health and safety. The Occupational Safety Health and Administration (OSHA) has established that the maximum exposure level of the ethanol to be 1000 ppm, since exposure to ethanol vapor causes health problems such as difficulties in breathings, headaches, drowsiness, irritation of the eyes, and liver damage [36,37,38,39]. In this regard, considerable efforts have been done to improve the ethanol sensing performances such as sensitivity and selectivity of WO3 sensors due to its electrochromic properties [38]. Moreover, heterojunction formation between the n-WO3 and p-CrO3 improve the gas sensing properties of WO3 nanorods for ethanol detection [39]. In literature, proposed many techniques of deposition of WO3 and doped WO3 sensing layer for detection of polluting gases have been proposed, such as hydrothermal method [3,6], sonochemical preparation [8], pulsed laser deposition [7,23], electrospinning [26], and sol-gel [38].
Since the thin film is a sensing material, which is the main part of the sensor, the aim of this paper is to study the structural, morphological, and optical characterization of the Fe-doped WO3 film. In literature, there are few papers regarding the Fe-doped WO3 as a sensing layers [17,20] and its characterization is required. The deposition method proposed here was pulsed laser deposition (PLD). The crystallinity, composition, and morphology of the film were investigated using XRD in thin film configuration (GIXRD–grazing incidence X-ray), SEM coupled with EDX and Raman spectroscopy. The optical properties (diffuse reflectance) were measured with Analytik Jena Specord 210 plus spectrophotometer equipped with an integrating sphere and a Spectralon reflectance standard.

2. Methods

The WO3 and Fe-doped WO3 thin films were deposited on the silicon substrates by PLD. PLD consists of a YAG:Nd laser working on the fifth harmonic frequency (215 nm, 10 ns pulse length, 10 Hz repetition rate). The laser radiation was focused on a rotating target of WO3 (purity 99.9%) and Fe (purity 99.8%), respectively, in sectorial deposition. The sectorial deposition was used to introduce Fe in the thin film. The substrate was rotating during deposition and the laser fluence was kept constant at 3 J/cm2. The first sample S1, Fe-doped WO3 film was deposited to a substrate temperature of 600 °C and then it was annealed to 700, 720, and 750 °C, respectively. The second sample S2, Fe-doped WO3 film has been deposited at 720 °C and it was annealing at 750 °C. The oxygen pressure during deposition and annealing treatment was kept constant at 14.66 Pa. The distance between target and substrate was 4.5 cm. The thickness of the film was 150 nm. The deposition and the annealing treatment were done 4 h in oxygen atmosphere. The temperature rate in annealing treatment was 15 °C/min similar to the deposition process.
The crystalline structure of the WO3 and Fe-doped WO3 samples was investigated by X-ray diffraction Szimadzu 6000 system, the morphology surface by SEM coupled with EDX, a Hitachi system, (Hitachi Ltd., Tokyo, Japan). The Raman spectroscopy (Renishaw system, Renishaw UK Ltd., New Mills, UK) with an excitation laser source of 785 nm, presented the vibration mode of WO3 and Fe-doped WO3 samples. The optical properties of the Fe-doped WO3 film samples were investigated by Analytik Jena Specord 210 plus (Analytik-Jena AG, Jena, Germany) in the wavelength region of 200 to 1200 nm.

3. Results

The results of this paper are presented in the following way: S1–the Fe-doped WO3 thin film was deposited at oxygen pressure of 14.66 Pa and temperature of 600 °C; this sample was subsequently annealed at temperature of 700, 720, and 750 °C, respectively; S2–the Fe-doped WO3 thin film was deposited at oxygen pressure of 14.66 Pa and temperature of 720 °C and then it was annealed at 750 °C. To compare the phase structure results of Fe-doped WO3 films, the WO3 thin film was deposited in oxygen pressure 14.66 Pa and temperature of 600 °C.
The phase and crystalline structure of the WO3 and Fe-doped WO3 samples, respectively presented in Figure 1, have been identified by GIXRD. The X-ray pattern of WO3 thin film is indexed to γmonoclinic structure (JCPDS card 01-083-0950), with the diffraction peaks characteristic to reflection planes (002), (020), and (200), respectively. The diffraction peaks of the WO3 corresponding to reflection planes of (002), (020), and (200), respectively, are in good agreement with reported diffractograms for γ-WO3 and δ-WO3 (triclinic) phases. These two phases are very similar with respect to angular distortions and atomic distances, and they are expected to have similar thermodynamic stability [40]. The positions of the reflection planes of WO3 film are 2θ = 23.13° (002), 2θ = 23.6° (020) and 2θ = 24.42° (200), respectively.
The S1 sample grown at temperature of 600 °C show a γ-monoclinic crystalline phase with main preferential orientation along the (002) plane direction, slightly shifted to 23.54 degree [7,24]. The S1 sample was annealed at temperature of 700 °C and the peaks are observed at 2θ = 23.54° and 24.28°, closely to the crystalline γ-monoclinic phases with (002) and (200) planes, respectively. The dominant peak at 2θ = 24.28° could be a mixture of (020) and (200) planes. Increasing the annealing temperature of the S1 sample at 720 °C the crystalline structure presents a mixture of γ-monoclinic and hexagonal phases. The diffraction peaks corresponding to (100) and (200) plane, respectively belong to hexagonal structure (JCPDS card 00-007-2322). When the temperature is increased to 720 °C, the film still holds the stable γ-monoclinic phase indicating a well-defined crystalline structure and the presence of stoichiometric hexagonal is appeared. Further annealing of the S1 sample to 750 °C show a hexagonal phase while the γ-monoclinic phase almost disappeared.
The thin film of S2 sample grown at temperature of 720 °C show a hexagonal phase with (002) and (110) planes, respectively. The plane (002) is slightly shifted to γ-monoclinic; therefore, a minor mixing of the hexagonal and monoclinic phase could occur merely for the film grown to 720 °C. Increasing the annealing temperature to 750 °C the hexagonal phase is presented and the γ-monoclinic phase nearly vanished. The structure of hexagonal WO3 is not maintained without some stabilizing ions or molecules in the hexagonal channels, and thus the existence of strictly stoichiometric hexagonal WO3 is uncertain [41]. Consequently, the S1 sample annealed at temperature of 700 °C show the same crystalline structure as WO3 film. Similar crystalline structures were observed between pure ZnO and Fe-doped ZnO, where the small amount of Fe did not change the structure of ZnO but has a small influence on the crystallinity [41]. In the present case, Fe-doped WO3 thin film contains a small amount of Fe according to EDX measurements. Therefore, it is supposed that the Fe enter in the WO3 host lattice, and traces of any diffraction peaks associated with crystalline structure of iron or iron oxides are not observed. The small shift observed in the present GIXRD, for the S1 to 600 and 720 °C, at 2θ = 23.54° is due to amount of Fe-doped WO3. The structure and temperature for deposition and annealing of the samples S1, S2, and WO3 film, respectively, are presented in Table 1.
Since the radius of Fe3+ is almost similar to that of W6+, a small distortion in the crystal lattice is observed to the WO3 doped with Fe. In this case a small shift in the diffraction peaks is present and a number of defects in the film could also be produced, making this film a better candidate for a gas detecting [22,42]. Likewise, the W6+ is octahedral coordinated with O2−, and in the crystal of iron oxide the field stabilization energy of Fe3+ is higher for octahedral orientation than for tetrahedral orientation. Consequently, Fe3+ can fulfil the same coordination as that of W6+, and the Fe-doped WO3 film presents the same crystal structure as WO3 film.
The surface morphology of the Fe-doped WO3 samples deposited at oxygen pressure of 14.66 Pa and various substrate temperature range between 600 and 750 °C, respectively, are shown in Figure 2. The surface morphology for the samples deposited at temperatures of 600 and 650 °C, is nearly similar presented in nanoneedles shape. An increase in the temperatures to 700 and 720 °C, respectively, during deposition the nanoneedles on the sample surface, are distorted into nanowires grown on small nanobands. At the high temperature of 750 °C, the nanobands became thicker and shorter and started to agglomerate, making a porous surface morphology.
Besides the growing of Fe-doped WO3 at various temperatures, an annealing of the surface morphology for S1 sample deposited at oxygen pressure of 14.66 Pa and temperature of 600 °C has been investigated by SEM. The S1 sample annealed at temperature of 700 and 720 °C, respectively, presented in Figure 3b,c, shows a porous nanostructured surface that is well crystallized and consistent with the GIXRD measurements. There is not a large modification in the morphology surface between the samples annealed at 700 and 720 °C, respectively, since the temperature difference is small. Further annealing treatment at 750 °C change the surface morphology significantly, long, and thinner nanowires started to grow on the nanostructured surface as shown in Figure 3d.
The nanowires are grown over the nanostructured surface and they could be done by the surface oxidation during the annealing process. The S2 sample grown at temperature of 720 °C in oxygen pressure of 14.66 Pa presented in Figure 3e, show a morphology surface formed by nanobands and very thin nanowires. An annealing of the S2 sample to a temperature of 750 °C changed the surface morphology to large and small nanobands. During annealing process at high temperature of 750 °C the nanowires vanished and thicker and shorter nanobands occurred. In the annealing treatment at this temperature, the oxygen atmosphere encourages the coalescence process of the thin nanowires to grow into nanobands. The samples grown and annealed at various temperatures change not only the crystalline phases but could significantly modify the film surface morphology.
According to the EDX measurements on the elemental composition in atomic percentages over the entire Fe-doped WO3 film of S1 and S2 samples is: O 40.88 at.%, Fe 0.35 at.% and W 58.77 at.%, respectively.
The EDX measurements of the S1 sample annealed at 750 °C are presented in Figure 4a,b. Figure 4a shows the EDX mapping on the film surface among the nanowires. The film surface is mainly formed by WO3. The elemental composition presented in Table 2 show the existence of tungsten (W) and tungsten oxide on the film surface. The formation of nanowires appeared as a gradual structural transformation of the film involved nucleation on the oxidation film surface.
In the selected points on the long nanowires and among them, presented in Figure 4b, the WO3 composition is dominated. A tiny Fe distribution appeared only in few points. In the present work, we supposed that long nanowires are grown on the nucleation center after surface oxidation during annealing process. Under oxygen, the nucleation site is grown and results in crystalline nanowire configuration, according to GIXRD hexagonal phase. The Fe dopant did not represent a nucleation center. Table 3 presented the elemental composition of the several points on the film surface.
The EDX mapping made on a large nanoband, presented in Figure 5a shows a distribution of W and O, almost similar to the long nanowires grown on the S1 sample, but with a large difference amount of W and O. The elemental composition of the large nanoband is shown in Table 4.
The EDX mapping on the large nanoband and a slice of film surface, presented in Figure 5b, shows the Fe distribution merely on the film surface. The elemental composition film presented in Figure 5b is shown in Table 4, second column. The EDX mapping show that the film is gradually converted from the thin nanowires and small nanobands into large nanobands during annealing treatment in the oxygen. It seems that the temperature is a critical parameter of the surface morphology. The actual shape of the S1 and S2 samples morphology surface is strongly dependent on the temperature higher than 700 °C. Changes in surface morphology formation proposed that the surface film is mainly metallic W, because of its heavy mass and low diffusivity, and a small amount of WO on the surface exists, according to EDX measurements. Related to GIXRD measurements, at a temperature of 750 °C, the WO3 oxide exists on the film surface in a hexagonal phase, but in the present work, we do not exclude the formation of sub-stoichiometric WO3−x oxides.
Raman scattering is very sensitive to microstructure of nanocrystalline materials, so it was employed to determine the nanostructure of the Fe-doped WO3 films. The Raman spectra of WO3 show three vibration modes, at a lower wavenumber than 250 cm−1, to an intermediate wavenumber of 200 to 400 cm−1 and at higher wavenumber of 600 to 900 cm−1, respectively. The vibrational modes at a lower wavenumber than 250 cm−1 belong to WO3 lattice, i.e., translational and vibrational motion of WO6 octahedral in the same cell of unit. The vibrational modes in the intermediate wavenumber of 200 to 400 cm−1 are attributed to bending modes of O–W–O bridging bonds. At higher wavenumber region of 600 to 900 cm−1, the vibrational mode is attributed to the O–W–O stretching bonds [43].
The Raman peak presented in Figure 6a, for the WO3 film grown at 600 °C is located at 810 cm−1. In Figure 6a, the large Raman peak located to 810 cm−1 is observed for the S1 samples annealed at 700 and 720 °C, respectively. A very sharp Raman peak located at 810 cm−1 for the S2 sample grown at 720 °C is shown in Figure 6b. These features are characteristic to the O–W–O stretching vibration modes of γ-monoclinic WO3 phase, in accordance with GIXRD measurements. The presence of Raman peaks at low and intermediate wavenumber is not clearly noticeable by the Si and Si–O bonds of Si substrate. The sharp peak of Si is not presented in the Figure 6. The peaks observed at 622 and 670 cm−1, respectively, belong to the Si–O bonds of Silicon substrate [44]. Two Raman peaks are observed for the S1 and S2 samples annealed at 750 °C in Figure 6b. These peaks are located at 735 and 791 cm−1, respectively and correspond to O–W–O stretching vibration mode. For the S2 sample as-grown at 720 °C, presented in Figure 6b, a small Raman peak is centered at 136 cm−1 and is ascribed to WO3 lattice vibration. At higher wavenumber, Raman peaks located to 714 and 810 cm−1, respectively, are assigned to O–W–O stretching vibration modes of monoclinic γ-WO3 phase in accordance with the GIXRD measurements. In Figure 6b, in a higher wavenumber range, a shift of 21 cm−1 is observed between the Raman peaks of the S2 sample as-grown at 720 °C and samples S1 and S2 annealed at 750 °C. A comparison of the Raman peaks for both samples is presented in Table 5. Such a shift could be connected with the shortening of stretching vibration modes O–W–O, which corresponds either to small cell parameters of Fe-doped WO3 or most likely to the hexagonal phase of the Fe-doped WO3 annealed at 750 °C in agreement to GIXRD measurements. No vibration mode connected with the Fe2O3 is observed in the present measurements, since the quantity of Fe is low (less than 1 at.%) and enter in the structure of WO3 host lattice. The lack of the Raman peak at 950 cm−1 show a high crystallization nature of films.
The optical band gap of Fe-doped WO3 film has been estimated with the help of Tauc Plot: F ( R ) versus photon energy ( h ν ) , in which the Kubelka-Munk function,
F ( R ) = ( 1 − R ) 2 2 R ,
where R is the diffuse reflectance. The optical band gap is obtained by extrapolating the linear part of ( F ( r ) h ν ) 2 plots to the energy axis. The ( F ( r ) h ν ) 2 plots versus photon energy are presented in Figure 7.
Table 6 shows the Eg of the Fe-doped WO3 for annealed and as-grown films.
A decrease in the optical band gap, particularly by temperature in the presence of oxygen vacancies or other structural defects such as voids, is presented in Table 6. Moreover, WO3 with Fe as dopant results in a small distortion of the WO3 crystalline structure, which reduces the optical band gap. The optical band gap of deposited and annealing samples at 720 °C is smaller than optical band gap of undoped sample. This difference for the optical band gap could not be done merely to doping with Fe, but most likely to the effect of the surface oxidation during annealing in oxygen. In addition, the measurements of the optical band gaps show the effect of quantum confinement in the WO3 samples. This effect is obvious for Fe-doped WO3 annealed samples over 700 °C. Based on this effect, the optical band gap is reduced with the increase of grain sizes by annealing.

4. Conclusions

The crystalline structure and the morphology thin film depend on deposition and annealing conditions and are easily controlled by PLD. The GIXRD results indicate a γ-monoclinic structure for WO3 thin film and a preferred orientation growth of a γ-monoclinic crystalline structure for the S1 sample grown at 600 °C and annealed at 700 °C, respectively. The Fe-doped WO3 film annealed at 720 °C show a γ-monoclinic structure mixed with hexagonal phase. When the annealed temperature is increased to 750 °C, the film presented a hexagonal phase. The S2 sample grown at 720 °C maintained a small of γ-monoclinic phase mixed with a hexagonal phase. The morphology of films differs significantly, depending on the grown and annealed conditions. The film grown at 600 °C shows a nanoneedles surface image. A further annealing of the S1 sample at high temperature, the surface became nanostructured, porous, and finally presented long nanowires due to surface oxidation. The Fe-doped WO3 samples deposited various temperatures, showing a continuous transformation from nanoneedles to nanowires grown on nanobands. At a temperature of 750 °C a porous surface morphology nanobands is formed. Regarding the S2 sample grown at 720 °C, the surface image shows thin nanowires and nanobands. An annealing of this sample at 750 °C changed the surface in broad nanobands. The EDX mapping show a similar elemental composition in the long nanowires and large nanobands, particularly W and O. The formation of long nanowires and broad nanobands during annealing under oxygen was supposed to be done by nucleation center generated due to the oxygen migration on the surface. The main Raman peak of the WO3 thin film is located at 810 cm−1 at 600 °C being large than the peaks for the S1 and S2 samples. A sharp Raman peak located at 810 cm−1 is observed for the S2 sample grown at 720 °C. At high annealing temperature of 750 °C, the Raman peaks of S1 and S2 samples are shifted with 21 cm−1 unlike to the Fe-doped WO3 films grown and annealed at 720 °C. This shift was supposed to be of Fe as a dopant or to the hexagonal phase. The S2 sample grown at 720 °C shows the lattice vibration mode at 136 cm−1. Diffraction planes or Raman peaks associated to iron or iron oxides are not present in these measurements. The optical band gap of Fe-doped WO3 films was found to decrease with increasing of annealing temperatures. This decrease might be done to the crystalline small cell distortion by Fe doping, either to large oxidation sample surface due to annealing at 750 °C which also increased the grain sizes. These results are useful in understanding the influence of Fe on the morphology surface and on the optical band gap at various deposition and annealed parameters in studying the applicability of the obtained thin film as an active membrane for gas sensor.

Author Contributions

Conceptualization, N.C. and M.O.; investigation, M.O. and M.J.; resources, X.X.; data curation, M.J.; validation, M.O. and N.C.; methodology, N.C.; writing—original draft preparation, M.O.; writing—review and editing, M.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Romanian National Authority for Scientific Research, UEFISCDI: project no. PCCDI 15; project name manufacture, calibration and testing of advanced integrated sensor systems for societal security.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The GIXRD pattern for WO3 and Fe-doped WO3 thin films, respectively. (a) S1 sample: the red spectrum corresponds to S1 grown at T = 600 °C; the green, blue, and magenta spectra belong to S1 samples annealed at T = 700, 720, and T = 750 °C, respectively. (b) S2 sample: the dark green spectrum belongs to S2 grown at 720 °C and the pink spectrum corresponds to S2 annealed to 750 °C, respectively. The black spectrum corresponds to WO3 deposited at T = 600 °C. The dotted lines show the γ-monoclinic plane positions of WO3. The circles correspond to hexagonal phase.
Figure 1. The GIXRD pattern for WO3 and Fe-doped WO3 thin films, respectively. (a) S1 sample: the red spectrum corresponds to S1 grown at T = 600 °C; the green, blue, and magenta spectra belong to S1 samples annealed at T = 700, 720, and T = 750 °C, respectively. (b) S2 sample: the dark green spectrum belongs to S2 grown at 720 °C and the pink spectrum corresponds to S2 annealed to 750 °C, respectively. The black spectrum corresponds to WO3 deposited at T = 600 °C. The dotted lines show the γ-monoclinic plane positions of WO3. The circles correspond to hexagonal phase.
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Figure 2. The top view topography of the Fe-doped WO3: (a) the surface of the film grown at T = 600 °C, (b) T = 650 °C, (c) T = 700 °C, (d) T = 720 °C, (e) T = 750 °C.
Figure 2. The top view topography of the Fe-doped WO3: (a) the surface of the film grown at T = 600 °C, (b) T = 650 °C, (c) T = 700 °C, (d) T = 720 °C, (e) T = 750 °C.
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Figure 3. The top view topography of the Fe-doped WO3: (a) S1 sample–the surface of the film grown at T = 600 °C, (b) the film annealed at T = 700 °C, (c) the film annealed at T = 720 °C, (d) the film annealed at T = 750 °C, (e) S2 sample–the film grown at 720 °C, (f) the film annealed at 750 °C.
Figure 3. The top view topography of the Fe-doped WO3: (a) S1 sample–the surface of the film grown at T = 600 °C, (b) the film annealed at T = 700 °C, (c) the film annealed at T = 720 °C, (d) the film annealed at T = 750 °C, (e) S2 sample–the film grown at 720 °C, (f) the film annealed at 750 °C.
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Figure 4. The image of the S1 sample annealed at 750 °C; (a) the EDX mapping between the nanowires, (b) 10 EDX points selected on the surface.
Figure 4. The image of the S1 sample annealed at 750 °C; (a) the EDX mapping between the nanowires, (b) 10 EDX points selected on the surface.
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Figure 5. The EDX mapping of the S2 sample at T = 750 °C. (a) EDX mapping on a large nanoband; (b) EDX mapping made on a large nanoband and slice of the film surface.
Figure 5. The EDX mapping of the S2 sample at T = 750 °C. (a) EDX mapping on a large nanoband; (b) EDX mapping made on a large nanoband and slice of the film surface.
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Figure 6. Raman spectra of WO3 and Fe-doped WO3 films. (a) the black spectrum belongs WO3 film grown at 600 °C; the blue spectrum belongs to S1 sample grown at 600 °C; the red and green spectra belong to S1 sample annealed at 700 and 720 °C, respectively. (b) the red spectrum belongs to S1 sample annealed at 750 °C, the green spectrum to S2 sample as-grown at 720 °C and the black spectrum to S2 sample annealed at 750 °C.
Figure 6. Raman spectra of WO3 and Fe-doped WO3 films. (a) the black spectrum belongs WO3 film grown at 600 °C; the blue spectrum belongs to S1 sample grown at 600 °C; the red and green spectra belong to S1 sample annealed at 700 and 720 °C, respectively. (b) the red spectrum belongs to S1 sample annealed at 750 °C, the green spectrum to S2 sample as-grown at 720 °C and the black spectrum to S2 sample annealed at 750 °C.
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Figure 7. The Eg for WO3 film and Fe-doped WO3 film samples. (a) WO3 film grown at 600 °C (undoped); (b) Fe-doped WO3 as-grown at 720 °C; (c) Fe-doped WO3 annealed at 720 °C.
Figure 7. The Eg for WO3 film and Fe-doped WO3 film samples. (a) WO3 film grown at 600 °C (undoped); (b) Fe-doped WO3 as-grown at 720 °C; (c) Fe-doped WO3 annealed at 720 °C.
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Table
Table 1. The structure and temperature of S1, S2 film samples and WO3, respectively.
Table 1. The structure and temperature of S1, S2 film samples and WO3, respectively.
SampleTemperature Grown FilmTemperature Annealed FilmXRD Structure/Plane Positions
S1 (Fe-doped WO3)600 °C-γ-monoclinic
2θ = 23.54° (002)
-700 °Cγ-monoclinic
2θ = 23.54° (002), 2θ = 24.28° (200)
-720 °Cγ-monoclinic
2θ = 23.54° (002), 2θ = 24.28° (200)
hexagonal
2θ = 13.96° (100), 2θ = 28.13° (200)
-750 °Chexagonal phase
S2 (Fe-doped WO3)720 °C-γ-monoclinic
2θ = 23.54° (002), 2θ = 24.28° (200)
hexagonal
2θ = 13.96° (100), 2θ = 28.13° (200)
-750 °Chexagonal
2θ = 13.96° (100), 2θ = 28.13° (200)
WO3 film600 °C-γ-monoclinic
2θ = 23.13° (002), 2θ = 23.6° (020), 2θ = 24.42° (200)
Table
Table 2. The elemental composition of film surface presented in Figure 3a.
Table 2. The elemental composition of film surface presented in Figure 3a.
ElementOW
Weight %3.7396.27
Atomic %30.7869.22
Table
Table 3. The elemental composition of the S1 sample annealed at T = 750 °C, in the selected points.
Table 3. The elemental composition of the S1 sample annealed at T = 750 °C, in the selected points.
CompositionElementSpectrum
12345678910
Weight %O10.123.617.323.0313.826.393.943.332.653.11
Fe0.390.47---0.75---0.79
W89.4995.9292.6896.9786.1892.8696.0696.6797.3396.1
Atomic %O56.1529.8547.5926.4464.8343.5132.0228.3523.8626.58
Fe0.621.11---1.46---1.93
W43.2369.0452.4173.5635.171.4667.9871.6576.1471.49
Table
Table 4. The elemental composition of the S2 sample annealed at T = 750 °C, on the large nanoband, and large nanoband with a slice of the film surface.
Table 4. The elemental composition of the S2 sample annealed at T = 750 °C, on the large nanoband, and large nanoband with a slice of the film surface.
EDX Mapping on a Large NanobandEDX Mapping on a Nanoband and a Slice of Film Surface
ElementOWOWFe
Weight %12.1587.857.9491.870.19
Atomic %61.3838.6246.67500.33
Table
Table 5. Comparison of Raman peak positions of WO3 film and S1, S2 samples (Fe-doped WO3).
Table 5. Comparison of Raman peak positions of WO3 film and S1, S2 samples (Fe-doped WO3).
SamplesRaman Peaks Position (cm−1)
Stretching ModeLattice Mode
Undoped FilmWO3 film grown at 60 °C810--
S1Film grown at 600 °C810--
S1Film annealed at 700 °C810--
S1Film annealed at 720 °C810--
S1Film annealed at 750 °C791735-
S2Film grown at 720 °C810714136
S2Film annealed at 750 °C810714-
Shift-21 cm−121 cm−1-
Table
Table 6. A comparison of the E g for WO3 and S1 and S2 samples.
Table 6. A comparison of the E g for WO3 and S1 and S2 samples.
SampleEg(eV)Fe at.% According EDXSurface Morphology
S1Fe-doped WO3 annealed to 720 °C2.520.35Nanostructured film
S2Fe-doped WO3 grown at 720 °C2.320.35Nanowires and nanobands
Undoped FilmWO3 grown at 600 °C3.30-Nanostructured film

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Osiac, M.; Cioatera, N.; Jigau, M. Structural, Morphological, and Optical Properties of Iron Doped WO3 Thin Film Prepared by Pulsed Laser Deposition. Coatings 2020, 10, 412. https://doi.org/10.3390/coatings10040412

AMA Style

Osiac M, Cioatera N, Jigau M. Structural, Morphological, and Optical Properties of Iron Doped WO3 Thin Film Prepared by Pulsed Laser Deposition. Coatings. 2020; 10(4):412. https://doi.org/10.3390/coatings10040412

Chicago/Turabian Style

Osiac, Mariana, Nicoleta Cioatera, and Maria Jigau. 2020. "Structural, Morphological, and Optical Properties of Iron Doped WO3 Thin Film Prepared by Pulsed Laser Deposition" Coatings 10, no. 4: 412. https://doi.org/10.3390/coatings10040412

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