Structural, Morphological, and Optical Properties of Iron Doped WO 3 Thin Film Prepared by Pulsed Laser Deposition

: The iron doped tungsten-oxide (Fe and WO 3 ) thin ﬁlm with di ﬀ erent morphology and crystalline structures were obtained for di ﬀ erent substrate temperatures at the oxygen pressure of 14.66 Pa. The Fe-doped WO 3 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 di ﬀ raction) 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 WO 3 ﬁlm as-grown and after annealing treatment. Raman spectroscopy presented the main vibration mode of the Fe-doped WO 3 thin ﬁlm. The optical energy bandgap of the ﬁlms is decreasing as the substrate temperature increases. structure (JCPDS card 01-083-0950), with the di ﬀ raction peaks characteristic to reﬂection planes (002), (020), and (200), respectively. The di ﬀ raction peaks of the WO 3 corresponding to reﬂection planes of (002), (020), and (200), respectively, are in good agreement with reported di ﬀ ractograms for γ -WO 3 and δ -WO 3 (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 The positions of the reﬂection planes of WO 3 ﬁlm are 2 θ = 23.13 ◦ (002), 2 θ = 23.6 ◦ (020) and 2 θ = 24.42 ◦ (200), respectively. annealing 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 WO 3 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 WO 3 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 . Di ﬀ raction planes or Raman peaks associated to iron or iron oxides are not present in these measurements. The optical band gap of Fe-doped WO 3 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.


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 WO 3 , SnO 2 , ZnO and In 2 O 3 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 (WO 3 ) 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 SnO 2 , ZnO and In 2 O 3 , 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,

Results
The results of this paper are presented in the following way: S1-the Fe-doped WO 3 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 WO 3 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 WO 3 films, the WO 3 thin film was deposited in oxygen pressure 14.66 Pa and temperature of 600 • C.
The phase and crystalline structure of the WO 3 and Fe-doped WO 3 samples, respectively presented in Figure 1, have been identified by GIXRD. The X-ray pattern of WO 3 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 WO 3 corresponding to reflection planes of (002), (020), and (200), respectively, are in good agreement with reported diffractograms for γ-WO 3 and δ-WO 3 (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 WO 3 film are 2θ = 23.13 • (002), 2θ = 23.6 • (020) and 2θ = 24.42 • (200), respectively.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 13 Fe in the thin film. The substrate was rotating during deposition and the laser fluence was kept constant at 3 J/cm 2 . 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, Fedoped 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.

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.  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 monoclinic plane positions  of WO3. The circles correspond to hexagonal phase. The GIXRD pattern for WO 3 and Fe-doped WO 3 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 WO 3 deposited at T = 600 • C. The dotted lines show the γ-monoclinic plane positions of WO 3 . The circles correspond to hexagonal phase.
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 Coatings 2020, 10, 412 4 of 13 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 WO 3 is not maintained without some stabilizing ions or molecules in the hexagonal channels, and thus the existence of strictly stoichiometric hexagonal WO 3 is uncertain [41]. Consequently, the S1 sample annealed at temperature of 700 • C show the same crystalline structure as WO 3 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 WO 3 thin film contains a small amount of Fe according to EDX measurements. Therefore, it is supposed that the Fe enter in the WO 3 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 WO 3 . The structure and temperature for deposition and annealing of the samples S1, S2, and WO 3 film, respectively, are presented in Table 1. Table 1. The structure and temperature of S1, S2 film samples and WO 3 , respectively.

Sample
Temperature Grown Film Temperature Annealed Film XRD Structure/Plane Positions S1 (Fe-doped WO 3 ) Since the radius of Fe 3+ is almost similar to that of W 6+ , a small distortion in the crystal lattice is observed to the WO 3 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 W 6+ is octahedral coordinated with O 2− , and in the crystal of iron oxide the field stabilization energy of Fe 3+ is higher for octahedral orientation than for tetrahedral orientation. Consequently, Fe 3+ can fulfil the same coordination as that of W 6+ , and the Fe-doped WO 3 film presents the same crystal structure as WO 3 film.
The surface morphology of the Fe-doped WO 3 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.
Coatings 2020, 10, x FOR PEER REVIEW 5 of 13 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 Besides the growing of Fe-doped WO 3 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 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 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 WO 3 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 WO 3 . 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 WO 3 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.    Table 3. The elemental composition of the S1 sample annealed at T = 750 °C, in the selected points. 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.  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 WO 3 oxide exists on the film surface in a hexagonal phase, but in the present work, we do not exclude the formation of sub-stoichiometric WO 3−x oxides.
Raman scattering is very sensitive to microstructure of nanocrystalline materials, so it was employed to determine the nanostructure of the Fe-doped WO 3 films. The Raman spectra of WO 3 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 WO 3 lattice, i.e., translational and vibrational motion of WO 6 [43].
The Raman peak presented in Figure 6a, for the WO 3 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 WO 3 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 WO 3 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-WO 3 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 WO 3 or most likely to the hexagonal phase of the Fe-doped WO 3 annealed at 750 • C in agreement to GIXRD measurements. No vibration mode connected with the Fe 2 O 3 is observed in the present measurements, since the quantity of Fe is low (less than 1 at.%) and enter in the structure of WO 3 host lattice. The lack of the Raman peak at 950 cm −1 show a high crystallization nature of films.
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.
(a) (b) 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.

Samples
Raman peaks position (cm −1 ) stretching mode lattice mode Undoped Film WO3 film grown at 60 °C 810 --S1 Film grown at 600 °C 810 --S1 Film annealed at 700 °C 810 --S1 Film annealed at 720 °C 810 --S1 Film The optical band gap of Fe-doped WO3 film has been estimated with the help of Tauc Plot: ( ) versus photon energy(ℎ ), in which the Kubelka-Munk function, where is the diffuse reflectance. The optical band gap is obtained by extrapolating the linear part of ( ( )ℎ ) plots to the energy axis. The ( ( )ℎ ) plots versus photon energy are presented in Figure 7. . Raman spectra of WO 3 and Fe-doped WO 3 films. (a) the black spectrum belongs WO 3 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. Table 5. Comparison of Raman peak positions of WO 3 film and S1, S2 samples (Fe-doped WO 3 ).

Stretching Mode Lattice Mode
Undoped Film WO 3 film grown at 60 • C 810 --S1 Film grown at 600 • C 810 --S1 Film annealed at 700 • C 810 --S1 Film annealed at 720 • C 810 --S1 Film annealed at 750 The optical band gap of Fe-doped WO 3 film has been estimated with the help of Tauc Plot: F(R) versus photon energy (hν), in which the Kubelka-Munk function, where R is the diffuse reflectance. The optical band gap is obtained by extrapolating the linear part of (F(r)hν) 2 plots to the energy axis. The (F(r)hν) 2 plots versus photon energy are presented in Figure 7. Table 6 shows the Eg of the Fe-doped WO 3 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, WO 3 with Fe as dopant results in a small distortion of the WO 3 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 WO 3 samples. This effect is obvious for Fe-doped WO 3 annealed samples over 700 • C. Based on this effect, the optical band gap is reduced with the increase of grain sizes by annealing.  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.

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

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 WO 3 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 WO 3 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 WO 3 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 WO 3 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 WO 3 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 WO 3 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. 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.