Spray-deposited nanocrystalline WO3 thin films prepared using tungsten hexachloride dissolved in N-N dimethylformamide and influence of In doping on their structural, optical and electrical properties Ramnayan Mukherjee1 Ajay Kushwaha2 and P.P. Sahay1* Department of Physics, Motilal Nehru National Institute of Technology Allahabad-211 004, India 2 Department of Physics, Indian Institute of Technology Bombay Powai Mumbai- 400076, India
1
ABSTRACT Undoped and In-doped nanocrystalline tungsten oxide (WO3) thin films were prepared by chemical spray pyrolysis using tungsten hexachloride (WCl6) dissolved in N-N dimethylformamide as the host precursor solution and indium chloride (InCl3) as the source of dopant. XRD analyses confirm the monoclinic phase of the prepared films with the predominance of triplet (002), (020) and (200) in the spectra.
On indium doping, the
crystallinity of the films decreases and becomes minimum at 1.5 at% doping. EDX analyses confirm the incorporation of In dopants into the WO3 lattice network. SEM micrographs show non- spherical grains over the surface and the average grain size decreases with higher In doping. AFM images of the films exhibit large nicely separated conical columnar grains (except in 1 at%) throughout the surface with coalescence of some columnar grains at few places. UV-visible measurements reveal that the optical transmittance of the 1 at% In-doped film increases significantly throughout the wavelength range 300-800 nm relative to that of the undoped film. Room temperature photoluminescence spectra show pronounced enhancement in the peak intensity of NBE emission on In doping. Electrical conductivity has been found to increase on In doping.
Keywords: In-doped WO3 thin films; spray pyrolysis; structural and optical properties; electrical properties.
__________________________________________________________ * Corresponding author: P.P. Sahay, Tel: +91-532-2271260; fax: +91-532-2545341. E-mail address:
[email protected] 1
1. Introduction Tungsten oxide (WO3), nowadays, has a huge potential application in the various fields of science and technology such as electronics, optics, space science, aircraft science, defence and other industries [1-5]. Tungsten oxide having wide band gap energy (2.6–3.6 eV) is one of the most important n-type semiconducting oxide materials [6]. WO3 has drawn a great deal of attention of researchers due to its excellent electrochromic behaviour [7], photochromic behaviour [8], gasochromic behaviour [9] and stability towards UV and visible region,. Since the mid- 1970’s, WO3 has been also extensively used in large scale as a photoanode for photoelectrochemical water-splitting systems
due to its ability to generate solar
photocurrents, good transport properties and stability in acid [10-12].
Tungsten oxide in thin film form appears in newly invented gadgets like eye wear [1], electronic nose [13], thermal and temperature control windows in space vehicles, antidazzling rearview mirrors for cars, affirming its versatile applicability [14,15]. WO3 thin films have been investigated by a number of researchers as an electrochromic material [14 19]. Physical deposition techniques like vacuum evaporation [20], pulse laser deposition [21], RF sputtering [22], electron beam evaporation [23], anodic oxidation [12], and the chemical deposition techniques such as sol-gel [19], hydrothermal [24], spray pyrolysis [1], have been used to prepare WO3 thin films. Here, we have used chemical spray pyrolysis technique for our film growth as it is a simple and effective low cost process of generating good quality stoichiometric film. Moreover, spray pyrolysis set up does not require high state of art equipment and can be easily assembled in any laboratory.
The characteristics like structure, morphology, crystallite size, of a film are highly dependent on the growth technique as well as the precursor and the solvent used. Keeping in view the instability of the WO3 precursors, we have used tungsten hexachloride (WCl6) dissolved in 2
N-N dimethylforamide as the spray solution. Literature review shows that few researchers have investigated spray-deposited WO3 thin films, prepared using WCl6 dissolved in N-N dimethylformamide [25].
One important method to modify the characteristics of the films is the introduction of dopants in the parent system. Various dopants like Fe [23, 26], Nb [27], Li [1] , C [28], have been used to improve the desired characteristics of WO3 material for practical applications. As far as we know, the influence of In doping on the structural, optical, and electrical properties of the spray-deposited WO3 thin films have not yet been reported in literature. This is the rationale for carrying out such an investigation, the results of which are presented in this paper.
2. Experimental Procedure 2.1 Fabrication of WO3 thin film For fabrication of the undoped and In-doped WO3 thin films, tungsten hexachloride (WCl6) was used as the host precursor while indium chloride (InCl3) was taken as the source of dopant. Dopant concentration (In/W at%) was varied from 0 to 2 at%. The required amount of WCl6 was dissolved in N-N dimethylformamide to obtain 0.1 M concentration. InCl3 was then added into it. The solution thus prepared was used as the spray solution. The films were deposited on the micro glass slides by the chemical spray pyrolysis technique. Before deposition, the slides were cleaned successively by dipping in formic acid for 4 hrs to remove oils and grease over the surface, followed by ultrasonically cleaning by trichloroethylene for 30 mins, and then washed by laboline and deionized water repeatedly and finally dried in air. The schematic diagram of the spray pyrolysis unit is given in Fig. 1. The atomization of the solution into a spray of fine droplets was carried out by a glass nozzle, 3
with the help of compressed air as the carrier gas. During the course of spraying, the substrate was kept at a constant temperature of 400±10°C which was measured using a chromel-alumel thermocouple with the help of a Motwane digital multimeter (Model: 454). The various process parameters used in the film deposition are listed in Table 1. In our investigation, the film growth rate was maintained at ~ 84 Å /min throughout the process in all cases (undoped and In-doped WO3 films).
2.2 Characterization The prepared films were undergone for structural, optical, and electrical analyses. The XRD analyses of the films were done using Bruker AXS C-8 advanced diffractometer with CuKα radiation (λ = 1.5406 Å) as the X-ray source for phase identification and crystallite size determination. The surface morphologies of the films were examined using a JEOL high resolution scanning electron microscope (HRSEM) and a NTEGRA atomic force microscope (AFM). The optical properties of the films were investigated by Perkin Elmer Lambda 35 UV–Vis spectrometer (UK) in the spectral range 300–800 nm, and the photoluminescence (PL) spectra was carried out with VARIAN CARY eclipse fluorescence spectrophotometer. The excitation source was a Xenon-lamp (290 nm), and the sample temperature was kept at room temperature. The temperature dependence of the electrical conductivity of the films was studied using a Keithley System Electrometer (Model: 6517B). The film thickness was estimated by weight-difference method using an electronic precision balance (Citizen, Model: CX 165) and found to be in the range 400-450 nm.
3. Results and discussion All the films prepared have been found to be transparent and well adherent to the glass slides. When the precursor solution was sprayed in the form of a fine mist of very small droplets
4
onto the hot glass substrate, the small fine droplets started nucleation and simultaneously decomposed thermally to start crystal growth, resulting in the formation of WO3 thin films. The chemical reaction of the pyrolytic process in WO3 film formation may be as follows [29]: WCl6 +
3 O2 → WO3+ 3Cl2 2
….
….
..….
(1)
The films thus prepared are now undertaken for further investigation.
3.1 Structural characterization 3.1. 1 XRD analyses The XRD spectra of the undoped and In-doped WO3 thin films are presented in Fig. 2. The XRD spectra have been found in close match with the JCPDS card no. 83-0951, and all the peaks have been indexed in agreement with the same card. No phase corresponding to indium or other indium compound is detected in the XRD spectra. The diffractogram shows a narrow peak with highest intensity along (200) direction for the undoped sample and (002) for the doped samples. The predominance of triplet (002), (020) and (200) in the spectra establishes the monoclinic phase of the prepared films [7].
It has been found that the crystallinity of the films decreases significantly on 1.5 at% indium doping relative to that of the undoped film. Decrease in crystallinity is attributed to the fact that In incorporation in the host WO3 system enables more nucleation sites which, in turn, inhibit the growth of crystal grains, resulting in an increase in the lattice strain [30]. It has also been observed that there is a slight variation in the diffraction peak positions of the WO3 films on In doping. Also, the lattice parameters (a, b, c and β) have been found to vary a little bit from the standard values a = 7.301 Å , b = 7.538 Å , c = 7.689 Å , and β = 90.893° [JCPDS card no. 83-0951]. These variations have been attributed to the incorporation of In3+ ions into
5
the WO3 lattice network, and the ionic radii of W6+ and In3+ are 0.074 nm and 0.094 nm, respectively. The lattice parameters (a, b, c and β), the crystallite size (D) and the lattice strain (ε) of the films, listed in Table 2, have been determined by using the equation (2) [31], the DebyeScherrer formula (3) [32] and the tangent formula (4) [32]:
1
d
2 hkl
D
2 2 2 2 l 2hl cos h k sin 2 2 2 2 ac sin a b c
1
0 .9 cos
4 tan
….
..….
(2)
…
….
….
….
(3)
….
….
….
….
(4)
where λ, β and θ are the x-ray wavelength, the full width at half maximum (FWHM) of the diffraction peak and the Bragg’s diffraction angle, respectively.
The most prominent peak observed in the XRD spectra of the undoped WO3 film corresponds to (200) plane, followed by the next prominent peak corresponding to (002) plane. On 1 at% indium doping, the peak intensity corresponding to (002) improves and becomes most intense while that along (200) plane decreases significantly. On further doping, the peak intensity along (002) begins to decrease slowly, while those along (020) and (002) improve. The change in preferential orientation with the indium dopant concentration is shown in Fig. 3 in terms of the peak intensity ratios [I(200)/I(002)] and [I(200)/I(020)].
The texture coefficient (TC) of the films has been determined using the equation [1]:
TC
hkl
I I 1 N I I hkl N
0 hkl
N 1
hkl
….
….
….
….
(5)
0 hkl
where, the measured intensity of the (h k l) plane is Ihkl and the standardized intensity of the (h k l) plane from the JCPDS card no 83-0951 is I0hkl . N is the number of peaks found in the 6
XRD spectra of the films. It has been found that the larger the value of TChkl deviating from unity, the higher the chances of desirable growth [1]. The crystallite size of the films has been found to decrease with doping up to 1.5 at%. However, on further doping i.e. at 2 at%, the crystallite size increases but still remains less than that of the undoped film.
3.1.2 SEM and AFM analyses The surface morphology using high resolution scanning electron microscope (HRSEM) of the undoped and In-doped WO3 films are presented in Fig. 4. The undoped and the doped film has many arbitrary non-spherical shaped grains all over the space. It has also been seen that the film surface is uniform and netted with crystal grains. Due to uniform temperature gradient maintained on the all the substrates, the regular distribution of grains takes place throughout the surface. No visible holes or faulty zones on the film surface are observed. The grain sizes have been estimated using image J picture tools, and the average grain size is listed in Table 2. The histogram plot of grains sizes is shown in Fig. 5. The inset contains EDX graph which confirms the incorporation of indium ions into WO3 lattice network in the doped films.
The two-dimensional (2D) and three-dimensional (3D) AFM images of the undoped and Indoped WO3 thin films are shown in Figs. 6A(a–d) and 6B(a–d), respectively scanned over an area of (1μm x 1μm). The 3D images of the undoped and doped samples exhibit large nicely separated conical columnar grains (except in 1 at%) throughout the surface with coalescence of some columnar grains at few places. The average surface roughnesses of the films are listed in Table 2.
3.2.1. UV-Visible spectra analyses
7
The optical band gap values of the undoped and In-doped films have been determined from the UV-visible transmittance spectra, shown in Fig 7. Since 550 nm is more sensitive to human eye and has highest absorption in the solar spectrum region [1], the variation in the transmittance for the wavelength 550 nm as a function of In dopant concentration has been depicted in the inset of Fig. 7. It has been found that the transmittance increases to 94% in the case of 1 at% In-doped film.
The optical band gap has been calculated for both direct and indirect transitions as WO3 is reported to have both these values because light is absorbed by both direct and indirect interband transitions in this oxide material [1, 27]. For allowed electronic transition in materials, the absorption coefficient α is given by equation [33]:
h K (h Eg ) p
…..
…..
…..
(6)
where p has discrete values like 1/2, 3/2, 2, or more depending on whether the transition is direct or indirect, and allowed or forbidden. In the direct and allowed cases, p = 1/2 whereas for direct but forbidden cases it is 3/2. But for the indirect and allowed cases p = 2 and for the forbidden cases it will be 3 or more. K is a constant given by equation [33]:
K [e 2 /(ncme h 2 )](2mr ) 3 / 2 *
…..
…..
…..
(7)
where me* and mr are the effective and reduced masses of charge carriers, respectively. Eg is the optical bandgap.
Fig. 8a shows the plot of (Ahν)2 vs (hν) for direct band gap. The optical band gap is obtained by extrapolating the linear portion of the plot to the energy axis. The optical band gap of the undoped WO3 thin film has been found to be 3.26 eV. On indium doping, the band gap increases and becomes equal to 3.55 eV in the case of 2 at% In-doped film. The increase in the value of Eg on indium doping thus shows the blue shift [1]. The increase in the value of Eg 8
may be attributed to the smaller crystallite size of the doped films as compared to the undoped one. In addition, surface roughness of the film as well as defects in the film also affects the optical properties of the film.
The indirect band gap (Egind) and the phonon energy (Ep) values have been estimated from the plot of (Ahν)1/2 vs hν, shown in Fig. 8b. Each plot may be resolved into two distinct straight line portions. The straight line obtained at lower photon energies corresponds to the phonon absorption process and cuts the energy axis at (Egind - Ep), whereas the line in the relatively higher energy range corresponds to the phonon emission process and cuts the energy axis at (Egind + Ep) [34, 35]. From these two simultaneous equations, the values of Egind and Ep have been calculated, and are listed in Table 3. The values of Egind for the undoped and In-doped WO3 films have been found to be less than those of the corresponding values of Eg. Also, Egind shows the blue shift on indium doping. Blue shift in the indirect band gap of WO3 thin films has also been reported by Subrahmanyam and Karuppasamy [36].
The absorption coefficient α near the fundamental absorption edge is found to be exponentially dependent on the incident photon energy and obeys the well-known Urbach relation expressed as [37]
o exp(
h
….
)
….
….
…..
(8)
Eo
where o is a constant and E o is a parameter describing the width of the tail of localized state in the band gap. In terms of absorption, the eq. (8) can be written as A A o exp(
h
….
)
Eo
9
….
….
…..
(9)
where Ao is another constant. E o is estimated from the slope of the linear relationship ln A against h , shown in Table 3.
3.2.2 Photoluminescence studies
The photoluminescence (PL) of nanostructured WO3 is less investigated. Zhao et al. [38] have observed an UV emission peak at 369 nm and a blue emission peak at 423 nm in the PL spectra of tungsten oxide nanowires. They have assigned the UV emission peak to be due to band-to-band transition while the blue emission peak is attributed to the localized states induced by the presence of oxygen vacancies or defects in the nanostructure [39, 40]. Zhang et al. [41] have reported a blue emission peak at 438 nm and a shoulder peak at 425 nm in the PL spectra of the WO3 nanorods. Rajagopal et al. [42] have found two emission peaks at 325 and 420 nm in the PL spectra of WO3 nanostructures.
Fig. 10 shows the room temperature PL spectra of the undoped and In-doped WO3 thin films in the wavelength range 300 nm–600 nm. The undoped WO3 film shows one strong emission centered at 396 nm (3.13 eV) and a weak blue emission peak at 422 nm (2.94 eV). The emission peak at 396 nm is due to the recombination of free excitons and referred to as near band edge emission (NBE). The weak blue emission at 422 nm may be attributed to the various luminescent centers such as defect energy levels arising due to oxygen vacancies or defects in the nanostructures.
It is obvious from the normalized PL spectra (inset of Fig. 10) that there is a blue-shift of the NBE peak in the In-doped films, which is usually caused by band structure deformation resulting from lattice disorder [43]. The difference in the ionic radii of W6+ and In3+ induces lattice disorder and strain on replacement of W6+ ions by In3+ ions in the In-doped films. The peak intensity of NBE emission in the In-doped films has been found to enhance relative to 10
that of the undoped film. Density of free excitons is the major factor affecting the intensity of NBE emission [44]. However, no significant variation in the peak intensity of NBE emission has been observed with the varying In dopant concentration. Kovendhan et al. [1] also did not observe variation in peak intensity at 420 nm in the PL spectra of Li-doped WO3 thin films with varying Li content.
3.3. Electrical studies The variation of dark electrical resistance of the undoped and In-doped films with temperature was measured in the range 30–350°C. High conducting silver paste was used for making ohmic contact at both ends of the films. Fig. 11 presents the logarithmic plot of electrical conductivity of the films as a function of inverse of absolute temperature. It has been observed that in the case of undoped and 1 at% doped film, the electrical conductivity first increases slowly with the rise in temperature in the range 30 – 150°C and then increases rapidly, whereas the 1.5 at% and 2 at% doped film exhibit slow increase in conductivity with the increasing temperature in the range 30 – 200°C, and after that there is a rapid increase in the conductivity with temperature. The slow and rapid increase in the conductivity with temperature in different regions is attributed to the effect of dominance of the thermal excitation of electrons over the oxygen adsorption on the film surface. In fact, two competing processes of thermal excitation of electrons and oxygen adsorption occur simultaneously. The films exhibit different activation energies in different temperature regions. The activation energy, ΔE, for thermal excitation of electrons has been determined using the relation:
R R
0
E exp kT
….
….
….
…..
(10)
where the symbols have their usual significance. The activation energies thus obtained are listed in Table 4.
11
4. Conclusions Undoped and In-doped WO3 thin films prepared by chemical spray pyrolysis technique have been found to be polycrystalline with the monoclinic structure having the predominance of triplet (002), (020) and (200) in the XRD spectra. The films show the change in preferential orientation depending on the In dopant concentration. The crystallite sizes have been found to be in the range 29-40 nm. SEM micrographs show that the film surface is uniform and netted with non-spherical crystal grains.
The UV-visible spectroscopy studies show a direct optical band gap of 3.26 eV and an indirect band gap of 2.36 eV in the undoped film. Both direct and indirect band gap have been found to increase with In dopant concentration. The room temperature PL spectrum of the undoped film shows one strong emission at 396 nm and a weak blue emission at 422 nm. On In doping, a blue-shift in the peak position is observed, which is usually caused by band structure deformation resulting from lattice disorder. Electrical conductivity of the film increases on In doping. The degree of increase in the conductivity with temperature in different temperature regions depends on the dominance of the thermal excitation of electrons over the oxygen adsorption on the film surface.
Acknowledgements The authors are grateful to Dr. E. Mohandas, Head, Materials Synthesis and Structural Characterization Division, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam India, for providing XRD facilities. They would wish to further express their gratitude to Professor M. Aslam, Department of Physics, Indian Institute of Technology Bombay, India for providing SEM, AFM, and PL measurement facilities. Kind support extended by Mr. Saikat Chakraborty, Department of Physics, Banaras Hindu University, Varanasi, India is 12
also acknowledged. Financial support provided by the University Grants Commission, New Delhi, India, in the form of a major research project (No. 40-450/2011 (SR)) is gratefully acknowledged.
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Table 1: Process parameters of the spray deposition of the films Process parameters
Optimum value/Item
Nozzle
Glass
Nozzle-substrate distance
20 cm
Substrate
Micro glass slides
Substrate temperature
400±10°C
Host precursor
WCl6
Dopant precursor
InCl3
Solvent
N-N dimethayl formamide
Host precursor solution concentration
0.1 M
Dopant concentration (In/W at%)
0 to 2 at%
Solution spray rate
~ 2 ml/min
Carrier gas
Compressed air
Carrier gas pressure
2.75 kg/cm2
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Table 2: Structural parameters of the undoped and In-doped WO3 thin films XRD analyses
AFM analyses
HRSEM analyses
Average roughness (nm)
Lattice strain (%)
a (Å )
b (Å )
c (Å )
ß (degree)
Average Crystallite size: 40.39 nm
0.0040 0.0026 0.0022 0.0046 0.0064 0.0032 0.0018 0.0051 0.0078 0.0013 0.0021 0.0034 0.0032 0.0021
7.340
7.556
7.713
89.736
11.75
Average grain size (nm) 53.13
70.39 46.97 47.03 28.47 47.98 36.03 48.09 29.50 43.00 50.82 15.44 19.48
Average Crystallite size: 40.26 nm
0.0025 0.0036 0.0035 0.0049 0.0025 0.0033 0.0025 0.0033 0.0020 0.0016 0.0045 0.0038
7.324
7.531
7.697
90.196
11.62
51.26
2.1923 0.7827 0.5614 1.0582 0.6412 1.2753 1.0238 0.8196 0.6904
62.57 35.22 20.15 23.73 18.00 37.64 38.12 12.88 13.00
Average Crystallite size: 29.03 nm
0.0028 0.0048 0.0082 0.0059 0.0067 0.0023 0.0021 0.0060 0.0057
7.340
7.543
7.684
90.064
9.97
47.83
2.0355 1.5421 1.4222 0.7535 1.1078 0.8426 0.6694 0.5323 0.7451 0.2908 0.8341 1.2246
80.44 46.97 47.02 11.81 23.73 47.98 48.09 24.59 25.08 25.34 25.41 26.00
Average Crystallite size: 36.03 nm
0.0022 0.0036 0.0035 0.0127 0.0059 0.0025 0.0025 0.004 0.0035 0.0032 0.0032 0.0029
7.328
7.539
7.699
89.647
10.40
38.10
WO3 film
Position
dSpacing observed
dhkl Spacing JCPDS
TC
Crystallite size (nm)
Undoped
23.045 23.528 24.231 26.452 28.717 33.193 34.039 41.620 45.615 47.156 49.760 50.469 53.433 55.748
3.8563 3.7782 3.6701 3.3668 3.1062 2.6969 2.6318 2.1682 1.9872 1.9258 1.8309 1.8069 1.7134 1.6476
3.8441 3.7694 3.6502 3.3493 3.1171 2.6914 2.6222 2.1776 1.9822 1.9221 1.8251 1.8112 1.7123 1.646
002 020 200 120 -112 022 202 -222 -321 004 400 -114 024 142
2.4008 0.7833 3.3835 0.3625 0.9406 0.4353 0.5692 0.4029 2.0639 0.7503 0.4811 0.3099 0.1647 0.8661
43.31 65.02 74.41 32.70 21.90 37.96 66.57 19.46 11.51 69.45 40.10 23.46 23.75 36.01
1 at% In
23.091 23.607 24.285 28.702 33.227 33.705 34.089 41.565 47.176 50.435 53.444 55.488
3.8486 3.7657 3.6622 3.1077 2.6942 2.6571 2.6280 2.1710 1.9250 1.8080 1.7131 1.6547
3.8495 3.7694 3.6502 3.1171 2.6914 2.6678 2.6222 2.1776 1.9221 1.8112 1.7123 1.6587
002 020 200 -112 022 -202 202 -222 004 -114 024 -402
2.7526 0.5358 0.4326 1.5344 0.6618 1.3870 0.2745 0.6410 1.5296 1.1250 0.4432 0.6824
1.5 at% In
23.131 23.570 24.240 28.740 33.249 47.253 50.480 53.578 55.561
3.8422 3.7716 3.6691 3.1038 2.6924 1.9220 1.8065 1.7091 1.6527
3.8441 3.7694 3.6502 3.1171 2.6914 1.9221 1.8112 1.7087 1.6510
002 020 200 -112 022 004 -114 -331 -142
2 at% In
23.086 23.584 24.272 26.621 28.787 33.244 34.133 41.585 47.195 49.795 50.517 55.703
3.8495 3.7693 3.6641 3.3459 3.0988 2.6928 2.6247 2.1699 1.9243 1.8297 1.8052 1.6488
3.8441 3.7694 3.6502 3.3493 3.1171 2.6914 2.6222 2.1776 1.9221 1.8251 1.7982 1.651
002 020 200 120 -112 022 202 -222 004 400 114 -142
18
Table 3: Optical parameters of the undoped and In-doped WO3 thin films Sample
Direct optical band gap, Eg (eV)
Indirect optical band gap determination Egind-Ep Egind+Ep Egind (eV) (eV) (eV)
Ep (meV)
Width of the localized states,E0 (eV)
Undoped_WO3
3.26
2.29
2.42
2.36
60.00
0.411
In (1.0 at%)_WO3
3.37
2.57
2.65
2.61
41.20
0.472
In (1.5 at%)_WO3
3.48
3.01
3.07
3.04
28.35
0.386
In (2.0 at%)_WO3
3.55
3.16
3.21
3.19
25.45
0.467
Table 4: Activation energies of the undoped and In-doped WO3 thin films Sample
Activation energies (eV) 30-150
Temperature range (°C) 150-250
250-350
Undoped_WO3
0.0192
0.2549
0.4073
In (1.0 at%)_WO3
0.0002
0.1084
0.5237
In (1.5 at%)_WO3
0.0058
0.0542
0.5760
In (2.0 at%)_WO3
0.0073
0.0260
0.5614
19
Figure captions Fig. 1: Schematic diagram of the spray pyrolysis unit. Fig. 2: XRD spectra of the undoped and In-doped WO3 thin films. Fig. 3: Variation of peak intensity ratios [I(200)/I(002)] and [I(200)/I(020)] as a function of In-dopant concentration. Fig. 4: HRSEM images of the undoped and In-doped WO3 thin films. Fig. 5: Histogram plot of grain sizes of the undoped and In-doped WO3 thin films. Fig. 6A(a-d): 2D AFM images of the undoped and In-doped WO3 thin films. Fig. 6B(a-d): 3D AFM images of the undoped and In-doped WO3 thin films. Fig. 7: Transmittance spectra of the undoped and In-doped WO3 thin films. Fig. 8a: Plot of (Ahυ)2 versus hυ. Fig. 8b: Plot of (Ahυ)1/2 versus hυ. Fig. 9: Plot of lnA versus hυ. Fig. 10: PL spectra of the undoped and In-doped WO3 thin films. Fig. 11: Variation of electrical conductivity with temperature.
20
1500 1000 500 0
20
30
2
40
21
(degrees)
50
(-142)
(400) (-114)
(004)
(-222)
(022) (202)
(-114)
(-114)
(004)
(-222)
(022) (-202)
(-112)
1.5 at% In_WO3
(-142)
(-114)
(004)
(-222)
(022)
(-112)
(002) (020) (200)
1500 1000 500 0
(002) (020) (200)
1500 1000 500 0
(002) (020) (200)
(-142)
(400) (-114)
(004)
(-222)
(022) (202)
(-112)
(002) (020) (200)
1500 1000 500 0
(-112)
Intensity (a.u.)
Fig. 1: Schematic diagram of the spray pyrolysis unit.
2 at% In_WO3
1 at% In_WO3
Undoped_WO3
60
Fig. 2: XRD spectra of the undoped and In-doped WO3 thin films.
2
4
I(200)/I(002) I(200)/I(020)
1
2
I(200)/I(020)
I(200)/I(002)
3
1
0
0 0.0
0.5
1.0
1.5
2.0
In-dopant (at%)
Fig. 3: Variation of peak intensity ratios [I(200)/I(002)] and [I(200)/I(020)] as a function of In-dopant concentration.
Fig. 4: HRSEM images of the undoped and In-doped WO3 thin films. 22
Undoped_W03
In (1.0 at%)_WO3
14
12
12
10
10
Number of counts
Number of counts
14
8 6 4 2
8 6 4 2
0
0 20
30
40
50
60
70
80
90
100
20
30
40
Size of particles (nm)
In (1.5 at%)_WO3
60
70
80
90
100
In (2.0 at%)_WO3
14
12
12
10
10
Number of counts
Numer of counts
14
50
Size of particle (nm)
8 6 4
8 6 4 2
2
0
0 20
30
40
50
60
70
80
90
100
20
30
40
50
60
70
Size of particles (nm)
Size of particles (nm)
Fig. 5: Histogram plot of grain sizes of the undoped and In-doped WO3 thin films.
23
80
90
100
Fig. 6A(a-d): 2D AFM images of the undoped and In-doped WO3 thin films.
Fig. 6B(a-d): 3D AFM images of the undoped and In-doped WO3 thin films. 24
100
Undoped_WO3 In (1.0 at%)_WO3 In (1.5 at%)_WO3
60
100
T ransm ittance at 550nm (% )
Transmittance (%)
80
40
20
In (2.0 at%)_WO3
80
60
40
20
0 0.0
0.5
1.0
1.5
2.0
D o p in g C o n ce n tra tio n (a t% )
0 400
500
600
700
800
wavelength (nm)
Fig. 7: Transmittance spectra of the undoped and In-doped WO3 thin films.
40 3.6
35 30
3.4
3.3
3.2
20
0.0
2
(Ah) (eV)
2
25
Band gap (eV)
3.5
0.5
1.0
1.5
2.0
In-doping concentration (at%)
15
Undoped_WO3 In (1.0 at%)_WO3
10
In (1.5 at%)_WO3
5
In (2.0 at%)_WO3
0 1.5
2.0
2.5
3.0
h (eV)
Fig. 8a: Plot of (Ahυ)2 versus hυ.
25
3.5
3 Undoped_WO3 In (1.0 at%)_WO3 In (1.5 at%)_WO3 In (2.0 at%)_WO3
1
0 2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
h (eV)
Fig. 8b: Plot of (Ahν)1/2 versus hυ.
0.0
Undoped_WO3 In (1.0 at%)_WO3
-0.2
In (1.5 at%)_WO3 In (2.0 at%)_WO3 In A
(Ah)
1/2
(eV)
1/2
2
-0.4
-0.6
-0.8 3.30
3.35
3.40
3.45
h (eV)
Fig. 9: Plot of lnA versus hυ.
26
3.50
3.8
Intensity (a.u.)
Normalised intensity (a.u.)
Undoped_WO3 In (1.0 at%)_WO3 In (1.5 at%)_WO3 In (2.0 at%)_WO3
350
400
450
Wavelength (nm)
Undoped_WO3 In (1.0 at%)_WO3 In (1.5 at%)_WO3 In (2.0 at%)_WO3
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 10: PL spectra of the undoped and In-doped WO3 thin films.
Undoped_WO3
1.0
In (1.0 at%)_WO3
0.5
In (1.5 at%)_WO3 In (2.0 at%)_WO3
-1
(In (mho m )
0.0 -0.5 -1.0 -1.5 -2.0 -2.5 0.0015
0.0020
0.0025
0.0030
0.0035
-1
1/T (K )
Fig. 11: Variation of electrical conductivity with temperature.
27
