- Email: [email protected]
Structural and optical characterisation of undoped and chromium doped tin oxide prepared by sol–gel method
Applied Surface Science 271 (2013) 260–264
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural and optical characterisation of undoped and chromium doped tin oxide prepared by sol–gel method N.B. Ibrahim ∗ , M.H. Abdi, M.H. Abdullah, H. Baqiah School of Applied Physics, Faculty of Science and Technology, University Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 28 May 2012 Received in revised form 23 January 2013 Accepted 24 January 2013 Available online 8 February 2013 Keywords: Energy gap Cr doped SnO2 Thin films Sol–gel
a b s t r a c t Transparent semiconductor thin films, Cr doped SnO2 (Sn1−x Crx O2 : x = 0.0, 0.02, 0.04, 0.06, 0.10, 0.20) were deposited onto glass substrates by a sol–gel method. The thermal gravitational analysis showed that mass lost happened at 120 ◦ C due to evaporation of water and ethanol. X-ray diffraction analysis showed that the Cr doped SnO2 were polycrystalline with SnO2 phases. The crystalline sizes were in the range of 3.7–4.9 nm. The optical property measured using UV-Vis spectrophotometer showed that the transparency of all samples was more than 90% and the calculated band gaps were in the range of 3.84–3.96 eV which was due to the Cr dopant that increased the samples energy band gap. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxide semiconductors such as tin oxide (SnO2 ), zinc oxide (ZnO), and indium oxide (In2 O3 ) have attracted great researchers’ attention due to their variable optical and electrical properties [1–5]. SnO2 has many applications such as in solar cells, gas sensors, display devices, optoelectronics and electrochromic devices [6]. These oxide semiconductors have also been doped with other metals in order to enhance their applications. For example, SnO2 has been doped with Pd and Pt to enhance its sensitivity as a gas sensor [7] and doped with Sb or F to obtain high transparency and excellent electrical properties [8]. Numerous studies have also been carried out to study SnO2 doped with transition metal such as Co, Fe, Ni, V and Cr. Based on their study of the ferromagnetic properties of Cr doped SnO2 nanowires prepared by a catalyzer-assisted chemical vapour deposition method, Zhang et al. suggested that the ferromagnetic of the films are due to the oxygen vacancy and Cr dopant [9]. López-Navarrete et al. synthesised Cr-SnO2 violet pigments by a pyrolysis process at 600 ◦ C. The Cr concentration affects the colour properties of the pigment. The optimum pigment was obtained from the sample with Cr/Sn weight ratio in the range of 0.045–0.057 [10]. Zuo et al. reported on the ferromagnetic properties of Sn1−x Crx O2 films prepared onto silicon (1 1 1) substrates by a sol–gel method. Due to antiferromagnetic super-exchange interaction among the nearest neighbour in Cr ions, the magnetic moment
∗ Corresponding author. E-mail address: [email protected] (N.B. Ibrahim). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.171
per Cr decreases as the Cr content increases [11]. However none of these literatures report on the optical properties of Cr doped SnO2 prepared by a sol–gel method. SnO2 is a well known transparent conductor thus it is important to know its optical properties after doped with transition metal, Cr. In this work, we prepared SnO2 and chromium doped SnO2 films onto glass substrates by a sol–gel technique. The aims of the study were (1) to study the effect of Cr dopant onto the structural and optical properties of SnO2 films and (2) to study the effect of different weight percent of Cr dopant on the properties of SnO2 films.
2. Experimental SnCl2 ·2H2 O (tin (II) chloride dehydrate (Merk)) was dissolved in 10 ml ethanol (99% purity). This solution was distilled at room temperature (27 ◦ C) and refluxed at 80 ◦ C for 4 h. Then it was filtered and deposited onto cleaned glass substrates by a spin coating technique at room temperature (27 ◦ C). The obtained films were dried at 100 ◦ C to get rid of the solvent and annealed at 600 ◦ C in air. Cr doped SnO2 films have been prepared by adding CrCl2 ·6H2 O to SnCl2 ·2H2 O and dissolved by ethanol. The dopant was added up to 20 wt%. The undoped-solution was studied with thermogravimetry (TGA) model Mettler Toledo SDTA 821. The Xray-diffraction patterns of samples were recorded by an X-ray diffractometer D6 Advance Bruker system using Cu K␣ radiation, also film surface morphology and cross-section were studied by using a field emission scanning electron microscope (FESEM) model Zeiss Supra 55vp. The optical properties of the films were studied using Lambda-900 UV-VIS visible spectrophotometer.
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
261
Fig. 3. XPS spectrum of Sn0.90 Cr0.1 O2 film. Fig. 1. TGA curves of the dried SnO2 gel.
values were between 11 and 24. The calculated cell volumes and crytalline size decrease as the dopant weight percent increases. This ˚ with is due to the replacement of bigger ionic radius, Sn4+ (0.69 A) ˚ [9]. Fig. 3 shows the typical X-ray smaller ionic radius, Cr3+ (0.65 A) photoelectron spectroscopy (XPS) result of the films, confirming the Cr3+ oxidation state in the films. Fig. 4(a) shows the typical FESEM micrographs for film surface. The film has a good surface quality without any crack or pores on the surface. The film is dense with grains sizes less than 20 nm. These results explain the broad peaks obtained from the XRD spectra. The thickness of the films was measured from the films cross-section micrographs obtained using the FESEM. Fig. 4(b) shows the typical cross-section for all films and the values are tabulated in Table 1. The measured thicknesses are independent of the Cr content. Transmission electron microscope (TEM) model CM 12 Philips was also used to measure the grain size of the film. Fig. 5 shows
Fig. 2. XRD pattern of the polycrystalline Sn1−x Crx O2 film.
3. Results and discussion Fig. 1 shows TGA curve of the dried SnO2 gel. The mass loss occurs at about 110–120 ◦ C. The mass loss is due to the evaporation of ethanol and water. No any other mass loss can be observed until 600 ◦ C. X-ray diffraction patterns of the thin films were studied at 2 (values from 20◦ to 60◦ and the results are shown in Fig. 2. The peaks obtained for all films are belong to cassite tin oxide with tetragonal phase. The films are polycrystalline with single phase. The calculation of intensities ratio is tabulated in Table 1. The intensities of the main peaks change as the Cr percentage increases indicating that the doping process affects the crystal growth of the films. Liu et al. have also reported that Cr dopant slightly affects their reactive dc co-sputtering SnO2 films [12]. Table 1 summaries the lattice parameters and cell volumes of all films calculated by using a Rietveld refinement software. The RWP Table 1 The XRD parameters in tin oxide and chromium doped tin oxide. Sample name
˚ a (A)
˚ C (A)
Cell volume (A˚ 3 )
Crystalline size (nm)
Thickness (nm)
x = 0.0 x=2 x=4 x=6 x = 10 x = 20
4.7100 4.7112 4.6720 4.6826 4.7218 4.7127
3.1950 3.1995 3.1612 3.1956 3.1850 3.1947
71.0000 71.0185 69.0032 70.0706 71.0133 70.9556
4.8 4.9 4.0 4.45 3.7 4.2
83.69 131.00 94.52 124.3 76.67 116.86
Fig. 4. Typical FESEM micrographs of (a) film’s surface and (b) film’s cross-section.
262
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
Fig. 7. Optical absorption coefficient versus wavelength of Sn1−x Crx O2 (x = 0, 0.02, 0.04, 0.06, 0.20).
Fig. 5. Transmission electron micrograph of (a) undoped tin oxide and (b) 4% chromium doped tin oxide film.
the TEM micrographs of the undoped SnO2 and x = 4 wt% Cr films. Both samples have irregular grain size. Bagheri-Mohagheghi et al. have also reported on the grain size of SnO2 powder prepared by a sol–gel method, as small as 5 nm [13]. Fig. 6 shows the optical transparent spectra of samples as a function of wavelength. These films show high transparency (80–99%) in the visible region (300–850 nm). Fig. 7 shows the optical absorption coefficient, ˛ for all samples. In the visible wavelength region, the highest dopant gives the highest ˛ value due to the disorder in the SnO2 lattice is created by high doping process [12,14]. Similar results have been also reported by Liu et al. [13]. The transmission
Fig. 6. Optical transmission spectra of Cr doped SnO2 films for various dopants.
of their Cr doped SnO2 film prepared by a co-sputtering technique was not affected by the dopant until its value reached ≥0.4 wt% [12]. Bagheri-Mohagheghi et al. reported that the transmission of their Co doped SnO2 films is affected when the dopant was 2 wt% and the transmission of their Fe doped SnO2 reduced after doped with 2 wt% Fe [15]. Due to Liu et al. the reduction of transmission in the visible region could be due two mechanisms. First, the unfilled d orbital of Cr ion which have been incorporated in SnO2 lattice produce in-gap absorption and second, the scattering of lights due to the impurity in their film [12]. As in this study none impurities can be detected hence the reduction is due to the in-gap absorption. Optical band gap from transmittance spectra was obtained using the Tauc’s formula: ˛h = A(h − Eg )1/m that A is constant, ˛ is the absorption coefficient, h is the photon energy, Eg is the optical band gap and m is a number that characterises the optical transition process: m = 1/2 for a direct allowed transition, m = 3/2 for a direct forbidden transition, m = 2 for indirect allowed transition and m = 3 for an indirect forbidden transition. To determine the ‘m’ value the graph of (˛h)m versus h with m = 1/2, 3/2, 2 and 3 were plotted for each sample. The m was determined from the graph with the best regression value. Fig. 8 shows the graph of (˛h)m versus h with m = 1/2, 3/2, 2 and 3 for undoped sample. Fig. 9 shows the dependent of the Eg with the Cr wt%. In the quantum confinement effect if the grain size is similar to the Bohr radius, the energy gap of materials increases as the grain size decreases. The grain sizes measured from Fig. 5 are bigger than the Bohr radius for SnO2 (2.7 nm). Furthermore the grain sizes irregular thus it is difficult to relate the increment of the energy gap to the grain size effect. However, the energy gap in the films is larger than reported by Bagheri-Mohagheghi et al. for spray pyrolysis SnO2 films. Their films have bigger grain size (micrometre range) than ours (nanometre) [14]. Thus the large energy gap compared to the energy gap of spray pyrolysis film could be due to the grain size effect. The refractive index is calculated using the formula, reflective spectra, R = ((1 + n)/(1 − n))2 and n is refractive index. The extinction coefficient is calculated by using absorption coefficient that related by ˛ = 4k/A where ˛, k, and A are absorption coefficient, extinction coefficient and absorption spectra, respectively. Fig. 10 shows the refractive index versus wavelength for all the films. There is a peak between 400 and 550 nm due to the high transmittance in this area. The ‘n’ values saturate at wavelengths larger than 550 nm. They are constant in the visible region for the higher dopant films because the transparency value is constant in the region. The extinction coefficients in the visible region are very small (see Fig. 11), which is consistent with previous reports [16,17]. Optical conductivity was calculated using the formula = ˛nc/4 and the results are shown in Fig. 12. The conductivity of films increases sharply around 4 eV due to high absorbance of SnO2 thin films [18].
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
263
Fig. 11. Extinction coefficient versus wavelength of Sn1−x Crx O2 (x = 0, 0.02, 0.04, 0.06, 0.20) film.
Fig. 8. (˛h)m vs photon energy for different m of undoped film. Fig. 12. Optical conductivity versus photon energy for undoped tin oxide film.
4. Conclusion Sn1−x Crx O2 (x = 0.0, 0.02, 0.04, 0.06, 0.1, 0.2) films have been successfully prepared by sol–gel method without any impurities. The lattice parameters and volume cells altered with the increment of chromium dopant. However to confirm whether the solubility of Cr can reach up 20 wt%, further investigation using Mossbauer spectroscopy is in progress. The optical properties characterised using a UV-Vis spectrophotometer showed that all samples have semiconductor characterisation with various band gaps and have transparency between 80 and 99%. The band gap increased with the increment of Cr dopant and refractive index become stable ∼2 for > 550 nm. Fig. 9. The optical band gap versus Cr wt% of Cr-doped SnO2 films.
Acknowledgement This research is funded by Universiti Kebangsaan Malaysia research grant code UKM-GUP-2011-221. References
Fig. 10. Refractive index of Sn1−x Crx O2 (x = 0, 0.02, 0.04, 0.06, 0.20) film.
[1] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Progress in Surface Science 79 (2005) 47–154. [2] P. Saikia, A. Borthakur, P. Saikia, Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method, Indian Journal of Physics 85 (2011) 551–558. [3] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 291 (2001) 854. [4] B. Pandey, S. Ghosh, P. Srivastava, D.K. Avasthi, D. Kabiraj, J.C. Pivin, Synthesis and characterization of Ni-doped ZnO: a transparent magnetic semiconductor, Journal of Magnetism and Magnetic Materials 320 (2008) 3347–3351. [5] Y.M. Hu, Y.T. Chen, Z.X. Zhong, C.C. Yu, G.J. Chen, P.Z. Huang, W.Y. Chou, J. Chang, C.R. Wang, The morphology and optical properties of Cr-doped ZnO films grown using the magnetron co-sputtering method, Applied Surface Science 254 (2008) 3873–3878.
264
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
[6] H. Hartnagel, A. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, London, 1995. [7] T. Oyabu, Sensing characteristics of SnO2 thin film gas sensor, Journal of Applied Physics 53 (1982) 2785–2787. [8] T. Minami, Transparent conducting oxide semiconductors for transparent electrodes, Semiconductor Science and Technology 20 (2005) S35. [9] L. Zhang, S. Ge, Y. Zuo, J. Wang, J. Qi, Ferromagnetic properties in undoped and Cr-doped SnO2 nanowires, Scripta Materialia 63 (2010) 953–956. [10] E. López-Navarrete, A. González-Elipe, M. Ocana, Non-conventional synthesis of Cr-doped SnO2 pigments, Ceramics International 29 (2003) 385–392. [11] Y. Zuo, S. Ge, L. Zhang, S. Yan, X. Zhou, Y. Xiao, Room temperature ferromagnetism of Sn1−x Crx O2 films fabricated by sol–gel method, Journal of Alloys and Compounds 475 (2009) 60–63. [12] S.J. Liu, L.Y. Chen, C.Y. Liu, H.W. Fang, J.H. Hsieh, J.Y. Juang, Physical properties of polycrystalline Cr-doped SnO2 films grown on glasses using reactive dc magnetron co-sputtering technique, Applied Surface Science 257 (2011) 2254–2258. [13] M.M. Bagheri-Mohagheghi, N. Shahtahmasebi, M. Alinejad, A. Youssefi, M. Shokooh-Saremi, The effect of the post-annealing temperature on the
[14]
[15]
[16] [17]
[18]
nano-structure and energy band gap of SnO2 semiconducting oxide nanoparticles synthesized by polymerizing–complexing sol–gel method, Physica B: Condensed Matter 403 (2008) 2431–2437. M.-M. Bagheri-Mohagheghi, M. Shokooh-Saremi, The electrical, optical, structural and thermoelectrical characterization of n- and p-type cobalt-doped SnO2 transparent semiconducting films prepared by spray pyrolysis technique, Physica B: Condensed Matter 405 (2010) 4205–4210. M.M. Bagheri-Mohagheghi, N. Shahtahmasebi, M.R. Alinejad, A. Youssefi, M. Shokooh-Saremi, Fe-doped SnO2 transparent semi-conducting thin films deposited by spray pyrolysis technique: thermoelectric and p-type conductivity properties, Solid State Sciences 11 (2009) 233–239. H. Demiryont, K.E. Nietering, R. Surowiec, F. Brown, D. Platts, Optical properties of spray-deposited tin oxide films, Applied Optics 26 (1987) 3803–3810. E. Shanthi, V. Dutta, A. Banerjee, K. Chopra, Electrical and optical properties of undoped and antimony-doped tin oxide films, Journal of Applied Physics 51 (1980) 6243–6251. R.L. Mishra, S.G. Prakash, Structural and physico-chemical studies of nanocrystalline tin oxide thin film, Journal of Optoelectronics and Advanced Materials 11 (2009) 274–279.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural and optical characterisation of undoped and chromium doped tin oxide prepared by sol–gel method N.B. Ibrahim ∗ , M.H. Abdi, M.H. Abdullah, H. Baqiah School of Applied Physics, Faculty of Science and Technology, University Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 28 May 2012 Received in revised form 23 January 2013 Accepted 24 January 2013 Available online 8 February 2013 Keywords: Energy gap Cr doped SnO2 Thin films Sol–gel
a b s t r a c t Transparent semiconductor thin films, Cr doped SnO2 (Sn1−x Crx O2 : x = 0.0, 0.02, 0.04, 0.06, 0.10, 0.20) were deposited onto glass substrates by a sol–gel method. The thermal gravitational analysis showed that mass lost happened at 120 ◦ C due to evaporation of water and ethanol. X-ray diffraction analysis showed that the Cr doped SnO2 were polycrystalline with SnO2 phases. The crystalline sizes were in the range of 3.7–4.9 nm. The optical property measured using UV-Vis spectrophotometer showed that the transparency of all samples was more than 90% and the calculated band gaps were in the range of 3.84–3.96 eV which was due to the Cr dopant that increased the samples energy band gap. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxide semiconductors such as tin oxide (SnO2 ), zinc oxide (ZnO), and indium oxide (In2 O3 ) have attracted great researchers’ attention due to their variable optical and electrical properties [1–5]. SnO2 has many applications such as in solar cells, gas sensors, display devices, optoelectronics and electrochromic devices [6]. These oxide semiconductors have also been doped with other metals in order to enhance their applications. For example, SnO2 has been doped with Pd and Pt to enhance its sensitivity as a gas sensor [7] and doped with Sb or F to obtain high transparency and excellent electrical properties [8]. Numerous studies have also been carried out to study SnO2 doped with transition metal such as Co, Fe, Ni, V and Cr. Based on their study of the ferromagnetic properties of Cr doped SnO2 nanowires prepared by a catalyzer-assisted chemical vapour deposition method, Zhang et al. suggested that the ferromagnetic of the films are due to the oxygen vacancy and Cr dopant [9]. López-Navarrete et al. synthesised Cr-SnO2 violet pigments by a pyrolysis process at 600 ◦ C. The Cr concentration affects the colour properties of the pigment. The optimum pigment was obtained from the sample with Cr/Sn weight ratio in the range of 0.045–0.057 [10]. Zuo et al. reported on the ferromagnetic properties of Sn1−x Crx O2 films prepared onto silicon (1 1 1) substrates by a sol–gel method. Due to antiferromagnetic super-exchange interaction among the nearest neighbour in Cr ions, the magnetic moment
∗ Corresponding author. E-mail address: [email protected] (N.B. Ibrahim). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.171
per Cr decreases as the Cr content increases [11]. However none of these literatures report on the optical properties of Cr doped SnO2 prepared by a sol–gel method. SnO2 is a well known transparent conductor thus it is important to know its optical properties after doped with transition metal, Cr. In this work, we prepared SnO2 and chromium doped SnO2 films onto glass substrates by a sol–gel technique. The aims of the study were (1) to study the effect of Cr dopant onto the structural and optical properties of SnO2 films and (2) to study the effect of different weight percent of Cr dopant on the properties of SnO2 films.
2. Experimental SnCl2 ·2H2 O (tin (II) chloride dehydrate (Merk)) was dissolved in 10 ml ethanol (99% purity). This solution was distilled at room temperature (27 ◦ C) and refluxed at 80 ◦ C for 4 h. Then it was filtered and deposited onto cleaned glass substrates by a spin coating technique at room temperature (27 ◦ C). The obtained films were dried at 100 ◦ C to get rid of the solvent and annealed at 600 ◦ C in air. Cr doped SnO2 films have been prepared by adding CrCl2 ·6H2 O to SnCl2 ·2H2 O and dissolved by ethanol. The dopant was added up to 20 wt%. The undoped-solution was studied with thermogravimetry (TGA) model Mettler Toledo SDTA 821. The Xray-diffraction patterns of samples were recorded by an X-ray diffractometer D6 Advance Bruker system using Cu K␣ radiation, also film surface morphology and cross-section were studied by using a field emission scanning electron microscope (FESEM) model Zeiss Supra 55vp. The optical properties of the films were studied using Lambda-900 UV-VIS visible spectrophotometer.
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
261
Fig. 3. XPS spectrum of Sn0.90 Cr0.1 O2 film. Fig. 1. TGA curves of the dried SnO2 gel.
values were between 11 and 24. The calculated cell volumes and crytalline size decrease as the dopant weight percent increases. This ˚ with is due to the replacement of bigger ionic radius, Sn4+ (0.69 A) ˚ [9]. Fig. 3 shows the typical X-ray smaller ionic radius, Cr3+ (0.65 A) photoelectron spectroscopy (XPS) result of the films, confirming the Cr3+ oxidation state in the films. Fig. 4(a) shows the typical FESEM micrographs for film surface. The film has a good surface quality without any crack or pores on the surface. The film is dense with grains sizes less than 20 nm. These results explain the broad peaks obtained from the XRD spectra. The thickness of the films was measured from the films cross-section micrographs obtained using the FESEM. Fig. 4(b) shows the typical cross-section for all films and the values are tabulated in Table 1. The measured thicknesses are independent of the Cr content. Transmission electron microscope (TEM) model CM 12 Philips was also used to measure the grain size of the film. Fig. 5 shows
Fig. 2. XRD pattern of the polycrystalline Sn1−x Crx O2 film.
3. Results and discussion Fig. 1 shows TGA curve of the dried SnO2 gel. The mass loss occurs at about 110–120 ◦ C. The mass loss is due to the evaporation of ethanol and water. No any other mass loss can be observed until 600 ◦ C. X-ray diffraction patterns of the thin films were studied at 2 (values from 20◦ to 60◦ and the results are shown in Fig. 2. The peaks obtained for all films are belong to cassite tin oxide with tetragonal phase. The films are polycrystalline with single phase. The calculation of intensities ratio is tabulated in Table 1. The intensities of the main peaks change as the Cr percentage increases indicating that the doping process affects the crystal growth of the films. Liu et al. have also reported that Cr dopant slightly affects their reactive dc co-sputtering SnO2 films [12]. Table 1 summaries the lattice parameters and cell volumes of all films calculated by using a Rietveld refinement software. The RWP Table 1 The XRD parameters in tin oxide and chromium doped tin oxide. Sample name
˚ a (A)
˚ C (A)
Cell volume (A˚ 3 )
Crystalline size (nm)
Thickness (nm)
x = 0.0 x=2 x=4 x=6 x = 10 x = 20
4.7100 4.7112 4.6720 4.6826 4.7218 4.7127
3.1950 3.1995 3.1612 3.1956 3.1850 3.1947
71.0000 71.0185 69.0032 70.0706 71.0133 70.9556
4.8 4.9 4.0 4.45 3.7 4.2
83.69 131.00 94.52 124.3 76.67 116.86
Fig. 4. Typical FESEM micrographs of (a) film’s surface and (b) film’s cross-section.
262
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
Fig. 7. Optical absorption coefficient versus wavelength of Sn1−x Crx O2 (x = 0, 0.02, 0.04, 0.06, 0.20).
Fig. 5. Transmission electron micrograph of (a) undoped tin oxide and (b) 4% chromium doped tin oxide film.
the TEM micrographs of the undoped SnO2 and x = 4 wt% Cr films. Both samples have irregular grain size. Bagheri-Mohagheghi et al. have also reported on the grain size of SnO2 powder prepared by a sol–gel method, as small as 5 nm [13]. Fig. 6 shows the optical transparent spectra of samples as a function of wavelength. These films show high transparency (80–99%) in the visible region (300–850 nm). Fig. 7 shows the optical absorption coefficient, ˛ for all samples. In the visible wavelength region, the highest dopant gives the highest ˛ value due to the disorder in the SnO2 lattice is created by high doping process [12,14]. Similar results have been also reported by Liu et al. [13]. The transmission
Fig. 6. Optical transmission spectra of Cr doped SnO2 films for various dopants.
of their Cr doped SnO2 film prepared by a co-sputtering technique was not affected by the dopant until its value reached ≥0.4 wt% [12]. Bagheri-Mohagheghi et al. reported that the transmission of their Co doped SnO2 films is affected when the dopant was 2 wt% and the transmission of their Fe doped SnO2 reduced after doped with 2 wt% Fe [15]. Due to Liu et al. the reduction of transmission in the visible region could be due two mechanisms. First, the unfilled d orbital of Cr ion which have been incorporated in SnO2 lattice produce in-gap absorption and second, the scattering of lights due to the impurity in their film [12]. As in this study none impurities can be detected hence the reduction is due to the in-gap absorption. Optical band gap from transmittance spectra was obtained using the Tauc’s formula: ˛h = A(h − Eg )1/m that A is constant, ˛ is the absorption coefficient, h is the photon energy, Eg is the optical band gap and m is a number that characterises the optical transition process: m = 1/2 for a direct allowed transition, m = 3/2 for a direct forbidden transition, m = 2 for indirect allowed transition and m = 3 for an indirect forbidden transition. To determine the ‘m’ value the graph of (˛h)m versus h with m = 1/2, 3/2, 2 and 3 were plotted for each sample. The m was determined from the graph with the best regression value. Fig. 8 shows the graph of (˛h)m versus h with m = 1/2, 3/2, 2 and 3 for undoped sample. Fig. 9 shows the dependent of the Eg with the Cr wt%. In the quantum confinement effect if the grain size is similar to the Bohr radius, the energy gap of materials increases as the grain size decreases. The grain sizes measured from Fig. 5 are bigger than the Bohr radius for SnO2 (2.7 nm). Furthermore the grain sizes irregular thus it is difficult to relate the increment of the energy gap to the grain size effect. However, the energy gap in the films is larger than reported by Bagheri-Mohagheghi et al. for spray pyrolysis SnO2 films. Their films have bigger grain size (micrometre range) than ours (nanometre) [14]. Thus the large energy gap compared to the energy gap of spray pyrolysis film could be due to the grain size effect. The refractive index is calculated using the formula, reflective spectra, R = ((1 + n)/(1 − n))2 and n is refractive index. The extinction coefficient is calculated by using absorption coefficient that related by ˛ = 4k/A where ˛, k, and A are absorption coefficient, extinction coefficient and absorption spectra, respectively. Fig. 10 shows the refractive index versus wavelength for all the films. There is a peak between 400 and 550 nm due to the high transmittance in this area. The ‘n’ values saturate at wavelengths larger than 550 nm. They are constant in the visible region for the higher dopant films because the transparency value is constant in the region. The extinction coefficients in the visible region are very small (see Fig. 11), which is consistent with previous reports [16,17]. Optical conductivity was calculated using the formula = ˛nc/4 and the results are shown in Fig. 12. The conductivity of films increases sharply around 4 eV due to high absorbance of SnO2 thin films [18].
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
263
Fig. 11. Extinction coefficient versus wavelength of Sn1−x Crx O2 (x = 0, 0.02, 0.04, 0.06, 0.20) film.
Fig. 8. (˛h)m vs photon energy for different m of undoped film. Fig. 12. Optical conductivity versus photon energy for undoped tin oxide film.
4. Conclusion Sn1−x Crx O2 (x = 0.0, 0.02, 0.04, 0.06, 0.1, 0.2) films have been successfully prepared by sol–gel method without any impurities. The lattice parameters and volume cells altered with the increment of chromium dopant. However to confirm whether the solubility of Cr can reach up 20 wt%, further investigation using Mossbauer spectroscopy is in progress. The optical properties characterised using a UV-Vis spectrophotometer showed that all samples have semiconductor characterisation with various band gaps and have transparency between 80 and 99%. The band gap increased with the increment of Cr dopant and refractive index become stable ∼2 for > 550 nm. Fig. 9. The optical band gap versus Cr wt% of Cr-doped SnO2 films.
Acknowledgement This research is funded by Universiti Kebangsaan Malaysia research grant code UKM-GUP-2011-221. References
Fig. 10. Refractive index of Sn1−x Crx O2 (x = 0, 0.02, 0.04, 0.06, 0.20) film.
[1] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Progress in Surface Science 79 (2005) 47–154. [2] P. Saikia, A. Borthakur, P. Saikia, Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method, Indian Journal of Physics 85 (2011) 551–558. [3] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 291 (2001) 854. [4] B. Pandey, S. Ghosh, P. Srivastava, D.K. Avasthi, D. Kabiraj, J.C. Pivin, Synthesis and characterization of Ni-doped ZnO: a transparent magnetic semiconductor, Journal of Magnetism and Magnetic Materials 320 (2008) 3347–3351. [5] Y.M. Hu, Y.T. Chen, Z.X. Zhong, C.C. Yu, G.J. Chen, P.Z. Huang, W.Y. Chou, J. Chang, C.R. Wang, The morphology and optical properties of Cr-doped ZnO films grown using the magnetron co-sputtering method, Applied Surface Science 254 (2008) 3873–3878.
264
N.B. Ibrahim et al. / Applied Surface Science 271 (2013) 260–264
[6] H. Hartnagel, A. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, London, 1995. [7] T. Oyabu, Sensing characteristics of SnO2 thin film gas sensor, Journal of Applied Physics 53 (1982) 2785–2787. [8] T. Minami, Transparent conducting oxide semiconductors for transparent electrodes, Semiconductor Science and Technology 20 (2005) S35. [9] L. Zhang, S. Ge, Y. Zuo, J. Wang, J. Qi, Ferromagnetic properties in undoped and Cr-doped SnO2 nanowires, Scripta Materialia 63 (2010) 953–956. [10] E. López-Navarrete, A. González-Elipe, M. Ocana, Non-conventional synthesis of Cr-doped SnO2 pigments, Ceramics International 29 (2003) 385–392. [11] Y. Zuo, S. Ge, L. Zhang, S. Yan, X. Zhou, Y. Xiao, Room temperature ferromagnetism of Sn1−x Crx O2 films fabricated by sol–gel method, Journal of Alloys and Compounds 475 (2009) 60–63. [12] S.J. Liu, L.Y. Chen, C.Y. Liu, H.W. Fang, J.H. Hsieh, J.Y. Juang, Physical properties of polycrystalline Cr-doped SnO2 films grown on glasses using reactive dc magnetron co-sputtering technique, Applied Surface Science 257 (2011) 2254–2258. [13] M.M. Bagheri-Mohagheghi, N. Shahtahmasebi, M. Alinejad, A. Youssefi, M. Shokooh-Saremi, The effect of the post-annealing temperature on the
[14]
[15]
[16] [17]
[18]
nano-structure and energy band gap of SnO2 semiconducting oxide nanoparticles synthesized by polymerizing–complexing sol–gel method, Physica B: Condensed Matter 403 (2008) 2431–2437. M.-M. Bagheri-Mohagheghi, M. Shokooh-Saremi, The electrical, optical, structural and thermoelectrical characterization of n- and p-type cobalt-doped SnO2 transparent semiconducting films prepared by spray pyrolysis technique, Physica B: Condensed Matter 405 (2010) 4205–4210. M.M. Bagheri-Mohagheghi, N. Shahtahmasebi, M.R. Alinejad, A. Youssefi, M. Shokooh-Saremi, Fe-doped SnO2 transparent semi-conducting thin films deposited by spray pyrolysis technique: thermoelectric and p-type conductivity properties, Solid State Sciences 11 (2009) 233–239. H. Demiryont, K.E. Nietering, R. Surowiec, F. Brown, D. Platts, Optical properties of spray-deposited tin oxide films, Applied Optics 26 (1987) 3803–3810. E. Shanthi, V. Dutta, A. Banerjee, K. Chopra, Electrical and optical properties of undoped and antimony-doped tin oxide films, Journal of Applied Physics 51 (1980) 6243–6251. R.L. Mishra, S.G. Prakash, Structural and physico-chemical studies of nanocrystalline tin oxide thin film, Journal of Optoelectronics and Advanced Materials 11 (2009) 274–279.