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Surface and optical properties of nanocrystalline anatase titania films grown by radio frequency reactive magnetron sputtering
Applied Surface Science 195 (2002) 284–290
Surface and optical properties of nanocrystalline anatase titania films grown by radio frequency reactive magnetron sputtering Fa-Min Liu*, Tian-Min Wang Center of Material Physics and Chemistry, School of Science, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China Received 6 April 2002; received in revised form 3 June 2002; accepted 3 June 2002
Abstract Nanocrystalline anatase TiO2 films were deposited on glass and silicon substrates by using radio frequency reactive magnetron sputtering. Microstructural and optical properties were characterized by using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), Raman spectra, optical absorption and photoluminescence (PL). XRD and Raman spectroscopy detected a nanocrystalline anatase titania films for input power at 250 W, for argon/oxygen ratio at 50 sccm/ 5.0 sccm and for substrate temperature at 500 8C. Visible broad widened photoluminescence peak is interpreted as the emission of self-trapped excitons localized on TiO6 octahedra. # 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.-m; 78.40.-q; 78.30.-j; 74.25.Gz; 79.60.-i; 61.82.Rx; 61.10.-i Keywords: Nanocrystalline anatase TiO2 film; XRD; XPS; Raman spectra; Optical properties
1. Introduction It is well known that titania exhibits three crystal phase as naturally occurring minerals: rutile, anatase, and brookite. TiO6 octahedra are interconnected differently for each phase, leading to different structures and symmetries. Rutile is a tetragonal with lattice of a ¼ 0:4594 nm, c ¼ 0:2958 nm and with refraction index of 2.7. Anatase TiO2 is a tetragonal with lattice of a ¼ 0:3785 nm, c ¼ 0:9514 nm, with refraction index of 2.54 and with optical band gap of 3.2 eV at room temperature. Brookite is an orthorhombic * Corresponding author. Tel.: þ86-10-82317941; fax: þ86-10-82315933. E-mail addresses: [email protected], [email protected] (F.-M. Liu).
with lattice of a ¼ 0:9184 nm, b ¼ 0:5447, c ¼ 0:5145 nm [1]. Titanium dioxide is attracting much interest for its unique properties, such as high dielectric constant [2,3], high refractivity [4] and photocatalysis [5–7]. The occurrence of anatase and rutile phase depends significantly on the method and conditions of deposition as well as the substrate temperature [8,9]. Fabrication methods span a wide range and include such techniques as growth in sol–gel [10,11], chemical vapor deposition (CVD) [12], metal organic chemical vapor deposition (MOCVD) [13], reactive evaporation [4], pulsed laser deposition [14], and magnetron sputtering [15–17]. Among those techniques, reactive magnetron sputtering is one of the most applicable techniques to control the microstructure and the stoichiometry, with an advantage of large area deposition. So far, many works have been reported for
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 5 6 9 - X
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
various techniques to prepare TiO2 films and their properties. However, a few works deal with the surface properties and photoluminescence of nanocrystalline anatase TiO2 films. In this paper, nanocrystalline TiO2 films were deposited onto glass and silicon substrates by radio frequency reactive magnetron sputtering. Microstructure was characterized by X-ray diffraction. The surface properties of these films were investigated by using Raman scattering spectra and X-ray photoemission spectra. Visible photoluminescence is interpreted as the emission of self-trapped excitons localized on TiO6 octahedra.
2. Experimental procedure Nanocrystalline anatase TiO2 films were grown on glass and silicon substrates by radio frequency reactive magnetron sputtering at different Ar/O2 gas-flow ratio which are determined by mass-flow controlled regulators. A high purity (99.999%) Ti plate with 60 mm in diameter and 3 mm in thick was used as a target. Glass and silicon substrates (20 mm 20 mm 1 mm) was ultrasonically washed successively in acetone, alcohol, and deionized water to obtain a clean surface before being placed in a vacuum chamber. The target was separated from the substrate by 5 cm. When the chamber was evacuated to a pressure of 5 105 Pa, high purity mixing gas of argon (99.9999%) and oxygen (99.99%) were introduced. During sputtering, the chamber working pressure was maintained at 2 Pa. The power to the radio frequency sputter gun was 250 W. All runs started with inverse sputtering the target for 10 min before the high quality TiO2 films deposited. The film thickness d was determined by surface profilometry using a DEKTAK 3 Alpha-step instrument. In our experiment, the deposition rate was as large as 2.8 nm/min. The coil of electric resistance was used to heat the substrate ranging from 50 to 700 8C. The TiO2 films were characterized with X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy and Raman scattering. XRD studies were finished on a X-ray diffractometer (D/max-rB type) with Cu Ka radiation and a monochromator. The XPS measurements were carried out with a VG ESCALAB MK II spectrometer using a Mg Ka X-ray source with the analyzer mode set at a constant energy of 50 eV.
285
The X-ray source was run at 120 W and a takeoff angle of 808. All the peak energies in the spectra were referenced with respect to the C 1s (284.6 eV) peak. Room temperature Raman and photoluminescence spectra were recorded with an SPEX-1403 laser Raman spectrometer. The excitation source was an Arþ ion laser lines of 514.5 and 488.0 nm operated at 200 mW in the backscattering geometry for Raman scattering and photoluminescence, respectively. The resolution of this instrument was 1 cm1. The measured spectra have been corrected according to the system response. The optical measurements of transmission and absorption spectra, neglecting reflection losses, were performed on a Perkin-Elmer Lambda 9 double beam spectrophotometer. The wavelength was scanned over the range from 300 to 2500 nm with an accuracy of 1 nm at room temperature.
3. Results and discussion Fig. 1 shows X-ray diffraction patterns of nanocrystalline TiO2 film deposited at RF generator of 250 W, at substrate temperature of 500 8C and Ar/ O2 gas-flow ratio of 50 sccm/5.0 sccm. It appears five anatase typical peaks signed (1 0 1), (0 0 4), (2 1 1), (2 0 4), (2 2 0) and (2 1 5), which are basically in agreement with [18,19]. The mean size of TiO2 is about 13.5 nm estimated by Scherrer formula. Wicaksana et al. [19] studied that the oxygen mole fraction affects on the structures of the TiO2 films. They pointed out that the rutile phase was shown to grow easier in lower O2 mole fraction, and the anatase phase was shown at oxygen fraction of 40 and 50%. Roy and White [20] studied the phase diagram of Ti–O compound, and they reported that the rutile is formed at above 800 8C in equilibrium condition. In our experiment, the substrate was heated at 500 8C, it is still insufficient to grow the rutile phase. Figs. 2 and 3 show XPS Ti 2p and O 1s core levels of nanocrystalline TiO2 film. From Fig. 2, one can see that the core levels of Ti 2p1/2 and Ti 2p3/2 are at approximately 464.1 and 458.4 eV, respectively, which are assigned to the Ti4þ (TiO2), with a peak separation of 5.7 eV between those two peaks. Compared with [21,22], the core levels of Ti 2p1/2 and Ti 2p3/2 have a small shift of 0.1 eV. The core level of O 1s is at line of 529.7 eV from Fig. 3, which is attributed to titanium
286
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
Fig. 1. X-ray diffraction of nanocrystalline TiO2 film deposited on glass substrate by radio frequency reactive magnetron sputtering. The mean size of TiO2 is about 13.5 nm estimated by Scherrer formula.
dioxide at the surface. This data is approximately agreeable with the main peak at 529.9 eV of TiO2. It is well known that anatase TiO2 belongs to the space group D19 4h (I41amd) with two units formula per
units-cell and with six Raman active modes (A1g þ 2B1g þ 3Eg ) [23], whereas rutile TiO2 is tetragonal (D14 4h ) with two units and with four Raman active modes (A1g þ B1g þ B2g þ Eg ). The amorphous TiO2
Fig. 2. XPS Ti 2p core level of nanocrystalline TiO2 film.
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
287
Fig. 3. XPS O 1s core level of nanocrystalline TiO2 film.
is known to have no Raman peak [24]. Raman scattering spectra of nanocrystalline anatase TiO2 films are presented in Fig. 4. These films were deposited on glass substrate, at substrate temperature of 500 8C and at different Ar/O2 gas-flow ratio indicated. When Ar/O2 gas-flow ratio ranging from 50 sccm/4.0 sccm to 50 sccm/5.0 sccm, one can see that the four Raman scattering peaks of anatase TiO2 are at approximately 144, 400, 515 and 640 cm1, which are assigned two B1g modes and two Eg modes, respectively. These results are agreeable with anatase TiO2 films obtained by Tang et al. [25]. However, the two Eg modes are enhanced and widened when the Ar/O2 gas-flow ratio is at and over 50 sccm/6.0 sccm. In addition, one can see that small Raman peak appears at about 796.3 cm1. This demonstrates that excessive oxygen will affect the structure of TiO2 film. Another relatively strong Raman peak around 1100 cm1 is attributed to titanium–oxygen–silicon dioxide bond formation [26], indicating that there is a strong interaction at the interface between the TiO2 film and the SiO2 layer on the glass substrate. Fig. 5 shows optical absorption evolution of the TiO2 film with wavelength. The TiO2 film exhibits a sharp absorption edge at about 336.2 nm (3.687 eV). The fluctuation appearing in the curves were identified
to be due to the interference fringes of the film. According to the feature of equal thickness interference fringes, one can estimated the thickness of the film from the following formula [27]. t¼
l1 l 2 jnðl1 Þl2 nðl2 Þl1 j 2
(1)
where t represents the thickness of the film, l1 and l2 are the wavelength of the position of two maximum or minimum, respectively, and, n(l1) and n(l2) are the refractive index corresponding to l1 and l2, respectively. According to Fig. 5, the n(l1) and n(l2) were approximately taken [28] as 2.53 and 2.47 corresponding to 550 and 676 nm, respectively. And from formula (1), the thickness of the film is estimated about 528.5 nm, which is basically agreement with the data measured by using a DEKTAK3 Alpha-step instrument. Fig. 6 indicates photoluminescence of the TiO2 film excited by 488.0 nm line of Arþ laser at room temperature. The TiO2 film is deposited on silicon substrate, at Ar/O2 gas-flow ratio of 50 sccm/5.0 sccm and at substrate temperature of 500 8C. The thickness of the film is about 336 nm. From Fig. 6, one sees a broad band with the PL peak maximum at approximately 682.5 nm (1.82 eV) and with the full width at
288
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
Fig. 4. Raman scattering spectra of nanocrystalline TiO2 films deposited on glass substrate by radio frequency reactive magnetron sputtering at different Ar/O2 gas-flow ratio indicated.
Fig. 5. Optical absorption vs. wavelength.
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
289
Fig. 6. Photoluminescence of the TiO2 film excited by 488.0 nm line of Arþ laser at room temperature. The TiO2 film is deposited on silicon substrate by radio frequency reactive magnetron sputtering at RF generator of 250 W, at Ar/O2 gas-flow ratio of 50 sccm/5.0 sccm and substrate temperature of 500 8C.
half maximum (FWHM) of 155.3 nm (0.4 eV), indicating that there is no free exciton emission. De Haart et al. [29] studied the luminescence spectra of SrTiO3 and found broad emission bands, which can be interpreted as resulting from the fundamental Ti4þ to O2 charge transfer transition localized on the TiO6 octahedron. Then, Tang et al. [30] investigated the photoluminescence in TiO2 anatase single crystals and found visible broad band luminescence peaks, which was interpreted as the emission from the self-trapped excitons localized on TiO6 octahedra. For TiO2 anatase film, the visible broad PL peak (682.5 nm) also explained by the self-trapped excitons. These results suggest that excitons in anatase be in the self-trapped state, characterized by strong electron–photon coupling and small exciton band width [30]. In addition, the surface plasma resonance of the TiO2 nanoparticles also affects the symmetry of PL peak.
4. Conclusions In summary, we have grown nanocrystalline anatase TiO2 films on glass and silicon substrates by using radio frequency reactive magnetron sputtering. X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and Raman spectra show that it is a pure anatase phase film when Ar/O2 gas-flow ratio equalizing about
50 sccm/5.0 sccm and substrate temperature at 500 8C. And visible broad band photoluminescence is interpreted as the emission of self-trapped excitons localized on TiO6 octahedra.
Acknowledgements This paper was supported by the post-doctoral foundation of China, and partly supported by National Natural Science Foundation of China (No. 59982002) and the foundation of State Key Laboratory for Advanced Metals and Materials.
References [1] Y. Leprince-Wang, K. Yu-Zhang, Surf. Coatings Technol. 140 (2001) 155. [2] E.K. Kim, M.H. Son, S.K. Min, Y.K. Han, S.S. Yom, J. Cryst. Growth 170 (1997) 803. [3] P. Alexandrov, J. Koprinarova, D. Todorov, Vacuum 47 (1996) 1333. [4] D. Mergel, D. Buschendorf, S. Eggert, R. Grammes, B. Samset, Thin Solid Films 371 (2000) 218. [5] L. Shivalingappa, J. Sheng, T. Fukami, Vacuum 48 (1997) 413. [6] W. Choi, S.J. Hong, Y.S. Chang, Y. Cho, Environ. Sci. Technol. 34 (2000) 4810. [7] O. Heintz, D. Robert, J.V. Weber, J. Photochem. Photobiol. A: Chem. 135 (2000) 77.
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[8] M.H. Suhail, G. Mohan Rao, S. Mohan, J. Appl. Phys. 71 (1992) 1421. [9] P. Lobl, M. Huppertz, D. Mergel, Thin Solid Films 251 (1994) 72. [10] G. San Vicente, A. Morales, M.T. Gutierrez, Thin Solid Films 391 (2000) 133. [11] Q. Fan, B. McQuillin, A.K. Ray, M.L. Turner, A.B. Seddon, J. Phys. D: Appl. Phys. 33 (2000) 2683. [12] M.D. Wiggins, M.C. Nelson, C.R. Aita, J. Vac. Sci. Technol. A14 (1996) 772. [13] D.J. Won, C.H. Wang, H.K. Jang, D.J. Choi, Appl. Phys. A73 (2001) 595. [14] M.P. Moret, R. Zallen, D.P. Vijay, S.B. Desu, Thin Solid Films 366 (2000) 8. [15] V. Vancoppenolle, P.Y. Jouan, M. Wautelet, J.P. Dauchot, M. Hecq, J. Vac. Sci. Technol. A17 (1999) 3317. [16] K. Okimura, Surf. Coatings Technol. 135 (2001) 286. [17] A. Kinbara, E. Kusano, S. Baba, J. Vac. Sci. Technol. A10 (1992) 1483. [18] K. Zakrzewska, A. Brudnik, M. Radecka, W. Posadowski, Thin Solid Films 343/344 (1999) 152. [19] D. Wicaksana, A. Kobayashi, A. Kinbara, J. Vac. Sci. Technol. A10 (1992) 1479.
[20] R. Roy, W.B. White, J. Cryst. Growth 13/14 (1972) 78. [21] J.F. Moudler, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer, Eden Praine, MN, 1992. [22] F. Zhang, G.K. Wolf, X. Wang, X. Liu, Surf. Coatings Technol. 148 (2001) 65. [23] M. Nicol, M.Y. Fong, J. Chem. Phys. 54 (1971) 3167. [24] L. Escobar-Alarcon, E. Haro-Poniatowski, M.A. CamachoLopez, M. Fernandez-Guasti, J. Jimenez-Jarquin, A. SanchezPineda, Appl. Surf. Sci. 137 (1999) 38. [25] H. Tang, K. Prasad, R. Sanjines, P.E. Schmid, F. Levy, J. Appl. Phys. 75 (1994) 2042. [26] T. Kamada, M. Kitagawa, M. Shibuya, T. Hirabo, Jpn. J. Appl. Phys., Part 1 30 (1991) 3594. [27] R. Islam, H.D. Banerjee, D.R. Rao, Thin Solid Films 266 (1995) 215. [28] J.M. Bennett, E. Pelletier, G. Albrand, J.P. Borgogno, B. Lazarides, C.K. Carniglia, R.A. Schmell, T.H. Allen, T. Tuttle-Hart, K.H. Guenther, A. Saxer, Appl. Opt. 28 (1989) 3303. [29] L.G.J. De Haart, G. Blasse, J. Solid State Chem. 61 (1986) 135. [30] H. Tang, H. Berger, P.E. Schmid, F. Levy, G. Burri, Solid State Commun. 87 (1993) 847.
Surface and optical properties of nanocrystalline anatase titania films grown by radio frequency reactive magnetron sputtering Fa-Min Liu*, Tian-Min Wang Center of Material Physics and Chemistry, School of Science, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China Received 6 April 2002; received in revised form 3 June 2002; accepted 3 June 2002
Abstract Nanocrystalline anatase TiO2 films were deposited on glass and silicon substrates by using radio frequency reactive magnetron sputtering. Microstructural and optical properties were characterized by using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), Raman spectra, optical absorption and photoluminescence (PL). XRD and Raman spectroscopy detected a nanocrystalline anatase titania films for input power at 250 W, for argon/oxygen ratio at 50 sccm/ 5.0 sccm and for substrate temperature at 500 8C. Visible broad widened photoluminescence peak is interpreted as the emission of self-trapped excitons localized on TiO6 octahedra. # 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.-m; 78.40.-q; 78.30.-j; 74.25.Gz; 79.60.-i; 61.82.Rx; 61.10.-i Keywords: Nanocrystalline anatase TiO2 film; XRD; XPS; Raman spectra; Optical properties
1. Introduction It is well known that titania exhibits three crystal phase as naturally occurring minerals: rutile, anatase, and brookite. TiO6 octahedra are interconnected differently for each phase, leading to different structures and symmetries. Rutile is a tetragonal with lattice of a ¼ 0:4594 nm, c ¼ 0:2958 nm and with refraction index of 2.7. Anatase TiO2 is a tetragonal with lattice of a ¼ 0:3785 nm, c ¼ 0:9514 nm, with refraction index of 2.54 and with optical band gap of 3.2 eV at room temperature. Brookite is an orthorhombic * Corresponding author. Tel.: þ86-10-82317941; fax: þ86-10-82315933. E-mail addresses: [email protected], [email protected] (F.-M. Liu).
with lattice of a ¼ 0:9184 nm, b ¼ 0:5447, c ¼ 0:5145 nm [1]. Titanium dioxide is attracting much interest for its unique properties, such as high dielectric constant [2,3], high refractivity [4] and photocatalysis [5–7]. The occurrence of anatase and rutile phase depends significantly on the method and conditions of deposition as well as the substrate temperature [8,9]. Fabrication methods span a wide range and include such techniques as growth in sol–gel [10,11], chemical vapor deposition (CVD) [12], metal organic chemical vapor deposition (MOCVD) [13], reactive evaporation [4], pulsed laser deposition [14], and magnetron sputtering [15–17]. Among those techniques, reactive magnetron sputtering is one of the most applicable techniques to control the microstructure and the stoichiometry, with an advantage of large area deposition. So far, many works have been reported for
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 5 6 9 - X
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
various techniques to prepare TiO2 films and their properties. However, a few works deal with the surface properties and photoluminescence of nanocrystalline anatase TiO2 films. In this paper, nanocrystalline TiO2 films were deposited onto glass and silicon substrates by radio frequency reactive magnetron sputtering. Microstructure was characterized by X-ray diffraction. The surface properties of these films were investigated by using Raman scattering spectra and X-ray photoemission spectra. Visible photoluminescence is interpreted as the emission of self-trapped excitons localized on TiO6 octahedra.
2. Experimental procedure Nanocrystalline anatase TiO2 films were grown on glass and silicon substrates by radio frequency reactive magnetron sputtering at different Ar/O2 gas-flow ratio which are determined by mass-flow controlled regulators. A high purity (99.999%) Ti plate with 60 mm in diameter and 3 mm in thick was used as a target. Glass and silicon substrates (20 mm 20 mm 1 mm) was ultrasonically washed successively in acetone, alcohol, and deionized water to obtain a clean surface before being placed in a vacuum chamber. The target was separated from the substrate by 5 cm. When the chamber was evacuated to a pressure of 5 105 Pa, high purity mixing gas of argon (99.9999%) and oxygen (99.99%) were introduced. During sputtering, the chamber working pressure was maintained at 2 Pa. The power to the radio frequency sputter gun was 250 W. All runs started with inverse sputtering the target for 10 min before the high quality TiO2 films deposited. The film thickness d was determined by surface profilometry using a DEKTAK 3 Alpha-step instrument. In our experiment, the deposition rate was as large as 2.8 nm/min. The coil of electric resistance was used to heat the substrate ranging from 50 to 700 8C. The TiO2 films were characterized with X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy and Raman scattering. XRD studies were finished on a X-ray diffractometer (D/max-rB type) with Cu Ka radiation and a monochromator. The XPS measurements were carried out with a VG ESCALAB MK II spectrometer using a Mg Ka X-ray source with the analyzer mode set at a constant energy of 50 eV.
285
The X-ray source was run at 120 W and a takeoff angle of 808. All the peak energies in the spectra were referenced with respect to the C 1s (284.6 eV) peak. Room temperature Raman and photoluminescence spectra were recorded with an SPEX-1403 laser Raman spectrometer. The excitation source was an Arþ ion laser lines of 514.5 and 488.0 nm operated at 200 mW in the backscattering geometry for Raman scattering and photoluminescence, respectively. The resolution of this instrument was 1 cm1. The measured spectra have been corrected according to the system response. The optical measurements of transmission and absorption spectra, neglecting reflection losses, were performed on a Perkin-Elmer Lambda 9 double beam spectrophotometer. The wavelength was scanned over the range from 300 to 2500 nm with an accuracy of 1 nm at room temperature.
3. Results and discussion Fig. 1 shows X-ray diffraction patterns of nanocrystalline TiO2 film deposited at RF generator of 250 W, at substrate temperature of 500 8C and Ar/ O2 gas-flow ratio of 50 sccm/5.0 sccm. It appears five anatase typical peaks signed (1 0 1), (0 0 4), (2 1 1), (2 0 4), (2 2 0) and (2 1 5), which are basically in agreement with [18,19]. The mean size of TiO2 is about 13.5 nm estimated by Scherrer formula. Wicaksana et al. [19] studied that the oxygen mole fraction affects on the structures of the TiO2 films. They pointed out that the rutile phase was shown to grow easier in lower O2 mole fraction, and the anatase phase was shown at oxygen fraction of 40 and 50%. Roy and White [20] studied the phase diagram of Ti–O compound, and they reported that the rutile is formed at above 800 8C in equilibrium condition. In our experiment, the substrate was heated at 500 8C, it is still insufficient to grow the rutile phase. Figs. 2 and 3 show XPS Ti 2p and O 1s core levels of nanocrystalline TiO2 film. From Fig. 2, one can see that the core levels of Ti 2p1/2 and Ti 2p3/2 are at approximately 464.1 and 458.4 eV, respectively, which are assigned to the Ti4þ (TiO2), with a peak separation of 5.7 eV between those two peaks. Compared with [21,22], the core levels of Ti 2p1/2 and Ti 2p3/2 have a small shift of 0.1 eV. The core level of O 1s is at line of 529.7 eV from Fig. 3, which is attributed to titanium
286
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
Fig. 1. X-ray diffraction of nanocrystalline TiO2 film deposited on glass substrate by radio frequency reactive magnetron sputtering. The mean size of TiO2 is about 13.5 nm estimated by Scherrer formula.
dioxide at the surface. This data is approximately agreeable with the main peak at 529.9 eV of TiO2. It is well known that anatase TiO2 belongs to the space group D19 4h (I41amd) with two units formula per
units-cell and with six Raman active modes (A1g þ 2B1g þ 3Eg ) [23], whereas rutile TiO2 is tetragonal (D14 4h ) with two units and with four Raman active modes (A1g þ B1g þ B2g þ Eg ). The amorphous TiO2
Fig. 2. XPS Ti 2p core level of nanocrystalline TiO2 film.
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
287
Fig. 3. XPS O 1s core level of nanocrystalline TiO2 film.
is known to have no Raman peak [24]. Raman scattering spectra of nanocrystalline anatase TiO2 films are presented in Fig. 4. These films were deposited on glass substrate, at substrate temperature of 500 8C and at different Ar/O2 gas-flow ratio indicated. When Ar/O2 gas-flow ratio ranging from 50 sccm/4.0 sccm to 50 sccm/5.0 sccm, one can see that the four Raman scattering peaks of anatase TiO2 are at approximately 144, 400, 515 and 640 cm1, which are assigned two B1g modes and two Eg modes, respectively. These results are agreeable with anatase TiO2 films obtained by Tang et al. [25]. However, the two Eg modes are enhanced and widened when the Ar/O2 gas-flow ratio is at and over 50 sccm/6.0 sccm. In addition, one can see that small Raman peak appears at about 796.3 cm1. This demonstrates that excessive oxygen will affect the structure of TiO2 film. Another relatively strong Raman peak around 1100 cm1 is attributed to titanium–oxygen–silicon dioxide bond formation [26], indicating that there is a strong interaction at the interface between the TiO2 film and the SiO2 layer on the glass substrate. Fig. 5 shows optical absorption evolution of the TiO2 film with wavelength. The TiO2 film exhibits a sharp absorption edge at about 336.2 nm (3.687 eV). The fluctuation appearing in the curves were identified
to be due to the interference fringes of the film. According to the feature of equal thickness interference fringes, one can estimated the thickness of the film from the following formula [27]. t¼
l1 l 2 jnðl1 Þl2 nðl2 Þl1 j 2
(1)
where t represents the thickness of the film, l1 and l2 are the wavelength of the position of two maximum or minimum, respectively, and, n(l1) and n(l2) are the refractive index corresponding to l1 and l2, respectively. According to Fig. 5, the n(l1) and n(l2) were approximately taken [28] as 2.53 and 2.47 corresponding to 550 and 676 nm, respectively. And from formula (1), the thickness of the film is estimated about 528.5 nm, which is basically agreement with the data measured by using a DEKTAK3 Alpha-step instrument. Fig. 6 indicates photoluminescence of the TiO2 film excited by 488.0 nm line of Arþ laser at room temperature. The TiO2 film is deposited on silicon substrate, at Ar/O2 gas-flow ratio of 50 sccm/5.0 sccm and at substrate temperature of 500 8C. The thickness of the film is about 336 nm. From Fig. 6, one sees a broad band with the PL peak maximum at approximately 682.5 nm (1.82 eV) and with the full width at
288
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
Fig. 4. Raman scattering spectra of nanocrystalline TiO2 films deposited on glass substrate by radio frequency reactive magnetron sputtering at different Ar/O2 gas-flow ratio indicated.
Fig. 5. Optical absorption vs. wavelength.
F.-M. Liu, T.-M. Wang / Applied Surface Science 195 (2002) 284–290
289
Fig. 6. Photoluminescence of the TiO2 film excited by 488.0 nm line of Arþ laser at room temperature. The TiO2 film is deposited on silicon substrate by radio frequency reactive magnetron sputtering at RF generator of 250 W, at Ar/O2 gas-flow ratio of 50 sccm/5.0 sccm and substrate temperature of 500 8C.
half maximum (FWHM) of 155.3 nm (0.4 eV), indicating that there is no free exciton emission. De Haart et al. [29] studied the luminescence spectra of SrTiO3 and found broad emission bands, which can be interpreted as resulting from the fundamental Ti4þ to O2 charge transfer transition localized on the TiO6 octahedron. Then, Tang et al. [30] investigated the photoluminescence in TiO2 anatase single crystals and found visible broad band luminescence peaks, which was interpreted as the emission from the self-trapped excitons localized on TiO6 octahedra. For TiO2 anatase film, the visible broad PL peak (682.5 nm) also explained by the self-trapped excitons. These results suggest that excitons in anatase be in the self-trapped state, characterized by strong electron–photon coupling and small exciton band width [30]. In addition, the surface plasma resonance of the TiO2 nanoparticles also affects the symmetry of PL peak.
4. Conclusions In summary, we have grown nanocrystalline anatase TiO2 films on glass and silicon substrates by using radio frequency reactive magnetron sputtering. X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and Raman spectra show that it is a pure anatase phase film when Ar/O2 gas-flow ratio equalizing about
50 sccm/5.0 sccm and substrate temperature at 500 8C. And visible broad band photoluminescence is interpreted as the emission of self-trapped excitons localized on TiO6 octahedra.
Acknowledgements This paper was supported by the post-doctoral foundation of China, and partly supported by National Natural Science Foundation of China (No. 59982002) and the foundation of State Key Laboratory for Advanced Metals and Materials.
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