- Email: [email protected]
Microstructure and visible-photoluminescence of titanium dioxide thin films fabricated by dual cathodic arc and nitrogen plasma deposition
Surface & Coatings Technology 201 (2007) 4897 – 4900 www.elsevier.com/locate/surfcoat
Microstructure and visible-photoluminescence of titanium dioxide thin films fabricated by dual cathodic arc and nitrogen plasma deposition A.P. Huang, Z.F. Di, Paul K. Chu ⁎ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Available online 9 August 2006
Abstract We report the concurrent use of plasma nitridation with cathodic arc deposition to fabricate N-doped TiO2 thin films on Si (100) wafers. The microstructures and optical properties are investigated. Our results reveal that the incorporation of a small amount of nitrogen in TiO2 enhances its visible photoluminescence (PL) significantly giving rise to peaks located at 472, 488, and 548 nm. Our study suggests that plasma nitridation in conjunction with cathodic arc deposition is an effective method to introduce N into TiO2 and to improve the optical properties of the thin films. The effects and underlying mechanism are discussed in details. © 2006 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 52.77.Dq; 78.55.-m Keywords: TiO2; Microstructure; Photoluminescence
1. Introduction Titanium dioxide thin films have attracted both industrial and academic attention due to potential applications as high efficiency photo-catalysts, optical coatings, and opto-electronic devices. The materials possess favorable physical, chemical and opto-electrical properties and high chemical stability under ultraviolet (UV) light [1–3]. Unfortunately, a high intrinsic band gap of TiO2 (3.2 eV for the anatase structure) allows only absorption of the ultraviolet part of the solar irradiation, which amounts to ∼4% of the incoming solar energy on the earth's surface [4,5]. Therefore, modification of TiO2 to render it more sensitive to visible light is an important goal to increase the utility of TiO2 in the optical and energy industry [6]. Recently, a variety of techniques have been explored to fabricate doped TiO2 thin films, such as radio-frequency and magnetron sputtering [7], spray pyrolysis [8], chemical vapor deposition (CVD) [9], sol–gel process [10], pulse laser deposition (PLD) [11] and so on. However, it has been quite difficult to effectively dope and achieve photoluminescence from TiO2 thin films in the visible range. In this work, we report the concurrent use of plasma nitridation with cathodic arc deposition to fabricate N-doped TiO2 thin films ⁎ Corresponding author. Tel.: +852 27887724; fax: +852 27889549. E-mail address: [email protected] (P.K. Chu). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.113
on Si (100) wafers. The microstructure and optical properties were investigated. Our results reveal that the incorporation of a small amount of nitrogen in TiO2 enhances its visible photoluminescence significantly. Our study suggests that plasma nitridation in conjunction with cathodic arc deposition is an effective method to dope TiO2 with nitrogen and to improve the optical properties of the thin films. The effects and underlying mechanism of nitrogen are also discussed. 2. Experimental details The nitrogen-doped TiO2 thin films were fabricated on p-type, 100 mm Si (100) wafers with resistivity of 4–7 Ω·cm using a filtered cathodic arc system. The experimental apparatus used in this study mainly included a magnetic duct and cathodic arc plasma source. A curved magnetic duct was inserted between the plasma source and the main chamber to remove macro-particles produced in the cathodic arc plasma. The cathodes used in our experiments were 99.9% pure Ti rods with a diameter of 1 cm, and oxygen gas was bled into the arcing region. The arc was ignited within the pulse duration of about 300 μs and repetition rate of 60 Hz. The titanium discharge was controlled by the main arc current between the cathode and anode. The cathodic arc plasma comprising titanium and oxygen was guided into the vacuum chamber by an electromagnetic field applied to the curved duct.
4898
A.P. Huang et al. / Surface & Coatings Technology 201 (2007) 4897–4900
Fig. 2. Raman shifts of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
Fig. 1. RBS spectra acquired from the TiO2 deposited at 500 °C: (a) without and (b) with nitrogen.
The duct was biased to −20 V to establish a lateral electric field while the external solenoid coils wrapped around the duct produced the axial magnetic field with the magnitude of 100 G. The substrate temperature which was controlled by a heating assembly mounted below the stainless steel substrate holder was measured by a chromel–alumel thermocouple attached to the backside of the Si substrate. Before deposition, the samples that were positioned about 15 cm away from the exit of the plasma stream were cleaned by argon plasma for 2 min using a sample bias of −500 V. To introduce nitrogen into the films, nitrogen gas was bled into the vacuum chamber at the vicinity of the exit of the metal arc discharge plume. In this way, the cathodic arc plasma was mixed with nitrogen to form hybrid metal–gas plasma to produce the nitrogen-doped TiO2 thin films on the silicon substrates. The vacuum chamber was about 1 × 10− 5 Torr and RF power of 100 W was applied for a deposition time of 120 min. Rutherford backscattering spectrometry (RBS) was carried out using a 2 MeV 4He+ beam and a backscattering angle of 170° to determine the composition as well as thickness of the thin films. The micro-Raman spectra of the TiO2 thin film and nitridated sample were acquired in the back-scattering mode using a DILORyISA LabRAM 010 system equipped with an unpolarized HeNe laser. The excitation line wavelength was 632.8 nm and the
laser power was 6.4 mW. The microstructure of the thin films was determined by X-ray diffraction (XRD) using a Siemens D500/ 501 thin film diffractometer with a Cu Kα source. Contact mode atomic force microscopy (AFM) was conducted on a Park Scientific Instrument (PSI) Autoprobe Research System to evaluate the surface morphology over a scanned area of 2 μm × 2 μm. Ti, O and N bonding information was acquired using X-ray photoelectron spectroscopy (XPS) employing monochromatic Al Kα radiation. Prior to the analyses, the sample surface was cleaned by 4 keVAr ion bombardment for 1 min to remove atmospheric contaminants. The PL spectra were recorded at room temperature by a Jobin Yvon Ramanor and 350 nm line of the Xe lamp was used as the excitation laser. 3. Results and discussion The elemental composition of the thin film influences the structural and optical properties and RBS was used to characterize our materials [12]. Fig. 1(a) and (b) display the
Fig. 3. XRD spectra of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
A.P. Huang et al. / Surface & Coatings Technology 201 (2007) 4897–4900
4899
RBS spectra showing both the experimental and fitted results acquired from the samples with different working gases. The results indicate that the ratio of Ti to O in the thin films deposited with pure oxygen is almost stoichiometric and that the composition of the thin films is quite uniform throughout the thickness. Our good results are in part due to the effective elimination of Ti macro-particles by the curved magnetic filter. The process efficacy can be evaluated according to the simulation results of the titanium contents in the layer [13]. As nitrogen was introduced into the chamber, about 3% atomic N in the doped sample was found according to the fitted RBS data as shown in Fig. 1(b). It indicates that using plasma nitridation in conjunction with cathodic arc deposition, a small amount of N can be incorporated into the TiO2 thin film. Besides, it can clearly be observed from the RBS spectra that the thicknesses of the thin films produced by plasma nitridation decrease. Fig. 2(a) and (b) show the Raman scattering spectra obtained from the TiO2 film and nitridated sample. The Raman peak around 150 cm− 1 appears from the pure TiO2 thin film, whereas only two Si Raman peaks at 305 cm− 1 and 521 cm− 1 can be
Fig. 5. Photoluminescence spectra of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
observed from the nitridated sample [14]. It is well known that anatase TiO2 belongs to the space group D4h (I41amd) with two units' formula per units-cell and with six Raman active modes,
Fig. 4. AFM images of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
Fig. 6. N1s core level XPS spectrum of the nitridated sample.
4900
A.P. Huang et al. / Surface & Coatings Technology 201 (2007) 4897–4900
which correspond to the Raman shifts at about 143 cm− 1, 169 cm− 1, 326 cm− 1, 392 cm− 1, 510 cm− 1, 633 cm− 1 and 789 cm− 1, respectively [15]. Among these peaks, the intensity of the Raman shifts at about 143 cm− 1 is the highest, whereas the amorphous TiO2 is known to have no Raman peak [16]. The results suggest that as nitrogen is introduced, the microstructure of the TiO2 thin film is modified at 500 °C. Fig. 3 shows the XRD results of the above samples. Diffraction peaks of pure TiO2 film at 25.28° and 55.06° emerge corresponding respectively to the (101) and (211) planes of the anatase TiO2 phase [17], and the nitridated sample exhibits no diffraction peaks signifying an amorphous phase, which is consistent with the Raman results. It means that a small amount of doped N can transform the microstructure of the thin film from the anatase to amorphous phase. The surface morphology and roughness of the thin films were assessed by atomic force microscopy (AFM). Fig. 4 depicts the AFM images of representative samples prepared with different working gases. Regular particles on the thin films are clearly seen in Fig. 4(a). With the addition of nitrogen, an amorphous morphology emerges as shown in Fig. 4(b). The surface roughness of the nitrogen-doped thin film decreases substantially and the particle size of the thin films is uniform. It further corroborates that the microstructure of the thin films can be transformed by addition of nitrogen during deposition. The effects of doped-nitrogen on the optical properties were investigated and the photoluminescence (PL) spectra of the TiO2 thin film and nitridated sample excited by the 350 nm line of the Xe lamp at room temperature are shown in Fig. 5(a) and (b), respectively. As shown in Fig. 5(a), only a strong PL band at 390 nm can be observed which can be attributed to the intrinsic emission of the TiO2 thin film. On the other hand, after a small amount of nitrogen has been introduced into the TiO2 thin film, a broad PL band centered at about 500 nm appears in the visible range as shown in Fig. 5 (b) [18]. The best Gaussian fit of the PL band gives three peaks situated at 472 nm, 488 nm, and 548 nm, respectively. Our results show the visible photoluminescence of TiO2 thin film can be enhanced significantly with plasma nitridation. To further fathom the effects of plasma nitridation on the microstructure and optical properties of the TiO2 thin films and to evaluate the N concentration, x, in the TiO2−xNx thin film, the film composition and Ti, O and N bonding information were determined by XPS. The results show that x is 0.165 in the Ndoped TiO2 thin film, which agrees with the RBS results. According to the report of Asahi et al. about sputtered nitrogendoped titanium oxides [19], slight N doping can change the crystal structure and optical properties of TiO2. Fig. 6 depicts the N1s core level XPS spectrum of the nitridated sample which has apparent asymmetry and gives two peaks located around 396 eV and 400 eV, respectively. It has been reported by Saha et al. that the N1s XPS peak at 396 eV arises from atomic β-N for substitutional N and those at 400 eV and 402 eV are molecular chemisorbed N–N or N–O for interstitial N [20]. In fact, the N atoms can occupy three potential positions, substitutional, interstitial, and both. Such molecular species are well screened
and can hardly interact with the band states of the TiO2 thin films. According to the recent report by Lee et al., the substitutional N in TiO2 might induce an N 2p states isolated above the valence-band maximum of TiO2 [21], which can result in the improvement of the visible photoluminescence observed in our experiments. Hence, it can be concluded that plasma nitridation in conjunction with cathodic arc deposition is an effective method to dope TiO2 with nitrogen in order to improve the PL properties in the visible range. 4. Conclusion TiO2 thin films exhibiting visible photoluminescence have been deposited using a dual cathodic arc and nitrogen plasma. The incorporation of N into TiO2 transforms the microstructure, and the optical properties of the thin films are substantially improved. Our study suggests that plasma nitridation in conjunction with cathodic arc deposition is an effective method to dope TiO2 with nitrogen and to improve the PL properties in the visible range. The new materials have high potential in photocatalysis and optoelectronics. Acknowledgments Our work was supported by City University of Hong Kong Direct Allocation Grant No. 9360110. The authors acknowledge SP Wong and WY Cheung of the Chinese University of Hong Kong for the RBS analysis. References [1] Fujishima, K. Honda, Nature (Lond.) 238 (1972) 37. [2] A.K. Ghosh, H.P. Maruska, J. Electrochem. Soc. 124 (1977) 1516. [3] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454. [4] Y.X. Zhang, G.H. Li, Y.X. Jin, Y. Zhang, J. Zhang, L.D. Zhang, Chem. Phys. Lett. 365 (2002) 300. [5] Y. Nakano, T. Morikawa, T. Ohwaki, Y. Taga, Appl. Phys. Lett. 86 (2005) 132104. [6] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243. [7] K. Okimura, Surf. Coat. Technol. 135 (2001) 286. [8] G. San Vicente, A. Morales, M.T. Gutierrez, Thin Solid Films 391 (2000) 133. [9] M.D. Wiggins, M.C. Nelson, C.R. Aita, J. Vac. Sci. Technol., A 14 (1996) 772. [10] Q. Fan, B. McQuillin, A.K. Ray, M.L. Turner, A.B. Seddon, J. Phys., D, Appl. Phys. 33 (2000) 2683. [11] M.P. Moret, R. Zallen, D.P. Vijay, S.B. Desu, Thin Solid Films 366 (2000) 8. [12] C. Jeynes, Z.H. Jafri, R.P. Webb, A.C. Kimber, M.J. Ashwin, Surf. Interface Anal. 25 (1997) 254. [13] R.K.Y. Fu, Y.F. Mei, L.R. Shen, G.G. Siu, P.K. Chu, W.Y. Cheung, S.P. Wong, Surf. Coat. Technol. 186 (2004) 112. [14] I.H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [15] M. Nicol, M.Y. Fong, J. Chem. Phys. 54 (1971) 3167. [16] L.E. Alarcon, E.H. Poniatowski, M.A. Camacho-lopez, M.F. Guasti, J.J. Jarquin, A.S. Pineda, Appl. Surf. Sci. 137 (1999) 38. [17] JCDS Powder Diffraction File Cards (1994), 21-1272 (TiO2-anatase). [18] M. Watanabe, S. Sasaki, T. Hayashi, J. Lumin. 87 (2000) 1234. [19] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [20] N.C. Saha, H.G. Tompkins, J. Appl. Phys. 72 (1992) 3072. [21] J.Y. Lee, J. Park, J.H. Cho, Appl. Phys. Lett. 87 (2005) 011904.
Microstructure and visible-photoluminescence of titanium dioxide thin films fabricated by dual cathodic arc and nitrogen plasma deposition A.P. Huang, Z.F. Di, Paul K. Chu ⁎ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Available online 9 August 2006
Abstract We report the concurrent use of plasma nitridation with cathodic arc deposition to fabricate N-doped TiO2 thin films on Si (100) wafers. The microstructures and optical properties are investigated. Our results reveal that the incorporation of a small amount of nitrogen in TiO2 enhances its visible photoluminescence (PL) significantly giving rise to peaks located at 472, 488, and 548 nm. Our study suggests that plasma nitridation in conjunction with cathodic arc deposition is an effective method to introduce N into TiO2 and to improve the optical properties of the thin films. The effects and underlying mechanism are discussed in details. © 2006 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 52.77.Dq; 78.55.-m Keywords: TiO2; Microstructure; Photoluminescence
1. Introduction Titanium dioxide thin films have attracted both industrial and academic attention due to potential applications as high efficiency photo-catalysts, optical coatings, and opto-electronic devices. The materials possess favorable physical, chemical and opto-electrical properties and high chemical stability under ultraviolet (UV) light [1–3]. Unfortunately, a high intrinsic band gap of TiO2 (3.2 eV for the anatase structure) allows only absorption of the ultraviolet part of the solar irradiation, which amounts to ∼4% of the incoming solar energy on the earth's surface [4,5]. Therefore, modification of TiO2 to render it more sensitive to visible light is an important goal to increase the utility of TiO2 in the optical and energy industry [6]. Recently, a variety of techniques have been explored to fabricate doped TiO2 thin films, such as radio-frequency and magnetron sputtering [7], spray pyrolysis [8], chemical vapor deposition (CVD) [9], sol–gel process [10], pulse laser deposition (PLD) [11] and so on. However, it has been quite difficult to effectively dope and achieve photoluminescence from TiO2 thin films in the visible range. In this work, we report the concurrent use of plasma nitridation with cathodic arc deposition to fabricate N-doped TiO2 thin films ⁎ Corresponding author. Tel.: +852 27887724; fax: +852 27889549. E-mail address: [email protected] (P.K. Chu). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.113
on Si (100) wafers. The microstructure and optical properties were investigated. Our results reveal that the incorporation of a small amount of nitrogen in TiO2 enhances its visible photoluminescence significantly. Our study suggests that plasma nitridation in conjunction with cathodic arc deposition is an effective method to dope TiO2 with nitrogen and to improve the optical properties of the thin films. The effects and underlying mechanism of nitrogen are also discussed. 2. Experimental details The nitrogen-doped TiO2 thin films were fabricated on p-type, 100 mm Si (100) wafers with resistivity of 4–7 Ω·cm using a filtered cathodic arc system. The experimental apparatus used in this study mainly included a magnetic duct and cathodic arc plasma source. A curved magnetic duct was inserted between the plasma source and the main chamber to remove macro-particles produced in the cathodic arc plasma. The cathodes used in our experiments were 99.9% pure Ti rods with a diameter of 1 cm, and oxygen gas was bled into the arcing region. The arc was ignited within the pulse duration of about 300 μs and repetition rate of 60 Hz. The titanium discharge was controlled by the main arc current between the cathode and anode. The cathodic arc plasma comprising titanium and oxygen was guided into the vacuum chamber by an electromagnetic field applied to the curved duct.
4898
A.P. Huang et al. / Surface & Coatings Technology 201 (2007) 4897–4900
Fig. 2. Raman shifts of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
Fig. 1. RBS spectra acquired from the TiO2 deposited at 500 °C: (a) without and (b) with nitrogen.
The duct was biased to −20 V to establish a lateral electric field while the external solenoid coils wrapped around the duct produced the axial magnetic field with the magnitude of 100 G. The substrate temperature which was controlled by a heating assembly mounted below the stainless steel substrate holder was measured by a chromel–alumel thermocouple attached to the backside of the Si substrate. Before deposition, the samples that were positioned about 15 cm away from the exit of the plasma stream were cleaned by argon plasma for 2 min using a sample bias of −500 V. To introduce nitrogen into the films, nitrogen gas was bled into the vacuum chamber at the vicinity of the exit of the metal arc discharge plume. In this way, the cathodic arc plasma was mixed with nitrogen to form hybrid metal–gas plasma to produce the nitrogen-doped TiO2 thin films on the silicon substrates. The vacuum chamber was about 1 × 10− 5 Torr and RF power of 100 W was applied for a deposition time of 120 min. Rutherford backscattering spectrometry (RBS) was carried out using a 2 MeV 4He+ beam and a backscattering angle of 170° to determine the composition as well as thickness of the thin films. The micro-Raman spectra of the TiO2 thin film and nitridated sample were acquired in the back-scattering mode using a DILORyISA LabRAM 010 system equipped with an unpolarized HeNe laser. The excitation line wavelength was 632.8 nm and the
laser power was 6.4 mW. The microstructure of the thin films was determined by X-ray diffraction (XRD) using a Siemens D500/ 501 thin film diffractometer with a Cu Kα source. Contact mode atomic force microscopy (AFM) was conducted on a Park Scientific Instrument (PSI) Autoprobe Research System to evaluate the surface morphology over a scanned area of 2 μm × 2 μm. Ti, O and N bonding information was acquired using X-ray photoelectron spectroscopy (XPS) employing monochromatic Al Kα radiation. Prior to the analyses, the sample surface was cleaned by 4 keVAr ion bombardment for 1 min to remove atmospheric contaminants. The PL spectra were recorded at room temperature by a Jobin Yvon Ramanor and 350 nm line of the Xe lamp was used as the excitation laser. 3. Results and discussion The elemental composition of the thin film influences the structural and optical properties and RBS was used to characterize our materials [12]. Fig. 1(a) and (b) display the
Fig. 3. XRD spectra of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
A.P. Huang et al. / Surface & Coatings Technology 201 (2007) 4897–4900
4899
RBS spectra showing both the experimental and fitted results acquired from the samples with different working gases. The results indicate that the ratio of Ti to O in the thin films deposited with pure oxygen is almost stoichiometric and that the composition of the thin films is quite uniform throughout the thickness. Our good results are in part due to the effective elimination of Ti macro-particles by the curved magnetic filter. The process efficacy can be evaluated according to the simulation results of the titanium contents in the layer [13]. As nitrogen was introduced into the chamber, about 3% atomic N in the doped sample was found according to the fitted RBS data as shown in Fig. 1(b). It indicates that using plasma nitridation in conjunction with cathodic arc deposition, a small amount of N can be incorporated into the TiO2 thin film. Besides, it can clearly be observed from the RBS spectra that the thicknesses of the thin films produced by plasma nitridation decrease. Fig. 2(a) and (b) show the Raman scattering spectra obtained from the TiO2 film and nitridated sample. The Raman peak around 150 cm− 1 appears from the pure TiO2 thin film, whereas only two Si Raman peaks at 305 cm− 1 and 521 cm− 1 can be
Fig. 5. Photoluminescence spectra of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
observed from the nitridated sample [14]. It is well known that anatase TiO2 belongs to the space group D4h (I41amd) with two units' formula per units-cell and with six Raman active modes,
Fig. 4. AFM images of TiO2 thin films prepared at 500 °C: (a) pure TiO2 and (b) nitridated sample.
Fig. 6. N1s core level XPS spectrum of the nitridated sample.
4900
A.P. Huang et al. / Surface & Coatings Technology 201 (2007) 4897–4900
which correspond to the Raman shifts at about 143 cm− 1, 169 cm− 1, 326 cm− 1, 392 cm− 1, 510 cm− 1, 633 cm− 1 and 789 cm− 1, respectively [15]. Among these peaks, the intensity of the Raman shifts at about 143 cm− 1 is the highest, whereas the amorphous TiO2 is known to have no Raman peak [16]. The results suggest that as nitrogen is introduced, the microstructure of the TiO2 thin film is modified at 500 °C. Fig. 3 shows the XRD results of the above samples. Diffraction peaks of pure TiO2 film at 25.28° and 55.06° emerge corresponding respectively to the (101) and (211) planes of the anatase TiO2 phase [17], and the nitridated sample exhibits no diffraction peaks signifying an amorphous phase, which is consistent with the Raman results. It means that a small amount of doped N can transform the microstructure of the thin film from the anatase to amorphous phase. The surface morphology and roughness of the thin films were assessed by atomic force microscopy (AFM). Fig. 4 depicts the AFM images of representative samples prepared with different working gases. Regular particles on the thin films are clearly seen in Fig. 4(a). With the addition of nitrogen, an amorphous morphology emerges as shown in Fig. 4(b). The surface roughness of the nitrogen-doped thin film decreases substantially and the particle size of the thin films is uniform. It further corroborates that the microstructure of the thin films can be transformed by addition of nitrogen during deposition. The effects of doped-nitrogen on the optical properties were investigated and the photoluminescence (PL) spectra of the TiO2 thin film and nitridated sample excited by the 350 nm line of the Xe lamp at room temperature are shown in Fig. 5(a) and (b), respectively. As shown in Fig. 5(a), only a strong PL band at 390 nm can be observed which can be attributed to the intrinsic emission of the TiO2 thin film. On the other hand, after a small amount of nitrogen has been introduced into the TiO2 thin film, a broad PL band centered at about 500 nm appears in the visible range as shown in Fig. 5 (b) [18]. The best Gaussian fit of the PL band gives three peaks situated at 472 nm, 488 nm, and 548 nm, respectively. Our results show the visible photoluminescence of TiO2 thin film can be enhanced significantly with plasma nitridation. To further fathom the effects of plasma nitridation on the microstructure and optical properties of the TiO2 thin films and to evaluate the N concentration, x, in the TiO2−xNx thin film, the film composition and Ti, O and N bonding information were determined by XPS. The results show that x is 0.165 in the Ndoped TiO2 thin film, which agrees with the RBS results. According to the report of Asahi et al. about sputtered nitrogendoped titanium oxides [19], slight N doping can change the crystal structure and optical properties of TiO2. Fig. 6 depicts the N1s core level XPS spectrum of the nitridated sample which has apparent asymmetry and gives two peaks located around 396 eV and 400 eV, respectively. It has been reported by Saha et al. that the N1s XPS peak at 396 eV arises from atomic β-N for substitutional N and those at 400 eV and 402 eV are molecular chemisorbed N–N or N–O for interstitial N [20]. In fact, the N atoms can occupy three potential positions, substitutional, interstitial, and both. Such molecular species are well screened
and can hardly interact with the band states of the TiO2 thin films. According to the recent report by Lee et al., the substitutional N in TiO2 might induce an N 2p states isolated above the valence-band maximum of TiO2 [21], which can result in the improvement of the visible photoluminescence observed in our experiments. Hence, it can be concluded that plasma nitridation in conjunction with cathodic arc deposition is an effective method to dope TiO2 with nitrogen in order to improve the PL properties in the visible range. 4. Conclusion TiO2 thin films exhibiting visible photoluminescence have been deposited using a dual cathodic arc and nitrogen plasma. The incorporation of N into TiO2 transforms the microstructure, and the optical properties of the thin films are substantially improved. Our study suggests that plasma nitridation in conjunction with cathodic arc deposition is an effective method to dope TiO2 with nitrogen and to improve the PL properties in the visible range. The new materials have high potential in photocatalysis and optoelectronics. Acknowledgments Our work was supported by City University of Hong Kong Direct Allocation Grant No. 9360110. The authors acknowledge SP Wong and WY Cheung of the Chinese University of Hong Kong for the RBS analysis. References [1] Fujishima, K. Honda, Nature (Lond.) 238 (1972) 37. [2] A.K. Ghosh, H.P. Maruska, J. Electrochem. Soc. 124 (1977) 1516. [3] T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454. [4] Y.X. Zhang, G.H. Li, Y.X. Jin, Y. Zhang, J. Zhang, L.D. Zhang, Chem. Phys. Lett. 365 (2002) 300. [5] Y. Nakano, T. Morikawa, T. Ohwaki, Y. Taga, Appl. Phys. Lett. 86 (2005) 132104. [6] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243. [7] K. Okimura, Surf. Coat. Technol. 135 (2001) 286. [8] G. San Vicente, A. Morales, M.T. Gutierrez, Thin Solid Films 391 (2000) 133. [9] M.D. Wiggins, M.C. Nelson, C.R. Aita, J. Vac. Sci. Technol., A 14 (1996) 772. [10] Q. Fan, B. McQuillin, A.K. Ray, M.L. Turner, A.B. Seddon, J. Phys., D, Appl. Phys. 33 (2000) 2683. [11] M.P. Moret, R. Zallen, D.P. Vijay, S.B. Desu, Thin Solid Films 366 (2000) 8. [12] C. Jeynes, Z.H. Jafri, R.P. Webb, A.C. Kimber, M.J. Ashwin, Surf. Interface Anal. 25 (1997) 254. [13] R.K.Y. Fu, Y.F. Mei, L.R. Shen, G.G. Siu, P.K. Chu, W.Y. Cheung, S.P. Wong, Surf. Coat. Technol. 186 (2004) 112. [14] I.H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [15] M. Nicol, M.Y. Fong, J. Chem. Phys. 54 (1971) 3167. [16] L.E. Alarcon, E.H. Poniatowski, M.A. Camacho-lopez, M.F. Guasti, J.J. Jarquin, A.S. Pineda, Appl. Surf. Sci. 137 (1999) 38. [17] JCDS Powder Diffraction File Cards (1994), 21-1272 (TiO2-anatase). [18] M. Watanabe, S. Sasaki, T. Hayashi, J. Lumin. 87 (2000) 1234. [19] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [20] N.C. Saha, H.G. Tompkins, J. Appl. Phys. 72 (1992) 3072. [21] J.Y. Lee, J. Park, J.H. Cho, Appl. Phys. Lett. 87 (2005) 011904.