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CdS quantum dots sensitized TiO2 photoelectrodes
Materials Chemistry and Physics 117 (2009) 26–28
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
CdS quantum dots sensitized TiO2 photoelectrodes K. Prabakar ∗ , Hyunwoong Seo, Minkyu Son, Heeje Kim Pusan National University, Department of Electrical Engineering, San 30, Jangjeong-Dong, Gumjeong-Ku, Busan 609 735, South Korea
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 28 May 2009 Accepted 29 May 2009 Keywords: Semiconductors Thin films Electron microscopy Electrical characterization
a b s t r a c t The CdS quantum dots were deposited on nanoporous TiO2 by chemical bath deposition technique to absorb the visible light in the longer wavelength region and are used as photo electrode in the dye sensitized solar cells (DSSC). The CdS quantum dots deposited on nanoporous TiO2 at 1 min deposition time showed improved short circuit current density (Jsc ) and open circuit voltage (Voc ) than films deposited at higher dipping times. The increase in the CdS dipping time reduces the amount of dye adsorbed on nanoporous TiO2 which limits the redox reactions in the electrolyte. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSC) based on the photosensitization of nanocrystalline TiO2 semiconductor electrodes by adsorbed dyes have been studied extensively with respect to improvements of the power conversion efficiencies and relatively inexpensive fabrication procedure compared with conventional inorganic solar cells [1–3]. The efficiency of a DSSC is mainly determined by the short circuit current density (Jsc ) and the open circuit voltage (Voc ). The former depends on the light harvesting property of the dye attached to the surface of the TiO2 nanocrystallite [4] and the later is the subject of optimization studies by means of molecular engineering of the semiconductor electrolyte interface [5]. The short circuit current density could be improved by the sensitization of nanoporous TiO2 using CdS [6–8], CdSe [9], and PbSe [10] nanoparticle quantum dots owing to the high absorption cross-section and ability to tune the particles sizes within the visible and near-infrared spectral range. Another advantage of the sensitizers over conventional dyes is their high extinction coefficient, which is known to reduce the dark current and increase the overall efficiency of a solar cell [11]. However, the solar cells energy conversion efficiency is limited due to the difficulty of assembling the sensitizers into the nanoporous TiO2 matrix to obtain a well-covered monolayer and the charge carrier injection into the sensitizer from the excited dyes is limited. Chemical bath deposition (CBD) is the most common method used for the synthesis of the CdS on TiO2 [12]. In the CBD process used to synthesize the semiconductor nanoparticle quantum dots onto nanoporous TiO2 films, the TiO2 films were dipped into aqueous solutions of the reactants. In the present study, a coating of CdS
absorber layer thicknesses of 40–100 nm were deposited by our CBD method on filling the pores and report our results by means of a cell configuration made with a nanoporous TiO2 film, a thin coating of CdS absorber. 2. Experimental Fluorine doped tin oxide (FTO) having sheet resistances of 10 /square was used to make both the working and counter electrodes. The working photo electrode of the DSSC was made by pasting 5 mm by 5 mm (0.25 cm2 active area) nanoporous TiO2 of about 20 m thicknesses on FTO by doctor blade method followed by annealing at 450 ◦ C for 30 min and the final thickness was 8 m after the solvent evaporation. The CdS films of different thicknesses were deposited on the above nanoporous TiO2 electrodes by our earlier method [7]. Dye sensitization of the CdS coated nanoporous TiO2 electrodes was carried out by soaking in N719 dye (Ruthenium 535 bis-TBA cis–bis (isothiocyanato) bis (2,2 -bipyridyl-4,4 -dicarboxylato)-ruthenium(II) bis-tetrabutyl ammonium) purchased from Solaronix dissolved in anhydrous ethanol solution. On counter electrode, platinum of about 120 nm thicknesses used to be the catalysis were sputtered by radio frequency (RF) sputtering at a RF power of 150 W and a sputtering pressure of 2.8 × 10−2 Torr. Finally, DSSC was completed by injecting the electrolyte solution consisted of iodolyte AN-50 (iodide-based low viscosity electrolyte with 50 mM of tri-iodide in acetonitrile) through the counter electrode pin-hole. Photo anode and counter electrodes were sealed by using thermoplast hotmelt sealing sheet of 60 m thickness in order to avoid evaporation of electrolyte. Field emission scanning electron microscopy (FE-SEM, S-4200, Hitachi) operated at 15 kV was used to characterize the microstructure of the films. The current–voltage characteristics of the DSSCs were performed under 1 sun illumination (AM 1.5G, 100 mW cm−2 ) with San Ei Electric (XES 301S, Japan) solar simulator having the irradiance uniformity of ±3% and Keithley 2400 source meter. The intensity of the incident solar illumination was adjusted to 1 sun condition using AIST certified silicon reference cell. In order to avoid the contribution from non-active area, shadow mask with an aperture slightly larger than that of TiO2 active layer was used on the front side of DSSCs.
3. Results and discussion ∗ Corresponding author. Tel.: +82 51 510 7334; fax: +82 51 513 0212. E-mail address: [email protected] (K. Prabakar). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.05.056
The CdS films deposited on FTO–TiO2 substrates were highly adherent and yellowish orange in colour. Optical properties of CdS
K. Prabakar et al. / Materials Chemistry and Physics 117 (2009) 26–28
Fig. 1. UV–vis absorbance spectra of CdS films deposited on TiO2 at (A) 1 min, (B) 2 min, (C) 5 min, and (D) 10 min dipping time.
films were measured by UV–vis spectrophotometer in the wavelength range of 350–550 nm to evaluate the amount of the CdS incorporated in a TiO2 film. The variation of the absorbance spectra with the CBD dipping time is shown in Fig. 1 for the films deposited on the FTO–TiO2 nanoporous films sintered at 450 ◦ C. The absorbance spectra increase with an increase of CBD dipping time, indicating an increased adsorption amount of CdS. Furthermore, it is seen that the absorption edge shifts towards longer wavelength with increasing dipping times implies the growth of the CdS nanoparticles [12]. The adsorption amount of CdS is increased until 5 min dipping time; however, it is saturated when deposited more than 5 min. It indicates that up to 5 min dipping time, higher surface area is available for the adsorption, attributable to the better penetration and wetting of the solution in the nanoporous TiO2
27
matrix. The CdS films could be formed well into the inner region of the nanoporous film, leading to a better-covered layer on the TiO2 surface during the first 5 min dipping time. If the dipping time was more than 5 min, the uncovered surface area of the TiO2 film decreases as and hence the amount of CdS absorbed is almost saturated. Fig. 2 shows the SEM images of the (A) doctor blade coated TiO2 on FTO substrates and CdS films deposited on TiO2 at (B) 1, (C) 2 and (D) 10 min dipping times respectively. The images of TiO2 films before and after 1 min dipping of CdS film deposition showed the presence of nanoparticles evenly distributed all over the nanoporous TiO2 crystals and the overall surface area has remarkably increased without affecting the surface nature. However, as the CdS dipping time was more than 1 min, the TiO2 nanopores are completely filled by the CdS deposition and form a chain-like continuous surface structure after 2 min deposition. At the CdS dipping time of 10 min, cracks are formed on the surface and the nanoporous structures of the TiO2 are completely destroyed. The photocurrent–voltage (I–V) characteristics of the DSSCs prepared by the standard Gratzel cells (S) and CdS-sensitized at different deposition times on TiO2 photoelectrodes (A–D) are shown in Fig. 3. It is observed contrary to the expectation that, CdS sensitization as wells as dipping time decreases the overall conversion efficiencies of the dye sensitized solar cells. The higher Jsc and fill factor values for the CdS sensitized cells were obtained in which the CdS deposition times were 1 min. It indicates that as the dipping time increases more than 1 min, CdS thickness increases along with complete coverage on the TiO2 surface and higher amount of CdS incorporated on the TiO2 film, which in turn reduces the amount of dye adsorbed on the TiO2 surfaces and hence limits the light harvesting as well as the redox reaction. As a function of CBD dipping time, the maximum overall efficiency was for 1 min (A), and then decreases for further increase in the dipping time. The decrease in efficiency is mainly caused by the reduction of the fill factor due to the blocking of the nanopores by the additional loading of the CdS as is in agreement with the earlier report [13]. Even though 1 and 2 min CdS dipped cells showed improved efficiencies, the overall
Fig. 2. SEM images of (A) nanoporous TiO2 pastes, CdS films deposited on nanoporous TiO2 at (B) 1 min, (C) 2 min and (D) 10 min dipping time.
28
K. Prabakar et al. / Materials Chemistry and Physics 117 (2009) 26–28
of CdS as sensitizer layer on TiO2 have been investigated. With increase in the CdS films thickness over nanoporous TiO2 , the short circuit current density as well as the open circuit voltage are decreased. The decrease in efficiency is mainly caused by the reduction of the fill factor due to the blocking of the nanopores by the additional loading of the CdS. Therefore, it is suggested that CdS films deposition should be carried while the nanoporous TiO2 were soaked in the dye in order to have optimized amount of the sensitizer as well the dye to increase the redox reaction. Acknowledgements
Fig. 3. I–V characterization of (S) standard Gratzel cell and CdS films sensitized on TiO2 at (A) 1 min, (B) 2 min, (C) 5 min and (D) 10 min dipping time.
The author K. Prabakar would like to thank the Pusan National University for the financial support and the grand-in-aid for basic research. The authors are grateful to Professor Young-Son Choe for the I–V characterization. References
performance of the cells were not increased significantly. It may be because of the limitation by the diffusion length of electrons along the longer pathways of the CdS phase. Another reason could be the increase of electron recombination at the interface between the nanoporous TiO2 and CdS films due to the increase of the interface pathway along the film and the open circuit voltage is greatly reduced [14]. It is evidenced from SEM measurement that at 1 min CdS dipped films, higher surface area was evidenced and further increase in the dipping time reduces the surface to be available for the dyes adsorption. At 10 min dipping time (D), the cells did not show standard I–V characterization because of the poor surface nature. From the experimental evidence, it is proposed that CdS sensitization should be done during the soaking of TiO2 films in the dyes to improve the cell efficiencies. If the CdS are deposited prior to soaking, less amount of dyes are adsorbed over TiO2 surfaces and the over all cell performance would be decreased due to either the increased surface recombination or the reduced redox reaction. 4. Conclusion In order to increase the incident photon to current conversion efficiency (IPCE) of the dye sensitized solar cells, the effects
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
B. O’Regan, M. Gratzel, Nature 353 (1991) 353. M. Gratzel, Nature 414 (2001) 338. A. Kay, M. Gratzel, Sol. Energy Mater. Sol. Cells 44 (1996) 99. Y. Chiba, A. Islam, R. Komiya, N. Koide, L. Han, Appl. Phys. Lett. 88 (2006) 223505. M. Nazeeruddin, R.H. Baker, P. Liska, M. Gradzel, J. Phys. Chem. B 107 (2003) 8981. G. Larramona, C. Chone, A. Jacob, D. Sakakura, B. Delatouche, D. Pere, X. Cieren, M. Nagino, R. Bayon, Chem. Mater. 18 (2006) 1688. K. Prabakar, T. Takahashi, T. Nakashima, Y. Kubota, A. Fujishima, J. Vac. Sci. Technol. A 24 (2006) 1613. K. Prabakar, T. Takahashi, T. Nezuka, K. Takahashi, Y. Kuboto, T. Nakashima, A. Fujishima, J. Vac. Sci. Technol. A 25 (2007) 1188. I.M. Sero, J. Bisquert, T.H. Dittrich, A. Belaidi, A.S. Susha, A.L. Rogach, J. Phys. Chem. C 111 (2007) 14889. R.D. Schaller, M. Sykora, J.M. Pietryga, V.I. Klimov, Nano Lett. 6 (2006) 424. P. Wang, C. Klein, R.H. Baker, S.M. Zakeeruddin, M. Gratzel, J. Am. Chem. Soc. 17 (2005) 808. S.-C. Lin, Y.-L. Lee, C.-H. Chang, Y.-J. Shen, Y.-M. Yang, Appl. Phys. Lett. 90 (2007) 143517. C.-H. Chang, Y.-L. Lee, Appl. Phys. Lett. 91 (2007) 53503. J. Xia, N. Masaki, K. Jiang, S. Yanagida, Chem. Commun. 138 (2007) 138.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
CdS quantum dots sensitized TiO2 photoelectrodes K. Prabakar ∗ , Hyunwoong Seo, Minkyu Son, Heeje Kim Pusan National University, Department of Electrical Engineering, San 30, Jangjeong-Dong, Gumjeong-Ku, Busan 609 735, South Korea
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 28 May 2009 Accepted 29 May 2009 Keywords: Semiconductors Thin films Electron microscopy Electrical characterization
a b s t r a c t The CdS quantum dots were deposited on nanoporous TiO2 by chemical bath deposition technique to absorb the visible light in the longer wavelength region and are used as photo electrode in the dye sensitized solar cells (DSSC). The CdS quantum dots deposited on nanoporous TiO2 at 1 min deposition time showed improved short circuit current density (Jsc ) and open circuit voltage (Voc ) than films deposited at higher dipping times. The increase in the CdS dipping time reduces the amount of dye adsorbed on nanoporous TiO2 which limits the redox reactions in the electrolyte. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSC) based on the photosensitization of nanocrystalline TiO2 semiconductor electrodes by adsorbed dyes have been studied extensively with respect to improvements of the power conversion efficiencies and relatively inexpensive fabrication procedure compared with conventional inorganic solar cells [1–3]. The efficiency of a DSSC is mainly determined by the short circuit current density (Jsc ) and the open circuit voltage (Voc ). The former depends on the light harvesting property of the dye attached to the surface of the TiO2 nanocrystallite [4] and the later is the subject of optimization studies by means of molecular engineering of the semiconductor electrolyte interface [5]. The short circuit current density could be improved by the sensitization of nanoporous TiO2 using CdS [6–8], CdSe [9], and PbSe [10] nanoparticle quantum dots owing to the high absorption cross-section and ability to tune the particles sizes within the visible and near-infrared spectral range. Another advantage of the sensitizers over conventional dyes is their high extinction coefficient, which is known to reduce the dark current and increase the overall efficiency of a solar cell [11]. However, the solar cells energy conversion efficiency is limited due to the difficulty of assembling the sensitizers into the nanoporous TiO2 matrix to obtain a well-covered monolayer and the charge carrier injection into the sensitizer from the excited dyes is limited. Chemical bath deposition (CBD) is the most common method used for the synthesis of the CdS on TiO2 [12]. In the CBD process used to synthesize the semiconductor nanoparticle quantum dots onto nanoporous TiO2 films, the TiO2 films were dipped into aqueous solutions of the reactants. In the present study, a coating of CdS
absorber layer thicknesses of 40–100 nm were deposited by our CBD method on filling the pores and report our results by means of a cell configuration made with a nanoporous TiO2 film, a thin coating of CdS absorber. 2. Experimental Fluorine doped tin oxide (FTO) having sheet resistances of 10 /square was used to make both the working and counter electrodes. The working photo electrode of the DSSC was made by pasting 5 mm by 5 mm (0.25 cm2 active area) nanoporous TiO2 of about 20 m thicknesses on FTO by doctor blade method followed by annealing at 450 ◦ C for 30 min and the final thickness was 8 m after the solvent evaporation. The CdS films of different thicknesses were deposited on the above nanoporous TiO2 electrodes by our earlier method [7]. Dye sensitization of the CdS coated nanoporous TiO2 electrodes was carried out by soaking in N719 dye (Ruthenium 535 bis-TBA cis–bis (isothiocyanato) bis (2,2 -bipyridyl-4,4 -dicarboxylato)-ruthenium(II) bis-tetrabutyl ammonium) purchased from Solaronix dissolved in anhydrous ethanol solution. On counter electrode, platinum of about 120 nm thicknesses used to be the catalysis were sputtered by radio frequency (RF) sputtering at a RF power of 150 W and a sputtering pressure of 2.8 × 10−2 Torr. Finally, DSSC was completed by injecting the electrolyte solution consisted of iodolyte AN-50 (iodide-based low viscosity electrolyte with 50 mM of tri-iodide in acetonitrile) through the counter electrode pin-hole. Photo anode and counter electrodes were sealed by using thermoplast hotmelt sealing sheet of 60 m thickness in order to avoid evaporation of electrolyte. Field emission scanning electron microscopy (FE-SEM, S-4200, Hitachi) operated at 15 kV was used to characterize the microstructure of the films. The current–voltage characteristics of the DSSCs were performed under 1 sun illumination (AM 1.5G, 100 mW cm−2 ) with San Ei Electric (XES 301S, Japan) solar simulator having the irradiance uniformity of ±3% and Keithley 2400 source meter. The intensity of the incident solar illumination was adjusted to 1 sun condition using AIST certified silicon reference cell. In order to avoid the contribution from non-active area, shadow mask with an aperture slightly larger than that of TiO2 active layer was used on the front side of DSSCs.
3. Results and discussion ∗ Corresponding author. Tel.: +82 51 510 7334; fax: +82 51 513 0212. E-mail address: [email protected] (K. Prabakar). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.05.056
The CdS films deposited on FTO–TiO2 substrates were highly adherent and yellowish orange in colour. Optical properties of CdS
K. Prabakar et al. / Materials Chemistry and Physics 117 (2009) 26–28
Fig. 1. UV–vis absorbance spectra of CdS films deposited on TiO2 at (A) 1 min, (B) 2 min, (C) 5 min, and (D) 10 min dipping time.
films were measured by UV–vis spectrophotometer in the wavelength range of 350–550 nm to evaluate the amount of the CdS incorporated in a TiO2 film. The variation of the absorbance spectra with the CBD dipping time is shown in Fig. 1 for the films deposited on the FTO–TiO2 nanoporous films sintered at 450 ◦ C. The absorbance spectra increase with an increase of CBD dipping time, indicating an increased adsorption amount of CdS. Furthermore, it is seen that the absorption edge shifts towards longer wavelength with increasing dipping times implies the growth of the CdS nanoparticles [12]. The adsorption amount of CdS is increased until 5 min dipping time; however, it is saturated when deposited more than 5 min. It indicates that up to 5 min dipping time, higher surface area is available for the adsorption, attributable to the better penetration and wetting of the solution in the nanoporous TiO2
27
matrix. The CdS films could be formed well into the inner region of the nanoporous film, leading to a better-covered layer on the TiO2 surface during the first 5 min dipping time. If the dipping time was more than 5 min, the uncovered surface area of the TiO2 film decreases as and hence the amount of CdS absorbed is almost saturated. Fig. 2 shows the SEM images of the (A) doctor blade coated TiO2 on FTO substrates and CdS films deposited on TiO2 at (B) 1, (C) 2 and (D) 10 min dipping times respectively. The images of TiO2 films before and after 1 min dipping of CdS film deposition showed the presence of nanoparticles evenly distributed all over the nanoporous TiO2 crystals and the overall surface area has remarkably increased without affecting the surface nature. However, as the CdS dipping time was more than 1 min, the TiO2 nanopores are completely filled by the CdS deposition and form a chain-like continuous surface structure after 2 min deposition. At the CdS dipping time of 10 min, cracks are formed on the surface and the nanoporous structures of the TiO2 are completely destroyed. The photocurrent–voltage (I–V) characteristics of the DSSCs prepared by the standard Gratzel cells (S) and CdS-sensitized at different deposition times on TiO2 photoelectrodes (A–D) are shown in Fig. 3. It is observed contrary to the expectation that, CdS sensitization as wells as dipping time decreases the overall conversion efficiencies of the dye sensitized solar cells. The higher Jsc and fill factor values for the CdS sensitized cells were obtained in which the CdS deposition times were 1 min. It indicates that as the dipping time increases more than 1 min, CdS thickness increases along with complete coverage on the TiO2 surface and higher amount of CdS incorporated on the TiO2 film, which in turn reduces the amount of dye adsorbed on the TiO2 surfaces and hence limits the light harvesting as well as the redox reaction. As a function of CBD dipping time, the maximum overall efficiency was for 1 min (A), and then decreases for further increase in the dipping time. The decrease in efficiency is mainly caused by the reduction of the fill factor due to the blocking of the nanopores by the additional loading of the CdS as is in agreement with the earlier report [13]. Even though 1 and 2 min CdS dipped cells showed improved efficiencies, the overall
Fig. 2. SEM images of (A) nanoporous TiO2 pastes, CdS films deposited on nanoporous TiO2 at (B) 1 min, (C) 2 min and (D) 10 min dipping time.
28
K. Prabakar et al. / Materials Chemistry and Physics 117 (2009) 26–28
of CdS as sensitizer layer on TiO2 have been investigated. With increase in the CdS films thickness over nanoporous TiO2 , the short circuit current density as well as the open circuit voltage are decreased. The decrease in efficiency is mainly caused by the reduction of the fill factor due to the blocking of the nanopores by the additional loading of the CdS. Therefore, it is suggested that CdS films deposition should be carried while the nanoporous TiO2 were soaked in the dye in order to have optimized amount of the sensitizer as well the dye to increase the redox reaction. Acknowledgements
Fig. 3. I–V characterization of (S) standard Gratzel cell and CdS films sensitized on TiO2 at (A) 1 min, (B) 2 min, (C) 5 min and (D) 10 min dipping time.
The author K. Prabakar would like to thank the Pusan National University for the financial support and the grand-in-aid for basic research. The authors are grateful to Professor Young-Son Choe for the I–V characterization. References
performance of the cells were not increased significantly. It may be because of the limitation by the diffusion length of electrons along the longer pathways of the CdS phase. Another reason could be the increase of electron recombination at the interface between the nanoporous TiO2 and CdS films due to the increase of the interface pathway along the film and the open circuit voltage is greatly reduced [14]. It is evidenced from SEM measurement that at 1 min CdS dipped films, higher surface area was evidenced and further increase in the dipping time reduces the surface to be available for the dyes adsorption. At 10 min dipping time (D), the cells did not show standard I–V characterization because of the poor surface nature. From the experimental evidence, it is proposed that CdS sensitization should be done during the soaking of TiO2 films in the dyes to improve the cell efficiencies. If the CdS are deposited prior to soaking, less amount of dyes are adsorbed over TiO2 surfaces and the over all cell performance would be decreased due to either the increased surface recombination or the reduced redox reaction. 4. Conclusion In order to increase the incident photon to current conversion efficiency (IPCE) of the dye sensitized solar cells, the effects
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
B. O’Regan, M. Gratzel, Nature 353 (1991) 353. M. Gratzel, Nature 414 (2001) 338. A. Kay, M. Gratzel, Sol. Energy Mater. Sol. Cells 44 (1996) 99. Y. Chiba, A. Islam, R. Komiya, N. Koide, L. Han, Appl. Phys. Lett. 88 (2006) 223505. M. Nazeeruddin, R.H. Baker, P. Liska, M. Gradzel, J. Phys. Chem. B 107 (2003) 8981. G. Larramona, C. Chone, A. Jacob, D. Sakakura, B. Delatouche, D. Pere, X. Cieren, M. Nagino, R. Bayon, Chem. Mater. 18 (2006) 1688. K. Prabakar, T. Takahashi, T. Nakashima, Y. Kubota, A. Fujishima, J. Vac. Sci. Technol. A 24 (2006) 1613. K. Prabakar, T. Takahashi, T. Nezuka, K. Takahashi, Y. Kuboto, T. Nakashima, A. Fujishima, J. Vac. Sci. Technol. A 25 (2007) 1188. I.M. Sero, J. Bisquert, T.H. Dittrich, A. Belaidi, A.S. Susha, A.L. Rogach, J. Phys. Chem. C 111 (2007) 14889. R.D. Schaller, M. Sykora, J.M. Pietryga, V.I. Klimov, Nano Lett. 6 (2006) 424. P. Wang, C. Klein, R.H. Baker, S.M. Zakeeruddin, M. Gratzel, J. Am. Chem. Soc. 17 (2005) 808. S.-C. Lin, Y.-L. Lee, C.-H. Chang, Y.-J. Shen, Y.-M. Yang, Appl. Phys. Lett. 90 (2007) 143517. C.-H. Chang, Y.-L. Lee, Appl. Phys. Lett. 91 (2007) 53503. J. Xia, N. Masaki, K. Jiang, S. Yanagida, Chem. Commun. 138 (2007) 138.