- Email: [email protected]
Electronic structure of perylene on Au studied by ultraviolet photoelectron spectroscopy and density functional theory
Synthetic Metals 151 (2005) 120–123
Electronic structure of perylene on Au studied by ultraviolet photoelectron spectroscopy and density functional theory S.J. Kang, Y. Yi, K. Cho, K. Jeong, K.-H. Yoo, C.N. Whang ∗ Institute of Physics and Applied Physics, Yonsei University, 134 Shinchon-dong Sudaemoon-ku, Seoul 120-749, Republic of Korea Received 28 February 2005; received in revised form 6 April 2005; accepted 11 April 2005 Available online 4 June 2005
Abstract The electronic structure of perylene was analyzed by using ultraviolet photoelectron spectroscopy to introduce the energy level alignment of perylene/Au interface. The energy level alignment was studied by the onset of the highest occupied molecular orbital level and the shift of the vacuum level of the perylene layer, which was deposited on Au surface by stages. The measured onset of the highest occupied molecular orbital energy level was 1.0 eV from the Fermi level of Au, and the vacuum level was shifted 0.2 eV toward higher binding energy side with additional perylene layer. Furthermore, the density functional theory calculation was performed to identify the valence band spectrum of perylene film. The good agreement between the experimental and theoretical valence band spectrum allows us to assign each peak of the valence band spectrum, which was obtained from the perylene/Au film. The representative molecular orbital shapes, which composed the valence band of perylene, are presented in this report. © 2005 Elsevier B.V. All rights reserved. PACS: 72.80.Le; 73.20.−r; 73.20.At; 85.30.Tv Keywords: Perylene; Ultraviolet photoelectron spectroscopy; Density functional theory
1. Introduction Organic light-emitting devices (OLEDs) and organic thin film transistors (OTFTs) have been attracted considerable interest up to now for their useful properties in applications.[1–4] There are many promising organic semiconductor materials in this field such as pentacene, 8hydroxyquinoline aluminum (Alq3 ), that pentacene is used as charge transfer layer in OTFTs and Alq3 is used as light emitting layer in OLEDs [5,6]. Recently, perylene has attracted attention since perylene was applicable to both OLEDs and OTFTs as a blue light emitting dopant and charge transfer layer. Moreover, perylene has been used as organic photoconductors and organic solar cell [7–10]. Though perylene can be used in various applications in semiconductor field, there is less attempt to investigate the ∗
Corresponding author. Tel.: +82 2 2123 3841; fax: +82 2 3127 090. E-mail address: [email protected] (C.N. Whang).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.04.005
electronic structure of perylene. The study of electronic structure is an important matter because the electronic structure affects directly to the charge transport in the semiconductor devices. Considering such an important point, the study of electronic structure should be executed in detail. Recently, the interface formation and the electronic structure of perylene on ruthenium was studied by Mao et al. [10]. The previous reports were valuable because they suggested the interface formation and electronic structure properties of perylene on single crystal substrates. However, to understand the charge transport mechanism of organic semiconductor devices, the study of the onset position of highest occupied molecular orbital (HOMO) level from the Fermi level and the vacuum level shift should be studied by analyzing the valence band spectrum in detail. Also, the nature of each peak in valence band spectrum should be studied by comparing the experimental spectrum and the theoretical calculated spectrum. In this paper, we analyzed the valence band spectrum of the perylene/Au interface obtained from the ultraviolet
S.J. Kang et al. / Synthetic Metals 151 (2005) 120–123
121
photoelectron spectroscopy (UPS). The barrier height for charge injection and the vacuum level shift was presented. The density functional theory was used to calculate the valence band structure and identify each peak that consists the spectrum.
2. Experiments The UPS measurements were performed using a PHI 5700 spectrometer equipped with a He I (21.2 eV) discharge lamp. The base pressure of UPS measurement chamber was 2 × 10−10 Torr. The sample was prepared in a preparation chamber, which is connected with the UPS measurements chamber. Au was deposited on the silicon substrate to find the Fermi level of the sample. On the Au, perylene was deposited in a stepwise manner to investigate the interface formation of electronic structures. During the film depositions, the pressure of preparation chamber was maintained 1 × 10−7 Torr. After each deposition of perylene, the valence band spectra were measured immediately with the sample bias −15 V. The density functional theory (DFT) calculations, which were done to identify the valence band spectrum, have been carried out using the Gaussian 03 package [11].
3. Results and discussion The valence band spectrum of perylene, which was obtained by UPS measurements, was shown in Fig. 1. The thickness of the perylene was 25.6 nm that was enough to observe the HOMO level (peak a) of perylene. As shown in Fig. 1, the UPS spectrum of perylene consists of six peaks (a–f) mainly, which compose the valence band structure. To identify each peak of UPS spectrum, we compared the UPS spectrum with the theoretically calculated valence band spectrum. To carry out the DFT calculation, the geometry of perylene was optimized with the nonlocal hybrid Becke threeparameter Lee–Yang–Parr correlation functional (B3LYP) method that provides a reliable geometry on organic materials. Then, the density of states calculation was also executed by using B3LYP with the basis set 6-31G (d) [12]. From the obtained density of states, all spectra are convolved with the Gaussian functions with the full-width at half-maximum of 0.4 eV to compare with the experimental valence band spectrum. The calculated density of state (DOS) and corresponding molecular orbital (MO) states are represented in Fig. 1. The UPS spectrum and calculated density of state shows fairly good agreement and allows us to assign each peak of the valence band spectrum, which was obtained from the UPS measurements. Fig. 2 shows the representative molecular orbital shapes of peak a–f in Fig. 1 that was obtained by the DFT calculations. From the molecular orbital shape, the peaks a–c, e, f indicate the molecular orbital states, which perform a significant role for the conduction of carrier in perylene because
Fig. 1. The valence band spectrum obtained by using UPS is shown with the theoretical spectrum, which was obtained by DFT calculations. The density of state (DOS) and corresponding molecular orbital (MO) states are represented. The orbital energies of each peak are indicated by vertical lines.
Fig. 2. The shapes of representative molecular orbital are shown. Each of the molecular orbital corresponds to the peak a–f of Fig. 1. (a–c, e and f) indicate the molecular orbital states while (d) shows the molecular orbital state.
122
S.J. Kang et al. / Synthetic Metals 151 (2005) 120–123
Fig. 3. The close up feature of the HOMO region is shown with the relative binding energy to the Fermi level of the Au layer.
Fig. 4. The spectra of the high binding energy cutoff region of Au, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8 and 25.6 nm of perylene are shown.
of the delocalized nature of the state. The main contribution of the molecular orbital states comes from the pz orbital (perpendicular to the plane of perylene molecule) of the C atoms in perylene. Among the molecular orbital states, the molecular orbital state related to the peak f shows the most delocalized nature. Therefore, the molecular orbital state related to the peak f contributes the most part of the conduction in perylene. On the other hand, the nodal plane and the internuclear axis of carbon atoms is perpendicular each other in the case of peak d [13]. The peak d indicates the molecular orbital state, which forms the back bone of the perylene. The UPS spectra measured immediately after deposition of perylene in a stepwise manner are shown in Figs. 3 and 4. The spectra were collected along the surface normal direction with a photon incidence angle of 30◦ . The close up feature of the HOMO region is shown in Fig. 3 with the relative binding energy to the Fermi level of the Au layer. The valence band of the perylene with the thickness of 0.4, 0.8, 1.6, 3.2, 6.4, 12.8 and 25.6 nm are shown in order with the Au valence band spectrum. The bottom spectrum indicates the valence band of 30 nm Au layer that shows the Fermi level and the feature of Au clearly. The small shoulder emerged at about 1.5 eV in Au UPS spectrum is due to the small amount of carbon contamination on the Au surface. The feature of Au was diminished while the feature of perylene was increased as the perylene was deposited on Au. The HOMO level of
perylene was appeared at 0.4 nm thick of perylene on Au and the HOMO level is shown clearly at the thickness of perylene 1.6 nm. After enough coverage of perylene (25.6 nm) on Au, the difference of Fermi level of Au and the HOMO onset was 1.0 eV. The difference of Fermi level of Au and the HOMO onset of perylene represents an important value at the interface between perylene and Au as the barrier height for the hole injection from Au to perylene in organic devices. The vacuum level of the perylene can be determined by linear extrapolation of the high binding energy cutoff region on the UPS spectra. To obtain clear signal of the high binding energy cutoff region, we measured each UPS spectrum with the sample bias −15 V. The spectra of the high binding energy cutoff region of Au, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8 and 25.6 nm of perylene are shown in Fig. 4. As the perylene was deposited on Au, the high binding energy cutoff was moved to higher binding energy position. At the thickness of perylene 1.6 nm, which shown the HOMO level clearly, the high binding energy cutoff shifted 0.1 eV toward higher binding energy. This means the vacuum level is lowered compared to the vacuum level of the Au. Finally, after enough coverage of the perylene (25.6 nm) on Au, the high binding energy cut off shifted 0.2 eV from the vacuum level of the Au. The shift of the high binding energy cut off shows that there exists an interface dipole at the interface between perylene and Au. The redistribution of electrons at the Au surface occurred as
S.J. Kang et al. / Synthetic Metals 151 (2005) 120–123
Fig. 5. Energy diagram of perylene/Au interface.
the perylene deposited on Au and caused the interface dipole to change the vacuum level [14]. From Figs. 3 and 4, we obtained the interfacial energy diagrams of perylene and Au as shown in Fig. 5. The ionization potential of perylene was obtained by using the equation as shown below. Ionization potential = hν − cutoff + EHOMO
(1)
The hν indicates the photon energy 21.2 eV, the cutoff means the high binding cut off energy and EHOMO means the onset of HOMO level. According to the Eq. (1), the ionization potential of perylene was obtained 5.1 eV that are similar value with the previous result of Hiramoto et al. [9]. The difference between the Fermi level of Au and HOMO onset of perylene was 1.0 eV. The energy of the lowest unoccupied molecular orbital (LUMO) of perylene can be determined by using the energy gap of perylene 2.5 eV [9]. Therefore, the barrier height of electron injection from Au to perylene is 1.5 eV. In addition, the vacuum level of perylene shifted 0.2 eV from that of Au.
4. Conclusion In conclusion, we obtained the electronic structure of perylene/Au interface by using UPS and analyzed the valence
123
band spectrum by DFT calculations. Each peak of the valence band spectrum was assigned by DFT calculations, and we found that the molecular orbital state related to the peak f contribute to the most part of the conduction in perylene due to the largest delocalized nature. The measured of the highest occupied molecular orbital energy onset was 1.0 eV from the Fermi level of Au, and the shift of the vacuum level was 0.2 eV toward higher binding energy. These observations provide a complete picture of electronic structure of perylene. This work is supported by BK21 project of the Korea Research Foundation (KRF). Additional support by the National Core Research Center for Nanomedical Technology is gratefully acknowledged.
References [1] J.M. Shaw, P.F. Seidler, IBM J. Res. Dev. 45 (2001) 3. [2] C.D. Dimitrakopoulos, D.J. Mascaro, IBM J. Res. Dev. 45 (2001) 11. [3] S.J. Kang, D.S. Park, S.Y. Kim, C.N. Whang, K. Jeong, S. Im, Appl. Phys. Lett. 81 (2002) 2581. [4] S.J. Kang, M. Noh, D.S. Park, H.J. Kim, C.N. Whang, C.H. Chang, J. Appl. Phys. 95 (2004) 2293. [5] D. Knipp, R.A. Street, J. Non Cryst. Solids 338 (2004) 595. [6] H. Tan, J. Yao, Proc. SPIE Int. Soc. Opt. Eng. 5280 (2004) 495. [7] S.Y. Ni, X.R. Wang, Y.Z. Wu, H.Y. Chen, W.Q. Zhu, X.Y. Jiang, Z.L. Zhang, R.G. Sun, Appl. Phys. Lett. 85 (2004) 878. [8] S.H. Kim, Y.S. Yang, J.H. Lee, J.-I. Lee, H.Y. Chu, H. Lee, J. Oh, L.-M. Do, T. Zyung, Opt. Mater. 21 (2003) 439. [9] H. Masahiro, T. Akinori, S. Kouji, Y. Masaaki, Appl. Phys. Lett. 85 (2004) 1852. [10] H. Mao, H. Huang, Q. Chen, N.V. Richardson, Y. Wu, J. Zhang, H. Li, P. He, S. Bao, J. Chem. Phys. 121 (2004) 6972. [11] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 03, Revision A. 1, Gaussian Inc., Pittsburgh, PA, 2003. [12] R.Q. Zhang, W.C. Lu, C.S. Lee, L.S. Hung, S.T. Lee, J. Chem. Phys. 116 (2002) 8827. [13] A.L. Companion, Chemical Bonding, McGraw-Hill Book Company, New York, 1964, p. 41. [14] H. Peisert, M. Knupfer, J. Fink, Appl. Phys. Lett. 81 (2002) 2400.
Electronic structure of perylene on Au studied by ultraviolet photoelectron spectroscopy and density functional theory S.J. Kang, Y. Yi, K. Cho, K. Jeong, K.-H. Yoo, C.N. Whang ∗ Institute of Physics and Applied Physics, Yonsei University, 134 Shinchon-dong Sudaemoon-ku, Seoul 120-749, Republic of Korea Received 28 February 2005; received in revised form 6 April 2005; accepted 11 April 2005 Available online 4 June 2005
Abstract The electronic structure of perylene was analyzed by using ultraviolet photoelectron spectroscopy to introduce the energy level alignment of perylene/Au interface. The energy level alignment was studied by the onset of the highest occupied molecular orbital level and the shift of the vacuum level of the perylene layer, which was deposited on Au surface by stages. The measured onset of the highest occupied molecular orbital energy level was 1.0 eV from the Fermi level of Au, and the vacuum level was shifted 0.2 eV toward higher binding energy side with additional perylene layer. Furthermore, the density functional theory calculation was performed to identify the valence band spectrum of perylene film. The good agreement between the experimental and theoretical valence band spectrum allows us to assign each peak of the valence band spectrum, which was obtained from the perylene/Au film. The representative molecular orbital shapes, which composed the valence band of perylene, are presented in this report. © 2005 Elsevier B.V. All rights reserved. PACS: 72.80.Le; 73.20.−r; 73.20.At; 85.30.Tv Keywords: Perylene; Ultraviolet photoelectron spectroscopy; Density functional theory
1. Introduction Organic light-emitting devices (OLEDs) and organic thin film transistors (OTFTs) have been attracted considerable interest up to now for their useful properties in applications.[1–4] There are many promising organic semiconductor materials in this field such as pentacene, 8hydroxyquinoline aluminum (Alq3 ), that pentacene is used as charge transfer layer in OTFTs and Alq3 is used as light emitting layer in OLEDs [5,6]. Recently, perylene has attracted attention since perylene was applicable to both OLEDs and OTFTs as a blue light emitting dopant and charge transfer layer. Moreover, perylene has been used as organic photoconductors and organic solar cell [7–10]. Though perylene can be used in various applications in semiconductor field, there is less attempt to investigate the ∗
Corresponding author. Tel.: +82 2 2123 3841; fax: +82 2 3127 090. E-mail address: [email protected] (C.N. Whang).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.04.005
electronic structure of perylene. The study of electronic structure is an important matter because the electronic structure affects directly to the charge transport in the semiconductor devices. Considering such an important point, the study of electronic structure should be executed in detail. Recently, the interface formation and the electronic structure of perylene on ruthenium was studied by Mao et al. [10]. The previous reports were valuable because they suggested the interface formation and electronic structure properties of perylene on single crystal substrates. However, to understand the charge transport mechanism of organic semiconductor devices, the study of the onset position of highest occupied molecular orbital (HOMO) level from the Fermi level and the vacuum level shift should be studied by analyzing the valence band spectrum in detail. Also, the nature of each peak in valence band spectrum should be studied by comparing the experimental spectrum and the theoretical calculated spectrum. In this paper, we analyzed the valence band spectrum of the perylene/Au interface obtained from the ultraviolet
S.J. Kang et al. / Synthetic Metals 151 (2005) 120–123
121
photoelectron spectroscopy (UPS). The barrier height for charge injection and the vacuum level shift was presented. The density functional theory was used to calculate the valence band structure and identify each peak that consists the spectrum.
2. Experiments The UPS measurements were performed using a PHI 5700 spectrometer equipped with a He I (21.2 eV) discharge lamp. The base pressure of UPS measurement chamber was 2 × 10−10 Torr. The sample was prepared in a preparation chamber, which is connected with the UPS measurements chamber. Au was deposited on the silicon substrate to find the Fermi level of the sample. On the Au, perylene was deposited in a stepwise manner to investigate the interface formation of electronic structures. During the film depositions, the pressure of preparation chamber was maintained 1 × 10−7 Torr. After each deposition of perylene, the valence band spectra were measured immediately with the sample bias −15 V. The density functional theory (DFT) calculations, which were done to identify the valence band spectrum, have been carried out using the Gaussian 03 package [11].
3. Results and discussion The valence band spectrum of perylene, which was obtained by UPS measurements, was shown in Fig. 1. The thickness of the perylene was 25.6 nm that was enough to observe the HOMO level (peak a) of perylene. As shown in Fig. 1, the UPS spectrum of perylene consists of six peaks (a–f) mainly, which compose the valence band structure. To identify each peak of UPS spectrum, we compared the UPS spectrum with the theoretically calculated valence band spectrum. To carry out the DFT calculation, the geometry of perylene was optimized with the nonlocal hybrid Becke threeparameter Lee–Yang–Parr correlation functional (B3LYP) method that provides a reliable geometry on organic materials. Then, the density of states calculation was also executed by using B3LYP with the basis set 6-31G (d) [12]. From the obtained density of states, all spectra are convolved with the Gaussian functions with the full-width at half-maximum of 0.4 eV to compare with the experimental valence band spectrum. The calculated density of state (DOS) and corresponding molecular orbital (MO) states are represented in Fig. 1. The UPS spectrum and calculated density of state shows fairly good agreement and allows us to assign each peak of the valence band spectrum, which was obtained from the UPS measurements. Fig. 2 shows the representative molecular orbital shapes of peak a–f in Fig. 1 that was obtained by the DFT calculations. From the molecular orbital shape, the peaks a–c, e, f indicate the molecular orbital states, which perform a significant role for the conduction of carrier in perylene because
Fig. 1. The valence band spectrum obtained by using UPS is shown with the theoretical spectrum, which was obtained by DFT calculations. The density of state (DOS) and corresponding molecular orbital (MO) states are represented. The orbital energies of each peak are indicated by vertical lines.
Fig. 2. The shapes of representative molecular orbital are shown. Each of the molecular orbital corresponds to the peak a–f of Fig. 1. (a–c, e and f) indicate the molecular orbital states while (d) shows the molecular orbital state.
122
S.J. Kang et al. / Synthetic Metals 151 (2005) 120–123
Fig. 3. The close up feature of the HOMO region is shown with the relative binding energy to the Fermi level of the Au layer.
Fig. 4. The spectra of the high binding energy cutoff region of Au, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8 and 25.6 nm of perylene are shown.
of the delocalized nature of the state. The main contribution of the molecular orbital states comes from the pz orbital (perpendicular to the plane of perylene molecule) of the C atoms in perylene. Among the molecular orbital states, the molecular orbital state related to the peak f shows the most delocalized nature. Therefore, the molecular orbital state related to the peak f contributes the most part of the conduction in perylene. On the other hand, the nodal plane and the internuclear axis of carbon atoms is perpendicular each other in the case of peak d [13]. The peak d indicates the molecular orbital state, which forms the back bone of the perylene. The UPS spectra measured immediately after deposition of perylene in a stepwise manner are shown in Figs. 3 and 4. The spectra were collected along the surface normal direction with a photon incidence angle of 30◦ . The close up feature of the HOMO region is shown in Fig. 3 with the relative binding energy to the Fermi level of the Au layer. The valence band of the perylene with the thickness of 0.4, 0.8, 1.6, 3.2, 6.4, 12.8 and 25.6 nm are shown in order with the Au valence band spectrum. The bottom spectrum indicates the valence band of 30 nm Au layer that shows the Fermi level and the feature of Au clearly. The small shoulder emerged at about 1.5 eV in Au UPS spectrum is due to the small amount of carbon contamination on the Au surface. The feature of Au was diminished while the feature of perylene was increased as the perylene was deposited on Au. The HOMO level of
perylene was appeared at 0.4 nm thick of perylene on Au and the HOMO level is shown clearly at the thickness of perylene 1.6 nm. After enough coverage of perylene (25.6 nm) on Au, the difference of Fermi level of Au and the HOMO onset was 1.0 eV. The difference of Fermi level of Au and the HOMO onset of perylene represents an important value at the interface between perylene and Au as the barrier height for the hole injection from Au to perylene in organic devices. The vacuum level of the perylene can be determined by linear extrapolation of the high binding energy cutoff region on the UPS spectra. To obtain clear signal of the high binding energy cutoff region, we measured each UPS spectrum with the sample bias −15 V. The spectra of the high binding energy cutoff region of Au, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8 and 25.6 nm of perylene are shown in Fig. 4. As the perylene was deposited on Au, the high binding energy cutoff was moved to higher binding energy position. At the thickness of perylene 1.6 nm, which shown the HOMO level clearly, the high binding energy cutoff shifted 0.1 eV toward higher binding energy. This means the vacuum level is lowered compared to the vacuum level of the Au. Finally, after enough coverage of the perylene (25.6 nm) on Au, the high binding energy cut off shifted 0.2 eV from the vacuum level of the Au. The shift of the high binding energy cut off shows that there exists an interface dipole at the interface between perylene and Au. The redistribution of electrons at the Au surface occurred as
S.J. Kang et al. / Synthetic Metals 151 (2005) 120–123
Fig. 5. Energy diagram of perylene/Au interface.
the perylene deposited on Au and caused the interface dipole to change the vacuum level [14]. From Figs. 3 and 4, we obtained the interfacial energy diagrams of perylene and Au as shown in Fig. 5. The ionization potential of perylene was obtained by using the equation as shown below. Ionization potential = hν − cutoff + EHOMO
(1)
The hν indicates the photon energy 21.2 eV, the cutoff means the high binding cut off energy and EHOMO means the onset of HOMO level. According to the Eq. (1), the ionization potential of perylene was obtained 5.1 eV that are similar value with the previous result of Hiramoto et al. [9]. The difference between the Fermi level of Au and HOMO onset of perylene was 1.0 eV. The energy of the lowest unoccupied molecular orbital (LUMO) of perylene can be determined by using the energy gap of perylene 2.5 eV [9]. Therefore, the barrier height of electron injection from Au to perylene is 1.5 eV. In addition, the vacuum level of perylene shifted 0.2 eV from that of Au.
4. Conclusion In conclusion, we obtained the electronic structure of perylene/Au interface by using UPS and analyzed the valence
123
band spectrum by DFT calculations. Each peak of the valence band spectrum was assigned by DFT calculations, and we found that the molecular orbital state related to the peak f contribute to the most part of the conduction in perylene due to the largest delocalized nature. The measured of the highest occupied molecular orbital energy onset was 1.0 eV from the Fermi level of Au, and the shift of the vacuum level was 0.2 eV toward higher binding energy. These observations provide a complete picture of electronic structure of perylene. This work is supported by BK21 project of the Korea Research Foundation (KRF). Additional support by the National Core Research Center for Nanomedical Technology is gratefully acknowledged.
References [1] J.M. Shaw, P.F. Seidler, IBM J. Res. Dev. 45 (2001) 3. [2] C.D. Dimitrakopoulos, D.J. Mascaro, IBM J. Res. Dev. 45 (2001) 11. [3] S.J. Kang, D.S. Park, S.Y. Kim, C.N. Whang, K. Jeong, S. Im, Appl. Phys. Lett. 81 (2002) 2581. [4] S.J. Kang, M. Noh, D.S. Park, H.J. Kim, C.N. Whang, C.H. Chang, J. Appl. Phys. 95 (2004) 2293. [5] D. Knipp, R.A. Street, J. Non Cryst. Solids 338 (2004) 595. [6] H. Tan, J. Yao, Proc. SPIE Int. Soc. Opt. Eng. 5280 (2004) 495. [7] S.Y. Ni, X.R. Wang, Y.Z. Wu, H.Y. Chen, W.Q. Zhu, X.Y. Jiang, Z.L. Zhang, R.G. Sun, Appl. Phys. Lett. 85 (2004) 878. [8] S.H. Kim, Y.S. Yang, J.H. Lee, J.-I. Lee, H.Y. Chu, H. Lee, J. Oh, L.-M. Do, T. Zyung, Opt. Mater. 21 (2003) 439. [9] H. Masahiro, T. Akinori, S. Kouji, Y. Masaaki, Appl. Phys. Lett. 85 (2004) 1852. [10] H. Mao, H. Huang, Q. Chen, N.V. Richardson, Y. Wu, J. Zhang, H. Li, P. He, S. Bao, J. Chem. Phys. 121 (2004) 6972. [11] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 03, Revision A. 1, Gaussian Inc., Pittsburgh, PA, 2003. [12] R.Q. Zhang, W.C. Lu, C.S. Lee, L.S. Hung, S.T. Lee, J. Chem. Phys. 116 (2002) 8827. [13] A.L. Companion, Chemical Bonding, McGraw-Hill Book Company, New York, 1964, p. 41. [14] H. Peisert, M. Knupfer, J. Fink, Appl. Phys. Lett. 81 (2002) 2400.