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Enhanced fluorescence from Dy3+ owing to surface plasmon resonance of Au colloid nanoparticles
Materials Letters 59 (2005) 1413 – 1416 www.elsevier.com/locate/matlet
Enhanced fluorescence from Dy3+ owing to surface plasmon resonance of Au colloid nanoparticles Jian Zhu* School of Science, Xi’an Jiaotong University, Xi’an 710049, China Received 24 July 2004; accepted 17 November 2004 Available online 12 January 2005
Abstract Gold colloidal containing rare earth ions Dy3+ were prepared at room temperature. Fluorescence spectra of Dy3+ ions and Au colloid containing Dy3+ were measured. For solution containing Dy3+, fluorescence emission occurs at visible light wavelength region. Two emission peaks are observed at 480 nm and 570 nm respectively when the corresponding excitation wavelength is at 350 nm. When Au colloids were added to the solution containing Dy3+, both these two fluorescence peaks were enhanced. Furthermore, with the increasing Au content, the fluorescence increases first and then decreases. We believe that this increased fluorescence are due to the local field enhancement around Dy3+ ions owing to the induced higher orders of surface plasmon resonance. D 2005 Elsevier B.V. All rights reserved. PACS: 78.67.Bf; 73.20.Mf; 36.40.Gk Keywords: Fluorescence; Au colloid; Dysprosium ions; Surface plasmon resonance; Local field
1. Introduction Rare earth ions in nanometer-size semiconductors or metallic low dimensional structures have received much attention in recent years due to their special electronic and optical properties [1–4]. Hussain et al. [1] have reported the fluorescence characteristics of the measured emission transitions of Eu3+ and Dy3+ doped laser liquids, by the application of the Judd-Ofelt intensity factors obtained from their absorption spectral profiles. Ishizaka and Kurokawa [2] have highly incorporated the rare earth ions into alumina by sol–gel method derived from aqueous AlCl3 solution. The absorption, emission and lifetime have been examined for each doped film at room temperature. In the letter of Brik et al. [3], optical spectra of Dy3+:LiYF4 (Dy3+:YLF) single crystal were analyzed using Judd-Ofelt theory and discrete variational multielectron method. Oscillator strengths of transitions up to 39,000 cm 1 in the absorption spectrum,
* Tel.: +86 29 2672529; fax: +86 29 2668004. E-mail address: [email protected]. 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.060
branching ratios and nonradiative transition rates were calculated; assignment of the absorption transitions was given. In the paper of Gu et al. [4], Dy3+ doped ZnO nanocrystals have been synthesized via a simple combustion method. The as-prepared cuboid-like ZnO nanocrystals appear to be single hexagonal crystalline phase with an average diameter of 20 nm. The characteristic luminescence of doped Dy3+ ions has been evaluated, and the highly enhanced photoluminescence of Dy3+ ions can be obtained by Li+ doping. In nanometer-size metallic particles, there are several classes of electromagnetic modes such as surface modes and local field. Surface plasmons are collective electromagnetic oscillations at metallic surfaces. It is widely accepted that for optical processes the primary role of the roughness in metals or the small size of noble metal particles is to enhance the local optical fields via localized plasmon resonance, and thus improve the efficiency of light absorption and emission [5–7]. Hayakawa et al. [8 9] have described the increase of europium ion fluorescence owing to resonant plasma oscillation of silver particles in silica glass, which were precipitated by annealing in a reducing
J. Zhu / Materials Letters 59 (2005) 1413–1416
2. Experimental Dysprosium oxide Dy2O3 (0.03 g) was dissolved in H2O (25 ml, including 20% HNO3). This synthesis was conducted under an ultrasonication at room temperature. The resultant solution was transparent and has no color. Gold colloid nanoparticles were prepared via electrochemical method [11 12]. The transmission electron microscopic (TEM) images are obtained by using a JEOL JEM-200CX TEM, the Au nanoparticles with mean diameter 15 nm have been observed, as shown in Fig. 1. The color of the resultant solution was red. At last, various amounts of gold colloid were added to the solution containing Dy3+ with gentle mixing. Then the resultant solution turns to light pink.
3. Results and discussion The UV–visible spectra are recorded on a Hitachi U2001 spectrophotometer. To a solution containing only
0.30 3+
0.28 0.26
solution containing Dy and Au nanoparticles 3+ solution containing Dy
350 nm
0.24
530 nm
atmosphere sol–gel derived SiO2 glass containing Ag+ and Eu3+ ions. They have come to the conclusion that the most probable mechanism for the fluorescence increase is a local field enhancement around Eu3+ ions, due to the induced surface plasmon resonance of Ag particles. Recently, they have also reported the enhanced photoluminescence of trivalent europium ions in the vicinity of nanometer-sized Au particles in glasses. They believed that Eu3+ ions were strongly excited by the energy transfer from CMO groups, which were placed in enhanced local fields near Au nanoparticles due to higher orders of surface plasmon resonances [10]. In this paper, we report on the fluorescence excitation and emission spectra properties of gold colloidal containing Dy3+. The relation between the fluorescence enhancement and the Au content has also been studied. Furthermore we discuss in detail the mechanisms of the enhanced fluorescence from Dy3+.
Absorbance /a.u.
1414
0.22 0.20 0.18 0.16 0.14 0.12 0.10 350
400
450
500
550
600
650
Wavelength /nm Fig. 2. Absorption spectra of solution containing Dy3+ and solution containing both Au nanoparticles and Dy3+.
Dy3+, a strong absorption peak was noted at around 350 nm, as shown in Fig. 2. When amount of Au colloid were added to the solution containing Dy 3+, this absorption peaks fixed at 350 nm increase. Furthermore, a new absorption peak fixed at 530 nm was observed, which is due to the surface plasmon resonance of Au nanoparticles. Both fluorescence excitation and emission spectra are recorded on a Perkin Elmer LS 55 spectrophotofluorometer. It is known that, to a solution containing Dy3+, two fluorescence emission peaks will take place at 483 and 576 nm respectively corresponding to 4F 9/2Y6H 15/2 and 4 F 9/2Y6H 13/2 transitions of the Dy3+ ion [1]. So the fluorescence excitation spectra are studied in order to find the sensitive excitation frequency. The excitation spectrum in Fig. 3 is the scanning excited wavelength from 200 to 450 nm when the detection wavelength was located at 483 nm. The excitation spectrum in Fig. 4 is the scanning excited wavelength from 200 to 550 nm when the detection wavelength was located at 576 nm. The experimental results in Figs. 3 and 4 show that both the fluorescence at 483 nm and 576 nm are sensitive to the excitation at 350 nm. It is 100 3+
solution containing Dy and Au nanoparticles
Intensity /a.u.
80
solution containing only Dy
3+
60 40 20 0 200
250
300
350
400
450
Wavelength /nm
Fig. 1. TEM image of gold colloidal nanospheres.
Fig. 3. Excitation spectra of solution containing only Dy3+ and solution containing both Au nanoparticles and Dy3+ (detection wavelength is 483 nm).
J. Zhu / Materials Letters 59 (2005) 1413–1416
1415
120
40 3+
high Au nanoparticle density medium Au nanoparticle density low Au nanoparticle density
solution containing Dy and Au nanoparticles 3+
Fluorescence Intensity /a.u.
solution containing only Dy
Intensity /a.u.
30
20
10
0 200
250
300
350
400
450
500
100
80
60
40
550 20 400
Wavelength /nm Fig. 4. Excitation spectra of solution containing only Dy and solution containing both Au nanoparticles and Dy3+ (detection wavelength is 576 nm).
important to note that when amount of Au colloid was added to the solution containing Dy3+, the corresponding excitation peaks increase, as shown in Figs. 3 and 4. Colloid gold nanoparticles, solution containing only Dy3+ and solution containing both Au nanoparticles and Dy3+ are excited at 350 nm. The corresponding emission spectra are compared in Fig. 5. For solution containing only Dy3+, two emission peaks fixed at 480 nm and 570 nm display. We believe that these two fluorescence peaks resulted from 4F 9/2Y6H 15/2 and 4F 9/2Y6H 13/2 transitions of the Dy3+. For Au colloid, no fluorescence peaks take place in this wavelength region. Whereas when amount of Au colloid were added to the solution containing Dy3+, both these two emission peaks increase. Furthermore, it is interesting to note that with the increasing Au content, the fluorescence increases first and then decreases, as shown in Fig. 6. We believe that these increased fluorescence are due to the local field enhancement around Dy3+ ions owing to the induced surface plasmon resonance of gold nanoparticles [8 9]. It is important to note that the incident light (at around 350 nm) for excitation used in this work does not
solution containing Dy and Au nanoparticles
110
solution containing only Dy solution containing only Au nanoparticles
100
550
600
650
Fig. 6. Emission spectra of solution containing both Au nanoparticles and Dy3+ with different Au content (exciting at 350 nm).
match with the resonance mode (at around 530 nm) of surface plasmon in gold nanoparticles. Nevertheless, the strong excitation produce hot electrons through the dYsp inter-band transition, resulting in an increasing number of density of conduction electrons within a very short time of electron-phonon coupling time in picosecond order [13]. This simultaneously means a probable blue-shift of plasmon frequency. Thus, we confidently propose that it is possible for the long UV light to derive a higher order of surface plasmon resonance for gold nanoparticles, in the vicinity of which luminescent centers such as rare earth ions can be strongly excited through the induced local field [10]. According to the theoretical research of the photoinduced luminescence and the surface enhanced secondharmonic generation from rough surfaces of noble metals, both the incoming and outgoing fields are propose to be enhanced via coupling to the local plasmon resonances [5,14,15]. Then the adsorbed Dy3+ will be easily stimulated into high energy level such as 4F 9/2. Relaxation of these electronic motions followed by the recombination of the stimulated electrons with holes in the low energy band such as 6 H 13/2 and 6 H 15/2 leads to the enhanced fluorescence emission.
3+
4. Conclusion
90 570 nm
80 70 60 50 40 30 20 10 400
500
3+
120
480 nm
Fluorescence Intensity /a.u.
130
450
Wavelength /nm
3+
450
500
550
600
650
Wavelength /nm Fig. 5. Emission spectra of Au colloid, solution containing only Dy3+ and solution containing both Au nanoparticles and Dy3+ (exciting at 350 nm).
Fluorescence spectrum characters of solution containing Dy3+ have been studied at room temperature. Two emission peaks are observed at 480 nm and 570 nm respectively. Both these two emission peaks are enhanced when amount of gold colloid is added to the solution containing Dy3+. Furthermore, with the increasing Au content, the fluorescence increases first and then decreases. We believe that these enhanced fluorescence of 4F 9/2 Y6H 15/2 and 4F 9/2Y6H 13/2 transitions of Dy3+ in the vicinity of Au nanoparticles are enhanced by a local field enhancement due to higher orders of surface plasmon resonance excitations.
1416
J. Zhu / Materials Letters 59 (2005) 1413–1416
References [1] N.S. Hussain, K. Annapurna, D.V.V. Rao, K.R. Reddy, G. Amaranath, S. Buddhudu, Spectrochimica Acta. Part A: Molecular and Biomolecular Spectroscopy 53 (1997) 761. [2] T. Ishizaka, Y. Kurokawa, Journal of Luminescence 92 (2001) 57. [3] M.G. Brik, T. Ishii, A.M. Tkachuk, S.E. Ivanova, I.K. Razumova, Journal of Alloys and Compounds 374 (2004) 63. [4] F. Gu, S.F. Wang, M.K. Lu, G.J. Zhou, D. Xu, D.R. Yuan, Langmuir 20 (2004) 3528. [5] G.T. Boyd, Th. Rasing, J.R.R. Leite, Y.R. Shen, Physical Review. B, 30 (1984) 519. [6] J. Zhu, Y.C. Wang, Y.M. Lu, Colloids and Surfaces. A, Physicochemical and Engineering Aspects 232 (2004) 155. [7] J. Zhu, Y.C. Wang, S.N. Yan, Chinese Physics Letters 21 (2004) 559.
[8] T. Hayakawa, S.T. Selvan, M. Nogami, Journal of Non-Crystalline Solids 259 (1999) 16. [9] T. Hayakawa, S.T. Selvan, M. Nogami, Applied Physics Letters 74 (1999) 1513. [10] T. Hayakawa, K. Furuhashi, M. Nogami, Journal of Physical Chemistry. B 108 (2004) 11301. [11] Y.Y. Yu, S.S. Chang, C.L. Lee, C.R.C. Wang, Journal of Physical Chemistry. B 101 (1997) 6661. [12] J. Zhu, Plasma Science and Technology 5 (2003) 1835. [13] H. Inouye, K. Tanaka, I. Takahashi, K. Hirao, Physical Review. B, 57 (1998) 1134. [14] G.T. Boyd, Z.H. Yu, Y.R. Shen, Physical Review. B, 33 (1986) 7923. [15] M.B. Mohamed, V. Volkov, S. Link, M.A. El-Sayed, Chemical Physics Letters 317 (2000) 517.
Enhanced fluorescence from Dy3+ owing to surface plasmon resonance of Au colloid nanoparticles Jian Zhu* School of Science, Xi’an Jiaotong University, Xi’an 710049, China Received 24 July 2004; accepted 17 November 2004 Available online 12 January 2005
Abstract Gold colloidal containing rare earth ions Dy3+ were prepared at room temperature. Fluorescence spectra of Dy3+ ions and Au colloid containing Dy3+ were measured. For solution containing Dy3+, fluorescence emission occurs at visible light wavelength region. Two emission peaks are observed at 480 nm and 570 nm respectively when the corresponding excitation wavelength is at 350 nm. When Au colloids were added to the solution containing Dy3+, both these two fluorescence peaks were enhanced. Furthermore, with the increasing Au content, the fluorescence increases first and then decreases. We believe that this increased fluorescence are due to the local field enhancement around Dy3+ ions owing to the induced higher orders of surface plasmon resonance. D 2005 Elsevier B.V. All rights reserved. PACS: 78.67.Bf; 73.20.Mf; 36.40.Gk Keywords: Fluorescence; Au colloid; Dysprosium ions; Surface plasmon resonance; Local field
1. Introduction Rare earth ions in nanometer-size semiconductors or metallic low dimensional structures have received much attention in recent years due to their special electronic and optical properties [1–4]. Hussain et al. [1] have reported the fluorescence characteristics of the measured emission transitions of Eu3+ and Dy3+ doped laser liquids, by the application of the Judd-Ofelt intensity factors obtained from their absorption spectral profiles. Ishizaka and Kurokawa [2] have highly incorporated the rare earth ions into alumina by sol–gel method derived from aqueous AlCl3 solution. The absorption, emission and lifetime have been examined for each doped film at room temperature. In the letter of Brik et al. [3], optical spectra of Dy3+:LiYF4 (Dy3+:YLF) single crystal were analyzed using Judd-Ofelt theory and discrete variational multielectron method. Oscillator strengths of transitions up to 39,000 cm 1 in the absorption spectrum,
* Tel.: +86 29 2672529; fax: +86 29 2668004. E-mail address: [email protected]. 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.060
branching ratios and nonradiative transition rates were calculated; assignment of the absorption transitions was given. In the paper of Gu et al. [4], Dy3+ doped ZnO nanocrystals have been synthesized via a simple combustion method. The as-prepared cuboid-like ZnO nanocrystals appear to be single hexagonal crystalline phase with an average diameter of 20 nm. The characteristic luminescence of doped Dy3+ ions has been evaluated, and the highly enhanced photoluminescence of Dy3+ ions can be obtained by Li+ doping. In nanometer-size metallic particles, there are several classes of electromagnetic modes such as surface modes and local field. Surface plasmons are collective electromagnetic oscillations at metallic surfaces. It is widely accepted that for optical processes the primary role of the roughness in metals or the small size of noble metal particles is to enhance the local optical fields via localized plasmon resonance, and thus improve the efficiency of light absorption and emission [5–7]. Hayakawa et al. [8 9] have described the increase of europium ion fluorescence owing to resonant plasma oscillation of silver particles in silica glass, which were precipitated by annealing in a reducing
J. Zhu / Materials Letters 59 (2005) 1413–1416
2. Experimental Dysprosium oxide Dy2O3 (0.03 g) was dissolved in H2O (25 ml, including 20% HNO3). This synthesis was conducted under an ultrasonication at room temperature. The resultant solution was transparent and has no color. Gold colloid nanoparticles were prepared via electrochemical method [11 12]. The transmission electron microscopic (TEM) images are obtained by using a JEOL JEM-200CX TEM, the Au nanoparticles with mean diameter 15 nm have been observed, as shown in Fig. 1. The color of the resultant solution was red. At last, various amounts of gold colloid were added to the solution containing Dy3+ with gentle mixing. Then the resultant solution turns to light pink.
3. Results and discussion The UV–visible spectra are recorded on a Hitachi U2001 spectrophotometer. To a solution containing only
0.30 3+
0.28 0.26
solution containing Dy and Au nanoparticles 3+ solution containing Dy
350 nm
0.24
530 nm
atmosphere sol–gel derived SiO2 glass containing Ag+ and Eu3+ ions. They have come to the conclusion that the most probable mechanism for the fluorescence increase is a local field enhancement around Eu3+ ions, due to the induced surface plasmon resonance of Ag particles. Recently, they have also reported the enhanced photoluminescence of trivalent europium ions in the vicinity of nanometer-sized Au particles in glasses. They believed that Eu3+ ions were strongly excited by the energy transfer from CMO groups, which were placed in enhanced local fields near Au nanoparticles due to higher orders of surface plasmon resonances [10]. In this paper, we report on the fluorescence excitation and emission spectra properties of gold colloidal containing Dy3+. The relation between the fluorescence enhancement and the Au content has also been studied. Furthermore we discuss in detail the mechanisms of the enhanced fluorescence from Dy3+.
Absorbance /a.u.
1414
0.22 0.20 0.18 0.16 0.14 0.12 0.10 350
400
450
500
550
600
650
Wavelength /nm Fig. 2. Absorption spectra of solution containing Dy3+ and solution containing both Au nanoparticles and Dy3+.
Dy3+, a strong absorption peak was noted at around 350 nm, as shown in Fig. 2. When amount of Au colloid were added to the solution containing Dy 3+, this absorption peaks fixed at 350 nm increase. Furthermore, a new absorption peak fixed at 530 nm was observed, which is due to the surface plasmon resonance of Au nanoparticles. Both fluorescence excitation and emission spectra are recorded on a Perkin Elmer LS 55 spectrophotofluorometer. It is known that, to a solution containing Dy3+, two fluorescence emission peaks will take place at 483 and 576 nm respectively corresponding to 4F 9/2Y6H 15/2 and 4 F 9/2Y6H 13/2 transitions of the Dy3+ ion [1]. So the fluorescence excitation spectra are studied in order to find the sensitive excitation frequency. The excitation spectrum in Fig. 3 is the scanning excited wavelength from 200 to 450 nm when the detection wavelength was located at 483 nm. The excitation spectrum in Fig. 4 is the scanning excited wavelength from 200 to 550 nm when the detection wavelength was located at 576 nm. The experimental results in Figs. 3 and 4 show that both the fluorescence at 483 nm and 576 nm are sensitive to the excitation at 350 nm. It is 100 3+
solution containing Dy and Au nanoparticles
Intensity /a.u.
80
solution containing only Dy
3+
60 40 20 0 200
250
300
350
400
450
Wavelength /nm
Fig. 1. TEM image of gold colloidal nanospheres.
Fig. 3. Excitation spectra of solution containing only Dy3+ and solution containing both Au nanoparticles and Dy3+ (detection wavelength is 483 nm).
J. Zhu / Materials Letters 59 (2005) 1413–1416
1415
120
40 3+
high Au nanoparticle density medium Au nanoparticle density low Au nanoparticle density
solution containing Dy and Au nanoparticles 3+
Fluorescence Intensity /a.u.
solution containing only Dy
Intensity /a.u.
30
20
10
0 200
250
300
350
400
450
500
100
80
60
40
550 20 400
Wavelength /nm Fig. 4. Excitation spectra of solution containing only Dy and solution containing both Au nanoparticles and Dy3+ (detection wavelength is 576 nm).
important to note that when amount of Au colloid was added to the solution containing Dy3+, the corresponding excitation peaks increase, as shown in Figs. 3 and 4. Colloid gold nanoparticles, solution containing only Dy3+ and solution containing both Au nanoparticles and Dy3+ are excited at 350 nm. The corresponding emission spectra are compared in Fig. 5. For solution containing only Dy3+, two emission peaks fixed at 480 nm and 570 nm display. We believe that these two fluorescence peaks resulted from 4F 9/2Y6H 15/2 and 4F 9/2Y6H 13/2 transitions of the Dy3+. For Au colloid, no fluorescence peaks take place in this wavelength region. Whereas when amount of Au colloid were added to the solution containing Dy3+, both these two emission peaks increase. Furthermore, it is interesting to note that with the increasing Au content, the fluorescence increases first and then decreases, as shown in Fig. 6. We believe that these increased fluorescence are due to the local field enhancement around Dy3+ ions owing to the induced surface plasmon resonance of gold nanoparticles [8 9]. It is important to note that the incident light (at around 350 nm) for excitation used in this work does not
solution containing Dy and Au nanoparticles
110
solution containing only Dy solution containing only Au nanoparticles
100
550
600
650
Fig. 6. Emission spectra of solution containing both Au nanoparticles and Dy3+ with different Au content (exciting at 350 nm).
match with the resonance mode (at around 530 nm) of surface plasmon in gold nanoparticles. Nevertheless, the strong excitation produce hot electrons through the dYsp inter-band transition, resulting in an increasing number of density of conduction electrons within a very short time of electron-phonon coupling time in picosecond order [13]. This simultaneously means a probable blue-shift of plasmon frequency. Thus, we confidently propose that it is possible for the long UV light to derive a higher order of surface plasmon resonance for gold nanoparticles, in the vicinity of which luminescent centers such as rare earth ions can be strongly excited through the induced local field [10]. According to the theoretical research of the photoinduced luminescence and the surface enhanced secondharmonic generation from rough surfaces of noble metals, both the incoming and outgoing fields are propose to be enhanced via coupling to the local plasmon resonances [5,14,15]. Then the adsorbed Dy3+ will be easily stimulated into high energy level such as 4F 9/2. Relaxation of these electronic motions followed by the recombination of the stimulated electrons with holes in the low energy band such as 6 H 13/2 and 6 H 15/2 leads to the enhanced fluorescence emission.
3+
4. Conclusion
90 570 nm
80 70 60 50 40 30 20 10 400
500
3+
120
480 nm
Fluorescence Intensity /a.u.
130
450
Wavelength /nm
3+
450
500
550
600
650
Wavelength /nm Fig. 5. Emission spectra of Au colloid, solution containing only Dy3+ and solution containing both Au nanoparticles and Dy3+ (exciting at 350 nm).
Fluorescence spectrum characters of solution containing Dy3+ have been studied at room temperature. Two emission peaks are observed at 480 nm and 570 nm respectively. Both these two emission peaks are enhanced when amount of gold colloid is added to the solution containing Dy3+. Furthermore, with the increasing Au content, the fluorescence increases first and then decreases. We believe that these enhanced fluorescence of 4F 9/2 Y6H 15/2 and 4F 9/2Y6H 13/2 transitions of Dy3+ in the vicinity of Au nanoparticles are enhanced by a local field enhancement due to higher orders of surface plasmon resonance excitations.
1416
J. Zhu / Materials Letters 59 (2005) 1413–1416
References [1] N.S. Hussain, K. Annapurna, D.V.V. Rao, K.R. Reddy, G. Amaranath, S. Buddhudu, Spectrochimica Acta. Part A: Molecular and Biomolecular Spectroscopy 53 (1997) 761. [2] T. Ishizaka, Y. Kurokawa, Journal of Luminescence 92 (2001) 57. [3] M.G. Brik, T. Ishii, A.M. Tkachuk, S.E. Ivanova, I.K. Razumova, Journal of Alloys and Compounds 374 (2004) 63. [4] F. Gu, S.F. Wang, M.K. Lu, G.J. Zhou, D. Xu, D.R. Yuan, Langmuir 20 (2004) 3528. [5] G.T. Boyd, Th. Rasing, J.R.R. Leite, Y.R. Shen, Physical Review. B, 30 (1984) 519. [6] J. Zhu, Y.C. Wang, Y.M. Lu, Colloids and Surfaces. A, Physicochemical and Engineering Aspects 232 (2004) 155. [7] J. Zhu, Y.C. Wang, S.N. Yan, Chinese Physics Letters 21 (2004) 559.
[8] T. Hayakawa, S.T. Selvan, M. Nogami, Journal of Non-Crystalline Solids 259 (1999) 16. [9] T. Hayakawa, S.T. Selvan, M. Nogami, Applied Physics Letters 74 (1999) 1513. [10] T. Hayakawa, K. Furuhashi, M. Nogami, Journal of Physical Chemistry. B 108 (2004) 11301. [11] Y.Y. Yu, S.S. Chang, C.L. Lee, C.R.C. Wang, Journal of Physical Chemistry. B 101 (1997) 6661. [12] J. Zhu, Plasma Science and Technology 5 (2003) 1835. [13] H. Inouye, K. Tanaka, I. Takahashi, K. Hirao, Physical Review. B, 57 (1998) 1134. [14] G.T. Boyd, Z.H. Yu, Y.R. Shen, Physical Review. B, 33 (1986) 7923. [15] M.B. Mohamed, V. Volkov, S. Link, M.A. El-Sayed, Chemical Physics Letters 317 (2000) 517.