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Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass
ARTICLE IN PRESS
Journal of Luminescence 124 (2007) 71–74 www.elsevier.com/locate/jlumin
Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass Haiping Hea,, Zhizhen Yea, Yuxia Wangb a
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
b
Received 20 July 2005; received in revised form 26 December 2005; accepted 7 January 2006 Available online 23 March 2006
Abstract Stable photoluminescence (PL) from AgI nanoparticles embedded in silica glass was investigated at room temperature. The Z1,2 excitonic emission of AgI exhibits fine structure with spacing of 0.20 eV (1610 cm 1), which is assigned to the frequency of vibration in interfacial water species. The PL excitation spectrum displays two newly observed bands at 3.45 and 4.35 eV associated with AgI–silica interaction. We suggest that the excitons in AgI are localized in the AgI/SiO2 interface region before radiative recombination. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55. m; 81.20. Fw Keywords: AgI nanoparticles; Photoluminescence; Exciton–phonon coupling
1. Introduction Silver iodide (AgI) has long been investigated due to its superionic property and to its use as sensitive detector in the visible range. Recently, there has also been increasing interest in nanometer AgI due to its novel structural, electrical, and optical properties different from bulk AgI [1–3]. It has been reported [4] that nanometer AgI may find applications as photocatalysts for solar energy conversion and as a medium for optical information or image storage. Although the superionic property of AgI and its composite glasses has been widely studied, data about optical properties of both bulk and microstructured AgI are rather poor. The optical properties of semiconductor nanocrystals provide information about the three-dimensional confinement of carriers and excitons. Among those optical analyses, photoluminescence (PL) is a useful tool for monitoring electronic changes at the nanoparticle surface. The confinement effect of excitons in AgI nanoparticles has been investigated by PL and optical absorption analyses as reported by several groups [5–8]. In Corresponding author. Tel.: 86 571 87953139; fax: 86 571 87952625.
E-mail address: [email protected] (H. He). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.365
most of these studies, AgI excitons were confined in crystal matrix, and the longitudinal optical (LO) phonon mode of AgI as well as unknown phonon modes were involved in the electron–phonon coupling. However, coupling of AgI excitons with vibrations of amorphous host material have not been observed. In this letter, we report for the first time the vibrational structured excitonic PL with spacing of about 1610 cm 1 from AgI nanoparticles embedded in silica glass. The radiative recombination involves vibrations of interfacial water species formed in silica during heat treatment. The current study shows a new type of stable light-emitting composite material and may throw light upon the application of AgI nanoparticles. 2. Experiments Tetraethoxysilane (TEOS), ethanol, and 0.1 M HNO3 solution were mixed slowly in a 1:3:0.004 molar ratio and the mixture was stirred for 30 min at 25 1C. Subsequently, 0.2 M AgNO3 solution was added into the mixture slowly. The final molar ratio of Ag+ ion to TEOS in the mixture was about 0.002. The clear liquid was then allowed to gel at 60 1C. After 8–12 h, the container was opened to the air and the excess liquid was decanted. The remaining gel was then
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air dried at 25 1C for at least a week before being encapsulated into an autoclave and heated at 180 1C for 4 h. This procedure produced a transparent rigid glass. The glass was then soaked in 1.0 M KI solution for 24 h and washed by de-ionized water. It became yellowish after soaking, indicating formation of AgI in the glass matrix. Finally, the yellowish glass was heated to 400 and 500 1C in air for 1–4 h and the final product became almost colorless. Transmission electron microscopy (TEM), PL, and PL excitation (PLE) spectra were measured to characterize AgI nanoparticles embedded in silica glass. The spectra were recorded on a Hitachi-850 fluorescence spectrophotometer. The excitation wavelength for the PL spectra is 230 nm, which was produced by a xenon lamp equipped with a grating monochromator. All of the measurements were carried out at room temperature. 3. Results and discussion
Z1,2 Intensity (a. u.)
Fig. 1. TEM images of AgI nanoparticles embedded in silica glass heated at 500 1C for 4 h.
Δν=1610cm-1
PL Intensity (a. u.)
TEM observations revealed that the distribution of AgI nanoparticles in silica glass is sparse, indicating a very small amount of AgI, consistent with the color change from yellowish to colorless after heat treatment. The color change is caused by the evaporation of AgI particles at high temperature. The particle size is about 5–20 nm, as illustrated in Fig. 1(a). Fig. 1(b) clearly shows the interface region between AgI nanoparticle and silica matrix. The width of the interface region is about 1.5–2 nm. Fig. 2 shows the room-temperature PL spectra of the samples heated at different temperatures and for different time. All the spectra exhibit two bands around 2.93 and 3.7 eV. The former is assigned to the Z1,2 excitonic emission of AgI [5], while the broad 3.7 eV band could be attributed to non-bridging oxygen hole centers (NBOHC) in silica [9]. For comparison, the spectrum for the sample that was not heated was also plotted. The spectrum only shows an emission band around 3.7 eV from the silica. The full-width at half-maximum (FWHM) of the Z1,2 emission is estimated to be 200 meV by using Gaussian fit. The inhomogeneous broadening is due to the size fluctuations and variations in surface structure of the nanoparticles. One can see from the aging test (inset of Fig. 2) that the Z1,2 emission is fairly stable. The most striking feature in these PL spectra is the presence of vibrational structure with energy spacing of about 0.20 eV (1610 cm 1). As the heating temperature and time increase, these shoulder peaks become more and more prominent. Such vibrational structure is strong evidence of phonon-assisted transition process. It is well known [5] that AgI is at the boundary between an ionic semiconductor and a covalent one. Because of the ionicity, AgI shows remarkable exciton–phonon coupling. Fig. 3 shows the PLE spectrum for the Z1,2 emission. Upon the basis of the band-to-band absorption in AgI, it exhibits four excitation bands at 3.45, 4.35, 4.94, and 5.70 eV. The excitation band around 4.9 eV (labeled E1) has been reported in AgI nanoparticles [3,10] and can be
2.0 1.5 1.0 0.5 0.0
0
2 1 3 Aging Time (month)
NBOHC
c Z1, 2
b a x5
2.0
2.5
3.0
3.5
4.0
4.5
Photon Energy (eV) Fig. 2. PL spectra of AgI nanoparticles embedded in silica glass heated at (a) 400 1C for 1 h, (b) 400 1C for 4 h, and (c) 500 1C for 4 h. The arrows show the vibrational structure with spacing of 1610 cm 1. The dashed curve represents spectrum for the sample that was not heated. The band around 3.7 eV could be attributed to non-bridging oxygen hole centers (NBOHC) in silica. Inset shows the stability of the Z1,2 emission.
ARTICLE IN PRESS H. He et al. / Journal of Luminescence 124 (2007) 71–74
account the following facts. For sol–gel-derived silica glass, it is well known that RSi–H and RSi–OH groups exist in the surface region. Heating the samples at high temperature results in break of RSi–OH, giving rise to the formation of RSi–O defects (NBOHC) [15]. This is evidenced by the growing intensity of the 3.7 eV PL with the heating temperature and increasing time. Thus, interfacial water molecules could be created during heat treatment by the following reaction as proposed by Glinka et al. [15] in silica nanoparticles:
monitored at 2.93 eV E1 PL Intensity (a. u.)
4.94
4.35 Z1, 2
73
3.45 5.70
(1)
3.0
3.5
4.0 4.5 5.0 Photon Energy (eV)
5.5
6.0
Fig. 3. PLE spectrum of AgI nanoparticles embedded in silica glass. Upon the basis of the band-to-band absorption in AgI, two bands around 3.45 and 4.35 eV are newly observed. The dash line indicates the Z1,2 energy of AgI.
attributed to the direct transitions at a point in the [1 1 1] direction of k space in most zincblende-like semiconductors [11], while the band at 5.70 eV is most probably due to the spin–orbital splitting of the E1 peak [3]. Laref et al. [12] and Goldmann [13] reported transitions with energies of 3.6 and 4.15 eV in AgI theoretically and experimentally, respectively. However, these transitions cannot be applied to identify the bands around 3.45 and 4.35 eV because the reported transitions take place between energy levels within the valence band of AgI. On the other hand, pure silica also does not exhibit absorptions with similar energies. The actual origins of these two newly observed bands are not clear; we suggest that they correlate to the interface states formed between AgI nanoparticles and SiO2 matrix during heat treatment, mainly due to the lattice mismatch, surface roughness, and variations in surface composition. An energy transfer from the excited interface states to AgI excitons during the excitation process could then be expected. The width of the interface region is comparable with the reported excitonic Bohr radius, 1.6–2.37 nm [6], as indicated in Fig. 1(b). Therefore, the disordered potential and the electronic interactions at the interface could induce exciton localization. The vibrational structure in PL as well as the PLE features indicates that the excitons in AgI nanoparticles are localized at the AgI/SiO2 interface and strongly couple with vibrations. However, the energy interval obtained in Fig. 3 is so large that it cannot be attributed to either AgI LO phonon (15.4 meV) or Si–O vibration (0.13–0.14 eV, 1100 cm 1). It is reminiscent of the O–H bond in water. The frequencies of bending vibrations for the liquid and gaseous water are 1640 and 1595 cm 1, respectively [14]. In order to clarify the center responsible for the 1610 cm 1 vibration, we take into
We suggest that the localized excitons in AgI could couple with the vibration of interfacial water species during radiative recombination, giving rise to the phonon replicas in the PL spectra. The coupling strength of electronic and vibrational excitation in nanoparticles is sensitive to surface structure [16]. As the heating temperature and time increase, the interaction between the AgI nanoparticles and the silica matrix becomes stronger and causes formation of interface states such as Si–O–Ag structure [3,17]. On the other hand, the amount of interfacial water species is also expected to increase. These factors would facilitate the exciton–phonon coupling, consistent with the results revealed in Fig. 2. It should be noted that trace amount of water exists inherently in our sample because the silica glass was synthesized by hydrolysis of TEOS. Therefore, it is rather difficult to obtain direct experimental evidences for the formation of interfacial water species. Although the interfacial water model still needs verification, our experimental results strongly support its validity. 4. Conclusion In summary, we have provided evidence that excitons in AgI nanoparticles are localized in the AgI/SiO2 interface region and then recombine radiatively, exhibiting a vibrational structure with spacing of 1610 cm 1. We assumed that the interfacial water species are responsible for the observed phonon frequency. The PLE spectrum revealed two newly observed excitation bands at 3.45 and 4.35 eV, which also confirm the strong interaction between AgI nanoparticles and the silica glass network. References [1] Y.H. Wang, C.H. Ye, G.Z. Wang, L.D. Zhang, Y.M. Liu, Z.Y. Zhao, Appl. Phys. Lett. 82 (2003) 4253. [2] Y.X. Wang, L. Huang, H.P. He, M. Li, Physica B 325 (2003) 357. [3] H.P. He, Y.X. Wang, Y.M. Zou, J. Phys.: Condens. Matter 15 (2003) 4869. [4] W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, K.M. Fang, Y. Xu, M.Z. Su, J.H. Lin, J. Appl. Phys. 83 (1998) 3811. [5] S. Mochizuki, K. Umezawa, Phys. Lett. A 228 (1997) 111.
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H. He et al. / Journal of Luminescence 124 (2007) 71–74
[6] G. Mshvelidse, O. Gogolin, E. Tsitsishvili, C. Ma¨rkle, J.S. Chadha, A. Hepting, F. Gindele, W. Petri, W. Send, D. Gerthsen, U. Woggon, C. Klingshirn, Phys. Status Solidi B 207 (1998) 369. [7] P.S. Kumar, C.S. Sunandana, Nano Lett. 2 (2002) 431. [8] O. Husberg, H. Vogelsang, W. von der Osten, J. Lumin. 96 (2002) 155. [9] B.D. Yao, H.Z. Shi, X.Y. Zhang, L.D. Zhang, Appl. Phys. Lett. 78 (2001) 174. [10] W. Chen, A.G. Joly, J. Roark, Phys. Rev. B 65 (2002) 245404. [11] M. Cardona, Phys. Rev. 129 (1963) 69.
[12] A. Laref, W. Sekkal, A. Zaoui, M. Certier, H. Aourag, J. Appl. Phys. 86 (1999) 4435. [13] A. Goldmann, Phys. Status Solidi B 81 (1977) 9. [14] G. Herzberg, Molecular Spectra and Molecular Structure: Infrared and Raman Spectra of Polyatomic Molecules, Krieger, New York, 1991. [15] Y.D. Glinka, S.H. Lin, Y.T. Chen, Appl. Phys. Lett. 75 (1999) 778. [16] Y. Kanemitsu, J. Lumin. 83–84 (1999) 283. [17] A.A. Ahmed, E.W.A. Allah, J. Am. Ceram. Soc. 78 (1995) 2777.
Journal of Luminescence 124 (2007) 71–74 www.elsevier.com/locate/jlumin
Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass Haiping Hea,, Zhizhen Yea, Yuxia Wangb a
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
b
Received 20 July 2005; received in revised form 26 December 2005; accepted 7 January 2006 Available online 23 March 2006
Abstract Stable photoluminescence (PL) from AgI nanoparticles embedded in silica glass was investigated at room temperature. The Z1,2 excitonic emission of AgI exhibits fine structure with spacing of 0.20 eV (1610 cm 1), which is assigned to the frequency of vibration in interfacial water species. The PL excitation spectrum displays two newly observed bands at 3.45 and 4.35 eV associated with AgI–silica interaction. We suggest that the excitons in AgI are localized in the AgI/SiO2 interface region before radiative recombination. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55. m; 81.20. Fw Keywords: AgI nanoparticles; Photoluminescence; Exciton–phonon coupling
1. Introduction Silver iodide (AgI) has long been investigated due to its superionic property and to its use as sensitive detector in the visible range. Recently, there has also been increasing interest in nanometer AgI due to its novel structural, electrical, and optical properties different from bulk AgI [1–3]. It has been reported [4] that nanometer AgI may find applications as photocatalysts for solar energy conversion and as a medium for optical information or image storage. Although the superionic property of AgI and its composite glasses has been widely studied, data about optical properties of both bulk and microstructured AgI are rather poor. The optical properties of semiconductor nanocrystals provide information about the three-dimensional confinement of carriers and excitons. Among those optical analyses, photoluminescence (PL) is a useful tool for monitoring electronic changes at the nanoparticle surface. The confinement effect of excitons in AgI nanoparticles has been investigated by PL and optical absorption analyses as reported by several groups [5–8]. In Corresponding author. Tel.: 86 571 87953139; fax: 86 571 87952625.
E-mail address: [email protected] (H. He). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.365
most of these studies, AgI excitons were confined in crystal matrix, and the longitudinal optical (LO) phonon mode of AgI as well as unknown phonon modes were involved in the electron–phonon coupling. However, coupling of AgI excitons with vibrations of amorphous host material have not been observed. In this letter, we report for the first time the vibrational structured excitonic PL with spacing of about 1610 cm 1 from AgI nanoparticles embedded in silica glass. The radiative recombination involves vibrations of interfacial water species formed in silica during heat treatment. The current study shows a new type of stable light-emitting composite material and may throw light upon the application of AgI nanoparticles. 2. Experiments Tetraethoxysilane (TEOS), ethanol, and 0.1 M HNO3 solution were mixed slowly in a 1:3:0.004 molar ratio and the mixture was stirred for 30 min at 25 1C. Subsequently, 0.2 M AgNO3 solution was added into the mixture slowly. The final molar ratio of Ag+ ion to TEOS in the mixture was about 0.002. The clear liquid was then allowed to gel at 60 1C. After 8–12 h, the container was opened to the air and the excess liquid was decanted. The remaining gel was then
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air dried at 25 1C for at least a week before being encapsulated into an autoclave and heated at 180 1C for 4 h. This procedure produced a transparent rigid glass. The glass was then soaked in 1.0 M KI solution for 24 h and washed by de-ionized water. It became yellowish after soaking, indicating formation of AgI in the glass matrix. Finally, the yellowish glass was heated to 400 and 500 1C in air for 1–4 h and the final product became almost colorless. Transmission electron microscopy (TEM), PL, and PL excitation (PLE) spectra were measured to characterize AgI nanoparticles embedded in silica glass. The spectra were recorded on a Hitachi-850 fluorescence spectrophotometer. The excitation wavelength for the PL spectra is 230 nm, which was produced by a xenon lamp equipped with a grating monochromator. All of the measurements were carried out at room temperature. 3. Results and discussion
Z1,2 Intensity (a. u.)
Fig. 1. TEM images of AgI nanoparticles embedded in silica glass heated at 500 1C for 4 h.
Δν=1610cm-1
PL Intensity (a. u.)
TEM observations revealed that the distribution of AgI nanoparticles in silica glass is sparse, indicating a very small amount of AgI, consistent with the color change from yellowish to colorless after heat treatment. The color change is caused by the evaporation of AgI particles at high temperature. The particle size is about 5–20 nm, as illustrated in Fig. 1(a). Fig. 1(b) clearly shows the interface region between AgI nanoparticle and silica matrix. The width of the interface region is about 1.5–2 nm. Fig. 2 shows the room-temperature PL spectra of the samples heated at different temperatures and for different time. All the spectra exhibit two bands around 2.93 and 3.7 eV. The former is assigned to the Z1,2 excitonic emission of AgI [5], while the broad 3.7 eV band could be attributed to non-bridging oxygen hole centers (NBOHC) in silica [9]. For comparison, the spectrum for the sample that was not heated was also plotted. The spectrum only shows an emission band around 3.7 eV from the silica. The full-width at half-maximum (FWHM) of the Z1,2 emission is estimated to be 200 meV by using Gaussian fit. The inhomogeneous broadening is due to the size fluctuations and variations in surface structure of the nanoparticles. One can see from the aging test (inset of Fig. 2) that the Z1,2 emission is fairly stable. The most striking feature in these PL spectra is the presence of vibrational structure with energy spacing of about 0.20 eV (1610 cm 1). As the heating temperature and time increase, these shoulder peaks become more and more prominent. Such vibrational structure is strong evidence of phonon-assisted transition process. It is well known [5] that AgI is at the boundary between an ionic semiconductor and a covalent one. Because of the ionicity, AgI shows remarkable exciton–phonon coupling. Fig. 3 shows the PLE spectrum for the Z1,2 emission. Upon the basis of the band-to-band absorption in AgI, it exhibits four excitation bands at 3.45, 4.35, 4.94, and 5.70 eV. The excitation band around 4.9 eV (labeled E1) has been reported in AgI nanoparticles [3,10] and can be
2.0 1.5 1.0 0.5 0.0
0
2 1 3 Aging Time (month)
NBOHC
c Z1, 2
b a x5
2.0
2.5
3.0
3.5
4.0
4.5
Photon Energy (eV) Fig. 2. PL spectra of AgI nanoparticles embedded in silica glass heated at (a) 400 1C for 1 h, (b) 400 1C for 4 h, and (c) 500 1C for 4 h. The arrows show the vibrational structure with spacing of 1610 cm 1. The dashed curve represents spectrum for the sample that was not heated. The band around 3.7 eV could be attributed to non-bridging oxygen hole centers (NBOHC) in silica. Inset shows the stability of the Z1,2 emission.
ARTICLE IN PRESS H. He et al. / Journal of Luminescence 124 (2007) 71–74
account the following facts. For sol–gel-derived silica glass, it is well known that RSi–H and RSi–OH groups exist in the surface region. Heating the samples at high temperature results in break of RSi–OH, giving rise to the formation of RSi–O defects (NBOHC) [15]. This is evidenced by the growing intensity of the 3.7 eV PL with the heating temperature and increasing time. Thus, interfacial water molecules could be created during heat treatment by the following reaction as proposed by Glinka et al. [15] in silica nanoparticles:
monitored at 2.93 eV E1 PL Intensity (a. u.)
4.94
4.35 Z1, 2
73
3.45 5.70
(1)
3.0
3.5
4.0 4.5 5.0 Photon Energy (eV)
5.5
6.0
Fig. 3. PLE spectrum of AgI nanoparticles embedded in silica glass. Upon the basis of the band-to-band absorption in AgI, two bands around 3.45 and 4.35 eV are newly observed. The dash line indicates the Z1,2 energy of AgI.
attributed to the direct transitions at a point in the [1 1 1] direction of k space in most zincblende-like semiconductors [11], while the band at 5.70 eV is most probably due to the spin–orbital splitting of the E1 peak [3]. Laref et al. [12] and Goldmann [13] reported transitions with energies of 3.6 and 4.15 eV in AgI theoretically and experimentally, respectively. However, these transitions cannot be applied to identify the bands around 3.45 and 4.35 eV because the reported transitions take place between energy levels within the valence band of AgI. On the other hand, pure silica also does not exhibit absorptions with similar energies. The actual origins of these two newly observed bands are not clear; we suggest that they correlate to the interface states formed between AgI nanoparticles and SiO2 matrix during heat treatment, mainly due to the lattice mismatch, surface roughness, and variations in surface composition. An energy transfer from the excited interface states to AgI excitons during the excitation process could then be expected. The width of the interface region is comparable with the reported excitonic Bohr radius, 1.6–2.37 nm [6], as indicated in Fig. 1(b). Therefore, the disordered potential and the electronic interactions at the interface could induce exciton localization. The vibrational structure in PL as well as the PLE features indicates that the excitons in AgI nanoparticles are localized at the AgI/SiO2 interface and strongly couple with vibrations. However, the energy interval obtained in Fig. 3 is so large that it cannot be attributed to either AgI LO phonon (15.4 meV) or Si–O vibration (0.13–0.14 eV, 1100 cm 1). It is reminiscent of the O–H bond in water. The frequencies of bending vibrations for the liquid and gaseous water are 1640 and 1595 cm 1, respectively [14]. In order to clarify the center responsible for the 1610 cm 1 vibration, we take into
We suggest that the localized excitons in AgI could couple with the vibration of interfacial water species during radiative recombination, giving rise to the phonon replicas in the PL spectra. The coupling strength of electronic and vibrational excitation in nanoparticles is sensitive to surface structure [16]. As the heating temperature and time increase, the interaction between the AgI nanoparticles and the silica matrix becomes stronger and causes formation of interface states such as Si–O–Ag structure [3,17]. On the other hand, the amount of interfacial water species is also expected to increase. These factors would facilitate the exciton–phonon coupling, consistent with the results revealed in Fig. 2. It should be noted that trace amount of water exists inherently in our sample because the silica glass was synthesized by hydrolysis of TEOS. Therefore, it is rather difficult to obtain direct experimental evidences for the formation of interfacial water species. Although the interfacial water model still needs verification, our experimental results strongly support its validity. 4. Conclusion In summary, we have provided evidence that excitons in AgI nanoparticles are localized in the AgI/SiO2 interface region and then recombine radiatively, exhibiting a vibrational structure with spacing of 1610 cm 1. We assumed that the interfacial water species are responsible for the observed phonon frequency. The PLE spectrum revealed two newly observed excitation bands at 3.45 and 4.35 eV, which also confirm the strong interaction between AgI nanoparticles and the silica glass network. References [1] Y.H. Wang, C.H. Ye, G.Z. Wang, L.D. Zhang, Y.M. Liu, Z.Y. Zhao, Appl. Phys. Lett. 82 (2003) 4253. [2] Y.X. Wang, L. Huang, H.P. He, M. Li, Physica B 325 (2003) 357. [3] H.P. He, Y.X. Wang, Y.M. Zou, J. Phys.: Condens. Matter 15 (2003) 4869. [4] W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, K.M. Fang, Y. Xu, M.Z. Su, J.H. Lin, J. Appl. Phys. 83 (1998) 3811. [5] S. Mochizuki, K. Umezawa, Phys. Lett. A 228 (1997) 111.
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[6] G. Mshvelidse, O. Gogolin, E. Tsitsishvili, C. Ma¨rkle, J.S. Chadha, A. Hepting, F. Gindele, W. Petri, W. Send, D. Gerthsen, U. Woggon, C. Klingshirn, Phys. Status Solidi B 207 (1998) 369. [7] P.S. Kumar, C.S. Sunandana, Nano Lett. 2 (2002) 431. [8] O. Husberg, H. Vogelsang, W. von der Osten, J. Lumin. 96 (2002) 155. [9] B.D. Yao, H.Z. Shi, X.Y. Zhang, L.D. Zhang, Appl. Phys. Lett. 78 (2001) 174. [10] W. Chen, A.G. Joly, J. Roark, Phys. Rev. B 65 (2002) 245404. [11] M. Cardona, Phys. Rev. 129 (1963) 69.
[12] A. Laref, W. Sekkal, A. Zaoui, M. Certier, H. Aourag, J. Appl. Phys. 86 (1999) 4435. [13] A. Goldmann, Phys. Status Solidi B 81 (1977) 9. [14] G. Herzberg, Molecular Spectra and Molecular Structure: Infrared and Raman Spectra of Polyatomic Molecules, Krieger, New York, 1991. [15] Y.D. Glinka, S.H. Lin, Y.T. Chen, Appl. Phys. Lett. 75 (1999) 778. [16] Y. Kanemitsu, J. Lumin. 83–84 (1999) 283. [17] A.A. Ahmed, E.W.A. Allah, J. Am. Ceram. Soc. 78 (1995) 2777.