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Influence of silver nanoclusters on formation of PbS quantum dots in glasses
Journal of Non-Crystalline Solids 357 (2011) 2428–2430
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Influence of silver nanoclusters on formation of PbS quantum dots in glasses Kai Xu a, Chao Liu a, Shixun Dai b, Xiang Shen b, Xunsi Wang b, Jong Heo a,⁎ a
Center for Information Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk 790-784, Republic of Korea b College of Information Science and Engineering, Ningbo University, Ningbo 315-021, China
a r t i c l e
i n f o
Article history: Received 9 July 2010 Received in revised form 7 October 2010 Accepted 2 November 2010 Available online 20 December 2010
a b s t r a c t Heat-treatment was used to precipitate PbS quantum dots (QDs) in silicate glasses doped with different amounts of Ag2O, and the influence of Ag2O on QDs was investigated. Under given heat-treatment conditions, the absorption coefficients and photoluminescence intensities of PbS QDs increased with the addition of Ag2O. Ag clusters formed by thermal treatment nucleated formation of PbS QDs in glasses. © 2010 Elsevier B.V. All rights reserved.
Keywords: PbS quantum dots; Silver clusters; Absorption; Photoluminescence; Thermal treatment
1. Introduction Over the past two decades, glasses doped with semiconductor quantum dots (QDs) have attracted considerable interest for optoelectronic applications due to the size-tunable optical properties of QDs. For example, glasses containing II–VI QDs have been used as sharp-cut filters and non-linear optical devices [1–3]. Recently, narrow-band IV–VI QDs embedded in glasses have also been investigated. These semiconductors have large exciton Bohr radii (PbS: 18 nm; PbSe: 46 nm) with the strong quantum confinement [4,5]. Therefore, glasses containing IV–VI QDs can have potential applications in optical switches and fiber-optic amplifiers for telecommunication [6,7]. Thermal treatment has been used to form QDs in glasses, but precise control of nucleation and growth of the QDs is difficult [8] and may require complicated procedures. A two-step heat-treatment method for the stepwise nucleation and growth of the nanocrystals can provide improvement to a certain degree. In some cases, a long annealing at low temperature helped to improve the uniformity of nucleation that led to the formation of small sized QDs with a narrow size distribution [9]. Nevertheless, precise control of the nucleation stage has been difficult and QDs precipitated inside glass matrices generally have a much broader size distribution than those prepared by colloidal precipitation [6]. Ag nanoclusters are known to act as nuclei and promote the formation of oxide nanocrystals in glass matrices [10]. This result ⁎ Corresponding author. Tel.: + 82 54 279 2147; fax: + 82 54 279 8653. E-mail address: [email protected] (J. Heo). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.091
suggests that Ag clusters in glasses may promote formation of QDs in glasses, and that adjusting Ag concentration may allow control of the number of nucleation sites. This paper reports the effect of Ag on the precipitation of PbS QDs, and the effects of heat-treatment temperature (T) on the absorption coefficients and photoluminescence (PL) characteristic of the glasses containing them. 2. Experimental procedures The nominal compositions of glasses were 50SiO2–35Na2O– 5Al2O3–8ZnO–2ZnS–1PbO (in mol %) with an additional Ag2O of 10, 20 and 30 ppm, respectively. After mixing, starting powders were melted in a platinum crucible at 1350 °C for 45 min under an ambient atmosphere. Melts were quenched by pouring onto a brass mold and pressing with another plate. The as-made glass containing 30 ppm of Ag2O contained defects of dark-colored stripes. Other compositions provided transparent glasses and they were annealed at 350 °C for 3 h, then heat-treated for 10 h at 440 ≤ T ≤ 480 °C to precipitate PbS QDs. Glasses were cut into sections (~1.5 mm thick) and optically polished. Optical absorption spectra of the glasses at wavelengths (λ) 300–2200 nm were recorded using a UV/Vis/NIR spectrophotometer (Perkin Elmer Lambda 750S). For the measurement of PL, an excitation source of 750 ≤ λ ≤ 790 nm from a continuous-wave Ti:Sapphire laser was used. A combination of a mechanical chopper of 50 Hz frequency, a 1/4 m monochromator, an InGaAs detector and a lock-in amplifier system was used to record PL spectra. All measurements were performed at room temperature. A high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2100F) was used under an
K. Xu et al. / Journal of Non-Crystalline Solids 357 (2011) 2428–2430
a
accelerating voltage of 200 kV to identify the crystal structure of the QDs.
2429
3
Absorption coefficient (cm-1)
As-made
3. Results 3.1. TEM analysis A glass containing 20 ppm of Ag2O which had been heat-treated at 480 °C was examined under HR-TEM (Fig. 1a). The QDs were nearly spherical with an average radius of approximately 3.3 nm and distribution of the size was ~10%. A TEM image (Fig. 1b) of a single nanocrystal was obtained and the fast Fourier transform (FFT) pattern was obtained for a portion of this image (Fig. 1b, inset). The diffraction pattern was consistent with that of a bulk PbS crystal which has a rock-salt crystal structure with a lattice constant of 5.9 Å. This result confirms that the QDs formed in the glasses are PbS.
440oC 450oC 460oC
2
o 470 C o 480 C
1
0 500
1000
1500
2000
Wavelength (nm)
b Absorption coefficient (cm-1)
3.2. Absorption and PL spectra In glasses without Ag, heat-treatment at 440 ≤ T ≤ 480 °C caused the absorption bands to appear at 695 ≤ λ ≤ 1580 nm (Fig. 2a). In glasses containing 20 ppm Ag2O, heat-treatment at 440 ≤ T ≤ 480 °C caused
8
As-made o 440 C
450oC 460oC
6
470oC o 480 C
4
2
0 500
1000
1500
2000
Wavelength (nm) Fig. 2. Absorption spectra of PbS QDs in glasses (a) without silver and (b) with 20 ppm of Ag2O after heat-treatment at various temperatures for 10 h.
these peaks to appear at 751 ≤ λ ≤ 1583 nm (Fig. 2b). At each concentration of Ag2O, the position of bands shifted to higher λ as temperature increased (Fig. 2; Table 1). This phenomenon demonstrates the strong quantum confinement effect in PbS QDs. The radii of the PbS QDs (Table 1) calculated using the parabolic model [11] increased with T. At a given T, the presence of Ag had little effect on the QD radius, but absorption coefficients for glasses containing 20 ppm of Ag2O were much higher than those for glasses without Ag. For instance, the absorption coefficient for the glass with 20 ppm of Ag2O heat-treated at 480 °C was ~ 5.0 cm− 1 while that for the glass without silver was 1.0 cm− 1 even subjected to the same heat-treatment (Table 1). In normalized PL spectra of PbS QDs in glasses containing 20 ppm of Ag2O the center wavelengths of PL from PbS QDs increased from 970 nm at T = 440 °C to 1615 nm at T = 480 °C (Fig. 3); this trend was
Table 1 Absorption peak position (λabs), absorption coefficient (α), and calculated average radius (R) of PbS QDs precipitated in glasses with 0, 10 or 20 ppm Ag2O by heat-treatment for 10 h at different temperatures. Treatment λabs (nm) temperature 0 10 (°C) ppm
Fig. 1. (a) TEM image of PbS nanocrystals precipitated in glass containing 20 ppm of Ag2O and heat-treated at 480 °C for 10 h (scale bar: 20 nm). (b) TEM image of one PbS nanocrystal and fast Fourier transform pattern (inset) obtained from the area in the circle.
440 450 460 470 480
α (cm− 1) 20 ppm
695 702 751 783 793 841 976 978 977 1232 1235 1256 1580 1587 1583
R (nm)
0
10 20 0 ppm ppm
10 ppm
20 ppm
0.05 0.16 0.28 0.75 1.00
0.14 0.29 0.71 1.42 2.10
1.2 ±0.1 1.5 ±0.1 1.9 ±0.2 2.4 ±0.3 3.4 ±0.3
1.4±0.1 1.6±0.2 1.9±0.2 2.5±0.4 3.4±0.3
0.48 0.94 1.91 3.65 5.05
1.2± 0.1 1.4± 0.2 1.9± 0.3 2.4± 0.3 3.4± 0.3
2430
K. Xu et al. / Journal of Non-Crystalline Solids 357 (2011) 2428–2430 440oC
1.0
o 460 C o 470 C o 480 C
Normalized PL
0.8
0.6
0.4
0.2
0.0 800
1000
1200
1400
1600
1800
Wavelength (nm) Fig. 3. Normalized PL spectra of PbS QDs in glasses containing 20 ppm of Ag2O after heat-treatment at various temperatures for 10 h.
3.0
absorption edge of glasses containing ZnS and PbO. Therefore, a glass that did not include ZnS and PbO was prepared to determine whether Ag clusters formed. This glass had a nominal composition (in mol %) of 50SiO2–35Na2O–5Al2O3–10ZnO and was doped with 20 ppm of Ag2O. An absorption band appeared at λ= 365 nm when the glass was heattreated at 460 °C for 10 h (Fig. 4). This absorption peak confirms the formation of Ag clusters such as Ag7 or Ag9 in this glass, and strongly suggests that they also formed in the glasses that included ZnS and PbO. The absorption coefficients and the PL intensities increased significantly with the addition of Ag2O (Fig. 5). The absorption coefficients are directly related to the concentration of PbS QDs [9], so the number of PbS QDs formed in glasses increased after Ag2O was added. We propose the following hypothesis to explain the results. First, increasing the concentration of silver increased the number of Ag clusters, which provide sites for nucleation of QDs. Second, this higher number of nucleation sites led to the formation of large numbers of PbS QDs in glasses. Third, the large number of QDs increased the absorption coefficients and the intensities of the PL spectra. Thus, Ag clusters act as nucleation sites and promote the growth of PbS QDs in glasses.
Absorption coefficient (cm-1)
As-made
2.5
5. Conclusions
460oC
PbS QDs were precipitated in glasses by heat-treatment and the effect of Ag2O content on the precipitation of the QDs was investigated. HR-TEM analysis confirmed the formation of the PbS QDs. Absorption coefficients and PL intensites of PbS QDs in glasses increased significantly as the amount of Ag2O increased. Silver clusters were formed during the heat-treatment and provided nucleation sites for PbS QDs.
2.0
1.5
1.0
~365 nm
0.5
Acknowledgements
0.0 300
400
500
600
700
Wavelength (nm) Fig. 4. Absorption spectra of glass without PbS QDs before and after heat-treatment at 460 °C for 10 h.
similar to the shift of the absorption bands to higher λ as T increased (Fig. 2).
Silver clusters such as Ag7 or Ag9 formed in glasses cause an absorption band at λ =~360 nm [12], but it is normally buried by the 4.0
460oC
3.5
50
20 Ag2O 10 Ag2O
3.0
No Ag2O
2.5
40 30
2.0 20
1.5
10
1.0 0.5
PL intensity (a. u.)
Absorption coefficient (cm-1)
References [1] [2] [3] [4]
4. Discussion
0.0 400
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0094037), and the Collaborative Research Project under the NRF—National Natural Science Foundation of China (NSFC) Cooperative Program (D00039).
0 600
800
1000
1200
1400
Wavelength (nm) Fig. 5. Absorption and PL spectra of PbS QDs in glasses containing different contents of Ag2O after heat-treatment at 460 °C for 10 h.
[5] [6] [7] [8] [9] [10] [11] [12]
N.F. Borrelli, D.W. Hall, H.J. Holland, D.W. Smith, J. Appl. Phys. 61 (1987) 5399. L.G. Zimin, Mater. Sci. Eng. B 9 (1991) 405. A.I. Ekimov, Phys. Scr. T39 (1991) 217. T. Okuno, A.A. Lipovskii, T. Ogawa, I. Amagai, Y. Masumoto, J. Lumin. 87 (2000) 491. F.W. Wise, Acc. Chem. Res. 33 (2000) 773. A.M. Malyarevich, K.V. Yumashev, A.A. Lipovskii, J. Appl. Phys. 103 (2008) 81307. J. Heo, C. Liu, J. Mater. Sci. Mater. Electron. 18 (2007) S135. R.E. Lamaëstre, H. Bernas, J. Appl. Phys. 98 (2005) 104310. S. Joshi, S. Sen, P.C. Ocampo, J. Phys. Chem. C 111 (2007) 4105. B. Zhu, Y. Liu, S. Ye, B. Qian, G. Liu, Y. Dai, H. Ma, J. Qiu, Opt. Lett. 34 (2009) 1666. I. Kang, F.W. Wise, J. Opt. Soc. Am. B 14 (1997) 1632. A. Henglein, Chem. Phys. Lett. 154 (1989) 473.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Influence of silver nanoclusters on formation of PbS quantum dots in glasses Kai Xu a, Chao Liu a, Shixun Dai b, Xiang Shen b, Xunsi Wang b, Jong Heo a,⁎ a
Center for Information Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk 790-784, Republic of Korea b College of Information Science and Engineering, Ningbo University, Ningbo 315-021, China
a r t i c l e
i n f o
Article history: Received 9 July 2010 Received in revised form 7 October 2010 Accepted 2 November 2010 Available online 20 December 2010
a b s t r a c t Heat-treatment was used to precipitate PbS quantum dots (QDs) in silicate glasses doped with different amounts of Ag2O, and the influence of Ag2O on QDs was investigated. Under given heat-treatment conditions, the absorption coefficients and photoluminescence intensities of PbS QDs increased with the addition of Ag2O. Ag clusters formed by thermal treatment nucleated formation of PbS QDs in glasses. © 2010 Elsevier B.V. All rights reserved.
Keywords: PbS quantum dots; Silver clusters; Absorption; Photoluminescence; Thermal treatment
1. Introduction Over the past two decades, glasses doped with semiconductor quantum dots (QDs) have attracted considerable interest for optoelectronic applications due to the size-tunable optical properties of QDs. For example, glasses containing II–VI QDs have been used as sharp-cut filters and non-linear optical devices [1–3]. Recently, narrow-band IV–VI QDs embedded in glasses have also been investigated. These semiconductors have large exciton Bohr radii (PbS: 18 nm; PbSe: 46 nm) with the strong quantum confinement [4,5]. Therefore, glasses containing IV–VI QDs can have potential applications in optical switches and fiber-optic amplifiers for telecommunication [6,7]. Thermal treatment has been used to form QDs in glasses, but precise control of nucleation and growth of the QDs is difficult [8] and may require complicated procedures. A two-step heat-treatment method for the stepwise nucleation and growth of the nanocrystals can provide improvement to a certain degree. In some cases, a long annealing at low temperature helped to improve the uniformity of nucleation that led to the formation of small sized QDs with a narrow size distribution [9]. Nevertheless, precise control of the nucleation stage has been difficult and QDs precipitated inside glass matrices generally have a much broader size distribution than those prepared by colloidal precipitation [6]. Ag nanoclusters are known to act as nuclei and promote the formation of oxide nanocrystals in glass matrices [10]. This result ⁎ Corresponding author. Tel.: + 82 54 279 2147; fax: + 82 54 279 8653. E-mail address: [email protected] (J. Heo). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.091
suggests that Ag clusters in glasses may promote formation of QDs in glasses, and that adjusting Ag concentration may allow control of the number of nucleation sites. This paper reports the effect of Ag on the precipitation of PbS QDs, and the effects of heat-treatment temperature (T) on the absorption coefficients and photoluminescence (PL) characteristic of the glasses containing them. 2. Experimental procedures The nominal compositions of glasses were 50SiO2–35Na2O– 5Al2O3–8ZnO–2ZnS–1PbO (in mol %) with an additional Ag2O of 10, 20 and 30 ppm, respectively. After mixing, starting powders were melted in a platinum crucible at 1350 °C for 45 min under an ambient atmosphere. Melts were quenched by pouring onto a brass mold and pressing with another plate. The as-made glass containing 30 ppm of Ag2O contained defects of dark-colored stripes. Other compositions provided transparent glasses and they were annealed at 350 °C for 3 h, then heat-treated for 10 h at 440 ≤ T ≤ 480 °C to precipitate PbS QDs. Glasses were cut into sections (~1.5 mm thick) and optically polished. Optical absorption spectra of the glasses at wavelengths (λ) 300–2200 nm were recorded using a UV/Vis/NIR spectrophotometer (Perkin Elmer Lambda 750S). For the measurement of PL, an excitation source of 750 ≤ λ ≤ 790 nm from a continuous-wave Ti:Sapphire laser was used. A combination of a mechanical chopper of 50 Hz frequency, a 1/4 m monochromator, an InGaAs detector and a lock-in amplifier system was used to record PL spectra. All measurements were performed at room temperature. A high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2100F) was used under an
K. Xu et al. / Journal of Non-Crystalline Solids 357 (2011) 2428–2430
a
accelerating voltage of 200 kV to identify the crystal structure of the QDs.
2429
3
Absorption coefficient (cm-1)
As-made
3. Results 3.1. TEM analysis A glass containing 20 ppm of Ag2O which had been heat-treated at 480 °C was examined under HR-TEM (Fig. 1a). The QDs were nearly spherical with an average radius of approximately 3.3 nm and distribution of the size was ~10%. A TEM image (Fig. 1b) of a single nanocrystal was obtained and the fast Fourier transform (FFT) pattern was obtained for a portion of this image (Fig. 1b, inset). The diffraction pattern was consistent with that of a bulk PbS crystal which has a rock-salt crystal structure with a lattice constant of 5.9 Å. This result confirms that the QDs formed in the glasses are PbS.
440oC 450oC 460oC
2
o 470 C o 480 C
1
0 500
1000
1500
2000
Wavelength (nm)
b Absorption coefficient (cm-1)
3.2. Absorption and PL spectra In glasses without Ag, heat-treatment at 440 ≤ T ≤ 480 °C caused the absorption bands to appear at 695 ≤ λ ≤ 1580 nm (Fig. 2a). In glasses containing 20 ppm Ag2O, heat-treatment at 440 ≤ T ≤ 480 °C caused
8
As-made o 440 C
450oC 460oC
6
470oC o 480 C
4
2
0 500
1000
1500
2000
Wavelength (nm) Fig. 2. Absorption spectra of PbS QDs in glasses (a) without silver and (b) with 20 ppm of Ag2O after heat-treatment at various temperatures for 10 h.
these peaks to appear at 751 ≤ λ ≤ 1583 nm (Fig. 2b). At each concentration of Ag2O, the position of bands shifted to higher λ as temperature increased (Fig. 2; Table 1). This phenomenon demonstrates the strong quantum confinement effect in PbS QDs. The radii of the PbS QDs (Table 1) calculated using the parabolic model [11] increased with T. At a given T, the presence of Ag had little effect on the QD radius, but absorption coefficients for glasses containing 20 ppm of Ag2O were much higher than those for glasses without Ag. For instance, the absorption coefficient for the glass with 20 ppm of Ag2O heat-treated at 480 °C was ~ 5.0 cm− 1 while that for the glass without silver was 1.0 cm− 1 even subjected to the same heat-treatment (Table 1). In normalized PL spectra of PbS QDs in glasses containing 20 ppm of Ag2O the center wavelengths of PL from PbS QDs increased from 970 nm at T = 440 °C to 1615 nm at T = 480 °C (Fig. 3); this trend was
Table 1 Absorption peak position (λabs), absorption coefficient (α), and calculated average radius (R) of PbS QDs precipitated in glasses with 0, 10 or 20 ppm Ag2O by heat-treatment for 10 h at different temperatures. Treatment λabs (nm) temperature 0 10 (°C) ppm
Fig. 1. (a) TEM image of PbS nanocrystals precipitated in glass containing 20 ppm of Ag2O and heat-treated at 480 °C for 10 h (scale bar: 20 nm). (b) TEM image of one PbS nanocrystal and fast Fourier transform pattern (inset) obtained from the area in the circle.
440 450 460 470 480
α (cm− 1) 20 ppm
695 702 751 783 793 841 976 978 977 1232 1235 1256 1580 1587 1583
R (nm)
0
10 20 0 ppm ppm
10 ppm
20 ppm
0.05 0.16 0.28 0.75 1.00
0.14 0.29 0.71 1.42 2.10
1.2 ±0.1 1.5 ±0.1 1.9 ±0.2 2.4 ±0.3 3.4 ±0.3
1.4±0.1 1.6±0.2 1.9±0.2 2.5±0.4 3.4±0.3
0.48 0.94 1.91 3.65 5.05
1.2± 0.1 1.4± 0.2 1.9± 0.3 2.4± 0.3 3.4± 0.3
2430
K. Xu et al. / Journal of Non-Crystalline Solids 357 (2011) 2428–2430 440oC
1.0
o 460 C o 470 C o 480 C
Normalized PL
0.8
0.6
0.4
0.2
0.0 800
1000
1200
1400
1600
1800
Wavelength (nm) Fig. 3. Normalized PL spectra of PbS QDs in glasses containing 20 ppm of Ag2O after heat-treatment at various temperatures for 10 h.
3.0
absorption edge of glasses containing ZnS and PbO. Therefore, a glass that did not include ZnS and PbO was prepared to determine whether Ag clusters formed. This glass had a nominal composition (in mol %) of 50SiO2–35Na2O–5Al2O3–10ZnO and was doped with 20 ppm of Ag2O. An absorption band appeared at λ= 365 nm when the glass was heattreated at 460 °C for 10 h (Fig. 4). This absorption peak confirms the formation of Ag clusters such as Ag7 or Ag9 in this glass, and strongly suggests that they also formed in the glasses that included ZnS and PbO. The absorption coefficients and the PL intensities increased significantly with the addition of Ag2O (Fig. 5). The absorption coefficients are directly related to the concentration of PbS QDs [9], so the number of PbS QDs formed in glasses increased after Ag2O was added. We propose the following hypothesis to explain the results. First, increasing the concentration of silver increased the number of Ag clusters, which provide sites for nucleation of QDs. Second, this higher number of nucleation sites led to the formation of large numbers of PbS QDs in glasses. Third, the large number of QDs increased the absorption coefficients and the intensities of the PL spectra. Thus, Ag clusters act as nucleation sites and promote the growth of PbS QDs in glasses.
Absorption coefficient (cm-1)
As-made
2.5
5. Conclusions
460oC
PbS QDs were precipitated in glasses by heat-treatment and the effect of Ag2O content on the precipitation of the QDs was investigated. HR-TEM analysis confirmed the formation of the PbS QDs. Absorption coefficients and PL intensites of PbS QDs in glasses increased significantly as the amount of Ag2O increased. Silver clusters were formed during the heat-treatment and provided nucleation sites for PbS QDs.
2.0
1.5
1.0
~365 nm
0.5
Acknowledgements
0.0 300
400
500
600
700
Wavelength (nm) Fig. 4. Absorption spectra of glass without PbS QDs before and after heat-treatment at 460 °C for 10 h.
similar to the shift of the absorption bands to higher λ as T increased (Fig. 2).
Silver clusters such as Ag7 or Ag9 formed in glasses cause an absorption band at λ =~360 nm [12], but it is normally buried by the 4.0
460oC
3.5
50
20 Ag2O 10 Ag2O
3.0
No Ag2O
2.5
40 30
2.0 20
1.5
10
1.0 0.5
PL intensity (a. u.)
Absorption coefficient (cm-1)
References [1] [2] [3] [4]
4. Discussion
0.0 400
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0094037), and the Collaborative Research Project under the NRF—National Natural Science Foundation of China (NSFC) Cooperative Program (D00039).
0 600
800
1000
1200
1400
Wavelength (nm) Fig. 5. Absorption and PL spectra of PbS QDs in glasses containing different contents of Ag2O after heat-treatment at 460 °C for 10 h.
[5] [6] [7] [8] [9] [10] [11] [12]
N.F. Borrelli, D.W. Hall, H.J. Holland, D.W. Smith, J. Appl. Phys. 61 (1987) 5399. L.G. Zimin, Mater. Sci. Eng. B 9 (1991) 405. A.I. Ekimov, Phys. Scr. T39 (1991) 217. T. Okuno, A.A. Lipovskii, T. Ogawa, I. Amagai, Y. Masumoto, J. Lumin. 87 (2000) 491. F.W. Wise, Acc. Chem. Res. 33 (2000) 773. A.M. Malyarevich, K.V. Yumashev, A.A. Lipovskii, J. Appl. Phys. 103 (2008) 81307. J. Heo, C. Liu, J. Mater. Sci. Mater. Electron. 18 (2007) S135. R.E. Lamaëstre, H. Bernas, J. Appl. Phys. 98 (2005) 104310. S. Joshi, S. Sen, P.C. Ocampo, J. Phys. Chem. C 111 (2007) 4105. B. Zhu, Y. Liu, S. Ye, B. Qian, G. Liu, Y. Dai, H. Ma, J. Qiu, Opt. Lett. 34 (2009) 1666. I. Kang, F.W. Wise, J. Opt. Soc. Am. B 14 (1997) 1632. A. Henglein, Chem. Phys. Lett. 154 (1989) 473.