- Email: [email protected]
Electric field-assisted Ag+ migration for PbS quantum dot formation in glasses
Journal of Non-Crystalline Solids 377 (2013) 254–256
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Electric field-assisted Ag + migration for PbS quantum dot formation in glasses Kai Xu, Jong Heo ⁎ Division of Advanced Nuclear Engineering, and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2012 Received in revised form 16 November 2012 Available online 31 January 2013 Keywords: Photoluminescence; Ag nanoparticles; Quantum dots; Electric-field assistance
a b s t r a c t Electric-field assisted solid-state diffusion of Ag+ was used to control precipitation of PbS quantum dots (QDs) inside glasses. Precipitation of PbS QDs was confirmed by transmission electron microscopy images and photoluminescence (PL) spectra. PL bands moved to longer wavelengths as applied voltage increased under the same subsequent heat-treatment, and as heat-treatment temperature increased under the same voltage. PbS QDs can preferentially precipitate at low temperature and photo-luminesce at longer wavelengths in Ag+-affected regions than in unaffected regions. Ag nanoparticles were also formed inside affected regions after thermal treatment, and provided nucleation sites for PbS QDs. Precipitation of PbS QDs and their spatial distribution in glasses can be controlled by electric field-assisted Ag+ migration. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Glasses doped with PbS quantum dots (QDs) can absorb and emit light at wavelength λ that can be controlled from ~ 0.7 ≤ λ ≤ ~ 2.0 μm by adjusting QDs' size from ~ 2 to ~ 10 nm [1–4]. They have possible applications as solid-state saturable absorbers for mode-locked lasers [5–7] and as fiber-optic amplifiers in optical communications [8,9]. The average size of PbS QDs and their size distribution inside glasses can be tailored by carefully controlling the temperature and duration of thermal treatment [1,4,10]. Using Ag nanoparticles (NPs) as nucleating agents to realize accurate control of PbS QD precipitation in glasses has been investigated [3,11,12]. In an early report, a few tens of parts per million of Ag+ ions were incorporated directly into glass melts and the glasses were heated to form PbS QDs [3]. Adding Ag+ ions allows control of the size and size distribution of PbS QDs, and their number density in glasses increases with the concentration of Ag+ ions added. This increase in PbS number density results in a large increase of absorption and photoluminescence (PL) intensities. Ag+ ions are also incorporated into the glasses using a Na+ ↔Ag+ ion-exchange process. The size of PbS QDs precipitated after thermal treatment is larger in the surface region affected by Ag+ ion-exchange than in the Ag+-free region. When the heat-treatment temperature is sufficiently low, PbS QDs precipitate preferentially only at the surface in which Ag+ ion-exchange has occurred [11,12]. These results indicated that the precipitation and size of PbS QDs in glasses can be controlled by manipulating Ag+ ions inside them. In previous work, Ag + ions were introduced into glasses through ion-exchange by immersing the specimens in an AgNO3 solution or a molten salt bath [11,12]. However, the long ion-exchange duration ⁎ Corresponding author. Tel.: +82 54 279 2147; fax: + 82 54 279 2399 E-mail address: [email protected] (J. Heo). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.003
in the AgNO3 solution causes the contamination of the glass surface. The migration of Ag + ions inside the AgNO3 melt was very fast, and it makes the control of amount of Ag difficult. Incorporation of Ag + ions into glasses can be also achieved using the solid-state Ag + diffusion method, in which Ag film or paste is directly deposited on the glass matrix as the ion sources. An electric field induces electrolysis of Ag + ions from neutral Ag (Ag 0) and assists Ag + ion migration into glasses [13–16]. The diffusion of Ag + ions into glasses can be exactly determined by controlling electric-field strength, process duration, or temperature. In addition, the distribution of Ag + ions inside glasses can be controlled with accuracy. In this study, an electric field was used to cause Ag + ions to migrate into glasses from Ag paste, and their effect on controlled precipitation of PbS QDs in glasses was investigated. The size of PbS QDs was controlled by changing applied voltages and subsequent heat-treatment temperatures. 2. Experimental procedures A glass with a nominal composition (mol%) of 50SiO2–35Na2O– 5Al2O3–8ZnO–2ZnS doped with 0.8 mol% of PbO was prepared using melt-quenching. The preparation procedures were similar to those used in previous studies [3,11,12]. Starting powders of ~25 g were finely ground in ethanol using ZrO2 balls. The mixtures were heated to remove ethanol and moisture, and then melted in a corundum crucible at ~1350 °C for 45 min. The melts were poured into a pre-heated brass mold and pressed to a thickness of ~1.5 mm using an iron plate. The glass was annealed at 350 °C for 3 h to release the thermal stress, and then cut into pieces of ~1.0 × 1.0 cm. The upper and lower surfaces of the pieces were optically polished until glass thickness was ~1.0 mm. Both surfaces of glass specimens were homogeneously coated with 0.1-mm-thick Ag paste, then placed between two electrodes.
K. Xu, J. Heo / Journal of Non-Crystalline Solids 377 (2013) 254–256
255
Electrodes were covered with Al foil to prevent parasite diffusion. Ag + diffusion was conducted under applied voltages of 2, 3, 6 or 10 kV/cm for 60 min at room temperature. Afterwards, the coated Ag paste on both surfaces was cleaned using acetone, and specimens were further heat-treated at 420, 430, 440 or 450 °C for 10 h to induce precipitation of PbS QDs. Formation of QDs was confirmed using a transmission electron microscope (TEM) under an accelerating voltage of 200 kV. PL spectra were recorded at room temperature using an 800-nm excitation beam from a continuous-wave Ti-sapphire laser; signals were collected and amplified using a combination of a mechanical chopper of 50-Hz frequency, a 1/4 m monochromator, an InGaAs detector and a lock-in amplifier system. Standard deviations of PL wavelengths were b 1.5% including instrumental and experimental errors. The depth of Ag penetration was analyzed using energy dispersive X-ray spectroscopy (EDX) after polishing a cross-section of the glass. 3. Results When the glass was subjected to electric voltage of 10 kV/cm and further heat-treated at 430 °C, the surface that contacted the positive electrode became dark brown, whereas the surface that contacted the negative electrode remained unchanged yellowish. A TEM specimen was prepared from the dark brown region using a focused ion beam method. Black dots with an average diameter of ~3.6 (±0.6) nm were uniformly distributed in the glass matrix (Fig. 1a). A high-resolution TEM image of one crystal was obtained; it had a fringe spacing of ~0.17 nm, which is similar to the (222) plane spacing of bulk PbS (Fig. 1b). PL spectra were recorded to demonstrate the effect of electric-field assisted Ag+ diffusion on the precipitation of PbS QDs. The exciting laser was focused on the positive electrode surface of the glass where Ag+ migrated into it and thus facilitated PbS QD formation. The laser beam was irradiated at an angle of 45° and the luminescence was also detected at a 45° angle to form a detection direction perpendicular to the incoming laser beam. In glasses subjected to 10 kV/cm, clear PL bands from PbS QDs were observed (Fig. 2a); the center wavelengths of PL were 1155, 1270, 1390 and 1525 nm at heat-treatment temperatures of 420, 430, 440 and 450 °C, respectively (Fig. 2a). In addition, when heat-treatment temperature was 430 °C, the PL peak shifted from 1175 to 1270 nm as applied voltage increased from 2 to 10 kV/cm (Fig. 2b). Therefore, the wavelength of emission from PbS QDs in glasses can be tuned by controlling the heat-treatment temperatures and the applied electric voltages. 4. Discussion When an electric-field is applied, the neutral Ag0 in paste is oxidized to Ag+ ions, which then migrate from the positive side into the body of the glass [13,15]. During thermal treatment, Ag+ ions inside glasses reduce to Ag0 by capturing electrons from impurities or non-bridging oxygens, then aggregate to form Ag NPs (nAg+ + ne− → nAg → Agn) [17,18]. Ag NPs were not clearly identified in the X-ray diffraction patterns or TEM micrographs, probably due to the low concentration and small size of NPs. Ag NPs that formed in silicate glasses normally have an absorption band at λ ≈ 400 nm [19], but the absorption by PbS QDs in our glasses also obscured this peak. To confirm that Ag NPs formed in our glasses, we prepared a glass free of PbS QDs with a composition (mol%) of 50SiO2–35Na2O–5Al2O3–8ZnO–2ZnS. The glass was subjected to 10 kV/cm voltage, and then heat-treated at 400 °C for 10 h. A weak absorption band was observed at λ ≈ 400 nm (Fig. 3), indicating that Ag NPs precipitated in the glass. Thus, we inferred that Ag NPs were also formed in glasses containing PbS QDs probably at temperature as low as 400 °C. PbS QDs cannot nucleate in the glasses at heat-treatment temperature ≤430 °C if Ag NPs are not present [3]. In the present study, PbS
Fig. 1. (a) TEM image of nano-dots precipitated in Ag+-migrated regions; inset: size dispersion of nano-dots. (b) High-resolution TEM image of one PbS nano-dot. 10 kV/cm voltage was applied for 60 min to the glass and then it was heat-treated at 430 °C for 10 h.
QDs precipitated after heat treatment at 420 °C but only in the area that contained Ag+, as indicated by the emission in Fig. 2a. When heat-treatment temperatures increased to 440 and 450 °C, center wavelengths of PbS QDs emission from Ag+-containing regions were ~1390 and ~1525 nm, respectively (Fig. 2a), longer than those from Ag+-free glasses as the published report that PL position red-shifted from ~1000 to ~1100 nm [12]. These results again confirmed that formation of Ag NPs caused increase in the size of the PbS QDs, similar to results reported previously [11,12]. Therefore, Ag NPs that formed in glasses after application of an electric field and thermal treatment provided nucleation sites for PbS QD formation. The sizes of PbS QDs can also be controlled by changing the applied voltage (Fig. 2b). When 10 kV/cm electric voltage was applied, Ag+ migrated up to ~3 μm into the glass, and showed a steep concentration gradient (Fig. 4). After subsequent heat treatment at 430 °C for 10 h, Ag+ diffused further into the glass to ~20 μm with approximately uniform distribution;
256
K. Xu, J. Heo / Journal of Non-Crystalline Solids 377 (2013) 254–256
10
a 1.0
(ii)
(iii)
(iv)
Normalized PL
0.8
(i) 420oC (ii) 430oC (iii) 440oC (iv) 450oC
10kV/cm o 10kV/cm+430 C 8
Atom (%)
(i)
0.6
0.4 1 0.2 0 0.0
0 1000
1200
1400
1600
1800
5
10
15
20
25
Depth from the surface ( m)
Wavelength (nm) Fig. 4. Ag concentrations in the glass after 10 kV/cm electric-field assistance for 60 min and then heat treatment at 430 °C for 10 h.
b 1.0
(i) 2kV/cm (ii) 3kV/cm (iii) 6kV/cm (iv) 10kV/cm
Normalized PL
0.8
(i) (ii)
0.6
The formation of PbS QDs was confirmed from the fringe spacing in a high-resolution TEM image. Peak wavelengths of the PL bands shifted from 1175 to 1270 nm as the applied voltage increased from 2 to 10 kV/cm under the same heat-treatment temperature of 430 °C. Wavelengths of PL bands recorded from the regions treated using a voltage of 10 kV/cm increased from 1155 to 1525 nm as temperatures of the subsequent thermal treatment increased from 420 to 450 °C. Penetration depth of Ag+ was ~20 μm after application of 10 kV/cm voltage and heat treatment at 430 °C. Ag NPs that precipitated inside affected regions formed the nucleation sites and induced preferential precipitation of PbS QDs at low temperatures. Electric field-assisted Ag+ migration can provide an alternative approach to controlling the precipitation and spatial distribution of PbS QDs inside glasses.
(iii) (iv)
0.4
0.2
0.0 1000
1200
1400
1600
Wavelength (nm) Fig. 2. (a) Normalized PL spectra of glasses subject to 10 kV/cm voltage for 60 min and then heat-treated for 10 h at (i) 420, (ii) 430, (iii) 440 and (iv) 450 °C. (b) Normalized PL spectra of glasses after the application of electric voltage of (i) 2, (ii) 3, (iii) 6 and (iv) 10 kV/cm for 60 min and then heat treatment at 430 °C for 10 h.
Acknowledgments
this result indicated that PbS QDs precipitated within this 20-μm layer from the glass surface at that condition.
This work was supported by the Basic Science Research (2010– 0022407), Priority Research Center (2009-0094036) and World Class University (WCU) (R31-30005) Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
5. Conclusions
References
Electric field-assisted Ag+ migration and subsequent thermal treatment were used to control precipitation of PbS QDs inside silicate glasses. 1.2
[5]
(i) As-made o (ii) 10kV/cm+400 C
Absorbance
1.0 0.8
[6] [7] [8] [9] [10] [11] [12] [13]
0.6 0.4
[14] [15] [16]
(ii)
0.2
(i) 0.0 300
[1] [2] [3] [4]
400
500
600
700
800
Wavelength (nm) Fig. 3. Absorption spectra recorded from (i) as-made PbS QDs-free glass and (ii) the glass after application of 10 kV/cm voltage for 60 min and then heat treatment at 400 °C for 10 h.
[17] [18] [19]
N.F. Borrelli, D.W. Smith, J. Non-Cryst. Solids 180 (1994) 25. N.O. Dantas, R.S. Silva, F. Qu, Phys. Status Solidi B 232 (2002) 177. K. Xu, C. Liu, S. Dai, X. Shen, X. Wang, J. Heo, J. Non-Cryst. Solids 357 (2011) 2428. P.A. Loiko, G.E. Rachkovskaya, G.B. Zacharevich, V.S. Gurin, M.S. Gaponenko, K.V. Yumashev, J. Non-Cryst. Solids 358 (2012) 1840. P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595. A.M. Malyarevich, M.S. Gaponenko, K.V. Yumashev, A.A. Lagatsky, W. Sibbett, A.A. Zhilin, A.A. Lipovskii, J. Appl. Phys. 100 (2006) 023108. A.M. Malyarevich, K.V. Yumashev, A.A. Lipovskii, J. Appl. Phys. 103 (2008) 081301. J. Heo, C. Liu, J. Mater. Sci. Mater. Electron. 18 (2007) S135. G. Dong, B. Wu, F. Zhang, L. Zhang, M. Peng, D. Chen, E. Wu, J. Qiu, J. Alloys Compd. 509 (2011) 9335. S. Joshi, S. Sen, P.C. Ocampo, J. Phys. Chem. C 111 (2007) 4105. K. Xu, J. Heo, J. Non-Cryst. Solids 358 (2012) 921. K. Xu, J. Heo, J. Am. Ceram. Soc. 95 (2012) 2880. S.I. Najafi, Introduction to Glass Integrated Optics, Artech House Inc., Norwood, MA, 1992. C. Kim, M. Tomozawa, J. Am. Ceram. Soc. 59 (1976) 321. A. Tervonen, S. Honkanen, M. Leppihalme, J. Appl. Phys. 62 (1987) 759. A. Abdolvand, A. Podlipensky, G. Seifert, H. Graener, O. Deparis, P.G. Kazansky, Opt. Express 13 (2005) 1266. P.W. Wang, Appl. Surf. Sci. 120 (1997) 291. J. Zhang, W. Dong, J. Sheng, J. Zheng, J. Li, L. Qiao, L. Jiang, J. Cryst. Growth 310 (2008) 234. U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer-Verlag, Berlin, 1995.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Electric field-assisted Ag + migration for PbS quantum dot formation in glasses Kai Xu, Jong Heo ⁎ Division of Advanced Nuclear Engineering, and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790784, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2012 Received in revised form 16 November 2012 Available online 31 January 2013 Keywords: Photoluminescence; Ag nanoparticles; Quantum dots; Electric-field assistance
a b s t r a c t Electric-field assisted solid-state diffusion of Ag+ was used to control precipitation of PbS quantum dots (QDs) inside glasses. Precipitation of PbS QDs was confirmed by transmission electron microscopy images and photoluminescence (PL) spectra. PL bands moved to longer wavelengths as applied voltage increased under the same subsequent heat-treatment, and as heat-treatment temperature increased under the same voltage. PbS QDs can preferentially precipitate at low temperature and photo-luminesce at longer wavelengths in Ag+-affected regions than in unaffected regions. Ag nanoparticles were also formed inside affected regions after thermal treatment, and provided nucleation sites for PbS QDs. Precipitation of PbS QDs and their spatial distribution in glasses can be controlled by electric field-assisted Ag+ migration. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Glasses doped with PbS quantum dots (QDs) can absorb and emit light at wavelength λ that can be controlled from ~ 0.7 ≤ λ ≤ ~ 2.0 μm by adjusting QDs' size from ~ 2 to ~ 10 nm [1–4]. They have possible applications as solid-state saturable absorbers for mode-locked lasers [5–7] and as fiber-optic amplifiers in optical communications [8,9]. The average size of PbS QDs and their size distribution inside glasses can be tailored by carefully controlling the temperature and duration of thermal treatment [1,4,10]. Using Ag nanoparticles (NPs) as nucleating agents to realize accurate control of PbS QD precipitation in glasses has been investigated [3,11,12]. In an early report, a few tens of parts per million of Ag+ ions were incorporated directly into glass melts and the glasses were heated to form PbS QDs [3]. Adding Ag+ ions allows control of the size and size distribution of PbS QDs, and their number density in glasses increases with the concentration of Ag+ ions added. This increase in PbS number density results in a large increase of absorption and photoluminescence (PL) intensities. Ag+ ions are also incorporated into the glasses using a Na+ ↔Ag+ ion-exchange process. The size of PbS QDs precipitated after thermal treatment is larger in the surface region affected by Ag+ ion-exchange than in the Ag+-free region. When the heat-treatment temperature is sufficiently low, PbS QDs precipitate preferentially only at the surface in which Ag+ ion-exchange has occurred [11,12]. These results indicated that the precipitation and size of PbS QDs in glasses can be controlled by manipulating Ag+ ions inside them. In previous work, Ag + ions were introduced into glasses through ion-exchange by immersing the specimens in an AgNO3 solution or a molten salt bath [11,12]. However, the long ion-exchange duration ⁎ Corresponding author. Tel.: +82 54 279 2147; fax: + 82 54 279 2399 E-mail address: [email protected] (J. Heo). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.003
in the AgNO3 solution causes the contamination of the glass surface. The migration of Ag + ions inside the AgNO3 melt was very fast, and it makes the control of amount of Ag difficult. Incorporation of Ag + ions into glasses can be also achieved using the solid-state Ag + diffusion method, in which Ag film or paste is directly deposited on the glass matrix as the ion sources. An electric field induces electrolysis of Ag + ions from neutral Ag (Ag 0) and assists Ag + ion migration into glasses [13–16]. The diffusion of Ag + ions into glasses can be exactly determined by controlling electric-field strength, process duration, or temperature. In addition, the distribution of Ag + ions inside glasses can be controlled with accuracy. In this study, an electric field was used to cause Ag + ions to migrate into glasses from Ag paste, and their effect on controlled precipitation of PbS QDs in glasses was investigated. The size of PbS QDs was controlled by changing applied voltages and subsequent heat-treatment temperatures. 2. Experimental procedures A glass with a nominal composition (mol%) of 50SiO2–35Na2O– 5Al2O3–8ZnO–2ZnS doped with 0.8 mol% of PbO was prepared using melt-quenching. The preparation procedures were similar to those used in previous studies [3,11,12]. Starting powders of ~25 g were finely ground in ethanol using ZrO2 balls. The mixtures were heated to remove ethanol and moisture, and then melted in a corundum crucible at ~1350 °C for 45 min. The melts were poured into a pre-heated brass mold and pressed to a thickness of ~1.5 mm using an iron plate. The glass was annealed at 350 °C for 3 h to release the thermal stress, and then cut into pieces of ~1.0 × 1.0 cm. The upper and lower surfaces of the pieces were optically polished until glass thickness was ~1.0 mm. Both surfaces of glass specimens were homogeneously coated with 0.1-mm-thick Ag paste, then placed between two electrodes.
K. Xu, J. Heo / Journal of Non-Crystalline Solids 377 (2013) 254–256
255
Electrodes were covered with Al foil to prevent parasite diffusion. Ag + diffusion was conducted under applied voltages of 2, 3, 6 or 10 kV/cm for 60 min at room temperature. Afterwards, the coated Ag paste on both surfaces was cleaned using acetone, and specimens were further heat-treated at 420, 430, 440 or 450 °C for 10 h to induce precipitation of PbS QDs. Formation of QDs was confirmed using a transmission electron microscope (TEM) under an accelerating voltage of 200 kV. PL spectra were recorded at room temperature using an 800-nm excitation beam from a continuous-wave Ti-sapphire laser; signals were collected and amplified using a combination of a mechanical chopper of 50-Hz frequency, a 1/4 m monochromator, an InGaAs detector and a lock-in amplifier system. Standard deviations of PL wavelengths were b 1.5% including instrumental and experimental errors. The depth of Ag penetration was analyzed using energy dispersive X-ray spectroscopy (EDX) after polishing a cross-section of the glass. 3. Results When the glass was subjected to electric voltage of 10 kV/cm and further heat-treated at 430 °C, the surface that contacted the positive electrode became dark brown, whereas the surface that contacted the negative electrode remained unchanged yellowish. A TEM specimen was prepared from the dark brown region using a focused ion beam method. Black dots with an average diameter of ~3.6 (±0.6) nm were uniformly distributed in the glass matrix (Fig. 1a). A high-resolution TEM image of one crystal was obtained; it had a fringe spacing of ~0.17 nm, which is similar to the (222) plane spacing of bulk PbS (Fig. 1b). PL spectra were recorded to demonstrate the effect of electric-field assisted Ag+ diffusion on the precipitation of PbS QDs. The exciting laser was focused on the positive electrode surface of the glass where Ag+ migrated into it and thus facilitated PbS QD formation. The laser beam was irradiated at an angle of 45° and the luminescence was also detected at a 45° angle to form a detection direction perpendicular to the incoming laser beam. In glasses subjected to 10 kV/cm, clear PL bands from PbS QDs were observed (Fig. 2a); the center wavelengths of PL were 1155, 1270, 1390 and 1525 nm at heat-treatment temperatures of 420, 430, 440 and 450 °C, respectively (Fig. 2a). In addition, when heat-treatment temperature was 430 °C, the PL peak shifted from 1175 to 1270 nm as applied voltage increased from 2 to 10 kV/cm (Fig. 2b). Therefore, the wavelength of emission from PbS QDs in glasses can be tuned by controlling the heat-treatment temperatures and the applied electric voltages. 4. Discussion When an electric-field is applied, the neutral Ag0 in paste is oxidized to Ag+ ions, which then migrate from the positive side into the body of the glass [13,15]. During thermal treatment, Ag+ ions inside glasses reduce to Ag0 by capturing electrons from impurities or non-bridging oxygens, then aggregate to form Ag NPs (nAg+ + ne− → nAg → Agn) [17,18]. Ag NPs were not clearly identified in the X-ray diffraction patterns or TEM micrographs, probably due to the low concentration and small size of NPs. Ag NPs that formed in silicate glasses normally have an absorption band at λ ≈ 400 nm [19], but the absorption by PbS QDs in our glasses also obscured this peak. To confirm that Ag NPs formed in our glasses, we prepared a glass free of PbS QDs with a composition (mol%) of 50SiO2–35Na2O–5Al2O3–8ZnO–2ZnS. The glass was subjected to 10 kV/cm voltage, and then heat-treated at 400 °C for 10 h. A weak absorption band was observed at λ ≈ 400 nm (Fig. 3), indicating that Ag NPs precipitated in the glass. Thus, we inferred that Ag NPs were also formed in glasses containing PbS QDs probably at temperature as low as 400 °C. PbS QDs cannot nucleate in the glasses at heat-treatment temperature ≤430 °C if Ag NPs are not present [3]. In the present study, PbS
Fig. 1. (a) TEM image of nano-dots precipitated in Ag+-migrated regions; inset: size dispersion of nano-dots. (b) High-resolution TEM image of one PbS nano-dot. 10 kV/cm voltage was applied for 60 min to the glass and then it was heat-treated at 430 °C for 10 h.
QDs precipitated after heat treatment at 420 °C but only in the area that contained Ag+, as indicated by the emission in Fig. 2a. When heat-treatment temperatures increased to 440 and 450 °C, center wavelengths of PbS QDs emission from Ag+-containing regions were ~1390 and ~1525 nm, respectively (Fig. 2a), longer than those from Ag+-free glasses as the published report that PL position red-shifted from ~1000 to ~1100 nm [12]. These results again confirmed that formation of Ag NPs caused increase in the size of the PbS QDs, similar to results reported previously [11,12]. Therefore, Ag NPs that formed in glasses after application of an electric field and thermal treatment provided nucleation sites for PbS QD formation. The sizes of PbS QDs can also be controlled by changing the applied voltage (Fig. 2b). When 10 kV/cm electric voltage was applied, Ag+ migrated up to ~3 μm into the glass, and showed a steep concentration gradient (Fig. 4). After subsequent heat treatment at 430 °C for 10 h, Ag+ diffused further into the glass to ~20 μm with approximately uniform distribution;
256
K. Xu, J. Heo / Journal of Non-Crystalline Solids 377 (2013) 254–256
10
a 1.0
(ii)
(iii)
(iv)
Normalized PL
0.8
(i) 420oC (ii) 430oC (iii) 440oC (iv) 450oC
10kV/cm o 10kV/cm+430 C 8
Atom (%)
(i)
0.6
0.4 1 0.2 0 0.0
0 1000
1200
1400
1600
1800
5
10
15
20
25
Depth from the surface ( m)
Wavelength (nm) Fig. 4. Ag concentrations in the glass after 10 kV/cm electric-field assistance for 60 min and then heat treatment at 430 °C for 10 h.
b 1.0
(i) 2kV/cm (ii) 3kV/cm (iii) 6kV/cm (iv) 10kV/cm
Normalized PL
0.8
(i) (ii)
0.6
The formation of PbS QDs was confirmed from the fringe spacing in a high-resolution TEM image. Peak wavelengths of the PL bands shifted from 1175 to 1270 nm as the applied voltage increased from 2 to 10 kV/cm under the same heat-treatment temperature of 430 °C. Wavelengths of PL bands recorded from the regions treated using a voltage of 10 kV/cm increased from 1155 to 1525 nm as temperatures of the subsequent thermal treatment increased from 420 to 450 °C. Penetration depth of Ag+ was ~20 μm after application of 10 kV/cm voltage and heat treatment at 430 °C. Ag NPs that precipitated inside affected regions formed the nucleation sites and induced preferential precipitation of PbS QDs at low temperatures. Electric field-assisted Ag+ migration can provide an alternative approach to controlling the precipitation and spatial distribution of PbS QDs inside glasses.
(iii) (iv)
0.4
0.2
0.0 1000
1200
1400
1600
Wavelength (nm) Fig. 2. (a) Normalized PL spectra of glasses subject to 10 kV/cm voltage for 60 min and then heat-treated for 10 h at (i) 420, (ii) 430, (iii) 440 and (iv) 450 °C. (b) Normalized PL spectra of glasses after the application of electric voltage of (i) 2, (ii) 3, (iii) 6 and (iv) 10 kV/cm for 60 min and then heat treatment at 430 °C for 10 h.
Acknowledgments
this result indicated that PbS QDs precipitated within this 20-μm layer from the glass surface at that condition.
This work was supported by the Basic Science Research (2010– 0022407), Priority Research Center (2009-0094036) and World Class University (WCU) (R31-30005) Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
5. Conclusions
References
Electric field-assisted Ag+ migration and subsequent thermal treatment were used to control precipitation of PbS QDs inside silicate glasses. 1.2
[5]
(i) As-made o (ii) 10kV/cm+400 C
Absorbance
1.0 0.8
[6] [7] [8] [9] [10] [11] [12] [13]
0.6 0.4
[14] [15] [16]
(ii)
0.2
(i) 0.0 300
[1] [2] [3] [4]
400
500
600
700
800
Wavelength (nm) Fig. 3. Absorption spectra recorded from (i) as-made PbS QDs-free glass and (ii) the glass after application of 10 kV/cm voltage for 60 min and then heat treatment at 400 °C for 10 h.
[17] [18] [19]
N.F. Borrelli, D.W. Smith, J. Non-Cryst. Solids 180 (1994) 25. N.O. Dantas, R.S. Silva, F. Qu, Phys. Status Solidi B 232 (2002) 177. K. Xu, C. Liu, S. Dai, X. Shen, X. Wang, J. Heo, J. Non-Cryst. Solids 357 (2011) 2428. P.A. Loiko, G.E. Rachkovskaya, G.B. Zacharevich, V.S. Gurin, M.S. Gaponenko, K.V. Yumashev, J. Non-Cryst. Solids 358 (2012) 1840. P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595. A.M. Malyarevich, M.S. Gaponenko, K.V. Yumashev, A.A. Lagatsky, W. Sibbett, A.A. Zhilin, A.A. Lipovskii, J. Appl. Phys. 100 (2006) 023108. A.M. Malyarevich, K.V. Yumashev, A.A. Lipovskii, J. Appl. Phys. 103 (2008) 081301. J. Heo, C. Liu, J. Mater. Sci. Mater. Electron. 18 (2007) S135. G. Dong, B. Wu, F. Zhang, L. Zhang, M. Peng, D. Chen, E. Wu, J. Qiu, J. Alloys Compd. 509 (2011) 9335. S. Joshi, S. Sen, P.C. Ocampo, J. Phys. Chem. C 111 (2007) 4105. K. Xu, J. Heo, J. Non-Cryst. Solids 358 (2012) 921. K. Xu, J. Heo, J. Am. Ceram. Soc. 95 (2012) 2880. S.I. Najafi, Introduction to Glass Integrated Optics, Artech House Inc., Norwood, MA, 1992. C. Kim, M. Tomozawa, J. Am. Ceram. Soc. 59 (1976) 321. A. Tervonen, S. Honkanen, M. Leppihalme, J. Appl. Phys. 62 (1987) 759. A. Abdolvand, A. Podlipensky, G. Seifert, H. Graener, O. Deparis, P.G. Kazansky, Opt. Express 13 (2005) 1266. P.W. Wang, Appl. Surf. Sci. 120 (1997) 291. J. Zhang, W. Dong, J. Sheng, J. Zheng, J. Li, L. Qiao, L. Jiang, J. Cryst. Growth 310 (2008) 234. U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer-Verlag, Berlin, 1995.