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Lead sulfide quantum dots in glasses containing rare-earth ions
NOC-16512; No of Pages 3 Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Lead sulfide quantum dots in glasses containing rare-earth ions Mi Ae Kim a, Yong Kon Kwon a, Chao Liu b, Jong Heo a,⁎ a Department of Materials Science and Engineering and Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea b State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology), Wuhan, Hubei 430070, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2012 Received in revised form 12 April 2013 Available online xxxx Keywords: Quantum dot; Rare-earths; Glass
a b s t r a c t The size and spatial distribution of PbS QDs in glasses need to be controlled in order to achieve desired optical characteristics. This paper reports on the use of several rare-earth oxides such as Ho2O3, Er2O3, and La2O3 to control the size of PbS QDs in silicate glasses. Additions of rare-earth oxides result in blue shifts of the absorption bands from 1344 nm to 930 nm, and the wavelengths of PL bands can be controlled between 1421 nm and 1035 nm. Additions of up to 0.8 mol% of Ho2O3 decreased the diameters of PbS QDs from 4.82 nm to 3.02 nm. The same tendencies were found for both Er2O3 and La2O3 additions, although the size of PbS QDs increased as the ionic radius of rare-earth ions increased. It is proposed that clusters of RE–O bonds act as nucleating agents for the precipitation of PbS QDs. © 2013 Published by Elsevier B.V.
1. Introduction A lead sulfide (PbS) semiconductor with a large exciton Bohr radius (18 nm) has a strong quantum confinement effect that is suitable for use in tunable infrared active optoelectronic materials [1,2]. PbS quantum dots (QDs) incorporated into glasses have possible photonic applications as saturable absorbers of ultrafast lasers [3] and in amplifiers for fiber-optic telecommunication [4]. The size and spatial distribution of QDs in glasses need to be controlled in order to achieve desired optical characteristics [5–7]. High-temperature fusion and subsequent thermal treatment of precursor glasses have been used to precipitate QDs, but this process normally results in an uncontrolled spatial distribution of QDs in the glass matrix [8]. Alternatively, femto second laser irradiation has also been used to control the growth of nanocrystals in glasses [6]. Irradiation induces not only a change in the size of QDs and an enhancement of their luminescent intensities, but it also leads to an increased size distribution. Photoluminescence spectra from rare earth ions (REs) show sharp peaks of wavelengths that are relatively immune to their environment. Glasses doped with rare-earth ions have applications for optical communication amplifiers [9,10]. On the other hand, one study found that the number density and size of CaF2 nanocrystals changed considerably when 2 mol% of Er 3 + ions was added into oxyflouride glasses, indicating that Er 3+ ions can affect the growth of crystals [11]. In this paper, we reported the size control of PbS QDs in silicate
⁎ Corresponding author at: Department of Materials Science and Engineering and Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea. Tel.: +82 54 279 2147; fax: +82 54 279 8653. E-mail address: [email protected] (J. Heo).
glasses using rare-earth oxides such as Er2O3, Ho2O3, and La2O3. Absorption and photoluminescence (PL) spectra after heat treatment were recorded to identify the effect of rare-earth ion concentrations on the sizes of QDs in glasses. The diameters of QDs decreased as the amount of rare-earth oxide increased. Comparative analyses of the various rare earth ions are also discussed. 2. Experimental procedure The nominal composition of the host glass is 50SiO2–25Na2O– 10BaO–5Al2O3–8ZnO–2ZnS–0.8PbO in mol%. ZnS and PbO were used as sources of PbS QDs and an extra 0–0.8 mol% (with a 0.2 mol% interval) of rare-earth oxides such as Ho2O3, Er2O3, or La2O3 was added. All specimens were prepared from high-purity powders (>99.9%). Starting powders were mixed with ethanol and milled with zirconia balls for 15 h. After drying the solvent in an oven at 110 °C for 15 h, resultant powders were melted in an alumina crucible at 1300 °C for 30 min. This melting condition was determined to avoid the excess volatilization of sulfur and the reduction of lead. The alumina crucible was capped with an alumina cover to reduce the volatilization of sulfur during the melting process. The melted frit was quenched by pouring it onto a brass mold and the glasses obtained were annealed for 3 h at a near glass-transition temperature. All glasses prepared were heat-treated at 500 °C for 10 h to precipitate the PbS QDs. The temperatures and durations of heat treatment remain fixed for all samples to focus on the effect of rare-earth concentrations on the size of PbS QDs precipitated. High-resolution scanning transmission electron micrographs (TEM, JEOL JEM-2100F) were recorded to identify the size and shape of the QDs. Absorption spectra at 800–1600 nm were recorded using a UV/Vis/NIR spectrophotometer (Lambda 750) at room temperature.
0022-3093/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030
Please cite this article as: M.A. Kim, et al., Lead sulfide quantum dots in glasses containing rare-earth ions, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030
PL spectra were measured using the 800 nm excitation source from a Ti–sapphire laser (Coherent 890 tunable laser) pumped by the second harmonic (532 nm) of an Nd:YVO4 diode laser (1064 nm) (Millennia Pro s-Series). A mechanical chopper, a monochromator (77200, Oriel), an InGaAs detector (DET 410, THORLABS), and a lock-in amplifier were used to record the PL at room temperature. 3. Results During the heat treatment process, the color of the glasses changed from transparent yellow to dark black. The dark black color appeared due to the interband absorption by the PbS QDs and thus provided indirect evidence of PbS QDs formation. Direct evidence was obtained from the TEM image in Fig. 1 similar to the previous report [12]. The nanocrystals in the TEM image are spherical in shape and approximately 5 nm in diameter. XRD patterns are important to show the presence of PbS quantum dots in the glasses. However, due to the low concentration and small size of precipitated PbS quantum dots in the glasses, it is very difficult to get clear diffraction patterns from these glasses. Absorption spectra recorded from all specimens showed a quantum confinement effect depending upon the concentration of rareearth oxides. For example, when concentrations of Er2O3 in the sample increased from 0.0 mol% to 0.8 mol%, the center wavelengths of the absorption bands moved to short wavelength regions from 1310 nm to 944 nm (Fig. 2a). Peaks at ~ 980 nm and ~ 1530 nm were induced by the absorption of Er 3+ ions (indicated by the black arrows in Fig. 2a). Similarly, the center wavelengths of the absorption bands decreased from 1300 nm to 1092 nm as the La2O3 concentration increased to 0.8 mol% (Fig. 2b), and Ho2O3 content led to similar changes in the peak wavelengths (Table 1). All numbers of the center wavelengths are illustrated in Table 1. Variations in the content of RE2O3 induced these significant changes in the wavelengths of the absorption bands while the heat treatment conditions remained constant. Different rare earths resulted in slightly different amounts of blue shifts in the absorption bands that will be discussed later. Photoluminescence (PL) spectra for 800 nm–1600 nm wavelengths show similar concentration dependencies. When the concentrations of Er2O3 increased from 0.0 mol% to 0.8 mol%, the center wavelengths of the PL bands became shorter from 1301 nm to 1035 nm (Fig. 3a). Similarly, wavelengths of the PL bands decreased from 1421 nm for a glass without La2O3 to 1219 nm with 0.8 mol% La2O3 addition (Fig. 3b). Again, all heat treatment conditions were the same and only the
Normalized absorbance (a.u.)
M.A. Kim et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
0.0 mol% 0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol%
(a) Er2O3 1.5
1.0
0.5
0.0 800
900
1000 1100 1200 1300 1400 1500 1600
Wavelength (nm)
Normalized absorbance (a.u.)
2
(b) La2O3
0.0 mol% 0.2 mol% 0.4 mol% 0.8 mol%
1.5
1.0
0.5
0.0 1000
1100
1200
1300
1400
1500
Wavelength (nm) Fig. 2. Normalized absorption spectra of glasses with different (a) Er2O3 and (b) La2O3 concentrations heat-treated at 500 °C for 10 h. Black arrows in (a) indicate the absorption peaks induced by Er3+ ions.
concentrations of RE2O3 changed, and blue shifts of PL bands with increasing RE2O3 content were observed for all rare-earth oxides (Table 1). 4. Discussion It is well known that band-gap energy can be calculated from the center wavelengths of absorption bands. Iwan et al. reported the following empirical equation Eg ¼ 0:41 þ
1 0:0252d2 þ 0:283d
ð1Þ
which can correlate the band-gap energies (Eg) with the diameters (d) of PbS QDs [13]. The calculated diameters of PbS QDs in our glasses using the center wavelengths of absorption bands are shown
Table 1 The center wavelengths of absorption and photoluminescence bands with the calculated diameters of PbS QDs in silicate glass containing Ho2O3, Er2O3, and La2O3. Note that the diameters decrease with increasing concentrations of rare-earth oxides. Concentrations of rare-earths [mol%] Center wavelengths of absorption bands [nm] (±5 nm) Center wavelength of PL bands [nm] (±5 nm)
Fig. 1. Transmission electron microscope image of a single PbS QD in glass containing 0.2 mol% of Er2O3.
Calculated diameters [nm] (±0.02 nm)
Ho2O3 Er2O3 La2O3 Ho2O3 Er2O3 La2O3 Ho2O3 Er2O3 La2O3
0.8
0.6
0.4
0.2
0.0
930 944 1092 1068 1035 1219 3.02 3.07 3.67
990 1068 1104 1102 1147 1301 3.25 3.57 3.72
1100 1140 1230 1218 1208 1319 3.70 3.88 4.28
1220 1230 1250 1256 1277 1401 4.23 4.23 4.37
1344 1310 1300 – 1301 1421 4.82 4.66 4.61
Please cite this article as: M.A. Kim, et al., Lead sulfide quantum dots in glasses containing rare-earth ions, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030
M.A. Kim et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
PL intensity (a.u.)
1.0
0.0 mol% 0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol%
(a) Er2O3
0.8 0.6 0.4 0.2
800
1000
1200
1400
1600
Wavelength (nm)
PL intensity (a.u.)
0.8 0.6 0.4
3.67 nm while that in Ho2O3-containing glass was only 3.02 nm, even though the rare-earth concentrations were the same. These differences can be explained by the differences in the ionic radius of various rare-earths. Radii of Ho 3+ and Er 3+ ions are nearly identical at 104.1 pm while the radius of La 3+ ion is 117.2 pm [14]. According to our previous work [12], PbS QDs were formed using the rare-earth oxide clusters as nucleating agents. Therefore, it is possible that the different ionic radius of rare-earth ions led to variations in the clusters' size, resulting in changes in the diameters of PbS QDs thus formed. 5. Conclusion
0.0
1.0
3
(b) La2O3 0.0 mol% 0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol%
We report that the sizes of PbS QDs in silicate glasses were controlled by increasing concentrations of rare-earth oxides under constant heat treatment conditions. Additions of Er2O3, La2O3, and Ho2O3 up to 0.8 mol% moved the absorption bands from 1344 nm to 930 nm and the wavelengths of PL bands could be controlled between 1421 nm and 1035 nm. The calculated diameters of PbS QDs decreased from 4.82 nm to 3.02 nm when 0.8 mol% of Ho2O3 was added. The same tendencies were observed for the addition of both Er2O3 and La2O3, although the size of PbS QDs increased as the ionic radius of rare-earth ion increased. It is proposed that clusters of rare-earth-oxygen bonds act as nucleating agents for the precipitation of PbS QDs.
0.2
Acknowledgments 0.0
1000
1200 1400 Wavelength (nm)
1600
Fig. 3. Normalized PL spectra of PbS quantum dots formed in glasses with (a) Er2O3 and (b) La2O3 heat-treated at 500 °C for 10 h.
in Table 1. For example, the diameters of PbS QDs changed from 4.82 nm to 3.02 nm as the concentrations of Ho2O3 in glass samples increased to 0.8 mol%. Similar decreases in diameters of PbS QDs were found for the rare-earth oxides investigated. Shim et al. proposed that PbS nanocrystals nucleate on the top of clusters made of Er–O bonds [12]. There were more clusters in the glasses containing higher concentrations of RE2O3. Since the amount of Pb and S in glasses remained the same, the statistical distribution of these components to all RE–O clusters will inevitably result in the formation of smaller PbS QDs. The variation of calculated diameters of PbS QDs among three glasses without RE2O3 remained within ~ 0.2 nm, which is relatively smaller than differences induced from different RE2O3 contents. There are notable differences in the calculated diameters of PbS QDs depending upon the species of RE2O3 used. For example, the diameter of PbS QDs in a specimen containing 0.8 mol% La2O3 was
This work was supported by the Basic Science Research (2010-0022407), the Priority Research Centers (2012-046983), and the World Class University (R31-30005) Programs through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology. References [1] T. Okuno, A.A. Lipovskii, T. Ogawa, I. Amagai, Y. Masumoto, J. Lumin. 87 (2000) 491. [2] F.W. Wise, Acc. Chem. Res. 33 (2000) 773. [3] P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595. [4] L. Bakueva, S. Musikhin, M.A. Hines, T.-W.F. Chang, M. Tzolov, G.D. Scholes, E.H. Sargent, Appl. Phys. Lett. 82 (2003) 2895. [5] K. Xu, J. Heo, J. Am. Ceram. Soc. 95 (9) (2012) 2880. [6] C. Liu, J. Heo, J. Am. Ceram. Soc. 93 (2010) 1221. [7] K. Xu, J. Heo, J. Non-Cryst. Solids 358 (2012) 921. [8] J. Heo, C. Liu, J. Mater. Sci. Mater. Electron. 18 (2007) S135. [9] Y. Wang, J. Ohwaki, Appl. Phys. Lett. 63 (1993) 3268. [10] S. Tanabe, H. Hayashi, T. Hanada, J. Am. Ceram. Soc. 85 (2002) 839. [11] D. Chen, Mater. Chem. Phys. 95 (2006) 264. [12] S.M. Shim, C. Liu, Y.K. Kwon, J. Heo, J. Am. Ceram. Soc. 93 (2010) 3092. [13] M. Iwan, ACS Nano 3 (2009) 10. [14] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.
Please cite this article as: M.A. Kim, et al., Lead sulfide quantum dots in glasses containing rare-earth ions, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Lead sulfide quantum dots in glasses containing rare-earth ions Mi Ae Kim a, Yong Kon Kwon a, Chao Liu b, Jong Heo a,⁎ a Department of Materials Science and Engineering and Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea b State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology), Wuhan, Hubei 430070, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2012 Received in revised form 12 April 2013 Available online xxxx Keywords: Quantum dot; Rare-earths; Glass
a b s t r a c t The size and spatial distribution of PbS QDs in glasses need to be controlled in order to achieve desired optical characteristics. This paper reports on the use of several rare-earth oxides such as Ho2O3, Er2O3, and La2O3 to control the size of PbS QDs in silicate glasses. Additions of rare-earth oxides result in blue shifts of the absorption bands from 1344 nm to 930 nm, and the wavelengths of PL bands can be controlled between 1421 nm and 1035 nm. Additions of up to 0.8 mol% of Ho2O3 decreased the diameters of PbS QDs from 4.82 nm to 3.02 nm. The same tendencies were found for both Er2O3 and La2O3 additions, although the size of PbS QDs increased as the ionic radius of rare-earth ions increased. It is proposed that clusters of RE–O bonds act as nucleating agents for the precipitation of PbS QDs. © 2013 Published by Elsevier B.V.
1. Introduction A lead sulfide (PbS) semiconductor with a large exciton Bohr radius (18 nm) has a strong quantum confinement effect that is suitable for use in tunable infrared active optoelectronic materials [1,2]. PbS quantum dots (QDs) incorporated into glasses have possible photonic applications as saturable absorbers of ultrafast lasers [3] and in amplifiers for fiber-optic telecommunication [4]. The size and spatial distribution of QDs in glasses need to be controlled in order to achieve desired optical characteristics [5–7]. High-temperature fusion and subsequent thermal treatment of precursor glasses have been used to precipitate QDs, but this process normally results in an uncontrolled spatial distribution of QDs in the glass matrix [8]. Alternatively, femto second laser irradiation has also been used to control the growth of nanocrystals in glasses [6]. Irradiation induces not only a change in the size of QDs and an enhancement of their luminescent intensities, but it also leads to an increased size distribution. Photoluminescence spectra from rare earth ions (REs) show sharp peaks of wavelengths that are relatively immune to their environment. Glasses doped with rare-earth ions have applications for optical communication amplifiers [9,10]. On the other hand, one study found that the number density and size of CaF2 nanocrystals changed considerably when 2 mol% of Er 3 + ions was added into oxyflouride glasses, indicating that Er 3+ ions can affect the growth of crystals [11]. In this paper, we reported the size control of PbS QDs in silicate
⁎ Corresponding author at: Department of Materials Science and Engineering and Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea. Tel.: +82 54 279 2147; fax: +82 54 279 8653. E-mail address: [email protected] (J. Heo).
glasses using rare-earth oxides such as Er2O3, Ho2O3, and La2O3. Absorption and photoluminescence (PL) spectra after heat treatment were recorded to identify the effect of rare-earth ion concentrations on the sizes of QDs in glasses. The diameters of QDs decreased as the amount of rare-earth oxide increased. Comparative analyses of the various rare earth ions are also discussed. 2. Experimental procedure The nominal composition of the host glass is 50SiO2–25Na2O– 10BaO–5Al2O3–8ZnO–2ZnS–0.8PbO in mol%. ZnS and PbO were used as sources of PbS QDs and an extra 0–0.8 mol% (with a 0.2 mol% interval) of rare-earth oxides such as Ho2O3, Er2O3, or La2O3 was added. All specimens were prepared from high-purity powders (>99.9%). Starting powders were mixed with ethanol and milled with zirconia balls for 15 h. After drying the solvent in an oven at 110 °C for 15 h, resultant powders were melted in an alumina crucible at 1300 °C for 30 min. This melting condition was determined to avoid the excess volatilization of sulfur and the reduction of lead. The alumina crucible was capped with an alumina cover to reduce the volatilization of sulfur during the melting process. The melted frit was quenched by pouring it onto a brass mold and the glasses obtained were annealed for 3 h at a near glass-transition temperature. All glasses prepared were heat-treated at 500 °C for 10 h to precipitate the PbS QDs. The temperatures and durations of heat treatment remain fixed for all samples to focus on the effect of rare-earth concentrations on the size of PbS QDs precipitated. High-resolution scanning transmission electron micrographs (TEM, JEOL JEM-2100F) were recorded to identify the size and shape of the QDs. Absorption spectra at 800–1600 nm were recorded using a UV/Vis/NIR spectrophotometer (Lambda 750) at room temperature.
0022-3093/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030
Please cite this article as: M.A. Kim, et al., Lead sulfide quantum dots in glasses containing rare-earth ions, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030
PL spectra were measured using the 800 nm excitation source from a Ti–sapphire laser (Coherent 890 tunable laser) pumped by the second harmonic (532 nm) of an Nd:YVO4 diode laser (1064 nm) (Millennia Pro s-Series). A mechanical chopper, a monochromator (77200, Oriel), an InGaAs detector (DET 410, THORLABS), and a lock-in amplifier were used to record the PL at room temperature. 3. Results During the heat treatment process, the color of the glasses changed from transparent yellow to dark black. The dark black color appeared due to the interband absorption by the PbS QDs and thus provided indirect evidence of PbS QDs formation. Direct evidence was obtained from the TEM image in Fig. 1 similar to the previous report [12]. The nanocrystals in the TEM image are spherical in shape and approximately 5 nm in diameter. XRD patterns are important to show the presence of PbS quantum dots in the glasses. However, due to the low concentration and small size of precipitated PbS quantum dots in the glasses, it is very difficult to get clear diffraction patterns from these glasses. Absorption spectra recorded from all specimens showed a quantum confinement effect depending upon the concentration of rareearth oxides. For example, when concentrations of Er2O3 in the sample increased from 0.0 mol% to 0.8 mol%, the center wavelengths of the absorption bands moved to short wavelength regions from 1310 nm to 944 nm (Fig. 2a). Peaks at ~ 980 nm and ~ 1530 nm were induced by the absorption of Er 3+ ions (indicated by the black arrows in Fig. 2a). Similarly, the center wavelengths of the absorption bands decreased from 1300 nm to 1092 nm as the La2O3 concentration increased to 0.8 mol% (Fig. 2b), and Ho2O3 content led to similar changes in the peak wavelengths (Table 1). All numbers of the center wavelengths are illustrated in Table 1. Variations in the content of RE2O3 induced these significant changes in the wavelengths of the absorption bands while the heat treatment conditions remained constant. Different rare earths resulted in slightly different amounts of blue shifts in the absorption bands that will be discussed later. Photoluminescence (PL) spectra for 800 nm–1600 nm wavelengths show similar concentration dependencies. When the concentrations of Er2O3 increased from 0.0 mol% to 0.8 mol%, the center wavelengths of the PL bands became shorter from 1301 nm to 1035 nm (Fig. 3a). Similarly, wavelengths of the PL bands decreased from 1421 nm for a glass without La2O3 to 1219 nm with 0.8 mol% La2O3 addition (Fig. 3b). Again, all heat treatment conditions were the same and only the
Normalized absorbance (a.u.)
M.A. Kim et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
0.0 mol% 0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol%
(a) Er2O3 1.5
1.0
0.5
0.0 800
900
1000 1100 1200 1300 1400 1500 1600
Wavelength (nm)
Normalized absorbance (a.u.)
2
(b) La2O3
0.0 mol% 0.2 mol% 0.4 mol% 0.8 mol%
1.5
1.0
0.5
0.0 1000
1100
1200
1300
1400
1500
Wavelength (nm) Fig. 2. Normalized absorption spectra of glasses with different (a) Er2O3 and (b) La2O3 concentrations heat-treated at 500 °C for 10 h. Black arrows in (a) indicate the absorption peaks induced by Er3+ ions.
concentrations of RE2O3 changed, and blue shifts of PL bands with increasing RE2O3 content were observed for all rare-earth oxides (Table 1). 4. Discussion It is well known that band-gap energy can be calculated from the center wavelengths of absorption bands. Iwan et al. reported the following empirical equation Eg ¼ 0:41 þ
1 0:0252d2 þ 0:283d
ð1Þ
which can correlate the band-gap energies (Eg) with the diameters (d) of PbS QDs [13]. The calculated diameters of PbS QDs in our glasses using the center wavelengths of absorption bands are shown
Table 1 The center wavelengths of absorption and photoluminescence bands with the calculated diameters of PbS QDs in silicate glass containing Ho2O3, Er2O3, and La2O3. Note that the diameters decrease with increasing concentrations of rare-earth oxides. Concentrations of rare-earths [mol%] Center wavelengths of absorption bands [nm] (±5 nm) Center wavelength of PL bands [nm] (±5 nm)
Fig. 1. Transmission electron microscope image of a single PbS QD in glass containing 0.2 mol% of Er2O3.
Calculated diameters [nm] (±0.02 nm)
Ho2O3 Er2O3 La2O3 Ho2O3 Er2O3 La2O3 Ho2O3 Er2O3 La2O3
0.8
0.6
0.4
0.2
0.0
930 944 1092 1068 1035 1219 3.02 3.07 3.67
990 1068 1104 1102 1147 1301 3.25 3.57 3.72
1100 1140 1230 1218 1208 1319 3.70 3.88 4.28
1220 1230 1250 1256 1277 1401 4.23 4.23 4.37
1344 1310 1300 – 1301 1421 4.82 4.66 4.61
Please cite this article as: M.A. Kim, et al., Lead sulfide quantum dots in glasses containing rare-earth ions, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030
M.A. Kim et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
PL intensity (a.u.)
1.0
0.0 mol% 0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol%
(a) Er2O3
0.8 0.6 0.4 0.2
800
1000
1200
1400
1600
Wavelength (nm)
PL intensity (a.u.)
0.8 0.6 0.4
3.67 nm while that in Ho2O3-containing glass was only 3.02 nm, even though the rare-earth concentrations were the same. These differences can be explained by the differences in the ionic radius of various rare-earths. Radii of Ho 3+ and Er 3+ ions are nearly identical at 104.1 pm while the radius of La 3+ ion is 117.2 pm [14]. According to our previous work [12], PbS QDs were formed using the rare-earth oxide clusters as nucleating agents. Therefore, it is possible that the different ionic radius of rare-earth ions led to variations in the clusters' size, resulting in changes in the diameters of PbS QDs thus formed. 5. Conclusion
0.0
1.0
3
(b) La2O3 0.0 mol% 0.2 mol% 0.4 mol% 0.6 mol% 0.8 mol%
We report that the sizes of PbS QDs in silicate glasses were controlled by increasing concentrations of rare-earth oxides under constant heat treatment conditions. Additions of Er2O3, La2O3, and Ho2O3 up to 0.8 mol% moved the absorption bands from 1344 nm to 930 nm and the wavelengths of PL bands could be controlled between 1421 nm and 1035 nm. The calculated diameters of PbS QDs decreased from 4.82 nm to 3.02 nm when 0.8 mol% of Ho2O3 was added. The same tendencies were observed for the addition of both Er2O3 and La2O3, although the size of PbS QDs increased as the ionic radius of rare-earth ion increased. It is proposed that clusters of rare-earth-oxygen bonds act as nucleating agents for the precipitation of PbS QDs.
0.2
Acknowledgments 0.0
1000
1200 1400 Wavelength (nm)
1600
Fig. 3. Normalized PL spectra of PbS quantum dots formed in glasses with (a) Er2O3 and (b) La2O3 heat-treated at 500 °C for 10 h.
in Table 1. For example, the diameters of PbS QDs changed from 4.82 nm to 3.02 nm as the concentrations of Ho2O3 in glass samples increased to 0.8 mol%. Similar decreases in diameters of PbS QDs were found for the rare-earth oxides investigated. Shim et al. proposed that PbS nanocrystals nucleate on the top of clusters made of Er–O bonds [12]. There were more clusters in the glasses containing higher concentrations of RE2O3. Since the amount of Pb and S in glasses remained the same, the statistical distribution of these components to all RE–O clusters will inevitably result in the formation of smaller PbS QDs. The variation of calculated diameters of PbS QDs among three glasses without RE2O3 remained within ~ 0.2 nm, which is relatively smaller than differences induced from different RE2O3 contents. There are notable differences in the calculated diameters of PbS QDs depending upon the species of RE2O3 used. For example, the diameter of PbS QDs in a specimen containing 0.8 mol% La2O3 was
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Please cite this article as: M.A. Kim, et al., Lead sulfide quantum dots in glasses containing rare-earth ions, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.030