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Luminescent properties of fluorophosphate glasses with lead chalcogenides molecular clusters
Journal of Luminescence 162 (2015) 36–40
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescent properties of fluorophosphate glasses with lead chalcogenides molecular clusters E.V. Kolobkova, D.S. Kukushkin, N.V. Nikonorov, T.A. Shakhverdov, A.I. Sidorov n, V.N. Vasiliev ITMO University, 4 Birzhevaya line, St. Petersburg 199034, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2014 Received in revised form 25 January 2015 Accepted 5 February 2015 Available online 14 February 2015
Fluorophosphate glasses containing lead, selenium, and sulfur exhibit an intense luminescence in the 400–620 nm spectral region when excited by the 240–420 nm radiation. This luminescence is due to the presence of (PbSe)n and/or (PbS)n molecular clusters in the glasses, which appear in the as-prepared glasses before quantum dots formation. The thermal treatment at temperatures less than the glass transition temperature results in the red-shift of the luminescence bands and in an increase in the luminescence intensity. Heating the thermally treated glass samples leads to the reversible thermal quenching of the luminescence. & 2015 Elsevier B.V. All rights reserved.
Keywords: Luminescence Molecular cluster Lead chalcogenide Fluorophosphate glass
1. Introduction Composite optical materials with PbSe and PbS quantum dots are used widely now in optics and photonics [1–12]. Such materials exhibit the luminescent properties in the near IR (see, for example, [4]) and also non-linear optical properties [5,6]. The procedure of synthesizing the lead chalcogenide quantum dots in glasses consists in the thermal treatment at a certain temperature higher than the glass transition one of a glass containing before the synthesis lead, selenium, and sulfur. The thermal diffusion of lead ions and chalcogen atoms leads to the formation and growth of quantum dots in glass. Variations in the thermal treatment temperature and duration make it possible to change the quantum dot dimensions in a wide range. It is evident that the lead chalcogenide structures should pass, in the course of their formation and growth, a stage at which their size remains less than 1 nm. This stage corresponds to the occurrence, in a glass, of lead chalcogenide structures in the form of subnanosized molecular clusters whose properties can considerably differ from the properties of quantum dots. For the last years, a scientific interest in the metal and semiconductor molecular clusters (MCs) has increased rapidly [13,14]. MCs are of interest also from the practical viewpoint
n Correspondence to: Kronverkskii pr. 49, St. Petersburg 197101, Russia. Tel.: +79119205938. E-mail address: [email protected] (A.I. Sidorov).
http://dx.doi.org/10.1016/j.jlumin.2015.02.009 0022-2313/& 2015 Elsevier B.V. All rights reserved.
because some of MC types exhibit an intense luminescence in the visible. In some cases, the MCs arise in glasses during the thermal treatment; in other cases, they are formed directly in the course of the glass synthesis. The luminescent properties of silver molecular clusters Agn in oxyfluorine and silicate glasses were investigated in Refs. [15–21]. In Refs. [22–24], the luminescent properties of (CuCl)n and (Cu2O)n clusters in potassium–alumina– borate and silicate glasses were studied. The investigation of silver and gold molecular clusters in the metal-organic compounds was performed in Refs. [25–30]. In spite of enormous amount of research works devoted to the investigation of lead and cadmium chalcogenide quantum dots, there are quite few sources describing the corresponding MCs properties. In Refs. [31,32] the structure of the stable (CdSe)n (n¼1–4) and (XCdS)n (n¼4–16, X¼Na, K, Cl, and Br) molecular clusters was investigated by the computer modeling. In Ref. [33], the structure of CdSe molecular clusters was studied by an NMR method. The structure of Cd2O2Se and CdO2Se molecular clusters was investigated in Ref. [34] by X-ray absorption spectroscopy. It was shown in Refs. [35–40] that the Se2 and Se2 molecular clusters are present in both glasses and sodalite. Such MCs exhibit the luminescence bands at 480 and 710 nm. To our opinion there are no publications in which the lead chalcogenide MCs properties are described. In our experiments for as-prepared fluorophosphate (FP) glasses containing lead, selenium, and sulfur we observed an intense luminescence in the visible, excited by the UV radiation before quantum dots formation. In the reference samples without lead only very weak luminescence was observed, which can be
E.V. Kolobkova et al. / Journal of Luminescence 162 (2015) 36–40
37
attributed to the defects of the glass network. The luminescence of lead and chalcogenide containing glasses indicates the presence, in the initial glasses, of (PbSe)n and (PbS)n molecular clusters that play the role of crystallization centers for quantum dots during the subsequent thermal treatment. The given work is aimed to study the luminescence properties of FP glasses with (PbSe)n and (PbS)n molecular clusters including the influence, on the MC luminescence, of thermal treatments at a stage preceding the quantum dot formation and luminescence thermal quenching.
2. Experimental FP glasses based on the Na2O–P2O5–ZnF2–Ga2O3–PbF2–AlF3– NaF system doped with ZnS or ZnSe were synthesized. Two groups of samples denoted further by FP(PbS) and FP(PbSe) were prepared, the samples containing (PbS)n or (PbSe)n molecular clusters, respectively. For synthesis, the reagent grade and chemically pure raw materials were used. The glass synthesis was conducted in an electric furnace with the air atmosphere at 950 1C. The glass transition temperature Tg was measured with a Perkin-Elmer STA6000 differential scanning calorimeter, its magnitude being found to be 400 1C. Samples to be investigated were prepared in the form of plane-parallel plates 1.5–2.0 mm thick. For the thermal treatment of the samples, Nabertherm muffle furnace was used. For minimizing the effects of gradual heating and cooling, the samples were placed, at once, into the muffle furnace heated beforehand up to temperature required and, after a desired exposure, extracted from the hot furnace into air. The optical density spectra of FP glass samples under study were recorded in the 300–1000 nm spectral region with a step of 1 nm using a Perkin-Elmer Lambda 650 spectrophotometer. For registering the luminescence and excitation spectra, a PerkinElmer MPF-44 A spectrofluorimeter was used, the entrance and exit slit widths being equal to 7 nm. The luminescence spectra measured were corrected taking into account the spectral sensitivity of the spectrofluorimeter. For the excitation spectra, no correction was conducted. For measuring the luminescence at a fixed excitation wavelength, a StellarNet EPP2000-UVN-SR fiber spectrometer was used, the luminescence being excited by a semiconductor laser (λ ¼405 nm). The optical density spectra were registered at room temperature.
3. Results and discussion Fig. 1 shows the influence of thermal treatment on the optical density spectra of FP(PbSe) glasses. The as-prepared glass has the absorption band edge at 410 nm and is practically colorless. The thermal treatment of the glass at 280–380 1C causes few changes in the optical density spectra, the main effect being a small rise in the absorption in the 450–1000 nm spectral region. However, the thermal treatment at 400 1C for 1 h leads to (i) a change in the sample color to dark brown and (ii) a substantial red-shift of the absorption band edge to the 550–600 nm region. These effects indicate the initial stage of PbSe quantum dots formation. An increase in the thermal treatment duration at 400 1C leads to the change in the sample color from brown to black. This is caused assumedly by an increase in the quantum dots sizes and their concentrations. Fig. 2 shows the luminescence and excitation spectra of FP(PbSe) glass before thermal treatment. As seen from Fig. 2, the as-prepared FP(PbSe) glass exhibits a broad-band luminescence in the visible from 400 to 660 nm. An increase in the excitation wavelength leads to the red-shift of the luminescence maximum. For the excitation
Fig. 1. Optical density spectra of FP(PbSe) glass before (1) and after (2–5) thermal treatment for 1 h. Thermal treatment temperatures: 2 – 280, 3 – 350, 4 – 380, and 5 – 400 1C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
wavelengths in an 310–360 nm region, the luminescence intensity maximum occurs at 480–500 nm. The luminescence measurements for the reference glass sample with no Pb ions have shown that the sample, when excited by UV radiation, exhibits quite weak luminescence in the visible. This luminescence can be attributed to the defects of the glass network [41–47]. The excitation spectra of FP (PbSe) glass samples are located in the 260–420 nm region, with maxima at 280–340 nm. The width of the excitation band and its complicated structure indicate that several types of luminescence centers are present in the glass. Thus, the occurrence of broad-band and intense luminescence of FP(PbSe) glass gives an evidence that the (PbSe)n MCs differing in n are present in the glass, being less than 1 nm in size. Many other types of MCs in glasses have similar optical properties as well (see, for example, [15–22]). It should be noted that the Se2 molecular clusters can contribute to the luminescence band at 480 nm [35–40]. During the thermal treatment at or above 400 1C, these MCs play the role of nucleation centers for the PbSe quantum dots. Fig. 3 illustrates the influence of thermal treatment on the luminescent properties of FP(PbSe) glass. An increase in the temperature of the thermal treatment up to 380 1C leads to the red-shift of the luminescence maximum and to an increase in its intensity by 5 times. The common reason for this luminescence intensity increase and the red-shift of the luminescence band is an increase in the sizes and concentrations of MCs under heating that occurs due to the thermal diffusion of Pb ions and Se atoms. After the thermal treatment at 400 1C, the luminescence intensity in the visible decreases considerably and a new weak luminescence band appears in the 900–1100 nm region. This indicates the initial stage of PbSe quantum dots formation. Fig. 4 shows the influence of sample temperature on the luminescence intensity. In this experiment, a sample treated preliminarily at 380 1C was used. An increase in the sample temperature from 20 to 250 1C leads to (i) a decrease in the luminescence intensity by 6.7 times and to (ii) a weak red-shift of the luminescence band. This effect is reversible and can be repeated many times. The thermal quenching the luminescence can be explained by the fact that, with an increase in temperature, electrons in the excited state can occupy high vibrational energy levels that intersect with the electronic ground state level at the configuration coordinate diagram [48]. This allows the vibrational relaxation of excited electrons to the ground state that occurs via the phonon release without the emission of radiation.
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Fig. 2. Luminescence (a) and excitation (b) spectra of FP(PbSe) glass before the thermal treatment. (a) Excitation wavelength is 330 nm. (b) Luminescence wavelength is 500 nm.
Fig. 3. Luminescence spectra of FP(PbSe) glass before (1) and after (2–5) thermal treatment for 1 h. Thermal treatment temperatures: 2 – 280, 3 – 350, 4 – 380, and 5 – 400 1C. Excitation wavelength is 405 nm. Inset: the maximum visible luminescence intensity vs. thermal treatment temperature.
Fig. 5. Optical density spectra of FP(PbS) glass before (1) and after (2–5) thermal treatment for 1 h. Thermal treatment temperatures: 2 – 280, 3 – 350, 4 – 380, and 5 – 410 1C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 4. Influence of temperature on the luminescence intensity of FP(PbSe) glass sample treated preliminarily at 380 1C. Measurement temperatures: 1 – 20, 2 – 50, 3 – 100, 4 – 150, 5 – 200, and 6–250 1C. Excitation wavelength is 405 nm. Inset: the maximum luminescence intensity vs. the sample temperature.
For the FP(PbS) glasses, the effects similar to those described above for the FP(PbSe) glasses are observed. The as-prepared FP (PbS) glass is colorless and have the absorption band edge at 390– 400 nm (Fig. 5). The thermal treatment of the glass at 280–350 1C
causes no changes in the optical density spectra, whereas that at 380 1C leads to (i) a change in the sample color to yellow and to (ii) a weak red-shift of the absorption band edge. These effects indicate the initial stage of PbS quantum dots formation. The thermal treatment at the greater temperature of 410 1C leads to a change in the sample color to red–brown and to a substantial redshift of the absorption band edge. This is caused assumedly by an increase in the quantum dots sizes and concentrations. Fig. 6 shows the luminescence and excitation spectra of FP(PbS) glass before the thermal treatment. As seen from Fig. 6, the asprepared FP(PbS) glass exhibits a broad-band luminescence in the visible from 420 to 620 nm with a maximum around 460–480 nm. The luminescence band of FP(PbS) glass is more narrow than that of FP(PbSe) one and its maximum exhibits the blue-shift compared to that for FP(PbSe) glass (see Fig. 2a and Fig. 6a), though, as follows from stated below, the general pattern of FP(PbS) glass spectra remains to be similar enough to that for FP(PbSe) glass. An increase in the excitation wavelength leads to the red-shift of the
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Fig. 6. Luminescence (a) and excitation (b) spectra of FP(PbS) glass before the thermal treatment. (a) Excitation wavelength – 340 nm. (b) Luminescence wavelength – 500 nm.
Fig. 7. Luminescence (a) and excitation (b) spectra of FP(PbS) glass after the thermal treatment at 380 1C. (a) Excitation wavelength is 340 nm. (b) Luminescence wavelength is 620 nm.
luminescence maximum. The excitation spectra of FP(PbSe) glass samples are located in the 240–420 nm region, their maxima lying at 320–340 nm. The complicated structure of the luminescence band indicates that several types of luminescence centers are present in the glass. Thus, the occurrence of broad-band and intense luminescence of FP(PbS) glass gives an evidence that the (PbS)n MCs differing in n and being less than 1 nm in size are present in the glass. Similar to the case of FP(PbSe) glass, the thermal treatment of FP(PbS) glass at temperatures less than the glass transition temperature results in an increase in the luminescence intensity (Fig. 7). The thermal treatment of FP(PbS) glass at 380 1C leads to the red-shift of the luminescence band maximum from 460–480 to 600–620 nm. The thermal treatment at 400 1C results in a substantial decrease in the luminescence intensity in the visible because of PbS quantum dots formation. Under an increase in the temperature of the thermally treated sample, the thermal
quenching of the luminescence occurs similar to the case of FP (PbSe) glasses.
4. Summary It is shown experimentally that before the quantum dots formation the fluorophosphate glasses containing lead, selenium, and sulfur exhibit an intense luminescence in the visible when excited by the UV radiation. This luminescence is assumed to be due to the presence of (PbSe)n and (PbS)n molecular clusters in the glasses. The thermal treatment of the glass samples at temperatures less than the glass transition temperature results in an increase in the luminescence intensity. In the course of thermal treatment around the glass transition temperature, the (PbSe)n and (PbS)n molecular clusters play a role of nucleation centers for the PbSe and PbS quantum dots growth. The heating of the
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thermally treated glass samples leads to the reversible thermal quenching of the luminescence. On the whole, luminescent fluorophosphate glasses with (PbSe)n and (PbS)n molecular clusters are shown to be promising materials for phosphors in photonic devices and down-converters of solar radiation in solar cells, and also in the luminescent fiber temperature sensors and dosimeters of the UV radiation. Acknowledgments This research was financially supported by the Russian Scientific Foundation (Agreement 14-23-00136). References [1] P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595. [2] I. Kang, F.W. Wise, JOSA B 14 (1997) 1632. [3] A. Lipovskii, E. Kolobkova, V. Petrikov, I. Kang, A. Olkhovets, T. Krauss, M. Thomas, J. Silcox, F. Wise, Q. Shen, S. Kycia, Appl. Phys. Lett. 71 (1997) 3406. [4] T. Okuno, A.A. Lipovskii, T. Ogawa, I. Amagai, Y. Masumoto, J. Lumin. 87–89 (2000) 491. [5] A.M. Malyarevich, V.G. Savitski, P.V. Prokoshin, N.N. Posnov, K.V. Yumashev, A.A. Lipovsky, E. Raaben, A.A. Zhilin, Proc. SPIE 4748 (2001) 375. [6] T. Akiyama, H. Kuwatsuka, T. Simoyama, Y. Nakata, K. Mukai, M. Sugawara, O. Wada, H. Ishikawa, IEEE J. Quant. Electron. 34 (2001) 1059. [7] N.O. Dantas, R.S. Silva, F. Qu, Phys. Status Solidi B 232 (2002) 177. [8] R.D. Schaller, M.A. Petruska, V.I. Klimov, J. Phys. Chem. B 107 (2003) 13765. [9] R.J. Ellingson, M.C. Beard, J.C. Johnson, P. Yu, O.I. Micic, A.J. Nozik, A. Shabaev, A.L. Efros, Nano Lett. 5 (2005) 865. [10] 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. [11] F. Pang, X. Sun, H. Guo, J. Yan, J. Wang, X. Zeng, Z. Chen, T. Wang, Opt. Express 18 (2010) 14024. [12] R. Lorenzi, S. Brovelli, F. Meinardi, A. Lauria, N. Chiodini, A. Paleari, J. NonCryst. Solids 358 (2012) 1840. [13] Y. Lu, W. Chen, Chem. Soc. Rev. 41 (2012) 3594. [14] L. Shang, S. Dong, G.U. Nienhaus, Nano Today 6 (2011) 401. [15] N.T. Cuong, V.K. Tikhomirov, L.F. Chibotaru, A. Stesmans, V.D. Rodríguez, M.T. Nguyen, V.V. Moshchalkov, J. Chem. Phys. 136 (2012) 174108. [16] A.S. Kuznetsov, V.K. Tikhomirov, V.V. Moshchalkov, Opt. Express 20 (2012) 21576. [17] J.J. Velázquez, V.K. Tikhomirov, L.F. Chibotaru, N.T. Cuong, A.S. Kuznetsov, V.D. Rodríguez, M.T. Nguyen, V.V. Moshchalkov, Opt. Express 20 (2012) 13582. [18] E.V. Kolobkova, N.V. Nikonorov, A.I. Sidorov, T.A. Shakhverdov, Opt. Spectrosc. 114 (2013) 236. [19] A.I. Ignatiev, N.V. Nikonorov, A.I. Sidorov, T.A. Shakhverdov, Opt. Spectrosc. 114 (2013) 769.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescent properties of fluorophosphate glasses with lead chalcogenides molecular clusters E.V. Kolobkova, D.S. Kukushkin, N.V. Nikonorov, T.A. Shakhverdov, A.I. Sidorov n, V.N. Vasiliev ITMO University, 4 Birzhevaya line, St. Petersburg 199034, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2014 Received in revised form 25 January 2015 Accepted 5 February 2015 Available online 14 February 2015
Fluorophosphate glasses containing lead, selenium, and sulfur exhibit an intense luminescence in the 400–620 nm spectral region when excited by the 240–420 nm radiation. This luminescence is due to the presence of (PbSe)n and/or (PbS)n molecular clusters in the glasses, which appear in the as-prepared glasses before quantum dots formation. The thermal treatment at temperatures less than the glass transition temperature results in the red-shift of the luminescence bands and in an increase in the luminescence intensity. Heating the thermally treated glass samples leads to the reversible thermal quenching of the luminescence. & 2015 Elsevier B.V. All rights reserved.
Keywords: Luminescence Molecular cluster Lead chalcogenide Fluorophosphate glass
1. Introduction Composite optical materials with PbSe and PbS quantum dots are used widely now in optics and photonics [1–12]. Such materials exhibit the luminescent properties in the near IR (see, for example, [4]) and also non-linear optical properties [5,6]. The procedure of synthesizing the lead chalcogenide quantum dots in glasses consists in the thermal treatment at a certain temperature higher than the glass transition one of a glass containing before the synthesis lead, selenium, and sulfur. The thermal diffusion of lead ions and chalcogen atoms leads to the formation and growth of quantum dots in glass. Variations in the thermal treatment temperature and duration make it possible to change the quantum dot dimensions in a wide range. It is evident that the lead chalcogenide structures should pass, in the course of their formation and growth, a stage at which their size remains less than 1 nm. This stage corresponds to the occurrence, in a glass, of lead chalcogenide structures in the form of subnanosized molecular clusters whose properties can considerably differ from the properties of quantum dots. For the last years, a scientific interest in the metal and semiconductor molecular clusters (MCs) has increased rapidly [13,14]. MCs are of interest also from the practical viewpoint
n Correspondence to: Kronverkskii pr. 49, St. Petersburg 197101, Russia. Tel.: +79119205938. E-mail address: [email protected] (A.I. Sidorov).
http://dx.doi.org/10.1016/j.jlumin.2015.02.009 0022-2313/& 2015 Elsevier B.V. All rights reserved.
because some of MC types exhibit an intense luminescence in the visible. In some cases, the MCs arise in glasses during the thermal treatment; in other cases, they are formed directly in the course of the glass synthesis. The luminescent properties of silver molecular clusters Agn in oxyfluorine and silicate glasses were investigated in Refs. [15–21]. In Refs. [22–24], the luminescent properties of (CuCl)n and (Cu2O)n clusters in potassium–alumina– borate and silicate glasses were studied. The investigation of silver and gold molecular clusters in the metal-organic compounds was performed in Refs. [25–30]. In spite of enormous amount of research works devoted to the investigation of lead and cadmium chalcogenide quantum dots, there are quite few sources describing the corresponding MCs properties. In Refs. [31,32] the structure of the stable (CdSe)n (n¼1–4) and (XCdS)n (n¼4–16, X¼Na, K, Cl, and Br) molecular clusters was investigated by the computer modeling. In Ref. [33], the structure of CdSe molecular clusters was studied by an NMR method. The structure of Cd2O2Se and CdO2Se molecular clusters was investigated in Ref. [34] by X-ray absorption spectroscopy. It was shown in Refs. [35–40] that the Se2 and Se2 molecular clusters are present in both glasses and sodalite. Such MCs exhibit the luminescence bands at 480 and 710 nm. To our opinion there are no publications in which the lead chalcogenide MCs properties are described. In our experiments for as-prepared fluorophosphate (FP) glasses containing lead, selenium, and sulfur we observed an intense luminescence in the visible, excited by the UV radiation before quantum dots formation. In the reference samples without lead only very weak luminescence was observed, which can be
E.V. Kolobkova et al. / Journal of Luminescence 162 (2015) 36–40
37
attributed to the defects of the glass network. The luminescence of lead and chalcogenide containing glasses indicates the presence, in the initial glasses, of (PbSe)n and (PbS)n molecular clusters that play the role of crystallization centers for quantum dots during the subsequent thermal treatment. The given work is aimed to study the luminescence properties of FP glasses with (PbSe)n and (PbS)n molecular clusters including the influence, on the MC luminescence, of thermal treatments at a stage preceding the quantum dot formation and luminescence thermal quenching.
2. Experimental FP glasses based on the Na2O–P2O5–ZnF2–Ga2O3–PbF2–AlF3– NaF system doped with ZnS or ZnSe were synthesized. Two groups of samples denoted further by FP(PbS) and FP(PbSe) were prepared, the samples containing (PbS)n or (PbSe)n molecular clusters, respectively. For synthesis, the reagent grade and chemically pure raw materials were used. The glass synthesis was conducted in an electric furnace with the air atmosphere at 950 1C. The glass transition temperature Tg was measured with a Perkin-Elmer STA6000 differential scanning calorimeter, its magnitude being found to be 400 1C. Samples to be investigated were prepared in the form of plane-parallel plates 1.5–2.0 mm thick. For the thermal treatment of the samples, Nabertherm muffle furnace was used. For minimizing the effects of gradual heating and cooling, the samples were placed, at once, into the muffle furnace heated beforehand up to temperature required and, after a desired exposure, extracted from the hot furnace into air. The optical density spectra of FP glass samples under study were recorded in the 300–1000 nm spectral region with a step of 1 nm using a Perkin-Elmer Lambda 650 spectrophotometer. For registering the luminescence and excitation spectra, a PerkinElmer MPF-44 A spectrofluorimeter was used, the entrance and exit slit widths being equal to 7 nm. The luminescence spectra measured were corrected taking into account the spectral sensitivity of the spectrofluorimeter. For the excitation spectra, no correction was conducted. For measuring the luminescence at a fixed excitation wavelength, a StellarNet EPP2000-UVN-SR fiber spectrometer was used, the luminescence being excited by a semiconductor laser (λ ¼405 nm). The optical density spectra were registered at room temperature.
3. Results and discussion Fig. 1 shows the influence of thermal treatment on the optical density spectra of FP(PbSe) glasses. The as-prepared glass has the absorption band edge at 410 nm and is practically colorless. The thermal treatment of the glass at 280–380 1C causes few changes in the optical density spectra, the main effect being a small rise in the absorption in the 450–1000 nm spectral region. However, the thermal treatment at 400 1C for 1 h leads to (i) a change in the sample color to dark brown and (ii) a substantial red-shift of the absorption band edge to the 550–600 nm region. These effects indicate the initial stage of PbSe quantum dots formation. An increase in the thermal treatment duration at 400 1C leads to the change in the sample color from brown to black. This is caused assumedly by an increase in the quantum dots sizes and their concentrations. Fig. 2 shows the luminescence and excitation spectra of FP(PbSe) glass before thermal treatment. As seen from Fig. 2, the as-prepared FP(PbSe) glass exhibits a broad-band luminescence in the visible from 400 to 660 nm. An increase in the excitation wavelength leads to the red-shift of the luminescence maximum. For the excitation
Fig. 1. Optical density spectra of FP(PbSe) glass before (1) and after (2–5) thermal treatment for 1 h. Thermal treatment temperatures: 2 – 280, 3 – 350, 4 – 380, and 5 – 400 1C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
wavelengths in an 310–360 nm region, the luminescence intensity maximum occurs at 480–500 nm. The luminescence measurements for the reference glass sample with no Pb ions have shown that the sample, when excited by UV radiation, exhibits quite weak luminescence in the visible. This luminescence can be attributed to the defects of the glass network [41–47]. The excitation spectra of FP (PbSe) glass samples are located in the 260–420 nm region, with maxima at 280–340 nm. The width of the excitation band and its complicated structure indicate that several types of luminescence centers are present in the glass. Thus, the occurrence of broad-band and intense luminescence of FP(PbSe) glass gives an evidence that the (PbSe)n MCs differing in n are present in the glass, being less than 1 nm in size. Many other types of MCs in glasses have similar optical properties as well (see, for example, [15–22]). It should be noted that the Se2 molecular clusters can contribute to the luminescence band at 480 nm [35–40]. During the thermal treatment at or above 400 1C, these MCs play the role of nucleation centers for the PbSe quantum dots. Fig. 3 illustrates the influence of thermal treatment on the luminescent properties of FP(PbSe) glass. An increase in the temperature of the thermal treatment up to 380 1C leads to the red-shift of the luminescence maximum and to an increase in its intensity by 5 times. The common reason for this luminescence intensity increase and the red-shift of the luminescence band is an increase in the sizes and concentrations of MCs under heating that occurs due to the thermal diffusion of Pb ions and Se atoms. After the thermal treatment at 400 1C, the luminescence intensity in the visible decreases considerably and a new weak luminescence band appears in the 900–1100 nm region. This indicates the initial stage of PbSe quantum dots formation. Fig. 4 shows the influence of sample temperature on the luminescence intensity. In this experiment, a sample treated preliminarily at 380 1C was used. An increase in the sample temperature from 20 to 250 1C leads to (i) a decrease in the luminescence intensity by 6.7 times and to (ii) a weak red-shift of the luminescence band. This effect is reversible and can be repeated many times. The thermal quenching the luminescence can be explained by the fact that, with an increase in temperature, electrons in the excited state can occupy high vibrational energy levels that intersect with the electronic ground state level at the configuration coordinate diagram [48]. This allows the vibrational relaxation of excited electrons to the ground state that occurs via the phonon release without the emission of radiation.
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Fig. 2. Luminescence (a) and excitation (b) spectra of FP(PbSe) glass before the thermal treatment. (a) Excitation wavelength is 330 nm. (b) Luminescence wavelength is 500 nm.
Fig. 3. Luminescence spectra of FP(PbSe) glass before (1) and after (2–5) thermal treatment for 1 h. Thermal treatment temperatures: 2 – 280, 3 – 350, 4 – 380, and 5 – 400 1C. Excitation wavelength is 405 nm. Inset: the maximum visible luminescence intensity vs. thermal treatment temperature.
Fig. 5. Optical density spectra of FP(PbS) glass before (1) and after (2–5) thermal treatment for 1 h. Thermal treatment temperatures: 2 – 280, 3 – 350, 4 – 380, and 5 – 410 1C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 4. Influence of temperature on the luminescence intensity of FP(PbSe) glass sample treated preliminarily at 380 1C. Measurement temperatures: 1 – 20, 2 – 50, 3 – 100, 4 – 150, 5 – 200, and 6–250 1C. Excitation wavelength is 405 nm. Inset: the maximum luminescence intensity vs. the sample temperature.
For the FP(PbS) glasses, the effects similar to those described above for the FP(PbSe) glasses are observed. The as-prepared FP (PbS) glass is colorless and have the absorption band edge at 390– 400 nm (Fig. 5). The thermal treatment of the glass at 280–350 1C
causes no changes in the optical density spectra, whereas that at 380 1C leads to (i) a change in the sample color to yellow and to (ii) a weak red-shift of the absorption band edge. These effects indicate the initial stage of PbS quantum dots formation. The thermal treatment at the greater temperature of 410 1C leads to a change in the sample color to red–brown and to a substantial redshift of the absorption band edge. This is caused assumedly by an increase in the quantum dots sizes and concentrations. Fig. 6 shows the luminescence and excitation spectra of FP(PbS) glass before the thermal treatment. As seen from Fig. 6, the asprepared FP(PbS) glass exhibits a broad-band luminescence in the visible from 420 to 620 nm with a maximum around 460–480 nm. The luminescence band of FP(PbS) glass is more narrow than that of FP(PbSe) one and its maximum exhibits the blue-shift compared to that for FP(PbSe) glass (see Fig. 2a and Fig. 6a), though, as follows from stated below, the general pattern of FP(PbS) glass spectra remains to be similar enough to that for FP(PbSe) glass. An increase in the excitation wavelength leads to the red-shift of the
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Fig. 6. Luminescence (a) and excitation (b) spectra of FP(PbS) glass before the thermal treatment. (a) Excitation wavelength – 340 nm. (b) Luminescence wavelength – 500 nm.
Fig. 7. Luminescence (a) and excitation (b) spectra of FP(PbS) glass after the thermal treatment at 380 1C. (a) Excitation wavelength is 340 nm. (b) Luminescence wavelength is 620 nm.
luminescence maximum. The excitation spectra of FP(PbSe) glass samples are located in the 240–420 nm region, their maxima lying at 320–340 nm. The complicated structure of the luminescence band indicates that several types of luminescence centers are present in the glass. Thus, the occurrence of broad-band and intense luminescence of FP(PbS) glass gives an evidence that the (PbS)n MCs differing in n and being less than 1 nm in size are present in the glass. Similar to the case of FP(PbSe) glass, the thermal treatment of FP(PbS) glass at temperatures less than the glass transition temperature results in an increase in the luminescence intensity (Fig. 7). The thermal treatment of FP(PbS) glass at 380 1C leads to the red-shift of the luminescence band maximum from 460–480 to 600–620 nm. The thermal treatment at 400 1C results in a substantial decrease in the luminescence intensity in the visible because of PbS quantum dots formation. Under an increase in the temperature of the thermally treated sample, the thermal
quenching of the luminescence occurs similar to the case of FP (PbSe) glasses.
4. Summary It is shown experimentally that before the quantum dots formation the fluorophosphate glasses containing lead, selenium, and sulfur exhibit an intense luminescence in the visible when excited by the UV radiation. This luminescence is assumed to be due to the presence of (PbSe)n and (PbS)n molecular clusters in the glasses. The thermal treatment of the glass samples at temperatures less than the glass transition temperature results in an increase in the luminescence intensity. In the course of thermal treatment around the glass transition temperature, the (PbSe)n and (PbS)n molecular clusters play a role of nucleation centers for the PbSe and PbS quantum dots growth. The heating of the
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