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Luminescence kinetics in silica gel doped with Tb3+ ions and ZnS:Mn2+ nanocrystals
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 921–923 www.elsevier.com/locate/jlumin
Luminescence kinetics in silica gel doped with Tb3+ ions and ZnS:Mn2+ nanocrystals Sebastian Mahlikb,, Magdalena Zalewskaa, Marek Grinbergb, Andrzej M. K"onkowskia, Marek Godlewskic,d a Faculty of Chemistry, University of Gdan´sk, Sobieskiego 18, 80-952 Gdan´sk, Poland Institute of Experimental Physics, University of Gdan´sk, Wita Stwosza 57, 80-952 Gdan´sk, Poland c Institute of Physics, Polish Academy of Sciences, Al. Lotniko´w 32/46, 02-668 Warsaw, Poland d Department of Mathematics and Natural Sciences College of Science, Cardinal S. Wyszyn´ski University, Warsaw, Poland b
Available online 8 December 2007
Abstract Luminescence kinetics and time-resolved luminescence spectra of SiO2, SiO2 doped with ZnS:Mn2+ nanocrystals and SiO2 doped with ZnS:Mn2+, and additionally co-doped with Tb3+, are presented. The purposes of the paper are the analysis of the kinetics of the Tb3+ and Mn2+ intra-shell luminescence and the elucidation of the energy-transfer mechanism between the ZnS:Mn2+ nanocrystals and the Tb3+ ions. We have found a blue luminescence related to defects in the ZnS nanocrystals and an intrinsic luminescence of the SiO2 lattice, which decays in few ns. A yellow luminescence related to the Mn2+ 4T1(G)-6A1 transition and yellow sharp lines related to the 5 D4-7F6, 7F5, 7F4 and 7F3 transitions in Tb3+ are found to decay in ms. A very effective energy transfer between ZnS:Mn2+ nanoparticles and Tb3+ ions has been observed. r 2008 Elsevier B.V. All rights reserved. Keywords: Sol–gel method; ZnS; Mn2+; Tb3+; SiO2
1. Introduction The interest in new phosphors that can transform blue or UV light into visible radiation results from many applications of highly effective luminescent materials. Standard applications, as screens and gas lamps phosphors, are still important; however, new light sources, mainly blue emitting diodes (LEDs), generate new challenges. To commercialize LED lamps, cheap, efficient and chemically stable phosphors that transform blue into white light are needed. One of the cheapest method for the synthesis of optically active materials is the sol–gel technique. To obtain high quantum efficiency and required spectral properties, frequently composite systems are used. We have focused on the silica gel doped with transition ions (rare earth or transition metal ions) and co-doped with nanocrystals of wide band semiconductors. In these systems, transition ions act as luminescent centers whereas Corresponding author. Tel.: +48 58 5232544; fax: +48 58 3413175.
E-mail address: fi[email protected] (S. Mahlik). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.12.015
semiconductor nanocrystals act as donors, which absorb the UV radiation and pass the excitation energy to the luminescent centers. In this contribution we analyze the luminescence kinetics and the energy-transfer process in the SiO2:ZnS:Mn2+, Tb3 system.
2. Experimental results and discussion The SiO2:ZnS:Mn2+, Tb3 samples were prepared by a sol–gel procedure [1,2]. ZnS:Mn2+ nanocrystals were obtained by the inverse micelle method. Then Tb3+ ions and ZnS:Mn2+ nanocrystals were embedded into the silica. Details of sample preparation are described in Ref. [3]. To obtain time-resolved spectra, the samples were excited using an OPG system that generates 30 ps laser pulses of 335 nm wavelength, with frequency 10 Hz. The luminescence kinetics and time-resolved spectra were measured using a Hamamatsu Streak Camera model C4334-01. The wavelength 335 nm was used because it effectively excites the Mn2+ 4T1(G)-6A1 luminescence.
ARTICLE IN PRESS S. Mahlik et al. / Journal of Luminescence 128 (2008) 921–923
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SiO2:ZnS:Mn2+, and is related to SiO2 and fast luminescence of ZnS:Mn2+ nanocrystals. In the luminescence spectrum integrated over the time from 100 ns to 100 ms (dotted curve) four sharp lines peaking at 488, 543, 585 and 620 nm are observed, related to the 5D4-7F6, 7F5, 7F4 and 7 F3, transitions of Tb3+, respectively. When the luminescence is integrated in the temporal range 0.1–2 ms (solid curve) only the Tb3+ emission is observed. One notices that the broadband emission related to the 4T1(G)-6A1 transition in Mn2+ in ZnS is not observed when the material has been co-doped intentionally with Tb3+. This is an empirical argument for the existence of an effective non-radiative energy transfer from ZnS:Mn2+ nanocrystals to Tb3+ ions. In Fig. 2 the fast luminescence spectra of SiO2:ZnS:Mn2+ (dashed curve) is compared with the fast luminescence of pure SiO2 (solid curve). The emission spectrum of pure SiO2 consists of a broad band peaking at about 450 nm. The luminescence spectrum of the SiO2:ZnS:Mn2+ is shifted towards shorter wavelengths and consists of two overlapping bands peaking at 440 and 420 nm. The peak positions are indicated by arrows in Fig. 2. The difference between SiO2:ZnS:Mn2+and SiO2 emission intensities is presented by a dashed–dotted curve (upper curve) in Fig. 2. This fast luminescence emission has been related to defects in the ZnS host [4]. To analyze a possible energy transfer between the SiO2 lattice and ZnS:Mn2+ nanocrystals, we have measured the luminescence decays. The luminescence decays of SiO2 and SiO2:ZnS:Mn2+ are presented in Fig. 3a. To obtain these decay curves the luminescence was integrated over the band at 420–440 nm. Since the decay of the SiO2:ZnS:Mn2+ emission is longer than that of pure SiO2, there is no non-radiative energy transfer from SiO2 to ZnS:Mn2+.
intensity (arb.units)
intensity (arb.units)
The luminescence spectra were collected by integration of the streak camera pictures over time intervals, whereas the luminescence decays were collected by integrating the streak camera pictures over the wavelength intervals. All spectra have been measured at room temperature. Time-resolved luminescence spectra of SiO2:ZnS:Mn2+ and SiO2:ZnS:Mn2+, Tb3+ are presented in Fig. 1a and b, respectively. In the case of SiO2:ZnS:Mn2+ (Fig. 1a) the emission integrated over time 0–2 ns ( dotted curve) consists of a broad blue band peaking at about 440 nm, and is related to the intrinsic luminescence of SiO2 and fast luminescence of ZnS:Mn2+ nanocrystals. In the emission spectrum integrated over the time from 100 ns to 100 ms (dashed curve) besides the blue luminescence, a broad yellow band peaking at about 600 nm appears. This yellow emission is more persistent and dominates the spectrum when it is integrated in time from 0.1 to 2 ms (solid curve). The band peaking at 600 nm is attributed to the internal 4 T1(G)-6A1 transition in tetrahedrally coordinated Mn2+ in ZnS. The sharp lines seen in the 550–650 nm spectral region are related to the uncontrolled Tb3+ impurity. In the case of SiO2:ZnS:Mn2+, Tb3 (Fig. 1b) the fast emission (dotted curve) is similar to that observed in
400
450
500 550 wavelength (nm)
600
650
Fig. 1. Time-resolved emission spectra of SiO2:ZnS:Mn2+ (a) SiO2:ZnS:Mn2+, Tb3+ (b). The applied delay times were: 2 ns (luminescence was integrated for the time interval 0–2 ns); 100 ms (luminescence was integrated for the time interval 0.01–100 ms); 2 ms (luminescence was integrated for the time interval 0.1–2 ms).
400
450
500 550 wavelength (nm)
600
650
Fig. 2. Luminescence of SiO2:ZnS:Mn2+ (dashed curve) and SiO2 (solid curve) integrated for the time interval 0–5 ns. Difference between SiO2:ZnS:Mn2+and SiO2 emission intensity is presented by dashed–dotted upper curve.
ARTICLE IN PRESS S. Mahlik et al. / Journal of Luminescence 128 (2008) 921–923
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104
SiO2:ZnS: Mn2+
103
Intensity (arb. units)
Intensity (arb.units)
ZnS:Mn2+in SiO2
103
2
102
SiO2 ZnS:Mn2+,Tb3+in SiO2 1 102 0
2
4
6
8
101
0
1
time (ns)
2 3 time (ms)
4
Fig. 3. Decay of the luminescence integrated in the band 420–440 nm for SiO2 (dashed curve) and SiO2:ZnS:Mn2+ (solid curve). (a) Decays of the persistent emission integrated in the band 580–630 nm for SiO2:ZnS:Mn2+ (dashed curve) and SiO2:ZnS: Mn2+, Tb3+ (solid curve 2). Decay of the SiO2:ZnS:Mn2+, Tb3+ luminescence monitored at 585 nm (5D4 -7F5 transition in Tb3+) is represented by solid curve 1.
The decays of the luminescence measured in the time scale from 0 to 10 ms are presented in Fig. 3b. The decays of the luminescence monitored at 488, 543, 585 and 620 nm, corresponding to Tb3+ are very similar and are represented by a single curve (solid curve 1). This decay is almost exponential and the decay time is equal to 1.0 ms. The decay of the SiO2:ZnS:Mn2+:Tb3+ luminescence integrated over 480 and 630 nm spectral region is presented by the solid curve 2. One can decompose this decay into two exponential decays. The lifetime of the longer component is equal to 1.04 ms and corresponds to the Tb3+ emission and the shorter lifetime is equal to 0.23 ms. This shorter decay has been attributed to the tail of the fast luminescence (mainly to the intrinsic SiO2 emission) that extends up to 650 nm. The decay of the SiO2:ZnS:Mn2+ emission integrated over the spectral region from 550 to 650 nm is presented by a dashed curve. It can be decomposed into three components characterized by decay constants: 0.23, 1.04 and 4.04 ms. The longest component has been attributed to 4 T1(G)-6A1 emission in SiO2:ZnS:Mn2+, whereas the shorter decays (0.23 and 1.04 ms) were attributed to the traces of intrinsic SiO2 emission and Tb3+ emission (uncontrolled dopand), respectively. 3. Conclusions Time-resolved luminescence spectra of SiO2, SiO2:ZnS:Mn2+ and SiO2:ZnS:Mn2+, Tb3+ have been
measured. A fast luminescence, related to SiO2 lattice and ZnS:Mn2+ nanocrystals, in the blue region has been observed in all samples, independently of the sample contains or not Tb3+. We have not observed the Mn2+ 4 T1(G)-6A1 luminescence in the time-resolved spectra for the time delays shorter than ms (see Fig. 1—dotted curves and Fig. 2). For longer delay times it has been found that the luminescence of SiO2:ZnS:Mn2+, Tb3+ does not contain the broad band related to the 4T1(G)-6A1 transition of Mn2+ in ZnS:Mn2+. This means that after relaxation almost all the energy absorbed by ZnS nanocrystals is transferred to Tb3+, before the relaxation of the system to the 4T1(G) state of Mn2+. Acknowledgment This paper has been supported by the Polish Scientific Research Council (Grant 1308/T09/2005/29). References [1] C.J. Brinker, G.W. Scherer, Sol–Gel Science, The Physics and Chemistry of Sol–Gel Processing, Academic Press, London, 1990. [2] M. Nogami, in: L.C. Klein (Ed.), Sol–Gel Optics, Kluwer Academic Publishers, Dordrecht, 1994, p. 329. [3] M. Zalewska, B. Kuklin´ski, E. Grzanka, S. Mahlik, J. Jezierska, B. Pa"osz, M. Grinberg, A.M. K"onkowski, J. Lumin., in press. [4] A.A. Bol, A. Meijerink, J. Lumin. 87–89 (2000) 315.
Journal of Luminescence 128 (2008) 921–923 www.elsevier.com/locate/jlumin
Luminescence kinetics in silica gel doped with Tb3+ ions and ZnS:Mn2+ nanocrystals Sebastian Mahlikb,, Magdalena Zalewskaa, Marek Grinbergb, Andrzej M. K"onkowskia, Marek Godlewskic,d a Faculty of Chemistry, University of Gdan´sk, Sobieskiego 18, 80-952 Gdan´sk, Poland Institute of Experimental Physics, University of Gdan´sk, Wita Stwosza 57, 80-952 Gdan´sk, Poland c Institute of Physics, Polish Academy of Sciences, Al. Lotniko´w 32/46, 02-668 Warsaw, Poland d Department of Mathematics and Natural Sciences College of Science, Cardinal S. Wyszyn´ski University, Warsaw, Poland b
Available online 8 December 2007
Abstract Luminescence kinetics and time-resolved luminescence spectra of SiO2, SiO2 doped with ZnS:Mn2+ nanocrystals and SiO2 doped with ZnS:Mn2+, and additionally co-doped with Tb3+, are presented. The purposes of the paper are the analysis of the kinetics of the Tb3+ and Mn2+ intra-shell luminescence and the elucidation of the energy-transfer mechanism between the ZnS:Mn2+ nanocrystals and the Tb3+ ions. We have found a blue luminescence related to defects in the ZnS nanocrystals and an intrinsic luminescence of the SiO2 lattice, which decays in few ns. A yellow luminescence related to the Mn2+ 4T1(G)-6A1 transition and yellow sharp lines related to the 5 D4-7F6, 7F5, 7F4 and 7F3 transitions in Tb3+ are found to decay in ms. A very effective energy transfer between ZnS:Mn2+ nanoparticles and Tb3+ ions has been observed. r 2008 Elsevier B.V. All rights reserved. Keywords: Sol–gel method; ZnS; Mn2+; Tb3+; SiO2
1. Introduction The interest in new phosphors that can transform blue or UV light into visible radiation results from many applications of highly effective luminescent materials. Standard applications, as screens and gas lamps phosphors, are still important; however, new light sources, mainly blue emitting diodes (LEDs), generate new challenges. To commercialize LED lamps, cheap, efficient and chemically stable phosphors that transform blue into white light are needed. One of the cheapest method for the synthesis of optically active materials is the sol–gel technique. To obtain high quantum efficiency and required spectral properties, frequently composite systems are used. We have focused on the silica gel doped with transition ions (rare earth or transition metal ions) and co-doped with nanocrystals of wide band semiconductors. In these systems, transition ions act as luminescent centers whereas Corresponding author. Tel.: +48 58 5232544; fax: +48 58 3413175.
E-mail address: fi[email protected] (S. Mahlik). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.12.015
semiconductor nanocrystals act as donors, which absorb the UV radiation and pass the excitation energy to the luminescent centers. In this contribution we analyze the luminescence kinetics and the energy-transfer process in the SiO2:ZnS:Mn2+, Tb3 system.
2. Experimental results and discussion The SiO2:ZnS:Mn2+, Tb3 samples were prepared by a sol–gel procedure [1,2]. ZnS:Mn2+ nanocrystals were obtained by the inverse micelle method. Then Tb3+ ions and ZnS:Mn2+ nanocrystals were embedded into the silica. Details of sample preparation are described in Ref. [3]. To obtain time-resolved spectra, the samples were excited using an OPG system that generates 30 ps laser pulses of 335 nm wavelength, with frequency 10 Hz. The luminescence kinetics and time-resolved spectra were measured using a Hamamatsu Streak Camera model C4334-01. The wavelength 335 nm was used because it effectively excites the Mn2+ 4T1(G)-6A1 luminescence.
ARTICLE IN PRESS S. Mahlik et al. / Journal of Luminescence 128 (2008) 921–923
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SiO2:ZnS:Mn2+, and is related to SiO2 and fast luminescence of ZnS:Mn2+ nanocrystals. In the luminescence spectrum integrated over the time from 100 ns to 100 ms (dotted curve) four sharp lines peaking at 488, 543, 585 and 620 nm are observed, related to the 5D4-7F6, 7F5, 7F4 and 7 F3, transitions of Tb3+, respectively. When the luminescence is integrated in the temporal range 0.1–2 ms (solid curve) only the Tb3+ emission is observed. One notices that the broadband emission related to the 4T1(G)-6A1 transition in Mn2+ in ZnS is not observed when the material has been co-doped intentionally with Tb3+. This is an empirical argument for the existence of an effective non-radiative energy transfer from ZnS:Mn2+ nanocrystals to Tb3+ ions. In Fig. 2 the fast luminescence spectra of SiO2:ZnS:Mn2+ (dashed curve) is compared with the fast luminescence of pure SiO2 (solid curve). The emission spectrum of pure SiO2 consists of a broad band peaking at about 450 nm. The luminescence spectrum of the SiO2:ZnS:Mn2+ is shifted towards shorter wavelengths and consists of two overlapping bands peaking at 440 and 420 nm. The peak positions are indicated by arrows in Fig. 2. The difference between SiO2:ZnS:Mn2+and SiO2 emission intensities is presented by a dashed–dotted curve (upper curve) in Fig. 2. This fast luminescence emission has been related to defects in the ZnS host [4]. To analyze a possible energy transfer between the SiO2 lattice and ZnS:Mn2+ nanocrystals, we have measured the luminescence decays. The luminescence decays of SiO2 and SiO2:ZnS:Mn2+ are presented in Fig. 3a. To obtain these decay curves the luminescence was integrated over the band at 420–440 nm. Since the decay of the SiO2:ZnS:Mn2+ emission is longer than that of pure SiO2, there is no non-radiative energy transfer from SiO2 to ZnS:Mn2+.
intensity (arb.units)
intensity (arb.units)
The luminescence spectra were collected by integration of the streak camera pictures over time intervals, whereas the luminescence decays were collected by integrating the streak camera pictures over the wavelength intervals. All spectra have been measured at room temperature. Time-resolved luminescence spectra of SiO2:ZnS:Mn2+ and SiO2:ZnS:Mn2+, Tb3+ are presented in Fig. 1a and b, respectively. In the case of SiO2:ZnS:Mn2+ (Fig. 1a) the emission integrated over time 0–2 ns ( dotted curve) consists of a broad blue band peaking at about 440 nm, and is related to the intrinsic luminescence of SiO2 and fast luminescence of ZnS:Mn2+ nanocrystals. In the emission spectrum integrated over the time from 100 ns to 100 ms (dashed curve) besides the blue luminescence, a broad yellow band peaking at about 600 nm appears. This yellow emission is more persistent and dominates the spectrum when it is integrated in time from 0.1 to 2 ms (solid curve). The band peaking at 600 nm is attributed to the internal 4 T1(G)-6A1 transition in tetrahedrally coordinated Mn2+ in ZnS. The sharp lines seen in the 550–650 nm spectral region are related to the uncontrolled Tb3+ impurity. In the case of SiO2:ZnS:Mn2+, Tb3 (Fig. 1b) the fast emission (dotted curve) is similar to that observed in
400
450
500 550 wavelength (nm)
600
650
Fig. 1. Time-resolved emission spectra of SiO2:ZnS:Mn2+ (a) SiO2:ZnS:Mn2+, Tb3+ (b). The applied delay times were: 2 ns (luminescence was integrated for the time interval 0–2 ns); 100 ms (luminescence was integrated for the time interval 0.01–100 ms); 2 ms (luminescence was integrated for the time interval 0.1–2 ms).
400
450
500 550 wavelength (nm)
600
650
Fig. 2. Luminescence of SiO2:ZnS:Mn2+ (dashed curve) and SiO2 (solid curve) integrated for the time interval 0–5 ns. Difference between SiO2:ZnS:Mn2+and SiO2 emission intensity is presented by dashed–dotted upper curve.
ARTICLE IN PRESS S. Mahlik et al. / Journal of Luminescence 128 (2008) 921–923
923
104
SiO2:ZnS: Mn2+
103
Intensity (arb. units)
Intensity (arb.units)
ZnS:Mn2+in SiO2
103
2
102
SiO2 ZnS:Mn2+,Tb3+in SiO2 1 102 0
2
4
6
8
101
0
1
time (ns)
2 3 time (ms)
4
Fig. 3. Decay of the luminescence integrated in the band 420–440 nm for SiO2 (dashed curve) and SiO2:ZnS:Mn2+ (solid curve). (a) Decays of the persistent emission integrated in the band 580–630 nm for SiO2:ZnS:Mn2+ (dashed curve) and SiO2:ZnS: Mn2+, Tb3+ (solid curve 2). Decay of the SiO2:ZnS:Mn2+, Tb3+ luminescence monitored at 585 nm (5D4 -7F5 transition in Tb3+) is represented by solid curve 1.
The decays of the luminescence measured in the time scale from 0 to 10 ms are presented in Fig. 3b. The decays of the luminescence monitored at 488, 543, 585 and 620 nm, corresponding to Tb3+ are very similar and are represented by a single curve (solid curve 1). This decay is almost exponential and the decay time is equal to 1.0 ms. The decay of the SiO2:ZnS:Mn2+:Tb3+ luminescence integrated over 480 and 630 nm spectral region is presented by the solid curve 2. One can decompose this decay into two exponential decays. The lifetime of the longer component is equal to 1.04 ms and corresponds to the Tb3+ emission and the shorter lifetime is equal to 0.23 ms. This shorter decay has been attributed to the tail of the fast luminescence (mainly to the intrinsic SiO2 emission) that extends up to 650 nm. The decay of the SiO2:ZnS:Mn2+ emission integrated over the spectral region from 550 to 650 nm is presented by a dashed curve. It can be decomposed into three components characterized by decay constants: 0.23, 1.04 and 4.04 ms. The longest component has been attributed to 4 T1(G)-6A1 emission in SiO2:ZnS:Mn2+, whereas the shorter decays (0.23 and 1.04 ms) were attributed to the traces of intrinsic SiO2 emission and Tb3+ emission (uncontrolled dopand), respectively. 3. Conclusions Time-resolved luminescence spectra of SiO2, SiO2:ZnS:Mn2+ and SiO2:ZnS:Mn2+, Tb3+ have been
measured. A fast luminescence, related to SiO2 lattice and ZnS:Mn2+ nanocrystals, in the blue region has been observed in all samples, independently of the sample contains or not Tb3+. We have not observed the Mn2+ 4 T1(G)-6A1 luminescence in the time-resolved spectra for the time delays shorter than ms (see Fig. 1—dotted curves and Fig. 2). For longer delay times it has been found that the luminescence of SiO2:ZnS:Mn2+, Tb3+ does not contain the broad band related to the 4T1(G)-6A1 transition of Mn2+ in ZnS:Mn2+. This means that after relaxation almost all the energy absorbed by ZnS nanocrystals is transferred to Tb3+, before the relaxation of the system to the 4T1(G) state of Mn2+. Acknowledgment This paper has been supported by the Polish Scientific Research Council (Grant 1308/T09/2005/29). References [1] C.J. Brinker, G.W. Scherer, Sol–Gel Science, The Physics and Chemistry of Sol–Gel Processing, Academic Press, London, 1990. [2] M. Nogami, in: L.C. Klein (Ed.), Sol–Gel Optics, Kluwer Academic Publishers, Dordrecht, 1994, p. 329. [3] M. Zalewska, B. Kuklin´ski, E. Grzanka, S. Mahlik, J. Jezierska, B. Pa"osz, M. Grinberg, A.M. K"onkowski, J. Lumin., in press. [4] A.A. Bol, A. Meijerink, J. Lumin. 87–89 (2000) 315.