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Luminescence of Mn2+ doped ZnS nanocrystallites
JOURNAL OF
LUMINESCENCE Journal of Luminescence66&67(1996)315-318
ELSEMER
Luminescence of Mn2 + doped ZnS nanocrystallites Chunming Jin *, Jiaqi Yu, Lingdong Sun, Kai Dou, Shanggong Hou, Jialong Zhao, Yimin Chen, Shihua Huang Laboratory qf Excited State Processes, Changchun Institute of Physics, Chinese Academy ofSciences. I, Yanhn Road, Changchun
130021, China
Abstract
Optical properties of Mn “-doped ZnS colloids are reported. The band to band excitation energy transfer to Mn2+ is more efficient compared to Mn2+ direct excitation, which is different from the case for bulk ZnS : Mn. Aging effects and the radiation-induced luminescence enhancement (RILE) effect are reported and an explanation for this behavior is presented.
1. Introduction Optical properties of semiconductor microcrystallites have been investigated extensively in recent years [l-3], since these materials have potential application to nonlinear optical devices. Most work has been devoted to understanding the intrinsic optical properties of such materials. Bhargava et al. [4] first synthesized ZnS semiconductor microcrystallites doped with Mn2 + ions. They found that the photoluminescence of Mn2+ ions in ZnS nanocrystals had a very high quantum efficiency with a luminescence decay several orders of magnitude faster than in the bulk crystals. This indicated that microcrystals doped with optically active luminescent centers may create new physics and applications. In this paper, we report the preparation and optical properties of Mn2+-doped ZnS semiconductor colloids. It was found that the Mn’ ’
*Corresponding
author.
0022-2313/96/$15.00 G 1996 SSDI 0022-23 13(95)00160-3
ElsevierScience
luminescent intensity was influenced by the presence of a surfactant. An aging effect and a radiation-induced luminescence enhancement (RILE) effect were observed for ZnS : Mn alcohol colloids.
2. Experiments The samples used in the experiments were prepared by the colloidal chemical method. The procedure is as follows: first Zn(CH2C00)2 and Mn(CH2C00)2 were dissolved into an alcohol solution. The molar concentration ratio of Mn2+ ions to Zn2+ was 1: 100 in the solution. Second, H2S gas was passed into the solution while stirring rapidly. The prepared ZnS : Mn alcohol colloid was quite transparent. The ZnS nanoparticle size can be controlled by altering the molar concentration of Zn2+ ions in the alcohol solution. A typical concentration of Zn’+ ions in alcohol is 1O-3 M, which gave about 3.6nm sized particles of ZnS: Mn. The photoluminescent emission and photoluminescent
B.V. All rights reserved
316
C. Jin rt (II. : Journal
of Luminescmce 66&67
(1996)
315-318
excitation spectra of Mn’+ in ZnS nanoparticles were measured at room temperature with a Hitachi F-4000 Spectra-fluormeter.
i
3. Results and discussion The photoluminescent emission spectra of three ZnS: Mn colloid samples (1,2,3) were measured and are shown in Fig. l(b). The Zn2+ ions concentrations are 10e3, 10m3 and lO_‘M in the three samples, respectively. There was a surfactant (polymethyl methacrylate) on both sample 2 (1O-3 M) and sample 3 (lo- ’ M). The yellow emission bands in the figures are attributed to the Mn2+ 4T, + “Ai transition of the ZnS nanocrystal host. The emission peaks are at 586.4, 585.8 and 587.4nm. The photoluminescent excitation (PLE) spectra were measured monitoring these emission peaks and are given in Fig. l(a). The excitation peaks are at 291.7, 297.4 and 311.4 nm. The ZnS : Mn nanocrystal size in sample 2 is about 3.6 nm for the 297.4 nm excitation peak [S]. The excitation bands come from band-to-band transitions of the ZnS nanocrystal. The excitation peaks are blue-shifted compared to the excitation peak (332nm) of the bulk material [4]. This energy shift is due to the quantum size effect in small ZnS particles. Larger blue shifts in the excitation spectra corresponds to smaller ZnS nanoparticle sizes. It was noticed that the excitation peaks from the ground state to higher excited states of Mn2+ were difficult to detect in the excitation spectra of the colloidal samples. This is very different from the situation in the bulk ZnS crystal, where the intensities of the band-to-band excitation and excitation from Mn2+ transitions are of the same order. This implies that the energy transfer from band-to-band excitations are more efficient in ZnS : Mn nanosize crystals than in Zns: Mn bulk crystals. From the comparison of the luminescence of ZnS: Mn bulk material and nanoparticles under the same excitation, we believe that nanoparticles of ZnS : Mn have higher luminescence intensities and efficiencies. It is possible that the efficient energy transfer is the result of electrons and holes being confined to be near Mn2 + ions in such small nanosize ZnS particles.
‘. I
240
280
320
I’
‘I
\
-._
36o.ioo
Wavelength
I
,
550
600
I
650
700
(nm)
Fig. 1. (a) Photoluminescence excitation spectra of three ZnS: Mn” alcohol colloidal samples: sample 1 (curve A). sample 2 (curve B), sample 3 (curve C). (b) Photoluminescence spectra of three ZnS : Mn’+ alcohol colloidal samples: sample I (curve A), sample 2 (curve B), sample 3 (curve C). The ratio of maximum Intensity of the curves is A : B : C = 10: 40 : 43.
An important aspect that influences the optical properties of colloids is the presence of a surfactant. We observed the influence of the surfactant (polymethyl methacrylate) on the Mn2+ luminescence of ZnS: Mn nanoparticles. It was found that the Mn2+ luminescent intensity is enhanced several or tens of times that of samples without surfactants. As an example, the luminescent intensity of sample 2 (with surfactant) is about four times that of sample 1 (without surfactant) under excitation at the peak wavelength of the excitation bands. The size difference between the two samples is small because the peaks of the excitation spectra are similar. We found that there is an aging effect for ZnS: Mn colloids. The Mn2+ ion luminescent intensity decreased after the colloids were kept at room temperature for a long time. As an example, the luminescent intensity of sample 2 decreased to a tenth of that of a fresh sample after it was kept for about two months. The luminescent peak is redshifted from 585.8 nm (fresh) to 590.4 nm (aged), as shown in Fig. 2(b). The excitation peak is shifted to lower energy, from 297 nm (fresh) to 3 12 nm (aged). The shape of the excitation spectrum is also changed, as shown in Fig. 2(a). The higher energy part of the excitation band decreased.
317
C. Jin etal. /Journal ofLuminescence ljri&67(1996)315-318
b
. . 240
280
320
360500
Wavelength
550
600
650
.
700
(nm)
Fig. 2. (a) Excitation spectra of sample 2: curve A (aged sample). curve B (fresh sample). (b) Photoluminescence spectra of sample 2: curve A (aged sample), curve B (fresh sample). The ratio of maximum intenstty of the curves is A : B = IO: 72.
We have also observed the radiation-induced luminescence enhancement (RILE) effect. The Mn2+ emission intensity of the aged sample 2 increased when it was exposed to UV-light. By changing the irradiation wavelength of a Xe lamp, the luminescence intensity enhancement is altered, as shown in Fig. 3(b). The most effective wavelength lies between 240and 330 nm as seen in Fig. 3(b). Besides an increase in luminescent intensity, the excitation peak of this exposed sample was shifted to higher energy, from 312 nm (unexposed) to 310-298.6 nm (exposed). As seen in Fig. 3(a), the shape of the excitation spectrum also changed, the higher-energy part increasing relative to the lowerenergy part. The shape of the luminescence spectra did not change, but the peak shifted from 590.4 to 585.6 nm for the sample exposed under 266 nm Xe lamp radiation. The size of the nanoparticles did not change after this irradiation as the absorption spectrum did not change. A tentative explanation for the aging effect and the RILE effect is as follows: a surface modification interaction by the surfactant reduces the nonradiative relaxation path for electrons excited into the conduction band, so samples with surfactant have higher luminescence intensity than samples without any surfactant. After storage for a long
240
280
320
360
Wavelength (nm) Fig. 3. (a) Photoluminescent excitation spectra of sample 2 (aged): curve A: unexposed, curve B: exposed under Xe lamp 266 nm radiation for 60 min. The ratio of maximum intensity of the curves is A : B = I : 19. (b) Photoluminescence intensity ratio for the sample exposed to several wavelengths of Xe lamp light for 60 min to that of the unexposed sample.
time, the surface modification deteriorated and nonradiative relaxation paths increase so the luminescence intensity decreases. With radiation exposure at a suitable wavelength, the surface modification by the surfactant was restored, so the luminescence recovers and can even be enhanced. Different nonradiative paths are connected with different states in the conduction band. The origin of these nonradiative paths may be surface states. The surface modification by the surfactant may fill these surface states with electrons, so the nonradiative paths are blocked. But these filled surface states are metastable. The higher the energy of the surface states, the faster the electron relaxation. So after a long time, the surface states with higher energy become unblocked and the higher part of the excitation bands become less efficient than the lower part of the excitation band. Irradiation with suitable wavelengths will refill these empty surface states with electrons. The luminescence recovers and is enhanced, and the efficiency of the higher part of the excitation band increases again.
318
C. Jin et al. J Journal of Luminescence 66&67 (1996) 315-318
Acknowledgements
References
The authors acknowledge the support of the National Natural Science Foundation of China and the Laboratory of the Excited State Processes, Changchun Institute of Physics, Chinese Academy of Sciences. The authors gratefully acknowledge the support of the K.C. Wong Education Foundation, Hong Kong.
Cl1 Y. Wang and N. Herron, J. Phys. Chem. 121A.D. Yoffe, Adv. Phys. 42 (1993) 173. c31 L. Brus, Appl. Phys. A 53 (1991) 465. r41 R.N. Bhargava,
95 (1991) 525.
D. Gallagher, X. Hong and A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. CSID. Gallagher, W.E. Heady, J.M. Racz and R.N. Bhargava, J. Crystal Growth 138 (1994) 970.
LUMINESCENCE Journal of Luminescence66&67(1996)315-318
ELSEMER
Luminescence of Mn2 + doped ZnS nanocrystallites Chunming Jin *, Jiaqi Yu, Lingdong Sun, Kai Dou, Shanggong Hou, Jialong Zhao, Yimin Chen, Shihua Huang Laboratory qf Excited State Processes, Changchun Institute of Physics, Chinese Academy ofSciences. I, Yanhn Road, Changchun
130021, China
Abstract
Optical properties of Mn “-doped ZnS colloids are reported. The band to band excitation energy transfer to Mn2+ is more efficient compared to Mn2+ direct excitation, which is different from the case for bulk ZnS : Mn. Aging effects and the radiation-induced luminescence enhancement (RILE) effect are reported and an explanation for this behavior is presented.
1. Introduction Optical properties of semiconductor microcrystallites have been investigated extensively in recent years [l-3], since these materials have potential application to nonlinear optical devices. Most work has been devoted to understanding the intrinsic optical properties of such materials. Bhargava et al. [4] first synthesized ZnS semiconductor microcrystallites doped with Mn2 + ions. They found that the photoluminescence of Mn2+ ions in ZnS nanocrystals had a very high quantum efficiency with a luminescence decay several orders of magnitude faster than in the bulk crystals. This indicated that microcrystals doped with optically active luminescent centers may create new physics and applications. In this paper, we report the preparation and optical properties of Mn2+-doped ZnS semiconductor colloids. It was found that the Mn’ ’
*Corresponding
author.
0022-2313/96/$15.00 G 1996 SSDI 0022-23 13(95)00160-3
ElsevierScience
luminescent intensity was influenced by the presence of a surfactant. An aging effect and a radiation-induced luminescence enhancement (RILE) effect were observed for ZnS : Mn alcohol colloids.
2. Experiments The samples used in the experiments were prepared by the colloidal chemical method. The procedure is as follows: first Zn(CH2C00)2 and Mn(CH2C00)2 were dissolved into an alcohol solution. The molar concentration ratio of Mn2+ ions to Zn2+ was 1: 100 in the solution. Second, H2S gas was passed into the solution while stirring rapidly. The prepared ZnS : Mn alcohol colloid was quite transparent. The ZnS nanoparticle size can be controlled by altering the molar concentration of Zn2+ ions in the alcohol solution. A typical concentration of Zn’+ ions in alcohol is 1O-3 M, which gave about 3.6nm sized particles of ZnS: Mn. The photoluminescent emission and photoluminescent
B.V. All rights reserved
316
C. Jin rt (II. : Journal
of Luminescmce 66&67
(1996)
315-318
excitation spectra of Mn’+ in ZnS nanoparticles were measured at room temperature with a Hitachi F-4000 Spectra-fluormeter.
i
3. Results and discussion The photoluminescent emission spectra of three ZnS: Mn colloid samples (1,2,3) were measured and are shown in Fig. l(b). The Zn2+ ions concentrations are 10e3, 10m3 and lO_‘M in the three samples, respectively. There was a surfactant (polymethyl methacrylate) on both sample 2 (1O-3 M) and sample 3 (lo- ’ M). The yellow emission bands in the figures are attributed to the Mn2+ 4T, + “Ai transition of the ZnS nanocrystal host. The emission peaks are at 586.4, 585.8 and 587.4nm. The photoluminescent excitation (PLE) spectra were measured monitoring these emission peaks and are given in Fig. l(a). The excitation peaks are at 291.7, 297.4 and 311.4 nm. The ZnS : Mn nanocrystal size in sample 2 is about 3.6 nm for the 297.4 nm excitation peak [S]. The excitation bands come from band-to-band transitions of the ZnS nanocrystal. The excitation peaks are blue-shifted compared to the excitation peak (332nm) of the bulk material [4]. This energy shift is due to the quantum size effect in small ZnS particles. Larger blue shifts in the excitation spectra corresponds to smaller ZnS nanoparticle sizes. It was noticed that the excitation peaks from the ground state to higher excited states of Mn2+ were difficult to detect in the excitation spectra of the colloidal samples. This is very different from the situation in the bulk ZnS crystal, where the intensities of the band-to-band excitation and excitation from Mn2+ transitions are of the same order. This implies that the energy transfer from band-to-band excitations are more efficient in ZnS : Mn nanosize crystals than in Zns: Mn bulk crystals. From the comparison of the luminescence of ZnS: Mn bulk material and nanoparticles under the same excitation, we believe that nanoparticles of ZnS : Mn have higher luminescence intensities and efficiencies. It is possible that the efficient energy transfer is the result of electrons and holes being confined to be near Mn2 + ions in such small nanosize ZnS particles.
‘. I
240
280
320
I’
‘I
\
-._
36o.ioo
Wavelength
I
,
550
600
I
650
700
(nm)
Fig. 1. (a) Photoluminescence excitation spectra of three ZnS: Mn” alcohol colloidal samples: sample 1 (curve A). sample 2 (curve B), sample 3 (curve C). (b) Photoluminescence spectra of three ZnS : Mn’+ alcohol colloidal samples: sample I (curve A), sample 2 (curve B), sample 3 (curve C). The ratio of maximum Intensity of the curves is A : B : C = 10: 40 : 43.
An important aspect that influences the optical properties of colloids is the presence of a surfactant. We observed the influence of the surfactant (polymethyl methacrylate) on the Mn2+ luminescence of ZnS: Mn nanoparticles. It was found that the Mn2+ luminescent intensity is enhanced several or tens of times that of samples without surfactants. As an example, the luminescent intensity of sample 2 (with surfactant) is about four times that of sample 1 (without surfactant) under excitation at the peak wavelength of the excitation bands. The size difference between the two samples is small because the peaks of the excitation spectra are similar. We found that there is an aging effect for ZnS: Mn colloids. The Mn2+ ion luminescent intensity decreased after the colloids were kept at room temperature for a long time. As an example, the luminescent intensity of sample 2 decreased to a tenth of that of a fresh sample after it was kept for about two months. The luminescent peak is redshifted from 585.8 nm (fresh) to 590.4 nm (aged), as shown in Fig. 2(b). The excitation peak is shifted to lower energy, from 297 nm (fresh) to 3 12 nm (aged). The shape of the excitation spectrum is also changed, as shown in Fig. 2(a). The higher energy part of the excitation band decreased.
317
C. Jin etal. /Journal ofLuminescence ljri&67(1996)315-318
b
. . 240
280
320
360500
Wavelength
550
600
650
.
700
(nm)
Fig. 2. (a) Excitation spectra of sample 2: curve A (aged sample). curve B (fresh sample). (b) Photoluminescence spectra of sample 2: curve A (aged sample), curve B (fresh sample). The ratio of maximum intenstty of the curves is A : B = IO: 72.
We have also observed the radiation-induced luminescence enhancement (RILE) effect. The Mn2+ emission intensity of the aged sample 2 increased when it was exposed to UV-light. By changing the irradiation wavelength of a Xe lamp, the luminescence intensity enhancement is altered, as shown in Fig. 3(b). The most effective wavelength lies between 240and 330 nm as seen in Fig. 3(b). Besides an increase in luminescent intensity, the excitation peak of this exposed sample was shifted to higher energy, from 312 nm (unexposed) to 310-298.6 nm (exposed). As seen in Fig. 3(a), the shape of the excitation spectrum also changed, the higher-energy part increasing relative to the lowerenergy part. The shape of the luminescence spectra did not change, but the peak shifted from 590.4 to 585.6 nm for the sample exposed under 266 nm Xe lamp radiation. The size of the nanoparticles did not change after this irradiation as the absorption spectrum did not change. A tentative explanation for the aging effect and the RILE effect is as follows: a surface modification interaction by the surfactant reduces the nonradiative relaxation path for electrons excited into the conduction band, so samples with surfactant have higher luminescence intensity than samples without any surfactant. After storage for a long
240
280
320
360
Wavelength (nm) Fig. 3. (a) Photoluminescent excitation spectra of sample 2 (aged): curve A: unexposed, curve B: exposed under Xe lamp 266 nm radiation for 60 min. The ratio of maximum intensity of the curves is A : B = I : 19. (b) Photoluminescence intensity ratio for the sample exposed to several wavelengths of Xe lamp light for 60 min to that of the unexposed sample.
time, the surface modification deteriorated and nonradiative relaxation paths increase so the luminescence intensity decreases. With radiation exposure at a suitable wavelength, the surface modification by the surfactant was restored, so the luminescence recovers and can even be enhanced. Different nonradiative paths are connected with different states in the conduction band. The origin of these nonradiative paths may be surface states. The surface modification by the surfactant may fill these surface states with electrons, so the nonradiative paths are blocked. But these filled surface states are metastable. The higher the energy of the surface states, the faster the electron relaxation. So after a long time, the surface states with higher energy become unblocked and the higher part of the excitation bands become less efficient than the lower part of the excitation band. Irradiation with suitable wavelengths will refill these empty surface states with electrons. The luminescence recovers and is enhanced, and the efficiency of the higher part of the excitation band increases again.
318
C. Jin et al. J Journal of Luminescence 66&67 (1996) 315-318
Acknowledgements
References
The authors acknowledge the support of the National Natural Science Foundation of China and the Laboratory of the Excited State Processes, Changchun Institute of Physics, Chinese Academy of Sciences. The authors gratefully acknowledge the support of the K.C. Wong Education Foundation, Hong Kong.
Cl1 Y. Wang and N. Herron, J. Phys. Chem. 121A.D. Yoffe, Adv. Phys. 42 (1993) 173. c31 L. Brus, Appl. Phys. A 53 (1991) 465. r41 R.N. Bhargava,
95 (1991) 525.
D. Gallagher, X. Hong and A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. CSID. Gallagher, W.E. Heady, J.M. Racz and R.N. Bhargava, J. Crystal Growth 138 (1994) 970.