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Photophysical properties of ZnS quantum dots
Journal of Physics and Chemistry of Solids 60 (1999) 13–15
Photophysical properties of ZnS quantum dots Yadong Li*, Yi Ding, Yue Zhang, Yitai Qian Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China Received 20 April 1998; accepted 6 July 1998
Abstract ZnS nanoparticles of ⱕ3 nm average diameter were synthesized by a solvothermal reaction with zinc acetate [Zn(CH3COO)2·2H2O] and thiourea at 120⬚C. Their photophysical properties were investigated. The UV–Vis spectrum showed an absorption shoulder at 276 nm, corresponding to the bandgap energy of 4.49 eV, and a weak shoulder at 320 nm (3.875 eV). The photoemissions at 365 nm (l ex 280 nm), 325 nm and 333 nm (l ex 220 nm) were observed, which may result from point defects in these ZnS quantum dots. 䉷 1998 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; B. Chemical synthesis; D. Defects; D. Luminescence
Semiconductor compounds have attracted more and more attention during the last few years because of their novel optical and transport properties which have great potential for many optoelectronic applications [1–4]. To fit the need of different temperatures, scientists are looking for large intrinsic bandgap semiconductor materials so as to expand the exhaustion regions. We know the extremely small size of these particles will result in quantum confinement of the photogenerated electron–hole pair, leading to a blue shift in the absorption spectrum [5, 6]. It is hence highly important to synthesize such low-dimensional nanosized semiconductor particles with a narrow size distribution. As the representatives of these semiconductor nanocrystals, ZnS and CdS have been synthesized by various methods. But most methods were carried out by direct contact of gaseous H2S to metal cations in solutions or on solid surfaces [7, 8]. The simpler mixing aqueous solutions of the metal salt and S 2⫺ has also been employed [9, 10], but it is inevitable that the inhomogeneity at an early stage of mixing might broaden the size distribution of the products for the methods mentioned above [11]. Recently, Ohtaki et al. reported a preparative technique of CdS nanoparticles by utilizing a homogeneous release of S 2⫺ from a controlled decomposition of P2S5 in non-aqueous solvents [11]. In this paper, we report a novel solvothermal synthesis of ZnS quantum dots
* Corresponding author.
of diameters no more than 3 nm from zinc acetate and thiourea at 120⬚C and its interesting photophysical properties. These seem to be a result of the strong quantum size effect and the effect of high concentration of point defects. Zinc and sulfur vacancy defects are thought to play an important role in the optical properties. The nanocrystalline ZnS sample was prepared by the solvothermal method [12, 13]. An appropriate amount of analytical grade thiourea was dissolved in absolute ethanol, then zinc acetate [Zn(CH3COO)2·2H2O] was added to the solution under continuous stirring. We chose the amount of thiourea in excess of 10% that of the zinc acetate so as to enable a complete reaction of zinc acetate. The solution was added into a Teflon-liner autoclave of 100 ml capacity until 70% of the total volume was filled. Then the autoclave was sealed into a stainless steel tank and maintained at 120⬚C for 5 h. After gradual cooling to room temperature, a white precipitate was obtained. The samples were filtered and washed with distilled water, then dried under infrared light. The samples were characterized by X-ray power diffraction (XRD) studies, using a Japan Rigaku Dmax rA X-ray diffractionmeter with graphite monochromatized Cu Ka ˚ ). Ultraviolet absorption spectroradiation (l 1.5418 A scopy was collected with a Shimadzu UV–visible Recording spectrophotometer (UV-240) at room temperature. The photoluminescent (PL) emission spectra were measured with a Hitachi 850 fluorescence spectrophotometer. The XRD pattern of ZnS nanoparticles is shown in Fig. 1.
0022-3697/99/$ - see front matter 䉷 1998 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(98)00247-9
14
Y. Li et al. / Journal of Physics and Chemistry of Solids 60 (1999) 13–15
Fig. 1. X-ray diffraction pattern of the obtained ZnS nanoparticles.
The three peaks at 28.6⬚, 47.8⬚, and 56.6⬚ correspond to the 111, 220, and 311 planes of cubic zinc sulfide, respectively. ˚ , which is close to the The lattice constant a 5.3906 A reported value of cubic ZnS (JCPDS card, No. 5-0566, a ˚ ). We also observed the typical broadening of the 5.4060 A three diffraction peaks, which indicated that the size of the ZnS particles was very small. Estimating from the Debye– Scherrer formula, the average size of the samples was 2.62 nm. From the XRD analysis, no characteristic peaks of impurity phases were observed. In the past few years, many metal-doped ZnS nanocrystals have been prepared, their optical properties reported and the doping metal cations found to mainly act as acceptor defects [5, 8, 14]. Usually, both Schottky defects and Frenkel defects are present in all solids; however, there is always a tendency for one type of defect to predominate since their energies of formation are usually unequal [15]. In cubic blende ZnS, Schottky defects are usually predominant [16]. It is known that most semiconductor properties are structure sensitive and highly dependent on the nature and amount of imperfections present in a crystal. In the ultraviolet absorption spectrum (Fig. 2), we found a shoulder at 276 nm, indicating that the bandgap energy (Eg) of ZnS
Fig. 2. UV–Vis absorption spectrum of ZnS nanoparticles.
Fig. 3. PL spectra of ZnS nanoparticles at different excitation wavelengths (l ex): (a) 280 nm; (b) 220 nm.
nanoparticles was about 4.49 eV, which was much larger than that of the macrocrystalline ZnS (3.66 eV). This blue shift was obviously caused by the strong quantum confinement effect. According to the effective mass model of Steigerwald and Brus [7], the particle size of ZnS was about 2.8 nm which is quite consistent with the XRD analysis. A further weaker shoulder appeared at 320 nm (3.875 eV) which might be caused by the sulfur vacancy defects which we will discuss in detail. Fig. 3 shows the PL spectra of ZnS nanoparticles at different excitation wavelengths. When the excitation wavelength was 280 nm (Fig. 3(a)), the photoemission was peaking at 365 nm which could be ascribed to a recombination of electrons at the sulfur vacancy donor level (Vsx) with holes trapped at the zinc vacancy acceptor level (Vzn 0). While the excitation wavelength was 220 nm (Fig. 3(b)), the photoemission spectrum became more complex. The emission at 325 nm has been termed ‘self-activated’ and is due to sulfur vacancies in the lattice [17]. In other words, it is the transition between the localized sulfur vacancy donor level (Vsx) and the top of the valence band of ZnS nanoparticles. The peak at 333 nm, we suppose, is caused by the dangling sulfur bonds in the interface of ZnS grains, since the thiourea was used excessively in the preparation of the ZnS. The weak interaction between the point defects and the dangling sulfur bond could lead to a localized energy
Y. Li et al. / Journal of Physics and Chemistry of Solids 60 (1999) 13–15
15
Fig. 4. The energy-level diagram of ZnS nanoparticles. DB refers to the localized level of the dangling sulfur bond.
level very close to the valence band [14, 18, 19]. The energy of the radiation corresponding to a wavelength of 307 nm is 4.04 eV, which corresponds to the difference between the conduction band and the localized zinc vacancy acceptor level. The energy-level diagram shown in Fig. 4 well illustrates the above phenomena. From the diagram, we could find that the electron trap depth, i.e. the difference between the conduction band and the sulfur vacancy donor level, is about 0.67 eV, which is very close to the reported 0.75 eV [20, 21]. In order to simplify the discussion, we did not consider any double ionized vacancy defects here, although they do exist in the ZnS nanoparticles. With the decrease of the crystal size, the concentration of point defects increased relatively, which results in some new phenomena in the optical properties [5, 20]. In the ZnS ( ⱕ 3 nm) nanoparticles, various kinds of defects other than Schottky vacancy defects were present, such as Znix, Zni䡠䡠, Si 00, and so on. They may read to the low-energy luminescence in the PL spectra (550–700 nm in Fig. 3(b)). In summary, we have succeeded in fabricating the ZnS quantum dots no more than 3 nm in size, using a controlled solvothermal synthesis method. Compared with other traditional methods, it was much easier to control the size of the product through changing the reaction temperature and time. The absorption and luminescence spectra showed some new characteristics, which, as we suggested, are caused by a strong quantum confinement effect and a high concentration of point defects in the lattice. Further studies are needed to investigate the effect that point defects have on the luminescence properties of these semiconductor nanoparticles. Acknowledgements We are grateful for the support of the National Natural Science Foundation of China and the National Nanometer Materials Climbing Program.
References [1] L.E. Brus, J. Phys. Chem. 90 (1986) 2555. [2] M.G. Bawendi, D.J. Carroll, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992) 946. [3] L. Spanhel, M. Haase, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [4] A. Hasselbarth, A. Eychmuller, H. Weller, Chem. Phys. Lett. 203 (1993) 271. [5] K. Sooklal, B.S. Cullum, S.M. Angel, C.J. Murphy, J. Phys. Chem. 100 (1996) 4551. [6] H. Weller, Angew. Chem., Int. Ed. Engl. 32 (1993) 41. [7] M.L. Steigerwald, L.E. Brus, Acc. Chem. Res. 23 (1990) 183. [8] J.M. Huang, Y. Yang, S.H. Xue, B. Yang, S.Y. Liu, J.C. Shen, Appl. Phys. Lett. 70 (18) (1997) 2335. [9] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322. [10] H.S. Zhou, I. Honma, H. Komiyama, J.W. Haus, J. Phys. Chem. 97 (1993) 895. [11] Ohtaki M., Oda K., Eguchi K., Arai H., Chem. Commun., 1209 (1996). [12] Y.D. Li, X.F. Duan, Y.T. Qian, L. Yang, M.R. Ji, C.W. Li, J. Am. Chem. Soc. 119 (23) (1997) 7869. [13] Y.D. Li, X.F. Duan, H.W. Liao, Y.T. Qian, Chem. Mater. 10 (1) (1998) 17. [14] Q.W. Chen, X.G. Li, Y.T. Qian, J.S. Zhu, G.E. Zhou, W.P. Zhang, Y.H. Zhang, Appl. Phys. Lett. 68 (25) (1996) 3582. [15] Leonid V. A., Introduction to Solids. McGraw-Hill, New York (1960). [16] West A. R., Solid State Chemistry and its Applications. Wiley, New York (1984). [17] W.G. Becker, A.J. Bard, J. Phys. Chem. 87 (1983) 4888. [18] J. Robertson, M.J. Powell, Appl. Phys. Lett. 44 (1984) 415. [19] J. Robertson, Philos. Mag. B. 63 (1991) 47. [20] W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, Appl. Phys. Lett. 70 (11) (1997) 1465. [21] Chen R., Kirsh Y., Analysis of Thermally Stimulated Processes. Pergamon, Oxford (1981).
Photophysical properties of ZnS quantum dots Yadong Li*, Yi Ding, Yue Zhang, Yitai Qian Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China Received 20 April 1998; accepted 6 July 1998
Abstract ZnS nanoparticles of ⱕ3 nm average diameter were synthesized by a solvothermal reaction with zinc acetate [Zn(CH3COO)2·2H2O] and thiourea at 120⬚C. Their photophysical properties were investigated. The UV–Vis spectrum showed an absorption shoulder at 276 nm, corresponding to the bandgap energy of 4.49 eV, and a weak shoulder at 320 nm (3.875 eV). The photoemissions at 365 nm (l ex 280 nm), 325 nm and 333 nm (l ex 220 nm) were observed, which may result from point defects in these ZnS quantum dots. 䉷 1998 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; B. Chemical synthesis; D. Defects; D. Luminescence
Semiconductor compounds have attracted more and more attention during the last few years because of their novel optical and transport properties which have great potential for many optoelectronic applications [1–4]. To fit the need of different temperatures, scientists are looking for large intrinsic bandgap semiconductor materials so as to expand the exhaustion regions. We know the extremely small size of these particles will result in quantum confinement of the photogenerated electron–hole pair, leading to a blue shift in the absorption spectrum [5, 6]. It is hence highly important to synthesize such low-dimensional nanosized semiconductor particles with a narrow size distribution. As the representatives of these semiconductor nanocrystals, ZnS and CdS have been synthesized by various methods. But most methods were carried out by direct contact of gaseous H2S to metal cations in solutions or on solid surfaces [7, 8]. The simpler mixing aqueous solutions of the metal salt and S 2⫺ has also been employed [9, 10], but it is inevitable that the inhomogeneity at an early stage of mixing might broaden the size distribution of the products for the methods mentioned above [11]. Recently, Ohtaki et al. reported a preparative technique of CdS nanoparticles by utilizing a homogeneous release of S 2⫺ from a controlled decomposition of P2S5 in non-aqueous solvents [11]. In this paper, we report a novel solvothermal synthesis of ZnS quantum dots
* Corresponding author.
of diameters no more than 3 nm from zinc acetate and thiourea at 120⬚C and its interesting photophysical properties. These seem to be a result of the strong quantum size effect and the effect of high concentration of point defects. Zinc and sulfur vacancy defects are thought to play an important role in the optical properties. The nanocrystalline ZnS sample was prepared by the solvothermal method [12, 13]. An appropriate amount of analytical grade thiourea was dissolved in absolute ethanol, then zinc acetate [Zn(CH3COO)2·2H2O] was added to the solution under continuous stirring. We chose the amount of thiourea in excess of 10% that of the zinc acetate so as to enable a complete reaction of zinc acetate. The solution was added into a Teflon-liner autoclave of 100 ml capacity until 70% of the total volume was filled. Then the autoclave was sealed into a stainless steel tank and maintained at 120⬚C for 5 h. After gradual cooling to room temperature, a white precipitate was obtained. The samples were filtered and washed with distilled water, then dried under infrared light. The samples were characterized by X-ray power diffraction (XRD) studies, using a Japan Rigaku Dmax rA X-ray diffractionmeter with graphite monochromatized Cu Ka ˚ ). Ultraviolet absorption spectroradiation (l 1.5418 A scopy was collected with a Shimadzu UV–visible Recording spectrophotometer (UV-240) at room temperature. The photoluminescent (PL) emission spectra were measured with a Hitachi 850 fluorescence spectrophotometer. The XRD pattern of ZnS nanoparticles is shown in Fig. 1.
0022-3697/99/$ - see front matter 䉷 1998 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(98)00247-9
14
Y. Li et al. / Journal of Physics and Chemistry of Solids 60 (1999) 13–15
Fig. 1. X-ray diffraction pattern of the obtained ZnS nanoparticles.
The three peaks at 28.6⬚, 47.8⬚, and 56.6⬚ correspond to the 111, 220, and 311 planes of cubic zinc sulfide, respectively. ˚ , which is close to the The lattice constant a 5.3906 A reported value of cubic ZnS (JCPDS card, No. 5-0566, a ˚ ). We also observed the typical broadening of the 5.4060 A three diffraction peaks, which indicated that the size of the ZnS particles was very small. Estimating from the Debye– Scherrer formula, the average size of the samples was 2.62 nm. From the XRD analysis, no characteristic peaks of impurity phases were observed. In the past few years, many metal-doped ZnS nanocrystals have been prepared, their optical properties reported and the doping metal cations found to mainly act as acceptor defects [5, 8, 14]. Usually, both Schottky defects and Frenkel defects are present in all solids; however, there is always a tendency for one type of defect to predominate since their energies of formation are usually unequal [15]. In cubic blende ZnS, Schottky defects are usually predominant [16]. It is known that most semiconductor properties are structure sensitive and highly dependent on the nature and amount of imperfections present in a crystal. In the ultraviolet absorption spectrum (Fig. 2), we found a shoulder at 276 nm, indicating that the bandgap energy (Eg) of ZnS
Fig. 2. UV–Vis absorption spectrum of ZnS nanoparticles.
Fig. 3. PL spectra of ZnS nanoparticles at different excitation wavelengths (l ex): (a) 280 nm; (b) 220 nm.
nanoparticles was about 4.49 eV, which was much larger than that of the macrocrystalline ZnS (3.66 eV). This blue shift was obviously caused by the strong quantum confinement effect. According to the effective mass model of Steigerwald and Brus [7], the particle size of ZnS was about 2.8 nm which is quite consistent with the XRD analysis. A further weaker shoulder appeared at 320 nm (3.875 eV) which might be caused by the sulfur vacancy defects which we will discuss in detail. Fig. 3 shows the PL spectra of ZnS nanoparticles at different excitation wavelengths. When the excitation wavelength was 280 nm (Fig. 3(a)), the photoemission was peaking at 365 nm which could be ascribed to a recombination of electrons at the sulfur vacancy donor level (Vsx) with holes trapped at the zinc vacancy acceptor level (Vzn 0). While the excitation wavelength was 220 nm (Fig. 3(b)), the photoemission spectrum became more complex. The emission at 325 nm has been termed ‘self-activated’ and is due to sulfur vacancies in the lattice [17]. In other words, it is the transition between the localized sulfur vacancy donor level (Vsx) and the top of the valence band of ZnS nanoparticles. The peak at 333 nm, we suppose, is caused by the dangling sulfur bonds in the interface of ZnS grains, since the thiourea was used excessively in the preparation of the ZnS. The weak interaction between the point defects and the dangling sulfur bond could lead to a localized energy
Y. Li et al. / Journal of Physics and Chemistry of Solids 60 (1999) 13–15
15
Fig. 4. The energy-level diagram of ZnS nanoparticles. DB refers to the localized level of the dangling sulfur bond.
level very close to the valence band [14, 18, 19]. The energy of the radiation corresponding to a wavelength of 307 nm is 4.04 eV, which corresponds to the difference between the conduction band and the localized zinc vacancy acceptor level. The energy-level diagram shown in Fig. 4 well illustrates the above phenomena. From the diagram, we could find that the electron trap depth, i.e. the difference between the conduction band and the sulfur vacancy donor level, is about 0.67 eV, which is very close to the reported 0.75 eV [20, 21]. In order to simplify the discussion, we did not consider any double ionized vacancy defects here, although they do exist in the ZnS nanoparticles. With the decrease of the crystal size, the concentration of point defects increased relatively, which results in some new phenomena in the optical properties [5, 20]. In the ZnS ( ⱕ 3 nm) nanoparticles, various kinds of defects other than Schottky vacancy defects were present, such as Znix, Zni䡠䡠, Si 00, and so on. They may read to the low-energy luminescence in the PL spectra (550–700 nm in Fig. 3(b)). In summary, we have succeeded in fabricating the ZnS quantum dots no more than 3 nm in size, using a controlled solvothermal synthesis method. Compared with other traditional methods, it was much easier to control the size of the product through changing the reaction temperature and time. The absorption and luminescence spectra showed some new characteristics, which, as we suggested, are caused by a strong quantum confinement effect and a high concentration of point defects in the lattice. Further studies are needed to investigate the effect that point defects have on the luminescence properties of these semiconductor nanoparticles. Acknowledgements We are grateful for the support of the National Natural Science Foundation of China and the National Nanometer Materials Climbing Program.
References [1] L.E. Brus, J. Phys. Chem. 90 (1986) 2555. [2] M.G. Bawendi, D.J. Carroll, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992) 946. [3] L. Spanhel, M. Haase, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [4] A. Hasselbarth, A. Eychmuller, H. Weller, Chem. Phys. Lett. 203 (1993) 271. [5] K. Sooklal, B.S. Cullum, S.M. Angel, C.J. Murphy, J. Phys. Chem. 100 (1996) 4551. [6] H. Weller, Angew. Chem., Int. Ed. Engl. 32 (1993) 41. [7] M.L. Steigerwald, L.E. Brus, Acc. Chem. Res. 23 (1990) 183. [8] J.M. Huang, Y. Yang, S.H. Xue, B. Yang, S.Y. Liu, J.C. Shen, Appl. Phys. Lett. 70 (18) (1997) 2335. [9] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322. [10] H.S. Zhou, I. Honma, H. Komiyama, J.W. Haus, J. Phys. Chem. 97 (1993) 895. [11] Ohtaki M., Oda K., Eguchi K., Arai H., Chem. Commun., 1209 (1996). [12] Y.D. Li, X.F. Duan, Y.T. Qian, L. Yang, M.R. Ji, C.W. Li, J. Am. Chem. Soc. 119 (23) (1997) 7869. [13] Y.D. Li, X.F. Duan, H.W. Liao, Y.T. Qian, Chem. Mater. 10 (1) (1998) 17. [14] Q.W. Chen, X.G. Li, Y.T. Qian, J.S. Zhu, G.E. Zhou, W.P. Zhang, Y.H. Zhang, Appl. Phys. Lett. 68 (25) (1996) 3582. [15] Leonid V. A., Introduction to Solids. McGraw-Hill, New York (1960). [16] West A. R., Solid State Chemistry and its Applications. Wiley, New York (1984). [17] W.G. Becker, A.J. Bard, J. Phys. Chem. 87 (1983) 4888. [18] J. Robertson, M.J. Powell, Appl. Phys. Lett. 44 (1984) 415. [19] J. Robertson, Philos. Mag. B. 63 (1991) 47. [20] W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, Appl. Phys. Lett. 70 (11) (1997) 1465. [21] Chen R., Kirsh Y., Analysis of Thermally Stimulated Processes. Pergamon, Oxford (1981).