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Structural and optical properties of the S-doped ZnO particles synthesized by hydrothermal method
Applied Surface Science 257 (2010) 1125–1128
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural and optical properties of the S-doped ZnO particles synthesized by hydrothermal method Yuanping Sun a,∗ , Tao He b , Hongying Guo a , Tao Zhang b , Weitian Wang a , Zhenhong Dai a a b
Institute of Science and Technology for Opto-electronic Information, Yantai University, Qianquan Road 32#, Laishan, Yantai, Shandong 264005, PR China School of Chemical and Biological Science and Engineering, Yantai University, Shandong 264005, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2010 Received in revised form 16 June 2010 Accepted 10 August 2010 Available online 17 August 2010 Keywords: ZnO Hydrothermal method Optical properties Structural properties
a b s t r a c t Sulfur-doped ZnO particles have been synthesized by hydrothermal method. The structural and optical properties were studied systematically by XRD, scanning electron microscopy (SEM), and photoluminescence. SEM results show that the particle is hexagonal and the average size decreases with increasing sulfur doping, which means a retardant effect of sulfur on the growth of S-doped ZnO. XRD results show that the lattice parameters increase with more sulfur, which means an effective sulfur doping and increasing strain. Optical characterization also shows that the effective sulfur doping will enhance the green emission and suppress the near bandgap emissions. © 2010 Elsevier B.V. All rights reserved.
Recently, ZnO have attracted much attention in the filed of light emitting devices due to its direct wide bandgap of 3.37 eV and a very large exciton binding energy of 60 meV, which makes the excitons thermally stable even at room temperature [1]. Great deal of applications has been found for ZnO, such as varistors [2], gas sensors [3], blue light optoelectronics [4,5] and other optical devices [6,7]. Many researches have been conducted both on the growth and device application [8–10]. Modification of ZnO properties by impurity doping is a current issue for the extensive application in optoand spin electronics [11,12]. Various elements have been selected as the dopant, such as Al [13,14], Mg [15,16] and other metal elements [17–19]. But the anion doping is rarely reported. As in same group, the physical and chemical properties of sulfur are very similar to oxygen. Under some certain conditions, oxygen can be easily substituted by sulfur to form sulfide, which could make the ZnO to show special electrical and optical properties. On the other hand, possible bandgap engineering of ZnO may be possible because of the larger Eg of ZnS than that of ZnO. Although there are such advantages for the doping of sulfur, rare research on the S-doped ZnO has been reported [20,21]. In this work, we reported the successful synthesis of sulfur-doped ZnO by hydrothermal method. Strain was introduced because of the substitution of S2− for O2− , which lead to smaller diffraction angle and enhanced green band emissions. The synthesis of ZnO particles including the removal of H2 O from zinc nitrate (Zn(NO3 )2 ·6H2 O) with phosphorous pentoxide (P2 O5 )
∗ Corresponding author. Tel.: +86 535 2102292. E-mail address: [email protected] (Y. Sun). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.041
and absolute ethanol with molecular sieve at first, and then dissolve the zinc nitrate in absolute ethanol with a ratio of 1 g to 5 ml to form a solution. 5 ml solution together with 15 ml ethanol and 3.3 g sodium dodecylbenzenesulfonate (SDBS) were put into the autoclave with Teflon liner. To study the effects of sulfur doping on the growth and properties of the formed ZnO particles, 0.042, 0.051 and 0.084 g sulfur powder had been added into the mixed solution and the resulted sample are denoted as A, B and C, respectively. The solution was heated to 90 ◦ C for 10 h and then at 180 ◦ C for 3 h. After cooling naturally to room temperature, the transparent supernatant was removed by pipette and black precipitates were left. The precipitates were placed in distilled water and incubated for 24 h, then separated by filtration and allowed to dry naturally; the surfactant templates were dissolved away in DI water for 3 times to obtain the ZnO particles, which were used as samples for characterizations. The morphologies of the as-grown samples were characterized by scanning electron microscopy (SEM, JEOL JSM-6300). The structural analysis was taken by X-ray diffraction meter (XRD) (Philips PW 1710 with Cu K␣ radiation, = 0.15418 nm). Optical characterizations were performed by using a He–Cd laser at the excitation wavelength of 325 nm and a He-cooling stage operating at a temperature range from 10 to 300 K. Fig. 1 shows the SEM images of the particles for samples A, B and C, respectively. As viewed from the cross-section, the profile of most particles obtained by hydrothermal synthesis is hexagonal. The particles form loose agglomerates with a size around 1 m. In fact, the size decreases with the increasing of sulfur content in reactant solutions with a sample sequence from A to C. The dominant
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Y. Sun et al. / Applied Surface Science 257 (2010) 1125–1128
Fig. 2. XRD pattern for samples A, B and C. All significant peaks in the XRD data can be indexed to ZnO with different shift.
Fig. 1. SEM images of ZnO particle samples with a magnification of 9000×. (a) Sample A, (b) sample B and (c) sample C.
size is 1.3, 1.0 and 0.7 m for samples A, B and C, respectively, as calculated from the SEM images. Because all samples were grown under the same conditions except for different sulfur amount, it can be concluded that larger amount of sulfur in reactant solution will retard or delay the growth process and lead to small size particles. In fact, there are two ways for sulfur to affect the growth of
ZnO: one is the growth rate and another is the nucleation rate. More works should be done to identify the real reasons. To obtain the structural properties of the hydrothermal synthesized ZnO samples, XRD studies have been performed and the results are shown in Fig. 2. All peaks in the figure can be indexed to the wurtzite structures of ZnO (JCPDS Card No. 36-1451) with certain shift. The lattice parameters calculated from the XRD data are 3.2526, 3.2646 and 3.2687 A˚ for a axis and 5.2124, 5.2274 and 5.2360 A˚ for c axis for samples A, B, and C respectively. As compared with the parameters of a = 3.2497 A˚ and c = 5.2066 A˚ (ZnO, JCPDS Card No. 36-1451), the lattice parameters of studied samples increased with the increasing amount of sulfur in solutions (A < B < C). On the other hand, the lattice parameters of ZnS are a = 3.820 A˚ and c = 6.260 A˚ [22]. For the larger size of S2− than O2− , the substitution of S for O will introduce strain in ZnO and then affect the lattice parameters. More strain will appear in the higher sulfur doping sample, which lead to smaller diffraction angle, as shown by the XRD data in Fig. 2. To study the influence of sulfur doping on the optical properties of ZnO particles, temperature-dependent PL has been performed with a temperature range from 10 to 300 K. The normalized spectra at 10 and 300 K for 3 samples are shown in Fig. 3. The main emission peak at 10 K (Fig. 3(a)) is at 499.3, 501.4 and 504.5 nm (2.483, 2.473 and 2.458 eV) for samples A, B and C, respectively. At 300 K (Fig. 3(b)), the main emission peak is at 500.5, 501.5 and 503.3 nm (2.477, 2.472 and 2.463 eV) for samples A, B and C, respectively. The samples give off strong green emission and the emission energies remain almost unchanged (<6 meV) with a temperature range from 10 to 300 K, which show a relatively stable optical properties. The green emission energy shows a slightly red-shift with increasing sulfur doping in samples, which can be explained by the strain-induced bandgap shrinkage [23]. For the near bandgap emissions of sulfur-doped ZnO at 10 K (Fig. 3(a)), it lies at 374.9, 369.8 and 375.0 nm (3.307, 3.353 and 3.306 eV), which are different with the reported bandgap value of pure ZnO: 3.4 eV [22]. As compared with the reported ultraviolet (UV) emissions of ZnO, the peak near 369.8 nm of sample B corresponds to D0 X, while the emission of samples A and C at UV range corresponds to the transitions of DAP [24]. At 300 K, the bandgap emission peak is at 380.0, 380.2 and 381.5 nm (3.263, 3.261 and 3.250 eV) for samples A, B and C, respectively. The disordered near bandgap peaks can be attributed to the competition of different recombination mechanisms, such as D0 X, A0 X, DAP, FX, etc. As compared with the green emission peaks, the bandgap emissions shift to its lower energy more than 40 meV, which show a strong temperature-dependent properties.
Y. Sun et al. / Applied Surface Science 257 (2010) 1125–1128
Fig. 3. The normalized PL spectra of samples A, B, and C at different temperatures. (a) 10 K and (b) 300 K.
Also we can conclude from Fig. 3 that the intensity ratio of near bandgap and green emission is decreased with more doping of sulfur. The ratio is 0.80, 0.38 and 0.04 at 10 K, 0.73, 0.28 and 0.07 at 300 K, for samples A, B and C, respectively. This trend agrees well with the effective doping of sulfur in ZnO, in which the sulfur doping level increases from sample A to C. Higher doping of sulfur will generate more defects in ZnO, which will enhance the green emission and weaken the UV emissions. Fig. 4 shows the normalized integrated intensity of near bandgap and green emission for samples A, B and C based on temperaturedependent PL data. For both emission band, the integrated intensity changed a little at lower temperature; while at higher temperature, it decreased very fast. As in III-nitride based materials, temperature dependence of integrated PL intensity can be fitted by the Arrhenius formula: I=
1+
I0
C exp(−EAi /kB T ) i=1 i
(1)
where EAi are the activation energy of the corresponding nonradiative recombination center and Ci are the fitting parameters related to the nonradiative to radiative recombination probability ratio [25]. The best fitting of the data gives the activation energies of 49.0, 43.8 and 35.5 meV for near ZnO bandgap emission and 49.5, 41.0 and 22.1 meV for green emission peaks for samples A, B and C, respectively. The activation energy of near ZnO bandgap and green emissions decrease with the increasing of sulfur doping, which means an increase of nonradiative recombination probability. As we have confirmed from XRD data that the doping of sulfur will introduce more defects and lead to the enhancement of nonradiative recombination.
1127
Fig. 4. The Arrhenius plot for the normalized integration PL intensity as a function of inverse temperature (dots) for (a) ZnO bandgap and (b) green emissions. The solid lines give best fitting according to formula (1).
In conclusion, the ZnO particles with different sulfur doping have been synthesized by hydrothermal method. The increasing of sulfur doping will retard the growth of ZnO particles and increase the lattice parameter, which will introduce strain in sample. The sulfur doping also affect the effective carrier recombination, which will enhance green band and reduce the near ZnO bandgap emissions. From temperature-dependent PL we find that activation energy for both emission bands decrease with increasing sulfur, shows an increase of nonradiative recombination probability. Acknowledgements This work is supported by the Excellent Young Scientist Scientific Research Encouragement Foundation of Shandong Province (No. 2006BS01240), Natural Science Foundation of Shandong Province (No. 2009VRA06063), National Natural Science Foundation of China with a grant number of 10704065 and The Natural Science Foundation for Distinguished Young Scholars of Shandong Province (No. 2008JQB01028). References [1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897–1899. [2] R.N. Viswanath, S. Ramasamy, R. Ramamoorthy, P. Jayavel, T. Nagarajan, Nanostruct. Mater. 6 (1995) 993–996. [3] J. Xu, Q. Pan, Y.a. Shun, Z. Tian, Sens. Actuators B: Chem. 66 (2000) 277–279. [4] X.-L. Guo, H. Tabata, T. Kawai, J. Cryst. Growth 237–239 (2002) 544–547. [5] M. Purica, E. Budianu, E. Rusu, Microelectron. Eng. 51–52 (2000) 425–431. [6] Z.L. Wang, J. Phys-Condens. Matter 16 (2004) R829–R858. [7] X.Y. Kong, Y. Ding, R. Yang, Z.L. Wang, Science 303 (2004) 1348–1351. [8] C.M. Shin, J.Y. Lee, J.H. Heo, J.H. Park, C.R. Kim, H. Ryu, J.H. Chang, C.S. Son, W.J. Lee, S.T. Tan, J.L. Zhao, X.W. Sun, Appl. Surf. Sci. 255 (2009) 8501–8505.
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[9] J.T. Chen, J. Wang, R.F. Zhuo, D. Yan, J.J. Feng, F. Zhang, P.X. Yan, Appl. Surf. Sci. 255 (2009) 3959–3964. [10] G.S. Kim, S.G. Ansari, H.K. Seo, Y.S. Kim, O.B. Yang, H.S. Shin, Appl. Surf. Sci. 253 (2007) 7197–7202. [11] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019–1022. [12] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466–2468. [13] T. Strachowski, E. Grzanka, W. Lojkowski, A. Presz, M. Godlewski, S. Yatsunenko, H. Matysiak, R.R. Piticescu, C.J. Monty, J. Appl. Phys. 102 (2007) 073513. [14] F. Zhuge, L.P. Zhu, Z.Z. Ye, D.W. Ma, J.G. Lu, J.Y. Huang, F.Z. Wang, Z.G. Ji, S.B. Zhang, Appl. Phys. Lett. 87 (2005) 092103–192103. [15] F.K. Shan, B.I. Kim, G.X. Liu, Z.F. Liu, J.Y. Sohn, W.J. Lee, B.C. Shin, Y.S. Yu, J. Appl. Phys. 95 (2004) 4772–4776. [16] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2005) 42–46.
[17] T. Wakano, N. Fujimura, Y. Morinaga, N. Abe, A. Ashida, T. Ito, Phys. E: LowDimens. Syst. Nanostruct. 10 (2001) 260–264. [18] D.B. Buchholz, R.P.H. Chang, J.H. Song, J.B. Ketterson, Appl. Phys. Lett. 87 (2005) 082503–082504. [19] V. Khranovskyy, U. Grossner, O. Nilsen, V. Lazorenko, G.V. Lashkarev, B.G. Svensson, R. Yakimova, Thin Solid Films 515 (2006) 472–476. [20] J.V. Foreman, J. Li, H. Peng, S. Choi, H.O. Everitt, J. Liu, Nano Lett. 6 (2006) 1126–1130. [21] B.Y. Geng, G.Z. Wang, Z. Jiang, T. Xie, S.H. Sun, G.W. Meng, L.D. Zhang, Synthesis and optical properties of S-doped ZnO nanowires, in: Applied Physics Letters, American Institute of Physics, 2003, p. 4791. [22] O. Madelung, Semiconductors: Data Handbook, 3rd ed., Springer, Berlin, 2004. [23] Y. Ishikawa, K. Wada, D.D. Cannon, J. Liu, H.-C. Luan, L.C. Kimerling, Appl. Phys. Lett. 82 (2003) 2044–2046. [24] S.-C. Chin, C.-Y. Chi, Y.-C. Lu, L. Hong, Y.-L. Lin, F.-Y. Jen, C.C. Yang, B.-P. Zhang, Y. Segawa, K.-J. Ma, J.-R. Yang, J. Cryst. Growth 293 (2006) 344–350. [25] S. Sanorpim, F. Nakajima, R. Katayama, K. Onabe, Y. Shiraki, Mater. Res. Soc. Symp. Proc. 744 (2003) M10.19.11.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural and optical properties of the S-doped ZnO particles synthesized by hydrothermal method Yuanping Sun a,∗ , Tao He b , Hongying Guo a , Tao Zhang b , Weitian Wang a , Zhenhong Dai a a b
Institute of Science and Technology for Opto-electronic Information, Yantai University, Qianquan Road 32#, Laishan, Yantai, Shandong 264005, PR China School of Chemical and Biological Science and Engineering, Yantai University, Shandong 264005, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2010 Received in revised form 16 June 2010 Accepted 10 August 2010 Available online 17 August 2010 Keywords: ZnO Hydrothermal method Optical properties Structural properties
a b s t r a c t Sulfur-doped ZnO particles have been synthesized by hydrothermal method. The structural and optical properties were studied systematically by XRD, scanning electron microscopy (SEM), and photoluminescence. SEM results show that the particle is hexagonal and the average size decreases with increasing sulfur doping, which means a retardant effect of sulfur on the growth of S-doped ZnO. XRD results show that the lattice parameters increase with more sulfur, which means an effective sulfur doping and increasing strain. Optical characterization also shows that the effective sulfur doping will enhance the green emission and suppress the near bandgap emissions. © 2010 Elsevier B.V. All rights reserved.
Recently, ZnO have attracted much attention in the filed of light emitting devices due to its direct wide bandgap of 3.37 eV and a very large exciton binding energy of 60 meV, which makes the excitons thermally stable even at room temperature [1]. Great deal of applications has been found for ZnO, such as varistors [2], gas sensors [3], blue light optoelectronics [4,5] and other optical devices [6,7]. Many researches have been conducted both on the growth and device application [8–10]. Modification of ZnO properties by impurity doping is a current issue for the extensive application in optoand spin electronics [11,12]. Various elements have been selected as the dopant, such as Al [13,14], Mg [15,16] and other metal elements [17–19]. But the anion doping is rarely reported. As in same group, the physical and chemical properties of sulfur are very similar to oxygen. Under some certain conditions, oxygen can be easily substituted by sulfur to form sulfide, which could make the ZnO to show special electrical and optical properties. On the other hand, possible bandgap engineering of ZnO may be possible because of the larger Eg of ZnS than that of ZnO. Although there are such advantages for the doping of sulfur, rare research on the S-doped ZnO has been reported [20,21]. In this work, we reported the successful synthesis of sulfur-doped ZnO by hydrothermal method. Strain was introduced because of the substitution of S2− for O2− , which lead to smaller diffraction angle and enhanced green band emissions. The synthesis of ZnO particles including the removal of H2 O from zinc nitrate (Zn(NO3 )2 ·6H2 O) with phosphorous pentoxide (P2 O5 )
∗ Corresponding author. Tel.: +86 535 2102292. E-mail address: [email protected] (Y. Sun). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.041
and absolute ethanol with molecular sieve at first, and then dissolve the zinc nitrate in absolute ethanol with a ratio of 1 g to 5 ml to form a solution. 5 ml solution together with 15 ml ethanol and 3.3 g sodium dodecylbenzenesulfonate (SDBS) were put into the autoclave with Teflon liner. To study the effects of sulfur doping on the growth and properties of the formed ZnO particles, 0.042, 0.051 and 0.084 g sulfur powder had been added into the mixed solution and the resulted sample are denoted as A, B and C, respectively. The solution was heated to 90 ◦ C for 10 h and then at 180 ◦ C for 3 h. After cooling naturally to room temperature, the transparent supernatant was removed by pipette and black precipitates were left. The precipitates were placed in distilled water and incubated for 24 h, then separated by filtration and allowed to dry naturally; the surfactant templates were dissolved away in DI water for 3 times to obtain the ZnO particles, which were used as samples for characterizations. The morphologies of the as-grown samples were characterized by scanning electron microscopy (SEM, JEOL JSM-6300). The structural analysis was taken by X-ray diffraction meter (XRD) (Philips PW 1710 with Cu K␣ radiation, = 0.15418 nm). Optical characterizations were performed by using a He–Cd laser at the excitation wavelength of 325 nm and a He-cooling stage operating at a temperature range from 10 to 300 K. Fig. 1 shows the SEM images of the particles for samples A, B and C, respectively. As viewed from the cross-section, the profile of most particles obtained by hydrothermal synthesis is hexagonal. The particles form loose agglomerates with a size around 1 m. In fact, the size decreases with the increasing of sulfur content in reactant solutions with a sample sequence from A to C. The dominant
1126
Y. Sun et al. / Applied Surface Science 257 (2010) 1125–1128
Fig. 2. XRD pattern for samples A, B and C. All significant peaks in the XRD data can be indexed to ZnO with different shift.
Fig. 1. SEM images of ZnO particle samples with a magnification of 9000×. (a) Sample A, (b) sample B and (c) sample C.
size is 1.3, 1.0 and 0.7 m for samples A, B and C, respectively, as calculated from the SEM images. Because all samples were grown under the same conditions except for different sulfur amount, it can be concluded that larger amount of sulfur in reactant solution will retard or delay the growth process and lead to small size particles. In fact, there are two ways for sulfur to affect the growth of
ZnO: one is the growth rate and another is the nucleation rate. More works should be done to identify the real reasons. To obtain the structural properties of the hydrothermal synthesized ZnO samples, XRD studies have been performed and the results are shown in Fig. 2. All peaks in the figure can be indexed to the wurtzite structures of ZnO (JCPDS Card No. 36-1451) with certain shift. The lattice parameters calculated from the XRD data are 3.2526, 3.2646 and 3.2687 A˚ for a axis and 5.2124, 5.2274 and 5.2360 A˚ for c axis for samples A, B, and C respectively. As compared with the parameters of a = 3.2497 A˚ and c = 5.2066 A˚ (ZnO, JCPDS Card No. 36-1451), the lattice parameters of studied samples increased with the increasing amount of sulfur in solutions (A < B < C). On the other hand, the lattice parameters of ZnS are a = 3.820 A˚ and c = 6.260 A˚ [22]. For the larger size of S2− than O2− , the substitution of S for O will introduce strain in ZnO and then affect the lattice parameters. More strain will appear in the higher sulfur doping sample, which lead to smaller diffraction angle, as shown by the XRD data in Fig. 2. To study the influence of sulfur doping on the optical properties of ZnO particles, temperature-dependent PL has been performed with a temperature range from 10 to 300 K. The normalized spectra at 10 and 300 K for 3 samples are shown in Fig. 3. The main emission peak at 10 K (Fig. 3(a)) is at 499.3, 501.4 and 504.5 nm (2.483, 2.473 and 2.458 eV) for samples A, B and C, respectively. At 300 K (Fig. 3(b)), the main emission peak is at 500.5, 501.5 and 503.3 nm (2.477, 2.472 and 2.463 eV) for samples A, B and C, respectively. The samples give off strong green emission and the emission energies remain almost unchanged (<6 meV) with a temperature range from 10 to 300 K, which show a relatively stable optical properties. The green emission energy shows a slightly red-shift with increasing sulfur doping in samples, which can be explained by the strain-induced bandgap shrinkage [23]. For the near bandgap emissions of sulfur-doped ZnO at 10 K (Fig. 3(a)), it lies at 374.9, 369.8 and 375.0 nm (3.307, 3.353 and 3.306 eV), which are different with the reported bandgap value of pure ZnO: 3.4 eV [22]. As compared with the reported ultraviolet (UV) emissions of ZnO, the peak near 369.8 nm of sample B corresponds to D0 X, while the emission of samples A and C at UV range corresponds to the transitions of DAP [24]. At 300 K, the bandgap emission peak is at 380.0, 380.2 and 381.5 nm (3.263, 3.261 and 3.250 eV) for samples A, B and C, respectively. The disordered near bandgap peaks can be attributed to the competition of different recombination mechanisms, such as D0 X, A0 X, DAP, FX, etc. As compared with the green emission peaks, the bandgap emissions shift to its lower energy more than 40 meV, which show a strong temperature-dependent properties.
Y. Sun et al. / Applied Surface Science 257 (2010) 1125–1128
Fig. 3. The normalized PL spectra of samples A, B, and C at different temperatures. (a) 10 K and (b) 300 K.
Also we can conclude from Fig. 3 that the intensity ratio of near bandgap and green emission is decreased with more doping of sulfur. The ratio is 0.80, 0.38 and 0.04 at 10 K, 0.73, 0.28 and 0.07 at 300 K, for samples A, B and C, respectively. This trend agrees well with the effective doping of sulfur in ZnO, in which the sulfur doping level increases from sample A to C. Higher doping of sulfur will generate more defects in ZnO, which will enhance the green emission and weaken the UV emissions. Fig. 4 shows the normalized integrated intensity of near bandgap and green emission for samples A, B and C based on temperaturedependent PL data. For both emission band, the integrated intensity changed a little at lower temperature; while at higher temperature, it decreased very fast. As in III-nitride based materials, temperature dependence of integrated PL intensity can be fitted by the Arrhenius formula: I=
1+
I0
C exp(−EAi /kB T ) i=1 i
(1)
where EAi are the activation energy of the corresponding nonradiative recombination center and Ci are the fitting parameters related to the nonradiative to radiative recombination probability ratio [25]. The best fitting of the data gives the activation energies of 49.0, 43.8 and 35.5 meV for near ZnO bandgap emission and 49.5, 41.0 and 22.1 meV for green emission peaks for samples A, B and C, respectively. The activation energy of near ZnO bandgap and green emissions decrease with the increasing of sulfur doping, which means an increase of nonradiative recombination probability. As we have confirmed from XRD data that the doping of sulfur will introduce more defects and lead to the enhancement of nonradiative recombination.
1127
Fig. 4. The Arrhenius plot for the normalized integration PL intensity as a function of inverse temperature (dots) for (a) ZnO bandgap and (b) green emissions. The solid lines give best fitting according to formula (1).
In conclusion, the ZnO particles with different sulfur doping have been synthesized by hydrothermal method. The increasing of sulfur doping will retard the growth of ZnO particles and increase the lattice parameter, which will introduce strain in sample. The sulfur doping also affect the effective carrier recombination, which will enhance green band and reduce the near ZnO bandgap emissions. From temperature-dependent PL we find that activation energy for both emission bands decrease with increasing sulfur, shows an increase of nonradiative recombination probability. Acknowledgements This work is supported by the Excellent Young Scientist Scientific Research Encouragement Foundation of Shandong Province (No. 2006BS01240), Natural Science Foundation of Shandong Province (No. 2009VRA06063), National Natural Science Foundation of China with a grant number of 10704065 and The Natural Science Foundation for Distinguished Young Scholars of Shandong Province (No. 2008JQB01028). References [1] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897–1899. [2] R.N. Viswanath, S. Ramasamy, R. Ramamoorthy, P. Jayavel, T. Nagarajan, Nanostruct. Mater. 6 (1995) 993–996. [3] J. Xu, Q. Pan, Y.a. Shun, Z. Tian, Sens. Actuators B: Chem. 66 (2000) 277–279. [4] X.-L. Guo, H. Tabata, T. Kawai, J. Cryst. Growth 237–239 (2002) 544–547. [5] M. Purica, E. Budianu, E. Rusu, Microelectron. Eng. 51–52 (2000) 425–431. [6] Z.L. Wang, J. Phys-Condens. Matter 16 (2004) R829–R858. [7] X.Y. Kong, Y. Ding, R. Yang, Z.L. Wang, Science 303 (2004) 1348–1351. [8] C.M. Shin, J.Y. Lee, J.H. Heo, J.H. Park, C.R. Kim, H. Ryu, J.H. Chang, C.S. Son, W.J. Lee, S.T. Tan, J.L. Zhao, X.W. Sun, Appl. Surf. Sci. 255 (2009) 8501–8505.
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