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Structure and visible luminescence of ZnO nanoparticles
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 9 (2006) 156–159
Structure and visible luminescence of ZnO nanoparticles W.Q. Peng, S.C. Qu, G.W. Cong, Z.G. Wang Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China Available online 20 February 2006
Abstract ZnO nanoparticles were synthesized in ethanolic solution using a sol–gel method. The structural and optical properties were investigated by X-ray diffraction, transmission electron microscopy, UV absorption, and photoluminescence. After annealing at 200 1C, the particle size is increased and the peak of defect luminescence in the visible region is changed. A yellow emission was observed in the as-prepared sample and a green emission in the annealed sample. The change of the visible emission is related to oxygen defects. Annealing in the absence of oxygen would increase oxygen vacancies. r 2006 Elsevier Ltd. All rights reserved. PACS: 61.46.+w; 78.67.Bf Keywords: Nanoparticles; Semiconducting materials; Photoluminescence
1. Introduction In recent years, semiconductor nanoparticles have attracted great interest [1–3]. This is stimulated mainly by physical probe into low-dimensional systems and potential applications for this class of materials. They always exhibit novel optical, electrical, and mechanical properties due to quantum confinement effects compared with their bulk counterparts, and thus can be applied in many areas, including luminescent devices, solar cells, chemical sensors, and biological labeling and diagnostics. Zinc oxide (ZnO), a wide-band-gap semiconductor (Eg3.37 eV), has been extensively investigated as an ideal candidate for optoelectronic applications such as light emitting diodes and lasers [4]. Its large Corresponding author. Tel.: +86 10 82304240.
E-mail address: [email protected] (W.Q. Peng). 1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.01.038
exciton binding energy (60 mV) also allows efficient excitonic emission even at room temperature, and thus gives promise for low-threshold and high-efficiency photonic devices [5]. Other applications include field emission displays, conductive electrodes, solar cell windows, and transistors [6]. Generally, ZnO exhibits UV emission and visible emission [7,8]. It has been recognized that the UV emission originates from the radiative recombination of excitons. However, the mechanisms of the visible emission are still an open problem, though it has been considered to be defect related. The visiblelight emission of ZnO is also of great importance for white-light LEDs [9]. In this paper, ZnO nanoparticles were synthesized in bulk quantities by a facile and inexpensive sol–gel technique. The structural and optical properties of the as-prepared and annealed nanoparticles were investigated. It was found that annealing treatment changes structural and optical properties
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of nanoparticles. After annealing, the yellow emission disappears. Instead, the green emission appears in the visible region. 2. Experimental ZnO nanoparticles were prepared in ethanol medium by the reaction of zinc acetate and sodium hydroxide. All chemicals used were AR grade and not further treated. The detailed preparation procedure is described as follows. The ethanolic solution of zinc acetate was placed in a flask. Then, the equimolar ethanolic solution of sodium hydroxide was slowly added into the flask in an ice bath (0 1C), along with continuous stirring. After centrifugally separated from the solution, washed, and then dried, a white powder of the nanoparticles was obtained. In order to examine the effect of heat treatment on structural properties, the as-prepared nanoparticles were annealed at 200 1C for 30 min under vacuum. The X-ray diffraction (XRD) patterns were recorded to characterize the phase and crystal structure of the nanoparticles using a multi-purpose XRD system (PANalytical) with a Cu Ka radiation source at 40 kV and 30 mA. The morphology of the nanoparticles was observed by a JEM-2010 transmission electron microscope (TEM) operated at 200 kV. Optical absorption was performed on an Agilent HP1100 diode-array UV-visible spectrophotometer. Room temperature photoluminescence (PL) of the samples was measured, using a He–Cd laser (325 nm) as excitation source. 3. Results and discussion Fig. 1 shows X-ray powder diffraction patterns of the as-prepared and annealed ZnO nanoparticles. In the as-prepared sample, the main diffraction peaks are broad and correspond to the hexagonal wurtzite structure of ZnO. After annealed at 200 1C, all diffraction peaks are consistent with those of wurtzite ZnO. In addition, these peaks become narrower and more intense, which indicates the increase of particle size and the improvement of crystallinity. Two extra peaks, which are denoted with asterisk in Fig. 1, also appear in the asprepared sample, but disappear in the annealed sample. It is suggested that these peaks are related with Zn(OH)2. After annealing treatment, Zn(OH)2 can be decomposed into ZnO.
Fig. 1. XRD patterns of the as-prepared and annealed ZnO nanoparticles.
Fig. 2. (a). TEM image of the annealed ZnO nanoparticles and (b) high-resolution TEM image of a single nanoparticle.
Morphology and size of the annealed ZnO nanoparticles were examined by TEM observation. A representative TEM image is presented in Fig. 2(a). The nanoparticles are quasi-spherical and have an average size of about 4 nm. By highresolution TEM observation, some characteristic lattice planes of wurtzite ZnO can be identified. As shown in Fig. 2(b), the lattice spacing of 0.247 nm is consistent with the (1 0 1) lattice spacing of bulk ZnO. Fig. 3 shows the optical absorption spectrum of the annealed ZnO nanoparticles. The exciton absorption was observed at about 341 nm, which is blueshifted compared with bulk ZnO due to quantum confinement effects. Based on the effective mass approximation theory [10], the particle size was calculated to be about 3.4 nm. This is in agreement with the results of TEM observation.
ARTICLE IN PRESS 158
W.Q. Peng et al. / Materials Science in Semiconductor Processing 9 (2006) 156–159
Fig. 4. Room-temperature visible PL spectra of the as-prepared and annealed nanoparticles. The wavelength range is between 400 and 800 nm. Fig. 3. UV-vis absorption spectrum of the annealed ZnO nanoparticles.
In ZnO nanoparticles, the luminescence behavior is of particular interest from viewpoints of both physical and applied aspects. For the visible emission, different mechanisms have been proposed. Vanheusden et al. [11] observed a green emission in ZnO and attributed it to the recombination between a transition between singly charged oxygen vacancy and photoexcited hole. However, some researchers suggested that such a green emission originates from Cu impurities [12]. On the other hand, a yellow emission in ZnO was reported and assigned to oxygen interstitial. Zwingel also proposed donor– acceptor recombination at lithium acceptors as the origin of the yellow emission [13]. In order to investigate the mechanisms of the defect-related visible emission, the room-temperature PL spectra of the as-prepared and annealed ZnO nanoparticles were recorded in the wavelength range between 400 and 800 nm. The results of PL measurement are shown in Fig. 4. It is found that different visible PL were observed in the asprepared and annealed ZnO samples. A yellow emission at around 580 nm was observed in the asprepared sample. Since no lithium source was used during synthesis, this yellow emission should originate from oxygen interstitial. However, after annealing treatment, the yellow emission disappears, and meanwhile a green emission peak around 490 nm occurs, which is commonly thought of the
transition related to oxygen vacancy [11]. Annealing could cause the increase of oxygen vacancies due to the absence of oxygen during annealing under vacuum. Therefore, this green emission should come from oxygen vacancies. Lu et al. also proved that annealing treatment would enhance the green emission [14]. In addition, the visible emission of the nanoparticles is decreased after annealing treatment, which may indicate the reduction of the defect number in the ZnO nanoparticles after annealing.
4. Conclusion ZnO nanoparticles were synthesized using a sol–gel method. The effect of annealing treatment on the structure and PL was investigated. XRD analysis demonstrates that the nanoparticles have the hexagonal wurtize structure and the particle size is increased after annealing. Due to quantum confinement effects, the absorption peak of the annealed nanoparticles is blueshifted compared with bulk material. The yellow and green emissions were observed in the visible region for the as-prepared and annealed samples, respectively. It is suggested that both visible emissions are related to oxygen defects. After annealing, oxygen vacancies were increased. Thus, the green emission appears and the yellow emission disappears.
ARTICLE IN PRESS W.Q. Peng et al. / Materials Science in Semiconductor Processing 9 (2006) 156–159
Acknowledgments The authors would like to thank the financial support from National Natural Science Foundation of China (Contract nos. 60276014 and 60476002) and Special Funds for Major State Basic Research Project of China (Contract nos. 2002CB311905).
References [1] Alivisatos AP. Science 1996;271:933–7. [2] Norris DJ, Yao N, Charnock FT, Kennedy TA. Nano Lett 2001;1:429. [3] Nirmal M, Brus L. Acc Chem Res 1999;32:407–14. [4] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, Weber E, Russo R, Yang P. Science 2001;292:1897.
159
[5] Wu X, Yamilov A, Liu X, Li S, Dravid VP, Chang RPH, Cao H. Appl Phys Lett 2004;85:3657. [6] Cooray NF, Kushiya K, Fujimaki A, Okumura D, Sato M, Ooshita M, Yamase O. Jpn J Appl Phys 1999;38:6213. [7] Xing YJ, Xi ZH, Xue ZQ, Zhang XD, Song JH, Wang RM, Xu J, Song Y, Zhang SL, Yu DP. Appl Phys Lett 2003; 83:1689. [8] Shan W, Walukiewicz W, Ager JW, Yu KM, Zhang Y, Mao SS, Kling R, Kirchner C, Waa A. Appl Phys Lett 2005; 86:153117. [9] Ohashi N, Ebisawa N, Sekiguchi T, Sakaguchi I, Wada Y, Takenaka T, Haneda H. Appl Phys Lett 2005;86:091902. [10] Brus LE. J Chem Phys 1984;80:4403. [11] Vanheusden K, Warren WL, Seager CH, Tallant DR, Voigt JA, Gnade BE. J Appl Phys 1996;79:7983. [12] Garces NY, Wang L, Bai L, Giles NC, Halliburton LE, Cantwell G. Appl Phys Lett 2002;81:622. [13] Zwingel D. J Lumin 1972;5:385. [14] Lu H, Chu S, Tan S. J Cryst Growth 2004;269:385.
Materials Science in Semiconductor Processing 9 (2006) 156–159
Structure and visible luminescence of ZnO nanoparticles W.Q. Peng, S.C. Qu, G.W. Cong, Z.G. Wang Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China Available online 20 February 2006
Abstract ZnO nanoparticles were synthesized in ethanolic solution using a sol–gel method. The structural and optical properties were investigated by X-ray diffraction, transmission electron microscopy, UV absorption, and photoluminescence. After annealing at 200 1C, the particle size is increased and the peak of defect luminescence in the visible region is changed. A yellow emission was observed in the as-prepared sample and a green emission in the annealed sample. The change of the visible emission is related to oxygen defects. Annealing in the absence of oxygen would increase oxygen vacancies. r 2006 Elsevier Ltd. All rights reserved. PACS: 61.46.+w; 78.67.Bf Keywords: Nanoparticles; Semiconducting materials; Photoluminescence
1. Introduction In recent years, semiconductor nanoparticles have attracted great interest [1–3]. This is stimulated mainly by physical probe into low-dimensional systems and potential applications for this class of materials. They always exhibit novel optical, electrical, and mechanical properties due to quantum confinement effects compared with their bulk counterparts, and thus can be applied in many areas, including luminescent devices, solar cells, chemical sensors, and biological labeling and diagnostics. Zinc oxide (ZnO), a wide-band-gap semiconductor (Eg3.37 eV), has been extensively investigated as an ideal candidate for optoelectronic applications such as light emitting diodes and lasers [4]. Its large Corresponding author. Tel.: +86 10 82304240.
E-mail address: [email protected] (W.Q. Peng). 1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.01.038
exciton binding energy (60 mV) also allows efficient excitonic emission even at room temperature, and thus gives promise for low-threshold and high-efficiency photonic devices [5]. Other applications include field emission displays, conductive electrodes, solar cell windows, and transistors [6]. Generally, ZnO exhibits UV emission and visible emission [7,8]. It has been recognized that the UV emission originates from the radiative recombination of excitons. However, the mechanisms of the visible emission are still an open problem, though it has been considered to be defect related. The visiblelight emission of ZnO is also of great importance for white-light LEDs [9]. In this paper, ZnO nanoparticles were synthesized in bulk quantities by a facile and inexpensive sol–gel technique. The structural and optical properties of the as-prepared and annealed nanoparticles were investigated. It was found that annealing treatment changes structural and optical properties
ARTICLE IN PRESS W.Q. Peng et al. / Materials Science in Semiconductor Processing 9 (2006) 156–159
157
of nanoparticles. After annealing, the yellow emission disappears. Instead, the green emission appears in the visible region. 2. Experimental ZnO nanoparticles were prepared in ethanol medium by the reaction of zinc acetate and sodium hydroxide. All chemicals used were AR grade and not further treated. The detailed preparation procedure is described as follows. The ethanolic solution of zinc acetate was placed in a flask. Then, the equimolar ethanolic solution of sodium hydroxide was slowly added into the flask in an ice bath (0 1C), along with continuous stirring. After centrifugally separated from the solution, washed, and then dried, a white powder of the nanoparticles was obtained. In order to examine the effect of heat treatment on structural properties, the as-prepared nanoparticles were annealed at 200 1C for 30 min under vacuum. The X-ray diffraction (XRD) patterns were recorded to characterize the phase and crystal structure of the nanoparticles using a multi-purpose XRD system (PANalytical) with a Cu Ka radiation source at 40 kV and 30 mA. The morphology of the nanoparticles was observed by a JEM-2010 transmission electron microscope (TEM) operated at 200 kV. Optical absorption was performed on an Agilent HP1100 diode-array UV-visible spectrophotometer. Room temperature photoluminescence (PL) of the samples was measured, using a He–Cd laser (325 nm) as excitation source. 3. Results and discussion Fig. 1 shows X-ray powder diffraction patterns of the as-prepared and annealed ZnO nanoparticles. In the as-prepared sample, the main diffraction peaks are broad and correspond to the hexagonal wurtzite structure of ZnO. After annealed at 200 1C, all diffraction peaks are consistent with those of wurtzite ZnO. In addition, these peaks become narrower and more intense, which indicates the increase of particle size and the improvement of crystallinity. Two extra peaks, which are denoted with asterisk in Fig. 1, also appear in the asprepared sample, but disappear in the annealed sample. It is suggested that these peaks are related with Zn(OH)2. After annealing treatment, Zn(OH)2 can be decomposed into ZnO.
Fig. 1. XRD patterns of the as-prepared and annealed ZnO nanoparticles.
Fig. 2. (a). TEM image of the annealed ZnO nanoparticles and (b) high-resolution TEM image of a single nanoparticle.
Morphology and size of the annealed ZnO nanoparticles were examined by TEM observation. A representative TEM image is presented in Fig. 2(a). The nanoparticles are quasi-spherical and have an average size of about 4 nm. By highresolution TEM observation, some characteristic lattice planes of wurtzite ZnO can be identified. As shown in Fig. 2(b), the lattice spacing of 0.247 nm is consistent with the (1 0 1) lattice spacing of bulk ZnO. Fig. 3 shows the optical absorption spectrum of the annealed ZnO nanoparticles. The exciton absorption was observed at about 341 nm, which is blueshifted compared with bulk ZnO due to quantum confinement effects. Based on the effective mass approximation theory [10], the particle size was calculated to be about 3.4 nm. This is in agreement with the results of TEM observation.
ARTICLE IN PRESS 158
W.Q. Peng et al. / Materials Science in Semiconductor Processing 9 (2006) 156–159
Fig. 4. Room-temperature visible PL spectra of the as-prepared and annealed nanoparticles. The wavelength range is between 400 and 800 nm. Fig. 3. UV-vis absorption spectrum of the annealed ZnO nanoparticles.
In ZnO nanoparticles, the luminescence behavior is of particular interest from viewpoints of both physical and applied aspects. For the visible emission, different mechanisms have been proposed. Vanheusden et al. [11] observed a green emission in ZnO and attributed it to the recombination between a transition between singly charged oxygen vacancy and photoexcited hole. However, some researchers suggested that such a green emission originates from Cu impurities [12]. On the other hand, a yellow emission in ZnO was reported and assigned to oxygen interstitial. Zwingel also proposed donor– acceptor recombination at lithium acceptors as the origin of the yellow emission [13]. In order to investigate the mechanisms of the defect-related visible emission, the room-temperature PL spectra of the as-prepared and annealed ZnO nanoparticles were recorded in the wavelength range between 400 and 800 nm. The results of PL measurement are shown in Fig. 4. It is found that different visible PL were observed in the asprepared and annealed ZnO samples. A yellow emission at around 580 nm was observed in the asprepared sample. Since no lithium source was used during synthesis, this yellow emission should originate from oxygen interstitial. However, after annealing treatment, the yellow emission disappears, and meanwhile a green emission peak around 490 nm occurs, which is commonly thought of the
transition related to oxygen vacancy [11]. Annealing could cause the increase of oxygen vacancies due to the absence of oxygen during annealing under vacuum. Therefore, this green emission should come from oxygen vacancies. Lu et al. also proved that annealing treatment would enhance the green emission [14]. In addition, the visible emission of the nanoparticles is decreased after annealing treatment, which may indicate the reduction of the defect number in the ZnO nanoparticles after annealing.
4. Conclusion ZnO nanoparticles were synthesized using a sol–gel method. The effect of annealing treatment on the structure and PL was investigated. XRD analysis demonstrates that the nanoparticles have the hexagonal wurtize structure and the particle size is increased after annealing. Due to quantum confinement effects, the absorption peak of the annealed nanoparticles is blueshifted compared with bulk material. The yellow and green emissions were observed in the visible region for the as-prepared and annealed samples, respectively. It is suggested that both visible emissions are related to oxygen defects. After annealing, oxygen vacancies were increased. Thus, the green emission appears and the yellow emission disappears.
ARTICLE IN PRESS W.Q. Peng et al. / Materials Science in Semiconductor Processing 9 (2006) 156–159
Acknowledgments The authors would like to thank the financial support from National Natural Science Foundation of China (Contract nos. 60276014 and 60476002) and Special Funds for Major State Basic Research Project of China (Contract nos. 2002CB311905).
References [1] Alivisatos AP. Science 1996;271:933–7. [2] Norris DJ, Yao N, Charnock FT, Kennedy TA. Nano Lett 2001;1:429. [3] Nirmal M, Brus L. Acc Chem Res 1999;32:407–14. [4] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, Weber E, Russo R, Yang P. Science 2001;292:1897.
159
[5] Wu X, Yamilov A, Liu X, Li S, Dravid VP, Chang RPH, Cao H. Appl Phys Lett 2004;85:3657. [6] Cooray NF, Kushiya K, Fujimaki A, Okumura D, Sato M, Ooshita M, Yamase O. Jpn J Appl Phys 1999;38:6213. [7] Xing YJ, Xi ZH, Xue ZQ, Zhang XD, Song JH, Wang RM, Xu J, Song Y, Zhang SL, Yu DP. Appl Phys Lett 2003; 83:1689. [8] Shan W, Walukiewicz W, Ager JW, Yu KM, Zhang Y, Mao SS, Kling R, Kirchner C, Waa A. Appl Phys Lett 2005; 86:153117. [9] Ohashi N, Ebisawa N, Sekiguchi T, Sakaguchi I, Wada Y, Takenaka T, Haneda H. Appl Phys Lett 2005;86:091902. [10] Brus LE. J Chem Phys 1984;80:4403. [11] Vanheusden K, Warren WL, Seager CH, Tallant DR, Voigt JA, Gnade BE. J Appl Phys 1996;79:7983. [12] Garces NY, Wang L, Bai L, Giles NC, Halliburton LE, Cantwell G. Appl Phys Lett 2002;81:622. [13] Zwingel D. J Lumin 1972;5:385. [14] Lu H, Chu S, Tan S. J Cryst Growth 2004;269:385.