CRYSTAL GROWTH & DESIGN
Temperature-Dependent Emission Shifts of Peanutlike ZnO Microrods Synthesized by a Hydrothermal Method
2007 VOL. 7, NO. 9 1686-1689
Zaiyin Huang,*,†,‡ Chunfang Chai,‡ and Bingqiang Cao§ School of Chemistry & Ecological Engineering, Guangxi UniVersity for Nationalities, Guangxi, Nanning 530006, School of Chemistry & Chemical Engineering, Guangxi UniVersity, Guangxi, Nanning 530004, and Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ReceiVed August 29, 2006; ReVised Manuscript ReceiVed June 13, 2007
ABSTRACT: Photoluminescence (PL) properties of the peanutlike ZnO microrods synthesized by a hydrothermal method were studied in the temperature range of 8-300 K. UV, violet, and green peaks, which exhibit blue, S-shaped (red-blue-red), and redshifts, respectively, and the intensity decrease, with an increase of the test temperature, are observed in the PL spectrum. The existence of Vo and Zni in these microrods is confirmed by X-ray electron spectrometry and electron paramagnetic resonance spectrum, respectively. The origin and different temperature dependence of the UV, violet, and green peaks are discussed. 1. Introduction ZnO, an important II-VI compound semiconductor with a wide band gap (3.3 eV) and high exciton binding energy (60 meV) at room temperature, displays excellent piezoelectric, catalysis, and novel optical properties.1 It is a good candidate for optoelectronics,2 sensors,3 and transistors4 and has wide applications in transparent conducting film,5 rheostat,6 and photocatalysis devices.7 Recently, many special ZnO nanostructures, such as nanohelices,8 nanocastles,9 nanocombs,10 and nanocages11 have been synthesized by various methods. The photoluminescence (PL) study on ZnO has been conducted for several decades, and the ultraviolet, violet, and green emissions have been researched systematically by many groups,12-20 but their proper emission mechanism is still under dispute, especially the defect-related emissions. It is well-known that the size, morphology, and synthesis methods of materials have great influence on their properties. In this article, novel peanutlike ZnO microrods were fabricated by a hydrothermal method and their low-temperature PL properties from 8 to 300 K in the range of 1.66-3.55 eV were tested for the first time. Three emission bands of these ZnO microrods, that is, UV, violet, and green emissions bands, were observed in the PL spectrum. Even though ZnO PL properties have been researched for tens of years, the special multipeaks of the UV emissions of the ZnO microrods synthesized by the hydrothermal method have rarely been reported. 2. Experimental Section A 0.01 mol amount of hydrated zinc acetate [Zn(Ac)2‚2H2O] and 0.005 mol of cetyltrimethyl ammonium bromide were dissolved in 40 mL of distilled water by vigorous stirring; then, 25% ammonia was added dropwise to adjust the pH to 10. After that, the solution was transferred into a stainless Teflon-lined 50 mL autoclave and kept at 150 °C for 24 h in the oven. After it was cooled to room temperature, * To whom correspondence should be addressed. Tel: +86-771-3262120. E-mail:
[email protected]. † School of Chemistry & Ecological Engineering, Guangxi University for Nationalities. ‡ School of Chemistry & Chemical Engineering, Guangxi University. § Chinese Academy of Sciences.
Figure 1. XRD pattern of as-synthesized samples.
the white precipitate was collected and washed with absolute ethanol and distilled water several times and then dried under 60 °C for 5 h. The obtained products were characterized with X-ray diffraction (XRD, Philips PW 1710 with Cu KR radiation), energy dispersive X-ray spectroscopy (EDS), and field-emission scanning electron microscopy (JEOL JSM-630). The PL spectra (Steady-State/Lifetime Spectrofluorometer, Fluorolog-3-TAU), in the range of 1.66-3.55 eV, over a temperature range from 8 to 300 K, were recorded under excitation with 325 nm light of a xenon laser. An X-ray electron spectrometer (XPS, ESCALAB-250) and electron paramagnetic resonance spectrometer (EPR, JES-FA200) were employed to further confirm the existence of Vo and Zni in the products.
3. Results and Discussion The XRD pattern of the as-prepared samples in Figure 1 shows several obvious diffraction peaks, which can be assigned to the wurtzite hexagonal ZnO with lattice constants of a ) 0.3249 nm and c ) 0.5206 nm. No others peaks can be found in the pattern, revealing that there is no impurity in the products.
10.1021/cg060577d CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007
Peanutlike ZnO Microrods
Figure 2. SEM images of ZnO microrods. (a) The representative peanutlike ZnO microrods: The inset is the EDS spectrum of the samples. (b) Magnified SEM image of the surface of the rod.
Figure 2a is a low-magnification SEM image that shows the high yield of the peanutlike ZnO microrods. The length and diameter of the rods are about 10 and 3 µm, respectively. Nevertheless, at the two ends, the diameter get shorter as the length extends. The EDS spectrum inset in Figure 2a shows the element signals of Zn and O, which further proves the pure ZnO composition of the samples. The molar ratio of Zn to O element is 52.18:47.82, indicating the existence of oxygen vacancies in the ZnO samples. The high-magnification SEM image (Figure 2b) shows the crude surface of the microrod. It seems that the rod consists of thousands of small triangular and quadrangular prisms with smooth surfaces and flat ends whose widths vary from tens to hundreds of nanometers. Figure 3a is the PL spectrum of the ZnO microrods tested from 1.66 to 3.55 eV over the temperature range of 8-300 K. Two main peaks were detected, located at ∼3.2 and ∼2.2 eV, which can be indexed to UV and green emissions, respectively. These UV peaks consist of several subminiature peaks, and the green peaks are wide and span the range of 1.7-2.7 eV. Furthermore, the intensity of UV peaks and green peaks decreases with the increase of the test temperatures. Further analysis was performed by fitting the spectrum with a Gaussian function. Besides the UV and green peaks, another peak located at ∼3.1 eV was found in the Gaussian-fitted PL spectrum, which is called violet emission. Figure 3b shows a typical Gaussianfitted spectrum for the full PL spectrum at 190 K. To analyze the shift characters of the three peaks in more detail, UV emission bands, Gaussian-fitted green emission bands, and violet emission bands were extracted from the general PL spectrum. Figure 4a is the magnified image of the UV emission bands. The spectrum in the range of 3.10-3.40 eV is
Crystal Growth & Design, Vol. 7, No. 9, 2007 1687
Figure 3. PL spectra of the ZnO peanutlike microrods. (a) PL spectrum at the temperature range of 8-300 K (all spectra are normalized by the green peak intensity and shifted in the vertical direction for clarity) and (b) typical Gaussian-fitting analysis of the PL spectrum at 190 K.
very complex. A sharp peak at 3.361 eV and a wide peak, composed of four weak peaks at 3.164, 3.210, 3.236, and 3.282 eV, are observed. With an increase in the test temperatures, the intensity of these peaks decreases in the different shrinking rates, and finally, four peaks incorporate to be a wide peak when the temperature increases up to 280 K. All of the UV peaks show a tiny redshift. Similar to the UV peaks, the intensity of the green peaks decreases when the test temperature varies from 8 to 300 K but shows a different blueshift action, shown in Figure 4b. Figure 4c exhibits the Gaussian-fitted violet emission bands of the microrods. With the temperature increase, the peak’s intensity decreases as the UV peaks and green peaks, but the shift action of the peaks is not monotonic. With the temperature increases from 8 to 130 K, the violet peaks show a redshift of about 0.09 eV; when the temperature increases from 130 to 160 K, the peaks blueshift about 0.07 eV, and when the temperature further increases to 300 K, another 0.04 eV redshift is detected. In a word, an S-shaped shift (red-blue-red) of the violet emission is found with the increase of the testing temperature. The temperature-dependent shift trends of the three bands are summarized in Figure 4d. It is generally accepted that the UV emissions originate from the emission of free exciton and banding exciton.21 According to previous research,12 the redshift of UV exciton emission with the increasing temperature is attributed to the temperatureinduced band gap shrinkage and changes of the electron-photon interaction, which could be predicted by the well-known Varshni relation.22 In our research, the UV line at 3.361 eV in Figure 4a can be attributed to the neutral donor-bound exciton complex
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Figure 4. (a) Magnified UV peaks, (b) Gaussian-fitting green emissions bands, and (c) Gaussian-fitting violet emissions bands spectrum at the temperature range of 8-300 K of the peanutlike ZnO microrods. (d) The temperature-dependent peak position for the three emission bands.
Figure 5. XPS and EPR spectra of ZnO microrods. Panels a, b, and c are a survey spectrum and Zn2p3/2 and O1s core level spectra, respectively. Panel d is the EPR spectrum.
Peanutlike ZnO Microrods
associated with defect pair (D0, X) emissions.23-25 Emissions at 3.282 and 3.205 eV are the 1-LO and 2-LO photon replicas (the energy of one LO phonon is ∼77 meV) of 3.361 eV, respectively.26 Emissions at 3.236 and 3.164 eV are the TO photon replicas (energy separation of 50.7 meV) of 3.282 and 3.210 eV, respectively.24 The violet emission could be attributed to electron-hole recombination between the Zni-shallow donor levels and holes in the valence band.12,27 The S-shaped shift of ZnO is the result from the competition between the temperature-induced localization effect and the band gap shrinkage.12 When the temperature increases from 8 to 130 K, the excitons gain sufficient thermal energy to transit to the lower energy level and recombine with the hollows at the valence band, so that the redshift takes place, which was also given another hand by the temperature-induced localization effect. In the range of 130-160 K, the 0.07 eV blueshift of the violet emission results from the thermalization of the excitons localized at the higher energy level and transitions from those levels to the valence band. The other redshift in the range of 160-300 K was mainly caused by the band gap shrinkage, which is similar to the former redshift in 8-130 K. It is generally accepted that the origin of the green PL is the electron-hole radiative recombination at oxygen vacancy, but the detailed transition process is still not very clear now. Here, we propose that two transitions could be employed to illuminate the green emission at near room-temperature: from the conduction band to Vo levels and from Zni to Vo levels.12 XPS and EPR spectra were employed to confirm the existence of Vo and Zni in the products, respectively. Figure 5a is the XPS survey spectrum, which is in accordance with the EDS spectrum of the products; Figure 5b,c shows the Zn2p3/2 and O1s core level spectra, respectively. The dissymmetrical spectrum line shown in panel b demonstrates that there is excessive Zn in the surface of ZnO.28 Figure 5c is the absorption band of O1s and its Lorentzian-fitted spectrum. Peaks located at 530.20, 531.58, and 532.80 eV can be indexed to O2- in the ZnO wurtzite structures, O2- in the ZnO crystal where O is insufficient, and O2- in H2O, which absorbed from the air, respectively.29,30 The EPR spectrum with g ∼ 1.9542 shown in Figure 5d confirmed the existence of the Zni shallow donors in the products.31,32 4. Conclusions In summary, peanutlike ZnO microrods with a length of ∼10 µm and a diameter of ∼3 µm were synthesized by a hydrothermal method. Their PL property was measured over the temperature range from 8 to 300 K. The intensities of the UV, violet, and green peaks decrease with the increase of the test temperatures. The different red, S-shaped, and blueshifts are observed in the UV, violet, and green emission bands, respectively. The redshift of the UV exciton emission is a result of the temperature-induced band gap shrinkage. The violet emission is ascribed to electron-hole recombination between the Zni defect level and the valence band, and its S-shaped shift is the result of competition between electron localization effect at the
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Zni level and band gap shrinkage. The blueshift of the green emission is attributed to the transitions of electrons from the conduction band to Vo levels and Zni to Vo levels. Acknowledgment. This work is financially supported by the Nature Science Foundation of Guangxi Zhuang Autonomous Region (No. 0575030 and No. 0639025). References (1) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P. X.; Hughes, W. L.; Yang, R. S.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 943. (2) Johnson, J. C.; Yan, H. Q.; Schaller, R. D.; Haber, L. H.; Saykally, R. J.; Yang, P. D. J. Phys. Chem. B 2001, 105, 11387. (3) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654. (4) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (5) Kaneko, D.; Shouji, H.; Kawai, T.; Kon-No, K. Langmuir 2000, 16, 4086. (6) Huang, M. H.; Wu, Y. Y.; Yang, P. D. AdV. Mater. 2001, 13, 113. (7) Wu, Y. J.; Yan, H. Q.; Yang, P. D. Top. Catal. 2002, 19, 197. (8) Gao, P. X.; Ding, Y.; Wang, Z. L. Science 2005, 309, 1700. (9) Wang, X. D.; Song, J. H.; Wang, Z. L. Chem. Phys. Lett. 2006, 424, 86. (10) Lao, C. S.; Gao, P. X.; Wang, Z. L. Chem. Phys. Lett. 2005, 417, 359. (11) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (12) Cao, B. Q.; Cai, W. P.; Zeng, H. B. Appl. Phys. Lett. 2006, 88, 161101-1. (13) Cao, B. Q.; Cai, W. P.; Zeng, H. B.; Duan, G. T. J. Appl. Phys. 2006, 99, 073516-1. (14) Cao, B. Q.; Cai, W. P.; Li, Y.; Sun, F. Q.; Zhang, L. D. Nanotechnology 2005, 16, 1734. (15) Cao, B. Q.; Cai, W. P.; Zeng, H. B.; Duan, G. T. Cryst. Growth Des. 2006, 6, 1091. (16) Boemarea, C.; Monteiroa, T.; Soaresa, M. J.; Guilhermeb, J. G.; Alves, E. Physica B. 2001, 308, 985. (17) Matsumoto, T.; Kato, H.; Miyamoto, K.; Sano, M. Appl. Phys. Lett. 2002, 81 (7), 1231. (18) Li, Y.; Meng, G. W.; Zhang, L. D.; Phillipp, F. Appl. Phys. Lett. 2000, 76, 2011. (19) Wang, Y. W.; Zhang, L. D.; Wang, G. Z.; Peng, X. S.; Chu, Z. Q.; Liang, C. H. J. Cryst. Growth 2002, 234, 171. (20) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Li, Y.; Xiao, Z. D. Chem. Lett. 2005, 34, 436. (21) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983. (22) Varshni, Y. P. Physica (Amsterdam) 1967, 34, 149. (23) Reynolds, D. C.; Look, D. C.; Jogai, B.; Litton, C. W.; Collins, T. C.; Harsch, W.; Cantwell, G. Phys. ReV. B 1998, 57, 12151. (24) Sherriff, R. E.; Reynolds, D. C.; Look, D. C.; Jogai, B.; Hoeisher, J. E.; Collins, T. C.; Cantwell, G.; Harsch, W. C. J. Appl. Phys. 2000, 88, 3454. (25) Matsumoto, T.; Kato, H.; Miyamoto, K.; Sano, M.; Zhukov, E. A.; Yao, T. Appl. Phys. Lett. 2002, 81, 1231. (26) Xie, J. S.; Wang, G. Z.; Han, X. H.; Fang, J. P.; Xu, B.; Yu, Q. X.; Liao, Y. J. Cryst. Growth 2004, 267, 223. (27) Zeng, H. B.; Cai, W. P.; Hu, J. L.; Duan, G. T.; Liu, P. S. Appl. Phys. Lett. 2006, 88, 171910. (28) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826. (29) Cebulla, R. J. Appl. Phys. 1998, 83, 1087. (30) Du, Y.; Zhang, M. S.; Hong, J.; Shen, Y.; Chen, Q.; Yin, Y. Appl. Phys. A 2003, 76, 171. (31) Vlasenko, L. S.; Watkins, G. D. Phys. ReV. B 2005, 71, 125201. (32) Leiter, F. H.; Alves, H. R.; Hofstaetter, A.; Hofmann, D. M.; Meyer, B. K. Phys. Status Solidi B 2001, 226, R4.
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