J. Phys. Chem. C 2007, 111, 13013-13015
13013
Planar Defects in Sn-Doped Single-Crystal ZnO Nanobelts Rui Deng,† XiTian Zhang,*,‡ E. Zhang,† Yao Liang,‡ Zhuang Liu,‡ HaiYang Xu,‡ and SuiKong Hark*,‡ Department of Physics, Harbin Normal UniVersity, Harbin, 150080, P. R. China, and Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, Hong Kong ReceiVed: May 14, 2007; In Final Form: July 4, 2007
Sn-doped ZnO nanobelts were synthesized by chemical vapor deposition. These single-crystalline nanobelts with a wurtzite structure grow along the direction. Transmission electron microscopy shows the existence of planar defects in the middle of each nanobelt, which contains antiphase and inversion boundaries. The appearance of inversion boundaries parallel to the (0001) plane is attributed to the release of structural strains, which result from the introduction of Sn+4 ions. In photoluminescence measurements, a strong ultraviolet peak and a weak visible band were observed at room temperature.
Introduction Zinc oxide (ZnO) is a wide direct band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV),1 which ensures efficient ultraviolet (UV) emission up to room temperature (RT). Therefore, it is a promising material for fabricating optoelectronic devices operating in the blue and UV region. Moreover, due to its superior conducting properties, it has also been investigated as a transparent conducting and piezoelectric material for use as solar cells, electrodes, and sensors.2 Doping in ZnO with selective elements can offer an effective method to modify their electrical, optical, and magnetic properties. It was reported that the Sn-doping induces a blue emission.2 In addition, Sn-doped ZnO nanowires exhibit significantly improved field emission characteristics in comparison with undoped ones3 and reduced resistance as the Sn content increases. Based on the above observations and factors i and ii below, we find Sn-doped ZnO nanostructures interesting: (i) Sn substituting Zn is a good candidate as an n-type dopant in ZnO; (ii) because the radius of Sn+4 (0.071 nm) is almost equal to that of Zn+2 (0.074 nm), it is highly dissolvable in ZnO. As far as we know, studies on Sn-doped ZnO nanobelts are rarely reported;4,5 we therefore synthesized one-dimensional (1D) Sndoped ZnO nanobelts with planar defects on the (0001) plane by a simple chemical vapor deposition method. In this paper, high-resolution transmission electron microscopy (HRTEM) is applied to investigate the nature of these planar defects. Optical properties of the Sn-doped ZnO nanobelts were also discussed. Experimental Section The Sn-doped ZnO nanobelts were grown on a Si (111) substrate, using a horizontal tube furnace heated by siliconmolybdenum rods. The synthesis details can be found elsewhere.6-8 A mixture of ZnO and Sn powders (at a mass ratio of 10:1) acts as the precursor. Prior to heating, the tube was pumped to a pressure of 25 Torr, which was maintained until the end of the growth process. Highly pure nitrogen gas was * Corresponding authors. E-mail:
[email protected] or
[email protected]. † Harbin Normal University. ‡ The Chinese University of Hong Kong.
introduced into the chamber as a carrier gas at a constant flow rate of 25 sccm. Then the furnace was heated to and held at 1400 °C for 30 min. After the system was naturally cooled down to RT, white products were found on the substrate. The as-deposited products were characterized by fieldemission scanning electron microscopy (FESEM, HITACHI4800), transmission electron microscopy (TEM, Philips CM 120) equipped with an energy-dispersive X-ray (EDX, Oxford) spectrometer, and high-resolution transmission electron microscopy (Philips Tecnai 20). Photoluminescence (PL) spectra of the synthesized products were taken at RT using the 325 nm line of a He-Cd laser as the excitation source. Results and Discussion Figure 1 is a representative SEM image of the as-deposited products, which shows a high density of nanobelts over the entire surface of the substrate. These nanobelts have a width of about 80-120 nm, a thickness of about 15 nm, and a length up to a few tens of micrometers. The width and thickness reported here are much smaller than those reported previously.4,5 The morphology and crystal structure of the nanobelts were further studied by TEM, as shown in Figures 2a and 2b. Figures 2c and 2d show an HRTEM lattice image and its corresponding Fourier transformed image, respectively, which indicate that the nanobelts have top/bottom surfaces ({21h1h0}, side surfaces ((0001), and a wurtzite structure and grow along the direction. From random TEM examination results, we found that the nanobelts have a narrow size distribution and each nanobelt has a sharp-contrast line corresponding to planar defects on the (0001) plane located in the middle of its top surfaces. The planar defects are parallel to the direction. Furthermore, this fact also indicates that presence of the planar defects is essential to forming ZnO nanobelts that grow along the direction. The EDX spectrum (the inset in Figure 2b) recorded from a nanobelt shows the presence of Zn and O elements at an atomic ratio of ∼48:50, besides minor Sn (2 atom %) (the Cu peaks come from the TEM sample grid), indicating that Sn was doped into the ZnO nanobelts. It is interesting to speculate on the nature of the planar defects. As is known, there is no center of inversion in the hexagonal crystal structure, and therefore, an inherent asymmetry that
10.1021/jp073668+ CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007
13014 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Figure 1. SEM image of the as-deposited products.
Figure 2. (a) Low-magnification and (b) high-magnification TEM images of Sn-doped ZnO nanobelts with clear planar defects in the middle of the nanobelts; the inset is an EDX spectrum of a nanobelt. (c) An HRTEM image of (b). (d) Fourier transform of (c).
allows anisotropic crystal growth along the direction (c-axis). Under thermodynamic equilibrium conditions, the surface energy of the {0001} planes is higher than those of {11h00} and {112h0} planes. So 1D nanowire growth along the c-axis is therefore preferred over other directions. However, such thermodynamic equilibrium could be disturbed, such as by incorporating impurities into the as-synthesized materials. Poisoning of the surface can easily rearrange the order of surface energies.9 In our experiment, metal Sn was doped into the ZnO nanobelts. The nanobelts therefore grow along the direction. Recently, Wang et al. reported the In-doped ZnO nanobelts, whose growth direction is , and they pointed out that nanobelts are always accompanied by planar defects.10 These defects were a stacking layer introduced by impurity atoms.10,11 In order to investigate the nature of the planar defects here, the HRTEM and the corresponding Fourier transform images are further analyzed. The resultant lattice fringe images of Figures 3a and 3b, which are obtained by only inversely Fourier transforming the (011h0)/(01h10) and the (0001)/( 0001h) pairs of diffraction spots in Figure 2d, reveal
Deng et al. that (i) each nanobelt contains two kinds of planar defects, (ii) the (011h0) fringes have an analogous translation at the corresponding defects (marked by arrowheads), and (iii) the {0001} fringes have an obvious difference in the two planar defects. Based on the information gathered, the left planar defect is not one of three types of stacking faults in a stoichiometric wurtzite structure.12,13 It is likely to be antiphase boundaries (APs)14 on the {21h1h0} plane, while the right planar defect, composed of three lattice fringes, shows an obvious contrast compared to the surrounding lattices as indicated by the intensity line scan image (Figure 3c) across the domain marked by the blue line. The latter indicates that the right planar defect may not be attributed to the stacking faults as reported in the literature.12,13 According to the model proposed by Daneu et al.,15 we deduce that the planar defect consists of three octahedral layers from O-2, Sn+4, and Zn+2 ions, which are so-called inversion boundaries (IBs). Since the atomic radius of Sn+4 ions is smaller than that of Zn+2 ions, the introduction of Sn+4 ions should cause a structural strain. Minimization of this structural strain results in the formation of the IBs. If we define the positive direction of the c-axis as pointing from an O atom to a Zn atom along the O-Zn bond, these IBs have a head-to-head (f | r) configuration. To satisfy a local charge balance, Daneu et al. proposed that the expected mole ratio of Zn+2:Sn+4 is 1:1 in the octahedral IB layer.15 Every IB plane itself consists of a mixed layer of Sn and Zn atoms that occupy octahedral sites of the boundary structure, where half of the interstices are occupied by Sn atoms and the rest by Zn atoms. Atoms arrange in an IB plane as follows: each Sn+4 ion is surrounded by four Zn+2 and two Sn+4 ions, and each Zn+2 ion is surrounded by four Sn+4 and two Zn+2 ions.15 In every IB plane, Sn and Zn atoms are ordered, but do not possess a symmetry plane. The final atomic arrangement model of a Sn-rich IB plane, which is attributed to the {0001} plane, is illustrated in Figure 4a (drawn by Material Studio software).15 However, if we observe the IB layer along family of crystallographic directions, the boundary consists of either separated Zn or Sn atoms, and we cannot observe a periodicity. Thus, the HRTEM image taken along the zone axis includes IBs and APs. Furthermore, Sn elemental mapping shows that the Sn content at the planar defects is much higher than that in other areas, where Sn is uniformly distributed (in Figure 4b). We therefore propose a structure model of the IBs in Figure 4c, which are the same defects shown in Figure 2c but magnified. The model is shown in Figure 4d. Figure 5 shows the room-temperature PL spectra recorded from the as-deposited Sn-doped ZnO nanobelts and undoped ZnO nanowires. There are a strong UV peak and a weak visible band in the PL spectrum of the undoped ZnO nanowires. The strong UV peak at 3.25 eV corresponds to the near-band-edge (NBE) emissions originated from the recombination of the free excitons of ZnO. The weak green luminescence band centered at 2.4 eV is due to the emissions related to oxygen vacancies with one electron.16 On the other hand, the NBE emission peak of Sn-doped ZnO nanobelts at 3.24 eV becomes broader and slightly shifts to lower energy compared with that of undoped ZnO nanowires, which are attributed to the introduction of Sn into the nanobelts. The UV emission peak for Sn-doped ZnO nanobelts is deconvoluted into two Gaussian components. The one at higher energy is the NBE emission of ZnO, and the other, centered at 3.17 eV, may be attributed to the incorporation of the Sn atoms. From ref 3, the Sn atoms replace Zn+2 ions (SnZn+2) in the ZnO structure and introduce a deep-energy level in band gap. Thus, the 3.17 eV emission peak is likely attributed
Planar Defects in Sn-Doped ZnO Nanobelts
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13015
Figure 3. (a and b) Inverse Fourier transforms of the (011h0)/(01h10) and (0001)/ (0001h) pairs of diffraction spots in Figure 2d, respectively. (c) Intensity line scan across the domain marked by the blue line.
Conclusions In summary, Sn-doped ZnO nanobelts with planar defects were prepared by a simple chemical vapor deposition method. The planar defects in the nanobelts include both APs and IBs, which may result from the minimization of structural strains. The introduction of Sn atoms might result in a rearrangement of the order of surface energies among {0001}, {011h0}, and {21h1h0} surfaces, causing the nanobelts to grow along the direction. The NBE emission peak at 3.24 eV for Sndoped ZnO nanobelts becomes broader and slightly shifts to a lower energy, compared with the emission of undoped ZnO nanowires, which are attributed to the introduction of Sn into the nanobelts.
Figure 4. (a) Possible atomic model of an octahedral IB layer. Yellow balls: Zn atoms. Red balls: O atoms. Light green balls: Sn. (b) Sn elemental mapping image of the nanobelt in Figure 2b. (c) Enlarged HRTEM image of a nanobelt. (d) Possible atomic model corresponding to the HRTEM image given in a white rectangle domain of Figure 4b. Cations in every IB layer are Zn or Sn along the orientation.
Figure 5. Photoluminescence spectra of Sn-doped ZnO nanobelts and undoped ZnO nanowires measured at room temperature. (Green curve is Gaussian fitted analysis for Sn-doped ZnO nanobelts.)
to the radiative recombinations of electrons in the deep-energy level with holes in the valence band.
Acknowledgment. This work was partially supported by the CUHK direct grants (Project codes 2060287, 2060305, and 2060308); Postdoctoral Start-up Fund of Personnel Bureau, Heilongjiang Province; the Project of Overseas talent, Education Bureau, Heilongjiang Province (1055HZO22); and the Science technology and Research Project of Education Bureau, Heilongjiang Province (10551095). References and Notes (1) Xu, L.; Su, Y.; Chen, Y. Q.; Xiao, H. H.; Zhu, L. A.; Zhou, Q. T.; Li, S. J. Phys. Chem. B 2006, 110, 6637. (2) Bae, S. Y.; Na, C. W.; Kang, J. H.; Park, J. J. Phys. Chem. B 2005, 109, 2526. (3) Li, S. Y.; Lin, P.; Lee, C. Y.; Tseng, T. Y.; Huang, C. J. J. Phys. D: Appl. Phys. 2004, 37, 2274. (4) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Li, Y.; Xiao, Z. D. Chem. Lett. 2005, 34, 436. (5) Chen, H. S.; Qi, J. J.; Huang, Y. H.; Liao, Q. L.; Zhang, Y. Acta Phys.sChim. Sin. 2007, 23 (1), 55. (6) Liang, Y.; Zhang, X. T.; Qin, L.; Zhang, E; Gao, H.; Zhang, Z. G. J. Phys. Chem. B 2006, 110, 21593. (7) Qin, L.; Zhang, X. T.; Liang, Y.; Zhang, E; Gao, H.; Zhang, Z. G. Acta Phys. Sin. 2006, 55, 3119. (8) Gao, H.; Zhang, X. T.; Zhou, M. Y.; Zhang, Z. G.; Wang, X. Z. Nanotechnology 2007, 18, 065601. (9) Zhang, X. T.; Liu, Z.; Wong, C. C.; Hark, S. K. Solid State Commun. 2006, 139, 387. (10) Ding, Y.; Kong, X. Y.; Wang, Z. L. Phys. ReV. B 2004, 70, 235408. (11) Cheng, B. C.; Xiao, Y. H.; Wu, G. S.; Zhang, L. D. AdV. Funct. Mater. 2004, 14, 913. (12) Potin, V.; Ruterana, P.; Nouet, G. J. Phys.: Condens. Matter 2000, 12, 10301. (13) Stampf, C.; Walle, C. G. V. D. Phys. ReV. B 1998, 57, R15052. (14) Bronikowski, M. J.; Wang, Y.; Hamers, R. J. Phys. ReV. B 1993, 48, 12361. (15) Daneu, N.; Recˇnik, A.; Walther, T.; Mader, W. Microsc. Microanal. 2003, 9, 286. (16) Zhang, X. T.; Liu, Y. C.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W.; Kong, X. G. J. Cryst. Growth 2003, 254, 80.
