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J. Phys. Chem. C 2008, 112, 17916–17919
Synthesis and Characterization of Sb-Doped ZnO Nanobelts with Single-Side Zigzag Boundaries Ya Yang,† Junjie Qi,† Qingliang Liao,† Yue Zhang,*,†,‡ Lidan Tang,† and Zi Qin† Department of Materials Physics, UniVersity of Science and Technology Beijing, Beijing 100083, China, and State Key Laboratory for AdVanced Metals and Materials, UniVersity of Science and Technology Beijing, Beijing 100083, China ReceiVed: July 21, 2008; ReVised Manuscript ReceiVed: August 24, 2008
Sb-doped ZnO nanobelts with single-side zigzag boundaries were synthesized by chemical vapor deposition with an Au catalyst. Transmission electron microscopy shows the existence of two types of periodic planar defects in each nanobelt, which are located on the (0001) and (022j1) planes, respectively. The growth of the nanobelts is suggested to be controlled by both the two planar defects. Raman scattering analysis shows that the Sb doping in ZnO depresses the Raman A1T mode of ZnO and induces the appearance of the additional peak at 761 cm-1. The near band edge emission peak in photoluminescence spectra has red-shifted as well as broadened seriously due to the heavy doping of Sb. Introduction
Experimental Section
Because of a direct wide band gap (3.37 eV) and a large exciton binding energy (60 mV),1 ZnO is one of the most promising materials for the fabrication of optoelectronic devices operating in the blue and ultraviolet regions.2 It has also been investigated as a transparent conducting3 and piezoelectric material in solar cells,4 electrodes,5 and sensors.6 To enhance n-type conduction, ZnO nanobelts were frequently doped with some dopants such as Ga, In, Sn.7 But p-type doping in ZnO is essential for device applications. Up to now, the obtainment of p-type ZnO thin films by doping Sb element has been reported,8-10 indicating that Sb is a good candidate as a p-type dopant in ZnO. Currently, the studies about fabrication of Sbdoped ZnO nanowires have also begun to appear in the literature.11 However, there has been no report about Sb-doped single-crystalline ZnO nanobelts with single-side zigzag boundaries.
Sb-doped ZnO nanobelts were synthesized by thermal evaporation with Au catalyst. The mixture of Zn, Sb2O3, and C powders with the mole ratio of 20:1:4 was placed in an Al2O3 boat inside a quartz tube as the evaporation source. A silicon substrate coated with 10 nm of Au was then positioned on the top of the source boat. Ar was used as the arrier gas, and O2 was the reaction gas. The flow rates of Ar/O2, the growth time, and the temperature were maintained as 300 sccm/3 sccm, 25 min, and 880 °C, respectively. After the reaction, a white-colored filmlike product was obtained on the Au-coated area of the substrate. The morphology and structure of the synthesized product were characterized using field emission scanning electron microscopy (FE-SEM, LEO1530, Japan), X-ray diffraction (XRD, D/MAX-RB), high-resolution transmission electron microscopy (HRTEM, JEOL-2010, Japan), and select area electron diffraction (SAED). Energy dispersive X-ray spectroscopy (EDS) was used to characterize the Sb content. The Raman backscattering measurements were performed at room temperature using a 200-mW Ar+ laser at 514 nm as the excitation source. Photoluminescence (PL) spectra of the synthesized products were taken at the room-temperature using the 325 nm line of a He-Cd laser as the excitation source.
In general, ZnO nanobelts usually grow along the c axis, and the side surfaces are {01j10} and/or {21j1j0} because of their lower energies than those of (0001).12 The ZnO nanobelts with the 〈01j10〉 growth direction were obtained by doping In or Sn ions in the growth process.13,14 Moreover, the ZnO nanobelts along the [21j1j0] growth direction were also synthesized.15 However, the single crystalline ZnO nanobelts with the other growth directions have not been reported. Herein, we report the synthesis of Sb-doped ZnO nanobelts with single-side zigzag boundaries growing along [022j1] direction with an Au catalyst. High-resolution transmission electron microscopy was applied to investigate the structures of the nanobelts. The microstructure characterization and the optical property of the nanobelts were also investigated by the Raman scattering and the photoluminescence spectra, respectively. * To whom correspondence should be addressed. E-mail: yuezhang@ ustb.edu.cn. † Department of Materials Physics, University of Science and Technology Beijing. ‡ State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing.
Results and Discussion The representative morphologies of the as-synthesized sample are revealed by FE-SEM in Figure 1a. These nanobelts have a width of about 50-100 nm and a length up to a few tens of micrometers. It is noticed that most of the nanobelts have zigzag side surfaces. The gold catalyst particles were clearly observed at the ends of the nanobelts, suggesting that the nanobelts may be formed by a vapor-liquid-solid (VLS) process.16 However, because of Zn powders used in the experiment, they can produce a very high vapor pressure at the high synthesis temperature (880 °C), facilitating the VS growth of the nanobelts.17 Thus the growth of the nanaobelts should be dominated by the VS process due to a result of high Zn vapor pressure. The phase of the as-grown nanobelts is identified by XRD. The result indicates that the nanobelts have the wurtzite structure, and no secondary
10.1021/jp8064213 CCC: $40.75 2008 American Chemical Society Published on Web 10/24/2008
Sb-Doped ZnO Nanobelts with Single-Side Zigzag Boundaries
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17917
Figure 1. (a) FE-SEM image of the Sb-doped ZnO nanobelts with single-side zigzag boundaries. (b) XRD pattern taken from the Sbdoped ZnO nanobelts.
phase was found from the XRD spectrum (Figure 1b). As compared to the standard ZnO powders (a ) 0.3246 nm and c ) 0.52 nm, Joint Committee on Powder Diffraction Standards (JCPDS) card number 36-1415), the Sb doping in ZnO causes significant change in lattice constants, resulting in measurable lower angle shift (about 0.05°) in the (100) and (101) peaks. The structure of the Sb-doped ZnO nanobelts was further characterized by TEM. Figure 2a shows a TEM image of an individual Sb-doped ZnO nanobelt and its corresponding SAED pattern in the inset, which indicates that the nanobelt has wurtzite ZnO structure and grows along the [022j1] direction. It is interesting that the nanobelt has periodic sharp contrast lines corresponding to planar defects on the (0001) and (022j1) planes. The HRTEM images recorded from the white rectangle zone are shown in Figure 2b. The measured d spacing of 0.266 nm between adjacent lattice planes corresponds to the distance between two (0002) crystal planes, which is slightly larger than that in pure ZnO (0.26 nm). It can be clearly seen that the nanobelt contains two planar defects, which are on the (0001) and (022j 1) planes, respectively. The angle between the two planar defects was measured to be about 76°, which is consistent with the angle relationship between the (0001) and (022j1) planes in the SAED pattern. It is observed in TEM that some thick nanobelts have more obvious zigzag side surfaces and that the planar defects on the (0001) and (022j1) planes are pushed to the side surfaces (inset in Figure 2c), suggesting that the growth of the nanobelts may be controlled by these planar defects. EDS measurement confirms that the average Sb content in the nanobelts, defined as Sb/(Zn+Sb) atomic ratio, reaches 8 at % (Figure 2c). To understand the origin of the novel single-side zigzag boundaries in the nanobelts, the morphology change with the concentration of the Sb element in the nanobelts was investigated. We found that most of the nanobelts had the single-side zigzag boundaries and wurtzite ZnO structures in the condition
Figure 2. (a) A low-magnification TEM image of a single Sb-doped ZnO nanobelt. The inset shows the corresponding SAED pattern of the nanobelt. (b) The HRTEM image from the rectangle zone in part a. (c) EDS of a Sb-doped ZnO nanobelt. The Cu peaks come from the surface of Cu grids used for TEM measurement. The inset is a lowmagnification TEM image of a single Sb-doped ZnO nanobelt with single-side zigzag boundaries.
with the mole ratio of 20:1:4 among Zn, Sb2O3, and C powders. However, the single-side zigzag boundaries and wurtzite ZnO structures can not be observed in the nanobelts when the mole ratio among these powders is up to 10:1:4. Moreover, when the concentration of the Sb element largely decreased (the mole ratio of 40:1:4), the single-side zigzag boundaries also disappeared in the nanobelts. These results indicate that the proper concentration (8%) of the Sb element is responsible for the formation of the single-side zigzag boundaries in the nanowires. The periodic zigzag boundaries correspond to the periodic planar defects. Since the radius of Sb3+ (0.078 nm) is much larger than that of Zn2+ (0.074 nm), the introduction of Sb3+ ions should cause a large structural strain. By combination of the reported similar planar defects in Sn-doped ZnO nanobelts,14 the release of the structural strain could result in the formation of the planar defects in this study. As is known, under thermodynamic equilibrium, the surface energy of the {0001} planes in ZnO is higher than those of {1j100} and {112j0} planes. So the ZnO nanobelt growth along the c axis is therefore preferred over other directions. However, such thermodynamic equilibrium could be disturbed by incor-
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Figure 3. Raman spectrum of (a) pure ZnO nanobelts grown on a silicon substrate and (b) Sb-doped ZnO nanobelts with single-side zigzag boundaries grown on a silicon substrate.
porating impurities into the raw material. The planar defects on the (0001) planes are considered to be the key factor for the growth of the Sn- and In-doped ZnO 〈01j10〉 nanobelts.13,14 However, in our experiment, the planar defects on (0001) planes did not independently induce the growth of the nanobelts along the [01j10] direction due to the appearance of the planar defects on (022j1) planes. So, we consider that both the planar defects on (0001) and (022j1) planes are responsible for the novel growth direction [022j1] of the Sb-doped ZnO nanobelts. At the beginning of the growth in the nanobelt, the planar defects on (0001) planes induce the growth of the nanobelt along the [01j10] direction, which is in accordance with the literature.13,14 In the growth process, due to the appearance of the planar defects on (022j1) planes, the nanobelt grows preferentially in the [0j110] direction but repressively in the direction perpendicular to the (022j1) planes. Hence, the two different growth states will reach a dynamic balance by minimization of the structural strains, resulting in highly elongated nanobelt along the [022j1] direction. Correspondingly, the nanobelts also grows preferentially in the direction parallel to the (022j1) planes but repressively in the direction perpendicular to the (0001) planes. So, the growth along the width direction of the nanobelts is periodically induced by the periodic planar defects, resulting in the formation of the zigzag side surfaces. Each zigzag structure corresponds to a couple of planar defects, which are pushed to the edge of the zigzag side surfaces as the nanobelt grows (inset in Figure 2c). The nanobelts are all accompanied by the periodic planar defects. To investigate the structure characterization of Sb-doped ZnO nanobelts with single-side zigzag boundaries further, the Raman backscattering measurements were carried out. Figure 3a shows the Raman spectrum of the pure ZnO nanobelts. The peaks at 99, 204, 332, 380, and 439 cm-1 correspond to the E2L, 2E2L, E2H-E2L, A1T, and E2H modes of the host ZnO modes,18,19 respectively. The peak at 520 cm-1 is attributed to the Si substrate. Figure 3b shows the Raman spectrum of the Sb-doped ZnO nanobelts. Correspondingly, the peaks at 97, 204, 332, 380, and 437 cm-1 correspond to the E2L, 2E2L, E2H-E2L, A1T, and E2H modes of the ZnO modes, respectively. As a comparison with the Raman spectrum of the pure ZnO nanobelts, the peak at 380 cm-1 is depressed and hardly observed in the Sb-doped ZnO nanobelts. For ZnO materials where close-packed layers of Zinc and oxygen are stacked alternately along the c axis, the lattice irregularity such as oxygen defects or the dopant atoms along the c axis would directly affect the displacement of ions in the A1T modes, which leads to a quick decrease in the intensity of A1T modes.20 In our experiment, both the O vacancies and the Sb dopant atoms are responsible for the depressed A1T mode of ZnO. The additional peak at 716 cm-1
Yang et al.
Figure 4. PL spectra of pure ZnO and Sb-doped ZnO nanobelts measured at room temperature.
is not well understood. However, it is reasonably here concluded that the additional peak is induced by the Sb doping in ZnO because the peak does not origin from the Si substrate, and it also does not belong to the modes of the pure ZnO nanobelts (Figure 3a). Figure 4 shows the room-temperature PL spectra recorded from the as-deposited pure ZnO and Sb-doped ZnO nanobelts. There are obvious differences between the PL spectra of doped and undoped ZnO nanobelts. Before doping, the pure ZnO exhibit the peak at 3.27 eV corresponding to the near band edge (NBE) peak that is responsible for the recombination of free excitons of ZnO.21,22 After doping with Sb, the new NBE peak at 3.15 eV could be attributed to the emission of heavily Sbdoped ZnO nanobelts and broadened seriously as well as redshifted strongly to lower energy as compared to the emission of pure ZnO. The broadening of the NBE peak can be interpreted by the formation of band tailing in the band gap, which is induced by the introduction of an impurity into the semiconductor. The red-shift of the NBE peak should be due to the narrowing of Eg. In particular, the Sb-doped ZnO nanobelts have a strong green emission band around 2.37 eV that originates from the recombination of holes with the electrons occupying the singly ionized O vacancies.23 Conclusions In summary, Sb-doped ZnO nanobelts with single-side zigzag boundaries were prepared by a simple chemical vapor deposition method. Each nanobelt includes two types of periodic planar defects, which are located on the (0001) and (022j1) planes, respectively. The growth of the nanobelts is dominated by both the two planar defects. The Raman A1T mode of ZnO is depressed in Sb-doped ZnO nanobelts and the Sb doping in ZnO induces the appearance of the additional peak at 761 cm-1. The PL of the doped ZnO nanobelts show the NBE peak has redshifted as well as broadened seriously as compared to that of pure ZnO because of the heavy doping of Sb. Acknowledgment. This work was supported by the National High Technology Research and Development Program of China (No. 2006AA03Z351), the Major Project of International Cooperation and Exchanges (No. 50620120439, 2006DFB51000), and the National Basic Research Program of China (No. 2007CB936201). 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) Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, I.; Koumoto, K. AdV. Mater. 2002, 14, 418.
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