Deterministic Synthesis of ZnO Nanorods
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Deterministic Synthesis of ZnO Nanorods Y. W. Heo1, V. Varadarajan1, M. Kaufman1, K. Kim1, F. Ren2, P. H. Fleming3, and D. P. Norton1 1 Department of Materials Science and Engineering, University of Florida, P.O. Box 116400, Rhines Hall, Gainesville, FL 32606 2 Department of Chemical Engineering, University of Florida, Gainesville, FL 32606 3 Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831 ABSTRACT The deterministic growth of ZnO nanorods using molecular beam epitaxy is reported. The process is catalyst-driven, as single crystal ZnO nanorod growth is realized via nucleation on Ag islands that are distributed on a SiO2-terminated Si substrate surface. Growth occurs at substrate temperatures on the order of 300-500˚C. The nanorods exhibit diameters of 15-40 nm and lengths in excess of 1 µm. Nanorod placement can be predefined via location of metal catalyst islands or particles. This, coupled with the relatively low growth temperatures needed, suggests that ZnO nanorods could be integrated on device platforms for numerous applications, including chemical sensors and nanoelectronics. INTRODUCTION The synthesis of nanoscale materials in the form of superlattices (2-D), nanowires (1-D) and nanodots (0-D) has become a topic of great interest in recent years [1]. Among the materials for functional nanodevices are semiconductors. Various means have been reported for the synthesis of semiconducting nanowires and nanorods [2-4]. Nano-device functionality has been demonstrated in the form of electric field-effect switching [5], single electron transistors [6], biological and chemical sensing [7], and luminescence for 1-D semiconducting structures [8]. Also of interest are semiconducting oxides, including Ga2O3 [9], In2O3 [10], and ZnO [11-12]. Zinc oxide is particularly interesting. ZnO has a hexagonal (wurtzite) crystal structure with a = 3.25 Å and c = 5.12 Å. Each Zn atom is tetrahedrally coordinated to four O atoms, where the Zn d-electrons hybridize with the O p-electrons. Layers occupied by zinc atoms alternate with layers occupied by oxygen atoms. ZnO is an n-type, direct bandgap semiconductor with Eg = 3.35 eV [13,14]. Electron doping via defects originates from Zn interstitials in the ZnO lattice. The intrinsic defect levels that lead to n-type doping lie approximately 0.05 eV below the conduction band. The room temperature Hall mobility in ZnO single crystals is among the highest for the oxide semiconductors, on the order of 200 cm2 V-1sec-1. The bandgap of ZnO can be modulated via Mg cation substitution, allowing for the formation of quantum-confined structures. The exciton binding energy is on the order of 60 meV in ZnO, yielding efficient luminescence at room temperature. Various applications would be attractive using ZnO nanorod materials. ZnO is routinely investigated as a gas sensor material based on the near-surface modification of charge distribution with certain surface-absorbed species [15]. The high surface-to-volume ratio in ZnO nanorods would provide significant enh
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