Growth and Characterization of ZnO Nanowires
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Growth and Characterization of ZnO Nanowires Jason B. Baxter, Ron E.M.W. Bessems and Eray S. Aydil Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, U.S.A. ABSTRACT Single crystal ZnO nanowires were grown by chemical vapor deposition using monodisperse 5 nm or 20 nm diameter gold nanoparticle catalysts to control the nanowire diameter and location. The nanowires reach several microns in length and grow only from the gold nanoparticles. The nanowires have narrowly dispersed diameters, albeit significantly larger than the diameter of the gold particles used for catalyzing the growth. The nanowires grow in the [ 10 1 0 ] or [ 10 1 1 ] directions normal to the lowest energy planes in ZnO. ZnO nanowires emit in the near ultraviolet region of the electromagnetic spectrum upon excitation with highenergy photons or electrons. Electron diffraction and absence of luminescence associated with oxygen vacancies indicate high quality crystalline ZnO nanowires. Cathodoluminescence emission along the entire length of the wire is consistent with a lack of non-radiative recombination sites associated with defects, lending further support for the high quality of these nanowires. INTRODUCTION ZnO is wide band gap semiconductor that can potentially enable a wide range of electronic and optoelectronic applications including diodes and lasers emitting in the ultraviolet (UV) region of the electromagnetic spectrum. In particular, ZnO has a band gap of 3.37 eV and exciton binding energy of 60 meV, making it well suited for room temperature UV lasers. In fact, lasing has already been reported from particles, thin films, and nanowires [1,2]. ZnO nanowires and nanorods have been grown using a variety of methods including vapor phase transport of Zn vapor generated by carbothermal or hydrogen reduction of ZnO [3,4], and metalorganic chemical vapor deposition [5]. In some vapor phase growth methods, gold thin films or gold nanoparticles were used to catalyze the growth of ZnO nanowires [3]. Thin gold films do not wet substrates such as Si and form nanometer sized islands. When gold is used as a catalyst, the nanowire growth is thought to proceed through the vapor-liquid-solid (VLS) mechanism [6] which is the accepted mechanism for growing other semiconducting nanowires such as Si and Ge [7]. In gold catalyzed growth of Si nanowires, a vapor source such as SiH4 gas dissociates on the gold surface to form a low temperature (~363 oC) Si-Au eutectic mixture. Supersaturation of the gold nanoparticle with Si results in nucleation of the Si nanowire from this mixture followed by axial growth through addition of Si to the wire at the Au-Si interface. In the case of Si, Lieber et al. have shown that nanowire diameter can be controlled by using colloidal gold particles with very narrowly dispersed diameters [8]. Si nanowires with diameters slightly larger than the gold nanoparticles are obtained and nanowire diameter distribution is similar to that of the starting gold colloids. Whether ZnO nanowi
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