In Situ Synchrotron X-ray Absorption Experiments and Modelling of the Growth Rates of Electrochemically Deposited ZnO Na
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In Situ Synchrotron X-Ray Absorption Experiments and Modelling of the Growth Rates of Electrochemically Deposited ZnO Nanostructures Bridget Ingham1,2, Benoit N. Illy3, Jade R. Mackay4, Stephen P. White1, Shaun C. Hendy1,4, and Mary P. Ryan3 1 Industrial Research Limited, Lower Hutt, New Zealand 2 Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA, 94025 3 Department of Materials, Imperial College London, London, SW7 2BP, United Kingdom 4 MacDiarmid Institute, School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand ABSTRACT ZnO is known to produce a wide variety of nanostructures that have enormous scope for optoelectronic applications. Using an aqueous electrochemical deposition technique, we are able to tightly control a wide range of deposition parameters (Zn2+ concentration, temperature, potential, time) and hence the resulting deposit morphology. By simultaneously conducting synchrotron x-ray absorption spectroscopy (XAS) experiments during the deposition, we are able to directly monitor the growth rates of the nanostructures, as well as providing direct chemical speciation of the films. In situ experiments such as these are critical to understanding the nucleation and growth of such nanostructures. Recent results from in situ XAS synchrotron experiments demonstrate the growth rates as a function of potential and Zn2+ concentration. These are compared with the electrochemical current density recorded during the deposition, and the final morphology revealed through ex situ high resolution electron microscopy. The results are indicative of two distinct growth regimes, and simultaneous changes in the morphology are observed. These experiments are complemented by modelling the growth of the rods in the transport-limited case, using the Nernst-Planck equations in 2 dimensions, to yield the growth rate of the volume, length, and radius as a function of time. INTRODUCTION ZnO is a material that has been the subject of great interest in recent years. As a bulk semiconductor it has a band gap of 3.4 eV that is conducive to optical applications. It forms a wide variety of nanostructures which can further tune the band gap and electronic properties, making ZnO nanostructured films a promising avenue for many optoelectronic applications such as sensors [1-2], optoelectronic devices [3-4] and piezoelectronic devices [5]. ZnO nanostructured films can be fabricated using a variety of physical deposition techniques, such as thermal evaporation [6-7], sol-gel [8], metal-organic chemical vapour deposition (MOCVD) [9], molecular beam epitaxy (MBE) [10] and pulsed laser deposition [11]. Electrochemical deposition can also be used to produce ZnO nanostructured films, and has several advantages over physical deposition techniques. It is a well-established technology with the possibility of scaling the process for industrial production. It is a low-temperature processing technique. The growth rate and morphology of the ZnO nanostruct
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