Optimising the Low Temperature Growth of Uniform ZnO Nanowires
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Optimising the Low Temperature Growth of Uniform ZnO Nanowires Nare Gabrielyan, Shashi Paul1 and Richard B.M Cross Emerging Technologies Research Centre, De Montfort University, Leicester, LE1 9BH, UK.
ABSTRACT Zinc oxide (ZnO) nanowires have been widely investigated and various different methods of their synthesis have been suggested. This work is devoted to the optimisation of the growth conditions for uniform and evenly distributed ZnO nanowire arrays. The nanowire growth process includes two steps: 1. Radio-frequency (RF) magnetron sputtering of a ZnO nucleation layer onto a substrate; 2. A hydrothermal growth step of ZnO nanowires using the aforementioned sputtered layer as a template. The optimisation process was divided into two sets of experiments: (i) the deposition of different thicknesses of the ZnO nucleation layer and the subsequent nanowire growth step (using the same conditions) for each thickness. The results revealed a strong dependence of the nanowire size upon the seed layer thickness and structural properties; (ii) the second set of experiments were based on growth solution temperature variation for the nucleation layers of the same thicknesses. This also showed nanowire size and distribution change with solution temperature variation. INTRODUCTION ZnO has numerous applications in optics, optoelectronics, sensors, solar cells, etc. due to its transparency, electrical conductivity, ferromagnetic, piezoelectric [1] and short-wavelength lightemitting properties [2]. And it is an important member of the II-VI group semiconductors with a direct band gap energy of ~3.37eV and strong exciton binding energy of 60 meV at room temperature. As the structural morphology is an important factor in determining the properties of the material, significant research has been focused on synthesizing different types of ZnO nanostructures such as nanowires, nanobelts, nanotubes and nanorods. Recently, devices based on nanostructured ZnO have been studied extensively such as room temperature lasers [3], gas sensors [4], transistors [5] and field emitters [6]. Furthermore, certain properties of ZnO, such as its nontoxicity, chemical stability, electrochemical activity, high electron transfer features and high isoelectric point (~9.5), render nanostructures of this material highly promising for applications such as biosensors. A number of methods for synthesising aligned one dimensional (1D) ZnO nanostructures have been reported previously. These include: the vapour–liquid–solid process (VLS) [7], metal– organic chemical vapour deposition (MOCVD) [8], thermal decomposition [9] and thermal evaporation [10]. Though there are a variety of the methods that have been developed, these often use metal catalysts to facilitate growth, which may cause unintentional doping of the nanostructures thereby affecting their physical properties. To negate this possibility, recent 1
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research has attempted to produce 1D ZnO structures using a two-step growth process [11]: i.e. th
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