Quantitative Investigation of the Factors Affecting the Hydrothermal Growth of Zinc Oxide Nanowires
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1174-V11-08
Quantitative Investigation of the Factors Affecting the Hydrothermal Growth of Zinc Oxide Nanowires Aron R. Rachamim1, Sharvari H. Dalal1, Sieglinde M.-L. Pfaendler1, Michael E. Swanwick1, Andrew J. Flewitt1 and William I. Milne1. 1
Electrical Engineering Division, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, U.K. ABSTRACT Zinc oxide (ZnO) nanowires (NWs) are receiving significant industrial and academic attention for a variety of novel electronic, optoelectronic and MEMS device applications due to their unusual combination of physical properties, including being optically transparent, semiconducting and piezoelectric. Hydrothermal growth is possible at significantly lower temperatures (and hence lower thermal budgets) compared with other NW growth methods, such as chemical vapour deposition. In this context, the hydrothermal growth of ZnO NWs on seeded substrates immersed in equimolar zinc nitrate/HMTA aqueous solution was investigated. NWs were grown on polished silicon (001) substrates, and the solution concentrations, temperatures and growth times were varied. Importantly, the NW diameter was found to depend only on concentration during hydrothermal growth for times up to 4 hours. The average diameter was 14 nm in 0.005 M solution and increased up to a maximum 150 nm at 0.07 M, when the NWs formed a continuous polycrystalline film. Concentration and temperature were all found to affect the axial growth rate of NWs in the [0001] direction. The growth rate was constant up to 4 hours (200 nm hr-1) for constant conditions (81 oC, 0.025 M). The growth rate was found to increase approximately linearly with concentration at a rate of 7840 nm M-1 hr-1 up to 0.06 M (81 oC solution). The growth rate also increased linearly with temperature at a rate of 4.9 nm hr-1 K-1 (0.025 M solution). This indicates that growth takes place close to the equilibrium point, found by linear regression to be 36 oC for 0.025 M solution. INTRODUCTION It has been reported that ZnO nanostructures have a higher piezoelectric constant than bulk material [1,2]. ZnO is also a wide bandgap (3.37 eV) II-VI semiconductor with a large exciton binding energy (60 meV). Therefore, it has been investigated in the fabrication of shortwavelength light emitting diodes [3], varistors and transparent semiconductors. ZnO NWs have also been shown to exhibit lasing [4] and have been investigated for application in solar cells [5]. A low temperature hydrothermal growth process is advantageous for commercial manufacture because of its low thermal budget, and hence low cost compared to other growth techniques. Understanding the hydrothermal growth process, so as to exert control over the morphology of grown NWs, is therefore essential in order to commercialize functioning devices that exploit fully the physical properties of ZnO NWs. In this context, the effects of modifying the hydrothermal growth parameters (substrate, solution concentration, growth time and temperature) were investigated.
EXPERIMENTAL DETAILS The hydrothermal g
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