Selective growth and kinetic study of copper oxide nanowires from patterned thin-film multilayer structures

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Selective growth of CuO nanowires on the etched face of Al2O3/Cu/Al2O3 thin-film multilayer patterns was achieved by ambient oxidation at 400 °C. The nanowires were observed to selectively grow only from the pattern edge with diameter limited by the thickness of Cu thin film. Transmission-electron-microscopy (TEM) characterization confirmed CuO nanowires of a monoclinic CuO growing in the [010] crystallographic direction. Nanowire growth kinetics was studied at 400 °C for different cumulative growth durations with initial growth rates of ∼1 nm/min. A base growth mechanism with kinetics limited by oxygen diffusion through defects of a scaling oxide film is consistent with observed kinetics. The oxygen diffusivity is found to be ∼10−11 cm2/s, consistent with the grain-boundary diffusion of oxygen through polycrystalline copper oxide.

I. INTRODUCTION

Nanowires of a wide variety of materials have numerous applications, including chemical sensors,1 electronic devices and interconnects,2,3 and etch masks.4 Realizing many of these applications requires development of techniques to control the location and diameter of nanowires for large-scale integration.5–8 An important specific challenge is to use conventional microfabrication techniques that result in nanowires suspended between photolithographically defined patterns.9–13 Required is precise control of the CNT/nanowire diameters. One interesting application is to use such suspended nanowire networks as shadow masks that can result in nanometer-scale gaps for molecular electronics.14–16 A major difficulty with CNTbased shadow mask lithography is the tendency of the nanowires to be highly curved as compared to the singlecrystalline nanowires. It is thus desired that growth of oxide nanowires be controlled in a suspended integrated architecture. Nanowire synthesis by variety of methods is widely reported in the literature, with significant focus on synthesis of semiconducting and oxide nanowires.17 Toward this goal, oxides of copper are very appealing due to their ease of synthesis and conductive properties.18–21 Since the 1950s, there has been significant research in the growth of copper oxide whiskers and nanowires.22–26 This includes solution-based growth,27 template-based

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0377 J. Mater. Res., Vol. 22, No. 10, Oct 2007

growth,28 and thermal oxidation of Cu29,30 and Cu2S.31 The thermal oxidation process is remarkably simple, where thick copper substrates [Cu transmission electron microscopy (TEM) grids, Cu foils, etc.] are heated to 400–600 °C in ambient air to grow CuO whiskers/ nanowires that are micrometers long and with submicrometer diameters (∼100–200 nm).24 The thick copper substrates32 effectively provide a large excess of copper during oxidative growth. But CuO nanowire growth from thin films of Cu has yet to be demonstrated. Earlier reports predict copper oxide nanowire growth supported by vapor–solid (V–S) mechanism involving the oxidation of copper metal.33–

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