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Controlled Catalytic Growth And Characterization of Zinc Oxide Nanopillars on A-Plane Sapphire

Using the vapor-liquid-solid (VLS) technique, we have grown well-aligned nanopillars on sapphire (a-plane) substrates at atmospheric pressure in a horizontal tube furnace employing gold catalyst seeds of different sizes and densities. It was the aim of the present work to find experimental conditions (source and template temperatures, temperature gradients, carrier gas flow, gold cluster size and density) under which controlled catalytic growth of nanopillars takes place. The VLS process is expected to result in a correlation of the zinc oxide (ZnO) pillar diameters with the gold catalyst cluster size.

This is indeed found to hold true except for very small gold clusters. A minimum value of the pillar diameter of about 20 nm on a-plane sapphire is obtained in our experiments, which apparently represents a general limit. Structure characterization relies on high-resolution x-ray diffraction, atomic force microscopy, and high-resolution scanning electron microscopy. Electronic characterization is done by Raman and standard luminescence measurements with large area (≈1 mm2) excitation. In particular, we have also studied individual nanopillars by spatially highly resolved cathodoluminescence spectroscopy. We find that luminescence averaged over large areas can be dominated by features which are not typical for well-shaped pillars but come from regions of irregular ZnO growth.


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