Patterns of Ensemble Variation of the Optical Properties of ZnO Nanowires Grown with Copper and Gold Catalysts

Understanding the source of variation of the optical properties within an ensemble of nanowires (NWs) is an important step toward fabrication of NWs with more uniform and better-controlled properties. This in turn will facilitate the development of complex architectures for device applications. Analysis of gold- and copper-catalyzed zinc oxide (ZnO) NWs grown in a high temperature tube furnace on a sapphire substrate shows that the structural and optical properties vary with NW growth sites on the substrate. Results show that there are systematic changes in the optical properties of the ZnO NWs from the center to the edge of the substrate, and also from "upstream" to "downstream" along the gas flow direction, implying changes in the availability of the source material and catalyst over the substrate surface during the growth. Photoluminescence microscopy has been used to map out these changes and unravel the growth patterns. We observe two distinct trends that are labeled the "edge" trend and the "gas flow" trend, respectively, and are linked to two growth phases. The first phase starts while the growth tube temperature and gas flow have not yet reached their final steady states. In this phase, due to the limited availability of the source material in gas phase, only metal nanodroplets that are located at the substrate edges have the advantage of early growth. While the edge NWs continue their growth, the second phase starts when the growth tube temperature and gas flow reach reasonable stabilities. Our results show that these two phases are more pronounced in the case of copper-catalyzed NWs. In the first phase, NWs have a better chance to grow at the substrate edges. In the second phase, copper diffuses downstream, causing an interesting variation in the optical properties of NWs. Numerous morphology examinations of NWs show that the variation in emission is related to the change in the ratio of the surface area to the bulk volume of the NWs within an ensemble of NWs. The patterns of the ZnO NW optical properties can be used as an indicator to follow the growth patterns of the NWs across the substrate.