Oct 13, 2009
Melting gold nanoparticles act as versatile catalyst
The extreme surface area to volume ratio of nanoscale particles allows them to melt at reduced temperatures, giving a single-element liquid catalyst for vapour-liquid-solid growth of ZnO nanowires. This mechanism provides a liquid catalyst independent of the substrate material, paving the way for nanowire growth on flexible, conductive, and low-cost substrates.
Semiconductor nanowires, in particular those made from wide-bandgap materials such as ZnO, have shown technological promise in a wide variety of areas, including large-area solar cells, light emitting diodes (LEDs), and lasers. These nanowires are often grown using the vapour-liquid-solid (VLS) mechanism, which typically relies on a liquid eutectic formed at the interface between a gold catalyst and a silicon substrate. However, because silicon from the substrate is necessary for the catalyst to liquefy, current VLS growth processes can be difficult to apply onto other substrates.
Researchers in the Narayanamurti Group at Harvard, US, have been studying methods for generalizing nanowire growth to arbitrary substrates including flexible, conductive, and low-cost materials. Recently, the team has demonstrated robust growth of ZnO nanowires on metal foil substrates using ultrasmall gold catalyst particles. The approach takes advantage of the nanoscale size-dependent melting point reduction of ultrasmall catalysts to provide a liquid growth site without any substrate interaction. In the case of gold, the bulk melting temperature of 1064 degC is reduced to 750-800 degC as the particle size shrinks to 5 nm. This reduced melting temperature allowed the researchers to grow ZnO nanowires on both Ti and Mo foil substrates.
This technique is expected to suit a variety of materials systems, catalysts, and substrates, and could have significant technological applications because metal foil substrates provide a lower-cost, higher-conductivity material for devices such as solar cells and light emitting diodes.
The researchers presented their work in the journal Nanotechnology.
About the authors
This work was conducted at Harvard University, US, and was supported by NSF/NNIN through the use of facilities at Harvard's Center for Nanoscale Systems (CNS). Eric Petersen is currently a PhD student at Columbia University. Edward Likovich is a PhD student at Harvard, and his research is supported by a U.S. Dept. of Homeland Security Fellowship. Kasey Russell is now a Post-Doctoral Researcher in the Evelyn Hu group at Harvard. Venkatesh Narayanamurti is the former Dean of the Harvard School of Engineering and Applied Sciences and is now a Professor of Technology and Public Policy at Harvard.