Apr 19, 2010
Semiconductor nanowires get in order
Semiconductor nanowires – quazi one-dimensional single crystals composed of elemental or compound semiconductors – have emerged as promising materials for fundamental studies in quantum-confined systems and applications in nanophotonics, electronics and energy sectors. A variety of proof-of-concept nanowire devices have been demonstrated, including transistors, light-emitting diodes, lasers and photovoltaic components. However, major challenges remain in controlling nanowire properties such as size, position and crystalline structure on the nanometre scale. This is mostly due to challenges in synthesizing both position- and diameter-controlled metal nanoparticles, which are used as seeds for the nanowire growth.
Researchers at MIT have developed a simple and efficient electrochemical process that combines a galvanic reaction and electron-beam lithography to selectively synthesize small-diameter gold nanoparticles, which are then used for growing ordered nanowire arrays with pre-defined diameter and position.
A unique feature of the synthesis technique is the spontaneous formation of localized etch pits during the galvanic reaction. The localized substrate etching, which the scientists observed exclusively on the lithography-defined surfaces, indicates that the substrate material is locally consumed during the galvanic reaction. By using electron and atomic force microscopy analysis, the group concludes that the etching is achieved under a particular condition: sufficient metallic ions are supplied on a limited area and the produced oxides are efficiently dissolved to maintain the reaction continuously on the semiconductor substrate.
Interestingly, the pits prevent gold diffusion during the high-temperature growth of nanowires, which allows the formation of well ordered and epitaxial nanowire arrays. The team's process is particularly important for the controlled synthesis of small-diameter (<20 nm) nanowires for which other catalyst-patterning and deposition combinations fail to provide necessary diameter and position control.
More broadly, galvanic processing coupled with electron beam lithography can facilitate the fabrication of other metal nanostructures for applications in nanoplasmonics or molecular electronics, among others. The researchers are currently exploring other types of lithography techniques for high-throughput synthesis of robust metal nanoparticle arrays.
The group presented its work in the journal Nanotechnology.
About the author
Silvija Gradecak is an assistant professor in the Department of Materials Science and Engineering at MIT. Her group is interested in the unique material properties of nanomaterials. The team combines synthesis, spectroscopic techniques, transport measurements and advanced electron microscopy techniques to directly correlate structural and physical properties on the nanometre scale.