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Chemical reactions can be easily squizzed and manipulated into a space with size down to 30nm, smaller than one millionth of table tennis ball. Researchers at Stanford University, California, have recently achieved unprecedented spatial control over growth of semiconductor nanowires and carbon nanotubes taking advantage of novel optical properties of metallic nanoparticles. (published in the most recent issue of Nano Lett.)
Metallic nanoparticles, especially noble metals like gold, silver and copper, can support light-induced surface plasmon-polaritons (SPPs), or collective electron oscillations. SPPs are electromagnetic ("light") waves that propagate along metal-dielectric interfaces and are coupled to the free electrons in the metal. When illuminated with an electromagnetic waves matching the surface plasmon, called as surface plasmon resonance (SPR), local electromagentic field at the proximity of metal nanoparticles and absorption of the particles to the light will be dramatically enhanced. Most of the absorbed energy will be subsequently converted to heat through a procedure called as plasmon damping (Landau damping ).
As metal nanostructures are used widely as catalysts in the chemical industry as well, the team, led by Mark L. Brongersma, an assistant professor affiliated with Department of Materials Science and Engineering at Stanford, has envisioned a golden opportunity to couple plasmonics and catalysis seeking a new pathway to control chemical reaction.
To find out, Brongersma and his colleagures put an assemly of gold nanoparticles into a flow of source gas, and illuminated the nanoparticles by a laser with power intensity carefully-controlled. The wavelenght of the laser (532nm) is chosn to be compatible with SPR absorption of the particles. They shown that the growth of silicon and germanium nanowires (NWs) and carbon nanotubes (NTs) can be initiated and confined at nanoscale-sized space and down to single NW or NT level. Neverthess, the growth can be positioned at arbitrarily specififed location moving the laser spot. Surprisingly, the laser power needed to initiate the growth (normally at ~500 degree C) is only at few milliwatt. "The strong, resonantly enhanced absorption by metallic nanostructures enables such efficient local heating that a low power laser pointer provides sufficient power to locally generate hundreds of degrees of temperature change." said Mark L. Brongersma.
As well as performing experiments, Brongersma and his team modelled the photothermal energy-conversion and heat conduction process in detail. The researchers came up with an result that indicates the heat generated by this techinique is highly confined into the illuminated area and the onset of heating or cooling can be finished in a scale of 1 ns (10-10 s ). "That means we are able to grow nanowires or nanotubes directly in devices architecture to make a nanodevices, and would be able to grow those materials in a controlled way monolayer by monlayer ", said Linyou Cao, a graduate student at Stanford and leading author of the paper. Most nanowires and nanotubes are currently grown in a globally heated furnace. Such procedures can damage pre-existing device structures, and hence device fabrication typically requires laborious post-growth processing.
" We anticipate that the versatility and simplicity of the technique will result in its broad adaptation by many researchers and engineers that require a nanoscale heating strategy", told Mark L. Brongersma to nanotechweb.org, " In general, the successful demonstration of high spatial and temporal control over nanoscale thermal environments inspires new pathways for manipulating a range of important thermally-stimulated processes and the development of novel photothermal devices. "
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