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Nanoparticles carve straight paths on semiconductor surface

Posted: 04 Jan 2016 ?? ?Print Version ?Bookmark and Share

Keywords:NIST? nanochannel? semiconductor? LED? nanowire?

A team of scientists at the National Institute of Standards and Technology (NIST) and IBM has revealed that tiny particles of gold can function like snow blowers, which could sweep through surface layers of an important class of semiconductors to dig accurately straight paths. According to the researchers, the trenching capability is an important addition to the toolkit of nature-supplied 'self-assembly ' methods that they aim to harness for developing useful devices.

Foreseeable applications include integrating lasers, sensors, wave guides and other optical components into so-called lab-on-a-chip devices now used for disease diagnosis, screening experimental materials and drugs, DNA forensics and more. Easy to control, the gold-catalysed process for creating patterns of channels with nanoscale dimensions could help to spawn entirely new technologies fashioned from ensembles of ultra-small structures.

Preliminary research results that began as lemons, a contaminant-caused failure that impeded the expected formation of nanowires, eventually turned into lemonade when scanning electron microscope images revealed long, straight channels.

Surface-directed nanochannels

Electron micrograph of surface-directed nanochannels formed on the surface of the semiconductor indium phosphide. Nanochannels are formed using a gold-catalysed vapour-liquid-solid etch process and their locations are defined by the deposited gold pattern. Credit: Marti/JILA

"We were disappointed, at first," said NIST research chemist Babak Nikoobakht. "Then we figured out that water was the contaminant in the process, a problem that turned out to be a good thing."

That's because, as determined in subsequent experiments, the addition of water vapour served to transform gold nanoparticles into channel diggers, rather than the expected wire makers. Beginning with studies on the semiconductor indium phosphide, the team teased out the chemical mechanisms and necessary conditions underpinning the surface-etching process.

First, they patterned the surface of the semiconductor by selectively coating it with a gold layer only a few nanometres thick. Upon heating, the film breaks up into tiny particles that become droplets. The underlying indium phosphide dissolves into the gold nanoparticles above, creating a gold alloy. Then, heated water vapour is introduced into the system. At temperatures below 300°C, the tiny gold-alloy particles, now swathed with water molecules, etch nanoscale pits into the indium phosphide.

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