An image of a nanocrawler, a horizontal nanowire that crawls along a graphene surface. (Image courtesy of Carnegie Mellon University.)
Electronics are getting smaller all the time, and their shrinkage necessitates ever smaller wires to carry electrical current. Nanowires, wires that are a thousand times slimmer than a human hair, enable current to flow in useful amounts at the increasingly small scales that we’re achieving.
A team of engineering researchers recently made a novel discovery about nanowires purely by accident: the team created nanowires that “crawled” along the surface of graphene film. These so-called nanocrawlers were surprising because the team was expecting the nanowires to grow vertically out of the graphene surface.
“We looked at the results, and they were kind of mind-blowing,” said Tzahi Cohen-Karni, a professor of materials science and engineering at Carnegie Mellon University who was involved in the research. “We set forth to understand why the nanowires crawled on the surface.”
An illustration of the process used to grow the nanowires and nanoparticles. The fraction of vertical nanowires to horizontal nanocrawlers (C) is controlled with hydrogen chloride gas: (i) HCl supplied through entire growth, (ii) HCl introduced after a 15 s delay during nucleation, (iii) No HCl. (Image courtesy of Nano Letters).
The team set out to grow germanium nanowires (GeNWs) on single-layer graphene surfaces, using a gold nanoparticles (AuNPs) catalyzed vapour-liquid-solid (VLS) growth mechanism. The process begins with a layer of copper foil that synthesizes a continuous monolayer graphene by a process called low pressure chemical vapour deposition (LPCVD). Then, a 2 nm gold film is evaporated on the graphene, and an annealing process breaks the film into AuNPs. Finally, in the growth and nucleation stage, GeNWs are synthesized by the VLS mechanism.
But in addition to the vertical GeNWs, the team found they had also synthesized horizontal nanocrawlers (GeNCs). By studying this result, the team determined they were able to control the amount of nanocrawlers on the graphene by introducing hydrogen chloride gas (HCl) during the nucleation phase of their experiment. By adjusting variables such as temperature and time lag before HCl introduction, the team could control the fraction of GeNCs compared to GeNWs.
Harvesting Light with Nanocrawlers
The team’s discovery is exciting because of the potential application of nanocrawlers as photodetectors. Nanocrawlers could prove to be much more sensitive to light – that is, more useful in converting it to electricity – than the vertical nanowires the team set out to create.
“Photodetectors made with nanocrawlers could potentially detect smaller amounts of light than their vertical nanowire counterparts,” said Cohen-Karni.
To learn more about the discovery, you can access the team’s paper in Nano Letters.
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