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A new, simple technique to produce ordered arrays of metal nanotubes has been developed by scientists in Moldova. The method works by electroplating in conductive nanotemplates routinely fabricated by anodic etching of semiconductor substrates in salty water. The result could be important for making plasmonic devices, photonic crystals, catalysts for energy conversion and chemical and biological sensors. 

 

Ion Tiginyanu and colleagues from the National Center for Materials Study and Testing have developed a cost-effective and environmentally friendly nanofabrication approach for making semiconductor nanotemplates.

The researchers showed that anodic etching of crystalline substrates of indium phosphide - a semiconductor compound used in modern electronics - in a neutral electrolyte based on an aqueous solution of sodium chloride leads to spatial nanostructuring of the material. In particular, it leads to the growth of self-organized ordered two-dimensional arrays of pores with transverse dimensions as low as 70 nm. "In fact, we have succeeded in producing high-quality nanotemplates by anodic etching of indium phosphide substrates in salty water from the Black Sea," Tiginyanu told nanotechweb.org.

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SEM images taken from cleaved porous n-InP samples with pore diameters of 70 nm after pulsed electrodeposition of Pt: general view of the 30-micron thick layer (a); top view after the sample was additionally cleaved along a plane perpendicular to pores (b). Credit: Electrochemistry Communications

Currently, two types of nanotemplates are widely used in nanofabrication: porous aluminium oxide and etched ion track membranes based either on inorganic materials or organic polymers. However, both of these materials have high resistivity and therefore only play a passive role in nanofabrication processes. "The great advantage of the semiconductor nanotemplates we used is their electrical conductivity, which can effectively be controlled during fabrication using light or applied electric fields, for example," said Tiginyanu.

Bringing order
The team found that pulsed electroplating of platinum leads to the formation of an ordered array of metal nanotubes embedded in semiconductor nanotemplates. "This is the result of high conductivity of the nanotemplate skeleton, which provides conditions for uniform deposition of the metal on the inner surface of the substrate pores without preliminary technological steps like surface sensitization and activation that often lead to severe contamination with impurities," explained Tiginyanu. In fact, the semiconductor nanotemplate with the embedded array of metal nanotubes behaves like a layered material and can therefore be easily cleaved into thin films consisting of a few rows of platinum nanotubes in an indium phosphide "envelope".

The technique could be used to develop plasmonic devices, negative-index materials based on metallo-dielectric periodic structures, catalysts for energy conversion, and chemical and biological sensors, added Tiginyanu.

The researchers reported their work in Electrochemistry Communications.

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