Silicon nanowire springs that stretch, keep their electrical properties

January 08, 2018 // By Julien Happich
In a paper titled "Deterministic Line-Shape Programming of Silicon Nanowires for Extremely Stretchable Springs and Electronics" published in the Nano Letters, an international team of researchers from Nanjing University, Peking University, and CNRS-Ecole Polytechnique has demonstrated a new route to designing silicon-based flexible electronics in a scalable way.

Rather than starting from Si wafers cut-out from drawn silicon ingots, they describe in their paper a nanoscale-version of the crystal-pulling methods used to grow silicon crystal ingots from molten silicon, albeit following wavy patterns pre-set in a layer of silicon oxide. The researchers started with a glass or wafer substrate, coated with a layer of silicon dioxide etched into wavy edge patterns. A small strip of indium at an extremity is then heated up so indium droplets form (under a hydrogen plasma at 300ºC), retained by the edges.

Submitted to a SiH 4 plasma at 150ºC, the full structure is deposited with an amorphous Si precursor thin film. The line-shape engineering of in-plane SiNWs, as the researchers describe it, takes place as the molten indium droplets are guided along the structure's arbitrarily-defined edges, absorbing amorphous Si precursor thin film to produce long c-Si NWs along the edges (precipitated crystalline silicon nanowires).

Such technique yielded crystalline silicon nanowires more than a millimetre long, shaped along patterns such as horseshoe shapes and a Peano curve, which has previously been shown to be one of the best fractal patterns for achieving large stretchability.

Using high resolution transmission electron microscopy, the researchers observed that their line-shaped engineered SiNW springs had a high quality mono-like crystallinity, while in-situ scanning electron microscopy stretching and current–voltage characterizations also demonstrated a super-elastic and robust electric transport, even when stretching over 200%.

Using this novel line-shape programming approach, the researchers envision that mature c-Si technology could be extended into stretchable electronics, very much like their organic counterparts but with much better electrical properties (higher charge mobility).

The researchers will now investigate techniques for transferring the silicon nanosprings from the growth substrate onto a softer surface.

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