While the potential to grow semiconducting nanowires from metal nanocatalyst particles is well established, it is only in the past few years that researchers have focused their attention on the possibility of growing new material phases by these methods, explains team leader Lincoln Lauhon. "Although the mechanism is not yet well understood, we have conclusively shown that new polytypes of silicon can be produced by nanowire growth processes, and that Raman spectroscopy can rapidly determine the yield of these new phases for a given growth condition," he told nanotechweb.org.

Polytypes are crystals with the same chemical formula but distinct electronic structures.

The Northwestern team grew silicon nanowires with 9R, 27T, 2H and other polytype structures and identified these using Raman microscopy, which is particularly useful in this case because it can discriminate between disordered and ordered stacking sequences. Transmission electron microscopy also showed that intrinsic stacking faults form the basic building blocks of these polytypes.

Distinct band gaps
Lauhon and colleagues suggest that the polytypes seed from incoherent twin boundaries in the material. The researchers are especially interested in finding out whether the expected differences in electronic properties of these structures could benefit applications such as solar cells, for example. "The distinct band gaps of different polytypes might be exploited to make electrically active junctions to separate and confine electrons and holes – a strategy that usually increases the lifetimes of these charge carriers and which is important for photovoltaics," said Lauhon. "There is also the possibility of enhanced light absorption towards the infrared region of the electromagnetic spectrum by these structures."

There might also be a potential connection between polytypes in face-centered cubic metals that arise from certain types of grain boundaries and the prevalent polytypes of silicon seen in this work, he adds. "If the connection holds up, we may be able to template the growth of these novel phases."

The work was reported in ACS Nano.