The unique characteristics of nanoscale materials presents both challenges and opportunities. But before these challenges can be met, or these opportunities realized, it is important to understand exactly how the shapes of nanoscale structures affects their properties. Now Donglai Yao, Gang Zhang and Baowen Li from Singapore1 have conducted a systematic computational investigation into the effect that the cross-sectional shape of silicon nanowires has on their electronic bandgap. Unexpectedly, their results suggest that the bandgap of a silicon nanowire of a given radius is not determined by its cross-sectional shape, but its surface to volume ratio.

Previous studies of the factors affecting the behavior of silicon and other semiconducting nanowires have focused on surface effects. This is not surprising given that a substantial fraction of the atoms in a nanowire can be considered as being part of its surface, especially for wires that are only a few nanometers in diameter. Now, it is well known that the local electronic structure at the surface of a semiconductor depends on the precise atomic structure and composition of this surface. Yet there have been systematic studies of the precise relationship between a nanowire's surface to volume ratio and its electronic structure.

In their study, Yao, Zhang and Li simulated silicon nanowires having three different cross-sectional shapes — triangular, square and hexagonal — and a range of widths between 1 and 7 nm diameter. For each nanowires structure they calculated its electronic bandgap — a key measure of semiconductor's electronic structure that determines both it's electrical and optical properties — and plotted this against diameter and surface to volume ratio.

Fig. 1: Unified linear dependence of the bandgap of triangular, square and hexagonal silicon nanowires as a function of surface to volume ratio (SVR).

They found that with increasing diameter, the bandgaps of all nanowires decreased, and that those with rectangular and hexagonal cross-sections showed different trends compared with nanowires with triangular cross-sections. As a function of surface to volume ratio, however, they found that the bandgaps of all three cross-sectional shaped nanowires followed the same straight line (Fig. 1).

The authors say that the universality of this relationship suggests that surface to volume ratio could be a much more useful criteria to use than the diameter when comparing the properties of different nanowires.