Silicon is appealing for nanotechnology applications because the techniques to make silicon-based devices are so well advanced. Different methods have been developed for the growth of silicon nanowires such as laser ablation, catalyst-free methods and solution techniques. Such methods produce nanowires with different orientations and diameters that are covered by an oxide sheath 1 to 3 nm thick. The smallest silicon nanowire made so far has a diameter of between 3 to 5 nm. However, theory predicts that significant quantum size effects only come into play at diameters of less than 3 nm.

Lee and co-workers fabricated silicon nanowires using an oxide-assisted growth method that produces wires with diameters in the range of a few to tens of nanometres. The resulting wires consist of a single crystalline silicon core and an oxide sheath, which is about one third of the diameter. The researchers removed this oxide layer and terminated the surface with hydrogen to produce an oxidation resistant wire.

The team then used scanning tunnelling spectroscopy to determine the electronic band gaps of the nanowires. They found that the band gap increases as the diameter decreases - from 1.1 eV for 7 nm diameter wires to 3.5 eV for 1.3 nm wires. This is in agreement with previous theoretical predictions and provides experimental evidence for the quantum size effect on the electronic density of states in silicon nanowires.

The researchers now hope to use these nanowires for use in light-emitting diodes and lasers. “We also want to apply our method to a host of other scientifically and technologically important semiconducting nanowires, such as zinc oxide, zinc sulphide, gallium nitride and germanium”, Lee told PhysicsWeb.