"Our results will come as a surprise to those who have said that growth direction can't be controlled, that you get what you get when you grow semiconductor nanowires," said researcher Peidong Yang. "Control over nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization and band gap may be used to tune the physical properties of nanowires made from a given material."

Yang and colleagues grew gallium nitride nanowires onto two types of substrate - single crystals of lithium aluminium oxide and magnesium oxide - by metal-organic chemical vapour deposition (MOCVD). The wires formed with a wurtzite-type crystal structure on regions of the substrate previously coated with a catalyst such as gold, nickel or iron.

On the (100) plane of a lithium aluminium oxide crystal, which has twofold symmetry, nanowires grew perpendicular to the substrate and in the [11bar0] crystal direction. The wires had cross-sections in the form of isosceles triangles, with the base of the triangles between 15 and 40 nm wide.

Nanowires deposited on the threefold symmetry of the (111) plane of a magnesium oxide crystal, on the other hand, grew in the [001] crystal direction, although they still formed at right angles to the substrate. These wires were hexagonal in cross-section.

"In nanowires made from the exact same gallium nitride material but grown on different substrates, the light emission was blue-shifted by 100 meV," said Yang. "We believe the emission difference is a clear manifestation of the different crystal growth directions."

The researchers have also controlled the growth orientation of nanowires of zinc oxide by using different substrates.

"Our goal is to put together a generic scheme for controlling the directional growth of all semiconductor nanowires," said Yang. "When we can do this we will be able to answer some important fundamental questions, such as how would the carrier mobility, light emission and thermoconductivity differ along different crystallographic directions for nanowires with the same compositions and crystal structures."

According to Yang, the use of MOCVD for growing gallium nitride nanowires will also enable the team to integrate nanowires and thin films of various compositions so that they can start making real devices.

The researchers reported their work in Nature Materials.