Self-assembly: Golden touch

The addition of Au nanopartiles is used to determine the shape of self-assembled Cu3Si nanostructures grown on silicon substrates.

Controlling the shape, size and composition of nanostructures is essential for devices applications. Now, materials researchers are particularly interested in silicide nanostructures as a means of producing electrical contacts to silicon devices.

A team of researchers in Singapore have used gold (Au) nanoparticles to control the shape and size of Cu₃Si—a silicide nanostructure. Interest in this silicide arises from the fact that the Cu/Si interface is important for both electronic and catalytic applications.

Fig. 1: Scanning electron microscope images of three Cu₃Si nanostructures formed on Si substrates of different orientation.

Tom Wu, Hongyu Chen and colleagues¹ at the Nanyang Technological University, Singapore, used a vapor transport technique to grow nanoscale silicides. Gold nanoparticles were dispersed on a silicon substrate which was then put into a tube furnace with CuO as the source of Cu. The researchers observed that depending on the substrate orientation used, nanotriangles (NTs), nanosquares (NSs) or nanowires (NWs) were formed (Fig. 1). Notably, the sides of NTs and NS and the axes of the NWs were all oriented along the same direction.

The presence of the Au nanoparticles was essential for the Cu₃Si growth itself, as confirmed by the absence of nanostructures when Au was not used. The team believes that the SiO₂ surface layer formed when the sample was heated, hindered Si diffusion and subsequent reaction with Cu. However, Au, besides absorbing Cu atoms, enabled the local diffusion of Si through the SiO₂ layer.

Finally, the team confirmed the importance of the Au nanoparticles by showing that the size of Au nanoparticles determined the final dimensions of the Cu₃Si nanoparticles as well.

“This synthesis method sits between the traditional epitaxial thin film growth and the relatively recent vapor-liquid-solid nanomaterials growth, and takes advantages of both approaches,” says Wu. “We expect that appropriate selection of materials combinations will trigger the self-assembled shape- and orientation-controlled growth of other systems beyond silicides. And we are also working on controlling the location of the nanomaterials on the substrates and integrating the nanoscale silicides with silicon-based devices and circuits.”