The enormous impact that even the smallest concentrations of gold in silicon have, makes it essential to understand the extent to which gold atoms diffuse in SiNWs and remain incorporated there and on the SiNW surface. This is especially true after it has been shown that gold atoms from the catalyst nanodroplets are very mobile and diffuse all over the SiNW surfaces and between SiNWs at temperatures as low as 600 °C thus leading to Ostwald ripening of gold nanoparticles at the SiNW sidewalls.

To provide some answers, researchers in Germany have studied the oxidation behaviour of SiNWs grown in an electron beam evaporation (EBE) reactor by the gold-catalyzed VLS growth process. The oxide layer thickness as well as oxide and SiNW morphology depend strongly on annealing conditions (time and temperature) as shown by varied oxidation processing and subsequent cross-sectional transmission electron microscopy (TEM) including high-resolution studies as well as scanning electron microscopy (SEM) studies.

Results strongly suggest that the SiNWs can be fully oxidized to form silica nanowires that can either keep their initial shape or, under certain annealing conditions do not keep their initial wire shape but assume a bamboo-like shape that forms most likely as a result of locally high stresses, which are related to the formation of nanocracks. These nanocracks form in the growing oxide layer mediated by the presence of gold nanoparticles that yield gold-enhanced SiNW oxidation and thus a faster oxidation rate, locally.

SiNWs that may be fully or partially oxidized are considered as potential active device materials for future nanoelectronics. Uses include light absorbers and emitters or wave guides or light absorbers, for example in solar cells, photodetectors or LEDs and as a large surface area to be functionalized for sensing applications. In all of these cases, the interfacial properties between the SiNW core and the SiO2 (silica) shell depend strongly on the gold presence there and are thus of key importance in terms of device functionality.

The researchers presented their work in the journal Nanotechnology.