Semiconductor nanowires offer a promising platform for sensing because of their high surface to volume ratio and their good electronic and mechanical properties. Semiconductor nanowires are often fabricated using nano-sized catalysts to crack precursor gases near the catalyst, creating the nanowire using the so-called vapor–liquid–solid (VLS) growth mechanism. VLS-grown nanowires usually grow in crystallographic <111>-type directions. With epitaxial templating, the orientation of the nanowire can be controlled; for example, free-standing nanowires can be grown perpendicular to a (111)-oriented substrate. In the work described in Nano Letters and on the cover of Nanotechnology, an ingenious structure is described, which can engineer the nanowire growth direction so that the nanowire grows against the substrate.

The structure utilized starts with a (001)-oriented, SOI substrate, which sandwiches a layer of SiO2 between two layers of single-crystalline silicon. A section of the top silicon layer is etched away to expose the underlying SiO2. A portion of the SiO2 is then etched isotropically to undercut the residual top silicon layer. This undercut portion of the top silicon layer is key to the engineered growth. Gold nanoparticles can then be deposited selectively on the undercut portion to act as the catalyst for the VLS, silicon nanowire growth. The nanowire grows toward the remaining SiO2 layer of the SOI substrate. When it impinges on the SiO2, it is forced to change direction and then grows along the surface in a <110>-type direction (the projection of the <111> direction on a (001) plane).

The nanowire continues to grow along the oxide surface even when it encounters a curved oxide section that changes the growth direction by 90°. Engineering the growth direction enables the routine fabrication of top-gated, nanowire, field-effect transistors with in situ-grown source and drain contacts and good device characteristics.

In the future, we plan to use the nearby substrate as a "back gate" to tune the characteristics of the sensor to increase its sensitivity and to integrate the sensor with other devices.