Signal enhancement in nano-Raman spectroscopy by gold caps on silicon nanowires obtained by vapour-liquid-solid growth.

Silicon nanowires grown by the vapour-liquid-solid growth mechanism with gold as the catalyst show gold caps approximately 50-400 nm in diameter with an almost ideal hemispherical shape atop a silicon column. These gold caps are extremely well suited for exploiting the tip or surface enhanced Raman scattering effects since they assume the right size on the nanometre scale and a reproducible, almost ideal hemispherical shape. On attaching a nanowire with a gold cap to an atomic force microscopy (AFM) tip, the signal enhancement by the gold nanoparticle can be used to spatially resolve a Raman signal. Applications of this novel nanowire based technical tip enhanced Raman scattering solution are widespread and lie in the fields of biomedical and life sciences as well as security (e.g. detection of bacteria and explosives) and in the field of solid state research, e.g. in silicon technology where the material composition, doping, crystal orientation and lattice strain can be probed by Raman spectroscopy. A prerequisite for obtaining this spatial resolution in nano-Raman spectroscopy is the attachment of a nanowire with a gold cap to an AFM tip. This attachment by welding a nanowire in a scanning electron microscope to an AFM tip is demonstrated in this paper.