Plasmonic Nanofabrication through Optical Heating

A temperature gradient can induce solutes to migrate from a hot to a cold region, and vice versa, in solution. This process, termed thermophoresis, has been applied to manipulate, transport, and separate various macromolecules and colloids by exploiting a microscale temperature gradient. Here we describe using a single gold nanoparticle (AuNP) as an efficient nanoscale heating source to promote thermophoresis. Moreover, on introducing a substrate with high thermal conductivity such as sapphire, a strong local temperature gradient can be shaped in the medium near the AuNP under continuous wave laser illumination. We observed molecules such as polyethylene glycol and sodium dodecyl sulfate being transported toward the AuNP and attaching to its surface, forming a gold core-organic shell structure within several tens of seconds of illumination. Spectroscopically, because of the gradual increasing encapsulation, progressive red shifts with enhanced scattering intensities were seen for the localized surface plasmon resonance bands of the AuNP with increasing cycles of illumination. Postmortem scanning electron microscopy provided direct evidence of shell formation. Our technique is relevant to nanofabrication based upon optical heating at the nanometer scale.