Mechanism of nanoparticle accumulation induced by thermophoresis around plasmonic nanostructure

When gold nanostructures are irradiated with laser light, various forces act on the surrounding nanoparticles. Localized surface plasmon resonance enhances the surrounding electric field, increasing optical radiation pressure. Simultaneously, photothermal conversion induces a temperature gradient that results in a thermophoretic force, while heat-induced natural convection produces viscous drag on the particles.

Experiments show that the accumulation pattern of nanoparticles varies with laser power, with a circular pattern (spot mode) appearing at low power and a ring-shaped pattern (ring mode) emerging at high power. In the spot mode, optical radiation pressure is considered dominant. However, the mechanism of ring mode formation remains unclear.

In this study, we aim to classify the mechanism of ring mode formation by numerically simulating the forces acting on polystyrene particles surrounding gold nanostructures under laser irradiation, and we found that the reversal of the sign of the Soret coefficient in thermophoresis plays a significant role in the ring mode formation. In addition, our simulation suggests that thermophoresis contributes more to the spot mode than optical radiation pressure. Furthermore, the contribution of natural convection was found to be negiligible.