Light is an efficient tool to activate and control a wide range of chemical reactions, from heterogeneous photocatalysis, to molecular switching, and pharmaceutical drug production. The advent of nanophotonics has further advanced these fields by allowing to focus, control and steer light at the nanoscale. In particular, the use of localized surface plasmon resonances, which are light-driven oscillations of free electrons in metal nanoparticles, is currently being explored to drive chemical processes such as nanoparticle syntheses, photocatalytic reactions, and nanolithographic patterning. These plasmon resonances can activate chemical reactions thanks to non-equilibrium “hot” charge carriers, near-field enhancement of the electromagnetic fields, and nanoscale temperature gradients.¹ The relative contribution of these different mechanisms is often quite difficult to disentangle due to the complex light propagation, poorly-understood quantum mechanical charge carrier generation and ejection, and complicated macroscopic and microscopic heat dissipation in irradiated reaction vessels.² Single nanoparticle studies can be used to elucidate the mechanism at play, as parameters such as illumination intensity, light polarization, and sample and illumination geometry can be accurately controlled.³ So far, however, it has been challenging to control the synthesis of individual nanostructures with desired chemical compositions and controlled morphologies.

Here, we show how we can selectively use plasmonic photothermal heating to activate and control the synthesis of individual Au@semiconductor core@shell nanoparticles. Under plasmon excitation of individual gold nanoparticles in a reactive flow cell, we activate the formation of conformal metal oxide and metal sulfide shells with growth rates that scale with the nanoparticle surface temperature. Given the large spectral sensitivity of plasmon resonances to the nanoparticle morphology, the shell growth reaction is self-limited in nature and can be monitored in-situ by tracking the photoluminescence spectra of the growing plasmonic nanostructure under laser irradiation. We demonstrate the versatility of plasmonic heating as a chemical activation mechanism by synthesizing CeO₂, ZnO and ZnS shells on gold nanoparticles of different sizes and shapes. The use of light as a tool to activate and control chemical reactions at the nanoscale can lead to the synthesis of shape- and size-controlled hierarchical nanostructures which are inaccessible with classical colloidal synthetic methods, with potential applications in nanolithography, catalysis, energy conversion, and photonic devices.