Ultrafast Thermal Imprinting of Plasmonic Hotspots

Plasmonic photochemistry is driven by a rich collection of near‐field, hot charge carrier, energy transfer, and thermal effects, most often accomplished by continuous wave illumination. Heat generation is usually considered undesirable, because noble metal nanoparticles heat up isotropically, losing the extreme energy confinement of the optical resonance. Here it is demonstrated through optical and heat‐transfer modelling that the judicious choice of nanoreactor geometry and material enables the direct thermal imprint of plasmonic optical absorption hotspots onto the lattice with high fidelity. Transition metal nitrides (TMNs, e.g., TiN/HfN) embody the ideal material requirements, where ultrafast electron–phonon coupling prevents fast electronic heat dissipation and low thermal conductivity prolongs the heat confinement. The extreme energy confinement leads to unprecedented peak temperatures and internal heat gradients (>10 K nm−1) that cannot be achieved using noble metals or any current heating method. TMN nanoreactors consequently yield up to ten thousand times more product in pulsed photothermal chemical conversion compared with noble metals (Ag, Au, Cu). These findings open up a completely unexplored realm of nano‐photochemistry, where adjacent reaction centers experience substantially different temperatures for hundreds of picoseconds, long enough for bond breaking to occur. Plasmonic heating ordinarily leads to isotropic heating of the material. However, through nanoparticle shaping, pulsed illumination, and the right choice of material (e.g., HfN), plasmonic absorption hotspots can be directly imprinted on the lattice as heat. This approach results in extreme thermal hotspots, lasting hundreds of picoseconds, which can boost the chemical reactivity of catalytic nanoreactors by orders of magnitude.