Amplified Generation of Hot Electrons and Quantum Surface Effects in Nanoparticle Dimers with Plasmonic Hot Spots

Plasmonic excitations in optically driven nanocrystals are composed of excited single-particle electron–hole pairs in the Fermi sea. In large nanostructures, most of the excited plasmonic electrons have relatively small excitation energies due to the conservation of linear momentum. However, small optically driven nanocrystals may have large numbers of hot electrons with large energies. In this study, we develop the concept of hot electron generation further by considering the effect of a plasmonic hot spot. Plasmonic hot spots are areas in a nanostructure with highly inhomogeneous and enhanced electric fields. In our model of a nanoparticle dimer, the hot spot region appears near the gap between the nanoparticles. We then apply the quantum formalism based on the density matrix to describe this system. We show that the electromagnetic enhancement and the nonconservation of linear momentum in the hot spot of the nanoparticle dimer lead to strongly increased rates of generation of energetic (hot) electrons. The rates of hot electron generation grow faster than the absorption cross section and the electromagnetic enhancement factor with the decrease of the gap between the nanoparticles. This happens due to the breaking of the linear momentum conservation of electrons in the hot spot regions. We also show that hot electron generation effect leads to the quantum mechanism of surface-induced absorption in nanocrystals that is an intrinsic property of any confined plasmonic system. The results obtained in this study can be useful for understanding and designing plasmonic photodetectors and hybrid materials for efficient photocatalysis.