Theory of photo-injection of hot plasmonic carriers in metal-semiconductor nanostructures

TL;DR

This paper presents a theoretical study of hot carrier injection in metal-semiconductor nanostructures, highlighting size-dependent distributions, optimal conditions for high-energy electron generation, and implications for opto-electronic device design.

Contribution

It develops a theory connecting hot carrier distributions in nanocrystals with size, excitation polarization, and injection efficiency, extending Fowler's bulk metal injection theory.

Findings

Small nanocrystals favor high-energy hot carrier generation.

Carrier distribution depends on nanocrystal size and momentum transfer.

Optimal excitation polarization enhances high-energy carrier injection.

Abstract

We investigate theoretically the effect of injection of plasmonic carriers from an optically-excited metal nanocrystal to a semiconductor contact or to attached molecules. The distributions of optically-excited hot carriers are dramatically different in metal nanocrystals with large and small sizes. In large nanocrystals, most carriers have very small energies and the hot carrier distribution resembles the case of a plasmon wave in bulk. In nanocrystals smaller than 20nm, the carrier distribution extends to larger energies and occupies the whole region E_{F}<E<omega. The physical reason for the above behaviors is non-conservation of momentum in a nanocrystal. Because of the above properties, nanocrystals of small sizes are most suitable for designing of opto-electronic and photosynthetic devices based on injection of plasmonic electrons and holes. The central parameter of the problem is DeltaN=omega/q_{L}*v_{F}, where q_{L} is the momentum transfer and v_{F} is the Fermi velocity. In gold nanocrystals, when DeltaN<7, the high-energy hot-electron generation is efficient. For larger parameters DeltaN, the number of high-energy electrons is greatly reduced. Another important factor is the polarization of the exciting light. For efficient excitation of carriers with high energies, the electric-field polarization vector should be perpendicular to a prism-like nanoantenna (slab or platelet) with a small width ~ 10-20 nm. We also show the relation between our theory for injection in plasmonic nanocrystals and the Fowler theory of injection from a bulk metal. Along with a prism geometry (or platelet geometry), we consider cubes. The results can be applied to design both purely solid-state opto-electronic devices and systems for photo-catalysis and solar conversion.