Plasmons in photocharged ZnO nanocrystals revealing the nature of charge dynamics

Localized surface plasmon resonances (LSPRs), known for their fascinating optical properties, have thus far been limited to nanostructures of materials with high steady-state charge carrier densities. Here, we show that even a nonequilibrium charge population can support a LSPR mode. Photocharged zinc oxide (ZnO) nanocrystals show an infrared LSPR, which can be dynamically turned off by discharging via redox activity. It is deduced that the photoinduced LSPR is a collective mode of as few as four conduction band electrons, the least observed thus far. The sustenance of a free-electron plasma in charged ZnO, supported by the LSPR observation, leads us to propose the existence of a many-body excitonic state and suggest a mechanism for previously unresolved charge trapping dynamics in ZnO. The LSPR, which serves as an optical signature of a charged state of the nanoparticle, is also demonstrated as a useful probe of surface redox reactions.