Plexcitons for Solar Energy Harvesting and Sensing Devices

Excitons and plasmons are both quantum quasiparticles. While excitons are bound electron-hole pairs, plasmons are the collective and coherent oscillations of conduction band electrons (typically in coinage metals) at metal-dielectric interfaces. Plasmons can interact with excitons in several different ways. When plasmonic and excitonic resonances are spatially and energetically close to one another, the two resonators can be strongly coupled. The strong coupling regime is characterized by a significant Rabi splitting and anti-crossing behavior.<br/>Strongly coupled plexcitonic states are entangled quantum systems which can be used to construct quantum sensors. Plasmons and excitons can also be weakly coupled to each other. The weak coupling regime is characterized by an anomalously broadened absorption band, and is particularly beneficial in light harvesting devices such as solar cells, photodetectors and photocatalysts. The Shankar Lab has found that strongly coupled plexcitons in gold nanoparticle (NP)-cyanine dye J-aggregate assemblies are excellent platforms for chemical sensing due to their sensitivity to the dielectric permittivity. The Rabi splitting (as large as 217 meV) decreases upon exposure to humidity and eventually disappears as the plexcitonic interaction moves into the weak coupling regime. Once humidity is expelled, the large Rabi splitting is recovered. Plasmons and excitons also interact through the PIRET effect, which stands for plasmon-induced resonance energy transfer, a plasmonic analog of the more well-known Förster resonance energy transfer. The PIRET effect involving Au NP plasmons and excitons in n-type carbon nitride NPs enabled a huge improvement in photoelectrochemical water-splitting performance, achieving a photocurrent density as high as 3 mAcm^-2.