Plasmonic Hot Carrier Transfer: Direction and Distance Dependence in Nanoparticle-on-Mirror Systems

The generation and transfer of hot charge carriers play a central role in plasmonic photocatalysis and photovoltaics. The decay of excited plasmons in gold nanoparticles (AuNPs) produces hot charge carriers, and their transfer to nearby molecules induces chemical reactions. Here, we investigate the direction of hot charge carrier transfer in nanoparticle-on-mirror (NPoM) systems, where two sources of hot carriers are available: AuNPs and gold films (AuFs). We also explore how far hot charge carriers can be delivered in space. To determine the transfer direction of hot carriers, we position the carboxyl group of 4-mercaptobenzoic acid (MBA) either facing the surfaces of bare AuNPs or AuFs. The decarboxylation reaction, driven by the transfer of hot carriers to the carboxyl group, occurs only when the carboxyl group is close to the AuNP surfaces, suggesting that hot carriers are transferred from AuNPs rather than AuFs in this coupled system. For distance-dependent hot carrier transfer, we adjust the location of the carboxyl group in the NPoM system using mixed self-assembled monolayers (SAMs) of MBA and alkanethiol spacers of varying lengths. The decarboxylation reaction yield significantly drops as soon as the carboxyl group is distanced from the AuNP surfaces by an alkanethiol spacer taller than the MBA SAMs. Hot carriers are unable to transfer to functional groups located just a few bond lengths away from the AuNP surfaces. This work identifies a critical condition for hot-carrier-driven reactions to occur, contributing to the design of highly efficient plasmonic catalysts.