Plasmonic Hot\nCarrier Transfer: Direction and Distance\nDependence in Nanoparticle-on-Mirror Systems

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