PNNL

Abstract

Chemical transformations near plasmonic metal nanostructures have attracted increasing attention in the past few years. Specifically, reactions occurring within plasmonic nanojunctions that can be detected via ultrasensitive surface and tip-enhanced Raman (SER and TER) scattering measurements were the focus of numerous reports. In this context, even though the transition between localized and tunneling (quantum) plasmons at nanojunctions is well documented, its implications on plasmonic chemistry remain poorly understood. We explore the latter through AFM-TER-current measurements. We use two molecules: (i) 4-mercaptobenzonitrile (MBN) that reports on the (non)local optical fields, and (ii) 4-nitrothiophenol (NTP) that features well-defined signatures of its neutral/anionic forms and dimerization product, 4,4’-dimercaptoazobenzene (DMAB). The transition from classical to quantum plasmons is clearly established through our initial measurements: it is marked by molecular charging and optical rectification. In the case of NTP, we observe both the parent and DMAB product beneath the probe in the classical regime. Further reducing the gap leads to the collapse of DMAB to form anionic NTP species. The process is shown to be reversible: anions subsequently recombine into DMAB. Our results have significant implications for ambient AFM-based TER measurements and their analysis, beyond the focused scope of this work. In effect, in cases where precise control over the junction is not trivial (e.g., in SER and ambient TER), both classical and quantum plasmons need to be considered in the design and analysis of plasmonic reactions.