Nonradiative decay of photoexcited plasmons generates energetic nonthermal charge carriers. These hot charge carriers play a major role in plasmonic photocatalysis and photovoltaics. Therefore, establishing the relationship between the hot carrier generation efficiency and the structural and chemical parameters of nanoparticles is crucial for developing highly efficient plasmonic catalysts and photovoltaic materials. In this study, we compare the quantum efficiency of hot carrier generation between gold (AuNPs) and silver nanoparticles (AgNPs), and spherical (AuNSs) and cubic gold nanoparticles (AuNCs). We construct nanoparticle-on-mirror (NPoM) systems where reactant molecules are positioned in the nanogaps between the nanoparticles and gold films. Excitation of the NPoM at 785 nm, followed by the detection of products using surface-enhanced Raman spectroscopy allows us to measure the plasmon-driven reaction yields. Dividing the reaction yield by the calculated absorption cross section at the excitation laser wavelength provides the efficiency of hot carrier generation per absorbed photon. We reveal that AgNPs are more effective at generating hot carriers than AuNPs, which is consistent with the higher electron-surface scattering rate of AgNPs. The hot carrier generation of AuNCs is marginally better than that of AuNSs, which can be attributed to the enhanced electric fields inside the AuNCs in the nanogap region. This study contributes to a rational design of plasmonic catalysts or photovoltaic materials of higher efficiencies.

Hot charge carriers created by plasmons break the C–B bond of 4-mercaptophenylboronic acid, transforming it into benzenethiol in nanogaps.

Narrow nanogaps formed between nanostructures act as hot spots, where the plasmonic properties are significantly enhanced. Consequently, the ability to create and control nanogaps is highly desirable for many plasmon-based applications. Nanoparticles-on-mirror (NPoM) is an attractive system that allows one to produce nanogaps on twodimensional surfaces with great flexibility. NPoM is formed by adsorbing gold nanoparticles (AuNPs) on self-assembled monolayers (SAMs) of molecules on Au substrates. The properties of the resulting nanogaps are defined by the SAM molecular spacer and the shape and size of the adsorbed AuNPs. In this paper, we present a method for controlling the spatial distribution of the nanogaps with micrometer resolution. UV irradiation of the SAMs leads to desorption of the thiol molecules from the surface via photooxidation, which hinders the subsequent adsorption of AuNPs on the surface. By applying spatioselective irradiation, spatially controlled NPoM patterns are constructed. Furthermore, filling the irradiated regions with different types of molecules leads to patterned nanogaps with two different sets of properties on a single Au substrate. The gap properties are measured with dark-field microscopy, scanning electron microscopy, and surface-enhanced Raman scattering. This method can be extended to the fabrication of more complex nanogap circuits with higher spatial resolution by applying advanced photolithography techniques.

We provide practical information about the basic properties of SAMs to prepare and modify NPoM systems.

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