Patterning Nanogaps: Spatial Control of the Distribution of Nanogaps between Gold Nanoparticles and Gold Substrates

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 two-dimensional 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.