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

Abstract

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.

Formation, Stability, and Replacement of Thiol Self‐Assembled Monolayers as a Practical Guide to Prepare Nanogaps in Nanoparticle‐on‐Mirror Systems

“…19 Reduction of HAuCl 4 with citrate produced the seed nanoparticles. We prepared the AuNPs with a diameter of 59 AE 6 nm using the seeded growth method.…”

“…We prepared the AuNPs with a diameter of 59 ± 6 nm using the seeded growth method . Reduction of HAuCl 4 with citrate produced the seed nanoparticles.…”

“…If the exchange reaction does occur, it may be useful to prepare mixed SAMs or to modify the SAMs after they are produced . This will allow us to prepare NPoM with mixed nanogap properties on a single substrate or even to change the gap properties after the NPoM is prepared …”

“…The prepared SAMs are stable in pure ethanol solvent, but are replaced by other thiol molecules in solution in less than 3 h. These findings provide a practical guide to preparing SAMs for NPoM structures and to changing the SAM molecules, with different ones. For example, spatioselective mixed SAMs and hence mixed nanogap properties can be realized if one can remove the SAMs from specific positions ( e.g ., using UV as in our prior work) and refill those areas with other thiol molecules. In that case, the immersion time in the second thiol for the refill must be between 30 min and 3 h.…”

Dark-Field Microscopy: Recent Advances in Accurate Analysis and Emerging Applications

“…Chaudhuri et al used DFM to qualitatively support detection of gold nanoparticle-based colorimetric analysis of multiple organisms (Figure A) . During the formation of the gold substrates for SERS, DFM was used to determine the states of the nanogaps . Similarly, in the investigation of the SERS of DNA origami-templated metamolecules, DFM was used to gain structural information, such as the nanoparticle size or clustering (Figure B) .…”