Survey of plasmonic gaps tuned at sub-nanometer scale in self-assembled arrays

Abstract

Creating nanoscale and sub-nanometer gaps between noble metal nanoparticles is critical for the applications of plasmonics and nanophotonics. To realize simultaneous attainments of both the optical spectrum and the gap size, the ability to tune these nanoscale gaps at the sub-nanometer scale is particularly desirable. Many nanofabrication methodologies, including electron beam lithography, self-assembly, and focused ion beams, have been tested for creating nanoscale gaps that can deliver significant field enhancement. Here, we survey recent progress in both the reliable creation of nanoscale gaps in nanoparticle arrays using self-assemblies and in the in-situ tuning techniques at the sub-nanometer scale. Precisely tunable gaps, as we expect, will be good candidates for future investigations of surface-enhanced Raman scattering, non-linear optics, and quantum plasmonics. Overview of plasmonicsLight-metal interactions were well exploited in a number of ancient artworks that are now on display around the world. One of the most common examples is the colorful window glass found in many ancient European churches, where the colors actually are a result of the scattering of metallic nanoparticles (NPs) [1]. Another even more famous example is the Lycurgus cup, which shows a strikingly red color when viewed in transmitted light, but turns green in reflected light, as shown in Fig. 1 section of scattering as illustrated in Fig. 1 counterparts [16,17]. Therefore, plasmonics holds great promise for penetration into chemical and biological sensors at tiny volumes, even more so than optoelectronic nanodevices were initially anticipated to accomplish [18,19]. Plasmonic gaps created by self-assemblyDue to strong near-field coupling, the intense electromagnetic field within the plasmonic gaps stimulates continuous development in several disciplines. The first method involves random aggregations triggered by some extraneous chemicals, including molecules, DNA, and ions, which can interrupt the surface charging onto metal NPs. When the suitable molecules or ions are added to the metallic colloids, a color change is visible to the naked eye [34][35][36][37]. Partial aggregations of gold colloids give rise to some plasmonic gaps, inducing the output of the SERS with extraordinarily large enhancements [38,39]. When plasmonic NPs are assembled at the two immiscible phases and even aggregated into volume colloids [40], the spatial distribution of the NPs can be tailored by changing their chemical environments [41,42]. A stepwise strategy to generate regular heterogeneous binary NP arrays has also been reported in the literature [43]. Nowadays, based on this principle of random aggregation, gold colloids have become commercially available in products used as biological detectors, such as oviposit and pregnancy test strips.In the second method, similar to the formation of a "coffee ring", a two-dimensional NP array can be achieved by self-assembly on a liquid/air interface [44]. The NP within the colloid will flow towards the contact ...

Mechanically Induced Anisotropic Fragments in Sn-Doped In2O3 Nanoparticle Films for Flexible Strain Sensing Based on Surface Plasmons

“…Metallic NP films are also known as nanogap-controlled plasmonic structures. The optical properties of metallic NP films are influenced by the physical and chemical control of interparticle gaps. , Several studies have reported the fabrication of plasmonic strain sensors using metallic NP films, , which can detect mechanical deformations based on strain-induced optical changes in interparticle gaps. However, the operating mechanism of the proposed plasmonic strain sensor differs from that of conventional metallic NP films.…”