Sequentially Self-Assembled Rings-in-Mesh Nanoplasmonic Arrays for Surface-Enhanced Raman Spectroscopy

Abstract

The discovery of the surface-enhanced Raman scattering (SERS) effect provided a revolutionary solution to the issue of low sensitivity in Raman spectroscopy. For more widespread application of SERS analysis, however, practical fabrication of well-defined ultrasmall nanostructures having large SERS signal enhancement capability and large-area signal uniformity is still a major challenge. Here, we report that rings-in-mesh Au nanostructures, which can be obtained by the consecutive self-assembly of polystyrene nanospheres and block copolymers (BCPs) with far different length scales, provide multiple advantages for SERS analysis in terms of signal amplification and measurement reproducibility. Significant signal enhancement is achieved by the hierarchical geometry composed of a submicrometer nanomesh and sub-10-nm nanogaps, which can be obtained from the self-assembly phenomena of polystyrene nanospheres and siloxane-based BCPs, respectively. Moreover, the two-dimensionally isotropic characteristics of the concentric nanoring structures eliminate the angular dependence of the SERS signal intensity and provide excellent reproducibility of measurement over a large area. ■ INTRODUCTIONRaman spectroscopy provides many advantages, such as high accuracy, high speed, and nondestructiveness for the characterization of a wide range of materials. However, despite these outstanding characteristics, 1−5 Raman spectroscopy fails to offer sufficient detection sensitivity, because of its weak signal intensity. 6 Surface-enhanced Raman scattering (SERS) is considered the most successful strategy for significantly enhancing the scattering cross-section and, consequently, the signal intensity. 7 The Raman intensity on noble metals such as gold and silver can be increased by several orders of magnitude by local enhancement of the electromagnetic field, 8−15 which can be controlled by the size, shape, and distance of metal nanostructures. Various metallic nanostructures such as nanoparticles, 6,16−18 nanogaps, 19−25 and sharp tips 26−28 have been suggested for maximizing the SERS signal intensity. Although previous studies reported a huge enhancement factor by controlling the size and arrangement of such nanostructures, facile and controlled creation of sub-10-nm plasmonic nanostructures with ultrahigh density, high throughput, and large-area uniformity remains a considerable challenge. 29−33 Moreover, the synergic combination of plasmonic nanostructures for maximizing signal intensity is not straightforward, because of process complexity.Compared to top-down photolithographic methods, selforganization of macromolecular materials is a promising fabrication route for well-controlled and well-ordered nanostructures with large-area uniformity, because of its high throughput, cost-effectiveness, and scalability. 21,32,34−38 In particular, the self-assembly of block copolymers (BCPs) is one of the most attractive candidates, because of the ability of BCPs to spontaneously assemble into a variety of structures at the nanometer length scale....

Ultralarge Area Sub-10 nm Plasmonic Nanogap Array by Block Copolymer Self-Assembly for Reliable High-Sensitivity SERS

“…Genuine parallel molecular assembly driven by the cooperative intra- and intermolecular interaction enables cost-effective, large-area scalability . More importantly, the typical feature size of BCP self-assembly in the tens-of-nanometers scale may offer the optimal plasmonic resonance wavelength in the visible light range for SERS applications. ,− …”

Hierarchical Order in Dewetted Block Copolymer Thin Films on Chemically Patterned Surfaces

“…The same degree of control over dewetted BCP films would allow hierarchical nanostructures to be developed for advanced functional materials (e.g. plasmonics, 39 superconductors, 40 metamaterials, 41 stretchable electronics, 42 or polymeric micelles 43 ). Nevertheless, the understanding of dewetting phenomena in BCP thin films is not trivial due to the high complexity of the system.…”

Scalable Fabrication of Metallic Nanogaps at the Sub‐10 nm Level

“…Feature shape (rings) is fixed. [ 156] Nanosphere lithography ≈5 M P L A monolayer of hexagonally packed nanospheres is deposited on a substrate and lightly etched using oxygen plasma to create a nanogap separation between the spheres. The spheres are used as an evaporative contact mask for metal deposition, resulting in nanogaps in the locations where the spheres previously touched.…”