Metallic nanostructures with nanogap features are proved to be highly effective building blocks for plasmonic systems, as they can provide ultrastrong electromagnetic (EM) fields and controllable optical properties. A wide range of fields, including surface enhanced spectroscopy, sensing, imaging, nonlinear optics, optical trapping, and metamaterials, are benefited from these enhanced EM fields. This review outlines the latest development of the fabrication methods for nanogap structures (metal nanoparticle assembly, nanosphere lithography, electron beam lithography (EBL), focused ion beam (FIB) lithography, oblique angle shadow evaporation, edge lithography, and so on), followed by a summary of their optical applications. The present review will inspire more ingenious designs and fabrications of plasmonic nanogap structures with lithography‐free fabrication techniques, and promote their applications in optics and electronics.

Sub-10 nm nanogaps are enantioselectively fabricated between two nanocrescents based on nanoskiving and show tailored circular dichroism (CD) activity. The mirror symmetry of the nanostructure is broken by subsequent deposition with different azimuthal angles. Strong plasmonic coupling is excited in the gaps and at the tips, leading to the CD activity. The dissymmetry g-factor of the chiral nanogaps with 5 nm gap-width is −0.055, which is 2.5 times stronger than that of the 10 nm gap-width. Moreover, the surface-enhanced Raman scattering (SERS) performance of L/D-cysteine absorbed on chiral nanogaps manifests as the emergence of enantiospecific Raman peaks and the appearance of distinct changes in SERS intensities, which affirms that chiral nanogaps can recognize specific cysteine enantiomers via standard Raman spectroscopy in the absence of circularly polarized light source and a chiral label molecule. The sub-10 nm chiral nanogaps with tailored chiroptical responses show great potential in a class of chiral applications, such as chiral sensing, polarization converters, labelfree chiral recognition, and asymmetric catalysis.

Plasmonic assemblies featuring high sensitivity that can be readily shifted by external fields are the key for sensitive and versatile sensing devices. In this paper, a novel fast-responsive plasmonic nanocomposite composed of a multilayer nanohole array and a responsive electrochromic polymer is proposed with the plasmonic mode appearance vigorously cycled upon orthogonal electrical stimuli. In this nanocomposite, the coaxially stacked plasmonic nanohole arrays can induce multiple intense Fano resonances, which result from the crosstalk between a broad surface plasmon resonance (SPR) and the designed discrete transmission peaks with ultrahigh sensitivity; the polymer wrapper could provide the sensitive nanohole array with real-time-varied surroundings of refractive indices upon electrical stimuli. Therefore, a pronounced pure electroplasmonic shift up to 72 nm is obtained, which is the largest pure electrotuning SPR range to our knowledge. The stacked nanohole arrays here are also directly used as a working electrode, and they ensure sufficient contact between the working electrode (plasmonic structure) and the electroactive polymer, thus providing considerably improved response speed (within 1 s) for real-time sensing and switching.

An in situ SERS study of plasmonic nanochemistry is realized on hierarchical Ag “hedgehog-like” arrays with strong surface plasmon resonance.

trapping, [5] single-molecule analysis, [6,7] surface plasmon (SP) enhanced spectroscopy. [6,8,9] To achieve the extremely concentrated and strong electric fields, lots of efforts are contributed to fabricate kinds of plasmonic structures, revealing that sub-10 nm nanogap and nanotip are the two key characteristics. [10,11] In theory, as predicted by Maxwell's equations, the electric fields would become stronger with the decreasing distance between metal nanostructures and can be confined in the gaps; [12][13][14][15][16][17][18][19][20][21] for nanotips, more electrons are distributed at the structural features with high curvature and thus lead to stronger electric fields. [22][23][24][25][26][27][28][29][30] Various plasmonic nanostructures with either nanogaps or nanotips have been fabricated, successfully enhancing the electric fields to several orders of magnitude and confining them in extremely small areas. To obtain better performances, a natural thought is to include both nanogaps and nanotips in one plasmonic nanostructure, i.e., integrated "hot spots." Impressive efforts have been made to develop the multifeature nanostructures. For example, spilt-wedge antenna structures with sub-5 nm gaps, [10] 3D sub-10 nm nanostar dimers gap, [6] and 3D sub-10 nm Ag/SiN x gap with an pair of uniform tips, [31] have been fabricated and highly boosted the local optical field intensity. The fabrication of most these structures has primarily relied on electron beam lithography (EBL) [6,[32][33][34] and focused-ion beam (FIB). [35] These techniques can facilely control the patterns and precisely tune the size and separation, but they suffer from the drawbacks of high-cost, time-consuming, and low-throughput. It is urgently needed to develop a simple and scalable process for their production.Nanoskiving, that combines the deposition of thin metal layers on substrates (flat or structured) and sectioning using a unity of ultramicrotome and diamond knife, would be an alternative fabrication technique for plasmonic nanostructures with both nanogaps and nanotips. [36][37][38][39] Compared with the conventional fabrication techniques (EBL and FIB), nanoskiving is uncomplicated, fast, and scalable, requiring no sophisticated equipment. Nanoskiving has been widely used in fabricating numerous nanostructures, showing the strong capability for nanostructures on-demand. In particular, our research group has reported 3D zig-zag nanowires that possess both nanogaps and nanotips via combining photolithography Plasmonic crescent nanogap arrays (CNGAs) integrated by nanoscale gaps and nanotips exhibit strong capability of light confinement and thus lead to extremely electric field enhancement. The CNGAs with tunable gap-width are fabricated via a low-cost and simple process combining colloidal lithography and nanoskiving techniques. Incident light is captured in the nanogaps and at the two tips of nanocrescent, where extremely strong electric fields are excited. The strong electric fields lead to greatly enhanced surface-enhanced Raman s...