Plasmonic metal nanostructures with extremely small features: new effects, fabrication and applications

Abstract

Surface plasmons in metals promise many fascinating properties and applications in optics, sensing, photonics and nonlinear fields. Plasmonic nanostructures with extremely small features especially demonstrate amazing new effects as the feature sizes scale down to the sub-nanometer scale, such as quantum size effects, quantum tunneling, spill-out of electrons and nonlocal states etc. The unusual physical, optical and photo-electronic properties observed in metallic structures with extreme feature sizes enable their unique applications in electromagnetic field focusing, spectra enhancing, imaging, quantum photonics, etc. In this review, we focus on the new effects, fabrication and applications of plasmonic metal nanostructures with extremely small features. For simplicity and consistency, we will focus our topic on the plasmonic metal nanostructures with feature sizes of sub-nanometers. Subsequently, we discussed four main and typical plasmonic metal nanostructures with extremely small features, including: (1) ultra-sharp plasmonic metal nanotips; (2) ultra-thin plasmonic metal films; (3) ultra-small plasmonic metal particles and (4) ultra-small plasmonic metal nanogaps. Additionally, the corresponding fascinating new effects (quantum nonlinear, non-locality, quantum size effect and quantum tunneling), applications (spectral enhancement, high-order harmonic wave generation, sensing and terahertz wave detection) and reliable fabrication methods will also be discussed. We end the discussion with a brief summary and outlook of the main challenges and possible breakthroughs in the field. We hope our discussion can inspire the broader design, fabrication and application of plasmonic metal nanostructures with extremely small feature sizes in the future.

Fig. 1. The schematic illustration of the plasmonic metal nanostructures with extremely small features.

Fig. 2. The full frequency dispersion relation of metal materials.

Fig. 3. (a) Scanning electron microscope (SEM) images of a typical Ag tip. (b and c) SEM images of a conical metallic tip with a grating coupler on the shaft and principle of the nonlocal excitation of the tip apex. (d) Micro-to nanoscale optical mode transformation on a tip. (e) Single-crystalline Au taper obtained from an electrochemical etching process. (f) Upon illumination of the grating coupler and SPPs propagate to the apex. (g and h) Schematic measurement principle and spatially resolved photoluminescence maps of Er upconversion emission. (i) SEM images of the sharp metallic pyramids (reproduced with permission from ref. and Copyright 2007, ref. Copyright 2011, ref. Copyright 2016, ref. Copyright 2010, ref. Copyright 2007, the American Chemical Society).

Fig. 4. The schematic illustration of general growth models of metal films.

Fig. 5. (a) Variation of RMS roughness for the 6 nm Ag film with seed layer thickness. (b) Comparison of total transmittance vs. sheet resistance of different flexible TEs. (c) Cross-sectional TEM image of the sample and the statistics of the film thickness. (d) Wavelength dependence of the Kerr coefficient and calculated quantized energy states and their corresponding wave functions (e). (f and g) Sketch of a Ag (111) film deposited on Si and HRTEM images of the transversal cross-section of the sample. (h) ARPES intensity as a function of electron energy relative to the Fermi energy and parallel wave vector (reproduced with permission from ref. Copyright 2013, ref. Copyright 2018, ref. Copyright 2019, the American Chemical Society, and ref. Copyright 2016, the Authors).

Fig. 6. (a) Extinction cross-section for the dipole resonance in a metal sphere. (b) Collection of normalized, deconvoluted EELS data from Ag particles. (c) Plot of the SPR energy versus particle diameter, with the inset depicting bulk resonance energies. (d) Schematic illustration of the “SPL” approach. (e) PL of Au nanodisks with different sizes (reproduced with permission from ref. Copyright 2014, ref. Copyright 2012 Springer Nature, ref. Copyright 2016 and ref. Copyright 2012, the American Chemical Society).

Fig. 7. The impact of quantum mechanical effects on plasmonic resonances (reproduced with permission from Copyright 2016, the Authors).

Fig. 8. (a and b) Experimental (a) and calculated (b) transmission spectra of disk (1), SRR (2), and SRR/D (3) arrays. (c and d) Measured SH emission and SH emission intensity versus the incident wavelength (c) and calculated scattering spectra (d) for the single split and perfect nanodisks (reproduced with permission from ref. Copyright 2013 and ref. Copyright 2016, the American Chemical Society).

Fig. 9. (a) Schematic diagram of the angle evaporation technique and the SEM of obtained nanostructures. (b) Schematic depiction of the two-step EBL process and TEM images of four representative dimers. (c) The 3D model and the electric-field distribution of heart-, rod-, triangle-, and disk-shaped Au nanoparticle dimers. (d) Nonclassical mesoscopic electrodynamics via d-parameters. (e) STEM images correlated with experimental plasmonic spectra and theoretical field profiles of a merging dimer. (f) Comparison of classical BEM EELS calculations with experimental EELS resonances. (g–i) Representative HAADF STEM images of the experimental samples with different junction geometries (g), experimental EELS spectra (h) and simulated spectra (i) of the dark modes of structures (reproduced with permission from ref. Copyright 2020 Wiley-VCH, ref. Copyright 2013, ref. Copyright 2010 and ref. Copyright 2012 the American Chemical Society, ref. Copyright 2014, Nature Publishing Group, ref. Copyright 2020, the Authors).

Fig. 10. (a) Schematic illustration of elevated Au bowties on top of Si posts, the calculated spatial distribution of the E field intensity, SEM image and SERS spectra of bowtie substrates. (b and c) The fabrication process (b) and SEM image (c) of AgAu 3D nanostar dimer. (d) The schematic illustration of atomic layer lithography. (e) The top-view SEM of a 5 nm-wide annular gap in a 200 nm–thick Ag film. (f) Schematic flow and corresponding SEM images in each stage of the fabrication process used for the coaxial nanocavity array using glancing-angle ion polishing. (g) Conceptual schematics of the self-folding of nanocylinders with plasmonic nanogaps for bimolecular sensing (reproduced with permission from ref. Copyright 2010, ref. Copyright 2014, ref. Copyright 2016, ref. Copyright 2020, the American Chemical Society, and ref. Copyright 2013, Nature Publishing Group).

Huimin Shi

Xupeng Zhu

Shi Zhang

Guilin Wen

Mengjie Zheng

Huigao Duan