Metal double layers with sub-10 nm channels

Pattern Recognition Directed Assembly of Plasmonic Gap Nanostructures for Single-Molecule SERS

Gold nanocubes (AuNCs) with tunable localized surface plasmon resonance properties are good candidates for plasmonic gap nanostructures (PGNs) with hot spots (areas with intense electric field localization). Nevertheless, it remains challenging to create shape-controllable nanogaps between AuNCs. Herein, we report a DNA origami directed pattern recognition strategy to assemble AuNCs into PGNs. By tuning the position and number of capture strands on the DNA origami template, different geometrical configurations of PGNs with nanometer-precise and shape-controllable gaps are created. The localized field enhancement in these gaps can generate hot spots that are in accordance with finite difference time domain simulations. Benefiting from the single Raman probe molecule precisely anchored at these nanogaps, the dramatic enhanced electromagnetic fields localized in hot spots arouse stronger single-molecule SERS (SM-SERS) signals. This method can be utilized in the design of ultrahigh-sensitivity photonic devices with tailored optical properties and SERS-based applications.

Wafer-Scale and Cost-Effective Manufacturing of Controllable Nanogap Arrays for Highly Sensitive SERS Sensing

The metallic nanogap has been proved as an efficient architecture for surface-enhanced Raman scattering (SERS) applications. Although a lot of nanogap fabrication methods have been proposed in the last few decades, the economical and high-yield manufacturing of sub-10 nm gaps remains a challenge. Here, we present a convenient and cost-effective fabrication method for wafer-scale patterning of metallic nanogaps, which simply combines photolithographic metal patterning, swelling-induced nanocracking, and superimposition metal sputtering without requiring expensive nanofabrication equipment. By controlling the swelling time and metal deposition thickness, the gap size can be precisely defined, down to the sub-10 nm scale. Furthermore, we demonstrate that the fabricated nanogap array can be used as an excellent SERS substrate for molecule measurements and shows a high Raman enhancement factor of ∼10⁸ and a high sensitivity for the detection of rhodamine 6G (R6G) molecules, even down to 10^-14 M, indicating an extraordinary capability for single-molecule detection. Due to its high controllability and wafer-scale fabrication capability, this nanogap fabrication method offers a promising route for highly sensitive and economical SERS detections.

Fabrication of plasmonic structures with well-controlled nanometric features: a comparison between lift-off and ion beam etching

After providing a detailed overview of nanofabrication techniques for plasmonics, we discuss in detail two different approaches for the fabrication of metallic nanostructures based on e-beam lithography. The first approach relies on a negative e-beam resist, followed by ion beam milling, while the second uses a positive e-beam resist and lift-off. Overall, ion beam etching provides smaller and more regular features including tiny gaps between sub-parts, that can be controlled down to about 10 nm. In the lift-off process, the metal atoms are deposited within the resist mask and can diffuse on the substrate, giving rise to the formation of nanoclusters that render the nanostructure outline slightly fuzzy. Scattering cross sections computed for both approaches highlight some spectral differences, which are especially visible for structures that support complex resonances, such as Fano resonances. Both techniques can produce useful nanostructures and the results reported therein should guide the researcher to choose the best suited approach for a given application, depending on the available technology.

Three-dimensional assembly of silver nanoparticles spatially confined by cellular structure of Spirulina, from nanospheres to nanosheets

Three-dimensional (3D) ordered construction of nanoparticles (NPs) has attracted much attention in wide applications, however, techniques with respect to cost effective nanofabrication of well defined functional architectures is still lacking. To address this specific issue, a bio-interface confinement approach is proposed that precisely replicates the complex cellular structural features of microbes and integrates silver NP (SNP) building blocks into their 3D framework in a precise, low cost and mass production way. Herein, the SNPs with nanospheres and nanosheets structure were synthesized by way of electroless deposition using Spirulina as template. Results showed that SNPs were orderly assembled along the cellular structure, and the spatially confinement of cellular texture induced the transformation of SNPs from sphere to flake morphology during their continuous growth. The silver assembly not only shows good antibacterial activity, but also exhibits excellent surface enhanced Raman scattering (SERS) performance with the enhancement factor as high as 5.95 × 10⁸ and good recuperability towards Rhodamine 6G. The fascinating SERS performance can be ascribed to the combined action of nanosheets morphology of SNPs, hierarchical nanostructure of the cellular structure, and the small interparticle spacing. This strategy provides an effective strategy for controllable and ordered 3D assembly of NPs by using the cellular texture.

Sub-5 nm Metal Nanogaps: Physical Properties, Fabrication Methods, and Device Applications

Sub-5 nm metal nanogaps have attracted widespread attention in physics, chemistry, material sciences, and biology due to their physical properties, including great plasmon-enhanced effects in light-matter interactions and charge tunneling, Coulomb blockade, and the Kondo effect under an electrical stimulus. These properties especially meet the needs of many cutting-edge devices, such as sensing, optical, molecular, and electronic devices. However, fabricating sub-5 nm nanogaps is still challenging at the present, and scaled and reliable fabrication, improved addressability, and multifunction integration are desired for further applications in commercial devices. The aim of this work is to provide a comprehensive overview of sub-5 nm nanogaps and to present recent advancements in metal nanogaps, including their physical properties, fabrication methods, and device applications, with the ultimate aim to further inspire scientists and engineers in their research.

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

Effective surface enhancement of Raman scattering (SERS) requires strong near-field enhancement as well as effective light collection of plasmonic structures. To this end, plasmonic nanoparticle (NP) arrays with narrow gaps or sharp tips have been suggested as desirable structures. We present a highly dense and uniform Au nanoscale gap array enabled by the customized design of NP shape and arrangement employing block copolymer self-assembly. Block copolymer self-assembly in thin films offers uniform hexagonally packed nanopost template arrays over the entire surface of a 2 in. wafer. Conventional evaporative metal deposition over the nanotemplate surface allows precise geometric control and positional arrangement of metal NPs, constituting tunable, strong plasmonic near-field enhancement particularly at the "hot spots" near interparticular nanoscale gaps. Underlying field distribution has been investigated by a finite-difference time-domain simulation. In the detection of thiophenol, our Au nanogap array shows a remarkable enhancement of Raman intensity greater than ∼10⁴, a standard deviation as small as 12.3% compared to that of the planar Au thin film. In addition, adenine biomolecules can be detected with a detection limit as low as 100 nM. Our approach proposes highly sensitive and reliable SERS on the basis of a scalable, low-cost bottom-up strategy.

High-Throughput Fabrication of Ultradense Annular Nanogap Arrays for Plasmon-Enhanced Spectroscopy

The confinement of light into nanometer-sized metallic nanogaps can lead to an extremely high field enhancement, resulting in dramatically enhanced absorption, emission, and surface-enhanced Raman scattering (SERS) of molecules embedded in nanogaps. However, low-cost, high-throughput, and reliable fabrication of ultra-high-dense nanogap arrays with precise control of the gap size still remains a challenge. Here, by combining colloidal lithography and atomic layer deposition technique, a reproducible method for fabricating ultra-high-dense arrays of hexagonal close-packed annular nanogaps over large areas is demonstrated. The annular nanogap arrays with a minimum diameter smaller than 100 nm and sub-1 nm gap width have been produced, showing excellent SERS performance with a typical enhancement factor up to 3.1 × 10⁶ and a detection limit of 10^-11 M. Moreover, it can also work as a high-quality field enhancement substrate for studying two-dimensional materials, such as MoSe₂. Our method provides an attractive approach to produce controllable nanogaps for enhanced light-matter interaction at the nanoscale.

Atomic-Layer-Deposition Assisted Formation of Wafer-Scale Double-Layer Metal Nanoparticles with Tunable Nanogap for Surface-Enhanced Raman Scattering

A simple high-throughput approach is presented in this work to fabricate the Au nanoparticles (NPs)/nanogap/Au NPs structure for surface enhanced Raman scattering (SERS). This plasmonic nanostructure can be prepared feasibly by the combination of rapid thermal annealing (RTA), atomic layer deposition (ALD) and chemical etching process. The nanogap size between Au NPs can be easily and precisely tuned to nanometer scale by adjusting the thickness of sacrificial ALD Al₂O₃ layer. Finite-difference time-domain (FDTD) simulation data indicate that most of enhanced field locates at Au NPs nanogap area. Moreover, Au NPs/nanogap/Au NPs structure with smaller gap exhibits the larger electromagnetic field. Experimental results agree well with FDTD simulation data, the plasmonic structure with smaller nanogap size has a stronger Raman intensity. There is highly strong plasmonic coupling in the Au nanogap, so that a great SERS effect is obtained when detecting methylene blue (MB) molecules with an enhancement factor (EF) over 10⁷. Furthermore, this plasmonic nanostructure can be designed on large area with high density and high intensity hot spots. This strategy of producing nanoscale metal gap on large area has significant implications for ultrasensitive Raman detection and practical SERS application.

Ultra sub-wavelength surface plasmon confinement using air-gap, sub-wavelength ring resonator arrays

Arrays of sub-wavelength, sub-10 nm air-gap plasmonic ring resonators are fabricated using nanoimprinting. In near infra-red (NIR) range, the resonator supports a single dipole mode which is excited and identified via simple normal illumination and explored through transmission measurements. By controlling both lateral and vertical confinement via a metal edge, the mode volume is successfully reduced down to 1.3 × 10⁻⁵ λ₀³. The advantage of such mode confinement is demonstrated by applying the resonators biosensing. Using bovine serum albumin (BSA) molecules, a dramatic enhancement of surface sensitivity up to 69 nm/nm is achieved as the modal height approaches the thickness of the adsorbed molecule layers.

Surface enhanced Raman scattering of gold nanoparticles supported on copper foil with graphene as a nanometer gap

Gaps with single-nanometer dimensions (<10 nm) between metallic nanostructures enable giant local field enhancements for surface enhanced Raman scattering (SERS). Monolayer graphene is an ideal candidate to obtain a sub-nanometer gap between plasmonic nanostructures. In this work, we demonstrate a simple method to achieve a sub-nanometer gap by dewetting a gold film supported on monolayer graphene grown on copper foil. The Cu foil can serve as a low-loss plasmonically active metallic film that supports the imaginary charge oscillations, while the graphene can not only create a stable sub-nanometer gap for massive plasmonic field enhancements but also serve as a chemical enhancer. We obtained higher SERS enhancements in this graphene-gapped configuration compared to those in Au nanoparticles on Cu film or on graphene-SiO2-Si. Also, the Raman signals measured maintained their fine features and intensities over a long time period, indicating the stability of this Au-graphene-Cu hybrid configuration as an SERS substrate.

Quantum dots-nanogap metamaterials fabrication by self-assembly lithography and photoluminescence studies

We present a new and versatile technique of self-assembly lithography to fabricate a large scale Cadmium selenide quantum dots-silver nanogap metamaterials. After optical and electron microscopic characterizations of the metamaterials, we performed spatially resolved photoluminescence transmission measurements. We obtained highly quenched photoluminescence spectra compared to those from bare quantum dots film. We then quantified the quenching in terms of an average photoluminescence enhancement factor. A finite difference time domain simulation was performed to understand the role of an electric field enhancement in the nanogap over this quenching. Finally, we interpreted the mechanism of the photoluminescence quenching and proposed fabrication method of new metamaterials using our technique.

Self-aligned colloidal lithography for controllable and tuneable plasmonic nanogaps

Au nanoparticles (NPs) deposited on a substrate function as ring shaped colloidal shadow masks. Using e-beam evaporation of gold, nanometer sized gaps are formed as a result. The size of these gaps can be accurately tuned by controlling the thickness of the gold deposition, thereby tuning the plasmonic coupling of the NPs with the substrate. The clean cavity produced between the Au NPs and the Au film provides an excellent SERS platform for trace molecule detection.