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

A strategy to fabricate nanostructures with sub-nanometer line edge roughness

Line edge roughness (LER) has been an important issue in the nanofabrication research, especially in integrated circuits. Despite numerous research studies has made efforts on achieving smaller LER value, a strategy to achieve sub-nanometer level LER still remains challenging due to inability to deposit energy with a profile of sub-nanometer LER. In this work, we introduce a strategy to fabricate structures with sub-nanometer LER, specifically, we use scanning helium ion beam to expose hydrogen silsesquioxane (HSQ) resist on thin SiNx membrane (∼20 nm) and present the 0.16 nm spatial imaging resolution based on this suspended membrane geometric construction, which is characterized by scanning transmission electron microscope (STEM). The suspended membrane serves as an energy filter of helium ion beam and due to the elimination of backscattering induced secondary electrons, we can systematically study the factors that influences the LER of the fabricated nanostructures. Furthermore, we explore the parameters including step size, designed exposure linewidth (DEL), delivered dosage and resist thickness and choosing the high contrast developer, the process window allows to fabricate lines with 0.2 nm LER is determined. AFM measurement and simulation work further reveal that at specific beam step size and DEL, the nanostructures with minimum LER can only be fabricated at specific resist thickness and dosage.

Periodic Arrays of Plasmonic Ag-Coated Multiscale 3D-Structures with SERS Activity: Fabrication, Modelling and Characterisation

Surface enhanced Raman spectroscopy (SERS) is gaining importance as sensing tool. However, wide application of the SERS technique suffers mainly from limitations in terms of uniformity of the plasmonics structures and sensitivity for low concentrations of target analytes. In this work, we present SERS specimens based on periodic arrays of 3D-structures coated with silver, fabricated by silicon top-down micro and nanofabrication (10 mm × 10 mm footprint). Each 3D-structure is essentially an octahedron on top of a pyramid. The width of the top part-the octahedron-was varied from 0.7 µm to 5 µm. The smallest structures reached an analytical enhancement factor (AEF) of 3.9 × 107 with a relative standard deviation (RSD) below 20%. According to finite-difference time-domain (FDTD) simulations, the origin of this signal amplification lies in the strong localization of electromagnetic fields at the edges and surfaces of the octahedrons. Finally, the sensitivity of these SERS specimens was evaluated under close-to-reality conditions using a portable Raman spectrophotometer and monitoring of the three vibrational bands of 4-nitrobenzenethiol (4-NBT). Thus, this contribution deals with fabrication, characterization and simulation of multiscale 3D-structures with SERS activity.

Advancing SERS as a quantitative technique: challenges, considerations, and correlative approaches to aid validation

Surface-enhanced Raman scattering (SERS) remains a significant area of research since it's discovery 50 years ago. The surface-based technique has been used in a wide variety of fields, most prominently in chemical detection, cellular imaging and medical diagnostics, offering high sensitivity and specificity when probing and quantifying a chosen analyte or monitoring nanoparticle uptake and accumulation. However, despite its promise, SERS is mostly confined to academic laboratories and is not recognised as a gold standard analytical technique. This is due to the variations that are observed in SERS measurements, mainly caused by poorly characterised SERS substrates, lack of universal calibration methods and uncorrelated results. To convince the wider scientific community that SERS should be a routinely used analytical technique, the field is now focusing on methods that will increase the reproducibility of the SERS signals and how to validate the results with more well-established techniques. This review explores the difficulties experienced by SERS users, the methods adopted to reduce variation and suggestions of best practices and strategies that should be adopted if one is to achieve absolute quantification.

Open-Nanogap-Induced Strong Electromagnetic Enhancement in Au/AgAu Monolayer as a Stable and Uniform SERS Substrate for Ultrasensitive Detection

Nanogap-based plasmonic metal nanocrystals have been applied in surface-enhanced Raman scattering detection, while the closed and insufficient electromagnetic fields as well as the nonreproducible Raman signal of the substrate greatly restrict the actual application. Herein, a highly uniform Au/AgAu monolayer with abundant nanogaps and huge electromagnetic enhancement is prepared, which shows ultrasensitive and reproducible SERS detection. Au/AgAu with an inner nanogap is first prepared based on Au nanotriangles, and the nanogap is opened from the three tips via a subsequent etching process. The open-gap Au/AgAu displays much higher SERS efficiency than Au and Au/AgAu with an inner nanogap on detecting crystal violet due to the open-gap induced electromagnetic enhancement and improved molecular absorption. Furthermore, the open-gap Au/AgAu monolayer is prepared via interfacial self-assembly, which shows further improved SERS due to the dense and strong hotspots in the nanocavities induced by the electromagnetic coupling between adjacent open gaps. The monolayer possesses excellent signal stability, uniformity, and reproducibility. The analytic enhancement factor and relative standard deviation reach to 2.12 × 108 and 4.65% on detecting crystal violet, respectively. Moreover, the monolayer achieves efficient detection of thiram in apple juice, biphenyl-4-thiol, 4-mercaptobenzoic, melamine, and a mixed solution of four different molecules, showing great promise in practical detection.

Periodic Folded Gold Nanostructures with a Sub-10 nm Nanogap for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy has emerged as a powerful spectroscopy technique for detection with its capacity for label-free, nondestructive analysis, and ultrasensitive characterization. High-performance surface-enhanced Raman scattering (SERS) substrates with homogeneity and low cost are the key factors in chemical and biomedical analysis. In this study, we propose the technique of atomic force microscopy (AFM) scratching and nanoskiving to prepare periodic folded gold (Au) nanostructures as SERS substrates. Initially, folded Au nanostructures with tunable nanogaps and periodic structures are created through the scratching of Au films by AFM, the deposition of Ag/Au films, and the cutting of epoxy resin, reducing fabrication cost and operational complexity. Periodic folded Au nanostructures show the three-dimensional nanofocusing effect, hotspot effect, and standing wave effect to generate an extremely high electromagnetic field. As a typical molecule to be tested, p-aminothiophenol has the lowest detection limit of up to 10-9 M, owing to the balance between the electromagnetic field energy concentration and the transmission loss in periodic folded Au nanostructures. Finally, by precisely controlling the periods and nanogap widths of the folded Au nanostructures, the synergistic effect of surface plasmon resonance is optimized and shows good SERS properties, providing a new strategy for the preparation of plasmonic nanostructures.

Fabrication of silver nanostructure array patterns (SNAPs) on silicon wafer for highly sensitive and reliable SERS substrates

Metal nanostructure arrays with large amounts of nano-gaps are important for surface enhanced Raman scattering applications, though the fabrications of such nanostructures are difficult due to the complex and multiple synthetic steps. In this research, we report silver nanostructure array patterns (SNAPs) on silicon wafer, which is fabricated with semiconductor manufacturing technology, Cu2O electrochemistry deposition, and Ag In-situ oxidation-reduction growth. Benefiting from the dense and uniform distribution of Ag nanowires, the fabricated SNAPs demonstrate a very strong and uniform surface-enhanced Raman scattering (SERS) effect. The efficiency of SNAPs was investigated by using rhodamine 6G (R6G) dye as an analyte molecule. The results show that the minimum detectable concentration of R6G can reach as low as 10-11 M, and the Raman signals in the random region show good signal homogeneity with a low relative standard deviation (RSD) of 4.77 %. These results indicate that the SNAPs perform a great sensitivity and uniformity as a SERS substrate. Furthermore, we used the SNAPs substrate to detect antibiotic sulfadiazine. The main peaks in sulfadiazine Raman and vibration modes assignments were obtained and the quantitative analysis model was established by principal component analysis (PCA). The detection and application results of sulfadiazine indicate that the SNAPs substrate can be applied for trace detection of antibiotics. In addition, we have cited the application of the SNAPs substrate in anti-counterfeiting labels. These practical applications demonstrate that the fabricated SNAPs can potentially provide a way to develop low-cost SERS platforms for environmental detections, biomedicine analysis, and commodities anti-counterfeiting.

Nanorod reassembling on a sprayed SERS substrate under confined evaporation inducing ultrasensitive TPhT detection

Triphenyltin is an estrogen like pollutant that poses significant environmental threats due to its highly accumulative toxicity. To improve regulation, a fast and sensitive detection method is urgently needed. SERS can capture fingerprint information and is capable of trace detection, making it an ideal solution. Here, we present a sprayed substrate comprised of lightconfining structures and gold nanorod assemblies that are easy to prepare, low-cost, and can form dense hotspots under confined evaporation. The substrates are three-layered: initially, a gold nanorod layer is sprayed as a support, then sputter Ag film on the surface to form a lightconfining structure, followed by another gold nanorod layer sprayed on the Ag film. The coupling of nanorod assembly with lightconfining Ag films leads to 10-fold sensitivity. In addition, sample droplet evaporation in a limited area called confined evaporation contributes to nanorod migration and reassembly on the corner of the substrate, enhancing analytes absorption, and substantially lowered the detection limits. By systematically evaluating the substrate performance, we were able to obtain an average enhancement factor of 3.31 × 106. After confined evaporation, the detection limit reached 10-18 M for R6G and for triphenyltin, it achieved 10-9 M. This novel method represents a significant advancement toward SERS application in detecting trace pollutants.

Spectroscopic analyses of particle and energy aggregations at the interface of silver nanoparticles and fluorescent carbon nanodots

We study the change in the surface electromagnetic field provided by photoexcited silver nanoparticles as the field is disturbed by fluorescent carbon nanodots. Fluorescent carbon nanodots with an appropriate quantity and quality of surface functional groups are used to mediate the aggregation of silver nanoparticles of matching size and shape to form available nano-size conical structures. Carbon nanodots in the composite absorb and transfer additional photoenergy to the silver surface, resulting in energy aggregation within the cone structure and enhancement of the electromagnetic field in proximity to the silver surface. This elevated energy state is manifested in the strengthening of the SERS signal of the analytical probe 4-aminophenyl disulfide and the mechanism involved is elucidated by additional molecular spectroscopy studies.

Flexible SERS Substrate with a Ag-SiO2 Cosputtered Film for the Rapid and Convenient Detection of Thiram

It is very important to build uniform large-area dense hotspots to improve the surface-enhanced Raman scattering (SERS) detection limit. In our research, we designed and prepared a new flexibile SERS substrate with ultradense hot spots that has the advantages of high sensitivity, good repeatability, easy fabrication, and low cost. Due to the special dense hot spot structure, the substrate reaches a SERS enhancement factor of 2.1 × 10¹¹. Because of the excellent physical stability of polydimethylsiloxane, the substrate can be bent at will, and the SERS performance will not change with bending. This is very important to extract effective detection objects on complex surfaces. The substrate has good light transmittance and softness and can be directly attached to the detected agricultural products to realize real-time and rapid SERS monitoring. This structure exhibits extraordinary performance for thiram detection in the ultralow concentration range of 10^-13 M.

Semiconductor-based surface enhanced Raman scattering (SERS): from active materials to performance improvement

Surface enhanced Raman scattering (SERS) is a powerful spectral analysis technique and has exhibited remarkable application prospects in various fields. The design and fabrication of high-performance SERS substrates is key to promoting the development of SERS technology. Apart from noble metal substrates, non-metal substrates based on semiconductor materials have received increasing attention in recent years owing to their unique physical, chemical, and optical properties. However, compared with noble metal substrates, most semiconductor substrates show weak Raman enhancement ability. Therefore, exploring effective strategies to improve the SERS sensitivity is an urgent task. Numerous reviews have outlined the research progress of semiconductor SERS substrates, which mainly focused on summarizing the material category of semiconductor substrates. However, reviews that systematically summarize the strategies for improving the SERS performance of semiconductor substrates are lacking. In this review, we comprehensively discuss the research on semiconductor SERS from the aspects of mechanism, materials, and modification. Firstly, the Raman enhancement mechanism of semiconductor substrates and the SERS-active materials are discussed. Then, we summarize several effective approaches to boost the SERS performance of semiconductor substrates. In conclusion, we propose some prospects for this field.

Inter-coffee-ring effects boost rapid and highly reliable SERS detection of TPhT on a light-confining structure

Triphenyltin chloride (TPhT) is a widely applied toxic compound that poses a significant threat to humans and the environment. Surface-enhanced Raman spectroscopy (SERS), capable of non-destructive, rapid, and trace detection, is desirable to better evaluate its distribution and content. However, a sensitive method with simple measuring protocols which maintains excellent reproducibility remains challenging. Here, we proposed an inter-coffee-ring effect to accelerate the sampling and measuring process while maintaining highly reproducible results. Two overlapping coffee-rings are formed through sequenced drying of gold nanorod colloids and a gold nanorod TPhT mixture on a superhydrophobic light-confining structure. Both the gold nanorods and the TPhT are enriched in the overlapping region. The gold nanorods reordered in such an area under the inter-coffee-ring effect yielded vast numbers of consistent hotspots at the sub-2 nm level. Such consistency leads to excellent SERS performance under the light-confining effect induced by the nanoarray substrates. The detection limits of the probe molecule R6G reached 10⁻¹² M, and TPhT reached 10⁻⁸ M while achieving excellent stability and reproducibility, and a linear regression coefficient above 0.99 was achieved for TPhT. Crucially, the visible nature of the inter-coffee-ring overlap enabled rapid measurements, thus providing robust support for detecting environmental pollutants.

Nanoparticles reassembling in the inter coffee-ring region simply through sequenced drying of two droplets enabled ultrasensitive and highly reliable SERS detection. A rapid test protocol is realized by exciting the visible inter-coffee-ring mark.