Optimizing electromagnetic enhancement of flexible nano-imprinted hexagonally patterned surface-enhanced Raman scattering substrates

Optimization of physical vapor deposition process for low background nanoimprinted SERS substrate in quantitative melamine analysis

Uniformity, sensitivity, reproducibility, and cost are the critical parameters of practical surface-enhanced-Raman-spectroscopy (SERS) substrates. Herein, we proposed a High-Aspect-Ratio-Nano-Pillar-Array (HARNPA) substrate deposited silver by physical vapor deposition (PVD) methods (e.g. E-beam evaporation, sputtering, and a two-stage intermittent sputtering) to fabricate high-performance SERS substrates. The substrate by the E-beam evaporation has a significant SERS effect, but the Raman background induced by the exposure of the polymer HARNPA limits the analyte choice. The substrate by the sputtering method has better step coverage of silver but a lower enhancement factor. Therefore, we proposed a process of two-stage intermittent sputtering to solve these limitations. In addition, we define a factor called the signal-to-background peak ratio (S/B peak ratio) to evaluate the influence of the Raman background from the SERS substrate. Finally, we accomplished a SERS substrate with an S/B peak ratio of 3.48 by intermittent sputtering, which has the best linearity (R2 = 0.97) of the melamine concentration curve and the lowest detection limit (LoD = 5.6 × 10-7 M) that meets the regulatory requirements for melamine detection (3.96 × 10-6 M). The benefits of our SERS substrates are easy fabrication, high sensitivity (EF = 1.44 × 107), high reproducibility (CV = 8.4 %), and excellent uniformity (CV = 7 % in 4″ area), which are beneficial for mass production in the future.

Development of an automated Raman system and use of principal component analysis to classify real and counterfeit liquors

We developed an automated Raman measurement platform for the customized design of various solution containers. We used the software LabVIEW to integrate the entire automatic measurement process. By designing an intuitive human-machine interface, the user only needs to input a few setting parameters and can efficiently operate the machine in automation mode for an array of solutions containing real or counterfeit liquors such as kaoliang liquor, vodka, rum, gin, rice wine, ethanol, and methanol. In this study, data from various alcoholic beverage solutions were subjected to principal component analysis (PCA) to distinguish from the low-concentration counterfeit liquors (methanol <50 g L-1). Moreover, several brands of liquors with the same alcohol concentration were successfully classified into different groups based on a combination of Raman spectroscopy and PCA analysis.

In situ detection of trace pollutants: a cost-effective SERS substrate of blackberry-like silver/graphene oxide nanoparticle cluster based on quick self-assembly technology

To realize fast detection of trace hazardous chemicals, a SERS substrate with the structure of a blackberry-like silver/graphene oxide nanoparticle cluster (Ag/GO NPC) has been designed and prepared through a quick capillarity-assistant self-assembly technology in this paper. Benefitting from the abundant "hot spots" and active oxygen sites brought by this Ag/GO NPC, the substrate shows good Raman performance for malachite green (MG), a common abusive germicide in aquaculture, with lowest limit of detection below 0.1 µg/L (3.48 × 10^-10 mol/L). Detailed analyses are taken on both the formation process and enhancement mechanism of this SERS substrate, and the finite-difference time-domain simulations are utilized as well to prove our hypotheses. Further constructing this structure on polyethylene terephthalate (PET) film, a translucent flexible SERS substrate can be obtained, realizing a fast in situ detection of trace MG in the fishpond subsequently. In consideration of the facile preparation process, good SERS enhancement and affordable materials (PET, Cu, Ag and GO, etc.), this substrate presents high cost performance and a promising application prospect.

Novel aluminum plasmonic absorber enhanced by extraordinary optical transmission

We report a theoretical and experimental study on a novel type of aluminum super absorber which exhibits a near perfect absorption based on the surface plasmon resonance in the visible and near-infrared spectrum. The absorber consists of Ag/SiO₂/Al triple layers in which the top Al layer is patterned by a periodic nano hole array. The absorption spectrum can be easily controlled by adjusting the structure parameters including the radius of the nano hole and the maximal absorption can reach 99.0% in theory. We completely analyze the SPP and LSP modes supported by the metal-dielectric-metal structure and their contribution to the ultrahigh absorption. On this basis, we find a novel method to enhance the absorption via the simultaneous excitation of SPP at different interfaces theoretically and experimentally. Moreover, for the first time we clarify the EOT caused by the nano hole array can enhance the absorption by experiment, which is not reported in previous works. This kind of absorber can be fabricated by low-cost colloidal sphere lithography and the use of stable Al overcomes the disadvantages brought by the noble metal, which make it a more appropriate candidate for photovoltaics, spectroscopy, photodetectors, sensing, and surface enhanced Raman scattering.

Advances in Nanoimprint Lithography

Nanoimprint lithography (NIL), a molding process, can replicate features <10 nm over large areas with long-range order. We describe the early development and fundamental principles underlying the two most commonly used types of NIL, thermal and UV, and contrast them with conventional photolithography methods used in the semiconductor industry. We then describe current advances toward full commercial industrialization of UV-curable NIL (UV-NIL) technology for integrated circuit production. We conclude with brief overviews of some emerging areas of research, from photonics to biotechnology, in which the ability of NIL to fabricate structures of arbitrary geometry is providing new paths for development. As with previous innovations, the increasing availability of tools and techniques from the semiconductor industry is poised to provide a path to bring these innovations from the lab to everyday life.

Plasmon enhanced Raman scattering effect for an atom near a carbon nanotube

Quantum electrodynamics theory of the resonance Raman scattering is developed for an atom in a close proximity to a carbon nanotube. The theory predicts a dramatic enhancement of the Raman intensity in the strong atomic coupling regime to nanotube plasmon near-fields. This resonance scattering is a manifestation of the general electromagnetic surface enhanced Raman scattering effect, and can be used in designing efficient nanotube based optical sensing substrates for single atom detection, precision spontaneous emission control, and manipulation.

Surface-enhanced Raman spectroscopy (SERS): progress and trends

Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.