Low-Background Tip-Enhanced Raman Spectroscopy Enabled by a Plasmon Thin-Film Waveguide Probe

Remote Excitation of Tip-Enhanced Photoluminescence with a Parallel AgNW Coupler

Tip-enhanced photoluminescence (TEPL) microscopy allows for the correlation of scanning probe microscopic images and photoluminescent spectra at the nanoscale level in a similar way to tip-enhanced Raman scattering (TERS) microscopy. However, due to the higher cross-section of fluorescence compared to Raman scattering, the diffraction-limited background signal generated by far-field excitation is a limiting factor in the achievable spatial resolution of TEPL. Here, we demonstrate a way to overcome this drawback by using remote excitation TEPL (RE-TEPL). With this approach, the excitation and detection positions are spatially separated, minimizing the far-field contribution. Two probe designs are evaluated, both experimentally and via simulations. The first system consists of gold nanoparticles (AuNPs) through photoinduced deposition on a silver nanowire (AgNW), and the second system consists of two offset parallel AgNWs. This latter coupler system shows a higher coupling efficiency and is used to successfully demonstrate RE-TEPL spectral mapping on a MoSe2/WSe2 lateral heterostructure to reveal spatial heterogeneity at the heterojunction.

Centrifugal Extraction-Assisted Fiber-Enhanced Raman Spectroscopy for Online Detection of Trace Furfural in Oil Power Equipment

Oil-paper insulated equipment is integral in power conversion and supports low-loss electricity transport. As a characteristic byproduct of the oil-paper insulation system, the realization of efficient detection of furfural in oil is crucial to the safe operation of the power grid. We proposed a novel approach using dual-enhanced Raman spectroscopy for sensing trace liquid components. This method employs a centrifugal extractor to separate and enrich the targeted components, achieving selective enhancement. The optimal phase ratio was determined to be 30:1. A liquid-core fiber was used to optimize the laser transmission efficiency and Raman signal collection efficiency, resulting in a nonselective signal enhancement of 44.86. It also investigated the impact of intermolecular interactions on the shift of Raman spectra, identifying the reasons for the differences in Raman signals between pure furfural, furfural in oil, and furfural in water. A batch of samples with furfural dissolved in insulation oil was measured using this system and achieved a limit of detection of 0.091 mg/L. The stability of the dual-enhanced Raman platform was experimentally verified with a spectral intensity fluctuation of 0.68%. This method is fast, stable, adaptable, and suitable for the detection of a wide range of liquid ingredients.

Progress of tip-enhanced Raman scattering for the last two decades and its challenges in very recent years

Tip-enhanced Raman scattering (TERS) has recently attracted remarkable attention as a novel nano-spectroscopy technique. TERS, which provides site-specific information, can be performed on any material surface regardless of morphology. Moreover, it can be applied in various environments, such as ambient air, ultrahigh vacuum (UHV), solutions, and electrochemical environments. This review reports on one hand progress of TERS for the last two decades, and on the other hand, its challenges in very recent years. Part of the progress of TERS starts with the prehistory and history of TERS, and then, the characteristics and advantages of TERS are described. Significant emphasis is put on the development of TERS instrumentation and equipment such as ultrahigh vacuum TERS, liquid TERS, electrochemical-TERS, and tip-preparations. Applications of TERS, particularly those with nanocarbons, biological materials, and surface and interface analysis, are mentioned in some detail. In the part on challenges, we focus on the very recent advances in TERS; progress in spatial resolution to the angstrom scale is the hottest topic. Recent TERS studies performed under UHV, for example chemical imaging at the angstrom scale and Raman detection of bond breaking and making of a chemisorbed up-standing single molecules at single-bond level, are reviewed. Of course, there is no clear border between the two parts. In the last part the perspective of TERS is discussed.

Experimental characterization techniques for plasmon-assisted chemistry

Plasmon-assisted chemistry is the result of a complex interplay between electromagnetic near fields, heat and charge transfer on the nanoscale. The disentanglement of their roles is non-trivial. Therefore, a thorough knowledge of the chemical, structural and spectral properties of the plasmonic/molecular system being used is required. Specific techniques are needed to fully characterize optical near fields, temperature and hot carriers with spatial, energetic and/or temporal resolution. The timescales for all relevant physical and chemical processes can range from a few femtoseconds to milliseconds, which necessitates the use of time-resolved techniques for monitoring the underlying dynamics. In this Review, we focus on experimental techniques to tackle these challenges. We further outline the difficulties when going from the ensemble level to single-particle measurements. Finally, a thorough understanding of plasmon-assisted chemistry also requires a substantial joint experimental and theoretical effort.