An analytical technique operating at the nanoscale must be flexible regarding variable experimental conditions while ideally also being highly specific, extremely sensitive, and spatially confined. In this respect, tip-enhanced Raman scattering (TERS) has been demonstrated to be ideally suited to, e.g., elucidating chemical reaction mechanisms, determining the distribution of components and identifying and localizing specific molecular structures at the nanometre scale. TERS combines the specificity of Raman spectroscopy with the high spatial resolution of scanning probe microscopies by utilizing plasmonic nanostructures to confine the incident electromagnetic field and increase it by many orders of magnitude. Consequently, molecular structure information in the optical near field that is inaccessible to other optical microscopy methods can be obtained. In this general review, the development of this still-young technique, from early experiments to recent achievements concerning inorganic, organic, and biological materials, is addressed. Accordingly, the technical developments necessary for stable and reliable AFM- and STM-based TERS experiments, together with the specific properties of the instruments under different conditions, are reviewed. The review also highlights selected experiments illustrating the capabilities of this emerging technique, the number of users of which has steadily increased since its inception in 2000. Finally, an assessment of the frontiers and new concepts of TERS, which aim towards rendering it a general and widely applicable technique that combines the highest possible lateral resolution and extreme sensitivity, is provided.

We report for the first time the tip-enhancement of resonance Raman scattering using deep ultraviolet (DUV) excitation wavelength. The tip-enhancement was successfully demonstrated with an aluminum-coated silicon tip that acts as a plasmonic material in DUV wavelengths. Both the crystal violet and adenine molecules, which were used as test samples, show electronic resonance at the 266-nm excitation used in the experiments. With results demonstrated here, molecular analysis and imaging with nanoscale spatial resolution in DUV resonance Raman spectroscopy can be realized using the tip-enhancement effect.

Localized surface plasmon resonances were controlled at deep-ultraviolet (DUV) wavelengths by fabricating aluminum (Al) nanostructures in a size-controllable manner. Plasmon resonances were obtained at wavelengths from near-UV down to 270 nm (4.6 eV) depending on the fabricated structure size. Such precise size control was realized by the nanosphere lithography technique combined with additional microwave heating to shrink the spaces in a close-packed monolayer of colloidal nanosphere masks. By adjusting the microwave heating time, the sizes of the Al nanostructures could be controlled from 80 nm to 50 nm without the need to use nanosphere beads of different sizes. With the outstanding controllability and versatility of the presented fabrication technique, the fabricated Al nanostructure is promising for use as a DUV plasmonic substrate, a light-harvesting platform for mediating strong light-matter interactions between UV photons and molecules placed near the metal nanostructure. V

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Tip-enhanced Raman spectroscopy (TERS) has recently become one of the most important tools for analyzing advanced nano-devices and nano-materials, because it allows strong enhancement of weak Raman signal from the nanometric volume of a sample. However, consistent enhancement in TERS is still an issue and scientists have been struggling to fabricate good tips for reliable, strong and reproducible enhancement. There is a strong need to study the morphology and the arrangement of metal nanostructures near the tip apex for efficient plasmonic enhancement in TERS. Here, we present a study on the metal grains attached to the tip surface for producing higher and much consistent enhancement in TERS. Our study shows that the plasmonic enhancement strongly depends on the number of grains and on the their separations. We found through simulations that multiple grains arranged closely but discretely on a dielectric probe act as an efficient plasmonic antenna and that enhancement in TERS is maximum for an optimized number of grains. The number of grains and the nano-gap between them are crucial for reproducible enhancement. This promising result, which we also demonstrate and prove by experiments, will bring TERS to a new level, where it can be utilized with more confidence of large reproducible enhancement for those nano-sized samples that have extremely weak Raman scattering.