Tip-enhanced Raman spectroscopy
Tip-enhanced Raman spectroscopy is a specialist approach to surface-enhanced Raman spectroscopy (SERS) in which enhancement of Raman scattering occurs only at the point of a near atomically sharp pin, typically coated with gold. The maximum resolution achievable using an optical microscope, including Raman microscopes, is limited by the Abbe limit, which is approximately half the wavelength of the incident light. Furthermore, with SERS spectroscopy the signal obtained is the sum of a relatively large number of molecules. TERS overcomes these limitations as the Raman spectrum obtained originates primarily from the molecules within a few  tens of nanometers of the tip. History. The earliest reports of tip enhanced Raman spectroscopy typically used a Raman microscope coupled with an atomic force microscope. Tip-enhanced Raman spectroscopy coupled with a scanning tunneling microscope (STM-TERS) has also become a reliable technique, since it utilizes the gap mode plasmon between the metallic probe and the metallic substrate. Equipment. Tip-enhanced Raman spectroscopy requires a confocal microscope, and a scanning probe microscope. The optical microscope is used to align the laser focal point with the tip coated with a SERS active metal. The three typical experimental configurations are bottom  illumination, side illumination, and top illumination, depending on which direction the incident laser propagates towards the sample, with respect to the substrate. In the case of STM-TERS, only side and top illumination configurations can be applied, since the substrate is required to be conductive, therefore typically being non-transparent. In this case, the incident laser is usually linearly polarized and aligned parallel to the tip, in order to generate confined surface plasmon at the tip apex. The sample is moved rather than the tip so that the laser remains focused on the tip. The sample can be moved systematically  to build up a series of tip enhanced Raman spectra from which a Raman map of the surface can be built allowing for surface heterogeneity to be assessed with up to 1.7 nm resolution. Subnanometer resolution has been demonstrated in certain cases allowing for submolecular features to be resolved. In 2019, Yan group and Liu group at University of California, Riverside developed a lens-free nanofocusing technique, which concentrates the incident light from a tapered optical fiber to the tip apex of a metallic nanowire and collects the Raman signal through the same optical fiber. Fiber-in-fiber-out NSOM -TERS has been developed. Applications. Several research have used TERS to image single atoms and the internal structure of the molecules. In 2019, the Ara Apkarian group at the Center for Chemistry at the Space-Time Limit, University of California, Irvine imaged vibrational normal modes of single porphyrin molecules using TERS. TERS-based DNA sequencing has also been demonstrated. TERS has also been used for ion-selective, atom-resolved imaging of a 2D CuN insulator using a functionalized tip.