In tip-enhanced Raman spectroscopy (TERS), a single nanoscale plasmonic antenna, e.g., a Au STM tip, under laser irradiation creates a strong and highly localized nearfield in the tip-sample gap. Target species located directly in this few-nanometer sized ‘hot spot’ give rise to particularly intense Raman scattering. The TERS antenna can be positioned strategically on the substrate surface to probe adsorption or reaction sites of specific interest. TERS offers extreme chemical spatial resolution in the range of a few nanometers or better, surface sensitivity down to single molecules and simultaneous topographic information through the retained STM imaging capability. While TERS in air has matured considerably as a surface nano-analysis tool over the past 15 years, its variant to study electrified solid/liquid interfaces, EC-TERS, has only recently been introduced.
Coherent anti-Stokes Raman spectroscopy (CARS)
One way to enhance the inherently weak Raman scattering from confined molecules is to enforce coherent scattering with short, intense laser pulses. In coherent anti-Stokes Raman spectroscopy (CARS), three photons interact with the sample molecules in a way that a fourth photon at the anti-Stokes energy is scattered. Through the coherent signal emission and nonlinear nature of the wave-mixing approach, the CARS signal is enhanced by three to five orders of magnitude compared to conventional Raman scattering, rendering CARS a valuable novel tool for in situ quantitative monitoring of dynamic molecular processes on the nanoscale. Commonly, CARS is employed in the field of bio(medical) imaging and has only recently been introduced to the field of materials science.
There is a rising interest in surface plasmon resonance imaging (SPRi) in electrochemical surface science after decades of a break, e.g., to study electroadsorption or electrocatalytic surface activity. Surface plasmons (plasmon polaritons) are collective charge density oscillations confined to a solid/dielectric interface that decay strongly in the direction normal to the interface and whose properties depend on the dielectric functions of the interface. Therefore, SPRi provides a sensitive tool for determining optical properties of an (electrochemical) interface, such as the refractive index and thickness of an adsorbate layer.
Electrochemical scanning tunneling microscopy and spectroscopy (EC-STM/STS)
Scanning tunnelling microscopy and spectroscopy (STM/STS) is routinely employed in surface science to investigate the electronic structure and apparent atomic-scale topography of surfaces in UHV or air. For the study of electrochemical interfaces, a bipotentiostat is used to independently control the potentials of the probe (tip) and substrate (and thus also the bias) in a four-electrode configuration. In this way, STM images and tunnelling spectra can be obtained under in-operando conditions, i.e. as a function of applied electrode potential.