Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy

Abstract

Molecule-surface interactions and processes are at the heart of many technologies, including heterogeneous catalysis, organic photovoltaics, and nanoelectronics, yet they are rarely well understood at the molecular level. Given the inhomogeneous nature of surfaces, molecular properties often vary among individual surface sites, information that is lost in ensemble-averaged techniques. In order to access such site-resolved behavior, a technique must possess lateral resolution comparable to the size of surface sites under study, analytical power capable of examining chemical properties, and single-molecule sensitivity. Tip-enhanced Raman spectroscopy (TERS), wherein light is confined and amplified at the apex of a nanoscale plasmonic probe, meets these criteria. In ultrahigh vacuum (UHV), TERS can be performed in pristine environments, allowing for molecular-resolution imaging, low-temperature operation, minimized tip and molecular degradation, and improved stability in the presence of ultrafast irradiation. The aim of this review is to give an overview of TERS experiments performed in UHV environments and to discuss how recent reports will guide future endeavors. The advances made in the field thus far demonstrate the utility of TERS as an approach to interrogate single-molecule properties, reactions, and dynamics with spatial resolution below 1 nm.

Interface Characterization and Control of 2D Materials and Heterostructures

“…The recent integration of UHV STM with magnetic spin resonance and radio‐frequency excitation is helping advance the temporal resolution and chemical sensitivity of STM. Similarly, the combination of STM and Raman spectroscopy has enabled UHV tip‐enhanced Raman spectroscopy as a tool to probe surface chemistry down to the single molecule limit . Finally, the application of artificial intelligence to materials research is beginning to reshape the discovery of new materials including 2D systems.…”

Selective excitation of one among the three peaks of tip‐enhanced Raman spectroscopy by a shaped ultrafast laser pulse

“…In the former sections, the probe point was placed at the coordinate origin (0, 0, 0) that was the center of the upper surface of the molecule layer. The light electric field was highly confined at the tip apex for the lateral incident laser . At different points, the intensities varied greatly, as shown in Figures and .…”

“…Figures and show the distributions of transient electric field intensity of the pump and the Stokes light. When the TERS is obliquely illuminated with a linearly polarized light, the electric field is confined at the tip apex . For the pump light at the time of 4,948.6 fs, Figure a shows that the transient intensity increases to 1.11 at the position of (0, 0, 0) and 3.19 at (0, 0, 1.5).…”

“…Combining the utility of scanning probe microscopy and Raman scattering, tip‐enhanced Raman spectroscopy (TERS) has become an ultrasensitive detection technique, and it is widely used in the fields of atomic structure, catalytic processes, and the redox behavior of single molecules . The selective Raman enhancement has been studied through the modifications and functionalization of TERS probes and controlling of surface plasmon polarizations and light polarizations . In order to break the limit of the scattering cross sections and reduce background noise, silver nanoparticles were employed as nanoantennas to couple free‐space excitation light to propagating surface plasmon polarizations in a sharp‐tip silver nanowire to excite Raman signals remotely .…”

“…Using electrochemical atomic force microscopy, the TERS intensity maps represented well the spatially dependent redox behavior of Nile Blue on Au nanoplate on indium tin oxide electrode . Moreover, Raman intensity in TERS depends strongly on the incident light polarization . Localized enhancement of electric field was studied using radially and linearly polarized light.…”