Tip-enhanced Raman spectroscopy: principles, practice, and applications to nanospectroscopic imaging of 2D materials

Abstract

Two-dimensional (2D) materials have been one of the most extensively studied classes of modern materials, due to their astonishing chemical, optical, electronic, and mechanical properties, which are different from their bulk counterparts. The edges, grain boundaries, local strain, chemical bonding, molecular orientation, and the presence of nanodefects in these 2D monolayers (MLs) will strongly affect their properties. Currently, it is still challenging to investigate such atomically thin 2D monolayers with nanoscale spatial resolution, especially in a label-free and non-destructive way. Tip-enhanced Raman spectroscopy (TERS), which combines the merits of both scanning probe microscopy (SPM) and Raman spectroscopy, has become a powerful analytical technique for studying 2D monolayers, because it allows very high-resolution and high-sensitivity local spectroscopic investigation and imaging, and also provides rich chemical information. This review provides a summary of methods to study 2D monolayers and an overview of TERS, followed by an introduction to the current state-of-the-art and theoretical understanding the spatial resolution in TERS experiments. Surface selection rules are also discussed. We then focus on the capabilities and potential of TERS for nanoscale chemical imaging of 2D materials, such as graphene, transition metal dichalcogenides (TMDCs), and 2D polymers. We predict that TERS will become widely accepted and used as a versatile imaging tool for chemical investigation of 2D materials at the nanoscale.

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

“…For the Stokes light at the time of 4,818.8 fs, the transient intensity increases to 0.72 at the position of (0, 0, 0) and 2.06 at (0, 0, 1.5). The electric field intensity increases rapidly as the probe point approaches to the tip because of the lightning rod effect . In the XY plane, the electric field distribution is not circularly symmetrical when the radius is larger than 8.0 nm because of the obliquely incident light and the propagation and coupling among multiple surface plasmon modes .…”

“…The electric field intensity increases rapidly as the probe point approaches to the tip because of the lightning rod effect . In the XY plane, the electric field distribution is not circularly symmetrical when the radius is larger than 8.0 nm because of the obliquely incident light and the propagation and coupling among multiple surface plasmon modes . However, in the highly focused region within a radius of 6.0 nm, the electric field intensity is circularly distributed.…”

“…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 .…”

“…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 . Giant enhancement and excellent spatial resolution from a monolayer MoS 2 stamped onto Au nanocluster arrays were demonstrated using gap mode of atomic force microscopy TERS . Due to strong hot electron doping through the plasmonic coupling between MoS 2 and Au nanoclusters, the TERS spectra indicated a structural change of MoS 2 from the 2H to the 1T phase.…”

“…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.…”