Visualization of subnanometric phonon modes in a plasmonic nano-cavity via ambient tip-enhanced Raman spectroscopy

Abstract

Phonons provide information on the physicochemical properties of a crystalline lattice from the material's vibrational spectrum. Optical phonons, in particular, can be probed at both micrometre and nanometre scales using light-based techniques, such as, micro-Raman and tip-enhanced Raman spectroscopy (TERS), respectively. Selection rules, however, govern the accessibility of the phonons and, hence, the information that can be extracted about the sample. Herein, we simultaneously observe both allowed and forbidden optical phonon modes of defect-free areas in monolayer graphene to study nanometre scale strain variations and plasmonic activation of the Raman peaks, respectively, using our home-built TERS system in ambient. Through TERS imaging, strain variations and nanometre-sized domains down to 5 nm were visualised with a spatial resolution of 0.7 nm. Moreover, such subnanometric confinement was found to activate not only the D and D' forbidden phonon modes but also their D + D' combination mode. With our TERS in ambient system, the full phonon characterisation of defect-free graphene and other 2D nanomaterials is now possible, which will be useful for subnanometre strain analysis and exploring the inherent properties of defect-free materials.

Electronic and vibrational surface-enhanced Raman scattering: from atomically defined Au(111) and (100) to roughened Au

“…There are indeed several reports on plasmon-mediated optical transitions. 21,71,72 As shown in Fig. 7, however, the evolution of the sharp features is more rapid than that of the plasmon resonance.…”

Atomic Point Contact Raman Spectroscopy of a Si(111)-7 × 7 Surface

“…Cooling the system to cryogenic temperature provides exceptional thermal stability. Remarkably, low-temperature tip-enhanced Raman spectroscopy (TERS) has recently proved the unprecedented chemical sensitivity with even submolecular spatial resolution. − TERS is a promising technique in wide-ranging fields including single-molecule spectroscopy, − electrochemistry, , heterogeneous catalysis, , biomolecular identification, − and 2D materials characterization, − bearing a great potential as nanoscale chemical microscopy. In most cases, however, the underlying enhancement mechanism of the Raman scattering intensity relies largely on the extreme field enhancement in plasmonic nanogaps. − This imposes a severe limitation on measurable systems, thus typically requiring a plasmonic substrate.…”

Graphene Confers Ultralow Friction on Nanogear Cogs

“…One can get a higher spatial resolution up to 25-40 nm and enhanced Raman scattering signals through tip-enhanced Raman spectroscopy for monitoring the contribution from the individual groove of the graphene covered textured surface. [55] This technique would be useful to investigate the change in the graphene lattice over the grooves and its associated electronic structure to be considered in near future.…”