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

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.