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

Balois, M. V.*; Hayazawa, Norihiko*; Yasuda, Satoshi   ; Ikeda, Katsuyoshi*; Yang, B. *; Kazuma, Emiko*; Yokota, Yasuyuki*; Kim, Y.*; Tanaka, Takuo*

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 Tip-enhanced Raman scattering (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.