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.

声子可根据材料的振动光谱提供有关晶格的物理化学性质的信息。尤其是，可以分别使用基于光的技术（例如微拉曼光谱和尖端增强拉曼光谱法（TERS））在微米和纳米尺度上探测光学声子。但是，选择规则支配着声子的可及性，因此支配着可以提取的有关样品的信息。在本文中，我们同时观察到单层石墨烯中无缺陷区域的允许和禁止的光学声子模式，以使用我们在环境中构建的TERS系统分别研究纳米级应变变化和拉曼峰的等离子体激元活化。通过TERS成像，应变分辨率和低至5 nm的纳米级域都可以以0的空间分辨率看到。7纳米。此外，发现这种亚纳米限制不仅激活了D和D'禁止声子模式，但也禁止D  +  D'组合模式。借助我们在环境系统中的TERS，现在可以对无缺陷的石墨烯和其他2D纳米材料进行完整的声子表征，这对于亚纳米应变分析和探索无缺陷材料的固有特性将非常有用。