Atomically resolved phonons localized at defects in monolayer graphene

Phonons at defects in materials have long attracted interest, not only because defects are inevitable in materials preparation, but also because defects can be used to engineer desirable properties. Advances in scanning transmission electron microscopy have recently enabled electron-energy-loss spectroscopy (EELS) with energy resolution similar to that of optical spectroscopies, with the extra advantage of atomic-scale spatial resolution. Here we report atomic-resolution EELS of phonons localized at various impurities in graphene, mapping the energy-loss intensity at the surrounding atomic shells, and analyze them using density-functional-theory (DFT) calculations of phonon densities of states (DOS) projected on individual atoms. The agreement between DOS and EELS enables the identification of the observed phonons and provides insights in the role of the defect atomic structures and local symmetries in controlling the localization of the various phonon modes. This research lays the groundwork for experimental measurements and theoretical understanding of phonons associated with defects that can play a role in thermal properties of materials such as thermal expansion, thermal conductivity etc.