Probing Optical and Electronic Properties of Individual Defects Through Scanning Transmission Electron Microscopy and First-Principles Theory

摘要

The successful correction of lens aberrations has greatly advanced the ability of the scanning transmission electron microscope (STEM) to provide direct, real space imaging at atomic resolution [1], both through improved spatial resolution and through enhanced signal to noise ratio. In monolayer materials such as BN, graphene or transition metal dichalcogenides, atom-by-atom characterization of atomic position is now a practical reality [2], even using accelerating voltages below the threshold for knock-on damage. Stable point defect complexes consisting of substitutional Si and N atoms lead to spatially localized enhancement of surface plasmon resonances at the subnanometer scale, acting as an atomic antenna in the petaHertz (1015 Hz) frequency range [3]. Contrary to expectations, images of such excitations obtained through electron energy loss spectroscopy (EELS) are highly localized, sometimes even exhibiting atomic resolution. EELS can also directly probe the nature of bonding at the single atom level [4]. Examples of such phenomena are presented below, together with a theoretical simulation of EELS images that includes both the formation of the STEM probe and solid state bonding effects for the first time [5]. This combined experimental/theoretical technique represents an atomic-resolution variant of optical absorption and promises to be a powerful probe of optical, electronic, and vibrational properties of individual defects in materials.