Imaging Dilute Atomic Impurities in a Monolayer Semiconductor by Conductive Atomic Force Microscopy

Two-dimensional semiconductors hold great potential for nanoelectronics. One of the key challenges for the development of two-dimensional semiconducting nanoelectronics is the lack of versatile and reliable techniques for characterizing dilute impurities at concentrations below 10¹¹ cm^-2, an important regime for conventional semiconductors. In this work, we demonstrate the use of conductive atomic force microscopy (c-AFM) to achieve characterization of dilute atomic impurities in monolayer WSe₂ in ambient condition. Through comprehensive correlated imaging of substitutionally vanadium-doped WSe₂ by c-AFM and scanning transmission electron microscopy (STEM), we show that c-AFM has sufficient sensitivity to detect individual atomic dopants, allowing rapid determination of their concentration and distribution at below 0.1% doping concentrations. A semi-quantitative model is proposed to explain the single impurity conduction mechanism, which is responsible for high contrast imaging. We further perform ambient tunnelling microscopy and spectroscopy using few-layer hexagonal boron nitride (hBN) barrier layer to probe the local electronic structure of individual impurities, revealing an expected enhanced band-edge density of states of single vanadium dopant.