Imaging Vacancy Defects in Single-Layer Chromium Triiodide.

As a van der Waals magnetic semiconductor, chromium triiodide (CrI) is widely considered for its high research value and potential applications. Defects in CrI are inevitably present and significantly alter the material properties. However, experimental identification of defects of CrI at the atomic level is still lacking. Here for the first time, we carried out a scanning tunneling microscopy (STM) study and density functional theory calculations to explore the intrinsic defects in monolayer CrI grown by molecular beam epitaxy. The three most common types of intrinsic point defects, i.e., I vacancy (V), Cr vacancy (V), and multiatom CrI vacancy (V) with distinct spatial distributions of the localized defect states, are identified and characterized by high-resolution STM. Moreover, defect concentrations are estimated based on our experiments, which agree with the calculated formation energies. Our findings provide vital knowledge on the types, concentrations, electronic structures, and migration mechanism of the intrinsic point defects in monolayer CrI for future defect engineering of this novel 2D magnet.