Defects on the atomic scale lead to dramatic local changes in materials that collectively transform the macroscopic properties of the host material. The characterization of defects in 2D materials and an understanding of their formation is therefore important in order to produce consistent material properties and to explore methods of precise tuning. Alternatively, the chaotic formation of defects can be exploited in order to make unclonable keys and security tags [1]. Atomic resolution imaging techniques provide an unparalleled insight into 2D material defects, particularly scanning probe microscopy methods [2,3] which can identify electronic properties unique to specific defect types.
Here, we show that defects can be identified with atomic resolution using conductive atomic force microscopy (cAFM) performed in an ambient environment on monolayer Transition Metal Dichalcogenide (TMD) samples prepared via mechanical exfoliation. We investigate the frequency of these defects on samples of MoS2, WSe2 and WS2 identifying preferences for specific defect types dependant on TMD material. By using AFM feedback for topography scans, whilst simultaneously measuring conductance, it is possible to achieve atomic resolution of defects within the bandgap of 2D material layers, suggesting that atomic resolution imaging of insulators such as hexagonal Boron Nitride may be possible. Through correlation with x-ray photoelectron, and photoluminescence, spectroscopy it is possible to gain further insight into the role of these defects on the optical properties relevant to security devices based on TMD materials.
1. Yameng Cao et al. "Optical identification using imperfections in 2D materials". 2D Mater. 4, 4, 045021 (2017).
2. Sara Barja et al. "Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides." Nat Commun 10, 3382 (2019).
3. Matthew R. Rosenberger et al. "Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS2." ACS Nano 12, 2, 1793?1800 (2018).