Conductive atomic force microscopy (c-AFM) has recently been used as an effective technique for visualizing point defects in transition metal dichalcogenides (TMDs) [1,2]. However, because monolayer TMDs are only three atoms thick, direct tunneling between AFM tip and conductive substrate produces a large background current, making defect-related signals difficult to distinguish. To suppress this background, we have focused on the use of hBN as a tunneling barrier (Fig. 1(a)). In this study, we systematically investigated the effects of the number of hBN layers on current contrast of point defects in monolayer WSe₂ as well as on its current–voltage characteristics.

Monolayer WSe₂ was grown on graphite by chemical vapor deposition (CVD), followed by the transfer of few-layer hBN onto the WSe₂. We first confirmed that increasing the number of hBN layers suppressed direct tunneling current between the tip and graphite. In the bilayer hBN/monolayer WSe₂/graphite heterostructure, clear current variations originating from point defects in monolayer WSe₂ were observed (Fig. 1(b)), whereas the defect contrast became weaker for four-layer hBN because of the reduced tunneling current. Furthermore, the dI/dV spectra measured through three- and four-layer hBN exhibited features reflecting the approximately 2 eV band gap of monolayer WSe₂. These results show that thinner hBN favors defect imaging, whereas thicker hBN is better suited for spectroscopy by suppressing substrate tunneling. Thus, hBN thickness control is important for c-AFM measurements of monolayer TMDs.

1. K. Xu, et al., ACS Nano 17, 24743 (2023). DOI: 10.1021/acsnano.3c05056
2. N. T. T. Vu, et al., ACS Nano 17, 15648 (2023) DOI: 10.1021/acsnano.3c02758