Observing the Layer-Number-Dependent Local Dielectric Response of WSe2 by Electrostatic Force Microscopy.

We investigate the layer-number-dependent dielectric response of WSe2 by measuring the phase shift (Φ) through an electrostatic force microscopy (EFM). The measured Φ results stem mainly from the capacitive coupling between the tip and WSe2 based on the plane capacitor model, leading to changes in the second derivative of the capacitance (C'') values, which increase in a few layers and saturate to the bulk value under an applied EFM tip bias. The C'' value is related to the dielectric polarization, reflecting the charge carrier concentration and mobility of WSe2 flakes with different numbers of layers. This implies that the dielectric constant of WSe2 shows layer-number-dependent behavior which increases with the number of layers, approaching the bulk value. Furthermore, we also construct a spatially resolved C'' map to observe the local dielectric response of WSe2 flakes. Our work could be significant in that it can improve the performance of novel electronic devices based on the controllable dielectric properties of 2D vdW semiconductor materials.