RESUMO
Nanoscale bubbles form inevitably during the transfer of two-dimensional (2D) materials on a target substrate due to their van der Waals interaction. Despite a large number of studies based on nanobubble structures with localized strain, the dielectric constant (κ) in nanobubbles of MoS2 is poorly understood. Here, we report κ measurements for nanobubbles on MoS2 by probing the polarization forces based on electrostatic force microscopy. Remarkably, higher κ values of 6-8 independent of the nanobubble size are observed for the nanobubbles as compared to flat regions with a κ of ≈3. We find that the charge carrier increase owing to the strain-induced bandgap reduction is responsible for the enhanced κ of the nanobubbles, where the measured κ is in good agreement with the calculations based on the Clausius-Mossotti relation. Our results provide fundamental information about the strain-induced local dielectric properties of 2D materials and a guide for the design and fabrication of high-performance optoelectrical devices based on 2D materials.
RESUMO
We report a local mapping photoresponse of WSe2 using a second-harmonic (2w) channel based on nondestructive electrostatic force microscopy (EFM). The 2w signals resulting from interaction between WSe2 and EFM tip are intrinsically related to the electrical conductivity of WSe2. The photoresponse images and rise/decay time constants of WSe2 are obtained by local mapping 2w signals under illumination. We observe that the local photoresponse signals of WSe2 increase with the positive tip gate voltage while the WSe2 shows a p-type behavior in dark conditions We find that the reduced mobility of the photogenerated charge carriers resulting from the enhanced carrier scattering in the accumulation regime of WSe2 is responsible for the gate-dependent photoresponse behavior. Our results provide a deep understanding the intrinsic optoelectrical properties of WSe2 and contribute to the developments in the optoelectronic devices based on van der Waals layered materials.
