The dielectric constant (ε) is a key parameter to consider when determining the fundamental electrical properties of the charging and screening of charges in optoelectronic devices based on two-dimensional (2D) layered materials with van der Waals interactions. In this study, we report a direct local mapping of the thickness-dependent ε of MoS2 nanoflakes using a nondestructive electrostatic force microscopy (EFM) imaging technique. EFM is used to simultaneously probe the thickness and local ε value of regions with different thicknesses by detecting a cantilever deflection while applying DC bias voltage between the conducting tip and the substrate. The measured ε increases with the thickness and saturates to the bulk value, consistent with previous theoretical and experimental results of dielectric constants for MoS2 at different thicknesses. Our works provide quantitative information pertaining to the thickness-dependent electric permittivity, which can be useful in quantitative design of high-performance and multifunctional nanoelectronic devices based on layered 2D materials.

Understanding the interlayer charge coupling mechanism in a two-dimensional van der Waals (vdW) heterojunction is crucial for optimizing the performance of heterostructure-based (opto)­electronic devices. Here, we report mapping the gate response of a multilayer WSe2/MoS2 heterostructure with locally different degrees of charge depletion through mobile carrier measurements based on electrostatic force microscopy. We observed ambipolar or unipolar behavior depending on the degree of charge depletion in the heterojunction under tip gating. Interestingly, the WSe2 on MoS2 shows gating behavior that is more efficient than that on the SiO2/Si substrate, which can be explained by the high dielectric environment and screening of impurities on the SiO2 surface by the MoS2. Furthermore, we found that the gate-induced majority carriers in the heterojunction reduce the carrier lifetime, leading to the enhanced interlayer recombination of the photogenerated carriers under illumination. Our work provides a comprehensive understanding of the interfacial phenomena at the vdW heterointerface with charge depletion.

The operation of highly transparent optical logic circuits composed of phototransistors with QDs/ZnO heterojunctions are demonstrated. Photoexcited charge transfer mechanism was confirmed for photoinduced carriers transfer at the QDs/ZnO interfaces.