Multimodal Kelvin Probe Force Microscopy Investigations of a Photovoltaic WSe₂/MoS₂ Type-II Interface

Abstract

Atomically thin transition-metal dichalcogenides (TMDC) have become a new platform for the development of next-generation optoelectronic and light-harvesting devices. Here, we report a Kelvin probe force microscopy (KPFM) investigation carried out on a type-II photovoltaic heterojunction based on WSe monolayer flakes and a bilayer MoS film stacked in vertical configuration on a Si/SiO substrate. Band offset characterized by a significant interfacial dipole is pointed out at the WSe/MoS vertical junction. The photocarrier generation process and phototransport are studied by applying a differential technique allowing to map directly two-dimensional images of the surface photovoltage (SPV) over the vertical heterojunctions (vHJ) and in its immediate vicinity. Differential SPV reveals the impact of chemical defects on the photocarrier generation and that negative charges diffuse in the MoS a few hundreds of nanometers away from the vHJ. The analysis of the SPV data confirms unambiguously that light absorption results in the generation of free charge carriers that do not remain coulomb-bound at the type-II interface. A truly quantitative determination of the electron-hole (e-h) quasi-Fermi levels splitting (i.e., the open-circuit voltage) is achieved by measuring the differential vacuum-level shift over the WSe flakes and the MoS layer. The dependence of the energy-level splitting as a function of the optical power reveals that Shockley-Read-Hall processes significantly contribute to the interlayer recombination dynamics. Finally, a newly developed time-resolved mode of the KPFM is applied to map the SPV decay time constants. The time-resolved SPV images reveal the dynamics of delayed recombination processes originating from photocarriers trapping at the SiO/TMDC interfaces.

Metal Chalcogenides on Silicon Photocathodes for Efficient Water Splitting: A Mini Overview

“…measured the surface potential image of the WSe 2 /MoS 2 van der Waals heterojunction using KPFM, from which the band alignment at the monolayer interface was obtained. They confirmed the impact of the defects to the photocarrier generation and the diffusion of negative charges near the heterojunction as well as the OCP value [115], by the surface photovoltage (SPV) image observation. Li et al demonstrated the more efficient charge separation of photogenerated carriers in n-Si/MoS 2 compared to that in p-Si/MoS 2 by using KFM.…”

Microscopy of defects in semiconductors

“…Layered materials such as NbSe2 and WSe2 which can be mechanically exfoliated to produce twodimensional (2D) semiconductor samples have long been attractive to researchers using STM to study defects since cleavage provides a straightforward means of producing clean, atomically-flat surfaces [132]. As the popularity of 2D materials has increased, so has the breadth of SPM studies, expanding to encompass not only mechanically exfoliated samples but also materials grown by chemical vapour deposition [133]. Furthermore, a wide range of SPM techniques have been applied, including C-AFM (which can resolve atomic-scale periodicity in samples imaged under a dry nitrogen atmosphere [134]), KPFM and differential surface photovoltage microscopy [133].…”

“…As the popularity of 2D materials has increased, so has the breadth of SPM studies, expanding to encompass not only mechanically exfoliated samples but also materials grown by chemical vapour deposition [133]. Furthermore, a wide range of SPM techniques have been applied, including C-AFM (which can resolve atomic-scale periodicity in samples imaged under a dry nitrogen atmosphere [134]), KPFM and differential surface photovoltage microscopy [133]. STM remains, however, an extremely powerful approach to the identification of defects in 2D materials and the assessment of their impact on electronic structure.…”

2D Transition Metal Dichalcogenide Thin Films Obtained by Chemical Gas Phase Deposition Techniques

“…Recently, as the popularity of TMDs has grown up, new tactics have been continuously developed. Mostly, lateral and vertical heterojunctions formed by TMDs and other 2DMs have been regarded as a promising way to obtain novel devices . Thus far, graphene/MoS 2 ‐ or /WS 2 ‐ or WSe 2 ‐based photodetectors have been reported .…”