Unveiling the delicate "hidden" interface conditions in WS2 flakes by advanced atomic force microscopy

TL;DR

This study uses advanced microscopy techniques to reveal complex interface conditions in WS2 flakes, uncovering how interfacial bonding and strain influence electrical properties and device performance.

Contribution

It introduces a combined microscopy approach to directly probe and understand the dynamic interface conditions in 2D WS2 materials, highlighting the role of puckering effects.

Findings

Contradiction between bandgap and conductivity measurements explained by puckering effect.

Interfacial bonding strength governs the dynamic puckering behavior.

Differences in interface conditions between open- and closed-ring regions of WS2.

Abstract

The delicate interfacial conditions and behaviors play critical roles in determining the valuable physical properties of two-dimensional materials and their heterostructures on substrates. However, directly probing these complex interface conditions remains challenging. Here, we reveal the coupled in-plane strain and out-of-plane bonding conditions in strain-engineered WS2 flakes by combining dual-harmonic electrostatic force microscopy (DH-EFM) and scanning microwave impedance microscopy (sMIM). A striking contradiction is observed between the compressive-strain-induced larger bandgap (lower electrical conductivity) detected by DH-EFM, and the enhanced conductivity probed by sMIM. Comparative measurements under different sMIM modes demonstrate that this contradiction originates from a tip-loading-force-induced dynamic puckering effect, which is governed by the interfacial bonding strength. Furthermore, the progressive accumulation and subsequent release of conductivity during forward/backward sMIM-contact scans further confirms this dynamic puckering behavior, revealing pronounced differences in interface conditions between the open- and closed-ring regions of WS2. This work resolves the correlation between electrical properties and interface conditions, and provides fundamental insights for interface-engineered devices.