Dynamics of Symmetry-Breaking Stacking Boundaries in Bilayer MoS2

TL;DR

This study demonstrates real-time atomic-resolution imaging of stacking boundary dynamics in bilayer MoS2, revealing how symmetry-breaking boundaries influence electronic properties and enabling potential control of valleytronics.

Contribution

It introduces a method to create and analyze atomically sharp stacking boundaries in bilayer MoS2, linking boundary dynamics to electronic and valleytronic properties.

Findings

Identification of metallic boundary states at stacking boundaries

Real-time tracking of boundary nucleation and expansion

Control of valley selectivity through engineered stacking boundaries

Abstract

Crystal symmetry of two-dimensional (2D) materials plays an important role in their electronic and optical properties. Engineering symmetry in 2D materials has recently emerged as a promising way to achieve novel properties and functions. The noncentrosymmetric structure of monolayer transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS2), has allowed for valley control via circularly polarized optical excitation. In bilayer TMDCs, inversion symmetry can be controlled by varying the stacking sequence, thus providing a pathway to engineer valley selectivity. Here, we report the in situ integration of AA' and AB stacked bilayer MoS2 with different inversion symmetries by creating atomically sharp stacking boundaries between the differently stacked domains, via thermal stimulation and electron irradiation, inside an atomic-resolution scanning transmission electron microscopy. The setup enables us to track the formation and atomic motion of the stacking boundaries in real time and with ultrahigh resolution which enables in-depth analysis on the atomic structure at the boundaries. In conjunction with density functional theory calculations, we establish the dynamics of the boundary nucleation and expansion and further identify metallic boundary states. Our approach provides a means to synthesize domain boundaries with intriguing transport properties and opens up a new avenue for controlling valleytronics in nanoscale domains via real-time patterning of domains with different symmetry properties.