Direct Optical-Structure Correlation in Atomically Thin Dichalcogenides and Heterostructures

TL;DR

This study demonstrates a novel electron beam induced cathodoluminescence technique in STEM to directly correlate optical properties with atomic-scale structures in atomically thin TMD heterostructures, revealing interface quality and intrinsic properties.

Contribution

First application of CL-STEM to measure optical-structural correlations in encapsulated monolayer TMDs, enabling direct analysis of buried interfaces and intrinsic optical properties.

Findings

Localized imperfections at interfaces affect optical coupling.

Encapsulation and annealing improve measurement accuracy.

CL-STEM effectively reveals structure-property relationships in TMD heterostructures.

Abstract

Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.