Strain Mapping of Two-Dimensional Heterostructures with Sub-Picometer Precision

TL;DR

This paper introduces a highly precise electron diffraction technique to map lattice strain and defects in 2D heterostructures at sub-picometer resolution, revealing strain relaxation mechanisms.

Contribution

It presents a novel 4D electron diffraction method using EMPAD to achieve sub-picometer strain mapping in 2D heterostructures, enabling detailed defect analysis.

Findings

Mapped lattice distortions with 0.3 pm precision

Identified dislocations and ripples affecting strain

Demonstrated technique on WS2 and WSe2 heterostructures

Abstract

Next-generation, atomically thin devices require in-plane, one-dimensional heterojunctions to electrically connect different two-dimensional (2D) materials. However, the lattice mismatch between most 2D materials leads to unavoidable strain, dislocations, or ripples, which can strongly affect their mechanical, optical, and electronic properties. We have developed an approach to map 2D heterojunction lattice and strain profiles with sub-picometer precision and to identify dislocations and out-of-plane ripples. We collected diffraction patterns from a focused electron beam for each real-space scan position with a high-speed, high dynamic range, momentum-resolved detector - the electron microscope pixel array detector (EMPAD). The resulting four-dimensional (4D) phase space datasets contain the full spatially resolved lattice information of the sample. By using this technique on tungsten disulfide (WS2) and tungsten diselenide (WSe2) lateral heterostructures, we have mapped lattice distortions with 0.3 pm precision across multi-micron fields of view and simultaneously observed the dislocations and ripples responsible for strain relaxation in 2D laterally-epitaxial structures.