Anomalous twin boundaries in two dimensional materials

A P Rooney,W Zhao,A Gholinia,A Kozikov,Z Li,G Auton,R J Young,S J Haigh,F Ding,R V Gorbachev

doi:10.1038/s41467-018-06074-8

Abstract

Twin boundary defects form in virtually all crystalline materials as part of their response to applied deformation or thermal stress. For nearly six decades, graphite has been used as a textbook example of twinning with illustrations showing atomically sharp interfaces between parent and twin. Using state-of-the-art high-resolution annular dark-field scanning transmission electron microscopy, we have captured atomic resolution images of graphitic twin boundaries and find that these interfaces are far more complex than previously supposed. Density functional theory calculations confirm that the presence of van der Waals bonding eliminates the requirement for an atomically sharp interface, resulting in long-range bending across multiple unit cells. We show these remarkable structures are common to other van der Waals materials, leading to extraordinary microstructures, Raman-active stacking faults, and sub-surface exfoliation within bulk crystals.

Highlights

Twin boundary defects form in virtually all crystalline materials as part of their response to applied deformation or thermal stress
For virtually all crystalline materials, macroscopic deformation is accommodated by the movement of dislocations and through the formation of twinning defects[3]; it is the geometry of the resulting microstructure that largely determines the mechanical and electronic properties
Our understanding of twinning in graphite has stemmed from the seminal work on twin structures by Friese and Kelly[4], nearly six decades ago, and has changed remarkably little since[9,16,17], recently there has been significant interest in the electronic transport properties of wrinkles, bends and kinks in graphene[2]

Summary

Introduction

Twin boundary defects form in virtually all crystalline materials as part of their response to applied deformation or thermal stress. Density functional theory calculations confirm that the presence of van der Waals bonding eliminates the requirement for an atomically sharp interface, resulting in long-range bending across multiple unit cells We show these remarkable structures are common to other van der Waals materials, leading to extraordinary microstructures, Raman-active stacking faults, and sub-surface exfoliation within bulk crystals. We demonstrate that different classes of microstructure are present in the deformed material and can be predicted from just the atomic structure, bend angle, and flake thickness We anticipate that this new knowledge of the deformation structure for 2D materials will provide foundations for tailoring transport behaviour[2], mechanical properties, liquidphase[5,6] and scotch-tape exfoliation[7,8], and crystal growth

Methods

Results

Conclusion