Formation of Defects in Two-Dimensional MoS₂ in the Transmission Electron Microscope at Electron Energies below the Knock-on Threshold: The Role of Electronic Excitations

Abstract

Production of defects under electron irradiation in a transmission electron microscope (TEM) due to inelastic effects has been reported for various materials, but the microscopic mechanism of damage development in periodic solids through this channel is not fully understood. We employ non-adiabatic Ehrenfest, along with constrained density functional theory molecular dynamics, and simulate defect production in two-dimensional MoS2 under electron beam. We show that when excitations are present in the electronic system, formation of vacancies through ballistic energy transfer is possible at electron energies which are much lower than the knock-on threshold for the ground state. We further carry out TEM experiments on single layers of MoS2 at electron voltages in the range of 20–80 kV and demonstrate that indeed there is an additional channel for defect production. The mechanism involving a combination of the knock-on damage and electronic excitations we propose is relevant to other bulk and nanostructured semiconducting materials.

Atomic-Scale Carving of Nanopores into a van der Waals Heterostructure with Slow Highly Charged Ions

“… 32 Bond weakening due to such a mechanism was recently identified as the reason for defect formation during low-energy electron irradiation. 33 …”

In Situ Characterization of Transformations in Nanoscale Layered Metal Chalcogenide Materials: A Review

“…At energies below the knock-on threshold, vacancies can form by electronic excitation and beam-induced chemical etching. [107] This indicates the possibility of enhanced vacancy concentrations in layered materials when being observed with electron microscopy techniques. Defects are not limited to vacancies, as adatoms also play a role in the transformation of these materials.…”

“…However, the formation of defects in nanoscale layered chalcogenides is not limited to knock‐on effects. At energies below the knock‐on threshold, vacancies can form by electronic excitation and beam‐induced chemical etching [107] . This indicates the possibility of enhanced vacancy concentrations in layered materials when being observed with electron microscopy techniques.…”

Recent progress of defect chemistry on 2D materials for advanced battery anodes

“…Despite the fact that both effective ways TEM and STM provide a structural image at the atomic-scale, they encounter a big challenges of complicated sample set-up and limited inspection surfaces. [74][75][76][77][78][79] On the other hand, a statistical process based on X-ray Photo-electron Spectroscopy presents the disadvantage of a limited area resolution. [80] In contrast, the optical spectroscopic techniques, especially Raman and photoluminescence spectroscopies, provide an efficient and nondestructive way to characterize defect on 2D materials.…”

“…For an in‐depth understanding the effective ways of introducing the above‐mentioned defects chemistry in 2D materials, three advanced characterization ways have been successfully exploited widely in recent years, namely the transmission‐electron microscope (TEM), Scanning‐Tunneling Microscope (STM) and X‐ray Photo‐electron Spectroscopy (XPS). Despite the fact that both effective ways TEM and STM provide a structural image at the atomic‐scale, they encounter a big challenges of complicated sample set‐up and limited inspection surfaces [74–79] . On the other hand, a statistical process based on X‐ray Photo‐electron Spectroscopy presents the disadvantage of a limited area resolution [80] .…”