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
posted on 2020-03-26, 16:33authored bySilvan Kretschmer, Tibor Lehnert, Ute Kaiser, Arkady V. Krasheninnikov
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.