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.

中文翻译：

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低于阈值的电子能量下，透射电子显微镜中二维MoS ₂缺陷的形成：电子激发的作用

对于各种材料，已经报道了由于非弹性作用而在透射电子显微镜（TEM）下在电子辐照下产生缺陷的原因，但是对通过该通道的周期性固体中损伤发展的微观机理尚未完全了解。我们采用非绝热的Ehrenfest以及受约束的密度泛函理论的分子动力学，并在电子束下模拟二维​​MoS _2中的缺陷产生。我们表明，当电子系统中存在激励时，在电子能量比基态的碰撞阈值低得多的情况下，通过弹道能量转移形成空位是可能的。我们进一步在MoS _2的单层上进行TEM实验电子电压在20–80 kV范围内，表明确实存在一个额外的缺陷产生通道。我们提出的涉及敲除损伤和电子激发的组合的机制与其他块状和纳米结构半导体材料有关。