The atomic structure of nanomaterials is often studied using transmission electron microscopy. In addition to image formation, the energetic electrons may also cause damage while impinging on the sample. In a good conductor such as graphene the damage is limited to the knock-on process caused by elastic electron-nucleus collisions. This process is determined by the kinetic energy an atom needs to be sputtered, ie, its displacement threshold energy. This is typically assumed to have a fixed value for all electron impacts on equivalent atoms within a crystal. Here we show using density functional tight-binding simulations that the displacement threshold energy is affected by the thermal perturbation of the atoms from their equilibrium positions. We show that this can be accounted for in the estimation of the displacement cross section by replacing the constant threshold value with a distribution. The improved model better describes previous precision measurements of graphene knock-on damage, and should be considered also for other low-dimensional materials.
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