Electron emission from a two-dimensional (2D) crystal with atomic thickness is theoretically studied with all the features associated with the low dimensionality and the atomic thickness being well considered. It is shown that, the atomic thickness results in quantum confinement of electrons in the crystal along thickness direction, and consequently two different ways of electron emission from it without and with quantum confinement of electrons normal to emission boundary: edge emission and surface emission. While electron emission from the edge of a 2D crystal can be described by the existing model, electron emission from the surface goes beyond its reach. Here, to describe the latter electron emission, a model based on the energy band theory with the quantum confinement along thickness direction being considered is proposed. It is shown that, the proposed model is a general one capable of describing not only electron emission with quantum confinement normal to an emission boundary but also electron emission without the regarded quantum confinement. The model is expected to advance the understanding and description of electron emission from a solid.
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