Conventionally, theoretical considerations in electron microscopy employ the weak phase approximation (WPA), which is only valid for weak scattering atomic elements (C, B, N) but not for transition metal dichalcogenide (TMD) materials. This leads to many exciting phenomena being overlooked. The present theoretical study goes beyond the weak phase approximation and thus the obtained results can be applied for two-dimensional (2D) crystals made of weakly as well of strongly scattering atoms, including the TMD materials. We show that the symmetry of an electron diffraction pattern, characterized by the Friedel’s pairs, is governed by the symmetry of the exit wave distribution. For an infinite periodic crystal, the exit wave is an infinite and periodic 2D distribution which can be assigned an exit wave unit cell. The latter is determined by both the chemical composition of the crystallographic unit cell and the distance between the atomic layers. For 2D crystals of identical atoms, such as graphene, the exit wave unit cell is symmetrical and, thus, a symmetrical diffraction pattern is expected. For TMD materials, the exit wave unit cell is not symmetrical and a non-symmetrical diffraction pattern is expected for both monolayer and bilayer. Conventionally asymmetry in diffraction patterns has been explained by presence of dynamical (multiple) scattering effects. Our study shows that the asymmetry of a diffraction pattern can be explained solely by the asymmetry of the exit wave unit cell. The exit wave unit cell can be asymmetrical even in kinematic (single) scattering model. Therefore, conclusions about dynamical (multiple) scattering effects in 2D materials cannot be made based solely on asymmetry of a diffraction pattern. We also show that for hexagonally arranged atoms the second-order diffraction peaks show perfectly symmetrical intensities independently on the symmetry of the exit wave unit cell distribution.

传统上，电子显微镜中的理论考虑采用弱相近似 (WPA)，它仅适用于弱散射原子元素 (C、B、N)，但不适用于过渡金属二硫属化物 (TMD) 材料。这导致许多令人兴奋的现象被忽视。目前的理论研究超越了弱相近似，因此所得结果可应用于由弱散射原子和强散射原子制成的二维 (2D) 晶体，包括 TMD 材料。我们表明，以弗里德尔对为特征的电子衍射图案的对称性受出射波分布的对称性控制。对于无限周期晶体，出射波是无限且周期性的二维分布，可以指定出射波晶胞。后者由晶胞的化学成分和原子层之间的距离决定。对于相同原子的二维晶体，​​例如石墨烯，出射波晶胞是对称的，因此预计会出现对称的衍射图案。对于 TMD 材料，出射波晶胞是不对称的，预计单层和双层的衍射图案都是非对称的。传统上，衍射图案的不对称性已通过动态（多重）散射效应的存在来解释。我们的研究表明，衍射图案的不对称性可以仅通过出射波晶胞的不对称性来解释。即使在运动学（单一）散射模型中，出射波单位单元也可能是不对称的。所以，不能仅基于衍射图案的不对称性得出关于 2D 材料中动态（多重）散射效应的结论。我们还表明，对于六边形排列的原子，二级衍射峰显示出完全对称的强度，独立于出射波晶胞分布的对称性。