Egerton R F, Watanabe M
Physics Department, University of Alberta, Edmonton T6G2E1, Canada.
Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015, United States.
Ultramicroscopy. 2018 Oct;193:111-117. doi: 10.1016/j.ultramic.2018.06.013. Epub 2018 Jun 28.
Fitted with a field emission source, aberration-corrected optics and an energy-dispersive X-ray detector of large solid angle, a modern analytical TEM can generate a current density high enough to chemically identify a single metal atom within a fraction of a second, if the atom remains stationary within the electron probe. However, atom motion will occur if the atomic binding energy is too low, the specimen temperature too high, or the electron accelerating voltage above a certain threshold. We discuss such motion in terms of thermal diffusion, beam-induced sputtering and beam-assisted surface migration. Calculations based on a Rutherford-scattering approximation suggest that when atomic displacement is possible, it drastically reduces the analytical signal and signal/noise ratio. For certain elements, electron energy-loss spectroscopy (EELS) provides a higher detectability than energy-dispersive X-ray (EDX) but suffers from the same problem of atomic displacement.
配备场发射源、像差校正光学器件和大立体角能量色散X射线探测器后，一台现代分析型透射电子显微镜能够产生足够高的电流密度，从而在几分之一秒内对单个金属原子进行化学识别，前提是该原子在电子探针内保持静止。然而，如果原子结合能过低、样品温度过高或电子加速电压高于某个阈值，原子就会发生移动。我们从热扩散、束流诱导溅射和束流辅助表面迁移等方面来讨论这种移动。基于卢瑟福散射近似的计算表明，当原子可能发生位移时，它会大幅降低分析信号和信噪比。对于某些元素，电子能量损失谱（EELS）比能量色散X射线（EDX）具有更高的检测能力，但也存在相同的原子位移问题。
