Max Planck Institute for Solid State Research, Heisenbergstrasse 1, DE-70569 Stuttgart, Germany.
Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.
ACS Nano. 2022 Nov 22;16(11):18568-18578. doi: 10.1021/acsnano.2c06897. Epub 2022 Nov 11.
Low-energy electron holography (LEEH) is one of the few techniques capable of imaging large and complex three-dimensional molecules, such as proteins, on the single-molecule level at subnanometer resolution. During the imaging process, the structural information about the object is recorded both in the amplitude and in the phase of the hologram. In low-energy electron holography imaging of proteins, the object's amplitude distribution, which directly reveals molecular size and shape on the single-molecule level, can be retrieved via a one-step reconstruction process. However, such a one-step reconstruction routine cannot directly recover the phase information encoded in the hologram. In order to extract the full information about the imaged molecules, we thus implemented an iterative phase retrieval algorithm and applied it to experimentally acquired low-energy electron holograms, reconstructing the phase shift induced by the protein along with the amplitude data. We show that phase imaging can map the projected atomic density of the molecule given by the number of atoms in the electron path. This directly implies a correlation between reconstructed phase shift and projected mean inner potential of the molecule, and thus a sensitivity to local changes in potential, an interpretation that is further substantiated by the strong phase signatures induced by localized charges.
低能电子全息术（LEEH）是少数几种能够在亚纳米分辨率下对蛋白质等大型复杂三维分子进行单分子水平成像的技术之一。在成像过程中，物体的结构信息既记录在全息图的振幅中，也记录在相位中。在蛋白质的低能电子全息成像中，可以通过一步重建过程来获取直接揭示单分子水平上分子大小和形状的物体振幅分布。然而，这样的一步重建程序不能直接恢复全息图中编码的相位信息。为了提取成像分子的全部信息，我们因此实现了一种迭代相位恢复算法，并将其应用于实验获取的低能电子全息图中，重建由蛋白质引起的相位偏移以及振幅数据。我们表明，相位成像可以根据电子路径中的原子数量来映射分子的投影原子密度。这直接意味着重建相位偏移与分子的投影平均内电势之间存在相关性，因此对局部电势变化具有敏感性，这种解释进一步得到了局部电荷引起的强烈相位特征的证实。
