Low-energy electron (20 - 300 eV) holography (or coherent imaging) [1] has been developing for the past three decades and has found a number of unique applications. One example is low-energy electron in-line holography [2], which is a promising tool for structural biology because it allows imaging of the structure of truly individual biological macromolecules [3], such as purple membrane [4, 5], DNA molecules [6,7,8], tobacco mosaic virus (TMV) [9, 10], bacteriophage [11], and BSA [12]. Another unique application of low-energy electron transmission microscopy is the imaging of the structure and properties of two-dimensional (2D) crystals such as graphene, where low-energy electrons can be applied for direct imaging of: adsorbates with charge that is less than elementary charge [13, 14], in situ alkali atoms (K, Li, ...) intercalation in bilayer graphene [15] (Lorenzo2018), electronic states and band structure of 2D crystals (graphene) [16], three-dimensional structure of corrugated graphene [17], atomic arrangement in twisted bilayer graphene [18]. The slow low-energy electrons interact with atoms more strongly than the high-energy electrons and therefore produce a much stronger contrast in the recorded images. For example, low-energy electrons can pick up phase shifts due to a fraction of the elementary charge of an individual adsorbate sitting on top of a 2D crystal [13, 14]. Low-energy electron holography was also realized at cryo-temperature (liquid helium temperature, 4.2 K) [19]. Diffraction patterns of individual macromolecules (carbon nanotubes) were acquired by using micro-lens for collimating the electron beam [20,21,22]. A low-energy electron microscope can also be modified to operate in scanning (similar to scanning tunnelling microscopy), as demonstrated in ref. [22].
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