Low-energy electron (30 –250 eV) in-line holography (also known as or point-projection imaging) is realized by placing a sample at a few tens of nanometers in front of an electron source (usually a sharp tungsten tip) where electrons are extracted by field emission [1]. When electron wave passes through the sample, part of the wave is scattered. The interference pattern measured in the far-field allows for reconstruction of the sample distribution. Three subjects will be discussed.
(1) Biological imaging: It has been demonstrated by the Biophysics group of Prof. Hans-Werner Fink at the University of Zürich, that individual biomolecules, such as DNA molecules, can withstand low-energy electrons radiation for hours without visible radiation damage [1]. Low- energy electron holograms of individual biological macromolecules and their reconstructions will be discussed.
(2) Imaging of charged impurities in graphene [2]: Some adsorbates on graphene transfer their charge to graphene thus forming a positively charged impurity which leads to a distinctive signature in the hologram – a bright spot. The strength of the charge can be evaluated from the intensity of the bright spot at a precision of a fraction of an elementary charge.
(3) Three-dimensional topography of graphene by divergent beam electron diffraction (DBED): DBED is a non-invasive, non-scanning and single-shot imaging technique that offers a possibility of direct visualisation of the three-dimensional topography of thin free-standing materials [3].
By employing electrons with low kinetic energy, ripples in graphene that are only 1 Angstrom in amplitude can be visualized..
[1] M. Germann et al., Phys. Rev. Lett. 104, 095501 (2010).
[2] T. Latychevskaia et al., Nano Lett. 16, 5469 (2016).
[3] T. Latychevskaia et al., Nat. Commun. 8, 14440 (2017).