Limits of elemental contrast by low energy electron point source holography

Abstract

Abstract. Motivated by the need for less destructive imaging of nanostructures, we pursue point-source in-line holography (also known as point projection microscopy, or PPM) with very low energy electrons (∼100 eV). This technique exploits the recent creation of ultrasharp and robust nanotips, which can field emit electrons from a single atom at their apex, thus creating a path to an extremely coherent source of electrons for holography. Our method has the potential to achieve atom resolved images of nanostructures including biological molecules. We demonstrate a further advantage of PPM emerging from the fact that the very low energy electrons employed experience a large elastic scattering cross section relative to many-kiloVolt electrons. Moreover, the variation of scattering factors as a function of atom type allows for enhanced elemental contrast. Low energy electrons arguably offer the further advantage of causing minimum damage to most materials. Model results for small molecules and adatoms on graphene substrates, where very small damage is expected, indicate that a phase contrast is obtainable between elements with significantly different Z-numbers. For example, for typical setup parameters, atoms such as C and P are discernible, while C and N are not.arXiv:1101.5135v1 [cond-mat.mtrl-sci]

Phase Reconstruction of Low-Energy Electron Holograms of Individual Proteins

“…The iterative reconstruction method thus shows a route toward identifying structural features related to local changes in potential within the imaged molecules. Furthermore, at high spatial resolution, the ability to identify and map localized potentials could be used to extract chemical information from holograms . However, because of the complex interaction of low-energy electrons with biological matter, further contributions likely need to be taken into account in the interpretation of the reconstructed phase maps.…”

“…However, assuming that the mean inner potential is the principal contribution to the reconstructed phase shift, the expected total phase shift can be estimated by calculating the phase shift induced by light atoms (C, N, O), which are the main elements composing a protein, and summing the resulting phase contributions according to the projected atomic density. Using a partial wave-based scattering algorithm for hologram simulation, 44 the phase shifts of individual carbon, nitrogen, or oxygen atoms are found to be in the range of 0.05 rad. For a β-galactosidase molecule in a flat orientation (projected atomic density approximately 10–30 atoms per pixel) one would hence expect a phase shift in the range of 0.5–1.5 rad.…”

“…Furthermore, at high spatial resolution, the ability to identify and map localized potentials could be used to extract chemical information from holograms. 44 However, because of the complex interaction of low-energy electrons with biological matter, further contributions likely need to be taken into account in the interpretation of the reconstructed phase maps. This could be elucidated by a comparison of the experimental phase data to simulations generated from quantitatively accurate potentials describing the investigated molecules.…”

Electrospray ion beam deposition plus low-energy electron holography as a tool for imaging individual biomolecules

“…The changes in the projected atomic density are reproduced by the measured phase shift, which is much lower in the flat molecule on the left than in the upright molecule on the right. A coarse theoretical estimate of the phase shift induced by a large protein such as β-galactosidase, obtained by summing the phase shifts calculated for individual light atoms (C, N, O) via a partial wave-based calculation [ 121 ] according to the amount of atoms in the electron path, yields a quantitative agreement with the measured phase shifts: the calculation yields a phase shift of approximately 0.05 rad per atom [ 98 ], corresponding to 0.5–1.5 rad for a molecule in flat orientation (projected atomic density 10–30 atoms per pixel) and 2–4 rad for a molecule in upright orientation (projected atomic density 40–80 atoms per pixel) [ 120 ]. This implies that LEEH phase imaging can be used to map projected atomic density and thereby local changes in the mean inner potential of biomolecules.…”

Low-energy electron holographic imaging of individual tobacco mosaic virions

“…more than five orders of magnitude higher than the critical dose in TEM, no radiation damage could be observed. This, combined with the fact that the deBroglie wavelengths associated with this energy range are between 0.7 and 1.7Å, makes low-energy electron microscopy an auspicious candidate for structural biology at the single molecule level 11,12 . During the last three decades, DNA, phages, viruses and individual ferritin proteins attached to carbon nanotubes were imaged by means of low-energy electron holography with nanometer resolution 9,10,[13][14][15] .…”