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-keV 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.Comment: 15 pages, 5 figure
