Point-source holographic imaging of nanostructures and interfaces with low energy electrons

Abstract

A lensless holographic in-line point source microscope was envisioned more than half a century ago, but its realization with electron waves has come short due to not only difficulties inherent in Fresnel-type reconstruction methods, but also to the lack of an adequate (spatially and temporally coherent) point source. With the recent creation of ultrasharp nanotips, which can field emit electrons from a single atom at their apex, an extremely coherent electron source is available that provides a great boost to the holographic method. The spatial coherence of such nanotips is a few Å, while their temporal coherence is characterized by a value of energy dispersion (FWHM) as low as 0.1 eV. In this work we ascertain the use of such a microscope in the imaging of nanoscale structures and interfaces. The method is suitable for two- and three- dimensional imaging of solid nanoparticles, thin crystals, and surfaces, but also for biological entities. We show how improvements in the reconstruction method can be made by applying the rigorous Fresnel-Kirchhoff diffraction theory adapted to Electron Optics. Sub-nanometer resolution is achievable for beam energy between 100–200 eV.