Limits of Elemental Contrast by Low Energy Electron Point Source Holography

TL;DR

This paper explores low energy electron holography using ultrasharp nanotips for high-contrast, minimally destructive imaging of nanostructures, demonstrating elemental contrast based on scattering differences.

Contribution

It introduces a low energy electron holography technique utilizing ultrasharp nanotips, enabling atom-resolved imaging and elemental contrast with minimal damage.

Findings

Elemental contrast varies with atom type due to scattering differences.

Atoms like C and P are distinguishable, while C and N are not.

Low energy electrons cause minimal damage, suitable for delicate nanostructures.

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-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.