Complementary LEEM and eV-TEM for imaging and spectroscopy

Rudolf M. Tromp,Peter S. Neu,Aniket Thete,Sense Jan van der Molen,Daniël Geelen

doi:10.1016/j.ultramic.2020.113199

Abstract

Transmission electron microscopy at very low energy is a promising way to avoid damaging delicate biological samples with the incident electrons, a known problem in conventional transmission electron microscopy. For imaging in the 0-30 eV range, we added a second electron source to a low energy electron microscopy (LEEM) setup, enabling imaging and spectroscopy in both transmission and reflection mode at nanometer (nm) resolution. The latter is experimentally demonstrated for free-standing graphene. Exemplary eV-TEM micrographs of gold nanoparticles suspended on graphene and of DNA origami rectangles on graphene oxide further establish the capabilities of the technique. The long and short axes of the DNA origami rectangles are discernable even after an hour of illumination with low energy electrons. In combination with recent developments in 2D membranes, allowing for versatile sample preparation, eV-TEM is paving the way to damage-free imaging of biological samples at nm resolution.

Highlights

Electron microscopy, in particular Transmission Electron Microscopy (TEM), has become a major tool for disciplines ranging from archae ology [1] to materials science [2] and biology [3]
The energy spread of 0.8 eV in eV-TEM is typical for the thermal barium-oxide disk emitter used (Kimball Physics, ES-015), while an energy spread of 0.25 eV is characteristic for the cold-field emission low energy electron microscopy (LEEM) gun
Thereby, eV-TEM circumvents the problem of electron beam damage to organic and biological samples to a large extent

Summary

Introduction

In particular Transmission Electron Microscopy (TEM), has become a major tool for disciplines ranging from archae ology [1] to materials science [2] and biology [3]. Modern TEM in struments accelerate electrons to hundreds of kilo-electron volts [4], where the electron Mean Free Path (MFP) increases with energy, enabling atomic resolution [5,6]. At such high energies beam damage is a problem, especially in organic molecules and biological specimens. Towards very low energies (i.e. below ~30 eV) the MFP increases, as suggested by the so-called ‘universal’ MFP vs energy curve [10] This allows for high electron transmission of sufficiently thin samples, with the potential of nanometer (nm) spatial resolution with minor damage. The energy spread of 0.8 eV in eV-TEM is typical for the thermal barium-oxide disk emitter used (Kimball Physics, ES-015), while an energy spread of 0.25 eV is characteristic for the cold-field emission LEEM gun

Resolution on few-layer graphene

Transmissivity and reflectivity of few-layer graphene

Few-Layer materials as a high-transmissivity support film for imaging

Gold nanoparticles on a graphene substrate

DNA origami on graphene oxide

Findings

Discussion