Extracting transverse electron mean free paths in graphene at low energy.

The LEEM-IV spectra of few-layer graphene show characteristic minima at specific energies, which depend on the number of graphene layers. For the same samples, low-energy TEM (eV-TEM) spectra exhibit transmission maxima at energies corresponding to those of the reflection minima in LEEM. Both features can be understood from interferences of the electron wave function in a purely elastic model. Inelastic scattering processes in turn lead to a finite, energy-dependent inelastic Mean Free Path (MFP) and a lower finesse of the interference features. Here we develop a model that introduces both an elastic and inelastic scattering parameter on the wave-function level, thus reconciling the models considered previously. Fitting to published data, we extract the elastic and inelastic MFP self-consistently and compare these to recent reports.

Complementary LEEM and eV-TEM for imaging and spectroscopy.

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.

Nanoscale measurements of unoccupied band dispersion in few-layer graphene.

The properties of any material are fundamentally determined by its electronic band structure. Each band represents a series of allowed states inside a material, relating electron energy and momentum. The occupied bands, that is, the filled electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on low-energy electron microscopy. It relies on the dependence of the electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only.

eV-TEM: Transmission electron microscopy in a low energy cathode lens instrument.

We are developing a transmission electron microscope that operates at extremely low electron energies, 0-40 eV. We call this technique eV-TEM. Its feasibility is based on the fact that at very low electron energies the number of energy loss pathways decreases. Hence, the electron inelastic mean free path increases dramatically. eV-TEM will enable us to study elastic and inelastic interactions of electrons with thin samples. With the recent development of aberration correction in cathode lens instruments, a spatial resolution of a few nm appears within range, even for these very low electron energies. Such resolution will be highly relevant to study biological samples such as proteins and cell membranes. The low electron energies minimize adverse effects due to radiation damage.