Johannes Jobst - Research

graphene Hall barIn the framework of my VENI grant, I investigate 'How Far can Electrons Fly in Graphene?' This is a pressing research question as future high-speed electronics will most likely be operated in the ballistic regime. This means that electrons are not scattered within an active device but fly through it rather undisturbed. A microscopic picture of this regime is widely missing because traditional methods to study electron transport rely on patterned contact pads and are therefore blind to local variations (see figure).

In oder to address this question, we develloped a method that allows us to map out the electronic potential in two-dimensional devices in situ with high lateral resolution. This technique is based on low-energy electron microscopy (LEEM) and provides deep insight into electronic charge transport on the nanoscale.

 

Low-energy Electron Microscopy

fig1 voltage dependence general 01 LEEM is a powerful electron microscopy technique where the sample surface is probed with a coherent beam of low energy electrons. This has two key advantages: information about the crystal structure can be obtained by low energy electron diffraction (LEED) and spectroscopic information about the material can be gained by varying the electron landing energy. Their energy-dependent coupling causes a complex shape of the IV-curve, i.e., the reflected electron intensity as a function of the electron energy.

 potential map grapheneIf a bias is applied over a sample, the landing energy of the electrons becomes position dependent (see figure). Therefore, the IV-curve also becomes a local quantity and is shifted according to the local potential at the surface. We utilize this shift as a probe in our novel technique, low-energy electron potentiometry.

The bottom image of the electronic potential shows in detail how the voltage drops over a graphene device. For example, it is clearly visible that the transition from monolayer graphene (left) to trilayer graphene (central S-shaped area) causes a steep potential drop. This allows us to identify grain boundaries as a major contribution to scattering in graphene.

Further reading: J. Kautz*, J. Jobst*, C. Sorger, R.M. Tromp, H.B. Weber & S.J. van der Molen, Low-Energy Electron Potentiometry: Contactless Imaging of Charge Transport on the Nanoscale. Scientific Reports 5, 13604 (2015).

 

Studying Novel Designer Materials

graphene van der WaalsIn addition, I study the new class of Van der Waals materials. These crystals are artificially composed of individual layers of two-dimensional materials. The building blocks are next to metallic graphene, semiconducting transitionmetal dichalcogenides, such as MoS2 and WSe2 and insulating boron nitride. This ever growing family and the posibility to stack them at ease enables a kind of material's LEGO (see figure).

To make these stacks a novel material, they have to form common electronic bands. The band structure of these artificial crystals is therefore a vivid research field that also provides insight into the coupling of the different layers.

ARES band structure graphene Next to ARPES, that is readily available in LEEM and can resolve occupied states, we therefore developed a novel technique to measure the unoccupied band structure. The latter utilizes the energy-dependent absorption of low-energy electrons in LEEM when energy and k-vector match an unoccupied band. We consequently named this technique ARRES, angle-resolved reflected-electron spectroscopy (article). Since the information is acquired from LEEM images, this new method has ~10nm latteral resolution and is therefore the only tool able to study unoccupied bands of Van der Waals crystals that typically are only available in small flakes. The figure shows, for example, the unoccupied bands for bilayer graphene on silicon carbide.

We use ARRES to study the interaction between layers of artificially created Van der Waals materials (article). Most recently on graphene on boron nitride, a widely used system for high-mobility graphene electronics.

Further reading:

Topological Insulators

Topological insulators also possess conductive states at the surface. The correlation between the topological properties and crystallographic details like grain boundaries, which is not resolved so far, can be investigated in LEEM. In combination with transport information we can provide valuable information for this field.

 

Available Projects