dr. Sense Jan van der Molen: Studying 'Van der Waals' systems by LEEM (Low-energy electron microscopy)

New materials can have a huge impact on society, as witnessed from the bronze age till today. Specifically appealing is the possibility to design materials with pre-programmed properties. One promising set are the so-called Van der Waals materials. These are layered systems, in which each single layer can be chosen from a series of atomically thin 2D systems. The latter include graphene (C), hexagonal BN, WS2 and MoS2.

In Leiden, we operate a unique low-energy electron microscope (LEEM), which currently holds the world record for lateral resolution, at 1.4 nm. I will discuss how we are extending this microscope to a versatile platform for condensed matter research and in particular for investigating quasi-2D systems. First, I will present a novel technique to study local charge transport. We demonstrate if for few-layer graphene, by mapping out the local potential at every point on a surface during a conductance measurement. Interestingly, a key role is played by unoccupied interlayer states to which the incoming ‘LEEM-electrons’ can couple resonantly. In fact, our data form an elegant demonstration of quantum physics, allowing one to project the wave properties of the electron onto a real-space sample surface. Second, I will discuss band structure measurements on graphene and BN. Using LEEM in an unconventional way, we can actually study dispersion relations of unoccupied electron bands. Specifically, we investigate if there is significant electronic coupling between BN and graphene layers, as they are key ingredients in future Van der Waals materials.