Electronic Properties of (Pseudo-) Two-Dimensional Materials

This thesis describes research into the interaction between electrons and various (pseudo) two-dimensional materials. This research is using two approaches: in Chapters 3 and 4 a low-energy electron microscope is used, and in Chapters 5 and 6 transport properties are studied. Chapter 1 introduces the concept of a two-dimensional material. First, the various kinds of such materials are illustrated. Secondly, the specific materials used in this thesis will be treated. We will see that two-dimensionality can be achieved in different ways: first of all top-down in a method where layers are peeled off a crystal until a single atomic layer remains. Secondly: bottom-up, in a method where a single layer is created from smaller components. Chapter 2 introduces the setup which was used for the measurements in Chapters 3 and 4. In these chapters, we will look at materials using electrons, in a low-energy electron microscope (LEEM). A regular microscope works by illuminating a sample with light. In a microscope, we observe bright and dark patches (corresponding to reflection and absorption of the light, respectively), as well as colors (corresponding to reflection and absorption of different wavelengths or energies of the light). We can also magnify objects using lenses. The LEEM works in a very comparable way, with the major difference that we do not use light (i.e. photons) but electrons to image the sample. An image is formed by electrons after interaction with the sample has taken place. This image can also be magnified, and contains bright and dark patches, from which the interaction of the material with the electrons can be established. Besides this, it is possible to change the electron energy in the setup, which makes it possible to measure the interaction at different energies. In the third Chapter we use the LEEM’s ability to measure the atomic orientation of thin layers of crystal. We look at graphene, a two-dimensional lattice of carbon atoms. This graphene was grown on a wafer. Contrary to peeling a crystal to atomically thin layers, this growth method is compatible with industrial processes, which require large slabs of graphene in predictable shapes. In developing these growing methods, it turns out to be difficult to grow large pieces of single-crystal material. With LEEM we look at differences in angular orientation in a layer of graphene. The motivation for this is that boundaries between such domains have a negative influence on the conductive properties of the material. In the fourth Chapter a method is extended to measure and visualize band structures in two-dimensional materials. We look specifically at molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN). The method (scanning ARRES) rapidly scans the electron bream across the first Brillouin zone. This gives a complete image of the band structure of these materials at energies above the Fermi level plus work function. The fifth and sixth Chapters concern single layer superstructures built out of nanocrystals. The building blocks are lead selenide (PbSe) single crystals in the form of a truncated cube, with a diameter of about 5 nm. By allowing these crystals to organize on a fluid surface, a single layer of crystals emerges. These crystals bond covalently in the direction of the atomic lattice. The material which emerges from this process can have multiple shapes, in this thesis we study the square structure. In Chapter 5 we study the conductance properties of such a structure at room temperature, under the influence of an ionic-fluid gate. This gate makes is possible to achieve high charge densities in these structures. We measure high mobilities for these systems, in the order of 1 cm2/Vs. In the sixth Chapter these samples are cooled to approximately 4 K. Despite the high mobilities measured in Chapter 5, the dependence of the conductance with temperature shows that transport is dominated by a hopping process and not by band transport, at the length scale of these samples.