2D Materials and Van der Waals Heterostructures

In this thesis we have studied several 2D materials as well as a few van der Waals heterostructures. The most famous and probably also most promising 2D material is graphene. Graphene occurs in Nature and the material is stable against ambient conditions, which is extremely beneficial for technological applications. Unfortunately, graphene also has one severe drawback: it is gapless and therefore pristine graphene cannot be used for field-effect based device applications. As an alternative for graphene we have studied in this proposal germanene. Germanene, the germanium analogue of graphene, is also a 2D Dirac material that hosts Dirac fermions, but the honeycomb lattice is buckled. This buckling allows the opening of bandgap by applying an external transversal electric field. This electric field results in a shift of charge from one triangular sub-lattice to the other sub-lattice.
In Chapter 3 we focus our attention on the synthesis and characterization of germanene on a molybdenum disulfide substrate (MoS₂). This is an interesting system because MoS₂ has a substantial bandgap of about 1 eV, which allows to keep the key electronics states of the germanene layer, which are located in the vicinity of the Fermi level, intact. The germanene layer will be grown using molecular beam epitaxy. In Chapter 4 we study the electronic properties of the germanene layer. Special attention will be paid to formation of charge puddles. In Chapter 5 we elaborate on the possibility to alter the quantum state of matter of a 2D Dirac material with a buckled honeycomb lattice using an external transversal electric field. We will show that beyond a critical transversal electric field the 2D Dirac material undergoes a topological phase transition from a 2D topological insulator to a normal band insulator. In Chapter 6 we present a detailed study of the structural and electronic properties of germanium selenide (GeSe). GeSe is a transition metal chalcogenide with a bandgap that is very comparable to the bandgap of MoS₂. The structure of both 2D materials is, however, different: GeSe has a rectangular unit cell, whereas MoS₂ has a hexagonal unit cell. Chapter 7 deals with the structural and electronic properties of graphene on a MoS₂ substrate. A graphene flake is placed on the substrate via the solvent transfer technique. In this chapter we also present a new technique to measure the Fermi velocity of graphene. This new technique relies on an analysis of the voltage dependent inverse decay length of the electrons that tunnel from tip to substrate, or vice versa. This is interesting as electron-electron interactions in conventional 2D electron gases result in a decrease of the Fermi velocity. We believe our newly developed technique is also applicable to many other 2D van der Waals heterostructure systems.