Controlling the Schottky barrier at MoS2|metal contacts by inserting a BN monolayer

Making a metal contact to the two-dimensional semiconductor MoS2 without creating a Schottky barrier is a challenge. Using density functional calculations we show that, although the Schottky barrier for electrons obeys the Schottky-Mott rule for high work function ($\gtrsim 4.7$ eV) metals, the Fermi level is pinned at 0.1-0.3 eV below the conduction band edge of MoS2 for low work function metals, due to the metal-MoS2 interaction. Inserting a boron nitride (BN) monolayer between the metal and the MoS2 disrupts this interaction, and restores the MoS2 electronic structure. Moreover, a BN layer decreases the metal work function of Co and Ni by $\sim 2$ eV, and enables a line-up of the Fermi level with the MoS2 conduction band. Surface modification by adsorbing a single BN layer is a practical method to attain vanishing Schottky barrier heights.Comment: 5 pages, 5 figure

Computational study of interfaces and edges of 2D materials

The discovery of graphene and its intriguing properties has given birth to the field of two-dimensional (2D) materials. These materials are characterized by a strong covalent bonding between the atoms within a plane, but weak, van derWaals, bonding between the planes. Such materials can be isolated as single or few atomic layers, or controllably grown by van der Waals epitaxy. Graphene has no band gap, but other 2D materials are natural semiconductors, such as the (group VI) transition metal dichalcogenides (TMDs) MX2, M = Mo;W, X = S; Se;Te. As the band gap of single layers of these TMDs is direct, they attract a large interest because of their potential applications in opto-electronics. This thesis concerns interfaces and edges of TMDs. It can be divided into two major parts. The first part, chapters 2,3, and 4, deals with the interfaces between these materials and metal contacts, which play a crucial role in the successful operation of electronic devices. In part two, chapters 5 and 6, we focus on the edges of 2D TMDs. Whereas the interior is semiconducting, many TMD edges are metallic and show a rich structure of electronic states with energies in the band gap. The metallicity is localized at the first few atomic rows along the edges, and it has one-dimensional character

One-dimensional electronic instabilities at the edges of MoS2

The one-dimensional (1D) metallic states that appear at the zigzag edges of semiconducting two-dimensional transition metal dichalcogenides (TMDCs) result from the intrinsic electric polarization in these materials, which for D3h symmetry is a topological invariant. These 1D states are susceptible to electronic and structural perturbations that triple the period along the edge. In this paper we study possible spin-density waves (SDWs) and charge-density waves (CDWs) at the zigzag edges of MoS2, using first-principles density-functional theory calculations. Depending on the detailed structures and termination of the edges, we observe either combined SDW/CDWs or pure CDWs, along with structural distortions. In all cases the driving force is the opening of a band gap at the edge. The analysis should hold for all group VI TMDCs with the same basic structure as MoS2