Computational Studies of Two-dimensional Crystals

Resumen

Nanoscale materials have become a field of high interest in research not only from a fundamental point of view, but also due to the potential of these materials in applied electronics, optoelectronics and spintronics. The present thesis is focused on the study of some of the most promising 2D materials. The research is conducted from an theoretical point of view. However, along the whole thesis we have been supported by experimental colaborations, which have allowed us to get a better comprehension of our results, enriching our understanding of the physics behind. Semiconductors of atomically thick monolayers, which can be combined to create van der Waals heterostructures where monolayers of multiple 2D materials are stacked vertically layerby- layer, can also be stitched together seamlessly in-plane to form lateral heterojunctions. Lateral interfacing of atomic monolayers has opened up unprecedented opportunities to engineer two-dimensional heteromaterials. Yet, little is known about the nature of these newly created interfaces. Here we turn our attention to, arguably, the most promising 2D crystal to date, a single layer of MoS2. In this thesis, we present a theoretical study of the electrical contact between the two most common crystallographic phases of MoS2 monolayer crystals: 2H (semiconductor) and 1T (metallic). The fabrication of van derWaals heterostructures, artificial materials assembled by individual stacking of 2D layers, is among the most promising directions in 2D materials research. Until now, the most widespread approach to stack 2D layers relies on deterministic placement methods, which are cumbersome and tend to suffer from poor control over the lattice orientations and the presence of unwanted interlayer adsorbates. Here, we present an extensive theoretical and experimental characterizations of franckeite which is a naturally occurring and air stable van der Waals heterostructure. As the bulk material is already composed of these alternating SnS2 and PbS layers, the exfoliation process minmizes stacking missorientation and avoids interlayer adsorbates in the isolated nanosheets of franckeite. Hence, franckeite can be considered as a naturally occurring vdW heterostructure analog of its synthetic cousin. One of the main problems that the scientific community is facing nowadays in nanostructured electronics is the heat dissipation issue that usually leads to device malfunction. Therefore, great efforts are being dedicated to find new materials that could circunvent this problem and to understand the thermal mechanisms working in nanoscale electronics. In this thesis, we investigate the electrical breakdown of TiS3 nanoribbon-based field-effect transistors (FETs) and the thermal mechanisms that lead to the devices breakdown. Furthermore, these results are compared with thermogravimetric analysis of bulk TiS3 degradation, as well as with density functional theory and Kinetic Monte Carlo simulations of surface oxidation and the subsequent desorption of sulphur atoms that lead to the creation of defects and could explain the FETs malfunction. The recent isolation of antimonene, a novel two-dimensional material, pushes even further the interest for this material which had already been theoretically predicted. Preliminary results on the electronic and electrical properties of few-layer antimonene are presented. Few-layer (> 2 BL) antimonene shows metallic characteristic. In particular, few-layer (> 6 BL) presented the gapless configuration of the surface bands give rise to a single Dirac cone (double degenerate), which is the signature of nontrivial topological order. H2O molecules on the surface broke the degeneracy of the Dirac cone giving rise to two Dirac cones at the ¡ point separated by 60 meV. The experimental electrical properties reported herein are in good agreement with theoretical calculations still ongoing, pointing to a conduction governed by topologically protected surface states in few-layer antimonene