The Environmental Stability And Design Of New Two-Dimensional Materials

Two dimensional (2D) materials have attracted tremendous interest and intense research effort because of their unique properties and great potential in applications. With the rising of new 2D materials discovered in recent years, researchers are facing increasing challenges as well as opportunities. At present, there are two main challenges in the area. On the one hand,although some 2D materials have remarkable virtues, they are not perfect. For a certain material there is always weakness such as environmental stability, poor device performance,high cost, etc. For example, few-layer black phosphorus (BP) is a 2D material with direct band gap, high carrier mobility and moderate on/off ratio which makes it an ideal material for field-effect transistor. However, it degrades rapidly in ambient condition which has become a major hurdle for BP-based devices. Therefore, finding an effective remedy for the weakness of existing 2D materials is an urgent requirement. On the other hand, the existing 2D materials cannot always fulfil the requirement, such as 2D topological insulator materials which are few in number and most of the designs are hard to synthesize. Designing new 2D materials is still in request. In this thesis, we focus on protecting few-layer BP from ambient degradation and designing new 2D topological insulators based on metal-organic complexes.The main conclusions are summarized below:1) The mechanism of light-induced ambient degradation of few-layer black phosphorus and its protection. The environmental instability of single- or few-layer black phosphorus has become a major hurdle for BP-based devices. The degradation mechanism remains unclear and finding ways to protect BP from degradation is still highly challenging.Based on ab initio electronic structure calculations and molecular dynamics simulations, a three-step picture on the ambient degradation of BP is provided: generation of superoxide under light, dissociation of the superoxide, and eventual breakdown under the action of water.The well-matched band gap and band-edge positions for the redox potential accelerates the degradation of thinner BP. Furthermore, in order to block the third step, it was found that the formation of P-O-P bonds can greatly stabilize the BP framework. A possible protection strategy using a fully oxidized BP layer as the native capping is thus proposed. Such a fully oxidization layer can resist corrosion from water and leave the BP underneath intact with simultaneous high hole mobility. In addition, in order to block the first step, we found that shifting down of conduction band minimum by Te doping will suppress the generation of superoxide and thereby improves the environmental stability of few-layer BP which has been confirmed to be feasible and effective by experiment.2) Improving the stability of few-layer black phosphorus by band-edge engineering via molecule intercalation. Distinguished from depositing capping layer on the surface, we propose a new strategy to improve BP's environmental stability by suppressing the production of superoxide which is the critical step of degradation process. Our first-principles calculations demonstrate that by enlarging the interlayer spacing can effectively shift the conduction band minimum down to suppress the generation of superoxide and the enlargement can be achieved by intercalating small molecules like H2 and He into BP.Moreover, the molecule intercalated BP has larger band gap and intact hole mobility, which makes it a better two-dimensional semiconductor for practical applications. The latest experiment results reveal that H2 treated BP device has great environmental stability as well as most of the carrier mobility and on/off ratio which confirms that the intercalation strategy is feasible and effective.3) Topological insulators based on 2D shape-persistent organic ligand complexes.We present a theoretical study on a new family of 2D nanomaterials based on the coordination of shape persistent organic ligands to heavy transition metal ions such as Pd2+ and Pte+. These 2D structures may be readily fabricated and are expected to be stable under normal atmospheric conditions. From first principles calculations and tight-binding model simulations carried out to characterize the bulk band structures, edge states, spin Chem numbers, and the Z2 topological invariants, we were able to identify candidates with non-trivial topological properties that may serve as topological insulators in real world applications.4) 2D topological insulators based on Au-organic complexes. We design two Au-based metal-organic complexes within kagome and decorated honeycomb lattice. Especially for decorated-honey comb-Au structure, it is the first time to find a topological insulator material in this lattice. By employing first principles calculations and single-orbital tight-binding model, we calculate the band structures, spin Chem numbers, edge states and Z2 topological invariant of the Au-organic complexes and identify them with non-trivial topological characteristic which makes them good candidates of topological insulators in real world applications. We demonstrate that topological insulators with different lattices can be achieved from same metal atom and similar organic ligands which reveals the flexibility and effectivity of metal-organic complexes.