First-principles Studies Of Stress On Two-dimensional Materials

Graphene,a crystalline carbon film of a single atom thickness,is a typical two-dimensional material.It has an extremely high carrier mobility,high mechanical strength,and excellent electrical conductivity and thermal conductivity.At present,two-dimensional photoelectric materials mainly include graphene,topological insulator,transition metal sulfur compounds,black phosphorus,etc.As new materials continue to be produced,members of the two-dimensional material family continue to grow.Two-dimensional materials have excellent mechanical strength and flexibility due to their strong planar covalent bond and their own atomic thickness,which proves to have excellent mechanical properties.Based on the first-principles calculation method of density functional theory,we study the uniaxial strain of two-dimensional material double-layer black phosphorus and double-layered CrI₃ under different stress compression effects.The structure and stability of the double-layer black phosphorus and double-layered CrI₃materials and the electronic structure properties have been studied in depth.The main contents of this paper are as follows:1)Structural changes and energy bands of double-layer black phosphorus under elastic strain.Based on the density functional theory calculation,we studied the change process of the electronic structure of double-layer black phosphorus under the action of large stress.We have studied the two processes of double-layer black phosphorus undergoing strain compression and strain recovery,and the electronic structure of double-layer black phosphorus has a different tendency from that of single-layer black phosphorus.The theoretical research work of the predecessors shows that when the uniaxial stress acts in the zigzag direction,the influence of the increase of the stress on the single-layer black phosphorus band gap increases,but when the uniaxial stress acts in the armchair direction,As the stress increases,the single layer does not have much influence on the black phosphorus band gap.In order to study the effect of stress on the electronic and optical properties of double-layered black phosphorus,we compared the total free energy and band gap of the five stacked structures of double-layer black phosphorus,and the most stable AB stacked double-layer black phosphorus from the structure.As an initial structure,the historical process of compression and recovery of the two-layer structure in the zigzag direction was studied.We have found that the stacking pattern of double-layer black phosphorus in the zigzag direction evolves with the increase and release of pressure,and the transition from direct band gap to indirect band gap can be realized.Moreover,after undergoing a complete strain"load-release"process,the stack structure of the double-layer black phosphorus is converted from the AB stack to the AA stack,and is converted into the AE stack when the stress is completely released.This implies that a simple uniaxial stress process can effectively modulate the stack structure of the double-layer black phosphorus.This is likely to be a pervasive method for structural control of two-dimensional materials.Further basic research on the performance-switching conversion of phosphorene-based electronic devices is of great significance.2)Structural changes,energy bands and magnetic properties of double-layer CrI₃under elastic strain.Based on the first-principles calculation method,we studied the structure and stability of the double-layered chromium iodide material by uniaxial strain on the double-layered CrI₃ zigzag direction and the armchair direction.In the process,we obtain the conversion of direct band gap and indirect band gap,which can be realized by adjusting the strain compression process in different directions.The band gap changes closely with the stress.Relationship,we also calculated the effect of stress on the magnetic properties of the double-layered CrI₃,but the double-layered CrI₃ magnetism applied in two different directions did not change throughout the calculation.We can achieve the performance we need by adjusting the corresponding structure of the double-layered CrI₃in different directions.This research has broad application prospects.