The Study Of The Theory Of The Two-dimensional Material

Graphene, a two-dimensional (2D) single layer of carbon atoms, has been the focus of recent research efforts, due to its unusual electronic and structural properties. Besides graphene, some other2D materials (such as BN,2D B) draw a lot of attention as well.There is a growing interest in exploring the structures and properties of2D Boron-Carbon nanostructures because both boron and carbon possess a richness of chemistry. Despite the recent discovery of some novel2D B-C nanostructures, a complete understanding of the structures and properties of2D B-C compounds with a wide range of boron concentrations is still elusive. This stems from the fact that the2D boron sheet differs from graphene, making the prediction of the structures of2D B-C compounds extremely difficult.In this work, we propose a general global optimization method to predict2D nanostructures based on the particle swarm optimization (PSO) technique as implemented in the Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) code to study the structure and properties of2D B-C compounds.Our earlier PSO algorithm is specially designed for3D crystal structure prediction. Here, we have, for the first time, applied the PSO algorithm to2D systems. In this application, we only consider single atomic layer2D systems. And our extensive tests show that the new method is very efficient in finding the stable2D nanostructures.In the PSO simulations, we use density functional theory (DFT) to relax the structures and calculate the energies. In the DFT plane-wave calculations, we use the local density approximation (LDA). The ion-electron interaction is treated using the projector augmented wave (PAW) technique as implemented in the Vienna ab-initio simulation package (VASP).Utilizing our method, we systematically study2D BxCy compounds with several B concentrations. Our simulations reveal new2D ground-state structures of BC5, BC2, BC, B2C, B3C, and B5C. We show that2D C-rich B-C compounds adopt the graphene-like honeycomb structure and therefore can be treated as B-doped graphene. The B-rich compounds have less similarity with that of boron α-sheet, although they both consist of different arrangements of hexagons and triangles. It is also interesting to see that a common feature of B-rich B-C compounds is that they all have similar C2v-like ptC motifs.