First-principles Study Of Two-dimensional Novel Structures Derived From Non-layered Materials

The exploration of novel two-dimensional（2D）materials has attracted considerable attentions.Due to the strong quantum confinement effects,unusual quantum phenomena such as topology,superconductivity,ferromagnetism,and charge density wave will occur in 2D materials.The research on 2D materials are mainly concentrated on 2D structures which could be exfoliated from their three-dimensional（3D）layered bulk crystals.In contrast,non-layered binary compounds in the 2D limit could possess unusual chemical stoichiometry and atomic configuration and display extraordinary electronic properties,which greatly expands the opportunities for future research.This broad class of 2D materials,however,remains largely unexplored.Using first-principles method,this thesis aims to search for 2D structures of non-layered materials and explore their interesting physical properties.One intriguing direction is to study novel 2D materials composed of heavy elements.Contributed to strong spin-orbit coupling effects,heavy elements might offer rich quantum effects such like top ology,magnetism,etc.In light of the recent successful growth of stanene and bismuthene and discoveries of 2D topological and superconducting phases therein,this thesis focuses on the two elements:tin and bismuth,and study their thin films and 2D bina ry compounds.Firstly,we investigated stanene,the tin counterpart of graphene.Benefitting from the strong spin–orbit coupling,stanene shows extraordinarily large quantum spin Hall gap,feasible for room-temperature use.Recent experiments have successfully fabricated stanene,but none of them have yet observed topological states due to unfavourable substrate effects.Our first-principles calculations confirmed a high-quality 2D ultraflat stanene fabricated on Cu（111）,is characterized by an s-p topological band inversion and a spin-orbit coupling induced bandgap at theΓpoint.In addition,we predicted a distinct class of ultraflat graphene-like materials displaying topological features.Secondly,we discussed 2D Sn-Bi binary compounds on silicon substrate.Using an efficient method based on the particle swarm optimization algorithm for predicting the structures of 2D systems,we confirmed the right geometry of Sn₂Bi atomic layer which has been experimentally fabricated on Si（111）.Because of the unique honeycomb configuration,the heavy elements,and the energy-dependent hybridization between Sn and Bi,2D Sn ₂Bi not only shows strong spin-orbit coupling effects but also exhibits high ele ctron-hole asymmetry:Nearly free hole bands and dispersionless flat electron bands coexist in the same system.Here,in this novel 2D semiconductor,Sn ₂Bi,we also found that there are up to five charge states realized in a single defect,although multiple charge states in solitary defects is usually hard to be realized ow ing to the quite large Coulomb repulsion.This unusual charging behavior is achieved by the delocalized defect states.Finally,we studied other 2D structures composed of Sn and Bi with different chemical stoichiometries.According to our calcul ations,large Rashba effect has been observed in these materials,and other quantum propertises such as topology were also discussed.We believe that these 2D structures of non-layered materials composed of heavy elements,could be used to explore novel quantum phy sics and device applications.