First-principles Study On Property Control Of Quasi-two-dimensional Semiconductor Materials

With exotic electronic,optical,mechanical,and transport properties,two-dimensional(2D)semiconductor materials are thought to be the leading candidates for next-generation electronic devices.Due to their special atomic structures,it is convenient to tune the physical properties of 2D semiconductor materials by means of van der Waals heterostructures,substrate,alloying,external field,and dielectric screening.Based on first-principles calculations,we study the tuning of solid solubility,Berry curvature,and topological surface state of 2D semiconductors.Alloying of materials is important for optical applications.Boron nitride(BN)is a wide bandgap semiconductor(?6e V)with high quantum efficiency.Since 2D BN was discovered as a promising material for deep ultraviolet solid-state lighting,tremendous efforts have been developed to alloying graphene into BN forming BNC solid solutions in order to realize a tunable band gap from 0 to 6e V.However,the poor solid solubility(SS)of BNC alloys has restricted their applications.In our study,a novel mechanism of selective orbital coupling between 2D alloy wrong-bond states and surface states mediated by the specific substrate has been proposed to stabilize the wrong bonds and in turn significantly enhance their SS.We demonstrate that five ordered alloys can spontaneously be formed when(BN)_1-(C₂)is grown on hcp-phase Cr and the disordered(BN)_1-(C₂)can be achieved with a hugely reduced miscibility temperature from?5600 to?1200 K.We find BNC alloys have excellent optical properties and extend the applications of solid-state lighting materials.Two-dimensional antiferromagnetic(AFM)materials have the advantages such as robustness against perturbation of external magnetic fields,the absence of a stray field,and ultrafast dynamics and promise exciting possibilities for future spintronic devices.Based on first-principles calculations,we find that the dual-gate technology can well tune the electronic and topological properties of AFM even septuple-layer(SL)Mn Bi₂Te₄thin films.Under an out-of-plane electric field that breakssymmetry,the Berry curvature of the thin film could be engineered efficiently,resulting in a huge change of anomalous Hall(AH)signal.Beyond the critical electric field,the double-SL Mn Bi₂Te₄thin film becomes a Chern insulator with a high Chern number of 3.We further demonstrate that such 2D material can be used as an AFM switch via electric-field control of the AH signal.These discoveries inspire the design of a low-power memory prototype for future AFM spintronic applications.Because of weak Coulomb screening of 2D semiconductors,the orbital interactions are much more sensitive to dielectric screening by the material layer itself and that pro-vided by its surrounding environment.Here,we investigate different kinds of orbital interactions on gapped topological surface state(TSS)of quasi-2D AFM topological ma-terials Mn Bi₂Te₄.We find local orbital interactions will tune the bandgap of TSS by-(9orbitals hybridization while the nonlocal orbital interactions by the change of space dis-tribution of TSS.We also find the quasi-2D surface orbital interactions will significantly tune the bandgap of TSS.