Electronic Properties Modulation And Quantum Transport Of Novel Low-Dimensional Semiconductor Materials From First Principles

Low-dimensional materials,a new class of material with one or more physical dimension(s)constrained to the nanometer scale,have attracted great attention in recent years.Compared with traditional bulk phases,low-dimensional materials exhibit unique electronic,optical and mechanical properties due to quantum confinement,surface and interface effects,etc.Therefore,low-dimensional materials give rise to remarkable potential for technological applications,especially nanoelectronic devices,and new directions of unexplored fundamental science.Meanwhile,great progress has been made in theoretical physic,chemistry and computer science,which promoted many quantum computing methods.Among them,the first principles method based on density functional theory(DFT)made it possible to solve Schrodinger equation through numerical ways and then obtain the physical and chemical properties which are comparable to experimental results.Theoretical simulation can predict the unknown properties of materials and provide reference for experimental preparation,measurement and application,etc.,so as to save resources and improve efficiency.In light of this,the theme of this thesis is to seek ways to overcome the bottleneck of the development of nanoelectronic devices,using first principles methods,regarding experimental systems,to conduct comprehensive research on the electronic properties and modulation,surface and interface interaction,quantum transport,and potential applications of some new low-dimensional materials.Around this theme,we have two entry points:use low-dimensional materials to design new logic devices,and conduct theoretical calculations on the properties of new low-dimensional semiconductor materials.The work of this thesis follows three main lines:(1)from experimental systems to simulation models;(2)from one-dimensional(ID)systems built with zero-dimensional(OD)quantum dots to two-dimensional(2D)systems;(3)from simulating small and middle size systems using traditional first principles methods to calculating large systems with spin-orbital coupling(SOC)using newly developed first principles code.The main results are as follows:(1)We report theoretical modeling of spin-dependent quantum transport properties of dangling bond wires(DBWs)on the Si(100)-2×1:H surface.A single spin-polarized dangling bond center(DBC)near the DBW may strongly affect transport as characterized by anti-resonances or dips in the transmission spectra.Such spin-dependent gating can be effective up to a distance of 1.5 nanometer between the DBW and the DBC.At the energies where anti-resonances occur,the scattering states of the system are found to be"attracted" to the DBC-rather than moving forward to the existing electrode.The variety of gating effects can be well organized by a physical picture,i.e.a strong hybridization or interaction between the spin-polarized DBW and DBC occurs with the same spin polarization(at DBW and DBC)and at similar energy.The sharp spin-resolved anti-resonance in the DBW gives rise to a spin-filtering effect up to 100%efficiency.(2)The recent demonstration of the growth of two-dimensional(2D)antimony-arsenic alloys provides an additional degree of freedom to tailor the basic properties of the emerging group-V 2D materials.With this perspective,herein,we propose and conduct a comprehensive first-principles investigation on this 2D group-V antimony arsenide(2D AsxSby),in both free-standing form as well as on the common substrates of Ge(111),Si(111),bilayer graphene,and bilayer hexagonal boron nitride(h-BN).Structural and electronic properties of the 2D AsxSby are evaluated for different compositions,different types of atomic arrangements for each composition,and different lattice matched interfacial configurations of the composite heterostructures for the four substrates.These systematic studies provide property benchmarks for this new class of group-V 2D materials.This analysis reveals microscopic origins of the interfacial interactions,orbital hybridization,charge transfer,and the resulting electronic structures of the 2D alloy.We predict that a change in the frontier states leads to an indirect-direct bandgap transition according to atomic arrangements in the monolayer AsxSby.On substrates,the relatively strong interfacial interaction between Ge or Si with AsxSby suppresses the semiconducting properties exhibited in free layers,while the weak van der Waals interaction between graphene or h-BN with AsxSby preserves the bands of the alloy.We conclude that 2D group-V alloys AsxSby give a large material phase-space with very interesting electronic properties.(3)Interlayer twisting in two-dimensional(2D)van der Waals(vdW)heterostructures results in the formation of moire patterns which may cause significant structural reconstruction and unltra-flat bands.One can build a periodic moire superlattice(MSL)with tunable length scale by controlling the rotation angle θ.Here we report structural reconstruction and electronic transition in moire patterns of twisted bilayer antimonene.Using first-principle density functional theory calculations,we find a gradual transition from a rigid moire structure to an array of high-symmetry stacking domains with soliton boundaries through vortex-like reconstruction.For θ≤6.01°,the impact of atomic reconstruction on the electronic structure become significant,which lead to the appearance of flat bands at the valence band edge.Unlike bilayer graphene,no magic angles are required for the flat bands formation.Both inhomogeneous interlayer hybridization and local strains are found to be responsible for the formation of flat bands.And the origin of flat bands can be switched by local strains.