Investigation On The Design Of 2D Nanoscale Materials And Quantum Spin Hall Effects

Based on the first-principles calculation of density functional theory(DFT),the geometric structure and electronic properties of several kinds of new two-dimensional(2D)nanomaterials are investigated by projector-augmented wave method with exchange-correlation functional in the Perdew-Berke-Ernzerhof(PBE)form within the generalized-gradient approximation(GGA),as implemented in VASP package.Through the stress,external electric field,functional modification and so on to achieve multi-degree of freedom to control the properties of 2D material,revealing the coupling mechanism between them and exploring it's applications in low-dimensional nano-technology.Firstly,we focus on arsenene composed by arsenic which in the same group of phosphorus,and explored two kinds of configurations,including hexagonal honeycomb-like buckled arsenene(B-arsenene)and puckered arsenene(P-arsenene)similar to the phosphene.As for B-arsenene,the external biaxial stress drives topological phase transition,the band gap decreases with increasing strain and changes from indirect to direct,and then the s-p band inversion takes place at the ? point as the tensile strain is larger than 11.14%,which leads to a nontrivially topological state.A single pair of topologically protected helical edge states is established for the edge of arsenene,and their quantum spin Hall(QSH)states are confirmed with the nontrivial topological invariant Z2 = 1.We also propose high-dielectric BN as an ideal substrate for the experimental synthesis of arsenene,maintaining its nontrivial topology.While the P-arsenene has a structure similar to phosphene,is an indirect bandgap semiconductor with 0.76 eV bandgap,it presents intrinsic anisotropic and topological trivial in the equilibrium state.Applying uniaxial stress in both a?and b?directions,the bandgap will change along with the stress.Unfortunately,the uniaxial stress does not cause the structure undergoing a topological phase transition.In order to further explore the topological properties of B-arsenene and make it better in experiment and application,we saturated the B-arsenic by hydrogen and oxygen atoms,respectively,and tried to enhance the band gap.We predict a new ?-type Dirac cone related to the px,y orbitals of As atoms in hydrogenation arsenene,dependent on in-plane tensile strain.The key is to separate the in-plane px,y and out-plane pz orbitals via hydrogenation and strain.Spin-orbit coupling(SOC)can open a nontrivial QSH gap of 193 meV at the Dirac cone.A single pair of topologically protected helical edge states is established and a QW encapsulating this system between the h-BN sheet on each side,maintaining a nontrivial QSH state with the Dirac cone lying within the band gap of cladding BN sheet.Likewise,the oxidation of B-arsenic is an intrinsic large bandgap quantum spin Hall insulator.By oxidation to As atoms,the out-plane pz is filtered from p orbitals,thus the strength of SOC on As-px,y orbitals is enhanced significantly.Furthermore,a single pair of topologically protected helical edges is established.By sandwiching AsO between BN sheets,the BN/AsO/BN quantum well remains topologically nontrivial with a sizeable band gap,suggesting the robustness of its band topology against the effect of the substrate.These findings demonstrate that these materials may be an innovative platform for QSH device design and fabrication operating at room temperature.Besides 2D materials of V group,we also explored the Sn film at Group IV.We predict that DB-SnCH3 is stable and behaves as a 2D topological insulators(TI),with a bulk-gap as large as 148 meV,which can be further tuned by external strain.The origin of QSH effect is demonstrated by s-px,y band inversion,topological invariant Z2,and helical gapless edge within bulk band-gap.We also propose SiC,h-BN,and Bi2Te3 as appropriate substrates to support DB-SnCH3 to realize QSH effect,indicating the high possibility for room-temperature QSH effect in spintronics.We theoretically explore the electric structures and topological properties of a 2D PbPo monolayer,and find that it is a 2D topological crystalline insulator(TCI)with crystalline-protected Dirac states at the edges.This nontrivial topological phase stems from the strong crystal field effect in the monolayer,which lifts the degeneracy between Po-px,y and Pb-pz orbitals,resulting in a px,y–pz band inversion.As compared to the corresponding narrow band gap in bulk PbPo,the quantum confinement of the 2D film leads to a larger band gap of 364.77 meV,making it viable for the practical realization of the TCI phase at room temperature.Additionally,its nontrivial quantum topology is preserved in QW structures formed by sandwiching a PbPo monolayer between NaI layers.This novel 2D TCI with a large bulk gap is a potential candidate in future spintronic devices with ultralow dissipation.Finally,we explored the 2D organic nanomaterials.Demonstrating the possibility of realizing the intrinsic quantum anomalous Hall(QAH)effect in 2D Kagome lattice,and predict that the Mn-DCA lattice is such a good candidate.The Curie temperature estimated from Monte Carlo simulations within the Ising model is about 253 K.Also,the nontrivial properties in Kagome bands are confirmed by a nonzero chern number,quantized Hall conductivity,and gapless chiral edge state.A tight-binding(TB)model is constructed to explain the origin of nontrivial topology.Such 2D materials are much easier to synthesize and much more homogeneous than inorganic materials,therefore enabling Mn-DCA lattice more promising platform for the realizing low-dissipation quantum and spintronics devices.