First-principles Studies On Structure Design And Surface-states Manipulation Of Topological Insulators

Using first-principles computational methods,several studies have been carried out in this thesis,including the design of two-dimensional?2D?materials,manipulation of the topolog-ical surface states of topological insulators?TIs?,understanding interfacial atomic structures,determination of bulk crystal structures,prediction of electronic structures,and the impurity properties of semiconductors.Among these,six studies are summarized in the following.1)The elemental 2D materials such as graphene,silicene,germanene,and black phospho-rus have attracted considerable attention due to their fascinating physical properties.Structurally they possess the honeycomb or distorted honeycomb lattices,which are composed of six-atom rings.Here we find a new structure of 2D allotropes of group V elements composed of eight-atom rings,which we name as the octagonal tiling?OT?structure.First-principles calculations show that these allotropes are dynamically stable and are also thermally stable at temperatures up to 600 K.These allotropes are semiconductors with band gaps ranging from 0.3 to 2.0 eV,thus they are potentially useful in near-and mid-infrared optoelectronic devices.Furthermore,OT-Bi is a 2D TI with a band gap of 0.33 eV,which is the largest among the reported elemental2D TIs,and this gap can be increased further by applying compressive strains.2)A magnetic exchange energy gap on the topological surface states of 3D topological insu-lators is essential for observing many exotic phenomena.It was theoretically predicted to occur in a topological insulator/magnetic insulator?TI/MI?heterostructure,however real samples are often n-type due to the interfacial band bending.In this study a charge compensation method is proposed to solve this problem.By substituting the interfacial Se atoms in Bi₂Se₃/MnSe het-erostructures with P or As atoms,a global energy gap opens at the Dirac surface states.The interfacial charges are compensated by the interfacial substitutions,and band bending is mostly corrected.This compensation mechanism also applies to other TI/MI heterostructures,and the full gaps in our studied systems range from 19 to 48 meV.The spin textures of the resulting massive Dirac fermion states are studied in details,and a model Hamiltonian is introduced to help understand the magnetic exchange coupling.The charge compensation approach in this work may facilitate the realization of full energy gaps in experimental TI/MI systems.3)To realize device applications of 2D TIs,the materials need to possess large energy gaps,and to retain their topological properties when fabricated on substrates.Although many2D TIs have been theoretically proposed,only the HgTe/CdTe quantum well is experimentally confirmed.One important reason may be that the substrate effects destroy the topological phase.Here based on first-principles calculations we find that the Bi?110?bilayer,which is metallic in its pristine form,can transform into 2D TIs through hydrogenation and halogenation.Their energy gaps are among the largest for 2D TIs,ranging from 0.45 to 0.93 eV.A p_z-?bonding-antibonding splitting mechanism and a charge-transfer mechanism are proposed to understand them.These topologically insulating films exhibit excellent dynamical and thermal stability,and could retain their nontrivial topological properties against large strains and substrates in-teraction.On MoSe₂and black phosphorus substrates,the functionalized Bi films form nearly freestanding 2D TIs and show large global energy gaps,suitable for room temperature applica-tions.4)In theory the proximity effect between an s-wave superconductor?SC?and the surface states of a TI creates Majorana fermions in the vortex.Wang et al[Science 336,52?2012?]have fabricated the high-quality NbSe₂/Bi₂Se₃heterostructure,where the superconducting gap as well as the topological surface states coexist.However,the in-plane lattice mismatching of these two materials is as large as 20%.To understand the growth process of the high-quality NbSe₂/Bi₂Se₃,their interfacial structure has been investigated by experimentally collaborating with Wang et al.Studies based on first-principles calculations and STM images show that a BiSe bilayer forms as the interfacial layer when Bi and Se atoms are codeposited on the NbSe₂substrate.BiSe bilayer has a NaCl-like structure,forming a chain-like pattern on the substrate to lower energy.This interfacial BiSe provides good lattice matching to Bi₂Se₃along one lattice vector,helpful to the obtaining of high-quality NbSe₂/Bi₂Se₃.5)?-PtBi₂has attracted much attention due to its gaint linear magneto-resistance.Both theoretical and experimental studies on this material are based on the?-PtBi₂structure with a P?3 group symmetry,which is called?₁-PtBi₂here.Structural relaxation and phonon spectrum all show the instability of this phase.On the contrary,a?₂-PtBi₂phase with P31m group symmetry is found to be very stable.Both the STM topography and ARPES images indicate?-PtBi₂has two different cleavage surfaces,in agreement with the case in?₂-PtBi₂,and ARPES bands also agree well with the first-pinciples bands of?₂-PtBi₂.Therefore,?₂-PtBi₂must be the correct structure of the experimental?-PtBi₂.Calculations also find that a cylinder-shaped Fermi surface exists in?-PtBi₂along??,????30^?,12^??direction,implying strong De Haas-Van Alphen oscillation in this direction.6)The mechanism of room temperature?RT?ferromagnetism in C-doped ZnO and the trend that the higher the C concentration,the smaller the saturation magnetic moment?M_s?,remain controversial and puzzling for a long time.Using density-functional theory calculations with hybrid functional,we qualitatively understand these experimental phenomena.The sub-stitutional C_Odefects dominate in Zn-rich conditions and n-type materials.They carry 1 or 2?_B/C,depending on the position of the Fermi level.In low C concentrations and at RT,the C_Odefects are kept isolated and prefer to couple ferromagnetically,thus the M_sis larger.As the C concentration increases,nonmagnetic C₂dimers form through binding of two C_Odefects or binding of one C_Odefect with one interstitial C,thus decreasing the M_s.Our results also suggest that the ferromagnetism from C_Odefects can be quenched by higher annealing temper-atures.Furthermore,the p-type conductivity from carbon dopant is not supported in the present study.