| Nanomaterials constitute a focus of current research efforts and display a wide range of important applications. A number of experimental and theoretical researches have been undertaken in this area. Such research enthusiasm is ignited by the peculiar geometry structure and unique electronic properties. Among these nanomaterials, graphene, a flat monolayer of carbon atoms tightly sp2-packed into a honeycomb lattice, is the most important one, mostly because of its unusual physical and chemical properties including high mobility of charge carriers, robust mechanical strength and quantum spin Hall effect. After the intense focus on graphene, the other two-dimensional (2D) materials are now attracting increasing interest. The goal of the research for other2D materials is to get some intriguing properties that are different from, complementary or even better than that of graphene. To date, tremendous of2D materials have been experimentally synthesized or theoretically predicated, such as BN, ZnO, snlicene, germanane, MoS2, MoSe2, WS2, GaS and so on. These systems display interesting physical and chemical properties, and have important potential applications in many areas.Besides2D materials, other nanomaterials such as organometallic nanowires are also of particular interest due to their unique properties that are useful for applications including catalysis, spintronics, optics and molecular sieves, especially spintronics. Organometallic nanowires, when used as components of spintronic devices, have significant advantages over inorganic ones. For example, the spin-orbit and hyperfine interactions are weak, leading to considerably long spin relaxation length and spin lifetime. With the deepening of research, searching for ways to effectively modulate the electronic properties of nanomaterials is urgent to solve. Furthermore, getting a deep insight into the underlying physical mechanisms is becoming another research focus. These research would be of great importance for the actual applications of nanomaterials.In this dissertation, we systematically investigate the electronic and magnetic behaviors of a series of organometallic nanowires,2D materials and other systems, as well as the manipulation of the relevant properties of these materials by means of doping, adsorption, strain, electronic field, and substrate. And the possible underlying physical mechanisms are discussed in detail. The dissertation is divided into seven chapters. In the first chapter, we introduce the research background and progress of organometallic nanowires and2D materials in the relevant fields. In the second chapter, we briefly review the basic concept of density functional theory and introduce the first-principles software package. In the third chapter, we investigate the electronic and magnetic properties of a series of molecular nanowires, as well as the modulation of the band alignment of graphene nanoribbon via N doping. In the forth chapter, we study the electronic and magnetic properties of a series of2D materials, as well as the manipulation of the related properties of these materials via doping, adsorption, strain, electronic field, or substrate. In the fifth chapter, we explore the manipulation of the electronic properties and related properties of some other materials. In the sixth chapter, we further explore the Rashba effect in2D BiTeX (X=Br, I), SrFBiS2and BiOBiS2, as well as the effect of strain or electronic field on the Rashba effect of these materials. In the seventh chapter, the research contents in this dissertation are summarized and some theoretical problems that need to be solved urgently as well as the further research directions are pointed out. The main content and conclusions are listed as follows:(1) We investigate the electronic and magnetic properties of transition metal phthalocyanine nanowire (M-PcNW, M=Cr, Mn, Co, Ni, Cu and Zn). We show that except for M=Ni and Zn being of nonmagnetic (NM) ground states, the other frameworks are magnetic. The magnetic moments of Cr, Mn, Co, Ni, Cu, and Zn are approximately4.0,3.0,1.0,0.0,1.0, and0.0μB-Further magnetic coupling calculations demonstrate that for M=Co, the framework is paramagnetic, while for M=Cr and Cu, the coupling is antiferromagnetic. Surprisingly, we find that the Mn-PcNW framework favors long-ranged ferromagnetic spin ordering and displays half-metallic.(2) We systematically investigate the electronic and magnetic properties of novel one-dimensional staircaselike organometallic wires (MSWNs, M=V, Cr, Mn, Fe, Co, and Ni) constructed with metallocenes. We find that, except for FeSNWs being nonmagnetic, the other wires are magnetic. And the magnetic moments of V-Ni are3.0,2.0,1.0,0.0,1.0, and2.0μB, respectively. These systems with ordered spin arrangement are privileged for spintronics. Further magnetic coupling calculations show that for TM=V, Cr, Mn, and Co, MSWNs are ferromagnetic, while for TM=Ni, the framework is antiferromagnetic. Additionally, we propose that the mechanism of the magnetism can be explained by employing the competition mechanisms of both through-bond and through-space exchange interactions.(3) A comprehensive analysis of the electronic and magnetic properties of a novel variety of sandwich inorganic molecular wires [(P)5TM]∞(TM=Ti, V, Cr, Mn, Fe, and Co)has been carried out using first-principles calculations. The formation of stable [(P)5TM]∞involves the transfer of one electron from each TM atom to the P5ligand forming [(P)5-V+(P)5-V+]∞structure. We find that [(P)5V]∞,[(P)5Cr]∞and [(P)5Mn]∞display ferromagnetic character, while for [(P)5Ti]∞,[(P)5Fe]∞and [(P)5Co]∞, the magnetic coupling is antiferromagnetic.(4) We present a comprehensive analysis of the effects of N substitution on the band gap and band alignment of chevron-shaped graphene nanoribbon (CGNR). We find that substitution of nitrogen for edge carbon at the peak, even with a significant substitution ratio, shows little effect on the size of the gap for CGNR. In contrast, the substitution of nitrogen for inner carbon would introduce significant band gap narrowing. This attributes to the fact that the edge carbon atoms at the peak have negligible contribution to the p-electron system, while the inner carbon atoms play a role as the edge on the electronic structure. More remarkably, by increasing edge substitution ratio, a linearly downshifting of the band alignment of CGNR occurs. Our results provide a new perspective on band alignment:material’s band alignment can be continuously and precisely shifted, without affecting the magnitude of the band gap, via substitution of well-selected impurity atoms for well-selected sites.(5) We discuss manipulation of the magnetic property of half-fluorinated single layers of BN, GaN and graphene via strain. First-principles calculations reveal that the energy difference between ferromagnetic and antiferromagnetic couplings increases significantly with strain increasing for half-fluorinated BN, GaN and graphene sheets. More surprisingly, the half-fluorinated BN and GaN sheets exhibit intriguing magnetic transitions between ferromagnetism and antiferromagnetism by applying strain. It is found that the magnetic coupling as well as the strain-dependent magnetic transition behavior arise from the combined effects of both through-bond and through-space exchange interactions.(6) We discuss manipulation of the electronic and magnetic property of ab (ab=SiC, GeC, SnC, BN, A1N, and GaN) via half-fluorination and half-hydrogenation. We demonstrate that the half-hydrogenated ab and half-fluorinated GaN sheets are expected to be ferromagnetic (FM). While half-fluorinated ab sheets (except for half-fluorinated GaN sheet) are predicated to be antiferromagnets (AFM). For half-fluorinated ab F-ab (half-hydrogenated ab H-ab) sheets, energy difference between the FM and AFM states decrease (increase) in the order F-SiC> F-GeC> F-SnC and F-AlN> F-GaN (H-SiC<H-GeC<H-SnC and H-AlN<H-GaN). Accordingly, the electronic and magnetic properties of the semidecorated sheets can be precisely modulated by controlling the adsorbed atoms on the a sites.(7) We investigate the effect of halogens on the electronic properties of germanene and snlicene. We find that the linear band structures are deformed and a large gap would be obtained at the Dirac point upon chemisorption of halogens. Our results demonstrate that, compared with pure germanene, the bands of germanene and snlicene adsorbed with Cl, Br and I remain crosses at one point at the Fermi level-despite former Dirac point being deformed. Except the fluorinated germanene, upon chemisorption of F, C1, Br, and I, germanene and snlicene remains to be gapless materials.(8) We present systematically the electronic and magnetic properties of one novel polymer (referred to as C4H) without and with strain-modifying, vacancy-doping, and nonmetal element (B, N, and P) doping. It is found that:(a) the C4H sheet is a nonmagnetic semiconductor with a wide indirect band gap;(b) The binding energies and electronic properties of the C4H sheet could be significantly modified by applying strain;(c) Vacancy defects can lead to intrinsic magnetism in C4H and, surprisingly, the induced spin polarization has large spatial extension;(d) Substitution of B, P and N at the unhydrogenated C site could form a local magnetic moment, whereas no spin-polarization could be induced for that with N at the hydrogenated C site.(9) The geometric and electronic structures of graphene adsorption on MoS2, MoSe2monolayers are studied. It is found that graphene is bound to MoS2, MoSe2monolayers with a large interlayer spacing and with a low binding energy, indicating a weak interaction between graphene and MoS2, MoSe2monolayers. A detailed analysis of the electronic structure indicates that the nearly linear band dispersion relation of graphene can be preserved in graphene accompanied by a small band-gap opening due to the variation of on-site energy induced by MoS2, MoSe2monolayers. Besides, decreasing the interlayer distance could increases the gap.(10) We report a systematic investigation of manipulation of the electronic properties of wrinkled germanane via external electric field (E-field). Our results demonstrate that a minuscule E-field can largely and continuously reduce the energy gap of wrinkled germanane. This phenomenon results from the spatial separation of the charge carries of conduction band edge and valence band edge in the existence of an E-field. It is worth noting that to tune the band gap of such a wrinkled germanane system only a tiny E-field is required, which can be easily realized in the applications. More interestingly, we also show that promising band inversion in wrinkled germanane can be induced by changing the strength of the tiny E-field.(11) First-principles calculations are performed to investigate the electronic and magnetic properties of graphene adsorbed on the (111) surface of diamond. Although graphene is loosely bonded to the diamond surface, the electronic structures of graphene can be significantly affected by the diamond surface. The graphene adsorbed on the diamond surface is a semiconductor with a finite gap. Magnetism can arise "intrinsically" in graphene because of the exchange proximity interaction between electrons in graphene and localized electrons on the diamond surface.(12) We systematically investigate the electronic and magnetic properties of VX2(X=S, Se) mono layers, as well as the intercoupling between the strain and magnetic properties. Our results unveil that VX2monolayers exhibit exciting ferromagnetic behavior, offering evidence of the existence of magnetic behavior in pristine2D monolayers. Furthermore, interestingly, both the magnetic moments and strength of magnetic coupling increase rapidly with increasing isotropic strain. It is proposed that the strain-dependent magnetic moment is related to the strong ionic covalent bonds, while both the ferromagnetism and the variation in strength of magnetic coupling with strain arise from the combined effects of both through-bond and through-space interactions.(13) The electronic and magnetic properties of perfect, vacancy-doped, and nonmetal element (H, B, C, N, O, and F) adsorbed MoSe2, MoTe2and WS2monolayers are systematically investigated. It is found that:(1) MoSe2, MoTe2and WS2exhibit surprising confinement-induced indirect-direct-gap crossover;(2) among all the neutral native vacancies of MoSe2, MoTe2and WS2monolayers, only the Mo vacancy in MoSe2can induce spin-polarization;(3) adsorption of nonmetal elements on the surface of MoSe2, MoTe2and WS2monolayers can induce local magnetic moments; H-absorbed MoSe2, MoTe2and WS2monolayers and F-adsorbed WS2and MoSe2monolayers show long-range antiferromagnetic coupling between local moments.(14) We present first-principles calculations to investigate systematically the electronic behavior and the electron energy low-loss spectra (EELS) of monolayer, bilayer, four-layer, and bulk configurations of periodic GaX (X=S, Se), as well as the effect of mechanical strain on the electronic properties of the GaX monolayer. We find that the GaSe changes its electronic properties from a direct semiconductor in the bulk phase to an indirect semiconductor in the monolayer; while for GaS, it retains its indirect gap nature with the change from the bulk to the monolayer phases. Furthermore, GaX varies drastically with the number of layers in a sheet. Aside from these features, more specifically, we find that the band gap of GaX monolayer can be widely tuned by applying mechanical deformation.(15) We present a theoretical study on the electronic and magnetic properties of the novel tetragonal transition-metal-based7,7,8,8-tetracyanoquinodimethane molecule coordination single sheets (referred to as TM@TCNQ, TM=Cr-Co). Our results unveil that, in TM@TCNQ, two valence electrons would transfer from each TM atom to TCNQ molecules, making them more stable. It is found that all the studied2D tetragonal frameworks are magnetic, carrying magnetic moments of4.0(8.0),3.0(6.0),2.0(4.0), and1.0(2.0) μB for Cr-Co in the R (S) configuration, respectively. The magnetic properties can be controlled by employing different combinations of the TM atoms. Further magnetic coupling calculations show that, except Co@TCNQ being nonmagnetic, the free-standing TM@TCNQ covalent networks favor robut antiferromagnetic spin arrangement. Additionally, to explain the magnitude of the magnetic moments, we construct a simple model, i.e.,"4+1splitting".(16) We investigate the geometric and electronic properties of janugraphene and chlorographene. We predict that two supposedly ordinary materials feature Dirac points in their band structure, which are located between Γ and X’,(-0.277,0,0) and (-0.215,0,0) respectively. The orbitals near the Dirac points are predominately by the carbon orbitals of the layer plane and are almost not related to the chlorine atoms or the phenyl ligands. The Dirac fermions of these materials are rather robust in response to external electric field.(17) We performe first-principles calculations to study the geometric, electronic and magnetic behaviors of2D Co2C18H12lattice structure and2D Ni2C18H12lattice structure. We find that2D Co2C18H12lattice structure has two favorable configurations corresponding to low-buckled Co2C18H12and high-buckled Co2C18H12, while2D Ni2C18H12lattice structure has only one stabe configuration. It is found that2D Ni2C18H12lattice structure and low-buckled Co2C18H12lattice structure favor spin-polarized ground states, while high-buckled Co2C18H12is spin-unpolarized. In addition, both Ni2C18H12lattice structure and low-buckled Co2C18H12lattice structure are predicted to possess a half-metallic Dirac point Fermi surface.(18) We report that β-InSe endowed with external strain realizes a novel three dimensional topological insulator (TI). In particular, only strain is capable of stabilizing a robust band inversion in β-InSe even without considering SOC. However, SOC is indispensable for breaking the incompatibility symmetry of the inverted bands to yield a band gap at the crossing points. At6%strain, the SOC yields a gap on the order of121meV, which approaches room temperature. Additionally, our detailed calculations have shown that the band inversion originates predominately from intralayer interaction, with a weak contribution of interlayer interaction.(19) The electronic structure of non-transition-metal element (Be, B, C, N, O and F)-doped CdS is studied based on spin-polarized density function theory. Our results show that the substitutional Be, B and C for S in CdS generates local magnetic moments2.0,3.0and2.0μB, respectively. Whereas doping with N, O and F in CdS does not induces spin polarization. The results indicate the electronegativity difference between the dopant and the anion of the host semiconductor plays an important role for the magnetism of such doped semiconductors. If difference is positive, the bond formed between the dopant with the nearest Cd is relative weak compared with the native bond between S and Cd, resulting in localized atomic-like2p states of dopant and stable magnetic ground states. Otherwise, the bond formed between the dopant with the nearest Cd is relative strong compared with the native bond between S and Cd, leading to the2p states of the dopant reside within the host bands, thus the spin magnetic moment decreases and eventually vanishes.(20) First-principles calculations are performed to study the adsorption of Ag at Cd-terminated CdS (0001) and S-terminated CdS (0001) surfaces. Our results reveal that Ag adsorption at Cd-terminated (0001) has a large binging energy than at S-terminated (0001) surface. For single Ag adsorption at CdS (0001), the overlapping between the localized Ag5s and localized Cd5s states indicates some ionic-like component of Ag-Cd bond. While for single Ag adsorption on CdS (0001), Ag4d states are more delocalized and hybridized with S3p states revealing that Ag-S bonding is covalent. For small Ag clusters (Ag2, Ag4, and Ag7) adsorbed Cd-terminated (0001) surface, adsorption energies are obviously lower than that of a single Ag adsorption, and the cluster adsorption energy decreases with the cluster size increasing.(21) We examine the adsorption of ammonia molecule on diamond (100) surface. We find that the adsorption energy increase with the increase of the ammonia molecule coverage due to the interaction among these molecules, which can account for that even the one-step amination method can only increase the maximum coverage of NH2groups on the surface to12%.(22) We investigate the electric polarity in BiTeX (X=Br and I) monolayers and the giant Rashba spin splitting. We find that, owing to the broken inversion symmetry and the markedly different constitution between the opposite outmost layers, BiTeX monolayer acquires large polar electric fields along the normal direction to the monolayer plane in their nature structure, making them polar monolayers. Calculations of formation energy and phonon spectrum confirm that the freestanding monolayer structures of BiTeBr and BiTel can be stable. We predict that the polar BiTeX monolayer can produce a strong Rashba spin splitting. In addition, the strength of Rashba effect in BiTeX monolayer can be effectively modulated with external strain.(23) We report on a giant Rashba-type spin splitting in SrFBiS2and BiOBiS2nanosheets originated from their hidden local polar atomic configurations. We find that in SrFBiS2(BiOBiS2) nanosheets can be described in terms of a ionic-like model, where the covalent coupling of Sr and F (Bi and O) atoms forms positively (Sr2F2)2+[(Bi2O2)2+] layers, whereas the covalent coupling of Bi and S atoms forms negatively (BiS2)-layers; hence, the Sr2F2-BiS2(Bi2O2-BiS2) contacts could be considered to be ionic. Thus, the crystal structure possesses two ionic-like strong polar field along the stacking direction, from the Sr2F2to BiS2layers and Bi2O2to BiS2layers for SrFBiS2and BiOBiS2nanosheets respectively. In both materials, we found that they hold two remarkable Rashba spin splittings from opposite nanosheet surfaces, which are degenerated as a result of the strong inversion symmetry. Owing to their peculiar structure, different from most of the previous studied Rashba systems, we could obtain two sets of Rashba spin splittings with a small perturbation, such as a tiny electric field.The research results unveil the underlying physical mechanisms of the electronic, magnetic and related properties of a series of organometallic nanowires,2D materials and other systems, as well as the manipulation of the relevant properties of these materials by means of doping, adsorption, strain, electronic field, and substrate. These systemic theoretical studies would broaden our knowledge of the related properties of these systems, which could provide the theoretical foundation for their applications in spintronics. |
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