| Low-dimensional nanoscale materials have attracted a great deal of attention owing to their distinct properties. At this scale, strong quantum effect entails the materials unique functions that are not available in bulk counterparts. However, the study on coupling between material properties and external physical fields is still in infancy. Especially, the function modulation of finite nanosystems settled on substrates remains to be studied. In this thesis, using tight-binding approximation as well as first principles calculations based on density functional theory, we systematically study the physical mechanical properties of a series of quasi-one dimensional nanomaterials, including carbon and boron nitride nanotubes, graphene and boron nitride nanoribbons, and organometallic sandwich nanowires. In particular, we get deep insight into the change in structures, electronic and magnetic properties of these nanoscale materials upon the applications of external physical fields, such as mechanical field, electric field, magnetic field, and charge- or substrate-induced potential field. Ample phenomenen from the coupling of strain, electronic properties and magnetism are revealed in these materials. The findings are birefly concluded below:(1) Electromagnetic effect in strained carbon nanotubes and energy gap modulation in boron nitride nanoribbons by electric field. We set up the tight binding model for studying the couling effects of uniaxial and torsional strains on the electronic and magnetic properties of single-walled carbon nanotubes. The strain-induced peaks of susceptibility are found in the carbon nanotubes; especially, paramagnetic-diamagnetic transition takes place at certain strains. The critical magnetic flux for semiconductor- metal transition changes linearly with strains depending on the chiralities of the tubes and the types of strains, mainly due to the tuning of the Van Hove singularities by the coupling of strains and magnetic flux.We reveal by first-principles calculations that the BN nanoribbons remain insulators with large band gap, regardless of the ribbon width and edge structure, thereby limiting their functions for applications. However, by applying a transverse electric field, the energy gap of all BN nanoribbons can be significantly reduced and even completely closed at a critical field, which decreases with increasing ribbon width. So these findings promise practical applications. (2) Study of stability, electronic and magnetic properties of boron nitride nanotubes. Boron nitride nanotube (BNNT) is usually regarded as an insulator and not suitable for designing electronic devices. By semi-empirical molecular dynamics simulations and ab initio total energy calculations, we predict that a freestanding (3,0) BNNT with a diameter of 2.7 ? can be stable well over room temperature, with remarkably higher stability than the experimentally reported (2,2) carbon nanotube. Importantly, small diameter BNNTs have become semiconductor and their electronic properties and work functions strongly depend on their chirality and diameter, exhibiting distinguished electronic properties from their large insulated family members. Moreover, we find that fluorine atoms topologically adsorbed on BNNTs can induce long-ranged ferromagnetic spin ordering along the tube, offering strong spin polarization around the Fermi level. The spin polarization increases significantly with decreasing tube radius, even giving rise to half-metal when the tube diameter is reduced to 3.3 ?. Applying radial strain to the fluorinated nanotube can efficiently modulate the ferromagnetic ordering, which enables the fluorinated BNNTs to function as piezomagnetic nanotubes. As normal-sized BNNTs can behave as excellent insulators, we propose a coaxial nanocable model consisting of carbon nanotube core and boron nitride nanotube sheath by ab initio calculations. We find the optimal interwall distance to be about 0.35 nm and the conductivity of the core carbon nanotube and the insulation of the boron nitride nanotube sheath are found to be rather tolerant to mechanical deformation. As such hybrid nanocable is impractical for mass production, it would be highly desirable to find a homogenous coaxial nanocable. Using first-principles calculations, we find that double-walled BNNTs could be natural homogeneous nanocables as injected electrons prefer abnormally to concentrate on the inner tube while the outer tube remains insulating. The ratio of extra electrons on the inner tube to total electron carriers in the double-walled BNNTs can be tuned widely by changing either the tube diameter or the local tube curvature through radial deformation.(3) Magnetoelectric effect and bias-induced modulation of electronic properties in graphene nanoribbons on silicon substrates. Zigzag graphene nanoribbons have recently attracted much attenation, but the substrate is unavoidable in designing practical devices. By first-principles calculations, we find that the electronic properties of single-layered zigzag graphene nanoribbons (Z-GNRs) adsorbed on Si(001) substrate strongly depend on ribbon width and adsorption orientation. Only narrow Z-GNRs with even rows of zigzag chains across their width adsorbed perpendicularly to the Si dimer rows possess an energy gap, while wider Z-GNRs are metallic due to width-dependent interface hybridization. Moreover, we predict a magnetoelectric effect in bilayer graphene nanoribbons on silicon substrates. It is shown that an applied bias voltage can produce strong linear ME effect by driving charge transfer between the nanoribbons and substrate, thus tuning the exchange splitting of magnetic edge states; moreover, the bias induced n-to-p-type transition in the ribbon layer can switch the ME coefficient from negative to positive due to the unique symmetry of band structures. Also, the band gap of the top ribbon layer can be effectively modulated by the applied bias voltage, which can lead to a semiconductor-to-metal transition in the top magnetic semiconductor layer. In all above mentioned systems, the magnetic moment is highly concentrated on the GNR, how to introduce spins in the silicon substrate remains elusive. We find that high spin-polarization can be achieved on the Si(111)-(2×1) surface via chemisorption of graphene nanoribbons. The total magnetic moment on the Si surface strongly depends on the ribbon width and is thus tunable upon controlling lateral ribbon separation. The Si surface magnetization can sustain considerable vertical compression to the ribbons but also can be functionally switched at high ribbon deformation.(4) Carrier-tunable magnetic ordering in vanadium-naphthaline sandwich nanowires. Organometallic sandwich nanowires (SWNs) have recently undergone a flurry of research interest due to their promising potential for future spintronic application. However, how to regulate the magnetic coupling of magnetic SWNs is little explored. We predicted from first-principles calculation the novel structures of NpTM2 SWNs (Np = naphthalene, TM = V, Mn, Ti, Nb, Sc). We show that the magnetic ordering in the NpV2 nanowire can be adjusted by changing its charge state. Its intrinsic antiferromagnetic ordering can be switched to ferromagnetic one by injecting electrons whereas injecting holes to the nanowire can further stabilize the antiferromagnetic state. In addition, the NpMn2 nanowire is ferromagnetic, the NpTi2, NpV2 and NpNb2 nanowires are antiferromagnetic, and the NpSc2 nanowire is nonmagnetic.
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