Physical Mechanics Of Mech-electro-magnetic Coupling In Quasi-one Dimensional Multielements Nanomaterials

Quasi-one-dimensional nanomaterials include nanotubes, nanowires, nanoribbons and so on. As a material with size limitations in two dimensions, it has a very significant quantum effects, exceptional properties and novel functions, and thus subjects to the general concern of the international scientific community. Wherein, the light elements(including carbon, boron, nitrogen, silicon, etc.) systems become the most widely studied materials, due to their universality of the breadth of their existence, the simplicity of the preparation, good stability and universal application. At the same time, the diversification of the material will make it richer nature. Except for intrinsic diversified material, doping is an effective way to change the properties of materials and achieve a diversified material. Interest in the current alignment of one-dimensional light-elements nanomaterials continues to heat up. However, the mech-electro-magnetic coupling study on it is still in the relativitily preliminary stage. In particular, research on the nature and function of the role of diversified materials formed by different doping is even scarcer. In this thesis, using the first principle density functional theory calculations, we systemically studied the physical and mechanical properties of a series quasi-one dimensional multi-light-elements nanomaterial, such as C-BN hetero-nanotubes, graphene nanoribbon with single atom doping, silicon nanowires with carbon atom interstitial doping and the intrinsic semiconductor nanowires, etc. The main content is as follows:(1)Research on stability and electronic properties of the C-BN axial hetero-nanotubes: We amply described the structure of C-BNNT: the heteronanoribbon comprised of local graphene and BN segments by C-N bonds or C-B bonds is curled into the heteronanotube by the method as a curled CNT. For the lattice mismatch between graphene and BN leading to structural distortion around the junction between segments, the mean radiuses for the semicircles formed by C, B, and N atoms are different. The merit of this structure is that the electronic properties of heteronanotubes can be easier modulated by changing the ratio between carbon and boron nitride. The thermodynamic stability, relative stability compared to CNTs and BNNTs and dynamic stability of C-BNNTs is studied. It is proved that free-standing C-BNNTs can be stable well at or even much above room temperature, which suggests the possibility for their preparation in reality. Concerning the electronic properties, zigzag C-BNNTs are found to be direct gap semiconductors with the band gaps varying depending on the tube diameters and the ratio of carbon dimer lines with respect to BN ones. In contrast, armchair C-BNNTs are metallic except for those with diameters less than 0.6 nm or 1-2 axisoriented zigzag carbon atomic chains around the circumferences, which become semiconducting. And the band gaps of semiconducing armchair C-BNNTs hardly change with the increase of tube diameter. The versatile electronic properties in these heteronanotubes originate from an intricate interplay between the quantum confinement effect and the local tube curvature effect.( 2) Research on electronic and transport properties of the C-BN helical hetero-nanotubes: We amply described the structure of C-BNNT: the tube is formed by a graphite strip and a BN strips interleaved winding in a helical manner in the space on the wall of the tube with a B-C or N-C bond connected between them. According to their chirals and winding manners, C-BN helical hetero-nanotubes can be divided into four categories. zz Cm-(BN)n-mNTs are conductors regardless of their size. Only when there is a very small proportion of carbon, the band gap of them will be opened. aa Cm-(BN)n-mNTs and az Cm-(BN)n-mNTs are semiconductors whose band gaps monotonously decrease with the tube diameter or the ratio of carbon composition increasing. When the size is very small or the proportion of component C is higher than BN, za Cm-(BN)n-mNTs show the properties of the metal. And for the rest cases, they are semiconductors with the band gaps oscillating with the increase of tube diameters, separated into three groups.The work functions(WF) of helical tubes can be compared with that of carbon nanotubes which have the same diameters. Due to the net polarization field experienced by the electrons, the WF is different at two ends of zigzag helical tubes. However for the nonpolar armchair helical tubes, the WFs are the same at both ends. All of the four types of C-BN helical hetero-nanotubes show well transport properties. The relations of current and voltage in zz Cm-(BN)n-mNTs which are conductors are excellently consistent with Ohm’s law. In semiconductor tubes, the ‘ON/OFF’ state and magnitude of current can be wonderfully modulated by bias voltage. The transmission peaks of current are absolutely derived from the helicoid carbon strips or C-BN boundaries, giving rise to a spiral current analogous with an energized nano-solenoid. According to Ampere’s Law, the energized nano-solenoid can generate uniform and tremendous magnetic field over 1.5 tesla. Morevoer, the magnitude of magnetic field can be easily modulated by bias voltage, providing great promise for a nano-inductor to realize electromagnetic conversion at the nanoscale.(3) Reasearch on the electronic, magnetic and transport properties of helical twisted zigzag graphene nanoribbons(ZGNR): The electronic and magnetic properties of helical twisted ZGNRs are analogical with those of pristine ZGNRs. The band gap will increase with the increase of ribbon width and oscillate with the increase of curling diameter. Electrons with two spin orientations will occupy two zigzag ends of ZGNRs, respectively, which induces the antiferromagnetic ground state. The most important case is that the transport-half-metallicity exhibits in both ferromagnetic and antiferromagnetic states of helical twisted Z-ZGNR. This property is not dependent on the electronic properties of helical Z-ZGNR but derives from the structural characteristic of zero-dimensional parallelogram carbon nanoflakes. If the ribbon width or curling diameter increases, the transport-half-metallicity will not disappear, although the energy width of zero transmission region for spin down channel will decrease. However, A-ZGNRs do not possess this spin selected transport property, since the bias will only modulate the half-metallicity between two zigzag edges. All the results suggest Z-ZGNRs are promising materials for spintronic applications, which can realize 100% spin polarization for transport in a large bias voltage region.(4) Research on the structural, electronic and magnetic properties of zigzag graphene nanoribbon with single atom doping introduced instrinsic strain and applying extrinsic strain: B, N, Be, O, Al, Si and P doping zigzag graphene nano ribbon can introduce different intrinsic strain. Because B and N atoms have similar structures and size with C atoms, they will cause minimum strain by doping. The deformation of the zigzag granphene nanoribbon structure with doping by the other atoms can be divided into three categories according to the charge state of the dopant: p-type doping significantly elongated the length of X-C bonds without causing the deformation in the thickness direction; the n-type doping induced the fold of ribbon at the doping site; neutral Si atom doping in the edge of the ribbon results in the fold deformation and in the middle position results in Si-C bond elengtion. We have proposed a model of volume change unexpected contrary with the common expectation to explain this unusal behavior in ribbon structure, which divides the volume change into the change caused by the size difference of atoms and the change caused by the electronic environment. Applying extrinsic strain at the same time will results in different deformation in system with different doping charge states And the bond length of the system and the variation of bending degree is also very different with the strain applied along the growth direction and the width direction. The doped systems are all antiferromagnetic ground state. The edge doping with p-type dopants induced the valence band shifting upwards and with n-type dopants induced the conduction band shifting downwards, so that the bands cross through the Fermi level and generates the spin splitting, which makes graphene nanoribbon become conductors. Simultaneously the magnetic of the edge with doping disappeared, so that the magnetic moments of the system are only contributed by the electrons of C atoms in the non-doped edge. The magnetic moment is still caused by the electrons of C atoms at both ribbon edge with doping in the center. A energy band generated by impurity atoms will cross through the Fermi level resulting in a conductive properties of the system. However, only Pand Si-doped systems keep the semiconductive properties because of the deeper level of impurity states in spin splitting.(5) Research on magnetism modulation in silicon nanowires with carbon atom interstitial doping by extrinsic strain: We reported on the electronic and magnetic properties of hydrogen-passivated silicon nanowires(Si NWs) doped with an interstitial carbon atom. It is found that the doped nanowire can have a novel ferromagnetic ground state when a surface dangling bond is created by removing a terminated H atom. In particular, when interstitial carbon is inserted at the core hollow sites, the nanowires with dangling bond can become half-metallic with 100% spin polarization, independent of wire size and the doped concentration of C atoms. It is interesting that the half-metallicity and ferromagnetic states can be enhanced when the nanowires are axially tensioned but can be quenched when compressed. This magnetic modulation is derived from the interplay between the localization of the defect states and pairwise π-π interaction among unsaturated surface Si dangling bond states, carbon atom and its neighboring Si atoms. Our results highlight a new physical coupling between the doped states and surface dangling bonds in silicon nanowires, and open a new opportunity for the development of nanoscale spintronics.(6)Mechanic-electronic coupling research in binary semiconducted nanowires: Based on Zn O and Cd S, we have studied the effect of uniform and non-uniform strain on the electronic properties of binary semiconducted nanowires in detail. First, the electronic properties of [0001] Zn O nanowires(Zn ONWs) with rectangular cross sections under uniaxial, lateral and shear strain are systemically calculated. The results show that all the Zn ONWs are semiconductors whose band-gaps will decrease(increase) with increasing tensile(compressive) uniaxial strain. The tensile(compressive) lateral strain on {10`10} surfaces will improve(reduce) the band-gaps for Zn ONW with clearly nonlinear characteristic, while the change trend of band-gaps for Zn ONW with lateral strain on {1`210} surfaces is basically opposite. When we enhance shear strain on Zn ONWs, the band-gaps are reduced. The increasing shear strain along [10`10] direction will sharply reduce the band-gap and the curve is nonlinear, while the band-gap decreases nearly linearly with the increase of shear strain along [1`210] direction. Next, when subject to bending strain, the band gap of II-VI semiconductor nanowire such as Zn O and Cd S will be reduced, and the larger strain will induce the smaller bandgap. It results of localized tensile/compressive strain distribution caused by the bending deformation, which will induce staggered edge-band arrangement arising between the different strained areas. Moreover, the higher-order terms of the strain--strain gradient is an important reason leading to the decrease of band gaps. Experimental spectra through the cathode can be good to prove that there is a well linear relationship between the strain gradient and band gap, which is called as flexoelectronic effect.