Boron Nitride Nanotubes Filled Structure, Doping And Adsorption, Electronic And Magnetic Properties

In recent years, nanomaterials such as nanotubes have been widely investigated for their unusual physical, chemical and mechanical properties as well as their great potential applications in nanodevices. The modifications of doping can effectively tune nanotube electronic structure and expand their applications. Therefore, in this paper, we focus on the boron nitride nanotubes'(BNNTs) filling, doping and adsorption, and the structural, electronic and magnetic properties of doped BNNT hybrid structures have been systematically investigated using first-principle calculations. The main conclusions can be summarised as follows.(1) When the hcp transition metal TMn (TM=Fe, Co, Ni, n=4,10,13) nanowires are encapsulated inside carbon nanotubes (CNTs)(m,0)(m=7-12) and the TM13nanowires are capsulated inside (12,0) BNNT, the relaxed geometry structures of TM/nanotube systems have only slightly changed. Binding energy analysis shows that the combining processes of TM/nanotube systems are exothermic, except from the systems TM4/C(7,0), TM,3/C(12,0) and Fe13/BN(12,0) which are endothermic. The BNNTs are better than CNTs for the thicker nanowires'encapsulation. But for the bcc structural TMn (n-5,9,13) encapsulated in (8,8) zinc oxide nanotube (ZnONT), due to strong interaction between inner TM13nanowires and outer ZnONT, not only a near square cross section shape is formed for outer ZnONT but also an anticlockwise rotation about their common axis takes place for the TM13nanowires. The combining processes of all TMn/Zn0(8,8) composites are exothermic and the TM13/Zn1(8,8) composites are the most stable hybrid structures. The charges are transferred from TM nanowires to more electronegative nanotubes, and the transferred charge increases with decreasing nanotube diameter or increasing nanowire thickness. The formed TM-C and TM-N bonds have covalent bond characteristics, but the TM-0bonds are ionic bonds. The TM nanowires'encapsulation makes not only the TM/nanotube hybrid systems have metallic characteristics but also exhibit strong ferromagnetism, and the magnetism is mainly originated from the TM nanowires. The magnetic moment per TM atom of TM/nanotube system increases with increasing CNT diameter or decreasing nanowire thickness. The magnetic moments of hybrid systems are smaller than those of the freestanding TM nanowires, especially for the atoms on the outermost shell of the nanowires. However, the composites still have larger magnetic moments, and almost no any reduction in the magnetic moments of loosely wrapped composites compared with that of corresponding freestanding nanowires.(2) When the Fe(1-X)Cox alloy nanowires are encapsulated in (10,0) CNT and BNNT, the combining processes of all Fe1-xCox/CNT systems are exothermic; but the combining processes of Fe-Co/BNNT composites are endothermic when Co concentration x<0.6and exothermic when x>0.6, and the most stable Fe-Co/BNNT composite is at Co concentration x=0.8. The charges are transferred from the Fe(1-X)Cox nanowires to the more electronegative nanotubes, and the formed Fe-C/Co-C (Fe-N/Co-N) bonds have polar covalent bond characteristics. Although the (10,0) CNT and BNNT are semiconductor and have not any magnetism, the metallic characteristics and a decrease in the magnetic moment are found after Fe-Co nanowires encapsulated inside (10,0) nanotubes. Both the spin polarization and the total magnetic moment of Fe(1-x)Cox/nanotube system are smaller than those of the corresponding freestanding Fe(1-X)Cox nanowire, and the magnetic moment of Fe(1-x)Cox/nanotube system decreases monotonously with increasing Co concentration, but the Fe(1-X)Cox/nanotube systems still have large magnetic moment implying they can be utilized in high density magnetic recording devices.(3) The substitution of a single boron atom or two atoms boron and nitrogen in (4,4) and (8,0) BNNTs by3d transition metal (TM=V, Cr or Mn) atoms has been systemically studied. All optimized TM-doped BNNT systems have obvious local deformation of the nanotubes, and almost all TM atoms protrude to the exterior of the wall. The substitution of TM atoms induces some impurity states within the band gaps of the pure BNNTs. The B site V-doped (4,4) BNNT system is half-metal while the BN1site TM-doped (4,4) BNNT systems are metallicity. All the TM-doped (8,0) BNNT systems are still semiconductor. Overall charges are transferred from the TM atoms to more electronegative BNNTs. The substituted doping of TM atoms gives rise to magnetic moments which mainly stem from TM atoms. The TM-doped and especially BN2site doped BNNT systems have very larger magnetic moments. Therefore, the substitution of doping by TM enhances the chemical reactivity and significantly increases the conductivity of pure BNNT, and renders the TM-doped BNNT systems have larger magnetic moments. All these show the TM-doped BNNT systems have potential applications in hydrogen storage, gas sensing, catalysts, spintronics and magnetic data storage devices.(4) The structural, magnetic and electronic properties of CO and NO molecules adsorption on transition metals (TM=V, Cr, Mn, Fe, Co or Ni) doped (8,0) BNNT have been investigated. The combining processes of all gas adsorptions on TM-doped BNNT are exothermic, accompanying with larger formation energies and charges transfer showing that both CO and NO molecules present strong chemical interaction with the TM-doped BNNT. The adsorption of NO is more stable than CO, and the adsorption of NO on N site is more stable than that of on B site. The most stable adsorption mode is the C or N atoms (of NO) pointing to TM, while O atoms away from the TM. The presence of CO molecule almost does not change the magnetic properties of the TM-BNNT systems. But the adsorption of NO gas on different sites of different TM doped BNNT has different magnetic moment. The adsorption of CO and NO molecules on TM-doped BNNTs leads to different electronic structure properties of BNNTs. Therefore, the TM-doped BNNT can be used as CO and NO gas sensor manufacturing raw materials, and it may be a potential material for nanodevice applications.(5) Structural, electronic and magnetic properties of six3d transition metals (TM=V, Cr, Mn, Fe, Co and Ni) linear monoatomic chains adsorbed on (5,5) boron nitride nanotube (BNNT) at five different sites have been investigated. The results indicate all TM chains can be spontaneously adsorbed on the outer surface of the BNNT. The stable adsorption sites are different for different TM chains. All TM chains can be adsorbed on the N site, while the adsorption on the Z site is unstable. The dispersion character occurs in energy band curves of stable TM/BNNT systems and bring about the band gap disappearance in comparison with that of pure (5,5) BNNT. Interestingly, the TM/BNNT systems with nearly half-filled3d metals V and Cr at H and N sites, as well as Mn at A site show a half-metal character and are usable in spintronics devices. The different electronic properties of BNNT can also be achieved through decorations of the same TM chain on different sites. The TM chain adsorbed BNNT systems exhibit high stability, promising electronic properties and high magnetic moments, which may be useful for a wide variety of next-generation nanoelectronic device components.