X. H. Yan

Effects of intertube coupling and tube chirality on thermal transport of carbon nanotubes

We study the thermal conductivity of single-walled carbon nanotube bundles and multi-walled carbon nanotubes. It is shown that, for an individual single-walled carbon nanotube, its thermal conductivity is both diameter and chirality dependent. In defect-free bundles and multi-walled carbon nanotubes, the phonon essentially does not transcend the bonds between the constituent walls due to the weak intertube interaction. If, however, the intertube coupling is strong, a substantial reduction in the thermal conductivity maybe implied. Such a low thermal conductivity can be found in several thermal transport experiments of carbon nanotube mats.

Electron transport of L-shaped graphene nanoribbons

L-shaped graphene nanoribbons (LGNRs) are important components of nanoelectronics and nanocircuits. By using the Green’s function method, we study the transport properties of LGNRs, which consist of a semi-infinite armchair edged nanoribbon (AGNR) and a semi-infinite zigzag edged nanoribbon (ZGNR). The width of AGNR determines whether the LGNR is metallic or not. The LGNR with a small included angle has high reflectance to the electrons, while the LGNR with a large included angle is nearly reflectionless. These are opposite to the transport characteristics of the LGNRs, which consist of two semi-infinite ZGNRs. As to the right-angle LGNR, its transport properties are associated with the width of ZGNR. The increase of width will decrease the conductance around the Fermi energy and simultaneously induce sharp conductance dips. In addition, an interesting spatially resolved local density of state is found in the right-angle LGNR.

Size-dependent strain effects on electronic and optical properties of ZnO nanowires

The electronic and optical properties of ZnO nanowires under uniaxial strain are investigated using first-principles calculations. The results show that the electronic band gap for the ultrathin ZnO nanowires displays a nonmonotonic relationship with the strain, while the gap is inversely proportional to strain and shows a linear relationship for the nanowires with diameter larger than 2.4 nm. Optical properties calculations show that the dielectric function peaks for ultrathin nanowires display a redshift with decreasing uniaxial strain, this energy shift decreases with increasing diameter and vanishes as the diameter increases to 2.4 nm.

Half-metallic properties of perovskite BaCrO3 and BaCr0.5Ti0.5O3 superlattice: LSDA+U calculations

Under the local-spin density approximation method plus on-site coulomb interaction U correction (LSDA+U), the BaCrO3 in the perovskite structure is found to be half-metallic ferromagnet with an integral magnet moment of 2.000 Bohr magnetons (μB) per unit. The feasibility of constructing the BaCr0.5Ti0.5O3 superlattice to stabilize the perovskite phase for the BaCrO3 is theoretically explored. Additionally, the influence of screened parameters U on the electronic structures and magnet moments of the perovskite BaCrO3 and BaCr0.5Ti0.5O3 superlattice is expounded.

Phonon spectrum and specific heat of silicon nanowires

Based on lattice dynamics theory and molecular dynamics simulations, we have investigated the geometrical structures, phonon dispersion relations, and specific heat of silicon nanowires with Stillinger-Weber potential. It was shown that the original Stillinger-Weber potential can reproduce the well-established four acoustical branches. With the calculated spectra, we calculated specific heats of silicon nanowires. It is found that the specific heats of thin nanowires are much higher than those of bulk silicon. According to the partial density of states of surface atoms, the enhancement of specific heats of silicon nanowires can be attributed to the surface effect and phonon confinement effect.

Electronic and transport properties of T-graphene nanoribbon: Symmetry-dependent multiple Dirac points, negative differential resistance and linear current-bias characteristics

Based on the tight-binding method and density functional theory, band structures and transport properties of T-graphene nanoribbons are investigated. By constructing and solving the tight-binding Hamiltonian, we derived the analytic expressions of the linear dispersion relation and Fermi velocity of Dirac-like fermions for armchair T-graphene nanoribbons. Multiple Dirac points, which are triggered by the mirror symmetry of armchair T-graphene nanoribbons, are observed. The number and positions of multiple Dirac points can be well explained by our analytic expressions. Tight-binding results are confirmed by the results from density functional calculations. Moreover, armchair T-graphene nanoribbons exhibit negative differential resistance, whereas zigzag T-graphene nanoribbons have linear current-bias voltage characteristics near the Fermi level.

Luminescence properties of rare earth doped YF3 and LuF3 nanoparticles

Eu3+ doped and Yb3+/Ho3+ codoped LuF3 and YF3 nanoparticles with a size distribution of 200–300 nm have been prepared by adopting a combustion-fluorization method. The luminescence spectra of Eu3+ and Ho3+ ions in the YF3 and LuF3 nanoparticles have been investigated through comparison. It is found that the Eu3+ and Ho3+ ions in the two hosts show different luminescence properties although the sites occupied by the rare earth (RE) ions in the LuF3 and YF3 hosts are of the same symmetry. The different luminescence properties may be ascribed to the difference in the RE-F bond nature in the YF3 and LuF3 hosts.

Spin-polarized current generated by carbon chain and finite nanotube

Inspired by recent progress of experimental fabrication of carbon structure [Borrnert et al., Phys. Rev. B 81, 085439 (2010)], we proposed a scheme to generate spin-polarized current based on an all-carbon system consisting of carbon nanotube and chain. The transmission spectra are calculated based on density functional theory combined with nonequilibrium Green’s function method. It is found that the spin-polarized current can be achieved in the proposed system by partial contact between nanotube and chain, without using the dopants, ferromagnetic electrodes, and external electric field. Moreover, our results show that the device containing carbon nanotubes with large length and diameter can produce the current with 100% spin polarization, which is essential for spintronic devices. Physical mechanisms and the comparison with the results of graphene are also discussed.

CrO2 thin films epitaxially grown on TiO2 (001): Electronic structure and magnetic properties

CrO2 thin films epitaxially grown on the rutile TiO2 (001) substrate are studied via density function theory. Due to the strain from the substrate, a semiconductor to half-metal transition with the film growth is observed. It is found that, as the film is thicker than three atomic layers, the half-metallic property can be retained with an antiferromagnetic feature which reduces the total magnetic moment. With the help of ionic and the double exchange picture, the physics behind the half-metallic rebuilding process is revealed.

Effects of radial strain on the desorption of hydrogen from the surface of palladium-doped carbon nanotubes

By using the first-principles method, the authors study the strain effects on hydrogen desorption on the surface of single-walled carbon nanotubes (SWCNTs). It is found that hydrogen chemisorbed on the surface of carbon nanotubes doped with Pd can be released under large radial strain, for the desorbed hydrogen atoms are molecularly bound by Pd atoms with several tenths of an eV. The method of desorption for the chemisorbed hydrogen can be expected to release the residual hydrogen on the surface of SWCNTs.