Xiaoling Lü

Optical properties of boron-doped diamond

We report optical reflectivity study on pure and boron-doped diamond films grown by a hot-filament chemical vapor deposition method. The study reveals the formation of an impurity band close to the top of the valence band upon boron doping. A schematic picture for the evolution of the electronic structure with boron doping was drawn based on the experimental observation. The study also reveals that the boron doping induces local lattice distortion, which brings an infrared-forbidden phonon mode at 1330 cm(-1) activated in the doped sample. The antiresonance characteristic of the mode in conductivity spectrum evidences the very strong coupling between electrons and this phonon mode.

Compositional and structural modifications of amorphous carbon nitride films induced by thermal annealing

Amorphous carbon nitride films deposited by rf magnetron sputtering were annealed up to 900°C in vacuum for 1 h. The variations of composition and bonding structure of the films were investigated by Fourier transformation infrared, Raman spectroscopy and x-ray photoelectron spectroscopy. The results showed that a great loss of N content was induced by annealing in the films surface, which dropped abruptly from 26.4 to l.5 at.% with annealing temperature rising to 900°C. In addition, it was found that annealing led to disruptions of most C–N bonds and the conversion from sp3 C to sp2 C. As a result, graphitization occurred and a large fraction of sp2 C bonds was formed in the CNx films surface. Surface etching of post-annealed films was carried out to study the change in the film interior layer. Approximately 7 at.% nitrogen atoms were found to still remain in the film interior layer at the annealing temperature 900°C. These remaining N atoms were mainly bound to sp3 C in CNx films instead of N–sp2 C bonds, which indicates the N–sp3 C bonds have higher thermal stability than N–sp2 C bonds.

Spin polarized electron transport through a graphene nanojunction

The electronic transmission spectrum of a graphene nanojunction formed by interconnecting two armchair-edged graphene nanoribbons is obtained by the first principle calculation. We find that in such a simple structure the electronic transmission is remarkably spin-polarized when its size is not very small. By calculating the local density of states and electron occupation numbers on some typical atoms at the scattering region, we conclude that the origin of the spin-polarization is the antiresonance effect, generated by the edge state localized at the zigzag-edged shoulder of the nanojunction.

Characteristics of electric parameters in aluminium alloy MAO coating process

Characteristics of electric parameters in the microarc oxidation (MAO) process of aluminium alloy at constant voltage were studied by a homemade data collecting system. The experimental results show that (1) the variations of the cathodic and anodic current amplitudes and the effective working current reflect obviously five different stages in the course of treatment and (2) variations of the dynamic forward resistance and electric resistivity of coatings have different stages too, while changes of the dynamic backward resistance and resistivity with treatment time are not evident. During the MAO process, the dynamic forward resistance is not equal to the backward resistance at any time, and the former is generally greater than the latter. Scanning electron microscopy analysis shows that these changes are attributed to variations of the coating porous structure during different treating times.

RKKY interaction in AB-stacked multilayer graphene

The RKKY interaction between two magnetic impurities absorbed on the surface layer of half-filled AB-stacked multilayer graphene (ABSMLG) is theoretically studied based on the lattice Greens function technique. In comparison with the case of monolayer graphene, the RKKY interaction in such multilayer graphene presents distinct properties in some aspects. Firstly, from the numerical results, we find that the thickness of the ABSMLG influences the RKKY interaction in a complicated manner, depending on the odd/even parity of the number of layers and the sublattice attribution of the positions of the two magnetic impurities. Then, we derive the asymptotic expressions of the RKKY interactions in ABSMLG in the long-distance limit. For even-layered ABSMLG, we find that the RKKY interactions of the 1A-1A, 1B-1A and 1B-1B couplings fall off as 1/R(2), 1/R(4) and 1/R(6) (1A and 1B stand for, respectively, the sublattice points in the surface layer, which are positioned directly on the plaquette and on a lattice point of the layer underneath). On the other hand, in odd-layered ABSMLG, the decays of these interactions follow the 1/R(2), 1/R(3) and 1/R(3) power laws respectively. In addition, we also find that these analytical expressions are quantitatively valid to describe the RKKY interaction in ABSMLG when the distance between the two magnetic impurities is larger than the lattice constant of graphene by one order of magnitude.

Structural, electronic and magnetic properties of transition-metal embedded zigzag-edged graphene nanoribbons

By means of ab initio calculations within density-functional theory, the structural, electronic and magnetic properties of a zigzag-edged graphene nanoribbon (ZGNR) with 3d transition-metal atoms (TMAs) (Sc–Zn) embedded in the periodically distributed single vacancies are systematically studied. Different from the pristine ZGNR, all of these composite structures show the subband structures with nontrivial spin polarizations, regardless of the type and the embedding position of the TMA. Embedding one kind of these atoms (V, Cr, Ni, Cu or Zn) near one ribbon edge can cause a notable edge distortion. Except for the cases of Sc, Fe and Co doping, other kinds of TMAs embedded near an edge of the ribbon can suppress the inherent magnetism of the zigzag edge. By further analysis, we find that two effects are responsible for the suppression of edge magnetism. One is the variation of the occupied spin-polarized subbands due to the hybridization of the edge state of the ZGNR and 3d atomic states of the dopant. The other is the delocalization of the edge state caused by the exotic TMA. The unilateral magnetism of these TMA-embedded ZGNRs can be utilized to realize the spin-polarized electronic transport, which is the key electronic property in the context of spintronics applications of carbon-based materials.

A valley-filtering switch based on the Stone-Wales defect array in carbon nanotube

We theoretically demonstrate that valley-filtering switch can be realized in a carbon nanotube (CNT) embedded with a series of Stone-Wales defects. It is due to the fact that such a CNT shows two peculiar subbands which span two Dirac valleys without nontrivial dispersion, just like the flat-bottomed subbands of a zigzag-edged graphene nanoribbon. Considering that similar defects have been experimentally patterned in a CNT in a controllable way, this kind of CNT is a promising device prototype for valleytronic applications.

Quasi-one-dimensional electronic states induced by an extended line defect in graphene: an analytic solution.

Analytic solutions of the quasi-one-dimensional (q1D) electron states around an extended line defect in a graphene lattice are derived within the tight-binding model. Then, the electronic properties of this kind of boundary state in graphene are studied in detail. It is found that one subband composed of the even-parity boundary states emerges in the vicinity of the Dirac point. In particular, when the bulk band is gapped, such a one-dimensional subband remains in the bandgap, spanning two inequivalent valleys. In addition, this boundary state subband exhibits nontrivial dispersion, which can carry the valley polarized charge current flowing along the extended line defect. As a result, the line defect behaves like a one-dimensional channel for electronic transport. Moreover, its appearance in graphene or a hexagonal boron nitride sheet provides a promising way to print electric circuits in these two-dimensional materials.