Huaizhe Xu

Variation of structural and magnetic properties with Co-doping in Zn1−xCoxO nanocrystals

The Zn1−xCoxO (x=0–0.20) nanocrystals synthesized via sol-gel method have been systematically investigated by using x-ray diffraction, transmission electronic microscopy (TEM), Raman spectrometer (Raman), UV-vis spectrometer (UV-vis), and vibrating sample magnetometer. It is found that the crystal lattice constant (a) and the cell volume of Zn1−xCoxO are irrespective of the Co composition x when x 0.05 is attributed to the formation of the Co3O4 phase in Zn1−xCoxO due to the self-purification mechanism in nanocrystals growth process. The formation of the antiferromagnetic Co3O4 well explains the decrease in Zn1−xCoxO ferromagnetism with the increase in Co-doping when x>0.05.

The variation of Mn-dopant distribution state with x and its effect on the magnetic coupling mechanism in Zn1?xMnxO nanocrystals

Zn1?xMnxO (x = 0.0005, 0.001, 0.005, 0.01, 0.02) nanocrystals are synthesized by using a wet chemical process. The coordination environment of Mn is characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, and its X-ray absorption fine structure. It is found that the solubility of substitutional Mn in a ZnO lattice is very low, which is less than 0.4%. Mn ions first dissolve into the substitutional sites in the ZnO lattice, thereby forming Mn2+O4 tetrahedral coordination when x ? 0.001, then entering into the interstitial sites and forming Mn3+O6 octahedral coordination when x ? 0.005. All the samples exhibit paramagnetic behaviors at room temperature, and antiferromagnetic coupling can be observed below 100 K.

Variation of the coordination environment and its effect on the white light emission properties in a Mn-doped ZnO-ZnS complex structure

Mn-doped ZnO-ZnS complex nanocrystals were fabricated through coating of dodecanethiol on Mn-doped ZnO nanocrystals. The relationship between the component of white light emission and the coordination environments of Mn-dopants were experimentally investigated. It was shown that Mn ions mainly formed Mn(3+)O6 octahedra in as prepared Mn-doped ZnO, while the Mn(3+) ions on the surface of ZnO transferred into Mn(2+) ions at the interface between ZnO and ZnS after dodecanethiol coating. The Mn(2+)S4 tetrahedron density and the orange emission intensity increased upon enhancing the dodecanethiol content. These results provide an alternative way to optimize the white emission spectrum from nanocrystals of Mn-doped ZnS-ZnO complex structures through modulation of the coordination environment of Mn ions.

Enhancement of ferromagnetism in Zn0.95Co0.05O films by lithium codoping

Zn0.95−xCo0.05LixO (x=0–0.12) thin films were synthesized by sol-gel method. Magnetic investigation showed that the ferromagnetism of the Zn0.95−xCo0.05LixO films increased with Li doping within the solubility limit of 9% but dropped sharply when x>9% owing to the presence of the second-phase LiCoO2. X-ray diffraction, UV-visible absorption spectrum (UV-vis), Raman spectrum (Raman), and electrical resistivity investigation indicated that the Li-doping process in Zn0.95Co0.05O was of three stages. The structure and magnetic properties of Zn0.95−xCo0.05LixO films had different change trends in different stages. It was found that not only LiZn′ related defects but also Li+ interstitials played important roles in enhancing the magnetic interaction between Co2+ ions, which was supported by the bound magnetic polaron model.

Gap opening in single-layer graphene in the presence of periodic scalar and vector potentials within the continuum model

Gap opening at the Dirac point of the single-layer graphene with periodic scalar and vector potentials has been theoretically investigated under the continuum model. The symmetry analysis indicates that the two-fold degeneracy at the Dirac point can be lifted when the potentials break both the chiral symmetry and the time-reversal symmetry. A gap equation at the Dirac point is obtained analytically with perturbation theory. It is shown that a mass term at the Dirac point would be generated by coupling of vector and scalar potentials. This gap equation could be considered as a criterion for gap opening at the Dirac point, which is confirmed by the numerical calculation. Furthermore, the bandgap from the gap equation agrees well with the exact result, when the applied potentials are weak.

The effect of transverse wave vector and magnetic fields on resonant tunneling times in double-barrier structures

The effect of transverse wave vector and magnetic fields on resonant tunneling times in double-barrier structures, which is significant but has been frequently omitted in previous theoretical methods, has been reported in this paper. The analytical expressions of the longitudinal energies of quasibound levels (LEQBL) and the lifetimes of quasibound levels (LQBL) in symmetrical double-barrier (SDB) structures have been derived as a function of transverse wave vector and longitudinal magnetic fields perpendicular to interfaces. Based on our derived analytical expressions, the LEQBL and LQBL dependence upon transverse wave vector and longitudinal magnetic fields has been explored numerically for a SDB structure. Model calculations show that the LEQBL decrease monotonically and the LQBL shorten with increasing transverse wave vector, and each original LEQBL splits to a series of sub-LEQBL which shift nearly linearly toward the well bottom and the lifetimes of quasibound level series (LQBLS) shorten with incre...

Manipulating coupling state and magnetism of Mn-doped ZnO nanocrystals by changing the coordination environment of Mn via hydrogen annealing

Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H2/Ar atmosphere with different H2/Ar ratios. It is found that hydrogen annealing could change the coordination environment of Mn in ZnO lattice and manipulate the magnetic properties of Mn-doped ZnO. Mn ions initially enter into interstitial sites and a Mn3+O6 octahedral coordination is produced in the prepared Mn-doped ZnO sample, in which the nearest neighbor Mn3+ and O2 ions could form a Mn3+–O2−–Mn3+ complex. After H2 annealing, interstitial Mn ions can substitute for Zn to generate the Mn2+O4 tetrahedral coordination in the nanocrystals, in which neighboring Mn2+ ions and H atoms could form a Mn2+–O2−–Mn2+ complex and Mn–H–Mn bridge structure. The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn3+–O2−–Mn3+ complex, while the annealed samples exhibit their ferromagnetism, which originates from the Mn–H–Mn bridge structure and the Mn–Mn exchange interaction in the Mn2+–O2−–Mn2+ complex.

Al doped ZnO nanogranular film fabricated by LBL method and its application for gas sensors

The Al doped ZnO nanogranular films were fabricated by layer-by-layer (LBL) self-assembly method and the gas sensitivity of ZnO films were investigated. The results show that sensitivity of Al:ZnO gas sensors is up to 12 against 20000 ppm hydrogen, while the response time and recovery time of the sensor are only 8 s and 9s respectively. The high sensitivity and fast response-recovery time are attributed to the high surface-to-volume ratio of nanogranular films.