Zhuxia Zhang

Geometric and electronic structures of new endohedral fullerenes: Eu@C72

The geometric and electronic structures of rare earth metallofullerenes Eu@C72 were investigated using density functional theory (DFT) within a generalized gradient approximation (GGA). The geometric optimization revealed that the most favorable endohedral site for Eu is off-center along the C2 axis on the σv plane pointing to the (5, 5) bond at the fusion of two pentagons. Calculations for electronic structures show that two 6s electrons in Eu transfer to the lowest-unoccupied-molecular orbitals of C72 while 4f electrons remain in Eu.

First principle study of cysteine molecule on intrinsic and Au-doped graphene surface as a chemosensor device

To search for a high sensitivity sensor for cysteine, we investigated the adsorption of cysteine on intrinsic and Au-doped graphene sheets using density functional theory calculations. Binding energy is primarily determined by the type of atom which is closer to the adsorbed sheet. Compared with intrinsic graphene, Au-doped graphene system has higher binding energy value and shorter connecting distance, in which strong Au-S, Au-N and Au-O chemical bond interaction play the key role for stability. Furthermore, the density of states results show orbital hybridization between cysteine and Au-doped graphene sheet, but slight hybridization between the cysteine molecule and intrinsic graphene sheet. Large charge transfers exist in Au-doped graphene-cysteine system. The results of DOS and charge transfer calculations suppose that the electronic properties of graphene can be tuned by the adsorption site of cysteine. Therefore, graphene and Au-doped graphene system both possess sensing ability, except that Au-doped graphene is a better sensor for cysteine than intrinsic graphene.

Tribological behaviour of CrAlN coatings at 600°C

Abstract Chromium aluminium nitride (CrAlN) coatings were prepared by a modified ion beam enhanced magnetron sputtering system in this study. Friction and wear tests were carried out by a (pin on disc) tribometer with Si3N4 (at 600°C) balls as a counter material without lubricant. The results show that CrAlN coating deposited at −120 V has a higher Al content and is more resistant to high temperatures than that deposited at 0 V. During the wear test at 600°C, the friction coefficient of CrAlN coating deposited at −120 V is higher than that of CrAlN coating deposited at 0 V, but the increased hardness, fracture toughness and improved microstructure in CrAlN coating deposited at −120 V can be attributed to its increased wear resistance and its wear rate is nearly unchanged. The wear resistance of the CrAlN coated H13 steel specimen has been greatly improved compared to the uncoated reference H13 steel specimen.

Transformation of the θ-phase in Mg-Li-Al alloys: a density functional theory study

In Mg-Li-Al alloys, θ-phase MgAlLi2 is a strengthening and metastable phase which is liable to be transformed to the equilibrium phase AlLi on overaging. While the structural details of the θ-phase MgAlLi2 and the microscopic transformation are still unknown. In this paper, the structure of MgAlLi2 unit cell was determined through X-ray powder diffraction simulation. Microscopic transformation process of θ-phase MgAlLi2 was discussed in detail using first principles method.

Morphological evolution of ZnO microspheres from Zn5(OH)8Ac2·2H2O by ultrasonic irradiation method

Monodispersed ZnO microspheres were synthesized through ultrasonic irradiation using zinc acetate as a Zn source and triethanolamine as the structure-directing and alkalinity control agent. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermo-gravimetric analyses were used to describe the as-prepared products and understand the formation mechanism. The results indicate that the morphology of the products is strongly dependent on the precursor Zn5(OH)8Ac2·2H2O. As the reaction time increased, the precursor, Zn5(OH)8Ac2·2H2O, decomposed into ZnO nanoparticles and simultaneously, ZnO nanoparticles assembled into a spherical structure under ultrasonic irradiation until the precursor, Zn5(OH)8Ac2·2H2O, completely disappeared. UV-vis absorption measurements of the as-prepared products at different stages show that the absorption peaks became sharper and blue-shifted from 422 to 364 nm with the change of composition and morphology. Photoluminescence (PL) measurements show that the intensity of the UV emission increased and that the wavelength first red-shifted and then blue-shifted with the increase of the reaction time, which may be related to the size and crystallinity of the ZnO particles.

Metallofullerenes Encaging Mixed‐Metal Clusters: Synthesis and Structural Studies of GdxHo3−xN@C80 and GdxLu3−xN@C80

Metallofullerenes of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 encapsulating mixed-metal nitride clusters were synthesized. Spectroscopic characterization of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 was employed to reveal their structural and vibrational properties. The structural properties of these species were analyzed by using theoretical calculations. The studies of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 laid the foundations for these species to be used as multifunctional molecules.

Performance enhancement of GaN-based light-emitting diodes by surface plasmon coupling and scattering grating

Using the analysis of the evanescent surface plasmon polariton (SPP) mode at the GaN/Ag interface as basis, we propose a light-emitting diode (LED) structure with a plasmonic Ag nanostructure and sapphire grating to enhance external quantum efficiency. The 2D finite-difference time-domain method is used to study the spectral properties of the hybrid structure and the effects of structural parameters on light emission enhancement. The plasmonic Ag nanostructure couples recombination energy to the SPP modes at the GaN/Ag interface, whereas the sapphire grating scatters photons out of the LED chips with high extraction efficiency. Under optimal parameters, external quantum efficiency enhancement increases to approximately eighteen times the original value at a relatively long wavelength.

Improvement of light extraction efficiency of GaN-based light-emitting diodes using Ag nanostructure and indium tin oxide grating.

Based on the analysis of the evanescent wave from total internal reflection, a light-emitting diode (LED) structure with a plasmonic Ag nanostructure and indium tin oxide (ITO) grating was proposed to enhance the extraction efficiency. The two-dimensional finite-difference time-domain method was used to study the spectral properties of the hybrid structure and the effects of structure parameters on extraction enhancement. The results demonstrate that the plasmonic Ag nanostructure can couple the evanescent wave to a propagation wave around the GaN/ITO interface, and then the photons are scattered out of the LED chips by the ITO grating with high extraction efficiency. Under the optimal parameters, the light extraction efficiency can reach approximately three times the original value at a relatively longer wavelength.

Theoretical studies on transforming a GaN semiconductor into a photonic crystal under a periodic external magnetic field

In this study, the properties of light propagation for a GaN semiconductor when subjected to a periodic external magnetic field were calculated by means of a transfer matrix. The dielectric function which was modulated periodically by the periodic external magnetic field caused changes of photonic band gap (PBG). In the reflection spectra, photons can be localized in the narrow bands. The position and intensity of PBGs can be modulated by changing the periodic external magnetic field and angle of the incident electromagnetic wave. All of these results have shown that a semiconductor has functions similar to conventional photonic crystals subjected to periodic external magnetic field.

Effects of GaxZn1−xO nanorods on the photoelectric properties of n-ZnO nanorods/p-GaN heterojunction light-emitting diodes

A Ga-doped ZnO nanorod array has been synthesized on a pGaN/Al2O3 substrate by a hydrothermal method at low temperature. The structures and morphologies of the samples were measured by XRD and FE-SEM. The results show that the Ga-doped ZnO nanorods have excellent crystallinity and good epitaxial relationships with the substrate. With increasing Ga doping, the ZnO nanorods will grow along the [001] direction rapidly resulting in a decreasing average diameter. At the same time, the incorporation of Ga also signicantly affects the optical and electrical properties of the n-ZnO nanorods/p-GaN heterojunction light-emitting diode. From the photoluminescence spectrum, it was found that Ga doping can be effectively regulated by the UV and visible emission peak intensity ratio of the ZnO nanorods. The I–V characteristics curve indicates that the n-ZnO nanorods/p-GaN heterojunction light-emitting diode will have better conductivity with increasing Ga doping concentration. Finally, the heterojunction light-emitting diode achieves green light emission under forward bias.