First-principle study on the surface atomic relaxation properties of sphalerite

The surface properties of sphalerite （ZnS） were theoretically investigated using first principle calculations based on the density functional theory （DFT）. DFT results indicate that both the （110） and the （220） surfaces of sphalerite undergo surface atom relaxation after geometry optimization, which results in a considerable distortion of the surface region. In the normal direction, i.e., perpendicular to the sur- face, S atoms in the first surface layer move outward from the bulk （dl）, whereas Zn atoms move toward the bulk （d2）, forming an S-enriched surface. The values of these displacements are 0.003 nm for dl and 0.021 nm for d2 on the （110） surface, and 0.002 nm for dl and 0.011 nm for d2 on the （220） surface. Such a relaxation process is visually interpreted through the qualitative analysis of molecular mechanics. X-ray photoelectron spectroscopic （XPS） analysis provides the evidence for the S-enriched surface. A polysulphide （S n^2- ） surface layer with a bind- ing energy of 163.21 eV is formed on the surface of sphalerite after its grinding under ambient atmosphere. This S-enriched surface and the S 2- surface layer have important influence on the flotation properties ofsphalerite. Keywords：

WHAT ARE THE DIFFERENCES OF POLYMER SURFACE RELAXATION FROM THE BULK?

The polymer surface relaxation in thin films has been a long debating issue. We report a new method on studying surface relaxation behaviors of polymer thin films on a solid substrate. This method involved utilizing a rubbed polyimide surface with a pretilting angle in a liquid crystalline cell. Due to the surface alignment, the liquid crystals were aligned along the rubbing direction. During heating the liquid crystalline cell, we continuously monitored the change of orientation of the liquid crystals. It is understood that at a temperature where the orientation of liquid crystal is lost, the surface relaxation on the glass substrate takes place to lose the polyimide surface orientation. It was found that the relaxation temperature at which the liquid crystals lose their orientation depends on the film thickness of the polyimide. A quantitative linear relationship between the relaxation temperature and reciprocal of the film thickness can be observed. Furthermore, different topologies of the rubbed and relaxed thin films were amplified using the polyethylene decoration method and observed using atomic force microscopy.

Surface deformation-dependent mechanical properties of bending nanowires:an ab initio core-shell model

An ab initio core-shell model is proposed to evaluate the surface effect in bending nanowires,in which the elastic modulus depends on the surface relaxation and deformation induced by external loading.By using first-principles calculations based on the density functional theory(DFT),the surface and bulk properties are calculated for Ag,Pb,and Si nanowires.The obtained theoretical predictions of the effective Young’s modulus of nanowires agree well with the experimental data,which shows that the fixed-fixed nanowire is stiffened and the cantilevered nanowire is softened as the characteristic size of the cross section decreases.Furthermore,the contrastive analysis on the two kinds of nanowires demonstrates that increasing the nanowire aspect ratio would enhance the surface effect.The present results could be helpful for understanding the size effect in nanowires and designing nanobeam-based devices in nanoelectromechanical systems(NEMSs).

Multiscale energy density algorithm and application to surface structure of Ni matrix of superalloy

Multiscale materials modeling as a new technique could offer more accurate predictive capabilities. The most active area of research for multiscale modeling focuses on the concurrent coupling by considering models on disparate scales simultaneously. In this paper, we present a new concurrent multiscale approach, the energy density method（EDM）, which couples the quantum mechanical（QM） and the molecular dynamics（MD） simulations simultaneously. The coupling crossing different scales is achieved by introducing a transition region between the QM and MD domains. In order to construct the energy formalism of the entire system, concept of site energy and weight parameters of disparate scales are introduced.The EDM is applied to the study of the multilayer relaxation of the Ni（001） surface structure and is validated against the periodic density functional theory（DFT） calculations. The results show that the concurrent EDM could combine the accuracy of the DFT description with the low computational cost of the MD simulation and is suitable to the study of the local defects subjected to the influence of the long-range environment.

First-principles of wurtzite ZnO(0001) and (0001)surface structures

The surface structures ofwurtzite ZnO（0001） and（0001） surfaces are investigated by using a first-principles calculation of plane wave ultra-soft pseudo-potential technology based on density functional theory（DFT）.The calculated results reveal that the surface energy of ZnO-Zn is bigger than that of ZnO-O,and the ZnO-Zn surface is more unstable and active.These two surfaces are apt to relax inward,but the contractions of the ZnO-Zn surface are smaller than the ZnO-O surface.Due to the dispersed Zn4s states and the states of stronger hybridization between the Zn and O atoms,the ZnO-Zn surface shows n-type conduction,while the O2p dangling-bond bands in the upper part of the valence cause the ZnO-O surface to have p-type conduction.The above results are broadly consistent with the experimental results.