Direct observation of oxygen vacancy formation and migration over ceria surface by in situ environmental transmission electron microscopy

The extremely high structural tolerance of ceria to oxygen vacancies(Ov)has made it a desirable catalytic material for the hydrocarbon oxidation to chemicals and pharmaceuticals and the reduction of gaseous pollutants.It is proposed that the formation and diffusion of Ov originate from its outstanding reduction property.However,the formation and diffusion process of Ov over the surface of ceria at the atomic level is still unknown.Herein,the structural and valence evolution of CeO_(2)(111)surfaces in reductive,oxidative and vacuum environments from room temperature up to 700℃was studied with in situ aberration-corrected environmental transmission electron microscopy(ETEM)experiments.Ov is found to form under a high vacuum at elevated temperatures;however,the surface can recover to the initial state through the adsorption of oxygen atoms in an oxygen-contained environment.Furthermore,in hydrogen environment,the step-CeO_(2)(111)surface is not stable at elevated temperatures;thus,the steps tend to be eliminated with increasing temperature.Combined with first-principles density function calculations(DFT),it is proposed that O-terminated surfaces would develop in a hypoxic environment due to the dynamic diffusion of Ov from the outer surface to the subsurface.Furthermore,in a reductive environment,H2 facilitates the formation and diffusion of Ov while Ce-terminated surfaces develope.These results reveal dynamic atomic-scale interplay between the nanoceria surface and gas,thereby providing fundamental insights into the Ov-dependent reaction of nano-CeO_(2) during catalytic processes.

Effects of in-plane stiffness and charge transfer on thermal expansion of monolayer transition metal dichalcogenide

The temperature dependence of lattice constants is studied by using first-principles calculations to determine the effects of in-plane stiffness and charge transfer on the thermal expansions of monolayer semiconducting transition metal dichalcogenides.Unlike the corresponding bulk material,our simulations show that monolayer MX2（M = Mo and W;X = S,Se,and Te） exhibits a negative thermal expansion at low temperatures,induced by the bending modes.The transition from contraction to expansion at higher temperatures is observed.Interestingly,the thermal expansion can be tailored regularly by alteration of the M or X atom.Detailed analysis shows that the positive thermal expansion coefficient is determined mainly by the in-plane stiffness,which can be expressed by a simple relationship.Essentially the regularity of this change can be attributed to the difference in charge transfer between the different elements.These findings should be applicable to other two-dimensional systems.

Ultrathin Carbon Nanotubes for Efficient Energy Storage： a First-Principles Study

On the basis of first-principles density functional calculations, the present study sheds theoretical insight on ultrathin carbon nanotube （UCNT） and hydrogenated ultrathin carbon nanotube （HUCNT） for use as potential materials not only for Li-ion battery anode but also for high-capacity hydrogen storage. The highest Li storage capacities in UCNT and HUCNT can be of LiC4 and LiC4H2, respectively, which are higher than that in graphite and LiC6. Binding between Li （Ca） atoms and these materials are found to be enhanced considerably. Each Li （Ca） atom may bind multi-hydrogen molecules, and the adsorption energies are ideally suited for storing hydrogen under ambient conditions, and the predicted weight percentage of molecular hydrogen are in the range of 6.4-12 wt% exceeding the target set by the United States Department of Energy.

Synthetic Spin-Orbit Coupling in Two-Level Cold Atoms

We theoretically and computationally show the simplest realization of SOC using two-level cold atoms interacting with only one laser beam.The underlying mechanism is based on the non-adiabatic nature of laser-atom interaction,with the Rabi frequency being not much larger than the kinetic energy of the atom.We use Zitterbewegung oscillation to further illustrate the effects of the synthesized SOC on the quantum dynamics of the two-level cold atoms.We expect our proposal to be of experimental interest in the quantum simulation of SOC-related physics.

Electronic and Magnetic Properties Modulated by Adsorption of 3d Transition Metal Atoms in Monolayer and Bilayer MoS_2 Sheets

The adsorption effects of 3d transitional metal （TM） adatoms on electronic and magnetic properties of monolayer and bilayer MoS2 sheets have been investigated by using first-principle calculations based on the density functional theory. The calculated results suggest that both monolayer and bilayer MoS2 sheets have power abilities of absorbing 3d TM atoms. The interlayer adsorption of bilayer MoSa is relatively more stable than the surface adsorption of monolayer MoS2. The 3d TM adatoms and the neighboring S atoms behave a clear covalent-binding character. It was found that TM adatoms induce certain impurity states within the band gap of the pristine MoS2 sheet which result in the systems magnetically semiconducting or half metallic. The adsorbed systems for Cr and Co on the surface of monolayer MoS2 sheet, as well as Sc, Cr and Fe in the interlayer of bilayer MoSa sheet exhibit half-metallic behavior. And the other 3d TM- adsorbed systems are magnetic semiconductor except for Ni species.