J.L. Nie

Hydrogen adsorption, dissociation and diffusion on the α-U(001) surface

First-principles pseudopotential plane-wave calculations based on density functional theory and the generalized-gradient approximation have been used to study the adsorption, dissociation, and diffusion of hydrogen on the α-U(001) surface. Weak molecular chemisorption was observed for H2 approaching with its molecular axis parallel to the surface. The optimization of the adsorption geometries on the threefold hollow sites yields final configurations with H2 molecules moving towards the top site at both coverages considered, 0.25 and 0.5 monolayers. A low dissociation barrier of 0.081 eV was determined for H2 dissociated from the onefold top site with the H atoms falling into the two adjacent threefold hollow sites. The analysis of the density of states along the dissociation paths shows that the hybridization of U 5f and H 1s states only occurs when the H2 molecule is dissociated.

Dehydrogenation: a simple route to modulate magnetism and spatial charge distribution of germanane

Two-dimensional (2D) materials recently emerged as a new type of nanostructures exhibited large potential for application in nanoscale devices. Nevertheless, many proposed applications require efficient modulation of magnetism and spatial charge distribution within these 2D nanostructures. Here we, via density functional theory (DFT), demonstrated that both magnetism and spatial charge distribution of recently experimentally realized germanane can be effectively modulated via a simple dehydrogenating process. Both single-sided and double-sided H vacancy clusters due to the unpairing of Ge-4p electrons can make germanane obtain magnetism with designated magnitudes. Charges of valence band maximum (VBM) and conduction band minimum (CBM) in germanane that contains single-sided H vacancy clusters can be effectively separated, and charges of VBM and CBM in germanane that contains double-sided H vacancy clusters can be effectively assembled. Thus, our results provide a new perspective on precisely modulating magnetism and spatial charge distribution of germanane, which are fundamentally important for application of germanane in quantum information and optoelectronic fields.

Calculation of exchange integrals and Curie temperature for La-substituted barium hexaferrites.

As the macro behavior of the strength of exchange interaction, state of the art of Curie temperature Tc, which is directly proportional to the exchange integrals, makes sense to the high-frequency and high-reliability microwave devices. Challenge remains as finding a quantitative way to reveal the relationship between the Curie temperature and the exchange integrals for doped barium hexaferrites. Here in this report, for La-substituted barium hexaferrites, the electronic structure has been determined by the density functional theory (DFT) and generalized gradient approximation (GGA). By means of the comparison between the ground and relative state, thirteen exchange integrals have been calculated as a function of the effective value Ueff. Furthermore, based on the Heisenberg model, the molecular field approximation (MFA) and random phase approximation (RPA), which provide an upper and lower bound of the Curie temperature Tc, have been adopted to deduce the Curie temperature Tc. In addition, the Curie temperature Tc derived from the MFA are coincided well with the experimental data. Finally, the strength of superexchange interaction mainly depends on 2b-4f1, 4f2-12k, 2a-4f1, and 4f1-12k interactions.

Chemisorptions of atomic oxygen and its replacement by hydrogen on the diamond (100) surface studied by first principles

The initial oxidation process of a clean diamond (100) surface was studied by first-principles calculations. The O-bridge with C–O–C bond chemisorption, O-on-dimer chemisorption with epoxy structure, and O-on-top chemisorption with C=O bond structure are found to be stable on the diamond (100) surfaces. The epoxy structure is more stable than the O-bridge structure. The calculation also shows that the oxygen atom can be replaced by hydrogen in the oxidized diamond (100) surface.

ADSORPTION OF Li ON Mo(110) SURFACE: A FIRST-PRINCIPLES STUDY

Periodic, self-consistent, density functional theory calculations have been performed to investigate Li adsorption on Mo(110) surface. It turns out that the long-bridge site is the most stable site and the Li–Mo surface alloy forms easily at high coverage with the substitution of Mo by Li atoms in the outermost layer. Work function analysis showed that the work function decreases dramatically as the coverage from 0 to 0.5 ML, and finally increases again at the coverage of 1 ML, which agrees well with the experimental finding of Kroger et al. (Surf. Sci. 449 (2000) 227–235). Vibrational properties, diffusion barrier of Li along the Mo(110) surface, and the energy of formation of the surface have also been investigated for Li adsorption at various coverages.

FIRST-PRINCIPLES STUDY OF Ni ADSORPTION ON Mo(110)

The adsorption of Ni atom on the Mo(110) surface has been studied within the density functional theory framework. It turned out that Ni–Mo surface alloy was formed with Ni atoms substituting Mo atom in the outermost layer. The subsurface site adsorption was found to be not preferred. Geometric analysis showed that the rumpling between substitutional Ni and Mo in the first alloy layer was about 0.108 A at medium and low coverage (Θ). In addition, the diffusion of Ni on bare and Ni-substitutional Mo(110) surface has been investigated. It was shown that the diffusion energy barrier was reduced as the increase of coverage on bare Mo(110) surface, which supports the switch of growth mode layer-by-layer to Stranski–Krastanov as the function of coverage. Substitutional Ni atom only slightly increases the energy barrier for Ni diffusion on Mo(110) surface.

First Principles Study on the Adsorption of Alkali Metal on C(100)( 2×1)

First-principles calculations based on density functional theory, using PW91 functional have been performed to study the adsorption of Alkali metal (AM) on C(100)(2×1) surface. The stable geometries, adsorption energies for all adsorption configurations have been calculated on half a monolayer and one monolayer. The preferred binding sites have been determined to be valley-bridge sites at the coverage of 0.5ML. At higher coverage of 1ML, two AM adsorbates were found to reside in pedestal site and valley-bridge site, respectively. Work function analysis showed that when AMs are adsorbed on C(100)(2×1) surface, the work function decreases linearly with increasing coverage and reaches a minimum at Θ=0.5ML. At higher coverage, the work function is increased again, which may be caused by depolarization effect of the adsorbate.