Search

Structural and electronic properties of the λ-MnO2(001) surface are investigated applying density functional theory approach. The calculations show that all Mn ions at unreconstructed smooth surface preserve the +4 oxidation state observed in the bulk. Upon the λ-MnO2(001) reconstruction, one fourth of Mn ions at the surface undergo a change of the oxidation state from +4 to +3, although the reconstruction does not change the Mn coordination number with oxygen. This is accompanied with the filling of initially empty 3dz2 states localized on cations with one electron denoted by two neighboring O atoms. Although the reconstruction leads to an energy gain of 0.04 eV per surface unit cell, it is not a spontaneous process since it proceeds with an activation energy of 0.12 eV. [ABSTRACT FROM AUTHOR]

Very recently, half-hydrogenated germanene has been achieved in an experiment. In this paper, we investigate the effects of tetracyanoquinodimethane (TCNQ) molecular adsorption and strain on the electronic properties of half-hydrogenated germanene through first-principles. As an electron-acceptor molecule, TCNQ is exploited to non-covalently functionalize the half-hydrogenated germanene. However, this physical adsorption induces a ferromagnetic semiconductor–metal transition in half-hydrogenated germanene due to charge transfer from the substrate to the TCNQ molecule. More importantly, the superstructure of half-hydrogenated germanene/TCNQ is extremely sensitive to biaxial tensile strain. Under the biaxial tensile strain of 0.25%, the ferromagnetic semiconductor–metal transition induced by molecular adsorption can surprisingly be overturned. Meanwhile, a strong p-type doping is exhibited. Remarkably, it would return from a ferromagnetic semiconductor to a metal again when the biaxial tensile strain increases to 1.5%. Our analysis based on the structural and electronic properties of half-hydrogenated germanene/TCNQ indicates that such metal–semiconductor–metal transition in half-hydrogenated germanene/TCNQ under biaxial tensile strain may originate from the strong local deformation, resulting in the energy of the valence band maximum decreasing below or increasing above the Fermi level. [ABSTRACT FROM AUTHOR]

Li2MnO3 compound is one essential component of the Li-rich solid solution cathode material (mLi2MnO3⋅nLiMO2, M=Mn, Ni, Co, etc.) for lithium ion batteries. The intrinsic insulating nature of the electronic structure determines that the electronic conductivity of the Li2MnO3 compound is low, which is unfavorable to the fast charge/discharge performance of the battery. In this paper, we demonstrate that F doping can create polaron states in the Li2MnO3 lattice from first principles calculations. The small polaron migration energy barrier in the Li2MnO3 lattice is about 0.27 eV. It is also observed that the polaron state is strongly trapped by the F atom, which decreases the efficiency of the doping enhanced electronic conductivity. [ABSTRACT FROM AUTHOR]

The evolution of the structural and electronic properties from sheet to silicene is studied by using first-principles density functional theory. It is found that the planar configurations of the - monolayer systems are basically kept except for the increase of the buckling of the planar structure when the substitution ratio of Si increases. Band gaps of the - monolayer system decrease gradually when the substitution ratio of Si atoms ranges from 0% to 100%. The energy and type of the band gaps are closely related with the substitution ratio of Si atoms and the - order. Further analysis of density of states reveals the orbital contribution of and atoms near the Fermi level. The discussion of the electronic evolution from SiC sheet to silicene would widen the application of the - monolayer systems in the optoelectronic field in the future nanotechnology. [ABSTRACT FROM AUTHOR]

The O2 dissociation and O atoms adsorption on free-standing germanene are studied by using first-principles calculations in this paper. Compared with the extremely active silicene in oxygen atmosphere, germanene is found to be less active due to an energy barrier for dissociation of about 0.57 eV. Moreover, the dissociated oxygen atom follows two opposite migration pathways on the germanene surface, which is quite different from the case of silicene. Furthermore, the migration and desorption of O atoms at room temperature are relatively difficult due to the strong Ge-O bonding, resulting in the formation of germanium oxides. Our results reveal the interplay between germanene and O2 and suggest the enhanced stability of germanene in oxygen atmosphere compared with silicene. [ABSTRACT FROM AUTHOR]

Recently, Yoo and Zhou [ACS Nano 5, 3020-3026 (2011)] reported that graphene can be directly used as electrode for Li-air batteries, suggesting that graphene itself can be served as catalyst for O2 dissociation reaction. In this work, we show from density functional theory calculations that the O2 dissociation reaction energy barrier is substantially decreased on the graphene surface comparing to that of in vacuum. Furthermore, N-doping can further decrease the energy barrier from 2.39 eV (undoped case) to 1.20 eV. The O2 molecule physical adsorption, O atom chemical adsorption at the graphene, and N-doped graphene surfaces are also simulated. [ABSTRACT FROM AUTHOR]

The lithium ion dynamics in graphite, an important anode material for lithium ion batteries, is strongly related with the layer-to-layer distance (d-spacing) and the stacking modes of graphite. We studied these relationships by first-principles calculations. It is found that a larger d-spacing results in the easier transformation of the stacking mode from A-B stacking to A-A stacking when lithium atoms are inserted. This transformation is unfavorable to the lithium diffusion because of the larger diffusion energy barrier for lithium in A-A stacking graphite compared to that in A-B one. On the other hand, as the d-spacing increases, the diffusion energy barrier for lithium in A-A stacking graphite decreases substantially, thus being favorable to the lithium diffusion. These results give a better understanding of the lithium ion dynamics in graphite, and show that it is possible to optimize the lithium ion dynamics in graphite by properly adjusting the d-spacing. [ABSTRACT FROM AUTHOR]

In this paper, we studied the structural and electronic properties of core/shell nanocables composed of cubic silicon carbide nanowires (β-) and boron nitride nanotubes (BNNT) using first-principles pseudopotential plane wave method within density functional theory. Our results show that the β-/BNNT heterojunction structures are metallic, which primarily originates from the contributions of the BNNTs and the surfaces of s. The BNNTs exhibit metallic characters after the nanowires are inserted. The transition of the BNNTs is attributed to the charge transfer between BNNTs and s. [ABSTRACT FROM AUTHOR]

The platinum copper alloy nanocrystals(NCs) have generated muchinterest because of their wide applications in fuel cells due primarilyto their good catalytic performance and to decreasing sensitivitytoward CO poisoning. The exact atomic-level morphology of platinumcopper alloy NCs is still not clear in the literature, and researchto understanding the poisoning mechanism is still insufficient todate. In this article, we report on density functional calculationsof small PtnCu clusters and their adsorptionof a CO molecule that provide evidence for degrading adsorption ofthe CO molecule compared to pure platinum clusters. The lowest-energygeometries of PtnCu and PtnCuCO clusters have been identified. The CO moleculeprefers to be adsorbed on the nearest platinum atom by the C-end-onmode, forming linear or quasi-linear O–C–Pt structures.The adsorption energies indicate that the introduction of a copperatom decreases the adsorption ability of the CO molecule. The localdensity of states of the representative clusters is used to characterizethe adsorption properties of the CO molecule on the PtnCu clusters. Results from our theoretical calculationscan be helpful for understanding the poisoning mechanism of the COmolecule on the platinum copper alloy NCs. [ABSTRACT FROM AUTHOR]