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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]

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]

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]

Fully-hydrogenated germanene, named germanane, represents a new nanostructured material for a variety of potential applications, such as electronics and optoelectronics. However, a critical requirement for developing practical and reliable electronic devices based on germanane consists of achieving a flexibly controllable charge carrier and doping level. Different to the conventional doping methods such as ion implantation and diffusion, by first-principles calculations we demonstrate that tetracyanobenzene (TCNB) molecular adsorption could introduce effective p-type doping in germanane due to the combination of germanane with electroactive acceptor molecule TCNB. The corresponding energy difference between the empty band minimum of the dopant and the valence band maximum for electron excitation is 0.173 eV. More importantly, this nondestructive p-type doping could be linearly tuned under an external E-field. Analysis of charge transfer by means of the equivalent capacitor model and the shift of energy levels in the superstructure of germanane/TCNB further reveals that the superposition of the external E-field and molecular adsorption-induced internal E-field plays a key role in the charge transfer between TCNB and germanane, especially in achieving a controllable p-type molecular doping level in germanane. Such convenient and flexible E-field-engineering of p-type molecular doping in germanane would be very helpful for potential applications of germanane-based electronic and optoelectronic devices in the future. [ABSTRACT FROM AUTHOR]

The effects of both molecular adsorption and external electric field on the electronic properties of germanene are investigated through first-principles calculations in this work. The molecular adsorption-induced intrinsic electric field by small molecules (SMs) breaks the symmetry of two sublattices in germanene, leading to a band gap opening from zero to 3.2 meV, even up to 81.0 meV. Under an external electric field, a wide ranging linearly tunable band gap (0–69.39 meV for the germanene/methane system and 37.66–134.17 meV for the germanene/ammonia system) can be realized, which is merely determined by the strength of the composited electric field independent of its direction. More importantly, the band gap of germanene/SMs can be closed and reopened under a critical electric field. On the other hand, the mechanism of charge transfer between germanene and SMs under an external electric field is revealed by an equivalent capacitor model to explain the tunable charge transfer. The composited field-induced charge transfer could be promoted when an external electric field and a molecular adsorption-induced internal electric field have the same direction. Otherwise, the charge transfer will be inhibited. Particularly, the tunable electronic properties of germanene are sensitive to the concentration of molecular adsorption under an electric field, representing multiple-effects that would significantly enhance the performance of germanene for electronic devices in the future. [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]

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 stability of free-standing silicene in O2 is an open question. In this letter, the O2 dissociation and O2-dissociation–induced O atoms adsorption on free-standing silicene are studied by using first-principles calculations. Our results show that the O2 molecule dissociates on the free-standing silicene surface easily from both the thermodynamic and kinetic points of view, which is different from the case of graphene. The dissociation reaction is an exothermic process, and the dissociated O atoms form strong bonds with Si atoms, which lowers the energy of the system substantially. On the other hand, the dissociation reaction occurs spontaneously on the free-standing silicene without overcoming any energy barrier. Furthermore, the migration and desorption of O atoms are relatively difficult under room temperature due to the strong Si-O bonds in the O-adsorbed silicene, which is in favor of forming silicon oxides. Our results provide convictive evidence to show that free-standing silicene is unstable in O2. [ABSTRACT FROM AUTHOR]

The effect of the tensile strain on the electronic structure of the silicene is studied by using first-principles density functional theory. It is found that a semimetal-metal transition occurs when an in-plane strain larger than 7.5% is applied in silicene. The downward movement of the lowest conduction band at Γ-point, which originates from the weakened interaction between neighboring Si atoms, leads to the transition. The proposed mechanical control of the electronic properties will widen the application of the silicene in Si-based nanotechnology. [ABSTRACT FROM AUTHOR]