Electric-field-tunable molecular adsorption on germanane.

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.

Strain-tunable molecular doping in germanane: a first-principles study.

Germanane, fully hydrogenated germanene, has recently attracted great interest, both theoretical and experimental. In this paper we thoroughly study strain-tunable n/p-type doping in germanane by adsorption of tetrathiafulvalene (TTF)/tetracyanoquinodimethane (TCNQ) molecules through first-principles calculations. The results show that both TTF and TCNQ molecules can non-covalently functionalize the electronic properties of germanane. Not surprisingly, TTF molecular adsorption induces n-type doping in germanane because the TTF molecule is a typical electron donor. Moreover, a linearly tunable band gap of germanane and differing n-type doping strengths can be realized by a biaxial strain ranging from -3% to 3%. Analysis indicates that tensile strain would promote the doping effect whereas compressive strain would inhibit it. Comparatively, TCNQ molecular adsorption induces a germanane/TCNQ system which exhibits metallic characteristics. Surprisingly, however, under a tensile strain of 2.5%, a strong p-type doping effect is achieved in germanene/TCNQ. In particular, with increasing tensile strain over the range 2.5%-3%, the strain-tunable p-type doping effect decreases gradually. Such a multiple effect of molecular adsorption and strain on the electronic properties of germanane could be helpful for potential future applications of germanane-based electron devices.

Multiple Dirac Points and Hydrogenation-Induced Magnetism of Germanene Layer on Al (111) Surface.

A continuous germanene layer grown on the Al (111) surface has recently been achieved in experiment. In this work, we investigate its structural, electronic, and hydrogenation-induced properties through first-principles calculations. We find that despite having a different lattice structure from its free-standing form, germanene on Al (111) still possesses Dirac points at high-symmetry K and K' points. More importantly, there exist another three pairs of Dirac points on the K(K')-M high-symmetry lines, which have highly anisotropic dispersions due to the reduced symmetry. These massless Dirac Fermions become massive when spin-orbit coupling is included. Hydrogenation of the germanene layer strongly affects its structural and electronic properties. Particularly, when not fully hydrogenated, ferromagnetism can be induced due to unpaired local orbitals from the unsaturated Ge atoms. Remarkably, we discover that the one-side semihydrogenated germanene turns out to be a two-dimensional half-semimetal, representing a novel state of matter that is simultaneously a half-metal and a semimetal.

Comparison of the stability of free-standing silicene and hydrogenated silicene in oxygen: a first principles investigation.

The stability of free-standing silicene in oxygen is worthwhile discussing. In this letter, the oxygen adsorption and dissociation on free-standing silicene is studied using first principles. It is found that free-standing silicene is not stable in oxygen because O2 molecules can be easily adsorbed and dissociated into O atoms on a silicene surface without overcoming any energy barrier. Moreover, dissociated oxygen atoms are difficult to migrate on and desorb from silicene surfaces, leading to the formation of Si-O compounds. To enhance the stability of free-standing silicene in oxygen, fully hydrogenated silicene is used as a stabiliser. Interestingly, compared with no energy barrier on pristine silicene, there are two minor energy barriers of O2 molecule adsorption and dissociation on fully hydrogenated silicene, indicating that hydrogenated silicene has higher stability than free-standing silicene in oxygen. However, once the O2 molecule dissociates into two O atoms on hydrogenated silicene, desorption of O atoms will be very difficult due to its high energy barrier. This work will be helpful to understand the detail of O2 molecule dissociation and dissociation-induced O atoms adsorption on free-standing and hydrogenated silicene in oxygen and will be useful to the application of silicene.

The role of Cu in degrading adsorption of CO on the PtnCu clusters.

The platinum copper alloy nanocrystals (NCs) have generated much interest because of their wide applications in fuel cells due primarily to their good catalytic performance and to decreasing sensitivity toward CO poisoning. The exact atomic-level morphology of platinum copper alloy NCs is still not clear in the literature, and research to understanding the poisoning mechanism is still insufficient to date. In this article, we report on density functional calculations of small PtnCu clusters and their adsorption of a CO molecule that provide evidence for degrading adsorption of the CO molecule compared to pure platinum clusters. The lowest-energy geometries of PtnCu and PtnCuCO clusters have been identified. The CO molecule prefers to be adsorbed on the nearest platinum atom by the C-end-on mode, forming linear or quasi-linear O-C-Pt structures. The adsorption energies indicate that the introduction of a copper atom decreases the adsorption ability of the CO molecule. The local density of states of the representative clusters is used to characterize the adsorption properties of the CO molecule on the PtnCu clusters. Results from our theoretical calculations can be helpful for understanding the poisoning mechanism of the CO molecule on the platinum copper alloy NCs.

Transition from Mn(4+) to Mn(3+) induced by surface reconstruction at λ-MnO(2)(001).

Structural and electronic properties of the λ-MnO(2)(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-MnO(2)(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 3d(z(2) ) 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.