Intrinsic defects and dopants in LiNH2: a first-principles study.

The lithium amide (LiNH(2)) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH(2) using density functional theory. We find that both Li-related defects (the positive interstitial Li(i)(+) and the negative vacancy V(Li)(-)) and H-related defects (H(i)(+) and V(H)(-)) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3-0.5 eV, they diffuse rapidly at moderate temperatures. V(H)(-) corresponds to the [NH](2-) ion. It is the dominant species available for proton transport with a diffusion barrier of ∼0.7 eV. The equilibrium concentration of H(i)(+), which corresponds to the NH(3) molecule, is negligible in bulk LiNH(2). Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH(2) lattice, which may affect the crystal morphology on the nano-scale.

Germanene: the germanium analogue of graphene.

Recently, several research groups have reported the growth of germanene, a new member of the graphene family. Germanene is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled and composed of two vertically displaced sub-lattices. Density functional theory calculations have revealed that free-standing germanene is a 2D Dirac fermion system, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, which is the relativistic variant of the Schrödinger equation. Germanene is a very appealing 2D material. The spin-orbit gap in germanene (~24 meV) is much larger than in graphene (<0.05 meV), which makes germanene the ideal candidate to exhibit the quantum spin Hall effect at experimentally accessible temperatures. Additionally, the germanene lattice offers the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene based field-effect devices. In this topical review we will (1) address the various methods to synthesize germanene (2) provide a brief overview of the key results that have been obtained by density functional theory calculations and (3) discuss the potential of germanene for future applications as well for fundamentally oriented studies.

Interfering Bloch waves in a 1D electron system.

Using low-temperature scanning tunnelling spectroscopy we have studied the spatial variation of confined electronic states between neighbouring atomic chains on a Ge(001)/Pt surface. The quasi-one-dimensional electronic states reside in the troughs between the atomic chains and exhibit a profound Bloch character along the chain direction. In the proximity of defects an enhancement of the oscillatory standing wave pattern in the density of states is found. The spatial variation of the standing wave pattern can be explained by an interference of incoming and reflected Bloch waves.

Doping graphene with metal contacts.

Making devices with graphene necessarily involves making contacts with metals. We use density functional theory to study how graphene is doped by adsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au, and Pt, while preserving its unique electronic structure, can still shift the Fermi level with respect to the conical point by approximately 0.5 eV. At equilibrium separations, the crossover from p-type to n-type doping occurs for a metal work function of approximately 5.4 eV, a value much larger than the graphene work function of 4.5 eV. The numerical results for the Fermi level shift in graphene are described very well by a simple analytical model which characterizes the metal solely in terms of its work function, greatly extending their applicability.

Graphite and graphene as perfect spin filters.

Based upon the observations (i) that their in-plane lattice constants match almost perfectly and (ii) that their electronic structures overlap in reciprocal space for one spin direction only, we predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. The spin filtering is quite insensitive to roughness and disorder. The formation of a chemical bond between graphite and the open d-shell transition metals that might complicate or even prevent spin injection into a single graphene sheet can be simply prevented by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin-injection property.

Parameter-free quasiparticle calculations for YH3.

Electronic structure calculations for YH3 within the local density approximation result in a metallic ground state with the bands at the Fermi energy overlapping by more than 1 eV, whereas a band gap of 2.8 eV is deduced from optical experiments. Here, we report the results of parameter-free GW calculations which predict a fundamental gap of 1 eV. When we take into account electric dipole matrix elements a large optical gap of almost 3 eV is obtained. A combination of photoemission and inverse photoemission spectroscopy could test the prediction of a small fundamental band gap.

CaB6: a new semiconducting material for spin electronics.

Ferromagnetism was recently observed at unexpectedly high temperatures in La-doped CaB6. The starting point of all theoretical proposals to explain this observation is a semimetallic electronic structure calculated for CaB6 within the local density approximation. Here we report the results of parameter-free quasiparticle calculations of the single-particle excitation spectrum which show that CaB6 is not a semimetal but a semiconductor with a band gap of 0.8+/-0.1 eV. Magnetism in La(x)Ca1-xB6 occurs just on the metallic side of a Mott transition in the La-induced impurity band.