2,168 research outputs found
Controlling the Schottky barrier at MoS2|metal contacts by inserting a BN monolayer
Making a metal contact to the two-dimensional semiconductor MoS2 without
creating a Schottky barrier is a challenge. Using density functional
calculations we show that, although the Schottky barrier for electrons obeys
the Schottky-Mott rule for high work function ( eV) metals, the
Fermi level is pinned at 0.1-0.3 eV below the conduction band edge of MoS2 for
low work function metals, due to the metal-MoS2 interaction. Inserting a boron
nitride (BN) monolayer between the metal and the MoS2 disrupts this
interaction, and restores the MoS2 electronic structure. Moreover, a BN layer
decreases the metal work function of Co and Ni by eV, and enables a
line-up of the Fermi level with the MoS2 conduction band. Surface modification
by adsorbing a single BN layer is a practical method to attain vanishing
Schottky barrier heights.Comment: 5 pages, 5 figure
Formation of Pt induced Ge atomic nanowires on Pt/Ge(001): a DFT study
Pt deposited onto a Ge(001) surface gives rise to the spontaneous formation
of atomic nanowires on a mixed Pt-Ge surface after high temperature annealing.
We study possible structures of the mixed surface and the nanowires by total
energy (density functional theory) calculations. Experimental scanning
tunneling microscopy images are compared to the calculated local densities of
states. On the basis of this comparison and the stability of the structures, we
conclude that the formation of nanowires is driven by an increased
concentration of Pt atoms in the Ge surface layers. Surprisingly, the atomic
nanowires consist of Ge instead of Pt atoms.Comment: 4 pages, 3 figure
Large potential steps at weakly interacting metal-insulator interfaces
Potential steps exceeding 1 eV are regularly formed at metal|insulator
interfaces, even when the interaction between the materials at the interface is
weak physisorption. From first-principles calculations on metal|h-BN interfaces
we show that these potential steps are only indirectly sensitive to the
interface bonding through the dependence of the binding energy curves on the
van der Waals interaction. Exchange repulsion forms the main contribution to
the interface potential step in the weakly interacting regime, which we show
with a simple model based upon a symmetrized product of metal and h-BN wave
functions. In the strongly interacting regime, the interface potential step is
reduced by chemical bonding
From spin-polarized interfaces to giant magnetoresistance in organic spin valves
We calculate the spin-polarized electronic transport through a molecular
bilayer spin valve from first principles, and establish the link between the
magnetoresistance and the spin-dependent inter- actions at the metal-molecule
interfaces. The magnetoresistance of a Fe|bilayer-C70|Fe spin valve attains a
high value of 70% in the linear response regime, but it drops sharply as a
function of the applied bias. The current polarization has a value of 80% in
linear response, and also decreases as a function of bias. Both these trends
can be modelled in terms of prominent spin-dependent Fe|C70 interface states
close to the Fermi level, unfolding the potential of spinterface science to
control and optimize spin currents.Comment: 13 pages, 5 figure
Band gaps in pseudopotential self-consistent GW calculations
For materials which are incorrectly predicted by density functional theory to
be metallic, an iterative procedure must be adopted in order to perform GW
calculations. In this paper we test two iterative schemes based on the
quasi-particle and pseudopotential approximations for a number of inorganic
semiconductors whose electronic structures are well known from experiment.
Iterating just the quasi-particle energies yields a systematic, but modest
overestimate of the band gaps, confirming conclusions drawn earlier for CaB_6
and YH_3. Iterating the quasi-particle wave functions as well gives rise to an
imbalance between the Hartree and Fock potentials and results in bandgaps in
far poorer agreement with experiment.Comment: 5 pages, 2 figures, 2 table
Electronic structure and correlations in pristine and potassium doped Cu-Phthalocyanine molecular crystals
We investigate the changes in the electronic structure of copper
phthalocyanine (CuPc) crystals that is caused by intercalation with potassium.
This is done by means of {\it ab initio} LSDA and LSDA+U calculations of the
electronic structure of these molecular crystals. Pristine CuPc is found to be
an insulator with local magnetic moments and a Pc-derived valence band with a
width of 0.32 eV. In the intercalated compound the additional
electrons that are introduced by potassium are fully transferred to the
states of the Pc-ring. A molecular low spin state results, preserving, however,
the local magnetic moment on the copper ions. The degeneracy of the
levels is split by a crystal field that quenches the orbital degeneracy and
gives rise to a band splitting of 110 meV. Molecular electronic Coulomb
interactions enhance this splitting in to a charge gap of 1.4 eV.
The bandwidth of the conduction band is 0.56 eV, which is surprisingly large
for a molecular solid. This is line with the experimentally observation that
the system with additional potassium doping, , is a metal
as the unusually large bandwidth combined with the substantial carrier
concentration acts against localization and polaron formation, while strongly
promoting the delocalization of the charge carriers.Comment: 5 pages, 7 figures embedde
Band gaps in incommensurable graphene on hexagonal boron nitride
Devising ways of opening a band gap in graphene to make charge-carrier masses
finite is essential for many applications. Recent experiments with graphene on
hexagonal boron nitride (h-BN) offer tantalizing hints that the weak
interaction with the substrate is sufficient to open a gap, in contradiction of
earlier findings. Using many-body perturbation theory, we find that the small
observed gap is what remains after a much larger underlying quasiparticle gap
is suppressed by incommensurability. The sensitivity of this suppression to a
small modulation of the distance separating graphene from the substrate
suggests ways of exposing the larger underlying gap
Ab initio study on the effects of transition metal doping of Mg2NiH4
Mg2NiH4 is a promising hydrogen storage material with fast (de)hydrogenation
kinetics. Its hydrogen desorption enthalpy, however, is too large for practical
applications. In this paper we study the effects of transition metal doping by
first-principles density functional theory calculations. We show that the
hydrogen desorption enthalpy can be reduced by ~0.1 eV/H2 if one in eight Ni
atoms is replaced by Cu or Fe. Replacing Ni by Co atoms, however, increases the
hydrogen desorption enthalpy. We study the thermodynamic stability of the
dopants in the hydrogenated and dehydrogenated phases. Doping with Co or Cu
leads to marginally stable compounds, whereas doping with Fe leads to an
unstable compound. The optical response of Mg2NiH4 is also substantially
affected by doping. The optical gap in Mg2NiH4 is ~1.7 eV. Doping with Co, Fe
or Cu leads to impurity bands that reduce the optical gap by up to 0.5 eV.Comment: 8 pages, 4 figure
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