223 research outputs found
Geometric, electronic and magnetic structure of FeO clusters
Correlation between geometry, electronic structure and magnetism of solids is
both intriguing and elusive. This is particularly strongly manifested in small
clusters, where a vast number of unusual structures appear. Here, we employ
density functional theory in combination with a genetic search algorithm,
GGA and a hybrid functional to determine the structure of gas phase
FeO clusters. For FeO cation clusters we also
calculate the corresponding vibration spectra and compare them with
experiments. We successfully identify FeO, FeO,
FeO, FeO and propose structures for
FeO. Within the triangular geometric structure of
FeO a non-collinear, ferrimagnetic and ferromagnetic state are
comparable in energy. FeO and FeO are
ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization.
FeO is ferrimagnetic due to the odd number of Fe atoms. We
compare the electronic structure with bulk magnetite and find
FeO, FeO, FeO to be mixed
valence clusters. In contrast, in FeO and FeO
all Fe are found to be trivalent.Comment: 14 pages, 21 figure
Low work function of the (1000) Ca2N surface
Polymer diodes require cathodes that do not corrode the polymer but do have
low work function to minimize the electron injection barrier. First-principles
calculations demonstrate that the work function of the (1000) surface of the
compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs.
2.87 eV). Moreover its reactivity is expected to be smaller. This makes Ca2N an
interesting candidate to replace calcium as cathode material for polymer light
emitting diode devices.Comment: 3 pages, 4 figures, accepted by J. Appl. Phy
Interrelation of work function and surface stability: the case of BaAl4
The relationship between the work function (Phi) and the surface stability of
compounds is, to our knowledge, unknown, but very important for applications
such as organic light-emitting diodes. This relation is studied using
first-principles calculations on various surfaces of BaAl4. The most stable
surface [Ba terminated (001)] has the lowest Phi (1.95 eV), which is lower than
that of any elemental metal including Ba. Adding barium to this surface neither
increases its stability nor lowers its work function. BaAl4 is also strongly
bound. These results run counter to the common perception that stability and a
low Phi are incompatible. Furthermore, a large anisotropy and a stable
low-work-function surface are predicted for intermetallic compounds with polar
surfaces.Comment: 4 pages, 5 figures, to be published in Chem. Ma
NMR shieldings from density functional perturbation theory: GIPAW versus all-electron calculations
We present a benchmark of the density functional linear response calculation
of NMR shieldings within the Gauge-Including Projector-Augmented-Wave method
against all-electron Augmented-Plane-Wavelocal-orbital and uncontracted
Gaussian basis set results for NMR shieldings in molecular and solid state
systems. In general, excellent agreement between the aforementioned methods is
obtained. Scalar relativistic effects are shown to be quite large for nuclei in
molecules in the deshielded limit. The small component makes up a substantial
part of the relativistic corrections.Comment: 3 figures, supplementary material include
Band Offsets at the Interface between Crystalline and Amorphous Silicon from First Principles
The band offsets between crystalline and hydrogenated amorphous silicon
(a-Si:H) are key parameters governing the charge transport in modern silicon
hetrojunction solar cells. They are an important input for macroscopic
simulators that are used to further optimize the solar cell. Past experimental
studies, using X-ray photoelectron spectroscopy (XPS) and capacitance-voltage
measurements, have yielded conflicting results on the band offset. Here we
present a computational study on the band offsets. It is based on atomistic
models and density-functional theory (DFT). The amorphous part of the interface
is obtained by relatively long DFT first-principles molecular-dynamics (MD)
runs at an elevated temperature on 30 statistically independent samples. In
order to obtain a realistic conduction band position the electronic structure
of the interface is calculated with a hybrid functional. We find a slight
asymmetry in the band offsets, where the offset in the valence band (0.30 eV)
is larger than in the conduction band (0.17 eV). Our results are in agreement
with the latest XPS measurements that report a valence band offset of 0.3 eV
[M. Liebhaber et al., Appl. Phys. Lett. 106, 031601 (2015)]
Band Offsets at the Interface Between Crystalline and Amorphous Silicon from First Principles
The band offsets between crystalline and hydrogenated amorphous silicon (a−Si∶H) are key parameters governing the charge transport in modern silicon heterojunction solar cells. They are an important input for macroscopic simulators that are used to further optimize the solar cell. Past experimental studies, using x-ray photoelectron spectroscopy (XPS) and capacitance-voltage measurements, have yielded conflicting results on the band offset. Here, we present a computational study on the band offsets. It is based on atomistic models and density-functional theory (DFT). The amorphous part of the interface is obtained by relatively long DFT first-principles molecular-dynamics runs at an elevated temperature on 30 statistically independent samples. In order to obtain a realistic conduction-band position the electronic structure of the interface is calculated with a hybrid functional. We find a slight asymmetry in the band offsets, where the offset in the valence band (0.29 eV) is larger than in the conduction band (0.17 eV). Our results are in agreement with the latest XPS measurements that report a valence-band offset of 0.3 eV [M. Liebhaber et al., Appl. Phys. Lett. 106, 031601 (2015)]
First-principles study of the optical properties of MgxTi(1-x)H2
The optical and electronic properties of Mg-Ti hydrides are studied using
first-principles density functional theory. Dielectric functions are calculated
for MgxTi(1-x)H2 with compositions x = 0.5, 0.75, and 0.875. The structure is
that of fluorite TiH2 where both Mg and Ti atoms reside at the Ti positions of
the lattice. In order to assess the effect of randomness in the Mg and Ti
occupations we consider both highly ordered structures, modeled with simple
unit cells of minimal size, and models of random alloys. These are simulated by
super cells containing up to 64 formula units (Z = 64). All compositions and
structural models turn out metallic, hence the dielectric functions contain
interband and intraband free electron contributions. The former are calculated
in the independent particle random phase approximation. The latter are modeled
based upon the intraband plasma frequencies, which are also calculated from
first-principles. Only for the models of the random alloys we obtain a black
state, i.e. low reflection and transmission in the energy range from 1 to 6 eV.Comment: 7 pages, 8 figure
Modeling and analysis of the three-dimensional current density in sandwich-type single-carrier devices of disordered organic semiconductors
We present the results of a modeling study of the three-dimensional current density in single-carrier sandwich-type devices of disordered organic semiconductors. The calculations are based on a master-equation approach, assuming a Gaussian distribution of site energies without spatial correlations. The injection-barrier lowering due to the image potential is taken into account, so that the model provides a comprehensive treatment of the space-charge-limited current as well as the injection-limited current (ILC) regimes. We show that the current distribution can be highly filamentary for voltages, layer thicknesses, and disorder strengths that are realistic for organic light-emitting diodes and, that, as a result, the current density in both regimes can be significantly larger than as obtained from a one-dimensional continuum drift-diffusion device model. For devices with large injection barriers and strong disorder, in the ILC transport regime, good agreement is obtained with the average current density predicted from a model assuming injection and transport via one-dimensional filaments.
Ab initio simulations of liquid systems: Concentration dependence of the electric conductivity of NaSn alloys
Liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80%
sodium) are studied using density functional calculations combined with
molecular dynamics(Car-Parrinello method). The frequency-dependent electric
conductivities for the systems are calculated by means of the Kubo-Greenwood
formula.
The extrapolated DC conductivities are in good agreement with the
experimental data and reproduce the strong variation with the concentration.
The maximum of conductivity is obtained, in agreement with experiment, near the
equimolar composition.
The strong variation of conductivity, ranging from almost semiconducting up
to metallic behaviour, can be understood by an analysis of the
densities-of-states.Comment: LaTex 6 pages and 2 figures, to appear in J.Phys. Cond. Ma
Ab initio simulations of liquid NaSn alloys: Zintl anions and network formation
Using the Car-Parrinello technique, ab initio molecular dynamics simulations
are performed for liquid NaSn alloys in five different compositions (20, 40,
50, 57 and 80 % sodium). The obtained structure factors agree well with the
data from neutron scattering experiments. The measured prepeak in the structure
factor is reproduced qualitatively for most compositions. The calculated and
measured positions of all peaks show the same trend as function of the
composition.\\ The dynamic simulations also yield information about the
formation and stability of Sn clusters (Zintl anions) in the liquid. In our
simulations of compositions with 50 and 57 % sodium we observe the formation of
networks of tin atoms. Thus, isolated tin clusters are not stable in such
liquids. For the composition with 20 % tin only isolated atoms or dimers of tin
appear, ``octet compounds'' of one Sn atom surrounded by 4 Na atoms are not
observed.Comment: 12 pages, Latex, 3 Figures on reques
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