360 research outputs found
Confinement effects in ultra-thin ZnO polymorph films: electronic and optical properties
Relying on generalized-gradient and hybrid first-principles simulations, this
work provides a complete characterization of the electronic properties of ZnO
ultra-thin films, cut along the Body-Centered-Tetragonal(010), Cubane(100),
h-BN(0001), Zinc-Blende(110), Wurtzite(100) and (0001) orientations.
The characteristics of the local densities of states are analyzed in terms of
the reduction of the Madelung potential on under-coordinated atoms and surface
states/resonances appearing at the top of the VB and bottom of the CB. The gap
width in the films is found to be larger than in the corresponding bulks, which
is assigned to quantum confinement effects. The components of the high
frequency dielectric constant are determined and the absorption spectra of the
films are computed. They display specific features just above the absorption
threshold due to transitions from or to the surface resonances. This study
provides a first understanding of finite size effects on the electronic
properties of ZnO thin films and a benchmark which is expected to foster
experimental characterization of ultra-thin films via spectroscopic techniques
Role of the environment in the stability of anisotropic gold particles
International audienceDespite the long-lasting interest in the synthesis control of nanoparticles (NPs) in both fundamental and applied nanosciences, the driving mechanisms responsible for their size and shape selectivity in an environment (solution) are not completely understood, and a clear assessment of the respective roles of equilibrium thermodynamics and growth kinetics is still missing. In this study, relying on an efficient atomistic computational approach, we decipher the dependence of energetics, shapes and morphologies of gold NPs on the strength and nature of the metalâenvironment interaction. We highlight the conditions under which the energy difference between isotropic and elongated gold NPs is reduced, thus prompting their thermodynamic coexistence. The study encompasses both monocrystalline and multi-twinned particles and extends over size ranges particularly representative of the nucleation and early growth stages. Computational results are further rationalized with arguments involving the dependence of facet and edge energies on the metalâenvironment interactions. We argue that by determining the abundance and diversity of particles nucleated in solution, thermodynamics may constitute an important bias influencing their final shape. The present results provide firm grounds for kinetic simulations of particle growth
The Adsorption of H2O on TiO2 and SnO2(110) Studied by First-Principles Calculations
First-principles calculations based on density functional theory and the
pseudopotential method have been used to investigate the energetics of HO
adsorption on the (110) surface of TiO and SnO. Full relaxation of all
atomic positions is performed on slab systems with periodic boundary
conditions, and the cases of full and half coverage are studied. Both molecular
and dissociative (HO OH + H) adsorption are treated,
and allowance is made for relaxation of the adsorbed species to unsymmetrical
configurations. It is found that for both TiO and SnO an unsymmetrical
dissociated configuration is the most stable. The symmetrical molecularly
adsorbed configuration is unstable with respect to lowering of symmetry, and is
separated from the fully dissociated configuration by at most a very small
energy barrier. The calculated dissociative adsorption energies for TiO and
SnO are in reasonable agreement with the results of thermal desorption
experiments. Calculated total and local electronic densities of states for
dissociatively and molecularly adsorbed configurations are presented and their
relation with experimental UPS spectra is discussed
Theoretical Analysis of STM Experiments at Rutile TiO_2 Surfaces
A first-principles atomic orbital-based electronic structure method is used
to investigate the low index surfaces of rutile Titanium Dioxide. The method is
relatively cheap in computational terms, making it attractive for the study of
oxide surfaces, many of which undergo large reconstructions, and may be
governed by the presence of Oxygen vacancy defects. Calculated surface charge
densities are presented for low-index surfaces of TiO, and the relation of
these results to experimental STM images is discussed. Atomic resolution images
at these surfaces tend to be produced at positive bias, probing states which
largely consist of unoccupied Ti 3 bands, with a small contribution from O
2. These experiments are particularly interesting since the O atoms tend to
sit up to 1 angstrom above the Ti atoms, so providing a play-off between
electronic and geometric structure in image formation.Comment: 9 pages, Revtex, 3 postscript figures, accepted by Surf. Scienc
Oxygen-induced transformations of an FeO(111) film on Pt(111): A combined DFT and STM study
International audienceThe structural stability of an FeO(111) film supported on Pt(111) was studied by density functional theory (DFT) as a function of oxygen pressure. The results showed formation of O-rich phases at elevated O-2 pressures and revealed a site specificity of the oxidation process within the coincidence (Moire) structure between FeO(111) and Pt(111), ultimately resulting in an ordered pattern of O-Fe-O trilayer islands, as observed by scanning tunneling microscopy (STM). In addition, high resolution STM images revealed a (root 3 x root 3)R30 degrees superstructure of the FeO2 islands with respect to pristine FeO(111). This structure is rationalized by DFT in terms of strong relaxations within the Fe sublayer and can be considered as an intermediate state of the FeO(111) transformation into an Fe2O3(0001) film
Origin of electrochemical activity in nano-Li2MnO3; Stabilization via a 'point defect scaffold'
Molecular dynamics (MD) simulations of the charging of Li2MnO3 reveal that the reason nanocrystalline-Li2MnO3 is electrochemically active, in contrast to the parent bulk-Li2MnO3, is because in the nanomaterial the tunnels, in which the Li ions reside, are held apart by Mn ions, which act as a pseudo 'point defect scaffold'. The Li ions are then able to diffuse, via a vacancy driven mechanism, throughout the nanomaterial in all spatial dimensions while the 'Mn defect scaffold' maintains the structural integrity of the layered structure during charging. Our findings reveal that oxides, which comprise cation disorder, can be potential candidates for electrodes in rechargeable Li-ion batteries. Moreover, we propose that the concept of a 'point defect scaffold' might manifest as a more general phenomenon, which can be exploited to engineer, for example, two or three-dimensional strain within a host material and can be fine-tuned to optimize properties, such as ionic conductivity
Built-in and induced polarization across LaAlO/SrTiO heterojunctions
Ionic crystals terminated at oppositely charged polar surfaces are inherently
unstable and expected to undergo surface reconstructions to maintain
electrostatic stability. Essentially, an electric field that arises between
oppositely charged atomic planes gives rise to a built-in potential that
diverges with thickness. In ultra thin film form however the polar crystals are
expected to remain stable without necessitating surface reconstructions, yet
the built-in potential has eluded observation. Here we present evidence of a
built-in potential across polar \lao ~thin films grown on \sto ~substrates, a
system well known for the electron gas that forms at the interface. By
performing electron tunneling measurements between the electron gas and a
metallic gate on \lao ~we measure a built-in electric field across \lao ~of 93
meV/\AA. Additionally, capacitance measurements reveal the presence of an
induced dipole moment near the interface in \sto, illuminating a unique
property of \sto ~substrates. We forsee use of the ionic built-in potential as
an additional tuning parameter in both existing and novel device architectures,
especially as atomic control of oxide interfaces gains widespread momentum.Comment: 6 pages, 4 figures. Submitted to Nature physics on May 1st, 201
Charge redistribution at Pd surfaces: ab initio grounds for tight-binding interatomic potentials
A simplified tight-binding description of the electronic structure is often
necessary for complex studies of surfaces of transition metal compounds. This
requires a self-consistent parametrization of the charge redistribution, which
is not obvious for late transition series elements (such as Pd, Cu, Au), for
which not only d but also s-p electrons have to be taken into account. We show
here, with the help of an ab initio FP-LMTO approach, that for these elements
the electronic charge is unchanged from bulk to the surface, not only per site
but also per orbital. This implies different level shifts for each orbital in
order to achieve this orbital neutrality rule. Our results invalidate any
neutrality rule which would allow charge redistribution between orbitals to
ensure a common rigid shift for all of them. Moreover, in the case of Pd, the
power law which governs the variation of band energy with respect to
coordination number, is found to differ significantly from the usual
tight-binding square root.Comment: 6 pages, 2 figures, Latex; Phys.Rev. B 56 (1997
The Small Unit Cell Reconstructions of SrTiO3 (111)
We analyze the basic structural units of simple reconstructions of the (111)
surface of SrTiO3 using density functional calculations. The prime focus is to
answer three questions: what is the most appropriate functional to use; how
accurate are the energies; what are the dominant low-energy structures and
where do they lie on the surface phase diagram. Using test calculations of
representative small molecules we compare conventional GGA with higher-order
methods such as the TPSS meta-GGA and on-site hybrid methods PBE0 and TPSSh,
the later being the most accurate. There are large effects due to reduction of
the metal d oxygen sp hybridization when using the hybrid methods which are
equivalent to a dynamical GGA+U, which leads to rather substantial improvements
in the atomization energies of simple calibration molecules, even though the
d-electron density for titanium compounds is rather small. By comparing the
errors of the different methods we are able to generate an estimate of the
theoretical error, which is about 0.25eV per 1x1 unit cell, with changes of
0.5-1.0 eV per 1x1 cell with the more accurate method relative to conventional
GGA. An analysis of the plausible structures reveals an unusual low-energy
TiO2-rich configuration with an unexpected distorted trigonal biprismatic
structure. This structure can act as a template for layers of either TiO or
Ti2O3, consistent with experimental results as well as, in principle, Magnelli
phases. The results also suggest that both the fracture surface and the
stoichiometric SrTiO3 (111) surface should spontaneously disproportionate into
SrO and TiO2 rich domains, and show that there are still surprises to be found
for polar oxide surfaces.Comment: 14 pages, 4 Figure
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