1,222 research outputs found
Tailoring electronic and optical properties of TiO2: nanostructuring, doping and molecular-oxide interactions
Titanium dioxide is one of the most widely investigated oxides. This is due
to its broad range of applications, from catalysis to photocatalysis to
photovoltaics. Despite this large interest, many of its bulk properties have
been sparsely investigated using either experimental techniques or ab initio
theory. Further, some of TiO2's most important properties, such as its
electronic band gap, the localized character of excitons, and the localized
nature of states induced by oxygen vacancies, are still under debate. We
present a unified description of the properties of rutile and anatase phases,
obtained from ab initio state of the art methods, ranging from density
functional theory (DFT) to many body perturbation theory (MBPT) derived
techniques. In so doing, we show how advanced computational techniques can be
used to quantitatively describe the structural, electronic, and optical
properties of TiO2 nanostructures, an area of fundamental importance in applied
research. Indeed, we address one of the main challenges to TiO2-photocatalysis,
namely band gap narrowing, by showing how to combine nanostructural changes
with doping. With this aim we compare TiO2's electronic properties for 0D
clusters, 1D nanorods, 2D layers, and 3D bulks using different approximations
within DFT and MBPT calculations. While quantum confinement effects lead to a
widening of the energy gap, it has been shown that substitutional doping with
boron or nitrogen gives rise to (meta-)stable structures and the introduction
of dopant and mid-gap states which effectively reduce the band gap. Finally, we
report how ab initio methods can be applied to understand the important role of
TiO2 as electron-acceptor in dye-sensitized solar cells. This task is made more
difficult by the hybrid organic-oxide structure of the involved systems.Comment: 32 pages, 8 figure
Renormalization of Optical Excitations in Molecules near a Metal Surface
The lowest electronic excitations of benzene and a set of donor-acceptor
molecular complexes are calculated for the gas phase and on the Al(111) surface
using the many-body Bethe-Salpeter equation (BSE). The energy of the
charge-transfer excitations obtained for the gas phase complexes are found to
be around 10% lower than the experimental values. When the molecules are placed
outside the surface, the enhanced screening from the metal reduces the exciton
binding energies by several eVs and the transition energies by up to 1 eV
depending on the size of the transition-generated dipole. As a striking
consequence we find that close to the metal surface the optical gap of benzene
can exceed its quasiparticle gap. A classical image charge model for the
screened Coulomb interaction can account for all these effects which, on the
other hand, are completely missed by standard time-dependent density functional
theory.Comment: 4 pages, 3 figures; revised versio
Analytical model of non-Markovian decoherence in donor-based charge quantum bits
We develop an analytical model for describing the dynamics of a donor-based
charge quantum bit (qubit). As a result, the quantum decoherence of the qubit
is analytically obtained and shown to reveal non-Markovian features: The
decoherence rate varies with time and even attains negative values, generating
a non-exponential decay of the electronic coherence and a later recoherence.
The resulting coherence time is inversely proportional to the temperature, thus
leading to low decoherence below a material dependent characteristic
temperature.Comment: 19 pages, 3 figure
Graphene on metals: a Van der Waals density functional study
We use density functional theory (DFT) with a recently developed van der
Waals density functional (vdW-DF) to study the adsorption of graphene on Al,
Cu, Ag, Au, Pt, Pd, Co and Ni(111) surfaces. In constrast to the local density
approximation (LDA) which predicts relatively strong binding for Ni,Co and Pd,
the vdW-DF predicts weak binding for all metals and metal-graphene distances in
the range 3.40-3.72 \AA. At these distances the graphene bandstructure as
calculated with DFT and the many-body GW method is basically unaffected
by the substrate, in particular there is no opening of a band gap at the
-point.Comment: 4 pages, 3 figure
An efficient parametric algorithm for octree traversal
An octree is a well known hierarchical spatial structure which is widely used in Computer Graphics algorithms.
One of the most frequent operations is the computation of the octree voxels intersected by a
straight line. This has a number of applications, such as ray-object intersection tests speed-up and visualisation
of hierarchical density models by ray-casting. Several methods have been proposed to achieve this
goal, which differ in the order in which intersected voxels are visited. In this paper we introduce a new
top-down parametric method. The main difference with previously proposed methods is related to descent
movements, that is, the selection of a child sub-voxel from the current one. This selection, as the algorithm,
is based on the parameter of the ray and comprises simple comparisons. The resulting algorithm is easy to
implement, and efficient when compared to other related top-down and bottom-up algorithms for octrees.
Finally, a comparison with Kelvin’s method for binary trees is presented
Entanglement of formation for a class of -dimensional systems
Currently the entanglement of formation can be calculated analytically for
mixed states in a -dimensional Hilbert space. For states in higher
dimensional Hilbert space a closed formula for quantifying entanglement does
not exist. In this regard only entanglement bounds has been found for
estimating it. In this work, we find an analytical expression for evaluating
the entanglement of formation for bipartite ()-dimensional mixed
states.Comment: 5 pages, 4 figures. Submitted for publicatio
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