559 research outputs found
Modification of anatase TiO(001) surface electronic structure by Au impurity
We have used density functional theory calculations based on the projector
augmented wave method to investigate the electronic structure of
Au-incorporated anatase TiO(001) surface. Due to the coordination with
several level oxygens, Au atoms can be encapsulated inside TiO slab. Au is
adsorbed over the surface Ti--O bond, so called the bridge site on anatase
TiO(001)--11 surface. However, for 0.25 ML coverage, Au atoms
energetically prefer to stay at 0.64 {\AA} above the midpoint of the two
surface oxygens which is significantly closer to the surface layer. When
implanted inside the slab for full coverage, Au forms parallel metallic wires
inside TiO lattice where interlayer distances increase due to local
segregation. Au brings half-filled impurity states into the band gap leading to
metallization, in addition to other filled surface and impurity bands within
the gap. These Au-driven Fermi-level-pinning gap states are close to, or even
in some cases inside, the conduction band of the host slab. On the other hand,
if Au is substituted for the surface Ti atom, Fermi level falls lower in the
gap closer to the valence band top.Comment: 10 pages, 4 figure
Variable and reversible quantum structures on a single carbon nanotube
The band gap of a semiconducting single wall carbon nanotube decreases and
eventually vanishes leading to metalization as a result of increasing radial
deformation. This sets in a band offset between the undeformed and deformed
regions of a single nanotube. Based on the superlattice calculations, we show
that these features can be exploited to realize various quantum well structures
on a single nanotube with variable and reversible electronic properties. These
quantum structures and nanodevices incorporate mechanics and electronics.Comment: 7 pages, 4 figures, To be appear in PR
Range separated hybrid density functional study of organic dye sensitizers on anatase TiO nanowires
The adsorption of organic molecules coumarin and the donor--acceptor
type tetrahydroquinoline (C2-1) on anatase (101) and (001) nanowires have been
investigated using screened Coulomb hybrid density functional theory
calculations. While coumarin forms single bond with the nanowire surface, C2-1
additionally exhibits bidentate mode giving rise to much stronger adsorption
energies. Nonlinear solvation effects on the binding characteristics of the dye
chromophores on the nanowire facets have also been examined. These two dye
sensitizers show different electronic charge distributions for the highest
occupied and the lowest unoccupied molecular states. We studied the electronic
structures in terms of the positions of the band edges and adsorbate related
band gap states and their effect on the absorption spectra of the dye-nanowire
combined systems. These findings were interpreted and discussed from the view
point of better light harvesting and charge separation as well as in relation
to more efficient charge carrier injection into the semiconductor nanowire.Comment: 8 pages, 4 figures, and 1 tabl
Adsorption of Pt and Bimetallic Pt-Au clusters on the Partially Reduced Rutile (110) TiO2 Surface: A First-Principles Study
Cataloged from PDF version of article.An extensive study of the adsorption of small Ptn (n = 1−8) and bimetallic Pt2Aum (m = 1−5) clusters on the partially reduced rutile (110) TiO2 surface has been nperformed via total energy pseudopotential calculations based on density functional theory. Structures, energetics, and electronic properties of adsorbed Ptn and Pt2Aum clusters have been determined. The surface oxygen vacancy site has been found to be the nucleation center for the growth of Pt clusters. These small Pt clusters strongly interact with the partially reduced surface and prefer to form planar structures for n = 1−6 since the cluster−substrate interaction governs the cluster growth at low Pt coverage. We found a planar-to-threedimensional structural transition at n = 7 for the formation of Ptn clusters on the reduced TiO2 surface. GGA+U calculations have also been performed to get a reasonable description of the reduced oxide surface. We observed significant band gap narrowing upon surface−Ptn cluster interaction which leads to the formation of gap localized Pt states. In the case of bimetallic Pt−Au clusters, Aum clusters have been grown on the Pt2−TiO2 surface. The previously adsorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. The presence of the Pt dimer remarkably enhances the binding energy and limits the migration of Au atoms on the titania surface. The charge state of both individual atoms and clusters has been obtained from the Bader charge analysis, and it has been found that charge transfer among the Pt atoms of Ptn clusters and the metal oxide surface is stronger compared to that of Au clusters and the Pt2−TiO2 system
Ab initio study of neutral (TiO2)n clusters and their interactions with water and transition metal atoms
Cataloged from PDF version of article.We have systematically investigated the growth behavior and stability of small stoichiometric (TiO2)n
(n = 1–10) clusters as well as their structural, electronic and magnetic properties by using the
first-principles plane wave pseudopotential method within density functional theory. In order to find out
the ground state geometries, a large number of initial cluster structures for each n has been searched via
total energy calculations. Generally, the ground state structures for the case of n = 1–9 clusters have at
least one monovalent O atom, which only binds to a single Ti atom. However, the most stable structure of
the n = 10 cluster does not have any monovalent O atom. On the other hand, Ti atoms are at least
fourfold coordinated for the ground state structures for n ≥ 4 clusters. Our calculations have revealed
that clusters prefer to form three-dimensional structures. Furthermore, all these stoichiometric clusters
have nonmagnetic ground state. The formation energy and the highest occupied molecular orbital
(HOMO)–lowest unoccupied molecular orbital (LUMO) gap for the most stable structure of (TiO2)n
clusters for each n have also been calculated. The formation energy and hence the stability increases as
the cluster size grows. In addition, the interactions between the ground state structure of the (TiO2)n
cluster and a single water molecule have been studied. The binding energy (Eb) of the H2O molecule
exhibits an oscillatory behavior with the size of the clusters. A single water molecule preferably binds to
the cluster Ti atom through its oxygen atom, resulting an average binding energy of 1.1 eV. We have also
reported the interaction of the selected clusters (n = 3, 4, 10) with multiple water molecules. We have
found that additional water molecules lead to a decrease in the binding energy of these molecules to the
(TiO2)n clusters. Finally, the adsorption of transition metal (TM) atoms (V, Co and Pt) on the n = 10
cluster has been investigated for possible functionalization. All these elements interact strongly with this
cluster, and a permanent magnetic moment is induced upon adsorption of Co and V atoms. We have
observed gap localized TM states leading to significant HOMO–LUMO gap narrowing, which is
essential to achieve visible light response for the efficient use of TiO2 based materials. In this way,
electronic and optical as well as magnetic properties of TiO2 materials can be modulated by using the
appropriate adsorbate atom
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