Modification of anatase TiO2_2(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$_2$(001) surface. Due to the coordination with several level oxygens, Au atoms can be encapsulated inside TiO$_2$ slab. Au is adsorbed over the surface Ti--O bond, so called the bridge site on anatase TiO$_2$(001)--1$\times$1 surface. However, for 0.25 ML coverage, Au atoms energetically prefer to stay at 0.64 {\AA} above the midpoint of the two surface oxygens which is significantly closer to the surface layer. When implanted inside the slab for full coverage, Au forms parallel metallic wires inside TiO$_2$ 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

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

Effect of impurities on the mechanical and electronic properties of Au, Ag, and Cu monatomic chain nanowires

Cataloged from PDF version of article.Ordered arrays of subwavelength hydrogen silsesquioxane (HSQ) nanorods on glass substrates are fabricated using room temperature nanoimprint lithography and anodized aluminum oxide membranes. Moth-eye type nanorod arrays exhibited superior omnidirectional antireflection characteristics in visible wavelengths. The ellipsometric measurements revealed that average specular reflection is remaining below 1% up to 55 degrees incidence angles. Transmission measurements at normal incidence resulted in significant increase in transmitted light intensity with respect to plain glass. Simulations showed that up to 99% transmission could be obtained from double sided tapered HSQ nanorod arrays on HSQ thin film and glass substrates. Achieving large-area, broadband and omnidirectional antireflective surfaces on glass pave the way for applications including photovoltaics. (C) 2011 American Institute of Physics

The integer quantum Hall effect of a square lattice with an array of point defects

Cataloged from PDF version of article.The electronic properties of a square lattice under an applied perpendicular magnetic field in the presence of impurities or vacancies are investigated by the tight-binding method including up to second nearest neighbor interactions. These imperfections result in new gaps and bands in the Hofstadter butterfly even when the second order interactions break the bipartite symmetry. In addition, the whole spectrum of the Hall conduction is obtained by the Kubo formula for the corresponding cases. The results are in accordance with the Thouless-Kohmoto-Nightingale-den Nijs integers when the Fermi energy lies in an energy gap. We find that the states due to the vacancies or impurities with small hopping constants are highly localized and do not contribute to the Hall conduction. However, the impurities with high hopping constants result in new Hall plateaus with constant conduction of sigma(xy) = +/- e(2)/h, since high hopping constants increase the probability of an electron contributing to the conduction

Hall conductance in graphene with point defects

Cataloged from PDF version of article.We investigate the Hall conductance of graphene with point defects within the Kubo formalism, which allows us to calculate the Hall conductance without constraining the Fermi energy to lie in a gap. For pure graphene, which we model using a tight-binding Hamiltonian, we recover both the usual and the anomalous integer quantum Hall effects depending on the proximity to the Dirac points. We investigate the effect of point defects on Hall conduction by considering a dilute but regular array of point defects incorporated into the graphene lattice. We extend our calculations to include next nearest neighbor hopping, which breaks the bipartite symmetry of the lattice. We find that impurity atoms which are weakly coupled to the rest of the lattice result in gradual disappearance of the high conductance value plateaus. For such impurities, especially for vacancies which are decoupled from the lattice, strong modification of the Hall conductance occurs near the E = 0 eV line, as impurity states are highly localized. In contrast, if the impurities are strongly coupled, they create additional Hall conductance plateaus at the extremum values of the spectrum, signifying separate impurity bands. Hall conductance values within the original spectrum are not strongly modified

Hofstadter butterfly of graphene with point defects

Cataloged from PDF version of article.We investigate the structure of Hofstadter's butterfly of graphene with point defects under a perpendicular magnetic field. We use a tight-binding method with interactions up to second-nearest neighbors. First of all, we present the Hofstadter butterfly spectrum of pure graphene, including all four valence orbitals with second-order hopping. To model defects, we perform calculations within an enlarged unit cell of seven carbon atoms and one defect atom. We find that impurity atoms with smaller hopping constants result in highly localized states which are decoupled from the rest of the system. The bands associated with these states form a nearly E = 0 eV line. On the other hand, impurity atoms with higher hopping constants are strongly coupled with the neighboring atoms. These states modify the Hofstadter butterfly around the minimum and maximum values of the energy by forming two self-similar bands decoupled from the original butterfly. We also show that the bands and gaps due to the impurity states are robust with respect to the second-order hopping