Density Of States For Cluster Research Articles

Titanium-oxo cluster with 9-anthracenecarboxylate antennae: a fluorescent and photocurrent transfer material.

Attention has been paid to titanium-oxo clusters (TOCs) modified with functional molecules, because they can be considered as model systems for dye-sensitized titanium oxides in terms of their information in structures and electron transfer. We select 9-anthracenecarboxylate (9-AC) as a photoactive ligand and prepare two model compounds, [Ti6O6(O(i)Pr)6(9-AC)6] (1) and [Ti6O4(O(i)Pr)6(cat)4(9-AC)2] (2) (where cat = catecholate). Structures of the TOCs and the dye-TOC linkage are characterized by single-crystal analysis. Solvent-induced fluorescence change is observed for the cluster solution, and the fluorescence can be turned off by irradiating and on by oxygen bubbling. Photoinduced Ti(III) is responsible for the fluorescence extinction. The photocurrent conversion property of the clusters is examined by use of a three-electrode cell with cluster-coated indium tin oxide (ITO) electrodes. The results indicate that 9-AC is an effective photosensitizer and cluster 1 shows higher photocurrent intensity for its multiantenna structure in comparison with that of 2. Density of states for cluster 1 is calculated, in which the discrete energy bands of Ti6O24 include a number of new energy levels for the contribution of 9-AC molecules.

Morphology and electronic properties of thermally stable TiOxnanoclusters on Au(111)

TiO${}_{x}$ clusters on atomically flat Au(111) films have been investigated by means of scanning tunneling microscopy and scanning tunneling spectroscopy. We present measurements of both the morphology and the electronic properties of these clusters. First, pure Ti clusters are produced in the gas phase and are deposited onto a clean Au(111) surface under controlled ultrahigh vacuum conditions. Next, the clusters are oxidized. Then, scanning tunneling microscopy measurements are performed before and after annealing up to high temperatures (970 K), indicating a high thermal stability of the deposited clusters. Scanning tunneling spectroscopy reveals an $n$-type semiconductorlike gap in the TiO${}_{x}$ cluster density of states. Local influence of the clusters on the electronic surface state of the Au(111) substrate is observed, which is relevant for catalytic cycles where certain steps of the cycles take place at the metal-oxide interface.

Electronic structure of small sodium clusters

We have calculated the electronic structure of three small clusters of sodium consisting of 9, 15 and 27 atoms. These clusters have the cubic symmetry of the first three shells of atoms in body-centered cubic metallic sodium and were not allowed to relax. The total energy per atom was found to decrease as one goes from Na9 to Na27, with the difference between Na15 and Na27 being smaller than the difference between Na9, and Na15. The charge density and density of states (DOS) of these clusters was also examined. The charge density of the interior atoms was found to become more spherical as more shells are added to the cluster. The cluster densities of states do not resemble closely the free electron like DOS of bulk metallic sodium.

Direct observation of the non-supported metal nanoparticle electron density of states by X-ray photoelectron spectroscopy

Synchrotron-based X-ray photoelectron spectroscopy on copper and silver cluster beams created by a magnetron-based gas-aggregation source has allowed mapping the electron density of states (DOS) of free metallic nanoparticles. The cluster DOS profiles obtained in the experiments strongly resemble the infinite solid DOS shapes, but the extracted cluster work-functions are lower than those for the bulk metal. The latter observation is explained by the initial negative charge on most of the clusters, created by the source.

Palladium clusters on graphite: Evidence of resonant hybrid states in the valence and conduction bands.

The electronic structure of Pd clusters deposited on polycrystalline graphite has been investigated by x-ray photoemission, Auger spectroscopy, and bremsstrahlung isochromat spectroscopy. Initial- and final-state shifts of energy levels as a function of the average cluster size R are small, but important modifications of the density of filled and empty states at the Fermi level are observed. With decreasing particle size, the d portion of the conduction band broadens and shifts to higher energy while its valence counterpart shows the opposite shift. A simple analytical model is developed which accounts in some detail for the measured shifts and linewidths and also explains earlier x-ray-absorption data. The size dependences are explained in terms of mixing of Pd 4d and graphite * continua with a squared coupling constant V2 proportional to R-1. As a consequence a gap opens up across the Fermi level in the cluster density of states and unoccupied hybrid resonances arise as observed experimentally. Initial-state energy shifts and cluster charging are estimated and found to play a minor role for the present system.

Transition metal impurities in copper

The electronic structure of magnetic transition metal impurities (Fe, Co, Ni) in Cu is calculated by means of a 13 and 19 atom cluster approach. The local cluster density of states (CDOS) for the Cu clusters agrees very well with the respective bulk and surface DOS of Cu metal. The local Fe moment of 3μ B is in good agreement to experiment and Green's function calculations. The spin density shows a negative polarization of the conduction electrons beside the strong localized impurity moment.

Effect of alloy fluctuations on the surface states of ternary semiconductor alloys: One-dimensional models

Abstract States which are localized to a surface or to a defect in a semiconductor alloy should be more sensitive to local fluctuations in site occupation than are the bulk states of the semiconductor valence and conduction bands. To illustrate the effects of alloy fluctuations on surface states and to assess the sensitivity of these effects to the localization of the states, calculations are presented for the local electronic surface densities of states of semi-infinite one-dimensional chains. The chains are used to model ternary semiconductor alloys with randomly occupied cation sites. Results are presented for the surface and adsorbate states of ideal chains with unrelaxed ends and for the surface states of chains with relaxed ends. The embedded cluster approach is employed to incorporate the alloy effects on states localized near a chain end. These results are compared with results obtained using the coherent potential approximation (CPA). For surface states which are well localized, the CPA reproduces the structure in the embedded cluster density of states resulting from fluctuations in the occupation of the two cation sites nearest the chain end and from the broadening due to fluctuations at other cation sites.

Cluster densities of states of nonrandom substitutionally disordered alloys

The embedded-cluster method is used to calculate the density of states (DOS) of nonrandom substitutionally disordered alloys. This method is based on the calculation of the Green's function for a cluster of atoms embedded in an effective medium. The effect of increasing cluster size as well as of different choices of the effective medium is investigated numerically in terms of one-dimensional alloys with various scattering strengths and degrees of short-range-order (SRO). A method for the self-consistent treatment of SRO in terms of the pair distribution function is proposed and in many cases is found to lead to results in much better agreement with exact DOS's than those obtained when SRO is not treated self-consistently.

Local-environment fluctuations and densities of states in substitutionally disordered alloys

We report calculations of densities of states (DOS) associated with specific atomic configurations of compact clusters in substitutionally disordered alloys describable by a Hamiltonian with nonoverlapping muffin-tin potentials. The method of calculation consists in evaluating the Green function associated with a cluster of atoms embedded in a translationally invariant effective medium such as the one determined in the Korringa-Kohn-Rostoker, coherent-potential-approximation (KKR-CPA) method. This method yields well-defined, analytic, and physically meaningful results and takes proper account of the crystal structure of the lattice. Numerical results are reported for near-neighbor clusters in both one-dimensional model muffin-tin alloys, as well as in realistic ${\mathrm{Ag}}_{c}{\mathrm{Pd}}_{1\ensuremath{-}c}$ alloys. The DOS associated with many cluster configurations exhibit more structure than the DOS calculated in the single-site KKR-CPA method, which can be understood in terms of fundamental physical quantities, such as the relative scattering strength of the alloy constituents, and of the symmetry of the lattice. Several possible applications of cluster DOS in calculating the physical properties of substitutionally disordered alloys are discussed.

The influence of hydrogen saturation on the local densities of states in small Si, Ge and GaAs clusters

Abstract The influence of the “cluster surface” on cluster densities of states (DOS) of small Si, Ge and GaAs clusters is analysed by means of SCF-Xα-SW calculations. The saturation of the “dangling bonds” is simulated by adding hydrogen atoms at the nearest neighbour sites of the outer atoms. This results in the disappearance of a cluster surface state in the local densities of states (LDOS) on the central atom sites. The relation of experimental solid state data with these local densities of states, together with cluster Auger line shapes and cross-relaxation energies, are discussed.

The influence of hydrogen saturation on the local densities of states in small Si, Ge AND GaAs clusters

The influence of the “cluster surface” on cluster densities of states (DOS) of small Si, Ge and GaAs clusters is analysed by means of SCF-Xα-SW calculations. The saturation of the “dangling bonds” is simulated by adding hydrogen atoms at the nearest neighbour sites of the outer atoms. This results in the disappearance of a cluster surface state in the local densities of states (LDOS) on the central atom sites. The relation of experimental solid state data with these local densities of states, together with cluster Auger line shapes and cross-relaxation energies, are discussed.

Calculations of bandstructure of intermetallic compounds using the multiple scattering Xα cluster method and k dependent boundary conditions

The spherical Wigner-Seitz boundary condition (1933, 1934) for approximately treating electronic states in solids by atomic calculations can be readily extended to include molecular clusters. If, in addition, matching conditions for cluster wavefunctions at the outer sphere radius R are taken to be wavevector dependent, some approximate representation of the energy bands En(k) is obtained. The self-consistent multiple scattering (SCMS) X alpha method with modified boundary conditions has been applied to calculations of the electronic structure of Cu metal and Al2Au and Al2Cu alloys. The results for Cu agree well with SC X alpha bandstructures, as to bandwidths and positions, giving quite reasonable density of states (DOS) curves. The cluster DOS are compared with X-ray emission and photoelectron (ESCA) spectra of the alloys. The results show that the model can describe the main features of electronic structure of intermetallic compounds, with very little computational effort.