bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synthesis and Optical Properties of Cadmium- and Lead-Chalcogenide Semiconductor Clusters (Diameter < 5 Nm) In Liquid Solutions and Silicate Glasses
Authors: Micic O. I. , Rajh T. , Comor M. I. , Zec S. , Nedeljkovic J. , Patel R. C. 
Published: March 2011

ABSTRACTSmall-particle clusters of CdS, CdSe, CdTe, PbS, and PbSe have been synthesized in aqueous solution. Absorption spectra with very sharp maxima were obtained for extremely small clusters (diameters < 30 Å). However, when clusters grow the excitonic absorption bands gradually decrease and the sharp maxima are replaced by broad shoulders. Cadmium chalcogenide clusters capped with 3-mercapto-l, 2-propanediol can be collected as stable solids with the same properties as original clusters. A method for the incorporation of semiconductor clusters in transparent silicate glasses has been developed. This technique relies on the low temperature sol-gel process, and silane alkoxide as the starting material. The absorption spectra of the clusters in the glasses are identical to those of the starting aqueous solutions. Excess charge carriers on PbS and CdS clusters lead to a blue shift in the optical absorption edge of semiconductors. The appearance of this shift depends critically on the method of colloid preparation and surface chemistry.
Structure and Dynamics of Aluminum Clusters
Authors: Bernholc J. , Yi Jae-Yel , Oh Dirk J. , Sullivan D. J. 
Published: March 2011

ABSTRACTThe energetics and structures of metallic clusters containing up to 55 atoms are studied by the Car-Parrinello (quantum-molecular dynamics) method with Al chosen as a paradigm. It is found that the structural energy differences between small cluster (icosahedral) and bulk (fee) structures are surprisingly small, indicating that the icosahedron-fee structural transition can occur very early in Al clusters. Simulated annealing studies for 13- and 55-atom clusters, where both perfectly symmetric icosahedral and cuboctahedral (fee) structures exist, show that the distortions from the ideal structures are substantial. For the 55-atom cluster several inequivalent but energetically nearly degenerate structures are found, which is likely to lead to floppiness at finite temperatures as well as a low melting point. Although the structure factors for the annealed structures are nearly identical, they are very different from those of either of the ideal structures. The computed IPs and EAs differ by less than 0.1 eV for weakly annealed cuboctahedral and icosahedral clusters, making the structural identification on the basis of IP and EA measurements very difficult. The quantum-mechanical results are also used to develop a classical potential for long-time molecular dynamics simulations of large clusters.
Interplay Between Local Environment Effect and Electronic Structure Properties in Close Packed Structures
Authors: Turchi P. E. A. 
Published: March 2011

ABSTRACTWithin a tight binding framework, the interplay between local atomic arrangement, orbital directionality and phase stability properties is discussed for a series of tetrahedrally close packed (tcp) structures. The common features which characterize the local densities of states of tcp phases can be explained by a moment analysis up to fourth order terms. On the contrary, it is shown that structural energy differences between simple and tcp crystalline structures requires the knowledge of at least the fifth order moments, a fact which has been underestimated in the past. Finally, the relative stability of various representative geometries, including the ones of simple crystalline structures is examined as a function of the number of valence electrons.
Observation of Melting in 30 ÅDiameter CdS Nanocrystals
Authors: Goldstein A. N. , Colvin V. L. , Alivisatos A. P. 
Published: March 2011

ABSTRACTTemperature dependent electron diffraction studies on 30 Å diameter CdS nanocrystals are described. The linear thermal expansion coefficient of the nanocrystals is 2.75 * 10−5 Å/K,and the melting point is 575 K. These data are in contrast to bulk CdS which has a melting point of 1750 K and a linear expansion coefficient of 5.5 * 10−6Å/K. The observed depression in the melting point of these semiconductor clusters is similar to effects observed in metals and molecular crystals, indicating that the phenomenon of reduced melting point in small systems is a general one regardless of the type of material. The observation of melting point depression in these clusters also has far reaching implications for the preparation of highly crystalline clusters of CdS, as well as for the use of these nanocrystals as precursors to thin films.
Development of an Inert Gas Condensation Based Ion Source for the Generation of Strong Beams of Large Cluster Ions
Authors: Sarkas H. W. , Kidder L. H. , Eaton J. G. , McHugh K. M. , Bowen K. H. 
Published: March 2011

ABSTRACTWe report on the further development of an ion source for producing intense, continuous beams of large positive and negative cluster ions comprised of high temperature materials. This device, the Smoke-Ion Source, is the result of combining inert gas condensation methods with techniques for injecting electrons directly into expanding jets. We demonstrate the capability of this ion source to generate strong beams of cluster ions comprised of materials including metals, semiconductors, and metal oxide ceramics.
Simulation of Gas Phase Clustering of Nanocrystals in Sputter Discharges
Authors: Choi Seung J. , Kushner Mark J. 
Published: March 2011

ABSTRACTThe preparation of nanocrystalline particles or clusters (sizes 1–50 nm) is of interest to the study of small systems and for use in sintering or compacting of high purity bulk materials. Recently, a method whereby these crystals can be fabricated using a sputter discharge has been reported. We have developed a computer model to simulate the formation of homogeneous (e.g., Si, Cu, Ti) gas phase clusters in these devices as precursors to larger nanocrystals. The model combines Monte Carlo and drift-diffusion algorithms to simulate the sputtering of atoms from the target, their thermalization in the buffer gas, and gas phase nucleation reactions. Densities of clusters having one to many hundreds of atoms are obtained as a function of position in the discharge. We find that the experimentally observed particle sizes cannot be explained by clustering involving solely neutral reactants due to their short residence times in the plasma. Negatively charged clusters which are trapped in the plasma have correspondingly longer residence times and most likely are responsible for the growth of large particles. Scaling laws for the growth of homogeneous clusters will be presented based on the results of the model.
A New Type of Cluster-Ion Source for Thin Film Deposition
Authors: Haberland Hellmut , Karrais Martin , Mall Martin 
Published: March 2011

ABSTRACTAtoms are gas discharge sputtered from a solid target. They are condensed to form clusters using the gas aggregation technique. An intense beam of clusters of all solid materials can be obtained. Up to 80 % of the clusters can be ionised without using additional electron impact ionisation. Total deposition rates vary between 1 and 1000 Å per second depending on cluster diameter, which can be varied between 3 and 500 nm. Thin films of Al, Cu, and Mo have been produced so far. For non accelerated beams a weakly adhering mostly coulored deposit is obtained. Accelerating the cluster ions this changes to a strongly adhering film, having a shiny metallic appearance, and a very sharp and plane surface as seen in an electron microscope. The advantages compared to Kyoto ICB-method are: easy control of the cluster size, no electron impact ionisation, high degree of ionisation, and sputtering is used instead of thermal evaporation, which allows the use of high melting point materials.
Controlled Formation of Quantum Size Metal and Semiconductor Particles from Aqueous Solutions
Authors: Patel Ramesh C. , Nedeljkovic Jovan , Micic Olga 
Published: March 2011

ABSTRACTPhotochemical and chemical reduction methods are described for the controlled formation of metallic silver in aqueous solutions. The former approach is capable of generally depositing a number of metals. When spherical silver bromide particles are the substrates, the resulting silver coated composite particles exhibit optical absorption spectra which vary with the coat thickness as theoretically predicted. In the case of spherical silica particles of uniform size, it was possible to produce both quantum size silver particles supported on silica, as well as a silver coat of variable thickness, depending on the rate of the deposition process. In addition to silica, substrates such as latex and chromium hydroxide could be used.CdS particles with two different particle diameters (50–200 Å and < 30 Å) were subjected to 308 nm excimer laser irradiation at 77 K, and the subsequent charge carrier processes studied by ESR. Dramatic differences in the ESR signals as a function of decreasing particle size could be observed, consistent with the localization of charge carriers on numerous surface sites.
The Physical Properties of Ge Clusters and Ultrathin Films
Authors: Yu R. Q. , Fortner J. , Chase S. J. , Lannin J. S. 
Published: March 2011

ABSTRACTRaman scattering and ultraviolet photoelectron spectroscopy on Ge clusters and ultrathin films before and after H chemisorption reveal the significant changes in the vibrational and electronic densities of states with size. These effects are attributed to the large fraction of surface dangling bonds and the important role of short range order changes on physical properties.
Solid C60 – How we Found It
Authors: Huffman Donald R. , Krätschmer Wolfgang 
Published: March 2011

ABSTRACTIn May of 1990, crystals of a new form of carbon were observed crystallizing from a solution of the newly-produced macroscopic quantities of fullerene molecules. This discovery followed many years of work we had done in producing and measuring the optical properties of small particles of graphitic carbon, a work which was aimed, in part, at a better understanding of the various spectral features of interstellar material. Among three distinctly different types of carbon particulates, distinguished by their differing ultraviolet and Raman spectra, was one that later proved to contain macroscopic quantities of C60 and C70. Evidence for fullerenes came from mass spectra, infrared spectra, and X-ray and electron diffraction. More, recent experiments with our collaborators have included scanning tunneling microscopy and gas phase emission spectroscopy in the infrared. At the moment, there is no obvious connection with interstellar spectral features.
Photoelectron Spectroscopy of Si2
Authors: Kitsopoulos Theofanis N. , Neumark Daniel M. 
Published: March 2011

ABSTRACTIn this report we discuss the photoelectron spectrum of Si2− obtained at 355 nm. The spectrum shows a series of transitions to the low-lying triplet states of Si2 as well as transitions to the higher-lying singlet states. Several possible assignments of the anion and neutral triplet states are presented.
Modelling of The Intramolecular Structure Of Monolayer C60 Molecules Observed with Scanning Tunneling Microscopy
Authors: Chen T. , Howells S. , Gallagher M. , Yi L. , Sarid D. , Lichtenberger D. L. , Nebesny K. W. , Ray C. D. 
Published: March 2011

ABSTRACTImages of monolayer coverages of C60 on gold substrates, using scanning tunneling microscopy, reveal intermolecular arrangement and intramolecular features. In this paper we concentrate on the interpretation of these internal features, present a model showing that they arise from local charge density variations of high-lying π orbitals on approximately every other carbon atom, and discuss also other possible mechanisms for this effect.
Vibrational Properties of Si Microclusters and Comparison to the Amorphous Material
Authors: Feldman Joseph L. , Kaxiras Efthimios 
Published: March 2011

ABSTRACTThe lattice vibrational properties of two clusters of Si atoms are studied using the Stillinger-Weber phenomenological interatomic potential to calculate the dynamical matrix. The clusters contain 33 and 45 atoms, and their structure was postulated from considerations of surface reconstructions in analogy to crystalline Si surfaces. A remarkable similarity to the vibrational density of states of amorphous Si is demonstrated especially for the 45 atom cluster.
Optical (Hyper)Polarizabilities of Small Silicon Clusters
Authors: Rantala Tapio T. , Stockman Mark I. , Jelski Daniel A. , George Thomas F. 
Published: March 2011

ABSTRACTElectronic contributions to the optical (hyper)polarizabilities of small silicon clusters are theoretically determined. Geometries and the electronic structures of the clusters are established using the tight-binding model. The nonlinear polarizabilities are found to depend primarily on the symmetry of the cluster and prove to be high for the low-symmetry clusters. Possible experiments and applications are discussed.
Proposal of Metastable Spin-Polarized He as a Probe of Antiferromagnetic Transition Metal Oxide Surfaces
Authors: Swan Anna , Franzen W. , El-Batanouny M. , Martini K. M. 
Published: March 2011

ABSTRACTWe suggest a new application for elastic scattering of a metastable spin-polarized atomic helium beam at thermal energy. We demonstrate how angle-resolved measurements of the Bragg peaks of scattered surviving metastable atoms can give information about the spin-ordering of an antiferromagnetic (AF) transition metal oxide surface. In this paper, we discuss the feasibility of such measurements for NiO(100) and MnO(100), based on available information about their electronic structure and the properties of spin-polarized metastable helium. On impact with a surface, the survival probability of metastables is generally very low (<10-2). There are two possible decay mechanisms for metastables, a resonance ionization followed by auger neutralization, or an auger de-excitation process. For AF surfaces which fulfill certain requirements on their electronic structure, spin-selection rules will inhibit the decay of the metastable atoms from a favourably aligned magnetic sublattice. The survival probability will then be dramatically enhanced from the chosen sublattice, and the coherently scattered surviving metastables will reflect the periodicity of that magnetic sublattice. In contrast to other methods currently applied to magnetic systems, this method does not rely on difference spectra. Consequently, reversal of spinorientation is not necessary for the observation of magnetic ordering.
Evidence of Quantum Motion of Hydrogen on Pd(111) in He-Diffraction data
Authors: Hsu C-H. , Larson B. E. , El-Batanouny M. , Willis C. R. , Martini K. M. 
Published: March 2011

ABSTRACTWe present elastic He-beam scattering data of the Pd(111)/H system. Diffraction intensities were measured as a function of surface temperature in the range 140°K–320°K. Two remarkable features are observed : the first is the presence of C3v symmetry at (1 × 1) saturation coverage (140°K) and its transformation to C6v symmetry at lower coverages (270°K). The second feature is the anomalous attenuation of the specular He beam accompanying this transformation. Taken together these features provide strong evidence of a fundamental change in the surface charge density corrugation. A classical interpretation of the motion of hydrogen either fails to reproduce the measured attenuation or leads to contradictory and unphysical conclusions regarding the H-metal bond length or surface equilibrium. An alternative quantum mechanical interpretation is developed and is shown to provide consistent and satisfactory explanation of the measurements.
Use of Atom Beama Scattering to Study Structures of and Phase Transitions in Ultra-Thin Films
Authors: Vidali G. , LI W. , Lin J.-S. , Moses C. 
Published: March 2011

ABSTRACTWe have used neutral helium beam scattering to probe structural and thermodynamic properties of in-situ deposited ultra-thin films of metals on well characterized substrates. We show the capabilities of this technique by presenting some recent results of our studies of Hg adsorption of Cu(001) and on monolayer phases of Pb on Cu(001).
The Electronic Structure of Grain Boundaries in NB
Authors: Sowa Erik C. , Gonis A. , Zhang X. -G. 
Published: March 2011

ABSTRACTWe present first-principles calculations of the electronic structure of Nb grain boundaries. These are the first such calculations for a bcc metal using the real-space multiple-scattering theory (RSMST). Local densities of states near a Σ5 twist grain boundary are compared to those for bulk Nb.
Defect Generation and Motion in Polyethylene-Like Crystals, Analyzed by Simulation With Supercomputers
Authors: Wunderlich Bernhard , Xenopoulos A. , Noid D. W. , Sumpter B. G. 
Published: March 2011

ABSTRACTDefects in polymers were observed by high resolution electron microscopy and inferred from their mechanical and dielectrical behavior. The details of their generation was not known, however, in the past. During the last few years we have been able to extend the molecular dynamics simulation of polyethylene to crystals containing up to 6100 atoms and to times as long as 100 ps. The major observation was that single bond rotations of more than 90° become possible already more than 100 K below the melting temperature. These defects have lifetimes of only a few ps. By coupling to kinks (2g1) they can extend their lifetime considerably. Addition of a thermal, mechanical or dielectric free energy gradient to the thermally created defects seems to be able to account for the microscopic motion needed to explain the macroscopically observed annealing, deformations and relaxation effects. Key to the mechanical and dielectric properties is thus the existence of conformational disorder (condis crystal).
The Surface Green's Function in Semiconductors by the Tight-Binding Linear Muffin-Tin Orbital Method
Authors: Šob M. , Kudrnovský J. 
Published: March 2011

ABSTRACTA computationally efficient method for the determination of the Green's function (GF) of an ideal semi-infinite semiconductor crystal within the first-principles TB-LMTO approach is developed. Our sample is described by a semi-infinite stack of principal layers with only nearestneighbor interaction between them. The projection of the GF of the ideal semi-infinite crystal onto the top principal layer is the surface GF (SGF), which is evaluated from the condition of removal invariance: by adding (removing) a principal layer of bulk atoms to (from) the semi-infinite crystal we recover the same semi-infinite crystal. This approach avoids the knowledge of the bulk GF and the surface-normal k-integration common in other treatments. Our method is illustrated on the evaluation of k11-resolved densities of states, both on the surface and deep in the sample, for the (110) and (001) faces of typical elemental and AIIIBV semiconductors.
Simulation of the Crystal-To-Amorphous Transformation in Irradiated Quartz
Authors: Jain Uma , Powell Adam C. , Hobbs Linn W. 
Published: March 2011

ABSTRACTQuartz and other crystalline polymorphs of silica transform from the crystalline to an aperiodic state under irradiation. There is a need to understand the structural changes involved during this amorphization process. We have built an engineering modelwhich simulates the growth of amorphous regions within a crystalline matrix during the crystal-to-amorphous transition in irradiated quartz. The resulting crystal structure is displayed on the computer screen or plotted on a printer with the orthogonal coordinates of all the atoms in the cluster and the interatomic distances stored in a file. We find the bond lengths increase by about 3%, which is a reasonable value to expect since quartz expands 14% by volume during the amorphization. The results also show the crystal structure surrounding the strained region to be somewhat disturbed, consistent with what is observed experimentally.
Defects in Materials: Their Characterization and Simulation.
Authors: Windsor Colin G. 
Published: March 2011

ABSTRACTMaterials research does not necessarily need to eliminate defects, but rather to characterize them, and to understand and control their effects. In most cases chacterization of defects means making structural or dynamic measurements of their properties. To understand these measurements in order to predict material and defect properties outside the range of the measurements is a much harder problem. Ideally a theory is required. However in the materials examples considered in this review, point defects in uranium oxide, copper clusters in steel, grain boundary aggregations, and stress concentrations, a true analytic theory is beyond our capabilities. Here computer modelling is often able to make the progress needed. This review considers the complementary nature of experimental characterization and computer simulation in our understanding of defects in materials.
Effect of Grain Boundary Structure on Grain Boundary Diffusivities in the Au/Ag System
Authors: Ma Qing , Balluffi R. W. 
Published: March 2011

ABSTRACTGrain boundary chemical diffusivities for a series of symmetric [001] tilt boundaries in the Au/Ag system were measured by the surface accumulation method using newly developed thin-film multi-crystal specimens, in which the grain boundaries feeding the accumulation surface were all of the same type. Possible effects due to segregation at the grain boundaries and surfaces were avoided. CSL boundaries of low-Σ ( i.e., 5, 13, 17, 25) and also more general boundaries with tilt angles between the low-Σ orientations were selected. The diffusivities were found to vary monotonically with tilt angle ( i.e., no cusps at low-Σ's were found) in a manner consistent with the Structural Unit model.
Photoconductivity of Highly Disordered Carbon Fibers
Authors: Kuriyama Kazuyoshii , Dresselhauss Mildred.S. 
Published: March 2011

ABSTRACTA highly disordered carbon material, activated carbon fibers, is investigated through bulk conductivity and photoconductivity measurements. This material has a high density of defect states introduced by an aqtivation process that leads to a huge specific surface area of up to 2000m2/g. The conductivity increases by a factor of 4–10 with increasing temperaturefrom 30K to 290K. In contrast, the photoconductivity decreases by a factor of three with increasing temperature. The relaxation time of the photoconductivity is rather long(on the order 10−1 sec), indicating that the recombination process proceeds through localized states. flopping processes are used to interpret the transport properties of this material.