bluuebunny/crossref_metadata_2025 huggingface dataset:
A Synchrotron Radiation Study of Nonlinear Diffusion in Cu-Au
Authors: Menon E. S. K. , Huang P. , Kraitchman M. , Hoyt J. J. , Chow P. , Fontaine D. De
Published: March 2011

Alternate layers of pure copper and gold were vapor deposited on a sodium chloride substrate, the average concentration of the films being Cu-16at% Au and the layering periodicity (modulation wavelength) being 3.31 nm. The composition modulation gives rise to satellite diffraction peaks around the (200) Bragg reflections. Synchrotron radiation at SSRL was able to detect up to third order satellite intensity the evolution of which was measured as a function of annealing time at 515 K. Although the first order satellite intensity decayed as expected exponentially with time, intensities of both second and third order satellites decreased very rapidly at first, then increased before decaying exponentially. These results are in conformity with theoretical models of satellite evolution during annealing in a onedimensional modulated system governed by a nonlinear diffusion equation.
Kinetics of Surface Ordering on Au-Rich Cu3Au(001)
Authors: Rivers S. B. , Unertl W. N. , Hung H. H. , Liang K. S.
Published: March 2011

we report grazing-incidence x-ray scattering measurements of the order-disorder phase transition near a Au-rich Cu3Au (001) surface. The bulk transition temperature TB = 648 K is lower than for stoichiometric Cu3Au(001). Surface ordering persists above TB in a layer several monolayers thick up to Ts = 667 K. Kinetics of the ordering were studied by quenching from Ts + 9 K to final temperatures of TF = 661 K and 645 K. Short-range order is established in the bulk within the quench time and does not appear to change subsequently. The bulk short-range order differs from that reported for stoichiometric Cu3Au. For TB < TF = 661 K < TS, the (100) in-plane diffraction beam intensity grows approximately as t1/4 for t > 1000 s and the width continues to narrow. For TF = 645 K < TB, power law growth is not observed for the longest times studied. In both cases the growth is substantially slower than predicted.
Evidence of a Correlation Between the Transition Temperature for Ordering and the Activation Energy for Ordering Kinetics in Ni75Al25-xFex Superalloys
Authors: Kozubski R. , Soltys J. , Cadeville M.C. , Pierron-Bohhes V.
Published: March 2011

Kinetics of long-range ordering was studied in the alloys Ni75Al25-xFex by means of in situ resistometry. The alloys under consideration exhibit a dramatic decrease of the "order-disorder" transition temperature Ttr with increasing X. After the existing theoretical models, the activation energy of long-range ordering should depend on one hand on the energy of ordering - related to Ttr -, and on the other hand, on the current value of the LRO degree. The performed measurements yield an experimental evidence of the above relationships. In addition, a considerable effect of magnetic order on LRO kinetics has also been registered. The results are qualitatively discussed in terms of the model of LRO kinetics in L12 structure proposed by Schoijet and Girifalco.
Antiphase Domain Structures and Analytical Electron Microscopy of α-Bainite in a Cu-Zn-Al Alloy.
Authors: Hamada Y. , Wayman C.M. , Wu M.H.
Published: March 2011

Antiphase domain (APD) structures and associated composition analyses of the α l-bainite which forms during isothermal heat-treatment of a Cu-26.7wt%Zn-4.0wt%Al alloy have been examined by TEM dark field imaging and energy dispersive X-ray microanalysis. At an early stage both nearest-neighbor (NN) and next-nearest-neigbor (NNN) APD structures of the β-parent phase are inherited by the α1-bainite having an M18R structure. The EDX analyses indicate that there is a difference in composition between the α 1-bainite and the β-parent phase at this stage in spite of the inheritence of both NN and NNN APD structures. Local depletion of Zn concentration at dislocations is also observed in the parent phase. With increasing aging time, the α 1-bainite grows in a manner that the NN APD structure still remains inherited but the NNN APB structure changes into a NNN disordered structure at outer regions of the plate. In addition, Zn and Al rich regions also form adjacent to the interface in the parent phase. The observed growth of APD structures in association with compositional analyses strongly suggest that the initial bainitic transformation is by a shear mechanism at solute depleted defects although subsequent plate growth is diffusion dominated.
A Calorimetric Study of the Kinetics of Al3Ni Nucleation and Growth During Reactions in Al/Ni Thin Films
Authors: Ma E. , Thompson C.V. , Clevenger L.A.
Published: March 2011

This study provides information regarding how an intermetallic phase nucleates and grows at an interface between elemental components during solid-state reactions. The reaction of Al/Ni multilayer films was chosen as a model case. The main experimental technique employed was differential scanning calorimetry (DSC), with assistance of other analytical tools such as crosssectional transmission electron microscopy/microanalysis (XTEM/STEM) and thin-film x-ray diffraction. Al/Ni multilayer films were prepared by electron-beam evaporation in high and ultrahigh vacuum systems. We show evidence that interdiffusion of Al and Ni precedes the formation of Al3Ni, forming metastable solid solutions. Using isothermal calorimetry and modeling, we find that Al3Ni subsequently nucleates in the interdiffused region at preferred sites, and that the nucleation sites are quickly consumed in the early stages of Al3Ni formation. The nucleation site density strongly depends on the grain sizes of the deposited films, suggesting that only certain types of intergranular defects can serve as nucleation sites. After coalescence into a continuous layer, Al3Ni thickens through a diffusion-limited process. A kinetic model is applied which yields calculated calorimetric traces in good agreement with experimental data. The results are discussed in light of the the calculated free energy-composition diagram for the Al-Ni system. The implications of our results to the phase selection during thin-film reactions are discussed.
Phase Formation, Kinetics and Microhardness of the Nitride Layer in Ion Nitrided 316L Stainless Steel
Authors: Blucher J.T. , Marchev K. , Gokhale S. , Giessen B.C.
Published: March 2011

316 L stainless steel (nominally 18% Cr, 8% Ni, 2% Mo, 1% Mn, balance Fe) has been ion nitrided in a N2/H2 atmosphere. Studies of nitride layer depth, structure, composition and microhardness show uniform layer formation with a sharp layer-to-matrix interface and a hardness increase over the bulk value by 600%. The structures of the nitride layers vary with treatment temperature, falling into two regimes over the temperature range studied. Generally, parabolic layer growth was found, and the growth coefficients were determined at five temperatures.
Pressure-Enhanced Solid Phase Epitaxy: Implications for Point Defect Mechanisms
Authors: Lu Guo-Quan , Nygren Eric , Aziz Michael J.
Published: March 2011

We have measured the effects of hydrostatic pressure on the solid phase epitaxial growth (SPEG) rates of undoped Ge(100) and Si(100) into their respective self-implanted amorphous phases. We found that pressure enhances the growth process in both Si and Ge, with activation volumes equal to -3.3 ± 0.3 cm3/mole for Si and -6.3 ± 0.60 cm3/mole for Ge. The results of this and other experiments are inconsistent with all bulk point-defect mechanisms, but are consistent with all interface point-defect mechanisms, proposed to date for thermal SPEG. A kinetic analysis of the Spaepen-Turnbull dangling bond mechanism shows it to be a highly plausible model for the growth process.
Monte Carlo Modeling of Interphase Boundaries in Cu-Ag and Cu-Ag-Au Alloys
Authors: Bacher P. , Wynblatt P.
Published: March 2011

Monte Carlo simulation, in conjunction with the embedded atom method, has been used to model the composition and structure of a semicoherent (001) interphase boundary separating coexisting Cu-rich and Ag-rich phases in a binary Cu-Ag alloy. The results are compared with earlier simulations of the same boundary in a Cu-Ag-Au alloy, in which Au was found to segregate to the interface, and the boundary was found to be unstable with respect to break-up into {111} facets. The boundary in the binary system is also unstable to faceting, but displays both {100} as well as {111} facets. It is concluded that Au segregation in the ternary alloy plays an important role in stabilizing the {111} facets. The interplay between the misfit dislocations present at the interface, and the compositional features of the boundary are also discussed.
The Pseudostable Phase
Authors: Kikuchi Ryoichi , Mohri Tetsuo , Fultz Brent
Published: March 2011

The basic nature of kinetic processes is examined theoretically. When an isothermal development of order is studied theoretically, sometimes an ordered phase different from the final equilibrium one transiently predominates, while the free energy (Fsys) decreases monotonically although very slowly. There is another category of kinetic changes for which the convenient interpretation is that the Fsys surface itself changes in time. In this second category, a state can initially be a local minimum of Fsys and thus can be called a ‘metastable’ state, but after a certain time, as the Fsys surface itself changes, a kinetic path appears along which Fsys decreases and the system can change along it. To include both categories, the concept of the PSEUDOSTABLE phase is proposed. It is shown that even in the nucleation process, Fsys for the entire system decreases monotonically, although the local free energy may fluctuate.
Kinetic Roughening of Interfaces in Driven Systems
Authors: Grossmann Bruno , Guo Hong , Grant Martin
Published: March 2011

We study the dynamics of an interface driven far from equilibrium in three dimensions. The relationship of the phenomena to self-organized critical phenomena is discussed. Numerical results are obtained for three models which simulate the growth of an interface: the Kardar-Parisi-Zhang equation, a discrete version of that model, and a solid-on-solid model with asymmetric rates of evaporation and condensation. We show that the three models belong to the same dynamical universality class by estimating the dynamical scaling exponents and the scaling functions. We confirm the results by a careful study of the crossover effects. In particular, we propose a crossover scaling ansatz and verify it numerically. Furthermore, the discrete models exhibit a kinetic roughening transition. We study this phenomenon by monitoring the surface step energy which shows a drastic jump at a finite temperature for a given driving force. At the same temperature, a finite size scaling analysis on the bond energy fluctuation shows a diverging peak [1].
Ionization Enhanced Solid Phase Epitaxy of Amorphous Silicon with Aluminum Impurities
Authors: Jeon Young-Jin , Becker M. F. , Walser R. M.
Published: March 2011

In this work we measured the functional dependence of the solid phase epitaxial regrowth (SPER) of amorphous silicon on NAI, the concentration of implanted aluminum (p-type). The SPER rates of self-ion amorphized layers in silicon wafers with (100) substrate orientation were measured by in situ high precision, isothermal, cw laser interferometry for temperatures from 470 °C to 550 °C, and concentrations in the range 3×1018 cm−3 ≤NAI≤ 4.7×1020 cm−3 obtained from samples implanted with three different doses.

In the concentration range 3×1018 cm−3 ≤NAI≤ 2.3×1019 cm−3, we observed a “compensation effect” in which, with increasing NAI, the SPER rate decreased below the regrowth rate in intrinsic silicon and the activation energy of SPER increased to 2.85 eV, compared to 2.72 eV for intrinsic silicon. In the range 3.3×1019 cm−3 ≤NAI≤ 5.6×1019 cm−3, the regrowth rate increased linearly with NAI as previously observed for SPER in boron, phosphorus, and arsenic implanted samples. However, due to the compensation effect, the aluminum data could not be fit to the normalized equation; V/Vi = 1 + N/Ni, as was done previously for data obtained for boron, phosphorus, and arsenic. The regrowth rate increased nonlinearly to the maximum implanted concentration of 4.7× 1020 cm−3 at which the regrowth rate was more than double the previously observed maximum rate in boron doped silicon. In the high concentration range, the SPER rate enhancement could be fit by a quadratic equation whose curvature was positive as was the case for boron. This contrasts with the negative curvature required to fit the nonlinear dependence of the SPER rate on the concentration of donor impurities such as phosphorus and arsenic.
Thermionic Emission from Hot Tungsten Clusters
Authors: Leisner T. , Athanassenas K. , Kandler O. , Kreisle D. , Recknagel E. , Echt Olof
Published: March 2011

We observe thermionic emission from tungsten clusters Wn, n ≥ 5, following excitation by a Q-switched Nd:YAG laser. The ion yield from this process dominates over prompt ionization if the laser fluence is chosen properly. Clusters originating from thermionic emission are significantly colder than expected if evaporative cooling would prevail.
Clusters-a New Source for Atomically Engineered Materials
Authors: Jena P. , Khanna S. N. , Rao B. K. , Manninen M.
Published: March 2011

The electronic structure and properties of small clusters are strongly dependent on their size. These size specific properties are illustrated by confining the discussion to three different topics (1) interaction of hydrogen with clusters (2) effect of temperature and magnetic field on the magnetization of clusters and (3) reaction of clusters with gas atoms. It is shown that the electronic energy levels that are strongly dependent on cluster size give rise to the size specific electronic properties of clusters. In cluster hydrides certain elements are found to absorb more hydrogen per atom in cluster form than in a crystalline form. The magnetization of clusters increases as a function of cluster size and externally applied magnetic field. In clusters reacting with gas atoms, it is possible to find the product cluster in an electronically excited state. The energetics of these states are strongly dependent on the cluster size and the reacting gas.
Production and Characterization of Deposited Mono-Sized Clusters
Authors: Cox Donald M. , Kessler Barbara , Fayet Pierre , Eberhardt Wolfgang , Sherwood Rex D. , Kaldor Andrew
Published: March 2011

Using high energy rare gas ion sputtering of metal targets, we are able to produce nanoamps of mass selected transition metal clusters. Mono-sized cluster ions are deposited at low kinetic energy upon substrates, e.g. silica or carbon, and are then characterized using UV and x-ray photoemission. In this paper we will discuss photoemission measurements of the 4f7/2core level energies of Au (1–5,7 atom samples) clusters deposited on silica. From such studies we are beginning to understand how electronic structure, cluster stability and mobility depend on (deposited) cluster size, surface coverage, and substrate temperature.
Cluster/Surface Interactions and Cluster/Matrix Deposition
Authors: Ross Mark M. , Callahan John H. , King Fred L. , Wysocki Vicki H. , Bach Stephan B.H. , Nelson Herbert H. , Baronavski Andrew P. , McElvany Stephen W.
Published: March 2011

This paper reports the most recent results from the cluster chemistry program at the Naval Research Laboratory, in which our efforts in the characterization of gas-phase cluster properties have been extended to studies of condensed-phase species. First, in an attempt to investigate the fundamental interactions between mass-selected cluster ions and surfaces, two tandem mass spectrometers were constructed or modified, and the results of the initial experiments will be discussed. The emphasis will be on illustrating the general utility of ion/surface collisions to study fragmentations, reactions, and deposition. Second, clusters were deposited into a matrix in order to perform spectroscopic analyses. The initial experiments have been directed at optimization of the techniques. Finally, recent measurement of the ionization potentials of large carbon clusters will be reported. These results are especially significant because of the new developments in large-scale production, isolation, and characterization of these species.
“Softlanding” of Monodispersed Small Metal Clusters in Rare Gas Matrices.
Authors: Harbich W. , Fedrigo S. , Buttet J. , Lindsay D.M.
Published: March 2011

We report on the successful softlanding of size selected metal clusters in rare gas matrices. Cluster-ions are sputtered from a metal target, energy filtered and mass selected by a quadrupole mass filter. Deposited clusters are detected by optical spectroscopic methods ( absorption, excitation, fluorescence). Our results show that for dimers and trimers fragmentation decreases with decreasing landing energy. In the case of Ag2 for example fewer than 25% of the dimers fragment when they are deposited at 20 eV. Triatomic clusters appear to fragment less easily than the dimers. New spectroscopic transitions have been found for both diatomic and triatomic clusters. Our results, when compared with previous studies on conventionally prepared clusters, show that some dimer and trimer features are incorrectly assigned.
Thin Film Deposition and Growth Processes by Ionized Cluster Beams
Authors: Yamada I. , Takaoka G.H. , Usui H. , Koh S.K.
Published: March 2011

Atomic scale imaging by STM and TEM of the initial stages of film growth of Ag and Au on graphite substrates indicate that the film nucleation processes are markedly different for ionized cluster beam (ICB) and molecular beam (MBE) deposition. Recent results on measurements of cluster size and formation of epitaxial metal-semiconductor layers by ICB are also discussed.
Cluster Beam Deposition of High Temperature Material
Authors: Herron William J. , Garvey James F.
Published: March 2011

We present our latest attempts to utilize a Smalley-type cluster beam source, to generate novel thin films. This technique employs entraining, within a high pressure molecular beam expansion, the products generated from laser ablation of a rotating target rod. We will show how such a cluster beam source can be used to generate a high temperature material within a molecular beam and deposit it intact on a relatively cool substrate. By tailoring the various expansion conditions (ie., expansion pressure, laser fluence, type of carrier gas, pulse delay, etc...) one can drastically effect the morphology and chemical nature of the surface generated. This technique has the promise that it may be able to fabricate a wide variety of thin films with obvious industrial applications (superconducting thin films, diamond-like carbon films, patterned or multi-layered thin films, etc...)
Surface-Plasma Absorption in Silver Induced by a Granular Dielectric Overcoating
Authors: Yamaguchi T. , Senda H. , Truong V-V.
Published: March 2011

It is shown that when transparent dielectric particles are deposited on metallic surfaces, excitation of surface plasma waves (SPW) becomes possible. The coupling of light with SPW is done via mirror-image effects. A drop in reflectance is observed at the resonance frequency in a similar fashion as in surface roughness experiments. Experimental evidence is proved by depositing very thin layers of hafnium oxide as well as tin oxide on silver surfaces.
Large Angle X-Ray Diffraction Study of Nanocrystalline Pd
Authors: Fitzsimmons M. R. , Eastman J. A. , Müller-Stach M. , Wallner G.
Published: March 2011

Quantitative x-ray diffraction measurements of ultrafine-grained (nanocrystalline) Pd and a coarse-grained polycrystalline reference foil were obtained using synchrotron radiation. The Bragg reflection intensity profiles of nanocrystalline Pd were found to be considerably better represented by Lorentzian functions than by Gaussian functions, indicating that a large fraction of the intensity from the Bragg peaks is located in the tails of the reflections. The intensity that is in the Lorentzian-shaped Bragg peaks differs only slightly for different grain-sized materials; thus, the atomic relaxations in the vicinity of grain boundaries do not produce broadly distributed diffuse scattering other than that found in the Bragg reflections. The results of the present work do not support the previously proposed existence of either a “gas-like” grain boundary phase, or large quantities of vacancies or voids within grains of nanocrystalline Pd.
Xrd and Hrem Studies of Nanocrystalline Cu and Pd
Authors: Nieman G. W. , Weertman J. R. , Siegel R. W.
Published: March 2011

Consolidated powders of nanocrystalline Cu and Pd have been studied by x-ray diffraction (XRD) and high resolution electron microscopy (HREM) as part of an investigation of the mechanical behavior of nanocrystalline pure metals. XRD line broadening measurements were made to estimate grain size, qualitative grain size distribution and average long range strains in a number of samples. Mean grain sizes range from 4-60 nm and have qualitatively narrow grain size distributions. Long range lattice strains are of the order of 0.2-3% in consolidated samples. These strains apparently persist and even increase in Cu samples after annealing at 0.35 Tm (498K) for 2h, accompanied by an apparent increase in grain size of e2x. Grain size, grain size distribution width and internal strains vary somewhat among samples produced under apparently identical processing conditions. HREM studies show that twins, stacking faults and low-index facets are abundant in as-consolidated nanocrystalline Cu samples. Methodology, results and analysis of XRD and HREM experiments are presented.
Nucleotides as Structural Templates for The Self-Assembly of Quantum-Confined Cds Crystallites
Authors: Coffer Jeffery L. , Chandler Robin R.
Published: March 2011

We report here the use of nucleotides as stabilizers in the formation of quantum-confined (‘Q-size’) CdS, with the size and composition of the nucleotide exerting a significant effect on the resultant CdS structure. In general, CdS formed from equimolar Cd+2 and S2− (6 × 10−4 M) in the presence of a number of nucleotides yields clusters possessing similar absorption spectra but which differ significantly with respect to emissive behavior and overall physical stability. CdS/polynucleotide colloids (DNA, poly [A], poly[C]) exhibit strong trap luminescence and are stable on a timescale of months, but analogous CdS prepared from the mononucleotides ATP and AMP are virtually nonemissive and flocculate within hours, even upon stabilization at lower temperatures (5 to −60°C). In addition to their preparation and spectroscopic properties, the results of TEM, AFM, and computer modeling studies on these CdS/nucleotide colloids are discussed.
Compacted Binary Mixtures of Nanometer-Sized Crystals
Authors: Herr U.
Published: March 2011

A preparation method for mixtures of chemically different kinds of crystals on a nanometer scale is described. The method is based on inert gas condensation followed by the consolidation by means of high pressure. Since the size of the crystals is in the range of 6–20 nm, an appreciable volume fraction of the material is made up by interphase boundaries between the different crystals. Investigations of the morphological homogeneity of compacted mixtures consisting of Fe crystals and Cu or Ag crystals respectively have been carried out by means of electron microscopy and small-angle X-ray scattering. The atomistic structure in the vicinity of the Fe atoms has been studied by Moessbauer spectroscopy; indications for the formation of alloy phases have been found even for the Ag-Fe system which exhibits no miscibility in the liquid state. Alloy formation is proposed to take place at the interphase boundaries due to the special thermodynamic conditions in the boundary regions. These conditions are attributed to the free volume of the boundaries as well as to the presence of stresses.
Molecular Clusters of 3D and Lower Magnetic Dimensionality
Authors: Papaefthymiou G. C.
Published: March 2011

Controlled polymerization of iron leads to the synthesis of molecular clusters of ever-increasing size, tending to extended structures. Polymerization of oxo-bridged octahedrally coordinated iron leads to clusters with 3D magnetic interactions between iron ions, while sulfide- and selenide-bridged tetrahedrally coordinated iron ions produce clusters of lower magnetic dimensionality. The magnetic properties of the resulting large molecular clusters with N ≥ 17 (where N = the number of iron ions in the cluster) are being investigated for the presence of collective magnetic correlations associated with the solid state.
Magnetic and Morphological Properties of Nanophase Metallic Particles of Fe, Co, and Ni
Authors: Gangopadhyay S. , Hadjipanayis G. C. , Sorensen C. M. , Klabunde K. J.
Published: March 2011

A vapor deposition technique has been used to prepare nano-size particles of Fe, Co and Ni using argon gas. The particles were passivated from further oxidation using a small volume of air. The range of particle size obtained in these systems was 47–200 Å. The saturation magnetization of Fe particles varied between 25–200 emu/g with the higher values corresponding to larger particles and the highest coercivity achieved at room temperature was 1050 Oe. In the case of Co and Ni, the magnetization varied in the range 35–100 emu/g and 14–45 emu/g, respectively. The highest room temperature coercivity was 1200 and 41 Oe for Co and Ni, respectively. A shell-type structure consisting of a metallic core surrounded by an oxide shell has been proposed for the particles.
A Comparative Study of the Nanocrystalline Material Produced by Sliding Wear and Inert Gas Condensation
Authors: Ganapathi S. K. , Aindow M. , Fraser H. L. , Rigney D. A.
Published: March 2011

A High Resolution Electron Microscopy (HREM) study of grain boundaries in nanocrystalline copper produced by sliding wear and by inert gas condensation is described. The results in both cases are essentially the same and similar to those in conventional polycrystals. Contrary to reports in the literature, these results do not indicate the presence of a fundamentally different structure in nanocrystalline materials. Measurements of the mechanical properties (such as yield strength, elastic modulus and strain rate sensitivity) of these materials using nanoindentation are also presented. These are found to be in good agreement with values expected from a polycrystalline material with a very fine grain size.
Spectroscopic and Photophysical Properties of the Soluble Cn Molecules, n = 60, 70, 76/78, 84
Authors: Whetten R. L. , Alvarez M. M. , Anz S. J. , Schriver K. E. , Beck R. D. , Diederich F. N. , Rubin Y. , Ettl R. , Foote C. S. , Darmanyan A. P. , Arbogast J. W.
Published: March 2011

A report is given on molecular carbon research in Los Angeles, starting with review of the molecular precursor approach to carbon materials.1 Regarding larger molecules, mixtures of carbon molecules, Cn, are extracted2 in 25–40 percent yield from graphite evaporated under helium. Gram quantities of the mixture are then separated into pure fractions – C60, C70, C76/C78, C84, and higher (n > 88),3,4 and characterized by mass spectrometry, liquid chromatography, and 13CNMR spectroscopy. Examination of the spectroscopic and photophysical5 properties of the molecules, with particular focus on C60 in solution, reveals: (i) the first transitions appear at high energy, are electronically forbidden (in C60, the homo-lumo hu - t1u transitions), and exhibit resolved vibronic structure; (ii) the spectra sharpen in cryogenic media, allowing vibronic assignment; and (iii) relaxation occurs with near-unity quantum yield to a long-lived triplet, which also transfers energy efficiently to acceptors including O2.
The Ionizations of C60 in the Gas Phase and in Thin Solid Films.
Authors: Lichtenberger Dennis L. , Jatcko Mark E. , Nebesny Kenneth W. , Ray Charles D. , Huffman Donald R. , Lamb Lowell D.
Published: March 2011

The high-resolution He I photoelectron spectrum of C60 in the gas phase is reported and compared with the photoelectron spectrum of C60as a thin film prepared by vapor deposition (one to three monolayers) on gold. The spectra show low valence ionization bands that are very sharp and well-separated for a molecule of this size, consistent with the highly symmetric truncated icosahedral structure and theoretical calculations. The total band widths of the valence ionizations from the thin film samples are comparable to those from the gas phase species, showing that the electronic interactions between the molecules and with the surface do not significantly influence these measurements of the molecular electronic structure. The gas phase photoelectron spectra also show vibrational fine structure in the first and second ionization bands with spacings that are consistent with the two totally symmetric vibrational modes of C60. The first vertical ionization energy relative to the vacuum level is determined to be 7.61 ± 0.02 eV from these gas phase measurements.
Solid State Dynamics of C60 And C70 Revealed by Carbon-13 NMR
Authors: Tycko R. , Dabbagh G. , Haddon R. C. , Douglass D. C. , Mujsce A. M. , Kaplan M. L. , Kortan A. R.
Published: March 2011

Carbon-13 NMR spectra of powder samples of pure C60 and of a mixture of C60 and C70 provide information about molecular motions in the solid state. At room temperature, C60 molecules rotate rapidly and isotropically. The transition from a rapidly rotating to a stationary (on the time scale of the experiment) system occurs over a temperature range from 120 K to 60 K, suggesting a distribution of activation energies. C70 molecules also rotate rapidly at room temperature, but more anisotropically.
Solution and Solid State NMR Studies of the Structure and Dynamics of C60 and C70
Authors: Johnson R. D. , Yannoni C. S. , Salem J. , Meijer G. , Bethune D. S.
Published: March 2011

We have investigated the structure and dynamics of C60 and C70 with 13C NMR spectroscopy. In solution, high-resolution spectra reveal that C60 has a single resonance at 143 ppm, indicating a strained, aromatic system with high symmetry. This is strong evidence for a C60 “soccer ball” geometry. A 2D NMR INADEQUATE experiment on 13C-enriched C70 reveals the bonding connectivity to be a linear string, in firm support of the proposed “rugby ball” structure with D5h symmetry, and furnishes resonance assignments. Solid state NMR spectra of C60 at ambient temperatures yield a narrow resonance, indicative of rapid molecular reorientation. Variable temperature T1 measurements show that the rotational correlation time is ∼ 10−9s at 230 K. At 77 K, this time increases to more than 1 ms, and the 13C NMR spectrum of C60 is a powder pattern due to chemical shift anisotropy (tensor components 220, 186, 40 ppm). At intermediate temperatures a narrow peak is superimposed on the powder pattern, suggesting a distribution of barriers to molecular motion in the sample, or the presence of an additional phase in the solid state. A Carr-Purcell dipolar experiment on C60 in the solid state allows the first precise determination of the C60 bond lengths: 1.45 and 1.40Å.
Aerosol Deposition and Film Formation using Silicon Powder
Authors: Gadepally Kamesh V. , Tennal K. B. , Hawk R. M. , Brown W. D.
Published: March 2011

An electrostatic deposition process which utilizes charged particle motion in an electric field has been successfully developed as a method for depositing coatings of silicon films. Coatings have been deposited on conducting, Insulating, and semiconducting surfaces after making suitable changes in deposition parameters. This process along with parameters Important to the deposition process which Include particle charge and size characteristics are discussed. These coatings are subsequently heat treated. Experimental results for depositions on silicon substrates are presented. Characterization in terms of chemical, electrical and physical properties of the film are presented.
Raman Scattering in Semiconductor Nanocrystals
Authors: Tu A. , Persans P. D.
Published: March 2011

We combine the principles of phonon and electron quantization in confined systems and breakdown of wavevector conservation in the photon-electron and phonon-electron couplings to describe the Raman spectrum of nanocrystals. The approach proposed here is consistent with experimental results and models developed for Raman scattering from quantum wells and superlattices.
The Performance of Channel Cut Collimators for Precision X-Ray Diffraction Studies of Epitaxial Layers
Authors: Loxley Neil , Bowen D. Keith , Tanner Brian K.
Published: March 2011

Replacement of the pinhole collimator on a double axis X-ray diffractometer with a device incorporating a channel-cut crystal permits the beam to be pre-conditioned in angular divergence. We examine the merits of such devices, known as channel-cut collimators (CCC's), of different materials and reflections. The experimental performance of InP 004 and Si 022 CCC's is presented.

With a reference crystal on the first axis, set in the dispersive peometry with respect to the CCC, conditioning in wavelength spread is achieved. Dispersion broadening is effectively eliminated and no resetting of the reference crystal is required when changing specimen materials or reflections. The devices have extremely low background and reduced Bragg tails. Application of the 4-reflection CCC to rocking curve analysis of thin epitaxial layers, ultra-low angle scattering from biological systems, grazing incidence reflectometry and triple axis diffraction of semi-conductors is discussed.
Principles and Performance of a PC-Based Program for Simulation of Double-Axis X-Ray Rocking Curves of Thin Epitaxial Films
Authors: Bowen D. Keith , Loxley Neil , Tanner Brian K. , Cooke Lynne , Capano Michael A.
Published: March 2011

PC-based software for the rapid simulation of double-axis X-ray rocking curves from epitaxial thin films by solution of the Takagi-Taupin equations is described. The principles of the mathematical model are discussed. Graded layers and interface roughness are treated by piece-wise approximation to linear or quadratic functions and fractional relaxation of each epilayer may be included. The reliability of the data-bases incororated are examined and the requirement for internal consistency demonstrated. Bench-mark tests are reported for various PC-compatible microcomputers. The shift in epilayer peak position, which occurs experimentally on reducing layer thickness at constant composition, is predicted in the simulations and compared with other simulated data in the literature. Detailed studies of generated data have been undertaken and compared with independent simulations and experimental data. Agreement is excellent.
Application of a Desk-Side Double-Axis X-Ray Diffractometer for Very Large Area Epilayer Characterization
Authors: Loxley Neil , Bowen D. Keith , Tanner Brian K.
Published: March 2011

A new desk-side double-axis X-ray diffractometer capable of rapid, automatic measurement of lattice mismatch between epitaxial thin films and substrate in a two dimensional grid 150 mm square has been built. The design principles behind the five independent axis systems, specimen loading, and the fail-to-safety X-ray shutter are elucidated, and examples of typical data from substrate material and thin epitaxial films of III-V compounds are presented.
Non-Destructive Analysis of Thin III-V Epitaxial Layers Using a Tabletop Double Crystal X-Ray Diffractometer
Authors: Sacks R. N.
Published: March 2011

Some of today's most promising and interesting semiconductor devices use only a few thin epitaxial layers of III-V materials, where each layer may be only 100 to 1,000A thick. There is a need for fast, accurate, non-destructive analysis techniques for these structures. Double-crystal x-ray diffraction has proven to be an excellent method for measuring composition, thickness, interface sharpness, and overall crystalline quality of III-V heterostructures. Data is presented on the use of a Bede QC1 automated table-top double-crystal diffractometer for the analysis of (AI,Ga)As, (ln,Ga)As, and GaAs epitaxial layers grown by Molecular Beam Epitaxy (MBE). It is shown that this technique can directly detect and analyze single layers of (In,Ga)As as thin as 200A, and in some cases, can indirectly detect layers of GaAs or (AI,Ga)As as thin as 100A without unusual measures such as glancing angle diffraction. The rocking curve results are compared with values predicted by RHEED intensity oscillation measurements, and with computer simulations using a commercial software package.
New Method for a Lensless Electron Microscope: Achieving High Resolution and Overcoming Effects of Multiple Scattering
Authors: Tong S. Y. , Huang H. , Li Hua
Published: March 2011

We present the processes necessary for the development of a lensless electron microscope with a resolution of ≤ 0.6Å and an image position accuracy of ≤ 0.3Å. The technique inverts interference fringes of emitted electrons from localized sources embedded in a material. Unlike conventional methods which use the Helmholtz-Kirchhoff integral theorem, the new process starts with 3-dimensional Fourier transformation with phaseshift correction, SWEEP for forward-scattering electrons and REEP for back-scattering electrons. These processes are applicable to strong multiple scattering systems as well as systems whose source atoms are buried below the surface. Materials with long-range or local order can be studied; coupled to a small spot-sized incident beam, images of disordered materials can be formed.
Electron Channeling Analysis of Strained Iron Aluminide Films
Authors: Keller R. R. , Angelo J. E. , Wowchak A. M. , Cohen P. I. , Gerberich W. W.
Published: March 2011

Relaxation modeling for strained epitaxial films depends strongly on the acquisition of accurate structural information. Electron channeling provides such information from bulk samples in a scanning electron microscope and hence is not subject to thin specimen artifacts. Electron channeling patterns (ECPs) were obtained from thin (∼100Å), partially relaxed iron aluminide films grown by MBE on thick (∼1mm) AlAs/GaAs (100) substrates. These systems exhibited low critical thicknesses as determined by in-situ reflection high energy electron diffraction (RHEED). Variation of SEM accelerating voltage provided ECPs from depths ranging from film only to film + substrate. High diffraction order lines were analyzed kinematically to determine lattice parameters in the strained film. ECP and RHEED indicated an elastic strain in the film of approximately 1.2%. A linear elastic finite element approach was then used to estimate the degree of strain redistribution into the substrate in a thinned TEM specimen. Results indicated that reducing the substrate thickness to below ten times the film thickness can decrease the elastic strain in the film by 0.5%.
Analysis of the Strain Profile in Thin Au/Ni Multilayers by X-Ray Diffraction
Authors: Chaudhuri J. , Gondhalekar V. , Jankowski A. F.
Published: March 2011

A dynamical x-ray diffraction theory has been used to obtain microscopic strain profiles in thin Au/Ni multilayers. Depth profiles of strains in these multilayers, with repeat periodicities varying from 0.82 nm to 9.0 nm, are obtained by an iterative fitting of the calculated diffraction pattern with the experimental one. Interfacial coherency is found to play an important role in understanding the origin of the supermodulus effect in metallic multilayers.
Determination of Composition and Linear Lattice Expansion Coefficient in Si1−x Gex/Si Thin Films by Simulation of X-Ray Rocking Curves
Authors: Cembali F. , Fabbri R. , Servidori M. , Zani A. , Iyer S.
Published: March 2011

By simulation of X-ray rocking curves of Si-Ge alloys grown on Si by molecular beam epitaxy and of Ge implanted samples, the Ge composition, the linear lattice expansion coefficient, the strain depth-distribution and the static Debye-Waller factor in the MBE alloys have been determined.
Reconstructed Structure of SiO2/Si(111) Interface
Authors: Hirosawa Ichiro , Nizuki Jun'ichiro , Tatsumi Toru , Akimoto Koichi , Matsui Junji
Published: March 2011

In order to investigate the initial oxidation process Qf the Si (111) surface, we have studied the molecular beam deposited Si0 2/Si(111)-7×7 interface structure using grazing incidence X-ray diffraction geometry. We suggest a three-fold symmetry structural model composed of stacking fault layer, dimer layer and additional ordered atoms. The three-fold symmetry structure comes from the preference for oxidation in the faulted half of the 7×7 structure.
Rheed Study of the Surface Structure of Cu(110) From 153 To 973 K
Authors: Liao Da-Zhong , Cao Y. , Montano P. A.
Published: March 2011

We have studied the surface structure of Cu(110) from 153 to 973 K. Reflection high energy electron diffraction (RHEED) was employed to analyze the surface modifications taken place at the crystal surface. We performed careful spot profile intensity measurements as a function of temperature.The typical characteristics of forward scattering, the short wavelength at high energy, and the small grazing incidence angle in RHEED provide strong surface sensitivity and are therefore ideal to study surface structures and especially surface roughness. We observe at low temperatures strong diffraction streaks in the patterns as well as Kikuchi lines. The diffraction intensities reduce slowly when the temperature is raised from 153 K to 473 K, then fall off more rapidly after that, and from 573 K, the intensities decrease drastically. High resolution LEED measurements indicate a reduction in the coherence length at high temperatures.
Spectral Transformation of Exelfs Data and a Structural Examination of Nitrogen on Cu(100) and Cu(110) Surfaces
Authors: Dai Q. , Gellman A. J.
Published: March 2011

EXtended Electron Energy Loss Fine Structure (EXELFS) spectra have been obtained above the K-edge of nitrogen atoms adsorbed on Cu. The radial distribution functions obtained from the fine structure indicate N-Cu bond lengths of 1.84(±.03) Å and 1.81(±.03) Å for nitrogen on the Cu(100) and Cu(110) surfaces respectively. As in previous EXELFS measurements, the actual measurements made are of N"(E) rather than direct measurement of N(E), the electron energy loss distribution. The EXELFS spectra from these two surfaces are used to illustrate the influence of this collection scheme on the radial distribution function obtained by Fourier transformation of the raw data. To obtain the direct analog of the radial distribution function found from the EXAFS experiment one must either resort to spectral integration or appropriate scaling of the distribution function found in the EXELFS experiment.
Molecular Dynamics Studies of Defects in Si
Authors: Duesbery M.S. , Michel D.J. , Kaxiras Efthimios , Joos B.
Published: March 2011

The efficacy of three modern empirical potentials in the molecular dynamic simulation of the configurations, energies and mobility of dislocation cores and their excitations is assessed in the light of recent literature. The results are found to be inconsistent both between different potentials and with experimental evidence. It is argued that the discrepancies are rooted in the limited databases which have been used in the construction of empirical potentials. The reason for the discrepancies is demonstrated by comparing empirical and density functional calculations of the generalized stacking fault energy.
On the Nature of Grain Boundary Structures in Nanophase Materials
Authors: Siegel R. W. , Thomas G. J.
Published: March 2011

A significant fraction of the atoms in nanophase materials are associated with grain boundaries. Hence, the nature of the grain boundary structures in these materials is interesting in its own right as well as ultimately important to the properties of nanophase materials. Considerable uncertainty has surrounded the atomic structure of nanophase boundaries, but recent experiments have shed some light on these interesting structures. These investigations have included direct HREM observations combined with image simulations, as well as Raman spectroscopy and SANS. The emerging picture is one of nanophase boundaries that are not dissimilar in structure to those normally encountered in conventional polycrystals. The connections between this picture, of nanophase materials as ultrafine-grained polycrystals, and experimental observations on these materials are discussed.
The Free Energy Simulation Approach to Grain Boundary Segregation In Cu-Ni
Authors: Wang H. Y. , Najafabadi R. , Srolovitz D. J. , Lesar R.
Published: March 2011

A new, accurate method for determining equilibrium segregation to defects in solids is employed to examine the segregation of Cu to grain boundaries in Cu-Ni alloys. The results are in very good agreement with the ones given by Monte Carlo. This method is based upon a point approximation for the configurational entropy, an Einstein model for vibrational contributions to the free energy. To achieve the equilibrium state of a defect in an alloy the free energy is minimized with respect to atomic coordinates and composition of each site at constant chemical potential. One of the main advantages this new method enjoys over other methods such as Monte Carlo, is the efficiency with which the atomic structure of a defect, segregation and thermodynamic properties can be determined. The grain boundary free energy can either increase or decrease with increasing temperature due to the competition between energetic and configurational entropy terms. In general, the grain boundary free energy increases with temperature when the segregation is strongest.
Simulating Vacancy, Impurity And Electronic Defect States In Mgo,Lici And La2Cuo4 Using Quantum Cluster And Classical Lattice Simulation Techniques In A Consistent Manner.
Authors: Grimes Robin W. , Catlow C. R. A. , Shluger A. L. , Pandey R. , Baetzold R. , Harker A. H.
Published: March 2011

We calculated defect energies by using a Hartree-Fock method to model an inner region that includes the defect site and oneor two shells of lattice ions. This is surrounded by an embedding region that is described by a classical Mott-Littleton calculation. Electrostatic multipole consistency between the two jegions is maintained throughout.

The results of defect calculations on MgO are examined as a function of basis set. We find that a sophisticated basis set is required before a reliable defect formation energy can be guaranteed. This experience has allowed us to develop a new model for the off-center relaxation of the exciton in LiC1. These calculations have required the use of pseudo potential cores for anions and floating functions to model the diffuse electron. Lastly, we report on recent calculations concerning the stability of hole states in La2CuO4. We find considerable delocalization of the hole over both the Cu2+ ions and the 02− ions that form the Cu-O planes.
Electron Channelling Contrast Imaging (ECCI) of Dislocations in Bulk Specimens
Authors: Czernuszka J.T. , Long N.J. , Boyes E.D. , Hirsch P. B.
Published: March 2011

Dislocations have been imaged in bulk specimens of Si using a FEG SEM operating at 30keV and 100keV, without using an energy filter, but by image processing of the back scattered electron signal collected by a highly efficient detector. The dislocation contrast is greater at 30 keV than at 10OkeV. Ilowever, the depth to which useful information may be obtained is greater at 1O0keV (˜210nm) than at 3OkeV (˜95nm). The final depth to which dislocations can be imaged is strongly dependent on image processing conditions.
Diffusion Mechanisms in Bcc-Zr:A Molecular Dynamics Approach
Authors: Willaime F. , Massobrio C.
Published: March 2011

Basing our calculations on a realistic N-body interatomic potential for Zr, we study the vacancy migration mechanism and determine the related diffusion coefficient in the bcc phase. The form of the potential energy along the nearest-neighborjump migration path is single-peaked. The vacancy jump rate determined by molecular dynamics simulations has a perfectly Arrhenian behavior and its activation energy is very close to the static value of the vacancy migration energy, both being very low (≈ 0.3 eV) . The diffusion coefficient is in very satisfactory agreement with experiments.