bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

ABSTRACTThis 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

ABSTRACTWe 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

ABSTRACTAtomic 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

ABSTRACTWe 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

ABSTRACTIt 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

ABSTRACTQuantitative 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

ABSTRACTConsolidated 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

ABSTRACTWe 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

ABSTRACTA 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

ABSTRACTControlled 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

ABSTRACTA 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

ABSTRACTA 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

Abstract: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

ABSTRACTThe 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

ABSTRACTCarbon-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

ABSTRACTWe 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

ABSTRACTAn 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

ABSTRACTWe 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

ABSTRACTReplacement 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

ABSTRACTPC-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

ABSTRACTA 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

ABSTRACTSome 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

ABSTRACTWe 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

ABSTRACTRelaxation 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%.