bluuebunny/crossref_metadata_2025_split huggingface dataset:
Excitons in Low-Dimensional Systems: Conjugated Polymers and Semiconductor Surfaces
Authors: Rohlfing Michael 
Published: April 2011

ABSTRACTWe investigate the formation of excitons in low-dimensional semiconducting systems. To this end a recently developed ab initio approach is employed to determine the excitation energies and optical transition matrix elements of bound and unbound excitonic states. From these results the optical spectrum can be evaluated. We discuss two classes of low- dimensional prototype materials: conjugated polymers and a semiconductor surface. Due to the reduced dimensionality, the excitonic binding energies are much larger than in con- ventional semiconductors.
Crystallization of TiO2 in Sol-Gel Derived SiO2-TiO2 System: Formation of TiO2(B) Nanocrystallites
Authors: Kogure T. , Umezawa T. , Kotani Y. , Matsuda A. , Tatsumisago M. , Minami T. 
Published: April 2011

AbstractThe crystallization of TiO2 in monolithic SiO2-TiO2 (SiO2>TiO2) gels by annealing has been investigated. The temperatures of the crystallization of TiO2 to anatase and anatase-to-rutile phase transition were considerably raised with the addition of SiO2. It is supposed that the anatase-torutile phase transition is primarily controlled by the grain size of the crystals. TiO2(B), one of the TiO2 polymorphs, was formed as one of the first crystalline phase in SiO2-TiO2 gels at 800-900°C. Transmission electron microscope observations revealed that TiO2(B) nanocrystallites with a size of 5-10 nm were dispersed in amorphous SiO2 matrix. TiO2(B) nanocrystallites did not grow larger but transformed to anatase at higher temperatures. It is supposed that the nucleation and stability of TiO2(B) are enhanced with the presence of surrounding SiO2, presumably by a low interfacial energy.
Nucleation Behavior and Solid-Liquid Interfacial Energy of Polytetrahedral Phases
Authors: Holland-Moritz Dirk 
Published: April 2011

AbstractAccording to classical nucleation theory the nucleation- and phase-selection behavior of undercooled meltallic melts is strongly dependent on the solid-liquid interfacial energy.A structural approach to the modelling of the interfacial energy for simple melt-crystal interfaces (fcc, hcp, bcc) was developed a number of years ago [1-4]. This approach is extended to polytetrahedral phases using numerical simulation. Results of these calculations are presented for different polytetrahedral structures: the tetragonal σ-phase in Ni-V, the monoclinic phase λ-Al13Fe4, the orthorhombic phase μ-Al5Fe2 and the icosahedral quasicrystalline I-phase in Al-Pd-Mn. The numerically estimated values for the solid-liquid interfacial energy are compared with results from experiments on the undercooling- and phase-selection behavior.
Sputtering of Au Induced by Single Xe Ion Impacts
Authors: Birtcher R. C. , Donnelly S. E. 
Published: April 2011

AbstractSputtering of Au thin films has been determined for Xe ions with energies between 50 and 600 keV. In-situ transmission electron microscopy was used to observe sputtered Au during deposition on a carbon foil near the specimen. Total reflection and transmission sputtering yields for a 62 nm thick Au thin film were determined by ex-situ measurement of the total amount of Au on the carbon foils. In situ observations show that individual Xe ions eject Au nanopanicles as large as 7 nm in diameter with an average diameter of approximately 3 nm. Particle emission correlates with crater formation due to single ion impacts. Nanoparticle emission contributes significantly to the total sputtering yield for Xe ions in this energy range in either reflection or transmission geometry.
X-Ray Standing Wave Assisted Layer Deposition and Crystal Growth (Xswdg)
Authors: Meyer D. C. , Paufler P. 
Published: April 2011

AbstractWe propose to combine layer deposition or crystal growth with an X-ray standing wave field to influence the structure of growth products in-situ on an atomic scale. By placing the periodic local density distribution of energy of these wave fields properly, photo-fragmentation of precursor gases and other relevant processes can be supported at definite atomic sites at the surface of substrates. In certain cases also a local variation of the probability of desorption can be favorable to remove atoms from unwanted adsorption sites. A large number of growth processes is conceivable, which can be affected intentionally that way. The general physical principle behind this proposal is the availability of non-thermal energy contributions on a sub-nanometer scale. The fundamentals of production and application of these wave fields are communicated as well as results of theoretical estimations.
Synthesis of SiC on Si by Seeded Supersonic Beams of Fullerenes
Authors: Ciullo G. , Biasioli F. , Podestá A. , Milani P. , Toccoli T. , Iannotta S. 
Published: April 2011

AbstractA well characterized C60 supersonic seeded beam has been used to synthesize SiC films on Si (111) 7×7. The control of beam parameters such as energy and flux distributions is shown to be important to improve quality of films in terms of morphology, defect density and structure. We demonstrate that a kinetic energy of a few eV of the C60 precursor is enough to induce carbidization at moderate substrate temperature. Kinetic energy activated SiC formation at 750°C is achieved with a strong reduction of the dimensions and density of defects. The films show a reduced roughness of about 2.5 nm (root mean square).
Relationship Between Structure and Luminescent Properties of Epitaxial Grown Y2O3:Eu Thin Films on LaAlO3 Substrates
Authors: Gao H-J. , Duscher G. , Fan X.D. , Pennycook S.J. , Kumar D. , Cho K.G. , Holloway P.H. , Singh R.K. 
Published: April 2011

AbstractCathodoluminescence images of individual pores have been obtained at nanometer resolution in europium-activated yttrium oxide (Y2O3:Eu) (001) thin films, epitaxially grown on LaA1O3 (001) substrates. Comparison with Z-contrast images, obtained simultaneously, directly show the “dead layer” to be about 5 nm thick. This “dead layer” is the origin of the reduced emission efficiency with increasing pore size. Pore sizes were varied by using different substrate temperatures and laser pulse repetition rates during film growth. These films are epitaxially aligned with the substrate, which is always Al terminated.
Structure Refinement of S-Phase Precipitates in Al-Cu-Mg Alloys by Quantitative HRTEM
Authors: Kilaas R. , Radmilovic V. , Dahmen U. 
Published: April 2011

AbstractThe crystal structure of the Al2CuMg S-phase precipitate in an Al matrix has been determined by quantitative high resolution electron microscopy. This work combines techniques of image processing and quantitative comparison between experimental and simulated images with automatic refinement of imaging and structural parameters.
Interfacial Interaction Between Cr Thin Films and Oxide Glasses
Authors: Jiang N. , Silcox J. 
Published: April 2011

AbstractThe interfacial interaction between Cr thin films and multi-component oxide glasses has been observed by means of high spatial resolution electron energy loss spectroscopy. Besides a partially oxidized Cr thin layer, a ∼5nm wide Cr diffusion layer is seen. Chromium oxidation at the interface results from the difference between the heats of oxide formation. Ion exchange between Cr2+ and alkaline earth ions then causes the formation of the diffusion layer. The electronic states of the Cr in this diffusion layer are different from that in the oxidized layer. Strong interaction between Cr and O in the diffusion layer suggests that such a layer could be responsible for the formation of a strong Cr/glass interface
X-ray Elemental Mapping of Multi-Component Steels
Authors: Papworth Adam J. , Williams David B. 
Published: April 2011

AbstractSteels used in steam turbine applications are susceptible to temper embrittlement if operated at temperatures above 400°C. The cause of this embrittlement is the segregation of impurity elements, mainly phosphorus, to the prior austenite grain boundaries. Molybdenum can act as an effective scavenger for phosphorus, but the scavenging effect is lost when the molybdenum is precipitated in carbides during service at these elevated temperatures. Thus, the very slow temper embrittlement is controlled by the rates of alloy carbide formation, rather than by the diffusion of phosphorus. The presence of vanadium apparently retards the embrittlement process even more by interfering with the formation of the molybdenum-rich carbides. Vanadium carbonitrides, are small, only a few nanometers in size, and difficult to see by standard TEM contrast mechanisms. Analysis of segregating elements to the grain boundaries has to be carried out on areas which are devoid of precipitates, especially small V precipitates or other precipitates that could be interpreted as segregation rather than precipitation. One method of detecting precipitates in the matrix is by X-ray mapping areas of interest. This paper describes a new method of X-ray mapping of multi-component steels that have overlapping lines. The method has been found to be effective in removing thickness effects from the maps and discriminating between real and artificial compositional components, which are created by the overlapping lines.
High Sensitivity Convergent Beam Electron Diffraction for the Determination of the Tetragonal Distortion of Epitaxial Films
Authors: Schuer C. , Leicht M. , Marek T. , Strunk H.P. 
Published: April 2011

AbstractWe have optimized the sensitivity of convergent beam electron diffraction (CBED) by orienting the specimen such that the central (000) diffraction disc shows a pattern of defect lines that are most sensitive to tetragonal distortion. We compare the position of these lines in the experimentally obtained patterns with results from computer simulations, which need to be based on dynamical diffraction theory. In both experimental and simulated patterns the positions of the defect lines are determined by applying a Hough transformation. As a result of this optimized approach, we can measure the tetragonal distortion of a low temperature grown GaAs layer as low as 0.04%.
Electronic Structures of Single-Walled Carbon Nanotubes Studied by NMR
Authors: Tang X.-P. , Kleinhammes A. , Shimoda H. , Fleming L. , Bennoune K. Y. , Bower C. , Zhou O. , Wu Y. 
Published: April 2011

ABSTRACTAn individual single-walled carbon nanotube (SWNT) has been shown [1-4] to exhibit remarkable electronic properties which depend on its diameter and chirality. In this work, the 13C nuclear magnetic resonance technique is used to measure quantitatively the electronic density-of-state (DOS) at the Fermi level in a bulk SWNT sample. Two types of 13C nuclear spins are observed with drastically different nuclear spin-lattice relaxation time (Tl). About onethird of the 13C nuclear spins with shorter Tl, are identified to reside at the metallic SWNTs and two-thirds can be associated with the semiconducting SWNTs. For the metallic SWNTs, the DOS at the Feimi level is measured quantitatively, which is about 0.022 states/(eV·atom·spin). The measured electronic DOS at the Fermi level agrees with the theoretical prediction for metallic tubes. This study also found that the semiconducting SWNTs in bundles, in fact, possess weak metallic characters. This indicates that tube-tube interactions within SWNT bundles could change the electronic properties.
Electrostatic Measurement of Plasma Plume Characteristics in Pulse Laser Ablated Carbon
Authors: Mayo R. M. , Newman J. W. , Sharma A. , Yamagata Y. , Narayan J. 
Published: April 2011

ABSTRACTA triple Langmuir probe measurement has been implemented to investigate plasma plume character in low fluence (∼ 3.0 J/cm2) pulsed laser evaporation (PLE) discharges and has been found to be an extremely valuable tool. Absolute plasma plume density estimates are found to reside in the range 1.0 × 1013–2.0 × 1014 cm−3 for vacuum pulses. A simple heavy particle streaming model for vacuum pulses allows estimates of the plume ionization fraction of ∼ 10%. This is consistent with typical deposition inventory suggesting that high kinetic energy ions play an important role in DLC film deposition. Electron temperature is found to consistently reside in the range 0.5-3.0 eV, and appears to be uninfluenced by operating conditions and large variations in Ar and N2 fill gas pressure. Consistent with strong plume ion and neutral particle coupling to the background fill, constancy of Te suggests expulsion of background gas by the energetic plume. The leading edge ion plume speed is measured via temporal displacement of spatially separated probe signals on consecutive PLE pulses. Flow speeds as high as 5.0 × 104 m/s are observed, corresponding to ∼ 156 eV in C+. The ion flow speed is found to be a strongly decreasing function of fill pressure from an average high of ∼ 126 eV in vacuum to ∼ 0.24 eV at 600 mTorr N2. Raman scattering spectroscopy indicates DLC film quality also degrades with fill pressure suggesting the importance of high ion kinetic energy in producing good quality films. Optical emission indicates an increase in C2 molecular light intensity with fill gas pressure implying a reduced, if any, role of these species in DLC production. Ion current signal anomalies in high pressure pulses indicate the formation of high mass carbon clusters during plume evolution in the presence of background gas. Mass diffusivity estimates, based on density decay, suggest the presence of under these conditions. Demonstration and control of such cluster formation may provide method(s) for controlling novel advanced materials properties.
Direct Ion Beam Deposition of Diamond-Like Films From RF Inductively Coupled (IC) Plasma Source: Mechanical, Electrical, and Optical Properties
Authors: Druz B. , Zaritskiy I. , Williams K. , Hayes A. , Polyakov V.I. , Kchomich A.V. , Li Xiodung , Bhushan Bharat 
Published: April 2011

ABSTRACTDiamond-like carbon (DLC) films were deposited on various substrates using direct ion beam deposition from an RF IC hydrocarbon plasma source. Combinations of gases such as CH4. CH4–N2 were used to form plasma. The mechanical, electrical and optical properties of the films were examined as a function of deposition conditions and N2 content in gas mixture. A small amount of N2 (<8 sccm) did not markedly change hardness and stress, while electrical conductivity was significantly increased. In addition, a small amount of N2 improved wear performance of the films reducing amount of debris and wear track size. Introduction of high N2 flow into the system significantly deteriorates value of these parameters. It was found that N2 essentially increases absorption coefficient, and reduces optical band gap. Analysis of the experimental results shows that observed effects can be explained by incorporation of N2 into carbon-strained network that induces structural changes and. in turn, leads to an increase of sp2 fraction in the DLC films.
Raman Signature of Bonding and Disorder in Carbons
Authors: Ferrari A C , Robertson J 
Published: April 2011

ABSTRACTThe factors controlling the position and intensity of the G3 and D peaks of the Raman spectra of disordered and amorphous carbons are separated in terms of a 3-stage model. The Raman spectra are shown to depend fundamentally on the degree of ordering of the sp2 sites, and only weakly or indirectly on the fraction of sp3 sites. Three factors control the G and D peaks; the perfection of graphitic order, replacing aromatic rings with olefinic chains and increasing the sp2 content. These rules allow us to state when the G3 peak position can be related reliably to sp3 content.
Fabrication and Characterization of Functionally Gradient Diamond-Like Carbon Coatings
Authors: Wei Q. , Sharma A. K. , Yamolenko S. , Sankar J. , Narayan J. 
Published: April 2011

ABSTRACTPure diamondlike carbon thin films largely bonded by four-fold coordination suffer from a large internal compressive stress that gives rise to a serious adhesion problem. In this work, functionally gradient (FG) diamondlike carbon thin coatings were prepared by pulsed laser deposition in a high vacuum chamber as an alternative approach to address the adhesion problem of diamondlike films. Copper, silver and titanium were incorporated into the growing films with their concentration as a function of the distance from the substrate surface. The top of the thin coating is pure DLC of about 400 nm in thickness. The total thickness of the functionally graded superhard DLC coatings can exceed 1.0 μm without buckling. Visible micro-Raman spectroscopy was used to characterize the bonding structure of the layers which contain alloy atoms. High resolution transmission electron microscopy was employed to study the microstructure of the coatings. Nanoscale mechanical characterizations using Nanoindenter XPTM were carried out to study the mechanical behavior of the functionally gradient DLC films.
An Electrochemical Impedance Evaluation and Laser Irradiation Effects on the Electronic Structure of Silicon Containing Diamond-Like Carbon Coating
Authors: Fountzoulos Costas G. , Kelley Jack V. , Halada Garry P. 
Published: April 2011

ABSTRACTHard, adherent, 2 μm thick lubricious silicon containing diamond-like carbon coatings (Si-DLC) were synthesized by 40 keV Ar+ ion beam assisted deposition (IBAD) of tetraphenyltetramethyl-trisiloxane oil on two, 5 cm by 5 cm by 0.64 cm thick 4340 steel substrates. Two different substrate surface finishes were examined, I polished and 1 600 grit finish (unpolished). The corrosion resistance of the Si-DLC coating was evaluated by Electrochemical Impedance Spectroscopy (EIS) in a 0.005N concentration sodium chloride (NaCI) solution. Low frequency impedance data from each of the coatings were compared with those of bare steel. The Si-DLC coating deposited on the polished substrate performed slightly better than the one deposited on the unpolished surface. Overall the Si-DLC coating did not appear to offer very much corrosion protection to the steel. This was mainly attributed to the presence of pinholes in the coating. Furthermore, to study the effect of radiation on the electronic structure of the Si-DLC coating, three Si-DLC coatings synthesized under the same deposition conditions on silicon substrates at various oil precursor temperatures were irradiated by a 355 nm wavelength, 0.37 Watts, pulsed YAG Laser at 35 kHz. Corrosion and irradiation results and procedures to minimize the pinhole density in the Si-DLC coating will be discussed in detail
Effect of Catalyst Supports in the Synthesis of Graphite Nanostructures
Authors: Anderson P. E. , Rodriguez N. M. 
Published: April 2011

ABSTRACTIt is well established that the structural characteristics of graphite nanofibers can be controlled by several factors including, the chemical nature of the catalyst, the composition of the reactant gas mixture and temperature at which the growth process is performed. In the current investigation we have endeavored to modify the behavior of the catalyst by dispersing the active metals on different support media. We have found that the strength of the metal-support interaction exerts a significant impact not only on the average size of the nanofibers generated by such a procedure, but also results in major changes in the architecture of the carbon materials. The support imposes certain geometrical constraints on the metal particles and these features are manifested in modifications in the degree of crystalline perfection and arrangement of the graphite sheets constituting the nanofibers. In addition, there is also the possibility that the support can induce electronic perturbations in the metal particles, a feature that will be most pronounced with a conductive carrier.
Electrical and Structural Properties of Carbon-Nitrogen Films
Authors: Kurt R. , Sanjinesa R. , Karimi A. 
Published: April 2011

ABSTRACTTo investigate the role of nitrogen in the structure formation and electrical properties of CNx thin films, several carbon thin films containing variable concentrations of nitrogen were deposited using a magnetron sputtering in a reactive N2/ Ar discharge. The chemical composition of the as-deposited films was investigated by means of Rutherford backscattering (RBS) and a [N]/[C] ratio up to 0.5 was obtained. TEM studies confirmed that the deposits are mainly amorphous and contain nanocrystalline CNx compounds. Additional STM investigations reveal a cluster-like microstructure of the surface showing a maximum cluster size of 10 nm in thin films deposited at a N2 / Ar pressure ratio of 30% in the plasma. The electrical resistivity ρ was studied as a function of temperature between 10 K and 320 K. All deposited films show semiconducting behaviour. The lowest ρ values were observed in films containing no nitrogen, whereas the films deposited with 30% N2in the sputter plasma reveal strongest semiconducting properties. The temperature dependence of the measured resistivity will be discussed according to Mott's law and the activation energy for the electrical transport estimated at room temperature will be given.
Bulk Synthesis of Nanotube-Like Carbon Material
Authors: Jacob Michael , Palmqvist Ulrik , Alberius-Henning Peter , Ekström Thommy 
Published: April 2011

ABSTRACTNanoporous carbons have been synthesised through a selective etching reaction, performed by halogenisation of metal carbides. The structures obtained can be controlled by choice of starting materials and reaction parameters, and here one example, chlorination of aluminium carbide at 700 °C, is given. The produced material is nanotube-like with agglomerates of short and wide interconnected tubular structures, which are here described as nanobarrels. The synthesis process gives a pure product with high-yield, and may be scaled up to produce bulk amounts.
Recent Developments in Superplasticity
Authors: McFadden S. X. , Mishra R. S. , Mukherjee A. K. 
Published: April 2011

AbstractThe phenomenon of superplasticity is explored in the range of high strain rate for both nanocrystalline and microcrystalline materials. True tensile superplasticity has been demonstrated in nanocrystalline grain size range. The difference in the details of such superplasticity between nanocrystalline and microcrystalline state is emphasized.
Simulation of Microstructural Evolution during Superplastic Deformation
Authors: Kim B.-N. , Hiraga K. 
Published: April 2011

AbstractSuperplastic tensile deformation is simulated in 2 dimensions by incorporating grain boundary diffusion and concurrent grain growth derived from static and dynamic growth mechanisms. The following relationship is found between microstructural changes and deformation behavior for constant stress conditions. Grain boundary diffusion produces an increase in the aspect ratio of the matrix grains during deformation and the increased aspect ratio causes a change in creep rate parameters: the stress exponent is decreased from the initial value of 1.0 for equiaxed grains and the grain size exponent is increased from the initial value of 3.0. Accelerated grain growth is also found by the present simulation.
Hyperplastic Forming: Process Potential and Factors Affecting Formability
Authors: Daehn Glenn S. , Vohnout Vincent J. , Datta Subrangshu 
Published: April 2011

AbstractThis paper has two distinct goals. First, we argue in an extended introduction that high velocity forming, as can be implemented through electromagnetic forming, is a technology that should be developed. As a process used in conjunction with traditional stamping, it may offer dramatically improved formability, reduced wrinkling and active control of springback among other advantages. In the body of the paper we describe the important factors that lead to improved formability at high velocity. In particular, high sample velocity can inhibit neck growth. There is a sample size dependence where larger samples have better ductility than those of smaller dimensions. These aspects are at least partially described by the recent model of Freund and Shenoy. In addition to this, boundary conditions imposed by sample launch and die impact can have important effects on formability.