bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ oriented crystal growth in a ceramic nanostructured system
Authors: Ribeiro Caue , Lee Eduardo J. H. , Giraldi Tânia R. , Aguiar Rosiana , Longo Elson , Leite Edson R.
Published: January 2005

We report here on an in situ experimental analysis of the growth processes of tin dioxide nanoparticles (with mean particle radius of less than 5nm) and the oriented attachment of these particles onto SnO2 nanobelts, used as single crystalline substrates, through electron irradiation by transmission electron microscopy. Characterization by high resolution transmission electron microscopy indicates that grain growth occurs in the solid state by a two-step process consisting of grain rotation followed by coalescence. The grain rotation is likely induced by the thermal energy provided by the electron beam. Coalescence or oriented attachment occurs preferentially on particles with similar crystallographic orientations.
Effect of the Ar–Ni(s) potential on the cross section for Ar+CH4/Ni{111} collision-induced desorption and the need for a more accurate CH4/Ni{111} potential
Authors: Sun Lipeng , Peterson Kirk A. , Alexeev Yuri , Windus Theresa , Kindt James , Hase William L.
Published: January 2005

In a previous paper [L. Sun, P. de Sainte Claire, O. Meroueh, and W. L Hase, J. Chem. Phys. 114, 535 (2001)], a classical trajectory simulation was reported of CH4 desorption from Ni{111} by Ar-atom collisions. At an incident angle θi of 60° (with respect to the surface normal), the calculated collision-induced desorption (CID) cross sections are in excellent agreement with experiment. However, for smaller incident angles the calculated cross sections are larger than the experimental values and for normal collisions, θi=0°, the calculated cross sections are approximately a factor of 2 larger. This trajectory study used an analytic function for the Ar+Ni(s) intermolecular potential which gives an Ar–Ni{111} potential energy minimum which is an order of magnitude too deep. In the work reported here, the previous trajectory study is repeated with an Ar+Ni(s) analytic intermolecular potential which gives an accurate Ar–Ni{111} potential energy minimum and also has a different surface corrugation than the previous potential. Though there are significant differences between the two Ar+Ni(s) analytic potentials, they have no important effects on the CID dynamics and the cross sections reported here are nearly identical to the previous values. Zero-point energy motions of the surface and the CH4–Ni(s) intermolecular modes are considered in the simulation and they are found to have a negligible effect on the CID cross sections. Calculations of the intermolecular potential between CH4 and a Ni atom, at various levels of theory, suggest that there are substantial approximations in the ab initio calculation used to develop the CH4+Ni{111} potential. The implication is that the differences between the trajectory and experimental CID cross sections may arise from an inaccurate CH4+Ni{111} potential used in the trajectory simulation.
Magnetic properties of bulk Zn1−xMnxO and Zn1−xCoxO single crystals
Authors: Kane M. H. , Shalini K. , Summers C. J. , Varatharajan R. , Nause J. , Vestal C. R. , Zhang Z. J. , Ferguson I. T.
Published: January 2005

Manganese and cobalt-doped ZnO have been produced using a modified melt-growth technique. X-ray diffraction measurements indicate that the samples are high-quality single crystals with ω−2θ full width at half maximum values of 78 arc sec for the undoped ZnO and 252 arc sec for Zn1−xMnxO (x=0.05). The lattice parameter of the Zn1−xMnxO was observed to increase with Mn concentration. Transmission measurements showed systematic variations dominated by absorption from interatomic Mn2+ and Co2+ transitions. No evidence of diluted magnetic semiconductor mean-field ferromagnetic behavior was observed in any of these nominally noncarrier-doped samples. The magnetic properties instead showed paramagnetic behavior for Zn1−xMnxO dominated by an antiferromagnetic Mn–Mn exchange interaction at low temperatures. Zn1−xCoxO showed hysteresis that was attributed to superparamagnetic Co clusters embedded in a diamagnetic ZnO matrix. It has been shown that in the bulk single-crystal form, intrinsic and noncarrier-doped Zn1−xTMxO is not ferromagnetic; thus creative processing and doping techniques are necessary to achieve practical ferromagnetism in these materials.
Ion-beam luminescence of thin-film CdS on glass formed by pulsed-laser deposition
Authors: Khanlary M. , Townsend P. , Ullrich B. , Hole D. E.
Published: January 2005

CdS thin films on glass have been produced with infrared pulsed-laser deposition. Three emission bands, in the green, yellow, and red spectral regions were observed in ion-beam luminescence (IBL) spectra of these as-deposited films. The temperature dependence of the IBL spectra of the films was investigated. The variations of the spectra with various species of implanted ions were also studied. It was found that for light ions (H+ or H2+) the intensity of all the luminescence bands was increased by decreasing the sample temperature. The relative intensities of the peaks changed with temperature so that the overall peak shapes appear to move as the temperature decreased. Comments on the origin of the signals are included.
Vacancy-type defects in strained-Si layers deposited on SiGe∕Si structures probed by using monoenergetic positron beams
Authors: Uedono Akira , Hattori Nobuyoshi , Naruoka Hideki , Ishibashi Shoji , Suzuki Ryoichi , Ohdaira Toshiyuki
Published: January 2005

Vacancy-type defects in strained-Si layers deposited on Si0.75Ge0.25∕graded-SiGe∕Si structures were probed by using monoenergetic positron beams. The Doppler broadening spectra of the annihilation radiation and the lifetime spectra of the positrons were measured for samples before and after annealing (800–1050 °C). For an as-received sample, the defects in the strained-Si layer were identified as vacancy-type defects coupled with Ge. The mean open size of these defects was estimated to be close to that of a divacancy. The line-shape parameter, S, corresponding to the positron annihilation in the strained-Si layers decreased with increasing annealing temperature, but no large change in the positron lifetime was observed. From a comparison between the Doppler broadening profiles for the strained-Si films and those calculated using the projector augmented-wave method, it was found that the number of Ge atoms forming a complex by coupling with a defect increased with increasing annealing temperature. The number was estimated to be three or four after annealing at 1050 °C. Since the defect complexes were stable even after annealing at such a high temperature, the defects detected by the positrons could be part of chainlike vacancy clusters.
Effect of oblique incidence on the spectral response of quantum well infrared photodetectors
Authors: Ribet-Mohamed I. , Guérineau N. , Suffis-Carretero S. , Tauvy M. , De Rossi A. , Costard E. , Bois P.
Published: January 2005

A full understanding of the effect of oblique incidence on the response of the pixels of a focal plane array is required for calibrated radiometric imaging. Measuring the angular response of infrared detectors requires an extremely complex experimental setup. This paper presents two complementary experimental setups that enable us to study the effect of the oblique incidence of light on the response of cooled infrared phototdetectors based on different technologies. The angular response measurement setup provides spectrally integrated measurements over a wide angular range (−80°–+80°) with a low background level and a low stray light. In contrast, the spectral response measurement setup provides the spectrally resolved response for incidence angles lower than 30°. Both experimental setups were used to characterize single-quantum well infrared photodetectors with crossed gratings. Relatively large pixel arrays (100×100μm2) were used to reduce finite-size effects. However, the comparison of our experimental spectral responses with calculations based on the modal expansion method, which assumes a grating of infinite extension, reveals discrepancies at high wavelengths. In order to determine if these discrepancies are due to finite pixel size effect, a more complicated model based on a finite element code is used. These calculations are in far better agreement with the experimental results and allow us to identify the effect of finite pixel size.
Optical and structural properties of siliconlike films prepared by plasma-enhanced chemical-vapor deposition
Authors: Cremona A. , Laguardia L. , Vassallo E. , Ambrosone G. , Coscia U. , Orsini F. , Poletti G.
Published: January 2005

Amorphous siliconlike thin films (Si:Ox:Cy:Hz), deposited by plasma-enhanced chemical-vapor deposition using hexamethyldisiloxane as monomer and Ar as feed gas, have been investigated for their optical and structural properties as a function of the deposition power, in the range of 100–400 W. The films have been analyzed by Fourier transform infrared spectroscopy (FTIRS), UV-VIS-NIR spectrophotometry, and atomic force Microscopy (AFM). From the analysis of FTIR spectra it results that films assume a marked inorganic character as the power increases. Indeed, at high power, Si–O–Si groups prevail over Si(CH3)x groups, so that the film chemistry approaches the SiO2 one. Reflectance∕transmittance spectra, acquired in the range of 200–2500 nm, allow us to describe the film absorption edge for interband transitions. The relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition power has been investigated. The reduction of the optical energy gap from 3.86 to 3.61 eV and the broadening of the optical-absorption tail with the increase of power from 100 to 400 W are ascribed to the growth of thermal and structural disorders. Moreover, the refractive index has been evaluated and related to the film morphology. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become flatter and more compact when power increases. We consider the densification of the film responsible for the growth of the refractive index from 1.90 to 1.97 in the power range 100–400 W.
Voltage-induced broad-spectrum reflectivity change with surface-plasmon waves
Authors: Wang Yu , Russell Stephen D. , Shimabukuro Randy L.
Published: January 2005

Voltage-induced broad-spectrum reflectivity change with surface-plasmon waves is reported. When white light is incident at a metal/electro-optical material interface, surface-plasmon waves can be excited under phase match conditions. This surface-plasmon resonance depends on the dielectric constants of both the metal and the electro-optical material. Photons in the surface-plasmon resonance wavelength range are absorbed by the interface. Since metals have large imaginary parts of their dielectric constants, the surface-plasmon resonances are broad and may cover all visible wavelengths. Applying voltage to the electro-optical material to change its dielectric constant can result in a change in the reflectivity at the interface. Experimental results showed a reflectivity change from almost 0% to about 40% under an applied voltage using a liquid-crystal and nickel film structure, and the results had good agreement with theoretical calculations. The theoretical calculations also predicted a 90% reflectivity recovery by exciting surface-plasmon waves at a Rh–Al/electro-optical material interface. These results demonstrate that a high efficiency white light modulator can be built using surface-plasmon excitations.
Cluster formation in two-Yukawa fluids
Authors: Liu Yun , Chen Wei-Ren , Chen Sow-Hsin
Published: January 2005

We present a different and efficient method for implementing the analytical solution of Ornstein–Zernike equation for two-Yukawa fluids in the mean spherical approximation. We investigate, in particular, the conditions for the formation of an extra low-Q peak in the structure factor, which we interpret as due to cluster formation in the two-Yukawa fluid when the interparticle potential is composed of a short-range attraction and a long-range repulsion. We then apply this model to interpret the small angle neutron scattering data for protein solutions at moderate concentrations and find out that the presence of a peak centered at Q=0 (zero-Q peak) besides the regular interaction peak due to charged proteins implies an existence of long-range attractive interactions besides the charge repulsion.
Structure and energetics of two- and three-dimensional neutral, cationic, and anionic gold clusters Au5⩽n⩽9Z (Z=0,±1)
Authors: Remacle F. , Kryachko E. S.
Published: January 2005

Low-energy structures are found on the potential energy surfaces of the neutral, cationic, and anionic gold clusters Au5⩽n⩽8Z(Z=0,±1) and on the neutral potential energy surface of Au9. These structures provide insights on the two to three dimensional (2D⇒3D) transition in small neutral and charged gold clusters. It is demonstrated that the size threshold for the 2D-3D coexistence is lower for cationic than neutral gold clusters: the 2D-3D coexistence develops for Au5+ and Au7+ on the cationic potential energy surfaces while only for Au9 on the neutral. Two metastable long-lived dianions of gold clusters are also reported.
Optical properties of a nanoporous array in silicon
Authors: Tian L. , Ram K. Bhargava , Ahmad I. , Menon L. , Holtz M.
Published: January 2005

We demonstrate an approach for producing an array of nanopores on a silicon surface. The methods used combine nonlithographic pattern transfer and chlorine plasma etching to produce ∼60nm diam holes up to 1μm in depth. The near-normal specular optical reflectance of these systematically modified surfaces is found to decrease dramatically with pore depth across the entire 2.0–6.0eV photon energy range studied. We adapt an effective medium approximation to model specular reflectance taking into account diffuse scattering by the nanopatterned surface. Micro-Raman measurements show a systematic intensity increase with pore depth. The observed dependence suggests that both insertion and extraction are enhanced by the nanopatterning.
Effects of post–deposition annealing on the material characteristics of ultrathin HfO2 films on silicon
Authors: Puthenkovilakam R. , Lin Y. -S. , Choi J. , Lu J. , Blom H.-O. , Pianetta P. , Devine D. , Sendler M. , Chang J. P.
Published: January 2005

High quality HfO2 films were deposited on p-type Si(100) wafers by an atomic layer deposition scheme. The deposited films were smooth, amorphous, and stoichiometric, as determined by atomic force microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy, respectively. The as-deposited films showed a very small interfacial layer between the HfO2 and silicon. The index of refraction of the film was determined to be slightly smaller than that of bulk HfO2 by spectroscopic ellipsometry. The films annealed in O2 and NH3 at 800°C showed a growth of the interfacial layer, which resembles a SiO2-rich dielectric layer. Short-range order in the as-deposited films was determined to be monoclinic by the extended x-ray absorption fine structure measurements, and signs of crystallization were observed in the O2 annealed sample. The films appeared to be polycrystalline upon high temperature (800°C) annealing as confirmed by high-resolution transmission electron microscopy. Annealing in the forming gas (450°C) resulted in a more substantial growth of the interfacial layer, though the film remained amorphous.
Correlation between Y2O3 concentration and photoluminescence excitation spectra of Tb3+ impurity in Y2O3-stabilized ZrO2
Authors: Nakajima Hiromitsu , Mori Toshiyuki
Published: January 2005

Room temperature photoluminescence (PL) spectra from Tb ions included as unintentional impurities in yttria-stabilized zirconia (YSZ) were measured at various concentrations of Y2O3. Two PL excitation bands were observed from the Tb at around 380 nm for all of the YSZ samples. These two bands can be attributed to the D35-F67 transition of Tb3+. We found that the energy separation between the two Tb bands decreased as the concentration of Y2O3 increased. This relationship can be explained by considering the oxygen vacancy concentration and the effective charge in the YSZ, and we found that the effective charge in YSZ can be evaluated from the energy separation. The relative strength of the crystal field in the YSZ samples could also be estimated from the energy separation. Furthermore, the phase boundary of the YSZ could be determined from the change in slope of the plots of energy separation against Y2O3 concentration.
Design and characteristics of strained InAs/InAlAs composite-channel heterostructure field-effect transistors
Authors: Lin H.-K. , Kadow C. , Bae J.-U. , Rodwell M. J. W. , Gossard A. C. , Brar B. , Sullivan G. , Nagy G. , Bergman J.
Published: January 2005

We report composite-channel heterostructure field-effect transistors (HFETs) with an InAs channel and an In0.9Al0.1As subchannel. The HFETs are grown on antimonide buffer layers. Two composite-channel structures with different planar Te doping schemes are designed, fabricated, and characterized. High radio-frequency transconductances of above 0.9 S/mm and ∼55GHz current gain cutoff frequencies are achieved in devices with 500 nm gates. Planar Te doping in the buffer layers reduces the high kink-effect currents otherwise found in InAs/AlSb HFETs, an effect which can be attributed to either increased breakdown field in the In0.9Al0.1As subchannel or to suppression of hole blocking in the buffer. The present limitations to device performance and suggested approaches for their elimination are discussed.
Monolayer growth modes of Re and Nb on 4H–SiC(0001) and 4H–SiC(0001¯)
Authors: Bryant K. W. , Bozack M. J.
Published: January 2005

Auger electron spectroscopy (AES) and secondary electron emission have been used to specify the room temperature monolayer adsorption characteristics of Re and Nb on the basal, polar surfaces of 4H–SiC. Measurement of the secondary electron emission current relates the absorbed crystal current to the change in AES peak-to-peak signal intensities for both substrate and adsorbate. For the 4H–SiC(0001) Si-face surface, both metals grow by formation of a single monolayer followed by growth of simultaneous monolayers. For the 4H–SiC(0001¯) C-face surface, both metals grow layer by layer (Frank–van der Merwe). Measurements of the coefficient of attenuation, inelastic mean free path, and coverage versus Auger intensities are also reported.
Scattering of g-process longitudinal optical phonons at hotspots in silicon
Authors: Sinha S. , Schelling P. K. , Phillpot S. R. , Goodson K. E.
Published: January 2005

Transistors with gate lengths below 100 nm generate phonon hotspots with dimensions on the order of 10 nm and peak power densities of about 50W∕μm3. This work employs molecular dynamics to investigate the impact of lattice energy density on phonon scattering at the hotspot. The hotspot studied in this work consists of longitudinal optical phonons involved in the g-type intervalley scattering of conduction electrons in silicon. A comparison of the decay modes in hotspots with high and moderate energy densities reveals that the decay mechanisms are the same but the relaxation rates differ. Scattering occurs through a three phonon process of the form LO→LA+TA, involving the zone-edge transverse acoustic modes. An increase in the energy density from a moderate value of 5 to 125W∕μm3 changes the relaxation time from 79 to 16 ps, approximately proportional to the the maximum initial amplitude of the phonons. This work improves the accuracy of the scattering rates of optical phonons and helps in advancing the electro-thermal modeling of nanotransistors.
Influence of the dipolar interactions in the magnetization reversal asymmetry of hard–soft magnetic ribbons
Authors: Rivas M. , García J. A. , Tejedor M. , Bertrán E. , Céspedes J. G.
Published: January 2005

Partial crystallization of the metallic glass Co66Si16B12Fe4Mo2 was performed by annealing at temperatures between 500 and 540 °C for 10–20 min, resulting in crystallite volume fractions of (0.7–5)×10−3 and sizes of 50–100 nm. This two-phase alloy presents a remarkable feature: a hysteresis loop shift that can be tailored by simply premagnetizing the sample in the adequate magnetic field. Shifts as large as five times the coercive field have been obtained which make them interesting for application as magnetic cores in dc pulse transformers. The asymetrical magnetic reversal is explained in terms of the magnetic dipolar field interaction and the observed hysteresis loops have been satisfactorily simulated by a modification of Stoner-Wohlfarth’s model of coherent rotations.
Theory of the Bloch oscillating transistor
Authors: Hassel J. , Seppä H.
Published: January 2005

The Bloch oscillating transistor (BOT) is a device in which single electron current through a normal tunnel junction enhances Cooper pair current in a mesoscopic Josephson junction, leading to signal amplification. In this article we develop a theory in which the BOT dynamics is described as a two-level system. The theory is used to predict current–voltage characteristics and small-signal response. The transition from stable operation into the hysteretic regime is studied. By identifying the two-level switching noise as the main source of fluctuations, the expressions for equivalent noise sources and the noise temperature are derived. The validity of the model is tested by comparing the results with simulations and experiments.
Path integral evaluation of the quantum instanton rate constant for proton transfer in a polar solvent
Authors: Yamamoto Takeshi , Miller William H.
Published: January 2005

The quantum instanton approximation for thermal rate constants, a type of quantum transition state theory (QTST), is applied to a model proton transfer reaction in liquid methyl chloride developed by Azzouz and Borgis. Monte Carlo path integral methods are used to carry out the calculations, and two other closely related QTST’s, namely, the centroid-density and Hansen–Andersen QTST, are also evaluated for comparison using the present path integral approach. A technique is then introduced that calculates the kinetic isotope effect directly via thermodynamic integration of the rate with respect to hydrogen mass, which has the practical advantage of avoiding costly evaluation of the activation free energy. The present application to the Azzouz–Borgis problem shows that the above three types of QTST provide very similar results for the rate, within 30% of each other, which is nontrivial considering the totally different derivations of these QTSTs; the latter rates are also in reasonable agreement with some other previous results (e.g., obtained via molecular dynamics with quantum transitions), within a factor of ∼2(7) for the H(D) transfer, thus significantly diminishing the possible range of the exact rates. In addition, it is revealed that a small but nonnegligible inconsistency exists in the parametrization of the Azzouz-Borgis model employed in previous studies, which resulted in the large apparent discrepancy in the calculated rates.
Photopyroelectric measurement of thermal conductivity of metallic powders
Authors: Rombouts M. , Froyen L. , Gusarov A. V. , Bentefour E. H. , Glorieux C.
Published: January 2005

The effective thermal diffusivity of metal powders in air at room temperature is measured by the photopyroelectric technique. The thermal conductivity is calculated from the diffusivity, the relative density, and the specific heat obtained from literature. Maxwell’s model is a good prediction but underestimates the measured effective thermal conductivity, especially for irregular particles. Due to the large difference between the thermal conductivity of metals and air, the effective conductivity is mainly determined by the relative density of the powder bed but not by the properties of the powder material. A theoretical model showing the influence of grain size and gas pressure is presented. The dependence on the particles’ size and pressure is explained by the gradual transition from the free molecular to conductive mechanism of heat transfer in gaps between particles. The theory gives a precise estimation of effective thermal conductivity for metallic powders with a narrow size distribution of spherical particles and it underestimates this value in the case of a wide size distribution or in the case of irregular particles.
Nanoporous Au: A high yield strength material
Authors: Biener Juergen , Hodge Andrea M. , Hamza Alex V. , Hsiung Luke M. , Satcher Joe H.
Published: January 2005

The plastic deformation of nanoporous Au under compressive stress was studied by depth-sensing nanoindentation combined with scanning electron microscope characterization. The nanoporous Au investigated in the current study exhibits a relative density of 42%, and a spongelike morphology of interconnecting ligaments on a length scale of ∼100nm. The material is polycrystalline with a grain size on the order of 10–60nm. Microstructural characterization of residual indentation impressions reveals a localized densification via ductile (plastic) deformation under compressive stress and demonstrates the ductile behavior of Au ligaments. A mean hardness of 145(±11)MPa and a Young’s modulus of 11.1(±0.9)GPa was obtained from the analysis of the load-displacement curves. The hardness of investigated np-Au is ∼10 times higher than the hardness predicted by scaling laws of open-cell foams thus potentially opening a door to a class of high yield strength—low-density materials.
Polarity-dependent forming in ion bombarded amorphous silicon memory devices
Authors: Gateru R. G. , Orwa J. O. , Shannon J. M.
Published: January 2005

Polarity-dependent forming in ion bombarded metal–semiconductor–metal (MSM) memory devices of hydrogenated amorphous silicon is reported. It is shown that prior to ion bombardment, current transport in the MSM devices is asymmetric and is controlled by the Schottky barriers at two MS junctions. Upon bombardment, however, there is a bulk component to the current and the I–V characteristics of the devices become symmetric at low bias voltages. The forming voltage in the bombarded devices shows polarity dependence. For positive bias applied on the top contact, we find that devices form at the same electric field independent of the thickness of the amorphous silicon while for negative voltage on the top contact, the electric field needed for forming increases with the thickness. A model involving the difference in energy deposition and heat sinking for the two polarities is proposed.
Diamondlike carbon film deposition on a polycarbonate-tube inner wall using a cylindrical electrode with radio frequency glow discharge plasma
Authors: Ohgoe Yasuharu , Hirakuri Kenji K.
Published: January 2005

Radio frequency (rf) plasma chemical vapor deposition (CVD) process is very useful for film deposition. However, common rf plasma CVD techniques using planar electrodes make it difficult to apply to three-dimensional insulator structures. In this study, to deposit diamondlike carbon (DLC) films on a polycarbonate-tube inner wall (ϕ 11mm, length 100mm), we have developed the cylindrical electrode plasma process. This process could be adapted to cylindrical substrates such as polymeric tubes. In investigating the availability of this process, under helium (He) gas pressures (50, 100, and 150Pa), the plasma states of the rf power at 30W were measured using a double-probe method. The He plasma was generated bright as a stable, hollow cathode discharge inside the polycarbonate tube. As a result, the cylindrical electrode process was expected to have applications for cylindrical materials such as polymeric tubes. In actual deposition of DLC film (CH4 gas at 10Pa, rf power at 30W, and deposition time at 10min), the DLC film was deposited completely on the polycarbonate-tube inner wall. The distribution of the film thickness was 300–350nm, and the structures of the DLC film were uniform. It was observed that the DLC film was deposited uniformly. The cylindrical electrode process is expected to apply to three-dimensional insulator structures such as polymeric tubes.
Enhancement of electron mobility in nanocrystalline silicon∕crystalline silicon heterostructures
Authors: Chen X. Y. , Shen W. Z. , He Y. L.
Published: January 2005

We report on an effective way to obtain high electron mobility (∼103cm2∕Vs) in lowly doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films by constructing nc-Si:H∕crystalline Si (c-Si) heterostructures. The enhancement has been demonstrated in a comparative study on nc-Si:H thin films grown on p- and n-type c-Si, as well as insulating glass substrates through temperature- and magnetic-field-dependent Hall-effect measurements. The effect has been attributed to the ordered structure and narrow boundaries between the nanograins, with the help of microstructure pictures from high-resolution transmission electron microscopy. In addition to the detailed individual transport information of carriers in the nc-Si:H systems, we observe clear evidence for the formation of a two-dimensional electron gas at the nc-Si:H∕c-Si interfaces. These results are discussed within the framework of the Boltzmann theory and the quantum interference theory.
Ab initio calculations, potential representation and vibrational dynamics of He2Br2 van der Waals complex
Authors: Valdés Álvaro , Prosmiti Rita , Villarreal Pablo , Delgado-Barrio Gerardo
Published: January 2005

An intermolecular potential energy surface for He2Br2 complex in the ground state is calculated at the levels of fourth-order (MP4) Møller–Plesset and coupled-cluster [CCSD(T)] approximations, using large-core pseudopotential for Br atoms and the aug-cc-pV5Z basis set for He. The surface is characterized by three minima and the minimum energy pathways through them. The global minimum corresponds to a linear He–Br2–He configuration, while the two other ones to “police-nightstick” and tetrahedral structures. The corresponding well depths are −90.39/−89.18, −81.23/−80.78 and −74.40/−74.02 cm−1, respectively, at MP4/CCSD(T) levels of theory. It is found that results obtained by summing three-body parametrized HeBr2 interactions and the He–He interaction are in very good accord with the corresponding MP4/CSSD(T) configuration energies of the He2Br2. Variational calculations using a sum of three-body interactions are presented to study the bound states of the vdW He2Br2 complex. The binding energy D0 and the corresponding vibrationally averaged structure are determined for different isomers of the cluster and their comparison with the available experimental data is discussed.
Vibrational overtone spectroscopy of jet-cooled methanol from 5000 to 14 000 cm−1
Authors: Rueda David , Boyarkin Oleg V. , Rizzo Thomas R. , Chirokolava Andrei , Perry David S.
Published: January 2005

Spectra of jet-cooled methanol in the overtone and combination region from 5000 to 14 000 cm−1 have been obtained by means of infrared laser-assisted photofragment spectroscopy. Many of the observed features are assigned to combination bands of the type nν1+ν6, nν1+ν8, and nν1+ν6+ν8 (n=1,2,3), where ν1 is the OH stretch, ν6 is the OH bend, and ν8 is the CO stretch. These bands show sharp torsion-rotation structure with features as narrow as 0.1 cm−1. We also observe CH stretch overtones that are weaker than the OH containing combination bands and lack distinct torsion-rotation structure above vCH=2. The extent of observed structure on these bands allows us to place limits on the intramolecular vibrational energy redistribution decay rates in the upper vibrational states. We report a global fit of the observed band centers to a simple expression involving low-order anharmonicity constants.
Pressure effect on the magnetization of Sr2FeMoO6 thin films grown by pulsed laser deposition
Authors: Fix T. , Versini G. , Loison J. L. , Colis S. , Schmerber G. , Pourroy G. , Dinia A.
Published: January 2005

Thin films of Sr2FeMoO6 (SFMO) are grown on SrTiO3 (001) substrates by pulsed laser deposition. The best films provide 3.2μB∕f.u. at 5K, a Curie temperature above 400K, low roughness, high crystallinity, and low splashing. Therefore, the use of such SFMO electrodes in magnetic tunnel junctions patterned with conventional lithography is promising. Pseudomorphic epitaxial growth is obtained for thicknesses under 50nm. Above this thickness the films do not relax homogeneously. A coherent and systematic variation of the magnetization with the deposition conditions is obtained, which highlights a high reproducibility. Under a reasonable O2 partial pressure to avoid parasite phases, the limiting factor for high magnetization is the total pressure or the deposition rate. Therefore, the deposition rate is suspected to have a strong influence on the Fe∕Mo ordering. Highly magnetic samples are obtained under a low gas flow of either a 20% O2+N2 or a 0.3% O2+Ar.
Extended wavelength emission to 1.3μm in nitrided InAs∕GaAs self-assembled quantum dots
Authors: Kita T. , Mori T. , Seki H. , Matsushita M. , Kikuno M. , Wada O. , Ebe H. , Sugawara M. , Arakawa Y. , Nakata Y.
Published: January 2005

We have studied long-wavelength emission in 1.3-μm optical communication range from nitrided InAs∕GaAs quantum dots (QDs). Atomic-layer nitridation just after the growth of InAs QDs realizes a remarkable redshift of the photoluminescence peak by more than 150nm. Growth temperature plays a key role in achieving room-temperature emission from the QDs. The emission wavelength can be adjusted by controlling the growth conditions of the initial InAs QDs. As compared with conventional QDs grown without nitridizing the QD surface, it is found that the nitridation results in larger dots with a large aspect ratio.
Optical absorption in semiconductor nanorings under a lateral terahertz electric field
Authors: Zhang T. Y. , Cao J. C.
Published: January 2005

The optical absorption of GaAs nanorings under dc and ac electric fields with frequency at the terahertz range applied in the plane containing the nanoring is investigated. The excitonic effects are essential to describe even qualitatively the absorption spectra. The applied lateral electric fields change the optical absorption spectra prominently. In the presence of terahertz electric fields, the optical absorption spectra may be very different for different frequencies. In certain frequencies, there arise other absorption peaks in the below-band-gap spectrum.
Photoelectron spectroscopic investigation of in-vacuum-prepared luminescent polymer thin films directly from solution
Authors: Dam N. , Beerbom M. M. , Braunagel J. C. , Schlaf R.
Published: January 2005

Thin films of the luminescent polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were deposited in high vacuum directly from toluene solution on Ag substrates using a homemade electrospray (ES) deposition system. The films were deposited in multiple steps without breaking the vacuum and characterized in situ using photoemission spectroscopy. The x-ray photoemission spectroscopy measurements indicate that the deposited layers are essentially contamination free and that subsequent depositions can be performed using the ES system without dissolving the previously deposited layers. Additional ultraviolet photoemission spectroscopy measurements showed the development of the highest occupied molecular-orbital structure as the MEH-PPV layer increased in thickness. This allowed the determination of the charge injection barriers (orbital alignment) at the Ag∕MEH-PPV interface.
Adsorption dynamics of CO2 on Zn-ZnO(0001): A molecular beam study
Authors: Wang J. , Burghaus U.
Published: January 2005

Presented are initial S0 and coverage Θ dependent, S(Θ), adsorption probability measurements, respectively, of CO2 adsorption on the polar Zn-terminated surface of ZnO, parametric in the impact energy Ei, the surface temperature Ts, the impact angle αi, varied along the [001] azimuth, the CO2 flux, and the density of defects, χ(Ar+), as varied by rare gas ion sputtering. S0 decreases linearly from 0.72 to 0.25 within Ei=0.12–1.33 eV and is independent of Ts. Above Ei=0.56 eV, S0 decreases by ∼0.2 with increasing αi. The shape of S(Θ) curves is consistent with precursor-mediated adsorption (Kisliuk shape, i.e., S∼const) for low Ei; above Ei=0.56 eV, however, a turnover to adsorbate-assisted adsorption (S increases with Θ) has been observed. The initial slope of S(Θ) curves decreases thereby with increasing αi, χ(Ar+), and Ts, i.e., the adsorbate-assisted adsorption is most distinct for normal impact on the pristine surface at low Ts and is independent of the CO2 flux. The S(Θ) curves have been parametrized by analytic precursor models and Monte Carlo simulations have been conducted as well. The temperature dependence of the saturation coverage shows two structures which could be assigned to adsorption on pristine and intrinsic defect sites, respectively, in agreement with a prior thermal desorption spectroscopy study. The heat of adsorption Ed for the pristine sites amounts to 34.0–5.4Θ, whereas for adsorption on the intrinsic defect sites Ed of ∼43.6 kJ/mol could be estimated. Thus, a kinetic structure-activity relationship was present.
A surface x-ray study of the structure and morphology of the oxidized Pd(001) surface
Authors: Stierle A. , Kasper N. , Dosch H. , Lundgren E. , Gustafson J. , Mikkelsen A. , Andersen J. N.
Published: January 2005

The oxidation of Pd(100) and the formation of PdO was studied in situ using surface x-ray diffraction. A bulklike, epitaxial PdO film is formed at oxygen partial pressures beyond 1 mbar and sample temperatures exceeding 650 K. The main orientation is PdO(001)/Pd(001), based upon bulk reflections from the PdO film. By comparing with measurements from the Pd crystal truncation rods, we estimate an rms surface roughness of 6 Å, in good agreement with previous high pressure scanning tunneling microscopy measurements. Finally, we observed the transformation from the (√5×√5) surface oxide to PdO bulk oxide at 675 K and 50 mbar O2 pressure.
Ab initio potential energy and dipole moment surfaces for H5O2+
Authors: Huang Xinchuan , Braams Bastiaan J. , Bowman Joel M.
Published: January 2005

Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H5O2+. Tens of thousands of coupled-cluster [CCSD(T)] and second-order Møller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H5O2+ PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm−1 for the entire data set up to 110 000 cm−1. This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H2O+H3O+, with De=11 923.8 cm−1. Geometrical and vibrational properties of the monomer fragments are presented. The corresponding global DMS fit (MP2 based) involves 3844 polynomial coefficients and also dissociates correctly.
Origin of the magic numbers of water clusters with an excess electron
Authors: Lee Han Myoung , Suh Seung Bum , Tarakeshwar P. , Kim Kwang S.
Published: January 2005

Electron-bound water clusters [e−(H2O)n] show very strong peaks in mass spectra for n=2, 6, 7, and (11), which are called magic numbers. The origin of the magic numbers has been an enigma for the last two decades. Although the magic numbers have often been conjectured to arise from the intrinsic properties of electron-bound water clusters, we attributed them not to their intrinsic properties but to the particularly weak stability of the corresponding neutral water clusters (H2O)n=2,6,7, and (11). As the cluster size increases; this nonsmooth characteristic feature in stability of neutral water clusters is contrasted to the smooth increase in stability of e−-water clusters. As the magic number clusters have significant positive adiabatic electron affinities, their abundant distributions in atmosphere could play a significant role in atmospheric thermodynamics.
Polarizabilities and hyperpolarizabilities for the atoms Al, Si, P, S, Cl, and Ar: Coupled cluster calculations
Authors: Lupinetti Concetta , Thakkar Ajit J.
Published: January 2005

Accurate static dipole polarizabilities and hyperpolarizabilities are calculated for the ground states of the Al, Si, P, S, Cl, and Ar atoms. The finite-field computations use energies obtained with various ab initio methods including Møller–Plesset perturbation theory and the coupled cluster approach. Excellent agreement with experiment is found for argon. The experimental ᾱ for Al is likely to be in error. Only limited comparisons are possible for the other atoms because hyperpolarizabilities have not been reported previously for most of these atoms. Our recommended values of the mean dipole polarizability (in the order Al–Ar) are ᾱ/e2a02Eh−1=57.74, 37.17, 24.93, 19.37, 14.57, and 11.085 with an error estimate of ±0.5%. The recommended values of the mean second dipole hyperpolarizability (in the order Al–Ar) are γ̄/e4a04Eh−3=2.02×105, 4.31×104, 1.14×104, 6.51×103, 2.73×103, and 1.18×103 with an error estimate of ±2%. Our recommended polarizability anisotropy values are Δα/e2a02Eh−1=−25.60, 8.41, −3.63, and 1.71 for Al, Si, S, and Cl respectively, with an error estimate of ±1%. The recommended hyperpolarizability anisotropies are Δγ/e4a04Eh−3=−3.88×105, 4.16×104, −7.00×103, and 1.65×103 for Al, Si, S, and Cl, respectively, with an error estimate of ±4%.
Improving the orbital-free density functional theory description of covalent materials
Authors: Zhou Baojing , Ligneres Vincent L. , Carter Emily A.
Published: January 2005

The essential challenge in orbital-free density functional theory (OF-DFT) is to construct accurate kinetic energy density functionals (KEDFs) with general applicability (i.e., transferability). During the last decade, several linear-response (LR)-based KEDFs have been proposed. Among them, the Wang-Govind-Carter (WGC) KEDF, containing a density-dependent response kernel, is one of the most accurate that still affords a linear scaling algorithm. For nearly-free-electron-like metals such as Al and its alloys, OF-DFT employing the WGC KEDF produces bulk properties in good agreement with orbital-based Kohn-Sham (KS) DFT predictions. However, when OF-DFT, using the WGC KEDF combined with a recently proposed bulk-derived local pseudopotential (BLPS), was applied to semiconducting and metallic phases of Si, problems arose with convergence of the self-consistent density and energy, leading to poor results. Here we provide evidence that the convergence problem is very likely caused by the use of a truncated Taylor series expansion of the WGC response kernel. Moreover, we show that a defect in the ansatz for the first-order reduced density matrix underlying the LR KEDFs limits the accuracy of these KEDFs. By optimizing the two free parameters involved in the WGC KEDF, the two-body Fermi wave vector mixing parameter γ and the reference density ρ* used in the Taylor expansion, OF-DFT calculations with the BLPS can achieve semiquantitative results for nine phases of bulk silicon. These new parameters are recommended whenever the WGC KEDF is used to study nonmetallic systems.
A theoretical study on the inner-valence photoelectron spectra lying between 21 and 26 eV of the O2 molecule
Authors: Takeshita Kouichi , Sadamatu Yosiki , Tanaka Kiyoshi
Published: January 2005

Ab initio calculations were carried out to study the structures of the photoelectron spectra of the O2 molecule using complete active space self-consistent field and multireference single and double excitation configuration interaction methods. We focused our attention primarily on the inner-valence ionized states lying below 26 eV. Vibrational levels were calculated using adiabatic potential curves and vibrational wave functions were used to obtain Franck–Condon factors. We show that not only discrete vibrational levels of (3) 2Πu, but also the continuum of nuclear motion in (1) 2Σu− contribute to the bands of vibrational progression and the broad peak of the continuum between 22.5 and 26 eV. Other states with weak intensity lying between 19 and 26 eV are also discussed.
A femtosecond study of the infrared-driven cis-trans isomerization of nitrous acid (HONO)
Authors: Schanz Roland , Boţan Virgiliu , Hamm Peter
Published: January 2005

We investigate the dynamics and mechanism of the IR-driven cis-trans isomerization of nitrous acid (HONO) in a low-temperature krypton matrix applying ultrafast time resolved IR spectroscopy. After excitation of the OH-stretching mode the trans HONO state decays biexponentially on a 8 and 260 ps time scale. The initially excited cis HONO state decays on a 20 ps time scale. Cis HONO isomerizes with 10% quantum yield on a 20 ps time scale to trans HONO. The quantum yield we observe is significantly smaller than the previously reported 100%, which could imply that additional, much slower reaction channels exist. We furthermore developed a four-dimensional model of the system, which includes the three proton intramolecular degrees of freedom of HONO fully quantum mechanically and one intermolecular translational degree of freedom of the molecule in the crystal cage. We find that cis-trans isomerization necessarily is accompanied by a translation of the molecule as a whole in the crystal cage. The translational degree of freedom tunes the intramolecular proton states of HONO with respect to each other. When resonances occur, the proton states might couple and transfer population. We suggest a possible reaction pathway, where the cis OH-stretch excited state first couples to a high cis torsional mode, which then may transfer almost instantaneously to the trans side. The model qualitatively explains all experimental observations.
Electron drift and the loss balance of charged particles in planar-unbalanced dc magnetron discharge
Authors: Seo Sang-Hun , In Jung-Hwan , Chang Hong-Young
Published: January 2005

The electron drift phenomenon is investigated in the downstream region of an unbalanced dc magnetron argon discharge. The spatially resolved measurements of the electron velocity distribution function (EVDF) using a planar probe reveal the existence of a strong on-axis electron drift parallel to magnetic field in spite of a very small axial variation less than 1V in the plasma potential. The average drift velocities calculated from the asymmetry of the measured EVDFs show that there exists a significant electron drift from cathode to substrate with a maximum speed of about 1×106m∕s, which is comparable to the bulk electron temperature. The magnetic mirror force which is driven by the axial gradient of the magnetic field (i.e., the parallel ∇B force) is suggested as a possible source for the parallel electron drift. Carrying out a scaling of current densities with the measured data, it is found that the parallel ∇B force can produce the electron current enough to balance the discharge current, implying that the electron transport in the downstream region is determined not by the classical diffusion model in which electron motion toward the anode is diffusion and mobility dominated but by the modified diffusion model in which electron motion is drift dominated.