bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron energy loss spectroscopy of interfacial layer formation in Gd2O3 films deposited directly on Si(001)
Authors: Botton G. A. , Gupta J. A. , Landheer D. , McCaffrey J. P. , Sproule G. I. , Graham M. J.
Published: July 2002

High-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS) were used to study the interfacial layers formed in Gd2O3 films on Si(001) during rapid thermal annealing at 780 °C in an O2 ambient. Oxygen diffuses through the films and reacts with the substrate to form a SiO2 layer and an intermediate layer containing Gd2O3 and SiO2. Singular value decomposition was used to profile the Si, SiO2, and Gd2O3 components through the film from the characteristic spectra observed in the Si L2,3, Gd N4,5 and O K-edge EELS. The profiling results support the results of x-ray photoelectron spectroscopy sputter profiling measurements and contribute to a complete picture of the chemical bonding within the films. The ab initio multiple scattering method was used to simulate the observed EELS spectra at the Si L2,3 and O K edges and provide further insight into the chemical bonding of the film and the origin of spectral features. For SiO2, the Si L2,3 EELS is due only to the first-neighbor oxygen atoms. The O K-edge EELS for Gd2O3 is also due only to Gd first neighbors, while the SiO2 EELS is very sensitive to the number of O second neighbors as well as the Si first neighbors.
In situ imaging of field emission from individual carbon nanotubes and their structural damage
Authors: Wang Zhong L. , Gao Rui Ping , de Heer Walt A. , Poncharal P.
Published: July 2002

Field emission of individual carbon nanotubes was observed by in situ transmission electron microscopy. A fluctuation in emission current was due to a variation in distance between the nanotube tip and the counter electrode owing to a “head-shaking” effect of the nanotube during field emission. Strong field-induced structural damage of a nanotube occurs in two ways: a piece-by-piece and segment-by-segment pilling process of the graphitic layers, and a concentrical layer-by-layer stripping process. The former is believed owing to a strong electrostatic force, and the latter is likely due to heating produced by emission current that flowed through the most outer graphitic layers.
On the efficiency of energy transfer in collisional activation of small peptides
Authors: Laskin Julia , Futrell Jean H.
Published: July 2002

We present a study of the efficiency of the internal excitation of small peptide ions upon multiple-collision activation with Ar and ion-surface interaction with self-assembled monolayers of fluorinated alkylthiol on gold. Internal energy distributions are extracted from RRKM modeling of collision energy-resolved fragmentation efficiency curves for protonated tri-, tetra-, and penta-alanine, and polyltetra-alanine. The efficiency of T→V transfer in surface collisional activation decreases for larger peptides. This is readily rationalized by the corresponding decrease in the center-of-mass collisions energy. For all peptides except protonated dialanine, energy transfer upon multiple-collision activation is rather insensitive to the peptide size and composition. The average energy deposited into protonated dialanine is substantially lower than the excitation level achieved for other peptides. Master equation modeling revealed that energy-transfer efficiency in peptide collisions with Ar is the same for all peptides excluding (AA)H+. The results suggest that protonated dialanine has a more extended structure than larger peptides studied in this work.
Magnetic and electronic nature of layered manganese oxysulfide Sr2CuMnO3S
Authors: Kamihara Y. , Matoba M. , Kyomen T. , Itoh M.
Published: July 2002

We have synthesized and investigated the magnetic and electronic nature of layered manganese oxysulfide Sr2CuMnO3S which crystallizes in an unusual intergrowth structure with Cu2S2 and pyramidal MnO5 layers separated by Sr ions. A Cu 2p x-ray photoemission spectroscopy (XPS) spectrum of Sr2CuMnO3S indicates that the Cu ion is in the monovalent state, or Cu+. This feature of the Mn 2p XPS spectrum of Sr2CuMnO3S is similar to that of the parent compound La3+Mn3+O3 of colossal magnetoresistance materials. The MnO5 cluster model analysis suggests that the electronic nature of the pyramidal MnO5 layers belongs to a strongly correlated electron system, and the system is the so-called Mott insulator resulting from the strongly correlated electron nature. Magnetic susceptibility (χ) exhibits low-dimensional antiferromagnetic ordering of Mn3+ in pyramidal MnO5 layers. Fitting the χ data to Curie–Weiss law provides evidence of strong second neighbor Mn–Mn interaction.
The development of chaotic advection
Authors: Aref Hassan
Published: July 2002

The concept of chaotic advection was developed some twenty years ago as an outgrowth of work on advection by interacting point vortices. The term “chaotic advection” was first introduced in the title of an abstract for the 35th annual meeting of the American Physical Society (APS) Division of Fluid Dynamics (DFD) in 1982. The main reference, a Journal of Fluid Mechanics paper published in 1984, may be the true “birthdate” of the term. Earlier work from the 1960s by Arnol’d and Hénon on advection by steady three-dimensional flows already contained closely related ideas and results but was not widely appreciated. The present paper, based on the 2000 Otto Laporte Memorial Lecture delivered at the 53rd APS/DFD annual meeting, traces these and other precursors and the development of chaotic advection over the past two decades. Some exciting recent developments, such as application to fluid mixing in micro-electromechanical systems (MEMS) and to materials processing, and the introduction of topological methods of analysis, are highlighted. On balance, chaotic advection is now established as a subtopic of fluid mechanics with wide ramifications and continued promise for theory, experiment and applications.
Investigation into the charge distribution and barrier profile tailoring in AlGaN/GaN double heterostructures by self-consistent Poisson–Schrödinger calculations and capacitance–voltage profiling
Authors: Zervos M. , Kostopoulos A. , Constantinidis G. , Kayambaki M. , Georgakilas A.
Published: July 2002

The two-dimensional electron gas (2DEG) distribution and conduction-band profile tailoring of the AlxGa1−xN/GaN/AlyGa1−yN/GaN double heterostructure (DH) has been studied in detail by self-consistent Poisson–Schrödinger (SCPS) calculations. We show that a 2DEG is always created at the AlyGa1−yN/GaN interface beyond the GaN quantum well but the latter will not be occupied if the lower barrier thickness or Al content exceed those of the top barrier. These findings were confirmed by capacitance–voltage profiling of a 5 nm Al0.3Ga0.7N/5 nm GaN/AlyGa1−yN/GaN DH grown by molecular beam epitaxy on n+ GaN, where the lower barrier thickness was varied between 10 and 40 nm. A maximum 2DEG density of 1.0×1013 cm−2 was achieved for a 40 nm lower barrier and y=0.2. We found good agreement between the integrated carrier concentration versus depth curve and the calculated equilibrium 2DEG density. The bias required to bring about a flatband condition at the lower AlyGa1−yN/GaN interface increased with the thickness of the lower barrier up to −9 V for 40 nm and y=0.2, in agreement with SCPS calculations of the subband depletion under negative bias. We discuss the potential use of an AlGaN/GaN DH with a thick GaN well for the realization of a 2DEG isolated from the buffer by strong barrier enhancement.
A Hamiltonian electrostatic coupling scheme for hybrid Car–Parrinello molecular dynamics simulations
Authors: Laio Alessandro , VandeVondele Joost , Rothlisberger Ursula
Published: July 2002

We present a fully Hamiltonian and computationally efficient scheme to include the electrostatic effects due to the classical environment in a Car–Parrinello mixed quantum Mechanics/molecular mechanics (QM/MM) method. The polarization due to the MM atoms close to the quantum system is described by a Coulombic potential modified at short range. We show that the functional form of this potential has to be chosen carefully in order to obtain the correct interaction properties and to prevent an unphysical escape of the electronic density to the MM atoms (the so-called spill-out effect). The interaction between the QM system and the more distant MM atoms is modeled by a Hamiltonian term explicitly coupling the multipole moments of the quantum charge distribution with the classical point charges. Our approach remedies some of the well known deficiencies of current electrostatic coupling schemes in QM/MM methods, allowing molecular dynamics simulations of mixed systems within a fully consistent and energy conserving approach.
Responsivity of quantum well infrared photodetectors at terahertz detection wavelengths
Authors: Gadir M. A. , Harrison P. , Soref R. A.
Published: July 2002

A first-principles model of the photocurrent in quantum well infrared photodetectors (QWIPs) is derived. The model examines the responsivity, carrier capture probability and quantum efficiency. It is found that the QWIP sensitivity reaches a plateau below the 10 μm detection wavelength and remains nearly constant from 10 to 50 μm.
1.3 μm InAs quantum dot laser with To=161 K from 0 to 80 °C
Authors: Shchekin O. B. , Deppe D. G.
Published: July 2002

Data are presented on the influence of p-type modulation doping on the gain characteristics of 1.3 μm InAs quantum dot lasers. The improvement in optical gain leads to very high characteristic temperatures for the lasing threshold that reach 161 K in the temperature range between 0 and 80 °C. 1.3 μm ground state lasing is obtained up to a temperature of 167 °C.
Examination of magnetostatic waves by new optical and induction probes
Authors: Vlannes Nickolas P. , Morgenthaler Frederic R.
Published: July 2002

New optical and induction probes have been developed and used to investigate microwave (1–4 GHz) magnetostatic waves (MSW) propagating in yttrium iron garnet (YIG) thin films (5–50 μm). The optical probe uses the magnetooptic properties of YIG, and light from a 45 mW, 1.15 μm HeNe laser, to examine rfmagnetization of MSW. Focused light for localized probing has achieved a spatial resolution of 100 μm. The induction probe senses the rf magnetic field, which fringes from the surface of YIG thin films, with metal loops of either gold wire wrapped around a plastic support or aluminum photolithographically fabricated on glass. The highest spatial resolution was attained with 45μm aluminum loops. The optical probe has been used to examine the amplitude of linear and nonlinear forward volume waves (MSFVW). Nonlinear effects are characterized by MSFVW saturation and then a decline as input power to the wave increased. Localized probing of nonlinear properties showed, that over small regions, a monotonic increase of light modulation with increasing microwave input power to MSFVW does not always occur. MSFVW in the linear regime were studied for evolution of the amplitude profile as it propagated from the launching antenna. Magnetostatic surface (MSSW) and backward volume (MSBVW) waves have been explored with the induction probe for amplitude and dispersion relation. In a uniform bias field, an apparent form of spatial focusing was exhibited by MSSW. Direct measure of wavelengths of MSSW and MSBVW gave the dispersion relations.
Ion bombardment energy distributions in radio-frequency glow-discharge systems
Authors: Thompson Brian E. , Allen Kenneth D. , Richards Albert D. , Sawin Herbert H.
Published: July 2002

The measurement and interpretation of ion bombardment energies in rf discharges of SF6, CF3Cl, and CF3Br from 0.2 to 1.0 Torr, are discussed. Errors in ion sampling due to disturbances of the electric field and neutral density around the sampling orifice are shown to be most important at higher pressures and with larger orifices. The dependence of the ion bombardment energy distribution on the electric-field-to-pressure ratio is reviewed. Combining this relation with a simple electrical model of a plasma gives estimates of the ion bombardment energies in collisional sheaths. The bombardment energy is proportional to (rf current)/(electrode area×frequency ×pressure) with the proportionality constant for a particular system depending on the collision cross sections and relative ion-to-netural mass ratio. The constants found for the three gases studied experimentally are close to theoretical estimates.
Stress effects on III-V solid-liquid equilibria
Authors: Larché F. C. , Cahn J. W.
Published: July 2002

The general equations that govern the equilibrium of a stressed III-V ternary compound with a melt are derived and applied to the case of liquid-phase epitaxy. The stresses are analyzed in detail and the thin-film approximation is assessed. The deviations of the solid and liquid compositions from their phase diagram values are obtained in the vicinity of the solid composition that gives lattice matching with the substrate. A quantity Q is defined that can be used as an index of the magnitude of the stress effect (sometimes called lattice pulling). The intrinsic properties and growth conditions that may affect Q are discussed.
Spin-wave instabilities and their revival by nonlinear mechanics (invited)
Authors: Suhl H. , Zhang X. Y.
Published: July 2002

Spin-wave instabilities in high signal power ferromagnetic resonance, first noted more than 30 years ago, have in recent years been reexamined in the light of modern dynamics. Both experiments and computer simulations have demonstrated limit cycles, period doubling, quasiperiodicity, and chaos as the rf power is progressively increased above the original threshold value. A distinctive feature of this problem is the participation of a vast number of spin-wave modes in the original instability, driving the system to a new fixed point with high excitation of an entire manifold. The next instability is a Hopf bifurcation in which a pair of collective modes grows into a limit cycle, and an accompanying spatial correlation pattern. Center manifold theory is used to establish these results. Prospects for some approximate analytic (as distinct from computational) treatment of the succession of higher bifurcations are discussed.
Coercivity in ThMn12 -type magnets
Authors: Schultz L. , Wecker J.
Published: July 2002

Fe-rich and RE-containing phases with the ThMn12 structure show interesting intrinsic magnetic properties. The single-phase composition Fe80Mo12Sm8 has a saturation magnetization of 10 kG, a Curie temperature of about 200 °C, and an anisotropy field of about 90 kOe, with the saturation magnetization and the Curie temperature being further improved by a Co substitution. In this contribution we report on the first experiments to create coercivity in these samples. As preparation techniques we applied mechanical alloying and rapid quenching which both result in microcrystalline material. The highest coercivity (3.8 kOe) so far was obtained for a multiphase sample prepared by mechanical alloying and a successive heat treatment at 800 °C. The dependence of coercivity on composition for these samples is discussed. Results for rapidly quenched material are also presented.
A void growth-based failure model to describe spallation
Authors: Rajendran A. M. , Dietenberger M. A. , Grove D. J.
Published: July 2002

A new dynamic failure model to describe void nucleation, growth, and coalescence in ductile metals is reported. The model is based on a pressure-dependent yield criterion for compressible plastic flow. This three-dimensional, plasticity-based continuum damage model is incorporated into a finite difference, wave propagation code. A procedure to determine the failure model parameters is proposed. In this procedure, the model parameters are calibrated based on the ability to match the experimental free-surface velocity history with code simulations. Model parameters for oxygen-free high-conductivity copper have been determined successfully using this procedure.
High-speed liquid-crystal modulators using transient nematic effect
Authors: Wu Shin-Tson , Wu Chiung-Sheng
Published: July 2002

The transient nematic effect, together with multiple pass technique and optimal temperature effect, was employed to demonstrate high-speed liquid-crystal modulators. As a result, nematic liquid-crystal modulators with frame time less than 50 μs have been achieved in the visible spectral region.
Photoluminescence and electroluminescence mechanisms at polycrystalline CdS in air and in contact with aqueous electrolytes
Authors: Ferrer I. J. , Salvador P.
Published: July 2002

Room-temperature photoluminescence (PL) and electrolyte electroluminescence (EL) spectra of polycrystalline CdS samples, in air and in contact with aqueous electrolytes, have been studied as a function of the atmosphere and temperature of annealing, light excitation intensity and wavelength, applied bias, electrolyte composition, and photoetching treatments. Both luminescence spectra are characterized by two broad bands centered at about 770 and 920 nm, associated with a red (R) and an infrared (IR) emission, respectively. According to our experimental results, the R band can be attributed to radiative recombination between electrons trapped at sulfur vacancies, localized at about 0.79 eV below the bottom of the conduction band, and valence-band free holes. The IR emission, on the other hand, is associated with a radiative recombination process involving electrons trapped at sulfur vacancies and holes trapped at cadmium vacancies located at about 0.26 eV above the top of the valence band. The luminescence spectrum shape is shown to depend on the rate of hole injection into the semiconductor and on the concentration of cadmium and sulfur vacancies. From the analysis of the R and IR band intensities, in both PL and EL spectra, valuable information about the generation and spatial distribution of vacancies under different annealing treatments can be obtained. Such information is of interest for the fabrication of high-performance liquid-junction solar cells based on CdS polycrystalline electrodes.
Quantum calculations of the change of refractive index due to free carriers in silicon with nonparabolic band structure
Authors: Huang H. C. , Yee S. , Soma M.
Published: July 2002

A quantum-mechanical approach used to calculate the change of refractive index, Δn, due to free carriers in various doped silicon is presented. This approach uses a numerical Kramers–Kronig relation to analyze a calculated carrier-related absorption spectrum below or near the energy-band gap. The absorption spectrum is obtained by considering the optical transitions between energy bands and impurity bands, and the free-carrier absorption due to acoustic phonons, optical phonons, and ionized impurities in a spherical nonparabolic band model. Values of Δn at wavelength λ=1.3 and 1.6 μm for different doping levels are obtained. The results are applicable to both the integrated-optics applications and optical-probing applications in silicon.
Structural investigation of silica gel films by infrared spectroscopy
Authors: Almeida Rui M. , Pantano Carlo G.
Published: July 2002

Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c-Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO2 grown on c-Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO2 gels and v-SiO2. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm−1 is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a state of strain in the Si—O—Si bridges of the gels near the surface of the pores. The gel films have also a stronger shoulder near 1200 cm−1, whose intensity decreases with heat treatment of the gel. Its nature is related to the longitudinal optical (LO) component of the high-frequency vibration of SiO2 and it is suggested that the gel pores may account for the activation of the LO component.
Laser ultrasonic monitoring of ceramic sintering
Authors: Telschow K. L. , Walter J. B. , Garcia G. V.
Published: July 2002

Noncontacting laser-ultrasonic measurements of the sintering of ceramics in real-time are described. Lasers are used for both generation and detection of ultrasonic waves propagating through the material. A pulsed laser generates the ultrasonic wave from thermoelastic absorption at the material surface. Detection is accomplished by a confocal Fabry–Perot interferometer, which is insensitive to the speckle nature of the scattered light from the sample surface and can obtain measurements from samples with rough or diffusely reflecting surfaces. The densification of the ceramic sample is determined by changes in the bulk longitudinal wave velocity and the sample shrinkage.
Mechanism of carrier transport in highly efficient solar cells having indium tin oxide/Si junctions
Authors: Kobayashi H. , Ishida T. , Nakato Y. , Tsubomura H.
Published: July 2002

The carrier transport mechanism of the Si solar cells having n-Si/indium tin oxide (ITO) junctions has been studied by use of the current-voltage and capacitance-voltage measurements and x-ray photoelectron spectroscopy. An 11-Å-thick nonstoichiometric Si oxide layer is formed when ITO is deposited by spray pyrolysis on a Si electrode etched with hydrofluoric acid. In this case, the tunneling probability of majority carriers through the oxide layer is high, and the thermionic emission current over the energy barrier in Si takes a dominant part of the dark current. On the other hand, for a Si electrode where a Si oxide layer is intentionally interposed between ITO and Si, the thermionic emission current is suppressed, and trap-assisted multistep tunneling through the depletion layer becomes dominant. By making a mat-structure treatment on the Si surface, a solar energy conversion efficiency of 13% and the photocurrent density of 42.5 mA cm−2 were attained under AM 1 100 mW cm−2 illumination.
The investigation of a dc induced transient optical 30-Hz element in twisted nematic liquid-crystal displays
Authors: Takahashi Seiki
Published: July 2002

Using a simulation experiment, transient optical 30-Hz elements which respond to dc signal changes in thin-film transistor liquid-crystal display are investigated. After dc voltage is applied, even without dc voltage, the 30-Hz element appears and decays to the level in the steady state. This is explained by the generation of an electric double layer. The generation mechanism of the layer is described by an equation with a creation time constant τ1 and a disappearance time constant τ2, and these values are estimated to be approximately 1400 and 500 s, respectively. The relaxation process is described by the empirical model in which relaxation time constant is initially 60 s and increases as a function of time.
Photoisomerization of azobenzene derivatives in polymeric thin films
Authors: Sekkat Z. , Morichère D. , Dumont M. , Loucif-Saïbi R. , Delaire J. A.
Published: July 2002

The photoisomerization of disperse red 1 (DR1) in polymethylmethacrylate thin films is demonstrated by monitoring the refractive index variations by the attenuated total reflection method. Absorption spectrum measurements give a confirmation of this phenomenon. The kinetics of photoisomerization and of thermal relaxation are nonexponential. Some other molecules are studied. As in solution, azobenzene derivatives (N—N) produce reversible isomerization while dimethylaminonitrostilbene, a stilbene derivative (C—C), exhibits no thermal cis→trans relaxation.
Bleaching mechanism of silver halide photochromic glasses
Authors: Caurant D. , Gourier D. , Vivien D. , Prassas M.
Published: July 2002

Thermal bleaching of silver halide photochromic glasses is studied by electron paramagnetic resonance spectroscopy of photoinduced CuII centers. During exposure to ultraviolet light, the only stable CuII species is the (CuIIVAg)A center, which is a CuII-silver vacancy complex with the vacancy in a nearest position. In the dark, this center rapidly decays via two parallel channels. The first involves the dissociation of the complex by displacement of the vacancy along a [110] direction, with an activation energy E3=0.44 eV and a frequency factor k30=3.4×105 s−1. The second channel involves the conversion of the (CuIIVAg)A center into a (CuIICl−VAg)B center, where the silver vacancy is in the next nearest position along the [100] direction. This process occurs with an activation energy E1=0.44 eV and a frequency factor k10=3.1×105 s−1. The (CuIICl−VAg)B center slowly decays by a vacancy hopping mechanism, with an activation energy E2=0.22 eV and a frequency factor k20=4.6 s−1. To explain these two decay channels, it is proposed that the (CuIIVAg)A and (CuIICl−VAg)B centers annihilate via the formation of a CuI ion and a neutral complex (AgIIVAg)A which migrates to the surface of the silver halide particle, where electron-hole recombination occurs.
Transient charging current in nematic liquid crystals
Authors: Naito Hiroyoshi , Yoshida Kazuyuki , Okuda Masahiro , Sugimura Akihiko
Published: July 2002

Transient charging currents in nematic liquid crystals have been measured with a single- and a double-voltage pulse technique to elucidate the role of impurity ions incorporated in the materials in the transient charging processes. In the single pulse experiment, a peak in the current transient is observed in the nematic phase. It is shown that occurrence of the peak is due to the alignment of the director of the liquid crystal molecules in the direction of applied electric field. In the double pulse experiment, a voltage pulse (prepulse) is applied to the liquid crystal before the measurement of the transient current. It is observed that the application of the prepulse significantly alters the shape of the current. The changes in the current can be explained by considering the impurity ion distribution, modified by the prepulse, in the nematic liquid crystals. It is concluded that the double pulse experiment is a powerful tool for the examination of the influence of impurity ions in liquid crystals on the current transients.
Influence of buffer layers on the deposition of high quality single crystal GaN over sapphire substrates
Authors: Kuznia J. N. , Khan M. Asif , Olson D. T. , Kaplan Ray , Freitas Jamie
Published: July 2002

Recently several research groups, including ours, have reported on the deposition of extremely high quality single crystal GaN layers over sapphire substrates. One of the keys to obtaining the high quality was the use of a thin AlN or GaN buffer layer between the sapphire substrate and the grown film. In this communication, we discuss the crystallinity and the influence of the buffer layer in controlling the crystalline, optical, and the electrical properties of the GaN depositions. We also compare the use of GaN and AlN as the buffer layer material. Our results indicate that the buffer layer thickness and the total film thickness are the key factors controlling the electrical, optical, and crystalline properties of the GaN depositions over sapphire substrates.
Electrical conductivity of Mn–Zn ferrites
Authors: Ravinder D. , Latha K.
Published: July 2002

The electrical conductivity of polycrystalline mixed ferrite system having the chemical formula MnxZn1−xFe2O4 (where x=0.0, 0.2, 0.4, 0.6, and 0.8) was investigated from room temperature to the neighborhood of the Curie temperature by the two-probe method. The electrical conduction in these ferrites is explained on the basis of the hopping mechanism. Plots of log (σT) versus 103/T are almost linear and show a transition near the Curie temperature. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region.
Density of states in CuIn(SSe)2 thin films from modulated photocurrent measurements
Authors: Herberholz R. , Walter T. , Schock H. W.
Published: July 2002

Results of modulated photocurrent measurements in the temperature range between 180 and 300 K on polycrystalline CuIn(S,Se)2 thin films are presented. Modeling of the obtained phase shifts implies a continuous energetic distribution of traps in the band gap. A superposition of an exponential energetic distribution of traps with a characteristic energy of 60 meV and a Gaussian peak with a depth of 250 meV can explain the obtained phase shift and amplitudes between 250 and 300 K. For lower temperatures a change of the transport path, reducing the effective depth of the peak in the density of states and reducing the ‘‘attempt to escape frequency’’ is likely. At low temperatures the phase shift depends on the photon flux due to the separation of the demarcation levels whereas in the case of the higher temperatures no significant dependence on the light intensity could be detected.
Comments on the use of the force mode in atomic force microscopy for polymer films
Authors: Aimé J. P. , Elkaakour Z. , Odin C. , Bouhacina T. , Michel D. , Curély J. , Dautant A.
Published: July 2002

Atomic force microscopy (A.F.M.) was first described as a powerful technique for studying insulating, hard surfaces. Since then, it has also been considered as an appropriate technique for investigating, at a submicromic scale, elastic and viscoelastic properties of soft materials as polymer films. An attempt is made to show how macroscopic models can be fruitfully employed in order to interpret the force curves obtained in AFM on polymer films. Through an analysis of the slope variation and the way the instability occurs at the tip-sample contact, it is shown that a macroscopic approach is a useful way to explain most of the features of the force curves. Furthermore the importance is underlined of the initial conditions. It is shown that for polymer samples which have a stiffness within the range of the probe one, drastic changes of the force curve shape can occur when the initial conditions vary. Finally, this approach should allow one to clarify the conditions for which the macroscopic approach fails.
The properties of magneto-optical MnBiR (R=In, Ge, and Sn) thin films
Authors: Jin Liu , Ying Ning , Tingjun Ma , Ruiyi Fang
Published: July 2002

The crystal structures and the magneto-optical properties of MnBiR (R=Ge, In, and Sn) thin films are reported. We found that doping MnBi films with Ge enhances their Kerr rotation angle θk(θk=2.1°, at λ=633 nm). It is thus possible to consider MnBi film doped with Ge a good candidate of magneto-optical recording material. In contrast, MnBi films doped with In or Sn loose the original NiAs-type hcp structure and correspondingly the Kerr rotation angle θk, is decreased.
Magnetic properties of γ-Fe2O3 nanoparticles obtained by vaporization condensation in a solar furnace
Authors: Martínez B. , Roig A. , Obradors X. , Molins E. , Rouanet A. , Monty C.
Published: July 2002

The magnetic properties of γ-Fe2O3 nanoparticles synthesized by vaporization condensation in a solar image furnace have been studied using both magnetic measurements and Mössbauer spectroscopy. The mean size of the particles turns out to be easily controlled by changing the pressure conditions in the growth chamber. The particles exhibit superparamagnetic behavior at room temperature. Magnetic measurements show the appearance of magnetic hysteresis in the low-temperature range and from the evolution with temperature of the ferromagnetic ratio, MR/MS, we have determined the distribution of the blocking temperatures for the smallest particles that is fitted to a log-normal distribution leading to a mean blocking temperature 〈TB〉=38±15 K. The size distribution of the magnetic unit is also determined from this fitting, as well as from the Mössbauer spectra, obtaining a mean particle volume of about 3.5×105 Å3.
Powder x-ray diffraction of two-dimensional materials
Authors: Yang D. , Frindt R. F.
Published: July 2002

An analytic solution for the normalized intensity for powder x-ray diffraction has been obtained for a simple two-dimensional lattice using a linear approximation for the interference function. The analytic solution, where the Bragg peaks are strongly asymmetric, is compared to computer simulations using the Debye formula, and is shown to be in closer agreement than earlier numerical solutions by Warren and others. For a two-dimensional structure consisting of more than one monolayer of atoms, the shape of the Bragg peaks is modulated by the structure factor. This structure factor modulation provides a continuous plot of the structure factor over the range of the diffraction tail and thus provides valuable information about the structure of the layer. It is demonstrated that because of structure factor modulation the Warren expression which relates the width of Bragg peaks to layer size cannot be used for a two-dimensional sheet with more than one layer of atoms, and it is proposed in such cases that measuring the low-angle side width of half-maximum intensity can be used for determination of the layer size. Single molecular layer MoS2 and WS2 suspensions, prepared by exfoliation, provide excellent randomly oriented two-dimensional systems for demonstrating the unique features of powder x-ray diffraction patterns of two-dimensional materials and for structure identification using Bragg peak profiles.
Swift-heavy-ion induced damage in germanium: An evaluation of defect introduction rates
Authors: Marie P. , Levalois M. , Paumier E.
Published: July 2002

N-type lightly doped germanium samples were irradiated at 300 K with swift heavy ions at the Grand Accelerateur National d’Ions Lourds. Induced damage was studied from in situ conductivity and Hall mobility measurements. By using previously reported deep-level-transient-spectroscopy results, simulations of experimental curves were performed. The introduction rates of the different induced defects are extracted from these simulations and analyzed in terms of the nuclear and electronic stopping powers of the ions. A partial annealing of the divacancies appears for the highest values of the electronic stopping power.
Aging behavior of photoluminescence in porous silicon
Authors: Fukuda Yoshio , Furuya Kazuo , Ishikawa Nobuhiro , Saito Tetsuya
Published: July 2002

The aging phenomena of porous silicon (PS) over a 192 day time span have been studied using photoluminescence (PL) spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy. As-prepared PS exhibits red PL peaking near 650 nm. The PL intensity increases with time to some equilibrium value. The PL peak shows no blueshift during aging. Infrared analysis reveals that the Si–O bond content in PS increases with time and correlates to the PL intensity. The PL lifetime increases with aging and its values range from 3 to 37 μs. Transmission electron microscopic observations indicate the presence of Si nanoparticles and amorphous substances in the PS layer. These results suggest that the PL from aged PS might be attributed to the combined effects of quantum confinement in Si nanoparticles and some defect in Si oxide covering the nanoparticles.
Device physics of single layer organic light-emitting diodes
Authors: Crone B. K. , Campbell I. H. , Davids P. S. , Smith D. L. , Neef C. J. , Ferraris J. P.
Published: July 2002

We present experimental and device model results for electron only, hole only, and bipolar organic light-emitting diodes fabricated using a soluble poly (p-phenylene vinylene) based polymer. Current–voltage (I–V) characteristics were measured for a series of electron only devices in which the polymer thickness was varied. The I–V curves were described using a device model from which the electron mobility parameters were extracted. Similarly, the hole mobility parameters were extracted using a device model description of I–V characteristics for a series of hole only devices where the barrier to hole injection was varied by appropriate choices of hole injecting electrode. The electron and hole mobilities extracted from the single carrier devices are then used, without additional adjustable parameters, to describe the measured current–voltage characteristics of a series of bipolar devices where both the device thickness and contacts were varied. The model successfully describes the I–V characteristics of single carrier and bipolar devices as a function of polymer thickness and for structures that are contact limited, space charge limited, and for cases in between. We find qualitative agreement between the device model and measured external luminance for a thickness series of devices. We investigate the sensitivity of the device model calculations to the magnitude of the bimolecular recombination rate prefactor.
Kinetics of scanned probe oxidation: Space-charge limited growth
Authors: Dubois Emmanuel , Bubendorff Jean-Luc
Published: July 2002

This article proposes an enhanced oxidation model for scanning probe microscope (SPM) nanolithography that reproduces the power-of-time law reported for tip-induced anodic oxidation. It is shown that the space charge resulting from nonstoichiometric states strongly limits the oxidation rate. The direct relationship between the oxide thickness and time is provided by integration of the oxide rate equation. Measurements on SPM-induced oxides generated on a titanium surface are compared to theory. The predominant role of the space charge is corroborated by electrical measurements on oxide barriers that exhibit current fluctuations due to Coulombic effects.
Localization of an excess electron in water clusters
Authors: Wallqvist A. , Thirumalai D. , Berne B. J.
Published: July 2002

Simulation of e−–(H20)n for n=1,2,3 done at 5 K, using path integral Monte Carlo methods shows that a water dimer as well as a water trimer in the linear configuration can bind an electron in a diffuse surface state. The binding energy of the electron dimer and the electron trimer is estimated to be between 3–6 and 4–9 meV, respectively. The results indicate that the electron does not alter the structure of the water dimer but does induce observable changes in the water trimer. It is also shown that an electron does not bind to a monomer. These results are discussed in connection with recent molecular beam experiments.
Reinvestigation of the aluminum hydride (AlH+ and AlD+) cation radicals by ESR in argon matrices at 4 K: Generation by reactive laser sputtering
Authors: Knight Lon B. , Cobranchi S. T. , Gregory B. W. , Earl Edward
Published: July 2002

The ESR spectra previously assigned to the AlH+ radical ion (X 2Σ) [J. Chem. Phys. 71, 3991 (1979)] actually belong to the divalent neutral aluminum radical AlHOH whose charge distribution and electronic structure can be described as AlH+OH− with 90% of the unpaired electron on the AlH+ part of the molecule. Reactive laser sputtering and photoionization of AlH(g) were used to generate the AlH+ and AlD+ cation radicals, whose ESR spectra have been observed for the first time. A detailed analysis of the ESR results for argon matrices at 4 K reveal unusually large Al hyperfine interaction (hfi) with Aiso and Adip values of 1586(2) and 49(1) MHz, respectively. The H hfi is essentially isotropic with Aiso=442(2) MHz. The observed nuclear hyperfine A tensors for Al and H show excellent agreement with ab initio CI theoretical calculations. The results for AlH+ are compared with the isoelectronic neutral radical MgH, and the similar AlF+ cation radical. The Al hfi is slightly larger in AlD+ relative to AlH+. This interesting isotopic effect is qualitatively explained on the basis of electronic structure dependence on small changes in the bond distance for the two isotopic radicals.
Bimodal angular hopping model for molecular rotations in liquids
Authors: Kivelson Daniel , Miles Danny
Published: July 2002

We examine an assortment of properties associated with molecular rotational relaxation in supercooled liquids, in particular relaxation of a spherical harmonic of order l. First, we have fitted the observed phenomena by a procedure employing a two-exponential memory function. We assume strong torques, and we can then obtain semiquantitative descriptions of the librational oscillations, of the low viscosity (αβ) relaxation, and of the high viscosity α and β relaxations; we find the α relaxation to be independent of l, as measured, and we predict the β relaxation to be strongly dependent upon l and to be the dominant (αβ) process at low viscosity. Second, we have associated one of the exponentials in the memory function, the larger more rapidly decaying one, with the relaxation of strong effective torques which are associated with β relaxation and with the librational motions; the other exponential is associated with weak effective torques and is responsible for the α relaxation. In order to gain insight, we have then introduced an ‘‘effective’’ anisotropic potential with two components, one with shallow, angularly closely spaced and narrow wells, which is associated with the strong ‘‘effective’’ torques, and one with deeper, angularly widely spaced and broad wells, which is associated with the weak effective torques.
Vibrational frequencies and intensities of H-bonded and Li-bonded complexes. H3N⋅⋅HCl and H3N⋅⋅LiCl
Authors: Szczȩśniak M. M. , Kurnig Ingrid J. , Scheiner Steve
Published: July 2002

The geometries, energetics, and vibrational spectra are calculated for the two complexes at the SCF and correlated MP2 levels using the 6-31G** basis set, augmented by a second set of d functions on Cl. While correlation represents an important factor in the binding of H3 N⋅⋅HCl, it contributes little to the stronger Li bond. Unlike the HCl stretch νs which decreases substantially in frequency and is greatly intensified in H3 N⋅⋅HCl, the frequency of the LiCl stretch undergoes an increase and little change is noted in its intensity, conforming to prior spectral measurements. The intensities of the intramolecular stretching modes of NH3 are greatly strengthened by formation of a H bond and even more so for a Li bond. These intensity patterns are analyzed via atomic polar tensors which reveal that formation of a H bond dramatically lessens the ability of the electron density to shift along with the proton. A stretch of H–Cl hence leads to a large increase in molecular dipole moment. This ‘‘freezing’’ of the electron cloud is much smaller in the Li bond and its effect on the νs intensity is counteracted by a much reduced Li atomic charge in the complex. Another distinction between the H and Li bonds relates to the destination of charge transferred from the NH3 subunit which accumulates on Cl in the former case but on Li in the latter.
Vibrational analysis of heterocyclic polymers: A comparative study of polythiophene, polypyrrole, and polyisothianaphtene
Authors: Faulques E. , Wallnöfer W. , Kuzmany H.
Published: July 2002

We present for the first time a comparative vibrational study of polythiophene, polypyrrole, and polyisothianaphthene. The calculation is performed at k=0 in the plane of the chains for both aromatic-like and quinoid-like structures. The Ag species which are present in the Raman spectra are more particularly investigated. The frequency changes, the atomic displacements, and the influence of the interring distance r and the interring force constant F(r2) are analyzed when the quinoid character is varied. We demonstrate that the ring linkage vibrations C–C or C=C give frequencies in the range 1450–1800 cm−1. It is found that a similar dependence of these frequencies on F(r2) and r exists for all the forms except for aromatic polyisothia naphthene. Also, analogies between the three polymers in the experimental Raman spectra are evidenced. Refined sets of force constants very close to those of the monomers are suitable for explaining the main features of these spectra and can be used to determine the degree of aromaticity and the degree of bond alternation.
Magnetic field effects on triplet exciton fission and fusion in a polydiacetylene
Authors: Austin R. H. , Baker G. L. , Etemad S. , Thompson R.
Published: July 2002

We have studied the origin and decay dynamics of triplet excitons in the conjugated polymer poly(4BCMU) in its sol(yellow) and gel(red) phases. Wavelength and intensity dependencies of the triplet yield show that the triplet exciton cannot be produced by excitation into the singlet exciton edge but only from higher lying states. The observed lifetime of the triplet state, coupled with the magnetic field dependence of the triplet state production and decay, indicate that the triplet state is created by a fission process from the excited singlet. The time and magnetic field dependence of the triplet exciton decay indicate that the triplet exciton decay in the red phase occurs by diffusive bimolecular fusion, but in the yellow phase the triplet exciton decay is nondiffusive. We postulate that the unimolecular decay of the triplet exciton in the yellow phase results from exciton pinning by conformational disorder.
Mathematical modeling of kinetic oscillations in the catalytic CO oxidation on Pd(110): The subsurface oxygen model
Authors: Bassett M. R. , Imbihl R.
Published: July 2002

Experimental investigations of the catalytic CO oxidation on a Pd(110) surface revealed that the temporal oscillations in the reaction rate (measured in a range 10−3 Torr<p<1 Torr) can be traced back to a periodic formation and depletion of oxygen in the subsurface region. Oscillations in the reaction rate arise because the variation in the subsurface oxygen concentration modulates the oxygen sticking coefficient and hence the catalytic activity. Based on the proposed reaction mechanism, which has been well supported by experimental data, a set of three coupled differential equations was established describing the variations in the absorbate coverages ΘO and ΘCO and in the subsurface oxygen concentration for the system Pd(110)/CO+O2. Numerical solutions from the mathematical model reproduce the essential qualitative and quantitative features of the experiment. Characteristic features in the experiment that indicate the presence of subsurface oxygen, such as a reversal of the usual clockwise hysteresis in the reaction rate, are also found in the simulation. The model reproduces the existence region for kinetic oscillations in good agreement with the experimental data and it exhibits similar bifurcation behavior to that observed in the experiment. The remaining, mainly quantitative, differences can be traced back to simplifications made in the formulation of the kinetics.