bluuebunny/crossref_metadata_2025 huggingface dataset:
A new approach for studying nucleation phenomena using molecular simulations: Application to CO2 hydrate clathrates
Authors: Radhakrishnan Ravi , Trout Bernhardt L.
Published: July 2002

We use an order-parameter formulation, in conjunction with non-Boltzmann sampling to study the nucleation of clathrate hydrates from water–CO2 mixtures, using computer simulations. A set of order parameters are defined: Φigg (i=1,2,…,n and gg for guest–guest), which characterize the spatial and orientational order of the CO2 molecules, and Φihh (hh for host–host), which govern the ordering of the water molecules. These are bond-orientational order parameters based on the average geometrical distribution of nearest-neighbor bonds. The free-energy hypersurface as a function of the order parameters is calculated using the Landau–Ginzburg approach. The critical cluster size that leads to the nucleation of the clathrate phase is determined accurately by analyzing the free energy surface. We find that the nucleation proceeds via “the local structuring mechanism,” i.e., a thermal fluctuation causing the local ordering of CO2 molecules leads to the nucleation of the clathrate, and not by the current conceptual picture, called “the labile cluster hypothesis.” The local ordering of the guest molecules induces ordering of the host molecules at the nearest- and next-to-nearest-neighbor shells, which are captured by a three-body host–host order parameter, ζhh; these thermodynamic fluctuations lead to the formation of the critical nucleus. Our results are significant in understanding the proposed sequestration of CO2 by direct ocean injection in order to mitigate the greenhouse effect.
Local composition fluctuations in strongly nonideal binary mixtures
Authors: Murarka Rajesh K. , Bagchi Biman
Published: July 2002

We study the static and dynamical aspects of small length scale composition fluctuations in strongly nonideal binary mixtures of two different type by isothermal–isobaric (NPT) ensemble molecular dynamics simulations. Both the two models studied consider strong attractive interaction between dissimilar species which discourage the phase seperation in the binary mixtures. We find that the joint probability distribution of the composition fluctuations within molecular sized volumes is nearly Gaussian. However, the distributions are rather broad in both the binary mixtures. This is to be contrasted with the density fluctuation which is nearly negligible. Spontaneous fluctuation in one component is found to be anticorrelated with the fluctuations in it’s counterpart. The investigation of the dynamical correlation functions of the fluctuations reveal nonexponential relaxation with a slow long time tail. The cross-correlation function relaxes at a slower rate than the two pure correlations.
A model for mechanochemical transformations: Applications to molecular hardness, instabilities, and shock initiation of reaction
Authors: Luty Tadeusz , Ordon Piotr , Eckhardt Craig J.
Published: July 2002

A basic theoretical structure for mechanochemical transformations based on prior models for solid-state reactions and HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) gap closing produces the concept of distortion-induced molecular electronic degeneracy (DIMED) of the highest occupied and lowest unoccupied molecular orbitals of an energetic molecule. Both intermolecular and intramolecular charge transfer are involved. The resulting distortion-induced local instability, a mechanochemical effect, leads to chemical transformations and can be analyzed by renormalization of the molecular hardness through the molecular deformation energy. Linear combinations of normal modes are shown to be useful for description of the mechanically induced reaction path. Numerical calculations for the RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) molecule are used to construct a path for initiation of a reaction by shock. They show the breaking of a single N–N bond as the primary step. DIMED is shown to be a kind of “inverse Jahn–Teller effect” leading to the general conclusion that distortion-induced instabilities and mechanically induced reactions require some, but not necessarily complete, HOMO–LUMO gap closure. This indicates that large local strains due to defects or cracks will contribute to DIMED. The DIMED concept, because of its generality, has wide applicability in solid-state chemistry.
Detection of hydrogen atoms in SiH4–H2 radio-frequency plasmas using two-photon laser-induced fluorescence
Authors: Hertl Michael , Jolly Jacques , Baravian Gérard
Published: July 2002

Two-photon laser-induced fluorescence spectroscopy is used to detect ground-state atomic hydrogen in highly diluted SiH4–H2 radio-frequency discharges currently used for microcrystalline silicon thin film deposition. Along with the fluorescence coming from the hydrogen atoms created in the discharge, additional atoms arising from photodissociation of silicon containing species at 205 nm are observed. This parasitic effect is identified as resulting mainly from the disilane molecules formed in the plasma. Hydrogen atoms created in the plasma and originating from the photodissociation are discriminated by a spectral analysis of the laser-induced fluorescence line profile and relative densities are deduced. Some results on the hydrogen atom and disilane concentrations as functions of the silane dilution, radio-frequency power, gas pressure, and gas temperature are given to illustrate the potentiality of such a diagnostic in SiH4–H2 discharges containing up to 6% of silane.
Triplex molecular layers with nonlinear nanomechanical response
Authors: Tsukruk V. V. , Ahn H.-S. , Kim D. , Sidorenko A.
Published: July 2002

The molecular design of surface structures with built-in mechanisms for mechanical energy dissipation under nanomechanical deformation and compression resistance provided superior nanoscale wear stability. We designed robust, well-defined trilayer surface nanostructures chemically grafted to a silicon oxide surface with an effective composite modulus of about 1 GPa. The total thickness was within 20–30 nm and included an 8 nm rubber layer sandwiched between two hard layers. The rubber layer provides an effective mechanism for energy dissipation, facilitated by nonlinear, giant, reversible elastic deformations of the rubber matrix, restoring the initial status due to the presence of an effective nanodomain network and chemical grafting within the rubber matrix.
Atom-radical reaction dynamics of O(3P)+C3H5→C3H4+OH: Nascent rovibrational state distributions of product OH
Authors: Park Jong-Ho , Lee Hohjai , Kwon Han-Cheol , Kim Hee-Kyung , Choi Young-Sang , Choi Jong-Ho
Published: July 2002

The reaction dynamics of ground-state atomic oxygen [O(3P)] with allyl radicals (C3H5) has been investigated by applying a combination of crossed beams and laser induced fluorescence techniques. The reactants O(3P) and C3H5 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor allyl iodide, respectively. A new exothermic channel of O(3P)+C3H5→C3H4+OH was observed and the nascent internal state distributions of the product OH (X 2Π:υ″=0,1) showed substantial bimodal internal excitations of the low- and high-N″ components without Λ-doublet and spin–orbit propensities in the ground and first excited vibrational states. With the aid of the CBS-QB3 level of ab initio theory and Rice–Ramsperger–Kassel–Marcus calculations, it is predicted that on the lowest doublet potential energy surface the major reaction channel of O(3P) with C3H5 is the formation of acrolein (CH2CHCHO)+H, which is consistent with the previous bulk kinetic experiments performed by Gutman et al. [J. Phys. Chem. 94, 3652 (1990)]. The counterpart C3H4 of the probed OH product in the title reaction is calculated to be allene after taking into account the factors of reaction enthalpy, barrier height and the number of intermediates involved along the reaction pathway. On the basis of population analyses and comparison with prior calculations, the statistical picture is not suitable to describe the reactive atom-radical scattering processes, and the dynamics of the title reaction is believed to proceed through two competing dynamical pathways. The major low N″-components with significant vibrational excitation may be described by the direct abstraction process, while the minor but extraordinarily hot rotational distribution of high N″-components implies that some fraction of reactants is sampled to proceed through the indirect short-lived addition-complex forming process.
Quench–jump sequence in phase separation in polymer blends
Authors: Fiałkowski Marcin , Hołyst Robert
Published: July 2002

A two-step process of phase separation–mixing is analyzed for binary mixtures. The system is first quenched into the thermodynamical instability region (temperature T), where the mixture undergoes a process of spinodal decomposition, characterized for short times by the growth of the Cahn peak of a scattered intensity at fixed scattering wave vector. Next we heat up a system (make a temperature jump to temperature T1) above the spinodal line (temperature Ts) and compute the decay of this peak. The peak intensity decreases and the peak position moves toward short wave vectors. The integrated peak intensity decreases exponentially at short times with a characteristic decay time that depends on T, T1, and Ts. The increase of the Euler characteristic from large negative values toward zero suggests that the shift of the peak toward short wave vectors is associated with the disappearance of small connections in a bicontinuous structure formed in the early stages of spinodal decomposition. Slow decay of the surface area indicates that the domains keep their shape for a long time, despite the fast decay of the saturation of the concentration field inside them.
Self-trapping of the amide I band in a peptide model crystal
Authors: Edler J. , Hamm P.
Published: July 2002

A femtosecond pump-probe study of the peculiar amide I band of acetanilide, a molecular crystal with hydrogen bonded chains of peptide units, is presented. The almost perfect harmonicity of the 1666 cm−1 subpeak is related to significant delocalization of this state at low enough temperatures (93 K). The “anomalous” peak (1650 cm−1), on the other hand, is strongly anharmonic, and hence assigned to a self-trapped state. This assignment is in agreement with a more indirect previous work. With increasing temperature, thermal disorder localizes the 1666 cm−1 band (Anderson localization) and at the same time destroys the self-trapping mechanism. Both the self-trapped state and the delocalized state decay on a 2 ps time scale into states outside the spectral window of this study. The excitation energy reappears on a much slower 35 ps time scale in the form of an increased lattice temperature.
Conductivity relaxation and ion transport processes in glassy electrolytes
Authors: Baskaran N.
Published: July 2002

The ac conductivity and dielectric relaxation properties of silver selenomolybdate glasses, Y% AgI–(1–Y)%{MAg2O–F[0.4SeO3–0.6MoO3]} with varying dopant salt concentration, Y=20–80% have been investigated as a function of frequency and temperature. The dc conductivity is found to increase by more than two orders of magnitude with increasing AgI concentration. The variation of conductivity with AgI concentration has been rationalized by considering “mixed cluster tissue to amorphous AgI aggregate” model for glass structure. The presence of positional disorder of Ag+ ions in the AgI aggregate region is found to be responsible for the observed enhancement in ionic conduction and also to the variation of conductivity with AgI concentration. The ac conductivity is frequency independent at low frequencies and follows an apparent power law, σ(ω)∝ωn at the high-frequency region. The observed dispersion in conductivity with frequency is attributed to the high probability for the correlated forward–backward hopping of mobile ions in the high-frequency region. The temperature dependence of ac conductivity at fixed frequencies is analyzed using biexponential law with two distinct activation energies. The origin of the two activation energies observed in the ac conductivity is explained in the light of the jump relaxation model. The electrical modulus which describes the dielectric relaxation behavior of the glasses is fitted to a stretched exponential φ(t)=exp[−(t/τ)β] and it obeys time-temperature superposition.
Correlation spectrum analyzer for direct measurement of device current noise
Authors: Ferrari Giorgio , Sampietro Marco
Published: July 2002

This article analyzes the realization and the performance of a correlation spectrum analyzer specifically conceived to directly measure the current noise produced by electronic devices with maximum sensitivity. The text describes in detail and gives the design rules of the instrument input amplifiers taking into consideration noise, dynamic range, stability, and bandwidth, together with the effects that a device under test (DUT) having complex impedance introduce. This article shows that the proposed scheme may allow current noise measurements with a sensitivity improved by few orders of magnitude with respect to a standard spectrum analyzer and to a correlation analyzer in voltage scheme whenever the DUT has an impedance larger than few 10 kΩ. Such a sensitivity makes the proposed instrument ideal for the characterization of advanced devices, such as ultrashort channel metal-oxide-semiconductor field effect transistors, mesoscopic junctions, or spin dependent electron transfer devices where it may be necessary to detect noise levels as low as fA/Hz.
Effects of Nb doping on the TiO2 anatase-to-rutile phase transition
Authors: Arbiol J. , Cerdà J. , Dezanneau G. , Cirera A. , Peiró F. , Cornet A. , Morante J. R.
Published: July 2002

We study the influence of Nb doping on the TiO2 anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped TiO2 samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the TiO2 rutile nanoparticles.
Measurement of mechanical properties of three-dimensional nanometric objects by an atomic force microscope incorporated in a scanning electron microscope
Authors: Fukushima Kimitake , Kawai Shigeki , Saya Daisuke , Kawakatsu Hideki
Published: July 2002

An atomic force microscope that mounts on a sample stage of a scanning force microscope (SEMAFM) was developed. It was implemented for measurement of static mechanical properties of three-dimensional nanometric objects. The sample stage of the AFM was equipped with piezoelectric actuators enabling raster scanning as well as three degrees of freedom positioning with sub nm resolution and mm movable range. This enabled centering the AFM tip to the field of view of the SEM. Measurement of the spring constant and rupture force of three-dimensional nanometric structures was made possible. The SEMAFM also functioned as a conventional AFM.
Interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes
Authors: Arellano J. S. , Molina L. M. , Rubio A. , López M. J. , Alonso J. A.
Published: July 2002

Density functional theory has been used to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Static calculations allowing for different degrees of structural relaxation are performed, in addition to dynamical simulations. Molecular physisorption inside and outside the nanotube walls is predicted to be the most stable state of those systems. The binding energies for physisorption of the H2 molecule outside the nanotube are in the range 0.04–0.07 eV. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state, with the molecule dissociated and the two hydrogen atoms bonded to neighbor carbon atoms, has also been found. However, reaching this dissociative chemisorption state for an incoming molecule, or starting from the physisorbed molecule, is difficult because of the existence of a substantial activation barrier. The dissociative chemisorption deforms the tube and weakens the C–C bond. This effect can catalyze the shattering and scission of the tube by incoming hydrogen molecules with sufficient kinetic energy.
Buried superconducting layers comprised of magnesium diboride nanocrystals formed by ion implantation
Authors: Zhai H. Y. , Christen H. M. , White C. W. , Budai J. D. , Lowndes D. H. , Meldrum A.
Published: July 2002

Superconducting layers of MgB2 were formed on Si substrates using techniques that are widely used and accepted in the semiconductor industry. Mg ions were implanted into boron films deposited on Si or Al2O3 substrates. After a thermal processing step, buried superconducting layers comprised of MgB2 nanocrystals were obtained which exhibit the highest Tc reported so far for MgB2 on silicon (Tconset≈33.6 K, ΔTc=0.5 K, as measured by current transport). These results show that our approach is clearly applicable to the fabrication of superconducting devices that can be operated at much higher temperatures (≈20 K) than the current Nb technology (≈6 K) while their integration with silicon structures remains straight-forward.
Effects of N-induced heterogeneous nucleation and growth of cavities at the CoSi2/polycrystalline–silicon interface
Authors: Alberti A. , La Via F. , Rimini E.
Published: July 2002

N-induced cavity nucleation at the CoSi2/Si interface has been investigated for different doses and its influence on improving the thermal stability of the silicide layer up to 1075 °C has been detailed. The N-implant energy and doses have been chosen in such a way that the projected range (Rp) was near the interface and the underlying polycrystalline silicon substrate was completely amorphized. After a thermal treatment of 975 °C, it has been found that the cavity density depends on the dose, but the cavity size is dose independent. The cavity density should be enough to saturate the silicide grain boundaries but not so high to neglect ripening and coalescence. A density of 2.5×1010 cavities/cm2 has been measured at a nitrogen dose of 7×1015/cm2 corresponding to a partial saturation of the silicide grain boundaries and to the higher stability gain (150 °C).
The optimal tunneling path for the proton transfer in malonaldehyde
Authors: Tautermann Christofer S. , Voegele Andreas F. , Loerting Thomas , Liedl Klaus R.
Published: July 2002

The proton tunneling reaction in malonaldehyde at low temperatures is investigated. The principal aim of this study is to find the optimal tunneling path at 0 K in the framework of the semiclassical theory with a global optimization method. An amount of 11366 ab inito points was determined in the reaction swath (i.e., the conformational space enclosed by the minima and the transition state) of malonaldehyde. With a simulated annealing approach, the path with the smallest integral of the imaginary action through the swath from minimum to minimum was determined. Surprisingly the optimal tunneling path was found to be quite far off the large curvature tunneling path [i.e., the straight connection of the two minima large-current tunneling (LCT path)]. At the beginning, it is following the minimum energy path (MEP) (i.e. the path with the lowest energy connecting the two minima and passing through the transition state), and then it is describing a curved path through the reaction swath. This curve was determined several times with different annealing schemes, which ended up with the same result—the tunneling path is proceeding close to the MEP rather than to the LCT path. Along the optimal tunneling path, the ground-state tunneling splitting was calculated with a new semiclassical method introduced in an accompanying study [C. S. Tautermann, A. F. Voegele, T. Loerting, and K. R. Liedl, J. Chem. Phys. 117, 1967 (2002), following paper]. Another focus of investigation was the influence of deformation of the tunneling paths and a general scheme of determining an approximated optimal tunneling path at 0 K is introduced.
Triode-type field emission array using carbon nanotubes and a conducting polymer composite prepared by electrochemical polymerization
Authors: Jin Y. W. , Jung J. E. , Park Y. J. , Choi J. H. , Jung D. S. , Lee H. W. , Park S. H. , Lee N. S. , Kim J. M. , Ko T. Y. , Lee S. J. , Hwang S. Y. , You J. H. , Yoo Ji-Beom , Park Chong-Yun
Published: July 2002

We report a method to fabricate carbon nanotubes (CNT)/conducting polymer composite films for the application of CNTs to field emission displays. The composite was prepared by a combination of electrochemical polymerization of pyrrole and electrophoretic deposition of CNTs. We obtained a uniform CNT/conducting polypyrrole polymer composite film. The CNTs were mainly coated on protrusions of the polypyrrole film and emitted electrons without rubbing and stretching. We realized a triode-type field emission array (FEA) using the CNT/polypyrrol composite. This FEA showed that the emission current was modulated by gate voltage of 30 V. The film morphology and emission characteristics of the CNT/conducting polymer composite were studied using optical microscopy, scanning electron microscopy, and an emission test in vacuum.
Temperature dependence of the absorption saturation relaxation time in light- and heavy-ion-irradiated bulk GaAs
Authors: Mangeney J. , Stelmakh N. , Aniel F. , Boucaud P. , Lourtioz J.-M.
Published: July 2002

The absorption saturation relaxation time in light- and heavy-ion-irradiated GaAs saturable absorbers has been measured as a function of the temperature in the range from 7 to 300 K. For both types of samples, the relaxation time is shorter than 4 ps at 7 K. A regular increase of this time with temperature is observed for light-ion-irradiated samples, a value of 9.5 ps being reached at room temperature. In contrast, an almost temperature-independent relaxation time is found for heavy-ion-irradiated samples. The results are interpreted on the basis of a simplified relaxation model accounting for capture and emission from defect levels. We suggest that light-ion irradiation creates shallow centers whereas heavy-ion irradiation creates deep centers.
Interfacial structure of epitaxial MgB2 thin films grown on (0001) sapphire
Authors: Tian W. , Pan X. Q. , Bu S. D. , Kim D. M. , Choi J. H. , Patnaik S. , Eom C. B.
Published: July 2002

The microstructure and interfacial atomic structure of MgB2 thin films fabricated on the (0001) Al2O3 substrate were characterized by transmission electron microscopy. It was found that the MgB2 films grow epitaxially on the substrate with an orientation relationship with respect to the substrate as: (0001)MgB2∥(0001)Al2O3 and [112̄0]MgB2∥[101̄0]Al2O3. At the film/substrate interface, both MgO and MgAl2O4 phases were observed, which also grow epitaxially on the (0001) Al2O3 substrate. The formation of these intermediate phases is ascribed to the existence of oxygen during the annealing.
Excited state dynamics of Cl2O in the near ultraviolet
Authors: Aures Robert , Gericke Karl-Heinz , Maul Christof , Trott-Kriegeskorte Gundula , Kawasaki Masahiro , Nakano Yukio
Published: July 2002

The state-resolved and isotope-specific detection of nascent ClO generated from the photodissociation of Cl2O parent molecules is performed by observing single-color (2+1) resonance enhanced multiphoton ionization (REMPI) spectra following excitation in the wavelength range from 336 to 344 nm; additionally state-resolved detection of nascent ClO is performed by observing single-color two photon laser-induced fluorescence. The REMPI spectrum is assigned to the ClO(C 2Σ−,v′=0←X 2ΠΩ,v=0) transition. The population of rotational states up to J=130 is evidence of large rotational excitation induced by a strong dependence of the excited potential energy surface (PES) on the Jacobi angle γ. Formation of the Π3/22 spin-orbit state is preferred: P(2Π1/2):P(2Π3/2)=0.30±0.05 suggesting significant radiationless transfer from the excited PES to closely lying neighboring states. The anisotropy parameter for the transition is determined to be β=0.35 independent of the ClO quantum state. The principal excited electronic state is assigned to a B21←A11 transition, in agreement with recent ab initio calculations. The measured β value is smaller than the limiting value of 1.0, suggesting dynamic reasons and simultaneous excitation to more than one PES. A fast and direct fragmentation process is deduced, where the upper limit for the lifetime of the excited dissociative state is calculated to be 300 fs.
Vibrational energy relaxation of polyatomic molecules in liquids: The solvent’s perspective
Authors: Deng Yuqing , Stratt Richard M.
Published: July 2002

Vibrationally excited polyatomic molecules can relax in a variety of different ways in solution; the excess energy can be dissipated directly to the solvent, or it can be redistributed between any number of different intramolecular modes, with the liquid absorbing (or supplying) just enough energy to make the process work. What we consider here is how the solvent participates in these mechanistic choices. Using the prototypical example of a symmetric linear triatomic molecule, we compare the molecular origins of the vibrational friction for the direct vibrational cooling of the symmetric and antisymmetric stretching modes and contrast both of those with intramolecular vibrational energy transfer between these two modes. Instantaneous-normal-mode analysis reveals that a solid-statelike perspective is a plausible starting point for understanding these processes; the solvent does define a band of intermolecular vibrations, and it is only when the energy being transferred falls within that band that the solvent can easily accept energy from a solute. However, it is also possible to discern some more liquid-state-specific details. Despite their different symmetries and different kinematic requirements, all of the different relaxation pathways are apparently driven by the dynamics of the same instantaneously nearest solvents.
Well-width and doping-density dependence of 1.35 μm intersubband transition in InGaAs/AlAsSb quantum wells
Authors: Gopal A. V. , Yoshida H. , Simoyama T. , Georgiev N. , Mozume T. , Ishikawa H.
Published: July 2002

We report 1.35 μm intersubband absorption, in InGaAs/AlAsSb multiquantum wells, obtained by introducing a one monolayer AlAs layer at the well-barrier interface. The observed peak covers the useful communication wavelength range of 1.2–1.6 μm. Polarization-resolved, room-temperature absorption spectra and 77 K photoluminescence spectra measured on a series of samples reveal this short wavelength intersubband absorption peak (≈1.35 μm) in 7 and 9 monolayer wells that are doped in the well region to a density ⩽2×1018 cm−3. For heavier doping and in narrower wells, we do not observe this transition.
Small band gap bowing in In1−xGaxN alloys
Authors: Wu J. , Walukiewicz W. , Yu K. M. , Ager J. W. , Haller E. E. , Lu Hai , Schaff William J.
Published: July 2002

High-quality wurtzite-structured In-rich In1−xGaxN films (0⩽x⩽0.5) have been grown on sapphire substrates by molecular beam epitaxy. Their optical properties were characterized by optical absorption and photoluminescence spectroscopy. The investigation reveals that the narrow fundamental band gap for InN is near 0.8 eV and that the band gap increases with increasing Ga content. Combined with previously reported results on the Ga-rich side, the band gap versus composition plot for In1−xGaxN alloys is well fit with a bowing parameter of ∼1.4 eV. The direct band gap of the In1−xGaxN system covers a very broad spectral region ranging from near-infrared to near-ultraviolet.
Comparison of electromagnetic scattering measurements to simulation for microelectronic structures
Authors: Paul David P. , Conrad Edward W. , Cole Daniel C. , Barouch Eytan
Published: July 2002

We present an electromagnetic-based method that enables prediction of microlithographic structures of 100 nm and below, for analyzing manufacturability of next-generation microchip technology in real time, in situ. The method is robust, versatile, precise, and fast, and experimentally verified by using both scanning electron microscopy and atomic force microscopy.
Magnetization reversal and configurational anisotropy of dense permalloy dot arrays
Authors: Zhu Xiaobin , Grütter P. , Metlushko V. , Ilic B.
Published: July 2002

Electron beam patterned permalloy circular dots of 700 nm diameter with small separations were studied by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. Images in the demagnetized state show that the dot is in a vortex state with a vortex core (singularity) in the center. Local hysteresis loops, measured by cantilever frequency shift in an external field, indicate that the magnetization reversal of individual disks is a vortex nucleation and annihilation process. By carefully doing MFM, nucleation and annihilation fields without MFM tip stray field distortions are obtained. Configurational anisotropy originated from magnetostatic coupling is found through hysteresis loops.
Accurate oxygen-content determination method for decreased sample amounts of superconductive and other functional oxides
Authors: Yasukawa Yukiko , Yamauchi Hisao , Karppinen Maarit
Published: July 2002

In a conventional wet-chemical redox analysis applied to the determination of oxygen contents of nonstoichiometric functional oxides such as superconductive copper oxides, the sample amount required for a single experiment is typically 50–100 mg. Here we demonstrate that an improved coulometric titration method works to precisely analyze oxygen contents of decreased amounts of various complex oxides containing, e.g., copper, cobalt, or iron at mixed valence values. The improved titration method is found to yield the oxygen content of the YBa2Cu3O7−δ reference sample with high reproducibility, i.e., |Δδ|<0.01, for samples with a mass of only ∼10 mg.
Competition between ferromagnetic metallic and paramagnetic insulating phases in manganites
Authors: Li G. , Zhou H.-D. , Feng S. J. , Fan X.-J. , Li X.-G. , Wang Z. D.
Published: July 2002

La 0.67 Ca 0.33 Mn 1−x Cu x O 3  (x=0 and 0.15) epitaxial thin films were grown on the (100) LaAlO3 substrates, and the temperature dependence of their resistivity was measured in magnetic fields up to 12 T by a four-probe technique. We found that the competition between the ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases plays an important role in the observed colossal magnetoresistance (CMR) effect. Based on a scenario that the doped manganites approximately consist of phase-separated FM and PI regions, a simple phenomenological model was proposed to describe the CMR effect. Using this model, we calculated the resistivity as functions of temperature and magnetic field. The model not only qualitatively accounts for some main features related to the CMR effect, but also quantitatively agrees with the experimental observations.
Electron-hole pair generation energy in gallium arsenide by x and γ photons
Authors: Bertuccio Giuseppe , Maiocchi Diego
Published: July 2002

The mean energy necessary to generate an electron-hole pair in gallium arsenide by x and γ photons has been measured in the 230–320 K temperature range. The experimental apparatus consists of a Schottky junction on a high-quality epitaxial GaAs, a silicon detector that generates a reference charge signal and highly stable low-noise electronics. The resolution of the system in measuring the generation energy is better than 2 meV. An apparent value of ε=4.216 eV has been measured at 300 K, lower than previously published values of 4.35 and 4.27 eV for α particles and 4.57 eV for electrons. Charge trapping in the GaAs detector, although very weak (less than 1%), has been observed and a model, based on the Hecht theory, has been developed to derive the true value of ε=4.184±0.025 eV at 300 K. The dependence of ε on the temperature, in the 230–320 K range, has been found linear, ε=4.55−0.00122×T [eV]. The dependence of ε on the band gap has been determined ε=2.76×Eg+0.257 [eV], in good agreement with the Shockley-Klein theory. At 300 K, the found value of ε in GaAs, together with those reported for germanium and silicon, yields a linear dependence of ε on the band-gap energy ε=1.83×Eg+1.6 [eV], with a correlation coefficient of 0.9997.
Tungsten nanowires and their field electron emission properties
Authors: Lee Yun-Hi , Choi Chang-Hoon , Jang Yoon-Taek , Kim Eun-Kyu , Ju Byeong-Kwon , Min Nam-Ki , Ahn Jin-Ho
Published: July 2002

We report the fabrication of tungsten nanowires, by simple thermal treatment of W films, that behave as self-catalytic layers and their excellent electron field emission properties as well. The obtained nanowires have a diameter ranging from 10 to 50 nm, showing perfect straightness and neat appearance. Typical turn-on field for the electron emission is about 5 V/μm, and the field enhancement factor β becomes 38 256, which is very close to that of the high efficient single-wall carbon nanotube emitters. The most exciting result is the possibility of easy fabrication of perfectly straight nanowires as promising building blocks for terabit-level interconnection and nanomachine components without the intentional use of any heterogeneous catalysts.
Selective-area growth of indium nitride nanowires on gold-patterned Si(100) substrates
Authors: Liang C. H. , Chen L. C. , Hwang J. S. , Chen K. H. , Hung Y. T. , Chen Y. F.
Published: July 2002

This letter reports the synthesis of indium nitride (InN) nanowires on gold-patterned silicon substrates in a controlled manner using a method involving thermal evaporation of pure indium. The locations of these InN nanowires were controlled by depositing gold in desired areas on the substrates. Scanning electron microscopy and transmission electron microscopy investigations showed that the InN nanowires are single crystals with diameters ranging from 40 to 80 nm, and lengths up to 5 μm. Energy dispersive x-ray spectrometry showed that the ends of the nanowires are composed primarily of Au, and the rest of the nanowires were InN with no detectable Au incorporations. The Raman spectra showed peaks at 445, 489, and 579 cm−1, which are attributed to the A1(transverse optical), E2, and A1(longitudinal optical) phonon modes of the wurtzite InN structure, respectively. Photoluminescence spectra of the InN nanowires showed a strong broad emission peak at 1.85 eV.
Structure and bonding in a cubic phase of SiAlON derived from the cubic spinel phase of Si3N4
Authors: Ouyang Lizhi , Ching W. Y.
Published: July 2002

The structure and electronic bonding in the spinel SiAlON (Si6−zAlzOzN8−z, z=1) derived from the cubic c-Si3N4 are studied by a first-principles density functional method. Al prefers the octahedral site of the spinel lattice. The small energy difference between the four possible structural configurations indicates that the real SiAlON may be a random solid solution. The lowest energy configuration of c-Si5AlON7 is a semiconductor with a direct LDA band gap of 2.29 eV.
Platinum–hydrogen complexes in silicon observed by measurements of optical absorption and electron spin resonance
Authors: Fukata N. , Mchedlidze T. , Suezawa M. , Saito K. , Kasuya A.
Published: July 2002

Platinum–hydrogen (Pt–H) complexes in Si doped with Pt and H by heating at 1000–1300 °C followed by quenching in water were investigated from the measurements of optical absorption at 5 K and electron spin resonance (ESR) at 8 K. Optical absorption peaks at 1909.1 and 1910.3 cm−1 were observed in addition to the peaks due to the PtH and PtH2 complexes. The H doping temperature dependence of these peaks showed that the number of H atoms in the complex responsible for the 1909.1 cm−1 peak is larger than that for the 1910.3 cm−1 peak. We also observed ESR signals due to the PtH3 complex. The annealing behaviors of the 1910.3 cm−1 peak and the ESR signals were almost the same. Based on these results, the 1909.1 and 1910.3 cm−1 peaks are, respectively, assigned to the PtH4 complex and the PtH3 complex.
Improvement of the dosimetric properties of chemical-vapor-deposited diamond films by neutron irradiation
Authors: Bruzzi Mara , Menichelli David , Pini Silvia , Bucciolini Marta , Mólnar József , Fenyvesi András
Published: July 2002

The performance of chemical-vapor-deposited (CVD) diamond films as on-line dosimeters has been substantially improved after irradiation with fast neutrons up to a fluence of 5×1014 n/cm2. This is correlated to a decrease of more than one order of magnitude in the concentration of deep levels with activation energy in the range 0.9–1.4 eV, as observed by thermally stimulated current and photoinduced current transient spectroscopy. As a consequence, a fast and reproducible dynamic response is observed during irradiation with a 6 MV photon beam from linear accelerator and with a Co60 source. A quasilinear dependence of the current on the dose rate is obtained in the range of interest for clinical applications (0.1–10 Gy/min). The resulting sensitivity is definitely higher than that of standard ionization chambers, and compares favorably with those of standard silicon dosimeters and of best-quality natural and CVD diamond devices.
Soft magnetism of Fe–Co–N thin films with a Permalloy underlayer
Authors: Sun N. X. , Wang S. X.
Published: July 2002

A systematic investigation was carried out on the Fe–Co–N thin films with varying Permalloy underlayer thicknesses (in short Py\Fe–Co–N films), which show significantly improved magnetic softness when a very thin Permalloy underlayer is applied. The easy axis coercivities of the Py\Fe–Co–N films drop from 1.3 kA/m (16 Oe) to 640 A/m (8 Oe) with an appropriate Permalloy underlayer; and the corresponding hard axis coercivity drops from 320 A/m (4 Oe) to less than 80 A/m (1 Oe). The highest transverse permeability is observed in the Py\Fe–Co–N film with a Permalloy underlayer of 3.5 nm, which also shows the smallest dispersion angle α50 among the Py\Fe–Co–N films. The changes in the mean grain size, texture, and magnetostriction are determined to not be responsible for the observed magnetic softness in the Py\Fe–Co–N films. In contrast, a tantalum underlayer does not reduce the coercivities and dispersion angles in the Ta\Fe–Co–N films. Based on these results, an exchange-induced ripple reduction mechanism is proposed to explain the effects of the Permalloy underlayer on the Py\Fe–Co–N films.
Electron collision cross sections for tetraethoxysilane
Authors: Morgan W. Lowell , Winstead Carl , McKoy Vincent
Published: July 2002

We describe the development of a consistent set of low-energy electron-collision cross sections for tetraethoxysilane, also known as tetraethylorthosilicate. Ab initio calculations are used to obtain the elastic and momentum-transfer cross sections. These are combined with measured values of the ionization cross section and with model vibrational and electronic excitation cross sections to form a preliminary cross section set, which is then adjusted to achieve consistency with measured swarm parameters.
Dynamical chaos and low-temperature surface diffusion of small adatom clusters
Authors: Kovalev A. S. , Landau A. I.
Published: July 2002

The nonlinear one-dimensional dynamics of diatomic adatom clusters on an atomically grooved crystal surface is investigated by a numerical simulation method. It is shown that for initial conditions such that a given dynamic Hamiltonian of the system possesses chaotic behavior in phase space, the dynamical chaos is manifested in the coordinate space as diffusive motion of the cluster. The process investigated is fundamentally different from ordinary thermal diffusion and can apparently be manifested at low temperatures.
Lattice dynamics and heat capacity of a two-dimensional monoatomic crystal on a substrate
Authors: Antsygina T. N. , Poltavsky I. I. , Poltavskaya M. I. , Chishko K. A.
Published: July 2002

A model is proposed which gives an analytical description of the dynamics of collective excitations of two-dimensional close-packed atomic crystal lattices (atomic monolayers on substrates). The model takes into account both the interaction between atoms of the layer and the interaction of the layer with the substrate. The phonon spectra are found for an ideal triangular lattice and for a triangular lattice with a uniform distortion along one of the close-packed directions in the plane of the layer. The temperature dependence of the heat capacity is constructed for crystalline structures of both the commensurate and incommensurate types (in relation to the substrate). The theoretical results obtained are used for a detailed discussion and interpretation of the published experimental data on the spectra of lattice excitations and the heat capacity of monolayers of rare gases, including He3 and He4, on various types of substrates.
High-purity carbon nanotubes synthesis method by an arc discharging in magnetic field
Authors: Anazawa Kazunori , Shimotani Kei , Manabe Chikara , Watanabe Hiroyuki , Shimizu Masaaki
Published: July 2002

We developed a synthesis method of multiwalled carbon nanotubes (MWNTs), in which an arc discharging was controlled by a magnetic field. Using this method, we can obtain high-purity MWNTs (purity >95%) without purification which disorders walls of MWNTs. The current–voltage measurements show that the carriers would transport ballistically through our defect-free MWNTs with the maximum current density of ∼1011 A/m2. Therefore, our method provides defect-free/high-purity MWNTs as nanosized electric wires for device fabrication.
Effect of internal bias field on 180° domain switching in telluric acid ammonium phosphate crystal
Authors: Matyjasek K.
Published: July 2002

The depolarization effect in a telluric acid ammonium phosphate crystal has been studied by direct imaging of the domain structure by nematic liquid-crystal decoration technique. The stabilization mechanism of 180° domains appears to have a correlation to a defect—induced internal field in the crystal. The internal field acts as a driving field causing nucleation and growth of domains. The strong effect of the poling voltage parameters (duration and amplitude) on the depolarization process is reported.
Self-assembled fabrication of aluminum–silicon nanowire networks
Authors: Paulose Maggie , Grimes Craig A. , Varghese Oomman K. , Dickey Elizabeth C.
Published: July 2002

Uniquely structured two-dimensional aluminum–silicon alloy nanowire networks are fabricated on glass and silicon substrates by dealloying an aluminum–silicon thin film through selective chemical etching. The nanowire network is comprised of 3–6 nm diameter wires with lengths of 50–200 nm, and a wire density of approximately 1010 wires/cm2. The spatial extent of the fabricated nanowire network is limited only by the substrate dimensions. Current–voltage measurements reveal the metallic nature of the nanowires, with current propagating throughout the nanowire network. The fabrication route offers the possibility for achieving billion wire/cm2 devices as a relatively low cost production commodity.
From nucleation to coalescence of Cu2O islands during in situ oxidation of Cu(001)
Authors: Yang J. C. , Evan D. , Tropia L.
Published: July 2002

The nucleation, growth, and coalescence of Cu2O islands due to oxidation of Cu(001) films were visualized by in situ ultrahigh-vacuum transmission electron microscopy. We have previously demonstrated that the nucleation and initial growth of copper oxides is dominated by oxygen surface diffusion. These surface models have been extended to quantitatively represent the coalescence behavior of copper oxidation in the framework of the Johnson–Mehl–Avrami–Kolmogorov theory. An excellent agreement exists between the experimental data of nucleation to coalescence with the surface model. The implication could be an alternate paradigm for passivation and oxidation, since classic theories assume uniform film growth.
(
Zr,Ti
)
O
2
interface structure in ZrO2–TiO2 nanolaminates with ultrathin periodicity
Authors: Aita C. R. , DeLoach J. D. , Yakovlev V. V.
Published: July 2002

A mixed cation interfacial structure in ZrO2–TiO2 nanolaminate films with ultrathin bilayer periodicity grown by sputter deposition at 297 K was identified by x-ray diffraction and nonresonant Raman spectroscopy. This structure consists of an amorphous phase at a ZrO2-on-TiO2 bilayer interface, followed by an extensive crystalline monoclinic (Zr,Ti)O2 solid solution predicted by Vegard’s law. Monoclinic (Zr,Ti)O2 has previously been reported only once, in bulk powder of a single composition (ZrTiO4) at high pressure. Its stabilization in the nanolaminates is explained by the Gibbs–Thomson effect. This complex interfacial structure is shown to be a means of accommodating chemical mixing in the absence of a driving force for heteroepitaxy.
Effects of titanium and tantalum adhesion layers on the properties of sol-gel derived SrBi2Ta2O9 thin films
Authors: Leu Ching-Chich , Lin Hung-Tao , Hu Chen-Ti , Chien Chao-Hsin , Yang Ming-Jui , Yang Ming-Che , Huang Tiao-Yuan
Published: July 2002

The effects of titanium (Ti) and tantalum (Ta) adhesion layers on the ferroelectric and microstructural properties of sol-gel-derived SrBi2Ta2O9 (SBT) films are investigated. It is found that the atoms from the adhesion layer play a significant role in the resultant microstructures and physical properties of SBT films. The electron spectroscopy for chemical analysis clearly indicates that both the Ti and Ta atoms of adhesion layers have out-diffused behavior onto the surface of bottom-electrode Pt films after a thermal treatment of 750 °C, 1 min. Various out-diffused species do cause the distinct properties of SBT films, which are confirmed by the results of surface analysis and polarization-electric field (P-E) measurements. The formation of undesirable second phase compounds near the SBT/PT interface has been observed in specimens with Ti layer, and it is speculated to be the significant degradation of spontaneous polarization. On the contrary, Ta species are found to exhibit the pure bismuth-layered structure (BLS) phase and the good ferroelectric properties of SBT films, which have been identified by x-ray diffraction spectra and P-E measurements. The remnant polarization values of SBT films on top of Pt/Ti/SiO2/Si and Pt/Ta/Ti/SiO2/Si substrates are 11.1 and 14.2 μC/cm2 at 5 V, respectively. The same phenomenon was also observed at specimens with an increased annealing temperature of 800 °C. Consistently, the SBT film with Ta adhesion layer displays a better polarization value of 16.2 μC/cm2 than that with Ti adhesion layer (12.6 μC/cm2). The fatigue tests of all samples depict the near-fatigue-free characteristics except for the specimens with Ti adhesion layer crystallized at 800 °C. It is suggested that the employment of the Ta element for the adhesion layer in the SBT process is more favorable to obtain a desirable BLS phase and to improve the ferroelectric properties.
Nanoscopic measurements of surface recombination velocity and diffusion length in a semiconductor quantum well
Authors: Malyarchuk V. , Tomm J. W. , Talalaev V. , Lienau Ch. , Rinner F. , Baeumler M.
Published: July 2002

We use a near-field microscopic technique to probe photoluminescence from the edge area of a quantum well. Near the edge, surface recombination gives rise to a gradual variation of the photoluminescence signal on a micrometer length scale. The overall shape in this transition region depends strongly on the excitation intensity. From solving two dimensional diffusion equations, we deduce the surface recombination velocity and the diffusion length. It is shown that the surface recombination velocity decreases with increasing intensity due to the saturation of nonradiative defect states.