bluuebunny/crossref_metadata_2025 huggingface dataset:
Blue- and green-emitting SrS:Cu electroluminescent devices deposited by the atomic layer deposition technique
Authors: Ihanus Jarkko , Ritala Mikko , Leskelä Markku , Soininen Erkki , Park Wounjhang , Kaloyeros Alain E. , Harris William , Barth Karl W. , Topol Anna W. , Sajavaara Timo , Keinonen Juhani
Published: December 2003

Observed structural, compositional, and optical differences between blue- and green-emitting SrS:Cu thin films deposited by the atomic layer deposition technique were studied. The use of hydrogen in the deposition process resulted in green-emitting electroluminescent devices. The green-emitting devices had also the highest potassium (K) content, but the exact role of K was not resolved. The green emission is thought to originate from copper pairs and isolated off-centered copper ions at octahedral sites, whereas blue emission is suggested to originate from copper ions at tetrahedral interstitial sites. The brightest blue-emitting device, measured at 60 Hz, emitted 17 cd/m2 at 40 V above the threshold voltage (L40) with x=0.17 and y=0.30 as the Commission Internationale de l’Eclairage color coordinates. Conversely, the brightest green-emitting device emitted 44 cd/m2 with x=0.33 and y=0.58. The blue- and green-emitting devices also showed different time-resolved electroluminescence behavior, which suggests reduced dynamic space charge within the green-emitting phosphors resulting in the higher emission intensity in the green-emitting devices.
Effect of electromechanical coupling on the strain in AlGaN/GaN heterojunction field effect transistors
Authors: Jogai B. , Albrecht J. D. , Pan E.
Published: December 2003

The strain in AlGaN/GaN heterojunction field-effect transistors (HFETs) is examined theoretically in the context of the fully coupled equation of state for piezoelectric materials. Using a simple analytical model, it is shown that, in the absence of a two-dimensional electron gas (2DEG), the out-of-plane strain obtained without electromechanical coupling is in error by about 30% for an Al fraction of 0.3. This result has consequences for the calculation of quantities that depend directly on the strain tensor. These quantities include the eigenstates and electrostatic potential in AlGaN/GaN heterostructures. It is shown that for an HFET, the electromechanical coupling is screened by the 2DEG. Results for the electromechanical model, including the 2DEG, indicate that the standard (decoupled) strain model is a reasonable approximation for HFET calculations. The analytical results are supported by a self-consistent Schrödinger–Poisson calculation that includes the fully coupled equation of state together with the charge-balance equation.
Monte Carlo study of kink effect in short-channel InAlAs/InGaAs high electron mobility transistors
Authors: Vasallo B. G. , Mateos J. , Pardo D. , González T.
Published: December 2003

A semiclassical two-dimensional ensemble Monte Carlo simulator is used to perform a physical microscopic analysis of the kink effect in short-channel InAlAs/InGaAs lattice-matched high electron mobility transistors (HEMTs). Due to the small band gap of InGaAs, these devices are very susceptible to suffer impact ionization processes, with the subsequent hole transport in the channel, both supposedly implicated in the kink effect and easy to be implemented in a Monte Carlo simulation. The results indicate that for high enough VDS, holes, generated by impact ionization, tend to pile up in the channel under the source side of the gate due to the attracting potential caused by the surface charge at the recess and, mostly, by the gate potential. Due to this pile up of positive charge, the potential barrier controlling the current through the channel is lowered, so that the channel is further opened and ID increases, leading to the well known kink effect in the current–voltage characteristics. The microscopic understanding of this phenomenon provides valuable information to conceive the optimum fabrication process for kink-effect-free HEMTs.
Quasi-adiabatic clocking of quantum-dot cellular automata
Authors: Mandell Eric S. , Khatun Mahfuza
Published: December 2003

We present a theoretical study of quasi-adiabatic clocking of quantum-dot cellular automata (QCA). Quasi-adiabatic clocking refers to periodical modulation of interdot potential barriers in order to keep the cells of a QCA device near their ground state throughout the entire switching process. The barrier modulation has been studied through the use of a trapezoidal-shaped, periodic, time-dependent, electric field. The time-dependent electric field has been calculated for arrays of linear charged rods. A continuous traveling maximum in the electric field represents the flow of information from one zone to the next. For a QCA device where the zones are set up, such that the flow of information is linear, a line of electrostatically charged rods can quasi-adiabatically clock the system.
Tunable fractional Fourier transformer for ballistic electrons
Authors: Dragoman Daniela , Dragoman Mircea
Published: December 2003

We show that a two-dimensional electron gas in a magnetic field can act as a tunable fractional Fourier transformer for the wave function of ballistic electrons traveling in the direction of the magnetic field. The degree of fractionality of this device, a graded-index electron waveguide, can be controlled through the magnetic field. This unique feature is not encountered in graded-index optical waveguides.
Lateral ion transport in nematic liquid-crystal devices
Authors: Neyts K. , Vermael S. , Desimpel C. , Stojmenovik G. , Verschueren A. R. M. , de Boer D. K. G. , de Boer D. K. G. , Snijkers R. , Machiels P. , van Brandenburg A.
Published: December 2003

Liquid crystals used in electronic displays usually contain small amounts of ions that move under the influence of the varying applied electric field. It is well known that the motion of ions perpendicular to the substrates may lead to modified electric fields resulting in image sticking effects. During operation, the modulation in the director tilt angle can also lead to a net residual lateral component of the ion motion, parallel with the glass plates. A sustained ac driving voltage will accumulate the lateral ion motion and may result in image sticking effects near the pixel edge. Such effects have indeed been observed in supertwisted nematic cells.
Investigation of charging phenomena in silicon nanocrystal metal–oxide–semiconductor capacitors using ramp current–voltage measurements
Authors: Ioannou-Sougleridis V. , Nassiopoulou A. G.
Published: December 2003

Current–voltage (I–V) measurements with different ramp rates were used to investigate charge trapping phenomena in silicon nanocrystal metal–oxide–semiconductor capacitors, fabricated by low-pressure chemical vapor deposition of very thin silicon layers on a tunneling silicon oxide and subsequent high-temperature thermal oxidation. The nanocrystal size was in the range of 1–2 nm. Capacitance–voltage curves deduced from the ramp I–V measurements showed an N-shaped peak after a threshold voltage, from which the charge trapped in the structure was calculated.
Formation of low resistance nonalloyed Al/Pt ohmic contacts on n-type ZnO epitaxial layer
Authors: Kim Han-Ki , Kim Kyoung-Kook , Park Seong-Ju , Seong Tae-Yeon , Adesida Ilesanmi
Published: December 2003

We have investigated nonalloyed Al/Pt ohmic contacts on n-type ZnO:Al (nd=2.0×1018 cm−3). It is shown that the as-deposited Al/Pt contacts produce a specific contact resistivity of 1.2×10−5 Ω cm2. Auger electron spectroscopy and x-ray photoelectron spectroscopy depth profile results show interdiffusion between oxygen and aluminum, resulting in an increase of carrier concentrations near the ZnO surface. The increase of the carrier concentration at the surface region of ZnO is attributed to the low resistance of the nonalloyed Al/Pt contact.
Thermo-optic properties of sol-gel-fabricated organic–inorganic hybrid waveguides
Authors: Wang Xianjiang , Xu Lei , Li Dongxiao , Liu Liying , Wang Wencheng
Published: December 2003

Organic/inorganic hybrid glass waveguides were fabricated by the sol-gel technique followed by direct ultraviolet patterning. The thermo-optic coefficient (dn/dT) of the hybrid material was measured by a prism-coupling method and was found to be −1.44×10−4/°C, comparable to those of typical polymer materials. In order to investigate the possibility of using this hybrid glass material as a thermo-optic waveguide material, a 2×2 Mach–Zehnder interferometric optical switch was fabricated on a silicon wafer. At the wavelength of 1.55 μm, the switch response time was measured to be less than 4.2 ms, and the switching power was 9.3 mW. Experimental results show that this hybrid material is very suitable for application in integrated thermo-optic components.
Composition dependence of the interband critical points in Si1−xGex alloys: A fractional–dimensional space approach
Authors: Tao Keyu , Zhang Yueli , Mo Dang , Sano Naokatsu , Kaneko Tadaaki
Published: December 2003

The fractional–dimensional space approach is used to analyze the compositional dependence of direct interband transitions in Si1−xGex. The pertinent critical point (CP) parameters are obtained, and a CP structure, identified as E1′, is found. With regard to the CP energy, general agreement with the results obtained by the standard treatment is achieved. The other CP parameters, such as dimensionality, lifetime broadening, and amplitude, directly show an effect of a residual oxide overlayer. Our research shows that the fractional–dimensional space approach has the advantages of directness, flexibility, and sensitivity which provide a straightforward and rapid analysis of critical points, especially useful for deformed optical spectra.
Persistent photoconductivity and optical quenching of photocurrent in GaN layers under dual excitation
Authors: Ursaki V. V. , Tiginyanu I. M. , Ricci P. C. , Anedda A. , Hubbard S. , Pavlidis D.
Published: December 2003

Persistent photoconductivity (PPC) and optical quenching (OQ) of photoconductivity (PC) were investigated in a variety of n-GaN layers characterized by different carrier concentrations, luminescence characteristics, and strains. The relation between PPC and OQ of PC was studied by exciting the samples with two beams of monochromatic radiation of various wavelengths and intensities. The PPC was found to be excited by the first beam with a threshold at 2.0 eV, while the second beam induces OQ of PC in a wide range of photon energies with a threshold at 1.0 eV. The obtained results are explained on the basis of a model combining two previously put forward schemes with electron traps playing the main role in PPC and hole traps inducing OQ of PC. The possible nature of the defects responsible for optical metastability of GaN is discussed.
Characterization of atomic force microscope probes at low temperatures
Authors: Radenovic Alexandra , Bystrenova Eva , Libioulle Laurent , Valle Francesco , Shubeita George T. , Kasas Sandor , Dietler Giovanni
Published: December 2003

Different types of atomic force microscopy (AFM) probes were characterized under ultrahigh vacuum conditions and at low temperatures. Properties of AFM probes, such as the resonance frequency, the spring constant and quality factor of cantilevers, depend on temperature. A typical shift in the resonance frequency as a function of temperature was observed for all kinds of cantilevers studied. This was related to the change in temperature of Young’s modulus of the cantilever material. Moreover, force–distance curves acquired at low temperatures and on different substrates, elucidate the importance of the hydrophobicity of the sample surface and that of the tips for lowering adhesion forces. Finally, all of the probes were imaged in a scanning electron microscope as a function of the temperature. A bending of the coated cantilever at low temperatures was observed, which explains the peculiar force–distance curves. As a consequence, the use of uncoated cantilevers for low-temperature applications is recommended.
Epitaxial C49–TiSi2 phase formation on the silicon (100)
Authors: Yang Jun-Mo , Park Ju-Chul , Park Dae-Gyu , Lim Kwan-Yong , Lee Soun-Young , Park Sung-Wook , Kim Youn-Joong
Published: December 2003

The crystallographic characteristics of an epitaxial C49–TiSi2 island formed on the Si (100) substrate were investigated by high-resolution transmission electron microscopy (HRTEM). The analysis results clearly showed that the optimum epitaxial relationship between the C49–TiSi2 phase and the Si substrate is [001]C49//[011]Si and (010)C49//(100)Si. We found that the interfacial energy at the C49–TiSi2/Si interface is relaxed by the formation of misfit dislocations and/or atomic steps, and consequently the epitaxial C49 phase is thermally stable so it is not transformed to the C54 phase even after high-temperature annealing above 900 °C. Further, the mechanism on the formation of the epitaxial C49 phase on the Si substrate and the atomic arrangement of stacking faults lying on the C49 (020) plane are discussed through the analysis of HRTEM images.
Influence of phosphine on Ge/Si(001) island growth by chemical vapor deposition
Authors: Kamins T. I. , Medeiros-Ribeiro G. , Ohlberg D. A. A. , Williams R. Stanley
Published: December 2003

When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge to form distinctive, three-dimensional islands. The shape of the islands is determined by the energies of the surface facets, facet edges, and interfaces. When phosphorus is added during chemical vapor deposition of Ge, the surface energies change, modifying the island shapes and sizes. Three different island shapes are found for doped layers, as for undoped layers; however, each doped island type is smaller than the corresponding undoped island type. The intermediate-size doped islands are of the same family as the undoped multifaceted “dome” structures, but are considerably smaller; they also have a narrow size distribution. The largest doped islands are related to the defective “superdomes” found for undoped islands, but are bounded by a smaller number of facets, creating pyramidal-shaped structures with their edges aligned along 〈110〉 directions. The distribution of Ge among the different island types depends on the phosphine partial pressure. Phosphorus appears to act as a mild surfactant, suppressing small islands at high PH3 partial pressures. Within the assumptions made, the segregation enthalpy is estimated to be −0.4 eV. Phosphine decreases the Ge deposition rate because of competitive adsorption; however, the steady-state surface coverage (as indicated by the Ge deposition rate) is not reached for thin layers.
Modification of (Pb,La)(Zr,Ti)O3 thin films during pulsed laser liftoff from MgO substrates
Authors: Tsakalakos L. , Sands T. , Carleton E. , Yu K. M.
Published: December 2003

Pulsed excimer laser irradiation of an absorbing thin film through a transparent substrate has been employed to separate epitaxial (Pb,La)(Zr,Ti)O3 (PLZT) thin films from their MgO growth substrates. The morphology, microstructure, and dielectric properties of the laser-modified PLZT layer generated at the original PLZT/MgO interface by this laser liftoff (LLO) process was investigated. Atomic force microscopy measurements of the film surface after LLO indicated average surface roughness on the order of 90–100 nm, complementing previously reported scanning electron microscopy studies that revealed what appeared to be a rough glassy layer. The surface roughness is thought to form as the original growth substrate is ejected from the films transferred towards the end of the 38 ns pulse, pulling the molten near-interfacial portion of the PLZT film via surface tension, and followed by quenching of the liquid phase. Transmission electron microscopy of PLZT films transferred to Si wafers showed the surface layer consists of an irregular amorphous layer ranging in thickness from 50 to 100 nm, although some surface features protruded as far as 300 nm. Enhanced levels of lead were found to be present in the amorphous phase by Rutherford backscattering spectrometry, implying PbO evaporation and subsequent condensation during LLO. Electrical measurements determined the relative dielectric constant of the surface layer to be in the range of 40 – 70. Subsequent ion milling of the layer demonstrated that the ferroelectric and dielectric properties of the original PLZT films were largely recovered, while the surface roughness was reduced to below 40 nm. The observed differences between the dielectric and ferroelectric properties of the film transferred after complete removal of the amorphous layer compared to those of the film on the original MgO substrate are attributed to the removal of substrate clamping.
Phase behavior of hard-core lattice gases: A fundamental measure approach
Authors: Lafuente Luis , Cuesta José A.
Published: December 2003

We use an extension of fundamental measure theory to lattice hard-core fluids to study the phase diagram of two different systems. First, two-dimensional parallel hard squares with edge-length σ=2 in a simple square lattice. This system is equivalent to the lattice gas with first and second neighbor exclusion in the same lattice, and has the peculiarity that its close packing is degenerated (the system orders in sliding columns). A comparison with other theories is discussed. Second, a three-dimensional binary mixture of parallel hard cubes with σL=6 and σS=2. Previous simulations of this model only focused on fluid phases. Thanks to the simplicity introduced by the discrete nature of the lattice we have been able to map out the complete phase diagram (both uniform and nonuniform phases) through a free minimization of the free energy functional, so the structure of the ordered phases is obtained as a result. A zoo of entropy-driven phase transitions is found: one-, two- and three-dimensional positional ordering, as well as fluid-ordered phase and solid-solid demixings.
A global electric dipole function of ammonia and isotopomers in the electronic ground state
Authors: Marquardt Roberto , Quack Martin , Thanopulos Ioannis , Luckhaus David
Published: December 2003

A global analytical representation of the electric dipole hypersurface for ammonia and isotopomers is developed as a function of bond lengths and bond angles. Its simple and general form allows for the simultaneous description of all three dipole moment components using a small number of parameters. The parameter values are determined by adjustment to dipole moment values obtained from ab initio calculations at the second-order Møller–Plesset and multiconfiguration self-consistent field level of theory. The dipole function is used to calculate six-dimensional transition moments for NH3, using wave functions obtained from a variational calculation and a global analytical representation of the potential hypersurface. The comparison with experiment demonstrates a fairly reliable description of the electric dipole hypersurface for molecular structures with potential energy equivalent to up to 10 000 cm−1. At higher excitations a qualitatively correct asymptotic behavior of the dipole function is assured by construction.
A comparison of stable carbonyls formed in the gas-phase reaction between group 10 atomic anions and methanol or methoxy radicals: Anion photoelectron spectroscopy and density functional theory calculations on HNiCO−, PdCO−, and PtCO−
Authors: Chatterjee Bappaditya , Akin F. Ahu , Jarrold Caroline Chick , Raghavachari Krishnan
Published: December 2003

The 3.49 eV photoelectron spectra of HNiCO−, PdCO−, and PtCO− generated from the gas-phase reaction of atomic anions with methanol are presented and interpreted in the context of new density functional calculations. The calculated electron affinities (EAs) for PdCO (0.58 eV) and PtCO (1.13 eV) are in excellent agreement with experimental values [0.606(10) and 1.212(10) eV, respectively] while for HNiCO, the calculated EA (1.64 eV) is 0.48 eV lower than the experimental value. However, in all three cases, the vibrational frequencies and spectral profiles are consistent with calculated structures, frequencies, and normal coordinates. HNiCO− and HNiCO are predicted to be linear, with Σ+1 and Δ2 ground states, respectively. Calculations on PdCO− agree with recent calculations by Andrews and co-workers [J. Phys. Chem. A 104, 3905 (2000)] which predict a bent A′2 ground state. The ground state of PtCO− is calculated to be the linear Σ+2 state. Both PdCO and PtCO are predicted to have Σ+1 ground states. For all species, backdonation of dπ-electron density into the CO π* orbital is an important component of the metal carbonyl bond, with the effect being more pronounced in the anions.
A high-resolution neutron powder diffraction study of ammonia dihydrate (ND3⋅2D2O) phase I
Authors: Fortes A. D. , Wood I. G. , Brodholt J. P. , Alfredsson M. , Vočadlo L. , McGrady G. S. , Knight K. S.
Published: December 2003

We have measured the thermal expansivity of ammonia dihydrate (ND3⋅2D2O) phase I from 4.2 to 174 K at ambient pressure, and the incompressibility at 174 K from 0 to 0.45 GPa, using time-of-flight neutron powder diffraction. The unit cell volume as a function of temperature, V(T), was fitted with a Grüneisen approximation to the zero-pressure equation of state (with the lattice vibrational energy calculated from a double-Debye model fitted to heat capacity data) having the following parameters at zero pressure and temperature: V0,0=356.464±0.005 Å3, (K0,0/γ)=7.163±0.024 GPa, and K0,0′=5.41±0.33 (where VP,T is the unit cell volume at pressure P and temperature T, KP,T is the isothermal bulk modulus, KP,T′ is its first pressure derivative, and γ is the Grüneisen ratio). The two Debye temperatures are θDA=165±3 K and θDB=729±4 K. The unit cell volume at 174 K as a function of pressure, V(P), was fitted with a third-order Birch–Murnaghan equation of state having the following parameters: V0,174=365.69±0.16 Å3, K0,174=7.02±0.25 GPa, and K0,174′=9.56±1.28. The volume thermal expansion coefficient, αV, at 174 K and atmospheric pressure is 281.3×10−6 K−1. The proton disorder manifested at high homologous temperatures is seen to be frozen in, on the time scale of these experiments, down to 4.2 K. A high-pressure polymorph of ammonia dihydrate was observed following melting of the sample at 179 K and 0.46 GPa.
Energy estimators for random series path-integral methods
Authors: Predescu Cristian , Sabo Dubravko , Doll J. D. , Freeman David L.
Published: December 2003

We perform a thorough analysis on the choice of estimators for random series path integral methods. In particular, we show that both the thermodynamic (T-method) and the direct (H-method) energy estimators have finite variances and are straightforward to implement. It is demonstrated that the agreement between the T-method and the H-method estimators provides an important consistency check on the quality of the path integral simulations. We illustrate the behavior of the various estimators by computing the total, kinetic, and potential energies of a molecular hydrogen cluster using three different path integral techniques. Statistical tests are employed to validate the sampling strategy adopted as well as to measure the performance of the parallel random number generator utilized in the Monte Carlo simulation. Some issues raised by previous simulations of the hydrogen cluster are clarified.
Ab initio study of the HO2+NO reaction: Prediction of the total rate constant and product branching ratios for the forward and reverse processes
Authors: Zhu R. S. , Lin M. C.
Published: December 2003

The mechanisms for HO2+NO and its reverse reactions have been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the B3LYP/6-311+G(3df,2p) level and their energies refined by single-point calculations with the highest scheme of the modified Gaussian-2 method. Ab initio results show that formation of HO+NO2 from HO2+NO by the direct fragmentation of the peroxynitrous acid, HOONO intermediate, is predominant; the alternative path occurring by the isomerization of HOONO to HONO2 is 5.2 kcal/mol less favorable. The stepwise formation of HNO+O2 from HOONO is energetically unfavorable; the barriers for the direct H abstract reactions via singlet and triplet paths are found to be rather high also. Rate constant calculations show that the forward reaction is pressure independent below 10 atm; the reverse OH+NO2 reactions producing HONO2 and HOONO appear to be strongly pressure dependent; below 1 atm, the yield of HOONO from HO+NO2 is <2.5% at 300–400 K, it reaches 12.2% and 9.1%, respectively, at 300 and 400 K at 3.8×104 Torr pressure. The low- and high-pressure rate constants with He as a third-body for the formation of HOONO and HONO2 from the HO+NO2 reaction can be expressed by k0 (HOONO)=3.15×102 T−12.3 exp(−585/T), k0 (HONO2)=3.32×10−6 T−8.8 exp(−1569/T)cm6 molecule−2 s−1 and k∞ (HOONO)=1.71×10−10 T−0.24 exp(100/T) and k∞ (HONO2)=4.74×10−9 T−0.82 exp(21/T) cm3 molecule−1 s−1, respectively, in the temperature range of 200–2000 K. The unimolecular decomposition rate constant (in Ar) of HNO3 can be expressed as kd∞ (HNO3)=2.30×1023 T−2.27 exp(−26317/T) s−1 and kd0 (HNO3)=1.27×1015 T−6.55exp(−26038/T) cm3 molecule−1 s−1, respectively. The predicted values are all in close agreement with experimental data for both forward and reverse processes.
Combined crossed-beam studies of C(3PJ)+C2H4→C3H3+H reaction dynamics between 0.49 and 30.8 kJ mol−1
Authors: Geppert Wolf D. , Naulin Christian , Costes Michel , Capozza Giovanni , Cartechini Laura , Casavecchia Piergiorgio , Gualberto Volpi Gian
Published: December 2003

The reaction C(3PJ)+C2H4(X 1A1)→C3H3+H(2S1/2) has been studied using complementary crossed molecular beam techniques. Integral cross sections have been obtained in the range of relative translational energies ET=0.49–24.9 kJ mol−1 in experiments conducted with pulsed supersonic beams coupled with laser-induced fluorescence detection of H(2S1/2) atoms. The major reaction pathway leading to HCCCH2 (propargyl)+H has been found without any barrier, with relative integral cross sections that are proportional to (ET)−0.60±0.03 below 8 kJ mol−1. Threshold for a minor pathway, leading also to H formation, occurs around 6 kJ mol−1; the relative importance of this second pathway increases with relative translational energy. Differential cross sections have been obtained at three relative translational energies: ET=9.1, 17.2, and 30.8 kJ mol−1 in experiments conducted with continuous supersonic molecular beams coupled with universal mass spectrometric detection and time-of-flight analysis. At the lowest ET of 9.1 kJ mol−1 formation of HCCCH2 (propargyl)+H is observed to be the dominant channel with a nearly forward–backward symmetric angular distribution in the center-of-mass (cm) frame; about 35% of the total available energy is channeled into translation indicating that the propargyl radical is highly internally excited; formation of less stable C3H3 isomer(s) is minor (2%). As ET increases, formation of appreciable, increasingly larger fractions of less stable propyn-l-yl and/or cyclopropenyl isomers is also observed. These findings are consistent with the integral cross-section measurements. While formation of propargyl is thought to proceed via an osculating complex mechanism following addition of C(3PJ) to the double bond of ethylene, the dynamics of formation of the less stable isomers is going through a long-lived complex, as witnessed by an isotropic cm angular distribution. The H2 elimination channel leading to C3H2 formation has not been found to occur, which suggests that inter-system-crossing to the ground singlet C3H4 potential energy surface manifold has low probability and/or the H2-elimination process on the triplet surface is characterized by a very large exit potential barrier.
Temperature evolution of structural and magnetic properties of transition metal clusters
Authors: Fthenakis Zacharias , Andriotis Antonis N. , Menon Madhu
Published: December 2003

We report an extension of our tight binding molecular dynamics method [Phys. Rev. B 57, 10069 (1998)] by incorporating the Nosé-bath and the multiple histogram approximations, so as to be applicable to cluster studies at finite temperatures in an efficient way. This generalization allows one to calculate the caloric curve for the cluster and use this to study the effect of temperature on the structural, electronic, and magnetic properties of clusters. The method is used to study the variation of structural and magnetic properties with temperature as well as to obtain the caloric curves of the Ni13 cluster. The results are compared with those obtained using classical potentials to describe the interatomic interactions.
Factorized time correlation diagram analysis of Raman induced Kerr effect spectroscopy using noisy light
Authors: Turner Daniel B. , Ulness Darin J.
Published: December 2003

The factorized time correlation (FTC) diagram analysis of Raman induced Kerr effect spectroscopy from noisy light is presented. The complete set of FTC diagrams is given. This process requires a more sophisticated FTC diagram analysis than is typical because of the important role color-locking plays for this spectroscopy. A physical understanding of the terms that give rise to the slow decaying asymmetric interferograms seen in the experimental literature can be gleaned from these FTC diagrams. The interferograms carry information about the decay of the librational coherences.
Self-consistent field theory of brushes of neutral water-soluble polymers
Authors: Baulin Vladimir A. , Zhulina Ekaterina B. , Halperin Avi
Published: December 2003

The self-consistent field theory of brushes of neutral water-soluble polymers described by two-state models is formulated in terms of the effective Flory interaction parameter χeff(T,φ) that depends on both temperature, T, and the monomer volume fraction, φ. The concentration profiles, distribution of free ends and compression force profiles are obtained in the presence and in the absence of a vertical phase separation. A vertical phase separation within the layer leads to a distinctive compression force profile and a minimum in the plot of the moments of the concentration profile versus the grafting density. The analysis is applied explicitly to the Karalstrom model. The relevance to brushes of Poly(N-isopropylacrylamide)(PNIPAM) is discussed.
Liquid–liquid demixing in a binary polymer blend driven solely by the component-selective crystallizability
Authors: Hu Wenbing , Mathot Vincent B. F.
Published: December 2003

On the basis of an extended Flory–Huggins expression for the mixing free energy of the long-chain binary polymer blend, we showed that liquid–liquid demixing can be driven solely by the propensities of crystallization differing with respect to components. Demixing prior to crystallization on cooling was demonstrated by dynamic Monte Carlo simulations of the lattice model for a symmetric polymer blend where only one component is crystallizable. We discussed the implications of this principle to several experimental observations including the (in)compatibility of polymer chains with different stereochemical compositions, the memory effect of polymer crystallization, and the spinodal decomposition at an early stage of polymer crystallization.
Fluid–solid equilibria of flexible and linear rigid tangent chains from Wertheim’s thermodynamic perturbation theory
Authors: Blas Felipe J. , Sanz Eduardo , Vega Carlos , Galindo Amparo
Published: December 2003

An extension of Wertheim’s first-order thermodynamic perturbation theory is proposed to describe the global phase behavior of linear rigid tangent hard sphere chains. The extension is based on a scaling proposed recently by Vega and McBride [Phys. Rev. E 65, 052501 (2002)] for the equation of state of linear chains in the solid phase. We have used the Einstein-crystal methodology, the Rahman–Parrinello technique, and the thermodynamic integration method for calculating the free energy and equation of state of linear rigid hard sphere chains with different chain lengths, including the solid–fluid phase equilibria. Agreement between the simulation data and theoretical predictions is excellent in all cases. Once it is confirmed that the proposed theory can be used to describe correctly the equation of state, free energy, and solid–fluid phase transitions of linear rigid molecules, a simple mean-field approximation at the level of van der Waals is included to account for segment–segment attractive interactions. The approach is used to determine the global phase behavior of fully flexible and linear rigid chains of varying chain lengths. The main effect of increasing the chain length in the case of linear rigid chains is to decrease the fluid densities at freezing, so that the triple-point temperatures increase. As a consequence, the range of temperatures where vapor–liquid equilibria exist decreases considerably with chain length. This behavior is a direct result of the stabilization of the solid phase with respect to the liquid phase as the chain length is increased. The vapor–liquid equilibria are seen to disappear for linear rigid chains formed by more than 11 hard sphere segments that interact through an attractive van der Waals mean-field contribution; in other words, long linear rigid chains exhibit solid–vapor phase behavior only. In the case of flexible chains, the fluid–solid equilibrium is hardly affected by the chain length, so that the triple-point temperature reaches quickly an asymptotic value. In contrast to linear rigid chains, flexible chains present quite a broad range of temperatures where vapor–liquid equilibria exist. Although the vapor–liquid equilibria of flexible and linear rigid chain molecules are similar, the differences in the type of stable solid they form and, more importantly, the differences in the scaling of thermodynamic properties with chain length bring dramatic differences to the appearance of their phase diagrams.
The influence of steps on the dissociation of NO on Pt surfaces: Temperature-programmed desorption studies of NO adsorption on Pt{211}
Authors: Mukerji R. J. , Bolina A. S. , Brown W. A.
Published: December 2003

Temperature-programmed desorption (TPD) has been used to investigate the adsorption of NO on Pt{211} at 300 K and 120 K. Results show that NO dissociation occurs readily on Pt{211}, as evidenced by the observation of N2 and N2O in the TPD spectrum. Following adsorption at 120 K three NO TPD peaks at 338, 416, and 503 K are observed, in agreement with previous observations. In combination with data acquired in a recent reflection absorption infrared spectroscopy and density functional theory investigation of NO/Pt{211}, these peaks are assigned to the desorption of NO from an O–NO complex, the recombinative desorption of N and O atoms, and to desorption of a step-bridged NO species, respectively. These assignments are in disagreement with previous work, where the high-temperature NO peak was assigned to the desorption of step bound NO and the two low-temperature peaks were assigned to the desorption of NO from terrace sites. TPD spectra recorded following adsorption at 300 K, with a heating rate of 1 K s−1, show similar features to those recorded following 120 K adsorption. This is also in disagreement with previous observations, where only two NO TPD peaks were observed following adsorption at room temperature. This disagreement can be accounted for by the different heating rates used in the two experiments.
Ultrahigh vacuum and high-pressure coadsorption of CO and H2 on Pd(111): A combined SFG, TDS, and LEED study
Authors: Morkel Matthias , Rupprechter Günther , Freund Hans-Joachim
Published: December 2003

Sum frequency generation (SFG) vibrational spectroscopy was carried out in conjunction with thermal desorption spectroscopy, low-energy electron diffraction, and Auger electron spectroscopy to examine the coadsorption of CO and H2 on Pd(111). Sequential dosing as well as various CO/H2 mixtures was utilized to study intermolecular interactions between CO and H2. Preadsorbed CO effectively prevented the dissociative adsorption of hydrogen for CO coverages ⩾0.33 ML. While preadsorbed hydrogen was able to hinder CO adsorption at low temperature (100 K), hydrogen was replaced from the surface by CO at 150 K. When 1:1 mixtures of CO/H2 were used at 100 K, hydrogen selectively hindered CO adsorption on on-top sites, while above ∼125 K no blocking of CO adsorption was observed. The observations are explained in terms of mutual site blocking, of a CO–H phase separation, and of a CO-assisted hydrogen dissolution in the Pd bulk. The temperature-dependent site blocking effect of hydrogen is attributed to the ability (inability) of surface hydrogen to diffuse into the Pd bulk above (below) ∼125 K. Nonlinear optical SFG spectroscopy allowed us to study these effects not only in ultrahigh vacuum but also in a high-pressure environment. Using an SFG-compatible ultrahigh vacuum-high-pressure cell, spectra of 1:10 CO/H2 mixtures were acquired up to 55 mbar and 550 K, with simultaneous gas chromatographic and mass spectrometric gas phase analysis. Under reaction conditions, CO coverages ⩾0.5 ML were observed which strongly limit H2 adsorption and thus may be partly responsible for the low CO hydrogenation rate. The high-pressure and high-temperature SFG spectra also showed indications of a reversible surface roughening or a highly dynamic (not perfectly ordered) CO adsorbate phase. Implications of the observed adsorbate structures on catalytic CO hydrogenation on supported Pd nanoparticles are discussed.
Surface structure of Co–Pd bimetallic particles supported on Al2O3 thin films studied using infrared reflection absorption spectroscopy of CO
Authors: Carlsson A. F. , Bäumer M. , Risse T. , Freund H.-J.
Published: December 2003

Infrared reflection absorption spectroscopy of CO has been used as a surface-sensitive probe for the available binding sites of Co–Pd bimetallic particles supported on a thin alumina film. A high-coverage state is obtained on the Co particles in addition to atop sites, attributable to a M(CO)n species. Bridge and threefold sites are not detected from CO stretching on the Co particles. When both metals are deposited sequentially at 300 K, Pd easily forms a shell on existing Co particles. In the reverse order, much more Co is required to coat Pd particles, because it nucleates between Pd particles as well as on top of them. For both metals, atop sites are better preserved at various bimetallic compositions because they are statistically less vulnerable than threefold hollow sites. The stretching frequency of CO to a given site is nearly independent of the bimetallic composition.
Interaction of sulfur dioxide with titanium–carbide nanoparticles and surfaces: A density functional study
Authors: Liu Ping , Rodriguez José A.
Published: December 2003

In the control of environmental pollution, metal carbides are potentially useful for trapping and destroying sulfur dioxide (SO2). In the present study, the density functional theory was employed to study the surface structures and electronic properties of the adsorbed SO2 on titanium carbides: metcar Ti8C12, nanocrystal Ti14C13, and a bulk TiC(001) surface. The geometries and orientations of SO2 were fully optimized on all these substrates. Our calculations show that, in spite of the high C/Ti ratio and C2 groups, metcar Ti8C12 exhibits extremely high activity towards SO2. The S–O bonds of SO2 spontaneously break on Ti8C12. The products of the decomposition reaction (S, O) interact simultaneously with Ti and C sites. The C atoms are not simple spectators, and their participation in the dissociation of SO2 is a key element for the energetics of this process. Nanocrystal Ti14C13 also displays a strong interaction with SO2. Although the dissociation of SO2 on Ti14C13 cannot proceed as easily as that on Ti8C12, it could occur by thermal activation even at very low temperature. SO2 is weakly bonded with the bulk TiC(001) surface. By thermal activation the dissociation of SO2 on a TiC(001) surface may also take place but it should be much more difficult than that on Ti14C13. Therefore, we suggest that the carbide nanoparticles (Ti8C12 and Ti14C13) should have special chemical activity towards SO2 removal associated with their “magic” structures.
Charge recombination in a poly(para-phenylene vinylene)-fullerene derivative composite film studied by transient, nonresonant, hole-burning spectroscopy
Authors: Offermans Ton , Meskers Stefan C. J. , Janssen René A. J.
Published: December 2003

Transient, nonresonant, hole-burning spectroscopy has been used to study the charge recombination process in poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV): methanofullerene (PCBM) composite films. The position and intensity of the spectral hole in the absorption band of MDMO-PPV have been monitored as a function of time in the 10 ns–10 μs time range. A time-dependent redshift is observed. The intensity of the spectral hole decays with time according to a power law (∝t−α). The exponent α≈0.5 is found to be nearly independent of the excitation fluence in the range 0.05–2 mJ/cm2. The depth of the spectral hole depends sublinearly on the excitation fluence (I) and can be described by (∝Γ−β) with β∼0.5. The time-dependent redshift and the power-law type time decay can be reproduced by numerical simulations. The Monte Carlo method is used to simulate the hopping dynamics of the photoinduced charges in a lattice of energetically disordered sites before they eventually recombine at the MDMO-PPV:PCBM interface. The results indicate that charge separation is assisted by disorder and that, in the 10 ns–10 μs time range, the recombination rate is limited by the detrapping of the cationic charge carriers in MDMO-PPV.
Quantum and classical aspects of activated surface diffusion
Authors: Shushin Anatoly I. , Pollak Eli
Published: December 2003

The specific features of classical and quantum activated diffusion of a particle over a surface, modeled by a one-dimensional periodic potential, are analyzed in the low-to-moderate friction limit, in which the kinetics of the process is determined by the energy relaxation. Different models for the energy transition probability are considered with special emphasis on the exponential model which leads to significant simplification of the problem. New expressions are presented for the escape rate, mean squared path length and diffusion coefficient of an activated particle whose energy exchange dynamics is described by an exponential kernel. A universal behavior pj∼j−3/2 exp(−Δj) (where Δ depends only on the friction strength) is found for the distribution pj of diffusive hopping lengths j. It is identical for classical and quantum activated diffusion, does not depend on the details of the model used or on the characteristic energy loss of the particle to the bath. Quantum effects (tunneling) demonstrate themselves only in the absolute values of hopping rates, which for the weak damping regime considered in this paper, lead to a decrease of rates and, thus, the diffusion coefficient. This quantum suppression of diffusion is shown to be equivalent to an effective increase in the activation barrier, caused by quantum above barrier-reflection.
NO
2
and CO gas adsorption on carbon nanotubes: Experiment and theory
Authors: Santucci S. , Picozzi S. , Di Gregorio F. , Lozzi L. , Cantalini C. , Valentini L. , Kenny J. M. , Delley B.
Published: December 2003

In this work a combined experimental and theoretical study on carbon nanotube (CNT) based system for gas sensing applications is reported. Carbon nanotubes thin films have been deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates provided with Pt electrodes. Microstructural features as determined by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy highlight the growth of defective tubular carbon structures. The electrical resistivity of the CNT film shows a semiconductinglike temperature dependence and a p-type response with decreasing electrical resistance upon exposure to NO2 gas (100 ppb). No response has been found by exposing the film to CO gas in the temperature range between 25 and 250 °C. In order to obtain a theoretical validation of the experimental results, the equilibrium position, charge transfer, and density of states are calculated from first principles for the CNT+CO and CNT+NO2 systems. Our spin-unrestricted density functional calculations show that NO2 retains its spin-polarized state upon adsorption. Both CO and NO2 molecules adsorb weakly on the tube wall, with essentially no charge transfer between the tube and molecules. The electronic properties of CNTs are sensitive to the adsorption of NO2, due to an acceptorlike peak close to the tube valence-band maximum, while they are insensitive to the CO adsorption. According to the experimental findings, our theoretical results suggest that gas-induced modification of the density of states close to the Fermi level might significantly affect the transport properties of nanotubes.
The reaction of OH with acetone and acetone-d6 from 298 to 832 K: Rate coefficients and mechanism
Authors: Yamada Takahiro , Taylor Philip H. , Goumri A. , Marshall Paul
Published: December 2003

The pulsed laser photolysis/pulsed laser-induced fluorescence technique has been applied to obtain rate coefficients for OH+CH3C(O)CH3 and CD3C(O)CD3 of kH(298–832 K)=(3.99±0.40)×10−24T4.00 exp(453±44)/T and kD(298–710 K)=(1.94±0.31)×10−21T3.17 exp(−529±68)/T cm3 molecule−1 s−1, respectively. Three pathways were characterized via the CBS–QB3 ab initio method to obtain complete basis set limits for coupled-cluster theory. Addition to form CH3C(O)(OH)CH3, followed by dissociation to CH3+CH3C(O)OH, is negligibly slow. Variational transition state theory reveals that the dominant products are CH3C(O)CH2+H2O formed by direct abstraction at higher temperatures and via a hydrogen-bonded complex below about 450 K. Inclusion of tunneling gives good accord with the observed kinetic isotope effect down to about 250 K.
Nonlinear optical response properties of molecules in condensed phases using the coupled cluster/dielectric continuum or molecular mechanics methods
Authors: Kongsted Jacob , Osted Anders , Mikkelsen Kurt V. , Christiansen Ove
Published: December 2003

In this work we present the first derivation and implementation of quadratic response theory as described within the combined coupled cluster/dielectric continuum (CC/DC) and the combined coupled cluster/molecular mechanics (CC/MM) methods. In the former approach, the solvent is represented as a homogeneous dielectric medium, whereas the latter approach accounts for the discrete nature of the solvent molecules. Furthermore, the CC/MM model includes polarization effects. The CC models implemented are CC2 and CCSD. Sample calculations are performed on liquid water and solvent effects on the first hyperpolarizability of water are found to be significant. In particular, the experimental observed sign change in the first hyperpolarizability of water is reproduced in both the CC/DC and CC/MM descriptions though larger basis sets are needed in the former approach.