bluuebunny/crossref_metadata_2025 huggingface dataset:
Bridge mediated two-electron transfer reactions: Analysis of stepwise and concerted pathways
Authors: Petrov E. G. , May V.
Published: February 2004

A theory of nonadiabatic donor (D)–acceptor (A) two-electron transfer (TET) mediated by a single regular bridge (B) is developed. The presence of different intermediate two-electron states connecting the reactant state D−−BA with the product state DBA−− results in complex multiexponential kinetics. The conditions are discussed at which a reduction to two-exponential as well as single-exponential kinetics becomes possible. For the latter case the rate KTET is calculated, which describes the bridge-mediated reaction as an effective two-electron D–A transfer. In the limit of small populations of the intermediate TET states D−B−A, DB−−A, D−BA−, and DB−A−, KTET is obtained as a sum of the rates KTET(step) and KTET(sup). The first rate describes stepwise TET originated by transitions of a single electron. It starts at D−−BA and reaches DBA−− via the intermediate state D−BA−. These transitions cover contributions from sequential as well as superexchange reactions all including reduced bridge states. In contrast, a specific two-electron superexchange mechanism from D−−BA to DBA−− defines KTET(sup). An analytic dependence of KTET(step) and KTET(sup) on the number of bridging units is presented and different regimes of D–A TET are studied.
Size dependent ion hydration, its asymmetry, and convergence to macroscopic behavior
Authors: Rajamani Sowmianarayanan , Ghosh Tuhin , Garde Shekhar
Published: February 2004

The packing and orientation of water molecules in the vicinity of solutes strongly influence the solute hydration thermodynamics in aqueous solutions. Here we study the charge density dependent hydration of a broad range of spherical monovalent ionic solutes (with solute diameters from ∼0.4 nm to 1.7 nm) through molecular dynamics simulations in the simple point charge model of water. Consistent with previous experimental and theoretical studies, we observe a distinct asymmetry in the structure and thermodynamics of hydration of ions. In particular, the free energy of hydration of negative ions is more favorable than that of positive ions of the same size. This asymmetry persists over the entire range of solute sizes and cannot be captured by a continuum description of the solvent. The favorable hydration of negative ions arises primarily from the asymmetric charge distribution in the water molecule itself, and is reflected in (i) a small positive electrostatic potential at the center of a neutral solute, and (ii) clear structural (packing and orientation) differences in the hydration shell of positive and negative ions. While the asymmetry arising from the positive potential can be quantified in a straightforward manner, that arising from the structural differences in the fully charged states is difficult to quantify. The structural differences are highest for the small ions and diminish with increasing ion size, converging to hydrophobiclike hydration structure for the largest ions studied here. We discuss semiempirical measures following Latimer, Pitzer, and Slansky [J. Chem. Phys. 7, 108 (1939)] that account for these structural differences through a shift in the ion radius. We find that these two contributions account completely for the asymmetry of hydration of positive and negative ions over the entire range of ion sizes studied here. We also present preliminary calculations of the dependence of ion hydration asymmetry on the choice of water model that demonstrate its sensitivity to the details of ion–water interactions.
Turnaround of hysterisis for capacitance–voltage characteristics of hafnium oxynitride dielectrics
Authors: Wang J. C. , Shie D. C. , Lei T. F. , Lee C. L.
Published: February 2004

The capacitance–voltage (C–V) characteristics of hafnium oxynitride gate dielectrics for silicon metal-oxide-semiconductor (MOS) capacitors with different sweep voltage were investigated. It was found that, for the p-type substrate MOS capacitor, the C–V hysterisis has a turnaround characteristic as the applied voltage exceeds −3.0 V. The phenomenon is explained by electron trappings at the low electric field and hole trappings, which resulted from the impact ionization, at the high electric field in the dielectrics.
Spectroscopic interpretation: The high vibrations of CDBrClF
Authors: Jung C. , Mejia-Monasterio C. , Taylor H. S.
Published: February 2004

We extract the dynamics implicit in an algebraic fitted model Hamiltonian for the deuterium chromophore’s vibrational motion in the molecule CDBrClF. The original model has four degrees of freedom, three positions and one representing interbond couplings. A conserved polyad allows in a semiclassical approach the reduction to three degrees of freedom. For most quantum states we can identify the underlying motion that when quantized gives the said state. Most of the classifications, identifications, and assignments are done by visual inspection of the already available wave function semiclassically transformed from the number representation to a representation on the reduced dimension toroidal configuration space corresponding to the classical action and angle variables. The concentration of the wave function density to lower dimensional subsets centered on idealized simple lower dimensional organizing structures and the behavior of the phase along such organizing centers already reveals the atomic motion. Extremely little computational work is needed.
State-to-state dynamics of the Cl+CH3OH→HCl+CH2OH reaction
Authors: Bechtel Hans A. , Camden Jon P. , Zare Richard N.
Published: February 2004

Molecular chlorine, methanol, and helium are co-expanded into a vacuum chamber using a custom designed “late-mixing” nozzle. The title reaction is initiated by photolysis of Cl2 at 355 nm, which generates monoenergetic Cl atoms that react with CH3OH at a collision energy of 1960±170 cm−1 (0.24±0.02 eV). Rovibrational state distributions of the nascent HCl products are obtained via 2+1 resonance enhanced multiphoton ionization, center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the CH3OH co-products is deduced by measuring the spatial anisotropy of the HCl products. The majority (84±7%) of the HCl reaction products are formed in HCl(v=0) with an average rotational energy of 〈Erot〉=390±70 cm−1. The remaining 16±7% are formed in HCl(v=1) and have an average rotational energy of 〈Erot〉=190±30 cm−1. The HCl(v=1) products are primarily forward scattered, and they are formed in coincidence with CH2OH products that have little internal energy. In contrast, the HCl(v=0) products are formed in coincidence with CH2OH products that have significant internal energy. These results indicate that two or more different mechanisms are responsible for the dynamics in the Cl+CH3OH reaction. We suggest that (1) the HCl(v=1) products are formed primarily from collisions at high impact parameter via a stripping mechanism in which the CH2OH co-products act as spectators, and (2) the HCl(v=0) products are formed from collisions over a wide range of impact parameters, resulting in both a stripping mechanism and a rebound mechanism in which the CH2OH co-products are active participants. In all cases, the reaction of fast Cl atoms with CH3OH is with the hydrogen atoms on the methyl group, not the hydrogen on the hydroxyl group.
Rotational dynamics of colloidal spheres probed with fluorescence recovery after photobleaching
Authors: Lettinga M. P. , Koenderink G. H. , Kuipers B. W. M. , Bessels E. , Philipse A. P.
Published: February 2004

We report a polarized fluorescence recovery after photobleaching (pFRAP) method to measure the rotational dynamics of fluorescent colloids over a wide dynamic range. The method is based on the polarization anisotropy in the fluorescence intensity, generated by bleaching of fluorescently labeled particles with an intense pulse of linearly polarized laser light. The rotational mobilities of the fluorescent particles can be extracted from the relaxation kinetics of the postbleach fluorescence polarization anisotropy. Our pFRAP setup has access to correlation times over a range of time scales from tens of microseconds to tens of seconds, and is highly sensitive, so very low concentrations of labeled particles can be probed. We present a detailed description of the theoretical background of pFRAP. The performance of the equipment is demonstrated for fluorescent colloidal silica spheres, dispersed in pure solvents as well as in fd-virus suspensions.
Ab initio calculation of electrostatic multipoles with Wannier functions for large-scale biomolecular simulations
Authors: Sagui Celeste , Pomorski Pawel , Darden Thomas A. , Roland Christopher
Published: February 2004

It has long been known that accurate electrostatics is a key issue for improving current force fields for large-scale biomolecular simulations. Typically, this calls for an improved and more accurate description of the molecular electrostatic potential, which eliminates the artifacts associated with current point charge-based descriptions. In turn, this involves the partitioning of the extended molecular charge distribution, so that charges and multipole moments can be assigned to different atoms. As an alternate to current approaches, we have investigated a charge partitioning scheme that is based on the maximally localized Wannier functions. This has the advantage of partitioning the charge, and placing it around the molecule in a chemically meaningful manner. Moreover, higher order multipoles may all be calculated without any undue numerical difficulties. Tests on isolated molecules and water dimers, show that the molecular electrostatic potentials generated by such a Wannier-function based approach are in excellent agreement with the density functional-based calculations.
Angular resolution and range of dipole–dipole correlations in water
Authors: Mathias Gerald , Tavan Paul
Published: February 2004

We investigate the dipolar correlations in liquid water at angular resolution by molecular-dynamics simulations of a large periodic simulation system containing about 40 000 molecules. Because we are particularly interested in the long-range ordering, we use a simple three-point model for these molecules. The electrostatics is treated both by Ewald summation and by minimum image truncation combined with a reaction field approach. To gain insight into the angular dependence of the simulated dipolar ordering we introduce a suitable expansion of the molecular pair distribution function into a set of two-dimensional correlation functions. We show that these functions enable detailed insights into the shell structure of the dipolar ordering around a given water molecule. For these functions we derive analytical expressions in the particular case in which liquid water is conceived as a dielectric continuum. Comparisons of these continuum models with the correlation functions derived from the simulations yield the key result that liquid water behaves like a continuum dielectric beyond distances of about 15 Å from a given water molecule. We argue that this should be a generic property of water independent of our modeling. By comparison of the results of the two different electrostatics treatments with the continuum description we show that the boundary artifacts occurring in both methods are isotropically distributed and are locally small in the respective boundary regions.
Improvement of crystalline silicon surface passivation by hydrogen plasma treatment
Authors: Martı́n I. , Vetter M. , Orpella A. , Voz C. , Puigdollers J. , Alcubilla R. , Kharchenko A. V. , Roca i Cabarrocas P.
Published: February 2004

A completely dry low-temperature process has been developed to passivate 3.3 Ω cm p-type crystalline silicon surface with excellent results. Particularly, we have investigated the use of a hydrogen plasma treatment, just before hydrogenated amorphous silicon carbide (a-SiCx:H) deposition, without breaking the vacuum. We measured effective lifetime, τeff, through a quasi-steady-state photoconductance technique. Experimental results show that hydrogen plasma treatment improves surface passivation compared to classical HF dip. Seff values lower than 19 cm s−1 were achieved using a hydrogen plasma treatment and an a-SiCx:H film deposited at 300 °C.
Mechanism of enhanced luminescence in InxAlyGa1−x−yN quaternary epilayers
Authors: Chen C. H. , Chen Y. F. , Lan Z. H. , Chen L. C. , Chen K. H. , Jiang H. X. , Lin J. Y.
Published: February 2004

We report firm evidence for the underlying mechanism of the enhanced luminescence in InxAlyGa1−x−yN quaternary epilayers. Photoluminescence, Raman scattering, field emission scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDS), and cathodoluminescence (CL) measurements have been employed to study the correlation between optical and structural properties in these alloys. The phonon replica structures accompanying luminescence line, InGaN-related phonon modes in Raman spectra, SEM images, element composition analysis by EDS, and localized CL spectra provide the evidence to show that the existence of InGaN-like nanoclusters is responsible for the enhanced luminescence in InxAlyGa1−x−yN quaternary alloys. Our result therefore gives an excellent demonstration showing that because of the existence of nanoclusters a very defective alloy can exhibit a strong emission even at room temperature.
Bulk-acoustic waves radiated from low-loss surface-acoustic-wave resonators
Authors: Knuuttila J. V. , Vartiainen J. J. , Koskela J. , Plessky V. P. , Hartmann C. S. , Salomaa M. M.
Published: February 2004

The bulk-acoustic conductance in low-loss surface-acoustic-wave filters utilizing leaky surface-acoustic waves is significant for the device operation. Here we directly measure the bulk acoustic wave radiation pattern on the backside of the piezoelectric substrate with the help of a scanning laser-interferometer probe. For the case studied, a leaky surface-acoustic wave resonator on 36°YX–LiTaO3, a numerical calculation is carried out and the different bulk-wave modes arriving at the substrate bottom are identified by comparing the measured and computed energy-flow angles. The results are expected to lead to improved models for describing the operation of low-loss surface-acoustic-wave filters.
Comparative study of defect energetics in HfO2 and SiO2
Authors: Scopel W. L. , da Silva Antônio J. R. , Orellana W. , Fazzio A.
Published: February 2004

We perform ab initio calculations, based on density functional theory, for substitutional and vacancy defects in the monoclinic hafnium oxide (m-HfO2) and α-quartz (SiO2). The neutral oxygen vacancies and substitutional Si and Hf defects in HfO2 and SiO2, respectively, are investigated. Our calculations show that, for a large range of Hf chemical potential, Si substitutional defects are most likely to form in HfO2, leading to the formation of a silicate layer at the HfO2/Si interface. We also find that it is energetically more favorable to form oxygen vacancies in SiO2 than in HfO2, which implies that oxygen-deficient HfO2 grown on top of SiO2 will consume oxygen from the SiO2.
Thermal stability of Schottky contacts on strained AlGaN/GaN heterostructures
Authors: Lin Zhaojun , Kim Hyeongnam , Lee Jaesun , Lu Wu
Published: February 2004

The thermal stability of Ni Schottky contacts on strained Al0.3Ga0.7N/GaN heterostructures and on n-type bulk GaN was investigated after various thermal stressings using capacitance–voltage and current–voltage characterization techniques. The reverse leakage current decreases after thermal treatment at up to 800 and 600 °C for Schottky contacts on the strained Al0.3Ga0.7N/GaN heterostructures and bulk n-GaN, respectively. Ni Schottky contacts on the heterostructure with 30-min thermal stressing at 700 °C exhibit lower reverse leakage current by more than three orders of magnitude lower than the control sample. However, decrease in two-dimensional electron gas sheet carrier concentration at the Al0.3Ga0.7N and GaN interface possibly due to interactions between Ni and AlGaN surface was observed with increase of stressing temperature and time. Ni Schottky contacts on bulk n-GaN layers degrade at lower annealing temperature and their rectifying property practically disappears after 700 °C annealing, while Al0.3Ga0.7N/GaN heterostructures still exhibit nice Schottky behavior after 800 °C annealing. The better thermal stability of Ni Schottky contacts on the heterostructures than those on bulk n-GaN can be attributed to the presence of piezoelectric polarization.
Correlation between oxidation potential and open-circuit voltage of composite solar cells based on blends of polythiophenes/ fullerene derivative
Authors: Gadisa Abay , Svensson Mattias , Andersson Mats R. , Inganäs Olle
Published: February 2004

The photovoltaic parameters of donor/acceptor blend organic solar cells are highly influenced by several parameters, such as the strength of the acceptor species, the morphology of the film due to the solvent, and the mobility of the free charge carriers. In this work, the open-circuit voltage (Voc) of solar cells based on series of conjugated polythiophene polymers were measured and compared. In every cell, the donor polymer was blended with an electron acceptor fullerene molecule. The devices were constructed in a sandwich structure with indium tin oxide (ITO)/metallic polymer (PEDOT:PSS) acting as an anode and Al or LiF/Al acting as a cathode. Comparing the Voc of all the cells shows that this important photovoltaic parameter is systematically varying with the polymer. The variation of photovoltage is attributed to the variation of the oxidation potential of the donor conjugated polymers after due consideration of the different injection conditions in the varying polymers.
Microwave radiation force on a parallel-plate resonator
Authors: Makarov S. , Kulkarni S.
Published: February 2004

A simulation method is proposed and tested in order to determine the radiation force on metal targets whose size is comparable to wavelength. The method is based on the method of moments solution of the electric-field integral equation, accurate calculation of the near field, and removal of the self-interaction terms responsible for the pinch effect. The method is used to determine the local force distribution for a parallel-plate metal resonator. It is observed that, at the resonance, the individual metal plates may experience large force densities, despite the fact that the net radiation force on the resonator still remains very small. A potential use of this observation is discussed, which is directed toward possible excitation of acoustic vibrations.
DNA detection on transistor arrays following mutation-specific enzymatic amplification
Authors: Pouthas F. , Gentil C. , Côte D. , Bockelmann U.
Published: February 2004

An integrated array of silicon field-effect transistor structures is used for electronic detection of label-free DNA. Measurements of the dc current–voltage characteristics of the transistors gives us access to reproducible detection of single- and double-stranded DNA, locally adsorbed on the surface of the device. We combine this approach with allele-specific polymerase chain reaction, to test for the 35delG mutation, a frequent mutation related to prelingual nonsyndromic deafness.
Ruthenium films by digital chemical vapor deposition: Selectivity, nanostructure, and work function
Authors: Dey Sandwip K. , Goswami Jaydeb , Gu Diefeng , de Waard Henk , Marcus Steve , Werkhoven Chris
Published: February 2004

Ruthenium electrodes were selectively deposited on photoresist-patterned HfO2 surface [deposited on a SiOx/Si wafer by atomic layer deposition (ALD)] by a manufacturable, digital chemical vapor deposition (DCVD) technique. DCVD of Ru was carried out at 280–320 °C using an alternate delivery of Bis (2,2,6,6-tetramethyl-3,5-heptanedionato)(1,5-cyclooctadiene)Ru (dissolved in tetrahydrofuran) and oxygen. The as-deposited Ru films were polycrystalline, dense, and conducting (resistivity ∼20.6 μΩ cm). However, Rutherford backscattering spectroscopy, x-ray photoelectron spectroscopy, and high-resolution electron microscopy results indicate the presence of an amorphous RuOx at the Ru grain boundaries and at the DCVD–Ru/ALD–HfO2 interface. The estimated work function of DCVD–Ru on ALD–HfO2 was ∼5.1 eV. Moreover, the equivalent oxide thickness, hysteresis in capacitance–voltage, and leakage current density at −2 V of the HfO2/SiOx dielectric, after forming gas (95% N2+5% H2) annealing at 450 °C for 30 min, were 1.4 nm, 20 mV, and 7.4×10−7 A cm-2, respectively.
Phase stability of diamond nanocrystals upon pulsed-laser-induced liquid-solid interfacial reaction: Experiments and ab initio calculations
Authors: Wang Cheng-Xin , Yang Yu-Hua , Liu Qiu-Xiang , Yang Guo-Wei , Mao Yu-Liang , Yan Xiao-Hong
Published: February 2004

To clarify different phase formations of diamond nanocrystals upon pulsed-laser-induced liquid-solid interfacial reaction (PLIIR), we preformed the first-principles investigation of the total energy of cubic and hexagonal diamond. Interestingly, by a comparison of the structure stabilities of cubic and hexagonal diamond, we found that both cubic and hexagonal structures could form, simultaneously or one by one, upon those processes like as far-from equilibrium growth processes, such as PLIIR, due to the relatively small different of total energies and similar crystalline structures. Eventually, these calculations were in good agreement with the experimental results.
Enhanced Faraday rotation in all-garnet magneto-optical photonic crystal
Authors: Kahl S. , Grishin A. M.
Published: February 2004

Research on magneto-optical photonic crystals has so far been focused on theoretical investigations, because suitable multilayers of iron garnet, the most promising material, have not been readily available. We report the preparation and characterization of a one-dimensional magneto-optical photonic crystal composed of 17 heteroepitaxial layers of bismuth iron garnet and yttrium iron garnet. The magneto-optical Faraday rotation was increased by 140% while transmission decreased by just 16% at the design wavelength of 750 nm as compared with a single-layer bismuth iron garnet film of equivalent thickness. The sample is free of cracks, and good agreement of simulated and experimental spectra of optical transmission and Faraday rotation indicate high quality of the sample and robustness of the preparation technique.
Effect of nitric oxide annealing on the interface trap density near the conduction bandedge of 4H–SiC at the oxide/(112̄0) 4H–SiC interface
Authors: Dhar S. , Song Y. W. , Feldman L. C. , Isaacs-Smith T. , Tin C. C. , Williams J. R. , Chung G. , Nishimura T. , Starodub D. , Gustafsson T. , Garfunkel E.
Published: February 2004

Nitric oxide postoxidation anneal results in a significant decrease of defect state density (Dit) near the conduction bandedge of n-4H–SiC at the oxide/(112̄0) 4H–SiC interface. Comparison with measurements on the conventional (0001) Si-terminated face shows a similar interface state density following passivation. Medium energy ion scattering provides a quantitative measure of nitrogen incorporation at the SiO2/SiC interface.
Pulsed-electron-beam deposition of transparent conducting SnO2 films and study of their properties
Authors: Choudhary R. J. , Ogale S. B. , Shinde S. R. , Kulkarni V. N. , Venkatesan T. , Harshavardhan K. S. , Strikovski M. , Hannoyer B.
Published: February 2004

Transparent conducting films of SnO2 are grown on single-crystal R-Al2O3 substrates by the pulsed electron beam deposition (PED) technique, and their electrical, optical, and microstructural properties are compared with the films grown by pulsed-laser deposition (PLD). In PED, the pulsed electron beam used for ablation is generated by a channel-spark system with discharge voltage of 12 kV, current ∼1 kA, and pulse duration of ∼100 ns. The PED films are highly oriented, show good epitaxy, and are highly transparent with transmittance of ∼80% in the visible and IR range. The measured band gap is close to 3.9 eV. It is shown that the PED film properties compare well with those of PLD films, with the Sn119 Mossbauer spectroscopy showing comparable local environments in the two.
Influence of layer thickness on the formation of In vacancies in InN grown by molecular beam epitaxy
Authors: Oila J. , Kemppinen A. , Laakso A. , Saarinen K. , Egger W. , Liszkay L. , Sperr P. , Lu H. , Schaff W. J.
Published: February 2004

We have used a low-energy positron beam to identify In vacancies in InN layers grown on Al2O3 by molecular beam epitaxy. Their concentration decreases from ∼5×1018 to below 1016 cm−3 with increasing layer thickness (120–800 nm). The decrease in the vacancy concentration coincides with the increase in the electron Hall mobility, suggesting that In vacancies act as electron scattering centers.
Field emission from gallium-doped zinc oxide nanofiber array
Authors: Xu C. X. , Sun X. W. , Chen B. J.
Published: February 2004

Gallium-doped nanostructural zinc oxide fibers have been fabricated by vapor-phase transport method of heating the mixture of zinc oxide, gallium oxide, and graphite powders in air. The zinc oxide fibers grew along [002] direction, forming a vertically aligned array that is predominantly perpendicular to the substrate surface. With a gallium doping concentration of 0.73 at. %, the corresponding carrier concentration and resistivity were 3.77×1020 cm−3 and 8.9×10−4 Ω cm, respectively. The field emission of these vertically aligned ZnO fiber arrays showed a low field emission threshold (2.4 V/μm at a current density of 0.1 μA/cm2), high current density, and high field enhancement factor (2317). The dependence of emission current density on the electric field followed Fowler–Nordheim relationship. The enhanced field emission is attributed to the aligned structure, good crystal quality, and especially, the improved electrical properties (increased conductivity and reduced work function) of the nanofibers due to gallium doping.
Ultrasound-modulated optical computed tomography of biological tissues
Authors: Li Jun , Wang Lihong V.
Published: February 2004

An optical imaging technique called ultrasound-modulated optical computed tomography is demonstrated for tomographic imaging of biological tissues. Ultrasound-modulated optical signals are extracted from scattered light to provide projection data for the image reconstruction. A filtered back-projection algorithm is implemented to reconstruct an image reflecting optical tissue properties from angular and linear scans of an ultrasonic beam across a sample. This reconstruction-based imaging technique provides a way to obtain images of cross sections containing the scanned ultrasonic axis in biological tissues, which enables three-dimensional ultrasound-modulated optical imaging. The technique combines the contrast advantage of optical waves and the resolution advantage of ultrasonic waves.
Switchable cantilever for a time-of-flight scanning force microscope
Authors: Lee DongWeon , Wetzel Adrian , Bennewitz Roland , Meyer Ernst , Despont Michel , Vettiger Peter , Gerber Christoph
Published: February 2004

We have developed a cantilever device for applying a time-of-flight scanning force microscope (TOF–SFM) system. The cantilever device consists of a switchable cantilever with an integrated bimorph actuator, an integrated extraction electrode to minimize the ion extraction voltage, and an interlocking structure for precise tip–EE alignment. The TOF–SFM with the cantilever device allows quasisimultaneous topographical and chemical analyses of solid surfaces to be performed in the same way as with the conventional scanning probe technique. The switching properties of the bimorph actuator are demonstrated for use in two operating systems. Field emission measurements and a TOF analysis of a Pt-coated tip are conducted with the TOF–SFM.
Anomalous charge transport behavior of Fullerene based diodes
Authors: Matt G. J. , Sariciftci N. S. , Fromherz T.
Published: February 2004

We observed an anomalous voltage–current (V–I) characteristics of fullerene based diodes in the low temperature regime. The diodes exhibit a negative differential resistance and voltage hysteresis for opposite current sweep directions. This behavior is directly observable at temperatures below 95 K and indicates the formation of highly conductive filaments in the fullerene thin films.
Physical properties of a single polymeric nanofiber
Authors: Tan E. P. S. , Lim C. T.
Published: February 2004

The nanostructural and elastic properties of a single polymeric nanofiber extracted from a nanofibrous scaffold are investigated using atomic force microscopy (AFM). AFM imaging of the nanofibers reveals a “shish-kebab” structure. A portion of the nanofiber is suspended over a microscale groove etched on a silicon wafer. A nanoscale three-point bend test is performed to obtain the elastic modulus. This elastic modulus is found to be 1.0±0.2 GPa for fibers less than 350 nm but decrease with increase in fiber diameter in excess of 350 nm. This is due to the significance of shear deformation as the length to diameter ratio decreases.
Switching current versus magnetoresistance in magnetic multilayer nanopillars
Authors: Urazhdin S. , Birge Norman O. , Pratt W. P. , Bass J.
Published: February 2004

We study current-driven magnetization switching in nanofabricated magnetic trilayers, varying the magnetoresistance in three different ways. First, we insert a strongly spin-scattering layer between the magnetic trilayer and one of the electrodes, giving increased magnetoresistance. Second, we insert a spacer with a short spin-diffusion length between the magnetic layers, decreasing the magnetoresistance. Third, we vary the angle between layer magnetizations. In all cases, we find an approximately linear dependence between magnetoresistance and inverse switching current. We give a qualitative explanation for the observed behaviors, and suggest some ways in which the switching currents may be reduced.
Thin-film field-effect transistors based on La-doped SrTiO3 heterostructures
Authors: Pan Feng , Olaya David , Price John C. , Rogers Charles T.
Published: February 2004

Field-effect transistors have been fabricated from epitaxial perovskite strontium titanate heterostructures. Lanthanum-doped SrTiO3 was used as the semiconducting channel while insulating SrTiO3 was used as a gate insulator. Both depletion and accumulation effects in the channel have been studied from 300 to 4 K. Hall effect measurements indicate a temperature independent n-type carrier density around 5×1019 cm−3. At 300 K, typical mobilities are 2–3 cm2 V−1 s−1 while low temperature mobilities are as high as 15 cm2 V−1 s−1.
Properties of Ir-based Ohmic contacts to AlGaN/GaN high electron mobility transistors
Authors: Fitch R. C. , Gillespie J. K. , Moser N. , Jenkins T. , Sewell J. , Via D. , Crespo A. , Dabiran A. M. , Chow P. P. , Osinsky A. , La Roche J. R. , Ren F. , Pearton S. J.
Published: February 2004

Measurement of the electrical characteristics of 250 devices on the same 2 in. diameter wafer shows that Ti/Al/Ir/Au Ohmic contacts on AlGaN/GaN high electron mobility transistors (HEMTs) have lower average specific contact resistance after annealing at 850 °C for 30 s (4.6×10−5 Ω cm2) compared to more standard Ti/Al/Ni/Au contacts (2×10−4 Ω cm2). HEMTs with these Ir-based contacts also show average interdevice isolation currents approximately a factor of 2 lower, higher peak transconductance (134 mS/mm compared to 121 mS/mm), and higher device breakdown voltage (31 V compared to 23 V) than the devices with Ni-based contacts. This Ir-based contact metallurgy looks promising for applications requiring extended thermal stability of the HEMTs.
Thermal stability of nitrogen incorporated in HfNxOy gate dielectrics prepared by reactive sputtering
Authors: Kang J. F. , Yu H. Y. , Ren C. , Li M.-F. , Chan D. S. H. , Hu H. , Lim H. F. , Wang W. D. , Gui D. , Kwong D.-L.
Published: February 2004

In this letter, we report the thermal stability of nitrogen incorporated in HfOxNy gate dielectrics prepared by reactive sputtering using x-ray photoelectron spectroscopy, secondary ions mass spectrometry, and electrical characterization. The results indicate that the bulk Hf–N bonds in reactive-sputtered HfOxNy are not stable during the postdeposition annealing and can be easily replaced by oxygen, resulting in significant loss of nitrogen from the bulk film. However, N at the HfOxNy/Si interface forms N–Si bonds, contributing to the excellent electrical stability of reactive sputtered HfOxNy gate dielectrics during the post deposition annealing.
Optical amplification and electroluminescence at 1.54 μm in Er-doped zinc silicate germanate on silicon
Authors: Baker C. C. , Heikenfeld J. , Yu Z. , Steckl A. J.
Published: February 2004

Optical amplification and electroluminescence at 1.5 μm is reported in Er-doped Zn2Si0.5Ge0.5O4 (ZSG:Er) on silicon. ZSG:Er films were deposited by rf sputtering from a composite target in Ar/O2 mixtures. Channel waveguides were fabricated by plasma etching with Cl/Ar. The refractive index of ZSG:Er was found to be 1.75 at 1.54 μm. Signal enhancement greater than 13 dB and an internal gain of ∼2 dB have been achieved by optically pumping a 4.7 cm ZSG:Er amplifier. Electroluminescence at 1.5 μm was achieved using an ac device structure with a ZSG:Er central layer and upper and lower dielectric layers.
Broadband emission and low absorption in microdisks with AlGaAs quantum wells
Authors: Kipp T. , Petter K. , Heyn Ch. , Heitmann D. , Schüller C.
Published: February 2004

In microphotoluminescence measurements at room temperature on Al0.2Ga0.8As/Al0.4Ga0.6As multiple quantum well microdisks, we observe a large number of whispering gallery modes in a remarkably broad energy range of about 250 meV. This is in contrast to microdisks containing InGaAs or GaAs quantum wells, where typically only one or two whispering gallery modes are observed, but similar to the behavior of microdisks containing self-organized InAs quantum dots. We conclude that impurity levels below the recombination energy of free electron-hole pairs inside the alloy quantum wells lead to an efficient internal broadband emitter with low absorption.
Electrical properties of Pt contacts on p-GaN activated in air
Authors: Lin Yow-Jon , Wu Kuo-Chen
Published: February 2004

In this study, the electrical properties of Pt contacts on p-type GaN (p-GaN) activated in air were investigated. From the observed photoluminescence result, it is suggested that the hydrogenated Ga vacancies (i.e., VGaH2) were formed during the activation process. However, VGaH2 in p-GaN near the surface was transformed into VGa after Pt deposition, because Pt strongly absorbed hydrogen. A large number of VGa at the Pt/p-GaN interface would lead to the pinning of the Fermi level at 0.3 eV above the valence-band edge, as well as the formation of the low barrier at the interface, and the formation of the nonalloyed ohmic contacts due to the occurrence of the tunneling transmission for holes at the interface.
Energy-level splitting of ligand-stabilized Au55 clusters observed by scanning tunneling spectroscopy
Authors: Zhang H. , Hartmann U. , Schmid G.
Published: February 2004

A monolayer of Au55 clusters stabilized by [P(C6H5)3]12Cl6 ligands was investigated at 7 K using a low-temperature ultrahigh vacuum scanning tunneling microscope. The topography of single clusters shows the actual arrangement of the C6H5 rings of ligand molecules. Characteristic charge-quantization phenomena usually obtained for metal particles were observed by current–voltage measurements. Spectroscopic data acquired at different locations within a cluster reveal energy levels with average spacing of 170 meV which can be attributed to the Au55 core.
Electron-beam-induced current observed for dislocations in diffused 4H-SiC P–N diodes
Authors: Maximenko S. , Soloviev S. , Cherednichenko D. , Sudarshan T.
Published: February 2004

The electron-beam-induced current (EBIC) method was employed to investigate the electrical activity of dislocations in silicon carbide Schottky and diffused p–n diodes. Dislocations in Schottky diodes appear as dark spots with the EBIC current signal at the dislocations reduced with respect to the background. However, in p–n diodes, the same dislocations exhibited characteristic bright halos, with the EBIC current higher than that of the background. These bright halos were attributed to a nonuniform impurity distribution around dislocations caused by the high-temperature (∼2000 °C) diffusion process.
Vertical alignment liquid crystal cell with optically compensated splay configuration of the liquid crystal
Authors: Lee S. H. , Kim S. J. , Kim J. C.
Published: February 2004

We have observed a phenomenon associated with a transition from vertical alignment to an optically compensated splay structure. With rubbed homeotropic alignment in parallel directions, the device shows vertical alignment but the liquid crystals (LCs) are twisted 180° in the absence of an electric field. Depending on the voltage applied, two different configurations of LCs are possible. After applying a critical voltage, the LC configuration becomes splayed such that the middirector lies parallel to the substrate and around it, and a hybrid structure forms symmetrically. A method for obtaining the transition and the electro-optic characteristics of the device is discussed.
H
2
nuclear magnetic resonance spectroscopy of deuterium adsorption on single-walled carbon nanotubes
Authors: Shen Kai , Pietraß Tanja
Published: February 2004

H 2 nuclear magnetic resonance (NMR) spectroscopy was employed to study the interaction between deuterated hydrogen gas and single walled carbon nanotubes before and after purification. Transmission electron micrographs revealed strong bundling of the tubes. After purification, very little amorphous carbon and no graphitic particles were present, implying that the interactions observed are truly due to the nanotubes. In the parent material, the NMR signal is dominated by interaction of hydrogen with residual metal catalyst particles. For purified material, hydrogen in the gas phase is discernible from adsorbed hydrogen. The two phases do not exchange with each other on a ms time scale. The hydrogen molecules move among different adsorption sites, presumably outer tube surfaces and interstitial channels. This process is diffusion limited in the pressure range investigated.
Organic thin-film transistors fabricated by microcontact printing
Authors: Leufgen M. , Lebib A. , Muck T. , Bass U. , Wagner V. , Borzenko T. , Schmidt G. , Geurts J. , Molenkamp L. W.
Published: February 2004

We have fabricated organic thin-film transistors (OTFTs) using a microcontact printing technique (μCP) that employs thin polydimethylsiloxane stamps on a rigid silicon substrate in order to reduce macroscopic distortions. Systematic variation of the printing pressure, printing time, and concentration of eicosanethiol, the “molecular ink” in the μCP process, permits the fabrication of devices with smaller channel lengths (Leff) than nominally defined by the stamp. Interdigitated Ti/Au electrode structures with Leff down to 100 nm could be fabricated which, after additional surface treatment and vacuum deposition of αα′-dihexylquaterthiophene, yield OTFTs with excellent characteristics.
High-impedance reflectivity and surface-wave band gaps in metamaterials
Authors: Zhou Lei , Chan C. T.
Published: February 2004

We studied the surface wave and reflective properties of two types of metamaterials, one that combines a material with dispersive μ with a metal film, and another that possesses arbitrary ε and μ. Generic surface wave phase diagrams with respect to ε and μ are given. For the first type, we find that the simultaneous appearance of high-impedance reflectivity and a surface-wave gap requires additional mechanisms such as Bragg scattering, while metamaterials of the second type can possess high surface impedance and a surface-wave gap simultaneously.
Resonance Raman scattering of boron carbonitride nanotubes
Authors: Zhi C. Y. , Bai X. D. , Wang E. G.
Published: February 2004

Resonance Raman spectra of boron carbonitride (BCN) nanotubes synthesized by hot-filament chemical vapor deposition were investigated. The intensity of the D band is insensitive to laser excitation energy (Elaser), while the intensity of the G and G′ bands increases as Elaser increases, and saturates at Elaser=2.67 eV. This particular resonance behavior is ascribed to an electronic transition process different from the π–π* transition that occurs in carbon materials. The dispersive behavior of the D-related bands also shows an inflection at Elaser=2.67 eV and different ∂ω/∂Elaser compared to carbon materials. These results indicate there are special electronic and phonon structures in BCN nanotubes.
Experimental confirmation of negative refractive index of a metamaterial composed of Ω-like metallic patterns
Authors: Huangfu Jiangtao , Ran Lixin , Chen Hongsheng , Zhang Xian-min , Chen Kangsheng , Grzegorczyk Tomasz M. , Kong Jin Au
Published: February 2004

A one-dimensional metamaterial is realized using three connected Ω rings printed back-to-back and reversed on two sides of a dielectric substrate. Both transmission and prism experiments are reported, yielding concordant results of the presence of a left-handed frequency band. Experiments show reduced losses and an enlarged left-handed frequency band.
Self-assembled InAsSb quantum dots on (001) InP substrates
Authors: Qiu Yueming , Uhl David
Published: February 2004

Self-assembled InAsSb quantum dots (QD) on (001) InP substrates have been grown using metalorganic vapor phase epitaxy. The dot density and size are found to be strongly dependent on the presence of arsine. Direct deposition of InSb on InP and GaSb substrates formed large islands of InSb with low density of less than 5×109/cm2, however, InAsSb QDs of density as high as 4×1010/cm2 could be self-assembled by alternating group III and group V precursors, and high density almost pure InSb QDs were achieved on In0.53Ga0.47As/InP. The formation of high density InAsSb QDs is a result of a local nonequilibrium process and a reduction in mobility of In adatoms on the growth surface due to the presence of arsenic atoms, and in the case of high density almost pure InSb QDs on InGaAs/InP, the InAs interface layer is believed to be responsible. Photoluminescence shows that InSb QDs emit light at room temperature in the range of 1.7–2.2 μm.
λ∼4–5.3 μ
m
intersubband emission from InGaAs–AlAsSb quantum cascade structures
Authors: Revin D. G. , Wilson L. R. , Zibik E. A. , Green R. P. , Cockburn J. W. , Steer M. J. , Airey R. J. , Hopkinson M.
Published: February 2004

The In0.53Ga0.47As–AlAs0.56Sb0.44 materials system, lattice matched to InP, is an attractive candidate for short wavelength quantum cascade lasers due to the very large conduction band discontinuity (∼1.6 eV) and compatibility with well established quantum cascade laser waveguide design and fabrication technology. In this letter we report the operation of In0.53Ga0.47As–AlAs0.56Sb0.44 quantum cascade structures emitting in the wavelength range λ∼4–5.3 μm. Clear intersubband electroluminescence peaks are observed close to the design wavelengths, with full widths at half maximum in the range of ∼30–40 meV.
Nanotube magnetism
Authors: Sui Y. C. , Skomski R. , Sorge K. D. , Sellmyer D. J.
Published: February 2004

FePt and Fe3O4 nanotubes are produced by hydrogen reduction in nanochannels of porous alumina templates and investigated by electron microscopy, x-ray diffraction analysis, and magnetic measurements. Loading the templates with a Fe chloride and Pt chloride mixture followed by hydrogen reduction at 560 °C leads to the formation of ferromagnetic FePt nanotubes in the alumina pores. Using a Fe nitrate solution, thermally decomposed at 250 °C and reduced in hydrogen for 2.5 h at the same temperature, yields Fe3O4 tubes. The length of the nanotubes is about 50 μm and their diameters range from about 150 to 220 nm, depending on the thickness of the template film and the pore diameter distribution. Reflecting the different magnetocrystalline anisotropies of the compounds, the coercivities range from 0.61 kOe for Fe3O4 to 20.9 kOe for FePt. The hysteresis is explained in terms of a tubular random-anisotropy model, which yields a diameter and anisotropy dependent transition from a curling-type mode (Fe3O4) to a localized mode (FePt).
Slow relaxation of field-induced piezoelectric resonance in paraelectric barium stannate titanate
Authors: Wei Xiaoyong , Feng Yujun , Yao Xi
Published: February 2004

A kind of slow relaxation of field-induced piezoelectric resonance is observed in paraelectric barium stannate titanate ceramics. The piezoelectric resonance peaks vanish in several minutes when bias field is either applied or removed. These phenomena may be universal for dielectrics and can be explained by a model of slow dipoles composed of injected homocharges. With this model, the authors predict a negative piezoelectric modulus of d33 when just the bias field is removed, and verify it by experimental results.
Measurements of charge mobility and diffusion coefficient of conjugated electroluminescent polymers by time-of-flight method
Authors: Tseng Hao-En , Jen Tzu-Hao , Peng Kang-Yung , Chen Show-An
Published: February 2004

Charge mobilities (μ) and diffusion coefficients (D) of hole (μh, Dh) and electron (μe, De) of the conjugated electroluminescent polymers, poly(phenylene vinylene)s and polyfluorenes, have been measured by fitting of a theoretical photocurrent transient equation to time-of-flight photocurrent transients. The μ so obtained are in agreement with those from inflection points of photocurrent transients. The D value lumps all factors together that cause the dispersion of carriers, and the parameter Dq/μkT can be used as an indicator of the degree of dispersion. This fitting method allows extracting μ and D from highly dispersive photocurrent transients, even for the case in which no inflection point appears.
Field emission from vertically aligned conductive IrO2 nanorods
Authors: Chen Reui-San , Huang Ying-Sheng , Liang Ya-Min , Hsieh Chim-Sung , Tsai Dah-Shyang , Tiong Kwong-Kau
Published: February 2004

We report on the preparation and field-emission properties of vertically aligned conductive IrO2 nanorods. The unique geometrical features of IrO2 nanorods, including nanosized structure and self-assembled sharp tip, exhibit a strong effect on field enhancement (β∼40 000), which result in a low threshold field (Eth∼0.7 V/μm) defined at the beginning of emission. A low turn-on field for driving a current of 10 μA/cm2 is about 5.6 V/μm, which is comparable with the carbon nanotube, diamond, and amorphous carbon. The potential of using IrO2 nanorods as an emitter material has been demonstrated.
Electron scattering in AlGaN/GaN structures
Authors: Syed S. , Manfra M. J. , Wang Y. J. , Molnar R. J. , Stormer H. L.
Published: February 2004

We present data on mobility lifetime τt, quantum lifetime τq, and cyclotron resonance lifetime τCR, of a sequence of high-mobility two-dimensional electron gases in the AlGaN/GaN system, covering a density range of 1 to 4.5×1012 cm−2. We observe a large discrepancy between τq and τCR(τq∼τCR/6), and explain it as the result of density fluctuations of only a few percent. Therefore, only τCR—and not τq—is a reliable measure of the time between electron-scattering events in these specimens. The ratio τt/τCR increases with increasing density in this series of samples, but scattering over this density range remains predominantly in the large-angle scattering regime.
Electroluminescence of single silicon nanocrystals
Authors: Valenta Jan , Lalic Nenad , Linnros Jan
Published: February 2004

We report on measurements of room-temperature electroluminescence from single silicon nanocrystals. The electrically driven emission reveals typical characteristics of single-nanocrystal luminescence: the peak wavelength variations, narrowing of spectral bands, a high degree of linear polarization, and intensity fluctuations (blinking) observed on a scale of minutes. From the count rate statistics of individual nanocrystals, we conclude that the yield of radiative emission is as high as 19%. These findings may open a route to highly efficient all-silicon light emitters.