bluuebunny/crossref_metadata_2025 huggingface dataset:
Compaction of granular quartz under hydrothermal conditions: Controlling mechanisms and grain boundary processes
Authors: van Noort Reinier , Spiers Christopher J. , Pennock Gill M.
Published: December 2008

We report isostatic compaction experiments performed on granular quartz under hydrothermal conditions (3–129 μm of initial grain size, 300–600°C, 200 MPa of fluid pressure, and 25–100 MPa of effective pressure). From microstructural evidence, it was determined that, whereas microcracking controlled precompaction at room temperature, pressure solution was the main mechanism during hydrothermal compaction, although a role of microcracking could not be excluded entirely. Our mechanical data, together with theoretical pressure solution rate models, further indicated that pressure solution was controlled by interface kinetics, dissolution being the most likely rate‐controlling mechanism. An empirical relation of the form = 10−7.8 (ϕ/ϕ0)10.0σ3.4d−1 exp (−105,000/RT) was fitted to our data to describe experimental compaction rates. Electron backscatter diffraction (EBSD) analysis performed on one sample showed limited evidence for plastic deformation (Dauphiné twinning and lattice bending) at grain contact points under high stress. Contact microstructures formed during compaction were studied using quartz single crystal discs or a mica plate as reference surfaces. This showed that, in all contacts, microstructures were rough, with a micrometer scale roughness. Many contacts also showed internal microcracking, and it is inferred that microcracking is likely an important mechanism for creating and maintaining rough contacts during quartz pressure solution, at least under the present experimental conditions. Extrapolation of our empirical equation for compaction rates to natural conditions is consistent with previous observations that, during burial and diagenesis of a sandstone, pressure solution starts to operate at depths of about ∼1.5–2 km, becoming the dominant compaction mechanism at depths greater than ∼2.5–3 km. The extrapolation gives good agreement with porosity‐depth trends reported for natural, arenitic sandstones.
Cross‐validation of Advanced Microwave Sounding Unit and lidar for long‐term upper‐stratospheric temperature monitoring
Authors: Funatsu Beatriz M. , Claud Chantal , Keckhut Philippe , Hauchecorne Alain
Published: December 2008

A comparison between NOAA‐16 Advanced Microwave Sounding Unit (AMSU) and lidar upper‐stratospheric temperature measurements for the period from 2001 to 2007 for purposes of temperature monitoring is presented. Monthly means of lidar data from the Observatoire de Haute‐Provence and AMSU data over Western Europe were found to have high correlation, particularly in winter months, when based on measurements for overlapping nights but lower correlation when different sets of days were used for the monthly mean calculation. This result implies that temperatures from lidar measurements are representative of an area relatively larger than its location, in a monthly mean timescale. However, the effect of temporal sampling arising from the fact that lidar measurements are only made in nights without visible cloud cover introduces discrepancies that propagate on the calculation of temperature tendencies. The estimated cooling rate based on lidar and AMSU data sets are in good agreement, although they are more negative than trends previously found based on the Stratospheric Sounding Unit data record from 1979 to 2005 for the middle‐upper stratosphere. In addition, the effect of NOAA satellite drift was to produce a difference (between monthly means for day and night passes) of up to 3 K near the stratopause, which must be accounted for particularly when a longer time series will be available.
Nitrogen cycling and metabolism in the thalweg of a prairie river
Authors: Dodds W. K. , Beaulieu J. J. , Eichmiller J. J. , Fischer J. R. , Franssen N. R. , Gudder D. A. , Makinster A. S. , McCarthy M. J. , Murdock J. N. , O'Brien J. M. , Tank J. L. , Sheibley R. W.
Published: December 2008

Nutrient dynamics in rivers are central to global biogeochemistry. We measured ammonium (NH4+) uptake, metabolism, nitrification, and denitrification in the thalweg, the river region of greatest flow, of the Kansas River (discharge = 14,360 L/s). We estimated gross and net uptake with a depleted 15N‐NH4+ release, metabolism with diel O2 measurements, and denitrification with dissolved N2 measurements. Net ecosystem production was negative. Net NH4+ uptake length was 2.1 km when concentrations were elevated, and gross uptake length was 1.9 km at ambient concentrations. Gross uptake rate measurements were comparable to estimates made using extrapolations from data obtained from streams (systems with 1/10th or less the discharge). Calculated lengths were maximal because the isotope pulse was primarily confined to the thalweg and not the shallow side channels or backwaters. Denitrification and nitrification rates were below detection. In the Kansas River, rates of N cycling are driven by heterotrophic processes, and considerable processing of N, particularly NH4+ uptake, occurred over a few kilometers of river length, with net uptake rates of NH4+ increasing with greater NH4+ concentrations.
Using a horizontal precipitation model to investigate the role of turbulent cloud deposition in survival of a seasonal cloud forest in Dhofar
Authors: Hildebrandt Anke , Eltahir Elfatih A. B.
Published: December 2008

This paper investigates a feedback between vegetation height and cloud interception in a seasonal semiarid cloud forest in Dhofar, Oman. In this forest, cloud interception by tree canopies (horizontal precipitation) constitutes a substantial fraction of available water. Owing to cattle browsing on tree canopies the forest is gradually degrading to grassland. We investigated if tree removal could reduce cloud interception to the extent that natural reestablishment of trees is inhibited. For this, we included a model describing turbulent cloud deposition as a module into a dynamic vegetation model. The model allows for estimation of cloud deposition based on cloud properties and dynamically changing vegetation structure. Cloud properties were estimated, using an inverse solution of the cloud deposition model, based on measured precipitation and meteorological data. When applying the model to the Dhofar region, we found that equilibrium vegetation depended on the initial vegetation condition. For most of the range of assumed cloud properties, equilibrium vegetation tended toward grassland, when the initial condition was grassland, and to forest, when the model was initialized with forest. However, the difference between the equilibrium vegetation condition emerging from different initial vegetation types depended on the assumed cloud properties. According to these modeling results, land degradation in this semiarid cloud forest might lead to irreversible destruction of the forest biome.
Sensitivity of the glacial ocean to Heinrich events from different iceberg sources, as modeled by a coupled atmosphere‐iceberg‐ocean model
Authors: Levine Richard C. , Bigg Grant R.
Published: December 2008

We introduce explicit icebergs from a dynamic and thermodynamic iceberg model into an intermediate complexity climate model, which includes the coupled atmosphere‐ocean system. This modeling approach allows iceberg meltwater to be injected into the ocean on the basis of thermodynamical considerations along the iceberg trajectories. Icebergs are seeded from known ice sheets in both hemispheres. Adding icebergs to the present‐day climate model has a minimal impact, but during the Last Glacial Maximum (LGM), Atlantic overturning strength is reduced by a third, while producing a model state that is consistent with a steady state climate. We test the sensitivity of the model at the LGM to additional Heinrich event‐scale fluxes of icebergs from three possible sources: Hudson Strait, the Gulf of Saint Lawrence, and the Norwegian Channel Ice Stream (NCIS). The sensitivity of the ocean is similar for all locations, with differences dominated by the length of the iceberg meltwater pathways to the main ocean convection region. The NCIS events result in more variability and a distinctly different, more northerly, salinity anomaly. We compare these results to a more typical modeling approach, whereby meltwater is injected directly into the ocean at the iceberg source locations, and find that these floods overestimate the oceanic response compared to the iceberg events. Our results suggest that 0.3–0.4 Sv of additional freshwater flux, either as icebergs or freshwater, is required to shut down the North Atlantic meridional overturning, a larger freshwater flux than sometimes suggested because of the localized nature of the release of the freshwater.
Impacts of irrigation and anthropogenic aerosols on the water balance, heat fluxes, and surface temperature in a river basin
Authors: Biggs Trent W. , Scott Christopher A. , Gaur Anju , Venot Jean‐Philippe , Chase Thomas , Lee Eungul
Published: December 2008

Changes in both land cover and the atmosphere have impacted the heat fluxes of south Asia in ways that may have altered the timing and magnitude of the monsoon. Century‐long budgets of water and energy in the Krishna Basin (258,948 km2) in southern India demonstrate that irrigation impacted the sensible heat flux of the land surface (H) as much as or more than did the atmospheric brown cloud (ABC) over 1960–2005. Annual discharge of the Krishna River fell from 226 mm during pre‐irrigation land cover (1901–1960) to 64 mm by 1990–2005, when 14–20% of the basin area was irrigated. Over the same period, annual evaporation increased by 166 ± 32 mm (+28%) causing H to decrease by 12.7 ± 2 W m−2 (−18%) compared to a decrease of 11.2 ± 1.8 W m−2 caused by the atmospheric brown cloud (ABC). The rate of change in H during irrigation expansion (1960–1990) was between −3.4 and −5.0 W m−2 per decade (da−1) due to irrigation expansion and −1.8 to −2.3 W m−2 da−1 due to the ABC. The trend in H caused by irrigation was negligible over 1990–2005 as irrigated area and evaporation stabilized. Previous work using the Parallel Climate Model estimated that the ABC decreased the latent heat flux by 2.4 W m−2; this decrease was more than offset by irrigation, resulting in a net increase in the latent heat flux of 12.9 W m−2. The maximum surface air temperature (Tmax) either decreased or remained the same in areas experiencing irrigation expansion but increased in a majority of unirrigated areas during the post‐monsoon season. The results provide observational evidence that irrigation changed both the basin‐scale sensible heat flux and surface air temperatures.
Continuous measurement and modeling of CO2 losses from a peatland stream during stormflow events
Authors: Dinsmore K. J. , Billett M. F.
Published: December 2008

While streams draining peatland and wetland systems are known to be supersaturated in CO2 with respect to the atmosphere, relatively little is known about short‐term temporal variability in response to extreme hydrological events. Here, we use submerged, nondispersive infrared (NDIR) sensors to make continuous measurements of CO2 concentrations during 18 storm events in a Scottish peatland stream. Individual storms exhibited 3 distinct types of hysteresis loop. We suggest that differences in loop form may be due to differences in the relative contributions of soil water or differences in the contributing catchment source area. We found a negative concentration‐discharge relationship over the full study period, suggesting that CO2‐rich deep peat/groundwater was the major source of aquatic CO2 under low flow conditions. By removing the effect of dilution and estimating additions and losses of CO2, we also show the importance of both surface peat CO2 inputs into the stream and evasion loss during stormflow. The best model of temporal variability in CO2 was achieved by separating the data set into “stormflow” and “dry periods.” Downstream CO2 export during the study period was dominated by stormflow events (71%), highlighting the importance of accurately accounting for high‐flow CO2 sources.
Characterization of BBB Permeability in a Preclinical Model of Cryptococcal Meningoencephalitis Using Magnetic Resonance Imaging
Authors: Pai Manjunath P , Sakoglu Unal , Peterson Steven L , Lyons C Richard , Sood Rohit
Published: December 2008

Cryptococcus neoformans meningoencephalitis (CNME) is a leading fungal cause of death among acquired immunodeficiency syndrome patients. Innovative preclinical systems that permit high throughput in vivo evaluation of novel agents are desperately needed. Magnetic resonance imaging (MRI) was evaluated as a tool to develop a rat model of CNME and to quantify noninvasively blood—brain barrier (BBB) disruption secondary to this disease. The aim of this study was to identify MRI changes compared with histopathology and fungal burden measurements as potential biomarkers. A well-characterized strain of C neoformans (CN) var grubii was used to infect rats using intravenous inoculation. An inoculum-finding study was performed by infecting rats with 103, 105, and 107 colony-forming units (CFUs). Animals underwent dynamic MRI on days 4 and 7 after inoculation. An inoculum-confirming study was performed by infecting rats with 105, 106, and 107 CFU and fungal burden was determined in the brain, lung, and spleen. Animals infected with 107 CFU of CN developed lesions that appeared hyperintense on T2-weighted images on day 4. The histopathology results correlated well with MRI data. Diffusion weighted and permeability estimates were 1.4 and 6.1-fold higher, respectively, in lesions compared with healthy tissue. Magnetic resonance imaging is a promising preclinical tool to evaluate effects of antifungal and adjunctive agents.
Carbon Storage: Integrating Experiments & Modelling to Quantify Trapping Capacity & Efficiency in the Subsurface
Authors: Pentland Christopher , Al-Mansoori Saleh , Iglauer Stefan , Qi Ran , Bijeljic Branko , LaForce Tara , Blunt Martin
Published: December 2008

This is a fundamental study of trapping of non-wetting fluids in porous media. When injecting CO~2~ into an aquifer for carbon storage, the non-wetting phase (CO~2~) is trapped due to capillary forces. This process is investigated in the laboratory for analogue fluids. We then design an injection strategy to maximise CO~2~ storage capacity and efficiency on the field scale - incorporating experimental and pore scale modelling results. A streamline based simulator is modified for this purpose.
Transcriptional ERRγ2-mediated activation is regulated by sentrin-specific proteases
Authors: Hentschke Moritz , Süsens Ute , Borgmeyer Uwe
Published: December 2008

Modification with SUMOs (small ubiquitin-related modifiers) has emerged as an important means of regulating the activity of transcription factors, often by repressing their activity. The ERRγ [oestrogen receptor-related receptor γ; ERR3 or NR3B3 (nuclear receptor subfamily 3, group B, gene3)] is a constitutively active orphan nuclear receptor. A PDSM, (phosphorylation-dependent sumoylation motif) is located in the close vicinity of the N-terminally located ERRγ2-specific AF-1 (activation function-1). Its function can be replaced by an NDSM (negatively charged amino acid-dependent sumoylation motif). A mutational analysis reveals that ERRγ2 activity is modulated through sumoylation of a lysine residue at position 40, which in turn is regulated by phosphorylation. Phosphorylation at the +5 position relative to the sumoylation target is directly visualized by a high-resolution EMSA (electrophoretic mobility-shift assay). Sumoylation represses the activity of ERRγ both with and without forced expression of the PGC-1β (peroxisome-proliferator-activated receptor γ co-activator-1β). Fusion proteins of a heterologous DNA-binding domain with the ERRγ2 N-terminus demonstrate the function of the PDSM as the RF-1 (repression function-1) for the neighbouring AF-1. De-repression is achieved by co-expression of sentrin/SENP (sentrin-specific protease) family members. Together, our results demonstrate reversible phosphorylation-dependent sumoylation as a means to regulate the activity of an orphan nuclear receptor.
Ag-modified silicon nanowires substrate for ultrasensitive surface-enhanced raman spectroscopy
Authors: Shao Ming-Wang , Zhang Ming-Liang , Wong Ning-Bew , Ma Dorothy Duo-duo , Wang Hui , Chen Weiwei , Lee Shuit-Tong
Published: December 2008

We report a unique substrate for surface-enhanced raman spectroscopy (SERS) based on silver nanoparticles-embedded silicon nanowires (SiNWs). The SiNWs were prepared by thermal evaporation of SiO powder via oxide-assisted growth, oxide removed with HF, and then used to reduce silver ions to form a highly decorated Ag-embedded surface. Such modified SiNWs substrates yielded ultrahigh SERS sensitivity, which could detect 25μl of 1×10−16M Rhodamine 6G, 1×10−16M crystal violet, and 1×10−14M nicotine in methanol solutions. An Ag-modified SiNW strand could also enable SERS detection of 25μl of 1×10−8mg∕ml calf thymus DNA. The possible mechanisms for the ultrahigh SERS sensitivity were discussed.
Stochastic oscillations of magnetic flux in the toroid sample of lead-doped lanthanum manganite
Authors: Rinkevich A. , Nosov A. , Vassiliev V. , Vladimirova E.
Published: December 2008

The dynamics of magnetic flux variations in a toroidal sample of the La0.6Pb0.4MnO3 manganite was investigated. Experiments were carried out when the ac magnetic field was applied perpendicular to the dc one. It is shown that under some conditions, several regimes of complex oscillations of magnetic flux can be excited. Evolution of these oscillations as a function of frequency of ac and magnitude of dc magnetic fields was investigated. The signs of existence of the stochastic regime of oscillations were found.
Analytical theory for favorable defects in tunnel diodes
Authors: Jandieri K. , Baranovskii S. D. , Stolz W. , Gebhard F.
Published: December 2008

Tunnel diodes play a decisive role in many semiconductor devices. Energy levels in the band gap of the diodes caused by impurities or defects are responsible for the device properties, particularly for the current/voltage characteristics. We present a simple analytical theory that provides a simple estimate for the position of energy levels in the band gap of tunnel diodes that contribute the highest electrical current at yet reliable low voltages. The latter demand is crucial for the efficient functioning of multilayered solar cells. Numerical calculations in the framework of the global transfer matrix technique show that our analytical criteria are sufficiently accurate. This makes our theory a valuable tool for the optimization of devices based on tunnel diodes.
Controlling protein molecular dynamics: How to accelerate folding while preserving the native state
Authors: Jensen Christian H. , Nerukh Dmitry , Glen Robert C.
Published: December 2008

The dynamics of peptides and proteins generated by classical molecular dynamics (MD) is described by using a Markov model. The model is built by clustering the trajectory into conformational states and estimating transition probabilities between the states. Assuming that it is possible to influence the dynamics of the system by varying simulation parameters, we show how to use the Markov model to determine the parameter values that preserve the folded state of the protein and at the same time, reduce the folding time in the simulation. We investigate this by applying the method to two systems. The first system is an imaginary peptide described by given transition probabilities with a total folding time of 1μs. We find that only small changes in the transition probabilities are needed to accelerate (or decelerate) the folding. This implies that folding times for slowly folding peptides and proteins calculated using MD cannot be meaningfully compared to experimental results. The second system is a four residue peptide valine-proline-alanine-leucine in water. We control the dynamics of the transitions by varying the temperature and the atom masses. The simulation results show that it is possible to find the combinations of parameter values that accelerate the dynamics and at the same time preserve the native state of the peptide. A method for accelerating larger systems without performing simulations for the whole folding process is outlined.
Short-range order and collective dynamics of poly(vinyl acetate): A combined study by neutron scattering and molecular dynamics simulations
Authors: Tyagi M. , Arbe A. , Alvarez F. , Colmenero J. , González M. A.
Published: December 2008

The structure and collective dynamics at local length scales of poly(vinyl acetate) (PVAc) have been investigated by combining neutron scattering on a deuterated sample and fully atomistic molecular dynamics simulations. The experimental data have provided microscopic information at molecular level and a critical check to validate the simulated cell. In order to gain deep insight on the structure factor, we have grouped the simulation results in terms of three molecular substructures: the main chain, the carboxyl group, and the methyl group. The study of the resulting static partial structure functions has revealed the origin of the diffraction peaks, and a real-space evaluation of the characteristic radial distribution functions has allowed separating intra- and interchain contributions to the total correlation functions. The extension of this study to the dynamic structure factor has revealed a coupling between the side-group motions and the main-chain dynamics at intermolecular distances. Moreover, a decrease in coherency of the main-chain motions at intermolecular level is observed with increasing temperature, leading to a rather decoupled behavior between the main chains at 200 K above the glass transition.
The remarkable influence of an “insignificant” quantity: How recoil orbital angular momentum determines product j distributions and (v;j) correlation in H+LH reactions
Authors: McCaffery Anthony J.
Published: December 2008

Reactions for which the reactant (r)-to-product (p) mass ratio (μr∕μp) is high, the well-known H+LH→HH+L processes, convert most of available energy to product rotation, while that disposed as recoil is often regarded as negligible. In angular momentum (AM) terms, however, this recoil orbital AM (lp) is shown to be a critical component of the overall AM balance. For products of light μp, the maximum value of lp is energy limited and as a result the formation of products in low rotational (jp) states is severely restricted. Here energy constraints on recoil orbital AM and the consequent restrictions on jp-state populations are quantified using novel diagrammatic methods that illustrate how constraints on lp determine the jp states that are allowed or forbidden by the need to conserve energy and AM for each state-to-state transition. The method accurately predicts jBaIj (v=0,1,2) peaks from crossed-beam Ba+HI experiments, providing a quantitative and physically transparent rationale for the observed BaI rotational distributions. Extension to a wider range of reactions having μr∕μp>1 shows that at least some jp are formally forbidden for each given reactant relative velocity or, more accurately, lr. The fraction of inaccessible product states for a given initial velocity rises rapidly with μr∕μp (>96% in Ba+HI). The method is also used to demonstrate that recoil orbital AM will be strongly aligned parallel to product rotational AM for high μr∕μp, although this correlation is generally lost in the low jp region as the parallel vector requirement is relaxed.
Al electrode dependent transition to bipolar resistive switching characteristics in pure TiO2 films
Authors: Do Young Ho , Kwak June Sik , Hong Jin Pyo , Jung Kyooho , Im Hyunsik
Published: December 2008

Stable bipolar resistive switching was demonstrated in polycrystalline TiO2 films involving two different top and bottom Al electrodes of two different structures (Al/TiO2/Pt and Pt/TiO2/Al) after a forming process. With an Al electrode, the transition to bipolar resistive switching was clearly observed, together with counterclockwise and clockwise switching directions, which depended on the position of the Al electrode. The transition from unipolar to bipolar resistive switching seems to be attributable to the redox reaction and trap/detrap at the interfaces between the Al electrode and TiO2 layer due to the migration of oxygen ions and electrons. However, current level analysis of devices reveals that the forming process method basically leads to the formation of conducting paths inside the TiO2 layers. The electrical device properties of the two different structures, the effects of compliance currents, and the operation voltages are also analyzed.
Detailed investigation of Ge–Si interdiffusion in the full range of Si1−xGex(≤x≤1) composition
Authors: Gavelle Mathieu , Bazizi El Mehdi , Scheid Emmanuel , Fazzini Pier Francesco , Cristiano Fuccio , Armand Claude , Lerch Wilfried , Paul Silke , Campidelli Yves , Halimaoui Aomar
Published: December 2008

Based on the recently developed MCs2+ secondary ion mass spectrometry methodology, the Ge–Si interdiffusion has been investigated, using Ge(:B) solid sources, for Ge concentrations between 0 and 100 at. %. A strong dependence of the interdiffusion with the Ge content of SiGe alloys, formed during annealing, has been shown. The Boltzmann–Matano method was used to extract the interdiffusivity values for all the temperatures studied (750, 800, 850, and 900 °C) in the full range of SiGe compositions. Two regimes of interdiffusion have been identified, both exhibiting an exponential increase in the interdiffusion coefficient as a function of the Ge concentration. The high Ge content regime (>65 at. %) is in good agreement with the values known in the “extreme” cases of Ge diffusion in Si (0 at. %), Ge self-diffusion, and Si diffusion in Ge (100 at. %), while in the low Ge content regime (<50 at. %), the presence and evolution of misfit dislocation can explain the important values of interdiffusivity found in this work. The observed results are perfectly reproduced by a simple empirical model in which the effect of the Ge concentration and the presence of misfit dislocations are taken into account. Based on the evolution of B delta layers (in Si) and Ge depth profiles during NH3 annealing, we showed that the Ge–Si interdiffusion is predominately assisted by a vacancy mechanism with a slightly interstitial contribution in the full range of Ge concentrations. We have estimated the interstitial fraction coefficient, fGe–SiI, to ∼0.17 at 900 °C. Finally, the effect of in situ B doping is studied, which is found to induce a retardation of the Ge–Si interdiffusion.
Splitting kinetics of Si0.8Ge0.2 layers implanted with H or sequentially with He and H
Authors: Nguyen Phuong , Bourdelle K. K. , Aulnette C. , Lallement F. , Daix N. , Daval N. , Cayrefourcq I. , Letertre F. , Mazuré C. , Bogumilowicz Y. , Tauzin A. , Deguet C. , Cherkashin N. , Claverie A.
Published: December 2008

We have performed systematic measurements of the splitting kinetics induced by H-only and He+H sequential ion implantation into relaxed Si0.8Ge0.2 layers and compared them with the data obtained in Si. For H-only implants, Si splits faster than Si0.8Ge0.2. Sequential ion implantation leads to faster splitting kinetics than H-only in both materials and is faster in Si0.8Ge0.2 than in Si. We have performed secondary ion mass spectrometry, Rutherford backscattering spectroscopy in channeling mode, and transmission electron microscopy analyses to elucidate the physical mechanisms involved in these splitting phenomena. The data are discussed in the framework of a simple phenomenological model in which vacancies play an important role.
Electrosorption selectivity of ions from mixtures of electrolytes inside nanopores
Authors: Hou Chia-Hung , Taboada-Serrano Patricia , Yiacoumi Sotira , Tsouris Costas
Published: December 2008

Grand canonical Monte Carlo (GCMC) simulations are employed to study the selective electrosorption of ions from a mixture of symmetric and asymmetric electrolytes confined in pores and results are compared to experimental observations obtained via cyclic voltammetry and batch electrosorption equilibrium experiments. GCMC simulations have the advantage over other Monte Carlo methods to unambiguously quantify the total number of ions in the pore solution. The exclusion parameter and selectivity factor are used to evaluate the selective capacity of pores toward different ionic species under various conditions. The number of coions inside the pore solution is determined by the proportion of different counterions present in the double-layer region. Because of the competitive effects resulting from asymmetries in charge and size associated with different ions, the electrosorption selectivity of small monovalent over large divalent counterions first decreases with increasing surface charge, passes through a minimum, and then increases with further increase in surface charge. At low and moderate surface charge densities, the fact that large divalent counterions preferentially screen the surface charge has a strong effect on pore occupancy; whereas at a very high surface charge density, size-exclusion effects dominate and determine the accessibility of different ions into the pores. Therefore, electrosorption selectivity of ions from a mixture of electrolytes could, in principle, be achieved via tuning the electrical double-layer formation inside the pores through the regulation of surface charge tailored for different ion characteristics. The findings of this work provide important information relevant to ion selectivity during separation processes and energy storage in supercapacitors.
Molecular dynamics simulation of imidazolium-based ionic liquids. I. Dynamics and diffusion coefficient
Authors: Kowsari M. H. , Alavi Saman , Ashrafizaadeh Mahmud , Najafi Bijan
Published: December 2008

Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium [amim]+ (alkyl=methyl, ethyl, propyl, and butyl) family with PF6−, NO3−, and Cl− counterions. The explicit atom transferable force field of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] is used in the simulations. In this first part, the dynamics of the ionic liquids are characterized by studying the mean-square displacement (MSD) and the velocity autocorrelation function (VACF) for the centers of mass of the ions at 400 K. Trajectory averaging was employed to evaluate the diffusion coefficients at two temperatures from the linear slope of MSD(t) functions in the range of 150–300 ps and from the integration of the VACF(t) functions at 400 K. Detailed comparisons are made between the diffusion results from the MSD and VACF methods. The diffusion coefficients from the integration of the VACFs are closer to experimental values than the diffusion coefficients calculated from the slope of MSDs. Both methods can show good agreement with experiment in predicting relative trends in the diffusion coefficients and determining the role of the cation and anion structures on the dynamical behavior of this family of ionic liquids. The MSD and self-diffusion of relatively heavier imidazolium cations are larger than those of the lighter anions from the Einstein results, except for the case of [bmim][Cl]. The cationic transference number generally decreases with temperature, in good agreement with experiments. For the same anion, the cationic transference numbers decrease with increasing length of the alkyl chain, and for the same cation, the trends in the cationic transference numbers are [NO3]−<[Cl]−<[PF6]−. The trends in the diffusion coefficient in the series of cations with identical anions are [emim]+>[pmim]+>[bmim]+ and those for anions with identical cations are [NO3]−>[PF6]−>[Cl]−. The [dmim]+ has a relatively low diffusion coefficient due to its symmetric structure and good packing in the liquid phase. The major factor for determining the magnitude of the self-diffusion is the geometric shape of the anion of the ionic liquid. Other important factors are the ion size and the charge delocalization in the anion.
Characterization of copper selenide thin film hole-injection layers deposited at room temperature for use with p-type organic semiconductors
Authors: Hiramatsu Hidenori , Koizumi Ikue , Kim Ki-Beom , Yanagi Hiroshi , Kamiya Toshio , Hirano Masahiro , Matsunami Noriaki , Hosono Hideo
Published: December 2008

Copper selenide, CuxSe(x∼2), was examined as a hole-injection layer for low-temperature organic devices. Crystalline CuxSe films grown at room temperature with atomically flat surfaces exhibited metallic conduction with a high electrical conductivity of 4.5×103 S/cm, a hole concentration of 1.4×1022 cm−3, and a mobility of 2.0 cm2/(V s). Analysis of the free carrier absorption using the Drude model estimated the effective mass of a hole as 1.0me. Photoemission spectroscopy measurements of the interfaces between CuxSe and organic hole transport layers, N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) and copper phthalocyanine (CuPc), verified that the hole-injection barriers of these interfaces (0.4 eV for NPB and 0.3 eV for CuPc) are smaller than that of a conventional indium tin oxide (ITO) hole-injection electrode/NPB interface (0.6 eV) but are comparable to that of an ITO electrode/CuPc interface (0.3 eV). Hole-only devices using the CuxSe layer as a hole-injection anode exhibited very low threshold voltages (0.4–0.5 V) and nearly Ohmic characteristics. The NPB layer on the CuxSe layer was found to be highly doped at 1017–1019 cm−3, probably due to copper diffusion, while the CuPc layer is nearly intrinsic with a doping concentration lower than 1015 cm−3. These results indicated that a CuxSe film combined with CuPc is a promising candidate for a low-voltage hole-injection anode or a buffer layer in low-temperature devices such as organic light-emitting diodes and thin film transistors.
Rapid thermal annealing effects on self-assembled quantum dot and quantum ring structures
Authors: Sanguinetti S. , Mano T. , Gerosa A. , Somaschini C. , Bietti S. , Koguchi N. , Grilli E. , Guzzi M. , Gurioli M. , Abbarchi M.
Published: December 2008

Low temperature photoluminescence spectroscopy is used to analyze the effects of the postgrowth thermal annealing on the electronic structure ad carrier dynamics of GaAs∕AlGaAs quantum dot and quantum ring structures grown by droplet epitaxy. All the samples show a large increase in the photoluminescence efficiencies after the thermal treatment due to sizeable reduction in the material defectivity. Modifications of the photoluminescence band, which depend on thermal annealing temperature, are found and quantitatively interpreted by means of a simple model based on the Al–Ga interdiffusion.
Design and characterization of combined function multipole magnet for accelerators
Authors: Sinha Gautam , Singh Gurnam
Published: December 2008

This paper presents the design and analysis of a multipurpose combined function magnet for use in accelerators. This magnet consists of three corrector magnets: (i) skew quadrupole, (ii) horizontal dipole, and (iii) vertical dipole magnets, along with the main sextupole magnet. The strength of the corrector magnets is smaller than that of the main sextupole magnet. The strength of all the four magnets can be varied independently. The excitation strength required to produce skew quadrupole gradient and the presence of various multipole components in the magnet are estimated using first order perturbation theory. The experimental data for the variation of the sextupole strength and its higher order multipoles in the presence of skew quadrupole excitations are presented and compared to the theoretical predictions. Simulation using two-dimensional fine element code, Poisson, is also done. Results obtained from all the above three methods are found to be in good agreement with each other. The variations of skew quadrupole gradient for different sextupole excitations are also measured. The validity of this theory is also checked for various combinations of excitations including the case where magnet gets saturated. The excitation strengths required for producing the horizontal and vertical dipole fields are estimated analytically along with the presence of various multipoles. Theoretical predictions of permissible multipoles are compared to the results obtained from simulation.
Correlation between carrier mobility of pentacene thin-film transistor and surface passivation of its gate dielectric
Authors: Cheng Kam Ho , Tang Wing Man , Deng L. F. , Leung C. H. , Lai P. T. , Che Chi Ming
Published: December 2008

The carrier mobility of pentacene thin-film transistor is studied by passivating the surface of its SiO2 gate dielectric in NH3 at different temperatures, namely, 900, 1000, 1100, and 1150 °C. Measurements demonstrate that the higher the annealing temperature, the higher the carrier mobility of the OTFT is. The device annealed at 1150 °C has a field-effect mobility of 0.74 cm2/V s, which is 35% higher than that of the device annealed at 900 °C. Energy-dispersive x-ray analysis, scanning-electron microscopy, and atomic-force microscopy show that the higher carrier mobility should be due to more nitrogen incorporated at the gate-dielectric surface which results in more passivated dielectric surface and larger pentacene grains for carrier transport.
ZnO nanowire arrays: Optical scattering and sensitization to solar light
Authors: Tena-Zaera Ramón , Elias Jamil , Lévy-Clément Claude
Published: December 2008

Arrays of ZnO nanowires with different lengths (0.5–2 μm) and diameters (100–330 nm) were electrodeposited to study the influence of the nanowire dimensions on light scattering. The nanowire length and diameter were found to be major parameters in modifying the intensity and the wavelength of the scattered radiation, respectively. A significant scattering for the whole visible wavelength range was attained in arrays of ZnO nanowires of ∼1.5 μm in length and ∼330 nm in diameter. ZnO nanowire arrays were sensitized to solar light with a conformally deposited thin CdSe layer. A clear correlation between light scattering before coating and absorption in the resulting ZnO/CdSe core-shell nanostructures was found. The enhancement in the scattering for wavelengths where CdSe exhibits a relatively low absorption coefficient resulted in an effective absorption, in the 400–725 nm range of the AM1.5 solar spectrum, as high as 88% with a CdSe shell thickness of ∼20 nm only.
Ablation and plasma emission produced by dual femtosecond laser pulses
Authors: Singha Sima , Hu Zhan , Gordon Robert J.
Published: December 2008

Pairs of 80fs, 800nm laser pulses were used to ablate Si, Cu, and CaF2 in air. The spectrally resolved plasma emission was measured as a function of laser fluence and pulse delay. After an initial dip, the fluorescence was found to increase monotonically with pulse delay, reaching a plateau after some tens of picoseconds, depending on the material and fluence. The enhancement ratio (defined as the ratio of the fluorescence produced by the pulse pair to that produced by a single pulse of the same total fluence) reaches a maximum value of 6 and 11 at a fluence of ∼6J∕cm2 for Si and Cu, respectively, and declines to a value below 2 at higher fluences. In contrast, the enhancement for CaF2 increases slowly from zero near threshold to a broad maximum value of 2 near 50J∕cm2. Using reflectivity and atomic force microscopy measurements as diagnostics, we interpret the Si and Cu behavior in terms of a two phase mechanism, in which the first pulse melts the surface of the crystal and the second pulse ablates the resulting liquid film. A qualitatively different mechanism initiated by multiphoton absorption is involved in CaF2 ablation.
Polarization orthogonalizer for a pair of laser beams with nearly equal frequencies
Authors: Yim Sin Hyuk , Yoon T. H. , Cho D.
Published: December 2008

We have demonstrated a device to produce an overlapping pair of orthogonally polarized laser beams with a 3 GHz frequency offset out of a single laser beam containing the two frequency components with the same linear polarization. Our design is based on a Michelson interferometer formed by a polarizing beam splitter and two quarter-waveplates. Such a device can be used to make the polarization states of a carrier and a sideband produced through modulation mutually orthogonal. An orthogonally polarized pair of coherent laser beams can be used for an interferometric measurement of a small displacement in a heterodyne scheme or to produce a large-contrast coherent population trapping signal from alkali metal atoms. As a demonstration we used the device to achieve 40% contrast for a coherent population trapping signal from a rubidium vapor cell.
Surface optical Raman modes in InN nanostructures
Authors: Sahoo Satyaprakash , Hu M. S. , Hsu C. W. , Wu C. T. , Chen K. H. , Chen L. C. , Arora A. K. , Dhara S.
Published: December 2008

Raman spectroscopic investigations are carried out on one-dimensional nanostructures of InN, such as nanowires and nanobelts synthesized by chemical vapor deposition. In addition to the optical phonons allowed by symmetry A1, E1, and E2 (high) modes, two additional Raman peaks are observed around 528 and 560 cm−1 for these nanostructures. Calculations for the frequencies of surface optical (SO) phonon modes in InN nanostructures yield values close to those of the new Raman modes. A possible reason for large intensities for SO modes in these nanostructures is also discussed.
Uniform InGaAs quantum dot arrays fabricated using nanosphere lithography
Authors: Qian X. , Li J. , Wasserman D. , Goodhue W. D.
Published: December 2008

We demonstrate the fabrication of optically active uniform InGaAs quantum dot arrays by combining nanosphere lithography and bromine ion-beam-assisted etching on a single InGaAs/GaAs quantum well. A wide range of lateral dot sizes was achieved from an oxygen plasma nanosphere resizing process. The increased lateral confinement of carriers in the dots results in low temperature photoluminescence blueshifts from 0.5 to 11 meV. Additional quantization was achieved using a selective wet-etch process. Our model suggests the presence of a 70 nm dead layer in the outer InGaAs radial edge, which we believe to be a result of defects and dislocations introduced during the dry-etch process.
Low-frequency mechanical spectroscopy study of conformational transition of polymer chains in concentrated solutions
Authors: Wu Xuebang , Xu Qiaoling , Shui Jiapeng , Zhu Zhengang
Published: December 2008

A low-frequency mechanical spectroscopy approach for liquids was proposed for studying conformational transition of polymer chains in concentrated solutions. The technique is applied to aqueous solutions of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer in order to determine if chain conformation is altered in response to temperature. Two transitions are revealed by mechanical spectra and verified by differential thermal analysis with increasing temperature, which may be related to the unimer-to-micelle transition and the phase separation, respectively. The transitions are also found to be much dependent on the concentration of the solution and the addition of NaCl. Moreover, it reveals that the PEO blocks play a more important role in the micellar crystallization process. This study may be helpful in understanding the dynamics of polymer chains in concentrated solutions.
Plasmonic photonic crystal with a complete band gap for surface plasmon polariton waves
Authors: Feng Liang , Lu Ming-Hui , Lomakin Vitaliy , Fainman Yeshaiahu
Published: December 2008

A dielectric plasmonic photonic crystal for manipulating surface plasmon polariton (SPP) fields has been designed, fabricated, and tested. The band structure of SPP fields inside the plasmonic photonic crystal has been calculated using the plane wave expansion method and validated by full wave numerical simulations. The fabricated device was characterized using our far-field SPP imaging technique. The transmittance of incident SPP waves is about 5% at 1520 nm (a designed band gap frequency), confirming the designed band structure. The results show, both experimentally and theoretically, a complete two-dimensional band gap for in-plane SPP waves. The SPP fields at frequencies within this specified 0.14ωa/2πc-wide band gap frequency range are forbidden to propagate through the plasmonic photonic crystal.
Glasslike state in PbFe1/2Nb1/2O3 single crystal
Authors: Kumar Ashok , Katiyar R. S. , Rinaldi Carlos , Lushnikov Sergey G. , Shaplygina Tatjana A.
Published: December 2008

Magnetic properties of PbFe1/2Nb1/2O3 single crystal showed onset of glasslike behavior at 120 K, saturation of onset at 28 K, and freezing of glasslike state at 27.6 K. Zero field cooled (ZFC) and field cooled magnetization curves displayed sharp cusp in ZFC. Almeida and Toulouse line illustrated stability over 1000 Oe with unstable onset for low field. Magnetic hysteresis was observed below glasslike freezing temperature having steplike behavior and low coercive field ∼296 Oe, which may be due to chemical inhomogeneity, strong anisotropy, and weak dipole interaction.
Electrically driven single InGaN/GaN quantum dot emission
Authors: Jarjour Anas F. , Taylor Robert A. , Oliver Rachel A. , Kappers Menno J. , Humphreys Colin J. , Tahraoui Abbes
Published: December 2008

Electroluminescence from single nitride-based quantum dots is reported. Clear single quantum dot emission is observed, which persists up to ∼85 K. This is achieved through the study of a quantum dot layer in the intrinsic region of a forward-biased vertical p-i-n diode. The current-voltage characteristic of the devices is examined at 4.3 K and observed to exhibit electrical bistability phenomena, which is explained in terms of charge accumulation in the InGaN layer. The dependence of the emission properties on current injection conditions are presented and related to the electrical properties of the device.
Temperature distribution and modification mechanism inside glass with heat accumulation during 250kHz irradiation of femtosecond laser pulses
Authors: Sakakura Masaaki , Shimizu Masahiro , Shimotsuma Yasuhiko , Miura Kiyotaka , Hirao Kazuyuki
Published: December 2008

Heat accumulation by high repetition rate femtosecond laser irradiation inside glass generates a much larger modification than that by a single pulse. In this study, we determined the temperature distribution due to heat accumulation and the characteristic temperature for heat modification inside a soda lime glass by analyzing the relationship between the radius of modification and glass temperature. The validity of the analysis was confirmed by reproducing the modification due to two-beam irradiation. The determined characteristic temperature suggested that the temperature distribution and the spatial dependence of the stress relaxation are important in the mechanism of heat modification.