Multiple Trapping Research Articles

Effect of hole scavenging and preirradiation on electron transport in polyvinylcarbazole

The results of experimental and theoretical studies of the effects of hole (majority carrier) scavengers, such as aromatic hydrazone, and preliminary irradiation on electron transport in polyvinylcarbazole are presented. It is shown that the scavenging effects can satisfactorily be described in terms of the multiple trapping model and the transport parameters of electrons—minority charge carriers—can simultaneously be determined (although approximately). The description of preirradiation effects required this model to be extended, in particular, with allowance for radiation-induced alteration in the electronic structure of the polymer at doses starting from the order of 100 Gy.

SIN NOMBRE VIRUS INFECTION OF DEER MICE IN MONTANA: CHARACTERISTICS OF NEWLY INFECTED MICE, INCIDENCE, AND TEMPORAL PATTERN OF INFECTION

Sin Nombre virus (SNV), hosted by the deer mouse (Peromyscus maniculatus), is the principal cause of hantavirus pulmonary syndrome (HPS) in North America. To improve our understanding of factors that contribute to the occurrence of HPS, we conducted an extensive field study of the characteristics of newly infected (as determined by recent acquisition of antibody) deer mice and the temporal pattern of antibody acquisition (seroconversion) from 1994 through 2004 in Montana, USA. We sampled 6,584 individual deer mice, of which 2,747 were captured over multiple trapping periods. Among these 2,747 deer mice, we detected 171 instances of seroconversion. There was no relationship between seroconversion and the acquisition of scars. However, recently infected Montana deer mice were more likely to be older, more likely to be males, and more likely to be in breeding condition. In addition, recently infected male deer mice gained less weight over the 1-mo period following seroconversion than did those that did not acquire antibody, suggesting that SNV infection may have negatively impacted the health of infected rodents. Incidence was highly variable among years, and timing of infections was primarily associated with the breeding season (generally early spring through late fall).

Characteristics of High Efficiency Dye-Sensitized Solar Cells

Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.

Fractional theory for transport in disordered semiconductors

The article is devoted to theoretical description of charge carrier transport in disordered semiconductors. The main idea of the approach lies in the use of fractional calculus. The physical reasons of introducing fractional derivatives in semiconductor theory are discussed, the process of derivation of fractional differential equations is demonstrated, the tied link of their solutions with non-Gaussian stable processes is shown. The last section of the article contains solutions of some concrete problems: multiple trapping, transient photocurrent and drift mobility, dispersive transport percolation model of semiconductors, transport in bilayer semiconductor and so on. Some numerical results are obtained and their agreement with experimental data is demonstrated.

Field-induced mobility degradation in pentacene thin-film transistors

Pentacene-based organic thin-field transistors (OTFTs) with bottom-gate, top-contact architecture were fabricated on alumina substrates. The devices were divided into two sets, depending on whether pentacene was deposited on bare alumina or on alumina modified with a fatty acid self-assembled monolayer (SAM). Previous analysis have shown that the modification of alumina with SAMs result in a substantial decrease of the size of the grains in hte polycrystalline films. Careful parameters extraction, including an original determination of the threshold voltage, and contact resistance extraction through the transfer line method (TLM), allowed us to estimate the gate-voltage dependent mobility in both series. The mobility is found to first increase at low bias, and then decrease at higher gate voltage (mobility degradation). The latter behavior is explained through an estimation of the distribution of charges across the accumulation layer, in which the pentacene film is modeled as a stack of dielectric layers, the distribution being calculated using basic equations of electrostatics and thermodynamics. A good agreement was found when the mobility in the layer next to the insulator was assumed to be negligible as compared to that in the bulk of the film. The initial rise of the mobility is interpreted in terms of multiple trap and release (MTR) with a distribution of traps located in the grain boundaries. Interestingly, the mobility corrected for both contact resistance and mobility degradation is found around 3 cm2/V s for pentacene deposited on bare alumina, and 5 cm2/V s when pentacene is deposited on SAM modified alumina. We conclude that the smaller grains grown on modified alumina are more regular, and hence less defective, than the larger grains deposited on bare alumina.

Recombination and Transport Processes in Dye-Sensitized Solar Cells Investigated under Working Conditions

The transport and recombination of electrons in dye-sensitized TiO(2) solar cells were studied by analysis of the current and voltage response to a small square-wave light-intensity modulation. Solar cells were studied under working conditions by using potentiostatic and galvanostatic conditions. An increase in applied voltage, that is, from 0 V toward open-circuit voltage, was found to lead to faster electron transport at low light intensities, while it slowed transport at higher light intensities. This observation seems to be conflicting with the multiple trapping model with diffusive transport. An effective diffusion length at the maximum power point was calculated, and it was shown that it decreases with increasing light intensity.

Identification of Major and Minor Constituents of Harpagophytum procumbens (Devil's Claw) Using HPLC-SPE-NMR and HPLC-ESIMS/APCIMS

The HPLC-SPE-NMR technique, supported by HPLC-MS measurements, was used to determine structures of major as well as some minor constituents of ethanol and petroleum ether extracts of Harpagophytum procumbens (Devil's claw) roots. This method was also shown to be applicable for rapid and precise on-line identification of secondary metabolites present in commercial herbal products of H. procumbens. A total of 15 compounds (1-14 and 17) were identified from the ethanol and petroleum ether extracts, including a novel Diels-Alder dimer 14. Optimization of the HPLC-SPE-NMR experiments included quantitative (1)H NMR measurements, determination of trapping and elution efficiency, effect of multiple trapping of analytes, use of various deuterated solvents for SPE cartridge elution, and effect of post-column dilution ratio of eluent with water. Linear accumulation of apolar and relatively polar analytes was demonstrated for at least 8-10 repeated trappings, resulting in greatly improved signal-to-noise ratios in NMR spectra and reduced acquisition times. Thus, the HPLC-SPE-NMR technique provides an efficient means of identification of multiple components of crude extracts. By allowing on-line generation of high-quality 2D NMR data without traditional purification of extract components, the HPLC-SPE-NMR methodology represents a paradigm shift in natural products research with respect to structure elucidation.

Interpretation of Apparent Activation Energies for Electron Transport in Dye-sensitized Nanocrystalline Solar Cells

Electron transport in dye-sensitized nanocrystalline solar cells appears to be a slow diffusion-controlled process. Values of the apparent electron diffusion coefficient are many orders of magnitude smaller than those reported for bulk anatase. The slow transport of electrons has been attributed to multiple trapping (MT) at energy levels distributed exponentially in the band gap of the nanocrystalline oxide. In the MT model, release of immobile electrons from occupied traps to the conduction band is a thermally activated process, and it might therefore be expected that the apparent electron diffusion coefficient should depend strongly on temperature. In fact, rather small activation energies (0.1-0.25 eV) have been derived from time and frequency resolved measurements of the short circuit photocurrent. It is shown that the MT model can give rise to such anomalously low apparent activation energies as a consequence of the boundary conditions imposed by the short circuit condition and the quasi-static relationship between changes in the densities of free and trapped electrons. This conclusion has been confirmed by exact numerical solutions of the time-dependent generation/collection problem for periodic excitation that provide a good fit to experimental data.

Occurrence of estrogens in the Scheldt estuary: A 2-year survey

Despite the increased research and regulatory interest in numerous bioactive agents, including natural hormones, xeno-hormones and pharmacological agents, little is known about the presence of these compounds in the estuarine and marine environment. In this study, the results of a 2-year survey on the occurrence of the natural female sex hormones, estradiol (E2) and estrone (E1) and the synthetic steroid, ethinylestradiol (EE2) in the Scheldt estuary (Belgium-The Netherlands) are presented. Chemical analysis of the water samples was performed using Speedisk™ extraction. Suspended matter samples were analyzed with accelerated solvent extraction (ASE) and detection was performed with gas chromatography coupled to multiple ion trap mass spectrometry. Detected concentrations were in the low ng L −1 range. E1 and βE2 ( β-isomer of E2) were detected in water and suspended matter, whereas concentrations of EE2 were below the limit of quantification (LOQ). E1 was observed most frequently and at concentrations up to 10 ng L −1 in water and up to 0.84 ng g −1 in suspended matter samples.

Enhanced sequence coverage of proteins in human cerebrospinal fluid using multiple enzymatic digestion and linear ion trap LC-MS/MS

The cerebrospinal fluid (CSF) provides a ready access into the health state of the central nervous system, and alterations in some CSF proteins have been documented in brain disease. However, the complete variety of proteins is not known and methods to identify protein components are still being developed. The goal of this study was to examine the sequence coverage obtained from human CSF digests produced with different proteases. Enzymatic digests of CSF proteins were obtained with arginine-C endopeptidase (ArgC), glutamic acid endopeptidase (GluC), chymotrypsin, trypsin and their combinations, and then examined using reverse phase chromatography and a Finnigan LTQ linear ion trap mass spectrometer. Peptide sequences were identified with BioWorks 3.1 and sequence coverage calculated for the 38 most confidently identified proteins. Trypsin and GluC yielded greater coverage than chymotrypsin, while ArgC had the least sequence coverage. Protein sequence coverage was affected only slightly over four orders of magnitude dynamic range of abundance. Combining the peptides derived from different proteases further increased the coverage. Maximal sequence coverage was achieved by combining digest results from both GluC and trypsin. These results have implications for future studies to identify CSF proteins and their post-translational modifications.

Modeling the emission red-shift in amorphous semiconductors and in organic-inorganic hybrids using extended multiple trapping

A model of thermal relaxation within localized states based on the extended multiple trapping framework is used to describe the red-shift of the emission maximum intensity as the excitation energy decreases. The model is applied to amorphous hydrogenated silicon (a-Si:H) and to organic-inorganic hybrids systems giving values for the energy gap, E0, (1.896–3.882 eV) and for the β (4.36–12.08 eV-1) parameter that characterizes the experimental decay of the density of localized states within the gap consistent with those achieved by some other recombination models previously reported for a-Si:H.. The thermal relaxation within localized states model is more physically detailed incorporating radiative and non-radiative transition mechanism for carriers relaxing into localized states that are explicitly absent in the previously reported theoretical descriptions

Modeling of the emission red-shift in organic–inorganic di-ureasil hybrids

The red-shift of the emission maximum intensity as the excitation energy decreases is modeled for organic/inorganic hybrids formed by a siliceous skeleton covalently grafted to oligopolyether chains by means of urea bridges (named di-ureasils). A new model of thermal relaxation within localized states based on the extended multiple trapping framework is derived and values for the energy gap and for the density of localized states are reported. The temporal dynamics that governs the emission is explicitly accounted assuming an effective energy independent lifetime.

Controlling LC–SPE–NMR systems

There are several stages of the LC-SPE-NMR process that should be monitored closely to ensure an efficient isolation and concentration of the target analyte, for instance analyte break-through and compound transfer from the LC-SPE to the NMR probe. In this study, analyte break-through monitoring was performed with a UV detector and a mass spectrometer placed after the SPE unit. Easy break-through was a problem when attempting multiple trapping of various compounds using C18 SPE cartridges with the original commercial system. However, on lowering the flow rate over the SPE system and using SPE cartridges packed with porous carbon, the number of trappings possible increased five-fold. To increase control over the on-line SPE-NMR transfer, a gradient pump-UV system was used to elute compounds trapped on an SPE to an NMR probe. The analyte band was placed in the active volume of the probe by a stop-flow mechanism. The modified LC-SPE system was also coupled with off-line NMR analysis for determination of a degradation product of the insecticide monuron, present in the low ppm range.

Temperature dependence of the electron diffusion coefficient in electrolyte-filledTiO2nanoparticle films: Evidence against multiple trapping in exponential conduction-band tails

The temperature and photoexcitation density dependences of the electron transport dynamics in electrolyte-filled mesoporous ${\mathrm{TiO}}_{2}$ nanoparticle films were investigated by transient photocurrent measurements. The thermal activation energy of the diffusion coefficient of photogenerated electrons ranged from 0.19--0.27 eV, depending on the specific sample studied. The diffusion coefficient also depends strongly on the photoexcitation density; however, the activation energy has little, if any, dependence on the photoexcitation density. The light intensity dependence can be used to infer temperature-independent dispersion parameters in the range 0.3--0.5. These results are inconsistent with the widely used transport model that assumes multiple trapping of electrons in an exponential conduction-band tail. We can also exclude a model allowing for widening of a band tail with increased temperature. Our results suggest that structural, not energetic, disorder limits electron transport in mesoporous ${\mathrm{TiO}}_{2}$. The analogy between this material and others in which charge transport is limited by structural disorder is discussed.

Optical sculpture: controlled deformation of emulsion droplets with ultralow interfacial tensions using optical tweezers

We report a technique for deforming micron-sized emulsion droplets that have ultralow interfacial tensions, by the manipulation of multiple optical trapping sites within the droplets.

The Einstein relation in systems with trap-controlled transport

The applicability of the Einstein relation considering diffusivity and mobility of charge carriers with respect to disordered systems has been a matter of serious discussion in recent years. Here the validity of the Einstein relation, using the generalized form, is checked for two specific density-of-states functions within a multiple trapping model and the mobility edge concept. It is shown that the classical Einstein relation is valid for almost all charge-carrier densities relevant for experimental purposes. It is demonstrated that this result is dependent on the charge-carrier mobility definition.

Comparison of analytical and semi-preparative columns for high-performance liquid chromatography–solid-phase extraction–nuclear magnetic resonance

The application of analytical and semi-preparative columns in reversed-phase liquid chromatography–solid-phase extraction–nuclear magnetic resonance (HPLC-SPE-NMR) was compared. The work was aiming at separating a higher sample amount in a single run and in this way to reduce the necessary NMR measurement time of separated compounds. Several parameters for compound separation and trapping procedures were optimised: flow rate of HPLC and make-up water pumps, choice of stationary phase cartridges and drying time. The separation and loadability of nine model compounds on analytical and semi-preparative columns was determined, as well as the focussing capacity of SH-type SPE cartridges. It was found that a semi-preparative column—or multiple peak trapping on analytical columns—gave better results than a standard 4.6 mm analytical column for non-polar compounds (e.g. flavonoid aglycones, sesquiterpene lactones, non-polar terpenes, log P > 2), but for polar compounds (log P < −2) did not offer any advantage over an analytical column, or was even disadvantageous. For intermediately polar compounds (−2 < log P < 2) this has to be investigated per compound.

Electrical and Optical Characterization of Thin Semiconductor Layers for Advanced ULSI Devices

An overview is given of analytical techniques for the characterization of the electrical and transport parameters in thin (&lt;1 µm) semiconductor layers. Some of these methods have been applied to the lifetime and diffusion length study in thin strain-relaxed buffer (SRB) layers of strained silicon (SSi) substrates, while a second group was dedicated to Silicon-on-Insulator (SOI) materials and devices. The employed techniques can be divided into two groups, whether a device structure (junction, MOS capacitor, MOSFET) is required or not. However, the MicroWave Absorption (MWA) technique can be used in both cases, making it a versatile tool to study both grown-in and processing-induced electrically active defects. The transport properties of SSi wafers are strongly determined by the density of threading and misfit dislocations, although the dependence of the recombination lifetime is weaker than expected from simple Shockley-Read-Hall (SRH) theory. This is related to the high injection regime typically employed, enabling the characterization of the 250-350 nm thick Si1-xGex layer only. At longer carrier decay times, multiple trapping events dominate that can be described by a stretched exponent approach, typical of disordered materials. For SOI substrates, transistor-based techniques will be demonstrated that enable to assess the generation or recombination lifetime in the thin silicon film (&lt;100 nm). The lifetime can be severely degraded by irradiation or hot-carrier degradation. Finally, it will be shown that Generation-Recombination (GR) noise spectroscopy as a function of temperature allows identifying residual ion-implantation-damage related deep levels, which are otherwise hard to detect even by Deep Level Transient Spectroscopy (DLTS).

Rapid Extract Dereplication Using HPLC-SPE-NMR: Analysis of Isoflavonoids from Smirnowia iranica

A novel hyphenated technique, HPLC-SPE-NMR, was used for accelerated identification of isoflavonoids from the roots of Smirnowia iranica. The extract constituents eluted from a HPLC column were automatically trapped on solid-phase extraction (SPE) cartridges, and NMR spectra were acquired with concentrated solutions after solvent change. The structures of 10 new isoflavonoids (1, 4, 5, 7-10, 12, 13, 16) and of seven previously described constituents (2, 3, 6, 11, 14, 15, 17) were elucidated from NMR spectra acquired in the HPLC-SPE-NMR mode. Multiple peak trapping on the same SPE cartridge increased analyte amounts and provided access to 2D NMR data. It was demonstrated that linear accumulation of material is possible in up to seven repeated trapping steps. The use of HPLC-SPE-NMR speeded up dereplication of the S. iranica extract considerably by providing detailed information about the constituents of a complex, essentially crude extract prior to their preparative-scale isolation or extract pre-fractionation, and the information obtained could be used to direct preparative isolation work. In connection with structure elucidation of isoflavonoids containing O-methylated 1,2,3-benzenetriol moieties as the B-ring, O-methylation-induced changes of chemical shifts of aromatic hydrogens were found to depend on the conformation of the resulting methoxy group, i.e., on the number of its ortho substituents. The recognized regularities will be useful in structure determination of partially O-methylated polyphenols based on 1D (1)H NMR spectra obtainable from HPLC-SPE-NMR experiments, diminishing dependence on 2D NMR data and (13)C NMR chemical shifts.

Hitler, the Allies, and the Jews

This book offers an analysis of the Holocaust as a multiple trap, its origins, and its final stages, in which rescue seemed to be possible. With the Holocaust developing like a sort of a doomsday machine set in motion from all sides, the Jews found themselves between the hammer and various anvils, each of which worked according to the logic created by the Nazis that dictated the behavior of other parties and the relations between them before and during the Holocaust. The interplay between the various parties contributed to the victims' doom first by preventing help and later preventing rescue. These help and rescue efforts proved mainly self defeating, and various legacies about them emerged during the Holocaust and are heatedly debated even today. Their real nature is uncovered here on the basis of newly opened archives worldwide.