Multiple Trapping Research Articles

Transient Photocurrents in Dye-Sensitized Nanocrystalline Solar Cells

The time varying photocurrent response of dye-sensitized solar cells to switching on illumination and then switching the illumination off after steady state has been reached has been measured and modeled with a multiscale Monte Carlo and continuum approaches within the multiple trapping picture, where electrons in the porous TiO2 electrodes undergo successive trapping and detrapping events. These models are more detailed than the more common formalism employing an effective diffusion coefficient as they take into account the large variation in conduction electron density with distance and time near the extracting electrodes. Agreement between the three sets of results using a trap concentration and trap energy distribution width from a separate set of measurements using charge extraction shows that the multiple trapping formalism holds.

Scanning x-ray microdiffraction of optically manipulated liposomes

We show optical tweezers manipulation of individual micron-sized samples investigating at the same time their inner nanostructure by synchrotron diffraction experiments. The validity of this technique is demonstrated for clusters of multilamellar liposomes trapped in single and multiple positions in the optical path of a microfocused x-ray beam and analyzed in a microscanning mode. The signal to background ratio of the first order peak shows that single liposome measurements are feasible. Multiple trapping by means of diffractive optical elements is demonstrated as an effective manipulation tool for future x-ray diffraction studies of the interaction between different sample entities.

Combining multiple optical trapping with microflow manipulation for the rapid bioanalytics on microparticles in a chip

An array of four independent laser traps is combined with a polydimethylsiloxane microfluidic chip to form a very compact system allowing parallel processing of biological objects. Strong three dimensional trapping allows holding objects such as functionalized beads in flows at speeds near 1 mm/s, enabling rapid processing. By pressure control of the inlet flows, the trapped objects can be put in contact with different solutions for analysis purpose. This setup, including a fluorescence excitation-detection scheme, offers the potential to perform complex biochemical manipulations on an ensemble of microparticles.

Hopping Transport of Electrons in Dye-Sensitized Solar Cells

A calculation of the chemical diffusion coefficient of electrons, Dn, in dye-sensitized solar cells, is presented, in the framework of the hopping model, where electron transport occurs by direct transitions between localized states in an exponential distribution. The Fermi-level dependence of Dn in the transport energy approximation is exactly the same as that in the multiple trapping model, but there are differences concerning the meaning of the parameters governing electron transport and the values of relevant energy levels inferred from experiments. The hopping model appears to describe well the experimental data of Dn from high-efficiency dye solar cells at various temperatures.

Numerical simulation of the transient photoconductivity in hydrogenated amorphous siliconincluding localized-state electron hopping

The transient photoconductivity (TPC) in hydrogenated amorphous silicon is studied bynumerical simulation. In addition to normal free carrier capture (emission) transitions into(from) localized states by the multiple-trapping (MT) process, we include theprocess of electron hopping (EH) through the conduction band tail states. Thedistribution of the dangling bond density is calculated by the defect pool model, whileexponential distributions are assumed for the conduction and valence band tails.The simulation results are in good agreement with previous theoretical results: ahopping transport energy level identified as the peak of the energy distribution ofthe hopping photoconductivity is in excellent agreement with the theoreticalhopping transport energy of Monroe. The simulated TPC is studied as a functionof temperature in order to determine the relative contribution of MT and EHtransport mechanisms. A smooth transition around 130 K between high temperatureextended-state conduction via MT and low temperature localized-state EH isdetermined.

Seasonal rhythms of net primary production and particulate organic carbon flux to depth describe the efficiency of biological pump in the global ocean

We investigate the functioning of the ocean’s biological pump by analyzing the vertical transfer efficiency of particulate organic carbon (POC). Data evaluated include globally distributed time series of sediment trap POC flux, and remotely sensed estimates of net primary production (NPP) and sea surface temperature (SST). Mathematical techniques are developed to compare these temporally discordant time series using NPP and POC flux climatologies. The seasonal variation of NPP is mapped and shows regional‐ and basin‐scale biogeographic patterns reflecting solar, climatic, and oceanographic controls. Patterns of flux are similar, with more high‐frequency variability and a subtropical‐subpolar pattern of maximum flux delayed by about 5 days per degree latitude increase, coherent across multiple sediment trap time series. Seasonal production‐to‐flux analyses indicate during intervals of bloom production, the sinking fraction of NPP is typically half that of other seasons. This globally synchronous pattern may result from seasonally varying biodegradability or multiseasonal retention of POC. The relationship between NPP variability and flux variability reverses with latitude, and may reflect dominance by the large‐amplitude seasonal NPP signal at higher latitudes. We construct algorithms describing labile and refractory flux components as a function of remotely sensed NPP rates, NPP variability, and SST, which predict POC flux with accuracies greater than equations typically employed by global climate models. Globally mapped predictions of POC export, flux to depth, and sedimentation are supplied. Results indicate improved ocean carbon cycle forecasts may be obtained by combining satellite‐based observations and more mechanistic representations taking into account factors such as mineral ballasting and ecosystem structure.

Getting the Drift: Examining the Effects of Timing, Trap Type and Taxon on Herpetofaunal Drift Fence Surveys

The evaluation of appropriate sampling methodologies is critical for accurately determining the distribution and status of herpetofaunal populations. We report the results of a year-long drift fence study, using multiple trap types (large pitfall traps, small pitfall traps and funnel traps), of a species-rich herpetofaunal community (59 species) surrounding an isolated wetland in the southeastern United States. Specifically, we determined the effects that timing, trap type and taxon had on capture rates of herpetofauna. We found that funnel traps captured the greatest number of herpetofaunal species, but a combination of funnel traps and large pitfall traps yielded the greatest number of individual captures due to complementary biases in capture efficiencies among herpetofaunal taxa. With little exception, small pitfall traps were relatively ineffective for sampling herpetofauna. We also found that the timing of drift fence monitoring affected herpetofaunal species accumulation rates but that seasonal effects were taxon-specific. Our study affirms that drift fences are exceptional tools for inventorying and monitoring diverse species and large numbers of herpetofauna and also demonstrates the important effects that season and taxon can have on capture rates. Therefore, we recommend a priori delineation of project goals and the use of multiple trap types with careful attention to the timing of drift fence monitoring to maximize sampling efficiency and minimize biases associated with data collection.

Passivation and reactivation of carriers in B- and P-doped Si treated with atomic hydrogen

The formation and annihilation of hydrogen (H)-related complexes were investigated in boron (B)- or phosphorus (P)-doped Si treated with high concentration of atomic H. The passivation and reactivation process of dopant carriers were significantly different between the p-type and n-type specimens. The differences are explained by the stable sites of the H atoms in the p-type and n-type specimens and, in turn, by the formation of H-related defects: i.e., H multiple trapping centers are formed by bond-breaking due to H atoms only in p-type B-doped Si. The formation of such defects retards the reactivation of B dopants by annealing.

Determination of the density of localized states in semiconductors from the pre-recombination transient photoconductivity

In this paper we present a new transient photoconductivity (TPC) inversion method for the determination of the density of localized states (DOS) energy distribution g( E) in thin film semiconductor materials with exact matrix solution for g( E). The method, derived from the multiple trapping model, is based on prior determination of the exact transient trap occupation function and applies to the pre-recombination time range of the TPC. It is demonstrated by application to simulated TPC data that high energy resolution can be achieved for the case of continuous DOS distribution, appropriate to amorphous semiconductors, as well as for discrete level DOS such as in crystalline semiconductors.

Modification of Nanostructured TiO2 Electrodes by Electrochemical Al3+ Insertion: Effects on Dye-Sensitized Solar Cell Performance

Nanostructured TiO2 films were modified by insertion with Al ions using an electrochem. process. After heat treatment these films were suitable as electrodes in dye-sensitized solar cells. By a catechol adsorption test and with photoelectron spectroscopy , the d. of Ti atoms at the metal oxide/electrolyte interface decreased due to Al modification. There is not a complete coverage of Al oxide onto the TiO2 - the results suggest either the formation of a mixed Al-Ti oxide surface layer or formation of a partial Al oxide coating. No new phase was detected. In solar cells with Al-modified TiO2 electrodes electron lifetimes and electron transport times increased. At high concns. of inserted Al ions, the quantum efficiency for electron injection decreased. Results are discussed according to a multiple trapping model which can explain slower kinetics by the creation of addnl. traps during Al insertion, and a surface layer model which explains the reduced recombination rate as well as the reduced injection efficiency by the formation of a blocking layer.

Novel designs for Penning ion traps

A number of alternative designs are presented for Penning ion traps suitable for quantum information processing (QIP) applications with atomic ions. The first trap design is a simple array of long straight wires, which allows easy optical access. A prototype of this trap has been built to trap Ca+ and a simple electronic detection scheme has been employed to demonstrate the operation of the trap. Another trap design consists of a conducting plate with a hole in it situated above a continuous conducting plane. The final trap design is based on an array of pad electrodes. Although this trap design lacks the open geometry of the other traps described above, the pad design may prove useful in a hybrid scheme in which information processing and qubit storage take place in different types of trap. The behaviour of the pad traps is simulated numerically and techniques for moving ions rapidly between traps are discussed. Future experiments with these various designs are discussed. All of the designs lend themselves to the construction of multiple trap arrays, as required for scalable ion trap QIP.

Physics-Based Percolation Model of Oxide Breakdown

A physics-based percolation model that relates defect generation and oxide breakdown is presented. The model is based on two elements: a compact model for the trap-assisted current through single and multiple trap paths and the statistical description of defect positions in these paths. This new percolation model overcomes the limitations of the standard geometrical implementation of the percolation model and is more easily linked to the experimental breakdown results in the ultra-thin oxide limit. This new picture reproduces all the important results of the traditional percolation model and is able to explain experimental results that the old model cannot even consider.

Analysis of charge transport in a polycrystalline pentacene thin film transistor by temperature and gate bias dependent mobility and conductance

The gate bias and temperature dependent field-effect mobility and conductance of a polycrystalline pentacene thin film transistor (TFT) were analyzed to study the charge transport in the material. Since both heating and cooling can obviously change the film morphology, a relatively narrow temperature range was adopted to rule out the possible influence of structure variation on the device characterization. Both mobility and conductance values increased with the gate bias and showed a thermally activated Arrhenius-like behavior, while the threshold voltage deceased with temperature. Several models were compared, and it was found that the observations could only be well interpreted by a multiple trapping model, which suggests that the temperature and gate bias dependences should be attributed to the increased free charge carrier density. The density of trap states in the band gap was evaluated by the field-effect mobility as well as the field-effect conductance data. The results disclose a possible field-effect mobility much higher than the present record in a polycrystalline pentacene TFT.

Evaluation of on-line solid-phase extraction parameters for hyphenated, high-performance liquid chromatography–solid-phase extraction–nuclear magnetic resonance applications

The hyphenated technique HPLC–SPE–NMR is proving to be a useful analytical tool for structure elucidation of mixture components, particularly for mass-limited samples where traditional isolation procedures are either time consuming or challenging. In this work, we investigated SPE trapping performance of 25 model natural products within a format corresponding to that of HPLC–SPE–NMR hyphenation. Six different silica-based bonded phases and two polymeric phases were evaluated. The trapping efficiency of polystyrene/divinylbenzene polymers was generally superior compared to silica bonded phases, which showed variable results and performed well only with hydrophobic analytes. Acetonitrile concentration in the loading solvent was critical for trapping on polymeric phase (Resin GP), as small changes of the organic solvent concentration (±3%) could alter the trapping efficiency significantly. Flow rate changes of the loading solvent within 0.8–5.0 mL/min did not affect trapping kinetics. Simulation of multiple trapping showed excellent performance of this approach for hydrophobic analytes, and moderate gain for more polar analytes that do not trap quantitatively in a single trapping step. Determination of 50% breakthrough levels by frontal chromatography analysis showed feasibility of accumulation of analyte amounts corresponding to about 0.5 μmol (10 mm × 2 mm i.d. Resin GP cartridge).

Extracting parameters from the current-voltage characteristics of polycrystalline octithiophene thin film field-effect transistors

Organic field-effect transistors (OFETs) in which the active semiconductor is made of polycrystalline octithiophene (8T) were fabricated and characterized. Several methods have been used to extract the parameters of the polycrystalline (8T) transistors as function of gate voltage at room and at low temperatures. These parameters such as the mobility, the threshold voltage, contact resistance and density of traps are extracted from the current-voltage characteristics of OFETs. The first method consists of deriving the drain current as function of gate voltage (transconductance), leading to the so-colled field effect mobility. It appears that the data must be corrected for the substantial source and drain contact resistance. In the second method, the carrier mobility has been corrected for the contact resistance. It is found to increase quasi linearly with gate voltage. Therefore, the contact resistances have been directly extracted from the experimental channel conductance. In the third method, data are interpreted according the multiple thermal trapping and release model using a power law dependence of the mobility with gate voltage together with a constant resistance. Finally, using a model where charge transport is limited by trapping and thermal release at localized states located at grain boundaries, the barrier height at grain boundaries in polycrystalline octhithiophene FETs has been estimated. It is shown that is gate bias dependent.

Effects of Optical Trapping and Liquid Surface Deformation on the Laser Microdeposition of a Polymer Assembly in Solution

A polymer microassembly is formed by focusing a near-infrared (NIR) laser beam in a thin film of a polymer solution. We have investigated the mechanism of laser microdeposition of a polyfluorene assembly by measuring the surface deformation of the solution film and the morphology of the deposited assembly. It is clearly observed that a rupture is formed at the laser focus in the solution film by using laser interferometric imaging. The time necessary for the rupture formation and the volume of the deposited microassembly are analyzed as a function of laser power. Experimental results suggest that the solution surface deformation induced by local laser heating and optical trapping effects determined the volume of the laser microdeposition. By combining this method with multiple optical trapping, a polymer microassembly with a polygonal morphology is formed on the glass substrate.

Fluxon collider for multiple fluxon–antifluxon collisions

We describe a device for generation and trapping of fluxon–antifluxon (FA) pairs in a long annular Josephson junction. The trapped fluxons and antifluxons experience multiple collisions and eventually decay into plasmons. We analyse the energy dissipation in the system and find the criteria for trapping of the FA pair. We describe a possible experiment for realization of multiple FA trapping and formation of a FA plasma. Similar to vortex liquids discussed recently by Anderson it might be a good candidate for another vortex liquid. The proposed device can be made of niobium or high temperature superconductors.

Dopant dependence on passivation and reactivation of carrier after hydrogenation

The formation of hydrogen (H)-related complexes and H effects on boron (B) and phosphorus (P) dopants was investigated in B- or P-doped silicon (Si) crystal treated with high concentration of H. The reactivation process of dopant carriers by annealing after hydrogenation was significantly different between the p-type and n-type specimens. The difference is likely to be attributable to the formation of H-related defects based on the stable sites of the H atoms, i.e., complicated H multiple trapping centers are formed by bond breaking due to H atoms in only p-type B-doped Si.

Neoclassical toroidal viscosity for an axisymmetric toroidal equilibrium with multiple trapping of particles

A method is illustrated on how to apply existing formulas to the neoclassical toroidal plasma viscosity for an axisymmetric toroidal equilibrium with an arbitrary aspect ratio that has multiple trapping states. The effect of the energy scattering on the viscosity in the ν regime is presented using an approximate collision operator. Here, ν is the collision frequency. The explicit expressions for the steady state toroidal flow speed resulting from the neoclassical toroidal flow damping in both the 1∕ν and the ν regime are also presented. The physics origin of the toroidal viscous force is the broken toroidal symmetry induced by the error fields and the amplification of these fields by stable or unstable resistive wall modes, or other asymmetric magnetic field perturbations.

New Results in the Theory of Multiple Mirror Plasma Confinement

Theory of plasma confinement in a multiple mirror trap is extended. Plasma lifetime is estimated for new regimes with partially devastated loss-cone, which occur at the stage of plasma decay in GOL-3 multiple mirror after termination of relativistic electron beam injection.