Multiple Trapping Research Articles

Two-Layer Multiple Trapping Model for Charge Transport in Molecularly Doped Polymers

Two-Layer Multiple Trapping Model for Charge Transport in Molecularly Doped Polymers

Band transport and mobility edge in amorphous solution-processed zinc tin oxide thin-film transistors

We report on charge transport phenomena in high-mobility solution-deposited amorphous zinc-tin oxide based thin-film transistors. At low carrier concentrations, the dominant transport mechanism is multiple trap and release, with the activation energy steadily decreasing with increasing carrier density. The activation energy decreases to zero and beyond a threshold carrier density, the mobility decreases with increasing temperature. This temperature dependence as well as the value of the mobility clearly indicates that transport is bandlike. Also observed is a clear mobility edge in accordance with the prediction of Mott’s model, which are normally observed in crystalline semiconductors.

Laser speckle field as a multiple particle trap

We demonstrate that a speckle pattern in the spatially coherent laser field transmitted by adiffuser forms a multitude of three-dimensional intensity micro-pockets acting asparticle traps for air-borne light absorbing particles. Confinement of up to a fewthousand particles in air with a unidirectional single beam has been achieved.Theoretical analysis of the speckle defined trapping volume is in a good agreement withexperimental results on capturing of aggregates of carbon nanoparticles in air.

Charge Transport and Interfacial Charge Transfer in Dye-Sensitized Nanoporous Semiconductor Electrode Systems

General characteristics of dye-sensitized nanoporous semiconductor electrode systems are summarized, with a particular emphasis on dye-sensitized solar cells. Properties of these electrode systems which distinguish them from conventional bulk semiconductor electrodes are highlighted. Current understanding of electron transport in dye-sensitized solar cells, in terms of the diffusion and multiple trapping models, is reviewed. Alternative transport and recombination theories are also briefly reviewed. Electron transfer at the semiconductor/electrolyte interface in dye-sensitized solar cells is reviewed and recent experimental results obtained by the authors are highlighted. As applicable, common techniques for characterization of electron transport and transfer in dye-sensitized solar cells are described, with reference to case studies where the electron diffusion length in dye-sensitized solar cells has been estimated. The steady-state aspects of the dye-regeneration process are also reviewed, together with the cross-surface percolation of holes in the dye monolayer and the finite-length diffusion of redox species in the electrolyte.

Imaging MS Methodology for More Chemical Information in Less Data Acquisition Time Utilizing a Hybrid Linear Ion Trap−Orbitrap Mass Spectrometer

A novel mass spectrometric imaging method is developed to reduce the data acquisition time and provide rich chemical information using a hybrid linear ion trap-orbitrap mass spectrometer. In this method, the linear ion trap and orbitrap are used in tandem to reduce the acquisition time by incorporating multiple linear ion trap scans during an orbitrap scan utilizing a spiral raster step plate movement. The data acquisition time was decreased by 43-49% in the current experiment compared to that of orbitrap-only scans; however, 75% or more time could be saved for higher mass resolution and with a higher repetition rate laser. Using this approach, a high spatial resolution of 10 μm was maintained at ion trap imaging, while orbitrap spectra were acquired at a lower spatial resolution, 20-40 μm, all with far less data acquisition time. Furthermore, various MS imaging methods were developed by interspersing MS/MS and MS(n) ion trap scans during orbitrap scans to provide more analytical information on the sample. This method was applied to differentiate and localize structural isomers of several flavonol glycosides from an Arabidopsis flower petal in which MS/MS, MS(n), ion trap, and orbitrap images were all acquired in a single data acquisition.

RETRACTED: The initial rise method extended to multiple trapping levels in thermoluminescent materials

The well known Initial Rise Method (IR) is commonly used to determine the activation energy when only one glow peak is presented and analysed in the phosphor materials. However, when the glow peak is more complex, a wide peak and some holders appear in the structure. The application of the Initial Rise Method is not valid because multiple trapping levels are considered and then the thermoluminescent analysis becomes difficult to perform. This paper shows the case of a complex glow curve structure as an example and shows that the calculation is also possible using the IR method. The aim of the paper is to extend the well known Initial Rise Method (IR) to the case of multiple trapping levels. The IR method is applied to minerals extracted from Nopal cactus and Oregano spices because the thermoluminescent glow curve's shape suggests a trap distribution instead of a single trapping level.

Ion Specificity in α-Helical Folding Kinetics

The influence of the salts KCl, NaCl, and NaI at molar concentrations on the α-helical folding kinetics of the alanine-based oligopeptide Ace-AEAAAKEAAAKA-Nme is investigated by means of (explicit-water) molecular dynamics simulations and a diffusional analysis. The mean first passage times for folding and unfolding are found to be highly salt-specific. In particular, the folding times increase about 1 order of magnitude for the sodium salts. The drastic slowing can be traced to long-lived, compact configurations of the partially folded peptide, in which sodium ions are tightly bound by several carbonyl and carboxylate groups. This multiple trapping leads to a nonexponential residence time distribution of the cations in the first solvation shell of the peptide. The analysis of α-helical folding in the framework of diffusion in a reduced (one-dimensional) free energy landscape further shows that the salt not only specifically modifies equilibrium properties but also induces kinetic barriers due to individual ion binding. In the sodium salts, for instance, the peptide's configurational mobility (or "diffusivity") can decrease about 1 order of magnitude. This study demonstrates the highly specific action of ions and highlights the intimate coupling of intramolecular friction and solvent effects in protein folding.

Concentration-Dependent Hole Mobility and Recombination Coefficient in Bulk Heterojunctions Determined from Transient Absorption Spectroscopy

A simple analytical function based on the multiple trapping model, is used to describe the biomolecular recombination of charge carriers in a bulk heterojunction (BHJ) film in the presence of an exponential energetic tail of localized hole states. The function is used to fit charge carrier decay data from an unannealed P3HT/PCBM film measured by transient absorption. The analysis assumes that only free holes participate in recombination and transport. This implies an effective recombination rate coefficient which varies with the ratio of free to trapped holes. The fit parameters yield a bimolecular recombination constant for free holes with free electrons (k(o) = 3.4 x 10(-12) cm(3) s(-1)) and information about the distribution of trap states (trap distribution parameter beta = 0.29) Assuming the Langevin recombination limit, the analysis yields a concentration dependent effective hole mobility saturating at mu(o) approximate to 7 x 10(-2) cm(2) V-1 s(-1). This approach should be useful to compare BHJs in a consistent and meaningful manner.

Structured light spots projected by a Dammann grating with high power efficiency and uniformity for optical sorting

We report a method for microfluidic multiple trapping and continuous sorting of microparticles using an optical potential landscape projected by a Dammann grating, enabling a high power-efficient approach to forming a composite two-dimensional spots array with high uniformity. The Dammann grating is fabricated in a photoresist by optical lithography. It is employed to create an optical lattice for multiple optical trapping and sorting in a mixture of polymer particles (n = 1.59) and silica particles (n = 1.42) with the same diameters of 3.1 μm. In addition to the exponential selectivity by the projected optical landscapes, the proposed microfluidic sorting system has advantages in terms of high power efficiency and high uniformity due to the Dammann grating.

Theory of bulk electron-hole recombination in a medium with energetic disorder

The kinetics of bulk recombination between electrons and holes in semiconductors that have energetic disorder is studied theoretically. The effect of energetic disorder in the medium is taken into account by using a multiple trapping model in which charges repeat trapping into and detrapping from trap sites with different trapping energies. We assume that recombination between an electron and a hole can occur only when at least one of them is detrapped. The distribution of trap sites with different trapping energies $E$ is assumed to be exponential: $\ensuremath{\sim}\text{exp}(\ensuremath{-}E/{E}_{0})$. A general theory for the kinetics of bulk recombination between electrons and holes is formulated which is valid for an arbitrary ratio of the recombination rate constant ${k}_{r}$ and the trapping rate constant ${k}_{t}$. In the cases of ${k}_{r}={k}_{t}$ and ${k}_{r}⪡{k}_{t}$ the kinetics of bulk recombination is solved analytically. The analytical result for the case of ${k}_{r}={k}_{t}$ agrees with the simulation results by Nelson [Phys. Rev. B 67, 155209 (2003)] for the same case. Our theory predicts that the number density of charges decays as ${t}^{\ensuremath{-}\ensuremath{\alpha}}$ at long times, where $\ensuremath{\alpha}={k}_{B}T/{E}_{0}$ and $T$ is the temperature. This result explains recent experimental observations on bulk recombination between electrons and holes in organic solar cells.

A clustered speckle approach to optical trapping

An in situ study of the clustered speckle 3D structure using an optical tweezer setup is presented. Clustered speckles appear when a coherently illuminated diffuser is imaged through a pupil mask with several apertures, properly distributed over a closed path, which is placed before the objective lens of a standard optical trapping system. Thus, light volumes are reduced several times when compared with standard speckles, being even smaller than the focus volume of a Gaussian beam commonly used to trap. Moreover, clustered speckles have odd statistical properties which differentiated it from standard speckles. Then, geometrically ordered multiple trapping arrays, with statistical random distribution of intensities, can be created with this technique. This fact could enable different studies concerning optical binding or new developments in coherent matter wave transport where Optical Trapping has been proven with standard speckles. In this work, a qualitative analysis of clustered speckles in an optical tweezer setup relative to the number of apertures in the mask and their size is carried on. Besides, in the Rayleigh regime, a general quantitative method to characterize the trapping capability of an optical field is proposed. Then, it is applied to clustered speckles. As a result, a relation between aperture size and the maximum size of the particles that could be trapped is found. This fact opens the possibility of engineering the statistic of the trapped particles by properly selecting the pupil mask.

Compact interferometric optical tweezer for patterned trapping and manipulation of polystyrene spheres and SWCNTs

We demonstrate a simple and compact optical interferometric unit combined with a conventional optical tweezer system for simultaneous multiple trapping and micromanipulation of mono-dispersed polystyrene spheres and aggregation of small-floating clusters of single-walled carbon nanotubes (SWCNTs). The interferometric unit was made compact by means of coating a thin layer of aluminum oxide on one side of the cubic beam splitter (CBS) which works as a static reference mirror and an adjustable mirror was mounted on a XYZ translational stage facing the other adjacent side of the CBS. Thus, the developed interferometric unit is quite analogous to a Michelson interferometer but is compact. Sinusoidal interference fringes with variable carrier frequency and orientations were generated. The interference fringes were then used for multiple trapping of polystyrene spheres along bright fringes resulting in pattern formation and also the aggregation of tiny floating clusters of SWCNTs. The proposed system is compact and easy to align because of its common-path geometry.

Automatic Storage and Analysis of Camera Trap Data

Automatic Storage and Analysis of Camera Trap Data

Investigation of electron trapping behavior in n-channel organic thin-film transistors with ultrathin polymer passivation on SiO 2 gate insulator

Electron trapping behavior at the interface between N, N′-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C13) film and thermal SiO 2 was investigated by utilizing ultrathin poly(methyl methacrylate) (PMMA) gate passivation layers. From the capacitance–voltage analysis for the PTCDI-C13/PMMA/SiO 2 interface, it is found that the electron tunneling appeared with PMMA thinner than 0.8 nm, and that the thickness of the gate passivation layer should be at least 1 nm for preventing injection-type hysteresis in the capacitance–voltage curve. The effective electron mobility of organic thin-film transistors (OTFTs) based on PTCDI-C13 with SiO 2 gate insulator was increased by suppressing shallow-level interface traps on SiO 2 with the PMMA layer, which can be partially accounted for by the multiple trap and release model. In this work, the thickness and the density of the PMMA layers were precisely controlled with a simple spin-coating process. Even 1.3-nm thick PMMA layer caused the improvements of the electron mobility and the air stability of the n-channel conduction.

3D multiple optical trapping of Au-nanoparticles and prokaryote E. coli using intra-cavity generated non-circular beam of inhomogeneous intensity

We report 3D multiple trapping of dielectric polystyrene (PS) beads and gold nano-particles (GNPs) in single beam optical tweezers system using an asymmetric beam of inhomogeneous intensity distribution. This special kind of beam of quasi-TEM11 profile was generated from intra-cavity CW-laser source operating at 532 nm. Multiple trapping of both the low refractive index rod-like Escherichia coli bacteria and 253 nm plasmonic GNPs dispersed in 1.025 μm PS beads which were homogenized in de-ionized water was realized utilizing this spatial beam. Laser-GNPs interaction rendered the enhancement of local surface plasmon resonance field around GNPs causing long-range aggregation of PS beads. The multiple trapping of plasmonic GNPs by the present simple method might find applications for micro- and nano-connectors, underlying physical processes in light-matter interaction assays for inter-particle force analysis, cancer diagnostic and photothermolysis, surface-enhanced Raman scattering (SERS) spectroscopy, and surface plasmon based biological and chemical sensors.

Two-Layer Mutiple Trapping Model for Universal Current Transients in Molecularly Doped Polymers

The mechanism of charge transport in molecularly doped polymers has been the subject of much discussion over the years. In this paper, data obtained from a new experimental variant of the time-of-flight (TOF) technique, called TOF1a, are compared to the predictions of a two-layer multiple trapping model (MTM) with an exponential distribution of traps. In the recently introduced TOF1a experimental variant, the charge generation depth is varied continuously, from surface generation to bulk generation, by varying the energy of the electron-beam excitation source. This produces systematic changes in the shape of the current transient that can be compared to predictions of the two-layer MTM. In the model, one additional assumption is added to the homogeneous MTM, namely: that there exists a surface region, on the order of a micrometer thick, in which the trap distribution is identical to the bulk, but has a higher trap concentration. We find that the characteristic experimental features of an initial spike, a flat plateau, and an anomalously broad tail, as well as the sometimes observed cusp or decreasing current occurring near the transit time, can all be described by such a two-layer model; that is, they can arise as a result of carriers delayed by a trap-rich surface layer. We find that we can semiquantitatively fit current transient data over the whole time range of the experiment, but only by using theoretical parameters that lie in a narrow range, the extent of which we quantify here.

Interpretation of small-modulation photocurrent transients in dye-sensitized solar cells – A film thickness study

Electron transport in dye-sensitized solar cells with varying mesoporous TiO 2 film thicknesses was investigated using experimental and computational methods. More specifically, photocurrent transients resulting from small-amplitude square-wave modulation of the incident light were recorded for a series of solar cells, whereby the dependence of the wavelength and direction of the illumination was investigated. The responses were compared to simulations using different models for diffusional charge transport and analyzed in detail. The photocurrent transients are composed of two components: an initial fast response in case of illumination from the working electrode side, or an initial apparent delay of photocurrent decay for illumination from the counter electrode side, followed by a single exponential decay at longer times, with a time constant that is identified as the electron transport time. The initial response depends on the thickness and the absorption coefficient of the film. Transport times for different films were compared at equal short-circuit current density, rather than at equal light intensity. Experimentally, the transport time showed a power-law dependence on the film thickness with an exponent of about 1.5. Analysis using the quasi-static multiple trapping (MT) formulation demonstrates that this behavior originates from differences in quasi-Fermi level in the TiO 2 films of different thickness when equal photocurrents are generated. The Fokker–Planck relation was used to derive expressions for the electrons flux in porous TiO 2 films with a position-dependent diffusion coefficient.

Data analysis and aging in phosphorescent oxygen-based sensors

The stretched exponential analysis of the photoluminescence (PL) decay curves of the oxygen-sensitive dye Pt octaethylporphyrin (PtOEP) embedded in a polystyrene (PS) film and used in gas-phase oxygen, dissolved oxygen (DO), glucose, and lactate sensors is discussed. Light emitting diodes (LEDs) and organic LEDs (OLEDs) served as the pulsed excitation sources for the PL. Typically, the stretched exponential analysis resulted in excellent fits of the oxygen-quenched PL decay curves, superior to the single exponential analysis (including an offset) and other models, in particular at the higher oxygen levels. While some previous studies of gas-phase oxygen sensors analyzed the decay curves with a single value of the stretching factor β (which was not suitable in this work), and other studies used the product of a single exponential and a stretched exponential with a fixed β, in this study only the stretched exponential term was used with β as a variable. As a result, β was found to decrease with increasing O 2 concentration ([O 2]), from β = 1, i.e., a simple exponential decay, at gas-phase [O 2] = 0 and [DO] = 0. The effect of doping the PtOEP:PS films with 360 nm titania particles (which enhance the PL) on the data analysis was also examined. In general, the TiO 2 increased the PL decay time and β. The results indicate that a distribution of O 2:dye collision rates, induced by the microheterogeneity of the sensor films, is responsible for the non-exponential decay kinetics. The [O 2]-dependent β is possibly associated with shallow multiple quencher trapping sites in the PS matrix that affect the frequency of dye:O 2 collisions. The TiO 2 particles may possibly increase O 2 trapping at their surface, reducing the effective concentration of O 2 molecules that collide with the dye, and thus increasing the PL decay time and β. Additionally, the long-term stability, data analysis, and detection sensitivity of the DO sensor during and following one-year aging, with the sensing film constantly immersed in water, are described. The findings impact commercial PL-based DO sensors.

Surface plasmon interference formed by tightly focused higher polarization order axially symmetric polarized beams

We numerically study the surface plasmon interference formed by tightly focused higher polarization order axially symmetric polarized beams (ASPBs) based on the vectorial diffraction theory. The definition of ASPBs is stated, and the optical setup for surface plasmon polariton (SPP) excitation and mathematical expressions for interfering SPP fields are proposed. The simulation results show that the interfering SPP fields present a multi-focal spot pattern. In addition, the number of spots is related to the polarization order of the incident beams P as 2\times(P-1), indicating potential utilization in near-field multiple optical trapping and near-field imaging and sensing. The unique interfering phenomenon is also explained.

Variable Temperature Mobility Analysis of n‐Channel, p‐Channel, and Ambipolar Organic Field‐Effect Transistors

AbstractThe temperature dependence of field‐effect transistor (FET) mobility is analyzed for a series of n‐channel, p‐channel, and ambipolar organic semiconductor‐based FETs selected for varied semiconductor structural and device characteristics. The materials (and dominant carrier type) studied are 5,5′′′‐bis(perfluorophenacyl)‐2,2′:5′,2″:5″,2′′′‐quaterthiophene (1, n‐channel), 5,5′′′‐bis(perfluorohexyl carbonyl)‐2,2′:5′,2″:5″,2′′′‐quaterthiophene (2, n‐channel), pentacene (3, p‐channel); 5,5′′′‐bis(hexylcarbonyl)‐2,2′:5′,2″:5″,2′′′‐quaterthiophene (4, ambipolar), 5,5′′′‐bis‐(phenacyl)‐2,2′: 5′,2″:5″,2′′′‐quaterthiophene (5, p‐channel), 2,7‐bis((5‐perfluorophenacyl)thiophen‐2‐yl)‐9,10‐phenanthrenequinone (6, n‐channel), and poly(N‐(2‐octyldodecyl)‐2,2′‐bithiophene‐3,3′‐dicarboximide) (7, n‐channel). Fits of the effective field‐effect mobility (µeff) data assuming a discrete trap energy within a multiple trapping and release (MTR) model reveal low activation energies (EAs) for high‐mobility semiconductors 1–3 of 21, 22, and 30 meV, respectively. Higher EA values of 40–70 meV are exhibited by 4–7‐derived FETs having lower mobilities (µeff). Analysis of these data reveals little correlation between the conduction state energy level and EA, while there is an inverse relationship between EA and µeff. The first variable‐temperature study of an ambipolar organic FET reveals that although n‐channel behavior exhibits EA = 27 meV, the p‐channel regime exhibits significantly more trapping with EA = 250 meV. Interestingly, calculated free carrier mobilities (µ0) are in the range of ∼0.2–0.8 cm2 V−1 s−1 in this materials set, largely independent of µeff. This indicates that in the absence of charge traps, the inherent magnitude of carrier mobility is comparable for each of these materials. Finally, the effect of temperature on threshold voltage (VT) reveals two distinct trapping regimes, with the change in trapped charge exhibiting a striking correlation with room temperature µeff. The observation that EA is independent of conduction state energy, and that changes in trapped charge with temperature correlate with room temperature µeff, support the applicability of trap‐limited mobility models such as a MTR mechanism to this materials set.