A model for charge transport in semicrystalline polymer thin films

A model for simulating the charge transport properties of semicrystalline polymer (SCrP) using Monte Carlo simulation is reinvented. The model is validated by reproducing the experimentally observed field and temperature dependence of mobility in Poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) thin films. This study also provides a new physical insight to the origin of much debated negative field dependence of mobility (NFDM) observed at low electric field strengths in P3HT thin films. The observed NFDM, which is not explainable with the mechanisms proposed earlier, is attributed to the weak dependence of transit time on the applied electric field strengths. In the semicrystalline films, the charge transport takes place mostly through the crystalline regions, in which the charge transport is weakly dependent on the strength of the applied electric field. In addition, a possible explanation for the origin of Arrhenius temperature dependence of mobility (lnμ ∝ 1/T) commonly observed in SCrP thin films is also proposed. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 137–141     

Electrochemical electron transfer and its relation to charge transport in single molecule junctions

Ability to control charge transport at nanometer scale lies in the heart of design of fast reliable electronic devices. Molecular electronics thrive to use functional molecules for such transport. If the molecule contains redox center(s), a diode-like or transistor-like behavior can be easily explored by controlling not only the voltage difference between two metallic contacts of the molecular junction but also the potential of one of the contacting electrodes with respect to some reference. Thus, one needs to understand the relationship between electrochemical electron transfer and charge transport in metal–molecule–metal junctions. This review presents latest theoretical approaches toward understanding of such relationship and discusses pivotal experimental works to validate them. Tunneling and hopping pathways may operate in parallel (two-channel model), but experimental conditions dictate the channel preference.     

A new tool for an old job: Using fixed cell scanning calorimetry to investigate dilute aqueous solutions

The development of fixed twin cell temperature scanning calorimeters has enabled the more efficient determination of heat capacities of dilute aqueous solutions with a precision comparable to that of the Picker flow heat capacity calorimeters developed nearly 40 years ago. Experiments require less than 0.5 cm³ of solution, and results can be obtained routinely over the temperature range (278 to 395) K at pressures up to a few bars. Multiple scanning of samples by both increasing and decreasing temperature allows assessment of instrument drift, solute stability, and reproducibility of results. Chemical calibration is essential to take full advantage of the precision and sensitivity of the calorimeters. The calorimetric output is a direct measure of the difference in heat capacity per unit volume of a solution and of a reference liquid, usually water. Thus, densities of the solution and reference liquid are needed to transform the results into heat capacities per unit mass of solution. Examples of solutes that have been investigated include a variety of inorganic and organic compounds that dissolve to give simple ionic or neutral species, or that produce complexes or species that exist in equilibrium distributions that can change as the temperature is scanned. Appropriate selection of the results from experiments on combinations of solutes allows calculation of standard state (zero concentration) thermodynamic quantities for chemical processes and reactions over the ranges of temperature scanned at the solution compositions investigated. Results for a few specific systems are presented and discussed for some representative classes of solutes that have been investigated in our laboratory since 1998.     

A new parameter to control heat transport in nanofluids: surface charge state of the particle in suspension

Although various conjectures have been proposed to explain the abnormal increase in thermal conductivity of nanofluids, the detailed mechanism has not been fully understood and explained. The main reason is due to the lack of knowledge of the most fundamental factor governing the mechanisms such as Brownian motion, liquid layering, phonon transport, surface chemical effects, and agglomeration. Applying a surface complexation model for the measurement data of hydrodynamic size, zeta potential, and thermal conductivity, we have shown that surface charge states are mainly responsible for the increase in the present condition and may be the factor incorporating all the mechanisms as well.     

A new route to charge distributions in ionic solids

A new empirical route for the calculation of charge distributions in inorganic solids with respect to the formal oxidation state is presented, together with distances and coordination numbers for the respective ions.     

Infrared intensities: A new tool in chemistry.

The interpretation of the infrared intensities in terms of atomic polar tensors and of electrooptical parameters allows to derive rich information on the charge distribution in the molecules. Using the results of several studies of this kind, it is now possible to derive information on intramolecular and intermolecular interactions even from “poor” data such as absolute intensities of whole regions in the spectrum or even from relative intensities.     

Porosity effects on electron transport in TiO2 films and its application to dye-sensitized solar cells

Porosity (P) of TiO2 film in dye-sensitized solar cells affects the light absorption coefficient and electron diffusion coefficient. A theoretical analytical expression of the intensity-modulated photocurrent spectroscopy (IMPS) response involving the light absorption coefficient and the electron diffusion coefficient as a function of the porosity has been proposed to investigate the influence of TiO2 film porosity on the characteristics of electron transport. The incident photon-to-current conversion efficiency (IPCE) and electron transit time depending on the porosity have been analyzed illuminating from both the electrolyte side (IE) and the substrate side (IS). The IPCE derived from the IMPS response reaches its maximum at a porosity of around 30% for IE and 41% for IS, respectively. Electron transit time increases with increasing the porosity for IE, while it declines when P < 0.41 for IS, which is attributable to the influence of the RC time constant. It has also been found that a larger RC time constant will lead to a longer transit time. The electron diffusion coefficient calculated from the transit time for IE corresponds to the results from the porosity reported in previous literature, which indicates that the dependence of the electron transit time tau(d) on the porosity is justifiable. The diffusion coefficient calculated for a larger RC time constant approaches the value from the literature when P > or = 0.41, while it is not practicable when P < 0.41 for IS.     

Modelling charge transport in organic semiconductors: from quantum dynamics to soft matter

The charge carrier dynamics in organic semiconductors has been traditionally discussed with the models used in inorganic crystalline and amorphous solids but this analogy has severe limitations because of the more complicated role of nuclear motions in organic materials. In this perspective, we discuss how a new approach to the modelling of charge transport is emerging from the alliance between the conventional quantum chemical methods and the methods more traditionally used in soft-matter modelling. After describing the conventional limit cases of charge transport we discuss the problems arising from the comparison of the theory with the experimental and computational results. Several recent applications of numerical methods based on the propagation of the wavefunction or kinetic Monte Carlo methods on soft semiconducting materials are reviewed.     

Contact angle measurements as a tool to investigate the filler-matrix interactions in polyurethane-clay nanocomposites from blocked prepolymer

Optically transparent polyurethane-clay nanocomposite films were prepared by dispersing 5 wt% of various commercial organo-clays in a mixture of cycloaliphatic amines used as chain extender-cross-linker of a blocked prepolymer. For the first time, vibration-induced equilibrium contact angle measurements were successfully employed to rank the selected organo-clays accordingly to their hydrophobicity order. Polymer-clay intercalation degree in the nanocomposites, as assessed from X-ray diffraction, was strongly correlated to the water-clay equilibrium contact angle. Moreover, as the clay intercalation degree increased, a decrease of the cross-linking degree of the polyurethane matrix occured.Uniaxial tensile tests under quasi-static and impact conditions, and isothermal thermogravimetric analysis were performed on both unfilled polyurethane matrix and nanocomposites. Secant tensile modulus, tensile energy to break, and thermal lifetime showed a non monotonic trend with a maximum as a function of the intercalation degree. This behaviour is discussed considering that two concomitant and contrasting effects develop as the polymer-clay intercalation degree increases: a positive improvement of the filler matrix interactions, and a negative reduction of the matrix cross-linking degree.     

A new approach to the prediction of RP-HPLC retention times in gradient elution from isocratic data

Summary A general chromatographic model has been set up starting from a set of equations based on the concept of the velocity of a solute along the column. The composition of the mobile phase is taken into account solely as a numerical factor entering into suitable equations and totally independent of the chemical-properties of the constituents. A few isocratic experimental runs are necessary as input data, and subsequently a small amount of computational effort is sufficient to make predictions of retention times under gradient elution conditions for solutes of whatever chemical structure. The prediction errors are dependent on the steepness of the linear gradient chosen but are, in any case, acceptably low.     

A new nebulization device with exchangeable aerosol generation mode as a useful tool to investigate sample introduction processes in inductively coupled plasma atomic emission spectrometry

A new sample introduction device has been designed in order to differentiate between the effects of the aerosol production and its following desolvation on analytical performances of an inductively coupled plasma optical spectrometer. This research tool allows to easily switch between the pneumatic and ultrasonic aerosol generation mode and to use a joint desolvation chamber. In this way, a real comparison between aerosol production systems may be attained and the influence of aerosol generation process on analytical figures clearly distinguished from that of the desolvation process. In this work, the separate effects of the aerosol generation and desolvation processes on analytical sensitivity and tolerance towards matrix effects have been investigated. Concerning sensitivity, it was found that both the processes play an important role in determining emission intensities, being the increase in sensitivity due to desolvation higher than that due to the improved aerosol generation efficiency. Concerning the matrix effects, a predominant role of the desolvation system was found, while the influence of the aerosol generation mode was much less important. For nitric acid, the decreasing effect was mitigated by the presence of a desolvation system, due to partial removal of the acid. On the contrary, the depressive effect of sulfuric acid was enhanced by the presence of a desolvation system, due to degradation of the solvent removal efficiency and to further decrease in the analyte transport rate caused by clustering phenomena. Concerning the interferences due to sodium and calcium, a depressive effect was observed, which is enhanced by desolvation.     

A novel charge extraction method for the study of electron transport and interfacial transfer in dye sensitised nanocrystalline solar cells

A novel charge extraction method has been developed to study the transport, trapping and back reaction of photogenerated electrons in dye sensitised nanocrystalline cells (Grätzel cells). The cell is illuminated at open circuit until a steady state is reached in which the rates of photogeneration and of back reaction of electrons with tri-iodide are equal. The illumination is then interrupted, and the electron density is allowed to decay for a given time in the dark before short circuiting the cell using a solid state switch. For high efficiency cells, the integrated current measured at short circuit corresponds closely to the remaining electronic charge in the film. Small corrections are required to account for back reaction and substrate charging. The delay time between interruption of the illumination and short circuit charge extraction is varied systematically to follow the decay of electron concentration. Analysis of the time dependence of the electron charge indicates that the back reaction of electrons with I₃⁻ is second order in electron density, which is consistent with the formation of I₂^−. as an intermediate. Simultaneous measurement of the charge and photovoltage decay curves shows that the density of trap states decreases exponentially with trap depth.     

Photoinduced electron transfer from chlorophyll-a to duroquinone in micellar solutions,charge separation through local electrostatic fields

A new method to achieve charge separation in light-driven redox reactions is presented. It is based on the principle of electrostatic interactions of the radical ions produced by the light with the local environment present in aqueous solutions of surfactant aggregates. Experimental data obtained from the flash photolysis of chlorophyll-a solubilized in anionic micelles in the presence of duroquinone substantiate the predictions of the model.     

High-rate charge-carrier transport in porphyrin covalent organic frameworks: switching from hole to electron to ambipolar conduction

Well conducted: a two-dimensional porphyrin covalent organic framework is described. Owing to the eclipsed stacking alignment, the framework is conductive and allows high-rate carrier transport through the porphyrin columns. The central metal in the porphyrin rings changes the conducting nature of the material from hole to electron, and to ambipolar conduction. It also drives the high on-off ratio photoconductivity of the framework.     

Backscattered electrons from gold surface films deposited on silicon substrates: a joint experimental and computational investigation to add new potentiality to electron microscopy

This paper addresses the problem of the thickness determination of thin gold overlayers deposited on silicon bulk substrates by looking at the electron backscattering coefficient involved in scanning electron microscopy (SEM). A Monte Carlo code, used to calculate the backscattering coefficient, together with a simple experimental setup, which uses a conventional SEM, allow to determine thin film thickness (in the range 25–200 nm) with an estimated accuracy of 20%. This adds obviously new potentiality to SEM. Copyright © 2012 John Wiley & Sons, Ltd.     

GISAXS: A versatile tool to assess structure and self-assembly kinetics in block copolymer thin films

Over the past two decades grazing incidence small-angle scattering (GISAXS) has morphed into a powerful tool for the determination of the structure and self-assembly kinetics of block copolymer thin films. An overview of the scattering process and the interpretation of GISAXS data is given and experimental requirements are discussed. The application of the technique for the characterization of block copolymer thin films is illustrated with selected examples.     

A way for evaluating parameters of electron transport in non-polar molecular liquids derived from analysis of the trapped electron recombination kinetics

The geminate recombination kinetics of electron-ion pairs produced by high energy radiation in liquid hydrocarbons is considered in the two state model of electron transport. The purpose of the study is to relate the trapped electron transient optical absorption, observed in the pulse radiolysis experiments, to fundamental parameters of electron transport in liquid. It is shown that measurements of the half-life time and amplitude of the trapped electron decay curve allow one to find the electron life time in a localized state.     

Low noise electron microscopy by merging multiple images digitized from conventional films with reference to the mouse kidney.

In a conventional transmission electron microscope system, the resolution is regarded as an absolute limitation, that is, 0.2 nm in theory and 0.6 nm in sections of biological materials. However, in an oversampled system, this limitation can be broken. In the present study, 60-nm-thick Epon sections from a mouse kidney were used. From these sections tight junctions located in the distal tubule were selected as test objects. Sets of up to 15 electron microscope images of the same target were recorded on negatives at x10,000, x13,000, and x63,000, respectively. The recorded films were digitized using a light microscope equipped with a digital camera. In each set the images were expanded, aligned, and merged into a more highly resolved output image. Each output image revealed details in the tight junction, which were not visible at the original magnifications. Two different sizes of colloidal gold particles (10 nm and 1 nm) conjugated with an immunoglobin G (IgG) served as references. With this improvement of resolution, it becomes possible to inspect some barely visible biologic (virus) particles and structures, such as glycogen and free ribosomes in their native environment.     

Fat crystals: A tool to inhibit molecular transport in W/O/W double emulsions

Water-in-oil-in-water (W/O/W) double emulsions are a promising technology for encapsulation applications of water soluble compounds with respect to functional food systems. Yet molecular transport through the oil phase is a well-known problem for liquid oil-based double emulsions. The influence of network crystallization in the oil phase of W/O/W globules was evaluated by NMR and laser light scattering experiments on both a liquid oil-based double emulsion and a solid fat-based double emulsion. Water transport was assessed by low-resolution NMR diffusometry and by an osmotically induced swelling or shrinking experiment, whereas manganese ion permeation was followed by means of T₂-relaxometry. The solid fat-based W/O/W globules contained a crystal network with about 80% solid fat. This W/O/W emulsion showed a reduced molecular water exchange and a slower manganese ion influx in the considered time frame, whereas its globule size remained stable under the applied osmotic gradients. The reduced permeability of the oil phase is assumed to be caused by the increased tortuosity of the diffusive path imposed by the crystal network. This solid network also provided mechanical strength to the W/O/W globules to counteract the applied osmotic forces.     

A new experimental design to investigate the concentration dependent diffusion of Eu(III) in compacted bentonite

The 'in-diffusion method was used to study the diffusion behavior of Eu(III) in compacted bentonite. The results (K _d, apparent and effective diffusion coefficients) derived from the capillary method are in good agreement with the literature data for similar bentonite dry densities and similar radionuclide concentrations, and fits the Fick's second law very well. The method was used to study the effect of solution concentration (10^-7-10^-3 mol/l) on the diffusion and sorption behavior in compacted bentonite. The experiments were carried out in synthetic groundwater, at room temperature. The results suggested that the diffusion of Eu(III) in the bentonite was independent on the density of bentonite, but dependent on the solution concentration. In agreement to the literature, the K _d values from the capillary experiments are in most cases lower than those from batch experiments, they are about one-half to one-third the value to those from batch experiments. The difference between the K _dvalues from capillary and batch experiments are a strong function of the bulk density of the bentonite. The results suggest that the content of interlaminary space plays a very important role to the transport of Eu(III) in compacted bentonite.