Separated Time Constants Research Articles

Exciton Dissociation, Charge Transfer, and Exciton Trapping at the MoS2/Organic Semiconductor Interface

Hybrid inorganic–organic semiconducting devices consisting of monolayer transition metal dichalcogenides (TMDs) represent a new frontier in advanced optoelectronics due to their high radiative efficiencies and capacity to form flexible p–n junctions with inherent device tunability. However, understanding how excitons and charges behave at the interface between TMDs and organic systems, a key requirement to advance the field, remains underexplored. Herein, a heterostructure consisting of a highly conjugated organic system, 9-(2-naphthyl)-10-[4-(1-naphthyl)phenyl]anthracene (ANNP), and monolayer molybdenum disulfide (MoS2) on quartz is elucidated via transient absorption and photoluminescence spectroscopies. Upon direct excitation of MoS2 at 532 nm, hole transfer to ANNP of ∼5 ps and a charge separation time constant of ∼2.4 ns are observed. When the sample is excited at 400 nm (where both ANNP and MoS2 absorb), a self-trapped exciton within ANNP is formed. The emission of the self-trapped exciton is long-lived compared to the exciton lifetime of ANNP, decaying within 20 ns. The trapping of the ANNP exciton is caused by structural deformities of the ANNP crystal lattice when grown on MoS2, which are removed by annealing the film. These observations highlight how exciton dissociation and charge transfer dominate at the interface of ANNP and MoS2 whereas the exciton dynamics within ANNP are prone to the formation of trap states brought about by crystal defects within the film. These insights will aid in future developments of TMD-containing optoelectronics.

Apparent disagreement between cyclic voltammetry and electrochemical impedance spectroscopy explained by time-domain simulation of constant phase elements

A selection of electrodes was analysed using cyclic-voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and a large apparent resistance was observed with CV that was absent with EIS. The explanation for this resistance anomaly was traced to the constant phase element (CPE) behaviour which is exhibited by the electrode double-layer capacitance. Computer simulations of the transient-response of an RQ network (where Q represents a CPE) to a voltage ramp revealed bi-exponential behaviour, with two separate time-constants. One is equal to the product of R and Q, but the other is fixed at about 0.3 s. This finding is supported by observation, by mathematical derivation, and by a novel mixed-domain five-component equivalent circuit model. In addition, example code is provided as a basis for transient simulations of constant phase elements with arbitrary voltage waveforms. This explanation assists in the correct interpretation of potentially misleading cyclic voltammetry results.

Designing of IMC-PID Controller for Higher-order Process Based on Model Reduction Method and Fractional Coefficient Filter with Real-time Verification

AbstractAn improved model reduction scheme is proposed here for higher-order processes and subsequently an enhanced IMC-PID controller is designed based on the obtained reduced model. In the proposed scheme, higher-order processes are estimated as first-order-plus-dead-time (FOPDT) model. Designed IMC controller includes a filter having two separate time constants with fractional order coefficients. Efficacy of the proposed model reduction scheme is verified in terms of closed loop performance evaluation for higher-order minimum and non-minimum phase process models in comparison with improved SIMC (iSIMC) controller (Grimholt. Optimal PI and PID control of first-order plus delay processes and evaluation of the original and improved SIMC rules. J Process Control. 2018;70:36–46). Overall performance enhancement for the proposed method is demonstrated through simulation study as well as real-time experimentation on a level control loop.

(Invited) Kinetic Parameters in Anion-Exchange Membrane Fuel Cells

Anion-exchange membrane fuel cells (AEMFC) are promising mainly due to the possibility to use none-PGM catalysts in the alkaline media [1]. However, before elaborating with alternative catalysts a better understanding is needed on which electrode is limiting and what the limitations are for the reference Pt/C catalyst. In fact, the rate constants for the ORR and HOR are not determined in the in the fuel cell [2]. On top of this the influence of water on the kinetics and water transport in and between the electrodes is debated [3]. In this study we have measured the influence of partial pressure in both a symmetrical H2/H2cell a H2/O2cell. Experimental I-V and impedance data has been modelled by two separate physic-based models. The experimental results show that the hydrogen reaction has two separate time-constants in the impedance spectra for the symmetrical cell, indicating a two-step reaction at the anode (see Figure below). From the O2/H2cell the ORR is found to be the limiting reaction at low currents densities, while the HOR has a greater impact at high currents densities. From the model, a limiting current is observed at the anode. The level of humidity is likely not determining the direct catalytic reaction kinetics but may affect indirectly through its impact on ionomer conductivity and swelling. Fig. Nyqvist plots of H2/H2symmetrical cell at OCP at different H2mixtures in Ar at 50°C, 95% RH, 125 ml/min. MEA: 0.4 mg/cm2Pt/C, AS4 ionomer and A201 Tokuyama membrane [4]. [1] D. R. Dekel. Journal of Power Sources, 375:158–169, 2018. [2] E. S. Davydova, S. Mukerjee, F. Jaouen, D. R. Dekel, ACS Catal. 8 , 665-6690, 2018. [3] B. Eriksson, H. Grimler, A. Carlson, H. Ekström, R. Wreland Lindström, G. Lindbergh, C. Lagergren, Int. J. Hydrognen Energy 44, 4930-4939, 2019. [4] A. CarlsonShapturenka, B. Eriksson, G. Lindbergh, C. Lagergren, R. Wreland Lindström, Electrochimica Acta 277 (2018) 151-160

Mechanical Properties of a Drosophila Larval Chordotonal Organ

Proprioception is an integral part of the feedback circuit that is essential for locomotion control in all animals. Chordotonal organs perform proprioceptive and other mechanosensory functions in insects and crustaceans. The mechanical properties of these organs are believed to be adapted to the sensory functions, but had not been probed directly. We measured mechanical properties of a particular chordotonal organ—the lateral pentascolopidial (lch5) organ of Drosophila larvae—which plays a key role in proprioceptive locomotion control. We applied tension to the whole organ in situ by transverse deflection. Upon release of force, the organ displayed overdamped relaxation with two widely separated time constants, tens of milliseconds and seconds, respectively. When the muscles covering the lch5 organ were excised, the slow relaxation was absent, and the fast relaxation became faster. Interestingly, most of the strain in the stretched organ is localized in the cap cells, which account for two-thirds of the length of the entire organ, and could be stretched by ∼10% without apparent damage. In laser ablation experiments we found that cap cells retracted by ∼100 μm after being severed from the neurons, indicating considerable steady-state stress and strain in these cells. Given the fact that actin as well as myosin motors are abundant in cap cells, the results point to a mechanical regulatory role of the cap cells in the lch5 organ.

Quantum dots use both LUMO and surface trap electrons in photoreduction process

Here, we explore a mechanism of quantum dots related photoreduction of two redox-active proteins, cytochrome c and ferredoxin, by detailed analysis of fluorescence decay and reconstruction of time-resolved emission spectra (TRES). We used two types of cadmium telluride quantum dots, with diameters of 2.6nm and 3.9 nm and maximum emissions at 550nm and at 650nm, respectively, which are known to be able to reduce proteins with different efficiencies. First, we observed that for a pure quantum dots solution, the fluorescence decay can be well fitted by three components. The average fluorescence lifetimes, as well as separate time constants, depend on the nanocrystal diameter. In the presence of proteins, fluorescence decay is faster and cytochrome c has a greater impact than ferredoxin. The TRES experiment showed that a fraction of the medium τ decay component is dominant in a pure quantum dot solution, with the maximum corresponding to the steady-state spectrum. The addition of ferredoxin does not change this pattern, while the presence of cytochrome c strongly promotes the shortest τ. Additionally, potassium iodide titration experiments were used to verify the origin of individual decay components. We propose that reduction occurs by electron transfer from both conductive band and surface trap states.

The extraordinary joint material of an articulated coralline alga. II. Modeling the structural basis of its mechanical properties.

By incorporating joints into their otherwise rigid fronds, erect coralline algae have evolved to be as flexible as other seaweeds, which allows them to thrive - and even dominate space - on wave-washed shores around the globe. However, to provide the required flexibility, the joint tissue of Calliarthron cheilosporioides, a representative articulated coralline alga, relies on an extraordinary tissue that is stronger, more extensible and more fatigue resistant than that of other algae. Here, we used the results from recent experiments to parameterize a conceptual model that links the microscale architecture of cell walls to the adaptive mechanical properties of joint tissue. Our analysis suggests that the theory of discontinuous fiber-wound composite materials (with cellulose fibrils as the fibers and galactan gel as the matrix) can explain key aspects of the material's mechanics. In particular, its adaptive viscoelastic behavior can be characterized by two, widely separated time constants. We speculate that the short time constant (∼14 s) results from the viscous response of the matrix to the change in cell-wall shape as a joint is stretched, a response that allows the material both to remain flexible and to dissipate energy as a frond is lashed by waves. We propose that the long time constant (∼35 h), is governed by the shearing of the matrix between cellulose fibrils. The resulting high apparent viscosity ensures that joints avoid accumulating lethal deformation in the course of a frond's lifetime. Our synthesis of experimental measurements allows us to draw a chain of mechanistic inference from molecules to cell walls to fronds and community ecology.

Dependence of quantum dot photocurrent on the carrier escape nature in InAs/GaAs quantum dot solar cells

This paper presents a theoretical study of the effect of the nature of the carrier escape from quantum dots (QDs) on the performance of InAs/GaAs QD solar cells (QDSCs), based on numerical simulations. Excitonic and non-excitonic dynamics of electrons and holes are considered in the modeling, by assuming identical or separate time constants for the intersubband carrier transfer processes in the ground and excited states. It is shown that the excitonic capture and escape allow us to explain the non-additive characteristic of the QD photocurrent, observed when the total photocurrent of the cell is much lower than the sum of the photocurrents contributed separately by the barrier and the nanostructures. This behavior is practically eliminated in the non-excitonic case. The correlated dynamics under the non-excitonic scenario is analyzed by calculating the device sensitivity to small changes introduced in the escape time of electrons. It is stated that the non-variation of the QD photocurrent with this parameter could be interpreted as a consequence of either a correlated electron-hole escape from QDs, or a dominant recombination over the other involved processes. The ideality factor of the different QDSCs studied is also calculated from simulations under both concentrated sunlight and dark conditions. The obtained results are in line with experimental measurements published in the literature.

Photoinduced Electron Transfer from PbS Quantum Dots to Cobalt(III) Schiff Base Complexes: Light Activation of a Protein Inhibitor

This paper describes the activation of a biologically inert Co(III) Schiff base [Co(III)-SB] complex to its protein inhibitor form by photoinduced electron transfer (PET) from a colloidal PbS quantum dot (QD, radii of 1.5-1.7 nm) to the cobalt center, with a charge separation time constant of 125 ns. Reduction of the Co(III)-SB complex initiates release of the native axial ligands, promoting replacement with the histidine mimic 4-methylimidazole. The rate of ligand displacement increases by a factor of approximately 8 upon exposure of the PbS QD/Co(III)-SB mixture to light with an energy greater than the energy of the first excitonic state of the QDs, from which PET occurs. These results suggest an approach for the preparation of inorganic therapeutic agents that can be specifically coupled to a biologically active site by cooperative redox binding ligation.

Effects of Hydrogen Treatment and Air Annealing on Ultrafast Charge Carrier Dynamics in ZnO Nanowires Under in Situ Photoelectrochemical Conditions

ZnO nanowires (NWs) grown by the hydrothermal method on fluorine-doped tin oxide glass substrate were characterized by scanning electron microscopy, optical absorption, photoelectrochemical photocurrent density, and ultrafast transient absorption pump probe spectroscopy. The as-grown NWs were annealed in air, pure hydrogen atmosphere, or annealed first with air annealing followed by hydrogen treatment. By TA spectroscopy, the samples exhibited a triple exponential transient bleach recovery with lifetimes on the fast (10–15 ps), medium (100–200 ps), and slow (>1 ns) time scale, attributed to shallow donor mixed with electron hole plasma recombination, donor bound recombination, and donor–acceptor pair recombination (DAP), respectively. The as-grown samples were dominated by donor and DAP recombination. Air annealing improved the crystal structure but had little effect on hole trapping. Significantly, the hydrogen-treated NWs showed a reduction in hole trapping and DAP recombination. Hole trapping was attributed to zinc vacancies (VZn) and hydrogen was proposed to passivate these defects. The photocurrent density of the air annealed and cotreated NWs were measured, the latter of which showed improved performance which was attributed to, in part, decreased hole trap states and improved electrical conductivity. In situ ultrafast TA spectroscopy was used to study the photoanodes under working conditions as a function of applied bias. For both samples, the medium time constant became faster with increasing applied bias. A model was proposed to extract the electron–hole separation time constant.

Modeling of surface exchange reactions and diffusion in composites including transport processes at grain and interphase boundaries

Surface exchange reactions and diffusion of oxygen in ceramic composites consisting of a dilute and random distribution of inclusions in a polycrystalline matrix (host phase) are modeled phenomenologically by employing the finite element method. The microstructure of the mixed conducting composite is described by means of a square grain model, including grain boundaries of the matrix and interphase boundaries between the inclusions and grains of the host phase. An instantaneous change of the oxygen partial pressure in the surrounding atmosphere may give rise to an oxygen exchange process, i.e., oxidation or reduction of the ceramic composite. Relaxation curves for the total amount of exchanged oxygen are calculated, emphasizing the role played by fast diffusion along the interfaces. The relaxation curves are interpreted in terms of effective medium diffusion, introducing appropriate equations for the effective diffusion coefficient and the effective surface exchange coefficient. When extremely fast diffusion along the grain and interphase boundaries is assumed, the re-equilibration process shows two different time constants. Analytical approximations for the relaxation process and relations for the separate relaxation times are provided for this limiting case as well as for blocking interphase boundaries. Furthermore, conductivity relaxation curves are calculated by coupling diffusion and dc conduction. In the case of effective medium diffusion, the conductivity relaxation curves do not deviate from those for the total amount of exchanged oxygen. On the contrary, the conductivity relaxation curves differ remarkably from the time dependence of the total amount of exchanged oxygen, when the different phases of the composite re-equilibrate with separate time constants.

Poroelastic stress-triggering of the 2005 M8.7 Nias earthquake by the 2004 M9.2 Sumatra–Andaman earthquake

The M9.2 Sumatra–Andaman earthquake (SAE) occurred three months prior to the M8.7 Nias earthquake (NE). We propose that the NE was mechanically triggered by the SAE, and that poroelastic effects were a major component of this triggering. This study uses 3D finite element models (FEMs) of the Sumatra–Andaman subduction zone (SASZ) to predict the deformation, stress, and pore pressure fields of the SAE. The coseismic slip distribution for the SAE is calibrated to near-field GPS data using FEM-generated Green's Functions and linear inverse methods. The calibrated FEM is then used to predict the postseismic poroelastic contribution to stress-triggering along the rupture surface of the NE, which is adjacent to the southern margin of the SAE. The coseismic deformation of the SAE, combined with the rheologic configuration of the SASZ produces two transient fluid flow regimes having separate time constants. SAE coseismic pore pressures in the relatively shallow forearc and volcanic arc regions (within a few km depth) dissipate within one month after the SAE. However, pore pressures in the oceanic crust of the down-going slab persist several months after the SAE. Predictions suggest that the SAE initially induced MPa-scale negative pore pressure near the hypocenter of the NE. This pore pressure slowly recovered (increased) during the three-month interval separating the SAE and NE due to lateral migration of pore fluids, driven by coseismic pressure gradients, within the subducting oceanic crust. Because pore pressure is a fundamental component of Coulomb stress, the MPa-scale increase in pore pressure significantly decreased stability of the NE fault during the three-month interval after the SAE and prior to rupture of the NE. A complete analysis of stress-triggering due to the SAE must include a poroelastic component. Failure to include poroelastic mechanics will lead to an incomplete model that cannot account for the time interval between the SAE and NE. Our transient poroelastic model explains both the spatial and temporal characteristics of triggering of the NE by the SAE.

Modelling of surface exchange reactions and diffusion in composites and polycrystalline materials

Abstract Surface exchange reactions and chemical diffusion in composites, consisting of a dilute distribution of inclusions in a matrix, and polycrystalline materials have been modelled by application of both a square grain and a spherical grain model. The diffusion equations have been solved numerically by employing a finite element approach in the case of the square grain model and the Laplace transform method involving numerical Laplace inversion with respect to the spherical grain model. The boundary conditions refer to oxygen exchange reactions between a gas phase and a mixed ionically–electronically conducting ceramic sample within the linear response regime, i.e. small variations of the oxygen partial pressure. Diffusion profiles as well as the time dependence of the total amount of exchanged oxygen (relaxation curves) have been calculated. A necessary requirement for effective medium diffusion is proposed, and appropriate relations for the effective chemical surface exchange coefficient and the effective chemical diffusion coefficient are derived. On the contrary, when the time constant for diffusion from the matrix into the inclusions of a composite exceeds considerably the relaxation time for effective medium diffusion, relaxation curves with two separate time constants are observed. Analogously, in the case of polycrystalline materials the overall transport process is determined by slow (rate-limiting) bulk diffusion from the grain boundaries into the grains. Adequate formulae for the relaxation times are given based on analytical approximations of the solution functions to the diffusion equations. In addition, the spherical grain model is applied to interpret the re-oxidation kinetics of the positive temperature coefficient of resistivity (PTC) ceramics based on conductivity relaxation experiments.

Surface exchange reactions and diffusion in composite materials: A finite element approach

A two-dimensional square grain model has been applied to model oxygen exchange processes between a gas phase and a ceramic composite consisting of two randomly distributed phases of equal grain size (side length of squares). Both average diffusion profiles for thin films and the time dependence of the total amount of exchanged oxygen (relaxation curves) have been calculated numerically by means of the finite element method. The boundary conditions refer to an instantaneous change of the oxygen partial pressure in the surrounding gas phase, which gives rise to surface exchange reactions as well as to diffusion in the composite. Both local equilibrium at the interface between different phases (host phase and inclusions) and blocking heterophase boundaries have been taken into account. The numerical results are compared with the analytical solution for diffusion in a homogeneous medium introducing effective diffusion and surface exchange coefficients. When the relaxation time for effective medium diffusion is considerably shorter than that for the transport process from the host phase into the inclusions, relaxation curves with two separate time constants are predicted. Based on analytical approximations, relaxation times for various limiting cases are given.

Typical auroral substorm: A bifurcated oval

Utilizing global auroral images obtained by Polar Visible Imaging System Earth Camera, we have analyzed the UV emissions from 116 auroral substorms. The events selected were fairly isolated and had to expand from a localized onset. It was found that essentially all auroral events fulfilling these simple criteria expand into the Akasofu (1964) bulge‐type aurora. Averaged auroral emission patterns were compiled for 11 time steps of the substorm covering 20 min prior to the onset until well into the recovery phase. This compilation required a three‐step normalization technique, one temporal based on the expansion time and two spatial, magnetic local time and latitude. These average patterns were then fit to either a single or double Gaussian distribution in latitude for each of 24 magnetic local times. On the basis of this analysis we made the following conclusions. The normalization technique is highly efficient in maintaining the key features in the individual auroral emission patterns, even though the individual events varied significantly in intensity, size, position, and lifetime. Thus our normalization results quantitatively validate the Akasofu (1964) assumption that key auroral features exist in the bulge‐type auroral substorm. After the substorm onset the auroral oval becomes clearly bifurcated consisting of two components: the oval aurora in the latitude range of the preonset oval and expanding primarily eastward postmidnight, and the bulge aurora, which emerges out of the oval, expanding poleward and both east and west in MLT. The oval aurora decay faster than the bulge emissions indicating that the decays have separate time constants. Owing to the pronounced difference in the spatiotemporal behavior of the two auroral components we suggest that their sources are quasi‐independent.

Towards explicit methods for differential algebraic equations

Explicit methods have previously been proposed for parabolic PDEs and for stiff ODEs with widely separated time constants. We discuss ways in which Differential Algebraic Equations (DAEs) might be regularized so that they can be efficiently integrated by explicit methods. The effectiveness of this approach is illustrated for some simple index three problems.

Phase separation in solid3He−4Hemixtures: Comparison with theory of homogeneous nucleation

NMR and pressure have been measured in a solid ${}^{3}\mathrm{He}{\ensuremath{-}}^{4}\mathrm{He}$ mixture as the temperature was lowered in steps through phase separation. The spin-echo method was used to detect the behavior typical for bounded diffusion and to estimate the diffusion coefficient, size and cluster concentration in the ${}^{3}\mathrm{He}$-enriched phase. The characteristic phase separation time constant of the mixture was found to decrease at lower temperatures. The results convincingly support homogeneous nucleation. From a comparison with theory, the surface tension at the boundary of the phase-separated clusters is found either from the cluster concentration, determined by NMR, or from the separation time constant, determined by pressure measurements. The results of the two independent determinations agree well and yield a surface tension coefficient of $1.27\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}$ $(1.27\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\mathrm{J}/\mathrm{m}}^{2}).$

Homogeneous nucleation in phase separation of solid 3He– 4He mixtures

NMR and pressure have been measured during phase separation in solid 3 He – 4 He mixtures. Spin echoes were used to observe bounded diffusion and to estimate the diffusion coefficient, size and nuclei concentration in the 3 He -enriched phase. The characteristic phase separation time constant of the mixture was found from pressure measurements. The results argue convincingly for homogeneous nucleation. The surface tension of the nuclei is found independently from NMR and from pressure measurements; the two determinations agree well and yield a surface tension coefficient of 4.9×10 −6 J m −2 .

A simple method for evaluating the transient thermal response of semiconductor devices

In this paper, a simple experimental method for the transient thermal characterisation of semiconductor packages is presented. The method is based upon the assumption that in many cases, device temperature evolution can be accurately described by a few exponential terms, as will be shown to be the case when transient thermal response has widely separated time constants. The proposed method, begins with the experimental determination of transient thermal response followed by numerical extraction of the time constants and amplitudes for the significant exponential terms, and is applied to a number of commercial Smartpack ® modules in order to obtain a dynamic model that can be used in circuit and numerical simulators, in order to predict the dynamic thermal behaviour of the device under any working condition.

Nonmonotonic temperature dependence of the mass transfer rate during isotopic phase separation of He3–He4 solid mixtures

It is discovered that the characteristic phase separation time constant of solid He3–He4 mixtures at low temperatures exhibits a nonmonotonic temperature dependence with a minimum. It means that the rate of mass transport slows down at very low temperatures, and the corresponding values of the effective diffusion coefficient also depend nonmonotonically on temperature and concentration and differ significantly from the spin diffusion coefficient measured earlier in NMR experiments on quantum diffusion. The discovered nonmonotonicity may be associated with the influence of the nonuniform field of elastic stresses in the crystal because of the difference in the molar volumes of the phases.