Distribution Of Diffusion Constants Research Articles

Ionic-Defect Distribution Revealed by Improved Evaluation of Deep-Level Transient Spectroscopy on Perovskite Solar Cells

One of the key challenges for future development of efficient and stable metal halide perovskite solar cells is related to the migration of ions in these materials. Mobile ions have been linked to the observation of hysteresis in the current--voltage characteristics, shown to reduce device stability against degradation and act as recombination centers within the band gap of the active layer. In the literature one finds a broad spread of reported ionic defect parameters (e.g. activation energies) for seemingly similar perovskite materials, rendering the identification of the nature of these species difficult. In this work, we performed temperature dependent deep-level transient spectroscopy (DLTS) measurements on methylammonium lead iodide perovskite solar cells and developed a extended regularization algorithm for inverting the Laplace transform. Our results indicate that mobile ions form a distribution of emission rates (i.e. a distribution of diffusion constants) for each observed ionic species, which may be responsible for the differences in the previously reported defect parameters. Importantly, different DLTS modes such as optical and current DLTS yield the same defect distributions. Finally the comparison of our results with conventional boxcar DLTS and impedance spectroscopy (IS) verifies our evaluation algorithm.

Proton diffusion in the catalytic layer for high temperature polymer electrolyte fuel cells.

The present study focuses on quasielastic neutron scattering (QENS) of the proton dynamics in phosphoric acid (PA) inside the catalytic layer of high-temperature polymer electrolyte fuel cells (HT-PEFCs). The nanosecond proton dynamics is investigated on the local length scale around operating temperatures (300 K–430 K) using neutron backscattering spectroscopy. We have investigated the catalyst doped with different amounts of PA in order to understand the distribution of PA inside the layer. Three approaches are considered for the description of proton dynamics: the random jump diffusion model, distribution of diffusion constants and, finally, the trap model. Due to adsorption of the PA on the Pt particles the diffusion of protons in the catalytic layer is different in comparison to the bulk acid. The proton dynamics in the catalytic layer can be described by the random jump diffusion with traps. This diffusion is significantly slower than the diffusion of free PA; this also results in a lower conductivity, which is estimated from the obtained diffusion constant.

Distribution of diffusion constants and Stokes-Einstein violation in supercooled liquids.

It is widely believed that the breakdown of the Stokes-Einstein (SE) relation between the translational diffusivity and the shear viscosity in supercooled liquids is due to the development of dynamic heterogeneity, i.e., the presence of both slow and fast moving particles in the system. In this study we directly calculate the distribution of the diffusivity for a model system for different temperatures in the supercooled regime. We find that with decreasing temperature, the distribution evolves from Gaussian to bimodal indicating that on the time scale of the typical relaxation time, mobile (fluid like) and less mobile (solid like) particles in the system can be unambiguously identified. We also show that less mobile particles obey the Stokes-Einstein relation even in the supercooled regime and it is the mobile particles which show strong violation of the Stokes-Einstein relation in agreement with the previous studies on different model glass forming systems. Motivated by some of the recent studies where an ideal glass transition is proposed by randomly pinning some fraction of particles, we then studied the SE breakdown as a function of random pinning concentration in our model system. We showed that degree of SE breakdown increases quite dramatically with increasing pinning concentration, thereby providing a new way to unravel the puzzles of SE violation in supercooled liquids in greater details.

Diffusion analysis of glucocorticoid receptor and antagonist effect in living cell nucleus

The diffusion properties of hGRα in living cells have been analyzed. The hGRα translocalized from the cytosol to the nucleus after addition of Dex just as RU486; however, the Brownian motions of the proteins in nucleus were different. In order to analysis microenvironment of the nucleus of living cell, four different tandem EGFPs were constructed. Diffusion of tandem EGFP was dependent on the length of the protein as a rod-like molecule in solution. We found two kinds of mobility, fast diffusional mobility as a major component and much slower diffusional mobility as a major component in living cells nucleoplasm. On the bases of this analysis, we compared the diffusion property of hGRα in the nucleus at the presence of Dex or RU486 by distribution of diffusion constants. Our result may suggest that EGFP-hGRα is activated by RU486 and kept the stage of binding cofactor, GRE and final complex. Finally this means that dimerization is not required for association with GRE, although it is required for stabilization of a complex of EGFP-hGRα.

Gelation and aggregation of mucin in relation to the stomach's protective barrier

AbstractDynamic light scattering was applied to study the effects of pH and concentration on pig gastric mucin. At low concentration (C = 2.0 mg/ml), the distribution of diffusion constants is independent of pH, with an average diffusion constant D = 4 × 10−8 cm2/s. At pH2, very large aggregates with 100‐time slower D (2 × 10−8cm2/s) are seen. Depolarized DLS can be observed at pH2 even at low concentration (C = 5.0 mg/ml). The rotational and translational diffusion constants were analyzed using the Kirkwood‐Riseman model for rods and show that at pH2 mucin is a rod‐like molecule of length 390 nm, diameter 55 nm, with an axial ratio (length/diameter) of 7. However, at pH7, no significant depolarized signals could be detected at any of the concentrations examined in the study. These results suggest that the mucin monomer undergoes a conformational change from a flexible to a rod‐like conformation upon lowering pH from 7 (neutral) to 2 (acidic). Such conformational change and aggregation of the gastric mucin molecule at acidic pH may provide a mechanism by which the gastric mucin layer prevents the stomach from digesting itself, since the back diffusion of acid from the lumen to the gastric epithelium is retarded by the gel layer.

Multiple wave-vector extensions of the NMR pulsed-field-gradient spin-echo diffusion measurement.

Multiple wave-vector extensions of NMR pulsed-field-gradient diffusion measurements are discussed in the context of diffractionlike effects of restricted diffusion. In the case of two independent wave vectors, it is shown that the dependence of the amplitude on the relative angle between the wave vectors carries information that is absent in the usual single-wave-vector amplitude. It is shown that a two-wave-vector measurement is sensitive to restricted diffusion even at small wave vectors, in contrast with the single-wave-vector case. It is proposed that the angular dependence noted above may be used to distinguish effects of restricted diffusion from those arising from a distribution of diffusion constants.