Poly(vinylidene fluoride) (PVDF) and poly(VDF-co-hexafluoropropylene) [P(VDF-HFP)] films having different polymorphisms and crystallite sizes but a similar crystal orientation (i.e., c-axes parallel to the film surface) were prepared by different film processing methods. Effects of polymorphism and crystallite size on the dipole reorientation behavior and electric energy storage/release were studied by electric displacement-electric field (D-E) loop measurements. Experimental results suggested that coupling interactions among ferroelectric domains, which could be adjusted by different polymorphisms and/or crystallite sizes, determined dipole reorientation/switching behaviors. Note that the ferroelectric domain coupling is realized via induced compensation polarizations from the media (either amorphous or crystalline PVDF) between aligned ferroelectric domains. A high β rather than R/δ content and a large crystallite size facilitated the coupling interactions among ferroelectric domains, and thus dipoles in highly coupled ferroelectric domains could be easily polarized, resulting in a high dielectric constant and a high stored energy density. However, strong coupling interactions impeded an easy dipole reversal to the so-called antiferroelectric-like (or random) state and thus reduced the discharged electric energy due to a high remanent polarization. Instead, the film with a high β content and a small crystallite size showed the highest discharged electric energy density, suggesting that the ferroelectric domain coupling could be weakened by confining them in nanoscale crystallites. These findings provide us useful guidance to achieve optimal crystalline morphology in PVDF copolymer films for high electric energy storage applications.

We analyze the spatiotemporal behavior of species concentrations in a diffusion-mediated conversion reaction which occurs at catalytic sites within linear pores of nanometer diameter. Diffusion within the pores is subject to a strict single-file (no passing) constraint. Both transient and steady-state behavior is precisely characterized by kinetic Monte Carlo simulations of a spatially discrete lattice-gas model for this reaction-diffusion process considering various distributions of catalytic sites. Exact hierarchical master equations can also be developed for this model. Their analysis, after application of mean-field type truncation approximations, produces discrete reaction-diffusion type equations (mf-RDE). For slowly varying concentrations, we further develop coarse-grained continuum hydrodynamic reaction-diffusion equations (h-RDE) incorporating a precise treatment of single-file diffusion in this multispecies system. The h-RDE successfully describe nontrivial aspects of transient behavior, in contrast to the mf-RDE, and also correctly capture unreactive steady-state behavior in the pore interior. However, steady-state reactivity, which is localized near the pore ends when those regions are catalytic, is controlled by fluctuations not incorporated into the hydrodynamic treatment. The mf-RDE partly capture these fluctuation effects, but cannot describe scaling behavior of the reactivity. Keywordscatalytic conversion, catalytic sites, continuum hydrodynamics, conversion reactions, discrete lattices, hydrodynamic treatment, kinetic Monte Carlo simulation, master equations, reaction diffusion, scaling behavior, single file diffusion, spatiotemporal behaviors, fluid dynamics Disciplines Mathematics | Physical Chemistry | Physics CommentsThe following article appeared in Journal of Chemical Physics 134,11 (2011) We analyze the spatiotemporal behavior of species concentrations in a diffusion-mediated conversion reaction which occurs at catalytic sites within linear pores of nanometer diameter. Diffusion within the pores is subject to a strict single-file (no passing) constraint. Both transient and steady-state behavior is precisely characterized by kinetic Monte Carlo simulations of a spatially discrete lattice-gas model for this reaction-diffusion process considering various distributions of catalytic sites. Exact hierarchical master equations can also be developed for this model. Their analysis, after application of mean-field type truncation approximations, produces discrete reaction-diffusion type equations (mf-RDE). For slowly varying concentrations, we further develop coarse-grained continuum hydrodynamic reaction-diffusion equations (h-RDE) incorporating a precise treatment of single-file diffusion in this multispecies system. The h-RDE successfully describe nontrivial aspects of transient behavior, in contrast to the mf-RDE, and also correctly capture unreactive steady-state behavior in the pore interior. However, steady-state reactivity, which is localized near the pore ends when those regions a...

The transient grating spectroscopy is widely used to determine the diffusion coefficients of valley excitons or spins in low-dimensional semiconductor materials. Here, we present the investigation on the diffusion dynamics of the valley excitons in a high-quality large-scale mechanically exfoliated tungsten diselenide (WSe2) monolayer by this technique at room temperature. Collinearly polarized laser excitation (at a photon energy of 1.66 eV resonant to the energy of valley A-excitons) was used to introduce a spatially periodic density of valley excitons. Through probing the spatial and temporal evolution of the initial density of valley excitons, we find that the signals of transient grating exhibit an nonexponential decay, and its decay rate is independent of the period of optical grating Λ. Combined with the transient reflection measurements, we show that the exciton-exciton annihilation plays a key role in decay processes of the transient grating spectroscopy, which results in the distortion of sinusoidal gratings. Based on Einstein relationship, we estimate the diffusion coefficient of valley exciton DX = 0.7 cm2/s.