Search query: effective negative diffusion

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abstract: Using diffraction limited ultrafast imaging techniques, we investigate the propagation of singlet and triplet excitons in single crystal tetracene. Instead of an expected broadening, the distribution of singlet excitons narrows on a nanosecond time scale after photoexcitation. This narrowing results in an effective negative diffusion in which singlet excitons migrate toward the high density region, eventually leading to a singlet exciton distribution that is smaller than the laser excitation spot. Modeling the excited state dynamics demonstrates that the origin of the anomalous diffusion is rooted in nonlinear triplet triplet annihilation (TTA) We anticipate that this is a general phenomenon that can be used to study exciton diffusion and nonlinear TTA rates in semiconductors relevant for organic optoelectronics.
author_names: Anton Matthijs Berghuis

T V Raziman

Alexei Halpin

Shaojun Wang

Alberto G Curto

Jaime Gomez Rivas
title: Effective Negative Diffusion of Singlet Excitons in Organic Semiconductors
venue: The journal of physical chemistry letters
year: 2021

abstract: While exciton relaxation in monolayers of transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton diffusion has received only a little attention in spite of being a key process for optoelectronics and having already shown interesting unconventional behaviours (e.g. spatial halos) Here, we study the spatiotemporal dynamics in TMD monolayers and track optically excited excitons in time, momentum, and space. In particular, we investigate the temperature dependent exciton diffusion including the remarkable exciton landscape constituted by bright and dark states. Based on a fully quantum mechanical approach, we show at low temperatures an unexpected negative effective diffusion characterized by a shrinking of the spatial exciton distributions. This phenomenon can be traced back to the existence of dark exciton states in TMD monolayers and is a result of an interplay between spatial exciton diffusion and intervalley exciton phonon scattering.
author_names: Roberto Rosati

Raul Perea-Causin

Samuel Brem

Ermin Malic
title: Negative effective excitonic diffusion in monolayer transition metal dichalcogenides.
venue: Nanoscale
year: 2019

abstract: We show that strong cation anion interactions in a wide range of lithium salt/ionic liquid mixtures result in a negative lithium transference number, using molecular dynamics simulations and rigorous concentrated solution theory. This behavior fundamentally deviates from that obtained using self diffusion coefficient analysis and explains well recent experimental electrophoretic nuclear magnetic resonance measurements, which account for ion correlations. We extend these findings to several ionic liquid compositions. We investigate the degree of spatial ionic coordination employing single linkage cluster analysis, unveiling asymmetrical anion cation clusters. We formulate a way to compute the effective lithium charge and show that lithium containing clusters carry a negative charge over a remarkably wide range of compositions and concentrations. This finding has significant implications for the overall performance of battery cells based on ionic liquid electrolytes. It also provides a rigorous prediction recipe and design protocol for optimizing transport properties in next generation highly correlated electrolytes.
author_names: Nicola Molinari

Jonathan P Mailoa

Boris Kozinsky
title: General Trend of a Negative Li Effective Charge in Ionic Liquid Electrolytes.
venue: The journal of physical chemistry letters
year: 2019

abstract: The electrophoretic mobilities m of all ion species in the lithium salt/ionic liquid mixtures LiTFSA/EmimTFSA and LiBF4/EmimBF4 are determined by 1H, 19F and 7Li electrophoretic NMR. The average drift direction of Li is identical to that of the anions TFSA or BF4 This proves a correlated ion motion of Li with the anions in negatively charged Li containing clusters in both systems. The effective charge of these clusters is determined as 1, or 2 in the system with TFSA or BF4, respectively, pointing at the existence of [Li(TFSA)2] or [Li(BF4)3]2 This behavior is described by a negative effective transference number of Li, resulting in a negative contribution of Li ions to the overall conductivity. Li effective transference numbers are in the range of 0.04 to 0.02, depending on Li salt concentration and anion type. Transference numbers thus clearly deviate from apparent transference numbers estimated from diffusion coefficients, as an effect of a vehicular transport mechanism. This has important implications for the mechanism of Li mass transport in Li ion batteries as the drift of charged clusters has to be overcompensated by diffusive mass transport of neutral, Li containing aggregates.
author_names: Martin Gouverneur

Florian Schmidt

Monika Schonhoff
title: Negative effective Li transference numbers in Li salt/ionic liquid mixtures: does Li drift in the "Wrong" direction?
venue: Physical chemistry chemical physics PCCP
year: 2018

abstract: Unrestricted Travel in Solar Cells In the past 2 years, organolead halide perovskites have emerged as a promising class of light harvesting media in experimental solar cells, but the physical basis for their efficiency has been unclear (see the Perspective by Hodes) Two studies now show, using a variety of time resolved absorption and emission spectroscopic techniques, that these materials manifest relatively long diffusion paths for charge carriers energized by light absorption. Xing et al. (p. 344) independently assessed (negative) electron and (positive) hole diffusion lengths and found them well matched to one another to the ~100 nanometer optical absorption depth. Stranks et al. (p. 341) uncovered a 10 fold greater diffusion length in a chloride doped material, which correlates with the material's particularly efficient overall performance. Both studies highlight effective carrier diffusion as a fruitful parameter for further optimization. Spectroscopy pinpoints facile carrier diffusion as a key factor in the efficiency of perovskite solar cells. [Also see Perspective by Hodes] Organic inorganic perovskites have shown promise as high performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence quenching measurements to determine the electron hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI3 xClx) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.
author_names: Samuel D Stranks

Giles E Eperon

Giulia Grancini

Christopher Menelaou

Marcelo J P Alcocer

T Leijtens

Laura M Herz

Annamaria Petrozza

Henry J Snaith
title: Electron Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber
venue: Science
year: 2013

abstract: Abstract The reduction of oxygen transfer resistance through porous components consisting of a gas diffusion layer (GDL) microporous layer (MPL) and catalyst layer (CL) is very important to reduce the cost and improve the performance of a PEFC system. This study involves a systematic examination of the relationship between the oxygen transfer resistance of the actual porous components and their three dimensional structure by direct measurement with FIB SEM and X ray CT. Numerical simulations were carried out to model the properties of oxygen transport. Moreover, based on the model structure and theoretical equations, an approach to the design of new structures is proposed. In the case of the GDL, the binder was found to obstruct gas diffusion with a negative effect on performance. The relative diffusion coefficient of the MPL is almost equal to that of the model structure of particle packing. However, that of CL is an order of magnitude less than those of the other two components. Furthermore, an equation expressing the relative diffusion coefficient of each component can be obtained with the function of porosity. The electrical conductivity of MPL, which is lower than that of the carbon black packing, is considered to depend on the contact resistance.
author_names: Gen Inoue

Kouji Yokoyama

Junpei Ooyama

Takeshi Terao

Tomomi Tokunaga

Norio Kubo

Motoaki Kawase
title: Theoretical examination of effective oxygen diffusion coefficient and electrical conductivity of polymer electrolyte fuel cell porous components
year: 2016

abstract: We propose a novel epidemic model based on two layered multiplex networks to explore the influence of positive and negative preventive information on epidemic propagation. In the model, one layer represents a social network with positive and negative preventive information spreading competitively, while the other one denotes the physical contact network with epidemic propagation. The individuals who are aware of positive prevention will take more effective measures to avoid being infected than those who are aware of negative prevention. Taking the microscopic Markov chain (MMC) approach, we analytically derive the expression of the epidemic threshold for the proposed epidemic model, which indicates that the diffusion of positive and negative prevention information, as well as the topology of the physical contact network have a significant impact on the epidemic threshold. By comparing the results obtained with MMC and those with the Monte Carlo (MC) simulations, it is found that they are in good agreement, but MMC can well describe the dynamics of the proposed model. Meanwhile, through extensive simulations, we demonstrate the impact of positive and negative preventive information on the epidemic threshold, as well as the prevalence of infectious diseases. We also find that the epidemic prevalence and the epidemic outbreaks can be suppressed by the diffusion of positive preventive information and be promoted by the diffusion of negative preventive information.
author_names: Zhishuang Wang

Chengyi Xia

Zengqiang Chen

Guan-Rong Chen
title: Epidemic Propagation With Positive and Negative Preventive Information in Multiplex Networks
venue: IEEE Transactions on Cybernetics
year: 2021

abstract: In this paper, we consider the backward problem for diffusion equation with space fractional Laplacian, i.e. determining the initial distribution from the final value measurement data. In order to overcome the ill posedness of the backward problem, we present a so called negative exponential regularization method to deal with it. Based on the conditional stability estimate and an a posteriori regularization parameter choice rule, the convergence rate estimate are established under a priori bound assumption for the exact solution. Finally, several numerical examples are proposed to show that the numerical methods are effective.
author_names: Guanghui Zheng

Quan-Guo Zhang
title: Determining the initial distribution in space fractional diffusion by a negative exponential regularization method
year: 2017

abstract: Polymer solid state electrolytes are actively sought for their potential application in energy storage devices, particularly lithium metal rechargeable batteries. Herein, we report a polymer with high concentration salts as a quasi solid state electrolyte used for lithium sulfur cells, which shows an ionic conductivity of 1.6 mS cm 1 at room temperature. The cycling performance of Li S battery with this electrolyte shows a long cycle life (300 cycles) and high coulombic efficiency >98% without any consuming additives in the electrolyte. Moreover, it also shows a remarkably decreased self discharge (only 0.2% after storage for two weeks at room temperature. The reason can be attributed to that the electrolyte can suppress polysulfide anions diffusion, due to the high ratio oxygen atoms with negative charges which induce an electrical repulsion to the polysulfide anions, and their relatively long chains which can provide additional steric hindrance. Thus, the polysulfide anions can be located around carbon particles, which result in remarkably improved overall electrochemical performance, and also the electrolyte have a function of suppress the formation of lithium dendrites on the lithium anode surface.
author_names: Hai Zhong

Chunhua Wang

Zhibin Xu

Fei Ding

Xinjian Liu
title: A novel quasi solid state electrolyte with highly effective polysulfide diffusion inhibition for lithium sulfur batteries
venue: Scientific reports
year: 2016

abstract: This is a report on the antibacterial activity of ZnO and CuO nanoparticles synthesized via sol gel method. The studies were performed on Gram positive Staphylococcus aureus and Gram negative Escherichia coli and Pseudomonas aeruginosa bacteria using disc and well diffusion methods, bioluminescence and optical density analysis. The results show a strong decline of bacterial strains after a short contact with nanoparticles. The modelling allowed clarifying the bacterial sensitivity of toxic agents at different stages of their population evolution kinetics. It was concluded that the bacterial suppression is most effective at the exponential growth phase while it is of a lower effectiveness at the lag and stationary phases. The CuO and ZnO nanoparticles showed comparable effectiveness at the exponential growth phase. In the same time, ZnO was almost inactive at the lag phase and of lower effectiveness at the stationary phase, at which CuO conserved a significant activity.
author_names: Rania Dadi

Rabah Azouani

Mamadou Traore

Christine Mielcarek

A Kanaev
title: Antibacterial activity of ZnO and CuO nanoparticles against gram positive and gram negative strains.
venue: Materials science engineering. C, Materials for biological applications
year: 2019