Homogenization of diffusion in multicomponent liquids.

Diffusion is a key kinetic factor determining chemical mixing and phase formation in liquids. In multicomponent systems, the presence of different elements makes it experimentally challenging to measure diffusivities and understand their mechanisms. Using a molecular dynamics simulation, we obtain the diffusion constants and the atomic process of a model Cantor alloy liquid made of five equimolar components. We show that the diffusivities conform remarkably well to the Arrhenius law in a wide range of temperature covering both the equilibrium and undercooled liquid regions. The activation energies for all the alloy elements with different bonding energies and atomic sizes are close to each other. The results suggest that the diffusivity in the multicomponent liquid tends to be homogenized by the components with marginal differences. This finding allows us to treat the different elements as a single type of atom, the pseudo-atom, for diffusional and maybe structural and physical properties in multicomponent liquids.

Thermally activated delayed fluorescence materials with aggregation-induced emission properties: a QM/MM study.

Organic molecules with thermally activated delayed fluorescence (TADF) and aggregation induced emission (AIE) properties have attracted increasing research interest due to their great potential applications in organic light emitting diodes (OLEDs), especially for those with multicolor mechanochromic luminescence (MCL) features. Theoretical research on the luminescence characteristics of organic TADF emitters based on the aggregation states is highly desired to quantify the relationship between the TADF properties and aggregation states. In this work, we study the 4,4'-(6-(9,9-dimethylacridine-10(9H)-yl)quinoline-2,3-dibenzonitrile (DMAC-CNQ) emitter with TADF and AIE properties, and calculate the photophysical properties in gas, solid and amorphous states by using the quantum mechanics and molecular mechanics (QM/MM) method. Our simulations demonstrate that the aggregation states enhance obviously the reverse intersystem crossing rates and transition dipole moments of the DMAC-CNQ emitter, and suppress the non-radiative rates from the lowest excited singlet state (S1) to ground state (S0). Specifically, the molecular stacking of DMAC-CNQ in solid phases can mainly restrict the geometric torsion of the DMAC moiety for decreasing non-radiative decay rates, and the torsion of the CNQ moiety for increasing the reverse intersystem crossing rates. As a result, the calculated fluorescence efficiencies of the DMAC-CNQ emitter in the crystal and amorphous states are 67% and 26% respectively, and in good agreement with the experimental results.

Chemical segregation in metallic glass nanowires.

Nanowires made of metallic glass have been actively pursued recently due to the superb and unique properties over those of the crystalline materials. The amorphous nanowires are synthesized either at high temperature or via mechanical disruption using focused ion beam. These processes have potential to cause significant changes in structure and chemical concentration, as well as formation of defect or imperfection, but little is known to date about the possibilities and mechanisms. Here, we report chemical segregation to surfaces and its mechanisms in metallic glass nanowires made of binary Cu and Zr elements from molecular dynamics simulation. Strong concentration deviation are found in the nanowires under the conditions similar to these in experiment via focused ion beam processing, hot imprinting, and casting by rapid cooling from liquid state. Our analysis indicates that non-uniform internal stress distribution is a major cause for the chemical segregation, especially at low temperatures. Extension is discussed for this observation to multicomponent metallic glass nanowires as well as the potential applications and side effects of the composition modulation. The finding also points to the possibility of the mechanical-chemical process that may occur in different settings such as fracture, cavitation, and foams where strong internal stress is present in small length scales.

High-performance cryo-temperature ionic thermoelectric liquid cell developed through a eutectic solvent strategy.

Ionic thermoelectric (i-TE) liquid cells offer an environmentally friendly, cost effective, and easy-operation route to low-grade heat recovery. However, the lowest temperature is limited by the freezing temperature of the aqueous electrolyte. Applying a eutectic solvent strategy, we fabricate a high-performance cryo-temperature i-TE liquid cell. Formamide is used as a chaotic organic solvent that destroys the hydrogen bond network between water molecules, forming a deep eutectic solvent that enables the cell to operate near cryo temperatures (down to -35 °C). After synergistic optimization of the electrode and cell structure, the as-fabricated liquid i-TE cell with cold (-35 °C) and hot (70 °C) ends achieve a high power density (17.5 W m-2) and a large two-hour energy density (27 kJ m-2). In a prototype 25-cell module, the open-circuit voltage and short-circuit current are 6.9 V and 68 mA, respectively, and the maximum power is 131 mW. The anti-freezing ability and high output performance of the as-fabricated i-TE liquid cell system are requisites for applications in frigid regions.

Polaron interfacial entropy as a route to high thermoelectric performance in DAE-doped PEDOT:PSS films.

Enhancing the thermoelectric transport properties of conductive polymer materials has been a long-term challenge, in spite of the success seen with molecular doping strategies. However, the strong coupling between the thermopower and the electrical conductivity limits thermoelectric performance. Here, we use polaron interfacial occupied entropy engineering to break through this intercoupling for a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate)) thin film by using photochromic diarylethene (DAE) dopants coupled with UV-light modulation. With a 10-fold enhancement of the thermopower from 13.5 µV K-1 to 135.4 µV K-1 and almost unchanged electrical conductivity, the DAE-doped PEDOT:PSS thin film achieved an extremely high power factor of 521.28 µW m-1 K-2 from an original value of 6.78 µW m-1 K-2. The thermopower was positively correlated with the UV-light intensity but decreased with increasing temperature, indicating resonant coupling between the planar closed DAE molecule and PEDOT. Both the experiments and theoretical calculations consistently confirmed the formation of an interface state due to this resonant coupling. Interfacial entropy engineering of polarons could play a critical role in enhancing the thermoelectric performance of the organic film.

Interaction of human synovial phospholipase A2 with mixed lipid bilayers: a coarse-grain and all-atom molecular dynamics simulation study.

Human secreted phospholipase A2s have been shown to promote inflammation in mammals by catalyzing the first step of the arachidonic acid pathway by breaking down phospholipids, producing fatty acids, including arachidonic acid. They bind to the membrane water interface to access their phospholipid substrates from the membrane. Their binding modes on membrane surfaces are regulated by diverse factors, including membrane charge, fluidity, and heterogeneity. The influence of these factors on the binding modes of the enzymes is not well understood. Here we have studied several human synovial phospholipase A2 (hs-PLA2)/mixed bilayer systems through a combined coarse-grain and all-atom molecular dynamics simulation. It was found that hydrophobic residues Leu2, Val3, Ala18, Leu19, Phe23, Gly30, and Phe63 that form the edge of the entrance of the hydrophobic binding pocket in hs-PLA2 tend to penetrate into the hydrophobic area of lipid bilayers, and more than half of the total amino acid residues make contact with the lipid headgroups. Each enzyme molecule forms 19-38 hydrogen bonds with the bilayer to which it binds, most of which are with the phosphate groups. Analysis of the root-mean-square deviation (rmsd) shows that residues Val30-Thr40, Tyr66-Gln80, and Lys107-Arg118 have relatively large rmsds during all-atom molecular dynamics simulations, in accordance with the observation of an enlarged entrance region of the hydrophobic binding pocket. The amino acid sequences forming the entrance of the binding pocket prefer to interact with lipid molecules that are more fluid or negatively charged, and the opening of the binding pocket would be larger when the lipid components are more fluid.

Melting and superheating in solids with volume shrinkage at melting: a molecular dynamics study of silicon.

The thermodynamics of homogeneous melting in superheated crystalline solids with volume shrinkage at melting is investigated using extensive molecular dynamics simulation in conjunction with a classical nucleation theory. A liquid-solid co-existing model is established to overcome the difficult in observing liquid phase formation in a superheated Si crystal. We found that melting is governed by two major factors, the volume change induced strain energy and the curvature of the interface between the liquid and the solid phases. The driving force for melting in superheating regime is lowered by the additional strain energy that restricts homogeneous nucleation of a liquid phase till temperature rises above the normal melting point, thus causing superheating. However, due to the abnormal behavior in the compressibility of the silicon liquid in the superheating regime, the degree of superheating in terms of the liquid nucleation gap becomes significantly reduced. More potential complications caused by the change of the atomic bonding in Si at melting are discussed.

The establishment and application of quality gain-loss function when the loss of primary and cubic term is not ignored and the compensation quantity is constant.

The traditional quality gain-loss function(QGLF) considers the case that the primary term loss cannot be ignored, does not consider the cubic term loss, but in practice the cubic term loss should not be ignored. In this paper, based on the existing QGLF model, the Taylor expansion is reserved to the third order, the determination of the quality loss coefficient is discussed and analyzed under the condition that the compensation quantity is constant, and the asymmetric cubic QGLF model is established, and uses an example of concrete mixture out of the machine slump during the dam concrete construction to analyze and discuss the relationship between the proposed model and the traditional quadratic QGLF, which verifies the rationality of the proposed model.

Ultrasensitive mechanical/thermal response of a P(VDF-TrFE) sensor with a tailored network interconnection interface.

Ferroelectric polymers have great potential applications in mechanical/thermal sensing, but their sensitivity and detection limit are still not outstanding. We propose interface engineering to improve the charge collection in a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) copolymer (P(VDF-TrFE)) thin film via cross-linking with poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonate (PEDOT:PSS) layer. The as-fabricated P(VDF-TrFE)/PEDOT:PSS composite film exhibits an ultrasensitive and linear mechanical/thermal response, showing sensitivities of 2.2 V kPa-1 in the pressure range of 0.025-100 kPa and 6.4 V K-1 in the temperature change range of 0.05-10 K. A corresponding piezoelectric coefficient of -86 pC N-1 and a pyroelectric coefficient of 95 µC m-2 K-1 are achieved because more charge is collected by the network interconnection interface between PEDOT:PSS and P(VDF-TrFE), related to the increase in the dielectric properties. Our work shines a light on a device-level technique route for boosting the sensitivity of ferroelectric polymer sensors through electrode interface engineering.

Anion Size Effect of Ionic Liquids in Tuning the Thermoelectric and Mechanical Properties of PEDOT:PSS Films through a Counterion Exchange Strategy.

Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) thermoelectric thin films have attracted significant interest due to their solution-processable manufacturing. However, molecular-level tuning or doping is still a challenge to synergistically boost their thermoelectric performance and mechanically stretchable capabilities. In this work, we report a counterion exchange between ionic liquid bis(x-fluorosulfonyl) amide lithium (Li:nFSI, n = 1, 3, 5) with different sizes of anions and a PEDOT:PSS-induced bipolaron network, which significantly boosted the thermoelectric power factor from 0.8 to 157 µW m K-2 at 235 °C and the maximum tensile strain from 3% to over 30%. The π-π* stacking of the PEDOT polymer chains was fine-tuned by the hydrophobic anions of nFSI-, providing a technical route for constructing a bipolaron network and inducing the transition from hopping transport to band-like transport. Furthermore, we found that the stretchable capabilities, that is, εmax, were connected to the gelation time of the PEDOT:PSS-Li:nFSI aqueous solution. Thus, more fluorine-containing groups resulted in longer gelation times and higher εmax values, which significantly improved the processability of the solution-derived films.

Sunlight-Coordinated High-Performance Moisture Power in Natural Conditions.

It is a challenge to spontaneously harvest multiple clean sources from the environment for upgraded energy-converting systems. The ubiquitous moisture and sunlight in nature are attractive for sustainable power generation especially. A high-performance light-coordinated "moist-electric generator" (LMEG) based on the rational combination of a polyelectrolyte and a phytochrome is herein developed. By spontaneous adsorption of gaseous water molecules and simultaneous exposure to sunlight, a piece of 1 cm2 composite film offers an open-circuit voltage of 0.92 V and a considerable short-circuit current density of up to 1.55 mA cm-2 . This record-high current density is about two orders of magnitude improvement over that of most conventional moisture-enabled systems, which is caused by moisture-induced charge separation accompanied with photoexcited carrier migration, as confirmed by a dynamic Monte Carlo device simulation. Flexible devices with customizable size are available for large-scale integration to effectively work under a wide range of relative humidity (about 20-100%), temperature (10-80 °C), and light intensity (30-200 mW cm-2 ). The wearable and portable LMEGs provide ample power supply in natural conditions for indoor and outdoor electricity-consuming systems. This work opens a novel avenue to develop sustainable power generation through collecting multiple types of natural energy by a single hybrid harvester.

Rapid artificial intelligence solutions in a pandemic-The COVID-19-20 Lung CT Lesion Segmentation Challenge.

Artificial intelligence (AI) methods for the automatic detection and quantification of COVID-19 lesions in chest computed tomography (CT) might play an important role in the monitoring and management of the disease. We organized an international challenge and competition for the development and comparison of AI algorithms for this task, which we supported with public data and state-of-the-art benchmark methods. Board Certified Radiologists annotated 295 public images from two sources (A and B) for algorithms training (n=199, source A), validation (n=50, source A) and testing (n=23, source A; n=23, source B). There were 1,096 registered teams of which 225 and 98 completed the validation and testing phases, respectively. The challenge showed that AI models could be rapidly designed by diverse teams with the potential to measure disease or facilitate timely and patient-specific interventions. This paper provides an overview and the major outcomes of the COVID-19 Lung CT Lesion Segmentation Challenge - 2020.

Water transport and purification in nanochannels controlled by asymmetric wettability.

Biomimetic asymmetric nanochannels have recently attracted increasing attention from researchers, especially in the aspect of the asymmetric wettability (a hydrophilic-hydrophobic system), which can be utilized to control the wetting behavior of aqueous media and to offer a means for guiding water motion. By using molecular dynamics simulations, a design for a potentially efficient water filter is presented based on (n, n) single-walled carbon nanotubes, where n = 6, 8, 10 and 12, asymmetrically modified with hydrophilic groups (carboxyl, -COOH) at one tip and hydrophobic groups (trifluoromethyl, -CF(3) ) at the other. The reduced water density on the hydrophobic sides of the functionalized nanotubes are observed in both pure water and aqueous electrolyte solution, except for the functionalized (6, 6) tube, due to the change of dipole orientation of the single-file water wire within it. The functionalized (8, 8) tube can significantly maintain the low water density on the hydrophobic side. Both (6, 6) and (8, 8) tubes have relatively high energy barriers at their tips for ion permeation, which can be obtained by calculating the potential of mean force. Such tip functionalization of a nanotube therefore suggests the great possibilities of water transport and filtration, dominated by asymmetric wettability. The functionalized (8, 8) tube could act as a nanofluidic channel for water purification, not only for ion exclusion but also as a stable water column structure.

Electronic structure of pyridine-based SAMs on flat Au(111) surfaces: extended charge rearrangements and Fermi level pinning.

Density functional theory calculations are used to investigate the electronic structure of pyridine-based self-assembled monolayers (SAMs) on an Au(111) surface. We find that, when using pyridine docking groups, the bonding-induced charge rearrangements are frequently found to extend well onto the molecular backbone. This is in contrast to previous observations for the chemisorption of other SAMs, e.g., organic thiolates on gold, and can be explained by a pinning of the lowest unoccupied states of the SAM at the metal Fermi-level. The details of the pinning process, especially the parts of the molecules most affected by the charge rearrangements, strongly depend on the length of the molecular backbone and the tail-group substituent. We also mention methodological shortcomings of conventional density functional theory that can impact the quantitative details regarding the circumstances under which pinning occurs and highlight a number of peculiarities associated with bond dipoles that arise from Fermi-level pinning.

An improved dynamic Monte Carlo model coupled with Poisson equation to simulate the performance of organic photovoltaic devices.

We describe a new dynamic Monte Carlo model to simulate the operation of a polymer-blend solar cell; this model provides major improvements with respect to the one we developed earlier [J. Phys. Chem. B 114, 36 (2010)] by incorporating the Poisson equation and a charge thermoactivation mechanism. The advantage of the present approach is its capacity to deal with a nonuniform electrostatic potential that dynamically depends on the charge distribution. In this way, the unbalance in electron and hole mobilities and the space-charge induced potential distribution can be treated explicitly. Simulations reproduce well the experimental I-V curve in the dark and the open-circuit voltage under illumination of a polymer-blend solar cell. The dependence of the photovoltaic performance on the difference in electron and hole mobilities is discussed.

Computational methods for design of organic materials with high charge mobility.

Charge carrier mobility is at the center of organic electronic devices. The strong couplings between electrons and nuclear motions lead to complexities in theoretical description of charge transport, which pose a major challenge for the fundamental understanding and computational design of transport organic materials. This tutorial review describes recent progresses in developing computational tools to assess the carrier mobility in organic molecular semiconductors at the first-principles level. Some rational molecular design strategies for high mobility organic materials are outlined.

Influences of dynamic and static disorder on the carrier mobility of BTBT-C12 derivatives: a multiscale computational study.

The role of dynamic and static disorder has been widely discussed for carrier transport in organic semiconductors. In this work, we apply a multiscale approach by combining molecular dynamics simulations, quantum mechanics calculations and kinetic Monte-Carlo simulations to study the influence of dynamic and static disorder on the hole mobility of four didodecyl[1]benzothieno[3,2-b]benzothiophene (BTBT-C12) isomers. It is found that the dynamic disorder of transfer integral tends to decrease the mobility for quasi-1D (quasi one-dimensional) BTBT1 and BTBT4 isomers and increase the mobility for 2D (two-dimensional) BTBT2 and BTBT3 isomers, while the dynamic disorder of site energy tends to decrease mobility for all the four isomers; however, the reduction in 2D molecules is much less than that in quasi-1D molecules. Results show that trap defects could reduce the mobility for both the quasi-1D and 2D molecular structures significantly, even to several orders of magnitude. In addition, our work also reveals that there might exist two kinds of oxidation defects of the scatter type for the concerned isomers, which thus leads to greater reduction in mobility for the quasi-1D molecular structures than the 2D molecular structures. The study shows that the 2D molecular structures are favored over the quasi-1D or 1D molecular structure, and it is expected that these results could be used to shed light on device design in organic electronics.

Surface Roughness Tuning at Sub-Nanometer Level by Considering the Normal Stress Field in Magnetorheological Finishing.

Although magnetorheological finishing (MRF) is being widely utilized to achieve ultra-smooth optical surfaces, the mechanisms for obtaining such extremely low roughness after the MRF process are not fully understood, especially the impact of finishing stresses. Herein we carefully investigated the relationship between the stresses and surface roughness. Normal stress shows stronger impacts on the surface roughness of fused silica (FS) when compared with the shear stress. In addition, normal stress in the polishing zone was found to be sensitive to the immersion depth of the magnetorheological (MR) fluid. Based on the above, a fine tuning of surface roughness (RMS: 0.22 nm) was obtained. This work fills gaps in understanding about the stresses that influence surface roughness during MRF.

Leaf-Inspired Flexible Thermoelectric Generators with High Temperature Difference Utilization Ratio and Output Power in Ambient Air.

The inherently small temperature difference in air environment restricts the applications of thermoelectric generation in the field of Internet of Things and wearable electronics. Here, a leaf-inspired flexible thermoelectric generator (leaf-TEG) that makes maximum use of temperature difference by vertically aligning poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and constantan thin films is demonstrated. Analytical formulae of the performance scales, i.e., temperature difference utilization ratio (φth) and maximum output power (Pmax), are derived to optimize the leaf-TEG dimensions. In an air duct (substrate: 36 °C, air: 6 °C, air flowing: 1 m s-1), the 10-leaf-TEG shows a φth of 73% and Pmax of 0.38 µW per leaf. A proof-of-concept wearable 100-leaf-TEG (60 cm2) generates 11 µW on an arm at room temperature. Furthermore, the leaf-TEG is flexible and durable that is confirmed by bending and brushing over 1000 times. The proposed leaf-TEG is very appropriate for air convection scenarios with limited temperature differences.

Rapid Artificial Intelligence Solutions in a Pandemic - The COVID-19-20 Lung CT Lesion Segmentation Challenge.

Artificial intelligence (AI) methods for the automatic detection and quantification of COVID-19 lesions in chest computed tomography (CT) might play an important role in the monitoring and management of the disease. We organized an international challenge and competition for the development and comparison of AI algorithms for this task, which we supported with public data and state-of-the-art benchmark methods. Board Certified Radiologists annotated 295 public images from two sources (A and B) for algorithms training (n=199, source A), validation (n=50, source A) and testing (n=23, source A; n=23, source B). There were 1,096 registered teams of which 225 and 98 completed the validation and testing phases, respectively. The challenge showed that AI models could be rapidly designed by diverse teams with the potential to measure disease or facilitate timely and patient-specific interventions. This paper provides an overview and the major outcomes of the COVID-19 Lung CT Lesion Segmentation Challenge - 2020.