Fast and automated identification of reactions with low barriers using meta-MD simulations

We test our meta-molecular dynamics (MD) based approach for finding low-barrier (<30 kcal/mol) reactions (SciPost Chem. 2021, 1, 003) on uni- and bimolecular reactions extracted from the barrier dataset developed by Grambow et al. (Scientific Data 2020, 7, 137). For unimolecular reactions the meta-MD simulations identify 25 of the 26 products found by Grambow et al., while the subsequent semiempirical screening eliminates an additional four reactions due to at an overestimation of the reaction energies or estimated barrier heights relative to DFT. In addition, our approach identifies an additional 36 reactions not found by Grambow et al., 10 of which are <30 kcal/mol. For bimolecular reactions the meta-MD simulations identify 19 of the 20 reactions found by Grambow et al., while the subsequent semiempirical screening eliminates an additional reaction. In addition, we find 34 new low-barrier reactions. For bimolecular reactions we found that it is necessary to ”encourage” the reactants to go to previously undiscovered products, by including products found by other MD simulations when computing the biasing potential as well as decreasing the size of the molecular cavity in which the MD occurs, until a reaction is observed. We also show that our methodology can find the correct products for two reactions that are more representative of those encountered in synthetic organic chemistry. The meta-MD hyperparameters used in this study thus appears to be generally applicable to finding low-barrier reactions.

Determinations of fission cross-sections and fission yields from proton induced fission reactions of 232Th, 233,235,236,238U, 237Np and 239Pu

In this work, the proton induced fission reaction cross-sections and fission yields are calculated for some actinides [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] using the fission barrier models of the TALYS 1.95 code. Cross-sections and fission yield calculations are carried out up to 100 MeV incident proton energies. The calculation results are compared with the available experimental data in the EXFOR library. In addition, a relative variance analysis of fission barrier models was done to determine the fission barrier model whose results best matched with the experimental results. Among the fission barrier models, the best agreement with the experimental data is obtained from the rotating-finite-range fission barrier model calculation for the [Formula: see text] reaction of the studied nuclei having the atomic mass number larger than 230. On the other hand, fission barrier heights for the studied reactions are determined using the same models.

Barrier properties and current conduction mechanism for metal contacts to lightly and highly doped p-type 4H-SiC

The barrier properties of Ti, Ni and Pt contact to lightly (9×1016 cm-3) and highly (9×1018 cm-3) doped p-type 4H-SiC were investigated. It is found that the barrier heights and ideality factors estimated from thermionic emission model for the lightly doped samples are non-ideal and abnormally temperature dependent. The anomalies have been successfully explained in terms of both pinch-off model and Gaussian distribution of inhomogeneous barrier heights. In addition, the evaluated homogeneous barrier heights are reasonably close to the average barrier heights from capacitance-voltage measurements. For the highly doped samples, thermionic field emission (TFE) is found to be the dominant carrier transport mechanism. The barrier heights estimated from TFE model are temperature independent. If the barrier inhomogeneities and tunneling effects are considered, the experimental results of the samples are in well agreement with the theoretical calculations.

Study on Li ion diffusion in LixV2O5 using first principle calculations and kinetic Monte Carlo simulations

We study the Li diffusion in LixV2O5 (0 < x < 1) - a potential cathode material for Lithium ion batteries. Different diffusion pathways in this material in dependence on the Li ion concentration are investigated by applying first-principles calculations. The results are used to obtain the corresponding diffusion coefficients by employing two complementary methodologies: Kinetic Monte Carlo (KMC) simulations and a statistical thermodynamics approach. The KMC simulations for two different crystal planes give new evidence that the diffusion occurs mainly along the [010] direction, while the corresponding diffusion coefficients show a temperature dependence obeying the Arrhenius' Law. The necessity of the consideration of concentration-dependent barrier heights in the KMC simulations are demonstrated by looking at the significant changes of the concentration-dependence of the diffusion cofficients. The simulated diffusion coefficients of the combined approach show a good quantitative agreement with experimental data reported previously.

Unraveling the origin of ferroelectric resistance switching through the interfacial engineering of layered ferroelectric-metal junctions

Ferroelectric memristors have found extensive applications as a type of nonvolatile resistance switching memories in information storage, neuromorphic computing, and image recognition. Their resistance switching mechanisms are phenomenally postulated as the modulation of carrier transport by polarization control over Schottky barriers. However, for over a decade, obtaining direct, comprehensive experimental evidence has remained scarce. Here, we report an approach to experimentally demonstrate the origin of ferroelectric resistance switching using planar van der Waals ferroelectric α-In2Se3 memristors. Through rational interfacial engineering, their initial Schottky barrier heights and polarization screening charges at both terminals can be delicately manipulated. This enables us to find that ferroelectric resistance switching is determined by three independent variables: ferroelectric polarization, Schottky barrier variation, and initial barrier height, as opposed to the generally reported explanation. Inspired by these findings, we demonstrate volatile and nonvolatile ferroelectric memristors with large on/off ratios above 104. Our work can be extended to other planar long-channel and vertical ultrashort-channel ferroelectric memristors to reveal their ferroelectric resistance switching regimes and improve their performances.

Schottky diodes based on 4H-SiC epitaxial layers

Forward and reverse current-voltage (IV) characteristics of Cr-SiC (4H) Schottky diodes based on epitaxial layers with doping (1-3)· 1015 cm-3 were studied in the temperature range of 300-550 K. It is shown that in many cases the IV characteristics are close to ideal, but a significant spread of the forward IV characteristics of diodes manufactured in the same way on the same epitaxial layer was found, probably due to the spread of the Schottky barrier heights reaching 0.3 eV. Heating of the diode, as well as packaging, can also change the Schottky barrier height. An alternative explanation suggests the presence of a powerful shunt.

Effects of cryo-EM cooling on structural ensembles

Structure determination by cryo electron microscopy (cryo-EM) provides information on structural heterogeneity and ensembles at atomic resolution. To obtain cryo-EM images of macromolecules, the samples are first rapidly cooled down to cryogenic temperatures. To what extent the structural ensemble is perturbed by the cooling is currently unknown. Here, to quantify the effects of cooling, we combined continuum model calculations of the temperature drop, molecular dynamics simulations of a ribosome complex before and during cooling with kinetic models. Our results suggest that three effects markedly contribute to the narrowing of the structural ensembles: thermal contraction, reduced thermal motion within local potential wells, and the equilibration into lower free-energy conformations by overcoming separating free-energy barriers. During cooling, barrier heights below 10 kJ/mol were found to be overcome resulting in reduction of B-factors in the ensemble imaged by cryo-EM. Our approach now enables the quantification of the heterogeneity of room-temperature ensembles from cryo-EM structures.

Surface Reactivity of Carbonaceous Nanoparticles: The Importance of Surface Pocket

The surface reactivity of carbonaceous nanoparticles is revealed from the barrier height and reaction enthalpy of hydrogen abstraction reaction by H radicals computed at the M06-2X/6–311g(d,p)//B3LYP/6-311G(d,p) level of theory. Small polycyclic aromatic hydrocarbon (PAH) clusters are selected as the model system of carbonaceous nanoparticles. The PAHs considered are naphthalene, pyrene, coronene, ovalene and circumcoronene. Cluster sizes range from dimer to tetramer with a parallel or crossed configuration. All results show similar values as that of monomers, but naphthalene dimers with a crossed configuration yield a lower barrier height and reaction enthalpy by ∼2 kcal/mol. A minor size dependence is noticed in the series of naphthalene clusters where a larger cluster exhibits a smaller barrier height. Larger homogeneous PAH clusters in a size range of 1.1–1.9 nm are later generated to mimic nascent soot surface. It is found that the barrier height decreases with the increase in particle size, and the averaged values are ∼2 kcal/mol lower than that of monomers. More importantly, a larger particle shows a wider spread in barrier heights, and low barrier heights are seen in the surface shallow regions (e.g., surface pockets). The lowest barrier height of ∼8.5 kcal/mol is observed at a C-H site locating in a surface pocket. A set of model systems are built to reveal the underlying mechanism of reduction in barrier height. It is shown that the reduction is caused by local interactions between the neighboring atoms and the local curvature. Further analysis on the average localized ionization potential shows that larger particles have higher reactivity, further supporting our findings from the barrier height of hydrogen abstraction reactions. Therefore, it is concluded that the surface reactivity depends on the particle size and the most reactive sites always locate at the surface pockets.