Accuracy and limitations of the bond polarizability model in modeling of Raman scattering from molecular dynamics simulations.

Calculation of Raman scattering from molecular dynamics (MD) simulations requires accurate modeling of the evolution of the electronic polarizability of the system along its MD trajectory. For large systems, this necessitates the use of atomistic models to represent the dependence of electronic polarizability on atomic coordinates. The bond polarizability model (BPM) is the simplest such model and has been used for modeling the Raman spectra of molecular systems but has not been applied to solid-state systems.

Identification of a Durability Descriptor for Molecular Oxygen Reduction Reaction Catalysts.

The development of durable platinum-group-metal-free oxygen reduction reaction (ORR) catalysts is a key research direction for enabling the wide use of fuel cells. Here, we use a combination of experimental measurements and density functional theory calculations to study the activity and durability of seven iron-based metallophthalocyanine (MPc) ORR catalysts that differ only in the identity of the substituent groups on the MPcs. While the MPcs show similar ORR activity, their durabilities as measured by the current decay half-life differ greatly.

Identification of High-Reliability Regions of Machine Learning Predictions Based on Materials Chemistry.

Progress in the application of machine learning (ML) methods to materials design is hindered by the lack of understanding of the reliability of ML predictions, in particular, for the application of ML to small data sets often found in materials science. Using ML prediction for transparent conductor oxide formation energy and band gap, dilute solute diffusion, and perovskite formation energy, band gap, and lattice parameter as examples, we demonstrate that (1) construction of a convex hull in feature space that encloses accurately predicted systems can be used to identify regions in feature space for which ML predictions are highly reliable; (2) analysis of the systems enclosed by the convex hull can be used to extract physical understanding; and (3) materials that satisfy all well-known chemical and physical principles that make a material physically reasonable are likely to be similar and show strong relationships between the properties of interest and the standard features used in ML. We also show that similar to the composition-structure-property relationships, inclusion in the ML training data set of materials from classes with different chemical properties will not be beneficial for the accuracy of ML prediction and that reliable results likely will be obtained by ML model for narrow classes of similar materials even in the case where the ML model will show large errors on the data set consisting of several classes of materials.

Origin of the Rarely Reported High Performance of Mn-doped Carbon-based Oxygen Reduction Catalysts.

Recent efforts to develop durable high-performance platinum-group metal (PGM)-free oxygen reduction reaction (ORR) electrocatalysts have focused on Fe- and Co-based molecular and pyrolyzed catalysts. While Mn-based catalysts have advantages of lower toxicity and higher durability, their activity has been generally poor. Nevertheless, several examples of high-performance Mn-based catalysts have been reported.

Application of Poly-L-Lysine for Tailoring Graphene Oxide Mediated Contact Formation between Lithium Titanium Oxide LTO Surfaces for Batteries.

When producing stable electrodes, polymeric binders are highly functional materials that are effective in dispersing lithium-based oxides such as LiTiO (LTO) and carbon-based materials and establishing the conductivity of the multiphase composites. Nowadays, binders such as polyvinylidene fluoride (PVDF) are used, requiring dedicated recycling strategies due to their low biodegradability and use of toxic solvents to dissolve it. Better structuring of the carbon layers and a low amount of binder could reduce the number of inactive materials in the electrode.

Understanding hydrazine oxidation electrocatalysis on undoped carbon.

Carbons are ubiquitous electrocatalytic supports for various energy-related transformations, especially in fuel cells. Doped carbons such as Fe-N-C materials are particularly active towards the oxidation of hydrazine, an alternative fuel and hydrogen carrier. However, there is little discussion of the electrocatalytic role of the most abundant component - the carbon matrix - towards the hydrazine oxidation reaction (HzOR).

CVD-Assisted Synthesis of 2D Layered MoSe on Mo Foil and Low Frequency Raman Scattering of Its Exfoliated Few-Layer Nanosheets on CaF Substrates.

Transition-metal dichalcogenides (TMDCs) are unique layered materials with exotic properties. So, examining their structures holds tremendous importance. 2H-MoSe (analogous to MoS; Gr.

A Multilevel Analytical Theory for Prediction of Ferroelectric Perovskite Oxide Properties from Composition.

Prediction of properties from composition is a fundamental goal of materials science that is particularly relevant for ferroelectric perovskite oxide solid solutions where compositional variation is a primary tool for material design. Design of ferroelectric oxide solid solutions has been guided by heuristics and first-principles and Landau-Ginzburg-Devonshire theoretical methods that become increasingly difficult to apply in ternary, quaternary, and quintary solid solutions. To address this problem, a multilevel model is developed for the prediction of the ferroelectric-to-paraelectric transition temperature (T ), coercive field (E ), and polarization (P) of PbTiO -derived ferroelectric solid solutions from composition.

A Predictive Theory for Domain Walls in Oxide Ferroelectrics Based on Interatomic Interactions and its Implications for Collective Material Properties.

Domain walls separating regions of ferroelectric material with polarization oriented in different directions are crucial for applications of ferroelectrics. Rational design of ferroelectric materials requires the development of a theory describing how compositional and environmental changes affect domain walls. To model domain wall systems, a discrete microscopic Landau-Ginzburg-Devonshire (dmLGD) approach with A- and B-site cation displacements serving as order parameters is developed.

Fe-N-C electrocatalysts in the oxygen and nitrogen cycles in alkaline media: the role of iron carbide.

Fe-N-C electrocatalysts hold a great promise for Pt-free energy conversion, driving the electrocatalysis of oxygen reduction and evolution, oxidation of nitrogen fuels, and reduction of N, CO, and NO. Nevertheless, the catalytic role of iron carbide, a component of nearly every pyrolytic Fe-N-C material, is at the focus of a heated controversy. We now resolve the debate by examining a broad range of FeC sites, spanning across many typical size distributions and carbon environments.

Modelling of high-temperature order-disorder phase transitions of non-stoichiometric MoC and TiC from first principles.

High-temperature order-disorder phase transitions play an important role in determining the structure and physical and chemical properties of non-stoichiometric transition metal carbides. Due to the large number of possible carbon vacancy arrangements, it is difficult to study these systems with first-principles calculations. Here, we construct a simple atomistic potential capable of accurately reproducing the energetics of the carbon vacancy arrangements in cubic MoC and TiC obtained from density functional theory calculations.

Sustainable existence of solid mercury (Hg) nanoparticles at room temperature and their applications.

Although liquid mercury (Hg) has been known since antiquity, the formation of stable solid nano forms of Hg at room temperature has not been reported so far. Here, for the first time, we report a simple sonochemical route to obtain solid mercury nanoparticles, stabilized by reduced graphene oxide at ambient conditions. The as-formed solid Hg nanoparticles were found to exhibit remarkable rhombohedral morphology and crystallinity at room temperature.

Control of Molecular Catalysts for Oxygen Reduction by Variation of pH and Functional Groups.

In the search for replacement of the platinum-based catalysts for fuel cells, MN molecular catalysts based on abundant transition metals play a crucial role in modeling and investigation of the influence of the environment near the active site in platinum-group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts. To understand how the ORR activity of molecular catalysts can be controlled by the active site structure through modification by the pH and substituent functional groups, the change of the ORR onset potential and the electron number in a broad pH range was examined for three different metallocorroles. Experiments revealed a switch between two different ORR mechanisms and a change from 2e to 4e pathway in the pH range of 3.

Tuning of ORR activity through the stabilization of the adsorbates by hydrogen bonding with substituent groups.

Metallocorroles and metalloporphyrins (M-N-C) are some of the best alternative molecular catalysts for the replacement of the expensive platinum-group metals (PGM) in oxygen reduction reaction (ORR) catalysis in polymer electrolyte membrane (PEM) fuel cells. To date, Co-based corroles have shown the best performance, but still suffer from the poor stability and the toxicity of the Co metal. Mn-N-C are more stable than Co-N-C, and are also less reactive towards peroxide formation.

Resonant domain-wall-enhanced tunable microwave ferroelectrics.

Ordering of ferroelectric polarization and its trajectory in response to an electric field are essential for the operation of non-volatile memories, transducers and electro-optic devices. However, for voltage control of capacitance and frequency agility in telecommunication devices, domain walls have long been thought to be a hindrance because they lead to high dielectric loss and hysteresis in the device response to an applied electric field. To avoid these effects, tunable dielectrics are often operated under piezoelectric resonance conditions, relying on operation well above the ferroelectric Curie temperature, where tunability is compromised.

First-Principles Investigation of the Formation of Pt Nanorafts on a MoC Support and Their Catalytic Activity for Oxygen Reduction Reaction.

We use first-principles calculations to study the formation of Pt nanorafts and their oxygen reduction reaction (ORR) catalytic activity on MoC. Due to the high Pt binding energy on C atoms, Pt forms sheet-like structures on the MoC surface instead of agglomerating into particles. We find that the disordered MoC surface carbon arrangement limits the Pt sheet growth, leading to the formation of 4-6 atom Pt nanorafts.

First-Principles Investigation of Black Phosphorus Synthesis.

Black phosphorus (BP) is a layered semiconductor with outstanding properties, making it a promising candidate for optoelectronic and other applications. BP synthesis is an intriguing task largely due to the insufficient understanding of the synthesis mechanism. In this work, we use density functional theory calculations to examine BP and its precursor red phosphorus as they are formed from P building blocks.

Slush-like polar structures in single-crystal relaxors.

Despite more than 50 years of investigation, it is still unclear how the underlying structure of relaxor ferroelectrics gives rise to their defining properties, such as ultrahigh piezoelectric coefficients, high permittivity over a broad temperature range, diffuse phase transitions, strong frequency dependence in dielectric response, and phonon anomalies. The model of polar nanoregions inside a non-polar matrix has been widely used to describe the structure of relaxor ferroelectrics. However, the lack of precise knowledge about the shapes, growth and dipole patterns of polar nanoregions has led to the characterization of relaxors as "hopeless messes", and no predictive model for relaxor behaviour is currently available.

Intrinsic ferroelectric switching from first principles.

The existence of domain walls, which separate regions of different polarization, can influence the dielectric, piezoelectric, pyroelectric and electronic properties of ferroelectric materials. In particular, domain-wall motion is crucial for polarization switching, which is characterized by the hysteresis loop that is a signature feature of ferroelectric materials. Experimentally, the observed dynamics of polarization switching and domain-wall motion are usually explained as the behaviour of an elastic interface pinned by a random potential that is generated by defects, which appear to be strongly sample-dependent and affected by various elastic, microstructural and other extrinsic effects.

Photoferroelectric and Photopiezoelectric Properties of Organometal Halide Perovskites.

Piezoelectrics play a critical role in various applications. The permanent dipole associated with the molecular cations in organometal halide perovskites (OMHPs) may lead to spontaneous polarization and thus piezoelectricity. Here we explore the piezoelectric properties of OMHPs with density functional theory.

Substantial bulk photovoltaic effect enhancement via nanolayering.

Spontaneous polarization and inversion symmetry breaking in ferroelectric materials lead to their use as photovoltaic devices. However, further advancement of their applications are hindered by the paucity of ways of reducing bandgaps and enhancing photocurrent. By unravelling the correlation between ferroelectric materials' responses to solar irradiation and their local structure and electric polarization landscapes, here we show from first principles that substantial bulk photovoltaic effect enhancement can be achieved by nanolayering PbTiO3 with nickel ions and oxygen vacancies ((PbNiO2)x(PbTiO3)(1-x)).

Asymmetric Response of Ferroelastic Domain-Wall Motion under Applied Bias.

The switching of domains in ferroelectric and multiferroic materials plays a central role in their application to next-generation computer systems, sensing applications, and memory storage. A detailed understanding of the response to electric fields and the switching behavior in the presence of complex domain structures and extrinsic effects (e.g.

Ultrafast Terahertz Gating of the Polarization and Giant Nonlinear Optical Response in BiFeO3 Thin Films.

Terahertz pulses are applied as an all-optical bias to ferroelectric thin-film BiFeO3 while monitoring the time-dependent ferroelectric polarization through its nonlinear optical response. Modulations in the intensity of the second harmonic light generated by the film correspond to on-off ratios of 220× gateable on femtosecond timescales. Polarization modulations comparable to the built-in static polarization are observed.