Phase-Field Model of Electronic Antidoping.

Charge carrier doping usually reduces the resistance of a semiconductor or insulator, but was recently found to dramatically enhance the resistance in certain series of materials. This remarkable antidoping effect has been leveraged to realize synaptic memory trees in nanoscale hydrogenated perovskite nickelates, opening a new direction for neuromorphic computing. To understand these phenomena, we formulate a physical phase-field model of the antidoping effect based on its microscopic mechanism and simulate the voltage-driven resistance change in the prototypical system of hydrogenated perovskite nickelates. Remarkably, the simulations using this model, containing only one adjustable parameter whose magnitude is justified by first-principles calculations, quantitatively reproduce the experimentally observed treelike resistance states, which are shown unambiguously to arise from proton redistribution-induced local band gap enhancement and carrier blockage. Our work lays the foundation for modeling the antidoping phenomenon in strongly correlated materials at the mesoscale, which can provide guidance to the design of novel antidoping-physics-based devices.

Simulation of Electrochemical Oxidation in Aqueous Environments under Applied Voltage Using Classical Molecular Dynamics.

Reactive molecular dynamics (MD) simulations of metal electrodes under an applied voltage in an explicit water environment were performed and compared to predictions from both other calculations and simulations and experimental measurements and observations. MD simulations using the third-generation charge-optimized many body (COMB3) potentials and the electrode COMB (eCOMB) approach allow for the simulation of an externally applied voltage by modifying the equations of motion during the charge equilibration step (QEq) of the MD simulation. Unlike previous work, which prevented charge transfer between the water and metal electrodes, this work coupled the water and metal together through the QEq, which leads to an accumulation of a negative charge on the water and a positive charge on the metal before any voltage is applied. The extent of this charge accumulation is characterized, and strategies to mitigate it are explored. Root mean square deviation plots of the metal surfaces are created for each simulation to compare the extent of oxidation and dissolution. Good agreement is found between the oxidation behavior of the simulated metal surfaces and the experimental observations.

Band Gap Narrowing in a High-Entropy Spinel Oxide Semiconductor for Enhanced Oxygen Evolution Catalysis.

High-entropy oxides (HEOs), which contain five or more metal cations that are generally thought to be randomly mixed in a crystalline oxide lattice, can exhibit unique and enhanced properties, including improved catalytic performance, due to synergistic effects. Here, we show that band gap narrowing emerges in a high-entropy aluminate spinel oxide, (Fe0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4 (A5Al2O4). The 0.9 eV band gap of A5Al2O4 is narrower than the band gaps of all parent spinel oxides. First-principles calculations for multicomponent AAl2O4 spinels indicate that the band gap narrowing arises from the broadening of the energy distribution of the 3d states due to variations in the electronegativities and crystal field splitting across the 3d transition-metal series. As a catalyst for the oxygen evolution reaction in an alkaline electrolyte, A5Al2O4 reaches a current density of 10 mA/cm2 at an overpotential of 400 mV, outperforming all of the single-metal end members at an applied potential of 1.7 V vs RHE. Catalyst deactivation occurs after 5 h at 10 mA/cm2 and is attributed, based on elemental analysis and grazing-incidence X-ray diffraction, to the formation of a passivating layer that blocks the high-entropy oxide surface. This result helps to validate that the HEO is the active catalyst. The observation of band gap narrowing in A5Al2O4 expands the scope of synergistic properties exhibited by high-entropy materials and offers insight into the question of how the electronic structure of multicomponent oxide materials can be engineered via a high-entropy approach to achieve enhanced catalytic properties.

Data-driven analysis of the electronic-structure factors controlling the work functions of perovskite oxides.

Tuning the work functions of materials is of practical interest for maximizing the performance of microelectronic and (photo)electrochemical devices, as the efficiency of these systems depends on the ability to control electronic levels at surfaces and across interfaces. Perovskites are promising compounds to achieve such control. In this work, we examine the work functions of more than 1000 perovskite oxide surfaces (ABO3) using data-driven (machine-learning) analysis and identify the factors that determine their magnitude. While the work functions of the BO2-terminated surfaces are sensitive to the energy of the hybridized oxygen p bands, the work functions of the AO-terminated surfaces exhibit a much less trivial dependence with respect to the filling of the d bands of the B-site atom and of its electronic affinity. This study shows the utility of interpretable data-driven models in analyzing the work functions of cubic perovskites from a limited number of electronic-structure descriptors.

Using C-DFT to develop an e-ReaxFF force field for acetophenone radical anion.

Increased electricity usage over the past several decades has accelerated the need for efficient high-voltage power transmission with reliable insulating materials. Cross-linked polyethylene (XLPE) prepared via dicumyl peroxide (DCP) cross-linking has emerged as the insulator of choice for modern power cables. Although DCP cross-linking generates the desired XLPE product in high yield, other by-products are also produced. One such by-product, acetophenone, is particularly intriguing due to its aromaticity and positive electron affinity. In this work, constrained density functional theory (C-DFT) was utilized to develop an e-ReaxFF force field suitable for describing the acetophenone radical anion. Initial parameters were taken from the 2021 Akbarian e-ReaxFF force field, which was developed to describe XLPE chemistry. Then, C-DFT geometry optimizations were performed wherein an excess electron was constrained to each atom of acetophenone. The resulting C-DFT energy values for the various electronic positions were added to the e-ReaxFF training set. Next, an analogous set of structures was energy-minimized using e-ReaxFF, and equilibrium mixture compositions for the two methods were compared at multiple temperatures. Iterative fitting against C-DFT energy data was performed until satisfactory agreement was achieved. To test force field performance, molecular dynamics simulations were performed in e-ReaxFF and the resulting electronic distributions were qualitatively compared to unconstrained-DFT spin density data. By expanding our e-ReaxFF force field for XLPE, namely, adding the capability to describe acetophenone and its interactions with an excess electron, we move one step closer to a comprehensive molecular understanding of XLPE chemistry in a high-voltage power cable.

Phase-Selective Solution Synthesis of Perovskite-Related Cesium Cadmium Chloride Nanoparticles.

All-inorganic metal halide perovskite-related phases are semiconducting materials that are of significant interest for a wide range of applications. Nanoparticles of these materials are particularly useful because they permit solution processing while offering unique and tunable properties. Of the many metal halide systems that have been studied extensively, cesium cadmium chlorides remain underexplored, and synthetic routes to access them as nanoscale materials have not been established. Here we demonstrate that a simple solution-phase reaction involving the injection of a cesium oleate solution into a cadmium chloride solution produces three distinct cesium cadmium chlorides: hexagonal CsCdCl3 and the Ruddlesden-Popper layered perovskites Cs2CdCl4 and Cs3Cd2Cl7. The phase-selective synthesis emerges from differences in reagent concentrations, temperature, and injection rates. A key variable is the rate at which the cesium oleate solution is injected into the cadmium chloride solution, which is believed to influence the local Cs:Cd concentration during precipitation, leading to control over the phase that forms. Band structure calculations indicate that hexagonal CsCdCl3 is a direct band gap semiconductor while Cs2CdCl4 and Cs3Cd2Cl7 have indirect band gaps. The experimentally determined band gap values for CsCdCl3, Cs2CdCl4, and Cs3Cd2Cl7 are 5.13, 4.91, and 4.70 eV, respectively, which places them in a rare category of ultrawide-band-gap semiconductors.

Optimized utilization of COMB3 reactive potentials in LAMMPS.

An investigation to optimize the application of the third-generation charge optimized many-body (COMB3) interatomic potential and associated input parameters was carried out through the study of solid-liquid interactions in classical molecular dynamics simulations. The rates of these molecular interactions are understood through the wetting rates of water nano-droplets on a bare copper (111) surface. Implementing the Langevin thermostat, the influence of simulation time step, the number of atoms in the system, the frequency at which charge equilibration is performed, and the temperature relaxation rate are all examined. The results indicate that time steps of 0.4 fs are possible when using longer relaxation times for the system temperature, which is almost double the typical time step used for reactive potentials. The use of the charge equilibration allows for a fewer atomic layers to be used in the Cu slab. In addition, charge equilibrium schemes do not need to be performed every time step to ensure accurate charge transfer. Interestingly, the rate of wetting for the nanodroplets is dominantly dependent on the temperature relaxation time, which is predicted to significantly change the viscosity of the water droplets. This work provides a pathway for optimizing simulations using the COMB3 reactive interatomic potential.

Effects of surface charge and cluster size on the electrochemical dissolution of platinum nanoparticles using COMB3 and continuum electrolyte models.

We study the site-dependent dissolution of platinum nanoparticles under electrochemical conditions to assess their thermodynamic stability as a function of shape and size using empirical molecular dynamics and electronic-structure models. The third-generation charge optimized many-body potential is employed to determine the validity of uniform spherical representations of the nanoparticles in predicting dissolution potentials (the Kelvin model). To understand the early stages of catalyst dissolution, implicit solvation techniques based on the self-consistent continuum solvation method are applied. It is demonstrated that interfacial charge and polarization can shift the dissolution energies by amounts on the order of 0.74 eV depending on the surface site and nanoparticle shape, leading to the unexpected preferential removal of platinum cations from highly coordinated sites in some cases.

Understanding the influence of defects and surface chemistry on ferroelectric switching: a ReaxFF investigation of BaTiO3.

Ferroelectric materials such as barium titanate (BaTiO3) have a wide range of applications in nano scale electronic devices due to their outstanding properties. In this study, we developed an easily extendable atomistic ReaxFF reactive force field for BaTiO3 that can capture both its field- and temperature-induced ferroelectric hysteresis and corresponding changes due to surface chemistry and bulk defects. Using our force field, we were able to reproduce and explain a number of experimental observations: (1) the existence of a critical thickness of 4.8 nm below which ferroelectricity vanishes in BaTiO3; (2) migration and clustering of oxygen vacancies (OVs) in BaTiO3 and a reduction in the polarization and the Curie temperature due to the OVs; (3) domain wall interaction with the surface chemistry to influence the ferroelectric switching and polarization magnitude. This new computational tool opens up a wide range of possibilities for making predictions for realistic ferroelectric interfaces in energy-conversion, electronic and neuromorphic systems.

Electrochemical stability and light-harvesting ability of silicon photoelectrodes in aqueous environments.

We study the factors that affect the photoactivity of silicon electrodes for the water-splitting reaction using a self-consistent continuum solvation model of the solid-liquid interface. This model allows us to calculate the charge-voltage response, Schottky barriers, and surface stability of different terminations while accounting for the interactions between the charge-pinning centers at the surface and the depletion region of the semiconductor. We predict that the most stable oxidized surface does not have a favorable Schottky barrier, which further explains the low solar-to-hydrogen performance of passivated silicon electrodes.

Voltage effects on the stability of Pd ensembles in Pd-Au/Au(111) surface alloys.

The catalytic performance of multimetallic electrodes is often attributed to a beneficial combination of ligand, strain, and ensemble effects. Understanding the influence of the electrochemical environment on the stability of the alloy surface structure is thus a crucial component to the design of highly active and durable electrocatalysts. In this work, we study the effects of an applied voltage to electrocatalytic Pd-Au/Au(111) surface alloys in contact with a model continuum electrolyte. Using planewave density functional theory, two-dimensional cluster expansions are parameterized and used to simulate dilute Pd-Au surface alloys under electrochemical conditions via Metropolis Monte Carlo within an extended canonical ensemble. While Pd monomers are stable at all potentials considered, different extents of surface electrification are observed to promote the formation of Pd dimers and trimers, as well as clusters of Pd monomers. We find that the relative proportion of monomer, dimer, and trimer surface fractions is in good agreement with in situ scanning tunneling microscopy measurements. The further development and refinement of the approaches described herein may serve as a useful aid in the development of next-generation electrocatalysts.

Vibrational probe of the origin of singlet exciton fission in TIPS-pentacene solutions.

We use native vibrational modes of the model singlet fission chromophore 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) to examine the origins of singlet fission in solution between molecules that are not tethered by a covalent linkage. We use the C-H stretch modes of TIPS side groups of TIPS-Pn to demonstrate that singlet fission does not occur by diffusive encounter of independent molecules in solution. Instead, TIPS-Pn molecules aggregate in solution through their TIPS side groups. This aggregation breaks the symmetry of the TIPS-Pn molecules and enables the formation of triplets to be probed through the formally symmetry forbidden symmetric alkyne stretch mode of the TIPS side groups. The alkyne stretch modes of TIPS-Pn are sensitive to the electronic excited states present during the singlet fission reaction and provide unique signatures of the formation of triplets following the initial separation of triplet pair intermediates. These findings highlight the opportunity to leverage structural information from vibrational modes to better understand intermolecular interactions that lead to singlet fission.

First-principles photoemission spectroscopy and orbital tomography in molecules from koopmans-compliant functionals.

The determination of spectral properties from first principles can provide powerful connections between microscopic theoretical predictions and experimental data, but requires complex electronic-structure formulations that fall outside the domain of applicability of common approaches, such as density-functional theory. We show here that Koopmans-compliant functionals, constructed to enforce piecewise linearity and the correct discontinuity derivative in energy functionals with respect to fractional occupation-i.e., with respect to charged excitations-provide molecular photoemission spectra and momentum maps of Dyson orbitals that are in excellent agreement with experimental ultraviolet photoemission spectroscopy and orbital tomography data. These results highlight the role of Koopmans-compliant functionals as accurate and inexpensive quasiparticle approximations to the spectral potential.

Piecewise linearity and spectroscopic properties from Koopmans-compliant functionals.

Density-functional theory is an extremely powerful and widely used tool for quantum simulations. It reformulates the electronic-structure problem into a functional minimization with respect to the charge density of interacting electrons in an external potential. While exact in principle, it is approximate in practice, and even in its exact form it is meant to reproduce correctly only the total energy and its derivatives, such as forces, phonons, or dielectric properties. Quasiparticle levels are outside the scope of the theory, with the exception of the highest occupied state, since this is given by the derivative of the energy with respect to the number of electrons. A fundamental property of the exact energy functional is that of piecewise linearity at fractional occupations in between integer fillings, but common approximations do not follow such piecewise behavior, leading to a discrepancy between total and partial electron removal energies. Since the former are typically well described, and the latter provide, via Janak's theorem, orbital energies, this discrepancy leads to a poor comparison between predicted and measured spectroscopic properties. We illustrate here the powerful consequences that arise from imposing the constraint of piecewise linearity to the total energy functional, leading to the emergence of orbital-density-dependent functionals that (1) closely satisfy a generalized Koopmans condition and (2) are able to describe with great accuracy spectroscopic properties.

Chemical Environment and Structural Variations in High Entropy Oxide Thin Film Probed with Electron Microscopy.

We employ analytical transmission electron microscopy (TEM) to correlate the structural and chemical environment variations within a stacked epitaxial thin film of the high entropy oxide (HEO) Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O (J14), with two layers grown at different substrate temperatures (500 and 200 °C) using pulsed laser deposition (PLD). Electron diffraction and atomically resolved STEM imaging reveal the difference in out-of-plane lattice parameters in the stacked thin film, which is further quantified on a larger scale using four-dimensional STEM (4D-STEM). In the layer deposited at a lower temperature, electron energy loss spectroscopy (EELS) mapping indicates drastic changes in the oxidation states and bonding environment for Co ions, and energy-dispersive X-ray spectroscopy (EDX) mapping detects more significant cation deficiency. Ab initio density functional theory (DFT) calculations validate that vacancies on the cation sublattice of J14 result in significant electronic and structural changes. The experimental and computational analyses indicate that low temperatures during film growth result in cation deficiency, an altered chemical environment, and reduced lattice parameters while maintaining a single phase. Our results demonstrate that the complex correlation of configurational entropy, kinetics, and thermodynamics can be utilized for accessing a range of metastable configurations in HEO materials without altering cation proportions, enabling further engineering of functional properties of HEO materials.

High-entropy engineering of the crystal and electronic structures in a Dirac material.

Dirac and Weyl semimetals are a central topic of contemporary condensed matter physics, and the discovery of new compounds with Dirac/Weyl electronic states is crucial to the advancement of topological materials and quantum technologies. Here we show a widely applicable strategy that uses high configuration entropy to engineer relativistic electronic states. We take the AMnSb2 (A = Ba, Sr, Ca, Eu, and Yb) Dirac material family as an example and demonstrate that mixing of Ba, Sr, Ca, Eu and Yb at the A site generates the compound (Ba0.38Sr0.14Ca0.16Eu0.16Yb0.16)MnSb2 (denoted as A5MnSb2), giving access to a polar structure with a space group that is not present in any of the parent compounds. A5MnSb2 is an entropy-stabilized phase that preserves its linear band dispersion despite considerable lattice disorder. Although both A5MnSb2 and AMnSb2 have quasi-two-dimensional crystal structures, the two-dimensional Dirac states in the pristine AMnSb2 evolve into a highly anisotropic quasi-three-dimensional Dirac state triggered by local structure distortions in the high-entropy phase, which is revealed by Shubnikov-de Haas oscillations measurements.

Donor and acceptor levels of organic photovoltaic compounds from first principles.

Accurate and efficient approaches to predict the optical properties of organic semiconducting compounds could accelerate the search for efficient organic photovoltaic materials. Nevertheless, predicting the optical properties of organic semiconductors has been plagued by the inaccuracy or computational cost of conventional first-principles calculations. In this work, we demonstrate that orbital-dependent density-functional theory based upon Koopmans' condition [Phys. Rev. B, 2010, 82, 115121] is apt for describing donor and acceptor levels for a wide variety of organic molecules, clusters, and oligomers within a few tenths of an electron-volt relative to experiment, which is comparable to the predictive performance of many-body perturbation theory methods at a fraction of the computational cost.

Atomic-scale polarization switching in wurtzite ferroelectrics.

Ferroelectric wurtzites have the potential to revolutionize modern microelectronics because they are easily integrated with multiple mainstream semiconductor platforms. However, the electric fields required to reverse their polarization direction and unlock electronic and optical functions need substantial reduction for operational compatibility with complementary metal-oxide semiconductor (CMOS) electronics. To understand this process, we observed and quantified real-time polarization switching of a representative ferroelectric wurtzite (Al0.94B0.06N) at the atomic scale with scanning transmission electron microscopy. The analysis revealed a polarization reversal model in which puckered aluminum/boron nitride rings in the wurtzite basal planes gradually flatten and adopt a transient nonpolar geometry. Independent first-principles simulations reveal the details and energetics of the reversal process through an antipolar phase. This model and local mechanistic understanding are a critical initial step for property engineering efforts in this emerging material class.

koopmans: An Open-Source Package for Accurately and Efficiently Predicting Spectral Properties with Koopmans Functionals.

Over the past decade we have developed Koopmans functionals, a computationally efficient approach for predicting spectral properties with an orbital-density-dependent functional framework. These functionals impose a generalized piecewise linearity condition to the entire electronic manifold, ensuring that orbital energies match the corresponding electron removal/addition energy differences (in contrast to semilocal DFT, where a mismatch between the two lies at the heart of the band gap problem and, more generally, the unreliability of Kohn-Sham orbital energies). This strategy has proven to be very powerful, yielding molecular orbital energies and solid-state band structures with comparable accuracy to many-body perturbation theory but at greatly reduced computational cost while preserving a functional formulation. This paper reviews the theory of Koopmans functionals, discusses the algorithms necessary for their implementation, and introduces koopmans, an open-source package that contains all of the code and workflows needed to perform Koopmans functional calculations and obtain reliable spectral properties of molecules and materials.

Strong electron-phonon coupling driven pseudogap modulation and density-wave fluctuations in a correlated polar metal.

There is tremendous interest in employing collective excitations of the lattice, spin, charge, and orbitals to tune strongly correlated electronic phenomena. We report such an effect in a ruthenate, Ca3Ru2O7, where two phonons with strong electron-phonon coupling modulate the electronic pseudogap as well as mediate charge and spin density wave fluctuations. Combining temperature-dependent Raman spectroscopy with density functional theory reveals two phonons, B2P and B2M, that are strongly coupled to electrons and whose scattering intensities respectively dominate in the pseudogap versus the metallic phases. The B2P squeezes the octahedra along the out of plane c-axis, while the B2M elongates it, thus modulating the Ru 4d orbital splitting and the bandwidth of the in-plane electron hopping; Thus, B2P opens the pseudogap, while B2M closes it. Moreover, the B2 phonons mediate incoherent charge and spin density wave fluctuations, as evidenced by changes in the background electronic Raman scattering that exhibit unique symmetry signatures. The polar order breaks inversion symmetry, enabling infrared activity of these phonons, paving the way for coherent light-driven control of electronic transport.