Tandem catalysis involves two or more catalysts arranged in proximity within a single reaction vessel, with the aim of synergistically aligning the catalysts' reaction pathways to maximize overall system performance. This study presents a proof of concept showing the integration of continuum transport modeling with design optimization in a simplified two-dimensional flow reactor setup for electrochemical CO2 reduction. Ag catalysts provide the CO2 -> CO reaction capability, and Cu catalysts provide the CO -> high-value products reaction capability. Given a set of input parameters, the optimization algorithm uses adjoint methods to modify the Ag/Cu surface patterning in order to maximize the current density toward high-value products, such as ethylene. The optimized designs yield significant performance enhancement especially at more negative applied voltages (i.e., stronger surface reactions) and for larger numbers of patterning sections. For an applied voltage of -1.7 V vs. SHE, the 12-section optimized design increases the current density toward ethylene by up to 65% compared to the unoptimized 2-section design. For the optimized cases, observed differences in the production and consumption of CO (the key intermediate species) and minimized zones of low CO reactant surface concentration on Cu sections explain the improved reactor performance.

Defect-driven phase instability critically influences the structural reliability of ultrawide bandgap oxides, yet direct nanoscale metrics linking local chemistry to structural transformation remain limited. Here, we introduce a coordination-sensitive atom probe tomography framework that quantitatively resolves reductions in local cation coordination and links them directly to defect-driven phase transformation. Using Si-doped beta-(AlxGa1-x)2O3 heterostructures with controlled Al composition (6-17

We show with hybrid density functional theory calculations that chalcogen donors other than oxygen (i.e. SN, SeN, and TeN) give rise to deep donor states in aluminum nitride. These donors trap a localized electron in their neutral charge state, leading to deep (+/0) donor levels that are 0.45 eV or more from the conduction-band edge. As such, this behavior is distinct from theDXbehavior leads to deep (+/-) levels which affects other donors such as ONand SiAl. We highlight how these results hint at the formation of small electron polarons in AlN, which are found to be unstable in the bulk, but metastable when bound to donor dopants like SiAland the chalocogens, with activation energies on the order of 0.2-0.3 eV. These results indicate that S, Se, and Te are not shallow donor dopants in aluminum nitride and identify origins of the experimentally observed ∼200-300 meV activation energies for dopant activation in donor-doped samples.

The electron self-interaction problem in density functional theory affects the accurate modeling of polarons, particularly their localization and formation energy. Charged and neutral density functional formulations have been developed to address this issue, yet their relationship remains unclear. Here, we demonstrate their equivalence in treating the many-body self-interaction of the polaron state. In particular, we connect with each other piecewise-linear functionals based on adding an extra charge to the supercell, the pSIC approach derived from the energetics of the neutral defect with polaronic distortions in a supercell, and the unit-cell method for polarons based on electron-phonon couplings. We show that these approaches lead to the same formal expression of the self-interaction corrected energy, which is fully defined by the energetics of the neutral charge state of the charged polaronic structure. Residual differences between these methods solely arise from the achieved polaronic structure, which is affected by different treatments of electron-screening and finite-size effects. We apply these methods to a set of prototypical small hole and electron polarons, including the hole polaron in MgO, the hole polaron in $β$-Ga$_2$O$_3$, the $V_\text{k}$ center in NaI, the electron polaron in BiVO$_4$, and the electron polaron in TiO$_2$. We show that the ground-state properties of polarons obtained using charged and neutral density functional formulations are in excellent agreement.

Catalysis at solid-liquid interfaces underpins many energy technologies, yet ab initio simulations that capture interfacial dynamics remain prohibitively expensive. Here we introduce Open Catalyst 2025 (OC25), the largest dataset for solid-liquid interfaces. To demonstrate OC25-trained models as practical tools for electrocatalysis, we investigate CO dimerization on Cu surfaces, a key step in CO_2 electroreduction. Using large cells (>800 atoms) and enhanced sampling up to 7 ns - the largest explicit-solvent CO dimerization study to date - we compute free-energy profiles under varied surface charge, cation identity, and surface facet. We find that dimerization is weakly sensitive to charge and cation identity, with appreciable stabilization only at the most negative charge densities, while extension to stepped Cu(310) reveals a more favorable pathway at modest reducing potentials. Our results demonstrate that OC25-trained models provide a scalable tool for investigating electrocatalytic transformations at solid-liquid interfaces, enabling simulations orders of magnitude beyond ab initio methods.

Defect spinel gamma-Ga2O3 is the least stable polymorph of Ga2O3, so its frequent appearance as a structural defect within or on the surface of monoclinic beta-Ga2O3 remains a mystery. Through first-principles calculations, we explore potential pathways for the phase transition from beta-Ga2O3 to gamma-Ga2O3, and examine two key driving forces: tensile strain and Ga deficiency. When configurational entropy contributions to phase energies are included, the gamma phase becomes energetically competitive with the beta phase, with the free energy difference between these phases diminishing even further under Ga-deficient conditions. Notably, a stability crossover occurs at room temperature at high vacancy concentrations ([V-Ga(3-)]>3%). A simple model beta ->gamma transformation pathway is identified, comprising two primary reactions, that enables the formation of the gamma phase via simultaneous migration of Ga atoms from tetrahedral lattice sites to octahedral interstitial positions. The transformation barriers are prohibitively large in pristine Ga2O3, but can be substantially reduced by: (1) the presence of Ga vacancies, (2) elongational strains along the crystallographic a-axis, and (3) when volumetric relaxations are possible during transformation. These results elucidate prior experimental observations, where gamma-Ga2O3 is seen on damaged surfaces or in highly n-type beta-Ga2O3 environments, which support Ga deficiency and mechanical strain. The insights into the driving forces and mechanisms of gamma-Ga2O3 formation enhance understanding of how localized strain and nonequilibrium defect concentrations may facilitate its formation from the beta phase.

Copper-based nanoparticles are key electrocatalysts for CO2 electrochemical reduction (CO2 ER) to liquid fuels and other value-added products. However, the copper catalyst can undergo rapid electrochemical corrosion, leading to a loss of catalyst material, fluctuations in the reaction conditions and increasing operational costs. We establish a mechanistic understanding of this detrimental process using in situ electrochemical electron microscopy and density functional theory (DFT). We find that copper corrosion can occur in the presence of CO2 in electroless conditions and before the onset potentials required for CO2 ER. The effects are isolated from pH changes resulting from dissolved CO2. Particles of corroded copper have oxidized surfaces, in contrast to copper surfaces exposed to CO2-free electrolytes. DFT calculations identify multiple routes by which CO2 can behave as a dissolution agent for copper and copper-oxide surfaces and suggest that formate-intermediates are a key driver of corrosion. This study highlights microenvironment-based factors that affect copper performance and degradation, facilitating strategies to inhibit and reverse copper degradation during CO2 ER.

Solar thermochemical hydrogen (STCH) production uses concentrated sunlight to produce hydrogen using reduction/oxidation of metal oxides. Typically, the metal oxide is heated to high temperature (>1400 o C) causing it to release oxygen, then it is cooled to a lower temperature (~<1000 o C) in steam whereby it re-oxidizes, stripping oxygen from water molecules and producing hydrogen. Due to improved stability, non-stoichiometric oxides that do not change phase during the STCH process are typically used, even though the reversible oxygen content is less than phase changing materials. In this presentation, recent progress in developing and experimentally validating new water splitting materials using a defect Graph Neural Network trained on crystal structures derived from density functional theory will be discussed. An experimental screening protocol used to evaluate predicted materials will be presented. Additionally, durability challenges and pathways to testing in reactors on-sun that meet DOE hydrogen production targets will be discussed. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. Part of the work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under contract no. DE-AC52-07NA27344.

The intensity of red Cr3+ photoluminescence (PL) in monoclinic gallium oxide (3-Ga2O3) is suppressed by n-type conductivity, an effect that has been attributed to a Cr deep acceptor level in the bandgap. In n-type material, such an acceptor level would be occupied, resulting in the Cr2+ oxidation state and the absence of Cr3+ PL. To test this model, n-type 3-Ga2O3 crystals co-doped with Cr and Zr (a donor) were grown from the melt. The samples show Cr3+ optical absorption bands and a high free-electron concentration of 4 x 1018 cm-3. If Cr were an acceptor, then it would be fully compensated and therefore would not exhibit the Cr3+ optical signature. Hybrid functional calculations indicate that Cr occupies the substitutional octahedral Ga(II) site and that the Cr2+ state is energetically unfavorable, i.e., Cr is not an acceptor. Weak Cr3+ PL was observed in the Cr/Zr co-doped samples. The quenching of PL may be caused by a transfer of energy to free electrons, a nonradiative process that would reduce the emission intensity.

β-Ga_2O_3 is a leading ultra-wide band gap semiconductor, but its performance depends on precise control over dopant incorporation and stability. In this work, we use first-principles calculations to systematically assess the diffusion behavior of eight potential deep-level substitutional acceptors (Au, Ca, Co, Cu, Fe, Mg, Mn, and Ni) in β-Ga_2O_3. We consider two key diffusion mechanisms: (i) interstitial diffusion under non-equilibrium conditions relevant to ion implantation, and (ii) trap-limited diffusion (TLD) under near-equilibrium thermal annealing conditions. Our results reveal a strong diffusion anisotropy along the b and c axes, with dopant behavior governed by competition between diffusion and incorporation (or dissociation) activation energies. Under interstitial diffusion, Ca^2+_i and Mg^2+_i show the most favorable combination of low migration and incorporation barriers, making them promising candidates for efficient doping along the b and c axes, respectively. In contrast, Au^+_i diffuses readily, but exhibits an incorporation barrier that exceeds 5 eV, rendering it ineffective as a dopant. From a thermal stability perspective, Co^2+_i shows poor activation but high diffusion barriers, which may suppress undesirable migration at elevated temperatures. Under trap-limited diffusion, the dissociation of dopant-host complexes controls mobility. Mg^2+_i again emerges as a leading candidate, exhibiting the lowest dissociation barriers along both axes, whereas Co^2+_i and Fe^2+_i display the highest barriers, suggesting improved dopant retention under thermal stress. Our findings guide dopant selection by balancing activation and thermal stability, essential for robust semi-insulating substrates.

Correction for “Atomate2: modular workflows for materials science” by Alex M. Ganose et al., Digital Discovery, 2025, 4, 1944–1973, https://doi.org/10.1039/D5DD00019J.

Compensating deep level defect states that influence the electrical properties of beta-phase gallium oxide (beta-Ga2O3) can be introduced intentionally through doping, or unintentionally through unwanted impurities and intrinsic defects. Understanding the complex behavior of these compensating centers is essential for optimizing beta-Ga2O3 for both high voltage and RF electronics. Applications in harsh environments, such as in space, add further complications due to defect creation resulting from high energy irradiation. Due to its inherent n-type conductivity, compensating deep acceptor states intentionally introduced by doping with Nitrogen, Iron or Magnesium to create semi-insulating regions of device structures are of great interest. However, inadvertent impurities such as Carbon are also predicted to create deep acceptors and can be problematic. The presence of intrinsic defects that form during growth or introduced by harsh environments makes the understanding of how defects impact material and device properties very complex. Here we use deep level transient (thermal) spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) to identify, compare and characterize individual deep acceptor states created by both extrinsic and intrinsic sources in beta-Ga2O3 and compare with theoretical predictions. Compensation efficiencies between acceptor choices are compared, and some of the more unusual behaviors of very deep states present below midgap are described, where both conduction and valence band transitions are observed and explained. Radiation studies used to differentiate extrinsic from intrinsic sources will be discussed in the context of unraveling the comprehensive impacts of defects in beta-Ga2O3.

Electrochemical CO2 reduction using copper catalysts is a promising approach to convert CO2 to higher-value fuels and chemicals, such as ethylene and ethanol, using renewable electricity. However, there is still significant room for improvement in the activity and selectivity of the desired products. CO2 electrolysis is a multiscale process, where atomistic, meso-scale, and cell-level phenomena are coupled together and need to be studied in tandem with one another. In this presentation, we present progress on our multiscale simulation approach, motivated by a custom flow cell that was recently designed to perform CO2 reduction with precise control over mass transport effects. First, we perform computational fluid dynamics (CFD) simulations of the flow cell with a simple passive scalar undergoing perfect reaction at the electrode to estimate the distribution of mass transport boundary layer thicknesses as a function of flow rate. Next, a one-dimensional (1D) simulation of the full electrochemical reactions is then conducted where the boundary layer thickness from CFD is prescribed. At the electrode, the CO2 reduction reactions are simulated using two separate microkinetic models: one that converts CO2 to CO, and one that converts CO to ethylene, ethanol, and methane; coupling between the two microkinetic models and the 1D mass transport simulation is done iteratively until convergence. By treating the microkinetic model in two parts, we can probe the role of intermediate CO on the overall reaction from CO2 to multi-carbon products. We conclude by discussing our preliminary comparisons between our multiscale simulation framework and experiments. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL release number: LLNL-ABS-871541.

One of the primary advantages of β-Ga2O3 over incumbent wide bandgap semiconductors is the ability to grow directly from the melt. Melt growth, using Czochralski or similar methods, results in impurities in the crystal which originate from the crucible, such as iridium and other transition metals like chromium. These impurities exhibit optoelectronic signatures useful for their identification and sensitive to the Fermi energy of a given crystal (i.e., signatures vary with the electrical conductivity of the matrix). In this work, we describe how laser Raman systems can be used to map and spatially correlate Cr3+ photoluminescence, electronic-coupled Raman scattering from Ir4+d–d internal transitions, and the Raman line attributed to hydrogenic shallow donors. Laser ablation inductively coupled plasma mass spectrometry directly measured spatially dependent relative metal concentrations and confirmed spectroscopic signals resulting from heterogeneities in impurity concentrations in β-Ga2O3 boules. Mapping of photoluminescence and Raman-related signatures is, thus, demonstrated as an effective and facile method for spatial measurement of chemical heterogeneities in both insulating and conductive melt-grown β-Ga2O3 crystals.

High-throughput density functional theory (DFT) calculations have become a vital element of computational materials science, enabling materials screening, property database generation, and training of "universal" machine learning models. While several software frameworks have emerged to support these computational efforts, new developments such as machine learned force fields have increased demands for more flexible and programmable workflow solutions. This manuscript introduces atomate2, a comprehensive evolution of our original atomate framework, designed to address existing limitations in computational materials research infrastructure. Key features include the support for multiple electronic structure packages and interoperability between them, along with generalizable workflows that can be written in an abstract form irrespective of the DFT package or machine learning force field used within them. Our hope is that atomate2's improved usability and extensibility can reduce technical barriers for high-throughput research workflows and facilitate the rapid adoption of emerging methods in computational material science.

The coherence of quantum dot qubits fabricated in semiconductors is often limited by charge noise from defects in gate dielectrics, which are material- and process-dependent. Characterizing these defects is an important step towards reducing their impact and improving qubit coherence. The identification of individual defects requires atomic-scale spatial resolution, however, and sufficient spectral sensitivity to determine their electronic structure. Electrostatic force microscopy (EFM) provides highly resolved maps of the surface potential of dielectrics, and importantly, is also sensitive to single-electron charging processes that reflect the spectral structure of underlying defects. In this work, we use cryogenic EFM to characterize aluminum oxide grown by atomic layer deposition (ALD) on bulk silicon. These measurements reveal defects close to the surface that exchange electrons with the EFM tip as they transition through different charge states. Detailed electrostatic modeling opens the door to powerful techniques for mapping tip-backgate charging voltages onto defect transition energies, allowing defects such as aluminum vacancies, and carbon, oxygen, or hydrogen impurities to be identified, by comparing to density functional theory (DFT). These results point towards EFM as a powerful tool for exploring defect structures in solid-state qubits.

Here we present a route towards designing transition metal oxide photocatalysts that offers tunability and suppression of photoexcited polarons. The coupling of photoexcited charge carriers with phonons in the crystal lattice can enable long timescale charge separation, but also causes carrier localization that results in limited carrier transport, which ultimately reduces device performance. Understanding the material properties that dictate the formation of photoexcited polarons in these highly polar photocatalysts would enable tunability of these properties for more efficient devices. Models that predict polaron formation and transport are limited to ground-state predictions and cannot accurately predict excited state dynamics, which limits exploration of new photocatalysts. Transient extreme ultraviolet (XUV) spectroscopy can probe element-specific, core-to-valence electronic-structural dynamics to provide insight into the nature of polaron formation in different transition metal oxide species. Here, we explore the phase space of polaron formation by measuring quantities related to the Hubbard-Holstein Hamiltonian. Specifically, we measured the excited state polaron dynamics in ErFeO 3 , where strong electron correlations and structural distortions slow polaron formation and compare it to GdFeO 3 where the effect of strong spin correlations suppress photoexcited polaron formation. We also consider the comparison between correlation effects in Fe(II) and Fe(III) oxides. The measurements of a range of transition metal oxide photocatalysts emphasize the importance of considering dynamic electron and spin correlations, in addition to lattice geometry, for tuning polarons. These measurements demonstrate pathways towards discovering new photocatalysts by providing a fundamental understanding of the underlying characteristics that dominate polaron formation and transport dynamics.

Cu-based catalysts for the electrochemical reduction of CO2 and CO exhibit a perplexingly unique reactivity toward multicarbon based products compared to other studied electrocatalysts. Here we use insights gained from a recent phenomenological 3-site microkinetic model and grand-canonical density functional theory calculations to clarify the importance of an underemphasized aspect critical to Cu's unique reactivity: a population of so-called "reservoir" sites. Using model Cu surface motifs, we discuss how these types can be represented by undercoordinated structural defects like step edges and grain boundaries which form a network of highly anisotropic migration channels. These pathways are found to be amenable for feeding *CO over time to reactive sites like Cu adatoms more active toward C-C coupling. These results highlight an often overlooked aspect of catalyst optimization: reservoir site engineering, which exploits surface mobility and presents an equally important avenue for electrocatalyst engineering oversimply maximizing active site densities.