David Anthony Muller

Thiospinel nanoparticles are promising catalytic materials due to their compositional tunability, electronic conductivity, and structural stability. While significant gains in catalysis have been made through material engineering, one continuously developing route is through the use of support materials to enhance performance. Here, we report a simple, scalable, low-temperature method to coproduce colloidal multimetal nanocrystals (<20 nm in diameter) and In2S3-based nanosheets (<1 nm thick), forming compositionally complex hybrid nanocrystal–nanosheet heterostructures in situ. The in situ-formed heterostructures demonstrate enhanced catalytic activity in the oxygen evolution reaction (OER) in alkaline conditions; peak performance was achieved in heterostructures formed between (Ni,Co,Fe)3S4 nanocrystals and (In,Fe,Mn)2S3 nanosheets with overpotentials as low as 283 mV at 10 mA/cm2. Heterostructures can also be produced ex situ by introducing nanosheets, postsynthetically, to the nanocrystals, reducing the overpotential of (Ni,Co,Fe)3S4 nanocrystals from 362 mV to 312 mV with only a 5 wt % addition. This improvement highlights synergistic interactions in the heterostructures. These results suggest that an optimum nanosheet loading that maximizes effective nanocrystal–nanosheet interfacial coupling while preserving access to the Ni/Co-rich catalytic domains is the relevant design parameter. Our findings provide a low-temperature, colloidal route for synthesizing compositionally complex and highly uniform (<15% dispersity) sulfide nanocrystals, nanosheets, or heterostructures of the two with tunable structures and enhanced electrocatalytic performance. To our knowledge, this is the first report of a compositionally complex hybrid material that is able to enhance performance when formed both in situ or ex situ and the use of In2S3 as a scaffold for a support material for enhancing OER performance.

Two-dimensional (2D) carbon allotropes have unique properties, synthetic tunability, and potential applications across several fields including catalysis, energy storage, and electronics. One such allotrope is the recently discovered graphullerite, which results from the oxidation and deintercalation of the layered rhombohedral fullerene polymer (Mg4C60)∞. The current synthetic approach to (Mg4C60)∞ is limited by low yield, phase impurity, and small crystal size, all of which preclude a comprehensive investigation of the physical and electronic properties of bulk graphullerite and its 2D derivative, graphullerene. Here, we demonstrate two chemical vapor transport methods that produce rhombohedral (Mg4C60)∞ and tetragonal (Mg2C60)∞ as millimeter-sized single crystals with near-complete conversion. Our methods spatially separate the starting materials to circumvent both the thermal decomposition of (Mg4C60)∞ and the significant disparity in vapor pressure between C60 and Mg. We find that using Mg foil results in a near-complete conversion of C60 into a mixture of (Mg4C60)∞ and (Mg2C60)∞. Replacing the foil with Mg2Si produces phase-pure (Mg4C60)∞ with an isolated crystalline yield of 55 to 62%. This high-yielding, phase-pure synthesis enables us to investigate the electrochemical intercalation and deintercalation of Mg2+ into the lattice.

Anion exchange membrane (AEM) fuel cells have gained attention in recent decades as they do not rely on platinum-group-metal catalysts, offering a potentially cheaper alternative to commercially used proton exchange membrane (PEM) fuel cells. Block copolymers have been widely leveraged in the pursuit of high-performance AEMs, as their ability to self-assemble affords well-defined morphologies that promote efficient ionic transport and mechanical integrity. Characterization techniques including conventional transmission electron microscopy and ensemble-averaged X-ray scattering have been crucial components in investigating AEM and PEM nanoscale morphology. These techniques, however, cannot definitively identify the location or architecture of ionic channels in the membranes. Here, we resolve the nanoscale ionic channel networks within ABC triblock copolymer AEMs with 2D-continuous and 3D- co -continuous microphase-separated morphologies by applying cryogenic electron energy loss spectroscopy in the scanning transmission electron microscope. This approach overcomes previous imaging limitations including surface artifacts in cross-sectioned samples and the need for elemental staining in low-contrast polymer materials. We investigate architectural properties including connectivity and tortuosity of ionic channel networks within 2D cross-sections, and their correlation with conductivity and mechanical durability. Our results demonstrate that both connectivity and tortuosity are critical design parameters that should be considered in the synthesis of AEMs.

For more than a century, the lattice thermal conductivity of crystalline solids has been known to decrease with increasing temperature, as the phonons that carry heat scatter more strongly via Umklapp processes. Here we report a pronounced violation of this rule in the layered charge-density-wave (CDW) metal 2H-TaSe₂. Its in-plane thermal conductivity is strongly nonmonotonic: it peaks sharply at the 122 K ordering transition, collapses to a deep minimum near 210 K, and then anomalously reverses, rising with temperature to at least 350 K. Such a high-temperature reversal, in which a crystal conducts heat more efficiently as it is heated, is rare in crystalline solids. Its thermal conductivity minimum lies deep in the disordered phase well above the ordering transition, unlike the weak, transition-centered anomalies of other charge-density-wave systems, pointing to heat conduction governed by fluctuating rather than established order. Combining inelastic X-ray scattering, selected-area electron diffraction, and synchrotron X-ray diffraction, we attribute the reversal to phonons scattering off persistent, fluctuating charge order that survives more than 150 K above the long-range transition, to at least 300 K. A phenomenological phonon–CDW scattering model, tied directly to the measured diffraction linewidth and intensity, quantitatively reproduces the entire nonmonotonic curve. We also reveal that the charge order melts not abruptly but through a fluctuation- and defect-driven, weak first-order process, leaving short-range correlations far above the transition. This work establishes fluctuating charge order as a sensitive control knob for thermal transport in quantum materials and thermal conductivity as a powerful probe of the hidden charge order.

We report high-current-density AlGaN pn diodes grown on bulk AlN substrates by molecular beam epitaxy. AlGaN layers with linearly graded Al composition between 70% and 95% are utilized for both n- and p-type regions. The dopant-free pn junction depletion region is accessed by impurity doped n- and p-contact layers. The diodes exhibit a low specific on-resistance of 0.14 mΩ cm2 and achieve current densities, J exceeding 20 kA/cm2 at 10 V, representing one of the lowest resistances reported for high Al-content AlGaN pn diodes. Capacitance–voltage measurements yield a built-in potential of 5.5 V and carrier concentrations of the order of 1018 cm−3 without impurity doping. Enabled by the low ideality factor ηmin = 1.64 of the diodes, analytical models for diffusion and Shockley–Read–Hall recombination current density could be fitted to the experimental J–V. Electroluminescence measurements reveal deep-ultraviolet emission with a peak wavelength of 240 nm, highlighting radiative recombination within high Al-content layers. These results demonstrate the potential of fully polarization-doped AlGaN pn junctions for high-power and high-frequency ultrawide-bandgap electronic and optoelectronic devices.

A major challenge in determining the physics of helimagnetic SrFeO3 is stabilizing the stoichiometric chemical phase over long enough time scales to conduct extensive measurements. Degradation in SrFeO3 manifests mainly as a crossover from metallic to insulating behavior. Using a combination of electronic transport and density functional theory, we show that this degradation is dominated by oxygen loss, possibly on the order of one percent. We further demonstrate that high quality SrFeO3 thin films can be stabilized long-term by combining a nanoscale band insulator capping layer with an ex situ ozone anneal. We show that this produces a nearly pristine cation sublattice and preserves metallicity for at least several weeks. These results establish a reliable pathway for producing chemically stable SrFeO3 thin films, enabling reproducible studies of its unusual helimagnetism.

Magnetic skyrmions are promising candidates for next‐generation spintronic memory and logic due to their nanoscale size and topological stability. However, skyrmion‐based devices remain limited by narrow operating temperature windows that typically exclude cryogenic regimes critical for quantum‐hybrid architectures. Here, we demonstrate that ion‐beam‐engineered disorder landscapes in B20‐phase FeGe thin films dramatically extend the temperature window over which the topological Hall‐like signal is observed, from in pristine films down to liquid‐helium temperatures () in the irradiated samples. The disorder‐broadened topological Hall phase exhibits doubled signal amplitude compared to that of the pristine films, consistent with enhanced chiral spin‐texture density. Simultaneously, we achieve systematic tunability of the anomalous Hall response by transitioning between distinct scattering regimes via ion‐beam modification. Scanning transmission electron microscopy and electron ptychography reveal that nanoscale point‐defect clusters could drive these emergent properties. These findings establish defect‐landscape engineering as a versatile strategy to extend the stability of the topological Hall phase, offering a general framework for tailoring topological spin textures in next‐generation cryogenic spintronic architectures.

Aqueous acidic batteries utilizing corrosion-resistant Sn metal anodes have emerged as promising candidates for safe energy storage systems. However, uncontrolled dendritic growth under high-rate operation remains a major obstacle. Here we showed that artificial layers acting solely as a physical barrier are insufficient to regulate Sn deposition. We instead introduced an active-site-rich porous carbon (PC) layer with abundant N-defect sites that simultaneously governs the interfacial charge distribution and Sn nucleation. This architecture enabled highly uniform Sn deposition both within the PC layer and on the underlying substrate. Crucially, we identified the formation of an H3O+-enriched electric double layer in acidic electrolytes as the key factor driving Sn nanoparticle deposition within the carbon layer. Consequently, this artificial layer strategy significantly improved the reversibility and stability of Sn anodes, achieving a high average Coulombic efficiency of 99.72% for 16,000 cycles in asymmetric cells and maintaining 83.5% capacity retention after 20,000 cycles in pouch cells.