David T. Limmer

Dialkyl esters, such as dibutyl sebacate (DBS), diethyl sebacate (DES), and diethyl adipate (DEA), are attractive, less-toxic alternatives to phthalate plasticizers, yet a mechanistic understanding of their behavior at soft interfaces, where wetting, adhesion, and physicochemical interactions are governed, remains limited. Resolving molecular orientation and organization at liquid–air interfaces is challenging with conventional techniques. Here, we combine sum frequency generation (SFG) vibrational spectroscopy with complementary infrared, Raman measurements, and molecular dynamics (MD) simulations to elucidate the interfacial structure of low-dielectric esters [DBS, DES, DEA, and diethyl oxalate (DEO)]. SFG reveals that terminal methyl group orientations vary substantially across the series, with DES exhibiting a pronounced tilt toward the surface plane relative to DBS, DEA, and DEO. The analysis of CH2/CH3 symmetric stretching intensity ratios indicates a higher population of gauche conformations and increased conformational disorder at the DES interface despite its shared sebacate backbone with DBS. MD simulations support these observations, showing differences in alkyl chain ordering, flexibility, and carbonyl–carbonyl separation, and yielding tilt angles broadly consistent with experiment for DES while highlighting sensitivity in SFG-derived values to optical and hyperpolarizability assumptions. Notably, the CH3 Fermi resonance ratio and surface tension follow similar trends (DES > DEA > DBS ≈ DEO) across the dialkyl ester series, linking enhanced conformational flexibility and intermolecular packing heterogeneity at the molecular scale to macroscopic interfacial properties. These results enhance our understanding of soft low-dielectric liquid interfaces more broadly.

Phase transitions in halide perovskites critically influence their optoelectronic performance and stability, yet the nanoscale pathways by which structural transformations proceed remain elusive because of their soft ionic lattice and electron-beam sensitivity. Here, we combine in situ heating with four-dimensional scanning transmission electron microscopy (4D-STEM) to investigate the phase-transition dynamics in single CsPbBrxI3–x nanowires, from the non-perovskite to the perovskite phase, with nanometer spatial and millisecond temporal resolution. We directly visualize and track the real-time propagation of the perovskite phase front along a nanowire, while mapping local crystallographic order and diffuse scattering during the transition. The emergence of amorphous diffraction rings indicates a breakdown of long-range crystalline order, and with molecular dynamics simulations, we can conclude that local atomic coordination is preserved. More broadly, this work establishes a general framework for resolving phase transition pathways in beam-sensitive materials beyond halide perovskites, with implications for stabilizing functional phases for future applications.

Solute-intercalation-induced phase separation creates spatial heterogeneities in host materials, a phenomenon ubiquitous in batteries, hydrogen storage, and other energy devices. Despite many efforts, probing intercalation processes at the atomic scale has been a significant challenge. By utilizing liquid-phase transmission electron microscopy (TEM), we study hydrogen (de)intercalation in palladium nanocrystals as a model system and have achieved unprecedented atomic-resolution imaging of hydrogen intercalation wave dynamics. Our observations reveal that intercalation wave mechanisms, instead of shrinking-core mechanisms, prevail at ambient temperature for palladium nanocubes ranging from ∼60 nm down to ∼10 nm. Systematic image analysis uncovers the atomic evolution of the hydrogen intercalation wave, transitioning from nonplanar and inclined boundaries to those closely aligned with {100} planes. Our kinetic Monte Carlo simulations demonstrate that the observed intercalation wave dynamics correspond to sorption pathways minimizing the lattice mismatch strain at the phase boundary. Unveiling the atomic intercalation pathways holds profound implications for engineering intercalation-mediated devices and advancements in energy sciences.

Atomic structures of nanomaterials are inherently dynamic and continuously reshaped through interactions with chemical species and external stimuli. Such dynamics are further amplified as the size and dimensionality of nanomaterials decrease. Despite advances in analytical methods, it remains challenging to capture the structural dynamics of nanomaterials in reactive environments with both atomic spatial resolution and commensurate temporal resolution. Here, we directly visualize atomic-scale dynamics of gold (Au) nanocrystals in reactive liquid environments with millisecond-speed liquid-cell electron microscopy (EM) and deep-learning denoising. We uncover reversible fluctuations in the local crystallinity of Au nanocrystals dependent on the surrounding chemical environment. These transient fluctuations, driven by interactions at nanocrystal-liquid interfaces, critically influence the dissolution kinetics and grain boundary relaxation. By overcoming the spatiotemporal limitations in conventional liquid-cell EM, our findings provide insights into how transient nanoscale structures dictate the stability and reactivity of nanomaterials.

Nanocrystals (NCs) serve as versatile building blocks for the creation of functional materials with NC self-assembly offering opportunities to enable novel material properties. Here, we demonstrate that PbS NCs functionalized with strongly negatively charged metal chalcogenide complex (MCC) ligands, such as Sn₂S₆^4- and AsS₄^3-, can self-assemble into all-inorganic superlattices with both long-range superlattice translational and atomic-lattice orientational order. Structural characterizations reveal that the NCs adopt an unexpected edge-to-edge alignment, and numerical simulation clarifies that orientational order is thermodynamically stabilized by many-body ion correlations originating from the dense electrolyte. Furthermore, we show that the superlattices of Sn₂S₆^4--functionalized PbS NCs can be fully disassembled back into the colloidal state, which is highly unusual for orientationally attached superlattices with atomic-lattice alignment. The reversible oriented attachment of NCs, enabling their dynamic assembly and disassembly into effectively single-crystalline superstructures, offers a pathway toward designing reconfigurable materials with adaptive and controllable electronic and optoelectronic properties.

Energy costs of information processing are growing exponentially. Bit erasure is a key problem in this energy-information nexus, and a number of seminal relationships have been deduced regarding the relationship between thermodynamic costs and memory storage. To continue making progress in the modern era, however, requires confronting thermodynamic costs in realistic physical systems, which operate away from equilibrium. Here, we explore the thermodynamic costs of bit erasure in a complementary metal-oxide-semiconductor model of two types of random-access memory. We find dynamic random-access memory dissipates the least amount of energy when operated in the quasistatic limit, where errors are also minimized. By contrast, static random-access memory is most efficiently operated in finite time due to the energy required to maintain the state of the bit. We demonstrate a numerically robust optimization scheme using mean field theory and automatic differentiation, finding optimal protocols compatible with electrical engineering insights. These results provide a framework for operating realistic circuits in thermodynamically advantageous ways.

Pathways and structural dynamics of phase transformations impact performance of materials in energy and information storage technologies. Palladium hydride (PdH_x) nanocrystals are an ideal model system for studying solute-induced phase transformations, where elastic energy from lattice mismatch between α-PdH_x and β-PdH_x phases is often considered a key to determining the transformation pathways. α/β-PdH_x interfacial elastic energy is affected by the confined geometry of a nanocrystal. However, how nanocrystal geometry influences phase transformation pathways is largely unknown. Using in situ liquid phase transmission electron microscopy, we directly visualize hydrogenation in Pd nanocrystals with two geometries, a nanocube and a hexagonal nanoplate. Both follow similar sequences of an initially curved nucleus, interface flattening, and reverse-stage nucleation; however, their evolving α/β-PdH_x interfaces exhibit geometry-dependent crystallographic alignments. In nanocubes, {100}-aligned configurations conform to static elastic energy ordering, representing a pathway that maintains a local mechanical equilibrium, whereas nanoplates display both {110}- and {211}-aligned interfaces. Theoretical simulations show that geometry determines the accessibility of alternative phase transformation pathways as the system is driven far from equilibrium during hydrogenation. These findings identify geometry as a fundamental parameter for directing phase transformation pathways, offering design principles for accessing atypical configurations and improving properties of intercalation-based devices.

We employ molecular dynamics simulations and quantum rate theories to elucidate the complex condensed-phase dynamics underpinning triplet-state formation in organic photosensitizers. Using models informed by first-principles calculations complete with a molecular representation of solvents of different polarities, we elucidate the interplay of the internal and environmental interactions underlying triplet yield. We find that triplet yields depend sensitively on the dielectric stabilization of the charge transfer intermediate that facilitates a transition into the triplet manifold. Our results illustrate the importance of molecularly detailed models in understanding the excited-state internal charge-transfer dynamics of photochemically relevant organic molecules.

We investigate ion pairing dynamics in electrolytes driven far from equilibrium using molecular simulations and nonequilibrium rate theory. Focusing on 0.5M LiPF6 in water and acetonitrile under uniform electric fields, we compute transition path theory observables, including reactive fluxes and mean first-passage times of ion pairing. Moreover, we introduce a dynamical proxy of free-ion population, where its field-induced change is strongly correlated with the nonlinear enhancement of conductivity, yielding an increase of 40% at 50 mV/Å in acetonitrile, compared to that of less than 10% in aqueous electrolytes. Further kinetic analysis elucidates that Onsager’s classical theory substantially overestimates field-induced enhancement of ion pair dissociation in molecular electrolytes. This discrepancy arises from solvent-mediated dynamical pathways and field-induced dielectric decrement that suppress ion pair dissociation within explicit solvents, highlighting that a faithful description of molecular details is essential. Our results provide a molecular interpretation of nonlinear electrolyte transport beyond continuum theories and establish a general framework for quantifying nonequilibrium reaction kinetics in condensed phase systems.

Electron transfer (ET) reactions underpin energy conversion and chemical transformations in both biological ^1,2 and abiological ^3–5 systems. The efficiency of any ET process relies on achieving a desired ET rate within an optimal driving force range. Marcus theory ^6,7 provides a microscopic framework for understanding the activation free energy—and therefore the rate—of ET in terms of a key parameter: the reorganization energy. For electrified solid–liquid interfaces, it has long been conventionally understood that only factors in the electrolyte phase are responsible for determining the reorganization energy and that the electronic density of states (DOS) of the electrode only serves to dictate the number of thermally accessible channels for ET ^5,8–12 . Here we show instead that the electrode DOS plays a central role in governing the reorganization energy, far outweighing its conventionally assumed role. Using atomically layered heterostructures, we tune the DOS of graphene and measure outer-sphere ET kinetics. We find the ensuing variation in ET rate arises from strong modulation in a reorganization energy associated with image potential localization in the electrode. Here we redefine the traditional paradigm of heterogeneous ET kinetics, revealing a deeper role of the electrode electronic structure in interfacial reactivity.