Controlling crystallographic registry is central to semiconductor heteroepitaxy, yet how epitaxial domains reorient and fluctuate once crystalline order first becomes detectable remains unresolved. Here, we use time-resolved transmission electron microscopy to simultaneously track the twist angle and coherently ordered area of individual MoS2 domains on graphene and the curved graphitic surfaces of carbon nanohorns, complemented by structural analyses spanning nanometre to submicrometre length scales. The smallest resolvable domains, with coherently ordered areas of approximately 1.5 nm2, preferentially adopt orientations near 30°. As their coherently ordered areas increase, individual domains progressively reorient towards 0° and their angular fluctuations diminish, revealing a size-dependent narrowing of the accessible orientational range. Under non-equilibrium formation conditions, however, near-30° orientations can persist to submicrometric dimensions, consistent with kinetic trapping of long-lived interfacial configurations. These observations establish coherent domain size as a key determinant of orientational preference and fluctuation amplitude, whereas kinetic pathways influence the persistence of non-equilibrium orientations. The crossover is consistent with finite-size competition between edge contributions and registry-dependent interfacial energetics, modulated by kinetic barriers. By resolving dynamic orientational pathways inaccessible to ensembleaveraged measurements, our results provide a mechanistic basis for twist-selective growth of twodimensional heterostructures.

While solution-phase supramolecular polymerization yields diverse organic semiconducting π-architectures, evaluating their intrinsic electronic properties at the single-assembly level in the solid state remains challenging. Here, we report the hierarchical supramolecular polymerization of a tetrabenzoporphyrin derivative into discrete pyramidal π-assemblies in solution and their subsequent integration into single-pyramid field-effect transistors. Kinetic and structural tuning during polymerization yielded macroscopic, easily handleable pyramids (> 50 µm in width) ideal for the device fabrication. Owing to the high dispersibility and structural stability of these pyramids, single-pyramid transistors were successfully fabricated via simple drop-casting and solvent evaporation, enabling the direct elucidation of individual p-type charge transport characteristics. This strategy unveils the intrinsic semiconductor properties of pre-organized supramolecular architectures, opening new avenues in supramolecular electronics.

An air- and moisture-stable, carbon-supported tungsten-dioxo single-site heterogeneous catalyst is more active and selective for alcohol dehydration than its Mo analog.

Diamond and adamantane (Ad) share a Td-symmetric carbon skeleton, but converting Ad to diamond has been challenging because it requires selective carbon-hydrogen (C-H) bond cleavage and monomer assembly into a diamond lattice. Our approach differs from the conventional high-temperature, high-pressure diamond syntheses. We electron-irradiated Ad submicrocrystals at 80 to 200 kilo-electron volts and 100 to 296 kelvin in vacuum for tens of seconds. This process yielded defect-free nanodiamonds (NDs) of cubic crystal structure, accompanied by hydrogen gas evolution. Time-resolved transmission electron microscopy revealed the initial formation of Ad oligomers transforming into spherical NDs. A sizable kinetic isotope effect indicates that C-H cleavage was rate-determining, and other hydrocarbons tested failed to form NDs.

Diamond and adamantane (Ad) share a Td-symmetric carbon skeleton, but converting Ad to diamond has been challenging due to selective C–H bond cleavage and monomer assembly into a diamond lattice. Electron irradiation (80–200 keV) of Ad sub-microcrystals in vacuum at 100 K for tens of seconds yielded defect-free nanodiamonds (NDs) of cubic crystal structure 2–4 nm in diameter, accompanied by hydrogen gas evolution. Time-resolved transmission electron microscopy revealed the initial formation of Ad oligomers transforming into spherical NDs. A sizable kinetic isotope effect confirmed C–H cleavage as rate-determining. Other hydrocarbons failed to form NDs, underscoring the unique suitability of Ad. This approach distinctively differs from the conventional high-temperature, high-pressure approach, exemplifying the value of controlled C–H activation for diamond synthesis.

De novo design of peptide nanoshapes is of great interest in biomolecular science since the local peptide nanoshapes formed by a short peptide chain in the proteins are often key to the biological activities. Here, we show that the de novo design of peptide nanoshapes with sub-nanometer conformational control can be realized using peptides consisting of N-methyl-L-alanine and N-methyl-D-alanine residues as studied by NMR, X-ray and XFEL crystallographic and computational analyses as well as by direct imaging of the dynamics of the peptide’s nanoshape using cinematographic electron microscopic technique. The conformation of N-methyl-L/D-alanine residue is largely fixed because of the restricted bond rotation, and hence can serve as a scaffold on which we can build a peptide into a designed nanoshape. The local shape control by per-residue conformational restriction by torsional strains starkly contrasts with the global shape stabilization of proteins based on many remote interactions. The oligomers allow the bottom-up design of diverse peptide nanoshapes with a small number of amino acid residues and would offer unique opportunities to realize the de novo design of biofunctional molecules, such as catalysts and drugs.

Traditionally, the study of polymorphism has relied on thermodynamics and mass-averaged measurements. This work introduces a novel approach by combining kinetic analysis and statistical mechanics with electron microscopic imaging to observe phase transitions directly. We demonstrate a remarkable impact of the crystal size on the kinetic stability of polymorphs at nanoscale domains, enabling in situ manipulation of phase transitions at 298 K through interface energy adjustments by size reduction. Starting with the B1 NaI polymorph, we synthesized the previously unknown B2 polymorph upon size reduction. Starting from the CsCl liquid phase, we produced B1, previously described only above 749 K, and then B2 via quick martensitic transformation.

Heterogeneous catalysts dominate the chemical industry but, unlike homogeneous catalysts, typically feature diverse, incompletely defined active sites. Thus, describing their structure-activity relationships remains challenging. In contrast, molecularly defined single-site heterogeneous catalysts (SSHCs) are poised to address these challenges and provide new avenues for catalysis research and development. The present study explores eco-friendly H2 production mediated by discrete MoO2 sites supported on carbon nano-horns (CNHs) and active for alcohol dehydrogenation. Although informative, detailed extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS,) kinetic measurements, and density functional theory (DFT) analysis alone cannot provide a full molecular picture of the reaction pathway. Here, using single-molecule atomic-resolution time-resolved electron microscopy (SMART-EM), we propose the identification of four key catalytic intermediates anchored to CNHs and uncover a new reaction pathway involving alkoxide/hemiacetal equilibration and acetal oligomerization. These intermediates are inferred through a combination of theory and SMART-EM, showcasing the potential of SMART-EM as a complementary tool for exploring mechanistic hypotheses in catalysis.

Well-defined heterostructures exhibit emergent properties distinct from their single-phase constituents, enabling advances across diverse technologies. Typically classified as self-assembly and epitaxy, heterointerface formation is generally assumed to proceed unidirectionally and irreversibly at bulk scales. Here we use in situ electron microscopy at 298 K to visualize the heterostructure formation from nanoscale mixtures of intrinsically immiscible salts at ambient conditions, NaCl and NaI. We find that both self-assembly and heteroepitaxy occur reversibly, governed by stochastic equilibria. During self-assembly, the system fluctuates between order and semiordered states, with kinetically favored iodide-graphene interactions and eventual formation of thermodynamically favored charge-neutral planes. The ions in the liquid state move rapidly. In the epitaxial regime, NaI layers repeatedly grow and exfoliate from a buffer layer. The observed motion of atoms driven by high interfacial energy occurs with an energy barrier of 18 kcal mol-1 or less as estimated from the Nyquist frequency of the frame rate of the movie. These observations reveal that heterointerface formation is governed by dynamic and stochastic processes, providing a framework for the understanding of the initiation stage of the heterointerface formation in high interface energy regimes such as surface and structural defects of bulk solids.

Phase diagrams and crystallography are standard tools for studying structural phase transitions, whereas acquiring kinetic information at the atomistic level has been considered essential but challenging. The η-to-θ phase transition of alumina is unidirectional in bulk and retains the crystal lattice orientation. We report a rare example of a statistical kinetics study showing that for nanoparticles on a bulk Al(OH)3 surface, this phase transition occurs nondeterministically through an ergodic equilibrium through the molten state, and the memory of the lattice orientation is lost in this process. The rate of the interconversion was found to be insensitive to the electron dose rate, and this process had a small Gibbs free energy of activation. These nondeterministic kinetics should be a key feature of crystal nucleation occurring in high-surface-energy regions of bulk crystals.

Chemical phenomena are inherently complex and stochastic, making them difficult to fully understand using conventional ensemble-averaged analytical methods. These methods primarily capture long-lived species and common structural features, limiting the study of transient intermediates and minute structural characteristics. In contrast, single-molecule time-resolved analysis using advanced microscopy techniques, particularly transmission electron microscopy, offers high spatial and temporal resolution to observe the nonequilibrium dynamics of molecules and their assemblies. This account discusses the authors' research on developing transmission electron microscopy techniques to visualize intricate and transient interactions within molecular systems, enhancing the understanding of chemical phenomena at atomic and molecular levels. Graphical Abstract Chemical phenomena are complex and stochastic, challenging traditional ensemble-averaged methods like nuclear magnetic resonance and X-ray diffraction. These techniques focus on long-lived species, missing transient intermediates. Single-molecule analysis, particularly via advanced TEM, offers high spatial and temporal resolution to observe nonequilibrium molecular dynamics. This account reviews the authors' TEM techniques for visualizing transient molecular interactions and advancing chemical understanding.

The full width at half maximum (FWHM) of emission spectra, which plays an important role in determining color purity, may not always receive sufficient attention in the design of emissive materials. Particularly for the red emitter, the traditional focus has been on emission maxima, yet broad FWHM values can significantly change the perceived color. For example, the red color (λ em =616–677 nm) emitted from reported Cu(I) complexes is perceived as orange to yellow if FWHM is large. To reduce FWHM, we incorporated a strained and rigid metalaphosphadicyclopenta[ a,f ]pentalene motif into Cu(I) complexes using trisphosphine ligands featuring a 1 H -indene-2,3-diyl backbone ( ITP ). Herein, we present the synthesis, structure, and emission properties of ITP-MX and the congeners, showcasing genuinely deep-red emission with narrow FWHM values of 56 nm. These materials exhibit color coordinates close to pure red on the CIE diagram, unlike reported broader-emitting counterparts. Measurement of the entropy of disorder of the emissive crystal by a recently reported statistical mechanical method revealed a quantitative correlation between FWHM and the increase in the number of microstates (the degree of freedom) of the crystals and suggested that mechanical stress can increase the entropy of the crystal, which results in emission broadening.

Heterogeneous catalysts dominate the chemical industry but typically feature diverse, incompletely defined active sites. Thus, describing structure-activity relationships, unlike homogeneous catalysts, remains challenging. In contrast, molecularly defined single-site heterogeneous catalysts (SSHCs), using appropriate tools, are poised to address these challenges and provide new avenues for catalysis research and development. The present study explores eco-friendly H2 production mediated by discrete MO2 sites supported on carbon nanohorns (CNHs) and active for alcohol dehydrogenation. While informative, detailed ensemble EXAFS/XANES, XPS, kinetic measurements, and DFT analysis alone cannot provide a full molecular picture of the reaction pathway. Here, using single-molecule atomic-resolution time-resolved electron microscopy (SMART-EM), we identify four key catalytic intermediates anchored to the CNHs and uncover a new reaction pathway involving alkoxide/hemiacetal equilibration and acetal oligomerization. These intermediates are identified solely by theory and SMART-EM, and this advance highlights the potential of SMART-EM to establish and verify mechanistic hypotheses in catalysis.

Traditionally, the study of crystal polymorphism has relied on thermodynamics and measurements averaged over time and the crystal's constituents. This work introduces a kinetic approach to phase identification─millisecond cinematographic electron microscopic imaging of the dynamics of phase transitions of crystals of a few nm in diameter. We demonstrate a remarkable impact of the interface energy on the relative stability of the nanocrystal's polymorphs, enabling in situ manipulation of phase transitions through size increase or decrease. Starting with the B1 NaI polymorph at 298 K, we identified the previously unknown B2 polymorph of a 1 s lifetime upon sublimation of the crystal. From the CsCl liquid phase, we produced the B1 phase, previously described only at 749 K.

Upon melting, the molecules in the crystal explore numerous configurations, reflecting an increase in disorder. The molar entropy of disorder can be defined by Bolzmann's formula dSd = Rln(Wd) where Wd is the increase in the number of microscopic states, so far inaccessible experimentally. We found that the Arrhenius frequency factor A of the electron diffraction signal decay provides Wd via an experimental equation A = AINTWd where AINT is an inelastic scattering cross-section. The method connects Clausius and Boltzmann experimentally and supplements the Clausius approach, being applicable to a femtogram quantity of thermally unstable and biomolecular crystals. The data also showed that crystal disordering and crystallization of melt are reciprocal, both governed by the entropy change, but manifesting in opposite directions.

Upon melting, the molecules in a crystal explore numerous configurations, reflecting an increase in disorder. The molar entropy of disorder can be defined by Boltzmann's formula ΔSd = Rln(Wd), where Wd is the increase in the number of microscopic states, so far inaccessible experimentally. We found that the Arrhenius frequency factor A of the electron diffraction signal decay provides Wd through an experimental equation A = AINTWd, where AINT is an inelastic scattering cross section. The method connects Clausius and Boltzmann experimentally and supplements the Clausius approach, being applicable to a femtogram quantity of thermally unstable and biomolecular crystals. The data also showed that crystal disordering and crystallization of melt are reciprocal, both governed by the entropy change but manifesting in opposite directions.

The advent of single-molecule atomic-resolution time-resolved electron microscopy (SMART-EM) has created a new field of 'cinematic chemistry,' allowing for the cinematographic recording of dynamic behaviors of organic and inorganic molecules and their assembly. However, the limited electron dose per frame of video images presents a major challenge in SMART-EM. Recent advances in direct electron counting cameras and techniques to enhance image quality through the implementation of a denoising algorithm have enabled the tracking of stochastic molecular motions and chemical reactions with sub-millisecond temporal resolution and sub-angstrom localization precision. This review showcases the development of dynamic molecular imaging using the SMART-EM technique, highlighting insights into nanomechanical behavior during molecular shuttle motion, pathways of multistep chemical reactions, and elucidation of crystallization processes at the atomic level.

We provide rare experimental evidence of ergodicity at the atomic level using high-resolution transmission electron microscopy. By examining the phase transition from η-alumina to θ-alumina in bayerite Al(OH)3 crystallites, we directly observe and quantify the ergodic nature of this transformation. The study demonstrates that at the nanoscale, the time a particle spends in various states during a phase transition aligns with the overall system's state distribution, thus confirming ergodic theory predictions. This research challenges traditional deterministic models of phase transitions in macroscopic crystals, suggesting a broader applicability of ergodic principles in understanding the dynamics of structural phase changes.

A ligand-controlled regiodivergence in Ni-catalyzed rearrangement of vinylcyclopropanes to 1,4- or 1,5-disubstituted cyclopentenes is reported. The 1,4- or 1,5-disubstituted cyclopentene is selectively obtained depending on the choice of ligands. Detailed kinetic studies and density functional theory calculations on the catalytic cycle revealed that the product selectivity is determined at the reductive elimination step from the six-membered η1 -allyl intermediate.

Carbon fiber (CF) obtained by pyrolysis of polyacrylonitrile (PAN-CF) surpasses metals in properties suitable for diverse applications such as aircraft manufacture and power turbine blades. PAN-CF obtained by pyrolysis at 1200-1400 °C shows a remarkably high tensile strength of 7 GPa, much higher than pitch-based CF (pb-CF) consisting of piles of pure graphene networks. However, little information has been available on the atomistic structure of PAN-CF and on how it forms during pyrolysis. We pyrolyzed an acrylonitrile 9-mer in a carbon nanotube, monitored the course of the reaction using atomic-resolution electron microscopy and Raman spectroscopy, and found that this oligomer forms a thermally reactive wavy graphene-like network (WGN) at 1200-1400 °C during slow graphitization taking place between 900 and 1800 °C. Ptychographic microscopic analysis indicated that such material consists of 5-, 6-, and larger-membered rings; hence, it is not flat but wavy. The experimental data suggest that, during PAN-CF manufacturing, many layers of WGN hierarchically pile up to form a chemically and physically interdigitated noncrystalline phase that resists fracture and increases the tensile strength─the properties expected for high-entropy materials. pb-CF using nearly pure carbon starting material, on the other hand, forms a crystalline graphene network and is brittle.