Understanding transient kinetic pathways governing the bottom-up synthesis of 2D MoS2 is critical for deterministic growth. Here, utilizing an atomically resolved graphene-based microreactor for in situ heating transmission electron microscopy, we directly visualize the interface relaxation pathways and stacking-registry evolution during the growth of MoS2, advancing beyond the mere observation of its continuous growth dynamics. We reveal that lateral expansion is driven by dynamic edge reconstructions, as kinetically metastable states spontaneously relax into thermodynamically favored zigzag configurations. During subsequent domain coalescence, the dynamic interface relaxation is achieved across three distinct modes: defect-forming gap healing, strain-regulated seamless stitching, and rigid-body oriented attachment, which robustly accommodate lattice misorientations. Furthermore, intrinsic in-plane mirror twin boundaries induce localized structural distortions that mediate out-of-plane interlayer sliding, transiently stabilizing nonequilibrium bilayer stacking registries. By resolving these dynamic atomic fluctuations, our work establishes a fundamental mechanistic framework linking growth kinetics to the synthesis of 2D materials.
Thermal decomposition of GeSn layers detrimentally affects device performance, while the governing mechanisms remain shrouded by the presence of SiO
2
passivation. In this work, we elucidate the atomic‐scale decomposition of GeSn during thermal degradation using in situ heating TEM. Two divergent decomposition modalities are identified: a
Laminar Receding
mode in defect‐free regions, characterized by uniform, layer‐by‐layer interfacial recession; and a
Stepwise Pinning
mode in the presence of stacking faults, where the decomposition boundary is intermittently arrested by defects and advances discretely upon overcoming local energetic barriers. First‐principles calculations reveal a preferred decomposition boundary along [11] crystallographic orientation as the kinetically preferred pathway due to its minimal energy barrier, while the [001] orientation provides thermodynamic stability with an exceptionally low formation energy of 0.08 eV/Å. The competitive interplay between these two orientations dictates the characteristic stepped boundary morphology, which is energetically optimized at a step length of approximately two atomic spacings. Furthermore, we demonstrate that surface defects impose a substantial energetic penalty on atom removal, providing an atomistic basis for the observed pinning effect. By correlating defect‐mediated anisotropic decomposition with its crystallographic dependence, this study establishes a foundational physical framework for enhancing the thermal stability of GeSn heterostructures.
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.
Yuxin Jiang, Hannah DeVyldere, Yin-Cheng Lin et al.· Journal of Physical Chemistr...· 0 citations
Phase transitions in transition metal oxides involving oxygen exchange underpin functionalities ranging from ionic electronics to infinite-layer superconductivity, yet pathway selection and transient intermediates remain difficult to capture. Here, we resolve the atomistic dynamics of oxygen-mediated topotactic transformations in strontium ferrite (SrFeO3-x) using in-situ high-resolution transmission electron microscopy combined with molecular dynamics (MD) simulations. Reduction of SrFeO3 at 450 °C proceeds through SrFeO2.5 and a previously unreported vacancy-ordered SrFeO2+σ phase that retains a less compact three-dimensional Fe-O framework and acts as a structural/chemical buffer before collapse to infinite-layer SrFeO2. During reverse oxidation, the pathway bifurcates: at 200 °C a direct interface-driven phase front converts the SrFeO2 film to perovskite SrFeO3, whereas deep undercooling to 20 °C induces a spatiotemporal competition between the SrTiO3 substrate and ambient oxygen reservoirs that stabilizes a cascade of vacancy-ordered intermediates, including SrFeO2+σ, SrFeO2.75, and SrFeO2.875. MD simulations further reveal a crossover from cooperative, stepwise oxygen diffusion to more continuous oxygen incorporation, rationalizing when staged metastable trapping versus near-direct transformation occurs. Density functional theory calculations show that these trapped intermediates are electronically and magnetically distinct phases rather than merely structural stepping stones, underscoring nonequilibrium pathway selection as a route to functional states beyond the equilibrium phase diagram.
Y. Xing, J. Byun, Kyeong Tae Kang et al.· Journal of the American Chem...· 0 citations
Progress in 2D memristive technologies is increasingly constrained by a limited understanding of how crystallographic anisotropy governs ion migration and resistive switching. Bi2O2Se offers a compelling model system in which in-plane and out-of-plane devices display strikingly different electrical behaviors, yet the atomic-scale origins of this disparity remain unknown. Here, we engineer orientation-defined Bi2O2Se nanodevices using focused ion beam fabrication coupled with in situ aberration-corrected transmission electron microscopy, enabling simultaneous electrical probing and real-time imaging of structural evolution under bias. Supported by density functional theory (DFT) calculations, we demonstrate that anisotropic migration barriers for O2- and Se2- ions give rise to two fundamentally distinct switching pathways. Vertical fields, constrained by strong interlayer electrostatic locking, lead to localized vertical migration and the formation of a reversible, ordered conductive D-Bi2O2Se phase, producing abrupt, threshold-type switching. By contrast, lateral fields enable long-range ion diffusion, generating extended Bi/Bi4+2nSe3/Bi2O2Se heterostructures through a topotactic sequence with continuously evolving Se concentration, yielding smooth and linear conductance modulation. These results establish the microscopic principles that underpin direction-dependent transport and phase transformation in Bi2O2Se memristors. By revealing how crystallographic orientation dictates functionality, our work provides a mechanistic foundation for the rational design of directionally engineered 2D neuromorphic and memory systems with enhanced versatility and integration potential. The development of two-dimensional memristors is limited by unclear links between crystal direction, ion motion, and switching. Qu et al. build orientation controlled Bi2O2Se devices and combine real time imaging and theory to reveal distinct direction dependent ion migration and switching pathways.
Ke Qu, Jian-Wei Zhang, Jianchu Chen et al.· Nature Communications· 0 citations
A unique relaxation mechanism in wet‐chemically synthesized Bi2Te3–Sb2Te3 lateral heterostructures is presented, combining insights from molecular statics/dynamics simulations with a novel characterization technique based on STEM imaging using a defocused probe. The molecular simulations indicate that these heterostructures undergo intrinsic structural relaxation due to lattice mismatch and elastic anisotropy, resulting in uniform bending into dome‐shaped geometries with radii of curvature on the micron scale. While conventional bright‐field TEM imaging experimentally shows symmetric bend contours due to uniform bending, we demonstrate that defocused HAADF‐STEM produces unique contrast patterns sensitive to the sign of the probe defocus. This allows for the acquisition of crystallographic information (both magnitude and sign of curvature) in real space over large fields of view. The underlying physical mechanism is elucidated through scattering theory, focusing on the matching of the incident electron wave phase front with the local crystal lattice orientation and is supported by comprehensive dynamical multislice simulations of electron channeling maps. These findings are further validated by electron tomography and momentum‐resolved 4D‐STEM experiments. These insights are not only relevant for deciphering the local crystallographic properties of the Bi2Te3–Sb2Te3 system, but also establish a STEM‐based approach to characterize crystal bending across different zone axes in 2D nanostructures. This methodology provides a robust, real‐space alternative for analyzing intrinsic or strain‐induced curvature over large fields of view using unconventional HAADF‐STEM.
Naveen Goyal, Koushik Jagadish, Ziria Herdegen et al.· Small Methods· 0 citations
In layered oxide cathodes, the utilization of high states of charge inevitably triggers complex phase transitions, accompanied by significant translational symmetry breaking. Although in situ X-ray diffraction has long been employed to monitor these transitions, quantitatively resolving the stochastic nature of planar defects remains a formidable challenge. In this work, we establish a robust analytical framework that correlates stacking faults with diffraction patterns, enabling the quantitative elucidation of one-dimensional stacking disorder. Taking the prototypical P2-to-O2 transition as a model, we demonstrate that this phase transition is a highly dynamic evolution governed by stochastic interlayer gliding. Through combined theoretical and experimental analyses, we reveal that the P2-to-O2 transition proceeds via an interlayer-asynchronized route, featuring an ordered OP6 intergrowth phase at the midpoint, which is an intermediate structure frequently misidentified as OP4 in the literature due to neglected unit cell periodicity constraints. Furthermore, by constructing and validating structural models that explicitly account for stacking defects, we establish a quantitative link among layer gliding, local coordination evolution, and subsequent transition-metal migration. Ultimately, the established descriptor framework serves as a standardized baseline amidst intricate structural evolutions, enabling the precise description and quantitative comparison of phase transition behaviors.
Guifan Zeng, Linhui Zeng, Yonglin Tang et al.· Journal of the American Chem...· 0 citations