Bin Chen

Ultrafast electron microscopy (UEM), also referred to as four-dimensional electron microscopy (4D-EM), has emerged as a transformative tool capable of recording atomic-scale dynamics with high spatiotemporal resolution. While the stroboscopic pump–probe mode has been extensively applied to reversible processes, many important phenomena are intrinsically irreversible and demand single-shot imaging. This review focuses on the principles and recent progress of single-shot 4D-EM for capturing non-repeatable events in space and time. We describe the two main single-shot modalities — single-frame and movie-mode (multi-frame) imaging — which rely on intense, ultrashort electron pulses to capture a transient following a single pump excitation. Representative applications are systematically surveyed, including laser-induced melting and rapid solidification, phase transitions, nanoparticle jumping and coalescence, redox and eutectic reaction dynamics, and liquid-phase behaviors. Key challenges, including the conflict between electron number and pulse duration imposed by space-charge effects, beam coherence, and detector limitations, are discussed, where correspondingly future perspectives and strategies are offered. The progress establishes single-shot 4D-EM as an indispensable platform for unraveling irreversible and stochastic processes across materials science, chemistry, physics, and biology.

All-inorganic perovskites, a representative class of water-sensitive materials, offer excellent thermal stability and photostability, marking them promising for tandem photovoltaics. Nevertheless, water-induced degradation constrains their performance, stability, and commercialization. Atomic-scale understanding of degradation pathways in perovskites has been elusive, largely due to technical challenge of synchronizing water exposure with real-time visualization. We employ liquid-cell transmission electron microscopy with a newly designed double-layer strategy to directly visualize water-induced degradation of CsPbBr ₃ nanocrystals at atomic-scale. Two kinetic regimes emerge: high water content triggers irreversible transformation to non-perovskite-structure CsPb ₂ Br ₅ with subsequent crystalline–amorphous oscillations; trace water induces a reversible cyclic pathway between CsPbBr ₃ and CsPb ₂ Br ₅ with involvement of intermediate molten phase (MP). Transformation frequency is size-dependent: smaller nanocrystals exhibit the higher rate, due to facile MP formation and effective erasure of accumulated defects. Our findings establish degradation as a pathway-selected transition governed by water content, crystal size, and transient disorder, offering a mechanistic framework for water-sensitive materials.

Interactive “visualization-manipulation” of membrane-regulated behavior with high spatiotemporal resolution remains challenging. Here, we in situ design and visualize inorganic nanocells from an exciting cinnabar semiconductor while simultaneously forming liquid-like membranes and Hg nanodroplets. A full picture of such membranes from birth to disappearance, including membrane-associated gating of mass transport either in a single nanocell or across multiple nanocells, is revealed at the atomic scale. Periodic reversible cross-feeding occurs among nanodroplets confined in a single nanocell, preventing the release of Hg-associated species to the surroundings. However, once the ionic balance of membranes is disturbed by nanobubbles or electrolytes, the nanodroplets collapse. The released species experience cell-to-cell transport over long distances through nanochannels and are crystallized into Hg(I/II) compounds. Ab initio molecular dynamics simulations suggest that the nanodroplet–membrane interface undergoes dynamic charge fluctuations, recognizing membrane-regulated mass transport in nanoconfined systems. The flexible membrane is stabilized through the balance between Hg atoms and ions, which can be destroyed by nanobubbles.

Heterostructures are widely employed in optoelectronic and catalytic applications due to the functional versatility offered by their inherent interfacial and junction effects. However, a critical concern lies in their susceptibility to ubiquitous etching when exposed to the operation environments involving humidity or liquid under external excitations. Despite the importance of addressing the etching response of heterostructures, achieving high spatiotemporal visualization to comprehensively map the full range of evolution pathways remains a marked challenge. Here, we reveal a complete picture of the etching pathways in water-/oil-phase CdSe-based heterostructures by liquid-cell transmission electron microscopy through the nanoscale-to-atomic understanding. Three distinct pathways are revealed in response to the intrinsic and extrinsic characteristics of heterostructures. The heterojunction region is instantaneously etched due to the local enhanced microenvironment induced by the charge separation effect. Subsequently, the etching is accelerated near the interface because of the existing inherent strain there. The necking occurs as the etching progresses, followed by the order-disorder transformation, which results in the oscillation of the etching rates. The extrinsic nanobubbles anisotropically accelerate the etching but with a slower onset upon comparison to the junction-/strain-induced reactions. Surprisingly, the AuSe and Au tips in the water-phase and oil-phase heterostructures can also be etched, but with distinct byproducts of Au2(SeO3)2(Se2O5) and Au2(SeO3)2(SeO4), respectively. The findings provide a versatile understanding of the etching characteristics in heterostructured systems, offering valuable insights into the rational design and manipulation of complex heterostructures with advanced functionalities and stability in various energy-related and catalytic fields.

Challenges in predicting and tuning nanoscale crystal growth stem from the fact that there are plenty of individual elementary steps occurring in the liquid-solid two phase zone involving phase transitions and that these steps are highly dynamic in nature. Direct visualization with high spatiotemporal resolution is indispensable to unveiling the important intermediate states underlying those alloying processes. Here, our in situ observations reveal that additional unexpected phase transformation between the crystalline and amorphous states also occurs periodically, even after the typical transition from the liquid to the precipitated solid nanoparticles (NPs) in the two-phase zone. The NPs demonstrate interesting oscillatory rotation in the liquid, followed by the anomalous Brownian motion type due to the nanoscale confinement. Such rotation further enhances the intermediate crystalline ↔ amorphous phase transition. The NPs finally become crystalline with a fixed orientation once the liquid region is nearly depleted. The underlying mechanism is discussed. The findings offer a fundamental understanding of the phase behaviors with dynamic intermediate structures in the complex liquid-solid zone under the input of nanoscale confinement and pave the way for optimizing the nanoalloy nucleation and growth with the desired properties.

Liquid-liquid phase separation (LLPS), often forming membraneless compartments, is prevalent in proteins and polymers, allowing the functionalities for biological activities and soft material engineering. Yet, direct visualization and manipulation of the full membrane-bound LLPS evolutions with high spatiotemporal resolution remains challenging and undefined. Inspired by dynamic modulation from biological membranes, we in-situ design inorganic nanocells from exciting nanoscale cinnabar with simultaneously forming flexible liquid-like membranes and dense Hg nanodroplets by the electron-responsive LLPS strategy. A full LLPS picture from birth to disappearance, including membrane-associated gating of mass transport either in single nanocell or across multiple nanocells is vividly revealed. Periodic reversible cross-feeding occurs among nanodroplets confined in the single nanocell, in contrast to the conventional Ostwald ripening or coalescence behavior. However, once the ionic balance of the membranes is disturbed by nanobubbles or electrolytes, the nanodroplets collapse. The released less dense species proceed cell-to-cell transport over long distances through nanochannels and are irreversibly crystallized into Hg(I/II) compounds. Ab initio molecular dynamics simulations suggest that the nanodroplet-membrane interface undergoes dynamic charge fluctuations, recognizing the unique membrane-bound LLPS in inorganic systems. The flexible membrane is stabilized through the balance between Hg atoms and ions, which can be destroyed by nanobubbles.

Nanodroplets at multiphase interfaces are ubiquitous in nature with implications ranging from fundamental interfacial science to industrial applications including catalytic, environmental, biological and medical processes. Direct observation of full dynamic evolutions of liquid metal nanodroplets at nanoscale multiphase interfaces offers indispensable insights, however, remains challenging and unclear. Here, we fabricate gas and liquid cells containing HgS nanocrystals through electrospinning and achieve the statistical investigations of full picture of Hg nanodroplets evolving at solid-gas and solid-liquid interfaces by in-situ transmission electron microscopy. In the gas cells, the voids nucleate, grow and coalesce into the crack-like feature along the direction, while Hg nanodroplets form, move rapidly on the ratchet surface and are evolved into bigger ones through the nanobridges. Distinctly, mediated by the solid-liquid interface, the liquid Hg with the ink-like feature jets in the liquid cells. Such ink-jetting behavior occurs multiple times with the intervals from several to several tens of seconds, which is modulated through the competition between reductive electrons and oxidative species derived from the radiolysis of liquids. In-depth understanding of distinct nanodroplets dynamics at nanoscale solid-gas and solid-liquid interfaces offers a feasible approach for designing liquid metal-based nanocomplexes with regulatory interfacial, morphological and rheological functionalities.

Wet-chemical reactions, covering almost all solution-based synthesis in either the oil-phase or water-phase microenvironment, lead to the as-formed products with distinct morphologies, structures, and functionalities. However, crystal nucleation and growth dynamics under those microenvironments for the same material system have not been clarified. Using in situ transmission electron microscopy on the classical II-VI CdSe-based heterostructures with atomic scale resolution, notably, we revealed the formation of Au on the CdSe surface in the oil phase while the AuSe product was nucleated in the water phase. The nucleation was analogous to the two-step amorphous-to-crystalline transition, followed by growth or coalescence into polycrystalline nanoparticles. During the ex situ growth, the majority of AuSe was polycrystalline (∼79%) in the water phase, in contrast to ∼52% in the oil phase. Surprisingly, the proportion of single crystals prevailed, which was significantly increased to ∼76% in the in situ case. Such distinct behaviors were further verified through the liquid-cell environment and elemental characterizations.

Nanodroplets at multiphase interfaces are ubiquitous in nature with implications ranging from fundamental interfacial science to industrial applications including catalytic, environmental, biological and medical processes. Direct observation of the full dynamic evolutions of liquid metal nanodroplets at nanoscale multiphase interfaces offers indispensable insights, however, remains challenging and unclear at the moment. Here, we have fabricated massive ready-to-use gas and liquid cells containing HgS nanocrystals through electrospinning and achieved the statistical investigations of full picture of Hg nanodroplets evolving at solid-gas and solid-liquid interfaces by in-situ transmission electron microscopy. Upon the electron-beam excitation of HgS in the gas cells, the voids nucleated, grew and then coalesced into the crack-like feature preferentially along the < 001 > direction through the bridges. Meanwhile, the Hg nanodroplets formed, moved rapidly on the ratchet surface with the velocity of several tens of nm/s and were finally evolved into bigger ones through the nanobridges with the relatively large gap of ~ 6 nm. Distinctly and surprisingly, mediated by the solid-liquid interface at nanoscale, the liquid Hg with the ink-like feature jetted in the liquid cells. Such ink-jetting behavior would occur multiple times with the intervals from several to several tens of seconds, which was modulated through the competition between the reductive electrons and the oxidative species derived from the radiolysis of liquid by the electron-beam. In-depth understanding of distinct nanodroplets dynamics at nanoscale solid-gas and solid-liquid interfaces offers a feasible approach of designing liquid metal-based nanocomplexes with regulatory interfacial, morphological and rheological functionalities.

Spiral inorganic perovskite nanowires (NWs) possess unique morphologies and properties that allow them highly attractive for applications in optoelectronic and catalytic fields. In popular solution‐based synthesis methodology, however, challenges persist in simultaneously achieving precise and facile control over morphological twisting and fantastic carrier lifetimes. Here, a cooperative strategy of concurrently employing selective etching and ligand engineering is applied to facilitate the formation of spiral CsPbBr ₃ perovskite NWs with an ultralong carrier lifetime of ≈2 µs. Specifically, a novel amine of 1‐(p‐tolyl)ethanamine is introduced to functionalize as both a selective etchant and the source of forming an effective ligand to passivate the exposed facets, favoring the structural twisting and the enhancement of carrier lifetimes. The twisting behaviors are dependent on the etch ratios, which are essentially associated with the densities of grain boundaries and dislocations in the NWs. The ultralong carrier lifetime and long‐term stability of the spiral NWs open up new possibilities for all‐inorganic perovskites in optoelectronic and photocatalytic fields, while the cooperative synthesis strategy paves the way for exploring complex spiral structures with tunable morphology and functionality.