Precise control over solid-liquid phase transitions at the nanoscale is a central pursuit in materials science. Traditional crystal nucleation theory primarily describes this process as a competition between the reduction in volumetric Gibbs free energy and the penalty of interfacial energy. However, in many natural phenomena and nanomaterial syntheses, phase transitions often occur within geometrically confined spaces, where the interaction between the critical nucleus and the surrounding interfaces fundamentally alters the energy landscape of the transition, significantly affecting phase stability, polymorph selection, and texture formation.

Despite the significance of geometrically confined phase transitions, real-time dynamic observation and precise manipulation of critical-size nanoclusters remain extremely challenging. On one hand, the critical nucleus is in a highly unstable thermodynamic state at the top of the free energy barrier, making it exceedingly difficult to isolate and observe stably. On the other hand, applying dynamically tunable nanoscale geometric confinement requires ultra-high-precision mechanical manipulation techniques. Furthermore, seamlessly integrating such nanoscale mechanical manipulation with atomic-resolution in situ transmission electron microscopy to capture the phase evolution among amorphous, liquid, and crystalline states in real time represents a critical technical bottleneck that remains to be overcome in this field.

To address the above challenges, a team comprising Wuhan University of Technology, Suzhou Laboratory, and other institutions conducted systematic research using ZEPTOOLS' in situ TEM measurement system. For the first time, this team achieved precise manipulation and real-time observation of individual critical-size bismuth nanoclusters within a nanogap at the atomic scale, revealing that aspect ratio, rather than volume, is the key mechanism governing reversible phase transitions and crystallographic texture selection under geometric confinement.

Title: Geometrically driven reversible solid-liquid phase transition at the atomic scale
Journal: Science
URL: https://doi.org/10.1126/science.aed6019

To tackle the difficulty of in situ capturing the phase transition of critical nuclei under confinement, the research team developed a voltage-driven combined with epitaxial growth method for the site-specific preparation of bismuth nanoclusters. Using focused ion beam to expose specific surfaces of bismuth telluride crystals, the van der Waals gaps were aligned perpendicular to the surface, serving as natural channels for rapid diffusion of bismuth cations. Under an applied bias, bismuth ions migrated along the van der Waals gaps and accumulated, epitaxially growing a high-purity bismuth crystalline layer at the interface. Subsequently, excess bismuth atoms underwent lateral diffusion, forming isolated bismuth nanodroplets. When these nanodroplets grew to a critical radius, they solidified into faceted crystalline nanoparticles according to classical nucleation theory, establishing a highly reliable sample foundation for subsequent single-particle phase transition studies under confined conditions.

In exploring the evolution of phase transitions under confinement, the team utilized ZEPTOOLS' in situ TEM sample holder to achieve atomic-scale mechanical and electrical control. Through the piezoelectrically controlled tip in this in situ holder, the researchers not only induced bismuth ion migration to generate isolated nanoclusters but also dynamically altered the gap distance between the tip and the substrate with nanometer precision, confining the bismuth nanoclusters within a tunable confined space. In situ TEM observations revealed that, under the precise manipulation enabled by this equipment, the confined bismuth nanoclusters could undergo highly reversible transitions among three phase states: quasi-amorphous nanodisks, single-crystalline nanowires, and liquid nanodroplets, unequivocally confirming the decisive influence of spatial confinement on phase transition pathways.

The study further revealed that aspect ratio, rather than volume, is the core thermodynamic descriptor governing the phase stability of confined nanoclusters. When bismuth nanowires were compressed to an aspect ratio of less than 0.4, the substantial interfacial energy and atomic-scale surface roughness forced atomic rearrangement, forming a quasi-amorphous structure driven by interfacial adaptation. Conversely, when the nanowires were stretched to an aspect ratio greater than 1.5, the sharp increase in lateral surface area triggered spontaneous melting into liquid nanodroplets. By combining a free-energy thermodynamic model with machine-learning force-field molecular dynamics simulations, the team constructed a three-phase diagram encompassing quasi-amorphous, single-crystalline, and liquid states, confirming that the phase boundaries are highly reproducible over a wide volume range, challenging the traditional volume-centric understanding of phase transitions.

Beyond phase control, spatial confinement also induced a significant crystallographic texture selection effect in bismuth nanowires. Statistical analysis and crystallographic projections indicated that unconfined nanoparticles exhibited free three-dimensional rotation in space, whereas the rotation of confined nanowires was strictly restricted to uniaxial rotation around a fixed growth axis, with specific crystal planes preferentially aligned parallel to the substrate. Density functional theory calculations further revealed that this preferred growth direction exposes side facets with extremely low surface energies, even lower than that of liquid bismuth, thereby completely eliminating the nucleation barrier and elucidating the thermodynamic mechanism by which geometric confinement and anisotropic surface energy synergistically regulate crystal texture.

ZEPTOOLS, as a Chinese domestic high-end precision instrument company, is a first-class supplier of in situ electron microscopy characterization solutions. Its PicoFemto series of in situ transmission electron microscopy characterization solutions have consistently provided technical support for groundbreaking research outcomes from both domestic and international users. Below is the ZEPTOOLS in situ TEM sample holder product used in this research: