Solid-liquid phase transition of tin particles observed by UHV high resolution transmission electron microscopy: pseudo-crystalline phase

Solid-liquid transition of fine tin particles having diameter of 2–10 nm is studied in-situ by high-resolution transmission electron microscopy under a ultra-high vacuum condition. Melting temperature is confirmed to decrease with the decrease of particle diameter. The particles less than the critical size, 2r _c?5 nm, are found to have a specific phase between the solid and the liquid phase. The particle in this “pseudo-crystalline” phase contains crystalline embryos in it. Particles larger than the critical size have sharp liquid-solid transition, which completed within the time resolution of our microscope observation, 33 ms upon heating or cooling process. Large solid particles have Wulff's polyhedron, while particles around the critical diameter have rather spherical shape. Structural anomaly at the critical size occurs all over the outer most surface layer slightly below the melting temperature. Origin of the “pseudo-crystalline” phase and surface pre-melting phenomena are discussed.