Exponential surface melting of Cu nanoparticles observed by in-situ TEM

In surface melting, a liquid film forms on the crystal surface even below its melting point (Tm). Currently, surface melting theory generally assumes a crystal with an infinite surface, and it is believed that the surface melting layer thickens according to ln [Tm/(Tm − T)] or (Tm − T)−r (r is a constant). In this work, we observed the melting of Cu nanoparticles with diameters of about 30 nm at 890 °C using in-situ heating high-resolution transmission electron microscopy (HRTEM). We found that the thickness of the melting layer on the surface of the Cu nanoparticles increased exponentially. The growth pattern of the melting layer was notably different from the results of previous simulations. This discovery may enhance our understanding of the melting mechanism of Cu nanomaterials.