It is well known that surface melting of metal materials is caused by vacancies, and melting proceeds layer by layer in theoretical predictions. However, the melting process has rarely been directly investigated in real time at atomic resolution. Herein, the (200) surface-melting process of Cu nanoparticles with sizes of about 50 nm at ${750}^{\circ }\mathrm{C}$ was first observed by in situ heating transmission electron microscopy. Initially, surface-melting nucleation occurs at the edge and corner of one side of the Cu(200) surface. Subsequently, the nucleated region size increases to a critical value (about 14 layers). Finally, collapse-type melting rapidly extends to the whole Cu(200) surface. This surface-melting process repeatedly occurs. This work will enhance the understanding of the surface-melting mechanism and provide a theoretical foundation to avoid the collapse of Cu nanomaterials during high-temperature applications.

• Received 13 March 2018
• Revised 9 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.045425