Survey of transient process during melting of silver below the equilibrium melting point

Understanding the melting behavior of metals at the microlevel and atomic level has been experimentally challenging due to the involvement of multiple phases at ultrafast time scale. By using the confocal scanning laser high-temperature microscope, differential scanning calorimetry, and environmental transmission electron microscope, we observed the transient process during melting of silver (Ag) nanoparticles below the equilibrium melting point. The melting point of Ag nanoparticles with the diameter of 60-120 nm is found to decrease by 100-400 °C, and the melting process is accompanied by a geometrical transformation at 840 °C, from an irregular polyhedron to a nearly spherical crystallinelike liquid with smooth facets. These results indicate that the melting of metal nanoparticles is not a direct sharp transformation from crystal to liquid but a gradual process via a certain intermediate state.