Size-Dependent Phase Stability of Silver Nanocrystals

Size dependences of three structural phase transitions, melting, evaporation, and solid-solid transition for silver (Ag) nanocrystals, were investigated with nanothermodynamics. The size-dependent melting temperature T-m(r) and evaporation temperature T-ev(r) of Ag nanocrystals were determined based on a root-mean-square-displacement (rmsd) model. It is found that both T-m(r) and T-ev(r) decrease with crystal size decreasing for the isolated Ag nanoparticles while T-m(r) increases for the Ag nanoparticles embedded in the Ni matrix. A nanothermodynamic model was also established to reveal the origin of size effect on the polymorphism (face-centered-cubic and 4H structures) behavior of Ag nanocrystals with the contributions of surface energy and surface stress to the total Gibbs free energy. It shows that the lower surface energy and surface stress coupled with a higher volume Gibbs free energy predominates the thermal stability of 4H-structured Ag in nanometer scale. The results may provide new insight into the fundamental understanding of high-performance nanostructural metals for their applications in nanodevices.