Structural relaxation in Ag-Ni nanoparticles: atomistic modeling away from equilibrium

Abstract

The out-of-equilibrium structural relaxation of Ag-Ni
 nanoparticles containing about 1000--3000 atoms was investigated
 computationally by means of molecular dynamics trajectories in which
 the temperature is decreased gradually over hundreds of
 nanoseconds. At low silver concentration of 10--30\%, the evolution
 of chemical ordering in Ni$_{\rm core}$Ag$_{\rm shell}$
 nanoparticles with different surface arrangements is found to
 proceed spontaneously and induce some rounding of the nickel core
 and its partial recristallization. Fast cooling of an initially hot
 metal vapor mixture was also considered, and it is shown to disfavor
 silver aggregation at the surface. Silver impurities are also
 occasionally produced but remain rare events under the conditions of
 our simulations.

Two-Steps Versus One-Step Solidification Pathways of Binary Metallic Nanodroplets

“…The majority of these simulations focused on systems with a tendency to intermixing between the two elements, such as AgAu, AlNi, AuPd, and NiCo. − Fewer studies were devoted to systems with a strong tendency toward phase separation. In particular, the freezing of AgCo, CuCo, CuNi, and AlFe nanodroplets was simulated ,, and, more recently, also the freezing of AgCu and AgNi nanoalloys. − The free-energy barrier for crystal nucleation in CuNi and CuPd nanoalloys has been calculated at fixed sizes and varying composition. , Melting has been studied in several simulations that revealed the occurrence of a two-step process. In fact, phase-separating nanoalloys such as AgCu, CuNi, AgNi, AuCo, AuNi, AgCo, AuFe, and several others adopt core–shell and quasi-Janus structures in their solid state, ,− and their shell can melt at a lower temperature than their core. − On the contrary, the occurrence of single-step or two-step freezing processes is not yet studied.…”

“…In particular, the freezing of AgCo, CuCo, CuNi, and AlFe nanodroplets was simulated 16 , 21 , 22 and, more recently, also the freezing of AgCu and AgNi nanoalloys. 23 − 25 The free-energy barrier for crystal nucleation in CuNi and CuPd nanoalloys has been calculated at fixed sizes and varying composition. 26 , 27 Melting has been studied in several simulations that revealed the occurrence of a two-step process.…”

Combined atomistic simulations to explore metastability and substrate effects in Ag–Co nanoalloy systems

“…When the components of nanoalloys are weakly miscible, especially in the case of Cu-Ag, Ni-Ag, and Co-Ag with high differences in cohesive energy, core-shell and quasi-Janus chemical orderings are expected at equilibrium in wide composition ranges. In coreshell nanoalloys, an external shell of the surface-segregating element (in these cases, Ag is expected) covers a core of the more cohesive element M (M being either Cu, Ni, or Co) [10][11][12][13][14] . In the quasi-Janus structure, the core is in off-center position, so that it is covered by a very thin layer of the shell element on one of its sides 12 .…”

“…In core-shell nanoalloys, an external shell of the surfacesegregating element (in these cases, Ag is expected) covers a core of the more cohesive element M (M being either Cu, Ni, or Co). [10][11][12][13][14] In the quasi-Janus structure, the core is in an off-center position, so that it is covered by a very thin layer of the shell element on one of its sides. 12 Among these, the AgCo system belongs to the class of materials composed by immiscible ferromagnetic and nonmagnetic elements, a mixture which raises interest as it can exhibit giant magnetoresistance effects.…”