STUDY OF ANHARMONIC EFFECTS IN THE PRESENCE OF ADPARTICLES AT THE SURFACES OF CRYSTALS OF NOBLE METALS

Abstract

The anharmonic effects at/near the surfaces of noble metals in the presence of adparticles have been presented at 300, 500 and 700 K temperatures. The Molecular Dynamics (MD) simulations are carried out for Cu(111), Cu(311), Cu(210) and Ag(111) surfaces using realistic many-body interatomic potentials obtained from the Embedded Atom Methods (EAM). In a comparative study of the structure and the dynamics of the close packed as well as open surfaces, the calculated shifts in the interlayer spacing, mean square displacement, surface diffusion and surface defects (fissure, dislocation and vacancy) show the onset of anharmonic effects at the characteristic temperatures. Fir the Cu(311) surface interlayer relaxations show a uniform damping in magnitude of oscillatory order (-, +, - , +, . . . ), while for the Cu(210) surface they reveal a non-uniform damping in magnitude with moving away from the surface. The plane registry relaxation at both (311) and (210) surfaces exhibits no order for damping. The diffusion of adatoms on the (111) surface of Ag and Cu has been studied to examine the surface anharmonicity. The diffusion coefficient, effective energy barrier and diffusion prefactor of Cu pentamer on Ag(111) are calculated. The MD simulations at 300, 500 and 700 K show that the diffusivity obeys the Arrhenius Law. An Arrhenius plot of the diffusion coefficients provides an effective energy barrier of 205.25 ±10 meV and a diffusion prefactor 5.549× 1012 Å2/s. A striking feature of a pop-up of single-atom at 700 K among 5-atom island is observed. The effective energy barrier obtained from the xiii Arrhenius plot is in excellent agreement with those extracted from scanning tunneling microscopy experiments. During the diffusion of Cu- and Ag-trimer on Cu- and Ag(111) surfaces at 300, 500 and 700 K temperatures, the constant energy MD simulation are consistent with anharmonic effects at the surface such as fissures, dislocations and vacancy generation in the presence of a island. The fissures and dislocations formed are in the range of 1.5–4 Å and 1–7 Å respectively from the island position. The Cu and Ag islands both diffuse easily on the Cu(111) surface, and indicate that diffusion is faster on the Cu surface as compared to the Ag surface. The process of breaking and opening of the island has been also observed. A surface atom is popped-up at 700 K by generating a vacancy near the Cu island at the Ag surface. The energy of vacancy generation at the Ag surface is found to be Ev = 1.078 eV, in the presence of a Cu trimer. The popped-up Ag adatom combines with Cu the trimer, making a mixed-tetramer (Cu3–Ag island). The adsorption energy of a copper atom from the mixed tetramer island into the substitutional site is Es = ∼– 0.813 eV, leaving a mixed-trimer (Cu2–Ag island) at Ag(111). The rate of diffusion increases with increase in temperature, both for homo and hetero cases. Furthermore, we comment on the effect of adparticles diffusion on the harmonic behavior of the surfaces in the anharmonic regime.