10 research outputs found
Structural, Vibrational and Thermodynamic Properties of AgnCu34-n Nanoparticles
We report results of a systematic study of structural, vibrational and
thermodynamical properties of 34-atom bimetallic nanoparticles from the
AgnCu34-n family using model interaction potentials as derived from the
embedded atom method and in the harmonic approximation of lattice dynamics.
Systematic trends in the bond length and dynamical properties can be explained
largely on arguments based on local coordination and elemental environment.
Thus increase in the number of silver atoms in a given neighborhood introduces
a monotonic increase in bond length while increase of the copper content does
the reverse. Moreover, based on bond lengths of the lowest coordinated (6 and
8) copper atoms with their nearest neighbors (Cu atoms), we find that the
nanoparticles divide into two groups with average bond length either close to
(~ 2.58 A) or smaller (~ 2.48 A) than that in bulk copper, accompanied by
characteristic features in their vibrational density of states. For the entire
set of nanoparticles, vibrational modes are found above the bulk bands of
copper/silver. Furthermore, a blue shift in the high frequency end with
increasing number of copper atoms in the nanoparticles is traced to a shrinkage
of bond lengths from bulk values. The vibrational densities of states at the
low frequency end of the spectrum scale linearly with frequency as for single
element nanoparticles, however, the effect is more pronounced for these
nanoalloys. The Debye temperature was found to be about one third of that of
the bulk for pure copper and silver nanoparticles with a non-linear increase
with increasing number of copper atoms in the nanoalloys.Comment: 37 pages, 12 figure
Structure, Dynamics and Themodynamics of a metal chiral surface: Cu(532)
The structure, vibrational dynamics and thermodynamics of a chiral surface,
Cu(532), has been calculated using a local approach and the harmonic
approximation, with interatomic potentials based on the embedded atom method.
The relaxation of atomic positions to the optimum configuration results in a
complex relaxation pattern with strong contractions in the bond length of atoms
near the kink and the step site and an equivalently large expansion near the
least under-coordinated surface atoms. The low coordination of the atoms on the
surface affects substantially the vibrational dynamics and thermodynamics of
this system. The local vibrational density of states show a deviation from the
bulk behavior that persist down to the 10th layer resulting in a substantial
contribution of the vibrational entropy to the excess free energy amounting to
about 90 meV per unit cell at 300K
Phonons of Metallic Vicinal Surfaces
We present an analysis of the vibrational dynamics of metal vicinal surfaces
using the embedded atom method to describe the interaction potential and both a
real space Green's function method and a slab method to calculate the phonons.
We report two main general characteristics : a global shift of the surface
vibrational density of states resulting from a softening of the force field.
The latter is a direct result of the reduction of coordination for the
different type of surface atoms; and an appearance of high frequency modes
above the bulk band, resulting from a stiffening of the force field near the
step atom. The latter is due to a rearrangement of the atomic positions during
the relaxation of the surface atoms yielding a large shortening of the nearest
neighbor distances near the step atoms.Comment: 6 figures, to appear in Sur. Sci. proceedings of VAS1
Structure and Vibrations of the Vicinal Copper (211) Surface
We report a first principles theoretical study of the surface relaxation and
lattice dynamics of the Cu(211) surface using the plane wave pseudopotential
method. We find large atomic relaxations for the first several atomic layers
near the step edges on this surface, and a substantial step-induced
renormalization of the surface harmonic force constants. We use the results to
study the harmonic fluctuations around the equilibrium structure and find three
new step-derived features in the zone center vibrational spectrum. Comparison
of these results with previous theoretical work and weith experimental studies
using inelastic He scattering are reported.Comment: 6 Pages RevTex, 7 Figures in Postscrip