39 research outputs found
Monte Carlo Simulation of Surface De-alloying of Au/Ni(110)
Based on BFS model and using Monte Carlo simulation we confirms the
de-alloying in immiscible Au/Ni(110) system, and the critical Au coverage when
de-alloying happens is also consistent with experiments. At the same time our
simulation show that the structural phase transition will lead to the
saturation of the number of alloying Au atoms.Comment: Submitted to Surface Science, 14 Pages, 6 Figure(.eps
Ag-Cu alloy surfaces in an oxidizing environment: a first-principles study
Recent experiments on model catalysts have shown that Ag-Cu alloys have
improved selectivity with respect to pure silver for ethylene epoxidation. In
this paper we review our first-principles investigations on the (111) surface
of this alloy and present new findings on other low index surfaces. We find
that, for every surface orientation, the presence of oxygen leads to copper
segregation to the surface. Considering the alloy to be in equilibrium with an
oxygen atmosphere and accounting for the effect of temperature and pressure, we
compute the surface free energy and study the stability of several surface
structures. Investigating the dependence of the surface free energy on the
surface composition, we construct the phase diagram of the alloy for every
surface orientation. Around the temperature, pressure and composition of
interest for practical applications, we find that a limited number of
structures can be present, including a thin layer of copper oxide on top of the
silver surface and copper-free structures. Different surface orientations show
a very similar behavior and in particular a single layer with CuO
stoichiometry, significantly distorted when compared to a layer of bulk CuO,
has a wide range of stability for all orientations. Our results are consistent
with, and help explain, recent experimental measurements
Spontaneous magnetization of aluminum nanowires deposited on the NaCl(100) surface
We investigate electronic structures of Al quantum wires, both unsupported
and supported on the (100) NaCl surface, using the density-functional theory.
We confirm that unsupported nanowires, constrained to be linear, show
magnetization when elongated beyond the equilibrium length. Allowing ions to
relax, the wires deform to zig-zag structures with lower magnetization but no
dimerization occurs. When an Al wire is deposited on the NaCl surface, a
zig-zag geometry emerges again. The magnetization changes moderately from that
for the corresponding unsupported wire. We analyse the findings using electron
band structures and simple model wires.Comment: submitted to PHys. Rev.
Effects of anharmonic strain on phase stability of epitaxial films and superlattices: applications to noble metals
Epitaxial strain energies of epitaxial films and bulk superlattices are
studied via first-principles total energy calculations using the local-density
approximation. Anharmonic effects due to large lattice mismatch, beyond the
reach of the harmonic elasticity theory, are found to be very important in
Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that
is the elastically soft direction for biaxial expansion of Cu and Ni, but it is
for large biaxial compression of Cu, Ag, and Au. The stability of
superlattices is discussed in terms of the coherency strain and interfacial
energies. We find that in phase-separating systems such as Cu-Ag the
superlattice formation energies decrease with superlattice period, and the
interfacial energy is positive. Superlattices are formed easiest on (001) and
hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the
formation energy of superlattices increases with period, and interfacial
energies are negative. These superlattices are formed easiest on (001) or (110)
and hardest on (111) substrates. For Ni-Au we find a hybrid behavior:
superlattices along and like in phase-separating systems, while for
they behave like in ordering systems. Finally, recent experimental
results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys,
immiscible in the bulk form, are explained in terms of destabilization of the
phase separated state due to lattice mismatch between the substrate and
constituents.Comment: RevTeX galley format, 16 pages, includes 9 EPS figures, to appear in
Physical Review