Atomistic simulation of Ag thin films on MgO(100) substrate: A template substrate
for heterogeneous adsorption
A. Ouahab and C. Mottet*
CRMCN-CNRS, Campus de Luminy, case 913, 13288 Marseille Cedex 9, France
J. Goniakowski
INSP, Campus de Boucicaut, 140 Rue de Lourmel, 75015 Paris, France
Received 10 January 2005; revised manuscript received 6 April 2005; published 8 July 2005
The nanostructuration of Ag thin films deposited on the MgO100 substrate is simulated by classical
molecular dynamics using a tight-binding many-body potential for the metal-metal bonds and a potential fitted
to ab initio calculations for the metal–oxide ones. Due to the lattice mismatch between the Ag deposit and the
MgO100 substrate, the silver film is strained. The stress is partially released by the introduction of misfit
dislocations at the interface. These dislocations form a network with a periodicity of about 10 nm, which varies
for ultrathin films according to the film thickness. The strain induced by the interfacial dislocation cores
propagates across the silver film up to the surface driving to the nanostructuration of the surface. The atomistic
results are compared to the predictions of the elasticity theory. The theoretical results are in a nice agreement
with recent experiments obtained by grazing incidence small angle x-ray scattering revealing a self-
organization of Co clusters adsorbed on a thin film of Ag/ MgO100F. Leroy, G. Renaud, A. Letoublon, R.
Lazzari, C. Mottet and J. Goniakowski unpublished. We show that the preferential Co adsorption site is
obtained on top of tensile surface sites and that the periodicity of the clusters’ self-organization can be tuned by
the Ag film thickness.
DOI: 10.1103/PhysRevB.72.035421 PACS numbers: 68.55.a, 68.47.Jn, 68.43.Bc
I. INTRODUCTION
Nanostructuration of surfaces used as templates for the
self-organized growth of clusters is a subject of intensive
research. The aim is to achieve ordered, homogeneous size
and shape, and high density clusters. Such a collection of
nanoclusters can present very interesting properties, either in
microelectronics as quantum dots,
2
or in magnetism
3
and
also in catalysis.
4
The self-organization and homogeneity of
the clusters’ distribution result from the nanostructuration of
the substrate due to long-range interactions that extend far
beyond the range of typical interatomic interactions.
5
Many
observations of self-organization have been reported con-
cerning a large variety of systems. Among them, we can
distinguish pure surfaces involving surface reconstruction,
6,7
heteroepitaxial systems driving to misfit dislocations
networks,
8,9
chemisorption on metallic surface,
10
or both of
the two last processes involved in the same system.
5
There
have been a lot of studies on the 111-oriented lattice-
mismatch heterogeneous metallic systems. Moiré superstruc-
tures have been predicted theoretically on the Ag/ Cu111
system
11
before to be experimentally observed on various
systems, such as Ag/Pt111,
12
Au/Ni111,
13
and
Ag/Cu111.
8,14,15
In the last two cases, triangle dislocation
loops in the underlying substrate surface were observed by
scanning tunneling microscope and confirmed by atomistic
simulations as a possible alternative to relax the stress in-
duced by the misfit. In any case, the periodic variation of the
strain/stress area due to such superstructures leads to a varia-
tion of adatom binding energy and makes them feasible can-
didates for island nucleation network.
16
Strain-relief patterns induced by misfit dislocation net-
works are mostly observed on metallic
16,17
and
semiconductor
18,19
systems, but usually less in metal–oxide
ones.
20
An Ag–MgO100 interface, is a model system that
has given rise to a large number of theoretical
21–30
and ex-
perimental studies.
31–37
It is now well established, from ex-
tended x-ray-absorption fine structure analysis
32
and grazing-
incidence x-ray-scattering GIXS and grazing-incidence
small-angle x-ray scattering GISAXS experiments
33–35
that
the epitaxial growth follows the cube-on-cube epitaxy with
respect to MgO100 with preferential adsorption of Ag at-
oms on top of oxygen sites, in agreement with most of the
theoretical studies.
23–25,27
Moreover, the GIXS have shown
that the lattice mismatch leads to a misfit dislocation network
with interfacial misfit dislocations oriented along the 110
direction.
33,35
Even in such a case of particularly small misfit
3%, we will see using atomistic simulations that the re-
sidual strain stored in the Ag layer, after the formation of
interfacial misfit dislocations, leads to a nanostructuration of
the Ag free surface. Such a surface strain field modulation
due to the buried incoherent interface has been described in
the framework of the elasticity theory.
38,39
It is notably found
that for a lattice-mismatch heterogeneous system with misfit
f = a
subs
- a
layer
/ a
layer
and interfacial dislocation network of
period = b / f , where b is the Burger’s vector amplitude, a
periodic strain field is created at the free surface which varies
as h
-1
in the vicinity of the core dislocation film thickness
h and as hexp-h for h . In the present study, we
will compare quantitatively the atomistic results with the
predictions of the classical theory of elasticity following the
development proposed by Bonnet and Verger-Gaugry
38
,
Bourret
39
and Willis et al.
40
The smaller the misfit 3%, the larger the interfacial dis-
location network about 10 nm square in the 110 direction,
PHYSICAL REVIEW B 72, 035421 2005
1098-0121/2005/723/03542110/$23.00 ©2005 The American Physical Society 035421-1