Surface Structure of an Ultrathin Alumina Film on Ni
3
Al111:
A Dynamic Scanning Force Microscopy Study
Guido Hamm,
1
Clemens Barth,
1
Conrad Becker,
2
Klaus Wandelt,
2
and Claude R. Henry
1,
*
1
CRMCN-CNRS, Campus de Luminy, Case 913, 13288 Marseille Cedex 09, France
†
2
Institut fu ¨r Physikalische und Theoretische Chemie, Universita ¨t Bonn, Wegelerstr. 12, 53115 Bonn, Germany
(Received 7 April 2006; published 21 September 2006)
The surface structure of an ultrathin alumina film on a Ni
3
Al111 substrate has been studied by
dynamic scanning force microscopy. The alumina film exhibits a hexagonal superstructure with a lattice
parameter of 4.14 nm and a 1=
3
p
1=
3
p
R30
substructure. Two domains rotated by 24
are present.
The film is terminated by a hexagonal lattice of oxygen ions with a lattice parameter of 0.293 nm, which is
rotated by 30
with respect to the substrate lattice. The nodes of the 4.14 nm superstructure and the
2.39 nm substructure are pinned on points of the substrate lattice, where the surface atomic lattice is
almost commensurable. The oxygen lattice is perfectly hexagonal close to these nodes and disordered in
the surrounding regions.
DOI: 10.1103/PhysRevLett.97.126106 PACS numbers: 68.55.a, 68.35.Bs, 68.37.Ps, 68.47.Gh
Ultrathin alumina films are important for several appli-
cations both in fundamental research like model catalysts
[1– 3] and in new technologies like protective coatings
against corrosion and mechanical wear, or dielectric bar-
riers in electronic devices. Although they are only a few
atomic layers thick permitting their characterization by
STM, these films generally show properties close to their
bulk counterparts [2]. Ultrathin alumina films are conven-
iently prepared by high temperature oxidation of NiAl
alloy substrates [4]. The most widely studied alumina
film is the one on the NiAl(110) surface [3,4]. Its atomic
structure has been very recently obtained by a combina-
tion of STM observations and ab initio calculations [5].
Alumina ultrathin films on Ni
3
Al111 are much less
studied despite the fact that they present remarkable fea-
tures [6 –10]. Rosenhahn et al. [8] have shown by STM that
two hexagonal superstructures are present on the surface.
A more recent study has shown that the larger structure,
with a lattice parameter of 4.16 nm, corresponds to the true
crystalline mesh which is commensurate with the Ni
3
Al
substrate, while the other superstructure with parameter of
2.4 nm is a 1=
3
p
1=
3
p
R30
sublattice [10]. LEED and
STM observations revealed the presence of two domains
rotated to each other by 24
[7,10]. A very interesting
property of this alumina film is that by vapor deposition,
metal particles nucleate preferentially on particular sites of
these superstructures, forming hexagonal arrays of clusters
[11,12]. Ag, Au, Cu, Pd nucleate on the nodes of the
4.16 nm unit cell while Mn and V nucleate on the nodes
of the 2.4 nm structure. So far, the surface structure and the
underlying processes determining particle nucleation on
these alumina films are relatively unknown. Maroutian
et al. [9] have shown, by low temperature scanning tunnel-
ling spectroscopy (STS), that the sites of the 4.16 nm
superstructure correspond to an electronic state
(2:0–2:6 eV) within the band gap of the film, but no atomic
structure was seen in the STM images. Conversely, Kelber
et al. [7] observed by STM an atomic lattice of 0.3 nm, but
this observation was too local to determine the relationship
with the previously observed superstructures. In this Letter,
we present dynamic scanning force microscopy (dynamic
SFM) measurements, which offers the possibility to image
surfaces of insulators with atomic resolution [13], allowing
a more complete analysis of the surface structure of the
ultrathin alumina film on Ni
3
Al111. For the first time, the
surface of the alumina film was imaged in situ at the atomic
level together with the two superstructures. Two types of
sites forming these superstructures appear differently and
clearly in the SFM images. An explanation for the origin of
the superstructures is presented.
A Ni
3
Al single crystal (MaTeck, Ju ¨lich) with a polished
surface that is oriented within 0.5
of the (111) direction
has been used as substrate. The Ni
3
Al111 surface was
cleaned by repeated cycles consisting of Ar
ion sputtering
(1.5 kV, 5 A) at room temperature, annealing at 1100 K
for 7 min and 1000 K for another 7 min. The oxide film was
prepared by exposing the sample at a temperature of
1000 K to 40 L of oxygen in an oxygen back pressure of
5 10
8
mbar. Following the oxygen treatment, the
sample was annealed for 5 min at 1050 K to equilibrate
the surface. The procedure of oxidation and subsequent an-
nealing was repeated 3 times until a continuous and highly
ordered oxide film was obtained. Measurements were per-
formed with a room temperature AFM/STM (Omicron
GmbH) equipped with a digital demodulator (Nanosurf).
The base pressure measured 4 10
10
mbar during SFM
experiments. The cantilever (Nanosensors) used for the
experiments shown here had a resonance frequency of
283.4 kHz, a spring constant of 38 N=m, and a peak-to-
peak amplitude of 20 nm. In order to measure surface
features at atomic scale with high precision, the x and y
scanner of the SFM was calibrated with atomic resolution
images of MgO(001), KBr(001), and KCl(001) [14].
The large-scale image in Fig. 1 shows the surface 7 hours
after the preparation of the alumina film and displays the
damping signal [15]. Two distinct features can be ob-
PRL 97, 126106 (2006)
PHYSICAL REVIEW LETTERS
week ending
22 SEPTEMBER 2006
0031-9007= 06=97(12)=126106(4) 126106-1 © 2006 The American Physical Society