### Surf. Sci. Letts., revised 7/01

**Structure of an ultrathin TiO~x~ film, formed by the strong metal**

**support interaction (SMSI), on Pt nanocrystals on TiO~2~(110).**

D. R. Jennison\*

Surface and Interface Sciences Department, Sandia National Laboratories,

Albuquerque, NM 87185-1415

and

O. Dulub, W. Hebenstreit, and U. Diebold

Department of Physics, Tulane University, New Orleans, LA 70118

Abstract

We use first-principles density functional theory to study ultrathin
TiO~x~ films on Pt(111). The preferred interface with Pt has Ti with O
as an overlayer. However, this ordering, preferred over Ti/O/Pt by 2.9
eV/Ti-O unit, produces \>10% stress. This explains a complex structure,
seen using STM, of TiOx bilayers encapsulating Pt(111) nanofacets: the
energetics of stress relief is about ten times that of differences in
the various possible O/Ti/Pt(111)-layer site occupations, thus favoring
dislocation formation. A structure is found that is stable. It consists
of a series of linear misfit dislocations at the relatively weak Ti/Pt
interface that are 6/7 Ti/Pt rows wide. In addition, strong interactions
at the O/Ti interface and O-layer strain also cause the Ti/Pt interface
to abruptly change from hcp- to fcc-site Ti, producing linear "canyons"
(Ti/Pt dislocation cores occur *between* these stripes). Furthermore,
alternating hcp- and fcc-site triangles, each with ten O-atoms, are
separated by bridging O in an abrupt O/Ti misfit dislocation, thus
producing a zigzag pattern. The above dislocations release strain along
both the x- and y-directions in the surface plane. However, we have been
unable to find a stable structure if the zigzag ends consist of O, but
stability is found if they consist of Ti. Finally, reverse bias STM
images indicate the ends might indeed different than the line portions
of the zigzag features.

Keywords: Density functional calculations, Insulating films, Surface
structure, Surface stress, Titanium oxide, Metal-insulator interfaces,
Catalysis, Pt.

\*Corresponding author: drjenni@sandia.gov

1\. Introduction

For several decades, numerous researchers in heterogeneous catalysis
have studied the so-called strong-metal support interaction (SMSI). The
acronym SMSI was coined by Tauster *et al*. \[1\] to account for the
changes in catalytic activity when the group VIII metals Fe, Ni, Rh, Pt,
Pd, and Ir, supported on TiO2 and other metal oxides (TaO5, CeO2, NbO,
etc.), are reduced at elevated temperature. Because SMSI allows one to
tailor the selectivity of a catalyst, it has caused widespread interest
\[2\].

It is now well established that a very thin, reduced titanium oxide
layer migrates onto the metal clusters upon catalyst reduction \[3\].
The same effect happens on model systems, i.e., metals vapor-deposited
onto reduced TiO2 single crystals, and annealed at high temperatures
\[4, 5\]. The composition and geometric structure of the encapsulation
layer needs to be known to develop a realistic picture of reaction
mechanisms \[6\]. A recent STM study by Dulub *et al*. on Pt
nanoclusters supported on TiO2(110) has revealed for the first time the
structure of this encapsulation layer \[7\].

An overview of the Pt clusters after the high-temperature annealing
procedure was given in Ref. \[7\]. Most of the clusters (type A) have a
hexagonal shape elongated along the substrate \[001\] direction, and
their (111) face parallel to the substrate. These are covered with the
crystalline layer shown in Figs. 1. A few have a square shape (type B).
Those are smaller and their surfaces exhibit no long-range order \[7\].
Possibly, they constitute the sides of the (111)-oriented clusters.
Low-energy He+ ion scattering (LEIS) data clearly showed that the
clusters are encapsulated

The complexity of the structure seen in Fig. 1 is certainly surprising.
In this Letter, we report calculations made to determine the physical
origin of the structure and, in the process, show one logical method of
analysis that can be used to better understand unusual ultrathin oxide
films on metallic substrates.

Concerning theory, recent advances in computing and in algorithms have
made possible the study of complex oxide systems from first principles.
For example, the κ-phase of Al~2~O~3~ was recently determined entirely
from density functional theory (DFT), and is in excellent agreement with
X-ray scattering data \[8\]. Also, analyses of ultrathin alumina film
structures on Ru(0001) substrates were done \[9,10\]. The interfacial
structure \[9\] is in agreement with recent spectroscopic data from
films on Ni~3~Al \[11\], and a prediction of Al ion first row
tetrahedral site preference \[9\] was recently observed in an X-ray
diffraction study of a film on NiAl \[12\]. The first-layer
Al-sublattice structure predicted in Ref. \[10\] also explains observed
faint half-order LEED spots seen in films on Ru(0001) \[13\].

Concerning titania, a complex surface rosette structure, proposed on the
basis of STM observations, was supported by DFT computations involving
over 250 atoms per unit cell \[14\]. This structure appears when
slightly reduced TiO~2~(110) is annealed over a relatively narrow range
of temperatures in an oxygen ambient \[15\].

While DFT can test a suggested complex structure for stability, and
describe properties such as geometry and local density of states (LDOS)
\[14\], it is still not computationally possible to predict such
structures for oxide/metal systems. This is because there are no
adequate model potentials for large-scale Monte Carlo or molecular
dynamics simulations. In contrast, DFT has long been known to be
accurate for the description of metallic systems, and more recent
evidence shows it is also accurate for the description of oxides.
However, it is still computationally prohibitive to use DFT in
simulations of the unit cell size needed here.

Below, we report a test of the proposed structure of Ref. \[7\] (Fig.
2), but it is immediately found to be unstable (Fig. 3a). Analysis shows
two problems, the first involving the Ti-O vs. O-O distances at the
surface and the second the proposed Ti/O/Pt bridge site O-stacking in
the zigzag structure, that appears unstable in the underlayer. Simply
reversing the stacking of the bridging zigzag atoms does not correct the
problem (Fig. 3b).

Therefore, we report a more systematic analysis of possible ultrathin
Ti-O structures on Pt(111), beginning with the nature of the interface
with Pt and the various possibilities for site occupation within the
film. We then address the surface stress and its energetics.
Large-unit-cell results are then presented for a likely structure (Fig.
4), built on a dislocated underlayer of Ti/Pt(111). This is found to be
stable with Ti atoms at the zigzag ends. However, if O atoms comprise
the ends, despite several attempts to stabilize them, we find they
"fall" into the linear canyons. Finally, we present evidence from
reverse bias STM that supports the conclusion that the end atoms are
indeed different than the line portions.

1.1. Description of the original model

A model to account for the zigzag structure displayed in Fig. 1 was
described in detail in Ref. \[7\] and is shown in Fig. 2. Here we
summarize the main experimental findings and the most important features
of the model.

Low energy He+ ion scattering (LEIS) results clearly show that the
clusters are encapsulated. The ratio of Ti:O on top of the clusters was
estimated as 1:2 \[7\]. This value was achieved with several
simplifications (coverage determination with STM without taking into
account cluster broadening by the tip, neglect of contribution from
square clusters and sides, neutralization probability the same for
scattering from encapsulated clusters and the uncovered TiO~2~ surface,
although the electronic structure is different). In agreement with these
previous XPS measurements \[5\], Ti *2p* spectra indicate the presence
of a lower oxidation state after the high-temperature treatment causing
the encapsulation. XPS taken with glancing take-off angles \[5\] were
performed on TiO2(110) samples with Pt coverages and thermal treatments
comparable with this work. The observed binding energy is consistent
with a Ti2+ oxidation state (which would correspond to a TiO layer) but,
based on relatively broad spectral features, the coexistence with more
reduced forms of Ti could not be ruled out \[5\]. Wang *et al*. observed
Pt-Ti alloy formation at higher annealing temperatures \[16\], and three
kinds of superstructure particles (Pt3Ti (cubic), Pt3Ti (hexagonal), and
PtTi) were identified with HREM. Formation of a PtTi3 alloy on
titania-impregnated Pt catalysts resulted in a Pt *4f7/2* peak shift of
0.3 eV \[17\]. We have not observed a shift or change in lineshape or
the Pt 4f peak and conclude that the clusters themselves remained Pt.

On top of the type A clusters, striped \'zigzags\' are visible (Fig. 1).
The stripes are approximately 15 Å wide and consist of bright spots
arranged in a hexagonal symmetry with a distance of \~3 Å. The bright
zigzag rows contain either 5 or 6 atoms along the close-packed
directions and separate triangular areas consisting of 10 atoms. The
apparent height of the zigzags is dependent on the STM bias voltage
(Fig. 1a vs. 1b). The empty-state image in Fig. 1a was taken with Vbias=
+1.12 V, and the apparent height of the bright zigzag atoms is \~0.5 Å.
Filled-state images with negative sample biases also showed the zigzag
structure (Fig. 1b top half), albeit with less resolution. STS on the
encapsulation layer on top of the Pt clusters shows a semiconductor-like
behavior. The black \'canyon\' separating the zigzag stripes is always
imaged black, indicating that the contrast is topographical in nature.

The atomic model proposed in Ref. \[7\] for the surface of type A
clusters is displayed in Fig. 2. It consists of two layers of titanium
oxide (side view Fig. 2), with misfit dislocations. The zigzag is
visible in both, empty and filled-state images (Fig. 1b), which
indicates higher positions of atoms in the zigzags. In empty-states
images of the clean TiO~2~(110) surface, Ti atoms are imaged bright
\[18\]. It is suggestive to associate bright spots on the zigzags with
Ti, and darker spots with O atoms. Six Ti atoms separate triangles of
ten oxygen atoms. The 20 oxygen and 10 Ti atoms per unit cell are
consistent with the (estimated) LEIS O:Ti intensity ratio.

Ti atoms in the second layer are placed in (approximately, see below)
3-fold hollow positions of the Pt fcc(111) substrate. The fcc and hcp
stacked domains contain eight titanium atoms each. (These are labeled B
and C and marked by forward and back-slashed regions in the side view of
Fig. 2.) They are separated by domain boundaries consisting of titanium
and oxygen atoms in bridge positions. Five oxygen atoms form zigzags,
which are covered by Ti atoms in the top layer. Rows of second layer
titanium atoms separate the stripe domains and appear as the dark
canyons in STM. Atoms in the top layer are placed in (approximately)
on-top (or A-A) positions above platinum atoms of the substrate (see
side view in Fig. 1). Ti atoms in zigzags reside in bridge positions
above the second layer oxygen atoms, and are 6-fold coordinated
(counting four oxygen neighbors in-plane and two underneath).

The registry of atoms on either side of the canyon suggests a change in
adsorption site with respect to the underlying fcc(111) lattice (see
line in Fig. 1a) This is achieved by expanding the top Ti-O layer
perpendicular to the stripes by a factor of 16/15. This uniaxial
expansion causes all the atoms to slightly shift from their perfect
hollow or bridge positions. Domains of differently stacked atoms and
partially incommensurate overlayers are common ways to accommodate
strain relief caused by lattice-mismatched ultrathin-film metals on
metal substrates. In the present case, the issue is the lattice constant
of a film with (almost) TiO stoichiometry placed onto a Pt(111)
substrate (containing 1.50 x 1015 atoms/cm2 per layer). The atomic
density of the surface layer in the model, 1.29 x 1015 atoms/cm2, is
reasonably close to the density of a TiO(111) layer (1.32 x 1015
atoms/cm2 per layer). The observed lattice constant, 3 Å, compares well
to interatomic distances on a TiO(111) surface (2.95 Å).

The model shown in Fig. 2, while consistent with experimental
observation, is necessarily speculative and leaves several questions
open. In particular we set to find out 1) if the proposed structure is
accurate in detail, and 2) what is the physical origin of the zigzag
cores and the black \'canyons\'.

1.2. Theoretical Analysis

The DFT \[19\] calculations used the Vienna *Ab initio* Simulations
Package (VASP) \[20\] in the local density approximation (LDA) \[21\].
The ultrasoft pseudopotentials of Vanderbilt \[22\] allowed good
convergence with a plane wave cutoff of only 270 eV. Because of
long-range electrostatic forces which occur with oxide, as opposed to
metallic systems, it is necessary to use a much larger vacuum gap
between the slabs (which repeat along the c-axis due to the plane wave
basis set). We have found \~ 16 Å or greater to be adequate. Because
these systems often display both hard and soft vibrational modes,
geometric relaxation must be done in small steps. Energetically, while
the damped molecular dynamics algorithm almost always proceeds
monotonically downhill, a poor first guess results in so much geometric
\"noise\" that equilibrium takes a prohibitively long time to be found.
Therefore, it is best to devote some time to making an accurate first
guess, especially with larger unit cells. In addition, it is advisable
to restrict the geometric relaxation to certain portions of the
structure (such as only the TiO overlayer) before relaxing the complete
structure (including the Pt substrate). Our first guess for atomic
positions was a composite of STM-derived coordinates for the in-plane
locations and LDA results (on the model systems described below) for the
vertical layer separations.

DFT in the LDA, as solved using the VASP code with Vanderbilt
pseudopotentials, is very accurate for geometry determination in bulk
systems related to the present study. The LDA lattice constant (vs.
experiment) for Pt is 3.910 (-0.3%), for TiO is 4.196 (+0.6%), and for
TiO~2~ is 6.461 (-0.1%) Å. It is normal to set the substrate lattice
constant at the LDA value to reduce an artificial source of stress.
However, in a heterogeneous system such as TiO/Pt, this choice
immediately induces an artificial stress of \~1% in the TiO overlayer,
as the LDA bulk errors have opposite sign. While small, this is a
potential source of error not present in calculations on more homogenous
systems.

As stated in the introduction, the structure proposed in Ref. \[7\] is
found not to be stable computationally. However, before we consider any
overall structures, we first report the results of model calculations
used to isolate the various components of the important energetics
concerning the film.

2\. Results

2.1. The Energetics of the Film

Beginning our systematic analysis, the nature of the interface of a Ti-O
two-atomic-layer film with the Pt(111) substrate was studied using
relaxed slabs. These had four atoms/layer and seven layers of Pt (with
the bottom four frozen at the bulk Pt LDA geometry) and 12 k-points were
used. The two overlayers were stacked first in the fcc progression with
either Ti in the first layer and O in the second, or visa versa. (A Ti
monolayer on Pt(111) prefers fcc to hcp hollows by a very small amount,
\<0.02 eV per Ti atom; in addition, the possible hollow-site stackings
for the O/Ti/Pt(111) system differ by \< 0.03 eV per Ti-O unit, see
below.) It was found that by a wide margin, 2.9 eV per Ti-O unit, the
interface preferred Ti in contact with Pt, with O as the top layer.
These results, with layer spacings and energetics seen in Table I, are
consistent with the triangular regions seen by Dulub *et al*. \[7\]
(Fig. 1) and their assignment of oxygen as the topmost atoms in these
areas (Fig. 2).

It is important to note that the Ti at the interface is *not* ionic, but
is metallic with a fractional positive charge. This is also true for
other systems, such as O(1x1)/Al(111) \[9, 23\] or O(1x1)/Ru(0001)
\[13\]: the surface oxygen, though negatively charged, obtains that
charge from a distribution of metal atoms that retain a metallic local
density of states (LDOS), even though charge transfer is evident from an
integration of the LDOS (e.g., see the computed interfacial LDOS in Ref.
\[9\], see also Ref. \[24\] and Fig. 5, described below).

Here, we have the complicating factor of the surface metallic layer
being of a different metal (Ti) than the deeper metallic layers (Pt). To
further examine the question of ionicity for the Ti layer, we compare
the interlayer spacing of O/Ti with those found in a TiO crystal (Table
I). We find significantly less separation between these atoms in the
commensurate O/Ti/Pt(111) film system than in the TiO compound, with the
interlayer spacing decreasing by over 0.2 Å, from 1.21 to 0.99 Å.
Furthermore, the Ti/Pt layer spacing is 2.15 Å, considerably larger than
would be obtained by assuming Ti were ionic. With a Pt metallic radius
of 1.39 Å and a Ti^2+^ ionic radius of 0.90 Å, the height would be 1.63
Å; in contrast, the computed spacing is significantly closer to the sum
of metallic radii (with a Ti radius of 1.47 Å, that height is 2.37 Å).
These numbers indeed suggest that Ti at the interface is not ionic, but
is a somewhat positive metallic species -- i.e. it is less oxidized than
+2.

Next, we varied the O/Ti/Pt stacking to include all ways to place the O
and Ti layers in hollows with respect to the underlying material. As
seen in Table II, the differences are very small compared with the
overall ordering reported above, showing it is easy energetically to
have both stackings, again supporting a proposal by Dulub *et al*.
\[7\], that the triangular oxygen regions alternate fcc- and hcp-hollow
occupancy (Fig. 2). Also shown in Table II is the energy required to
move O on Ti/Pt(111) to the bridge position. This was found by moving an
entire layer, as the STM image in Fig. 1 shows all surface atoms are at
about the same spacing. For comparison, we also show the energy for
making Ti bridge on Pt(111), rather than sit in the preferred fcc
hollow. These energies are to be used as estimates for the cost of
dislocation formation, discussed below.

In some ultrathin layers of oxide (e.g. Ref. \[10\]), a commensurate
relationship between the oxide and the substrate can be assumed due to
excellent lattice matching. In these cases, large regions of the real
surface can be expected to be free of dislocations. In the present case,
we cannot immediately estimate the surface stress (produced by the
lattice mismatch of the O/Ti bilayer with the Pt(111) substrate) because
film-Pt(111) system has no bulk analogue to use for a two layer film of
O/Ti having polar surfaces.

The surface stress associated with the O/Ti/Pt(111) system was computed
using a series of relaxed thin slab calculations (Table III); otherwise
the computations were the same as those described above. As expected,
two layer slabs of only Pt showed a reduced unit cell, with a lattice
constant -2.4% vs. the bulk. This illustrates the normal concept of
surface stress, analogous to surface tension, caused by reduced
coordination at the surface. When the two layer Pt slab was compared to
one with a layer of Ti above the hollows of a single layer of Pt,
surprisingly the lattice constant did not change within 0.2%. This shows
that the interface with the substrate does not in and of itself
contribute to stress in the film.

Next, three layer slabs of O/Ti/Pt were studied. A very large outward
expansion of the lattice constant was found, amounting to +7.0% compared
with the Pt bulk LDA spacing of 2.77 Å. This is solely due to the O/Ti
bilayer itself, as confirmed by a two-layer computation of same, showing
a +10.4% expansion. Such a large value explains several features
observed by Dulub, *et al*. \[7\], as *both* the separated triangular
oxygen islands and the linear missing rows of O atoms reduce the surface
stress, one along the vertical axis and one along the horizontal.

To explore the energetics of the stress and its relief, we report the
energy necessary to compress the O/Ti/Pt three layer and the O/Ti two
layer slabs to the LDA lattice constant of Pt (Table III). These
energies are 0.3 and 0.6 eV per Ti-O unit, respectively, and are
sufficient to explain the cost of making the complexities seen in the
structure, which are reflected in the energetics in Table II.

2.2. Zigzag Structure

Finally, we turn to the overall structure itself: With a unit cell of
over 100 atoms, a single k-point was used. The proposed structure (Fig.
2) was first studied with a unit cell of two layers of frozen Pt metal
under the oxide film. This structure incorporates several important
observations from the data (Fig. 1) including the overall alignment of
the zigzags and the directly opposed atoms across the missing rows. As
stated in the introduction, it was immediately found to disagree with
experiment (Fig. 3a). The reason became obvious: The distances between
the zigzag Ti atoms and the O atoms to the side are too long compared
with the optimal distance in TiO (2.9 vs. 2.1 Å). In addition, the Ti
atoms in the zigzag lines do not have symmetry in the direction
perpendicular to the lines, as fcc vs. hcp stacking exists on opposite
sides. This produces a small initial force to displace these atoms off
the lines, and the movement continues until the Ti atoms achieve near
optimal Ti-O spacings (Fig. 3a).

An obvious possible correction is to invert the stacking at the zigzag
dislocation cores so that O/Ti/Pt is the order throughout the structure.
Now, O-O repulsion stabilizes the zigzag pattern, as was immediately
seen in early stages of geometric relaxation, and it is now natural for
the zigzag-to-O distances to be about the same as O-O, as is evident
from the data (Fig. 1). However, further relaxation again introduced
large distortions vs. experiment (Fig. 3b). An examination of the second
layer showed significant movement of Ti atoms off the bridge positions
in the zigzag cores and suggested a source of the problem was in the
second layer, invisible of course to STM.

In Table IV, we show the computed adhesion energies of the O/Ti vs.
Ti/Pt interfaces from a series of slab calculations. Note that: 1) the
interfacial energies are locally determined and vary little depending on
the slabs used, and 2) the O/Ti adhesion is much stronger than that of
Ti/Pt. This means it costs little energy to relieve stress within the
O/Ti layers by creating a Ti misfit dislocation with respect to the
underlying Pt(111), along the y-axis in the figures, see Fig. 4. In
turn, this produces *uniform* missing rows, with a width in agreement
with experiment \[7\] and directly opposed atoms on either side of the
resulting "canyons" (described in more detail below).

The structure resulting from the above analysis was quite stable but
showed one final problem: The atoms at the zigzag ends (or "points") had
no stability against falling into the missing row canyons. This was true
even when the zigzag pattern was frozen in the x- and y-coordinates and
not released until the rest of the entire film was relaxed, and then
relaxed with only small steps in the damped molecular dynamics
algorithm.

Thus, they were replaced with Ti atoms and the required stability was
found. A new proposed structure is shown in Fig. 4. The yellow numbers
in Fig. 4 point out alignment features that agree with experiment (Fig.
1). In addition, to show the second layer Ti atoms, we have removed the
hollow site O atoms in the right portion of the picture: the second
layer has 6 vertical rows of Ti for each 7 rows of Pt. Therefore, based
on these calculations and the similarity in O-O and zigzag-O distances
in the top layer, we conclude that the zigzag atoms cannot be composed
entirely of Ti.

The structure does seem somewhat ad hoc, having only two Ti atoms in the
top layer, and the (positive-bias) STM images of Dulub et al. \[7\] show
a brightening at all the zigzag atoms. However, the reverse bias STM
image (Fig. 1b) does suggest there is something different about the end
point vs. the line zigzag atoms. Ti atoms in special positions might
very well be responsible for this increased reactivity observed in SMSI
systems \[1,2\]. While we do not have conclusive experimental evidence
for the proposed isolated Ti atoms embedded in a layer of oxygen atoms,
it is tempting to think of them as highly reactive sites.

We have also computed the local DOS and the charge density contained in
eigenvectors within 0.1 eV of a bias voltage of interest. This means
eigenvectors between E~f~ = 0 and -0.1 eV for the sample negative bias,
where the electrons come from near E~f~, and at +1 ± 0.1 and +2 ± 0.1 eV
for the sample positive bias. We find two things: 1) In sample negative
bias, the point Ti atoms have a higher density with respect to the
bridging O atoms than with the positive bias, and therefore should be
brighter (cf. Fig. 1b top half and Fig. 5 at E~f~); and 2) At the
relevant tip distances, the charge densities of the Ti and the O zigzag
atoms are comparable at the positive bias (cf. Fig. 1a). It was not
possible to be more definitive here because, at reasonable tip
distances, the details of the (extremely small) computed charge density
are lost in the considerable numerical noise. However, these trends are
at least consistent with the STM observations for both biases.

Concerning the electronic structure of the film, the LDOS of selected
atoms may be found in Fig. 5. Note both types of oxygen look very ionic
with little DOS between --3 eV and the Fermi level (at zero). The
bridging oxygen has its 2p band at higher energy due to reduced
coordination, but appears about as ionic (from the area) as the others.
The bandwidths are about 3 eV, much less than the 6 eV typical of bulk
oxides, again due to the reduced coordination. The Ti atoms show a
typical amount of LDOS mixing in the oxygen band for
reduced-coordination titanium oxide structures (cf. Ref. \[14\]), but
also show a rising DOS from about --3 eV to E~f~ for the underlayer.
This is atypical of ionic Ti and is due to the proximity of the Pt
atoms, indicating some metallic bonding between the somewhat positive Ti
and the Pt(111) surface. Meanwhile, even the top-most Ti has some DOS in
this region due to the thinness of the film.

It is worthwhile noting that counting downward in Fig. 4, along the
zigzag overall direction, one finds eight O atoms for ten Pt-Pt
distances, and 6 atoms for 7 distances along the Pt rows which intersect
the missing rows. Thus in the observed structure \[7\], stress
relaxation is in the 14-20% range, consistent with the magnitude of the
computed O/Ti stress.

While the overall structure agrees quite well with the experimental
result \[7\], some differences remain. At least partially, this is due
to inherent problems in computing systems of mixed materials, as noted
above. However, there is also a second conceivable reason: it is now
known that an ultrasoft vibrational mode is present on the TiO~2~(110)
surface that noticeably affects the geometry compared with DFT
calculations \[25\]. This finite temperature effect could quite
conceivably occur in the present case, especially involving the zigzag
atoms, and cause the STM results (at room temperature) to differ in
detail from the zero degree Kelvin LDA geometry.

Interestingly, an interfacial preference analogous to O/Ti/Pt occurs for
another Group VIIIc element. In a recent study of a Pd overlayer on a
film of Al~2~O~3~/Al(111), the interfacial energy associated with Pd/Al,
as opposed to Pd/O, actually overcame the cohesive energy of Al metal.
During a damped molecular-dynamics relaxation, aluminum was drawn into
the oxide film, creating a REDOX reaction that reduced Al ions near Pd;
thus, the preferred Pd/Al(1ML metallic)/Al~2~O~3~ interface began to
form \[10\]. The preference for cation species at the interface was also
recently seen in a study of vanadium oxide/Pd \[26\].

3\. Conclusions

Our computations have found a structure consistent with experimental STM
observations of Dulub, *et al*. \[7\] and the energetics of a TiO film
on Pt(111) determined from theory. It originates first in a strong
preference of Pt(111) for having Ti at the interface; secondly, in the
weak stacking preference within an ultrathin O/Ti film on Pt(111), thus
facilitating dislocation formation; thirdly, in the relative weakness of
adhesion at the Ti/Pt interface vs. the O/Ti interface, that causes the
former to distort more than the latter; and finally, and most
importantly, in the large surface stress produced by the polar
O-overlayer on Ti/Pt(111).

VASP was developed at the Institut für Theoretische Physik of the
Technische Universität Wien. Sandia is a multiprogram laboratory
operated by Sandia Corporation, a Lockheed Martin Company, for the
United States Department of Energy under Contract DE-AC04-94AL85000.
This work was partially supported by a Laboratory Directed Research and
Development project (Sandia) and by NSF-CAREER (Tulane).

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# Tables

Table I. The relative energies (in eV per Ti-O unit) and interlayer
separations (in Å) of fcc stacked O/Ti/Pt(111) vs. Ti/O/Pt(111). The
spacings are also compared to a TiO crystal and the sum of the metallic
radii of Ti and Pt.

#  O/Ti O/Ti Ti/Pt Ti/Pt O/Pt

∆E (film) (TiO) (film) (radii) (film)

O/Ti/Pt 0.0eV 0.99Å 1.21Å 2.15Å 2.35Å n/a

Ti/O/Pt 2.9eV 0.90Å 1.21Å n/a n/a 2.85Å

Table II. The relative energies per Ti-O unit of the various hollow- and
bridge- (bd-)-site stacking sequences in an O/Ti/Pt(111) film. (The
geometry was relaxed to within 0.002 eV of the minimum.) For comparison,
also shown is the bridge vs. fcc relative energy per atom for a
monolayer of Ti on Pt(111).

Stacking ∆E(eV)

# O(fcc)Ti(fcc) 0.000

# O(fcc)Ti(hcp) 0.000

O(hcp)Ti(hcp) 0.011

O(hcp)Ti(fcc) 0.024

O(bd)Ti(fcc) 0.656

Ti(bd) vs. Ti(fcc) 0.128

Table III. The relaxed lattice constant (*a*~o~) with respect to the LDA
value for bulk Pt, and the energetics of stress relief (per primitive
unit cell, i.e. per TiO unit) for several films, found by comparing the
relaxed film to one constrained to the LDA *a*~o~ of bulk Pt (3.91Å).

# Film relaxed *a*~o~ ∆E(eV) @ 3.91 Å

Pt/Pt -2.4% +0.05

Ti/Pt -2.3% +0.08

O/Ti/Pt +7.0% +0.3

#### O/Ti +10.4% +0.6

Table IV. Interfacial adhesion energies (in eV per primitive cell)
computed by comparing the total energies of various relaxed slabs (the
"Pt" slab had five layers, with the bottom three frozen at the bulk LDA
geometry).

Interface Slabs Used Adhesion

Ti/Pt O/Ti/Pt vs. O/Ti and Pt 2.95

Ti/Pt Ti/Pt vs. Ti and Pt 2.75

O/Ti O/Ti/Pt vs. O and Ti/Pt 9.57

O/Ti O/Ti vs. O and Ti 9.37

Figure Captions

1.  Atomically-resolved STM images (100 x 100 Å2) of an encapsulated
    Pt(111) - oriented cluster, showing the crystalline overlayer that
    is analyzed in this work: a) Empty-states (sample positive bias)
    constant-current topography; b) STM bias voltage was reversed at the
    arrow from positive (bottom half) to negative (top half). Note the
    change in contrast at the zigzag ends vs. the middle portions.

2\. Top and side view of the atomic model for the encapsulation layer
displayed in Fig. 2 that was proposed in Ref. \[7\].

3\. Relaxed unit cells in the LDA: a) starting from the proposed
structure (Fig. 2) \[7\], which has Ti/O/Pt bridge site stacking at the
zigzag cores; and b) starting from the same structure, but with O/Ti/Pt
stacking at the cores. Bridging surface Ti or O atoms are shown in blue,
surface oxygen in red, second layer Ti in gold, and Pt in pink.

4.  Extended view of the proposed structure, where the interfacial Ti
    atoms have a misfit with the underlying Pt(111); note the uniform
    narrow canyons, whose widths agree with experiment (Fig. 1) \[7\]. A
    portion of the surface has been removed on the right side to show
    the second layer Ti dislocation with respect to Pt(111). The surface
    Ti atom is in green, the zigzag bridging oxygen atoms in blue,
    surface hollow site oxygen in red, second layer Ti in gold, and Pt
    in pink. The yellow lines and numbers are features used to point out
    agreement with the STM data \[7\].

5.  Local density of states for selected atoms in the structure shown in
    Fig. 4. O-h is a hollow site oxygen (red in Fig. 4)), O-b bridging
    (blue), Ti-b bridging (green) and Ti-u is an underlayer Ti (gold).