Early stages of NiO growth on Ag(001): A study by LEIS, XPS, and LEED

Article abstract of DOI:10.1021/jp037805o

Ultrathin nickel oxide films were grown on the Ag(001) surface by evaporation of Ni in an O₂ atmosphere. The growth mechanism, composition, and structure of the NiO layers formed in the early stages of deposition have been investigated by LETS, XPS, XPD, and LEED. We found that at O₂ pressures in the 10^-7- 10^-6 mbar range and sample temperatures between room temperature and 373 K, a nickel oxide layer forms showing a (2 × 1) LEED pattern. This is a phase of monatomic thickness which gradually covers the substrate surface upon increasing the surface coverage. LETS and XPS data show that before the (2 × 1) layer is completed NiO(001) islands start to nucleate. The substrate surface is completely covered by NiO(001) for coverages higher than 2 ML. When NiO is deposited with the sample held at 453 K, we found that in the early stages of deposition metallic Ni is present in the subsurface region of the substrate. Upon further NiO deposition the oxide becomes the prevalent species on the surface. Under these conditions, the (2 × 1) phase is not observed. We found that the (2 × 1) phase is not stable upon annealing at 473-600 K and transforms into NiO(001). LEIS and XPD results indicate the formation of NiO islands 2 ML thick upon annealing. The structure of the (2 × 1) phase has been investigated by LEED intensity analysis (tensor LEED method). The best agreement with the experimental data is obtained for a structural model similar to the (111) surface of NiO. The effects of surface temperature and O ₂ pressure on nickel oxide growth are discussed in terms of the low sticking probability of O₂ on Ag(001) and of the interaction between nickel and silver.

Full text of DOI:10.1021/jp037805o

J. Phys. Chem. B 2004, 108, 9919-9926
9919
Early Stages of NiO Growth on Ag(001): A Study by LEIS, XPS, and LEED
†
†
†
,‡
§
M. Caffio, B. Cortigiani, G. Rovida, A. Atrei,* and C. Giovanardi
Dipartimento di Chimica, UniVersit a` di Firenze, Polo Scientifico di Sesto F.no. 50019, Sesto F.no (Fi), Italy,
Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi, UniVersit a` di Siena. 53100, Siena, Italy, and
3
INFM-National Research Center on NanoStructure and BioSystems at Surfaces (S ), 41100, Modena, Italy
ReceiVed: December 11, 2003; In Final Form: April 30, 2004
Ultrathin nickel oxide films were grown on the Ag(001) surface by evaporation of Ni in an O
2
atmosphere.
The growth mechanism, composition, and structure of the NiO layers formed in the early stages of deposition
-
7
-6
have been investigated by LEIS, XPS, XPD, and LEED. We found that at O pressures in the 10 - 10
2
mbar range and sample temperatures between room temperature and 373 K, a nickel oxide layer forms showing
a (2 × 1) LEED pattern. This is a phase of monatomic thickness which gradually covers the substrate surface
upon increasing the surface coverage. LEIS and XPS data show that before the (2 × 1) layer is completed
NiO(001) islands start to nucleate. The substrate surface is completely covered by NiO(001) for coverages
higher than 2 ML. When NiO is deposited with the sample held at 453 K, we found that in the early stages
of deposition metallic Ni is present in the subsurface region of the substrate. Upon further NiO deposition the
oxide becomes the prevalent species on the surface. Under these conditions, the (2 × 1) phase is not observed.
We found that the (2 × 1) phase is not stable upon annealing at 473-600 K and transforms into NiO(001).
LEIS and XPD results indicate the formation of NiO islands 2 ML thick upon annealing. The structure of the
(
2 × 1) phase has been investigated by LEED intensity analysis (tensor LEED method). The best agreement
with the experimental data is obtained for a structural model similar to the (111) surface of NiO. The effects
of surface temperature and O pressure on nickel oxide growth are discussed in terms of the low sticking
probability of O on Ag(001) and of the interaction between nickel and silver.
2
2
1
. Introduction
same in-plane parameter of the substrate) for a thickness ranging
from 5 to 10 ML, depending on the preparation conditions.3
,7,8
The geometric structure, electronic properties, and reactivity
For higher coverages, the strain is released by the formation of
of oxide films a few atomic layers thick deposited on metal
surfaces are expected to be different from those of the surfaces
of bulk oxides due to the nanometric thickness of the films and
the interaction with the substrate.1,2 In addition to that, the
morphology of the films, the presence of defects, may be
important in determining the physical and chemical properties
of these systems. The study of relatively simple oxides (such
as NiO and MgO) grown on metal surfaces is relevant in order
to understand fundamental aspects concerning the structure and
properties of ultrathin oxide layers on metal substrates. Well-
ordered, epitaxial nickel oxide films can be prepared by
evaporation of metallic nickel in an O2 atmosphere on substrates
having the appropriate lattice parameter. The Ag(001) surface
appears to be an ideal substrate for the epitaxial growth of NiO
films oriented along the (001) direction due to the small lattice
mismatch (2.3%) and the scarce reactivity of silver toward O2.
Indeed, several studies performed by means of various surface
science methods show that well-ordered NiO(001) films grow
on Ag(001) (in a wide range of sample temperatures, oxygen
pressures, and evaporation rates) for coverages higher than 2
3
5
misfit dislocations and NiO assumes its bulk lattice parameter.
The structure of a NiO film 2 ML thick has been determined
by low-energy electron diffraction (LEED) intensity analysis.4
This study confirmed the formation of a pseudomorphic
NiO(001) film and showed that O atoms are on top of Ag atoms
at the oxide-substrate interface. The results of these studies
are in good agreement with recent ab initio calculations
9
performed for this system. The situation is not so well-defined
for submonolayer coverages of nickel oxide. In the very early
stages of deposition, an oxidic phase exhibiting a (2 × 1)
3,10,11
periodicity is observed.
Upon increasing the NiO coverage
the (2 × 1) LEED pattern disappears and the (1 × 1) diffraction
diagram, corresponding to pseudomorphic NiO(001), is ob-
3
served. Recently, the effect of the deposition conditions (in
particular the O2 flux during Ni evaporation) on the formation
1
2
of the (2 × 1) phase has been studied by Giovanardi et al.
The (2 × 1) phase of NiO has been mainly investigated by
means of scanning tunneling microscopy (STM), and on the
basis of the STM images a structural model was proposed.1
However, a detailed spectroscopic characterization and a de-
termination of the composition for the (2 × 1) phase are missing.
0,11
3
-8
monolayer (ML).
completely cover the substrate at a coverage of 2 monolayers
ML), and above this thickness a layer-by-layer growth was
It was found that nickel oxide films
(
In the present work we investigated by means of X-ray
photoelectron spectroscopy (XPS) and low-energy ion scattering
3
observed. The NiO(001) is pseudomorphic (that is with the
(LEIS) the composition and the growth mechanism of the phases
*
Corresponding author. E-mail: atrei@unisi.it.
Universit a` di Firenze.
Universit a` di Siena.
formed in the early stages of nickel oxide deposition on Ag(001).
X-ray photoelectron diffraction (XPD) and LEED were used
for the structural characterization of the nickel oxide films. An
analysis of the LEED intensities (using the tensor LEED
†
‡
§
INFM-National Research Center on NanoStructure and BioSystems at
3
Surfaces (S ).
1
0.1021/jp037805o CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/08/2004

9920 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Caffio et al.
method) was carried out to determine the crystallographic
structure of the (2 × 1) precursor phase. The effect of substrate
temperature and O2 pressure on the composition and structure
of the NiO layers was investigated with the aim of understanding
the driving force leading to the formation of the precursor
(2 × 1) phase.
2
. Experimental Details
The experiments were carried out in an ultrahigh vacuum
-
10
(UHV) chamber with a base pressure in the low 10
mbar
range. The chamber was equipped with an ion gun, X-ray
source, hemispherical electron/ion energy analyzer, LEED
optics, and quadrupole mass spectrometer. Nonmonochroma-
tized Al KR radiation was used for XPS and XPD measure-
ments. The angle between the analyzer axis and the X-ray source
was 55°, and the semicone angle of acceptance of the analyzer
was 4°. The XPS spectra were measured with a fixed pass
energy of 44 eV. The binding energy (BE) scale was calibrated
13
setting the Ag3d5/2 peak to 368.3 eV. The inelastic background
14
in the spectra was subtracted by means of the Shirley method.
For LEIS we used a He ion beam with an energy of 1 keV
impinging on the surface at an angle of 45°, along the [100]
direction. The scattering angle was 135°. The sample was
mounted on a manipulator which permits polar rotation (around
the manipulator axis) and azimuthal rotation (around the normal
to the sample surface). The XPD curves were acquired by
monitoring the intensity of the Ni2p3/2 and Ag3d5/2 peaks as a
function of the polar emission angle in nonequivalent azimuthal
directions. The background was estimated by measuring simul-
taneously the intensity at a given peak maximum and the
intensity at kinetic energies 5-10 eV higher than the peak.
The LEED measurements were performed by means of three-
grid rear view LEED optics. The intensities versus accelerating
voltage (I-V) curves of the diffracted beams were collected by
means of a video LEED system. The LEED I-V curves were
measured at normal incidence of the electron beam in the energy
range of 30-250 eV. The I-V curves of symmetric beams were
averaged to compensate for minor differences due to small
deviations from normal incidence. The I-V curves were
background subtracted and normalized to constant incident
electron current. The LEED intensities were collected at room
temperature.
Figure 1. Ni2p XPS spectra measured for increasing amounts of nickel
oxide deposited on Ag(001). The films were prepared by evaporating
-6
Ni in an oxygen atmosphere (pO
2
3 × 10 mbar) with the sample
held at 373 K. For comparison the spectrum measured for 1 ML of Ni
deposited in vacuum is shown. For the NiO coverage of 0.8 ML, a
(
2 × 1) LEED pattern is observed. The intensity of each spectrum is
normalized to its maximum, and an offset has been added for a better
reading.
photoelectrons). Essentially the same thickness was obtained
using the Ag3d intensity measured for the clean Ag(001) surface
and the Ni2p intensity measured for a NiO film thick enough
that the Ag3d signal was barely visible. The thickness of the
film is expressed in ML, considering that 1 ML of NiO(001)
has a thickness of 2.1 Å. For a pseudomorphic NiO(001) film
2
this corresponds to a density of 0.12 Ni atoms/ Å . The
evaporation rate, estimated on the basis of the XPS data, was
of the order of 0.1 ML of NiO per min. With such an
evaporation rate the flux of oxygen molecules arriving at the
surface is much larger than the flux of nickel atoms, the former
being nearly 2 orders of magnitude higher than the latter at the
lowest oxygen pressure.
The sample was a disk with a diameter of ca. 10 mm cut and
polished along the (001) surface with an accuracy of (0.1°.
The temperature was measured by means of a chromel-alumel
thermocouple placed near the sample. The surface was prepared
by cycles of argon ion sputtering (600 eV) and annealing (700
K for 30 min) until no contamination was detectable by means
of XPS and LEIS and a sharp (1 × 1) LEED was visible.
Nickel (99.999% purity) was evaporated using an electron
3
. Results
3.1. XPS and LEIS Results. The XPS spectra measured for
increasing amounts of NiO deposited are shown in Figure 1.
The spectra shown in the figure are for NiO films prepared by
-
7
-6
-6
beam evaporator at O2 pressures in the 10 -10 mbar range.
For the evaporation rate calibration, Ni was evaporated under
conditions leading to the formation of a NiO(001) film. The
thickness of the layer was determined (assuming a film of
uniform thickness which completely covers the substrate surface)
from the Ni 2p and Ag 3d XPS peak areas.14 The attenuation
lengths of Ni2p and Ag 3d photoelectrons in nickel oxide and
in silver needed for the thickness calculation were calculated
using the Tanuma, Powell, and Penn method.15 The signals from
NiO(001) and Ag(001) crystals of semi-infinite thickness were
evaluated using the Ni2p and Ag3d photoionization cross
sections reported in ref 16 taking into account the effect of the
analyzer transmission function on the intensity (assumed to be
proportional to 1/EKin where EKin is the kinetic energy of the
evaporating nickel at an oxygen pressure of 3 × 10 mbar on
the substrate held at 373 K. Similar results were obtained with
the same oxygen pressure but the sample at room temperature
-
7
and with an oxygen pressure 5 × 10 mbar and the sample
held at 373 K. Under these conditions, in the early steps of
growth, a (2 × 1) LEED is observed. Significant changes in
the Ni2p peak shape and position are observed between 1 and
2 ML of NiO. In this range of coverages the transformation
4
from a (2 × 1) to a (1 × 1) LEED pattern is observed. For
NiO coverages below 1 ML (when the (2 × 1) LEED pattern
is observed), the Ni2p3/2 peak is located at a BE of 853.4 eV.
The Ni2p3/2 BE is intermediate between the values measured
for NiO and metallic Ni. The satellite features at 6-7 eV higher
BEs with respect to the main Ni2p3/2 and Ni2p1/2 peaks

Early Stages of NiO Growth on Ag(001)
J. Phys. Chem. B, Vol. 108, No. 28, 2004 9921
of the O1s region measured for the clean surface (see Figure
2
). However, the presence of a minor contribution to the O1s
peak from hydroxyl contamination (which would fall in this
1
3
BE region ), coming from water dissociation on the NiO film,
cannot be ruled out.
The stoichiometry of the deposited films was estimated using
the ratio of the O1s and Ni2p peak areas measured for the
thickest NiO layer (ca. 20 ML) grown on Ag(001). In order to
reduce as much as possible diffraction effects which may affect
O1s and Ni2p intensities in a different way, the XPS spectra
were measured at a polar angle of 10° and an azimuthal angle
of 22°. This emission angle is off the closed-packed directions
of Ag(001) and NiO(001) along which intensity enhancement
due to forward scattering of photoelectron occurs.21 The O1s/
Ni2p peak area ratio was corrected to take into account the
different depth probed by the O1s and Ni2p photoelectrons using
the following equation:
d
λO
1
- exp -
(
)
]
IO
IO
[
)
(I
)
d
λNi
INi
Ni ∞
1
- exp -
[
(
)
]
where IO/INi and (IO/INi)∞ are the O1s and Ni2p peak area ratios
for a NiO film with thickness d and a film with a thickness
much larger than the attenuation lengths, respectively. λO and
λNi are the attenuation lengths of the O1s (17 Å) and Ni2p (13
Å) photoelectrons in NiO calculated using the Tanuma, Powell,
Figure 2. O1s XPS spectra measured for amounts of NiO correspond-
ing to the Ni2p spectra shown in Figure 1. The spectrum of the O1s
region for the clean Ag(001) surface is reported to show the presence
of the Ag 2p1/2 peak (BE of 603 eV) excited by the Kâ radiation of the
Al X-ray source. The intensity of each spectrum is normalized to its
maximum, and an offset has been added. The spectrum of the clean
surface has been normalized to the maximum of the O1s spectrum
measured for 0.8 ML of NiO.
15
and Penn method.
The analysis of the XPS intensity shows that the O/Ni atomic
ratio for the (2 × 1) phase is the same as that obtained for thicker
NiO films, within an uncertainty of (10% due to the difficulties
in determining the O1s peak intensity at low coverage and
residual diffraction effects. The NiO films were studied also
by means of LEIS in order shed light onto the growth mode of
the oxide layer. Since LEIS is virtually sensitive to the
composition of the topmost layer only, the plot of the substrate
LEIS signal versus the film coverage provides direct information
characteristic of thick NiO layers are barely visible in the XPS
spectrum of the (2 × 1) phase. These features are interpreted
2+
17-19
as the unscreened 2p hole of the Ni ion in NiO.
The
lack of the satellites indicates that the screening of the Ni2p
core hole for the (2 × 1) oxidic phase is more efficient than for
the thicker NiO(001). This effect can be due to the different
electronic structures of the (2 × 1) and the NiO(001) films or
2
2,23
about the mechanism of growth of the layer.
The Ag LEIS
(more likely) to the valence electrons of the substrate which
signal as a function of the amount of deposited NiO is shown
in Figure 3. The Ag LEIS signal is proportional to the fraction
of substrate surface not covered by the NiO oxide layer. In the
early stages of growth, the linear decrease of the Ag LEIS signal
with NiO coverage indicates the formation of a layer of uniform
thickness. Extrapolating the linear decrease, we see that the Ag
LEIS signal goes to 0 at a coverage close to 1 ML. Hence, the
phase formed in the initial stages is a 2-dimensional phase, with
monolayer thickness. The substrate surface is completely
covered by NiO for coverages above 2 ML. These results are
in agreement with previous findings indicating that the Ag(001)
efficiently screen the Ni2p hole for nickel oxide films of
submonolayer thickness. When the Ni is evaporated with an
-
7
O2 pressure of 5 × 10 mbar and the sample held at 373 K,
after deposition of 0.1-0.2 ML of NiO, the Ni2p3/2 BE is lower
than that measured for the (2 × 1) phase and basically coincident
with the BE of metallic nickel. Upon increasing the NiO
coverages, a Ni2p XPS spectrum characteristic of the (2 × 1)
phase is obtained.
In the spectra measured for NiO films with a thickness above
2
ML, the Ni2p3/2 BE is 854.8 eV. This BE is in agreement
with the Ni2p3/2 BE values reported in the literature for
surface is entirely covered for NiO coverages higher than 2 ML
NiO.1
shape are observed. The most relevant one is the appearance
for thicknesses larger than 4-5 ML) of a shoulder on the high-
3,18,20
After 2 ML, only minor changes of the Ni2p peak
which corresponds to the onset of a layer-by-layer growth.
3,4
A
similar behavior of the substrate LEIS signal versus the NiO
coverages is observed for O2 pressures and sample temperatures
for which the (2 × 1) phase can be observed (see Figure 3).
The Ag LEIS signal versus NiO coverage curves can be
correlated with the changes observed by LEED and XPS. The
initial linear decrease of the Ag LEIS signal corresponds to the
growth of the (2 × 1) phase which forms 2-D islands. The
extraspots of this structure reach the maximum intensity in the
range of coverages at which the deviation from linearity sets
in. Upon increasing the film thickness, the reflexes of the (2 × 1)
(
BE side of the main Ni2p3/2 line. This feature, which is
interpreted in terms of nonlocal screening of the 2p hole in NiO,
1
8,19
is highly sensitive to the long range structure of the film.
The corresponding O1s spectra are shown in Figure 2. For the
(
2 × 1) phase and the 2 ML NiO film, the O1s BE is 529.7 eV
and shifts to 530.2 eV for the thickest NiO film. The O1s peak
at low NiO coverages presents a tail on the high-BE side,
whereas it is symmetric for the thickest film. Such a tail is due
mainly to the Ag 2p1/2 peak (BE of 603 eV) excited by the Kâ
radiation of the Al X-ray source as we can see from the spectrum
4
phase disappear and a sharp (1 × 1) LEED pattern is observed.
The deviation from linearity of the Ag LEIS signal versus NiO

9922 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Caffio et al.
Figure 3. LEIS signal of the substrate as a function of the NiO
coverage (in monolayers). The thickness was estimated on the basis of
the XPS data (see text). The LEIS intensity is normalized to the value
of the clean surface. The solid line corresponds to the decrease of the
Ag LEIS signal with NiO coverage expected for the growth of islands
of monolayer thickness which spread on the surface. Under the
conditions of temperature and oxygen pressure reported here a (2 × 1)
LEED pattern is observed for submonolayer coverages of NiO.
Figure 4. Ni2p XPS spectra measured for NiO films deposited by
-
7
evaporation of nickel with an oxygen pressure of 5 × 10 mbar and
the sample held at 453 K. The NiO coverage is nominal, estimated
from the evaporation rate and the deposition time. For each spectrum
(dots), the best-fit curve (solid line) obtained by mixing the spectrum
measured for one monolayer of nickel deposited in vacuum (dotted
line) with that measured for a NiO oxide film 2 ML thick (dashed
line) is shown.
coverage above 0.7-0.8 ML corresponds to the growth of
NiO(001)(1 × 1) islands, probably in the form of bilayers. This
4
interpretation is confirmed by the LEED results.
The XPS and LEIS results obtained evaporating Ni with the
O2 pressures reported above but with the sample at 453 K are
presented in Figures 4 and 5. When the NiO is deposited at
453 K, the XPS spectra show that metallic Ni is the main
component when nominal submonolayers of NiO are deposited.
The Ni2p XPS spectrum obtained after deposition of 0.7 ML
-
7
of NiO with an oxygen pressure of 5 × 10 mbar of oxygen
is shown in Figure 4, bottom curve. The Ni2p peak shape is
similar to that observed for the (2 × 1) phase. However, the
Ni2p3/2 BE (852.8 eV) is closer to that of metallic nickel and
the features characteristic of NiO are present in the spectrum
shown in Figure 4. Upon further deposition of NiO, the NiO
oxide components are more evident and those of metallic Ni
appear as a shoulder (Figure 4, top curve). These results clearly
indicate that on the surface both metallic nickel and nickel oxide
are present when NiO is deposited at 453 K. Indeed, a rather
good fit of the Ni2p peak can be obtained by mixing the
spectrum measured for metallic nickel deposited on Ag(001)
with that measured for a nickel oxide film 2 ML thick. The
fitting procedure allowed us to estimate the relative amount of
metallic and oxidized Ni. The results of the fitting indicate that
after a nominal deposition of 0.7 ML of NiO, on the surface
there is 60% of metallic Ni, the remaining fraction being nickel
oxide. Upon deposition of 1.5 ML, nickel oxide is the prevalent
component (60%). When higher nominal amounts of NiO are
deposited, only the NiO components are detectable in the XPS
spectra. The analysis of the XPS spectra obtained evaporating
Figure 5. LEIS signal of the substrate as a function of the nominal
NiO coverage for nickel evaporation on the Ag(001) sample held at
-
7
-6
4
53 K at O
2
pressures of 5 × 10 mbar and 3 × 10 mbar. The solid
line represents the Ag LEIS signal expected for the growth of a
monolayer of NiO.
-
6
Ni with an O2 pressure of 3 × 10 mbar at the same
temperature leads to similar results. The only difference is that,
with the same nominal amount of NiO, a larger fraction of nickel
oxide is present on the surface. For the NiO films prepared under
these conditions, a (1 × 1) LEED pattern was observed.
The Ag LEIS signal versus NiO coverage curves measured
for NiO films deposited with the sample at 453 K are shown in
Figure 5. The Ag LEIS signals decrease much slower with NiO
coverage than expected for the growth of a film with monolayer
thickness which gradually covers the substrate surface. This can
be explained taking into account the presence on metallic Ni.
The deposition of metallic Ni on the Ag(001) was studied in
2
4
detail by means of LEIS. Here we report the LEIS spectrum
of the clean Ag(001) surface and that measured after evaporation
in vacuum of 1 ML of nickel at room temperature. The Ag
LEIS signal basically does not decrease upon deposition of 1

Early Stages of NiO Growth on Ag(001)
J. Phys. Chem. B, Vol. 108, No. 28, 2004 9923
Figure 6. LEIS spectra measured for the clean Ag(001) surface and
the surface after deposition of 1 ML of nickel in vacuum at room
temperature. The magnified region (×10) of the Ni peak is shown.
Figure 7. Ni2p XPS spectra measured for the (2 × 1) phase prepared
-7
upon deposition of 0.8 ML of NiO (pO
2
5 × 10 mbar, 373 K) (bottom
curve), after annealing the (2 × 1) phase to 520 K for 30 min (middle
curve) and after a further 15 min of annealing at 620 K (top curve).
An offset has been added to the intensities of the spectra.
ML of metallic Ni, and the Ni signal is barely detectable (Figure
6). This is a clear indication that already at room temperature
metallic Ni is buried under silver in the subsurface region of
the substrate in agreement with the results of previous stud-
25,26
ies.
When Ni is evaporated in an oxygen atmosphere with
the substrate at high temperature the incorporation of Ni in the
subsurface region of the substrate is faster than the reaction with
oxygen. In other words, nickel atoms have a higher probability
of being incorporated in the substrate subsurface region than
of being hit by oxygen molecules impinging on the surface.
Hence, in the earlier stages of deposition, Ni is mainly in the
metallic state covered by (at least) one layer of Ag atoms and
only in part reacts with oxygen forming NiO. When the amount
of Ni in the subsurface region increases the incorporation rate
slows down so that Ni can be oxidized. The disappearance of
the metallic component of Ni in the XPS upon increasing the
evaporation time of Ni in oxygen atmosphere might be
interpreted considering that buried nickel can react with oxygen
and form NiO on the surface. When nickel oxide is the main
species on the surface, the slow decrease of the Ag LEIS signal
with NiO coverage is an indication that at this temperature NiO
grows forming 3-D islands.
3
.1.1. Effect of Annealing of the (2 × 1) Phase. Changes in
the XPS and LEIS spectra are observed after annealing the
2 × 1) oxidic phase in vacuum at temperatures ranging from
70 to 620 K. The satellites characteristic of NiO appear in the
(
4
Figure 8. LEIS spectra measured for the clean surface (solid line),
for the as-prepared (2 × 1) phase (dashed line), and after annealing at
6
20 K for 15 min (small circles). The intensities are normalized to the
Ni2p XPS spectrum after annealing times of the order of 10-15
min at ca. 470 K (Figure 7). For longer annealing times (or
higher temperatures) the Ni2p XPS spectrum gradually becomes
that typical of NiO. These changes are associated with the
disappearance of the (2 × 1) extraspots in the LEED pattern.
The transformation from the (2 × 1) oxidic phase to NiO is
complete upon annealing at 620 K for 15 min (see Figure 7,
top curve). At this stage a sharp (1 × 1) LEED pattern is
observed. It is interesting to note that the O/Ni atomic ratio
determined from the XPS data does not change (within an
accuracy better than 10%) during the transformation of the
maximum of the clean surface.
(2 × 1) layer into NiO. This is a further evidence that the
stoichiometry of the (2 × 1) oxidic phase is NiO.
The LEIS spectra reveal an increase of the fraction of
substrate surface not covered by the oxide upon annealing. The
fraction of free substrate surface which is 0.25 when only the
(2 × 1) phase is present increases to 0.62 upon annealing at
620 K for 15 min when NiO forms (Figure 8). This result can
be explained taking into account the formation of NiO islands
with a thickness of two layers. This interpretation is consistent

9924 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Caffio et al.
thickness, but the absence of significant intensity modulations
(at least in the range of polar angles accessible in our experiment
setup) prevents a further analysis involving the simulation of
the experimental data on the basis of structural models. Hence,
we carried out a LEED intensity analysis of the (2 × 1) phase
which is a more suitable method for the structural determination
of two-dimensional systems. The fraction order spots of the
(
2 × 1) phase are rather weak and have a measurable intensity
above the background (at least at room temperature) only in a
limited energy range. Moreover, as has been shown above,
before the (2 × 1) oxidic film covers the whole substrate surface
islands of NiO start to form. This prevents the use of integral
order spots in the analysis since they receive contributions from
the clean Ag(001) surface and from the NiO(001) (for coverage
4
around 1 ML). For these reasons a reduced data set had to be
used in the analysis. Therefore, the LEED intensity analysis
allows us only to rule out possible models for the (2 × 1)
structure and to determine the main features of the structural
model.
Figure 9. Ni 2p3/2 polar XPD curves measured for the as-prepared
2 × 1) phase, after annealing at 520 K for 30 min and for a NiO film
0 ML thick. The polar angle is measured with respect to the normal
to the surface. For each curve the maximum of the modulation
amplitude (defined as ((Imax - Imin)/Imax)%, where Imax and Imin are the
maximum and minimum of the highest peak, respectively) is reported.
A schematic model of the NiO(001) structure along the [100] and [110]
azimuthal direction is shown with Ni-scatterer directions corresponding
to the observed forward scattering peaks. Small circles are nickel atoms,
large circles are oxygen atoms.
(
2
The intensity analysis was performed by means of the tensor
LEED (TLEED) method. The TLEED method consists of a first
step in which a full dynamical calculation is performed for a
chosen reference structure to produce the elements of the tensor.
With this tensor, the I-V curves for various values of the
structural parameters in the neighborhood of the reference
structure can be quickly calculated, using a perturbation
with the results of the STM study reported in refs 10 and 11.
Because the determination by STM of the NiO island’s height
with respect to the substrate surface is influenced by electronic
effects, the formation of double-layer NiO(001) islands was
inferred from the halving of the surface area covered by the
3
0
method. The TLEED calculations were performed by means
of the Barbieri-Van Hove symmetrized automated tensor LEED
3
1
package (SATLEED). The Barbieri-Van Hove program
31
package was used for the calculation of the phase shifts. Ten
phase shifts (lmax ) 9) were used in the LEED calculations.
The phase shifts for silver and NiO were calculated from the
muffin-tin potentials of Ag and NiO. The LEED calculations
were performed between 30 and 300 eV. The imaginary part
of inner potential was set to 5 eV, whereas the real part was set
to 10 eV and optimized in the search procedure. The Debye
temperatures were set to 215, 300, and 500 K for Ag, Ni, and
O, respectively. These parameters are the same as those used
in a previous LEED analysis of a NiO(001) film 2 ML thick
1
0,11
oxide film upon annealing of the (2 × 1) phase.
3
.2. XPD Results. The polar Ni2p3/2 XPD curves measured
for the (2 × 1) phase along the [100] and [110] azimuth are
shown in Figure 9. These curves do not show any significant
intensity modulation as a function of the polar angle besides
the increase of intensity toward higher polar angles characteristic
of a phase with a thickness much lower than the photoelectron
27
attenuation length. Since the most intense intensity modulations
in the XPD curves correspond, for photoelectrons having
relatively high kinetic energies, to forward scattering along
emitter-scatterer directions, the nearly featureless XPD curves
4
deposited on Ag(001). In the automated search of the structural
parameters giving the best agreement with the experimental I-V
curves, three nonequivalent beams were used for a total energy
range of 390 eV. The Pendry reliability factor (Rp) was used in
the analysis to judge the agreement between the experimental
2
1
indicate that the (2 × 1) phase has a monatomic thickness.
Intensity modulations versus polar angle appear upon an-
nealing the (2 × 1) phase (Figure 9). Besides the peak along
the normal to the surface, the main features are located at θ )
5° in the [100] azimuth and θ ) 35° and 55° (considering the
centroid of the broad peak at the highest polar angle) in the
3
2
and calculated I-V curves.
4
First of all we performed the TLEED calculations for the
model proposed by Bertrams and Neddermeyer on the basis of
1
0
[110] azimuth. The XPD curves acquired after annealing the
(2 × 1) phase are very similar to those measured for a thick
(20 ML) NiO(001) film with changes in the relative intensities
the STM results. In this model there are rows of Ni atoms
occupying on-top and bridge positions on the Ag(001) surface,
whereas O atoms occupy the quasi-3-fold sites on the nickel
layer (Figure 10, model A). In the calculations we also
considered the same model but with a different registry, namely,
with the Ni atoms in 4-fold hollow sites and in bridge sites
(Figure 10, model B). These models were used as reference
structures and the coordinates of the atoms optimized in the
TLEED analysis in order to get the best fit with the experimental
I-V curves. The I-V curves calculated for both these models
give a very poor agreement with the experimental data, as shown
by the visual comparison and the Rp values of 0.7 and 0.5 for
model A and model B, respectively. We tested a different
structural model with Ni atoms occupying 4-fold hollow and
bridge sites and oxygen atoms sitting in bridge sites over the
Ni layer (model C). The I-V curves calculated for this structure
also gave a high R factor. The best agreement with the
of the main features due to the different thicknesses of the films.
The XPD curves for the thick NiO film are in good agreement
with those reported in the literature for NiO(001) single crystals
28,29
and thick NiO(001) film epitaxially grown on Ag(001).
Since
the forward scattering peaks are quite broad, the small shift of
these peaks upon increasing the film thickness found with other
5
,8
experimental techniques is not detectable in the present case.
The presence of the peak at θ ) 35° in the [110] azimuth shows
that there is a fraction of NiO islands with a thickness of three
layers. The XPD data are consistent with the present LEIS
results and with the previous STM studies of the annealing of
10,11
the (2 × 1) phase.
.3. LEED Intensity Analysis. The XPD analysis of the
2 × 1) film tells us that the precursor phase has a monatomic
3
(

Early Stages of NiO Growth on Ag(001)
J. Phys. Chem. B, Vol. 108, No. 28, 2004 9925
Figure 10. Top view of the schematic models for the reference
structures used in the TLEED analysis. Light gray circles: Ag atoms.
Gray circles: Ni Atoms. Dark circles: O atoms. For the structure shown
in D, the cross section along the a-a′direction is shown in Figure 11.
See text for details.
experimental I-V curves is obtained for a model having the
same structure of the nickel atoms in models and B and C but
a different geometry of the oxygen atoms (Figure 10, model
D). This structure is similar to that of NiO(111) although the
atoms are displaced from the arrangement in NiO(111) to
produce the (2 × 1) periodicity. Moreover, the NiO(111)
structure consists of alternating layers of nickel and oxygen
atoms (NiO(111) is a polar surface), whereas here Ni and O
atoms are almost coplanar. After optimization of the structural
parameters a minimum Rp of 0.35 was obtained. The coordinates
of the atoms for the best-fit structure are reported in Figure 11
with the comparison of the experimental and calculated I-V
curves. The error bars on the derived structural parameters ((0.1
Å for the interlayer spacings and (0.2 Å for the in-plane
coordinates) are rather large because of the limited data set. It
is assumed that a structural model is correct and the structural
parameters reliable when Rp values around 0.3 (or below) are
obtained. Hence, the model proposed here can be considered
essentially correct although some of the structural parameters
may not be fully optimized. The relatively low intensity of the
diffraction spots of the (2 × 1) phase (which makes the I-V
more sensitive to the background subtraction procedure) could
be responsible for the moderate level of agreement between the
experimental and calculated data.
Figure 11. Top part: Comparison of the experimental and calculated
I-V curves for model D shown in Figure 10. For each beam the value
of R is reported. Bottom part: cross section along the a-a′ direction
p
indicated in Figure 10D. The structural parameters corresponding to
the optimized models are reported. Large circles: Ag atoms. Gray
circles: Ni atoms. Dark circles: O atoms.
the thermal treatment of the (2 × 1) phase. The different
behavior cannot be explained in terms of surface and interface
2 36
energies. The surface energies of MgO(001) (1.16 J/m ) and
2 37
NiO (1.23 J/m ) are very close (and close to that of Ag(001),
2
38
1
.25 J/m ). The ab initio calculations reported in ref 9 indicate
that the interface energy is comparable for one monolayer of
MgO(001) and NiO(001) on Ag(001). Since the pseudomorphic
MgO film is more strained than NiO, the higher cost in energy
for increasing the thickness of the pseudomorphic film may
stabilize the monolayer of MgO(001)(1 × 1).
The situation for cobalt oxide on Ag(001) is less clear, and
a detailed study concerning the growth mechanism is missing.
However, for submonolayers of CoO, STM studies have shown
that a CoO precursor phase and CoO(001) islands grow
depending on the substrate temperature during deposition and
4
. Discussion and Conclusions
Despite the good structural matching with the substrate, the
1
1,39
evaporation of nickel in an oxygen atmosphere does not lead
to the formation of NiO(001) for submonolayer coverages.
Instead, in a wide range of conditions, a two-dimensional oxidic
phase with a (2 × 1) periodicity grows on the surface up to
a coverage close to 1 ML. It is interesting to compare this
behavior with the growth mechanism on Ag(001) of other oxides
having the same structure as NiO (rocksalt structure, lattice
parameter: 4.18 Å) and similar lattice parameters, such as
MgO (4.21 Å) and CoO (4.26 Å). For MgO, pseudomorphic
islands of MgO(001) are present since the earlier stages of
subsequent annealing.
In a more recent study by STM, a
hexagonal lattice was observed for the CoO precursor phase
4
0
by STM. This precursor phase of CoO may be equivalent to
the (2 × 1) structure of NiO on Ag(001).
The model for the (2 × 1) phase proposed on the basis of
the LEED analysis consists of a nickel layer and an oxygen
layer with a distorted hexagonal structure. Besides the distortion
of the hexagonal unit cell and the buckling of the Ni and the O
layer, this structure is similar to that of NiO(111). With
2
+
2-
alternating layers of Ni and O the (111) surface is not a
stable termination of NiO, and indeed the NiO(111) surface is
3,33-35
deposition.
MgO(001) deposited on Ag(001) forms islands
4
1
one atomic layer thick below 1 ML and grows with a nearly
layer-by-layer mode.3 On the contrary, a monolayer of
NiO(001) on Ag(001) seems to be unstable and tends to form
islands with a thickness of two (or more) layers as indicated by
the results concerning the deposition at higher temperature and
reconstructed. The electrostatic interaction of the oxide film
with metal substrate may stabilize this structure of the (2 × 1)
phase. However, this structure is metastable as indicated by the
transformation of the (2 × 1) phase into NiO(001) upon heating
at temperatures ranging from 400 to 500 K.
,35

9926 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Caffio et al.
Two important aspects are relevant in order to understand
References and Notes
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evaporated in an oxygen atmosphere (at the pressures used in
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implies that the oxidation process takes place among Ni atoms
that are adsorbed and O2 molecules that arrive at them from
the gas phase instead of being a reaction between Ni and O
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(
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3
atoms is relatively high (even at room temperature) so that
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9
(
38
is the much lower surface energy of Ag compared to Ni. Since
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1
(
7
0,24
(
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(
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(
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1
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Acknowledgment. This work has been funded by MIUR
under the Project PRIN 2003: "Natura, propriet a` e controllo di
difetti su superfici di ossidi: un approccio integrato verso
l’ingegneria dei difetti". The financial support by INFM under
the Advanced Research Project ISADORA is gratefully ac-
knowledged. The authors thank Alessandro Di Bona and Sergio
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