Reactive deposition of NiO ultrathin films on Pd(1 0 0)

Article abstract of DOI:10.1016/j.susc.2004.07.033

NiO ultrathin films have been grown on Pd(1 0 0) following a reactive deposition procedure. Ni has been dosed at room temperature on the substrate surface in an oxygen partial pressure of 4 × 10⁶ mbar. The electronic and structural evolution of the resulting NiO(1 0 0) ultrathin films has been followed by means of X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunnelling microscopy (STM). XPS, XPD and STM data indicate a 2D growth of the first NiO monolayer, while further growth leads to the nucleation of 3D islands, in a Stranskia?€ Krastanov growth scheme. Combined XPD and LEED data indicate an initially pseudomorphic growth, characterised by in-plane compressive tetragonal strain of the NiO film, with a consequent out-of-plane interlayer expansion. Partial strain relaxation occurs abruptly, very likely between the second and the third atomic layer of the 3D islands, while a completely bulk-like cubic environment is reached only gradually as a function of thickness. NiO(1 0 0) films even ～50 equivalent monolayers thick can be grown with good long-range order, as shown by (1 × 1) LEED images.

Full text of DOI:10.1016/j.susc.2004.07.033

Surface Science 569 (2004) 105–117
www.elsevier.com/locate/susc
Reactive deposition of NiO ultrathin films on Pd(100)
*
T. Orzali, S. Agnoli, M. Sambi , G. Granozzi
Unita` di Ricerca INFM and Dipartimento di Scienze Chimiche, Universita` di Padova, Via Marzolo 1, I-35131 Padova, Italy
Received 8 June 2004; accepted for publication 26 July 2004
Available online 12 August 2004
Abstract
NiO ultrathin films have been grown on Pd(100) following a reactive deposition procedure. Ni has been dosed at
room temperature on the substrate surface in an oxygen partial pressure of 4 · 10ꢀ6 mbar. The electronic and structural
evolution of the resulting NiO(100) ultrathin films has been followed by means of X-ray photoelectron spectroscopy
(XPS), X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunnelling micro-
scopy (STM). XPS, XPD and STM data indicate a 2D growth of the first NiO monolayer, while further growth leads to
the nucleation of 3D islands, in a Stranski–Krastanov growth scheme. Combined XPD and LEED data indicate an
initially pseudomorphic growth, characterised by in-plane compressive tetragonal strain of the NiO film, with a conse-
quent out-of-plane interlayer expansion. Partial strain relaxation occurs abruptly, very likely between the second and
the third atomic layer of the 3D islands, while a completely bulk-like cubic environment is reached only gradually as a
function of thickness. NiO(100) films even ꢁ50 equivalent monolayers thick can be grown with good long-range order,
as shown by (1 · 1) LEED images.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Nickel oxides; Palladium; Epitaxy; X-ray photoelectron spectroscopy; Low energy electron diffraction (LEED); Scanning
tunneling microscopy; X-ray scattering, diffraction, and reflection
1. Introduction
sues related to the electronic structure of 2D oxide
overlayers grown on an oxide or a metal substrate.
As a matter of fact, when dealing with metallic
substrates, the interplay between the reduced
dimensionality of the film, the influence of the im-
age charge potential at the interface and the
hybridization of oxide and substrate states near
the Fermi level have a role in determining the elec-
tronic properties and hence the reactivity of the
overlayer, which can differ dramatically from those
Ultrathin NiO films deposited on different sub-
strates have become an increasingly appealing field
of research in recent years [1–13]. The reasons for
such an interest are partly due to fundamental is-
*
Corresponding author. Tel.: +39 049 827 5189; fax: +39
049 827 5161.
E-mail address: mauro.sambi@unipd.it (M. Sambi).
0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.07.033

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T. Orzali et al. / Surface Science 569 (2004) 105–117
of the bulk oxide [14]. Moreover, when transition
metal oxides are concerned, the partial population
of d levels often leads to peculiar magnetic proper-
ties, which can be dramatically modified in the
ultrathin film regime. It has been shown recently
that a single monolayer (ML) NiO film grown on
Ag(100) shows strong dichroism in L2,3 X-ray
absorption (XAS) spectra, which is phenomeno-
logically similar to the linear magnetic dichroism
detected in anti-ferromagnetic (AF) thicker films,
but is rather originated by crystal field effects re-
lated to the lower symmetry C4v environment of
Ni2+ ions in the monolayer film, opposed to the
Oh environment for the cations in bulk NiO [15].
In addition, tetragonal strain deriving from match-
ing pseudomorphically the NiO film (whose bulk
phase is characterised by a fcc rocksalt structure)
to a lattice-mismatched substrate has been shown
to have consequences on the magnetic linear dichr-
oism of the L2 edge [16]. In particular, it has been
demonstrated that the dichroic signal can be re-
versed for NiO(100) ultrathin films grown on
materials with reversed lattice mismatch, like
oxidation during the PO treatment, which ulti-
mately leads to amorphous overlayers. Substrate
oxidation can be avoided by preventing the forma-
tion of a direct metal–metal interface. This can be
accomplished by presaturating the Pd surface with
p
p
oxygen, so to get the ( 5 · 5)-R27°-O/Pd(100)
p
structure [18,19]. When the ‘‘ 5’’ structure is used
as the starting point for the PO growth procedure,
NiO(100) ultrathin films with moderate long-
range order and with good compositional homoge-
neity as a function of thickness can be obtained,
with no substrate oxidation.
In the present paper we report on the alterna-
tive growth procedure consisting in reactive depo-
sition (RD), where Ni is dosed on the Pd substrate
at room temperature (RT) in the presence of an
oxygen background pressure. We show that NiO
ultrathin films up to several tens of ML thick
can be obtained following this route, characterised
by a better developed long-range order with re-
spect to the PO procedure. Information on the
growth mode and strain relaxation is also derived
by combining STM, photoelectron and low energy
electron diffraction measurements.
˚
˚
Ag(100) (aNiO = 4.176 A > aAg = 4.086 A, result-
ing in a +2.2% lattice mismatch), thus opening
new perspectives in the understanding of exchange
bias and in its applications.
2. Experimental
Recently, we have considered Pd(100) as an
alternative to Ag(100) for the growth of NiO
ultrathin films [17]. The NiO/Pd(100) system is
characterised by a larger reversed lattice mismatch
with respect to the NiO/Ag(100) interface
Sample treatments were performed in an UHV
preparation chamber at
a base pressure of
5 · 10ꢀ11 mbar. The Pd(100) crystal was cleaned
by repeated cycles of argon ion sputtering
(KE = 2 keV), and annealing at T = 970 K with
a final flash in 1 · 10ꢀ6 mbar O2 (uncorrected ion
gauge reading). The exposure to oxygen ensured
the removal of the carbon contaminant segregat-
ing to the surface during each annealing cycle,
and left the surface free from either C or O, as
judged by grazing angle XPS measurements. The
cleaning cycles were repeated until a sharp (1 · 1)
LEED (VG Microtech Rear View LEED—
RVL900) pattern was observed and a good anisot-
ropy developed in Pd 3d XPD scans (anisotropy
v = (Imax ꢀ Imin)/Imax, with Imax and Imin being
respectively the maximum and minimum values
of the photoelectron intensity detected in a scan).
NiO layers were grown following a RD proce-
dure: Ni was deposited at RT by means of an
˚
(aPd = 3.891 A, leading to a 7.3% lattice mismatch)
and Pd has a valence band (VB) composed prima-
rily by 4d states near the Fermi level, to be com-
pared to the 5sp nature of the Ag VB in the
same energy range.
Our previous work [17] was concerned with the
post-oxidation (PO) growth procedure, which con-
sists in depositing small amounts on metallic Ni in
UHV conditions on the Pd substrate, followed by
an annealing treatment in oxygen in order to pro-
duce the NiO overlayers. The obtained results
highlighted the surprising dependence of the ultra-
thin film composition and structure on the Ni dose
deposited at each step of the PO procedure. In par-
ticular, it has been shown that there is a critical Ni
dose which is able to initiate substantial substrate

T. Orzali et al. / Surface Science 569 (2004) 105–117
107
electron beam evaporator (Omicron EFM) in an
oxygen partial pressure of 4 · 10ꢀ6 mbar (residual
gas analyser reading). An annealing treatment at
473 K in an oxygen partial pressure of 5 · 10ꢀ7
mbar following the deposition was occasionally
performed for low coverages (ꢁ1 ML) in order
to improve the long-range order of the deposited
layer, leading to a c(4 · 2) LEED pattern (see be-
low). The deposition rate of NiO resulting from
such treatments was estimated to be ꢁ2.2 0.1
for the XPD experiments. The O KLL Auger line
has been used for monitoring oxygen because the
O 1s peak is partially superimposed to the Pd
3p1/2 feature of the substrate.
STM experiments were performed in a custom
designed three-chamber UHV system operating
at a base pressure of 1 · 10ꢀ10. The STM (Oxford
Instruments) has been operated at room tempera-
ture (RT) in a constant current mode.
˚
A/min, as determined by angle resolved XPS
[17,20] and quartz microbalance calibrations.
Thickness values expressed in ML equivalents
(MLeq) are calculated assuming an interlayer dis-
tance between adjacent planes in NiO(100) equal
3. Results
3.1. XPS data
˚
to 2.09 A. Thicknesses investigated in the present
work correspond to 1, 2, 5, 8, 10, 15, 20 and 46
MLeq.
Fig. 1 shows the Ni 2p photoemission line from
NiO deposited reactively on Pd(100) as a func-
tion of the overlayer thickness at near-normal
and grazing emission. Fig. 2 shows the Pd 3d
peak from the clean substrate (dashed line) and
from the Pd(100) crystal covered by a 5 MLeq
thick NiO film (full line), measured at grazing
emission.
XPS and XPD data were collected using a
modified VG ESCALAB MKII photoelectron
spectrometer. The sample was mounted on a
two-axis goniometer which allowed the sweeping
of the electron emission direction with an angular
resolution of 1° both in polar angle (h, defined
with respect to the surface normal) and in azi-
muthal angle (/, defined with respect to the
[011] direction on the surface). The acceptance
half-angle of the electron analyser was estimated
to be 3.5°.
All relevant XPS features were acquired at near
normal (h = 24°) and grazing (h = 72°) emission, in
order to probe the compositional homogeneity of
the NiO overlayers as a function of thickness. True
normal emission was avoided to prevent possible
distortions due to strong along-normal forward
scattering (FS) effects. The MgKa source with a
pass energy (PE) of 20 eV was used throughout
the XPS experiments. Sufficient resolution is there-
by achieved to be able to highlight the fine struc-
ture evolution in Ni 2p spectra as a function of
thickness.
The following observations can be made:
• The full width at half maximum (FWHM) of
the two main components of the Ni 2p doublet
reaches a maximum with the third MLeq, while
it decreases to an almost constant value starting
from the fifth MLeq.
• The Ni 2p3/2 component undergoes a shift of 1.8
eV (from 852.2 to 854.0 eV) on going from 1 to
46 MLeq. The Ni 2p1/2 component shifts by 2.4
eV (from 869.8 to 872.2 eV) in the same thick-
ness range. Most of the shift occurs within the
first 5 MLeq: while the binding energy (BE) of
the two components remains constant for the
first 2 MLeq, a clear shift is observed starting
from the third MLeq and the steady-state,
bulk-like value is reached with the fifth MLeq.
The measured shifts do not depend on the
acquisition angle.
XPD scans for structural analysis were obtained
for Pd 3d emission from the clean substrate and
for Ni 2p3/2 photoelectron and O KLL Auger
emission from NiO ultrathin films as a function
of overlayer thickness. Line intensities are reported
after a simple linear background subtraction.
Unmonochromatized AlKa radiation was used
• The BE separation between the Ni 2p3/2 photo-
emission line and its main satellite is 8.8 eV for 1
and 2 MLeq thick NiO films; it becomes 7.0 eV
for thicker layers. As far as the Ni 2p1/2 peak
and its satellite are concerned, the BE separa-
tion goes from 10.8 to 8.6 eV.

108
T. Orzali et al. / Surface Science 569 (2004) 105–117
Ni 2p, theta= 72
Ni 2p, theta= 24
˚
˚
46 MLeq
46 MLeq
20 MLeq
20 MLeq
15 MLeq
15 MLeq
10 MLeq
10 MLeq
8 MLeq
8 MLeq
5 MLeq
3 MLeq
5 MLeq
3 MLeq
2 MLeq
1 MLeq
2 MLeq
1 MLeq
840
850
860
870
880
890
840
850
860
870
880
890
Binding Energy (eV)
Binding Energy (eV)
Fig. 1. Ni 2p XPS photoemission lines acquired on NiO/Pd(100) as a function of thickness, at h = 24° (left) and at h = 72° (right) with
respect to the sample normal. Full vertical lines indicate the BE positions of the bulk doublet components; dashed vertical lines are
placed at the expected BE values for the bulk main satellite lines. The MgKa source was used for acquisition.
• The intensity ratio between the satellites and the
Pd 3d, clean Pd (100)
Pd 3d, 5 MLeqNiO/Pd (100)
theta= 72˚
respective main peaks grows as a function of
thickness, while the FWHM of the satellites
decreases. Stable values are reached with the
fifth MLeq.
• Multiplet splitting of the Ni 2p3/2 line, typical of
long-range ordered bulk-like NiO [9,17] is
absent in the 1 MLeq film. It starts to develop
in the 2 MLeq thick film, it is not easily discern-
ible in the 3 MLeq film due to the transition (to
be detailed later) occurring at this thickness,
and it finally reaches the appearance of bulk-
like NiO starting from the fifth MLeq.
• No substantial oxidation of the substrate is
apparent from data reported in Fig. 2 as the
deposition proceeds: the FWHM of both Pd
3d doublet components do not change with
NiO exposure. This points out an important dif-
ference from what happens with the PO growth
procedure, where substantial Pd oxidation
328
332
336
340
344
348
352
Binding Energy (eV)
Fig. 2. Pd 3d photoemission line at h = 72° acquired on the
clean substrate (dots) and on Pd covered by 5 MLeq of NiO (full
line). Spectra have been normalized to the same intensity in
order to point out directly possible changes in the FWHM.

T. Orzali et al. / Surface Science 569 (2004) 105–117
109
occurs if no oxygen presaturation of the sub-
strate surface is performed prior to the NiO
growth [17].
along with their anisotropy as a function of thick-
ness. Both PPD and APD scans show a gradual
development of the diffraction features peculiar
to a (100) oriented rocksalt structure [17]. In par-
ticular, curves referring to the 1 MLeq coverage are
almost structureless, thus confirming the 2D
growth mode of the first deposited monolayer.
The 2 MLeq polar scans, besides weak off-normal
features, show clearly the onset of along-normal
FS.
Important information is conveyed by the 3 and
5 MLeq Ni 2p polar scans along both azimuths
(Fig. 3): although FS peaks are very broad, which
is indicative of a complex mixture of locally slightly
different environments, a clear shift of the main off-
normal FS features toward more grazing angles is
observed in this thickness range. In particular, the
peaks which are found at h = 16° and 40° in the 3
3.2. XPD data
Figs. 3 and 4 show polar Ni 2p photoelectron
diffraction (PPD) scans along the [001] and [011]
main substrate azimuths, acquired as a function
of thickness with the AlKa source. Fig. 3 displays
the low coverage curves, up to 5 MLeq, normalized
with respect to the 1/cosh response typical of ultra-
thin films [20]. Curves referring to thicker layers
are reported in Fig. 4.
Ni 2p and O KLL azimuthal photoelectron dif-
fraction (APD) scans at the polar angle h = 45° as
a function of thickness are reported in Fig. 5,
PPD Ni 2p, [001]
PPD Ni 2p, [011]
5 MLeq
5 MLeq
3 MLeq
3 MLeq
2 MLeq
1 MLeq
2 MLeq
1 MLeq
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Polar angle (deg)
Polar angle (deg)
Fig. 3. Polar photoelectron diffraction (PPD) scans on Ni 2p along two main substrate azimuthal directions for the thickness interval
1–5 MLeq. Dashed lines indicate the occurrence of main forward scattering directions. Out-of-plane interlayer contraction between 3
and 5 MLeq is highlighted by bending the off-normal dashed lines. The kinetic energy of outgoing photoelectrons is KE ꢁ 635 eV.
Scans have been normalized with respect to the 1/cosh response typical of ultrathin films.

110
T. Orzali et al. / Surface Science 569 (2004) 105–117
PPD Ni 2p, [001]
PPD Ni 2p, [011]
46 MLeq
46 MLeq
20 MLeq
20 MLeq
15 MLeq
10 MLeq
15 MLeq
10 MLeq
8 MLeq
8 MLeq
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
Polar angle (deg)
Polar angle (deg)
Fig. 4. Polar photoelectron diffraction (PPD) scans on Ni 2p along two main substrate azimuthal directions for the thickness interval
8–46 MLeq. Dashed lines indicate the occurrence of main forward scattering directions in bulk NiO(100). KE ꢁ 635 eV.
MLeq curve along the [001] azimuth shift to
h = 20° and 45° respectively in the 5 MLeq curve.
Similarly, along the [011] substrate azimuth the
peaks centered at h = 32° and 50° shift to h = 35°
and 55°, respectively, which are the bulk-like values
for NiO(100). This shift is paralleled by a more
than 10% increase of azimuthal curves anisotropies
between 3 and 5 MLeq (see Fig. 5, right panel). Fur-
ther growth simply increases the anisotropy of po-
lar scans and enriches to some extent their fine
structure, with no further major changes in posi-
tion or relative intensities.
reported in Figs. 7 and 8. The energy of the elec-
tron beam is generally set at 70 eV, with a few
small exceptions. For the 2, 3 and 5 MLeq thick
films, data acquired at E = 88 eV are also reported.
The 1 MLeq pattern (Fig. 7a) shows a rather
sharp (1 · 1) periodicity. Avery faint c(4 · 2)
superstructure was observable in the very initial
stages of the LEED image acquisition, which
quickly faded away, presumably under the effect
of the electron beam. Interestingly enough, a
post-annealing in oxygen (partial pressure of
p = 5 · 10ꢀ7 mbar) at T = 473 K leads to a very
sharp and stable c(4 · 2) superstructure (Fig. 7b),
which is of the same pattern obtained by dosing
Finally, Fig. 6 shows the Ni 2p3/2 and O KLL
2p plots acquired on a 20 MLeq thick NiO film.
Both plots are characterised by fourfold symmetry
and by very similar diffraction features.
p
p
Ni on the ( 5 · 5)-R27°-O/Pd(100) structure
in the PO growth procedure with oxygen presatu-
ration [17]. Athorough investigation of the
c(4 · 2) phase, including STM data, will be dis-
cussed in a forthcoming paper [21].
3.3. LEED data
LEED images acquired on NiO ultrathin films
grown on Pd(100) as a function of thickness are
If RD is continued up to 2 MLeq (without prior
post-annealing of the 1 MLeq layer: Fig. 7c and d;

T. Orzali et al. / Surface Science 569 (2004) 105–117
111
APD Ni 2p, theta=45˚
APD O KLL, theta=45˚
46 MLeq
20 MLeq
15 MLeq
46 MLeq
20 MLeq
15 MLeq
10 MLeq
0.40
0.35
0.30
0.25
0.20
0.15
10 MLeq
8 MLeq
8 MLeq
5 MLeq
O KLL
Ni 2p
5 MLeq
3 MLeq
10
20
30
40
Thickness (ML)
3 MLeq
2 MLeq
1 MLeq
2 MLeq
1 MLeq
0
15 30 45 60 75 90 105120135150165180 0 15 30 45 60 75 90 105120135150165180
Azimuthal angle (deg)
Azimuthal angle (deg)
Fig. 5. Azimuthal photoelectron diffraction (APD) scans on Ni 2p (left, KE ꢁ 635 eV) and O KLL (center, KE ꢁ 510 eV), taken at
h = 45° as a function of the NiO ultrathin film thickness. Anisotropy v of Ni 2p and O KLL azimuthal scans at h = 45° as a function of
the overlayer thickness (right).
Fig. 6. (a) Ni 2p3/2 (KE ꢁ 632 eV) and (b) O KLL (KE ꢁ 510 eV) 2p plots from the 20 MLeq thick NiO film on Pd(100). The
maximum off-normal h value is 70°. The [001] substrate main azimuth corresponds to the horizontal radius pointing to the right.
after post-annealing, i.e. starting from the c(4 · 2)
phase: Fig. 7e) a (1 · 1) structure is observed,
which however shows a spot splitting towards
the specular beam (hidden by the electron gun in
our rear-view geometry). The splitting, which indi-
cates a shrinkage of the first Brillouin zone (BZ)

112
T. Orzali et al. / Surface Science 569 (2004) 105–117
Fig. 7. LEED patterns of NiO ultrathin films as a function of thickness in the 1–5 MLeq range: (a) 1 MLeq, E = 70 eV; (b) 1 MLeq
,
c(4 · 2) phase obtained by post-annealing at 473 K in 5 · 10ꢀ7 mbar O2, E = 134 eV; (c) 2 MLeq, E = 70 eV; (d) 2 MLeq, E = 88 eV; (e)
1 MLeq, c(4 · 2) phase + 1 MLeq, E = 93 eV; (f) 3 MLeq, E = 70 eV; (g) 3 MLeq, E = 88 eV; (h) 5 MLeq, E = 66 eV; (i) 5 MLeq, E = 88
eV.
for a number of atomic layers above a certain
value, is observed up to a thickness of 5 MLeq,
although the splitting becomes less resolved as
thickness increases (Fig. 7h and i). Aquantitative
measurement of the splitting is made difficult by
the rather low resolution of the standard LEED
equipment. However, a 3.5% shortening of the
reciprocal unit cell side can be estimated rather

T. Orzali et al. / Surface Science 569 (2004) 105–117
113
Fig. 9. STM image of 1.7 MLeq NiO deposited on Pd(100). 500
˚
˚
A · 500 A, I = 0.2 nA, V = ꢀ2.0 V.
Fig. 8. LEED patterns of NiO ultrathin films as a function of
thickness in the 10–46 MLeq range: (a) 10 MLeq, E = 70 eV; (b)
ML grows as 3D islands up to three atomic layers
thick.
15 MLeq, E = 70 eV; (c) 20 MLeq, E = 74 eV; (d) 46 MLeq
E = 67 eV.
,
4. Discussion
accurately from the 2 MLeq LEED image by meas-
uring the distance between the (ꢀ1,ꢀ1) and the
(1,1) and between the (ꢀ1,1) and (1,ꢀ1) split
spots. This contraction in the first BZ corresponds
to a 3.6% in-plane expansion of the real lattice.
The thicker layers observed with LEED (Fig.
8a–d) show a simple (1 · 1) structure with broad-
ened spots, which however is clearly seen even
for the thickest layer considered in our investiga-
tion (46 MLeq).
It has been shown recently that the Ni 2p XPS
threshold from a 2D NiO single ML on the
Ag(100) metallic substrate differs substantially
from the analogous feature from bulk NiO, being
more similar to—though with significant differ-
ences—the metal case [9]. Reasons for such a
behaviour can be traced back to the reduction of
Coulomb and charge transfer energies in the
metal-supported thin film with respect to the bulk
oxide due to the image charge potential, coupled
with the hybridization of the oxide electronic
structure with the substrate states close to the Fer-
mi level [14,22]. The Ni 2p feature measured from
our NiO 1 MLeq on Pd(100) resembles the analo-
gous data collected from the NiO ML deposited
on Ag(100) [9]. In particular, the BE energy of
the two main components are intermediate be-
tween the reference values for metallic Ni and
NiO, the separation between the main lines and
the respective satellites is larger than in metallic
3.4. STM data
Fig. 9 shows an STM image of 1.7 MLeq NiO
grown on Pd(100). The deposition was performed
at RT at a rate of 0.8 MLeq/min in an oxygen
background (p(O2) = 2 · 10ꢀ6 mbar) and followed
by two annealing cycles (at T = 473 and 573 K,
respectively) in oxygen (p(O2) = 5 · 10ꢀ7 mbar).
The wetting and long-range ordered c(4 · 2)
phase developed by the first Ni ML is clearly seen
in the image. The NiO dose exceeding the single

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T. Orzali et al. / Surface Science 569 (2004) 105–117
nickel, while the satellite intensities are considera-
bly weaker than in bulk nickel oxide. Moreover,
the main lines do not show the multiplet splitting
which, in bulk-like NiO is due to 2p–3d hole–hole
interactions and possibly to Ni–Ni intersite inter-
actions known as non-local screening [23]. In sum-
mary, the 1 MLeq thick NiO film deposited on
Pd(100), judging from the Ni 2p XPS feature, ap-
pears to grow as a wetting 2D overlayer. Further
evidence for this suggestion will be given by
XPD data.
The incipient appearance of the multiplet split-
ting in the Ni 2p3/2 line and the intensity increase
of the satellites already for the 2 MLeq thick over-
layer seems to point out a change in the growth
mode from 2D wetting to 3D clustering, a sugges-
tion which is strengthened by XPD data, as will be
seen below. The peculiar shape of the 3 MLeq spec-
trum, on the other hand, can be correlated to the
strain relaxation of the initially pseudomorphic,
tetragonally distorted NiO overlayer to a relaxed,
more bulk-like oxide lattice, which is demon-
strated by both LEED and XPD measurements.
The typical features of long-range ordered, bulk-
like NiO are henceforth maintained up to substan-
tial thickness values, and are always accompanied
by a clear (1 · 1) LEED pattern, albeit the more
diffuse the larger the thickness (Fig. 7).
substrate as soon as the reactants meet on the sur-
face, i.e. no stable Ni metallic clusters having the
critical size necessary to initiate substrate
oxidation.
XPD data reported in Figs. 3–5 give further
support to the Stranski–Krastanov growth mode
suggested by XPS results. Curves referring to the
single MLeq are almost structureless. The 1 MLeq
azimuthal scans reported in Fig. 5 are particularly
clear in this respect: no structure at all is seen in
both Ni 2p and O KLL azimuthal scans at
h = 45°, which indicates the absence of scatterers
in the plane above the photoelectron emitters at
a polar angle where strong FS events are expected
for a (100)-oriented NaCl lattice structure. The 1
MLeq polar scans normalized with respect to the
1/cosh response typical of ultrathin thin films give
similar information: no FS peaks are detected in
the whole polar angle range, thus confirming the
2D growth habit of the overlayer in the single
monolayer regime.
Ni 2p polar curves along the substrate main azi-
muths referring to 2 MLeq (Fig. 3), besides show-
ing the FS events which would be expected for
an actual thickness of 2 ML NiO(100) (at
h = 45° along [001], at h = 55° along [011] and
at h = 0° along the normal to the surface for an
undistorted, bulk-like arrangement; at h = 40°
along [001] and at h = 50° in the presence of sub-
stantial compressive tetragonal strain, as in the
present case), also display an additional faint peak
along the [001] polar scan at h ꢁ 16–18°, which
shifts to higher polar angles and becomes more in-
tense for thicker layers (see Fig. 4, left panel). In a
NaCl-like structure along the [001] azimuth, a Ni–
Ni FS event is expected at h = 18.4° and a weaker
Ni–O FS peak occurs at h = 26.6°. These FS fea-
tures can be expected only for layers at least 4
and 3 ML thick, respectively (as a matter of fact,
in polar scans from thicker layers—Fig. 4—a sin-
gle peak is detected at h ꢁ 22°, which is a convolu-
tion of the two mentioned features, given our
experimental angular resolution, which is esti-
mated to be approximately 3.5°). In addition,
in the 2 MLeq polar scan along the [011] azimuth
(Fig. 3, right panel), a faint peak is detected at
h ꢁ 30–32°, which corresponds to the Ni–Ni FS
peak at h = 35.3° in the bulk-like rocksalt environ-
An important observation is the absence of sub-
strate oxidation during the RD procedure, as op-
posed to the PO procedure without substrate
oxygen presaturation (Fig. 2). In our previous
work on the PO growth [17] we have shown that
Pd oxidation critically depends upon the quantity
of metallic nickel that is put in contact with the
clean Pd surface during the first Ni dose. When
such critical value is used, subsequent PO leads
to substantial substrate oxidation and eventually
to the amorphization of the interface. These obser-
vations gave convincing evidence that metallic Ni
at a particular coverage acts as a promoter of Pd
oxidation, which can therefore be avoided by pre-
venting the formation of a direct Ni/Pd interface,
notably by presaturating the Pd surface with the
p
p
( 5 · 5)-R27° oxygen superstructure. In the pre-
sent RD experiment no Pd oxidation is observed
even in the absence of oxygen presaturation, which
suggests that Ni oxidation occurs readily on the

T. Orzali et al. / Surface Science 569 (2004) 105–117
115
ment for layers at least 3 ML thick. In summary,
the 2 MLeq polar data support a strong clustering
of the second deposited equivalent monolayer, so
to give 3D features on the initially wetting mono-
layer. Adefinitive confirmation of the SK growth
mode is given by the STM image reported in Fig.
9, where up to three atomic layers can be counted
on the 3D islands nucleating on the wetting
c(4 · 2) phase for a nominal coverage of only 1.7
MLeq.
overlayer, while it becomes apparent in XPD after
the third MLeq, can be attributed to the different
sampling depth of the two techniques coupled
with the SK growth mode of the overlayer. While
in LEED almost only the two topmost layers con-
tributes to the detected intensity, all layers con-
tribute to the XPD signal at the given kinetic
energy of outgoing Ni 2p photoelectrons
(KE ꢁ 635 eV). PPD scans are therefore an aver-
age produced by the interfacial strained layers
and by the topmost relaxed layers. In order to
appreciate the contribution of the unstrained lay-
ers in a FS geometry, their weight should be com-
parable to the weight of the strained portion, and
this happens when the relaxed part is at least two
atomic layers thick and its population is a sub-
stantial fraction of the strained part. This explains
the delay with which one observes the transition
with XPD with respect to LEED. In fact, the lat-
ter is able to sample the transition when it involves
a sufficient population of single unstrained atomic
layers. Moreover, the fact that the spot splitting in
LEED is observed in a whole thickness range (at
least from 2 to 5 MLeq) can be reconciled with
the SK growth mode if one supposes that the tran-
sition starts at a precise thickness of the 3D is-
lands. XPD and LEED taken together seem to
suggest that this thickness is between 2 and 3
atomic layers: at 2 MLeq, given the SK growth
mode, there is already a sufficient population of
3 atomic layers thick islands (with the third layer
relaxed) capable of producing the spot splitting;
at 5 MLeq (and most likely also above this value),
there is still an appreciable fraction of 3 atomic
layers thick island portions producing the residual
spot splitting, which is however less resolved as
the thickness increases—an indication of increas-
ing structural disorder arising from the rather con-
tinuous relaxation of the residual strain in islands
thicker than 3 atomic layers. The smooth relaxa-
tion towards the bulk-like aspect ratio of the
NiO structure could also be responsible for the
rather broad FS peaks in PPD scans below
approximately 10 MLeq.
Ni 2p polar scans along both main substrate
azimuthal directions for 2, 3 and 5 MLeq films
(Fig. 3) and Ni 2p and O KLL azimuthal scans
at h = 45° (Fig. 5) should be read in parallel with
the LEED patterns referring to the same cover-
ages, reported in Fig. 7. The XPS Ni 2p peak line-
shapes of Fig. 1, already discussed above, should
also be considered. Aclear shift of the main FS
features in PPD scans along both main azimuths
at more grazing angles is observed on going from
3 to 5 MLeq, as already specified in Section 3.2.
At the same time, a spot splitting indicating the
shrinkage of the first BZ is observed in LEED
images, starting from the 2 MLeq patterns (Fig.
7c–d). In other words, there is converging evidence
for the occurrence of strain relaxation, with the
system evolving from a pseudomorphic NiO over-
layer affected by in-plane compressive strain and
out-of-plane interlayer expansion, towards a more
bulk-like NiO structural environment, character-
ised by a partial strain relaxation. The sudden in-
crease in the anisotropy of azimuthal scans
measured at h = 45° starting from the 5 MLeq thick
film (Fig. 5) is also explained by strain relaxation.
Asignificant fraction of Ni–Ni and O–O nearest
neighbour directions along the [001] and symme-
try-equivalent azimuths—initially found at angles
higher than h = 45° due to the interlayer expan-
sion—become closer to h = 45° after the strain
relaxation and thus contribute more effectively to
the modulation of the APD curves. A partial strain
relaxation is suggested by the quantitative LEED
estimate of a 3.6% in-plane lattice parameter
expansion at the transition, while the lattice mis-
match between bulk Pd(100) and bulk NiO(100)
amounts to 7.3%.
The stable structure for thicker layers is cer-
tainly the bulk-like NiO(100) lattice, with a
½001Šk½001Š epitaxial relationship with the sub-
strate, as summarised by Ni 2p and O KLL 2p
The fact that the in-plane collapse is obvious
from LEED already starting from the 2 MLeq

116
T. Orzali et al. / Surface Science 569 (2004) 105–117
plots reported in Fig. 6. As already discussed in
Ref. [17], the very similar XPD modulations of
the two signals in a FS regime marks the identity
of the structural environments sampled by photo-
emission from both anion and cation lattice sites
and are thus a reliable fingerprint of the NaCl-like
lattice structure of NiO. Good LEED patterns ob-
tained for layers several tens of ML thick give con-
vincing evidence for better quality NiO ultrathin
films obtained with the RD procedure with respect
to the PO route [17].
5. Conclusion
Reactive deposition in an oxygen atmosphere of
NiO ultrathin films on Pd(100) leads to epitaxial
NiO(100)/Pd(100), ½001Šk½001Š, characterised by
good long-range order up to thicknesses of several
tens of monolayers. The first deposited monolayer
grows as a 2D wetting film. Further deposition re-
sults in a SK growth mode, leading to the forma-
tion of 3D islands. Due to the rather high lattice
mismatch between NiO(100) and Pd(100), pseu-
domorphic growth is maintained only up to the
second atomic layer. Partial strain relaxation oc-
curs between the second and the third atomic lay-
ers, while residual strain is recovered within several
subsequent monolayers.
Finally, the results reported in the present paper
should be compared with the NiO/Ag(100) case,
where, due to the much lower lattice mismatch,
the critical thickness is ꢁ20 ML [3], and the
growth mode is of the layer-by-layer type [7]. A n-
other important difference with respect to the NiO/
Ag(100) system concerns the thermodynamic sta-
bility and the structure of the first deposited mono-
layer. While on Ag(100) the epitaxial relationship
is of the (1 · 2) type [7,8] for reactive growth in
oxygen and of the (1 · 1) type if a stronger oxidant
like NO2 is used as the reactant [9], the (1 · 1) rela-
tionship for the NiO(1 ML)/Pd(100) is established
for depositions at RT, but it is easily changed to a
c(4 · 2) superstructure by means of a post-anneal-
ing treatment in oxygen (see Figs. 7b and 9), which
is then stably maintained also by cooling down the
sample again at RT. This observation seems to
indicate that the (1 · 1) structure is formed at
RT on kinetic grounds, but is thermodynamically
metastable. It is worth noting that subsequent
growth of NiO(100) on the c(4 · 2) phase leads
to XPS, XPD and LEED results (not shown, with
the exception of the LEED image shown in Fig.
7e) which are virtually indistinguishable from
those reported in this paper—i.e. the presence of
subsequent atomic layers leads the interfacial
matching to the simple (1 · 1) relationship. Asim-
Acknowledgments
This work has been partially funded by the Ital-
ian Ministry of Instruction, University and Re-
search (MIUR) through the fund ‘‘Programmi di
ricerca di rilevante interesse nazionale’’ (PRIN),
project title: ‘‘Nature, properties and control of
oxide surface defects. An integrated approach to-
wards defect engineering’’. Prof. Falko P. Netzer
(Insitute for Experimental Physics, Karl-Franzens
University, Graz, Austria) is gratefully acknowl-
edged for allowing S.A. to perform in his labora-
tory the STM measurement reported in Fig. 9.
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