Surface Science 537 (2003) 36–54

www.elsevier.com/locate/susc

Growth of NiO ultrathin films on Pd(1 0 0) bypost-oxidation of Ni films: the effect of pre-adsorbed oxygen

M. Sambi *, R. Sensolo, G.A. Rizzi, M. Petukhov 1, G. Granozzi

Unit�aa di Ricerca INFM and Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universit�aa di Padova,

Via Loredan 4, 35131 Padova, Italy

Received 27 February 2003; accepted for publication 2 May 2003

Abstract

This paper reports on the optimisation of the growth parameters of NiO ultrathin films on Pd(1 0 0). Growth is

performed by means of UHV metal deposition and post-oxidation cycles. Chemical and structural characterisation of

the deposits is achieved by means of electron spectroscopy (X-ray photoelectron spectroscopy, XPS; angle resolved

XPS) and electron diffraction techniques (low energy electron diffraction, LEED; X-ray photoelectron diffraction,

XPD).

Three growth procedures have been investigated, which differ for the particular growth parameters adopted in each

case. We demonstrate that post-oxidation is effective in order to obtain epitaxial NiO only if the initial dose of Ni

evaporated on the clean Pd(1 0 0) substrate exceeds a critical value, corresponding approximately to two equivalent

monolayers. However, the overlayer thus obtained is strongly understoichiometric in oxygen close to the metal/oxide

interface and poorly ordered on the long range. When a Ni dose below this limiting first value is used, the layer evolves

toward polycrystalline NiO, due to substantial oxidation of the Pd substrate promoted by the presence of Ni, very likely

through a work function decrease upon direct metal/metal interface formation. On the contrary, epitaxial NiO(1 0 0)

layers of good structural quality, with limited oxygen deficiency at the interface, with negligible substrate oxidation and

with a good degree of long-range order are obtained if deposition and post-oxidation cycles are initiated on an oxygen

pre-saturated Pd surface, characterised by the (p5�p

5)-R27� O/Pd(1 0 0) LEED pattern. We therefore demonstrate

that oxygen can act either as an inhibitor or as a promoter of NiO epitaxial growth on Pd(1 0 0), depending on the way

it is used.

� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Nickel oxides; Oxidation; Palladium; Epitaxy; X-ray photoelectron spectroscopy; Low energy electron diffraction (LEED);

Growth

* Corresponding author. Tel.: +39-049-827-5189; fax: +39-

049-827-5161.

E-mail address: mauro.sambi@unipd.it (M. Sambi).1 Present address: IGNP, Russian Research Center, Kur-

chatov Institute, Kurchatov sq., 123182 Moscow, Russia.

0039-6028/03/$ - see front matter � 2003 Elsevier Science B.V. All r

doi:10.1016/S0039-6028(03)00641-1

1. Introduction

MgO and NiO ultrathin films grown on

Ag(1 0 0) are by now well-established model sys-

tems in which a simple and a strongly correlated

oxide, respectively, having analogous rocksalt

lattice structures and similar lattice parameters are

ights reserved.

M. Sambi et al. / Surface Science 537 (2003) 36–54 37

matched epitaxially to a fcc transition metal

characterised by a valence band of predominantly

5sp character close to the Fermi level [1]. Epitaxy

is favoured by the low lattice mismatch between

the overlayers and the metal for this particular

substrate choice. A rich debate has been under-taken in the literature in order to elucidate the

modifications of the oxides� structural and elec-

tronic properties and of their chemical reactivity

due to the presence of the strong compositional

gradient introduced by the overlayer/metal inter-

face. In this context, the importance of the image

charge potential induced by the ionic cores of the

overlayer in the substrate in evaluating the ener-getics of the interface has been stressed [2,3].

In this work, Pd(1 0 0) has been tested as an

alternative metal substrate for NiO epitaxy. Al-

though it is less appealing from the point of view

of the lattice mismatch, which is significantly lar-

ger than for silver (+7.3% vs +2.2%, respectively),

nonetheless it gives the opportunity of investigat-

ing the interfacial interactions between the oxideand a metal with a valence band of predominantly

4d character near the Fermi level. This is helpful

in order to assess the role and importance of

the hybridisation of overlayer and metal states at

the interface in determining the properties and the

reactivity of the system, also by comparison with

the qualitatively different Ag case. The NiO/

Pd(1 0 0) interface is also interesting from the pointof view of magnetic properties. Although peculiar

magnetisation properties have been demonstrated

for NiO/Pd superlattices (which show ferromag-

netic behaviour, while bulk Pd and NiO are

paramagnetic and antiferromagnetic, respectively)

[4,5], interestingly enough, no surface science in-

vestigation has been performed up to now to elu-

cidate in detail the structure of the NiO/Pdinterface.

For all these reasons, the first issue to be ad-

dressed is the establishment of a growth procedure

which leads reproducibly to epitaxial NiO ultra-

thin films of good quality on Pd(1 0 0).

Oxide layers are usually grown either via reac-

tive deposition (RD, i.e. metal evaporation in the

presence of oxygen or of another oxidising agent,e.g. NO2 [6]) or, less frequently, via post-oxidation

(PO) of small amounts of metal deposited on the

substrate in UHV conditions [7]. In the present

paper, we explore the PO route, which consists in

depositing small amounts of Ni on the clean sub-

strate at RT, followed by PO in oxygen at 450 K.

Results concerning the RD procedure will be the

argument of a forthcoming paper.Here we show that, when Pd(1 0 0) is used as the

substrate, the microstructure and the composition

of the overlayer are strongly dependent on the

deposition procedure. Both properties are essen-

tially driven by the surface chemistry of the sub-

strate itself towards oxygen (which shows a rather

complex phase diagram for oxygen adsorption)

and by the competition between Pd and Nitowards oxidation during the first stages of PO

deposition. In fact, four distinct oxygen chemi-

sorption phases on Pd(1 0 0) are known as a

function of coverage [8], ranging from p(2 · 2)

through c(2 · 2) and (5 · 5) to the saturation phase

(p5�p

5)-R27� (indicated for brevity as ‘‘p5

structure’’ in the following), while a complete ox-

idation of subsurface Pd layers is not attainablestarting from clean palladium in high vacuum

conditions (oxygen pressures up to 10�5 mbar).

However, this picture is substantially modified by

the presence of Ni, which––as will be shown––

promotes Pd oxidation in certain conditions, with

direct consequences on the quality of the epitaxial

overlayer.

By combining X-ray photoelectron spectro-scopy (XPS), angle resolved XPS (ARXPS) [9], an-

gle scanned X-ray photoelectron diffraction (XPD)

[10] and low energy electron diffraction (LEED)

measurements, in this work we explore such a

complex behaviour. Its understanding allows us to

set up a deposition procedure which leads to a

sharp interface between epitaxial and homoge-

neous NiO ultrathin films and metallic Pd(1 0 0).

2. Experimental

Sample treatments were performed in an UHV

preparation chamber at a base pressure of 5 · 10�11

mbar. The Pd(1 0 0) crystal was cleaned by re-

peated cycles of argon ion sputtering (KE¼ 2.4keV), and annealing at T ¼ 970 K with a final

flash in 1 · 10�6 mbar O2 (uncorrected ion gauge

38 M. Sambi et al. / Surface Science 537 (2003) 36–54

reading). The exposure to oxygen ensured the re-

moval of the carbon contaminant segregating 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 anisotropy de-

veloped in Pd 3d XPD scans.

NiO layers were grown with Ni metal deposi-

tion and PO cycles. Three different growth proce-

dures were followed, characterised by different Ni

doses at each cycle and/or by substrate pre-satu-

ration with oxygen. Common features to all pro-cedures were the following: Ni depositions were

performed at room temperature (RT) by means of

an electron beam evaporator (Caburn MDC,

model EB90). PO treatments were performed by

dosing 60 L of O2 at RT, followed by 150 L with

the substrate held at T ¼ 440 K. The oxygen dose

at RT was performed in order to initiate the

overlayer oxidation while avoiding as muchas possible Ni diffusion into the substrate. The

deposition rate of NiO resulting from such treat-

ments was estimated to be �2.15� 0.10 �AA/min, as

determined by XPS calibrations (see below).

Thickness values expressed in ML equivalents

(MLeq) are calculated assuming an interlayer dis-

tance between adjacent planes in NiO(1 0 0) equal

to 2.09 �AA. When oxygen pre-saturation of thesubstrate has been exploited, it has been per-

formed by dosing 600 L of O2 at T ¼ 570 K,

resulting in a (p5�p

5)-R27� O/Pd(1 0 0) LEED

pattern, as reported in the literature [11].

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 theelectron emission direction with an angular reso-

lution of �1� both in polar angle (h, defined with

respect to the surface normal) and in azimuthal

angle (/, defined with respect to the [0 1 1] direc-

tion on the surface). The acceptance half-angle of

the electron analyser was estimated to be 3.5�.ARXPS measurements were performed in order

to obtain an estimate of the growth rate andgrowth mode of the overlayer [9]. Intensities as a

function of the polar angle h were obtained for Pd

3d (BE� 335 eV for the 3d5=2 component) and Ni

2p (BE� 854 eV for the 2p3=2 component) photo-

electron signals.

XPD scans for structural analysis were obtained

for Pd 3d emission from the clean substrate and

for Ni 2p3=2 photoelectron and O KLL Augeremission from NiO ultrathin films as a function of

overlayer thickness. Line intensities are reported

after a simple linear background subtraction.

Unmonochromatised Al Ka radiation was used in

the experiment, except where explicitly stated.

3. Results and discussion

3.1. Deposition procedure I: polycrystalline NiO

layers

The first deposition procedure explored in the

present work consisted in 1 min of Ni deposition,

followed by PO, as specified above in the experi-

mental section. Metal deposition and PO cycleshave been repeated for a total deposition time of

14 min.

3.1.1. ARXPS data

ARXPS has been used to estimate the growth

rate and the growth mode resulting from the

adopted procedure. Fig. 1 reports the ratio between

the background-subtracted overlayer Ni 2p andsubstrate Pd 3d integral XPS peak intensities as a

function of the sine of the acquisition angle vs the

sample surface. The experimental data for different

deposition times (dots) are fitted with the standard

ARXPS equation for patched overlayer growth [9]

(lines). Data have been acquired after each minute

of deposition up to 6 min, and after each 2 min

thereafter. The ratio K between Ni 2p and Pd 3dintensities from, respectively, NiO and Pd samples

of semi-infinite thickness and inelastic attenuation

lengths for the overlayer and the substrate photo-

electrons in the NiO matrix (6.7 and 16 �AA, respec-

tively, calculated by means of the TPP-2 formula

[12] for Al Ka-excited emission) are used as input

data. The covered surface fraction c and the aver-

age islands thickness z are the fitting output.c and z as a function of deposition time are

reported in Fig. 2a. The error bars represent one

Fig. 1. Growth procedure I. Ratio between the background-

subtracted overlayer Ni 2p and substrate Pd 3d integral XPS

peak intensities as a function of the sine of the acquisition angle

vs the sample surface for increasing deposition times (from

bottom to top). The experimental data for different deposition

times (dots) are fitted with the standard ARXPS equation for

patched overlayer growth [9] (lines).

M. Sambi et al. / Surface Science 537 (2003) 36–54 39

standard deviation, as resulting from the fitting

procedure at each deposition time. The product

c � z, shown in Fig. 2b, is the overlayer volume per

surface unit area, which can be interpreted as the

average equivalent layer thickness (the thicknessthe layer would have if the growth were perfectly

bi-dimensional, i.e. with c ¼ 1). The linearity of

the plot in Fig. 2b confirms the stability in time

and the reproducibility of Ni evaporations, as well

as the absence of systematic errors in the deter-

mination of the growth rate and the growth mode,

e.g. due to diffraction effects. The slope of the line

in Fig. 2b is the deposition rate, corresponding to2.15� 0.10 equivalent �AA/min. In terms of MLeq of

crystalline NiO(1 0 0)––which however is not the

outcome of this particular growth route, but will

be shown to be the product of the epitaxial growth

of nickel oxide on Pd(1 0 0) following different

growth procedures (see Sections 3.2 and 3.3)––1

ML corresponds to the distance between two ad-

jacent lattice planes along [1 0 0], i.e. to half the

NiO unit cell: 2.09 �AA. The growth rate expressed inMLeq is thus 1.02� 0.05 MLeq/min.

Data reported in Fig. 2a also give valuable in-

formation on the NiO overlayer growth mode.

After 1 min (1 MLeq), the fraction of surface area

covered by the overlayer amounts to c ¼ 0:28,while the mean overlayer islands thickness is 6.8 �AA,

i.e. more than 3 ML. Following the first dose,

there is a substantial increase of c up to the fifth–sixth minute, while the increase of the mean island

thickness is moderate. After the sixth minute,

when the covered fractional area is c ¼ 0:75, the

slope of c as a function of time decreases, while the

increase of the mean island thickness is steeper.

These data can be interpreted in terms of a 3D

nucleation of NiO islands on the substrate surface

in the initial stages of deposition, which expandpredominantly in the plane of the surface up to

5–6 MLeq, until a large fraction of the substrate

surface is covered.

3.1.2. XPS data

It is well known that the NiO Ni 2p XPS region,

in the binding energy (BE) interval between 850

and 890 eV, is profoundly different from the cor-responding photoemission features in Ni metal

[13]. The two main components of the spin–orbit-

split doublet differ in BE position, line-shape, full-

width-at-half-maximum (FWHM) and mutual

separation. In addition, the satellites that accom-

pany the main thresholds are closer to and by far

weaker than the main lines in Ni metal than in the

oxide, whose main spectral features can be inter-preted by considering NiO as a charge transfer

insulator with a conductivity gap determined by

the charge transfer energy. Moreover, while the

main thresholds in the metal are structureless, in

NiO each main line is enriched by multiplet

structure due to 2p–3d hole–hole interactions and

possibly by Ni–Ni intersite interactions known as

non-local screening [14]. The latter are quenched inthe impurity limit (multiplet structure is not de-

tected when Ni is present at the impurity level in a

Fig. 2. Growth procedure I. (a) Surface fractional coverage c and average islands thickness z (�AA) vs deposition time (minutes) as a

result of fitting of curves in Fig. 1. Error bars are one standard deviation, as resulting from the fitting procedure at each deposition

time; (b) Product c � z (�AA), corresponding to the overlayer volume per surface unit area. It can be interpreted as the equivalent layer

thickness, i.e. the thickness the layer would have if the growth were strictly layer-by-layer. The slope of the line interpolating data

points is the deposition rate, corresponding to 2.15� 0.10 equivalent �AA/min (1.02� 0.05 MLeq/min, assuming an interlayer distance of

2.09 �AA in NiO(1 0 0), see text). Error bars are calculated from standard deviations on c and z by error propagation.

40 M. Sambi et al. / Surface Science 537 (2003) 36–54

MgO lattice [15]) and/or if long-range 3D trans-

lational order of the oxide lattice is not well de-

veloped. As a matter of fact, in the present work

we show preliminary evidence that the multiplet

structure can be correlated to the degree of long-

range order in epitaxial NiO multilayers.On the other hand, the Ni 2p XPS threshold

from a single 2D NiO ML on a metallic substrate

(Ag(1 0 0) is the most studied case to date) differs

dramatically from the analogous feature from bulk

NiO, being more similar to––though with signifi-

cant differences––the metal case. It is not our aim to

review here the reasons for such behaviour: excel-

lent discussions can be found in the literature [16].Suffice here to say that this particular feature of the

NiO ML precludes a simple identification of Ni

oxidation state in the overlayer solely on the basis

of BE values and line-shapes of the Ni 2p threshold.

Moreover, when Pd is used as the substrate, addi-

tional complications arise from the superposition

of the O 1s photoemission peak with the Pd 3p3=2

line, thus precluding a rigorous quantitative de-termination of the overlayer stoichiometry for very

thin layers. Having in mind these limitations, we

now proceed to the discussion of Ni 2p and Pd 3d

XPS data for the growth procedure I.

Fig. 3 reports selected Ni 2p XPS peaks as a

function of thickness and of the acquisition angle hwith respect to the sample normal. The main

findings can be summarised as follows:

• After the first minute of deposition, correspond-

ing to 1 MLeq, the satellites typical of NiO (in

the BE range 858–868 and 875–885 eV) are al-

ready developed, with a relative intensity with

respect to the main lines which is lower than

for bulk NiO, but larger than for the 2D NiO

ML. The bulk-like intensity ratio is achievedstarting approximately from 2 MLeq. Since the

2D NiO ML (e.g. on Ag(1 0 0) [16]) is character-

ised by much lower intensity satellite peaks, this

observation is in agreement with the 3D cluster-

ing in the initial stages of the growth evidenced

by ARXPS data. The substantial similarity of

the spectra acquired at grazing and near-normal

emission even for thinnest layers suggests agood compositional homogeneity of the deposit

as a function of thickness.

Fig. 3. Growth procedure I. Ni 2p XPS peaks for selected

thickness values measured at near-normal (h ¼ 24�, continuousline) and grazing (h ¼ 72�, dashed line) emission. Vertical da-

shed lines indicate the position of the Ni 2p doublet main

components in thick NiO overlayers.

Fig. 4. Growth procedure I. Pd 3d doublet from clean Pd(1 0 0)

(dashed line) and from the substrate covered by a 6 MLeq thick

NiO layer (continuous line). Data are acquired at h ¼ 60�.

M. Sambi et al. / Surface Science 537 (2003) 36–54 41

• The reduced thickness of 3D islands for 1 MLeq

affects the BE position, the line-shape and the

FWHM of the main lines of the doublet, which

are different than in thicker layers. Minor

changes are seen from 2 MLeq onwards. The

Ni 2p3=2 component undergoes an overall shift

toward higher BE equal to 0.8 eV (from 853.6

eV for 1 MLeq to 854.4 eV for 12 MLeq).• Although the Ni 2p spectrum referring to 12

MLeq has well-developed oxide satellites, no

multiplet structure is detected in the main lines.

We then checked the behaviour of the Pd sub-

strate upon Ni PO. Fig. 4 shows the Pd 3d doublet

from clean Pd(1 0 0) (dashed line) and from the

substrate covered by a 6 MLeq thick NiO layer. A

strong broadening of the doublet components to-

wards higher BE is observed––the FWHM of the

3d5=2 line goes from 1.4 to 2.2 eV––which gives

evidence for substantial substrate oxidation [17](we will show in Section 3.3 that simple oxygen

chemisorption up to the saturation level leads to a

much smaller broadening of the doublet compo-

nents). It should be remarked that in oxygen par-

tial pressures compatible with the UHV equipment

(i.e. up to 10�5 mbar), clean Pd(1 0 0) does not

undergo bulk oxidation for any temperature value

[8]. This means that the presence of Ni promotessubstrate oxidation. Possible reasons for this be-

haviour will be discussed in Section 3.4.

3.1.3. LEED data

LEED analysis gives valuable qualitative in-

formation on the initial stages of substrate oxida-

tion. Fig. 5a displays the (1 · 1) LEED pattern

from the clean Pd(1 0 0) substrate, acquired at akinetic energy (KE) of 55 eV. The image resulting

after the deposition of 1 MLeq of NiO is reported

in Fig. 5b (KE¼ 57 eV). The pattern is a mixture

of a two domains (2 · 1) structure and of thep5

structure (see Fig. 5c for reference), along with

streaks along the h001i directions and with an

increased background, associated to substantial

disorder upon deposition.While the (2 · 1) pattern is not known for O

chemisorption on clean Pd(1 0 0), and should

Fig. 5. Growth procedure I. (a) (1· 1) LEED pattern from the

clean Pd(1 0 0) substrate, KE¼ 55 eV; (b) LEED pattern after

the deposition of 1 MLeq of NiO, KE¼ 57 eV; (c) (p5�p

5)-

R27� O/Pd(1 0 0), KE¼ 64 eV; (d) LEED pattern after the de-

position of 2 MLeq of NiO, KE¼ 57 eV.

42 M. Sambi et al. / Surface Science 537 (2003) 36–54

therefore be due to a co-adsorption phase of Ni

and O on the substrate surface, thep5 structure

on clean Pd is obtained upon extensive O dosing

(600 L) at sample temperatures (around 570 K)

[8,11] which are substantially higher than the

temperature adopted upon PO in the present case

(150 L and 450 K). The LEED pattern thereforeconfirms that the presence of the first Ni dose on

the Pd surface increases the substrate reactivity

towards oxygen.

Fig. 5d is the LEED pattern detected after a 2

MLeq dose (KE¼ 57 eV). It consists in a very

diffuse c(4 · 2) structure associated to high back-

ground intensity, indicating substantial disruption

of long-range order. We show in Section 3.3 thatthe same structure, though sharper and more de-

fined, is detected for 1 MLeq dose on the oxygen

pre-saturated Pd surface, an observation which is

briefly discussed in Section 3.4. For doses larger

than 2 MLeq no LEED pattern is detected any-

more: long-range order is lacking.

3.1.4. XPD data

XPD measurements confirm in a definitive way

that structural order in the NiO overlayer grown

following growth procedure I is lacking even on a

short-range scale.

Fig. 6 shows Ni 2p3=2 polar scans (polar pho-

toelectron diffraction, PPD) from NiO ultrathin

films along the (a) [0 0 1] and (b) [0 1 1] substrate

main azimuthal directions as a function of theequivalent overlayer thickness. Fig. 7 displays

Ni 2p3=2 azimuthal scans (azimuthal photoelec-

tron diffraction, APD) at a polar angle h ¼ 45�,which––in a cubic NaCl-like lattice exposing the

(1 0 0) surface––corresponds to strong cation–

cation forward scattering (FS) events.

It can be immediately seen that there is no dif-

fraction modulation at all in APD curves, andnearly no modulation in PPD scans, with the ex-

ception of faint maxima along the surface normal

for thicker layers. We conclude that the clusters

nucleating on the substrate surface, at least for

doses larger than 2MLeq, have no translational and

orientational coherence at all, at least in the plane

parallel to the surface. Largely casual local orien-

tations wash out diffraction effects, with a resultingcomplete azimuthal isotropy of XPD scans.

The residual FS along the surface normal in

PPD scans may indicate a certain degree of uni-

axial preferential orientation of NiO crystallites

along the growth direction. For this reason we

indicate the layer as polycrystalline rather than as

completely amorphous.

3.2. Deposition procedure II: epitaxial, short-range

ordered, understoichiometric NiO layers

Growth procedure II differs form the first one

just for the Ni dose at each cycle, which is doubled

in the present case (2 min, corresponding to 2

MLeq), while the PO treatment is left unaltered.

The growth has been followed up to 18 MLeq.Although the present growth route leads to NiO

layers of poor structural quality and largely in-

homogeneous in composition as a function of

thickness, nevertheless it is helpful in elucidating

factors that influence the growth mechanisms. In

fact, it evidences a strong and surprising depen-

dence of the structural and chemical nature of the

overlayer on the Ni dose at each cycle, which istightly linked to the behaviour of the substrate

towards oxidation.

Fig. 6. Growth procedure I. Ni 2p3=2 PPD from NiO ultrathin films along the (a) [0 0 1] and (b) [0 1 1] substrate main azimuthal

directions as a function of the equivalent overlayer thickness.

M. Sambi et al. / Surface Science 537 (2003) 36–54 43

3.2.1. XPS data

Fig. 8 displays the Ni 2p XPS spectra for three

NiO overlayers of increasing thickness (2, 6 and 12MLeq, respectively) deposited on Pd(1 0 0), mea-

sured both at near-normal (h ¼ 24�) and grazing

(h ¼ 72�) emission. Spectra referring to the 18

MLeq deposit are substantially equivalent to those

for 12 MLeq, which can therefore be assumed to be

representative of a bulk-like situation. The rele-

vant observations are the following:

• for 2 MLeq, there is a shift towards higher BE of

the doublet components and an increase of the

satellite intensity with respect to main lines on

going from h ¼ 72� to 24�;• for a fixed emission angle, the Ni 2p3=2 compo-

nent undergoes a large FWHM increase on

going from 2 to 6 MLeq, while there is a slight

decrease from 6 to 12 MLeq;• the BE of the Ni 2p3=2 component shifts from

852.6 (853.0) eV at h ¼ 24� (72�) to 855.0

(855.2) eV for 12 MLeq––the DE is equal to

2.4 (2.2) eV towards higher BE. Similarly, the

Ni 2p1=2 component shifts from 869.6 (869.6)eV for 2 MLeq to 872.4 (872.8) eV for 12

MLeq, i.e. DE ¼ 2:8 (3.2) eV.

All these findings are indicative of a NiO

overlayer which is highly oxygen-defective at the

interface with the substrate, with an oxygen con-

tent that increases progressively toward the top-

most layer (i.e. toward the film/vacuum interface).The more effective oxidation of the topmost layers

is particularly evident for the 2 MLeq film: the

ARXPS measurement enhances the contribution

of the surface layers at grazing emission––the BE

shift, the increased FWHM and the increased

satellite intensity follow consequently. A slight BE

shift towards higher values and a minor decrease

in FWHM is found also for thicker layers (6 and12 MLeq) on moving from near-normal to grazing

emission. This is a consequence of the larger

Fig. 7. Growth procedure I. Ni 2p3=2 APD from NiO ultrathin

films at h ¼ 45� as a function of the equivalent overlayer

thickness.Fig. 8. Growth procedure II. Ni 2p XPS peaks for selected

thickness values measured at near-normal (h ¼ 24�, continuousline) and grazing (h ¼ 72�, dashed line) emission. Vertical da-

shed lines indicate the position of the Ni 2p doublet main

components in thick NiO overlayers.

44 M. Sambi et al. / Surface Science 537 (2003) 36–54

sampling depth at low h, which averages over

different Ni oxidation states found as a function ofthickness, while at grazing emission only the fully

oxidised layers contribute significantly to the

overall intensity.

Also the observed FWHM variation of the

main doublet components as a function of thick-

ness is consistent with an oxygen-gradient model.

The same conclusion can be drawn from the de-

pendence of the separation between the maindoublet components on both thickness and emis-

sion angle: the separation is closer to the value

pertaining to Ni metal for thinner layers and at

near-normal emission, while it shifts towards the

value typical for NiO for thicker layers, all the

more so at grazing emission.

Finally, it should be remarked that the multiplet

structure in the main thresholds is not observed forany thickness, which is in good agreement with the

lack of a LEED pattern for NiO overlayers grown

following the present deposition route.

If we now consider Fig. 9, which displays Pd 3d

lines from clean Pd(1 0 0) and from the substrate

covered by 6 MLeq of NiO, in analogy to Fig. 4 for

growth procedure I, we notice a substantial dif-

ference with respect to the preceding case: no

substrate oxidation is detected, at least within the

available energy resolution. This is consistent withthe only partial oxidation of interfacial Ni evi-

denced above. As a final remark, on the basis of

the reported data and of recent X-ray reflectivity

data [18], a likely cause for the partially ineffective

interfacial Ni oxidation is a certain degree of Ni–

Pd intermixing at the interface, in any case re-

stricted to a few monolayers.

3.2.2. XPD data

Fig. 10 reports the Ni 2p APD scans from NiO

layers of increasing thickness grown following

Fig. 9. Growth procedure II. Pd 3d doublet from clean

Pd(1 0 0) (dashed line) and from the substrate covered by a 6

MLeq thick NiO layer (continuous line). Data are acquired at

h ¼ 60�.

Fig. 10. Growth procedure II. Ni 2p3=2 APD from NiO ultra-

thin films at h ¼ 45� as a function of the equivalent overlayer

thickness. Main azimuthal directions on the substrate surface

are indicated.

M. Sambi et al. / Surface Science 537 (2003) 36–54 45

growth procedure II. Strong modulations of the

XPS signal as a function of the detection angle are

clearly seen, which reflect the fourfold symmetry of

the substrate: there are intensity maxima along

h001i directions of the substrate, along with local

maxima along h011i azimuths. Data therefore

indicate the presence of an overlayer ordered at

least on the short range, with a well-defined ori-

entational relationship with the substrate and

characterised by the same symmetry. It is worthobserving that intensity modulations for the first

deposition (2 MLeq) differ significantly from those

observed on thicker layers: intensity maxima along

main azimuthal directions are less pronounced and

richer in fine structure. From 4 MLeq onwards,

substantial changes in the shapes of azimuthal

scans are not detected any more.

Additional information on the overlayer latticestructure is given by Ni 2p PPD scans along the

main substrate (and overlayer) azimuths as a

function of equivalent thickness, reported in Fig.

11. Fig. 11a shows the evolution of the FS peaks

along the sample normal and at h ¼ 45� along the

[0 0 1] surface direction; the normal FS and a

broad maximum centred at h ¼ 55� with a side

feature at h ¼ 35� are found along the [0 1 1] azi-muth, as reported in Fig. 11b. Also in this case, the

curves referring to 2 MLeq differ significantly from

scans measured on thicker layers, since they show

only weak diffraction features. In particular, the

FS peak at h ¼ 45� along the [0 0 1] direction,

clearly seen in the APD cut of Fig. 10, is barely

perceptible in the associated PPD scan in Fig. 11a,

due to its low intensity coupled to the steepbackground.

In summary, on one side the overall symmetry

and FS peaks location both in h and in / for

thicker NiO layers are consistent with a NaCl-like

lattice exposing the (1 0 0) surface (this is also

confirmed by full Ni 2p and O KLL 2p plots, see

below). On the other side the evolution of these

characteristic features is only gradual as a functionof thickness, being fully developed after approxi-

mately the sixth MLeq. If this curves are compared

to their counterparts obtained on stoichiometric,

epitaxial NiO deposited via growth procedure III

(see Section 3.3.3), several differences are detected,

which can be attributed to the inhomogeneous

oxygen understoichiometry as a function of

thickness in the present case.In order to evidence in a more intuitive way the

overlayer orientation with respect to the substrate

Fig. 11. Growth procedure II. Ni 2p3=2 PPD from NiO ultrathin films along the (a) [0 0 1] and (b) [0 1 1] substrate main azimuthal

directions as a function of the equivalent overlayer thickness.

46 M. Sambi et al. / Surface Science 537 (2003) 36–54

and to confirm definitively its NaCl-like lattice

structure, Fig. 12 displays full 2p plots for Pd 3d

emission from the clean substrate (12a) and for Ni

2p3=2 (12b) and O KLL (12c) features from the 18

MLeq ultrathin film. Overlayer 2p plots have been

acquired using the Mg Ka primary excitationsource in order to reduce the electron inelastic

mean free paths, so to sample predominantly the

oxide layer far from the heavily understoichio-

metric overlayer/substrate interface.

From the comparison of Pd 3d to Ni 2p3=2 and

O KLL 2p plots, it becomes immediately evident

that the substrate and overlayer [0 0 1] azimuths

are aligned to each other. In fact, the four intenseFS events at h ¼ 45� in the Pd 3d plot, corre-

sponding to strong Pd–Pd nearest-neighbour FS

events in the fcc lattice of the metal, pin the h001isubstrate azimuths. Main FS maxima in both

overlayer plots are found along the same azi-

muthal directions and at the same polar angles

of the substrate (h ¼ 45�). This is explained by

assuming the NaCl structure for the overlayer,

with the main intensity maxima corresponding to

Ni–Ni (Ni 2p3=2 plot) and O–O (O KLL plot) FS

along the h001i surface azimuths of the overlayer.

In addition, the NaCl structure is demonstrated

directly by the close similarity between the Ni 2pand O KLL plots, which reflects the local identity

of lattice sites occupied by both anions and cations

in this particular arrangement (each ion is octa-

hedrally coordinated by a set of six NN counter-

ions).

In conclusion, growth procedure II leads to

short-range ordered, epitaxial NiO, with a

(1 0 0) k (1 0 0), [0 0 1] k [0 0 1] epitaxial relationshipwith the substrate, which is strongly understoi-

chiometric in the first 5–6 MLeq at the interface

with the substrate.

Metallic Ni on Pd(1 0 0) is known to grow via

3D nucleation of epitaxial, pseudomorphic Ni is-

lands in the first few ML [19,20]. In the present

case, such islands are too large to be completely

Fig. 12. Growth procedure II. (a) Pd 3d 2p plot from clean Pd(1 0 0) (KE¼ 1150 eV); (b) Ni 2p3=2 (KE¼ 398 eV) and (c) O KLL

(KE¼ 510 eV) 2p plots from a 18 MLeq NiO ultrathin film. h ¼ 0� (the surface normal) corresponds to the centre of each plot. The

outer circle corresponds to h ¼ 80� in (a) and to h ¼ 70� in (b) and (c). The intensity is given by the colour scale (black: minimum,

white: maximum). Main substrate azimuths are indicated.

M. Sambi et al. / Surface Science 537 (2003) 36–54 47

oxidised by the adopted PO procedure, but at the

same time prevent extensive substrate oxidation,

thus keeping at least a certain degree of epitaxialorder in the overlayer, which evolves only gradu-

ally towards NiO.

3.3. Deposition procedure III: epitaxial long-range

ordered and stoichiometric NiO layers

Growth route III has been devised in order to

optimise the Ni dose at each deposition cycle, so toinsure its complete oxidation, while at the same

time preserving as much as possible the substrate

from oxidation, since this seems to be the main

factor preventing the epitaxial growth of the oxide.

A dose of 1 MLeq/cycle (as the one used in growth

procedure I) satisfies the first requirement, but it

has also been shown to promote substrate oxida-

tion. In order to avoid the unwanted effect, wehave chosen to dose 1 MLeq of Ni on the Pd sur-

face which has been pre-treated with oxygen, in

order to avoid the formation of a direct interface

between metallic Ni and Pd (the reason for this

choice will be clarified in the general discussion,

Section. 3.4). Following this procedure, chemi-

sorbed oxygen on the substrate surface is poten-

tially available for the oxidation of the first Nidose as soon as Ni atoms reach the substrate

surface. In other words, the aim is bypassing the

stage of the metal/metal interface formation

through direct nucleation of NiO clusters in the

initial stages of deposition. This procedure there-fore differs from the growth route I only for oxy-

gen pre-saturation of the substrate surface before

the first Ni dose, resulting in thep5 structure of O

on Pd(1 0 0).

3.3.1. XPS data

Ni 2p XPS spectra for 2, 6 and 12 MLeq of NiO

on Pd(1 0 0) grown following growth procedure IIIare reported in Fig. 13, both for near-normal

(h ¼ 24�) and grazing (h ¼ 72�) emission. The

main remarks are the following:

• The oxide-related satellites are well developed

already after 2 MLeq, which suggests 3D cluster-

ing in the initial stages of growth, with good

compositional homogeneity throughout thelayer.

• The Ni 2p3=2 main threshold undergoes a minor

shift toward higher BE (from 853.6 to 854.0 eV

at h ¼ 24�, DE ¼ 0:4 eV) on going from 2 to

6 MLeq; its position remains unchanged for

thicker layers at both polar angles.

• For a fixed emission angle, the FWHM and the

line-shape of the main doublet componentsdoes not change as a function of thickness

(except possibly for the 2 MLeq layer, where

Fig. 13. Growth procedure III. Ni 2p XPS peaks for selected

thickness values measured at near-normal (h ¼ 24�, continuousline) and grazing (h ¼ 72�, dashed line) emission. Vertical da-

shed lines indicate the position of the Ni 2p doublet main

components in thick NiO overlayers.

Fig. 14. Ni 2p3=2 treshold referring to 12 MLeq for growth

procedure I (black circles) and III (white circles). The multiplet

structure is indicated by an arrow.

48 M. Sambi et al. / Surface Science 537 (2003) 36–54

the effects due to the reduced dimensionality

play a certain role). Spectra measured at h ¼24� show an incipient multiple structure in the

Ni 2p3=2 threshold, which can be associated to

a better-developed lattice order than in the pre-ceding cases. A higher resolution Ni 2p spec-

trum referring to 12 MLeq is reported in Fig.

14, where the multiplet structure, although less

developed than for epitaxial NiO layers on

Ag(1 0 0) [16,21], is clearly visible. (This finding

correlates well with the LEED pattern reported

in Fig. 16d, see below, which has to be com-

pared to the much sharper LEED pattern ofNiO layers on Ag(1 0 0) [16] 2.) As a useful com-

2 It will be shown in a forthcoming paper that even better

quality NiO films on Pd(1 0 0) can be obtained by means of RD.

In that case, the Ni 2p XPS multiple structure is still more

pronounced and similar to that found in bulk-like NiO.

parison, Fig. 14 also reports the equivalent

spectrum from growth procedure I, where no

multiplet structure is visible, although the

FWHM of the Ni 2p3=2 component is only mar-

ginally larger.

On the basis of these observations, we concludethat the NiO layers grown on the oxygen pre-

saturated Pd surface have a rather homogeneous

composition as a function of thickness and are

presumably characterised by a good degree of

medium-to-long-range order.

Pd 3d spectra taken before and after oxygen

pre-saturation are reported in Fig. 15a, while Fig.

15b compares Pd 3d spectra after O pre-saturationand after a Ni dose corresponding to 6 MLeq.

These results give a key to understanding the

successful outcome of the present growth route:

there is an only marginal FWHM increase

(DE ¼ 0:2 eV) of the Pd 3d5=2 line upon oxygen

pre-saturation, while further increase upon NiO

deposition is negligible: the substrate has been

Fig. 15. Growth procedure III. (a) Pd 3d doublet from clean

Pd(1 0 0) (dashed line) and from the substrate pre-saturated

with oxygen, giving thep5 structure (continuous line). (b) Pd

3d doublet from oxygen pre-saturated Pd(1 0 0) (dashed line)

and from the substrate covered by a 6 MLeq thick NiO layer

(continuous line). Data are acquired at h ¼ 60�.

Fig. 16. Growth procedure III. (a) LEED pattern after the first

Ni dose (1 min) on the pre-saturated Pd(1 0 0) surface and be-

fore the PO treatment, KE¼ 56 eV; (b) and (c) LEED pattern

after the first Ni dose (1 min) on the pre-saturated Pd(1 0 0)

surface and after the PO treatment, KE¼ 56 and 99 eV, re-

spectively; (d) LEED pattern of a 15 MLeq NiO film on

Pd(1 0 0), KE¼ 69 eV.

M. Sambi et al. / Surface Science 537 (2003) 36–54 49

effectively protected from extensive oxidation in

the present case.

3.3.2. LEED data

Fig. 16a shows the LEED pattern detected after

the first Ni dose (1 min) on the pre-saturated

Pd(1 0 0) surface and before the PO treatment. In-

terestingly, the image corresponds to a rather well-

defined c(4 · 2) structure, i.e. the same pattern

detected following the growth procedure I for the

second Ni dose after the PO cycle, but of betterquality. In other words, LEED suggests the for-

mation of NiO due to the reaction between the

evaporated metal and oxygen chemisorbed on the

substrate surface. The co-adsorption structure

thus formed has the same long-range symmetry

detected in the initial stages of growth following

growth procedure I. However, the lack of sub-

strate oxidation in the present case, together with

the uniform distribution of the oxidising agent on

the Pd surface (which makes it readily available for

reaction with upcoming Ni) produces a more ef-

fective long-range ordering of the overlayer. Fig.16b and c (measured at E ¼ 56 and 99 eV, re-

spectively) show the effect of the PO treatment on

the first Ni dose: the structure is still a c(4 · 2)––the

only effect of PO is a further improvement of the

long-range order in an already existing atomic

arrangement.

Finally, Fig. 16d shows the LEED pattern of a

15 MLeq NiO deposition on Pd(1 0 0) following thecurrent growth procedure. Though rather diffuse,

a (1 · 1) pattern due to NiO(1 0 0) is detected,

which was not attained with the previous growth

recipes: there is a fair degree of long-range order in

the present case.

3.3.3. XPD data

Ni 2p3=2 APD scans at h ¼ 45� from NiO filmsof increasing thickness, deposited following the

50 M. Sambi et al. / Surface Science 537 (2003) 36–54

growth route III, are reported in Fig. 17. Strong

intensity modulations are detected even for 2

MLeq. As was the case with the previous growth

route, these reflect the substrate fourfold symme-

try, with intensity maxima aligned with the h001idirections of the substrate surface. However, in thepresent case there is no substantial difference be-

tween the 2 MLeq azimuthal curve and those from

thicker layers.

Ni 2p3=2 PPD scans along the [0 0 1] and [0 1 1]

main azimuths are reported in Fig. 18. Vertical

dashed lines indicate the polar positions where the

main FS features are expected for a cubic NaCl

lattice exposing the (1 0 0) face. The experimentalfeatures follow nicely the expectations: all the main

peaks are completely developed above 4 MLeq. In

particular, the peak at h ¼ 21� along the [0 0 1]

direction is expected for a minimal thickness of 4

ML. Since it appears in the experiment between 4

and 6 MLeq, we conclude that the growth is largely

bi-dimensional in the present case, though unfor-

Fig. 17. Growth procedure III. Ni 2p3=2 APD from NiO ul-

trathin films at h ¼ 45� as a function of the equivalent overlayer

thickness.

tunately an ARXPS cross-check is made unreliable

by strong diffraction effects.

Fig. 19 shows the Ni 2p3=2 and O KLL full 2pplots from 18 MLeq NiO on Pd(1 0 0). FS peaks are

sharper and the fine structure is more defined than

in the corresponding plots of Fig. 12. This is aclear indication of a longer coherence length of

emitted photo- and Auger electrons inside the

overlayer, which is made possible by a more or-

dered lattice structure. Again, the close similarity

of Ni 2p3=2 and O KLL 2p plots is a fingerprint of

the NaCl-like lattice structure.

In conclusion, when Ni deposition and PO cy-

cles are initiated on a Pd (1 0 0) surface which hasbeen pre-saturated with oxygen, NiO islands

(more bi-dimensional than in the preceding cases)

are formed by direct reaction of Ni with chemi-

sorbed oxygen. The substrate is effectively pro-

tected from oxidation, and the resulting NiO layer

is ordered on the medium–long range and char-

acterised by a clear (1 0 0) k (1 0 0), [0 0 1] k [0 0 1]

epitaxial relationship with the substrate, with goodcompositional homogeneity as a function of

thickness.

3.4. Discussion

On the basis of the reported results, it is possi-

ble to put forward a hypothesis concerning the

factors which are responsible for the remarkabledependence of the overlayer quality on the adop-

ted growth procedure.

The substrate behaviour towards oxygen used

in PO treatments is found to be of paramount

importance for the structure of the deposited NiO

overlayer. As already remarked above, in high

vacuum conditions Pd(1 0 0) cannot be bulk-oxi-

dised at any temperature. As a matter of fact, whatwe see in the growth procedure I––by observing

the Pd 3d XPS spectra––is precisely a substantial

oxidation of the substrate upon deposition and

PO, as if metallic Ni were able to promote the

reaction between O and Pd. In addition, this ca-

pacity of promoting Pd oxidation depends not

only on the generic presence of Ni on the substrate

surface, but also on the precise amount of thedeposited metal. In growth procedure II, with the

Ni dose doubled with respect to the first attempt

Fig. 18. Growth procedure III. Ni 2p3=2 PPD from NiO ultrathin films along the (a) [0 0 1] and (b) [0 1 1] substrate main azimuthal

directions as a function of the equivalent overlayer thickness. Vertical dashed lines indicate the angles at which main FS directions are

expected for a bulk-like NaCl structure.

Fig. 19. (a) Ni 2p3=2 (KE¼ 631 eV) and (b) O KLL (KE¼ 510

eV) 2p plots from a 18 MLeq NiO ultrathin film. h ¼ 0� (the

surface normal) corresponds to the centre of each plot. The

outer circle corresponds to h ¼ 80�. The intensity is given by

the colour scale (black: minimum, white: maximum). Main

substrate azimuths are indicated.

M. Sambi et al. / Surface Science 537 (2003) 36–54 51

and with oxidation conditions kept constant, the

substrate is not significantly affected, though still

partially exposed (c is around 0.5 at 2 MLeq).

This behaviour can be correlated with the work

function (WF) lowering of the system, which is

generally observed in the first stages of a metal/

metal interface formation. The phenomenon is

particularly well studied for the deposition of al-kali metals on transition metal substrates [22],

where the WF lowering amounts even to several

eV, but is also documented for the formation of

interfaces between transition metals [23,24], where

the variation is more modest, but nonetheless sig-

nificant (jDWFj � 1 eV or less). In general, when

an overlayer is grown on a metal substrate, with

WFoverlayer <WFsubstrate, there is a steep decrease ofthe WF in the initial stages of deposition towards a

minimum, which is often lower than the WF of

both pure metals, followed by a plateau close to

the WF of the bulk overlayer. Unfortunately, no

52 M. Sambi et al. / Surface Science 537 (2003) 36–54

WF study upon the deposition of Ni on Pd is re-

ported in the literature, but it is reasonable to ex-

pect a behaviour which is similar to the one just

described. The important point is the following: if

the overlayer lowers the WF of the substrate, it

makes it more reactive towards oxidation––in fact,there is a rich bibliography on the alkali metals as

oxidative promoters on transition metal surfaces

[22].

We conclude that the most likely cause for the

substrate oxidation during Ni deposition on

Pd(1 0 0) and PO in growth procedure I is the WF

lowering upon the formation of a direct Ni/Pd

interface after the first Ni dose. Pure Pd has aslightly higher WF than pure Ni [25,26], nonethe-

less the former cannot be oxidised in high vacuum,

while oxidation of the latter readily occurs. But if

the first Ni dose (procedure I) corresponds ap-

proximately to the WF-vs-coverage minimum for

the Ni/Pd(1 0 0) system, we may reasonably expect

the substrate oxidation to be promoted. On the

contrary, when the WF minimum is overcome by alarger first Ni dose (growth procedure II), sub-

strate oxidation is made more difficult.

Let us now discuss the microstructural conse-

quences of the substantial oxidation of the Pd

substrate in growth procedure I. It seems likely

that palladium oxide, PdO, characterised by a te-

tragonal lattice structure (a ¼ 3:029 �AA, c ¼ 5:327�AA) with no simple epitaxial matching either to NiOor to Pd(1 0 0) lattice parameters due to the high

lattice mismatch resulting from any orientation,

leads to a rapid disordering of the interface. NiO

deposited in successive cycles is restricted to grow

on a poorly ordered substrate, with the conse-

quence of a quick loss of epitaxial coherency. This

loss of structural order can be bypassed by dosing

more Ni at the first deposition cycle, in order toovercome the WF minimum of the newly created

interface––but at the expense of a thorough oxi-

dation of the deposit during the PO treatment.

This second growth route therefore leads to a NiO

layer which is heavily understoichiometric at the

interface and poorly ordered on the long range.

An effective way to avoid the formation of a

direct Ni/Pd interface is the pre-saturation of thesubstrate surface with oxygen by means of the

p5

structure of O on Pd(1 0 0) (growth route III),

which has been shown to act as a protection to-

wards substrate oxidation. In energy terms, oxy-

gen pre-saturation leads to an increase of the

substrate WF [27]. In addition, evaporated Ni re-

acts with pre-adsorbed oxygen, thereby directly

producing NiO nuclei on the Pd surface, as dem-onstrated by LEED images referring to the growth

procedures I and III (Figs. 5d and 16a, respec-

tively), which show the formation of the same

c(4 · 2) superstructure at the first Ni dose after the

PO in case I and before PO in case III. Hence,

when O pre-saturation is performed, an interface

with a profoundly different energetics with respect

to the Ni/Pd(1 0 0) case is formed, which allowsus to avoid substrate oxidation.

4. Conclusions

This work has been devoted to the optimisation

of the growth parameters of NiO ultrathin films

on Pd(1 0 0) by means of metal deposition andpost-oxidation cycles, and to their chemical and

structural characterisation by means of surface

spectroscopies (XPS, ARXPS) and electron dif-

fraction techniques (LEED, XPD).

Three growth procedures have been investi-

gated, which differ for the particular growth pa-

rameters adopted in each case. On the basis of the

reported results, the following conclusions can bedrawn:

• NiO deposited on the Pd(1 0 0) surface displays

a degree of epitaxial ordering which depends

on the quantity of evaporated metallic Ni at

each dose. In particular, when 1 MLeq of NiO

is obtained at each deposition/PO cycle (growth

procedure I), a polycrystalline layer character-ised by a very low degree of short-range struc-

tural order is obtained. Moreover, the residual

ordering is limited exclusively to the direction

normal to the surface. If the Ni dose is increased

in order to grow 2 MLeq of NiO at each cycle

(growth procedure II), the resulting oxide layer

is moderately ordered, being characterised by

an epitaxial relationship with the substrate de-veloped on the short range. However, it is not

homogeneous from the compositional point of

M. Sambi et al. / Surface Science 537 (2003) 36–54 53

view: a strong gradient of oxygen understoichi-

ometry is found at the interface with the sub-

strate.

• In growth procedure I, XPS analysis reveals a

substantial oxidation of the Pd substrate upongrowth. The reaction between oxygen and the

substrate during the PO, very likely promoted

by Ni through a WF lowering, is found to be

competitive with the nucleation of epitaxial

Ni oxide, with a resulting polycrystalline over-

layer.

• The layer obtained following growth procedure

II is characterised by a strong gradient of oxy-gen defectivity in the interfacial region, which

is reduced only starting from an overlayer thick-

ness of approximately 6MLeq. We conclude that

the first Ni dose evaporated on the Pd surface,

which is certainly epitaxial and pseudomorphic

to the substrate [19,20], prevents the substrate

oxidation (presumably both because the Ni dose

is such to imply an insufficient decrease of theWF and through direct protection). However,

it also leads to the formation of too large Ni nu-

clei, which cannot be oxidised in the adopted

PO conditions.

• In growth procedure III, the oxide deposition is

not performed directly on the clean substrate,

but on the oxygen pre-saturated Pd surface in-

stead, i.e. on the (p5�p

5)-R27� O/Pd(1 0 0)structure. The first Ni dose reacts with pre-

adsorbed oxygen (as confirmed by LEED im-

ages) so to give a NiO layer characterised by a

fair degree of long-range order, which is simply

improved by the PO treatment. In addition,

there are indications from angle resolved XPS

spectra and from XPD scans, that this route

leads to a more distinctly bi-dimensional over-layer growth. In this case, there is no occurrence

of a direct Ni/Pd interface at the Ni dose close to

the WF minimum of the system, so that the sub-

strate oxidation is largely prevented. The result

is a homogeneous NiO film, ordered in the

medium-to-long range and characterised by an

epitaxial relationship with the substrate of the

type (1 0 0) k (1 0 0), [0 0 1] k [0 0 1]. The fair de-gree of long-range order is confirmed by the

multiplet structure found in the Ni 2p3=2 thresh-

old in XPS spectra, typical of bulk NiO.

In conclusion, the most relevant feature of the

NiO/Pd(1 0 0) system highlighted by the present

work is the importance of the substrate reactivity

towards oxygen in determining the chemical na-

ture and the structural quality of the oxide over-

layer. If one succeeds in preventing substrateoxidation through the choice of the right amount

of Ni per each deposition cycle, an oxide layer

ordered at least on the short range can be ob-

tained. Finally, if the substrate is protected from

oxidation (catalysed by metallic Ni) through oxy-

gen pre-saturation and the Ni dose is sufficiently

small (so to be completely oxidised during the PO

treatment), homogeneous and medium/long-rangeordered epitaxial NiO ultrathin films are pro-

duced. Oxygen––necessary as the oxidising agent––

may act either as an inhibitor or as a promoter of

the epitaxial growth of the overlayer, depending

on the way it is used.

Many questions are left open by this work. In

particular, the intricate evolution from thep5

structure, through the c(4 · 2), ending to the (1 · 1)LEED pattern in the initial stages of deposition

should still find an explanation, a rather difficult

task if one considers that, at present, there is no

commonly accepted atomic-scale description of

the surface reconstruction leading top5 structure

of O on Pd(1 0 0). A tensor LEED analysis, per-

formed some years ago [11] and recently refined

[28], has been questioned by very recent STM datacoupled to theoretical total energy calculations

[29]. An STM study of the initial stages of growth

would therefore be helpful in giving a more precise

picture of the structural and morphological evo-

lution of the system as a function of thickness.

In addition, though many different experimen-

tal observations are nicely explained in terms of

WF variation as a function of the Ni dose, nodirect WF data for the Ni/Pd(1 0 0) system are

available––WF measurements are therefore an-

other priority.

Acknowledgements

This work has been partially funded by Minis-tero della Ricerca Scientifica e Tecnologica

(MURST) through the fund ‘‘Programmi di

54 M. Sambi et al. / Surface Science 537 (2003) 36–54

ricerca di rilevante interesse nazionale’’ and is part

of the PRA ISADORA (INFM).

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