Comparative study of metal adsorption on the metal and the oxide surfaces

Article abstract of DOI:10.1016/S0038-1098(01)00511-7

Adsorption of Ti, Cr, Fe, Ni and Cu atoms at coverage not exceeding two monolayers on the surface of ultrathin (10-15 ?) alumina and magnesia films (γ-Al₂O₃(111) or α-Al₂O₃(1000) and MgO(111) grown on Mo(110) were studied in ultrahigh vacuum by means of electron spectroscopy techniques (Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), work function measurements and reflection absorption infrared spectroscopy (RAIRS)). At very low metal coverage and low substrate temperature (85 K) when the film can be viewed as consisting of separate adatoms and/or very small clusters the electronic properties of adatoms on the oxide films, on one hand, and on Mo(110) surface, on the other hand, are quite different. With increasing metal coverage, the properties on both the oxide and the metallic substrates change becoming similar at the coverage close to monolayer. On the Mo(110) surface the electronic properties change gradually with the metal coverage, whereas on the oxide there is a critical coverage of about 0.15 ML separating ionic and metallic adsorption of the metal species. It is shown that the lateral interaction of adatoms on the oxide surface plays a dominant role in the formation of the band-like structure of the adsorbed 2D film.

Full text of DOI:10.1016/S0038-1098(01)00511-7

PERGAMON
Solid State Communications 122 )2002ꢀ 341±346
www.elsevier.com/locate/ssc
Comparative study of metal adsorption on the metal
and the oxide surfaces
a,
b
T.T. Magkoev , G.G. Vladimirov , D. Remar , A.M.C. Moutinho
c,1
c
*
a
Department of Physics, University of North Ossetia, Kesaev 121-83, Vladikavkaz362020, Russian Federation
Department of Physics, University of St. Petersburg, Ulyanovskaya 1-1, St. Petersburg 198904, Russian Federation
b
c
Department of Physics, New University of Lisbon, Monte de Caparica, Lisbon P-2825, Portugal
Received 11 November 2001; accepted 10 December 2001 by C.N.R. Rao
Abstract
Ê
Adsorption of Ti, Cr, Fe, Ni and Cu atoms at coverage not exceeding two monolayers on the surface of ultrathin )10±15 Aꢀ
alumina and magnesia ®lms )g-Al O )111ꢀ or a-Al O )1000ꢀ and MgO)111ꢀ grown on Mo)110ꢀ were studied in ultrahigh
2
3
2
3
vacuum by means of electron spectroscopy techniques )Auger electron spectroscopy )AESꢀ, electron energy loss spectroscopy
EELSꢀ, high resolution electron energy loss spectroscopy )HREELSꢀ, low energy electron diffraction )LEEDꢀ, work function
)
measurements and re¯ection absorption infrared spectroscopy )RAIRSꢀꢀ. At very low metal coverage and low substrate
temperature )85 Kꢀ when the ®lm can be viewed as consisting of separate adatoms and/or very small clusters the electronic
properties of adatoms on the oxide ®lms, on one hand, and on Mo)110ꢀ surface, on the other hand, are quite different. With
increasing metal coverage, the properties on both the oxide and the metallic substrates change becoming similar at the coverage
close to monolayer. On the Mo)110ꢀ surface the electronic properties change gradually with the metal coverage, whereas on the
oxide there is a critical coverage of about 0.15 ML separating ionic and metallic adsorption of the metal species. It is shown that
the lateral interaction of adatoms on the oxide surface plays a dominant role in the formation of the band-like structure of the
adsorbed 2D ®lm. q 2002 Elsevier Science Ltd. All rights reserved.
PACS: 73.20.Mf; 73.30. 1 y; 73.90. 1 f
Keywords: A. Surfaces and interfaces; A. Thin ®lms; E. Electron emission spectroscopies; E. Electron energy loss spectroscopy
1
. Introduction
investigation on the adsorption of metal atoms, clusters and
thin ®lms on the surfaces of metal oxides. In this paper the
experimental results on adsorption of Ti, Cr, Fe, Ni and Cu
atoms on the surface of Mo)110ꢀ single crystal and alumi-
num and magnesium oxides ultrathin ®lms grown onto
Mo)110ꢀ are presented.
Investigation of the metal deposits on oxide surfaces has
attracted a lot of attention in recent years due to wide
involvement of the metal/oxide interfaces in various tech-
nologically important areas such as thin ®lm technology,
composites, heterogeneous catalysis, gas sensors and others
[
1±4]. Nevertheless, despite of the application importance,
and, hence, the very extensive research undertaken in this
eld over the last decade, there is still a limited knowledge
about the underlying physics and chemistry of such systems
2. Experimental
®
Measurements have been carried out in different ultra
high vacuum )UHVꢀ chambers )base pressure 2 £
210
10
[
5,6]. In relation to this, it is necessary to perform further
Torrꢀ enabling a combination of different techniques:
Auger electron spectroscopy )AESꢀ, electron energy loss
spectroscopy )EELSꢀ, high resolution electron energy loss
spectroscopy )HREELSꢀ, low energy electron diffraction
*
Corresponding author. Tel.: 17-867-242-4595; fax: 17-867-
75-4635.
E-mail address: magkoev@osetia.ru )T.T. Magkoevꢀ.
2
)
LEEDꢀ, low energy ion scattering )LEISꢀ, re¯ection
1
Permanent address: Department of Chemistry, University of
Boston, USA.
absorption infrared spectroscopy )RAIRSꢀ and work func-
tion measurements. For AES a double pass cylindrical
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098)01ꢀ00511-7

342
T.T. Magkoev et al. / Solid State Communications 122 ꢀ2002) 341±346
MgO)111ꢀ ultrathin ®lm structure, normally unstable for
bulk crystal, is formed due to the ordering in¯uence of
the Mo)110ꢀ substrate and holds up to six monolayers of
magnesia [12]. The ®lms of Ti, Cr, Fe, Ni and Cu were
deposited by a standard procedure of thermal evaporation
2
1
at a growth rate of 0.025±0.03 ML min . The sample
holder allowed to cool the sample to 85 K and heat it to
2600 K. Experimental techniques and procedures are
described in more detail elsewhere [12±15].
3
. Results and discussion
In adsorption of alumina and magnesia the work function
gradually decreases with the oxide coverage )uꢀ and exhibits
no noticeable change as the coverage exceeds the two mono-
molecular layers. Assuming that the work function of the
clean Mo)110ꢀ surface is equal to 5.0 eV [16], the stationary
values of f for aluminum and magnesium oxides were
measured to be 3.90 and 3.55 eV, respectively. At u higher
than two monolayers the Auger and HREEL spectra closely
resemble those characteristic for the bulk oxide crystals. As
an example, in an inset of Fig. 1 the HREEL spectrum of
MgO ®lm is shown. The one-to-one stoichiometry of MgO
®lm is con®rmed by the appearance of sharp optical phonon
losses, a characteristic loss pattern for single crystal MgO
Fig. 1. A series of retarding curves for deposition of Cu onto
MgO)111ꢀ ®lm on Mo)110ꢀ at different coverage )uꢀ of Cu; u,
ML: 3±0.4; 4±0.6 and 5±1.1. Curves 1 and 2 correspond to the
Ê
bare MgO surface at ®lm thickness of 15 and 100 A, respectively.
Ê
In an inset a HREELS spectrum of MgO ®lm at a thickness of 15 A
consisting of fundamental and multiple optical phonon losses is
shown.
mirror analyzer with coaxial gun operating at a primary
energy of 3 keV and beam current of 0.1 mA was used.
Electron energy loss spectra and LEED patterns were
observed by means of retarding hemispherical rear-view
four-grid system with normal incidence gun. The work
function change )Dfꢀ was measured by Anderson method
2
1
[17]: the fundamental loss at 670 cm and multiple losses
2
at 1350 and 1900 cm are evident in the spectrum. The
1
2
1
phonon losses at 400, 600 and 840 cm , which are close
to those measured for bulk alumina [18], are also evident in
the loss spectrum of ultrathin aluminum oxide ®lm. More-
over, the shape of the corresponding retarding curves in
Anderson method might indicate that the oxide ®lms exhibit
insulating properties. As an example in Fig. 1 )curve 1ꢀ, the
retarding curve for MgO/Mo)110ꢀ is shown, featuring
decrease of the current through the sample, covered by
oxide ®lm, as the bias voltage sweeps over the so called
`cut-off' point [7] D )part DEꢀ. Quite recently, a shape of
the iꢀU† curve similar to that shown in Fig. 1)1ꢀ was also
observed for LiF, a highly insulating material )band gap:
14 eVꢀ, deposited on the Si)111ꢀ surface [19]. Therefore,
we tend to attribute the decrease of the current through the
oxide/metal sample as due to the insulating properties of the
®lm.
[
7]. The properties of the probing NO molecules adsorbed
on the surface of metal deposits have been studied by
RAIRS. The latter was carried out using a conventional
Fourier-transform infrared spectrometer. The infrared light
was introduced into the UHV chamber through CaF2
window at an incidence angle of about 808. The absorption
spectrum was typically averaged over 300 scans at a
2
1
resolution of 4 cm . For HREELS an Ibach type double
pass spectrometer )HIB 1000, VSW Scienti®c Instrumentsꢀ
with monochromatized electron beam was used [8]. Most
spectra were collected in a specular direction at a scattering
angle of 608 with respect to the surface normal and at
incident beam energy of 6 eV. The typical resolution )full
width at half maximumꢀ of the elastic peak was about
2
1
6
composition of the topmost layer and the growth mode of
0 cm . The LEIS technique is extremely sensitive to the
Additional information on the properties of the oxide
®lms can be obtained by comparison of the values of the
work function and the coef®cient of elastic low energy
electron re¯ection )rꢀ of the ®lms and respective bulk
crystals. The latter can be de®ned in the present case as r ˆ
a=b )Fig. 1ꢀ. The values of f and r depend on the structure
and stoichiometry of the samples, therefore such a compari-
son may not provide as straightforward information as that
given by AES and HREELS. Nevertheless, a rough agree-
ment of these values measured for the ®lms )r ˆ 0:5; f ˆ
3:55 eV for MgO and r ˆ 0:35 and f ˆ 3:9 eV for Al O ꢀ
the ®lm [9]. In the present experiment the probe ions are
1
1
.0 keV He and the scattering angle is ®xed at 1358. Thin
alumina and magnesia ®lms were prepared by a well-known
procedure of exposing the substrate )Mo)110ꢀꢀ at room
temperature to Al and Mg vapor, respectively, in a con-
trolled oxygen atmosphere. Subsequently the ®lms were
annealed in an oxygen ambient at 1200 K for Al
9
2
O
3
and
00 K for MgO. According to the LEED results the oxide
®
lms grown in this way exhibit hexagonal structure, at least
at a coverage )uꢀ exceeding two monolayers. This corre-
sponds to the structure resembling either g-Al )111ꢀ
or a-Al O )1000ꢀ, and MgO)111ꢀ [10,11]. The polar
2
3
2
O
3
with those reported in the literature for bulk alumina
2
3
and magnesia crystals )r ˆ 0:6; f ˆ 3:8 eV for MgO and

T.T. Magkoev et al. / Solid State Communications 122 ꢀ2002) 341±346
343
These are: 0.7, 0.4, 0.3, 0.2 and 0.15 D for Ti, Cr, Fe, Ni, and
Cu on Mo)110ꢀ and 20.1, 20.2, 20.3, 20.4 and 20.6 D,
and 20.05, 0.15, 20.25, 20.3 and 20.4 D for Ti, Cr, Fe, Ni
and Cu on magnesium and aluminum oxide ®lms, respec-
tively. It is seen that not only the absolute value, but also the
sign of the dipole moment is different for adsorption on the
metal and oxide substrates. Such difference might be
attributed to quite different electronic state of adatoms at
low coverage on the metal and oxide surfaces. As the
metal coverage increases the dipole moments gradually
change and acquire the value close to zero for all adsorbates
and substrates studied at a coverage close to unity. Also,
there is a common feature that for both types of substrates
)
oxide and Mo)110ꢀꢀ the work function at a metal coverage
higher than unity has about the same value: 3.9, 4.25, 4.35,
.6 and 4.9 eV for Ti, Cr, Fe, Ni, Cu, respectively. This may
4
serve as an evidence that at very low metal coverage the
properties of the ®lms adsorbed on the oxide and Mo)110ꢀ
surface are quite different, whereas at higher coverage )close
or more than unityꢀ the properties are similar. Metal
electronic state transformation under adsorption also leads
to the change of the electronic structure of the substrate.
This manifests itself in the abovementioned change of the
shape of the retarding curves, indicating decrease of the
re¯ection coef®cient )Fig. 1, curves 3±5ꢀ. Decrease of
value of r suggests that additional channels of charge ¯ow
through the insulating oxide ®lm appear under metal atom
adsorption. Results of recent theoretical calculations [22]
that very intense structures in the density of states located
in the band gap of MgO ®lm emerge under adsorption of
transition metals, may provide a possible explanation of
such behavior.
Fig. 2. )aꢀ Work function versus coverage plots for Ti, Fe, Ni, Cu on
MgO)111ꢀ/Mo)110ꢀ and Mo)110ꢀ )dashed linesꢀ. )bꢀ Auger uptake
curves of Fe)LVV, 702 eVꢀ )1ꢀ and O)KVV, 503 eVꢀ )2ꢀ for Fe
deposition on MgO)111ꢀ/Mo)110ꢀ and oxygen LEIS intensity
2 3
)410 eVꢀ for Cu )3ꢀ and Ti )4ꢀ on Al O . In all cases substrate
temperature during metal deposition is 85 K.
r ˆ 0:4; f ˆ 4:2 eV for Al O [20,21]ꢀ supports the above
2
3
assumption that the properties of ultrathin ®lms resemble
those of the bulk oxides. After the oxide ®lms have been
formed on the Mo)110ꢀ crystal, the system was cooled
down to 85 K at which the metal atoms were successively
deposited onto the oxide ®lm surface. For all the metals
studied such a deposition results in the change of the retard-
ing curve shape in the way shown in Fig. 1 for Cu on
MgO)111ꢀ, featuring: )1ꢀ parallel shift of the linear part
The growth mode of metal overlayers has been evaluated
from the well-known procedure of analysis of the corre-
sponding Auger uptake curves [23,24]. An observed linear
change of the plots accompanied by a break indicates that
the layer-by-layer growth mode occurs, at least up to
1.4 ML. LEIS results corroborate this conclusion: in all
cases, except Cu on Al O , the metal LEIS intensity versus
)
CDꢀ and )2ꢀ gradual increase of the collected current at
2
3
positive bias voltages as metal coverage increases )part
DEꢀ. This behavior indicates that the sample undergoes a
work function increase and re¯ection coef®cient decrease
with the metal coverage. For all the metals deposited on
both oxide ®lms the work function gradually increases
until saturation at a coverage close to unity. As an example,
in Fig. 2)aꢀ there are work function versus coverage plots for
depositing of Ti, Fe, Ni and Cu atoms onto MgO)111ꢀ ®lm.
For comparison the corresponding plots for the same metals
on Mo)110ꢀ are also shown )dashed linesꢀ. It is seen that
f)uꢀ plots for oxide and Mo))110ꢀ substrates exhibit
markedly different trends: In the ®rst case the work function
increases, whereas in the latter case the work function
decreases. Assuming the validity of the dipole model of
the work function change, using the Helmholtz equation
coverage linearly decreases until the value close to zero is
obtained at the unity coverage. Such behavior indicates of
the complete monolayer formation. As an example, in
Fig. 2)bꢀ there are LEIS intensity versus coverage plots of
Cu and Ti on Al O ®lm )curves 3 and 4ꢀ. Thus, most of the
2
3
metals studied wet the oxide surfaces. This observation is,
however, in contrast with what is expected from thermo-
dynamic considerations: occurrence of a layer-by-layer,
3D-island or Stranski±Krastanov growth mode depends on
whether the surface free energy of the substrate )G ꢀ is
s
greater than )or equal toꢀ the sum of the surface energies
of the adsorbed ®lm )G ꢀ and the ®lm/substrate interface
f
)G ꢀ [25]. Generally, the surface energy of oxides is much
sf
lower than that of transition metals )for instance, G for
s
2
2
Al O is 690 erg cm , and G for Ti is 1600 erg cm and
22
2
3
f
2
G is about 60 erg cm [25], which makes the wetting of
2
ꢀ
Df ˆ 4puem†; one may estimate the initial dipole
sf
moments )mꢀ of single adsorbed atoms )that is, at u ! 0ꢀ.
the oxide surfaces by transition metals unfavorable.

344
T.T. Magkoev et al. / Solid State Communications 122 ꢀ2002) 341±346
two monolayers its properties in many aspects are close to the
properties of bulk metals [29]. By measuring the LVV/LMM
Auger intensity ratio this allows to obtain the value of charge
transfer. The Dq versus coverage plots for Ti, Cr, Fe, Cu on
Mo)110ꢀ are shown in Fig. 3. As an example in an inset the
Auger spectra of Cr on Mo)110ꢀ at a Cr coverage ranging from
0
.05to2 MLarealsoshown. SincetheshapeoftheAugerlines
does not change with the metal coverage, peak-to-peak height
was chosen as the Auger intensity. It isseenthatinall cases the
Dq gradually decreases with increasing adsorbate coverages
until a value close to zero is obtained at a coverage around
2
ML. The sign of Dq indicates that there is an electronic
charge transfer from adatom to the substrate, and the amount
of this charge transfer decreases in the sequence Ti±Fe±Cr±
Cu. By extrapolating these plots to zero coverage it is possible
to estimate the charges of single adsorbed atoms on Mo)110ꢀ
surface. These are )in electronic charge unitsꢀ: 10.2, 10.15,
Fig. 3. Charge transfer )Dqꢀ versus coverage plots for Ti, Cr, Fe and
Ni adsorption on Mo)110ꢀ surface. In an inset a series of Cr Auger
spectra in LVV±LMM region at Cr coverage ranging 0.05±2 ML is
shown.
10.1 and 10.05 for Ti, Fe, Cr and Cu, respectively. As the
coverage grows, these values decrease )Fig. 3ꢀ due to competi-
tion offorming lateral metal/metal bonds with metal/substrate
bonds. Quite recently the possibility of Ni 3d-orbital charge
transfer to Mo)110ꢀ was reported on the basis of ultraviolet
photoelectron spectroscopy measurements [30]. Also, theore-
tical calculations on the electronic properties of Fe and Pd on
Ta)110ꢀ surface [31] has shown that there is an important
redistributionof chargethat polarizesadatomelectrons toward
the metal/substrate interface, although the bond cannot be
described as ionic. The calculated bonding energy of 3±4 eV
per adatom implies formation of very strong interfacial metal/
substrate bond. Observed variation of the electronic properties
of adsorbed species )work function, dipole moment, charge
transferꢀ along the 3d-series suggests that d-orbitals play an
important role in the formation of chemisorption bonds with
the substrate.
However, in the present case, when the MgO)111ꢀ ®lm is
used as a substrate, the same thermodynamic argument may
favor complete transition metal monolayer formation, since
2
2
the surface energy of MgO)111ꢀ )4300 erg cm ꢀ [26] much
exceeds the corresponding value of transition metals. The
2 3
fact that for the metals )except Cuꢀ on Al O the observed
growth mode differs from thermodynamic predictions may
be explained assuming that the metal ®lms formed at 85 K
are not thermodynamically stable. At low substrate tempera-
ture kinetic barriers should limit surface diffusion of
adatoms and thus coalescence of them into 3D-clusters.
Qualitative similar behaviour was also reported recently
for Fe deposited on MgO)100ꢀ [27]. No superstructure of
the metal deposits on both alumina and magnesia was
observed by LEED for all the metals studied.
For metals adsorbed on alumina and magnesia oxides the
Rꢀu† plots are different from what is observed on Mo)110ꢀ
surface. )For Cr it was not possible to reliably measure such
a plot, since Cr)LMMꢀ and O)KVVꢀ Auger lines intersectwith
each otherꢀ. For all metals, except Ti on both alumina and
magnesia, the plots show similar trends. As an example,
Rꢀu† plot for Fe on MgO)111ꢀ is shown in Fig. 4. With increas-
ing coverage, at the Auger ratio abruptly increases from the
value of 1.3 to 1.6, and does not signi®cantly change as the
coverage grows further. This implies that at low coverage
For comparison of the electronic state of metal adatoms on
the oxide ®lms and the Mo)110ꢀ crystal, apart from the above
measurement of the work function and the dipole moment, we
also measured the plots of the metal LVV/LMM Auger inten-
sity )Iꢀ ratio versus the metal coverage. The LVV Auger inten-
sity is sensitive to the electronic charge at the valence level of
the metal atom. It was shown that for the ®rst-row transition
metals the following relationship holds [28]
R ˆ IꢀLVV†=IꢀLMM† ˆ Cqꢀq 2 1†
)
lower than 0.15 MLꢀ the valence electronic charge of the
When the metal adsorbs on the substrate surface the value of
electronic charge )qꢀ at the valence level may change, by the
valueofchargetransferDq, duetothe formationof chemisorp-
tion bonds. Inthiscase, the aboveequationshouldbe written in
the following form:
metal is shifted towards the oxide. This assumption is
supported by the RAIRS data. In an inset of Fig. 4 there are
vibrational spectra of NO molecules adsorbed on the Ni ®lm
deposited onto Al O surface at low Ni coverages. All spectra
2
3
correspond to the saturation coverage of NO: 10 L ꢀ1 L ˆ
2
6
1
0
Torr s†: Initially, at a Ni coverage of 0.01 ML a vibra-
R ˆ Cꢀq 1 Dq†ꢀq 1 Dq 2 1†
21
tional band at an energy 1812 cm appears )curve 1ꢀ. With
increasing u to 0.04 ML the intensity of this line grows with a
slight red shift )curve 2ꢀ. At a Ni coverage of 0.1 ML further
increase of the high-frequency band intensity is accompanied
by the appearance of a splitted band at a lower frequency
The value of C for each metal can be obtained assuming that at
coverage when RꢀQ† plot stabilizes ꢀQ ˆ 1:5±2 ML† the Dq is
equal to zero. The motivation of this assumption is that at a
coverage of adsorbed complete metal ®lm exceeding one to

T.T. Magkoev et al. / Solid State Communications 122 ꢀ2002) 341±346
345
Fig. 4. Fe LVV to LMM Auger intensity ratio versus Fe coverage on
MgO)111ꢀ/Mo)110ꢀ. In an inset a RAIRS spectra for NO molecules
Fig. 5. EEL spectra of Ti on Al O )2±4ꢀ and MgO )5 and 6ꢀ ®lms.
2
3
adsorbed on Ni ®lm on Al
Niꢀ, ML: 1±0.01; 2±0.04; 3±0.1 and 4±0.16. Substrate tempera-
ture: 85 K.
2
O
3
at different Ni coverage are shown; u
Curve 1 corresponds to EELS of Al O ®lm of about 10 AÊ thick. Ti
coverage, ML: 2±0.2; 3±0.6 and 4 to 6±1. MgO ®lm thickness, AÊ :
5±15 and 6±100.
2 3
)
2
1
)
1546±1565 cm ꢀ )curve 3ꢀ. As Ni coverage reaches
.16 ML the high-frequency line intensity is lowered whereas
transfer from the metal to the oxide at low metal coverage
)lower than 0.15 MLꢀ. As it is demonstrated earlier, the
same trend also holds for metals on Mo)110ꢀ surface.
However, unlike the metallic substrate, for which the charge
transfer gradually decreases with increasing metal coverage
and stabilizes at u . 2 ML )Fig. 3ꢀ, for oxides there is an
abrupt decrease of the adatom charge at a coverage of 0.15±
0.20 ML to the stationary value characteristic for metallic
adsorbate. This impliesthatatlow coverage the metal adatoms
are ionic )likely with a fractional chargeꢀ. As the coverage
increases, adatoms begin to combine with each other and/or
the very small cluster size increases, so that the lateral bonds
between adatoms form and the clusters acquire metallic prop-
erties manifesting themselves at the coverage higher than
0.15±0.20 ML. Thus, on theoxides, unlikethemetalsubstrate,
there is a critical coverage )0.15±0.20 MLꢀ separating ionic
and metallic adsorption.
0
thatofthelow-frequencybandisincreased)curve4ꢀ. Athigher
coverage the high-frequency band totally disappears, and only
the low-frequency band is evident in the spectrum. The latter
resembles the line apparent for NO adsorbed on the bulk Ni
crystal [32], and thus may be attributed to NO adsorbed on the
Ni islands formed on an oxide. In thisregard, it isreasonableto
expect that these islands may have adsorptive properties close
to those of the bulk metal. It should be noted that recently no
high-frequency line was detected for NO adsorbed on Ni ®lm
on W)110ꢀ at all submonolayer Ni coverages studied; only the
21
lines in the range 1480±1600 cm were detected depending
on Ni coverage and NO exposure [33]. This also differentiates
the metallic versus the oxide surface. To ®nd out the origin of
the high-frequency band at a wavenumber higher than
21
1
800 cm in the case of oxide substrate, it is informative to
recall the Blyholder model of NO chemisorption on the metal
surface [34]. It is noteworthy that a number of recent theore-
tical calculations [35,36] con®rmed the validity of this model,
proposed in 1964. In view of this model, the high-frequency
band can be explained if one assumes a limited, compared to
that of bulk Ni, backdonation from Ni valence orbitals to NO
EELS provides further support of the above consideration.
The main feature in electron energy loss spectra during the
metal adsorption on aluminum and magnesium oxide is the
appearance of the metal surface plasmon line already at a
coverage close to 0.2 ML. As an example, the corresponding
2 3
spectra for Ti deposited onto Al O are shown in Fig. 5. The
2
p antibonding orbital. Therefore, it is reasonable to expect
spectrum of aluminum oxide )curve 1ꢀ almost in detail resem-
bles that reported recently [38]. The main line at an energy of
about 20 eV corresponds to the collective motion of the
valence band electrons )bulk plasmonꢀ [38], whereas the
lower intensity line at 11 eV corresponds to the interband
transition. As Ti coverage increases above 0.2 ML, a new
band at a loss energy of about 10 eV emerges, of which
intensity increases with the Ti coverage accompanied by a
red shift. At unity coverage )curve 4ꢀ the spectrum roughly
agrees with that reported for metallic Ti [39] with the line at
8.6 eVbeingattributedtothesurfaceplasmonloss. Thisobser-
vation is in line with the above assumption that for a complete
thattherewillbea lackof electroniccharge atthevalencelevel
of adsorbed metal atoms at a very low coverage compared to
the bulk metal. This lack of charge can be reconciled assuming
the charge transfer from the metal orbital to the oxide occur-
ring upon chemisorption. This conclusion agrees with the
results of previous theoretical calculations [37], suggesting
that the dominating bonding scheme of transition metal
adatom with the aluminum oxide surface is the hybridization
of the metal d-orbitals with the oxygen nonbonding 2p-orbital
with the charge shift towards the oxygen. Thus, both the LVV/
LMMAugerintensityratioandRAIRSdatapointatthecharge

346
T.T. Magkoev et al. / Solid State Communications 122 ꢀ2002) 341±346
monolayer a certain properties of the metallic ®lms on oxides
are close to those of metals deposited onto the metallic
substrate, or even the bulk metals. To ®nd out how important
is the effect of the underlying metallic support )Mo)110ꢀꢀ on
the formation of the bulk like properties of the metal deposits,
we have carried out measurements at low and high oxide inter-
layer ®lm thicknesses. When magnesia coverage reaches
Acknowledgements
One of us )T.T.M.ꢀ gratefully acknowledges research
grants from European Union Science CommitteeÐ
INVOTAN fund, DAADÐGermany and the Ministry of
Science and Technology of Portugal. The authors greatly
thank Profs K. Christmann )Free University of Berlinꢀ,
Y. Murata )University of Tokyoꢀ and R. Lahaye )AMOLF
Institute, Amsterdamꢀ for their cooperation.
30 ML the slope of the linear part )CDꢀ of the corresponding
retarding curve changes )Fig. 1, curve 2ꢀ. We tend to attribute
this to the electric charging of the ®lm. The value of electric
resistance in the normal direction to the surface is determined
by the slope of the linear part of the curve, and can be deter-
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