Electronic structure of TiO2 monolayers grown on Al2O3 and MgO studied by resonant photoemission spectroscopy

Article abstract of DOI:10.1016/S0039-6028(02)01334-1

The electronic structure of the TiO₂-Al₂O₃ and TiO₂-MgO interfaces have been studied using resonant photoemission spectroscopy. To this end, respective TiO₂ monolayers have been grown on both substrat

Full text of DOI:10.1016/S0039-6028(02)01334-1

Surface Science 507–510 (2002) 672–677
www.elsevier.com/locate/susc
Electronic structure of TiO monolayers grown on Al O and
2
2
3
MgO studied by resonant photoemission spectroscopy
a
a,
a
b,c
b,c
*
M. S ꢀa nchez-Agudo ,L. Soriano
,C. Quir oꢀ s ,L. Roca
,V. P ꢀe rez-Dieste
,
a
J.M. Sanz
a
Departamento de F ꢀı sica Aplicada, Instituto de Ciencia de Materiales Nicol aꢀ s Cabrera, Universidad Aut oꢀ noma de Madrid,
Cantoblanco, E-28049 Madrid, Spain
b
Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain
c
LURE, Universit eꢀ Paris-Sud, B a^ timent 209d, F-91405 Orsay, France
Abstract
The electronic structure of the TiO
mission spectroscopy. To this end,respective TiO
2
–Al
2
O
3
and TiO
2
–MgO interfaces have been studied using resonant photoe-
2
monolayers have been grown on both substrates. Valence band
photoemission spectra through the Ti 2p ! 3d absorption edge,i.e. 455–470 eV,have been measured. The results have
been analysed in terms of the constant initial state curves. On-resonance minus off-resonance difference spectra have
been used to separate the contribution to the valence band of the Ti 3d states for both monolayers. From the com-
parison of the results obtained for each interface it is inferred that the covalent–ionic character of the substrate affects
the Ti–O bonding at the interface and the Ti 3d states’ contribution to the valence band. Ó 2002 Elsevier Science B.V.
All rights reserved.
Keywords: Near edge extended X-ray absorption fine structure (NEXAFS); Photoemission (total yield); Interface states; Aluminum
oxide; Titanium oxide; Coatings; Insulating films
1
. Introduction
case the effect was slightly weaker,suggesting that
the covalence of the substrate was a key parameter
to understand the mechanism of such interaction.
During the last few years a big effort has been
made in order to investigate the oxide–oxide in-
terfaces. Previously,we have published the study
2 3
In fact,in the case of the Al O substrate,we have
shown that the strong covalent character of the
Al–O bonding leads to a decrease of the covalence
of the Ti–O bonding at the interface [3]. Therefore,
2 2
of the TiO –SiO interface [1] by means of X-ray
absorption spectroscopy (XAS). It was shown that
strong overlayer–support interaction exists at the
interface. Similar effects were observed in the study
the study of the growth of TiO on a more ionic
2
oxide like MgO seems well justified. In this work
of the TiO
2
–Al
2
O
3
interface [2],although in this
we present a comparative study of respective TiO
monolayers grown on two substrates such as Al
and MgO,which clearly differ in ionicity.
2
3
2
O
*
Similar effects to those observed in the unoc-
cupied electronic states,as shown by the XAS
spectra,should also be reflected in the occupied
Corresponding author. Tel.: +34-913974192; fax: +34-
9
13973969.
E-mail address: l.soriano@uam.es (L. Soriano).
0
039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0039-6028(02)01334-1

M. S ꢀa nchez-Agudo et al. / Surface Science 507–510 (2002) 672–677
673
states in the valence band. To investigate this,we
have used resonant photoemission spectroscopy
resolution. The spectra were normalised to the
incident I current as measured from a gold grid
0
(
RPES),which has already proved its potential
located at the entrance of the chamber to correct
the beam intensity loss. The absolute energy scale
was calibrated according to the Fermi level of a Cu
sample.
in the analysis of interfaces [3]. RPES has been
widely used in the analysis of the electronic
structure of bulk compounds. In particular,RPES
has been used to isolate the cationic contribution
to the valence band in transition metal com-
pounds. More details can be found in the review
3. Results and discussion
2
by Davis [4]. TiO has been previously character-
ised by resonant photoemission at the 3p ! 3d
3.1. The Ti 2p XAS spectra
edge [5–7] and at the 2p ! 3d edge [8].
In this work we present and discuss the Ti 2p
The Ti 2p XAS spectra are usually interpreted
in terms of atomic multiplets projected in a crystal
field with the corresponding symmetry. These
spectra are site and symmetry selective and very
sensitive to the local environment of the atoms.
According to this,the Ti 2p XAS spectra of a
monolayer should reflect the local environment of
the Ti atoms located at the interface and conse-
quently the possible electronic interaction between
overlayer and substrate. This is clearly illustrated
in Fig. 1,where the spectra of the reference TiO 2
thin film grown at room temperature (b) and after
heating at 300 °C (a) are shown in comparison with
XAS spectra for respective TiO
Al O and MgO. They are compared to the spectra
2
monolayers on
2
3
of a bulk TiO thin film. Then,we present the
2
experimental valence band RPES through the Ti
2
monolayers on Al and MgO. The RPES spec-
p ! 3d absorption edge of the respective TiO
2
2
O
3
tra are analysed in terms of constant initial state
curves and on-resonance minus off-resonance dif-
ference spectra.
2
. Experimental
The Al substrate was prepared by thermal
oxidation of a high purity (99.999%) aluminium
those of the respective TiO
2
monolayers grown on
2
O
3
MgO (c) and Al (d). The spectra have been nor-
2 3
O
malised to their maximum intensity for compari-
son purposes.
foil at 350 °C for 30 min in an oxygen atmosphere
À5
(
by reactive evaporation of Mg in an oxygen at-
1 Â 10 Torr). The MgO substrate was prepared
2
The spectrum of the TiO thin film after an-
nealing at 300 °C (Fig. 1a) agrees well with other
À5
mosphere (1 Â 10 Torr) at room temperature.
published spectra for TiO [9,10]. This spectrum
2
TiO
in an oxygen atmosphere (5 Â 10 Torr) at room
2
was grown by reactive evaporation of Ti
À6
has already been interpreted in the literature and
has been theoretically simulated in terms of mul-
tiplets using a crystal field (10 Dq) of 1.8 eV [10].
temperature. The deposition rate was low enough
to allow a good control of the coverage. For long
2
The spectrum corresponding to the TiO thin film
ꢁ
evaporation time,a 200 A thick TiO
2
film was
grown. Then,it was submitted to thermal anneal-
grown at room temperature (Fig. 1b) shows sig-
nificant differences with respect to the previous
one. These differences can be explained in terms of
disorder of the TiO2 thin film grown at room
temperature [2].
À6
ing at 300 °C in an oxygen atmosphere (5 Â 10
Torr) for 30 min.
The photoemission measurements were per-
formed at the SU8 beam-line of the SuperAco
storage ring at LURE. This beam-line is equipped
with a plane grating–spherical mirror monochro-
mator (PGM-SM). The electron analyser was an
angle resolved analyser from VSW working at
constant pass energy. The exit slits were adjusted
in order to obtain an acceptable count rate and
On the other hand,the spectra of the respective
TiO
clearly differ from each other. Whereas the spec-
trum of the TiO monolayer on MgO (Fig. 1c)
resembles that of the as-grown TiO thin film,the
spectrum of the TiO monolayer on Al O (Fig.
2 2 3
monolayers grown on MgO and Al O
2
2
2
2
3
1d) differs significantly from the others showing

674
M. S ꢀa nchez-Agudo et al. / Surface Science 507–510 (2002) 672–677
metries [12]. The presence of metallic Ti can also
be discarded as otherwise the spectra should be
broader and shifted in energy as it is shown by
experimental data for metallic Ti [13]. Therefore,
the Ti 2p XAS spectra reveal that the more cova-
2 3
lent Al O substrate strongly affects the Ti atoms
of the TiO monolayer by reducing significantly
2
the crystal field whereas the effect of the more ionic
MgO substrate is weaker.
3.2. The valence band resonant photoemission spec-
tra
Fig. 2 shows the valence band resonant photo-
emission spectra through the Ti 2p ! 3d thresh-
Fig. 1. The Ti 2p XAS spectra of: (a) TiO
room temperature and annealed at 300 °C in an oxygen at-
mosphere (see text); (b) TiO thin film grown at room tem-
perature; (c) TiO monolayer grown on MgO; solid line
2
thin film grown at
2
2
4þ
represents the 2p ! 3d atomic multiplet calculation for Ti in
octahedral symmetry with a crystal field 10 Dq ¼ 1.3 eV; (d)
2 2 3
TiO monolayer grown on Al O .
only two main peaks with weaker structures at
the low energy side of the main peaks. This spec-
4þ
trum has already been explained in terms of Ti
species with an important reduction of the crystal
field of the Ti atoms located at the Al surface
2
O
3
(
10 Dq ꢀ 1.0 eV) with respect to bulk TiO (10
2
Dq ꢀ 1.8 eV) [2]. In the case of the TiO monolayer
2
on MgO the crystal field of the Ti atoms at the
interface,as inferred from the comparison of the
Ti 2p XAS spectrum with atomic multiplet calcu-
lations [10],is 1.3 eV (Fig. 1c). This value is in
between those of bulk TiO
layer on Al . It is important to note here that
these spectra cannot be interpreted in terms of
2 2
and the TiO mono-
2
O
3
3
þ
2þ
other oxidation states,i.e. Ti
have no resemblance at all with experimental XAS
spectra for Ti and TiO [11] and existing mul-
tiplet calculation for Ti atoms in d and d sym-
and Ti ,as they
Fig. 2. The valence band resonant photoemission spectra as a
2
O
3
function of the photon energy for a TiO
MgO (bottom) and a TiO monolayer grown on Al O
monolayer grown on
(top).
2
1
2
2
2 3

M. S ꢀa nchez-Agudo et al. / Surface Science 507–510 (2002) 672–677
675
old,i.e. 456–467 eV energy range for the respective
TiO monolayer on Al (top) and on MgO
bottom). Both valence bands are different because
their shapes are nearly those of the substrates,i.e.
Al and MgO,respectively. In both cases,the
2
2 3
O
(
2
O
3
valence band is formed by two main structures
labelled as A and B. These bands come from the
hybridisation of the O 2p states with Al 2p and Mg
2
p states,respectively,forming the r (A) and p (B)
bands in the valence band. Also the O 2p and Ti 3d
states of the TiO monolayer are present in these
spectra. For the TiO monolayer on Al we can
2
2
2 3
O
observe important changes in intensity of the
structures of the valence band,specially that la-
belled as B (p band). For the TiO monolayer on
2
MgO the changes in intensity of these structures
2 2 3
seem to be weaker than those for TiO –Al O . In
both cases,these changes in intensity correspond
to a resonant photoemission process from the
TiO
2
monolayer. This is an example of a Fano type
6
0
resonance mechanism involving the Ti 2p 3d !
5
1ꢁ
2
p 3d excited state which occurs when the in-
cident photon energy varies through the Ti 2p !
d threshold. We have also performed resonant
3
photoemission measurements of the substrates con-
firming that no resonance phenomena take place
Fig. 3. CIS curves for a TiO
TiO
layer grown on Al
2
thin film annealed at 300 °C (top);
2
monolayer grown on MgO (middle); and TiO
2
mono-
in this energy range for Al
Fig. 3 shows the constant initial states (CIS)
curves for the respective TiO monolayers and for
the TiO
thin film annealed at 300 °C for com-
2 3
O or MgO.
2
O
3
(bottom). Open circles (A) indicate the
intensity of the high binding energy structure of the valence
band and solid circles (B) indicate the intensity of the low
binding energy side of the valence band. For explanation see
text.
2
2
parison. The CIS curves have been generated by
plotting the intensities of the main features A and
B of the valence band as a function of the photon
energy. The values of the two,A and B,features
taken for each sample are: 8.5 and 5.2 eV for the
show that both structures A and B have the same
resonant behaviour,i.e. both structures resonate at
the same energies. However,for the TiO 2 mono-
layer on Al O only the lower binding energy
Al
and 8.0 and 5.7 eV for the TiO
tively. In the case of the TiO
2
O
3
substrate,8.6 and 6.0 for the MgO substrate
thin film,respec-
thin film,the two
2
2
3
2
feature (B) of the valence band,i.e. the p band,
resonates. This clearly indicates that the Ti 3d
states in the valence band for the TiO –Al O
main structures of the valence band present the
same resonant behaviour. This is due to the fact
that Ti 3d states are distributed throughout the
whole valence band in agreement with the density
of states (DOS) calculated by Munnix and
2
2
3
monolayer are distributed in a narrower energy
region. This is consistent with a reduction of the
covalence of the Ti–O bonding in the monolayer
due to the more covalent character of the Al O
Schmeits [14] for bulk TiO
incide with those reported by Prince et al. [8] for a
TiO single crystal which is a good indication of
2
. Our CIS curves co-
2
3
substrate [3]. In general,and as expected,there is a
2
good agreement between XAS spectra in Fig. 1
and CIS curves in Fig. 3 for the corresponding
monolayer.
the quality of our TiO thin film. The CIS curves
2
for the TiO monolayer on MgO in Fig. 3 also
2

676
M. S ꢀa nchez-Agudo et al. / Surface Science 507–510 (2002) 672–677
In order to separate the Ti 3d contribution to
distributed through the whole valence band with a
higher weight of the high binding energy side (r
band) in agreement with the difference spectrum
(crossed circles).
the valence band,the spectra have been analysed
in terms of on-resonance minus off-resonance dif-
ference spectra. This method has been commonly
used in the literature for transition metal com-
pounds [4]. Fig. 4 shows the on-resonance minus
2
In the case of the TiO monolayer on MgO,the
difference spectrum is also distributed throughout
the whole valence band. This spectrum roughly
reproduces that of the TiO thin film,although the
off-resonance difference spectra of the TiO thin
2
film after annealing at 300 °C as well as those of
2
the monolayers of TiO
In the case of the TiO
the Ti 3d DOS (solid line) calculated in [14]. As
mentioned above,it is seen that the Ti 3d states are
2
grown on both substrates.
thin film we have depicted
structures are broader and the spectral weight is
slightly shifted at lower binding energies with re-
spect to that of the thin film. This is consistent with
the small decrease of the crystal field for this
2
2
monolayer (1.3 eV) with respect to bulk TiO (1.8
eV) observed in the XAS spectra. However,for the
TiO monolayer on Al O the Ti 3d states are
2
2
3
mainly distributed at the lower binding energy
feature of the valence band (6.0 eV). This signifi-
cant change in the distribution of the Ti 3d states is
also consistent with the lowering of the crystal field
(down to 1.0 eV) observed in the Ti 2p XAS
spectrum of this monolayer.
These results confirm that not only the unoc-
cupied electronic states of the TiO monolayer,as
2
observed by XAS,are affected by the presence of
the substrate but also the occupied states in the
valence band as observed by RPES. It is clear that
whereas in the case of more ionic MgO substrate
the results obtained are closer to those obtained in
bulk TiO
strate,i.e. Al
2
,in the case of the more covalent sub-
,the electronic structure of the
2
O
3
TiO monolayer is strongly affected by the sub-
2
strate.
4. Conclusions
We have made a comparative study of the
2 2 3
electronic structure of the TiO –Al O and TiO–
MgO interfaces by means of the Ti 2p XAS and
the valence band resonant photoemission spectra.
From the comparison of the results obtained for
the respective TiO
MgO it is inferred that the more covalent Al
substrate leads to a reduction of the covalence in
the bonding of the TiO overlayer. This covalence
reduction of the bonding is evidenced by a reduc-
2 2 3
monolayers on Al O and
2
O
3
Fig. 4. On-resonance (solid circles) minus off-resonance (open
circles) difference spectra (crossed circles) for a TiO
annealed at 300 °C (top); TiO monolayer grown on MgO
middle); and TiO monolayer grown on Al (bottom). Solid
line shows the occupied Ti 3d states as calculated in [14] for
bulk TiO . Dotted lines are plotted as an eye guide.
2
thin film
2
2
(
2
2 3
O
4þ
tion of the crystal field on the Ti ions and a
different distribution of the Ti 3d states in the
2

M. S ꢀa nchez-Agudo et al. / Surface Science 507–510 (2002) 672–677
677
[
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