Available online at www.sciencedirect.com
Surface Science 602 (2008) 1599–1606
www.elsevier.com/locate/susc
A multi-technique investigation of TiO2 films prepared
by magnetron sputtering
M.B. Casu a,*, W. Braun b, K.R. Bauchspieß a, S. Kera a,1, B. Megner a, C. Heske c,
R. Thull b, E. Umbach a
b
a
University of Würzburg, Experimentelle Physik II, Am Hubland, 97074 Würzburg, Germany
University of Würzburg, Department for Functional Materials in Medicine and Dentistry, Pleicherwall 2, 97070 Würzburg, Germany
c
Department of Chemistry, University of Nevada, Las Vegas, NV89154-4003, USA
Received 19 November 2007; accepted for publication 21 February 2008
Available online 4 March 2008
Abstract
We have investigated seven samples of radio frequency magnetron-sputtered TiO2 thin films by using near-edge X-ray absorption fine
structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). A detailed analysis of the Ti
2p and O 1s NEXAFS edges allows determining the degree of order and the polymorphs of the investigated samples. We have found that
thin films grown by using a Ti target and lower deposition rates have a rutile character, while, by using a TiO2 target and higher deposition rates, amorphous/mixed thin films are obtained. We have also found that this correlation between phase and preparation conditions is perfectly reproducible leading to films with equal characteristics for the same preparation conditions. The NEXAFS results are
supported by complementary XRD results.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Magnetron sputtering; Titanium dioxide; Thin film structures; NEXAFS; XPS; XRD
1. Introduction
Despite the large number of metals and alloys that are
presently known, only few of them fulfil the requirements
making them suitable for a (even preliminary) consideration as implant materials. The relatively corrosive biological environment combined with the poor tolerance of the
body to even minute concentrations of most metallic corrosion products eliminates most metals from discussion. In
fact, corrosion and degradation have to be avoided in
materials for implantation, not only for the obvious need
*
Corresponding author. Present address: University of Tübingen,
Institute of Physical and Theoretical Chemistry, Auf der Morgenstelle 8,
72076 Tübingen, Germany. Tel.: +49 7071 29 76905; fax: +49 7071 29
5490.
E-mail address: benedetta.casu@uni-tuebingen.de (M.B. Casu).
1
Present Address: Faculty of Engineering, Chiba University, Yayoicho, Inage-ku, Chiba 263-8522, Japan.
0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2008.02.030
to be mechanically and chemically stable in time but also
because their occurrence can lead to toxic substances, poisoning the host organism. Among the possible metallic
candidates, tantalum and the noble metals do not have
suitable mechanical properties for the construction of most
orthopaedic tools and implants, while zirconium is in general too expensive. Stainless steels, cobalt chromium alloys,
titanium and titanium-based alloys constitute the material
groups that are mostly used for implants due to their specific characteristics.
In particular, titanium is a widely used material for medical implants [1]. Its use in dentistry is known for decades,
as well as its mechanical properties that resemble bone
properties better than those of other materials with special
regard to their strength per density. Titanium is highly
reactive, and in air, it is immediately passivated. This is
one of the aspects that makes titanium biocompatible since
the passivation prevents corrosion. The high Ti reactivity is
also the reason why titanium used in biological systems is
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M.B. Casu et al. / Surface Science 602 (2008) 1599–1606
passivated by a thin oxide layer, since a cell adsorbed on
the implanted metal is very sensitive to the metallic surface.
Thus, the importance of Ti oxides is straightforward in this
context.
Single crystals of TiO2 have been intensively studied.
Titanium oxide is the most investigated single crystalline
system among all metal oxides [2]. The potential of applications is enormous, ranging from medicine to electronics
[1,2]. TiO2 exists in three different crystallographic phases:
rutile, anatase, and brookite. Their electronic structure as
well as their surface structure is well understood. However,
nowadays, there is the need to go beyond the single crystal
characterization towards applications that require different
types of surfaces. TiO2, in polycrystalline form and as thin
film, has already attracted multiple attention in biology
and medicine, because of the standard methods (e.g. machined or sandblasted Ti) used to manufacture implant devices. In addition, titanium oxide thin films are also useful
for applications like catalysis, optical coatings for selective
filters, dye-sensitised photovoltaic cells, and electrochemical sensors [3–6]. A possible method to grow thin films of
titanium oxide is given by magnetron sputtering [7–14]. Despite the large number of applications, only a few studies
have been published investigating the electronic structure
of titanium oxide films [15,16], and not much has been
done on magnetron-sputtered films [11]. These investigations have been mainly focused on the electronic structure
of magnetron-sputtered thin films deposited on substrates
like Al2O3 and MgO [15,16], while X-ray photoelectron
spectroscopy (XPS) has been used to study the influence
of the plasma parameters on the film composition [11].
In this work, we present near-edge X-ray absorption fine
structure (NEXAFS) spectroscopy and XPS investigations
on TiO2 thin films prepared by r.f. magnetron sputtering,
with the purpose to investigate their structure and the Ti
oxidation states. In addition, X-ray diffraction (XRD)
has been used to cross-check the NEXAFS findings. In a
further study, we have grown thin and thick layers of the
amino acid L-cysteine on these thin oxide films in order
to investigate the bond with the passivated surface. The
reason for this investigation is based on the fact that biological cells can be immobilized by attaching them by an
amino acid sequence, but the bond between an amino acid
and a passivated surface is not completely understood. The
results of the amino acid interaction will be published in a
forthcoming paper.
ing-gas pressure 7 103 mbar, sputtering power 300 W.
Two different targets were used: TiO2 target for sputtering
in Ar+-atmosphere, and a Ti target for reactive sputtering
by adding oxygen. The partial pressure of oxygen and argon as well as the total pressure were controlled by the flow
rate. A ratio of 80 sccm argon to 30 sccm oxygen led to a
concentration of oxygen around 27%. The thickness of
the deposited titanium oxide layers were estimated by using
AFM measurements performed on thin films deposited under the same conditions. In order to determine the thickness, we used a method from the glass coating industry: a
small area of the substrate was marked with a felt-tip pen
before deposition, such that the oxide layer deposited in
this area could be easily removed by acetone because of
the absence of adhesion forces to the substrate. The height
of the resulting very sharp step was determined by AFM
and provided the nominal thicknesses. The detailed preparation conditions and the nominal thicknesses are given in
Table 1 for each of the two sets of samples investigated
here.
XPS and NEXAFS spectra were recorded at the PM-3
bending magnet beamline of the synchrotron light source
BESSY II (Berlin, Germany). The experimental end-station
has been described in detail elsewhere [17,18]. Before measuring, the samples were chemically cleaned in baths of isopropanol, and immediately introduced into the measuring
chamber. No sample charging was observed during our
experiments. The obtained XPS spectra collected in normal
emission (pass energy = 50 eV) were background subtracted (linear background) and normalised. The NEXAFS
signal was measured by collecting the Ti and O Auger electron yields (AEY) with a VG CLAM-II analyser. All
NEXAFS data shown here were collected by using incident
synchrotron radiation, which was linearly polarized in the
plane parallel to the surface. Finally, the spectra were normalised as discussed in [19].
XRD patterns were performed using a Siemens D5005
diffractometer with monochromatized copper Ka X-rays.
Data were collected in glancing incidence mode (2°) in
the 2h range between 20° and 60° with a step size of
0.02° and a normalized count time (The Bragg–Brentano
geometry did not allow to measure a signal because of
the film thickness). The phase composition was assigned
by means of JCPDS reference patterns for titanium (PDF
Ref. 44–1294), rutile (PDF Ref. 21–1276), anatase (PDF
Ref. 21–1272), and brookite (PDF Ref. 29–1360)
2. Experiment
3. Results and discussion
The titanium oxide thin films were deposited on polycrystalline Ti substrates by using r.f. magnetron sputtering
(13.56 MHz). In order to remove the native oxide layer before the sputter deposition, the substrates were cleaned
using Ar plasma etching (PAr = 7 103 mbar, 15 min,
200 W). 12 cm diameter targets with 99.9% purity were
used, the substrate–target distance was 15 cm. The sputtering conditions were: base pressure 3 106 mbar, work-
We investigated two sets of samples prepared in two different time periods in order to test our preparation technique as well as its reproducibility. During preparation
both, targets and deposition rates, were changed in order
to determine a recipe for thin films characterised by a specific polymorph. The recipe was therefore checked for its
reproducibility. The preparation conditions are listed in
Table 1 for each sample. Fig. 1 shows the Ti 2p NEXAFS
M.B. Casu et al. / Surface Science 602 (2008) 1599–1606
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Table 1
Preparation parameters of the samples investigated here
Set
Sample
1, subgroup 1
1, subgroup 2
2
2
2
Target
B1, B2
G1, G2
R1
Y1
BL1
Ti
TiO2
TiO2
Ti
Ti
Composition (sccm)
Ar:O = 80:30
100 Ar
95 Ar
Ar:O = 80:30
Ar:O = 80:30
spectra for the samples B1, B2, G1, and G2 (see Table 1).
They are dominated by several well-resolved prominent
features, labelled A, B, C, and D, that can be correlated
to the main features that characterise the Ti 2p NEXAFS
spectra of TiO2 single crystal surfaces. This immediately
indicates that we can interpret the thin film spectra by following the assignments that have been extensively used for
the spectra from single crystals and their surfaces. The latter are not different from those of the bulk, showing the absence of surface states [2]. In addition, this observation
provides useful information about the oxidation state of
Ti in these films.
In fact, NEXAFS spectra constitute a clear fingerprint
of titanium oxides. They show significant differences
depending on the crystallographic phase as well as on the
Ti reduction [20–26]. In particular, TiO2 single crystal spectra show well-resolved peaks in the range between 455 and
470 eV corresponding to the various transitions Ti 2p ? 3d
[20–24], as shown in Ref. [2]. The Ti 2p edge shows two
groups of peaks arising from the spin-orbit splitting of
Set 1
Ti 2p
NEXAFS (a. u.)
G2
G1
B2
C D
B
A
B1
450
460
470
480
Photon Energy (eV)
Fig. 1. Ti 2p NEXAFS spectra of sample set 1, subgroups 1 and 2, as
indicated. A vertical offset is used for clarity. The arrows evidence pre-edge
features, the capital letters assign main features as discussed in the text.
Time (min)
120
120
180
60
120
P (mbar)
3
7.7 10
7.2 103
7.5 103
7.7 103
7.6 103
Nominal thickness (nm)
50
100
180
25
50
the Ti 2p core level into 2p3/2 and 2p1/2 levels with 6 eV
energy separation. These levels are further split due to crystal-field effects [20,21]. This rich structure is absent in the
case of TiO and Ti2O3 [20]. Their spectra present broader
features, and the splitting due to crystal-field effects is
quenched by the fact that the Ti 3d spectral weight is redistributed by the core hole influence. An analogous behaviour is seen in the spectra of metallic Ti [24]. Comparing
our experimental results with the NEXAFS spectra of
TiO, TiO2, and Ti2O3 [20,23], as well as of metallic Ti
[24], we can confidently exclude that the spectral features
belong to films with Ti3+ or Ti2+ oxidation states or have
strong metallic contributions.
Now we consider the NEXAFS spectrum of sample B1
in Fig. 1 in more detail since it will be used as a model for
the overall results. Feature A is centred at around 458 eV,
in agreement with the results obtained for the TiO2 single
crystal, 458.2 eV [2,22]. The details of the spectrum also
resemble the single crystal case: the feature A is sharp,
while C is broader, probably because of the enhanced Coster–Kronig decay channel [22]. The energy difference between peaks A and C, which is due to spin-orbit splitting,
is 5.4 eV, the energy separation of peaks C and D is
2.3 eV. These values agree with those obtained at the Ti
2p edge of two TiO2 crystallographic phases: the spin-orbit
splitting is 5.4 eV for both rutile and anatase, and the difference between peaks C and D is 2.45 and 1.97 eV, for rutile and anatase, respectively [22]. Comparing spectra of
subgroup 1 (B1, B2) with those from subgroup 2 (G1,
G2), it is clear that peak B has a different shape for the
two cases. Again, it is useful to refer to the single crystal
phases. In particular, the second peak, B, appears different
in rutile and anatase. It has more intensity on the high than
on the low photon energy side in the rutile case, and shows
the opposite behaviour in anatase, i.e. higher intensity on
the low than on the high photon energy side. Although
the peak energy positions are almost the same for rutile
and anatase, the shapes of these peaks depend on the ligand coordination and are different for the two crystallographic phases [24]. The rutile characteristics are clearly
found in the subgroup 1 spectra, while the subgroup 2 spectra cannot be interpreted in a unique way.
Fig. 2 shows the Ti 2p NEXAFS spectra for set 2. They
have the same characteristics as discussed for the samples
of set 1. Also in this case we can derive that their features
belong to TiO2 thin films, i.e. the Ti4+ oxidation state is the
most likely to occur under the preparation conditions used
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M.B. Casu et al. / Surface Science 602 (2008) 1599–1606
XPS Ti 2p
Set 2
hν=550 eV
Intensity (a. u.)
Ti 2p
NEXAFS (a. u.)
Set 2
R1
BL1
BL1
Y1
468
464
460
456
Binding Energy (eV)
Fig. 3. Ti 2p XPS spectra for sample set 2. The two peaks are assigned to
Ti 2p3/2 at lower and Ti 2p1/2 at higher binding energy. Vertical offsets are
used for clarity.
Y1
Set 1
O1s
R1
450
460
470
480
G2
Fig. 2. Ti 2p NEXAFS spectra for sample set 2. As Fig. 1.
here. Note that in the R1 spectrum peak B differs in shape
from peaks B of the other two samples. Peak B in the R1
spectrum resembles the subgroup 2 feature, while peak B
in the Y1 and BL1 spectra is similar to peak B in the subgroup 1 spectra. Consequently, we conclude that the samples Y1 and BL1 also show rutile-like characteristics at
the Ti 2p edge. Finally, it is interesting to note that in the
spectra of set 1 as well as in those of set 2, typical TiO2
pre-edge features are visible, as indicated by arrows in Figs.
1 and 2. They may be assigned to a spin-orbit forbidden
transition (p3/2 ? d3/2) as in the single crystal case [22].
In order to cross-check the information obtained from
the NEXAFS spectra regarding the Ti oxidation states,
we performed XPS investigations on both sets of samples.
XPS is a powerful tool that gives information about the
chemical states of the investigated materials. As an example, the Ti 2p XPS spectra of set 2 are displayed in
Fig. 3. They show two photoemission lines at 459.4 and
465.1 eV, respectively. The energy positions are typical
for the Ti4+2p3/2 and Ti4+2p1/2 binding energies of TiO2
[11]. This demonstrates once more that the Ti4+ oxidation
states predominate. The results are also in agreement with
XPS investigations performed on samples with varying
oxygen concentration [11], which show that already for
an oxygen concentration of at least 16%, i.e. corresponding
to the range of conditions used in our work (27% oxygen
concentration, see Section 2), the oxidation state with the
highest expectation value is Ti4+.
Figs. 4 and 5 show the O 1s NEXAFS spectra for set 1 and
set 2, respectively. We focus again on the B1 spectrum, using
NEXAFS (a. u.)
Photon Energy (eV)
G1
A'
B'
B2
D' E'
F'
C'
B1
520
530
540
550
560
Photon Energy (eV)
Fig. 4. O 1s NEXAFS spectra for sample set 1, subgroups 1 and 2, as
indicated. The capital letters assign main features as discussed in the text.
it as a model, in order to interpret the spectra and to check if
they support the observation made at the Ti 2p edge. The B1
O 1s NEXAFS spectrum is characterised by two main features at 530.7 (peak A0 ) and 533.4 eV (peak B0 ), respectively.
Also in this case, their shape and their relative intensity allow
excluding, by direct comparison with NEXAFS spectra of
TiO and Ti2O3, that they contain significant contributions
from Ti3+ or Ti2+ oxidation states [20]. This holds true for
all samples, in agreement with the results obtained from
the analysis of the Ti 2p NEXAFS and XPS results. Several
features are visible in the higher photon energy range in the
B1 spectrum, at 539.6 (peak C0 ), 542.7 (peak D0 ), 545.3 (peak
E0 ), and 552.6 eV (peak F0 ). However, this large number of
clearly distinguishable peaks is a characteristic only of subgroup 1 (Fig. 4 B1 and B2).
M.B. Casu et al. / Surface Science 602 (2008) 1599–1606
Set 2
O1s
NEXAFS (a. u.)
BL1
Y1
R1
520
530
540
550
560
Photon Energy (eV)
Fig. 5. O 1s NEXAFS spectra for sample set 2.
Looking at the O 1s NEXAFS spectra of subgroup 2
(Fig. 4: G1 and G2), it is possible to distinguish several details that make them different from those of subgroup 1.
The two low energy features, A0 and B0 , although still representative of a Ti4+ oxidation state, are less resolved in the
G1 and G2 spectra: the ‘‘dip” between the two peaks is less
sharp than in B1 and B2 spectra. In addition, the features
in the high photon energy range are less structured. In particular, in the G2 spectrum the two structures C0 and F0 are
hardly discernible, in contrast to the G1 spectrum. An
analogous observation is made for set 2 (Fig. 5). The samples Y1 and BL1 show spectra with several structures, similar to subgroup 1, while, in contrast, the R1 spectrum is
characterised by only two structures in the high photon energy range. Referring again to the single crystal, it is established that also at the NEXAFS O 1s edge different
crystalline structures show different behaviours, especially
in the high photon energy range above 537 eV [20,22,24],
which is very sensitive to long range order. In particular,
the O 1s edge in rutile shows two NEXAFS peaks at
530.9 and 533.5 eV, respectively, which do not differ much
in the energy positions from the analogous peaks in the
anatase case, located at 531.1 and 533.7 eV. The two structures are due to transitions from O 1s orbitals to mixed O
2p and Ti 3d states with covalent nature. Their distance
gives a good estimation of the ligand-field splitting. The relative distance is identical, 2.6 eV, in both crystalline forms
[22,25]. Moreover, several features around 539.9, 542.5 and
544.9 eV are present in rutile spectra, while, in contrast,
anatase shows only two NEXAFS peaks at 539.1 and
544.6 eV [20,22]. The comparison thus leads to the clear
1603
conclusion that, also at the NEXAFS O 1s edge, B1, B2,
Y1, and BL1 show rutile-like features, while the other thin
films resemble at first glance an anatase phase but a detailed comparison indicates that they may consist of a mixture of various crystalline and perhaps amorphous phases
(see below).
As known from previous work on titanium oxides, the
NEXAFS and XPS spectra constitute a valuable background to understand the nature of the investigated films.
At this point, we have all the information necessary to address the question regarding the structural properties of the
investigated thin films and how these are related to their
preparation conditions.
First, according to the above observations, we can divide the samples into two main groups depending on the
characteristics of their NEXAFS spectra: B1, B2, Y1, and
BL1 show the same peculiarities at both Ti 2p and O 1s
edges. The remaining samples, G1, G2, and R1 have common characteristic features in their spectra, different from
those of the first group.
Second, we conclude that B1, B2, Y1, and BL1 can be
assigned to the rutile structure. The splitting and line shape
of the NEXAFS peak B at the Ti 2p edge together with the
richness of structures at the O 1s edge, are strong indicators
that these films have a rutile-like structure. Taking into account that the NEXAFS signal is averaged over the area
sampled by the incident beam, the rutile crystallographic
phase is clearly predominant in these films. If we compare
our results with NEXAFS spectra performed on titania sol,
previously characterised using X-ray diffraction, to clearly
quantify the mixture of anatase and rutile [21], we also find
clear evidence that B1, B2, Y1, and BL1 have a dominating
rutile nature, and that the anatase phases on average contribute very little, if anything at all. In particular, the two
components of peak B at the Ti 2p edge for phase mixtures
show equal intensity in agreement with the presence of
both forms [21], while structure B in B1, B2, Y1, and
BL1 has a clear rutile characteristic.
The situation is more complicated for G1, G2, and R1:
the O 1s edge seems to show characteristics in the higher
photon energy similar to the anatase phase, but the low
photon energy range is characterised by peaks A0 and B0
which are not well separated by a sharp dip, as it would
be expected if this was the case. In addition, peak B at
the Ti 2p edge does show a different splitting, with different
intensities of the components, and peaks A and C at the
same edge are much weaker. Therefore, we cannot unambiguously conclude that these thin films represent an anatase phase, but we can exclude that they are a simple
mixture of both polymorphs (because of the NEXAFS Ti
2p edge). Thus, we suggest that these films have a high degree of disorder.
We now concentrate on the Ti 2p edge and especially on
the shape of the peak B from a qualitative point of view.
We observe that for samples G1 and G2 the relative intensities of the two components of peak B resemble an anatase-like behaviour. Rutile and anatase have a B splitting
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M.B. Casu et al. / Surface Science 602 (2008) 1599–1606
Fit
Gauss1
Gauss2
B1 O1s NEXAFS
NEXAFS (a. u.)
NEXAFS (a. u.)
Fit
Gauss1
Gauss2
G2 O1s NEXAFS
528
530
532
534
536
528
530
532
534
536
Photon Energy (eV)
Photon Energy (eV)
Fig. 6. Fit analysis of O 1s NEXAFS spectra (open circles) for sample B1 (a) and G2 (b). Fit curves (black curves) together with the two Gaussian
components, Gauss1 and Gauss2 (grey curves), are shown. The fit shows marked differences between both samples in the region of the high-energy peak.
Note that two Gaussians are insufficient for a good representation of the experimental results.
consistent with the Ti 2p edge spectra, which also resemble
those of amorphous TiO2 grown on Al2O3 and MgO [16].
Summarising, we conclude from the analysis of the Ti 2p
and O 1s NEXAFS spectra that there is a distinction between the group of sample B1, B2, Y1, and BL1, having
a rutile-like structure, and the remaining samples, having
an amorphous/mixed nature, possibly with some anatase,
and perhaps brookite contributions.. The assigned structures are also given in Table 2.
To test the validity of our phase assignments, we performed XRD measurements on the set 2 samples. The
XRD patterns are shown in Fig. 7 for samples R1, Y1,
and BL1. The Y1 and BL1 patterns show similar characteristics, like in NEXAFS. These spectra are dominated by the
Ti peaks from the substrate, due to the fact that the two
TiO2 films are thin, but they show the preferential (1 0 1)
crystalline orientation of rutile, with the peak centred at
36.09°, clearly visible in both XRD patterns. The pattern
Y1
BL1
Intensity (a. u.)
of about 1 eV [22,25]. Due to the broader shape of peak B
in the investigated film the splitting is certainly larger than
in the single crystal, therefore the investigated films are
characterised by weak-field ligands in comparison with single crystal polymorphs. The increase of the splitting indicates a shorter distance between the ligands and the
central titanium atom, raising the hypothesis of a relevant
distortion of the octahedron of oxygen atoms surrounding
the titanium atoms. In particular, this distortion seems to
be stronger for G1 and G2 than for B1, supporting the idea
that the former (and by direct comparison sample R1) are
disordered systems.
We also performed a simple fit of the B1 (rutile-like) and
G2 O 1s NEXAFS spectra, focusing on the two main features in the low photon energy range. They are reproduced
by two Gaussian curves (Fig. 6a and b). Note that two
Gaussians are insufficient for a good representation of
the experimental results. However, they help in our qualitative approach. In the B1 case, the two Gaussian peaks have
1.4 and 2.9 eV width. In the G2 case, the two Gaussian
widths are 1.4 and 3.8 eV. As mentioned, this difference is
related with the ligand field. The broader peak B0 in the
spectrum of sample G1 further confirms the different environment of the titanium atoms in this case. This fact together with the structureless bump in the higher photon
energy range, are analogous to the characteristics seen in
the TiO2 NEXAFS spectra due to the changes induced
by Ar+ bombardment [20,24] or to disorder as observed
in the typical NEXAFS spectra of disordered TiO2 thin
films grown on Al2O3 [15]. These O NEXAFS spectra are
R1
Ti
R
A
Table 2
Structure attribution according to the NEXAFS analysis
Sample
Phase
B1, B2
G1, G2
R1
Y1
BL1
Rutile-like
Amorphous/mixed
Amorphous/mixed
Rutile-like
Rutile-like
B
20
25
30
35
40
45
50
55
60
2θ (deg)
Fig. 7. XRD patterns recorded for R1, Y1, and BL1, as indicated.
Titanium (Ti), rutile (R), anatase (A), and brookite (B) peaks are included
as bars for comparison.
M.B. Casu et al. / Surface Science 602 (2008) 1599–1606
obtained from R1 represents a mixed structure: several
peaks are assigned both to rutile (36.09° and 39.19°), and
anatase (25.29°, 36.95°, 37.80°, and 53.89°), some are
attributed to Ti (35.09° and 40.17°), and brookite (25.34°,
42.34°, and 52.01). In this case, the peaks due to the substrate contribution have intensities that are reduced with
respect to Y1 and BL1, because this TiO2 film is rather
thick. Finally, some further peaks are present which cannot
be assigned to any of the mentioned polymorphs or to Ti.
This is in agreement with the NEXAFS analysis that indicates an amorphous/mixed nature of the R1 film. The
agreement is an interesting finding because NEXAFS is
surface sensitive while XRD is a bulk sensitive technique,
indicating that the structural properties of the investigated
films are depth-independent.
In the following final section, we address the influence of
the different preparation conditions. Looking at Tables 1
and 2, it is immediately clear that the two main groups,
we have distinguished via the spectra analysis, are characterised by two different preparation conditions. B1, B2,
Y1, and BL1 have been prepared by using a Ti target, in
oxygen atmosphere, while the remaining samples (G1,
G2, and R1) have been prepared by using a TiO2 target.
Furthermore, the deposition rates are rather different:
two times higher for G1, G2, and R1 than for B1, B2,
Y1, and BL1. Therefore, there is a clear relationship between the preparation conditions and the obtained polymorphs of the investigated thin films. A plausible
explanation arises by considering the growth mechanisms.
The high deposition rates may lead to a kinetics-dominated
growth and may not allow an energy minimisation necessary for obtaining a well-ordered structure. On the contrary, a lower deposition rate, coupled with the use of a
Ti target in oxygen, gives a higher probability for the
occurrence of conditions close to the thermodynamic equilibrium, thus resulting in the growth of films with a rutilelike structure. Preliminary XRD experiments performed on
samples for which the preparation conditions have been
optimised according to the present findings seem to endorse this explanation.
tributions due to hydroxyl groups in the O 1s XPS spectra
does not influence the NEXAFS results, i.e. the thin film
electronic structure remains unaltered.
This is the first step made to understand and to optimise
the preparation protocol of TiO2 thin films deposited on Ti
substrates by using r.f. magnetron sputtering. Further
work needs to be done. To answer some of the still open
questions, regarding for example nucleation and growth
modes, it is necessary (1) to investigate the influence of
annealing, especially on amorphous/mixed films, (2) to discriminate between the influence of the target and the deposition rate, and (3) to investigate the film morphology
depending on the preparation conditions by using techniques like atomic force microscopy. This will give a wider
set of information that will allow a complete control of the
growth procedure.
Finally, this work clearly shows the extension of possibilities given by the use of a powerful tool like NEXAFS,
not only to investigate single crystals, but also TiO2 thin
films. The strong correlation between the electronic structure and the structural phase allows to correctly assign, directly from the NEXAFS spectra, the polymorphs even in
the case of very thin films for which diffraction techniques
may not provide unambiguous information and for the surface region, which is important for the interface properties
especially in the case of implants.
Acknowledgements
The authors like to thank the BESSY staff, in particular
Dr. T. Kachel, for technical support while measuring at the
PM3 beamline. Deutsche Forschungsgemeinschaft through
Schwerpunktprogramm SPP 1100, project Um 6/9-1 is
gratefully acknowledged. One of us (E.U.) likes to thank
the Fonds der Chemischen Industrie for support.
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In summary, we have investigated two sets of magnetron-sputtered TiO2 thin films by using NEXAFS, XPS,
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