Abstract Scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) were used to study the electronic structure of the reduced TiO 2 (1 1 0) surface. At the occupied part of the spectra some states at energies of about 1.1 and 0.6 eV below the Fermi level were found. At the unoccupied part of the spectra, the presence of a surface state at an energy of about 0.6 eV above the Fermi level was observed. Their presence has been ascribed to the appearance of Ti 2 O 3 regions on the TiO 2 (1 1 0) surface. High temperature spectroscopy measurements indicated smooth insulator-metal transition (I-M) caused by bands overlap in Ti 2 O 3 , which takes place at elevated temperatures. 

S Datta

S Pierzgalski

Z Klusek

CiteSeerX

Insulator–metal transition on heavily reduced TiO2(1 1 0)
surface studied by high temperature-scanning
tunnelling spectroscopy (HT-STS)
Z. Kluseka,b,*, S. Pierzgalskib, S. Dattaa
aAdvanced Materials Research Institute (AMRI), University of Northumbria, Ellison Building,
Ellison Place, Newcastle upon Tyne NE1 8ST, UK
bDepartment of Solid State Physics, University of Lodz, Pomorska 149/153,
90-236 Lodz, Poland
Received 21 February 2003; accepted 26 June 2003
Abstract
Scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) were used to study the electronic
structure of the reduced TiO2(1 1 0) surface. At the occupied part of the spectra some states at energies of about 1.1 and 0.6 eV
below the Fermi level were found. At the unoccupied part of the spectra, the presence of a surface state at an energy of about
0.6 eV above the Fermi level was observed. Their presence has been ascribed to the appearance of Ti2O3 regions on the
TiO2(1 1 0) surface. High temperature spectroscopy measurements indicated smooth insulator–metal transition (I–M) caused by
bands overlap in Ti2O3, which takes place at elevated temperatures.
# 2003 Published by Elsevier B.V.
PACS: 61.16Ch; 71.30.þh; 73.20At
Keywords: Scanning tunnelling microscopy; Scanning tunnelling spectroscopy; Insulator–metal transition; Transition-metal oxides; TiO2
1. Introduction
During last years, increasing experimental and the-
oretical efforts have been made to understand the
electronic conduction in oxides ([1] and references
therein]. In contrast to metals, the electric properties
of oxides show many characteristic features. One of
these is the insulator–metal transition (I–M) marked
by a change from the insulating behaviour to the
metallic behaviour at certain temperatures [1,2].
The mechanism of the I–M transition is complex
and is still not well understood. The complexity of
this phenomenon is a result of the many electron–
phonon interactions in oxides. As a result, theories for
many body problems in strongly coupled electron–
phonon systems must be considered to explain this
phenomenon [1,2].
Until now the I–M transition, which takes place at
temperatures higher than room temperature, has been
mainly studied using electron spectroscopy (UPS,
XPS) and resistivity measurement of a sample as a
Applied Surface Science 221 (2004) 120–128
* Corresponding author. Tel.: þ48-42-635-5704;
fax: þ48-42-679-0030.
E-mail address: zbklusek@mvii.uni.lodz.pl (Z. Klusek).
0169-4332/$ – see front matter # 2003 Published by Elsevier B.V.
doi:10.1016/S0169-4332(03)00877-8
function of temperature [1]. However, these techni-
ques do not allow extracting local information on the
spatial distribution of electron density of states on the
investigated surface, i.e. local density of states of the
surface (LDOS (E, x, y)). Thus, a technique that
provides local spectroscopic information at the atomic
level is required. The high temperature-scanning tun-
nelling spectroscopy (HT-STS) technique enables us
to obtain spectroscopic information from the tunnel-
ling current characteristics by showing spectral fea-
tures in agreement with other surface sensitive
techniques [3]. The advantage of these methods is
that they offer the possibility of examining both
occupied and unoccupied electronic states with atomic
selectivity and high-energy resolution. Recently, HT-
STS has been successfully used to in situ studies of the
effect of temperature on the electronic structure of
NixMn3O3xO3d thin films [4].
In this paper, we focus on the measurements of
electron density of states as a function of temperature
on heavily reduced TiO2(1 1 0) surface. In stoichio-
metric TiO2 crystal, the metal atom is in 3d
0 (Ti4þ)
electronic configuration; however, the surface can be
reduced by ion sputtering or heat treatment to create
Ti2O3 regions on TiO2(1 1 0) surface in which metal
atom is in 3d1 (Ti3þ) configuration [5,6]. At about
Tc ¼ 450 K, Ti2O3 exhibits a smooth insulator–metal
transition with increasing temperature [1,2,7–10].
Below Tc, Ti2O3 indicates the presence of a small
energy gap Eg ¼ 0:1 eV between the occupied a1g
band and the unoccupied epg þ epg band [10]. As
was established the change in the conductivity of
Ti2O3 above Tc is accompanied by an increase in
the c-axis lattice parameter and a decrease in the a-
axis lattice parameter with no change in lattice sym-
metry leading to a broad crossover between a1g and
epg þ epg bands [1,2,7–10].
The identification of the I–M transition on heavily
reduced TiO2(1 1 0) density of states on HT-STS data
is of great importance not only from the fundamental
point of view but also in the studies of early stages of
high temperature oxidation of alloys, especially TiAl.
Although, the scanning tunnelling microscopy (STM)
results for reduced TiO2(1 1 0) surface have been
presented previously ([11] and references therein),
no experimental results for high temperature tunnel-
ling spectroscopy have been published yet. The goal of
this paper is to bridge this existing gap.
2. Experimental
The STM/STS/current imaging tunnelling spectro-
scopy (CITS) experiments were performed at elevated
temperatures with a commercial VT-STM/AFM sys-
tem in UHV condition (Omicron, GmbH, Germany)
equipped with LEED/Auger spectrometer (SPECTA-
LEED) and a sputtering gun (ISE 5). The TiO2(1 1 0)
(Pi-Kem, UK) surface was prepared by repeated cycles
of Arþ ion sputtering (typically 30 min, 1 keV, current
4 mA) and annealing (typically 1050 K, 1 h).
During the STM/STS measurements performed, the
base pressure was 2  1010 mbar. The tips used were
prepared by mechanical cutting from the 90% Pt–10%
Ir alloy wires (Goodfellow). In current imaging tun-
nelling spectroscopy mode, the I/V curves were
recorded simultaneously with a constant current image
by the interrupted-feed-back-loop technique.
The experimental parameters chosen for the CITS
measurements were þ2.5 V for the sample bias and
1 nA for the tunnelling current set point and the energy
range of the spectroscopy was 2 eV. Based on these
measurements, the first derivative of the tunnelling
current with respect to voltage (dI/dV) was calculated.
The dI/dV spectra were normalised using the method
proposed by Feenstra et al. [12] where the differential
conductance is divided by the total conductance—
(dI/dV)/(I/V). The (dI/dV)/(I/V) quantity is a measure
of local density of states of the surface [12]. The
divergence problem in the case of (dI/dV)/(I/V) was
overcome by applying some amount of broadening
(DV) to the I/V values [13]. In all our cases, DV ¼ 1 V
value was chosen.
3. Scanning tunnelling microscopy results
The development of terraces, which occurred after
many cycles of heating and sputtering at various tem-
peratures of the TiO2(1 1 0) surface is demonstrated in
Fig. 1. Atomic layer steps of 0.32 nm as expected from
geometric structure separate the terraces.
Further annealing produced the (1  1) superstruc-
ture on the TiO2(1 1 0) surface (blue colour of the
sample). The (1  1) superstructure contains two types
of oxygen atoms, i.e. atoms at the bridging and in-plane
positions, and two types of titanium atoms which are
five- and six-fold co-ordinated. Oxygen atoms bridging
Z. Klusek et al. / Applied Surface Science 221 (2004) 120–128 121
and covering the six-fold co-ordinated Ti atoms form
ridges along the [0 0 1] axis. The five-fold co-ordinated
Ti atoms are exposed to the surface. Thus, the (1  1)
superstructure consists of alternative rows of the ex-
posed Ti atoms and the bridging O ridges. The O ridges
and Ti rows are aligned with a 0.65 nm separation,
respectively (see Fig. 2).
The STM image showing (1  1) superstructure of
TiO2(1 1 0) surface exhibiting bright atomic rows
parallel to the [0 0 1] direction and separated by about
0.7 nm is presented in Fig. 3. The image was recorded
at a positive sample bias of 1 V (1 eV above the Fermi
level), i.e. the lower conductance band of the TiO2 was
probed. From band electronic structure of the TiO2
(see Fig. 4), it is clear that the lower conductance band
consists of Ti dxy orbitals contributing to the metal–
metal interactions due to the s bonding of the Ti t2g–Ti
t2g states. These states are localised on the five-fold co-
ordinated Ti atoms exposed to the surface. As a result,
Ti atoms were imaged by STM as bright rows, thought
the Ti atoms are located 0.11 nm below the bridging
oxygen atoms. It confirms the well-known fact that
STM does not show an atomic structure of the surface
Fig. 1. The 30 nm2 STM image of lightly reduced TiO2(1 1 0) surface
(blue colour of the sample). The height of the terraces is 0.32 nm.
Fig. 2. Atomistic model of the (1  1) superstructure on TiO2(1 1 0) surface.
122 Z. Klusek et al. / Applied Surface Science 221 (2004) 120–128
Fig. 3. The 15 nm2 STM image of TiO2(1 1 0) surface showing (1  1) reconstruction.
Fig. 4. (a) The basic structural unit of TiO2 (octahedron—MO6); (b) energy level splitting of the Ti 3d orbitals in the octahedral positions;
(c) bands energy diagram for rutile TiO2; and (d) simple form of bands energy diagram for rutile TiO2.
Z. Klusek et al. / Applied Surface Science 221 (2004) 120–128 123
in crystallographic sense, but rather an atomic struc-
ture of the surface, which is strongly affected by the
local electronic structure.
Further sputtering and annealing of the sample
make the surface more reduced (black colour of the
sample). It results in loss of oxygen and rearrange-
ments of Ti3þ ions leading to formation Ti2O3-like
Ti3þ pairs [5,6]. When the density of pairs increases
areas of Ti2O3 start to form on the TiO2(1 1 0) surface.
Typical topography of heavily reduced surface is
presented in Fig. 5. Occasionally, on very small
regions of the sample the (1  1) superstructure can
be observed as presented on the inset in the Fig. 5. In
this case (1  1), superstructure is accompanied by the
presence of small bright areas, which can be attributed
to early stages of Ti2O3 formation on TiO2(1 1 0)-
(1  1) surface. On the regions of the surface, which
are completely reduced, no images with high resolu-
tions could be collected. Even though atomic resolution
Fig. 5. The 600 nm2 STM image of heavily reduced TiO2(1 1 0) surface (black colour of the sample). The inset shows atomic resolution on
the heavily reduced surface.
Fig. 6. The (dI/dV)/(I/V) curves recorded at two different spatial
positions on the lightly reduced TiO2(1 1 0) surface.
124 Z. Klusek et al. / Applied Surface Science 221 (2004) 120–128
was not achieved steps and terraces are well visible
(Fig. 5). It helped us to record tunnelling spectra far
from the step edges, which influence local electronic
structure.
4. Scanning tunnelling spectroscopy results
The (dI/dV)/(I/V) spectra recorded at room tem-
perature on lightly reduced TiO2(1 1 0) surface (see
Fig. 1 for topography) are presented in Fig. 6. The
figure shows clearly two surface states located below
the Fermi level, i.e. d and D surface states (herewith
we use Heise and Courths [6] notation). The observed
states are the extrinsic surface states caused by the
presence of defects considered in terms of oxygen
vacancies and formation of Ti3þ ions on the reduced
TiO2(1 1 0) surface. As was shown in [5], the origin of
the d state can be attributed to the presence of high
density of Ti3þ ions which start to interact with each
other and give rise to the formation of the Ti3þ pairs.
This process is interpreted in terms of the surface
phase transition. The D state starts to appear when the
density of Ti3þ pairs increases, and small areas of
Ti2O3 on the surface are formed [5,6]. The surface
Fig. 7. Bands energy diagrams for (a) TiO2; (b) Ti2O3; and (c) Ti2O3 above I–M phase transition.
Fig. 8. The (dI/dV)/(I/V) curves recorded at two different spatial
positions on the heavily reduced TiO2(1 1 0) surface.
Fig. 9. The I/V curve and its normalized form (dI/dV)/(I/V) recorded
at low energy range.
Z. Klusek et al. / Applied Surface Science 221 (2004) 120–128 125
phase transition from TiO2 towards Ti2O3 is accom-
panied by a change in the electronic structure as
presented in Fig. 7a and b.
The tunnelling spectra recorded on heavily reduced
TiO2(1 1 0) surface (see Fig. 5 for topography) are
presented in Fig. 8. When comparing these spectra
with the spectra recorded on lightly reduced surface
(Fig. 6), we conclude that energetic positions of the d
and D states agree. It is also clearly seen that the
amplitudes of the d and D states on heavily reduced
surface are much higher then the amplitudes measured
on lightly reduced surface—it is caused by larger
number of defects in this sample. Furthermore, on
heavily reduced sample, the D state was observed
more frequently in comparison with d state.
In our measurements, the D state has the same
energy (0.6 eV below the Fermi level) as the energy
of the valence band edge (a1g in pure Ti2O3 [14]. Thus,
the D state and a1g band edge state are the same states,
i.e. D  a1g (Fig. 7b). Since, Ti2O3 is treated as a
insulator with small energy gap between the valence
and the conductance band edges then the D  a1g state
should be accompanied by the presence of a symme-
trically located state at the unoccupied part of the
spectra which can be ascribed to the presence of the
edge of the conductance band, i.e. epg þ epg state. This
Fig. 10. The I/V curve and its normalized form (dI/dV)/(I/V) recorded at (a) 293 K; (b) 393 K; (c) 423 K; and (d) 473 K on the TiO2(1 1 0)
surface.
126 Z. Klusek et al. / Applied Surface Science 221 (2004) 120–128
can especially be seen from Fig. 9 in which we present
the I/V curve and its normalized form recorded in the
lower energy range where the energy gap, the valence
band edge (a1g), and the conductance band edge
(epg þ epg ) are well pronounced (see also Fig. 7b for
bands description). Especially, it is possible to esti-
mate the width of the band gap, which is 0.1 eV. This
value agrees very well with the theoretical calcula-
tions based upon the Mott–Hubbard regime giving
Eg ¼ 0:098 eV for 300 K [10]. This result clearly
shows that we are dealing with Ti2O3 regions on
the reduced TiO2(1 1 0) surface [15]. It should be
mentioned here that the tip-induced band bending
effect can be neglected in our experiments. This effect
is large only in the case of small number of charge
carriers, which is not the case with the heavily reduced
TiO2 crystal.
Finally, in Fig. 10, we present the I/V curves and
their normalized form (dI/dV)/(I/V) recorded on heav-
ily reduced TiO2(1 1 0) surface at four temperatures:
293; 393; 423 and 473 K. It is clearly seen the dis-
appearance of the energy gap accompanied by sub-
stantial decreasing of amplitude of the band edge
states (a1g, e
p
g þ epg ) with increasing temperature. It
indicates smooth insulator–metal transition, which
takes place on the surface. As was observed the
disappearance of the energetic gap was completely
reversible and it appeared when the temperature was
lowered. As was shown in [8,10], the I–M transition in
Ti2O3 can be explained by the competition between
electron–electron correlation energy and electron-
band entropy at elevated temperatures. As a result,
the a1g and e
p
g þ epg bands overlap producing smooth
transition with no change in crystal symmetry. Sketch
of the Ti2O3 bands model above the I–M transition is
shown in Fig. 7c. This clearly explains the disap-
pearance of the energetic gap on high temperature-
tunnelling spectroscopy measurements presented in
Fig. 10.
5. Conclusions
We have studied the electronic structure of the
heavily reduced TiO2(1 1 0) surface by scanning tun-
nelling spectroscopy. At the occupied part of the
spectra, we found the presence of a surface state at
energy of about 1.1 eV below the Fermi level (d surface
state). The presence of the d state is ascribed to
the appearance of Ti3þ pairs. When the density of
Ti3þ pairs increases, and small areas of Ti2O3 on the
surface are formed a new surface state starts to appear
(D surface state). The energy of the D state is about
0.6 eV below the Fermi level and is related to the
presence of the valence band edge in Ti2O3. At the
unoccupied part of the spectra, we found the presence
of a surface state at energy of about 0.6 eV above the
Fermi level. This state is ascribed to the conductance
band edge in Ti2O3. The result obtained clearly shows
that we are dealing with Ti2O3 regions on the reduced
TiO2(1 1 0) surface.
High temperature spectroscopy measurements
recorded on the TiO2(1 1 0) surface which contains
Ti2O3 regions showed disappearance of the energy gap
accompanied by substantial decreasing of amplitude
of the band edge states with increasing temperature. It
indicates smooth insulator–metal transition caused by
bands overlap in Ti2O3, which takes place at elevated
temperatures.
Acknowledgements
This work was carried out at the Advanced Material
Research Institute, University of Northumbria, sup-
ported by grants from the European Regional Depart-
ment Founding and the University Innovation Centre
for Nanotechnology.
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Insulator-metal transition on heavily reduced TiO 2 (1 1 0) surface studied by high temperature-scanning tunnelling spectroscopy (HT-STS)

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Insulator-metal transition on heavily reduced TiO 2 (1 1 0) surface studied by high temperature-scanning tunnelling spectroscopy (HT-STS)

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