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PHOTOLUMINESCENCE IN AMORPHOUS TiO2-PbO SYSTEM
E. R. Leite , N.L.V.Carreño, L.P.S. Santos, J. H. Rangel, and E. Longo
Centro Multidiciplinar de Desenvolvimento de Materiais Cerâmicos (CMDMC)Departamento de Química, Universidade Federal de São Carlos,
C. Postal 676, 13560-905 São Carlos, SP, Brazil
C.E.M.Campos, F.Lanciotti Jr. and P.S.Pizani
Departamento de Fisíca, Universidade Federal de São Carlos,
C. Postal 676, 13560-905 São Carlos, SP, Brazil
J. A. Varela
Centro Multidiciplinar de Desenvolvimento de Materiais Cerâmicos (CMDMC)-Instituto
de Química-UNESP
C. Postal 355, 14801-970 Araraquara, SP, Brazil
ABSTRACT
Photoluminescence (PL) at room temperature has been achieved in amorphous thin films
and powders of the TiO2-PbO system. They were prepared by the polymeric precursor
method with [PbO]/[TiO2] molar ratios ranging from 0.0 to 1.0. The energy position of
maximum PL emission and the PL intensity showed dependence on the Pb concentration.
The Pb addition suggests an increase of the number of non-bridging oxygens (NBO) in the
amorphous
TiO2
network.
These
results
support
the
relationship
between
photoluminscence and structure in TiO2 based amorphous materials.
KEY-WORDS:
Photoluminescence, sol gel, thin films, lead titanate
INTRODUCTION
The preparation of thin films by the deposition of a chemical solution appears
particularly as a promising process for the opto-eletronic device fabrication.1 Chemical
solution deposition presents a wide variety of applications including ferroelectric thin
films, high-density optical data storage and semiconductors.2,3
Besides, the chemical
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solution process allowed to obtain amorphous compounds that usually are not possible to
be prepared by the melting process.
Pizani et al.4 and Leite et al.5 have recently reported intense room-temperature
photoluminescence (PL) of amorphous PbTiO3 (a-PT) powders and thin films deposited on
Si (100). In those studies, a simple chemical method was used to process powders and thin
films at low temperatures (T < 400 oC).
The PL observed in a-PT showed a strict
relationship with the disorders in the perovskite structure. The study of the amorphous PT
structure by X-ray absorption near edge structure (XANES) showed that these compounds
are formed basically by six-fold oxygen Ti co-ordination (TiO6-octahedra) and five-fold
oxygen Ti co-ordination (TiO5-square-base pyramids).5 In the a-PT, approximately 80 % of
the Ti atoms are co-ordinated by six oxygen atoms, while the rest of the Ti atoms are coordinated by five oxygen atoms. It is noticeable that the TiO5-square-base pyramid
structure is not found in well-crystallized titanates. In that study it was postuled that the
amorphous materials are formed by a TiO2 network, and the Pb ions modify the TiO2
network, resulting in non-bridging oxygens (NBO).
In the PbTiO3 amorphous material, the charge of the Pb cation dopant must be
compensated by the negative charge of the oxygen (NBO). In addition, other defects may
be present in the amorphous structure, such as oxygen vacancies. These positively-charged
defects possibly stem from different states of valence of the Ti ion (Ti
4+
and Ti
3+ 5
). This
report presents strong room-temperature PL of pure and Pb doped amorphous TiO2
compounds, prepared by the polymeric precursor method.6-8 The PL peak position and
intensity are also displayed as a function of the Pb incorporation into the pure TiO2
network.
EXPERIMETAL PROCEDURES
Amorphous TiO2-PbO particles and thin films were processed by a c, using the
polymeric precursor method, which is based on the chelation of cations (metals) by citric
acid, in a water solution and ethylene glycol. The precursor solution containing metallic
cations, which form the TiO2-PbO system, was prepared with the PbO molar ratio varying
from 0.0 to 1.0. The polymeric precursors formed during the polymerization step can be
used to prepare thin films or powders, by means of their calcination at the 200-300 oC, in
an O2 atmosphere. These temperatures are sufficiently high to promote the polymer
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pyrolysis, but not crystallization. The resins were deposited on the glass substrate by the
spin-coating technique. The x-ray diffraction (Siemens, D5000) analyses showed that, after
calcination at low temperatures, an amorphous phase is formed. The results of atomic force
microscopy (AFM) Nano Scope IIIa (Digital Instrument), showed roughness values in the
range from 0.2 to 4.0 nm, for all thin films. Further details about the polymer precursor
synthesis of these compounds can be found in Ref. 6 and 7.
Photoluminescence spectra were performed by using a U1000 Jobin-Yvon double
monochomator coupled to a cooled GaAs photomultiplier and conventional photon
counting system. The 514.5 nm line of an argon ion laser were used as excitation light. All
measurements were taken at room temperature.
RESULTS AND DISCUSSION
Amorphous TiO2-PbO materials, whose [PbO]/[TiO2] molar ratio ranges from 0.0
to 1.0, were excited with argon laser at 514.5 nm. Their PL spectra are presented in Figure
1. Figure 1.a refers to amorphous powders annealed at 300 oC for 80 hours and Figure 1.b
is related to thin films heat treated at 200 oC for 8 hours. The values of PL intensity were
normalized for the film thickness (Fig. 1.b). An weak red photoluminescence is observed
in the visible region with a broad band for pure TiO2. The visible emission of the pure TiO2
can be associated with the intrinsic NBO or other defects such as oxygen vacancies. A
blue shift was detected in the PL spectra of Pb doped TiO2 samples. The PL intensity of
both powders and thin films shows a similar dependence on Pb concentration.
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PL Intensity
3
(10 cts/s)
(cts/s.nm)
12
10
8
6
4
2
0
c)
d)
1.a
b)
a)
c)
6
d)
4
1.b
b)
2
a)
0
560
640
720
800
Wavelength (nm)
Figure 1: Illustrative photoluminescence spectra of
amorphous TiO2-PbO materials
excited with the 514,5 nm argon laser line. Figure 1.a refers to powders annealed at 300 oC
during 80 hours. Figure 1.b deals with thin films annealed at 200 oC for 8 hours. Spectra
a), b), c) and d) are relative to 0.00, 0.25, 0.75 and 1.00 [PbO]/[TiO2] molar ratios,
respectively.
A maximum in the PL intensity was observed at 0.75 [PbO]/[TiO2] molar ratio for
powders and at the ratio value of 0.8 for thin films, as can be seen in Figure 2. This
behavior was shown for the PL spectra of powders with 514.5 nm exciting wavelengths.
The mechanism that leads to higher PL intensities in the Pb richer samples is not yet clearly
known, but it can been associated with higher defect concentrations promoted by the Pb
incorporation in the structure of the amorphous system.5,9 The PL intensity decrease above
the 0.75-0.8 [PbO]/[TiO2] molar ratio range may be attributed to an increased material
order (start of the crystallization process).5
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10
3
PL intensity (10 cts/s)
15
5
0
0.0 0.2 0.4 0.6 0.8 1.0
[PbO]/[TiO2] molar ratio
Figure 2. PL intensity of amorphous TiO2-PbO powders, full circles refer to the excitation
with the 514.5 nm argon laser line. Normalized PL intensity of TiO2-PbO thin films
annealed at 200 oC (full squares, excited with the in 514.5 nm argon laser line). In all the
cases PL is a function of PbO concentration, with the [PbO]/[TiO2] molar ratio varying
from 0,0 to 1.0.
Figure 3 presents the spectral dependence of the optical transmitance of the TiO2PbO amorphous thin films annealed at 200 oC during 8 hours. These measurements were
taken at room temperature using a Cary 5G equipment. It is noticeable the absorption edge,
which is characteristic of the amorphous semicondutors.10
The profiles of the four
amorphous thin films are not coincident. These differences are considered to be related to
different electronic states promoted by the Pb addition.
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Transmitance (a.u.)
30 de maio a 2 de junho de 2001 - Florianópolis – SC.
e
d
a
c
b
300
400
500
Wavelength(nm)
Figure 3. Spectral transmitance of the TiO2-PbO thin films a), b), c) and d) refer to
amorphous films annealed at 200 oC for 8 hours, presenting the [PbO]/[TiO2] molar
ratio of 0.00; 0.5; 0.75; 0.80 respectively and e) is PbTiO3 crystalline film, annealed
at 450 oC for 4 hours.
It is believed that the Pb incorporation into the TiO2 amorphous network results in an
increase of NBO. This interpretation corroborates the PL results for Pb doped TiO2
samples, since the PL efficiency increased with the Pb addition. The identified roomtemperature PL in the amorphous TiO2-PbO materials, which are prepared by low-cost
chemical methods and can be processed as thin film, is an important characteristic for
optic-electronic applications.
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CONCLUSIONS
The PL results at room temperature for pure and Pb doped TiO2 powders and thin
films, correlate directly with the Pb addition. This suggests an increase of non-bridging
oxygens (NBO) in the amorphous structure and an increase in the degree of disorder in the
TiO2-PbO system. Room temperature PL has been observed in the amorphous thin films of
the TiO2-PbO system deposited on glass subtrates.
ACKNOWLEDGMENTS
The authors acknowledge the following Brazilian funding support agencies:
FAPESP, CNPq/Pronex, and CAPES.
REFERENCES
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