Effects of annealing temperature on the structural and photoluminescence properties of nanocrystalline ZrO2 thin films prepared by solgel route

Article abstract of DOI:10.1016/j.jpcs.2011.02.012

Highly transparent nanocrystalline zirconia thin films were prepared by the solgel dip coating technique. XRD pattern of ZrO₂ thin film annealed at 400 °C shows the formation of tetragonal phase with a particle size of 13.6 nm. FT-IR spectra reveal the formation of ZrOZr and the reduction of OH and other functional groups as the temperature increases. The transmittance spectra give an average transmittance greater than 80% in the film of thickness 262 nm. Photoluminescence (PL) spectra give intense band at 391 nm and a broad band centered at 300 nm. The increase of PL intensity with elevation of annealing temperature is related to reduction of OH groups, increase in the crystallinity and reduction in the non-radiative related defects. The luminescence dependence on defects in the film makes it suitable for luminescent oxygen-sensor development. The Red shift of excitation peak is related to an increase in the oxygen content of films with annealing temperature. The Blue shift of PL spectra originates from the change of stress of the film due to lattice distortion. The defect states in the nanocrystalline zirconia thin films play an important role in the energy transfer process.

Full text of DOI:10.1016/j.jpcs.2011.02.012

Journal of Physics and Chemistry of Solids 72 (2011) 673–677
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Effects of annealing temperature on the structural and photoluminescence
properties of nanocrystalline ZrO thin films prepared by sol–gel route
2
n
K. Joy , I. John Berlin, Prabitha B. Nair, J.S. Lakshmi, Georgi P. Daniel, P.V. Thomas
Thin film Lab, Post Graduate and Research Department of Physics, Mar Ivanios College, Thiruvananthapuram 695015, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly transparent nanocrystalline zirconia thin films were prepared by the sol–gel dip coating
technique. XRD pattern of ZrO2 thin film annealed at 400 1C shows the formation of tetragonal phase
with a particle size of 13.6 nm. FT-IR spectra reveal the formation of Zr–O–Zr and the reduction of OH
and other functional groups as the temperature increases. The transmittance spectra give an average
transmittance greater than 80% in the film of thickness 262 nm. Photoluminescence (PL) spectra give
intense band at 391 nm and a broad band centered at 300 nm. The increase of PL intensity with
elevation of annealing temperature is related to reduction of OH groups, increase in the crystallinity
and reduction in the non-radiative related defects. The luminescence dependence on defects in the film
makes it suitable for luminescent oxygen-sensor development. The ‘‘Red shift’’ of excitation peak is
related to an increase in the oxygen content of films with annealing temperature. The ‘‘Blue shift’’ of PL
spectra originates from the change of stress of the film due to lattice distortion. The defect states in the
nanocrystalline zirconia thin films play an important role in the energy transfer process.
Received 16 April 2010
Received in revised form
1
7 January 2011
Accepted 16 February 2011
Available online 24 February 2011
Keywords:
A. Thin films
B. Sol–gel
C. X-ray diffraction
D. Luminescence
D. Defects
&
2011 Elsevier Ltd. All rights reserved.
1
. Introduction
properties. Advanced materials for luminescent oxygen-sensors
require modified surface with efficient oxygen exchange property
and transparency high enough for luminescent light output.
Photoluminescence studies provide information about a mate-
rial on electronic band transitions, structure, defects and chemical
composition. The exciton luminescence and intrinsic defects
related luminescence are known in a number of oxides. For
2
Thin films of ZrO have attracted special interest owing to their
advantageous properties of being highly homogeneous, transpar-
ent and their applications in optical storage elements, scintillators
and luminescent oxygen-sensors [1]. Of the many techniques
used to prepare thin films, sol–gel fabrication has gained much
interest because of its simplicity, low processing temperature,
stoichiometry control and its ability to produce uniform, chemi-
cally homogenous films over large areas that can provide integra-
tion with other circuit elements. The sol–gel method can be
successfully used for the preparation of pure oxide films applied
in electronics and optics [2]. The synthesis of advanced ceramic
materials as nanocrystals is one of the major fields in materials
processing technology. Advantages of nanocrystalline films are
superior phase homogeneity, sinterability and microstructure
leading to unique mechanical, electrical, dielectric, magnetic,
optical and catalytic properties [3,4].
2
ZrO , the photoluminescence is of much interest for both theore-
tical and experimental investigations [5–7]. Some researchers
observed a broad PL emission at 480 nm for bulk ZrO
385 nm for nanosized ZrO [10] and 390 nm for ZrO
films [11], induced by UV irradiation.
However, not many studies have been done on the origin and
the variation of PL properties of nanocrystalline ZrO thin films
2
[8,9],
2
2
thin
2
with annealing temperature, especially on the UV-blue PL proper-
ties. If tough films are used, the abrupt interfaces that will appear
in the multilayered devices will produce localized dielectric
breakdown at weak spots, especially when the layers are very
thin. When the films are not transparent, there is absorption and
scattering of the emitted light [12]. Because of this, the devices do
not have good resolution of PL peaks, contrast and efficiency. The
sol–gel zirconia contains a large amount of OH groups. The
existence of OH groups also results in shorter decay times and
quenching of luminescence. Several reports are available on the
optical properties of zirconia thin films [13–17], but in all these
reports the transparency of the films is low, which restricts their
use as good photoluminescent materials in optical applications.
Thermal annealing is a widely used method to improve crystal
quality and to study structural defects in materials. Due to
annealing, the structure and the stoichiometric ratio of the
material will change. Such phenomena can have major effects
on the optical properties, particularly the photoluminescence
n
Corresponding author. Tel.:+91 471 2531053; fax: +91 471 2530023.
E-mail address: jolly2jolly@gmail.com (K. Joy).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2011.02.012

6
74
K. Joy et al. / Journal of Physics and Chemistry of Solids 72 (2011) 673–677
In this paper, we report the preparation of nanocrystalline
transparent zirconia thin films by inorganic precursor route using
zirconium oxychloride octahydrate (ZrOCl O). The sol was
kept at an elevated temperature during deposition. An indigen-
ously fabricated dip coating apparatus was used for coating. We
investigated the structural and optical properties of the tetragonal
thin films with special attention on its PL properties.
T(111)
2
ꢀ 8H
2
T(220)
T(311)
2
. Experimental details
T(111)
500ºC
In the present study, inorganic precursor route was chosen for
T(220) T(311)
4
00ºC
the fabrication of nanocrystalline transparent zirconia thin films.
Zirconium oxychloride octahydrate (ZrOCl O) (Zigma-
ꢀ 8H
2
300ºC
2
Aldrich 99.5%) was used for the preparation of the precursor
solution [18]. A homogeneous solution (2 wt%) of zirconium
oxychloride octahydrate in 2-butanol and ethanol (in the ratio
10
20
30
40
50
60
70
2
theta (deg)
1
:1) was prepared, by mixing 1 mole of zirconium oxychloride
octahydrate in 1/3 of the total volume of mixed 2-butanol and
ethanol. The solution was stirred for 45 min using magnetic
stirrer. The water for hydrolysis and nitric acid for oxidation were
then added to the salt–alcohol solution. Subsequently, the rest of
Fig. 1. XRD pattern of nanocrystalline ZrO
glass annealed at 300, 400 and 500 1C.
2
thin films (12 coatings) on corning
2
-butanol and ethanol mixed with the required amount of
200ºC
acetylacetone, were added under vigorous stirring in a controlled
manner. The stirring was continued for another 90 min to get a
clear transparent sol–solution.
300ºC
The precursor solution thus prepared, was heated to 40 1C and
deposited onto clean and chemically etched corning glass sub-
strates, using the dip coating apparatus. The coating parameters
were optimized as 10 cm/min lifting speed and 901 vertical lifting,
depending on the viscosity and concentration of the precursor
solution. The dip coated films were dried at room temperature
and pre-fired at 150 1C for 1 h. This process of coating and drying
was repeated 12 times, as 12 coatings were required to obtain
useful data from X-ray diffraction characterization. The decom-
position temperature of zirconium organometallic compounds is
approximately 450 1C [19]. Four sets of temperatures 200, 300,
4
00ºC
00ºC
5
0
500 1000 1500 2000 2500 3000 3500 4000 4500
Wavenumber (cm-1
)
4
00 and 500 1C were selected for annealing the films. The samples
Fig. 2. FT-IR spectra of the gel heat- treated at different temperatures.
were heated at the rate 4 1C/min and held at these temperatures
for 1 h each, during the course of annealing. The films were then
cooled to room temperature at the same rate. The structural and
optical characteristics of these films were studied.
Crystalline phase of the Zirconia thin films was characterized
by X-ray diffraction (XRD) using the X-ray diffractometer (Mod-
el—PW 1710 PHILIPS). The FT-IR spectra of the gel powder have
(
1 1 1) plane. The intensity of the (1 1 1) peak was enhanced in
the film annealed at 500 1C. The grain size of the nanocrystalline
film was calculated from the FWHM of the diffraction peak (1 1 1)
using the Scherrer equation [20]
K
l
ꢁ
1
D ¼
ð1Þ
been recorded in the 4000–400 cm
range with the Perkin-
b
cos
y
Elmer 1600 Fourier Transform instrument using the KBr pellet
technique. Optical transmittance was studied using a spectro-
photometer (Model—JASCO-V550). PL spectra were recorded
using a Perkin-Elmer Fluorescence Spectrometer (Model-LS55)
with a 40 W Xenon Lamp as the excitation source and 2.5 nm
excitation and emission slit width.
where D is the grain size, K¼0.9 is a correction factor,
b
is the full
width at half maximum (FWHM) of the most intense diffraction
plane, is the wavelength of X-ray (CuK radiation
¼1.5405 A˚ )
and is the Bragg angle.
l
a
l
y
The grain size of the thin film increased from 13.6 nm at 400 1C
to 15.5 nm at 500 1C. The (1 1 1) peak position also shifted from
3
0.311 to 30.491. The calculated lattice constant(c) for the films
annealed at 400 and 500 1C are equal to 0.510 and 0.509 nm,
respectively. The changes in 2 angles of (1 1 1) peak and lattice
3
. Results and discussion
y
constants indicate a change in the stress [21] in zirconia thin film,
with the increase in temperature.
3.1. X-ray diffraction studies
2
The X-ray diffraction spectra of sol–gel derived ZrO thin films
(
12 layer coatings) on corning glass substrates annealed at
different temperatures 300, 400 and 500 1C are shown in Fig. 1.
The spectra revealed that ZrO coatings remained in the amor-
phous state up to 300 1C. In our study it was observed that ZrO
3.2. FT-IR spectroscopy
2
Fig. 2 shows the FT-IR spectra of the gel, heat treated at
different temperatures. The FT-IR spectrum, of the gel heat
treated at 200 1C, showed an absorption band at 465 cm . This
band was shifted to 484 cm
peaks 465 and 484 cm
2
ꢁ
1
thin films annealed at the relative low temperature of 400 1C
attained crystallinity, having tetragonal (T) phase of zirconia
ꢁ
1
on heating the gel at 500 1C. The
are attributed to Zr–O stretching in
ꢁ
1
(2
y
¼30.311, 50.551 and 60.591) with preferred orientations in T

K. Joy et al. / Journal of Physics and Chemistry of Solids 72 (2011) 673–677
675
tetragonal ZrO
2
[22,23]. This shift in the absorption band at
purity. It was also seen that the interference fringes shifted towards
the lower wavelength (blue shift) region with the increase in
annealing temperature, which is compatible with a lower optical
thickness, increase in oxidation and change in packing density
(Table 1) [28]. Broadening of peak fringes was also caused by the
decrease in thickness with temperature. As temperature was
increased, the refractive index of the films increased, getting closer
ꢁ
1
4
65 cm
to a higher frequency, with increase in annealing
temperature, is attributed to an increase of the Zr–O bond
strength. This can also be correlated to the crystallization with
increase in temperature as is evident in the XRD spectra. The
bands observed at about 1450–1578 cm
Zr–O–C species. The bands at 3200–3600 cm are characteristic
of OH groups. Absorption due to C–H vibrations was also
ꢁ
1
are characteristic of
ꢁ
1
2
to the value (2.2) reported for pure ZrO [29]. This behavior is
ꢁ
1
observed at around 2335 and 1381 cm
and that of C–C vibra-
distribution peak is shown at
029 cm . With thermal treatment, the absorption due to
compatible with the phase transformation (Fig. 1), which accounts
for the decrease in transparency due to the presence of tetragonal
ꢁ
1
tions at 1635 cm . The –CH
1
2
ꢁ
1
2
ZrO in the film [30]. The decrease of transmittance and the increase
Zr–O–C and OH groups decreased and eventually disappeared
while that of Zr–O–Zr increased with the temperature [24].
of refractive index can be ascribed to the decrease in porosity. Film
thickness decreased with temperature yielding the desired zirco-
nium oxide, deposited on the substrate. Optimum processing
temperature, which resulted in high quality zirconia films, is
3.3. Optical studies
500 1C in terms of minimum film thickness, minimum transparency,
minimum porosity and the desired phase formation (Table 1).
Fig. 3 shows the spectral transmittance of nanocrystalline ZrO
2
thin films (12 coatings) on corning glass, annealed at different
temperatures. The optical constants of zirconia thin films are
highly influenced by the structure[25], composition and method
of fabrication. Roughly, the spectrum can be divided into two
regions: a transparent oscillating region and a zone of strong
absorption, where the transmittance decreases drastically. If
thickness is not uniform or is slightly tapered, all the interference
effects will be smoothed out in the transmission curve. The
presence of these oscillations in the Fig. 3 is indicative of the
homogeneity and uniform thickness of the films [26,27].
3
.3.1. Photoluminescence
Fig. 4 shows the excitation and PL spectra, obtained for
2
nanocrystalline transparent ZrO thin films on corning glass with
different numbers of coatings, annealed at 500 1C. The PL spectra
of the films exhibit an intense peak at 391 nm and a broad band at
3
00 nm. Monitoring of the 391 nm emission band revealed that
excitation at 243 nm produced a large intensity. It is well
established that defects and vacancies in semiconductors and
insulators can lead to the formation of localized states near the
conduction band edge. The perturbations due to defects and
impurities are characterized by discrete energy levels that lie
within the band gap. When the concentrations of such (structural
or composition) defects become sufficiently large, they may either
segregate to form cavities or distribute in the atom network.
Intrinsic (delocalized) states may then be affected by the resulting
distortion of the lattice. The principal intrinsic defect in nano-
crystalline zirconia is anion vacancies. UV irradiation of this
dielectric insulator gives rise to broad absorption in the 240–
The film thickness, porosity and refractive index of nanocrystal-
2
line transparent ZrO thin films, at different annealing temperatures,
have been determined (Table 1) from the transmittance spectra
following Swanepoel’s envelope method [17,26]. An average trans-
mittance greater than 80% was observed for all the annealed
samples in the visible region. The high average value of transmit-
tance in the visible region is indicative of homogeneity and chemical
1
00
9
00 nm spectral region. This has been assigned to the formation
+
of electrons (F and F centers) [31]. Two different mechanisms
have been proposed to explain ZrO luminescence [32,33], an
2
impurity luminescence center model and a structure defect
+
model—electron trapping by the anion vacancies (F singly
ionized oxygen vacancies). The sol–gel process leads to the
existence of large amounts of surface defects (oxygen-vacancy)
8
6
0
0
2
in ZrO nano thin films due to their large surface area [34].
a
Currently, the structure defect model mechanism is generally
accepted.
b
c
d
Ex
PL
3
00 400
500 600
700
800 900
Wavelength (nm)
12 coatings
Fig. 3. Transmission spectrum of nanocrystalline ZrO
2
thin films on corning glass
(12 coatings) annealed at (a) 200 1C (b) 300 1C (c) 400 1C and (d) 500 1C.
Table 1
Thickness, refractive index, porosity and average transmittance (
l
¼550 nm) of the
3
coatings
nanocrystalline ZrO
2
thin films (12 coatings) annealed at different temperatures.
Annealing
Thickness
(nm)
Refractive
Porosity
Transmit-
Temperature T
a
(1C)
index n
tance T (%)
1 coating
450 500
200
250
300
350
400
2
3
4
5
00
00
00
00
514
505
338
262
2.01
2.02
2.15
2.18
0.208
0.197
0.056
0.022
88
83
83
82
Wavelength (nm)
Fig. 4. Excitation (left) and PL (right) spectra of nanocrystalline ZrO
corning glass annealed at 500 1C.
2
thin films on

6
76
K. Joy et al. / Journal of Physics and Chemistry of Solids 72 (2011) 673–677
The broad emission band at 300 nm can be ascribed to the
phase. It was seen that PL intensity increased as the number of
coatings increased. Further a nine-fold increase in the intensity of
the PL was affected by the increase of thickness of the film from
ꢂ35 to 262 nm (1 coating to 12 coatings). This increase in the
emission intensity is due to the increase in surface density of the
photoexcited electrons and holes, which governs the behavior of
the charge carriers in the intrinsic defect states.
radiative decay of self- trapped excitons, which is created by
optical excitation across the gap inducing an electron–hole pair.
Analogous nature has been proposed for a broad emission
observed from single crystals of zirconia [35]. The 391 nm
zirconia emission peak can be due to the capture of electrons by
oxygen vacancies from the conduction band [31]. In zirconia thin
film prepared by sol–gel method, the groups coordinated on the
surfaces of the films such as Zr(OH) could form electronic states
different from those of bulk component. They may form dangling
Zr bond during the condensation and dehydration process.
Because of these surface states, dangling bonds and other factors
2
Fig. 5a and b shows the PL spectra of nanocrystalline ZrO thin
films (12 coatings) on corning glass annealed at different tempera-
tures. It was seen that with the increase of temperature, the
luminescent peak intensity increases. The variation in the PL
intensity with temperature is effected by the crystallinity of the
films [22] and the quenching mechanism by OH groups [38]. This
clearly indicates the reduction of OH groups and increase in Zr–O–
Zr, by increasing the treatment temperature, as is evident in the
FT-IR spectrum (Fig. 2). In addition at low temperature the oxygen
sub-lattice may contain many defects, both inside the film and at
the surface [39]. Such defects would be expected to produce various
non-radiative centers and hence reduce light emission. After anneal-
ing at high temperature, these non-radiative related defects could be
reduced and re-structured with a consequent increase in PL.
(
2
such as the influence of O in the atmosphere), there are many
4
+
4+
Zr defect sites. The existence of such defect Zr sites (more
3
+
likely at irregular near-face sites) may photogenerate Zr centers
by electron capture. The excitation band corresponds to the
spectral range of the extrinsic absorption of ZrO
existing defects [36]. The energy gap of tetragonal ZrO
greater than 5 eV [37]. The excitation band at 243 nm (5.1 eV)
2
caused by pre-
2
phase is
corresponds to energy near the energy gap of ZrO
2
tetragonal
It can also be seen that the position of excitation peak exhibits
a red shift and the emission peak exhibits a blue shift from 397 to
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
5
00°C
391 nm, with increase in annealing temperature. A red shift in the
excitation peak of zirconia thin films, is observed when increasing
the annealing temperature [40,41]. This latter shift might be due
to an increase in the oxygen content of the films. Hence, the
increased oxygen content implies a lower number of such oxygen
vacancies. Less number of oxygen vacancies implies a decrease in
the band gap energy [31]. The blue shift of the emission peak for
the samples treated at lower temperature, is much larger. The
blue shift from 397 to 391 nm is believed to originate from the
change of stress in the thin film due to lattice distortions [39].
4
00°C
3
00°C
00°C
2
4. Conclusion
Homogeneous nanocrystalline ZrO
2
thin films have been
fabricated from ZrOCl O on corning glass substrates by
2
ꢀ 8H
2
220
240
260
280
300
320
340
sol–gel dip coating method by keeping the sol at an elevated
temperature. The films were crystallized in a pure tetragonal
structure at a low annealing temperature 400 1C. An average
transmittance greater than 80% was obtained for the film of
thickness 262 nm, annealed at 500 1C. The increase of PL intensity
with increase in annealing temperature is related to their crystal-
linity, reduction of OH groups and decrease in the non-radiative
related defects. The red shift in the excitation peak is due to the
reduction in oxygen vacancies. The blue shift in the PL peak is
because of the change of stress in the thin film. Thus the modified
sol–gel method allows one to fabricate highly transparent nano-
crystalline zirconia thin films with a refractive index 2.18 and
intense PL emission in the range 350–440 nm.
Wavelength (nm)
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
500°C
00°C
4
300°C
Acknowledgments
2
00°C
The authors are grateful for the financial assistance of KSCSTE,
Major Research Project (2009–2012).
260 280 300 320 340 360 380 400 420 440 460
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