Defect-effects on the photoluminescence of ZrO2 bulk, film and nanocrystals

Article abstract of DOI:10.1016/j.physb.2011.08.096

The photoluminescence (PL) properties of ZrO₂ have been measured at different temperatures between 7 and 300 K, using various kinds of ZrO ₂ specimens: bulk crystal melt-grown by a large solar furnace, thermally oxidized zirconium plate (ZrO₂ film crystal on Zr) and nanocrystals (surface area: 35-45m²/g, diameter: 20-30 nm). The results clarify the deep and shallow energy level structures in the energy gap. Reversible UV-laser-light-induced spectral changes are observed for all of the specimens in different specimen-atmospheres (vacuum and O₂ gas). The results elucidate the defect-effects of the PL properties and the PL enhancement mechanism in ZrO₂ nanocrystals.

Full text of DOI:10.1016/j.physb.2011.08.096

Physica B 407 (2012) 2911–2914
Contents lists available at SciVerse ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
Defect-effects on the photoluminescence of ZrO bulk, film and nanocrystals
2
n
Shosuke Mochizuki , Takashi Saito
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
a r t i c l e i n f o
a b s t r a c t
Available online 3 September 2011
The photoluminescence (PL) properties of ZrO2 have been measured at different temperatures between
7
and 300 K, using various kinds of ZrO
2
specimens: bulk crystal melt-grown by a large solar furnace,
film crystal on Zr ) and nanocrystals (surface area: 35–45
Keywords:
thermally oxidized zirconium plate (ZrO
2
Photoluminescence
Metal oxide
2
m /g, diameter: 20–30 nm). The results clarify the deep and shallow energy level structures in the
energy gap. Reversible UV-laser-light-induced spectral changes are observed for all of the specimens in
different specimen-atmospheres (vacuum and O2 gas). The results elucidate the defect-effects of the PL
Nanocrystals
Oxygen defect
properties and the PL enhancement mechanism in ZrO
2
nanocrystals.
2011 Elsevier B.V. All rights reserved.
&
1
. Introduction
melt-grown by a large solar furnace, thermally oxidized zirco-
nium plate (ZrO film crystal on Zr) and nanocrystals (surface
area: 35–45 m /g, diameter: 20–30 nm) of ZrO Both the
2
2
Zirconia (ZrO
2
) is a well known metal oxide with useful
2
.
mechanical, thermal, optical and electrical properties [1]. It
crystallizes at room temperature in the monoclinic crystal struc-
obtained time-resolved spectra and the excitation photon-energy
dependence clarify the defect effects, especially, deep and shallow
energy level structures in the forbidden gap.
2
ture. It has been reported that ZrO film displays a Stokes-shifted
intrinsic PL band and several extrinsic PL bands at about 4.3 and
below 3.5 eV [2]. Also, it has been well known that intense, long-
lasting and white luminescence is excited by photons with lower
2. Experimental
energies than the energy gap (E
g
¼5 eV), which is nothing other
ZrO
with a solar furnace and by cooling gradually. X-ray diffraction
XRD) analysis of the grown crystal blocks proves that they are
polycrystalline monoclinic ZrO . We named the bulk ZrO crystal
‘solar furnace’’ for short, in Figs. 1–3. Commercial high-purity
Aldrich 99.99%) ZrO powder was used as the purest ZrO
specimen. XRD analysis of the powder proves that the powder
is monoclinic ZrO . We named the purest ZrO powder ‘‘high
purity powder’’ for short, in Figs. 1–3. Commercial ZrO nano-
powder (Aldrich, 20–30 nm in size) were used as ZrO nanocrystal
specimen. XRD analysis of the nanopowder proves that the
powder is monoclinic ZrO . We named the ZrO nanopowder
‘nanopowder’’ for short, in Figs. 1–3. Zirconium plate was
2
bulk crystal was grown in air by melting a sintered disk
than in-gap excitation. Besides, since the PL intensity increases
considerably with decreasing size, the nanosized particles are
particularly interesting for the development of novel opto-elec-
tronic and sensor devices [3]. However, the detailed PL mechan-
ism (optical excitation and its relaxation processes) for the in-gap
(
2
2
‘
(
2
2
photoexcitation of ZrO
main reasons is that it is hard to grow ZrO
an indispensable reference specimen for studying the nanocrystal
nature. ZrO has a high value of the melting point (42973 K) and
therefore high-quality bulk crystal is hard to be grown without
any flux. Fortunately, using a large solar furnace, we grew ZrO
crystal in air, without any crucible and any flux. The second is the
difficulty in separating hafnium from zirconium. Usual ZrO
2
is not fully elucidated. The first of the
2
bulk crystal, which is
2
2
2
2
2
2
2
2
‘
2
oxidized at 1173 K in O
2
gas stream for 7 h. The XRD analysis of
films.
We named the oxidized Zr plate ‘‘oxidized surface’’ in short. Both
the ZrO powder, which is the starting material powder
20–30 nm in size) for the ZrO bulk crystal, and ZrO nanopowder
Aldrich, 25 nm in size) were used as ZrO nanocrystal specimen.
The PL and photo-induced PL spectral change experiments
were carried out with a continuous wave (CW) He–Cd laser line
wavelength
contains several percents of Hf atoms as impurity, which may
create some shallow levels and other levels in the energy gap. In
the oxidized Zr plates shows that they are monoclinic ZrO
2
2
009, we reported the outline of the PL properties [4]. To obtain
2
more correct conclusion, we have been carrying out more detailed
experiments until today. In the experimental studies, a compara-
tive study of PL properties of ZrO
(
(
2
2
2
2
has been carried out at
different temperatures between 7 and 300 K, using bulk crystal
(
l¼325 nm, photon energy hv¼3.81 eV). The same
n
He–Cd laser line excites luminescence. The emitted light is
dispersed and detected using a grating spectrograph equipped
Corresponding author. Tel./fax: þ81 3 5317 9771.
E-mail address: motizuki@physics.chs.nihon-u.ac.jp (S. Mochizuki).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2011.08.096

2
912
S. Mochizuki, T. Saito / Physica B 407 (2012) 2911–2914
6
0x103
6x103
7
K
36 K
nanopowder
0ns - 20ms
nanopowder
0ms - 20ms (x 4)
100µs - 20ms (x 4)
5
1
2
97 K
x 1
1
4
2
0
0
0
2
56 K
4
20ns - 20ms
10ns - 20ms
76 K
3
2
1
0
2
95 K
30ns - 20ms
3.0
3
.2
3.0
2.8
2.6
37 K
2.4
2.2
7 K
2.0
1.8
3
3
3
.2
.2
.2
2.8
2.6
2.6
2.6
2.4
2.2
2.0
1.8
6
0x103
1
solar furnace
x 2.4
150
0ns - 20ms
solar furnace
1
97 K
7 K
1
20
90
60
4
2
0
0
0
9
100µs - 20ms
2
38 K
1
ms - 21ms
10ms - 30ms
10ns - 20ms
2
78 K
3
0
0
2
98 K
3
3
3
.2
.2
.2
3.0
7
2.8
2.8
2.8
2.6
2.4
2.4
2.4
2.2
2.0
1.8
3.0
2.8
2.4
2.2
2.0
1.8
2
0x103
high purity powder
x 18
80
high purity powder
10ms - 20ms (x 20)
K
9
6 K
157 K
97 K
1
0ns - 20ns
1
1
5
0
5
0
6
4
2
0
0
0
0
1
2
0ns - 40ns
3.0
298 K
3.0
2.6
157 K
2.2
2.0
1.8
2.8
2.4
2.2
2.0
1.8
5x103
10
8
2
oxidized surface
x 31
oxidized surface
1ms - 2ms
2
1
1
0
5
0
5
0
2
2
78 K
37 K
7 K
6
0
ns - 10ns
4
2
98 K
2
3.0
2.6
2.2
2.0
1.8
0
3
.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
Photon energy (eV)
Photon energy (eV)
Fig. 1. PL spectra of various ZrO
7 KrTr298 K).
2
specimens at different temperatures
(
Fig. 2. Time-resolved PL spectra of various ZrO
2
specimens at 7 K.
with an image-intensified multichannel photodetection system.
The time-resolved PL measurements were made using the same
photodetection system to which two delay pulse generators were
in this figure, nanopowder exhibits the most intense lumines-
cence and the high purity powder specimen does relatively weak
luminescence. Except for nanopowder specimen, the spectral
shapes and temperature dependences of three kinds of specimens
indicate that their PL spectra consist of at least two kinds of
luminescence centers, which emit luminescence around about
2.4 and about 2.5 eV. The high purity powder displays lumines-
cence at photon energies higher than about 2.8 eV, while the
contribution of the broad luminescence component around about
2.5 eV (which was prominently observed for the nanopowder,
solar-furnace-grown crystal and oxidized Zr surface) is relatively
small. It suggests that the PL band around 2.5 eV is extrinsic one.
3
þ
attached and a Nd –YAG laser (
l¼355 nm, pulse width¼
4
–5 ns, repetition¼10 Hz). In the measurements of excitation
3
þ
photon energy dependence, another Nd –YAG laser (
l¼266 nm,
pulse width¼15 ns, repetition¼10 Hz) was used. Vacuum of
ꢀ
4
about 10 Pa and oxygen gas of about 0.1 MPa were used for
the atmosphere around the specimen. Evacuation was carried out
with an oil-free vacuum system consisting of a turbo-molecular
pump and a mechanical diaphragm pump.
2
With increasing temperature, the PL intensity of ZrO nanopow-
3
. Results and discussion
der increases slightly and then it reached a maximum of 36 K.
Such temperature dependence is frequently observed for a broad
band, which arises from several luminescence centers adjacent in
energy. With further increasing temperature, the PL intensity is
exponentially decreased up to about 298 K without changing
2
We measured the PL spectra of various kinds of ZrO speci-
mens at different temperatures between 7 K and about 298 K. The
results are shown in Fig. 1. The luminescence was excited by a CW
3
25 nm laser light. Since the energy gap of ZrO
2
is about 5 eV, the
the intensity peak energy. The activation energy (
DE) for the PL
ꢀ
1
3
25 nm photoexcitation corresponds to the in-gap excitation. The
intensity (I) decrease were calculated as 69.6 meV (¼561 cm ),
ꢀ
1
ꢀ1
spectra are appropriately rescaled in this figure. Each rescaling
factor is given in each figure, for example, x 1, x 2.4, etc. As shown
81.3 meV (¼656 cm ), 6.8 meV (¼54.8 cm ) and 68.4 meV
ꢀ1
(¼551 cm ) for the nanopowder, solar-furnace-grown crystal,

S. Mochizuki, T. Saito / Physica B 407 (2012) 2911–2914
2913
6x103
Fig. 3 shows the excitation wavelength dependence of PL
spectra of various ZrO specimens at room temperature. As shown
1
nanopowder
3
55 nm
2
in Fig. 3(a), clear excitation-laser-wavelength-dependences on PL
spectra are observed at photon energies higher than 2.6 eV for the
nanopowder specimen. The wavelengths, 355 and 266 nm, corre-
spond to 3.49 and 4.66 eV, respectively. These energies are close
to those of the extrinsic PL bands observed by French et al. [1] for
film specimen. The similar spectral structure is also observed for
other specimens, as shown in Fig. 3(b)–(d). This indicates that
there are many kinds of extrinsic luminescent levels in the energy
gap, though the wavelength dependence of the light-penetration
depth cannot be ignored. At the initial stage of photoexcitation,
the laser light pulse is seen at twice the wavelength, 532 nm
1
2
8
4
0
3
25 nm (x 1.2)
2
66 nm (x 2.1)
3
.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
0x103
1
solar furnace
8
6
4
2
0
3
55 nm
(
¼266 nm ꢁ 2¼2.33 eV), of each spectrum, which is common to
2
66 nm (x 1.3)
every grating monochromator.
3
25 nm (x 1.3)
Fig. 4(a) and (b) shows the reversible photo-induced spectral
change of ZrO
and O gas, respectively. The irradiation time is given by each
curve. Fig. 4(c) shows the irradiation time dependence of the
integrated PL intensity under vacuum (V) and oxygen (O ) atmo-
spheres. In the experiments, the 325 nm laser light irradiation in a
vacuum induces the desorption of O molecule creating various
oxygen-deficient defect structures, while that in O gas induces
the adsorption of O molecule at oxygen defect site with annihi-
2
nanopowder at room temperature in a vacuum
2
3
.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
2
1
2x102
high purity powder
3
55 nm
2
2
8
4
2
2
66 nm (x 1.3)
25 nm (x 0.4)
lating oxygen-deficient defect structures. Fig. 4(a) and (b) shows
clearly how the PL spectrum changes with oxygen desorption and
adsorption, for the first time. The PL band due to the oxygen
desorption corresponds to about 2.3 eV. In the previous report [4],
3
0
2
the PL intensity of ZrO specimens is increased by 325 nm laser
3
.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
light irradiation in a vacuum. Very recently, we have measured
2
0x103
oxidized surface
20x103
1
1
5
0
5
0
1
5 min
30 min
in a vacuum
15
10
5
3
55 nm
1
0 min
2
min
266 nm (x 1.4)
325 nm (x 1.5)
0
.5 min
0 min
2.6
3
.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
Photon energy (eV)
0
3
.2
3.0
2.8
2.4
2.2
min
2.0
1.8
Fig. 3. Excitation wavelength dependence of PL spectra for various ZrO
2
speci-
2
0x103
mens at room temperature.
0
in O gas
2
1
1
5
0
5
0
0
.5 min
high-purity powder and oxidized surface, respectively. Excepting
for the high purity powder specimen, the E of the white PL band
is nearly close. These energies are the heights of an average
potential barrier separating white luminescent states from the
ground state.
D
1
0 min
2 min
30 min
2.6 2.4
Photon energy (eV)
Fig. 2 shows the time-resolved PL spectra of various ZrO
2
3
.2
3.0
2.8
2.2
2.0
V
1.8
specimens at 7 K. The luminescence at energies higher than 2.7 eV
disappears rapidly within 30 ns after excitation, and the lumines-
cence at energies lower than 2.7 eV remains over 10 ms. However,
the long-lasting luminescence band is not clear whether time
resolved or not, but a faint spectral shift is seen. The shift is due to
the time integration effect [5]. The PL decay curves of the long-
lasting luminescence were measured at 7 K and room tempera-
ture for all of the specimens. The results for the white PL band
were examined with a power law and the multi-component-type
exponential one. It is found that the decay curves are well fitted
6
x106
V
V
V
V
4
2
0
O2
O2
O2
O2
O2
ꢀ
2
with a power law (Ipt ) for all of the specimens at 7 K and room
temperature. It indicates that the carriers photo-excited from
defect-related in-gap states walk randomly via shallow traps and
an accidental recombination with an ionized defect (and/or an
impurity) creates a long-lasting PL.
0
30 60 90 120 150 180 210 240 270 300
Irradiation time (min)
2
Fig. 4. Reversible photo-induced spectral transition of ZrO nanopowder at room
temperature.

2
914
S. Mochizuki, T. Saito / Physica B 407 (2012) 2911–2914
the PL spectra in a wide laser fluence range and found the
followings. Under intense excitation (41500 mW/cm ), with
excited from extrinsic in-gap states to the conduction band. The
electrons move randomly throughout specimens, experiencing
many trapping and releasing via shallow traps. Accidental recom-
bination of the donor (which releases electron) and randomly
walking electron produces a long-lifetime luminescence.
2
increasing irradiation time in a vacuum, the PL intensity increases
first and then it decreases. On the other hand, under weak
2
excitation (o100 mW/cm ), the PL intensity increases with
increasing irradiation time in a vacuum. The main reason of the
difference may be the phonon-assisted creation of non-radiative
traps (namely, thermal-damage effects).
Acknowledgments
This work is partially supported by Nihon University Strategic
Projects for Academic Research: Nanotechnology Excellence—
Nanomaterial based Photonic and Quantum Technologies. This
work is also partially supported by a Research Grant from the
College of Humanities and Sciences (Nihon University).
4
. Concluding remarks
In the present study, the detailed experiments of the PL
properties due to the in-gap photo-excitation were carried out
for various kinds of ZrO specimens. It is found that the bluish
white PL of ZrO consists of many luminescence components,
2
2
which arise from extrinsic luminescence centers (various oxygen-
deficient defect structures, Hf-related defect structures and other
References
impurities). Usually, ZrO
elements as impurities. Hf
lattice point, creating shallow trap levels below the conduction
band, while impurity transition elements ion and oxygen defects
create deep donor levels in the energy gap. Anyway, electrons are
2
contain Hf, C and transition metal
[
1] R.H. French, S.J. Glass, F.S Ohuchi, Phys. Rev. B 49 (1994) 5133.
[2] M. Kirm, J. Aarik, M. J u¨ rgens, I. Sildos, Nucl. Instrum. Methods A 537 (2005)
51.
3] C. Lin, C. Zhang, J. Lin, J. Phys. Chem. C 111 (2007) 3300.
4] F. Fujishiro, S. Mochizuki, Phys. Status Solidi C 6 (2009) 354.
4
þ
4þ
ions may occupy randomly Zr
2
[
[
[5] S. Mochizuki, F. Fujishiro, S. Minami, J. Phys.: Condens. Matter 17 (2005) 923.