Photoluminescence of thin films ZrO2 : Er3+ excited by soft X-ray

Article abstract of DOI:10.1016/j.ssc.2007.08.028

The photoluminescence (PL) properties and crystalline structure of undoped and erbium ion (Er³⁺)-doped zirconia (ZrO₂) that were prepared by sol-gel methods were examined. The ZrO₂ films exhibited a dopant concentration-dependent crystalline phase. As the Er³⁺ ions' concentration increases, the amount of monoclinic phases of ZrO₂ declines. However, the proportion of the tetragonal phase of ZrO₂ is enhanced. This fact suggests that Er³⁺ ions replace the Zr⁴⁺ ions and cause the phase transition in the host lattice of ZrO₂. Photoluminescence bands were observed by exciting the film at 300 eV with soft X-rays. Experimental results showed that the PL emission bands of Er⁺³-doped ZrO₂ films can be tuned by controlling the Er³⁺ concentration. In particular, with increasing Er³⁺ concentration, the blue bands are quenched whereas the green and red bands are enhanced. This phenomenon can be explained as a non-radiative energy transfer.

Full text of DOI:10.1016/j.ssc.2007.08.028

Solid State Communications 144 (2007) 181–184
www.elsevier.com/locate/ssc
3
+
Photoluminescence of thin films ZrO :Er excited by soft X-ray
2
a,∗
b
c
a
Lee-Jene Lai , Tieh-Chi Chu , Meng-I Lin , Yao-Kwang Lin
a
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30077, Taiwan
b
Department of Radiological Technology, Yuanpei Institute of Science and Technology, Hsinchu 30015, Taiwan
c
Department of Nuclear Science, National Tsing Hua University, Hsinchu 30013, Taiwan
Received 1 August 2007; accepted 29 August 2007 by P. Sheng
Available online 4 September 2007
Abstract
The photoluminescence (PL) properties and crystalline structure of undoped and erbium ion (Er )-doped zirconia (ZrO ) that were prepared
3
+
2
3
+
by sol–gel methods were examined. The ZrO films exhibited a dopant concentration-dependent crystalline phase. As the Er ions’ concentration
2
increases, the amount of monoclinic phases of ZrO declines. However, the proportion of the tetragonal phase of ZrO is enhanced. This fact
2
2
3
+
4+
suggests that Er ions replace the Zr ions and cause the phase transition in the host lattice of ZrO . Photoluminescence bands were observed
by exciting the film at 300 eV with soft X-rays. Experimental results showed that the PL emission bands of Er -doped ZrO films can be tuned
by controlling the Er concentration. In particular, with increasing Er concentration, the blue bands are quenched whereas the green and red
bands are enhanced. This phenomenon can be explained as a non-radiative energy transfer.
2
+3
2
3+
3+
ꢀ
c 2007 Elsevier Ltd. All rights reserved.
PACS: 78.20.-e; 78.55.-m; 78.66.-w; 81.20.Fw
Keywords: A. Photoluminescence; B. Zirconia; C. Thin film; D. X-ray excitation
1
. Introduction
applications in X-ray imaging systems and flat panel display
devices [8–10]. Compared with the conventional powder
phosphor display devices, the transparent thin-film phosphors
are of greater interest due to their higher contrast and resolution,
Zirconia (ZrO2)-based oxide ceramic is an excellent
material because of its physical properties, which include its
chemical stability, low thermal conductivity, low optical loss
and high refractive index. This explains its use in a variety of
applications, such as oxygen sensors, fuel cells and optically
transparent materials [1–3]. Zirconia has three well-defined
polymorphs, with monoclinic (ZrO2/M), tetragonal (ZrO2/T)
and cubic (ZrO2/C) phases. ZrO2/M is thermodynamically
3
+
minimal optical scattering and better adhesion [10]. Er
ion-activated ZrO2 is a promising candidate for making
scintillator materials. Since its structure stabilization [11,12]
has been used as a luminescence activator in monocrystalline
zirconia and polycrystalline zirconia film, charge compensation
associated with the substitution of Zr4 by a trivalent ion,
resulting in oxygen vacancies in the zirconia lattice, gives rise
to the many optical properties of doped zirconia [13,14].
+
◦
stable up to 1100 C; ZrO2/T exists in the temperature range
1
◦
◦
100–2370 C, whereas ZrO2/C is found above 2370 C [4].
In this work, Er3 -doped ZrO2 thin films were prepared
by a sol–gel method. The effect on the crystalline structure
and optical properties of zirconia thin films by the variation of
+
At room temperature, ZrO2 exists in the monoclinic form
since the cubic phase is metastable [5]. Furthermore, the
optical properties of zirconia are modulated by the crystalline
structure and by dopants [6,7]. Rare earth-doped luminescent
thin films have emerged over the last few years, with potential
the Er3 dopant concentration was investigated. Experimental
results demonstrate that photoluminescence of ZrO2 thin films
is observed using soft X-ray excitation, also, the emission bands
+
can be tuned by controlling the Er3 concentration. The process
conditions, consequent structures and luminescence properties
of thin-film phosphors were reported in detail.
+
∗
Corresponding author. Tel.: +886 3 5780281; fax: +886 3 5783813.
E-mail address: jene@nsrrc.org.tw (L.-J. Lai).
0
038-1098/$ - see front matter ꢀc 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2007.08.028

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L.-J. Lai et al. / Solid State Communications 144 (2007) 181–184
emission from the sample was focused using two lenses on the
entrance slit of the monochromator. The PMT was applied to
collect the photons from the exit slit of monochromator and
then, the signal from the PMT was transferred to the multimeter
for data acquisition using a computer. The PL measurements
were made at room temperature using a monochromator from
Acton Research Corporation (ARC) SpectraProTM VM-504, a
0
.39 m triple grating, and a photomultiplier tube (PMT) R955
from Hamamatsu Photonics K. K, Japan. All PL spectra were
normalized to the photon flux I0(E), and the X-ray diffraction
˚
(
XRD) data were obtained at λ = 1.5406 A using a Cu target
at room temperature.
3. Results and discussion
Fig. 2 presents XRD data for ZrO2 films doped with
–6.0 mol% ErCl3 on fused-silica substrates to elucidate how
0
the dopant influences the host ZrO2 structure. The films were
obtained directly using the sol–gel method at a sintering
◦
temperature of 1100 C. The background spectrum associated
with fused-silica was subtracted from all data. The XRD
spectrum of the undoped film reveals a pure monoclinic
structure of ZrO2 (ZrO2/M), and the main peaks of (111)M and
(
111)M are at 2θ = 28.34 and 31.56 degree, respectively, based
on PDF #36-0420, as presented in Fig. 2(a). However, the XRD
spectrum of the ZrO2 film with a high concentration, 6.0% of
dopant, reveals a pure tetragonal structure of ZrO (ZrO /T),
Fig. 1. Experimental setup for measuring soft X-ray-excited photolumines-
3+
cence from ZrO films doped with Er
ions on fused-silica substrates at
2
NSRRC 24A WR beamline.
2
2
and a main peak of (101)T at 2θ = 30.50 degree, according
2
. Experiment
to PDF #81-1545, as displayed in Fig. 2(e). When the ZrO2
The Er3 -doped ZrO2 films were obtained with various
+
film contained 2.0 mol% ErCl dopant, as in Fig. 2(c), the
3
Er3 concentrations using the sol–gel method. All chemicals,
+
XRD spectra exhibited not only the main monoclinic peaks of
(
111)M and (111)M , and a strong peak at 2θ = 28.97 degree,
but also a small tetragonal peak of (101)T . The strong peak at
θ = 28.97 degree corresponds to the cubic structure (222)C of
except the erbium (III) chloride (ErCl3, anhydrous 99.9%,
Strem Chemicals, Inc.), were reagent grade and were supplied
by Aldrich, Inc. The doping of erbium (III) chloride was done
with 0.5 mol%–6.0 mol%. Samples are called the undoped film,
the 0.5 mol% dopant film, the 6 mol% dopant film, and so
on. The samples were obtained using zirconium n-propoxide
2
Er2O3(Er2O3/C), according to the standard PDF #77-0463. As
the concentration of Er3 ions increases, the peaks of (111)M
and (111)M diminish and the peak of (101)T increases. Then,
the ZrO2/T becomes dominant as the dopant concentration
+
(
Zr(OPr)4) and erbium (III) chloride (ErCl3) as precursors.
increases, further suggesting that Er3 ions replace Zr ions
and cause a phase transition in host lattice of ZrO2. Meanwhile,
another crystalline phase of the cubic structure Er2O3/C was
observed at a dopant concentration of 2 mol%, and the
intensity of the peak of Er2O3(222)C then falls as the dopant
concentration increases to 4 mol%. This result differs from that
presented by Rosa-Cruz et al., [16], who found no Er2O3/C at
dopant ion concentrations of 0.16–3.46 mol%.
+
4+
All components were mixed in a solution of propanol (PrOH),
acetylacetone (AcAc) and deionized water. After stirring at
room temperature for 3 h, a clear precursor solution was
obtained. A spin-coating procedure was adopted to deposit
films on degreased fused-quartz substrates; these films were
further sintered in an oven at controlled temperatures 1100 C
for 6 h, then cooled in an oven until room temperature.
The Photoluminescence (PL) experiments were conducted
on ZrO2 films that were irradiated with soft X-rays at the 24 A
wide-range beamline in the third-generation 1.5 GeV storage
ring at the National Synchrotron Radiation Research Center
◦
Fig. 3 displays the photoluminescence (PL) spectra of ZrO2
films that were doped with 0–6.0 mol% Er3 ions on fused-
silica substrates when the samples were excited with 300
eV soft X-rays. No PL emission peak was observed from
the background of fused-silica substrates. For the undoped
sample, the strong broad emission bands at around 400–600 nm
centered at 485 nm with weak emission bands at 250–400 nm
were observed. For the 0.5 mol% dopant film, not only were the
main strong broad bands centered at approximately 485 nm and
the weak bands at 250–400 nm observed, but also minor sharp
peaks at around 500–570 nm and 650–700 nm appeared. For
+
(
NSRRC), Hsinchu, Taiwan. This beamline delivers photons
with energies from 10 to 1500 eV [15]. Fig. 1 schematically
depicts the system setup for the PL experiment. An incident
focused and monochromatic soft X-ray photon flux was
monitored using the photocurrent I0(E) derived from a copper
mesh, which is placed in front of the sample. A photon energy
of 300 eV was selected to excite the sample since this beamline
had the highest photon flux in 300 eV. Then, the luminescence

L.-J. Lai et al. / Solid State Communications 144 (2007) 181–184
183
3+
Fig. 2. XRD spectra of ZrO films, with various mole concentrations of Er
2
ions. (a) 0 mol%; (b) 0.5 mol%; (c) 2.0 mol%; (d) 4.0 mol%; (e) 6.0 mol%.
Fig. 4. Gaussian deconvolutions for the ZrO2 films in Fig. 3. (a) Undoped ZrO2
film; thick solid line: raw data, dashed line: fitted curves with the center peaks
at around 290, 355, 450, 510 and 555 nm; (b) Doped 2.0 mol% film; thick solid
line: raw data, dashed line: fitted curves with the broadened center peaks at
around 290, 355, 450, 510 and 555 nm; thin solid lines: fitted curves with the
sharp center peaks at around 525, 550, 560, 660 and 680 nm.
the fitted curves with dashed lines from undoped ZrO2 film,
including the broadened center peaks at around 290, 355, 450,
5
2
10 and 555 nm. Fig. 4(b) presents the fitted curves from
.0 mol% Er3 -doped film, including not only the broadened
+
center peaks, dashed lines, as undoped ZrO2 film, but also
the sharp center peaks, solid lines, at around 525, 550, 560,
6
4
60 and 680 nm. The main luminescence bands, centered at
85 nm, observed at 450, 510 and 555 nm from the undoped
film are associated with the non-radiative relaxation of each
electron, excited from the valence to the conductive band,
to each luminescence recombination site and the subsequent
luminescent relaxation from the luminescence recombination
site to the valence band. This assumption is similar to that
adopted by Rosa-Cruz et al. to account for the formation of
the PL broad band centered at 490 nm [17]. However, the
emission peaks at 290 and 355 nm from the undoped film are
attributed to the radiative decay of self-trapped excitons (STE)
and the capture of electrons by oxygen vacancies from the
conduction band (CB), respectively. The results are similar to
those of Kirm et al., [18] who found that the emission bands
of the ZrO2 film peaked at 4.3 and 3.5 eV, when the photon
excitation energy exceeded 5.8 eV, which is the bandgap of
ZrO2. The sharp peaks from the 2.0 mol% doped film at 525
and 550–560 nm corresponds to the green emission band of the
Fig. 3. PL spectra of ZrO films doped with 0, 0.5, 2.0, 4.0 and 6.0 mol% of
2
3+
Er ions, with excitation using 300 eV soft X-rays.
the 4 mol% dopant film, the main sharp peaks at 500–570 nm
and 650–700 nm were clearly observed, but the minor peaks
from 250 to 500 nm had almost disappeared. For the 6.0 mol%
dopant film, the PL intensities, not only from 250 to 400 nm but
also from 400 to 600 nm almost disappeared.
2
H11/2 → I15/2 and 4 S3/2 → I15/2 of Er transitions,
4
4
3+
respectively. However, the emission peaks at 660 and 680 nm
are very weak, corresponding to the red emission band of the
The PL spectrum from the samples can be fitted with
Gaussian deconvolution as shown in Fig. 4. Fig. 4(a) displays
4
4
3+
F9/2 → I15/2 of Er transition.

1
84
L.-J. Lai et al. / Solid State Communications 144 (2007) 181–184
demonstrate a strong fluorescence emission broad band cen-
tered at 485 nm from the undoped ZrO2 film. However, the
3+
Er -doped ZrO2 films exhibit not only a broad band centered
at 485 nm, but also sharp characteristic bands, indicating that
3
+
the ZrO2 absorbs the energy, and then transfers it to Er
,
the active ion, which then relaxes to produce the characteristic
emission band of the active ion. Non-radiative energy transfer
from the luminescence sites of the host to the active ion was
performed. Also, the active ions stabilized the tetragonal phase
of ZrO2 and quenched the fluorescence emission of the host.
Acknowledgements
The authors would like to thank the National Synchrotron
Radiation Research Center and the National Science Council of
the Republic of China, Taiwan, (Contract No. NSC 93-2113-M-
Fig. 5. PL intensity ratio for donor emission centered at 450 nm (I ) to
acceptor emission centered at 550 nm of Er (Ia), corresponding to erbium
ion dopant concentration.
d
3+
213-008) for financially supporting this research.
The emission spectrum from the Er3 -doped sample with
a broad band centered at around 485 nm corresponds to the
ZrO2 host emission, and several emission lines that correspond
to the erbium transitions, and proves that the energy transfer
from the host to the active ions, was obtained. The host absorbs
the energy, and then either emits the broad band that is centered
+
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+
1
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+
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3+
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4
. Conclusion
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3+
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(
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