Efficient photoluminescence of Dy3+ at low concentrations in nanocrystalline ZrO2

Article abstract of DOI:10.1016/j.jssc.2007.09.033

Nanocrystalline ZrO₂:Dy³⁺ were prepared by sol-gel and the structural and photoluminescence properties characterized. The crystallite size ranges from 20 to 50 nm and the crystalline phase is a mixture of tetragonal and monoclinic structure controlled by dopant concentration. Strong white light produced by the host emission band centered at ～460 nm and two strong Dy³⁺ emission bands, blue (488 nm) and yellow (580 nm), under direct excitation at 350 nm were observed. The highest efficiency was obtained for 0.5 mol% of Dy³⁺. Emission is explained in terms of high asymmetry of the host suggesting that Dy³⁺ are substituted mainly into Zr⁴⁺ lattice sites at the crystallite surface. Luminescence quenching is explained in terms of cross-relaxation of intermediate Dy³⁺ levels.

Full text of DOI:10.1016/j.jssc.2007.09.033

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 75–80
www.elsevier.com/locate/jssc
Efficient photoluminescence of Dy3+ at low concentrations in
nanocrystalline ZrO2
Ã
L.A. Diaz-Torresa, E. De la Rosaa, , P. Salasb, V.H. Romeroa, C. Angeles-Chavezb
´
aCentro de Investigaciones en Optica, A.P. 1-948, Leon, Gto. 37150, Mexico
´
Instituto Mexicano del Petroleo, L. Cardenas 152, Mexico D.F. 07730, Mexico
b
Received 11 May 2007; received in revised form 28 August 2007; accepted 24 September 2007
Available online 6 October 2007
Abstract
Nanocrystalline ZrO2:Dy3+ were prepared by sol–gel and the structural and photoluminescence properties characterized. The
crystallite size ranges from 20 to 50 nm and the crystalline phase is a mixture of tetragonal and monoclinic structure controlled by dopant
concentration. Strong white light produced by the host emission band centered at ꢀ460 nm and two strong Dy3+ emission bands, blue
(488 nm) and yellow (580 nm), under direct excitation at 350 nm were observed. The highest efficiency was obtained for 0.5 mol% of
Dy3+. Emission is explained in terms of high asymmetry of the host suggesting that Dy3+ are substituted mainly into Zr4+ lattice sites at
the crystallite surface. Luminescence quenching is explained in terms of cross-relaxation of intermediate Dy3+ levels.
r 2007 Published by Elsevier Inc.
Keywords: Dy3+; ZrO2; Photoluminescence; Nanocrystals; White light
1. Introduction
Tb3+ ions (in oxide hosts) that have allowed charge-
transfer absorption band (CTB) or 4f8–4f75d absorption
band in the UV region, respectively, the excitation
spectrum of Dy3+ consists of only narrow f–f transition
lines from 300 to 500 nm. Both the CTB and 4f9–4f85d
excitation band of Dy3+ are located below 200 nm [6]. As a
result, the photoluminescence (PL) of Dy3+ cannot be
excited with 254 nm UV light, and the excitation can occur
only by the f–f transitions with low oscillator strength
(10ꢁ6) due to their forbidden features by the parity
selection rule [1]. This drawback of Dy3+ luminescence
can be overcome by sensitization, such as host sensitization
in YVO4:Dy3+ [3] and ion sensitization in Ca2Gd8
(SiO4)6O2:Pb2+Dy3+ [7].
Nanocrystalline ZrO2 doped with different rare earths
has been demonstrated to present suitable optical proper-
ties for photonics applications due to its enhanced
luminescence properties [8–11], which in turn is due to
nanosized effects. The very low stretching frequency of
ZrO2 (470 cmꢁ1) opens up the possibility of higher efficient
luminescence of active ions incorporated into ZrO2 matrix.
Electrons confinement effect is not expected due to the
localization of electrons in atomic orbitals of active ions.
However, excitation dynamics is influenced by the nanoscopic
Rare earth ions have been playing an important role in
modern lighting and display fields due to the abundant
emission colors based on their 4f–4f or 5d–4f transitions [1].
In order to be excited efficiently, phosphors activated with
rare earth ions should have a strong and broad absorption
band in the UV or VUV region depending on the practical
application situation. For example, the tricolor fluorescent
lamp phosphors have a strong and broad absorption band
around 254 nm (UV) to meet the irradiation from the
discharge of low-pressure mercury vapor [1,2]. The visible
luminescence of trivalent dysprosium Dy3+ mainly consists
of narrow lines in the blue (470–500 nm, 4F9/2-6H15/2) and
yellow (4F9/2-6H13/2, 570–600 nm) wavelength region. The
latter one belongs to the hypersensitive transition (DL=2,
DJ=2), which is strongly influenced by the environment [3].
At a suitable yellow to blue intensity ratio, Dy3+ will emit
white light. Thus, luminescent materials doped with Dy3+
may be used as potential two-band phosphors [4,5].
However, unlike the most frequently used Eu3+ and
Ã
Corresponding author.
E-mail address: elder@cio.mx (E. De la Rosa).
0022-4596/$ - see front matter r 2007 Published by Elsevier Inc.
doi:10.1016/j.jssc.2007.09.033

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L.A. Diaz-Torres et al. / Journal of Solid State Chemistry 181 (2008) 75–80
76
interaction and has been reported a dependence of the
luminescence efficiency with particle size [12,13]. The
interest on this new rare earth doped nanophosphor is to
produce visible emission for application such as solid-state
lighting, displays and new generation television screen. In
particular, the development of UV LED open up new
possibilities to obtain visible emission at any wavelength
with the proper phosphor that must be developed [14].
Considering the promising optical properties of ZrO2 and
the spectral qualities of the Dy3+ UV excited emission, it is
interesting to evaluate the luminescence properties of
ZrO2:Dy3+ for white light generation applications. Only
few reports have been published on ZrO2:Dy3+ nanocrys-
tals. Recently, it has been reported the characterization
of such kind of nanocrystals prepared for different
methods. In those papers authors report that the PL
depends on the crystalline phase and that the highest
efficiency occur for samples doped at 2 mol% of Dy2O3
[15,16]. In this letter we present our study of the
luminescence characterization of this nanophosphor pre-
pared by sol–gel and its dependence on Dy3+ content.
Here, PL depends strongly on Dy3+ concentration instead
of crystalline phase. The highest efficiency was obtained for
0.5 mol% of Dy2O3 content contrary to results reported
previously by others authors.
3. Results and discussions
The crystalline phase of ZrO2:Dy3+ nanocrystals is a
mix of monoclinic (m) and tetragonal (t) phase being the
phase composition determined by the ion concentration,
see Fig. 1. The diffraction pattern for the 0.1 mol% Dy2O3
doped sample was dominated by the (ꢁ1,1,1)m and (1,1,1)m
reflections characteristic of the monoclinic phase. The
(0,1,1)t reflection is characteristic of the metastable tetra-
gonal phase and was much more smaller than monoclinic
reflections for dopant concentration lower than 1 mol%.
This indicates the dominant presence of the monoclinic
phase in relation to the tetragonal phase. As the Dy3+ ion
concentration increases the monoclinic peaks diminishes
whereas the tetragonal peak increases, being dominant for
the 2.0 mol% Dy2O3 content. This behavior in the phase
composition is consistent with previous reports on rare
earth stabilization of different ZrO2 crystalline phases
[17,18]. The phase composition, average crystallite size and
lattice parameters of both the monoclinic and tetragonal
structures were calculated for 0.5 and 2.0 mol% of Dy2O3
doped samples by using the Rietveld method [19], and
listed in Table 1. The major content of the tetragonal phase
was 52.8 wt% for the sample with 2.0 mol% Dy2O3
whereas for the sample with 0.5 mol% Dy2O3 the tetra-
gonal phase content was only 8.7%. This indicates the
enhancing effect of the impurity addition on the stability of
the tetragonal phase up to 1000 1C. Average crystallite sizes
of 22.8 and 40.0 nm for 0.5 and 2.0 mol% Dy2O3 content,
respectively, were estimated from the tetragonal phase.
And crystallite sizes of 47.2 and 36.4 nm for 0.5 and
2. Experimental procedure
The samples were prepared by using the sol–gel method
[17]. Undoped ZrO2 was obtained by mixing zirconium
n-propoxide as precursor in a solution of ethanol, nitric
and hydrochloric acid at room temperature and vigorous
stirring according to the molar rate 1:20:0.1:0.3, respec-
tively. ZrO2:Dy3+ samples were prepared by adding differ-
ent molar concentrations (0.1, 0.5, 1.0 and 2.0 mol%)
of Dy2O3 using dysprosium nitrates, corresponding to
Zr/Dy molar ratios of 1:0.001, 1:0.005, 1:.01, and 1:0.02,
respectively. CO2-free distilled water in the molar ratio of
1:4 was added dropwise to manipulate the hydrolysis
process. After gelation, the samples were aged at 120 1C for
12 h followed by calcinations at 400 1C for 5 h. Tempera-
ture was raised to 1000 1C at a rate of 5 1C/min; as soon as
the temperature was reached, sample was retired from the
furnace.
(0,1,1)t
(-1,1,1)m
(1,1,1)m
2.0% Dy O - ZrO
2
2
3
1.0% Dy O - ZrO
2
2
3
0.5% Dy O - ZrO
2
2
3
The crystalline structure and crystallite size of samples
were investigated by X-ray diffraction (XRD) using a
˚
Siemens D-500 equipment with l ¼ 1.5405 A, scanning in
0.1% Dy O - ZrO
2
3
2
the 15–801 2y range with 0.051 increments and 3 s swept.
The morphology and crystallite size was analyzed in a
field-emission electron microscope JEM-2200FS operating
at 200 kV. The microscope is equipped with an ultra
high resolution (UHR) configuration (Cs ¼ 0.5 mm; Cc ¼
1.1 mm; point-to-point resolution, 0.19 nm).
Both fluorescence excitation and emission spectra are
recorded on an Acton Research modular 2300 spectro-
fluorometer. The excitation source is a 75 W Xenon lamp.
The spectral resolution of the spectrofluorometer was 1 nm.
ZrO
2
10
20
30
40
50
60
70
80
2θ
Fig. 1. XRD patterns for undoped and Dy3+ doped nanocrystals.
Reflection peaks are denoted according to JCPDS 37-1484 (monoclinic)
and JCPDS 50-1089 (tetragonal).

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77
Table 1
Phase composition, crystallite size and lattice parameter obtained from Rietveld refinement
Dy2O3
(mol%)
Phase composition
(wt%74%)
Lattice parameters (A70.1%)
Cell volume
(A3)
Crystallite size
(nm72%)
ao
bo
co
b
0.5
2.0
Tetragonal (8.7)
Monoclinic (91.3)
Tetragonal (52.8)
Monoclinic (47.2)
3.598
5.149
3.598
5.151
3.598
5.202
3.598
5.204
5.181
5.314
5.183
5.314
67.09
142.37
67.13
22.8
47.2
40.0
36.4
99.02
98.9
142.50
a
2.0 mol% Dy2O3 content, respectively, were estimated for
the monoclinic phase. Both crystallite size and phase
composition obtained from XRD results and Rietveld
refinement analysis are in agreement with results obtained
from TEM and HRTEM images. The particle size
measured from TEM ranges between 20 and 50 nm and
all these nanoparticles are well faceted, see Fig. 2a. The
change in lattice parameters is induced by the difference of
ion size of Dy3+ (1.03 A) and Zr (0.84 A). Such changes
confirm that Dy3+ is substituting Zr4+ ion. The valence
difference of the substituted cations requires charge
compensation that leads to small local distortions of the
crystalline lattices, with an overall loss of symmetry in both
crystalline phases. Up to the annealing temperature of the
samples there was not observed Dy2O3 crystalline phase in
the bulk or surface of the zirconia nanoparticles suggesting
that the Dy3+ atoms remain within the crystallite lattices of
both phases. The HRTEM images show the atomic
resolution corresponding to the monoclinic zirconia phase
in the [211] direction and tetragonal zirconia phase in the
[100] direction, see Fig. 2b. From the HRTEM of both
phases, there is no evidence of the presence of Dy2O3
crystalline phase on the nanoparticles, confirming that
Dy3+ ions remain within the crystalline lattices.
4+
˚
˚
b
The excitation spectrum in Fig. 3 is the scanning excited
wavelength from 300 to 410 nm when the detection
wavelength was set at 488 nm, for the sample with
0.5 mol% of Dy2O3 content. The excitation maxima for
the 488 nm emission is located at 350 nm corresponding to
6
the transition from the ground level H15/2 to the
6
hypersensitive level P7/2, from where it relaxes nonradia-
4
tively to the F9/2 metastable level. Three secondary
excitation peaks at 326, 366, and 388 nm were assigned
and correspond to the transitions from the ground level
(6H15/2-6P3/2), (6H15/2-6P5/2), (6H15/2-4F7/2), respec-
tively. The general observed peaks distribution agrees with
the reported structure distribution spectra for ZrO2:Dy3+
nanocrystals [15]. Those broad bands indicate that the
Dy3+ ions are substituted mainly at Zr4+ sites with high
symmetry, that is, at the more symmetric eight-fold
coordination sites of the Zr4+ cation within the tetragonal
phase [15]. Thus, this excitation spectrum suggests that
most of the Dy3+ ions are substituted within the tetragonal
crystallites. The experimental results in Fig. 4 show the
fluorescence emission from 400 to 650 nm of the five
prepared samples annealed at 1000 1C, both undoped and
[0 0 1]
(0 0 2)
[2 1 1]
(0 1 1)
(0 1 -1)
(-1 2 0)
(0 -1 1)
(-1 1 1)
T
M
Fig. 2. (a) TEM, (b) HRTEM and FFT for Zr2O3:Dy3+ nanocrystal
doped at 0.5 mol%.

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78
transition associated to signal emitted are described in the
40
35
30
25
20
15
10
5
6H
6P
7/2
15/2
energy diagram from Fig. 5. It is also worth to notice that
the host band actually corresponds to the baseline for the
Dy3+ emission on the doped samples. Therefore, the
luminescence of the doped system simply corresponds to
the superposition of host plus the impurity emissions. This
fact indicates that there is no energy transfer processes
between the host and the Dy3+ ions in opposite of results
reported by Fu et al. [15]. That difference might be due to
the fact that in Ref. [15] high concentrations of Y3+ have
been added as codopants in order to stabilize different
crystalline phases, that in turn increases substantially the
number of oxygen vacancies which on time act as
sensitizers of Dy3+. In that sense, the absence of energy
transfer in our samples might be an indication of a reduced
amount of oxygen vacancies.
6H
6P
5/2
15/2
6H
4F
7/2
15/2
6H
6P
3/2
15/2
λems = 488nm
300
320
340
360
380
400
420
440
Wavelength [nm]
Fig. 3. Excitation spectrum of sample doped at 0.5 mol% of Dy3+
.
One can observe that the highest emission corresponds
to the 0.5 mol% Dy2O3 doped sample, which presents
mainly the monoclinic structure. For all samples the
emission is dominated by the magnetically allowed
(4F9/2-6H15/2) transition that hardly varies with variations
of the crystal field strength around the Dy3+ ion. The ratio
of the (4F9/2-6H15/2) transition to the hypersensitive
(4F9/2-6H13/2) transition keeps an almost constant value
of 1.2870.04 over the range of Dy3+ content. That
indicates that in spite of considerable changes on phase
content, the hypersensitive transition (4F9/2-6H13/2) senses
on average the same degree of asymmetry at the substituted
Dy3+ sites. This leads to assume that in spite that the
tetragonal content increases with Dy3+ concentration, a
great amount of such tetragonal crystallites have distorted
tetragonal cells and in consequence the Dy3+ ions on such
crystallites continue to sense reduced local symmetry, that
on turn lead to an increased luminescence emission. Loss
of local symmetry is also induced by surface defects
that in turn are a consequence of small crystallite size.
These results suggest that Dy3+ ions could be distributed
along the nanocrystals producing a gradient ion, being
0.1% Dy
50
λexc= 350nm
0.5% Dy
1.0% Dy
40
4F
6H
2.0% Dy
9/2
15/2
Pure ZrO2
30
20
10
0
4F
6H
13/2
9/2
400
450
500
550
600
650
700
Wavelength [nm]
Fig. 4. Photoluminescence spectra of undoped and Dy3+ doped
nanocrystals.
doped ZrO2 under excitation at 350 nm. The broad
emission band is dominated by two main peaks, the
X103 cm-1
strongest one in the blue region centered at 488 nm (4F9/2
-
6H15/2) and a less strong one in the yellow region centered
at 580 nm (4F9/2-6H13/2), both characteristic of Dy3+ in a
solid solution [20]. Notice that the 2.0 mol% Dy2O3 doped
sample had an emission almost indistinguishable from the
undoped sample. Such behavior indicates that the Dy3+
luminescence was completely quenched for a 2.0 mol%
content. It is also shown the emission of undoped ZrO2
which has a broad emission band extending from 425
to 650 nm peaking around 460 nm [21]. The shape of the
UV side of this emission band is defined by the
transmittance of the filter used to stop the excitation
wavelength. In this case, electrons promoted to the
conductive band decay non radioactively to the recombi-
nation band associated to traps produced by the presence
of defects [10]. From this metastable band electrons decay
to the valence band producing the broad emission. All
36
32
(Eg)
N
M
L
J
28
4G11/2
K
I
4I15/2
24
20
16
12
8
4F9/2
RB
488 nm
580 nm
6
6
H
5/2
6
6
6
F
3/2
5/2
H
F
7/2
F
7/2
460 nm
6
H
H
9/2
6
11/2
6
4
H
13/2
6
0
H
Dy3+
ZrO2
Fig. 5. Energy diagram indicating the transition involved in the visible
emission.

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L.A. Diaz-Torres et al. / Journal of Solid State Chemistry 181 (2008) 75–80
79
accumulated in few planes in the surface substituted at
some Zr4+ lattice sites. The absence of changes of hyper-
sensitive transition is in opposite to results reported by
Gu et al. [16] where they claim to observe changes in the
blue to yellow bands ratio as the phase composition
change. In addition, concentration quenching of the Dy3+
luminescence is observed as Dy3+ concentration increases
over 0.5 mol% Dy2O3 content. Luminescence of the
2.0 mol% Dy2O3 doped sample is almost identical to the
host luminescence. The most plausible explanation for such
behavior is cross relaxation among Dy3+ pairs to the
intermediate levels Dy3+ (6F3/2) and Dy3+ (6H9/2), from
where the ions decay non radiatively or by IR emission,
see energy diagram in Fig. 5 [22,23]. These quenching
transitions are mainly Dy3+ (4F9/2)+Dy3+ (6H15/2)-
Dy3+ (6F3/2)+Dy3+ (6H9/2). The quenching of the
Luminescence is best observed on Fig. 6 where one can
note the drastic reduction of the total emission from
0.5 mol% Dy2O3 to 2.0 mol% Dy2O3. Such behavior
indicates that for nanocrystalline ZrO2:Dy3+ the optimum
concentration is around 0.5 mol% Dy2O3 content, which is
to our knowledge the lowest concentration for maximum
Dy3+ PL in a ceramic phosphor.
The spectral characteristics of this new phosphor make it
a promising candidate for application on optical devices
and solid-state lighting for general illumination purposes.
Nanocrystalline ZrO2:Dy3+ shows its main peak emissions
in blue as well yellow spectral regions, and has a strong
broad host emission band extending from 400 to 650 nm.
Consequently, the emission color of nanocrystalline
ZrO2:Dy3+ is a white color tending slightly to yellowish.
Inset in Fig. 6 shows the PL emission observed under
excitation at 350 nm. It is important to consider the
relatively strong emission observed in spite of the low
excitation intensity coming from the spectrofluorometer
with slits set at 5 mm. The CIE coordinates measured with
a LS2000 Minolta colorimeter were x ¼ 0.36350 and
y ¼ 0.4077. The color coordinates were verified with the
help of a CIE chromaticity coordinates diagram confirming
the yellowish white color shown in Fig. 6.
4. Conclusions
In summary, nanocrystalline ZrO2:Dy3+ have been
synthesized by sol–gel method and highly efficient yellow-
ish white light emission at 0.5 mol% Dy2O3 was observed.
Dy3+ concentration determines both luminescence proper-
ties and crystalline phase composition. Results suggest that
nanocrystalline ZrO2:Dy3+(0.5%) is a promising phosphor
candidate for white lighting applications. The strong
quenching above 1.0 mol% content and the constant value
of the ratio (4F9/2-6H15/2)/(4F9/2-6H13/2) suggest that
Dy3+ ions are distributed along the nanocrystal but mainly
in few planes on the surface substituted at the Zr lattice
sites.
Acknowledgments
This works was partially supported by CONACyT
trough Grants 43168-F, 46971-F and scholarship for
V.H. Romero. The help of Dr. R. A. Rodriguez in the
color coordinate measurement is also acknowledged.
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