Hydrothermal synthesis and white luminescence of Dy3+-Doped NaYF4 microcrystals

Article abstract of DOI:10.1111/j.1551-2916.2011.04518.x

2 mol% Dy³⁺-doped NaYF₄ microcrystals with different morphologies and crystalline phases have been synthesized through a hydrothermal method without assistance of any surfactant, catalyst, or template by controlling reaction temperatures, reaction time, and molar ratios of reactants. For comparison, the samples were also synthesized by a direct coprecipitation method. The final products were characterized by X-ray diffraction, field emission scanning electron microscopy, photoluminescence excitation and emission spectra, and luminescent dynamic decay curves. Dy³⁺ exhibits bright white light under near ultraviolet 350 or 351 nm excitation and the white light was evaluated by chromaticity coordinates. The experimental results suggest that the obtained Dy³⁺-doped NaYF₄ microcrystals have potential applications in white light materials.

Full text of DOI:10.1111/j.1551-2916.2011.04518.x

J. Am. Ceram. Soc., 94 [10] 3405–3411 (2011)
DOI: 10.1111/j.1551-2916.2011.04518.x
r 2011 The American Ceramic Society
ournal
J
Hydrothermal Synthesis and White Luminescence of Dy31-Doped
NaYF4 Microcrystals
Chunyan Cao,z,y Hyun Kyoung Yang,z Jong Won Chung,z Byung Kee Moon,z Byung Chun Choi,z Jung Hyun
Jeong,w,z and Kwang Ho Kimz
zDepartment of Physics, Pukyong National University, Busan 608-737, Korea
yCollege of Mathematics and Physics, Jinggangshan University, Ji’an 343009, China
zSchool of Materials Science and Engineering, Pusan National University, Busan 609-735, Korea
2 mol% Dy31-doped NaYF4 microcrystals with different mo-
rphologies and crystalline phases have been synthesized through
a hydrothermal method without assistance of any surfactant,
catalyst, or template by controlling reaction temperatures, re-
action time, and molar ratios of reactants. For comparison,
the samples were also synthesized by a direct coprecipitation
method. The final products were characterized by X-ray diffrac-
tion, field emission scanning electron microscopy, photolumines-
cence excitation and emission spectra, and luminescent dynamic
decay curves. Dy31 exhibits bright white light under near ultra-
violet 350 or 351 nm excitation and the white light was evaluated
by chromaticity coordinates. The experimental results suggest
that the obtained Dy31-doped NaYF4 microcrystals have
potential applications in white light materials.
have been well reviewed by Li and Lin.25 The hydrothermal
treatment as a typical solution approach has been proven to be
effective and convenient in preparing various inorganic materi-
als with diverse controllable morphologies and architectures.30
Compared with the synthesis in organic solvents, a water-based
system provides a relatively green chemical alternative to pre-
pare various materials because it does not involve toxic organic
solvents. Therefore, the hydrothermal route is often adopted to
synthesize NaYF4.31,32 During the growth process of crystals,
not only their intrinsic structures but also a series of external
factors, such as reaction temperatures, reaction time, pH values
of precursor solution, and organic additives, drastically influ-
ence the crystallization process and shape evolution of particles
in a solution-based system.33,34
RE31-doped luminescent materials have attracted much at-
tention for their potential applications in flat-panel displays,35,36
solid-state lasers,37 solar cells,38 light-emitting devices,39 and flu-
orescent labels.40,41 More and more attention has been paid to
Dy31 luminescence due to its potential applications in electro-
luminescent displays, plasma display panels, field emission dis-
plays, and mercury-free lamps.42,43 Dy31 with a 4f 9 electronic
configuration generally has two dominant transitions in visible
range: the 4F9/2-6H15/2 transition corresponds to the blue emis-
I. Introduction
ARE earth compounds, such as hydroxides,1 oxides,2,3 phos-
R
phates,4,5 fluorides,6–10 and vanadates,11–13 have been
extensively studied because of their potential applications in
high-performance magnets, luminescent devices, catalysts, and
other functional materials based on the electronic, optical, and
chemical characteristics arising from 4f electrons. Among them,
the rare earth fluoride compounds with general formula
NaREF4 (RE 5 rare earth), possessing a high refractive index
and low phonon energy,14–17 have attracted more attention be-
cause they are regarded as excellent host matrixes for down-
conversion (DC) and up-conversion (UC) processes. In the in-
vestigated fluorides, NaYF4, as one of the most efficient DC and
UC host lattices, has attracted greater attention in the field of
materials science.9,18–24,25,26 The crystal structure of NaYF4 ex-
hibits two polymorphic forms, namely, cubic (a) phase and hex-
agonal (b) phase, depending on the synthesis conditions and
methods. The b-NaYF4 has three cation sites, one for Y31 (1a),
one for both Y31 and Na1 (1f ), and the third one for Na1 (2h).
Sites 1a and 1f have C3h symmetry, whereas site 2h has Cs sym-
metry.6,27 Previous investigations have shown that b-NaYF4 is a
much better host lattice than a-NaYF4 for the luminescence of
various optically active lanthanide ions.28,29 There are several
successful methodologies for preparation of b-NaYF4 which
4
sion and the F9/2-6H13/2 transition corresponds to the yellow
emission, respectively. It is well-known that a white lumines-
cence can be obtained by not only mixing the appropriate ratio
of the three primary red, green, and blue colors but also creating
the appropriate mixture of blue and yellow emissions. At a suit-
-
4
able lattice site, white light emitting can be fulfilled by F9/2
6
6H15/2, H13/2 emissions of Dy31 43–45
.
In the paper, via a mild hydrothermal route and a coprecip-
itation method, we study the effects of reaction temperatures,
reaction time, and molar ratios of NaF to RE31
(RE31 5 Y311Dy31) on crystal structures and morphologies
of the resultant materials without any surfactant, catalyst, or
template. Then we focus on the investigation of white lumines-
cent properties of 2 mol% Dy31-doped a- and b-NaYF4 micro-
crystals in an effort to adjust the white light of the final products
with near ultraviolet (near-UV) 350 or 351 nm radiation.
The results indicate that the chromaticity coordinates of ob-
tained white light vary with the phase and size changing of the
final products.
R. S. Hay—contributing editor
II. Experimental Details
2 mol% Dy31-doped NaYF4 samples were synthesized through
a mild hydrothermal method. The raw materials Y2O3 (99.99%,
Sigma-Aldrich, China), Dy2O3 (99.9%, Sigma-Aldrich, Aus-
tria), and NaF (99%, Sigma-Aldrich, USA) were used directly
without further purification. Rare earth (Y31:Dy31 5 98:2)
nitrate (RE(NO3)3) stock solution (0.1M) was prepared by
Manuscript No. 28988. Received December 8, 2010; approved February 18, 2011.
This study was financially supported by Pukyong National University in the 2010 Post-
Doc and this work was supported by the National Research Foundation of Korea (NRF)
grant funded by the Korea government (MEST) (No. 2010-0029885) Program also this
work was partially supported by NCRC (National Core Research Center) program through
the National Research Foundation of Korea funded by the Ministry of Education, Science
and Technology (2010-0001-226).
wAuthor to whom correspondence should be addressed. e-mail: jhjeong@pknu.ac.kr
3405

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Journal of the American Ceramic Society—Cao et al.
Vol. 94, No. 10
dissolving the proportional mixed Y2O3 and Dy2O3 in nitric acid
at elevated temperature. In a typical hydrothermal synthesis pro-
cedure, 16 mmol NaF (NaF:RE31 5 8:1) was dissolved into 25
mL deionized water forming a clear solution by stirring. Then
the NaF solution was added dropwise into a 20 mL RE(NO3)3
solution to form turbid solution while stirred. After stirred for
another 10 min, the as-obtained solution was transferred into a
70 mL Teflon bottle held in a stainless-steel autoclave, sealed,
and maintained at 1201C for 24 h. When the autoclave cools
down to room temperature naturally, the upper clear solution
was discarded and the precipitates were collected by centrifuga-
tion, washed with deionized water, and dried in air at 601C for 24
h. Other synthetic conditions, such as reaction temperatures (801,
1601, 2001, and 2401C), reaction time (12 and 48 h), and molar
ratio of NaF to RE31 (4:1), were selected to investigate the
effects of above factors on crystal structures, morphologies, and
optical properties of final products. For comparison, the samples
were synthesized through a coprecipitation method as well.
The crystalline structures of obtained samples were charac-
terized by X-ray diffraction (XRD) patterns using a Philips
Xpert/MPD system (Eindhoven, the Netherlands) with CuKa
the samples synthesized by hydrothermal method at 1201C
for different time (NaF:RE31 5 8:1) and corresponding stan-
dard data. For comparison, the XRD pattern of sample
synthesized by coprecipitation method is presented in the
figure as well. As shown in Fig. 1(b), the sample synthesized
through coprecipitation method is pure a-NaYF4, which is
consistent with the calculated standard data of a-NaYF4
ꢀ
(JCPDS No. 77-2042) with space group Fm3mð225Þ shown
in Fig. 1(a). After hydrothermally treated at 1201C for 12 h
Ã
(Fig. 1(c)), the sample is a mixture of a-NaYF4 (denoted as )
and b-NaYF4. The b-NaYF4 can be well indexed to the stan-
dard data (JCPDS No. 16-0334 card with space group P63/m)
shown in Fig. 1(f). Figure 1(d) gives XRD pattern of the
sample hydrothermally treated at 1201C for 24 h, which
presents b-NaYF4 phase and can be well indexed to stan-
dard data shown in Fig. 1(f ). Prolonging reaction time to 48
h can obtain pure b-NaYF4, too. The corresponding XRD
pattern is illustrated in Fig. 1(e), which can be exactly in-
dexed to standard JCPDS card No. 16-0334. The narrowed
peaks of XRD patterns indicate that the samples crystallized
well, which is beneficial for luminescence. The evolution of
XRD patterns proves that the samples change from a-NaYF4
to b-NaYF4 with reaction time prolongation while keeping
other conditions unchanged.
Figure 2 presents corresponding FE-SEM images which were
all captured with magnification of ꢀ 10 000 and scale bars
equaling to 1 mm for comparison. The sample synthesized by
coprecipitation method illustrates cubic-like morphology with
average size of 200–300 nm in length, which is a-NaYF4
according to the XRD pattern shown in Fig. 1(b). After
hydrothermally treated at 1201C for 12 h, the sample exhibits
ill-defined bulk morphology besides cubic-like morphology.
According to the XRD pattern in Fig. 1(c) (mixed a- and
b-NaYF4), we can infer that the ill-defined bulk material is
b-NaYF4. Figures 2(c) and (d) are the FE-SEM pictures of the
sample prepared at 1201C for 24 and 48 h, respectively. The cu-
bic morphology almost disappears and the material presents
mainly short-rod-like morphology in average lengths of about
2 mm and diameters of 800 nm with aspect ratio of 2.5. After
b-NaYF4 is formed, prolonging reaction time has little effect on
the morphology of final products.
˚
(l 5 1.54056 A) radiation at 40 kV and 30 mA. The 2y ranges of
all the data sets are from 151 to 701 with a step size of 0.021. The
morphology and size of the obtained phosphors were observed
with field emission scanning electron microscopy (FE-SEM,
JSM-6700F, JEOL, Tokyo, Japan). For optical property inves-
tigation, room-temperature photoluminescence (PL) excitation
and emission spectra were recorded by using a time-resolved
Photon Technology International (PTI, Birmingham, NJ) flu-
orimeter with a Xe-arc lamp of power 60 W as excitation source.
The luminescent dynamic decay curves were measured by using
a phosphorimeter attachment to the main system (PTI) with
Xe-flash lamp (25 W) as excitation source.
III. Results and Discussion
Samples synthesized through a hydrothermal method have a lot
of factors to control, such as reaction time, reaction temperatures,
pH values, raw materials, organic additives, etc. In this paper, we
will study effects of reaction time, reaction temperatures, and raw
material ratios on the structures, morphologies, and white lumi-
nescent properties of the final Dy31-doped products.
Temperature is another crucial role in controlling morphol-
ogy and phase of final products. In the study, when the molar
ratio of NaF to RE31 is 8:1 and other reaction conditions re-
main unchanged, the XRD patterns of samples synthesized by
hydrothermal method at different temperatures for 24 h as well
as standard data are illustrated in Fig. 3. As analyzed in the
above text, the sample synthesized through coprecipitation
method exhibits pure a-NaYF4. Figure 3(b) shows the XRD
pattern of sample hydrothermally treated at 801C for 24 h. As
presented in the figure, a-NaYF4 is the main sample, which can
(1) Crystalline Phase and Morphology
We firstly study the effects of reaction time on phase and
morphology of final products. Figure 1 is XRD patterns of
be indexed to standard data JCPDS 77–2042 shown in Fig. 3(a).
Ã
Besides a-NaYF4, a small amount of b-NaYF4 (denoted as
)
emerge. Figures 3(c)–(e) are XRD patterns of the final products
hydrothermally treated at 1201, 1601, and 2001C for 24 h, re-
spectively, which are all can be well indexed to the literature
data of b-NaYF4 JCPDS No. 16-0334) shown in Fig. 3(f). Ac-
cording to variations of XRD patterns, we believe that relatively
lower hydrothermal temperature is good for a-NaYF4 and
relatively higher hydrothermal temperature is favorable for
b-NaYF4.
The FE-SEM images of the final products synthesized at 801,
1201, 1601, and 2001C for 24 h are presented in Figs. 4(a)–(d),
respectively. The sample synthesized at 801C exhibits cubic-like
morphology with average length of about 300 nm, which is
mainly a-NaYF4. After hydrothermally treated at 1201C, the
final product is mainly rod-like morphology with length of
about 2 mm and diameter about 800 nm. Besides, there are
small particles in the sample. Considering the corresponding
XRD patterns, we can infer that the rods (b-NaYF4) evolve
from cubic particles (a-NaYF4). When hydrothermal tempera-
ture increases to 1601 and 2001C, the rod-like particles become
Fig. 1. XRD patterns of final products synthesized by coprecipitation
method and hydrothermal treatment at 1201C for different time
(NaF:RE31 5 8:1) and standard data of a-NaYF4 (JCPDS 77-2042)
and b-NaYF4 (JCPDS 16-0334) as references.

October 2011
Hydrothermal Synthesis and White Luminescence of Dy31
3407
Fig. 2. FE-SEM images of samples (NaF:RE31 5 8:1) synthesized by coprecipitation method (a) and hydrothermal treatment at 1201C for (b) 12 h,
(c) 24 h, and (d) 48 h, respectively (magnifications are all ꢀ 10000 and scale bars are all 1 mm).
slightly smaller in length and diameter remaining aspect ratio of
about 2.5, which will affect optical properties slightly.
synthesized by coprecipitation method presents cubic phase,
too. At lower hydrothermal temperatures (801 and 1201C), the
samples remain a-NaYF4, as presented in Figs. 5(c) and (d), re-
spectively. When the hydrothermal temperature is increased to
The reactant ratio of NaF to RE31 has great effects on the
phases of the final products, too. In the study, we only choose
two molar ratio of NaF to RE31 (8:1 and 4:1) to study their
effects on the final products. Figure 5 shows the corresponding
XRD patterns of the final products synthesized at different re-
action temperatures for 24 h and coprecipitation method with
NaF:RE31 equaling to 4:1 and other conditions unchanged. For
clarity, the corresponding standard data of a-NaYF4 (JCPDS
77-2042) (Fig. 5(a)), b-NaYF4 (JCPDS 16-0334) (Fig. 5(h)), and
orthorhombic phase YF3 (JCPDS 70-1935) (Fig. 5(i)) are listed
in the figure as references. As shown in Fig. 5(b), the sample
Ã
1601C (Fig. 5(e)), new materials b-NaYF4 (denoted as ) and
orthorhombic phase YF3 (denoted as H) emerge in the final
product. Increasing temperature cannot obtain pure materials.
Figures 5(f) and (g) are XRD patterns of the samples hydro-
thermally treated at 2001 and 2401C, respectively, which can be
indexed to mixtures of a-NaYF4, b-NaYF4, and orthorhombic
phase YF3. From intensity and position variations of diffraction
peaks, we can infer that contents of b-NaYF4 and orthorhombic
phase YF3 increase with hydrothermal temperature increasing
and there is nearly no a-NaYF4 when hydrothermal tempera-
ture is as high as 2401C.
Comparing the XRD patterns in Figs. 3 and 5, we learn that
only the molar ratios of NaF to RE31 in raw materials change
but great changes happen in the final resultant materials. The
samples synthesized by coprecipitation method are both a-
NaYF4 with the two molar ratios (NaF: RE31 5 8:1 and 4:1).
After the same hydrothermal condition reaction, the final prod-
ucts evolve differently as discussed above. With hydrothermal
temperature increasing, the excess Fꢁ (NaF) results in pure b-
NaYF4 while the stoichiometric Fꢁ (NaF) leads to mixtures of
a, b-NaYF4, and orthorhombic phase YF3. For clarity, the final
products and corresponding synthesis conditions including re-
action temperature (T), reaction time (t), and molar ratio of
NaF to RE31 (R), are summarized and listed in Table I.
(2) PL Properties
To illustrate Dy31 excitation and emission process well, repre-
sentative PL excitation and emission spectra of b-NaYF4
(1201C, 24 h, NaF:RE31 5 8:1) are presented in Fig. 6. The
4f–4f transitions from the ground state of Dy31 to different ex-
cited states or the reverse processes are found in the spectra.
Curve 6(a) is the PL excitation spectrum of the sample by mon-
Fig. 3. XRD patterns of samples synthesized at different reaction tem-
peratures for 24 h with NaF:RE31 equaling 8:1 and standard data of
a-NaYF4 (JCPDS 77-2042) and b-NaYF4 (JCPDS 16-0334) as refer-
ences.

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Journal of the American Ceramic Society—Cao et al.
Vol. 94, No. 10
Fig. 4. FE-SEM images of final products synthesized by hydrothermal method at (a) 801C, (b) 1201C, (c) 1601C, and (d) 2001C for 24 h, respectively
(magnification is ꢀ 20 000 for (a) and ꢀ 10000 for (b–d). Scale bars are all 1 mm).
4
itoring the emission at 573 nm corresponding to the F9/2
4
radiatively relaxes to populate the F9/2 level. The radiative
-
4
emissions take place from the F9/2 level to the lower states.
6H13/2 transition of Dy31. The characteristic linear excitations
6
originating from the ground state H15/2 of Dy31 to different
The strong yellow emission centering at 573 nm corresponds to
the hypersensitive transition 4F9/2-6H13/2 (DL 5 2, DJ 5 2) and
its intensity is strongly influenced by the surrounding environ-
ment around Dy31. Another strong blue emission peaking at
482 nm corresponds to the 4F9/2-6H15/2 transition, which is less
excited states 4K15/216P3/2 (B326 nm), 4M15/216P7/2 (B351 nm),
4
4I11/2 (B365 nm), M21/214M19/214K17/2 (B390 nm) are
detected in the excitation spectrum.45,46 According to the PL
excitation spectrum intensity, we choose 351, 365, 326, and 390
nm as excitation light. Curves 6(b-e) are the corresponding PL
emission spectra with above near-UV radiations as excitation
lights, respectively. For example, the 351 nm radiation excites
the Dy31 to the 4M15/216P7/2 levels and then Dy31 quickly non-
4
sensitive to the host. It is also known that the F9/2-6H15/2
transition is of magnetic dipole and the 4F9/2-6H13/2 transition
is of electric dipole origin. In general, when Dy31 is located at a
low symmetry site (without an inversion center), the yellow
emission is dominant while the blue emission is stronger when
Dy31 is located at a high symmetry site (with an inversion
center). As seen from the PL emission spectrum, the yellow
emission is slightly stronger than the blue emission in the Dy31
-
activated b-NaYF4. The integrated yellow intensity Iyellow (550–
605 nm) to blue intensity Iblue (463–525 nm) ratio b (Iyellow/Iblue),
known as the asymmetry ratio of Dy31, is 0.921 in the sample.
The PL emission spectrum and b-value suggest that the ligand
field does not deviate much from inversion symmetry in the b-
NaYF4. In addition, blue emission at 453 nm and red emission
at 662 nm are weak and they can be assigned to the transitions
of 4I15/2-6H15/2 and 4F9/2-6H11/2, respectively. With 365, 326,
and 380 nm excitation, the similar PL emission spectra are
observed except emission intensities.
Figure 7 shows PL emission spectra of the Dy31-doped
NaYF4 samples synthesized by hydrothermal method at 1201C
for different time (12, 24, and 48 h) and coprecipitation method
(NaF:RE31 5 8:1). Under 350 or 351 nm excitation, all samples
emit bright white light. The emission intensity and the asymme-
try ratio b (Iyellow/Iblue) values change greatly in the four sam-
ples. The sample synthesized by coprecipitation method has the
weakest emission intensity. With reaction time increasing, the
PL emission intensities become stronger. As illustrated by XRD
data, the samples evolved from pure a-NaYF4, mixed a, b-
NaYF4, to pure b-NaYF4 with the prolongation of reaction
time. b-NaYF4 is a much better host lattice than a-NaYF4 for
Fig. 5. XRD patterns of final products synthesized at different reaction
temperatures for 24 h with NaF:RE31 equaling 4:1 and standard data of
a-NaYF4 (JCPDS 77-2042), b-NaYF4 (JCPDS 16-0334), and ortho-
rhombic YF3 (JCPDS 70-1935) as references.

October 2011
Hydrothermal Synthesis and White Luminescence of Dy31
3409
Table I. Final Products, Molar Ratio (R) of NaF:RE31, Reaction Temperature T, Reaction Time t, Excitation Wavelength kex,
Emission Wavelength kem, Intensity Ratio b (Iyellow/Iblue), as well as Calculated Chromaticity Coordinates (x, y)
Final products
R
T (1C)
t (h)
lex (nm)
lem (nm)
b
(x, y)
a-NaYF4
a, b-NaYF4
b-NaYF4
b-NaYF4
a, b-NaYF4
b-NaYF4
b-NaYF4
a-NaYF4
a-NaYF4
a-NaYF4
8:1
8:1
8:1
8:1
8:1
8:1
8:1
4:1
4:1
4:1
4:1
4:1
4:1
Coprecipitation
350
351
351
351
351
351
351
350
351
351
351
351
351
479, 574
481, 573
482, 573
481, 573
479, 574
482, 573
482, 573
479, 574
479, 573
479, 573
478, 573
478, 573
478, 572
0.759
0.908
0.921
0.947
0.767
0.982
0.979
0.817
0.741
0.738
0.740
0.819
0.832
(0.299, 0.332)
(0.313, 0.359)
(0.314, 0.362)
(0.318, 0.367)
(0.301 0.339)
(0.322, 0.371)
(0.323, 0.371)
(0.308, 0.346)
(0.300, 0.338)
(0.299, 0.338)
(0.297 0.340)
(0.308, 0.355)
(0.305, 0.353)
120
120
120
80
160
200
12
24
48
24
24
24
Coprecipitation
80
120
160
200
240
24
24
24
24
24
a, b-NaYF4, YF3
a, b-NaYF4, YF3
b-NaYF4, YF3
optically active lanthanide ions.28,29 So emission intensities in-
crease with hydrothermal time prolongation. When b-NaYF4 is
formed, emission intensities change slightly due to little
change in morphology or aspect ratio of the sample. And
the cubic phase sample is more symmetrical than hexagonal
phase sample. Correspondingly, the blue (4F9/2-6H15/2) and
orange (4F9/2-6H13/2) emissions of Dy31 change in the four
samples.
From the above XRD data, we can learn that the reactant
ratio has great effects on the final products. It is reasonable that
the optical properties will change correspondingly for Dy31 is
sensitive to the local surrounding environment. Figure 9 shows
PL emission spectra of the samples synthesized at different tem-
peratures with NaF:RE31 equaling to 4:1. Different from the PL
emission spectra of samples synthesized with NaF:RE31 equal-
ing to 8:1, the emission intensities increase always with hydro-
thermal temperature increasing, which are mainly caused by
higher crystallinity of the final products and concentration in-
crease of b-NaYF4. The hydrothermal temperature-dependent
Iblue and Iyellow as well as b (Iyellow/Iblue) are plotted in the inset of
the figure, too. The Iblue and Iyellow increase with hydrothermal
temperature increasing, while the b firstly decreases then in-
creases for the phase transition and crystallization of the final
products.
As presented in all the PL emission spectra, white light
emitting may be fulfilled in Dy31-activated NaYF4. Here, we
investigate the luminescent color of the synthesized samples by
using the 1931 Commission International de I’Eclairage (CIE)
system.47,48 All samples give bright white light under 4f–4f
excitation. Figure 10 is the 1931 CIE diagram for all the sam-
ples. The obtained white light changes slightly in different final
products. Correspondingly, the calculated chromaticity coordi-
nates of obtained light change slightly, too.
Furthermore, optical properties of the final products as func-
tions of hydrothermal temperatures are studied, too. Figure 8
is PL emission spectra of the samples synthesized with
NaF:RE31 5 8:1. The sample synthesized by coprecipitation
method has the weakest emission intensity. After hydrothermal-
ly treated at 801C for 24 h, the sample has stronger emissions
intensity. Then there is abrupt increase of the emission intensity
in the sample synthesized at 1201C. In addition, the emission
4
6
peaks of F9/2-6H15/2, H13/2 split clearly. The corresponding
XRD data suggest that the final products change from a-NaYF4
to b-NaYF4 with hydrothermal temperature increasing. As
discussed in the above text, emission intensities increase and
blue and orange emission peaks split clearly. When hydrother-
mal temperatures increase to 1601 and 2001C, the emission in-
tensities of final products become weaker. The corresponding
FE-SEM images suggest that higher temperature results in
smaller rods which finally lead to weaker luminescence. To
illustrate the effects of hydrothermal temperature on optical
Detailed information, including excitation wavelength (lex),
properties well, the Iblue and Iyellow as well as the b (Iyellow/Iblue
as functions of hydrothermal temperatures are plotted in the
inset of the figure.
)
main emission peak wavelength (lem), asymmetric ratio b (Iyellow/
Iblue), and calculated chromaticity coordinates (x, y) correspond-
ing with the final products are also listed in Table I.
Fig. 7. PL emission spectra of 2 mol% Dy31-doped NaYF4 samples
synthesized by hydrothermal method at 1201C for different time and
coprecipitation method (NaF:RE31 58:1).
Fig. 6. PL excitation and emission spectra of 2 mol% Dy31-doped
b-NaYF4 (1201C, 24 h, NaF:RE31 5 8:1).

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Journal of the American Ceramic Society—Cao et al.
Vol. 94, No. 10
Fig. 8. Reaction temperature-dependent PL emission spectra of
2 mol% Dy31-doped NaYF4 synthesized with NaF:RE31 5 8:1. Insets
are integrated blue intensity Iblue, yellow intensity Iyellow, and integrated
intensity ratio b (Iyellow/Iblue) as functions of hydrothermal temperatures.
Fig. 10. 1931 CIE diagram for all the samples.
Fig. 9. PL emission spectra of the resultant materials synthesized with
NaF:RE31 5 4:1 dependence on reaction temperatures. Iblue, Iyellow, and
b (Iyellow/Iblue) corresponding to hydrothermal temperatures are plotted
in the insets.
Fig. 11. Luminescent dynamic decay curves for 2 mol% Dy31 acti-
vated a-NaYF4 (NaF:RE31 5 4:1, 801C, 24 h) and b-NaYF4
(NaF:RE31 5 8:1, 1201C, 24 h).
The dynamic decay curves for the luminescence of Dy31 are
studied, too. Figure 11 is the representative decay curves
4
monitoring at Dy31 F9/2-6H13/2 transitions for a-NaYF4
(NaF:RE31 5 4:1, 801C, 24 h) and b-NaYF4 (NaF:RE31 5 8:1,
1201C, 24 h) samples, respectively. The decay curves can be well
fitted by a single exponential function as I(t) 5 I0exp(-t/t), where
I0 is the initial emission intensity at t 5 0, t is the 1/e lifetime of
ucts. Further, these factors affect the luminescent properties of
Dy31 activated NaYF4 microcrystals greatly. Dy31-doped
NaYF4 samples exhibit bright white light under near-UV 350
or 351 nm excitation and the chromaticity coordinates of ob-
tained white light change for Dy31 is sensitive to local lattice
environment symmetry. The experimental results suggest that
the Dy31-doped NaYF4 is a promising white light material and
has potential applications in lighting technology.
4
Dy31. The lifetimes of Dy31 for F9/2-6H13/2 of a-NaYF4
(monitoring at 573 nm with 350 nm as excitation light) and b-
NaYF4 (monitoring at 573 nm with 351 nm as excitation light)
samples are 396711 and 36977 ms, respectively.
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