Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4:Ln3+ (Ln = Eu and Yb/Tm) microcrystals

Article abstract of DOI:10.1016/j.jallcom.2008.07.050

Cubic and hexagonal NaYF₄:Ln³⁺ (Ln = Eu and Yb/Tm) microcrystals were separately synthesized by an EDTA-assisted solvothermal method. Under 393-nm excitation, the emission spectra of NaYF₄:Eu³⁺ microcrystals sho

Full text of DOI:10.1016/j.jallcom.2008.07.050

Journal of Alloys and Compounds 475 (2009) 452–455
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Controlled synthesis and luminescence properties from cubic to hexagonal
NaYF4:Ln3+ (Ln = Eu and Yb/Tm) microcrystals
Guofeng Wang, Weiping Qin∗, Jisen Zhang, Lili Wang, Guodong Wei, Peifen Zhu, Ryongjin Kim
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Cubic and hexagonal NaYF4:Ln3+ (Ln = Eu and Yb/Tm) microcrystals were separately synthesized by an
EDTA-assisted solvothermal method. Under 393-nm excitation, the emission spectra of NaYF4:Eu3+ micro-
crystals show the characteristic Eu3+ emissions. In the excitation spectra of the 610-nm emission, the
Article history:
Received 10 June 2008
Received in revised form 9 July 2008
Accepted 10 July 2008
Available online 28 August 2008
7F0
→
5D3 is dominant for the cubic microcrystals, while the 7F0
→
5L6 is dominant for the hexagonal
3H6, 1I6 3F4, and 1D2 3H6) were
sample. In addition, unusually strong ultraviolet emissions (1I6
→
→
→
observed in the hexagonal NaYF4:Yb3+/Tm3+ microcrystals under 980-nm excitation. In comparison with
a cubic sample having the same chemical compositions, the hexagonal microcrystals had a markedly
enhanced ability of emitting ultraviolet up-conversion luminescence.
Keywords:
NaYF4
Rare earth
Up/down-conversion luminescence
© 2008 Elsevier B.V. All rights reserved.
In the present study, cubic and hexagonal NaYF4:Ln3+ (Ln = Eu
and Tm/Yb) microcrystals were synthesized by a facile solvother-
1. Introduction
Fluorides are efficient hosts for down-conversion (DC) and up-
conversion (UC) luminescence of rare earth (RE) ions due to their
low phonon energies and optical transparency over a wide wave-
length range [1]. Many applications of RE-doped fluorides have
been demonstrated, such as lasers [2], optical communications [3],
display devices [4], and so on [5].
mal method. The luminescence properties were studied in detail.
Under 393-nm excitation, the characteristic emissions of Eu3+ ions
were observed in NaYF4:Eu3+ microcrystals. In the excitation spec-
5
tra of the 610-nm emission, the 7F0 → D3 is dominant for the cubic
5
microcrystals, while the 7F0 → L6 is dominant for the hexagonal
sample. Under 980-nm excitation, unusually strong UV emissions
were observed in the hexagonal NaYF4:Yb3+/Tm3+ microcrystals.
In comparison with a cubic sample having the same chemical com-
positions, the hexagonal microcrystals can emit enhanced UV UC
fluorescence.
Among the fluorides reported, NaYF4, one of the most efficient
DC and UC host lattices, has attracted increasing attention [6–12].
It is well known that NaYF4 may be prepared in two polymor-
phic forms, namely, cubic and hexagonal phases [13,14]. The crystal
structure of the hexagonal NaYF4 was known to have three cation
sites, one for Y3+ ions, one for both Y3+ and Na+ ions, and the
third for Na+ ions [6]. The cubic NaYF4 is a fluorite structure, in
which Y3+and Na+ ions are randomly distributed in the cation sites.
Previous investigations have shown that the hexagonal NaYF4 is a
much better host lattice for the luminescence of RE ions than the
cubic NaYF4, which set off a new upsurge of synthesizing hexago-
nal NaYF4 [6]. However, the luminescence properties of NaYF4:Eu3+
have received little attention. Especially, the enhancement of the
ultraviolet (UV) UC luminescence in hexagonal NaYF4:Yb3+/Tm3+
has never been reported.
2. Experimental
2.1. Synthesis
Y2O3, Eu2O3, Yb2O3, and Tm2O3 (purity ≥ 99.999%) were supplied by Shang-
hai Chemical Reagent Company. Ethylenediamine tetra-acetic acid (EDTA), NaF, and
HNO3 were supplied by Beijing Chemical Reagent Company, and were of analytical
grade. All the reagents were used as received without further purification. Deionized
water was used to prepare solutions. Ln2O3 (Ln = Y, Eu, Yb, and Tm) were dissolved
in dilute HNO3 by heating to prepare the stock solution of Ln(NO3)3.
In a typical synthesis, 1 mL of 0.5 M Ln(NO3)3 aqueous solution and 0.5 mmol
of EDTA were dispensed into 15 mL of ethanol and magnetically stirred for 1 h,
forming a chelated Ln–EDTA complex. An aqueous solution of NaF was added into
the complex and vigorously stirred for 1 h. The mixture was then transferred into
a 50 mL autoclave and heated at 180 ◦C for 24 h. After cooling to room tempera-
ture, the suspension was centrifuged at 8000 rpm for 10 min. The product was then
washed thoroughly and dried in vacuum at 80 ◦C. To improve the crystallinity of the
nanocrystalline powders, all of the resultant products were annealed at 400 ◦C for
2 h.
∗
Corresponding author. Tel.: +86 431 85168240/8325;
fax: +86 431 85168240/8325.
E-mail address: wpqin@jlu.edu.cn (W. Qin).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.07.050

G. Wang et al. / Journal of Alloys and Compounds 475 (2009) 452–455
453
2.2. Characterization
The crystal structures and phase purity of the products were examined by a
Rigaku RU-200b X-ray powder diffractometer (XRD) using a nickel-filtered Cu K␣
radiation (ꢀ = 1.5418 Å) in the range of 20◦ ≤ 2Â ≤ 80◦. The size and morphology
of the products were characterized by scanning electron microscope (SEM, KYKY
1000B). SEM sample was prepared by placing droplets of the ethanol dispersion
on silicon wafer and letting the ethanol evaporate in air. The luminescence spec-
tra were recorded with a Hitachi F-4500 fluorescence spectrophotometer at room
temperature. For comparison of the luminescence properties of different samples,
the luminescence spectra were measured with the same instrument parameters
(2.5 nm for slit width and 400 V for PMT voltage). Raman spectra were taken by
using Renishaw Invia Raman spectrometer.
3. Results and discussion
3.1. Crystal structure and morphology
The structure of the NaYF4:Ln3+ microcrystals were controlled
by adjusting the molar ratio of Ln(NO3)3 to NaF in the reaction
solution. The XRD patterns of the NaYF4:Eu3+ microcrystals with
different phase structures are shown in Fig. 1. The diffraction peaks
in curve (a) for sample A (Ln(NO3)3:NaF = 1:8) can be indexed to the
pure cubic NaYF4 (JCPDS 77-2042). The diffraction peaks in curve
(b) for sample B (Ln(NO3)3:NaF = 1:12) are identified to the pure
hexagonal NaYF4 (JCPDS 16-0334). No other impurity peaks were
detected. Fig. 2a shows the SEM image of sample A, which is of
a spherical shape with mean diameter of 300 nm. The SEM image
of sample B is shown in Fig. 2b, which reveals that the hexagonal
microcrystals have relatively clean and smooth surfaces.
Fig. 2. SEM images of NaYF4:Eu3+(2%) microcrystals with different phase structures:
(a) cubic; (b) hexagonal.
3.2. DC luminescence properties of NaYF4:Eu3+ microcrystals
Fig. 3 presents the emission spectra of NaYF4:Eu3+(2%) micro-
crystals with different phase structures under 393-nm excitation.
7
netic dipole and electric dipole character [18]. The 5D0 → F4 is an
The 5D0 → FJ (J = 0, 1, 2, 3, 4) and 5D1 → F2 transitions were
observed. Because the 4f energy levels of Eu3+ are hardly affected
by the crystal field [17], there is no notable shift in the positions
of the emission peaks compared to other Eu3+-doped systems [6].
Some of these transitions satisfy magnetic dipole selection rules
7
7
electric dipole transition.
It is well known that the intensity ratio of 5D0 → F2 to 5D0 → F1
is strongly dependent on the local symmetry of the Eu3+ ions. There-
fore, Eu3+ ions are often used as probes to detect local environments
7
7
in a matrix. In a site with inversion symmetry the 5D0 → F1 is dom-
7
7
7
7
inating, while in a site without inversion symmetry the 5D0 → F2 is
(ꢁJ = 0, 1 except 0 ↔ 0). The 5D0 → F1 and 5D1 → F2 transi-
tions are magnetic-dipole-allowed and their intensities are almost
independent on the local environment around Eu3+ ions [18]. The
the strongest. According to the emission spectra displayed in Fig. 4,
the ratios of (5D0 → F2)/(5D0 → F1) were calculated to be 0.247 for
the cubic and 1.02 for the hexagonal, indicating that the symmetry
around Eu3+ in the hexagonal is lower than that in the cubic. Fur-
7
7
5D0 → F2 transition is electric-dipole-allowed due to an admixture
7
of opposite parity 4fn−15d states by an odd parity crystal-field com-
ponent [19]. Therefore, its intensity is sensitive to the local structure
thermore, in the emission spectrum of the hexagonal, the 5D0 → F1
7
7
emission was split into two sub-bands, which may be explained by
analyzing high resolution spectra and decay times of the lumines-
cence. However, these measurements cannot be performed because
of our experiment limitations.
around Eu3+ ions. The 5D0 → F3 transition exhibits a mixed mag-
Fig. 1. XRD patterns of NaYF4:Eu3+ microcrystals with different phase structures:
Fig. 3. Emission spectra of NaYF4:Eu3+(2%) microcrystals with different phase struc-
(a) cubic; (b) hexagonal.
tures excited at 393 nm: (a) cubic; (b) hexagonal.

454
G. Wang et al. / Journal of Alloys and Compounds 475 (2009) 452–455
Fig. 6. UC luminescence spectra (normalized to 1D2
→
3H6 transition) of
NaYF4:Yb3+(20%)/Tm3+(2%) microcrystals with different phase structures under
980-nm excitation: (a) cubic; (b) hexagonal.
Fig. 4. Excitation spectra of NaYF4:Eu3+(2%) microcrystals with different phase
structures monitored at 610 nm: (a) cubic; (b) hexagonal.
transfer transition of pure fluoride systems usually exists in the
vacuum ultraviolet (VUV) region as a result of high electroneg-
ativity. Under current experimental condition, we cannot further
explore the VUV-excited luminescence in the microcrystals.
Fig. 4 shows the excitation spectra (monitored at 610 nm) of the
NaYF4:Eu3+(2%) microcrystals with different phase structures. The
positions of these excitation peaks are practically identical to the
characteristic absorption bands for f–f intra-configuration transi-
tions in trivalent europium [15]. It is surprising to observe that the
3.3. UC luminescence properties of NaYF4:Yb3+/Tm3+
microcrystals
most intense peak is centered at 393 nm (7F0 → L6) for hexagonal
5
5
microcrystals and 409 nm (7F0 → D3) for cubic sample. Presum-
ably, two mechanisms might cause the spectral difference between
hexagonal and cubic samples. (1) The 393- and 409-nm absorption
transitions in the two samples may be different because the crystal
structure is changed from cubic to hexagonal. (2) The nonradiative
Under 980-nm excitation, unusually strong UV emissions were
observed in hexagonal NaYF4:Yb3+(20%)/Tm3+(2%) microcrystals.
In comparison with a cubic sample having the same chemical
compositions, the UV emissions from the hexagonal microcrys-
tals are greatly enhanced. Fig. 6 shows the UC spectra of the
cubic and hexagonal NaYF4:Yb3+(20%)/Tm3+(2%) microcrystals. The
5
relaxation of 5L6 → D3 for the hexagonal is more efficient than that
for the cubic. Since the nonradiative relaxation is closely related to
the phonon energies of the hosts, the Raman spectra of the samples
were studied (Fig. 5). For the hexagonal microcrystals, two Raman
bands centered at 230 and 520 cm−1 are observed. However, for the
cubic sample, only one broad Raman band centered at 260 cm−1
can be observed. The energy separation between 5L6 and 5D3 is
about 1000 cm−1. Two phonons (900–1200 cm−1) in the hexago-
nal sample exactly match the energy separation between 5L6 and
spectral peaks correspond to the following transitions of Tm3+
:
3
3
3
1I6 → H6 (∼290 nm), 1I6 → F4 (∼345 nm), 1D2 → H6 (∼361 nm),
3
3
3
1D2 → F4 (∼450 nm), 1G4 → H6 (∼477 nm), 1G4 → F4 (∼643 nm),
and 3H4 → H6 (∼802 nm). It is found that the luminescence inten-
3
sity at 290 nm is ∼2 times stronger than that at 477 nm for
the hexagonal microcrystals, while the luminescence intensity at
290 nm is less than 10% of the 477-nm emission in the cubic sample.
To investigate the fundamental UC mechanism of samples, the
pumping power dependence of the fluorescent intensity was inves-
tigated. For an unsaturated UC process, the emission intensity is
proportional to the nth power of the excitation intensity, and the
integer n is the number of the laser photons absorbed per upcon-
verted photon emitted [20]. Fig. 7 shows the power dependence of
the UC emission intensities: n = 4.68, 3.86, 2.77, and 2.06 for 345-,
361-, 477-, and 802-nm emissions, respectively. This means that
the populations of the states 1I6, 1D2, 1G4, and 3H4 come from five-
5
5D3, resulting in the effective nonradiative relaxation of 5L6 → D3.
5
Consequently, the excitation band of 7F0 → L6 is dominant for the
5
hexagonal, while the 7F0 → D3 is dominant for the cubic.
No notable excitation peak was observed in the range from 200
to 300 nm, which is similar to the excitation spectrum of YF3:Eu3+
nanocrystals [16]. In Eu3+-doped systems, charge transfer occurs
by electron delocalization from the filled 2p-shell of the ligand
to the partially filled 4f-shell of Eu3+. The transition energy is
determined by the electronegativity of the ligand [16]. The charge-
Fig. 5. Raman spectra of NaYF4:Eu3+(2%) with different phase structures: (a) cubic;
(b) hexagonal.
Fig. 7. Dependence of the UC emission intensity on the excitation power in cubic
NaYF4:Yb3+(20%)/Tm3+(2%) microcrystals.

G. Wang et al. / Journal of Alloys and Compounds 475 (2009) 452–455
455
For the excitation spectra of the 610-nm emission, the excitation
5
band of 7F0 → D3 is dominant for the cubic microcrystals, while
5
the 7F0 → L6 is dominant for the hexagonal sample, indicating that
5
the nonradiative relaxation of 5L6 → D3 for the hexagonal is more
efficient than that for the cubic.
3
3
3
Under 980-nm excitation, the 1I6 → H6, 1I6 → F4, 1D2 → H6,
3
3
3
3
1D2 → F4, 1G4 → H6, 1G4 → F4, and 3H4 → H6 emissions were
observed in NaYF4:Yb3+/Tm3+ microcrystals. In comparison with
the cubic sample having the same chemical compositions, the
hexagonal microcrystals can emit enhanced UV UC luminescence.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Grant Nos. 10474096 and 50672030).
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