YF3:Ln3+ (Ln = Ce, Tb, Pr) submicrospindles: Hydrothermal synthesis and luminescence properties

Article abstract of DOI:10.1039/c2dt30325f

YF₃:Ln³⁺ (Ln = Ce, Tb, Pr) microspindles were successfully fabricated by a facile hydrothermal method. X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), lifetimes, photoluminescence (PL) and low-voltage cathodoluminescence (CL) were used to characterize the resulting samples. The lengths and diameters of YF ₃:0.02Ce³⁺ microspindles are around 760 nm and 230 nm, respectively. Adding dilute acid and trisodium citrate (Cit^3-) are essential for obtaining YF₃ microspindles. A potential formation mechanism for YF₃ microspindles has been presented. PL spectroscopy investigations show that YF₃:Ce³⁺ and YF ₃:Tb³⁺ microcrystals exhibit the characteristic emission of Ce³⁺ 5d → 4f and Tb^{3+ 5}D₄ → ⁷F_J (J = 6-3) transitions, respectively. In addition, the energy transfer from Ce³⁺ to Tb³⁺ was investigated in detail for YF₃:Ce³⁺, Tb³⁺ microspindles. Under the excitation of electron beams, YF₃:Pr³⁺ show quantum cutting emission and YF₃:Ce³⁺, Tb³⁺ phosphors exhibit more intense green emission than the commercial phosphor ZnO:Zn.

Full text of DOI:10.1039/c2dt30325f

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PAPER
YF3:Ln3+ (Ln = Ce, Tb, Pr) submicrospindles: hydrothermal synthesis and
luminescence properties†
Chong Peng,a,b Chunxia Li,a Guogang Li,a,b Suwen Lia and Jun Lin*a,b
Received 14th February 2012, Accepted 10th May 2012
DOI: 10.1039/c2dt30325f
YF3:Ln3+ (Ln = Ce, Tb, Pr) microspindles were successfully fabricated by a facile hydrothermal method.
X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy
(TEM), high-resolution transmission electron microscopy (HRTEM), lifetimes, photoluminescence (PL)
and low-voltage cathodoluminescence (CL) were used to characterize the resulting samples. The lengths
and diameters of YF3:0.02Ce3+ microspindles are around 760 nm and 230 nm, respectively. Adding
dilute acid and trisodium citrate (Cit3−) are essential for obtaining YF3 microspindles. A potential
formation mechanism for YF3 microspindles has been presented. PL spectroscopy investigations show
that YF3:Ce3+ and YF3:Tb3+ microcrystals exhibit the characteristic emission of Ce3+ 5d → 4f and Tb3+
5D4 → 7FJ (J = 6–3) transitions, respectively. In addition, the energy transfer from Ce3+ to Tb3+ was
investigated in detail for YF3:Ce3+, Tb3+ microspindles. Under the excitation of electron beams, YF3:Pr3+
show quantum cutting emission and YF3:Ce3+, Tb3+ phosphors exhibit more intense green emission than
the commercial phosphor ZnO:Zn.
Yttrium trifluoride (YF3) is well-known as an excellent fluore-
1. Introduction
scent host lattice owing to its wide band gap and low vibrational
energies.1,11–13 The Y3+ sites can be easily replaced by trivalent
Lanthanide ions play an important role in modern technology for
their superior luminescent properties.1–5 They have a wide emis-
lanthanide ions with the same valence state and similar ionic
radius.14 The Ce3+ ion needs an inorganic lattice with a large
sion range from ultraviolet to infrared, and energy transfer
band gap, otherwise the emission would be quenched.15 The low
between different lanthanide ions is possible due to their various
energy levels. The 4f → 4f transition is an atomic-like transition
with long lifetime, sharp absorption lines, and excellent coher-
ence properties, because the 4f-electrons are well protected from
the surrounding interactions by the 5s2 and 5p6 outermost
shells.6,7 However, the 5d → 4f transition has a short lifetime,
broad absorption band, and large oscillator strength.8 The Ce3+
ion is known for its broad 5d → 4f transition band which is spin
allowed and very sensitive to the crystal field interaction. The
Tb3+ ion is frequently used as a green emitting dopant because
phonon energy minimizes the quenching of excited states of
lanthanide ions. An appreciable amount of reports have been
published on the photoluminescence of YF3 doped with different
lanthanide ions.11,16 However, the cathodoluminescence (CL)
properties are rarely reported. The CL properties of phosphors
are critical for field emission displays (FED), which are promis-
ing emissive displays with high resolution and low electric
power consumption.17–19 Nowadays, much attention is focused
on enhancing the luminescent intensity of phosphors. Besides
the energy transfer from sensitizer to activator, like Ce3+ to Tb3+,
another way to improve the efficiency of phosphors is a quantum
cutting process. The quantum cutting process has drawn much
attention for its potential applications in noble gas discharge
lamps,20,21 and was discovered in YF3:Pr3+ independently by
Piper and Sommerdijk in 1974.22,23 In theory, when each
vacuum ultraviolet (VUV) photon is absorbed, a two photon
emission is allowed in quantum cutting phosphors. As far as we
know, no quantum cutting process has been reported from
electron beams excitation. Because of the high energy of electron
beams, the lattice can be directly excited from the valence band
to conduction band. In this situation, a quantum cutting process
may be realized, which might be a promising process to enhance
the efficiency of FED phosphors.
the dominant 5D4
→
7F5 transition is located in the green
region.7 The Pr3+ ion can emit different colored light depending
on the host lattice. In some pervoskite lattices, the dominant tran-
sition is 1D2
→
3H4, giving red emission. However, in most
oxide lattices, the dominant emission is blue-green, correspond-
ing to the 3P0 → 3H4 transition.9,10
aState Key Laboratory of Rare Earth Resource Utilization, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, P. R. China. E-mail: jlin@ciac.jl.cn;
Fax: +86-431-85698041; Tel: +86-431-85262031
bGraduate University of Chinese Academy of Sciences, Beijing, 100049,
P. R. China
†Electronic supplementary information (ESI) available: FTIR spectrum
of as-prepared YF3: 0.02Ce3+ microspindles (Fig. S1), and the calculat-
ing method for spectral overlap. See DOI: 10.1039/c2dt30325f
It is well known that no size-dependent quantization effect is
found for lanthanide ion doped phosphors, and surface states of
8660 | Dalton Trans., 2012, 41, 8660–8668
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nanoparticles often act as non-radiative centers and have great
influence on luminescence properties.24 In this work, we have
synthesized YF3 submicrospindles by a facile hydrothermal
method and studied the energy transfer between Ce3+ and Tb3+
and the cathodoluminescence properties of YF3:Ce3+, Yb3+
microspindles. In particular, we investigated the quantum cutting
process in YF3:Pr3+ under the excitation of low-voltage electron
beams.
electron diffraction (SAED) were obtained from a FEI Tecnai G2
S-Twin transmission electron microscope with a field emission
gun operating at 200 kV. The photoluminescence (PL) measure-
ments were performed on a Hitachi F-7000 spectrophotometer
equipped with a 150 W xenon lamp as the excitation source. The
cathodoluminescence (CL) measurements were carried out in an
ultrahigh-vacuum chamber (<10−8 Torr), where the samples
were excited by electron beams at a voltage range of 0.5–3 kV
with different filament currents. The emission spectra were
recorded using a F-7000 spectrophotometer. The luminescence
decay curves were obtained from a Lecroy Wave Runner 6100
digital oscilloscope (1 GHz) using a tunable laser (pulse width =
4 ns, gate = 50 ns) as the excitation source (Continuum Suncite
OPO). All the measurements were performed at room tempera-
ture (RT).
2. Experimental
2.1 Materials and chemicals
The lanthanide oxides Y2O3 (99.999%), Pr6O11 (99.999%),
Tb4O7 (99.999%), La2O3 (99.999%) and Ce(NO3)3·6H2O
(99.99%) were purchased from Changchun Applied Chemistry
Science and Technology Limited, China. Sodium fluoroborate
(NaBF4), trisodium citrate (C6H5O7Na3·2H2O) and hydrochloric
acid (HCl) were purchased from the Beijing Chemical Company.
All chemicals were analytical grade reagents and used without
further purification. The lanthanide chloride stock solutions of
0.2 M were prepared by dissolving the corresponding metal
oxide in hydrochloric acid at elevated temperatures.
3. Results and discussions
3.1 Morphology and microstructure of the products
In the hydrothermal process, we found the pH value of the initial
solution had a great influence on the microstructure and morpho-
logy of YF3:0.02Ce3+. Fig. 1 and 2 show the representative XRD
patterns and SEM images of the as-obtained YF3:0.02Ce3+
samples prepared at different pH values of the initial solution,
which can be adjusted with the dilute HCl solution (1 M).
When no HCl solution was added, the pH of the initial solution
was 5. The XRD pattern shows that the products are composed
of mixed crystal phases of Na5Y9F32 and Na(Y1.5Na0.5)F6
(Fig. 1a). The products have a microspheric shape with an
average diameter of 240 nm, as presented in Fig. 2a. Further-
more, the surfaces of the microspheres are very coarse. When the
pH value of the solution was adjusted to 4, the obtained products
were pure YF3 phase (Fig. 1b) and took the shape of an irregular
truncated octahedron. The surfaces were smoother but the
average crystal size was larger (Fig. 2b). This truncated
2.2 Preparation
In a typical procedure for the preparation of YF3:Ln3+ (Ln = Ce,
Tb, Pr) microspindles, stoichiometric YCl3 and LnCl3 (Ln = Ce,
Tb, Pr) mixed solution (5 mL, 0.2 M) was added to 10 mL of
aqueous solution containing 1 mmol C6H5O7Na3·2H2O. After
vigorous stirring for 30 min, 15 mL of aqueous solution contain-
ing 12.5 mmol of NaBF4 was introduced into the solution.
The pH of the initial solution was adjusted to 1 with diluted HCl
(1 M). After additional agitation for 15 min, the as-obtained
transparent solution was transferred into a Teflon bottle held in a
stainless steel autoclave, sealed, and maintained at 180 °C for
24 h. The autoclave cooled to room temperature naturally,
the precipitates were then separated by centrifugation, washed
with ethanol and deionized water in sequence, and dried in air
at 80 °C for 12 h. Finally, YF3:Ln3+ (Ln = Ce, Tb, Pr) micro-
spindles were obtained. The reaction yield was about 88%, indi-
cating that the hydrolysis of tetrafluoroborate anions can generate
a sufficient amount of F− ions. LaF3:0.001Pr3+ samples were
synthesized by the same procedures. Additionally, different reac-
tion parameters were selected to investigate the morphological
evolution process of YF3.
2.3 Characterization
The X-ray diffraction (XRD) measurements were carried out on
a D8 Focus diffractometer (Bruker) with Cu-Kα radiation (λ =
0.15405 nm). Fourier transform infrared spectroscopy (FTIR)
spectra were measured with a Perkin-Elmer 580B infrared
spectrophotometer using the KBr pellet technique. The mor-
phology of the samples was inspected using a scanning
electron microscope (SEM, S-4800, Hitachi). Transmission elec-
tron microscopy (TEM) images, high-resolution transmission
electron microscopy (HRTEM) micrographs and selected area
Fig. 1 XRD patterns of the synthesized samples, with the pH values of
the initial solution pH = 5 [a, Na5Y9F32 and Na(Y1.5Na0.5)F6 mixture],
pH = 4 (b, YF3:0.02Ce3+), pH = 3 (c, YF3:0.02Ce3+), and pH = 1
(d, YF3:0.02Ce3+), and the standard data (JCPDS No. 32-1431) for YF3
as reference.
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Fig. 3 TEM (a) and HETEM (b) images of YF3:0.02Ce3+ microspin-
dles. Inset (a) shows the SAED of the microspindles.
Fig. 2 SEM images of the synthesized samples, with the pH values of
the initial solution pH = 5 [a, Na5Y9F32 and Na(Y1.5Na0.5) F6 mixture],
pH = 4 (b, YF3:0.02Ce3+), pH = 3 (c, YF3:0.02Ce3+), and pH = 1
(d, YF3:0.02Ce3+) (scale bar is 500 nm).
octahedron has been synthesized by Tao et al. using
Na2H2EDTA (ethylenediaminetetraacetic acid disodium salt) as a
capping reagent.25 This result indicates that changing the
pH value of the initial solution is essential for forming the YF3
phase. As we know, the hydrolysis of NaBF4 produces many dis-
sociated F− ions:26
BF4ꢀ þ 3H2O Ð H3BO3 þ 3HF þ Fꢀ
ð1Þ
The release of F− was favorable when the pH was high and
higher pH point to a higher concentration of F− in the solution.
Thus YF3 with a smaller F : Y ratio [compared with Na5Y9F32
and Na(Y1.5Na0.5) F6] could be obtained at low pH. As the
pH decreased to 3, the products consist of spindle-like micro-
crystals and a few truncated octahedrons (Fig. 2c). Only micro-
spindles are formed with continued decreases in the pH value to
1 (Fig. 2d). The average length and diameter of the micro-
spindles at pH = 1 are 760 nm and 230 nm, respectively. From
the SEM images and corresponding XRD patterns, we found
that the nanoparticles that form the microspindles get smaller as
the pH value of the initial solution decreases from 4 to 1. With
regard to the XRD analysis, the first and the fifth peaks in the
diffractograms [(011) and (210) reflections] present lower relative
intensities than those expected in comparison to the standard.
Moreover, this difference seems to be more pronounced when
the pH is high, which is easily visualized in the (210) peaks of
Fig. 1. This indicates that crystal growth in the (011) and (210)
directions is not favorable.
Fig. 4 SEM images of YF3:0.02Ce3+ samples, the amounts of the tri-
sodium citrate are 0 mmol (a), 0.25 mmol (b), 0.5 mmol (c), and
1.25 mmol (d) (scale bar is 500 nm).
crystallization of YF3:0.02Ce3+ crystalline grain. The distance
(d = 0.3704 nm) between the adjacent lattice fringes corresponds
to the interplanar distance of YF3 (011) planes, agreeing
well with the d (011) spacing of the literature value (JCPDS
No. 32-1431).
FTIR spectroscopy of the YF3:0.02Ce3+ microspindles in
Fig. 2d was performed (Fig. S1,† ESI). It can be seen that
although the as-prepared sample was washed several times with
water and ethanol, there were still some organic molecules (tri-
9−
sodium citrate, Cit3−) and polyborate group B3O9 on the
surface of the samples.26,27
The effect of Cit3− on the morphology of the products was
investigated further. Fig. 4 shows SEM images of the particles
obtained with different amounts of Cit3−. When no Cit3− was
added to the initial solution, there were a lot of irregular aggre-
Fig. 3 shows TEM and HRTEM images of the YF3:0.02Ce3+
microspindles from Fig. 2d. In Fig. 3a, the microspindles are
composed of closely linked nanoparticles. The SAED pattern in
the inset of Fig. 3a confirms that the microspindles are consti-
tuted of polycrystalline grains. Moreover, some grains compos-
ing the microspindles are slightly elongated and this might be
related to the unfavourable growth in the (011) and (210) direc-
tions (Fig. 1). Fig. 3b illustrates the HRTEM image of a single
crystalline grain in a YF3:0.02Ce3+ microspindle. Well-resolved
lattice fringes can be observed, which indicates the excellent
gated nanoparticles (Fig. 4a). With increasing amounts of Cit3−
,
these nanoparticles self-assembled into microspindles. Moreover,
the more Cit3− that was added, the more tightly the nanoparticles
connected to each other, as shown in Fig. 4d. These results indi-
cate that adding Cit3− is crucial to the formation of
microspindles.
To gain a better understanding of the evolution mechanism of
YF3 microspindles, we conducted a series of detailed time-
dependent experiments to obtain reaction intermediates. SEM
8662 | Dalton Trans., 2012, 41, 8660–8668
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Fig. 7 The (111) diffraction peaks of YF3:xCe3+ (x = 0, 0.01, 0.07,
0.10) microspindles.
Fig. 5 SEM images of YF3 microspindles obtained by a hydrothermal
route at pH = 1 for 15 min (a), 30 min (b), 1 h (c), and 3 h (d) in the
presence of citrate (scale bar is 500 nm).
size of crystallite became larger.28 The possible formation
mechanism for the YF3 microspindles can be addressed as
follows. First, the nuclei crystallite formed quickly and aggre-
gated to spindle-like microcrystals because of the effect of Cit3−
.
Next, as the time extended, the crystallites grow larger. This
mechanism is little different from that of YF3 nanospindles using
EDTA as a capping agent.29
In Fig. 7, with an increasing concentration of Ce3+, the posi-
tion of (111) diffraction peak of YF3:xCe3+ (x = 0, 0.01, 0.07,
0.10) microspindles shifts to lower angles. Moreover, the inten-
sity decreases and the FWHM increases. The slight shift of the
diffraction peak is attributed to the lattice expansion from the
larger ionic radius of doped Ce3+ with respect to that of Y3+.
In the orthorhombic structure with the Pnma space group, each
yttrium ion has eight fluorine neighbors at about 2.3 Å and
another at 2.6 Å.30 The ionic radii of Y3+, Ce3+, Pr3+ and Tb3+
ions in the nine-coordination site are 1.075, 1.196, 1.179,
1.095 Å, respectively.31 The changes in the intensity and the
FWHM of the diffraction peak indicate the crystallite size
decreases with increasing dopant concentration. All the results
indicate that the Ce3+ ion can be effectively doped into a YF3
lattice.
Fig. 6 XRD patterns of YF3 microspindles obtained by hydrothermal
route at pH = 1 for 15 min (a), 30 min (b), 1 h (c), and 3 h (d) in the
presence of citrate.
images of the corresponding intermediates prepared at the same
pH value (pH = 1) are shown in Fig. 5. All of the products were
obtained in the presence of citrate (1 mmol). At the beginning,
the solution turned clear after NaBF4 solution and dilute acid
3.2 Luminescence properties
3.2.1 Photoluminescence properties. Fig. 8a shows the exci-
tation (λem = 310 nm, left) and emission (λex = 255 nm, right)
spectra of the YF3:0.02Ce3+ microspindles obtained at pH = 1
for 24 h. Monitored with an emission wavelength of 310 nm, the
obtained excitation spectrum consists of a broad band with a
maximum at 255 nm and three shoulder bands. The emission
spectrum includes a broad band ranging from 275 nm to
375 nm, and peaking at 298 nm. To probe the energy structure of
the Ce3+ 5d and 4f levels in YF3:0.02Ce3+, the excitation and
emission spectra were analyzed by peak deconvolution into
Gaussian components. The excitation spectra consist of four
broad bands with maximum at about 203 nm, 215 nm, 235 nm
and 255 nm, which can be assigned to the transitions from the
was added into the mixed solution containing LnCl3 and Cit3−
.
After 15 min reaction in the oven, spindle-like structures formed
with a homogeneous shape, as shown in Fig. 5a. The mean par-
ticle size of the crystallite nanoparticle was 20 nm, as calculated
from the XRD pattern (Fig. 6). It can be deduced that the micro-
spindles are composed of nanoparticles. As the reaction time was
extended to 1 and 3 h, there were no further changes in mor-
phology, except that the surface of the microspindles became
rougher, which was caused by the growth of the crystalline
grain, as shown in Fig. 5c and 5d, respectively. This was also
affirmed by the XRD patterns. As shown in Fig. 6, the full width
at half maximum (FWHM) of the products obtained at different
times decreases with increases in the time, indicating that the
2
ground state F5/2 of Ce3+ to the different components of the
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The other lines in the longer wavelength region are character-
istic 4f → 4f transition lines within the Tb3+ (4f)8 configuration,
assigned as the transitions from the 7F6 ground state to the differ-
ent excited states of Tb3+, that is, 274 nm (5I7), 284 nm (5F4),
304 nm (5H6), 319 nm (5D0), 327 nm (5D1), 341 nm (5L7),
352 nm (5D2), 368 nm (5L10), and 378 nm (5D3), respectively.37
Upon excitation into the spin-allowed 4f → 5d transition at
201 nm, the obtained emission spectrum of YF3:0.02Tb3+ con-
sists of 4f → 4f transition lines within (4f)8 electron configur-
5
7
5
7
ation of Tb3+, i.e., D4 → F6 (491 nm), D4 → F5 (544 nm)
5
7
and D4 → F4 (588 nm). As shown in Fig. 8, there is spectral
overlap between the Ce3+ emission and Tb3+ absorption in the
range of 275–380 nm. Therefore, it is expected that an efficient
energy transfer can occur from Ce3+ to Tb3+.
The YF3:0.02Ce3+ microspindles doped with different amount
of Tb3+ ion show a strong green emission under UV excitation.
Fig. 9a gives the PL spectra of YF3:0.02Ce3+, xTb3+ micro-
spindles. Excitation at 255 nm into the Ce3+ absorption band
yields emission of Ce3+ (275–375 nm) and emission of Tb3+
(450–700 nm). Moreover, the intensities of the Tb3+ emission
increase and the intensities of Ce3+ emission decrease with
increases in the concentration of Tb3+ ions. This indicates that an
energy transfer from Ce3+ to Tb3+ occurs in the YF3:0.02Ce3+,
xTb3+ microspindles. The excitation spectra of the Tb3+ emission
at 545 nm were also presented in Fig. 9a. The excitation peaks
can be assigned to the transitions of Ce3+, and the intensities of
the excitation peaks increase with increases in the concentration
of Tb3+ ion.
To gain further insight in the energy transfer processes
between Ce3+ and Tb3+, luminescence decay curves were
recorded for the Ce3+ emission upon excitation to the 5d level of
Ce3+ at 285 nm (Fig. 9b). The lifetimes of Ce3+ ions decrease
when the YF3:0.02Ce3+ microspindles are codoped with Tb3+.
This shows that energy transfer from Ce3+ to Tb3+ occurs and
the transfer rate increases as the Tb3+ concentration is raised.
The non-exponential character of the decay curves reflects the
different distributions of Tb3+ ions around different Ce3+ ions,
which gives rise to a wide distribution of transfer rates between
Ce3+ and Tb3+ ions.38 Owing to the decays depart from single
exponential function, so we define an average fluorescence life-
time of Ce3+ as
Fig. 8 PL excitation and emission spectra of YF3:0.02Ce3+ microspin-
dles (a), and YF3:0.02Tb3+ microspindles (b). The inset shows the PL
emission intensity of Ce3+ as a function of its concentration.
excited 5d states of Ce3+ split by the crystal field.32 The asym-
metric band with a maximum at about 298 nm is fitted as the
sum of two components, corresponding to the transitions from
2
2
the lowest Ce3+ 5d excited state to the F5/2 and F7/2 spin–orbit
components of the Ce3+ 4f configuration, respectively.33 The
magnitude of the spin–orbit splitting is determined to be
1860 cm−1 from the separation between the fitted peak positions
ð1
τ ¼
IðtÞdt
ð2Þ
2
of 294 and 311 nm. The Stokes shift of the emission to the F5/2
0
state is 5202 cm−1, which is relatively large for Ce3+ compounds.
The inset shows that the intensities of the Ce3+ emission increase
with the dopant concentration. In our work, no quenching of
Ce3+ emission was observed. As the literature reported, this is
because CeF3 is a luminescent material with 100% activator
concentration.34
where I(t) is the fluorescence intensity at time t with normalized
initial intensity.39 Inset of Fig. 9b shows the lifetimes of Ce3+
calculated by eqn (1).
The ET efficiency (η) can be calculated by
τ
η ¼ 1 ꢀ
ð3Þ
Fig. 8b shows the excitation (λem = 544 nm, left) and emission
(λex = 201 nm, right) spectra of the YF3:0.02Tb3+ microspindles.
The excitation spectrum monitored at 544 nm emission of Tb3+
(5D4 → 7F5) consists of several components. It has been reported
that the first spin-allowed 4f → 5d transition is centered at about
190 nm, so the band edge at 201 nm is attributed to this tran-
sition.35 The two bands, one centered at 215 nm and the other in
the range of 220–270 nm with maximum at 255 nm, stem from
spin-forbidden 4f → 5d transitions within the Tb3+ ion.35,36
τ0
where τ and τ0 are the corresponding fluorescence lifetime of
Ce3+ in the presence and absence of Tb3+ for the same Ce3+ con-
centration, respectively.16,40 Thus the energy transfer efficiencies
from Ce3+ to Tb3+ in YF3:0.02Ce3+, xTb3+ microspindles are
determined, as shown in Fig. 9c. The increase in energy transfer
efficiency from Ce3+ to Tb3+ with Tb3+ concentration is due to
the decrease in the average distance between Ce3+ and Tb3+.
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Fig. 9 PL excitation and emission spectra of YF3:0.02Ce3+, xTb3+ (x = 0–0.17) (a), decay curves for Ce3+ in YF3:0.02Ce3+, xTb3+ (b), the calculated
ET efficiency from Ce3+ to Tb3+ (c), and the decay curve for 5D4 → 7F5 transition of Tb3+ in YF3:0.02Ce3+, xTb3+ (d). The insets of (b) and (d) show
the lifetimes of Ce3+ and Tb3+ as a function of the Tb3+ concentration in YF3:0.02Ce3+, xTb3+, respectively.
The decay curves of Tb3+ fluorescence are presented in
Fig. 9d, monitoring at 545 nm and upon 285 nm pulsed exci-
tation. The decay curves for the luminescence of Tb3+ can be
fitted by the single exponential function I = I0 exp(−t/τ), and the
where η0 and η are the luminescence quantum efficiencies of
Ce3+ in the absence and presence of Tb3+, respectively; the
values of η0/η can be approximately calculated by the ratio of
related luminescence intensities (IS0/IS), C is the total dopant
concentration of Ce3+ and Tb3+, and n = 6 and 8 correspond to
dipole–dipole and dipole–quadrupole interactions, respectively.
The IS0/IS − Cn/3 plots are further illustrated in Fig. 10a and 10b,
and they all show a linear relation when n = 6 and 8, indicating
that the energy transfer from the Ce3+ to the Tb3+ ions may
occur via the dipole–dipole or the dipole–quadrupole
mechanism.
To further confirm the dominating ET mechanism, we calcu-
lated the critical distance. The critical distance is defined as the
distance for which the transfer probability between the sensitizer
and activator equals the emission probability of the sensitizer.
Considering the dipole–dipole interaction, the critical distance
from a sensitizer to an acceptor is given by the spectral overlap
method. Hence, Rc can be obtained from the following simplified
formula:43
5
fluorescence lifetimes for the D4 state of Tb3+ in YF3:0.02Ce3+,
xTb3+ are determined, as shown in the inset of Fig. 9d. The life-
times of the 5D4 state decrease with increases in the concen-
tration of Tb3+. This is because of the cross-relaxation between
Tb3+ ions, (5D3, 7F6) → (5D4, 7F0).41
The energy transfer between lanthanide ions takes place by
means of an electric multipolar interaction. The transition of the
2
2
Ce3+ ion from the 5d excited state to the F5/2 and F7/2 ground
states are pure electric dipolar transitions, but the 4f → 4f tran-
sitions of Tb3+ ion simultaneously have dipolar and quadrupolar
characters. Therefore, both dipole–dipole and dipole–quadrupole
mechanisms may be involved in the energy transfer from the
Ce3+ to Tb3+ ions. On the basis of Dexter’s energy transfer
formula of multipolar interaction and Reisfeld’s approximation,
the following relation can be obtained:42
ð
η0
FSðEÞFAðEÞ
/ Cn=3
ð4Þ
R6c ¼ 3:024 ꢁ 1012fd
dE
ð5Þ
E4
η
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Fig. 10 Dependence of Is0/Is of Ce3+ on (a) C6/3 and (b) C8/3
.
where fd (≈ 10−6) is the electric dipole oscillator strength of the
Tb3+ ions,40 ∫FS(E)FA(E)dE/E4 represents the overlap between
the normalized shapes of Ce3+ ions emission FS(E) and Tb3+
ions excitation FA(E), and it is calculated to be about
0.00154 eV−5. The critical distance Rc was calculated to be
about 4.1 Å. This value is almost the critical distance of
exchange interaction (3–4 Å), so the electric dipole–dipole
interaction can be excluded. Therefore, we consider the electric
dipole–quadrupole interaction as a main mechanism for this ET
process. From the view of dipole–quadrupole interaction, Rc can
be calculated using the following equation:44
ð
FSðEÞFAðEÞ
Fig. 11 The CL spectra of YF3:0.02Ce3+, 0.14Tb3+ and commercial
phosphor ZnO:Zn (a). The inset shows the integrated intensity of
YF3:0.02Ce3+, xTb3+ (x = 0.02, 0.05, 0.08, 0.11, 0.14, and 0.17) and
ZnO:Zn (point A). The CL spectrum of YF3:0.001Pr3 (b). The inset
R8c ¼ 3:024 ꢁ 1012λS2fq
dE
ð6Þ
E4
here fq is the oscillator strength of the Tb3+ quadrupole transition.
The ratio fq/fd is about 10−3–10−2 when fd is applied to a forbid-
den dipole transition. λS is the emission wavelength of Ce3+.
By using the above values we calculate that the critical distance
Rc for a dipole–quadrupole interaction mechanism is 9.0–12.0 Å.
shows the CL spectrum of LaF3:0.001Pr3+
.
seen that the intensity (in height) of YF3:0.02Ce3+, 0.14Tb3+ is
higher than that of ZnO:Zn. However, the integrated area is
smaller than ZnO:Zn. The CL efficiency of a luminescent
material includes the radiation efficiency (η) and the luminous
efficiency (L, brightness).45 η and L can be compared roughly by
the emission peak areas and the CL intensity (in height) of
YF3:0.02Ce3+, 0.14Tb3+ and ZnO:Zn. It can be concluded that η
of YF3:0.02Ce3+, 0.14Tb3+ might be smaller than that of ZnO:
Zn, but L of YF3:0.02Ce3+, 0.14Tb3+ might be larger than that
of ZnO:Zn.
3.2.2. Cathodoluminescence properties. In this section, we
discuss the CL properties of YF3:Ce3+, Tb3+ and quantum
cutting process in YF3:Pr3+. For comparative purposes, we syn-
thesized LaF3:Pr3+ nanoparticles and investigated their CL pro-
perties. Under the excitation of low-voltage electron beams
(accelerating voltage = 2.5 kV, filament current = 83 mA), the
YF3:0.02Ce3+, xTb3+ phosphors show green luminescence and
the obtained CL spectrum (Fig. 11a) consists of the characteristic
emission lines of Tb3+: 5D4
→
7F6 (492 nm), 5D4
→
7F5
The CL emission spectrum of YF3:0.001Pr3+ under the exci-
tation of electron beams is shown in Fig. 11b. The strongest
emission corresponds to the 1S0 → 1I6 transition, the same as the
sample under VUV excitation.21 Using the method described in
ref. 46, we can determine that the emission quantum efficiency
of the Pr3+ 3P0 level, η(3P0), is 59%, which is much smaller than
(546 nm), 5D4 7F4 (590 nm) and 5D4
→
→
7F3 (624 nm).
However, the emission band of Ce3+ is absent. This is because
there is an optical waveguide fiber that connects the ultra-high
vacuum chamber and the spectrophotometer and the trans-
mission range of this fiber is 380–2100 nm. The emission band
of Ce3+ located between 275 nm and 375 nm, thus the emission
from Ce3+ cannot be found in the CL spectrum. The CL spec-
trum of commercial phosphor ZnO:Zn under the same excitation
conditions is also shown in Fig. 11a. The inset of Fig. 11a shows
the integrated emission area of YF3:0.02Ce3+, xTb3+ (x = 0.02,
0.05, 0.08, 0.11, 0.14, 0.17) and ZnO:Zn (point A). It can be
3
1
100%. This may be because the transition from P0 to D2 or
1G4 level cannot be observed in our experiment, or because of
3
cross relaxation between Pr3+ ions, such as (3P0, H4) → (1D2,
3
1
3
1
3F2), (3P0, H4) → (3F2, D2), and (3P0, H4) → (1G4, G4). The
CL spectrum shows that the quantum cutting process can also
8666 | Dalton Trans., 2012, 41, 8660–8668
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exist under the excitation of electron beams. Under the electron
beams excitation, the energy transfer process in lanthanide ion
doped phosphors can be described as follows. Firstly, excitons
can be generated under impact ionization of the host lattice
atoms by hot electrons. Then excitons can be trapped by impuri-
ties (lanthanide ions or other impurities), local or extended
defects, recombine and transfer their energy to a nearby lantha-
nide ion.47 In YF3:0.001Pr3+, the recombination energy resonant
with the 1S0 level is transferred non-radiatively to a nearby
1
Pr3+ ion. The electrons in the S0 level can return to the ground
1
state by a two step emission process. The first step is the S0 →
1I6 transition in the blue-violet spectral range around 409 nm.
3
3
3
3
The second step is P0 → H4 (485 nm), P0 → H5 (526 nm),
3P0 → H6, F2 (606 nm) transitions after the electron thermal
3
3
relaxation from 1I6 level to 3P0 level.46
The inset of Fig. 11b shows the CL spectrum of
LaF3:0.001Pr3+ under the same parameter with YF3:0.001Pr3+.
In contrast to YF3:0.001Pr3+, the emissions are all from the
3P0 level. This phenomenon has been observed under the X-ray
excitation.21,46 It has been reported that the luminescence of
LaF3:0.001Pr3+ is dominated by optical transitions originating
1
from the S0 level under the excitation of VUV.48,49 The differ-
ence emission of LaF3:0.001Pr3+ under the excitation of electron
beams and VUV can be explained as follows. Under VUV exci-
Fig. 12 The CIE chromaticity diagram for YF3:0.02Ce3+, 0.14Tb3+
1
tation, the electrons are excited to the S0 level or 5d states of
(1), ZnO:Zn (2), LaF3:0.001Pr3+ (3), and YF3:0.001Pr3+ (4).
Pr3+ ion, and then the two step emission occurs. But in electron
beam excitation, the lanthanide ions cannot be excited directly
by the electron beam. They are excited though the recombination
of excitons, and the host lattice acts as a sensitizer. In YF3, the
triplet and singlet transitions of self-trapped excitons (STE) are
two bands located at 4.4 eV and 5.6 eV, respectively.50,51 The
band located at 5.6 eV has a spectral overlap with the absorption
1
of S0 state (5.8 eV) of Pr3+, and the energy transfer between
1
STE and S0 state is effective. Therefore, quantum cutting emis-
sion can be observed in YF3. However, in LaF3, it was observed
that the emission of STE is a broad band centered near
360 nm.52 The spectral overlap between the recombination emis-
1
sion of STE (5 eV–2.5 eV) and the S0 state of Pr3+ is small or
negligible.21 The energy of STE in LaF3 is resonant with the
1
3PJ (J = 0, 1, 2) and I6 levels of Pr3+ ion. Thus only emissions
3
from the P0 level can be observed in LaF3:0.001Pr3+ under the
excitation of electron beams. The difference between the CL
spectra of YF3:0.001Pr3+ and LaF3:0.001Pr3+ is because of the
different energy of STE produced from electron beam excitation.
In past decades, the quantum cutting process has been investi-
gated under the excitation of VUV and X-rays.53 The quantum
cutting phosphors can produce more than one visible photon per
incident vacuum ultraviolet photon, which can significantly
improve the overall energy conversion of xenon-based fluore-
scent lamps. In the CL process, the phosphors are excited by
secondary electrons with larger energy than UV photons. If one
secondary electron can produce more than one photon, the lumi-
nescent quantum efficiency can be enhanced. In this work, we
observed the quantum cutting process under the excitation of
electron beams, indicating the quantum cutting process may be a
promising method to enhance the luminescent properties of FED
phosphors.
Fig. 13 The CL emission intensity for the YF3:0.02Ce3+, 0.14Tb3+
microspindles as a function of the accelerating voltage (a). The CL emis-
sion intensity for the YF3:0.02Ce3+, 0.14Tb3+ microspindles as a func-
tion of the filament current (b).
0.14Tb3+ and ZnO:Zn, the chromaticity coordinates are deter-
mined to be (0.291, 0.597) and (0.197, 0.447), respectively.
The chromaticity coordinates of YF3:0.02Ce3+, 0.14Tb3+ are
very near to the European Broadcasting Union (EBU)
primary green color (0.29, 0.60). The chromaticity coordinates
of LaF3:0.001Pr3+ and YF3:0.001Pr3+ are (0.198, 0.319) and
(0.190, 0.232), respectively.
The CL emission intensity for the YF3:0.02Ce3+, 0.14Tb3+
microspindles has been investigated as a function of the accele-
rating voltage and the filament current, as shown in part a and b
of Fig. 13, respectively. When the filament current is fixed at
85 mA, the CL intensity strengthens with an increase in the
In our present work, the chromaticity coordinates have been
calculated from the CL spectra (Fig. 12). For YF3:0.02Ce3+,
This journal is © The Royal Society of Chemistry 2012
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View Article Online
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which results in more lanthanide ions being excited and an
increase in the CL intensity.
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4. Conclusions
In summary, YF3:Ln3+ (Ln = Ce, Pr, Tb) microspindles were pre-
pared by a facile hydrothermal method. The energy transfer from
Ce3+ to Tb3+ in YF3 was effective and was demonstrated to be a
resonant type via a dipole–quadrupole mechanism. Under the
excitation of low-voltage electron beams, the YF3:Ce3+, Tb3+
exhibited green emission with good chromaticity coordinates
and YF3:0.001Pr3+ showed the quantum cutting process. Elec-
tron beams can be used to investigate the quantum cutting
process in phosphors. These phosphors may have potential appli-
cations in FED devices.
Acknowledgements
This project is financially supported by the National High Tech-
nology Program of China (2011AA03A407), the National Basic
Research Program of China (2010CB327704), the National
Natural Science Foundation of China (NSFC 20901074,
51172227, 21101149, 20921002), and the Science and Technol-
ogy Development Program of Jilin Province for Youths
(20100106).
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8668 | Dalton Trans., 2012, 41, 8660–8668
This journal is © The Royal Society of Chemistry 2012