Sr2Y8(SiO4)6O2:Bi3+/Eu3+: a single-component white-emitting phosphor via energy transfer for UV w-LEDs

Article abstract of DOI:10.1039/c5tc01993a

A series of Bi³⁺,Eu³⁺-doped Sr₂Y₈(SiO₄)₆O₂ (SYSO) phosphors were synthesized by the Pechini-type sol-gel route. XRD patterns, Rietveld refinement, FT-IR spectra, photoluminescence spectra (PL), fluorescence decay time, and quantum yields (QYs) were utilized to characterize the as-prepared samples. Intense blue-green emission due to the ³P₁ → ¹S₀ transition of Bi³⁺ ions is produced under UV excitation in Bi³⁺ singly doped SYSO samples. Spectral analysis illustrates that two kinds of Bi³⁺ ions are generated by occupying the two kinds of sites (4f and 6h) available for Y in SYSO, denoted as Bi³⁺(1) and Bi³⁺(2), corresponding to the two main emission bands at around 413 and 493 nm, respectively. A wide spectral overlap between Bi³⁺ emission and Eu³⁺ excitation spectra results in the energy transfer from Bi³⁺ to Eu³⁺. This has been demonstrated via the excitation spectra monitored at Bi³⁺ and Eu³⁺ emission bands, and the PL emission spectra and decay time of Bi³⁺ in SYSO:Bi³⁺,Eu³⁺ phosphors. The energy transfer mechanism is determined to be dipole-quadrupole interaction. The critical distance of energy transfer from Bi³⁺ to Eu³⁺ ions is calculated to be 12.78 ? based on the concentration quenching method, which is in good agreement with that of the spectral overlap route (10.03-13.37 ?). Moreover, the QYs, CIE chromaticity and thermal quenching properties have also been investigated. White emission color can be realized with the CIE coordinates (0.325, 0.311) and QY 52% for SYSO:0.08Bi³⁺,0.48Eu³⁺. The above results suggest that SYSO:Bi³⁺,Eu³⁺ can be a potential single-component white-emitting phosphor for UV w-LEDs.

Full text of DOI:10.1039/c5tc01993a

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Journal of
Materials Chemistry C
ARTICLE
3
+
3+
Sr Y (SiO ) O :Bi /Eu : a single-component white- emitting
2
8
4 6
2
phosphor via energy transfer for UV w-LEDs
Received 00th January 20xx,
Accepted 00th January 20xx
ab
a
a
a
a
Kai Li, Jian Fan, Mengmeng Shang, Hongzhou Lian* and Jun Lin*
DOI: 10.1039/x0xx00000x
3+
3+
A series of Bi , Eu -doped Sr
2
Y
8
(SiO
)
4 6
O
2
(SYSO) phosphors were synthesized by the Pechini-type sol-gel route. XRD
patterns and Rietveld refinement, FT-IR spectra, photoluminescence spectra (PL), fluorescent decay times, quantum yields
www.rsc.org/
3
1
(
QYs) were utilized to characterize the as-prepared samples. Intense blue-green emission due to the P
1
→ S
0
transition of
3
+
3+
Bi ions is produced under UV excitation in Bi singly-doped SYSO samples. Spectral analysis illustrates that two kinds of
3
+
3+
3+
Bi are generated by occupying the two kinds of sites (4f and 6h) available for Y in SYSO, denoted as Bi (1) and Bi (2),
3
+
corresponding to the two main emission bands around 413 and 493 nm, respectively. Wide spectral overlap between Bi
3
+
3+
3+
emission and Eu excitation spectra results in the energy transfer from Bi to Eu . This has been demonstrated via the
3
+
3+
3+
excitation spectra monitored at Bi and Eu emission bands, and the PL emission spectra and decay times of Bi in SYSO:
3+
3+
Bi , Eu phosphors. The energy transfer mechanism is determined to be dipole-quadrupole interaction. The critical
3
+
3+
distance of energy transfer from Bi to Eu ions is calculated to be 12.78 Å based on the concentration quenching
method, which is in good agreement with that of spectral overlap route (10.03-13.37 Å). Moreover, the QYs, CIE
chromaticity and thermal quenching properties have also been investigated. White emission color can be realized with the
3
+
3+
3+
3+
CIE coordinates (0.325, 0.311) and QY 52% for SYSO:0.08Bi , 0.48Eu . The above results suggest that SYSO:Bi , Eu can
be of potential as a single-component white-emitting phosphor for UV w-LEDs.
expenses. Besides, different attenuation situations of each
phosphor will give an unstable white light. Therefore, w-LEDs
are proposed to be fabricated by a UV/n-UV LED chip coated
3
1
. Introduction
Recently, white-light-emitting diodes (w-LEDs),
a next-
with a single-phase white-light-emitting phosphor, which has
been an active research topic in solid state lighting field. It is
well accepted that energy transfer from sensitizers to
acceptors in certain host can play an important role to
generate the tunable emission color including white. A large
number of single-phase white-light-emitting phosphors
suitable for UV-pumped w-LEDs have been reported, most of
generation of illumination source, have been drawing
worldwide attention due to their superior advantages such as
low power consumption, long operation lifetime, good stability
in physical and chemical properties and especial mercury-free
properties, which are considered to be the excellent
candidates to replace the current conventional fluorescent and
incandescent lamps. It is well known that the most effective
white lighting device can be realized by combining a blue-
emitting LED chip with yellow-emitting YAG:Ce (excited by
blue light) phosphors. This kind of phosphor-converted w-LED
fabrication involves some advantages such as low cost, a
1
2+
2+
3+
2+
2+
3+
2+
which are based on Eu -Mn , Ce -Mn , Eu -Tb -Mn ,
Ce -Tb -Mn and Ce -Dy systems because the Eu and
3+
3+
2+
3+
3+
4
2+
3
+
3+
Ce have the high emission intensity and broad band spectra
originated from their 4f-5d parity allowed electric dipole
transitions, and can effectively transfer their energy to
activators to enhance the emission of coactivators. However,
2
single-chip fabrication process, and circuit simplicity.
Regretfully, this method exhibits a poor color-rendering index
5
2+
2+
the red component from Mn 3d-3d transition in Mn -
contained white-emitting systems often presents wide
emission range, therefore, the color purity of red section is not
satisfactory, which may be partly overlapped by some blue and
(CRI) and a high correlated color temperature (CCT) attributed
to the deficiency of red component for warm white light,
which constricts its application for indoor lighting. Another
useful way is suggested to improve this issue by employing
near-ultraviolet (n-UV)/UV LED chips with red, green, and blue
phosphors. However, there exist the problems of trade-off
luminescent efficiency (originating from the reabsorption of
blue-emission by the green and red phosphors) and high
3+
green parts. As we know, Eu can yield dominant red emission
5
7
line due to its electric dipole
deviates from the inversion center of the crystal lattice. Some
0 2
D → F transition when it
3+
efforts have been done to enhance red emission of Eu based
3+
on the energy transfer effect from Bi to Eu ions. However,
3+
6
3
Bi can emit the blue-green band emission around 400-550
+
2
nm attributed to its 6s6p→6s transition, which also can be
expected to achieve white light emission when combined with
3
+
5
7
0 2
Eu red emission (mainly due to their D → F transition) in
single-component phosphor, just like the effect of above
systems. Nevertheless, few works have been devoted to this,
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C5TC01993A
ARTICLE
Journal Name
which gives us an inspiration to explore the luminescence constant stirring and heating to form a colorless solution of
3
+
3+
properties on the Bi /Eu co-doped phosphors.
Y(NO
Y(NO
3
)
)
3
and Eu(NO
3
)
3
, respectively. Stoichiometric amounts of
and Bi(NO ·5H O were dissolved
Silicate have been extensively investigated as the hosts for
rare earth ions incorporation because of their diverse crystal
structures and chemical and physical stabilities originated from
their strong and rigid frameworks with covalent Si–O bonds
which can provide a relatively distorted and complex
3
3
, Eu(NO ) , Sr(NO
3
3
3
)
2
3
)
3
2
in deionized water, with stirring for 10 min, after which the
citric acid with the fixed stoichiometric value [2:1 (mol/mol)
citric acid/metal ion] was added. Then, the pH of the solution
7
was adjusted to
stoichiometric amount of tetraethyl orthosilicate [Si(OC
dissolved in ethanol after 20 min stirring. At last, a certain
3
∼2 with HNO followed by the addition of a
coordination environment around the cations. Apatite-related
silicate compounds, a large family of silicates, have been used
as the hosts for phosphors because they can show good
2 5 4
H ) ]
photoluminescence (PL) properties when doped with rare amount of poly(ethylene glycol) (PEG; molecular weight =
earth (RE) ions for solid state lighting. These oxy-apatite host 20000, analytical reagant grade) was appended as a cross-
8
lattices consist of two cationic sites, that is, seven-fold linking agent (CPEG = 0.005 mol/L) after stirring for about 15
s
coordinated 6h sites with C point symmetry and the nine-fold
coordinated 4f sites with C point symmetry. Both two sites
3
min. The final mixtures were stirred for 2 h and then heated at
5 °C with a water bath to form homogeneous gels. Then, the
7
are demonstrated to be suitable and easily accommodated a
obtained gels were prefired at 600 °C for 4 h in air followed by
fully grinding and calcination at 1350 °C for 6h in a muffle
furnace to generate the final powder materials.
3
+
9
10
great variety of RE ions. Recently, Zhou etc. have
developed new single-phased silicate phosphor
MgY Si 10:Eu /Bi , showing tunable emission color
containing white light based on the energy transfer (ET) from
a
3
+
3+
2
3
O
3
+
3+
11
2.2 Measurement and Characterization.
Bi to Eu . In 2010, Shen etc. have reported the crystal
3
+
All the measurements were performed using the finely ground
powder. D8 Focus diffractometer at a scanning rate of
structure and PL properties of Eu -doped Sr
2
Y
8
(SiO
which a large amount of Eu can be introduced into the
Sr (SiO
belongs to the apatite style with the space group of P6
4 6 2
) O , in
3
+
−
1
(SYSO) compound. The structure of SYSO 10°min in the 2θ range from 10° to 110° with graphite-
/m, monochromatized Cu K radiation (λ = 0.15405 nm) was used
2
Y
8
4 6 2
) O
3
α
where 6h site are completely occupied by Y2 with seven-fold to check the phase purity of samples. Infrared spectra were
coordination, while other Sr1 and Y1 are randomly distributed
obtained on a VERTEX 70 Fourier transform infrared (FT-IR)
2
+
3+
2 8 4 6 2
in 4f site with nine-fold coordination. Sr Y (SiO ) O :Eu ,Eu
spectrometer (Bruker). The excitation source from
a
phosphor synthesized and studied by J. Sokolnick etc.
produced white light based on the two different sites of Y in
fluorescence spectrophotometer equipped with a 450 W
xenon lamp (Edinburgh Instruments FLSP-920) was used to
carry out the photoluminescence (PL) measurements. The
luminescence decay lifetimes were measured and obtained
from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz)
using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the
1
2
3+
this host. As is described above, the introduction of Bi into
silicate such as SYSO here with two different crystallographic
sites may be also highly expected to produce two different
emission bands. They can overlap the excitation spectrum of
3
+
3+
Eu and then sensitize it to enhance its emission when Bi
3
+
and Eu are co-doped into SYSO compound. Herein, we have excitation (Contimuum Sunlite OPO) source. PL quantum yields
successfully synthesized the SYSO:Bi , Eu phosphor via the (QYs) of as-prepared phosphors were obtained directly by the
Pechini-type sol-gel method. As a new single-component absolute PL QY measurement system (C9920-02, Hamamatsu
3
+
3+
3
+
3+
tunable emission phosphor SYSO:Bi , Eu , the crystal phase,
PL properties and Commission Internationale de L’Eclairage
Photonics K. K., Japan). All the measurements above were
performed at room temperature (RT) condition. What’s more,
the temperature-dependent (298-523 K) PL spectra were
(CIE) chromaticity coordinates and quantum yields have been
investigated in detail. This phosphor not only shows
3
characteristic Bi emission in blue-green area, but also the ET
+
measured on Edinburgh Instruments FLSP-920 with
temperature controller.
a
3
from Bi to Eu ions. Additionally, the ET mechanism and
+
3+
3
+
3+
critical distance from Bi to Eu ions have been discussed in
detail as well.
3
. Results and Discussion
The phase composition and purity have been examined by
GASA Rietveld refinements at room temperature for the
2
. Experimental
3
representative SYSO:0.08Bi (Fig. 1a) and SYSO:0.08Bi ,
+
3+
2.1 Materials and Synthetic procedures
3
+
Y2O3 (99.99%) and Eu O3 (99.99%) were purchased from 0.40Eu (Fig. 1b) samples. The original structure model and
2
Science and Technology Parent Company of Changchun crystallographic data for the refinement were referred to
Institute of Applied Chemistry, and other chemicals involved Sr Nd (SiO ) O (ICSD 155627) compound. All the atomic
2 8 4 6 2
were purchased from Beijing Chemical Company. All chemicals positions, fraction factors, and thermal vibrational parameters
were of analytical grade and used directly without further were converged and refined. We can observe that the
purification.
p
weighted profile R-factor (Rwp) and the expected R factor (R )
3
are 5.67% and 3.81% for SYSO:0.08Bi , and 6.77% and 4.12%
+
A series of objective samples Sr Y (SiO ) O (SYSO) and Sr Y
2 8-
2
8
4 6
2
3
+
3+
3+
3+
x-y(SiO ) O :xBi , yEu were prepared by the Pechini-type sol- for SYSO:0.08Bi , 0.40Eu
samples, respectively. This
gel route. The doping concentration of Bi and Eu are x = indicates the reflection conditions are well satisfied. Detailed
.01-0.40 and y = 0.04-0.56 in SYSO, respectively. First, Y O
information for the results is listed in Table 1. In addition, this
4 6
2
1
3
3+
3+
0
2
3
and Eu O were dissolved in dilute nitric acid (HNO ) under result confirms that the structure of SYSO is isostructural with
2
3
3
2
| J. Mater. Chem. C, 2015, 00, 1-10
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ARTICLE
(a)
Raw data
Calculated line
Background line
Differences
(a)
(b)
Rwp = 5.67%
Bragg positions
R
= 3.81%
= 4.517
p
2
χ
Fig. 2 Crystal structure of SYSO (a) and coordination
20
40
60
80
100
environment for Y1 and Y2 (b).
2θ
(degree)
3
+
3+
(b)
Raw data
Calculated line
SYSO:0.08Bi , 0.40Eu
Background line
Differences
Bragg positions
3+
SYSO:0.40Eu
Rwp = 6.77%
Rp = 4.12%
3+
SYSO:0.08Bi
2
χ
= 6.484
SYSO
ICSD 155627 Sr Nd (SiO ) O
2
2
8
4
6
2
0
40
60
80
100
2
0
30
40
50
60
2θ
(degree)
2
θ
(degree)
3
+
Fig.
SYSO:0.08Bi , 0.40Eu (b) samples.
1
Rietveld refinements for SYSO:0.08Bi
(a) and
3
Fig. 3 XRD patterns for SYSO host and SYSO: 0.08Bi , SYSO:
+
3
+
3+
3
+
3+
3+
.40Eu , and SYSO:0.08Bi , 0.40Eu samples, as well as the
0
standard pattern of Sr Nd (SiO ) O (ICSD 155627).
2
8
4 6 2
Sr Nd (SiO ) O . Therefore, SYSO crystallizes in hexagonal
4 6 2
2
8
space group P63
/
m
with two kinds of Y atom sites where 6h
3+
Eu -doped SYSO phosphors shift to higher degree relative to
Sr Nd (SiO ) O obviously. This is because the ionic radius of
3+
site are completely occupied by Y1 with seven-fold
coordination, while other Sr and Y are randomly distributed in
2
8
4 6 2
Y
[0.96 Å for coordination number (CN=7) and 1.10 Å for
4f
site with nine-fold coordination, as depicted in Fig. 2a and b,
3+
CN=9] is smaller than that of Nd (1.06 Å for CN=7 and 1.19 Å
for CN=9). However, their profiles are consistent with each
other, indicating that all the patterns are well assigned to pure
respectively. The XRD patterns for as-prepared SYSO host and
3
+
3+
Bi , Eu -doped SYSO phosphors, as well as standard patterns
of Sr Nd (SiO ) O (ICSD 155627) have been presented in Fig.
2
8
4 6 2
3+
3+
Sr Y (SiO )O phase. The Bi and Eu are expected to
2
8
4
2
3
. It is found that the diffraction positions of SYSO host and
3+
substitute the Y sites based on their similar ionic radii (Bi ,
3+
3
Bi ,
+
1
.07 Å for CN=7 and 1.15 Å for CN=9, Eu , 1.03 Å for CN=7 and
.10 Å for CN=9) and same charge. Furthermore, the
1
3
+
3+
introductions of Bi and Eu ions into SYSO compound do not
result in any other new phases.
Table 1 Crystallographic data and details in the data collection
3
and refinement parameters for the SYSO:0.08Bi and
+
3
+
3+
3
+
SYSO:0.08Bi , 0.40Eu samples.
3+
The FT-IR spectra of the SYSO host and Bi , Eu -doped
samples are presented in Fig. 4. We can observe that the
3
+
3+
SYSO:0.08Bi , 0.40Eu
3+
3
profiles of FT-IR spectra of SYSO:0.08Bi and SYSO:0.08Bi ,
+
3+
Sample
SYSO:0.08Bi
3
+
.40Eu samples are similar to that of the SYSO host shown in
0
Space group
Symmetry
a, Å
P6 /m
3
P6 /m
3
3
+
3+
Fig. 4a, indicating that the incorporations of Bi and Eu into
this host have made little change to the structure of SYSO,
consistent with the XRD analysis mentioned above. The two
hexagonal
9.3873(1)
9.3873(1)
6.8471 (2)
524.60(2)
1
hexagonal
9.3976(2)
9.3976(2)
6.8829 (2)
526.42(0)
1
b, Å
-1
bond vibration positions around 513 and 559 cm correspond
to the bending mode of the SiO tetrahedron, and the peak
around 931 cm is assigned to the asymmetric Si-O stretching
modes of the SiO tetrahedron. Those results are in good
agreement with the data [400-600 cm refers to δ(SiO
c, Å
3
V, Å
4
-1
Z
2θ-interval,°
10-110
5.67
10-110
6.77
4
-1
R
R
wp
4
) and
)] reported in the previous
paper. Another three absorption peaks of FT-IR spectra at
-1
9
00-1100 cm refers to ν(SiO
4
p
2
3.81
4.12
1
4
χ
4.517
6.484
-1
-
about 1630, 2924 and 3440 cm are attributed to the OH
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Journal Name
3
+
3+
3+
(2)
3+
Bi (1)
a) Bi
Bi (2)
SYSO:0.08Bi
(
3
+
Bi (2)
λex = 345 nm
λem = 493 nm
4
-
[
4
SiO ]
1630
3
+
SYSO
3+
Bi (1)
Bi (1)
3
+
(b)
SYSO:0.08Bi
2924
3+
3+
λex = 373 nm
SYSO:0.08Bi , 0.40Eu
3440
3+
559
λem = 413 nm
Bi (1)
513
931
2
00 250 300 350 400 450 500 550 600 650 700
5
00 1000 1500 2000 2500 3000 3500 4000
-1
Wavelength (nm)
Wavenumber (cm )
3
Fig. 4 FT-IR spectra of as-prepared SYSO, SYSO:0.08Bi and
+
3
+
3+
SYSO:0.08Bi , 0.40Eu samples.
1
(c)
3+
SYSO:0.08Bi
stretching vibration mode arising from the covered water on
the surface of samples under air condition.
τ1
= 517.81 ns, λem = 493 nm
0.1
.01
Fig. 5a and b shows the PL excitation and emission spectra
3
+
for SYSO:0.08Bi phosphor. The emission and absorption
spectra are expected to present broad bands, which strongly
depend on coordination number, covalence, bond volume
polarizability and current charge, consistent with previously
0
τ2
= 225.17 ns,
λem = 413 nm
2
+
2+ 15
3+
1
reported Eu and Pb . The ground state of Bi ions is S
0
2
with a 6s configuration, whereas the excited state is triplet
1
2
3
4
5
3
3
3
state ( P , P , P ) originating from its 6s6p configuration and
Decay Times (
ꢀs)
0
1
2
1
P singlet state with rising excitation energy. Generally, the Fig. 5 (a) PL excitation (monitored at 493 nm) and emission
1
1
3
S → P and S → P absorption of Bi locates at ultraviolet (excited at 345 nm) spectra for SYSO:0.08Bi . (b) PL excitation
1
3
3+
3+
0
0
0
1
area at room temperature. As depicted in Fig. 5a, the (monitored at 413 nm) and emission (excited at 373 nm)
3
+
spectra SYSO:0.08Bi . (c) Fluorescent decay curves monitored
at 413 and 493 nm with the excitation wavelength of 345 nm
for SYSO:0.08Bi .
excitation spectrum detected at 493 nm displays a main band
with the maximum at 331 nm and an unobvious one at 345 nm,
3
+
1
originating from the S → P transition of Bi (2). Upon 345
3
3+
0
1
nm excitation, the emission spectrum presents a broad
wavelength range extending from 350 to 650 nm with two
obvious emission bands around 413 and 493 nm, respectively.
4
13 and 493 nm and a minor emission around 375 nm, which
3
+
2-
may be attributed to the Bi -O charge transfer transition.
3+ This is because the transition energy of charge transfer
3
1
These two bands are ascribed to the P → S transition of Bi
1
0
1
1
3+ 18
transition is higher than that of P - S transition of Bi . Fig.
1
0
in Fig. 5a. It is easily accepted that the two main emission
3
5c shows the fluorescence decay curves of Bi monitored at
+
3
peaks can be assigned to the occupations of two kinds of Y by
+
3
413 [Bi (1)] and 493 nm [Bi (2)] with the excitation
+
3+
3
+
Bi . Based on the calculation of average bonds of Y-O, the
16
3
+
3
Y (1)-O is much longer than that of Y (2)-O, therefore, the
+
3+
wavelength of 345 nm in SYSO:0.08Bi , which can be well
fitted with the double-order exponential decay, as expressed
3
coordinated bond of Bi (1)-O is much weaker than that of
+
1
9
3
Bi (2)-O, . Thus, we can infer that the band around 413 nm
+
17
below:
3
can be ascribed to the P → S transition of Bi ions
1
3+
1
0
I
(
t
)
= A exp
(
−t /τ1
)
+ A exp
(
−t /τ2 + I0
)
1
2
occupying the Y(1) sites with nine-coordination, denoted as
(1)
(t) is the luminescence intensity at certain time t, A1
3
+
3+
Bi (1), while the other one attributes to Bi ions replacing Y(2)
where
I
3
+
ions with seven-coordination, denoted as Bi (2). Fig. 5b
illustrates the PL excitation spectrum monitored at 413 nm
and emission spectrum excited into 373 nm. Only one band
around at 413 nm from the emission spectrum can be
observed. The emission position is consistent with that in Fig.
and A are constants, τ and τ are rapid and slow times for the
2
2
1
exponential components, respectively. The effective decay
times can be approximately calculated as follow:
*
2
1
2
2
τ =
(
A
τ
+ A
τ
)
/
(
A
τ
+A
τ
)
3
1
1
2
1
1
2
2
(2)
5
1 0
a, therefore, this band is confirmed to be the P → S
3
transition of Bi (1). The two primary bands around 340 and According to the equation above, we can calculate that the
+
1
73 nm can be assigned to the S
3
3+
3
0 0
→ P transition of Bi (1). decay times at 413 nm and 493 nm are 225.17 ns and 517.81
Moreover, we can observe that a minor excitation band ns, respectively. It is evident that they are very different from
3
each other. This result validates that two kinds of Bi emission
+
around 260 nm occurs in both two excitations monitored at
3
bands exist in SYSO:0.08Bi phosphor.
+
4
| J. Mater. Chem. C, 2015, 00, 1-10
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Journal Name
Fig. 6 shows the PL emission spectra of SYSO:xBi upon 340
Jou Pr nl e aa ls oe fd Mo an to et r ai ad lj su s Ct hm ea mr g ii sn ts ry C
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DOI: 10.1039/C5TC01993A
ARTICLE
3
+
3
+
λex = 340 nm
nm excitation with different concentrations of Bi . It is
observed that the profiles of emission spectra are similar to
each other. The emission intensity first increases with the
x = 0.01
x = 0.04
x = 0.08
x = 0.12
x = 0.16
x = 0.24
x = 0.32
x = 0.40
3
+
increase of Bi concentration until the maximum x = 0.08,
after which the intensity begins to decrease monotonously
3
+
with further increasing Bi concentration attributed to the
concentration quenching effect, as illustrated in Fig. 6. It is
accepted that concentration quenching effect takes place due
3
+
to the energy transfer between Bi ions, whose probability
3
+
raises as the concentration of Bi increases. In order to
approximately assess the energy transfer mechanism between
350
400
450
500
550
600
650
700
Wavelength (nm)
3
Bi , it is necessary to know the critical distance (R ) between
+
c
3+
3
+
3+
activators such as Bi . The distance between Bi becomes Fig. 6 PL emission spectra of SYSO:xBi (x = 0.01-0.40) upon
3
+
3
40 nm excitation.
shorter and shorter with increasing Bi content, leading to the
increase of energy transfer probability. When the distance is
small enough, the concentration quenching happens and the
energy migration is blocked. The Rc can be calculated using the
4
.2
.0
4
2
0
3.8
concentration quenching method as given by the expression:
slope = -1.94
3
.6
.4
1
3
3
+

3V
X N

Bi (1)
3
Rc ≈ 2
3+




(3)
Bi (2)
fitting line
fitting line
4π
c
3.2
3
.0
.8
where
V and N correspond to the volume of the unit cell and
slope = -1.98
2
the number of available sites for the dopant in the unit cell,
respectively, and Xc is the critical concentration of dopant ions.
3 16
N = 1, V = 524.076 Å , and Xc is 0.08 for
2.6
2.4
For the SYSO host,
3
+
-1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3
Bi ; Consequently, the critical distance (Rc) was determined to
be about 23.21 Å.
Log (xBi3+
)
Since the exchange interaction generally occurs in the
forbidden transition whose Rc is typically 5Å, therefore, the
Fig. 7 Linear fitting of log(x) versus log(I/x) in various SYSO:
xBi phosphors with two kinds of Bi beyond the
3
+
3+
result above is much bigger than it, indicating little possibility concentration quenching (x > 0.08).
of this mechanism. The mechanism of radiation reabsorption is
only efficacious which requires the wide overlap between 6, indicating the dipole-dipole interactions are responsible for
3
+
3+
fluorescence and absorption spectra. We can find this is also the energy transfer between Bi in SYSO:Bi phosphors.
3
The PL properties of SYSO:0.40Eu sample are shown in Fig.
+
not considered to take place here. As a result, it can be
inferred that the electric multipole interaction contributes to 8a. The PL excitation spectrum monitored at 615 nm shows
the non-radiative concentration quenching between two one wide excitation band from 200 to 350 nm centered at 272
3
nearest Bi centers based on the Dexter theory:
+
21
nm and several sharp lines peaking at 363, 383, 396, 466 and
34 nm from 350 to 550 nm, which can be assigned to the
5
−
1
I
θ
)
3
3+ 2-
7
5
7
5


charge transfer (CT) transition of Eu -O and F
→ D
transitions of Eu ,
is Bi concentration respectively. The emission position of CT band varies relying
represents a constant for the on the different hosts. The PL emission spectra under 272 and
4 0 7
, F → L ,
=
1+
β
(
x
0


(4)
7
5
7
5
7
5
3+
x
F
0
→ L
6
,
F
0
22
→ D
2
,
and
0 1
F → D
3
+
where
I refers to the emission intensity, x
beyond the optimal value, and
β
given matrix under the identical excitation conditions. The 466 nm excitation are similar to each other, and they both
type of energy transfer mechanism of electric multipole include many emission lines in the region from 550 nm to 750
5
7
interaction can be determined by analyzing the constant
from this formula, in which the value of = 6, 8, and 10 2, 3, 4) of Eu . The peak at 538 nm is ascribed to the transition
correspond to dipole–dipole, dipole–quadrupole, and of D → F of Eu ion. The peaks at 589 and 595 nm originate
θ
0 J
nm, which are assigned to the transitions of D → F ( J = 0, 1,
3
+
θ
5
7
3+
1
1
5
7
quadrupole–quadrupole interactions, respectively. Fig.
illustrates the dependences of I/x on x on a logarithmic scale considered to be insensitive to site symmetry. The red
7
0 1
from the D → F magnetic-dipole transition. This transition is
3
+
5
7
for two kinds of Bi . The two approximately fitting linear emission D → F electric-dipole transition is produced due to
0
2
3
+
3+
3+
relations between log(I/x) and log(x) for Bi (1) and Bi (2) are the lack of inversion symmetry at the Eu site and can be
also given. The slopes of fitted straight lines are equal to -1.98 much stronger than that to the F level. When the Eu ions
7
3+
1
3
+
3+
and -1.94 = -
θ/3 for Bi (1) and Bi (2), respectively. As a locate at low local symmetry environment, the hypersensitive
5
7
consequence, the
θ
= 5.94 and 5.82 are approximately both as transition of D - F dominates their emission spectrum. Based
0 2
3
on the valence state and the ionic radius, the doped Eu ions
+
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ARTICLE
Journal Name
3
+
2-
3+
(a)
CT Eu -O SYSO:0.40Eu
λex = 345 nm
λem = 413 nm
λem = 493 nm
λem = 615 nm
λex = 396 nm
λex = 272 nm
λem = 615 nm
2
00 250 300 350 400 450 500 550 600 650 700
200
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
(
b)
Fig. 10 PL excitation spectra monitored at 413, 493 and 615
nm, and emission spectrum upon 345 nm excitation in
y = 0.04
y = 0.10
y = 0.16
y = 0.24
y = 0.32
y = 0.40
y = 0.48
y = 0.56
3
+
3+
SYSO:0.08Bi , yEu
3
SYSO:0.08Bi , 0.40Eu .
+
3+
λex = 466 nm
the phosphors also can be utilized as red phosphors with a
blue chip.
In order to explore the possibility of energy transfer from
3
+
Bi to Eu ions, the excitation spectrum monitored at 615 nm
3+
3
in SYSO:0.40Eu and emission spectrum excited at 345 nm in
+
3
+
SYSO:0.08Bi were detected and supplied in Fig. 9. A
significant spectral overlap can be easily observed. This result
5
00
550
600
650
700
3
+
3+
Wavelength (nm)
gives the probable energy transfer from Bi to Eu ions when
they are co-doped into SYSO host. The energy transfer
+
mechanism from Bi to Eu ions was expected to be the
Fig. 8 (a) PL (λex = 272 nm) and PLE (λem = 615 nm) spectra of
3
3+
3
+
SYSO:0.40Eu
SYSO:0.08Bi , yEu (y = 0.04, 0.10, 0.16, 0.24, 0.32, 0.40, 0.48
sample. (b) PL emission spectra for
2
4
3
+
3+
resonance type according to the proposal from Dexter’s. In
addition, the existence of energy transfer can be
demonstrated by the PL emission and excitation spectra in
3
+
and 0.56) in Eu doped concentration (λex = 466 nm).
3
+
3+
3
+
SYSO:0.08Bi , 0.40Eu sample, as illustrated in Fig. 10. Upon
345 nm excitation, the emission spectrum exhibits not only
3+
SYSO:0.08Bi , λex = 345 nm
3
+
SYSO:0.40Eu
, λem = 615 nm
blue-green area from Bi but also some sharp red component
3
from Eu . The excitation spectra monitored at 413 and 493
+
3
+
nm are well consistent with those obtained in Bi single doped
SYSO sample in Fig. 5. This demonstrates that the blue-green
3
+
band in emission spectrum is originated from Bi . Monitored
3
at 615 nm of Eu transition, the excitation spectrum consists
+
2
-
3+
3+
of O -Eu charge transfer band, characteristic Eu excitation
lines, and the obvious 331 and 345, and 373 nm bands from
3
+
3+
Bi , confirming the energy transfer from Bi (1) and Bi (2) to
3+
3
00 350 400 450 500 550 600 650
Wavelength (nm)
3
Fig. 9 Spectral overlap of Eu excitation spectrum monitored
+
y = 0
y = 0.04
λex = 345 nm
3
+
at 615 nm in SYSO:0.40Eu and emission spectrum excited at
3
45 nm in SYSO:0.08Bi .
+
y = 0.10
y = 0.16
y = 0.24
y = 0.32
y = 0.40
y = 0.48
y = 0.56
3
3
are apt to occupy the Y sites in SYSO, making the D
+
5
7
- F
0
2
3
+
emission of Eu at 615 nm be the prominent peak in the
7
5
emission spectrum. The ratio of I( D
5
7
0 1 0 2
→ F )/I( D → F ) strongly
3
depends on the local symmetry of Eu ions in the host. In
+
23
5
7
this case, the
0 2
D → F transition at 615 nm becomes the
3
dominant one, indicating that Eu ions occupy the site without
+
3
inversion symmetry. The PL emission spectra of SYSO:0.08Bi ,
+
350 400 450 500 550 600 650 700
3
yEu (y = 0.04, 0.10, 0.16, 0.24, 0.32, 0.40, 0.48 and 0.56) with
+
Wavelength (nm)
3
+
different Eu concentrations (λex = 466 nm) are shown in Fig. Fig. 11 PL emission spectra for SYSO:0.08Bi , yEu (y = 0, 0.04,
3+
3+
3
b. The characteristic emission of Eu ion here indicates that
+
8
0.10, 0.16, 0.24, 0.32, 0.40, 0.48 and 0.56) excited at 345 nm.
6
| J. Mater. Chem. C, 2015, 00, 1-10
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DOI: 10.1039/C5TC01993A
Journal Name
a)
ARTICLE
(
y = 0, τ = 225.17 ns
y = 0.04,
y = 0.16,
y = 0.32,
τ
τ
τ
= 220.22 ns
= 157.93 ns
= 114.58 ns
y = 0.48,
2
τ
= 79.05 ns
3
1
4
5
Decay times (ꢀs)
(b)
y = 0, τ = 517.81 ns
3
+
Fig. 14 CIE chromaticity diagram for SYSO: 0.08Bi , yEu
3+
y = 0.04,
y = 0.16,
y = 0.32,
τ
τ
τ
= 485.33 ns
= 389.93 ns
= 278.63 ns
samples (y = 0, 0.04, 0.10, 0.16, 0.24, 0.32, 0.40, 0.48 and 0.56).
concentration with the excitation wavelength of 345 nm. The
PL emission intensity of red component increases until the
3
maximum y = 0.40 with increasing Eu ion concentration. The
+
3
+
subsequent decrease of Eu emission intensity beyond y =
.40 takes place because of the general concentration
quenching effect. This result gives us a further confirmation of
y = 0.48,
2
τ
= 196.34 ns
3
0
1
4
5
3
+
3+
3+
3+
Decay times (
ꢀs)
existent energy transfer from Bi to Eu ions in SYSO:Bi , Eu .
To support the spectra analysis, the fluorescent decay curves
3
Fig. 12 Fluorescent decay curves for Bi (1) at 413 nm (a) and
+
3
Bi (2) at 493 nm (b) upon 345 nm excitation with increasing
+
3
+
3+
located at Bi (1) and Bi (2) emission bands were measured
with the excitation wavelength of 345 nm and plotted in Fig.
3
Bi concentration in SYSO:0.08Bi , yEu .
+
3+
3+
1
2, which can be fitted with the second-order exponential
0
.8
.7
.6
.5
.4
.3
.2
.1
.0
decay given above. The values of decay times are determined
3
0
+
to be 225.17, 220.22, 157.93, 114.58 and 79.05 ns for Bi (1) at
13 nm, and 517.81, 485.33, 389.93, 278.63 and 196.34 ns for
3+
0
4
λ
ex = 345 nm
0
Bi (2) at 493 nm corresponding to the y = 0, 0.04, 0.16, 0.32
and 0.48, respectively, indicating the monotonous decrease of
0
3
+
decay times with increasing Eu
concentration in
0
3
SYSO:0.08Bi , yEu samples. The result can strongly prove the
+
3+
0
3
+
3+
existence of energy transfer from Bi to Eu ions.
0
Moreover, in view of this host, the energy transfer efficiency
3
+
3+
0
(η ) from sensitizer Bi to acceptors Eu can be approximately
T
2
5
estimated using following equation:
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
3
+
Eu concentration (y)
IS
η =1−
3
Fig. 13 Energy transfer efficiency (η ) in SYSO:0.08Bi , yEu
+
3+
T
(5)
T
IS0
3
+
phosphors with different Eu concentrations.
where ηT represents the value of energy transfer efficiency, IS0
3+
3
+
Eu . Additionally, we can observe that the excitation and IS are the luminescence intensities of Bi ions without and
3
+
intensities at 331, 345 nm and 373 nm are all stronger than with the existence of Eu ions, respectively. Fig. 13 plots the
3
+
2-
3+
3+
that of Eu -O charge transfer band around 272 nm, which is curve of calculated value of ηT from Bi to Eu in
3
+
3+
3+
highly expected in Eu doped phosphors. The PL excitation SYSO:0.08Bi , yEu samples under 345 nm excitation as a
3
+
3+
3+
spectrum of Eu , Bi co-doped SYSO phosphor matches well function of Eu concentration. The calculated ηT value rises
3
+
with the UV chip, as observed from Fig. 10, indicating that this monotonously with increasing Eu concentration in this
phosphor can be potentially applied for UV-chip w-LEDs.
concentration interval, which can reach the maximum of 74%.
3
+
3+
3+
To further study the energy transfer properties from Bi to Therefore, the energy transfer from Bi to Eu ions is
3
+
3+
3+
3+
3+
Eu ions, a series of Bi , Eu co-doped SYSO samples were reckoned to be an efficient process in SYSO:Bi , Eu
synthesized. Fig. 11 shows the variation of PL emission spectra phosphors.
3
+
3+
as a function of Eu concentration in SYSO:0.08Bi , yEu
3+
3+
The CIE chromaticity diagram for SYSO:0.08Bi , yEu
3+
3
+
samples. We can find the emission intensity of Bi presents a samples (y = 0, 0.04, 0.10, 0.16, 0.24, 0.32, 0.40, 0.48 and 0.56)
monotonous decrease with gradually increasing Eu
3
+
is displayed in Fig. 14 with the corresponding value of
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Table 2 The variation of CIE chromaticity coordinates (x, y) and
Journal Name
4
.0
(c)
(
a)
(b)
3
+
QYs for SYSO:0.08Bi , yEu phosphors excited at 345 nm.
3+
3
.5
.0
.5
2.0
.5
.0
3
Sample Eu concentration
+
QY
CIE coordinates (x, y)
3
no.
(y mol)
(%)
2
A
B
C
D
E
F
0
(0.168, 0.255)
(0.177, 0.264)
(0.194, 0.274)
(0.212, 0.282)
(0.236, 0.291)
(0.265, 0.298)
(0.290, 0.305)
(0.325, 0.311)
(0.347, 0.312)
74.8
48.9
40.2
44.1
49.0
61.3
69.0
52.0
33.9
0.04
0.10
0.16
0.24
0.32
0.40
0.48
0.56
1
1
0.0 0.1 0.2 0.3 0.4 0
1
2
3
0 5 10 15 20 25
2
6
/3
8/3
CBi3+
10/3
10 CBi3+
×10
+Eu
C
3+
3+
3+
×
3+
G
H
I
Bi +Eu
+Eu
3
Fig. 15 Dependence of (I /I ) of Bi on (a) C (b) C and (c)
+
6/3
8/3
S0
S
1
0/3
C
.
calculated CIE chromaticity coordinates (x, y) in Table 2. With
3
1
5a and c, indicating that the best linear behavior occurs when
+
3+
3+
increasing Eu concentration in SYSO:0.08Bi , yEu samples,
the CIE chromaticity coordinate varies from (0.168, 0.255) to
α = 8. Consequently, the dipole-quadrupole mechanism
3+
3+
contributes to the energy transfer process from Bi to Eu
(0.347, 0.312) corresponding to y = 0 and 0.56, located at blue-
ions.
green and white areas, respectively. Therefore, the emission
3+
3
color can be tuned from blue-green to white in SYSO:Bi , Eu
+
In view of the dipole-quadrupole interaction, the Rc value
3
by adjusting the concentrations of Bi and Eu ions. The
+
3+
also can be obtained through the spectral overlap route which
2
has the formula displayed as follows:
8
7
3
obtained quantum yield (QYs) of SYSO:0.08Bi is 74.8%, and
+
3
the maximum can reach 69.0% for SYSO:0.08Bi , 0.40Eu
+
3+
12
2
S
4
(
8)
R = 3.024×10 λ f
∫ fS (
of Eu has not been obtained so
E
)
fA
(
E / E dE
)
c
q
3
+
3+
sample in Bi , Eu co-doped SYSO samples, as listed in Table 2.
As presented above, for this host, the is determined to be
2.78 Å by utilizing the equation (2), in which the different
value is denoted as the critical concentration of dopant ions
3
+
f
In our case, it is a pity that q
R
c
-3
-2
far. However, Verstegen et al. proposed that fq/fd = 10 -10
1
X
c
-6
can be reasonable, fd = 10 is the oscillator strength of the
2
electric dipole transitions. λS (4130 Å) and E are the
8
3
+
3+
(
total concentration of Bi and Eu , approximately 0.48), that
3
+
wavelength of strongest intensity of Bi and the energy
3
is, at which the luminescence intensity of Bi is half of that in
+
4
involved in the transfer (in eV); ∫fS(E)f (E)/E dE stands for the
3
+
the sample without the existence of Eu . The result also
excludes the mechanisms of exchange interaction and
radiation reabsorption based on analysis, indicating the
electric multipole interaction would be responsible for energy
A
spectral overlap between the normalized shapes of the
3
+
3+
sensitizer Bi emission fS(E) and the activator Eu excitation
fA(E), and in this condition it is determined to be about
−
5
.00198 eV . As a result, the value of Rc is calculated to be
3
+
3+
3+
3+
0
transfer from Bi to Eu ions in SYSO:Bi , Eu . According to
the Dexter’s energy transfer formula of multipolar interaction,
1
0.03-13.37 Å, which agrees well with that calculated by
2
6
concentration quenching method above (12.78 Å). This result
can further testify that the electric dipole-quadrupole
interaction would be responsible for the energy transfer
the following relationship can be adopted:
ηS 0
α /3
∝
C
ηS
(6)
3+
3+
3+
3+
mechanism from Bi to Eu ions in SYSO:Bi , Eu samples.
As an important parameter for LED fabrication and
application, thermal quenching properties of representative of
S
where ηS0 and η stand for the luminescence quantum efficiencies
3
+
3+
of the Bi ions with the absence and presence of the Eu ions,
3
+
3+
3+
3+
respectively.
ions concentrations. The values
dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole emission spectra and emission intensity of Bi and Eu have
interactions, respectively. However, the is difficult to be been plotted in Fig. 16a. We can observe the emission
determined and therefore it can be assessed instead by the intensity decreases monotonously with increasing operated
, where stand for the luminescence intensities of temperature. It is accepted that the reason for the decrease of
C
represents the total values of the Bi and Eu
SYSO: 0.08Bi , 0.40Eu sample has been investigated with the
α
= 6, 8 and 10 correspond to temperature range of 298-523 K. The measured variations of
3
+
3+
S0 S
η /η
I
S0
3
/
I
S
IS0 and I
S
+
3+
Bi ions without and with the existence of Eu ions, emission intensity is attributed to the thermal quenching via
respectively, accordingly, the above equation can be replaced the thermal activation through the crossing point between the
as follows:
ground and the excited states. At 150 ℃, the emission
3
+
3+
intensity of Bi and Eu keep about 65% and 41% of their
7) corresponding initial values, respectively. It is clear the
IS0
α
/3
∝
C
(
IS
3+
declined rate is different for Bi and Eu ions in SYSO: 0.08Bi ,
3+
3+
α/3
3+
The relationship between
I
S0
/
I
S
and
C
based on the above 0.40Eu . The reason for this is that the energy transfer
3
+
3+
equation are plotted in Fig. 15. One can find that the value of process from Bi to Eu ions is also affected by the
2
R with the linear fitting in Fig. 15b is bigger than those in Fig.
3+
temperature. Moreover, the light blue-shift of Bi emission
8
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DOI: 10.1039/C5TC01993A
ARTICLE
3
position also can be found, which is common in many doped into the SYSO, the efficient energy transfer from Bi to
+
29
3+
phosphors. In order to better understand the quenching Eu may occur, which is deduced by the spectral overlap
3
+
3+
3+
3+
phenomenon, the Arrhenius formula concerning the between Bi emission and Eu excitation in Bi and Eu
relationship between temperature and luminescence intensity singly doped SYSO, and demonstrated by the PL emission
3
0
was used, which can be copied as follow:
spectra under UV excitation and excitation spectra monitored
3+
3
+
3+
at Bi and Eu emissions, as well as the decay times of Bi
3
+
3+



I0 
Ea
emission bands in SYSO:Bi , Eu phosphors. The non-radiate
energy transfer mechanism is determined to be dipole-
quadrupole interaction, which is further illustrated via the
consistence of calculated critical distances of energy transfer
In
= InA−

(9)
I
kT

where
E
a
is the objective activation energy,
A
is a constant, T
3
+
3+
represents the temperature (K) and
k
is the Boltzmann from Bi to Eu ions by concentration quenching (12.78 Å)
0
constant (8.626×10 eV). I and I are the integrated intensity and spectral overlap methods (10.03-13.37 Å). The emission
-5
3
+
3+
at the room emission intensity and differently operated color of SYSO: Bi , Eu phosphor can be tuned from blue-
temperatures, respectively. Therefore, we plot the relationship green area (0.168, 0.255) to white region (0.347, 0.312) in our
3
of In(I /I-1) versus 1/kT activation energy graph for thermal experimental data by adjusting the concentrations of Bi and
+
0
3
quenching of SYSO: 0.08Bi , 0.40Eu phosphor in Fig. 16b. The Eu , white emission is realized for SYSO:0.08Bi , 0.48Eu
+
3+
3+
3+
3+
slope of fitting line is -0.2283, therefore, the activation energy with the CIE chromaticity coordinate (0.325, 0.311). The
Ea can be inferred to be approximate 0.2283 eV. The relative maximum value of quantum yield is determined to be 69% for
3
+
3+
3+
3+
SYSO:0.08Bi , 0.40Eu in Bi , Eu co-doped SYSO samples.
high value indicates its good thermal property.
3
+
3+
Moreover, the SYSO:0.08Bi , 0.40Eu shows good thermal
quenching properties. The results above indicate that
3
+
3+
Conclusions
SYSO:Bi , Eu phosphor possesses the potential as a single-
component tunable emission (including white) phosphor for
UV w-LEDs.
3
A series of Bi , Eu -doped SYSO phosphors were synthesized
+
3+
3
+
by Pechini-type sol−gel route. Bi singly -doped SYSO presents
intense blue-green emission with the wavelength ranging from
3
+
3+
50-650 nm under UV excitation. When Eu and Bi were co-
3
Acknowledgements
This project is financially supported by the Scientific and
3
+
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
Bi emission
(a)
Technological Department
of Jilin
Province (Grant
No.
3
+
Eu emission
2
98 K
20150520029JH), National Natural Science Foundation of
China (NSFC Grants 51472234, 51172227, 91433110), National
Basic Research Program of China (Grants 2014CB643803), and
Joint Funds of the National Natural Science Foundation of
China (Grant U1301242).
323 K
373 K
423 K
473 K
3
00 350 400 450 500 550
Temperature (K)
523 K
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DOI: 10.1039/C5TC01993A
TOC Graphic
3
+
3+
A series of tunable blue-green to white phosphors Sr Y (SiO ) O :Bi , Eu phosphors
2
8
4 6
2
prepared via Pechini-type sol-gel method have been investigated.
y = 0
y = 0.04
λex = 345 nm
y = 0.10
y = 0.16
y = 0.24
y = 0.32
y = 0.40
y = 0.48
y = 0.56
3
+
3+
SYSO: 0.08Bi , Eu
350 400 450 500 550 600 650 700
Wavelength (nm)