Enhanced red light emission from LaBSiO5:Eu3+,R 3+ (R = Bi or Sm) phosphors

Article abstract of DOI:10.1016/j.saa.2010.11.030

Polycrystalline LaBSiO₅:Eu³⁺,R³⁺ (R = Bi or Sm) phosphors have been synthesized by a facile sol-gel method. The phosphors have been characterized by thermogravimetric analysis/different scanning calorimeter, scanning electron microscopy, X-ray diffractometer and fluorescence measurements. It was found that the emission intensity of LaBSiO₅:Eu phosphors increases clearly and reaches a maximum at 30 mol% with increasing of Eu³⁺ concentration. The incorporation of Bi³⁺ ions and/or Sm³⁺ ions have greatly enhanced the emission intensity of Eu ³⁺ upon excitation with 391 nm light. The possible sensitization mechanisms of Sm³⁺ and/or Bi³⁺ on Eu³⁺ emission intensity have been investigated and discussed. The high brightness and short luminescence decay times make it promising red-emitting candidates for white light-emitting diodes.

Full text of DOI:10.1016/j.saa.2010.11.030

Spectrochimica Acta Part A 78 (2011) 607–611
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
3
+
3+
Enhanced red light emission from LaBSiO :Eu ,R (R = Bi or Sm) phosphors
5
∗
Y.N. Xue, F. Xiao, Q.Y. Zhang
MOE Key Lab of Specially Functional Materials and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, PR China
a r t i c l e i n f o
a b s t r a c t
Polycrystalline LaBSiO :Eu3+,R3+ (R = Bi or Sm) phosphors have been synthesized by a facile sol–gel
method. The phosphors have been characterized by thermogravimetric analysis/different scanning
calorimeter, scanning electron microscopy, X-ray diffractometer and fluorescence measurements. It was
found that the emission intensity of LaBSiO5:Eu phosphors increases clearly and reaches a maximum
Article history:
Received 10 May 2010
Received in revised form 9 October 2010
Accepted 29 November 2010
5
3
+
3+
3+
at 30 mol% with increasing of Eu concentration. The incorporation of Bi ions and/or Sm ions have
Keywords:
3+
greatly enhanced the emission intensity of Eu upon excitation with 391 nm light. The possible sensiti-
Luminescence
Energy transfer
Red phosphors
Light emitting diodes
3
+
3+
3+
zation mechanisms of Sm and/or Bi on Eu emission intensity have been investigated and discussed.
The high brightness and short luminescence decay times make it promising red-emitting candidates for
white light-emitting diodes.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
Herein, the main objective of this work is to carry out a
detailed study on the structural and luminescent properties of
LaBSiO5:Eu3+,R (R = Bi or Sm) phosphors by sol–gel synthesis
method. The high brightness red light has been obtained from
La0.68BSiO5:Eu0.30,Sm0.02, suggesting that the as-prepared phos-
phors are promising red luminescent materials for white LEDs.
3+
White light emitting diodes (LEDs) have high potential appli-
cation for replacement of the conventional incandescent and
fluorescent lamps due to their superior advantages such as high
efficiency, low energy consumption, long lifetime, environmental
benefit, and easy maintenance [1,2]. At present, the most com-
mon approach for fabricating the white LEDs is to combine a blue
3+
2. Experimental
LED with yellow-emitting phosphor, such as Y Al5O12:Ce [3].
3
Although this type of white LEDs has high luminous efficiency, it
exhibits a low color rendering index due to the deficient of red
light component [4]. Alternatively, the white light can be gener-
ated to apply a near-ultraviolet InGaN chip (380–410 nm) with blue,
green, and red phosphors [5,6]. To attain high color rendering white
LEDs, the red-emitting phosphors are urgently required. How-
ever, the currently commercial available red-emitting phosphors
Red phosphors of LaBSiO5:Eu3+,R3+ (R = Bi or Sm) were syn-
thesized by sol–gel method. The starting materials were Eu2O3
(99.99%), Bi2O3 (99.99%), Sm2O3 (99.99%), La2O3 (99.99%), H3BO3
(A.R.), Si(OC2H5)4 (TEOS) (A.R.), nitric acid and citric acid. Firstly,
La(NO3)3, Eu(NO3)3, Sm(NO3)3 and Bi(NO3)3 solutions were made
by dissolving their oxides completely in nitric acid. Then, stoichio-
metric amounts of nitrate solutions were added into the beaker
together with citric acid. Finally, the tetraethyl orthosilicate was
added dropwise under vigorous stirring to form transparent sols
which turned to yellow gels after drying at oven. The gels were cal-
3
+
such as Y O S:Eu are low efficiency and chemically unstable
2
2
[
7]. Therefore, it is necessary to develop more efficient red-
emitting phosphors suitable for the fabrication of white LEDs.
Recently, silicate-based phosphors have been investigated exten-
sively because of their high quantum efficiency, excellent chemical
and thermal stability, water resistance and low cost [8–12]. More-
over, interesting candidates for red phosphors include materials
◦
◦
cined at 700 C for 4 h and then 950 C for 4 h in air. After cooling to
room temperature, the phosphors were obtained.
The crystallization and phase transformation behavior of the
obtained phosphors were monitored by both thermogravimet-
ric analysis (TGA)/different scanning calorimeter (DSC) (Netzsch
STA449C, at a heating rate of 10 K/min) and X-ray powder diffrac-
tion (XRD) (Philips Model PW1830 diffractometer, Cu K␣). The
morphology and size of the synthesized phosphors were character-
ized by a scanning electron microscope (SEM) (JEOL-JSM-5610LV).
Room temperature excitation and emission spectra were recorded
on a TRIAX320 spectrofluorimeter (Jobin-Yvon Co.) with a 450 W
xenon lamp as the excitation source.
3
+
containing Eu , which is widely used as a red-emitting activator
5
7
via the D → F transition at about 614 nm. In order to enhance
0
2
3+
the red emission intensity of Eu , rare earth or transition metal
ions usually are codoped as sensitizers [13,14].
∗ Corresponding author. Tel.: +86 20 87113681.
E-mail address: qyzhang@scut.edu.cn (Q.Y. Zhang).
1
386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.11.030

6
08
Y.N. Xue et al. / Spectrochimica Acta Part A 78 (2011) 607–611
Fig. 1. TG/DSC curves of the LaBSiO5:Eu precursor. The inset presents SEM image of
the LaBSiO5:Eu phosphor.
Fig. 3. Excitation (ꢀem = 614 nm) and emission spectra (ꢀex = 391 nm) of
La0.7BSiO5:Eu0.3.
3
. Results and discussion
has a triclinic crystal structure with a space group of P31 and the
Fig. 1 presents TG/DSC curves of the LaBSiO5:Eu precursor. It
cell parameters are a = b = 6.874 A˚ , c = 6.717 A˚ . As shown in the
indicates the weight loss takes place in the TG curve from room
◦
inset of Fig. 2, two different tetrahedral groups, namely BO4 and
temperature to 550 C and decomposition process can be divided
◦
SiO , are interlinked by corner-sharing with each other to form a
into two processes. The first weight loss step below 200 C accom-
panied by a strong endothermic peak around 116 C is attributed
4
◦
six-membered ring system [15].
Fig.
3 gives the excitation and emission spectra of the
to dehydration of physical adsorbed water. The second weight
◦
◦
La0.7BSiO :Eu phosphor. The band extending from 250 to 325 nm
loss process from 200 C to 550 C corresponds to the burning of
the citrate acid ligand and the decomposition of metal nitric. The
5
0.3
2−
3+
is associated with O –Eu charge-transfer (CT) transition while
3+
7
5
◦
the sharp lines ascribe to transitions of Eu : F → L (358 nm),
exothermic peak at about 813 C is related to the crystallization
of LaBSiO5. Therefore, the calcination temperature of 950 C was
0
9
5
G3 (380 nm), 5L6 (391 nm), 5D3 (442 nm), 5
◦
D
(462 nm). Under
2
the excitation of 391 nm, the emission bands are caused by the
adopted to ensure the prepared LaBSiO5 phosphor with both a high
chemical homogeneity and a fine particle size. The inset illustrates
SEM image of the LaBSiO5:Eu phosphor. The average size of the
phosphor is about 1 ␮m.
4
(
(
f–4f transitions of Eu3+ corresponding to D → F (574 nm), F1
5
7
7
0
0
582 nm), 7F2 (600, 606, 614, 620 nm), F3 (641, 651 nm), and 7F4
7
5
7
684, 694 nm), respectively [16]. The D → F emission line of
0
2
3+
Eu splits into four peaks, which is determined by the energy-level
The XRD patterns of the samples La0.7BSiO5:Eu0.3
,
7
3+
splitting of the F2 state [17]. In Eu doped LaBSiO5 phosphor, the
lack of inversion symmetry and the break of parity selection rules
make the 5D → F electric dipole transition dominated among
La0.68BSiO5:Eu0.3,Bi0.02 and La0.68BSiO5:Eu0.3,Sm0.02 are shown in
Fig. 2. The patterns agree well with Joint Committee on Powder
Diffraction Standards (JCPDS) 87-2172. In addition, it can be visu-
7
0
2
alized that the limited doped Eu3+ ions do not cause any significant
changes in the host structure. The lanthanum borosilicate LaBSiO5
all these transitions [18]. The result is beneficial to obtain the red
phosphor with good color purity.
Fig. 2. XRD patterns of: (a) JCPDS 87-2172; (b) La0.7BSiO5:Eu0.3; (c)
La0.68BSiO5:Eu0.3,Bi0.02; and (d) La0.68BSiO5:Eu0.3,Sm0.02. The inset shows the
crystal structure of LaBSiO5.
Fig. 4. Eu3+ concentration dependence of the emission intensity of La1−xEuxBSiO5
(ꢀex = 391 nm) phosphor.

Y.N. Xue et al. / Spectrochimica Acta Part A 78 (2011) 607–611
609
enlarge figure in the inset of Fig. 5(a). Therefore, it is indicated
that an effective energy transfer from Bi3+ to Eu ions was hap-
3+
pened. The emission spectra for La0
BSiO5:Eu0.3,Bix phosphors
.7−x
under 391 nm excitation were presented in Fig. 5(b). Compared
with La0.7BSiO5:Eu0.3 phosphor, the luminescence intensity of
La0.68BSiO5:Eu0.3,Bi0.02 phosphor has increased by 2.5 times. How-
3+
ever, with increasing Bi concentration, the emission intensity
3+
starts decreasing due to the sensitization effect of the Bi ion on the
3
+
3+
Eu emission varies with the Bi concentration. For higher con-
3+
3+
centration of Bi ions, Bin aggregates may be formed which act as
trapping centers and dissipate the absorbed energy nonradiatively,
3+
instead of transferring it to the Eu activator ions [13]. The possible
energy transfer process from Bi3+ to Eu in La0
3+
BSiO5:Eu0.3,Bix
.7−x
phosphors has been illustrated in the energy level scheme as pre-
sented in Fig. 6.
Fig. 7(a) shows the excitation spectra of LaBSiO5:Eu, LaBSiO5:Sm
and LaBSiO5:Eu,Sm. It is exhibited that the excitation spectrum of
LaBSiO5:Eu,Sm including both the excitation band of LaBSiO5:Eu
and LaBSiO5:Sm, where the new peaks located at 346, 375, 404,
6
3
6
and 480 nm corresponding to the transitions H5/2 → H7/2, P7/2
,
4
4
4
3+
K11/2 and F5/2 + I
of Sm , respectively. The emission spectra
13/2
of La0
BSiO5:Eu0.3,Smx under 391 nm excitation were investi-
.7−x
gated and displayed in Fig. 7(b). The intensities of the emission
spectra of the Eu3+–Sm co-doped systems are much stronger than
3+
3+
3+
that of the single Eu -doped systems. It indicates that the Sm
ions can absorb and transfer energy to Eu3+ ions efficiently. The
luminescence reaches the strongest intensity at x = 2 mol%. Higher
3+
doping concentration of Sm can cause concentration quenching
of Sm3+ emission, and results in the decreasing of emission inten-
3
+
4
sity of Eu . As displayed in Fig. 8, the energy of the G5/2 state of
3+
5
3+
Sm ions is a little higher than that of the D state of the Eu ions,
0
3+
3+
which supposed that energy transfer from Sm to Eu dominated
by resonance takes place [14].
The decay curves for the 5D → F (614 nm) transition of Eu3+
7
0
2
ions in the phosphors are presented in Fig. 9. The values can be fit-
Fig. 5. (a) Excitation spectra of LaBSiO5:Eu3+ and LaBSiO5:Bi3+,Eu3+ phosphors, the
inset shows the enlarged figure of excitation band from 225 to 350 nm. (b) Emission
spectra of La0.7−xBSiO5:Eu0.3,Bix (ꢀex = 391 nm) phosphors.
ted well into single-exponential function as I = I exp(−t/ꢂ), in which
0
ꢂ is the decay lifetime. The lifetime ꢂ values for La BSiO5:Eu ,
0.3
0
.7
La0.68BSiO5:Eu0.3,Bi0.02, and La0.7BSiO5:Eu0.3,Sm0.02 phosphors are
determined to 2.79 ms, 2.54 ms and 2.17 ms, respectively. After the
Fig. 4 exhibits the relationship between emission intensity
3+
3+
3+
introducing of Bi or Sm ions, the faster decay time of Eu may
and concentration of Eu3 ion. With increasing Eu concentra-
tion, the photoluminescence intensities are gradually enhanced
up to the critical concentration of 30 mol% and then decrease
+
3+
3+
due to concentration quenching. When the concentration of Eu
ion increases, the distance between the Eu3+ luminescent centers
becomes smaller which results in the nonradiative energy transfer
3
+
3+
from one Eu to another Eu ion takes place easily. With respect to
the mechanism of energy transfer in phosphors, Blasse has pointed
out that the critical transfer distance (R ) at which the probability
C
of transfer is equal to the probability of radiative emission, can be
calculated using the formula [19]:
ꢀ
ꢁ
1/3
3
ꢁxcN
V
Rc ≈ 2
4
where V is the volume of the unit cell, xc is the critical concentration
of the activator ion, and N is the number of cations in the unit cell.
By taking the experimental and analytic values of V, xc, and N (274.9
3
3+
Å , 0.3, 3, respectively [15]), the critical transfer distance of Eu in
LaBSiO5 is calculated to be about 8.4 A˚ .
For enhancing the red emission intensity of Eu3+, Bi3+ and/or
Sm3+ was codoped into LaBSiO5 as sensitizers. Fig. 5(a) shows
3+
3+
3+
the excitation spectra of LaBSiO5:Eu and LaBSiO5:Bi ,Eu phos-
3+
phors. After introducing Bi into the phosphor, a rather broad band
due to the absorption of Bi3 was observed in the excitation band
from 240 to 340 nm which almost overlap with charge transfer
+
Fig. 6. Energy level diagrams of Bi3+ and Eu3+. Energy transfer process from Bi3+ to
3
+
band of O–Eu . The changes can be visualized clearly from the
3+
Eu is indicated.

6
10
Y.N. Xue et al. / Spectrochimica Acta Part A 78 (2011) 607–611
Fig. 9. The luminescence decay curves of the 614 nm ( D0 → 7F2) emissions of: (a)
La0.7BSiO5:Eu0.3; (b) La0.68BSiO5:Eu0.3,Bi0.02; and (c) La0.68BSiO5:Eu0.3,Sm0.02 phos-
phors (ꢀex = 391 nm).
5
Fig. 7. (a) Excitation spectra of LaBSiO5:Eu3+, LaBSiO5:Sm3+ and LaBSiO5:Sm3+,Eu3+
phosphors. (b) Emission spectra of La0.7−xBSiO5:Eu0.3,Smx (ꢀex = 391 nm) phosphors.
be ascribed to the more impurities which provides more nonradia-
tive recombination centers.
The Commission Internationale de I’Eclairage (CIE) 1931 chro-
maticity coordinates were calculated based on the corresponding
PL spectra and summarized in Table 1, and represented also in
Fig. 10. The CIE chromaticity coordinates of La0.7BSiO5:Eu0.3 are
Fig. 10. CIE color coordinates of La0.68BSiO5:Eu0.3,Sm0.02 (ꢀex = 391 nm) (G, R, B: CRTs
coordinates). The inset presents the picture of La0.7BSiO5:Eu0.3 phosphor under a UV
source.(For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
Fig. 8. Energy level diagrams of Sm and Eu3+. Energy transfer process from Sm3+
3+
3+
to Eu is indicated.

Y.N. Xue et al. / Spectrochimica Acta Part A 78 (2011) 607–611
611
Table 1
The CIE chromaticity coordinates and D0 → 7F2 relative intensity of phosphors (ꢀex = 391 nm).
5
5
D0 → 7F2 relative
Sample no.
Phosphor compositions
CIE chromaticity coordinates
intensity (a.u.)
ꢀex = 391 nm
x
y
a
b
c
La0.7BSiO5:Eu0.3
La0.68BSiO5:Eu0.3,Bi0.02
La0.68BSiO5:Eu0.3,Sm0.02
0.624
0.623
0.623
0.375
0.377
0.376
1.0
2.5
3.6
(
x = 0.624, y = 0375) under the excitation of 391 nm, the codoped
Acknowledgments
3+
3+
Bi or Sm ions has made no big change to the CIE coordinates.
The inset of Fig. 10 presents the picture of La0.7BSiO5:Eu0.3 phos-
phor with bright red emission from its surface under a UV-source.
Table 1 also lists the comparison of relative emission intensity of the
samples. It is indicated that La0.68BSiO5:Eu0.3,Sm0.02 phosphor with
CIE chromaticity coordinates of (x = 0.623, y = 0.376) is the best can-
didate for the application of white LEDs due to the 3.6 times higher
The authors would like to acknowledge the support from the
NSFC (Grant No. 50872036) and the Fundamental Research Funds
for the Central Universities, SCUT.
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4
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0
2
3
+
tion of Eu . It was worthy of mention here that the red emission
3
+
of Eu
2
is enhanced by a factor of 2.5 and 3.6 by co-doping
mol% Bi3+ and/or 2 mol% Sm , respectively. A bright red light
3+
[
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