The energy transfer phenomena and colour tunability in Y2O 2S:Eu3+/Dy3+ micro-fibers for white emission in solid state lighting applications

Article abstract of DOI:10.1039/c4dt00349g

This paper reports on the structural, optical and photometric characterization of an Eu³⁺/Dy³⁺ doped yttrium oxysulfide phosphor (Y₂O₂S:Eu³⁺/Dy³⁺) for near white emission in solid state lighting. A series of Y₂O₂S phosphors doped with Eu³⁺/Dy³⁺ were prepared by the hydrothermal method. The microstructures of the as-synthesized phosphors were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD results reveal that the obtained powder phosphors have a single-phase hexagonal structure and also indicate that the incorporation of the dopants/co-dopants did not affect the crystal structure. The SEM images reveal the morphology of the prepared phosphors as an intense interpenetrating network of interconnected micro-fibers with a diameter of about 0.15 μm. The band gap of the phosphors was calculated from diffuse reflectance spectra using the Kubelka-Munk function. The Eu³⁺, Dy³⁺ doped and Eu ³⁺/Dy³⁺ co-doped phosphors illuminated with ultraviolet light showed characteristic red luminescence corresponding to the ⁵D₀→⁷F_J transitions of Eu ³⁺ and characteristic blue and yellow luminescence corresponding to the ⁴F_9/2→⁶H_15/2 or ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺. The luminescence spectra, the energy transfer efficiency and the decay curves of the phosphors indicated that there exists a strong energy transfer from Dy³⁺ to Eu³⁺ and this was demonstrated to be a resonant type via a dipole-quadrupole reaction. Furthermore, the critical distance of the Eu³⁺ and Dy³⁺ ions have also been calculated. By utilizing the principle of energy transfer it was also demonstrated that with an appropriate tuning of the activator content the Y ₂O₂S:Eu³⁺/Dy³⁺ phosphors can exhibit a great potential to act as single-emitting component phosphors for white light emission in solid state lighting technology.

Full text of DOI:10.1039/c4dt00349g

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The energy transfer phenomena and colour
tunability in Y2O2S:Eu3+/Dy3+ micro-fibers for
Cite this: DOI: 10.1039/c4dt00349g
white emission in solid state lighting applications
S. Som,*a,b P. Mitra,b Vijay Kumar,a Vinod Kumar,a J. J. Terblans,a H. C. Swarta and
S. K. Sharma*b
This paper reports on the structural, optical and photometric characterization of an Eu3+/Dy3+ doped
yttrium oxysulfide phosphor (Y2O2S:Eu3+/Dy3+) for near white emission in solid state lighting. A series of
Y2O2S phosphors doped with Eu3+/Dy3+ were prepared by the hydrothermal method. The microstruc-
tures of the as-synthesized phosphors were investigated by X-ray diffraction (XRD) and scanning electron
microscopy (SEM). The XRD results reveal that the obtained powder phosphors have a single-phase hexa-
gonal structure and also indicate that the incorporation of the dopants/co-dopants did not affect the
crystal structure. The SEM images reveal the morphology of the prepared phosphors as an intense inter-
penetrating network of interconnected micro-fibers with a diameter of about 0.15 μm. The band gap of
the phosphors was calculated from diffuse reflectance spectra using the Kubelka–Munk function. The
Eu3+, Dy3+ doped and Eu3+/Dy3+ co-doped phosphors illuminated with ultraviolet light showed charac-
teristic red luminescence corresponding to the 5D0→7FJ transitions of Eu3+ and characteristic blue and
yellow luminescence corresponding to the F9/2→6H15/2 or F9/2→6H13/2 transitions of Dy3+. The lumine-
scence spectra, the energy transfer efficiency and the decay curves of the phosphors indicated that there
exists a strong energy transfer from Dy3+ to Eu3+ and this was demonstrated to be a resonant type via a
dipole–quadrupole reaction. Furthermore, the critical distance of the Eu3+ and Dy3+ ions have also been
calculated. By utilizing the principle of energy transfer it was also demonstrated that with an appropriate
tuning of the activator content the Y2O2S:Eu3+/Dy3+ phosphors can exhibit a great potential to act as
single-emitting component phosphors for white light emission in solid state lighting technology.
4
4
Received 3rd February 2014,
Accepted 21st February 2014
DOI: 10.1039/c4dt00349g
www.rsc.org/dalton
in practice since different emitting components require
different drive voltages and one can hardly be sure about the
Introduction
The research efforts to fabricate effective new phosphor consistency in the decay process for the different components,
materials for applications in next-generation displays and solid thus resulting in the change in color with time and the
state lighting have been the subject of intense research during complex behavior of the circuit.6,9,10 A more feasible approach
the last decades.1–5 In recent years, white light emission from is to find such a material that can produce white light emis-
single emitting component phosphors has attracted much sion via the combination of different color emissions from an
attention due to their low electric consumption, high bright- identical luminescence center.9 Recently, a lot of research
ness and colour quality and less complex characteristics.6–8 works have been carried out to develop the single-phased
Hence, there is growing interest in developing white light emit- white-emitting phosphors which are based on the mechanism
ting phosphors for applications in display and lighting. White of the energy transfer from the sensitizer to the activator, such
light can be achieved through the combination with the as Tb3+/Dy3+ (ref. 11) and Ce3+/Dy3+1 12
for white light appli-
,
recently available blue, green and red phosphors according to cations. Dy3+ doped luminescent materials have been known
the appropriate ratio.9,10 However, this method is hard to work to exhibit two principal emissions in the blue and yellow
regions corresponding to the 4F9/2→6H15/2 and 4F9/2→6H13/2
transitions. By adjusting the yellow-to-blue intensity ratio (Y/B)
aDepartment of Physics, University of Free State, P.O. Box 339, Bloemfontein,
appropriately, it is possible to obtain pure white light from
ZA 9300, South Africa
bDepartment of Applied Physics, Indian School of Mines, Dhanbad 826004, India.
Dy3+ activated luminescent materials.6 Owing to the forbidden
f–f transitions of the Dy3+ ions and the low absorption cross-
E-mail: sudipta.som@gmail.com, sksharma.ism@gmail.com;
section (∼0.01–1 cm−1), the emission intensity of the Dy3+
Fax: +91 27 58 718 444; Tel: +91 27 51 401 3852, +91 27 58 718 5308
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singly doped phosphor is poor under UV excitation.6 Moreover, tion of the prepared precursor was performed by a solid state
the quality of the white light emission from Dy3+ activated reaction using the concept of a topotactic reaction.16 In a topo-
luminescent materials is poor due to the lack of the red light tactic reaction, the morphology of the material does not
component.6 This necessitates the codoping of some red emit- change while the phase of the material changes. Sodium car-
ting materials in Dy3+ doped phosphors. It is well known that bonate (Na2CO3) and sodium thiosulphate (Na2S2O3·5H2O)
Eu3+ is a popular activator ion for many different host lattices were mixed and pulverized with the obtained precursor in a
for a rich red emission.12 Dy3+ ions are usually introduced into mortar pestle. This mixture was then calcined at a temperature
Eu3+ doped luminescence materials to enforce the Eu3+ emis- of 1200 °C for 1 hour. During heating, the crystalline lattice of
sion. Combining this red emission with Dy3+ emission it is the Y2O3 precursor is sulphureted yielding the Y2O2S phos-
possible to achieve white light with good quality. But for this phor. The high temperature synthesis reaction is:17,18
to happen, a good host also is required. Yttrium oxysulfide has
long been known as an excellent host material for tunable
ð5 ꢀ xÞNa2CO3 þ ð3x þ 1ÞNa2S2O3
! 2ðx þ 1ÞNa2SO4 þ 4Na2Sx þ ð5 ꢀ xÞCO2
colour emitting phosphors.13 Manashirov et al. developed and
commercialized yttrium oxysulfide based blue, yellow and
green phosphors and achieved white light from a triply doped
complex oxysulfide system.14 A white long-lasting phosphor
based on Y2O2S:Tb3+, Sm3+ is reported by Lin Lin et al.15
However, studies on the energy transfer (ET) process between
the Eu3+ and Dy3+ pair are rarely reported. No work has been
reported on the white light emission from the Y2O2S:Eu3+/Dy3+
single emitting phosphor component on the basis of energy
transfer (ET). Therefore, a considerable interest has been
grown to dope and co-dope Y2O2S phosphors by Eu3+ and Dy3+
ions. The luminescent behavior and the ET phenomenon of
1
2
Y2O3 þ Na2Sx ! Y2O2S þ O2 þ Na2Sxꢀ1
ð1Þ
After cooling, the reaction product was washed a few times
with water to remove the residual flux (Na2CO3) and the
desired phosphor was obtained. During the synthesis, sodium
thiosulphate was used to achieve the sulphuration of Y2O3 and
sodium carbonate was used as the flux.18
The characterization of the rare-earth doped/co-doped yttrium
oxysulfide phosphors
the doped and co-doped Eu3+ and Dy3+ in the Y2O2S phosphors The X-ray diffractogram of the prepared phosphors was
are discussed.
recorded in a wide range of Bragg angles 2θ (20° ≤ 2θ ≤ 90°)
using a Bruker D8 advanced X-ray diffraction (XRD) measuring
instrument with Cu target radiation (λ = 0.154056 nm). Scan-
ning electron microscopy (SEM) images were obtained using a
Shimadzu SSX-550 Superscan SEM at different magnifications
to examine the crystallinity and surface morphology of the pre-
pared phosphors. The diffuse reflectance (DR) spectra were
Experimental details
The synthesis of the rare-earth doped/co-doped yttrium
oxysulfide phosphors
The Eu3+ doped, Dy3+ doped and Eu3+/Dy3+ co-doped Y2O2S recorded using a Perkin-Elmer make Lambda 35 UV-VIS spec-
phosphor were prepared by a hydrothermal process followed trophotometer in the wavelength range 200–800 nm. The
by a solid state reaction.16 In the synthesis method, europium photoluminescence (PL) studies were carried out on a Hitachi
oxide (Eu2O3), dysprosium oxide (Dy2O3), yttrium oxide (Y2O3), Fluorescence Spectrometer F-2500 with a 150 W Xe lamp as an
nitric acid (HNO3), sodium carbonate (Na2CO3) and sodium excitation source in the range 220–650 nm. The decay kinetics
thiosulfate (Na2S2O3, 5H2O) were used as starting ingredients. were studied on a Quanta Master 40 fluorometer. All the
The stock solutions of Y(NO3)3, Eu(NO3)3 and Dy(NO3)3 were studies were carried out at room temperature.
prepared by dissolving Y2O3, Eu2O3 and Dy2O3 in nitric acid
and diluting with deionized water. The Dy doped samples were
prepared by mixing Y(NO3)3 and Dy(NO3)3 according to the
formula Y2−xDyxO2S (x = 0.02, 0.04, 0.06, 0.08, 0.10). Similarly,
Eu doped samples were prepared by mixing Y(NO3)3 and
Results and discussion
Phase identification and morphology
Eu(NO3)3 according to the formula Y2−yEuyO2S (y = 0.02, 0.06, Fig. 1 shows the XRD patterns of the undoped, Eu3+, Dy3+
0.10, 0.14, 0.20). The Eu, Dy co-doped samples were prepared (3 mol%) doped and Eu3+/Dy3+ (3 : 3) co-doped Y2O2S phos-
by mixing Y(NO3)3, Eu(NO3)3 and Dy(NO3)3 according to the phors. The XRD peaks of the undoped phosphor were identi-
formula Y2−y−xEuyDyxO2S and varying the x/y ratio. NaOH was fied and indexed according to the JCPDS file no. 24-1424.19
introduced drop-wise to the vigorously stirred mixed nitrate The sharp and single diffraction peaks of the XRD pattern
solution until the pH of the mixed solution was observed confirm the formation of a single phase compound. The (hkl)
between 9 to 10. The obtained white precipitate then trans- values were labeled on the peaks. This implies that the Y2O2S
ferred to a 100 mL Teflon-lined stainless steel autoclave and phosphors have a hexagonal structure with the space group
ˉ
heated at 200 °C for 14 hours. The autoclave was then naturally P3m1 and the lattice constants a = b = 3.784 Å, c = 6.589 Å; α =
cooled to room temperature.6 The resultant precursor was β = 90° and γ = 120°. No new peaks were observed in the XRD
washed several times with deionized water and absolute patterns after doping and codoping, which indicates that the
ethanol, and then dried at 70 °C for 5 hours. The sulphuriza- incorporation of Eu3+ and Dy3+ into the Y2O2S lattice did not
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The diameter for the fiber shaped particles were found to
obey the log normal nature;25
ꢀ
ꢁ
2
ˉ
1
ꢀ ln ðd=dÞ
2σ2
pffiffiffiffiffiffiffi
PðdÞ ¼
exp
ð2Þ
dσ 2π
Here d and σ are related to the average size and the size dis-
tribution of the particles. It was found that the average dia-
meter of the undoped phosphor was estimated to be 0.15 µm
with a narrow size distribution (σ = 0.20) and the average dia-
meter of the co-doped phosphor was estimated to be 0.17 µm
with a relatively wider size distribution (σ = 0.25). There is very
little difference in the SEM morphology and the average dia-
meter of the undoped and codoped Y2O2S indicating that
doping/codoping does not affect the structure and morphology
of the synthesized phosphors.26
The mechanism for the formation of the fibrous mor-
phology normally involves two steps, one is the nucleation and
growth of the particles and the second is self-assembly by
oriented attachment.27 The assembly of the particles in a
certain way is the main initiation step for the formation of the
fiber. The synthesis and growth mechanism in this case may
be summarized as follows: all the reactants were dissolved in
the aqueous solution under intensive stirring and the pH was
kept between 9 and 10 as shown in Fig. 3(a). A flocculent type
precipitate (Fig. 3(b)) was observed after dissolving the reac-
tants in an aqueous solution. This suspension was transferred
into a Teflon-lined stainless steel autoclave. The autoclave was
maintained at a fixed temperature of 200 °C for 14 h without
stirring or shaking. In these hydrothermal conditions, nuclei
were formed from the flocculent precipitates in the suspension
as shown in Fig. 3(c). Then the nuclei start to grow by consum-
ing the flocculent precipitate materials (Fig. 3(d)). Due to the
electrostatic gravitation and intermolecular force, the larger
nanoparticles fall to the bottom of the autoclave and start to
aggregate the particles (Fig. 3(e)). The smaller ones were
attached to the aggregate and can coalesce under hydrother-
mal conditions in an oriented attachment extended in a
certain way (Fig. 3(f)).28 This is because of the anisotropic
growth of the nanocrystals in a template-free method. It is gen-
erally related to the different surface energies of the different
crystal planes of the nanocrystals. These planes with a high
surface energy have a strong tendency to capture smaller nano-
particles from the reaction solution in order to reduce their
surface energy. This leads to growth along the almost perfectly
aligned crystalline lattice planes and the formation of the 1D
fibrous morphology as shown in Fig. 3(g).27–29 The resultant
precursor was washed several times with deionized water and
absolute ethanol, and then dried at 70 °C for 5 hours. The sul-
phurization of the prepared fibre bundles was performed by a
Fig. 1 The XRD patterns of the undoped, doped and co-doped Y2O2S
phosphors.
induce any significant changes in the structure of the host
material.20
To obtain information about the morphology, grain size
and shape of the synthesized phosphor, SEM studies were
carried out for the undoped and co-doped Y2O2S phosphors.
Fig. 2 shows the SEM micrograph of the undoped Y2O2S
(Fig. 2a) and the Y1.94Eu0.03Dy0.03O2S (Fig. 2b) with different
magnifications, as indicated. The SEM images show that the
Y2O2S consists of interconnected micro-fibers with a diameter
of about 0.15 μm, which forms an intense interpenetrating
core-spike network.21 The rough surface of the Y2O2S might
also be very useful for light harvesting due to the high surface
to volume ratio.21–24
solid state reaction using the concept of
reaction.16
a topotactic
Diffuse reflectance spectroscopy and bandgap
The DR spectra of the undoped, doped and co-doped Y2O2S
phosphors were measured against a reference standard spectra
Fig. 2 SEM images of the Y2O2S phosphors.
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Fig. 3 A schematic diagram for the formation of the fibrous morphology.
on a compound. In the diffuse reflectance spectra, a sharp reference sample is measured as a function of the wavelength
band at 213 nm was observed for all samples as shown in λ, R∞ = Rsample/Rreference
32 The relationship between the diffuse
.
Fig. 4. The band at 213 nm was due to the band gap of the reflectance of the sample (R∞), the absorption coefficient (K)
undoped, doped and co-doped Y2O2S phosphors. In the case and the scattering coefficient (S) is given by the Kubelka–
of the Y2O2S:Eu3+ phosphor a band edge was observed at Munk function F(R∞):
250 nm. This is due to the charge transfer from Eu3+ to O2−
.
2
ð1 ꢀ R Þ
K
S
1
1
Other weak bands beyond 300 nm were also observed in the
case of the doped and co-doped samples. These bands were
due to the meta-stable energy states formed between the
valence band and the conduction band by the RE3+ ions.30
These extra bands were absent in the undoped phosphor. The
Kubelka–Munk theory31 was used to calculate the band gap of
the undoped, doped and co-doped Y2O2S phosphors using the
DR spectra. In the DR spectra, the ratio of the light scattered
from a thick layer of the sample and an ideal non-absorbing
FðR Þ ¼
¼
ð3Þ
1
2R
The band gap Eg and linear absorption coefficient α of a
material are related through the well-known Tauc
relationship:33
n=2
αhv ¼ C1ðhv ꢀ EgÞ
ð4Þ
where ν is the photon energy and C1 is a proportionality con-
stant. When the material scatters in a perfectly diffuse manner
(or when it is illuminated at 60° incidence), the absorption
coefficient K becomes equal to 2α. Considering the scattering
coefficient S as constant with respect to the wavelength, and
using eqn (3) and (4), the following expression can be written:
2
n
½FðR Þhvꢁ ¼ C2ðhv ꢀ EgÞ
ð5Þ
1
The value of n is 1 for direct allowed transitions, 2 for non-
metallic materials, 3 for direct forbidden transitions, 4 for
indirect allowed transitions and 6 for indirect forbidden tran-
sitions. Among the plots of [F(R∞)hν]2, [F(R∞)hν], [F(R∞)hν]2/3
,
[F(R∞)hν]1/2 and [F(R∞)hν]1/3 as a function of the photon
energy hν, the best fitting was obtained with n = 1 in eqn (5),
i.e., [F(R∞)hν]2 as a function of hν indicating that the band
transitions that occurred were direct in nature, as suggested by
Tauc et al.32,33 From the plot of [F(R∞)hν]2 versus hν, the value
of Eg was obtained by extrapolating the linear fitted regions to
[F(R∞)hν]2 = 0.
The band gap calculated from the DR spectra using the
K–M function F(R∞) was found to be around 5.91 eV for the
undoped sample, 6.1 and 6.05 eV for the Eu3+ and Dy3+ doped
Fig. 4 The DR spectra of the undoped, doped and co-doped Y2O2S
phosphors.
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samples, respectively and 5.98 eV for the co-doped sample and (a). It exhibits distinct emission lines between 500–700 nm
shown in the inset of Fig. 4.
with the strongest emission at 614 nm. These emission lines
were observed due to transitions from the excited 5D1 to 7F1
and 5D0 to 7Fj ( j = 0–4) levels of the Eu3+ ions.20 The most
intense peaks at 614 and 631 nm correspond to the hypersen-
sitive transition 5D0→7F2 levels of the Eu3+ ion due to the
forced electric dipole transition mechanism. The weak emis-
sion in the vicinity of 591 nm (590–600 nm) is ascribed to the
magnetic dipole transition of D0 to F1. It is well known that
the 5D0→7F2 hypersensitive transition is strongly influenced by
the outside surroundings. When Eu3+ is located at a low sym-
metry local site, this emission transition is often dominate in
their emission spectra. The performance of a luminescent
material is influenced by the doping concentration and it is
very important to determine the optimum concentration for
luminescence applications.20 The emission spectra for
different Eu3+ concentrations (1 mol%–10 mol%) excited at
254 nm are shown in Fig. 5(b). The PL emission intensity of
Y2O2S:Eu3+ increased with an increase of the Eu3+ concen-
tration up to 5 mol% and then decreased due to the concen-
tration quenching phenomena as per the following cross-
relaxation mechanism:20
Photoluminescence spectroscopy and asymmetric ratio
Fig. 5(a) shows the photoluminescence excitation (PLE) and
photoluminescence emission (PL) spectra of the Eu3+ doped
Y2O2S phosphor. The PLE spectrum monitored at 614 nm exhi-
bits a wide band from 220–550 nm with a maximum intensity
at 254 nm, which is assigned to the transitions due to the
charge transfer band (CTB) from O2−–Eu3+. Another weak CTB
peak at ∼315 nm is observed due to the Eu3+←S2− transition.34
In addition to the 254 and 315 nm peaks; three more peaks
have also been identified at 394 nm, 466 nm and 533 nm due
to the absorption of the incident radiation by the Eu3+ ions
and lead to the excitation of electrons from the Eu3+ ground
state to its excited 4f levels for the intra-configurational 4f–4f
5
7
transition of Eu3+ in the host lattice i.e., the F0→5L6, F0→5D2
and 7F0→5D1 transitions, respectively.33 This indicates that
these phosphors could be excited both by the near-UV light at
394 nm and blue light at 466 nm, green light at 533 nm along
with the CTB, which might find applications in solid-state
lighting technology.12 The similarity in the location and shape
of the Eu3+ emission lines of this phosphor was observed
when excited with ∼254, 394, 466 and 533 nm excitation wave-
lengths excluding their relative intensity.35 For this reason, the
7
7
5
7
5
7
Eu3þð D1Þ þ Eu3þð F0Þ ! Eu3þð D0Þ þ Eu3þð F1Þ
It was reported that the ratio between the integrated inten-
emission spectrum of this Eu3+ doped phosphor recorded at sity of the transitions D0→7F2 and D0→7F1, known as asym-
5
5
an excitation wavelength of 254 nm is depicted alone in Fig. 5 metric ratio (R/O),12 can be used as a local crystal field probe
Fig. 5 (a) The PL emission and excitation spectrum for the Eu3+ doped phosphor, (b) the variation of the PL intensity with the Eu3+ concentration,
(c) the PL emission and excitation spectrum for the Dy3+ doped phosphor and (d) the variation of the PL intensity with the Dy3+ concentration.
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to measure the nature of the local Eu3+ surroundings. Thus
this ratio indicates the degree of distortion from the inversion
symmetry of the Eu3+ ion local environment. The emissions of
Eu3+ will also contain some emission component from the
upper excited levels of Eu3+, but compared with the emissions
5
5
of D0→7F2 and D0→7F1 of Eu3+ these contributions are very
small, and would not greatly affect the trends of the R/O of
Eu3+. The calculated asymmetric ratios were 6.50, 6.58, 6.62,
6.52 and 6.45 for 1, 3, 5, 7 and 10 mol% of doped Eu3+ concen-
trations, respectively, which is strong evidence that the Eu3+
ions mainly occupy the lattice site without inversion
symmetry.20
The PLE and PL spectra of the Dy3+ doped Y2O2S phosphor
are presented in Fig. 5(c). The emission spectrum of the Dy3+
doped phosphor was recorded at an excitation wavelength of
351 nm. It consists of two distinct peaks at 488 and 575 nm
6
which correspond to the 4F9/2→6H15/2 and H13/2 transitions,
respectively. The 351 nm radiation excites the Dy3+ ions to the
6P7/2 level and then quickly relaxes to the 4F9/2 level by emitting
non-radiative transitions. The strong yellow emission band
centered at 575 nm corresponds to the hypersensitive tran-
sition 4F9/2→6H13/2. Another blue emission band at 488 nm
Fig. 6 The spectral overlap between the PL spectra of Y2O2S:Dy3+
phosphor and the PLE spectra of Y2O2S:Eu3+ phosphor.
Moreover, the Eu3+ or Dy3+ solely doped Y2O2S emits red, blue
or yellow light, so it is possible to obtain white light emission
by tailoring the red, blue or yellow light in the Eu3+ and Dy3+
co-doped Y2O2S phosphor by varying the Dy3+/Eu3+ doping
concentration.
4
corresponds to the F9/2→6H15/2 transition,6 which is less sen-
sitive to the host. The excitation spectrum of the Dy3+ doped
Y2O2S was recorded keeping the emission wavelength at
575 nm. The spectra consist of several excitation bands of the
f–f transitions, which are ascribed to the different transitions
from the ground state 6H15/2 to the various excited states of the
The effect of codoping and energy transfer
4f9 electronic configuration of the Dy3+ ions. The excitation Fig. 7(a–b) show the emission spectra of the Y2O2S:Eu3+, Dy3+
maxima located at 351 nm corresponds to the hypersensitive phosphors as a function of the Eu3+ concentration with a fixed
6
6
transition from the ground H15/2 to P7/2 level. This excitation Dy3+ concentration of 3 mol%, and as a function of the Dy3+
spectrum matches well with that of the NUV-light.6,12 It is concentration with a fixed Eu3+ concentration of 5 mol%,
hence suggested that this phosphor is suitable for solid state respectively. Fig. 7(a) shows the emission spectra of the Eu3+,
lighting. When the Dy3+ concentration was varied from 1 mol Dy3+ co-doped phosphors taking different mass ratios of Dy3+/
% to 5 mol%, the emission intensity first increased up to Eu3+ = 3 : 0, 3 : 1, 3 : 2, 3 : 3, 3 : 4, 3 : 5. The corresponding inte-
3 mol% and then decreased concluding that the optimum grated PL intensity of the Eu3+ activator (or energy acceptor)
doping concentration of Dy3+ in the Y2O2S host is 3 mol% was observed to increase, whereas that of the Dy3+ sensitizer
(Fig. 5(d)). Beyond this optimum concentration, the intensity (or energy donor) is simultaneously found to decrease mono-
decreases due to the concentration quenching phenomena tonically with increasing the Eu3+ dopant content. On the
resulting from a resonance energy transfer between neighbour- other hand, by taking different mass ratios of Eu3+/Dy3+ = 5 : 0,
ing Dy3+ ions.6 The relative intensity ratio between the electric 5 : 1, 5 : 2, 5 : 3, 5 : 4, 5 : 5 (Fig. 7(b)) both the Eu3+ and Dy3+
dipole transition and the magnetic dipole transition is known characteristic peaks decreased. These above two phenomena
as an asymmetric ratio that is the measure of the structural indicate that the Eu3+ emission is enforced by the introduction
distortion around the Dy3+ ions. The calculated asymmetric of Dy3+ and this is attributed to the energy transfer phenom-
ratios for the various concentrations of the Dy3+ doped Y2O2S ena from the Dy3+ ion to the Eu3+ ion in the Y2O2S host
phosphors are found to be 1.57, 1.59, 1.60, 1.58 and 1.57, material. Here, Eu3+ acts as an activator and Dy3+ as a sensi-
respectively.12 This value is greater than 1 which indicates that tizer.20 Fig. 7(c) shows the energy level scheme of the Eu3+ and
the surroundings of the Dy3+ ion are distorted.
Dy3+ ions in the Y2O2S host and the schematic energy transfer
As shown in Fig. 5(a and c) and Fig. 6, the comparison of mechanism between the Dy3+ and Eu3+ ions.
the PLE and PL spectra for the Y2O2S:Eu3+ and Y2O2S:Dy3+
Thus, white light can be obtained by combining the blue-
phosphors reveals a significant spectral overlap between the yellow emission of the Dy3+ ions and the red emission of the
excitation band of Eu3+ and the Dy3+ emission transitions. Eu3+ ions in a single host lattice with properly tuning the rela-
Therefore, an effective energy transfer from Dy3+ to Eu3+ is tive ratio of the Eu3+ to Dy3+ ions through the principle of
expected.36 This type of energy transfer is quite common and energy transfer.
has been observed in several Eu3+ and Dy3+ co-activated phos-
phors such as in silicate glass37 and fluoride materials.12 the Dy3+ to Eu3+ ions, the PL decay curves of the Dy3+ ions in
To further identify the evidence of the energy transfer from
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Fig. 7 (a–b) The emission spectra of Y2O2S co-doped with various mass ratios of Eu3+ to Dy3+. (c) The energy transfer scheme from Dy3+ to Eu3+
(d) The schematic representation of the energy transfer.
.
Y2O2S:Eu3+, Dy3+ were measured with an excitation at 285 nm fitting parameters. The average lifetimes were calculated using
and monitored at 575 nm, as represented in Fig. 8(a). The the formula;38
decay curve for singly Dy3+-doped Y2O2S can be well fitted into
the bi exponential function as;
2
2
kτl ¼ ðA1τ1 þ A2τ2 Þ=ðA1τ1 þ A2τ2Þ
ð7Þ
On the basis of this equation and the decay curves, the life-
time values were determined to be 11.8, 10.6, 8.9, 7.8, 6.6 and
5.8 s for y = 0, 0.01, 0.02, 0.03, 0.04 and 0.05 in Y1.97−yEuy-
I ¼ y0 þ A1 expðꢀt=τ1Þ þ A2 expðꢀt=τ2Þ
ð6Þ
where τ1 (fast components) and τ2 (slow components) are the Dy0.03O2S, respectively.
attenuation times which determine the decay rate for the
The doping of the Eu3+ ions significantly modifies the fluo-
corresponding exponential components, and A1 and A2 are the rescent dynamics of the Dy3+ ions. The decay lifetime for Dy3+
Fig. 8 (a) The PL decay curve of Dy3+ in the Y2O2S:Eu3+/Dy3+ phosphor and (b) the dependence of the energy transfer efficiency in Y2O2S:Eu3, Dy3+
on the content of Eu3+
.
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was found to reduce on increasing the Eu3+ doping content, where N is the number of ions in the unit cell, V is the volume
which strongly demonstrated the energy transfer from Dy3+ to of the unit cell and χ is the total concentration of Eu3+ and
Eu3+, as reported by Ruelle et al.39 and Jia et al.,40 respectively. Dy3+. For the Y2O2S host, N = 5 and V = 97.44 Å3. Thus, RDy–Eu
These results give further evidence that the transfer process (in Å) is determined to be 9.8, 9.1, 8.6, 8.2 and 7.9 Å for y =
happens through a resonance-type energy transfer mechan- 0.01, 0.02, 0.03, 0.04 and 0.05, respectively, in Y1.97−yEuy-
ism.36 We are also interested in investigating the energy trans- Dy0.03O2S. The critical concentration (χc), at which the lumi-
fer efficiency (ηT) of Dy3+→Eu3+ and a simple operational nescence intensity of Dy3+ is half of that observed in the
definition as suggested by Paulose et al.,41 ηT can be expressed sample in the absence of Eu3+, is 0.048. This is clearly shown
by;
in the dependence of the relative emission intensity of Dy3+ on
the different Eu3+ dopant content and is represented in Fig. 9
(a). Therefore, the critical distance (Rc) of the energy transfer
was calculated to be about 9.4 Å which indicates that the mul-
tipole–multipole interaction is dominant, which is the major
cause for the energy transfer in the present case. It was also
observed that the radiative emission from Dy3+ prevails when
RDy–Eu ≫ Rc and the energy transfer from Dy3+ to Eu3+ domi-
nates when RDy–Eu < Rc.7,36
Another way to determine the main cause in a more
detailed way for the energy transfer phenomena is described
below. On the basis of the Dexter’s energy transfer expressions
of exchange interaction, the following relationship can be
given as;45
τS
ηT ¼ 1 ꢀ
ð8Þ
τS0
where τS0 is the intrinsic decay lifetime of the sensitizer (Dy3+)
and τS is the decay lifetime of the sensitizer (Dy3+) in the pres-
ence of the activator (Eu3+). The ηT for Dy3+→Eu3+ in Y2O2S:
Eu3+, Dy3+ was calculated and is illustrated in Fig. 8(b) as a
function of the Eu3+ concentration. The ηT was found to
increase gradually from y = 0 to 0.05 with the increase of the
Eu3+ content in Y1.97−yEuyDy0.03O2S.
Forster defined the energy transfer efficiency (ηT) in terms
of the fluorescence intensities in the presence and absence of
an acceptors as;20
ꢂ
ꢃ
I0
I
η
ηT ¼ 1 ꢀ ¼ 1 ꢀ
η0
I
I0
ln
/ C
ð11Þ
ð9Þ
where I0 is the intrinsic luminescence intensity of the Dy3+
ions, I is the luminescence intensity of the Dy3+ ions with the
activator (Eu3+) present and C is the doping concentration of
the Eu3+ ions. The ln(I0/I) − C plot shows a non-linear relation-
ship (Fig. 9(b)), indicating that the exchange interaction is not
a dominant energy transfer mechanism in Y2O2S:Eu3+, Dy3+.
Based on Dexter’s energy transfer formula of the multipolar
interactions and Reisfeld’s approximation, the following
relationship can be obtained;46
The calculated transfer efficiencies are summarized in
Table 1. There is a good agreement between the energy trans-
fer efficiencies obtained from the decay time data and those
determined from the emission spectra. This good agreement
suggests that the Dy3+→Eu3+ energy transfer through a radia-
tive mechanism is neglected, since the radiative energy trans-
fer does not cause any reduction in the sensitizer decay time.
This transfer of energy may occur via one of the following
mechanisms: an exchange interaction or a multipole–multi-
pole interaction.42,43 The type of the interaction mechanism
can be identified by knowing the critical distance (Rc) between
the donor and the acceptor rare earth ions. If the Rc is greater
than 5 Å, then the multipolar interaction dominates and if Rc
is less than 5 Å, the exchange interaction will be the main
mechanism for energy transfer. As suggested by Blasse,44 the
average separation RDy–Eu can be expressed by;
η0
η
I0
I
ꢂ
/ Cn=3
ð12Þ
where η0 and η are the luminescence quantum efficiencies of
the donor in the absence and presence of the acceptor,
respectively, and C is the sum of the donor and acceptor ion
concentrations. n = 6, 8 and 10, corresponds to the dipole–
dipole, dipole–quadrupole and quadrupole–quadrupole inter-
actions, respectively.17 The values (η0/η) can be found from the
ratio of I0/I where I0 and I are the fluorescence intensities of
ꢀ
ꢁ
1=3
3V
4πχN
RDyꢀEu ꢂ 2
ð10Þ
Table 1 The energy transfer efficiency, average donor–acceptor distances and probability of an energy transfer
ηT (%)
Cdonor (mol%)
Cacceptor (mol%)
RDy–Eu (Å)
I0
I
η/η0
From emission
From decay kinetics
PDy–Eu (s−1
)
3
0
1
2
3
4
5
—
134
—
—
—
—
—
1
—
12
27
35
45
52
—
11
25
33
42
49
—
68
150
260
450
692
9.8
9.1
8.6
8.2
7.9
118
98
87
74
64
0.88
0.73
0.65
0.55
0.48
—
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Fig. 9 (a) The dependence of the relative Dy3+ emission intensity in Y2O2S:Eu3+/Dy3+ on the content of Eu3+ and (b) the dependence of ln (I0/I) of
Dy3+ on CEu
.
Fig. 10 The dependence of the I0/I of Dy3+ on (a) C6/3, (b) C8/3 and (c) C10/3
.
the donor in the absence and presence of the acceptor,
When considering the dipole–quadrupole interaction, the
respectively. The I0/I − Cn/3 plots are further illustrated in energy transfer probability PSA (in s−1) from a sensitizer to an
Fig. 10(a–c), and the relationship is observed when n = 6, 8, accepter is given by the following formula of Dexter;7,36
and 10. A linear relationship was observed when n = 8. This
ð
λS2f q
R8τS
clearly indicates that the energy transfer mechanism from the
Dy3+ to Eu3+ ions is a dipole–quadrupole reaction. Therefore,
the electric dipole–quadrupole interaction predominates in
FSðEÞFAðEÞdE
PDyꢀEu ¼ 3:024 ꢃ 1012
ð13Þ
E4
the energy transfer mechanism from the Dy3+ to Eu3+ ions in where fq is the oscillator strength of the involved absorption
Y2O2S:Eu3+, Dy3+.7,36
transition of the acceptor (Eu3+), λS (in Å) is the wavelength
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position of the sensitizer’s emission, τS is the radiative decay
time of the sensitizer (in seconds), R is the sensitizer–acceptor
average distance (in Å), E is the energy involved in the transfer
Ð
(in eV) and FS(E)FA(E)/E4dE represents the spectral overlap
between the normalized shapes of the Dy3+ emission FS(E) and
the Eu3+ excitation FA(E), and in our case it is calculated to be
about 0.01565 eV−5. The critical distance (RC) of the energy
transfer from the sensitizer to the acceptor is defined as the
distance for which the probability of the transfer equals the
probability of the radiative emission of the donor, the distance
for which PDQτs = 1. Hence, RC can be obtained from eqn (13)
as;7,36,47
ð
FSðEÞFAðEÞdE
RC ¼ 3:024 ꢃ 1012λS2f q
ð14Þ
8
E4
In the wavelength region of the Dy3+ emission
(400–600 nm) the fd electric dipole oscillator strength of Eu3+
is very low (1.2 × 10−8),46 since the Eu3+ transition is more or
less forbidden as an electric dipole transition by the parity
selection rule.48 The fq electric quadrupole oscillator strength
was estimated from its relationship with fd, fq = (a/λa)2fd.1
Using a ≈ 1 Å (the radius of Eu3+) and λEu ≈ 4880 Å (the wave-
length of the absorbed light by the europium in the dyspro-
sium emission), and considering that the forbidden electric
dipole transitions are weaker than the allowed ones by a factor
of 10−4 to 10−5, then the ratio fq/fd is 4.2 × 10−3 for Eu3+.1 So,
the oscillator strength of the Eu3+ electric quadrupole tran-
Fig. 11 The variation of the colour co-ordinates with the Dy3+/Eu3+
mass ratio in Y2O2S:Eu3+/Dy3+
.
Table 2 The CIE parameters of Y2O2S:Eu3+/Dy3+ with different Eu3+
and Dy3+ doping ratios
Colour
coordinates
(Y2−x−yEuxDyy)2O2S x : y
CCT (K)
3 : 0
3 : 1
3 : 2
3 : 3
3 : 4
3 : 5
0 : 3
0.45
0.42
0.37
0.34
0.38
0.41
0.45
0.52
0.48
0.44
0.32
0.30
0.32
0.31
3600
3972
4597
6125
3294
2657
1871
sition ( fq) is about 5.04 × 10 .
−11 49 From the above equation, on
the basis of these values and the calculated spectral overlap,
the critical distance Rc was estimated to be 9.3 Å. This result is
in good agreement with that obtained using the concentration
quenching method. Using eqn (13) and (14), the probability of
energy transfer can be rewritten as;
by changing the Dy3+/Eu3+ mass ratios. A near white light was
achieved for a 3 mol% Dy3+/Eu3+ concentration with the colour
coordinates (0.34, 0.32). This colour coordinate is close to the
standard D65 daylight with coordinates of 0.3129 (x)/0.3292
(y).51 The CCT values of the studied phosphors were found to
vary from 1871–6125 K. Generally, a CCT value greater than
5000 K indicates the cold white light used for commercial light-
ing purpose. The obtained CCT value (6125 K) for the near white
light emitting phosphor agrees with the CCT value of standard
daylight at noon (D65, 6500 K) suitable for cold near white light
emission.52–54 The results of the 3 mol% Dy3+/Eu3+ co-activated
Y2O2S phosphor are in good agreement with the photometric
parameter data reported elsewhere51–54 and support the appli-
cation of this phosphor as a cold near white light source for
solid state lighting. The values of the CIE parameters for the
different co-doped phosphors are summarized in Table 2.
ꢂ
ꢃ
8
1
τs
RC
R
PDyꢀEu
¼
ð15Þ
and it is summarized in Table 1.
Photometric characterization
Fig. 11 illustrates the CIE (International Commission on Illu-
mination) chromaticity diagram of the Y2O2S:Eu3, Dy3+ phos-
phors for different Dy3+/Eu3+ mass ratios. The CIE parameters
such as the colour coordinates (x, y) and the colour correlated
temperature (CCT) were calculated to characterize the colour
emission.7 The colour coordinates were determined using the
Colour calculator software. The colour correlated temperature
(CCT) is given by the McCamy empirical formula;50
CCT ¼ ꢀ437n3 þ 3601n2 ꢀ 6861n þ 5514:31ð16Þ
ð16Þ
where n = (x − xe)/(y − ye) and the chromaticity epicentre is at
xe = 0.3320 and ye = 0.1858.6
From the chromaticity diagram (Fig. 11), it can be seen that
the colour coordinates traverse a wide range from the yellow to
Conclusions
the red region on varying the Dy3+/Eu3+ mass ratios. The A series of Eu3+ and Dy3+ co-activated Y2O2S phosphors were
results show that the tuning of the emission colour is possible synthesized by the hydrothermal route and the luminescence
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properties were investigated under photo-excitation. The
obtained phosphors exhibit a broad excitation band ranging
from 250 to 500 nm, which can perfectly match a near-UV
8 W. Lu, N. Guo, Y. Jia, Q. Zhao, W. Lv, M. Jiao, B. Shao and
H. You, Inorg. Chem., 2013, 52, 3007.
9 B. Liu, C. Shi and Z. Qi, Appl. Phys. Lett., 2005, 86, 191111.
light indicating the suitability of these materials for lighting 10 Z. Xia, J. Zhuang and L. Liao, Inorg. Chem., 2012, 51, 7202.
technology. The spectroscopic data indicate that the 11 S. Som, S. K. Sharma and T. Shripathi, J. Fluoresc., 2013, 23,
Dy3+→Eu3+ energy transfer process takes place in the host
439.
matrix of Y2O2S. The non-radiative character of this transfer is 12 V. R. Bandi, B. K. Grandhe, H.-J. Woo, K. Janga, D. S. Shin,
inferred from both a decrease of the Dy3+ overall emission and
S.-S. Yi and J.-H. Jeong, J. Alloys Compd., 2012, 538, 85.
a decrease in the decay rate of the Dy3+ emission, when the 13 P.-F. Ai, Y.-L. Liu, L.-Y. Xiao, H.-J. Wang and J.-X. Meng, Sci.
phosphor is co-doped with dysprosium and europium. From Technol. Adv. Mater., 2010, 11, 035002.
the spectroscopic data, it is inferred that a Dy3+ to Eu3+ energy 14 O. Y. Manashirov, A. N. Georgobiani, V. B. Gutan,
transfer via an electric dipole–quadrupole interaction appears
E. M. Zvereva and A. N. Lobanov, Inorg. Mater., 2012, 48,
to be the most probable energy transfer mechanism. The
721.
efficiency of this transfer is enhanced up to 52% for the phos- 15 L. Lin, C. Kun, W. Zhifang, Y. Baogui, C. Yonghu,
phors with the highest Eu3+ content. The critical energy trans-
Z. Weiping and S. Chaoshu, J. Rare Earths, 2008, 26, 648.
fer distance has also been calculated by the concentration 16 C. Cui, G. Jiang, P. Huang, L. Wang and D. Liu, J. Lumin.,
quenching and spectral overlap methods. The results obtained 2014, 145, 665.
from the two approaches are in good agreement. Furthermore, 17 E.-J. Popovici, L. Muresan, H.-S. Amalia, E. Indrea,
3+
it was also demonstrated that the Y2−x−yO2S:Eux3+, Dyy can
M. Vasilescu, M. Nazarov and J. D. Young, Opt. Mater.,
2004, 27, 559.
be systematically tuned to generate white light under ultra-
violet radiation and it has been shown to exhibit the potential 18 S. Som, A. Choubey and S. K. Sharma, J. Exp. Nanosci.,
to act as a single-emitting-component white-emitting phos-
phor for solid state lighting applications.
2013, 1.
19 X. Zhou, M. Xing, T. Jiang, Y. Fu, Y. Peng, H. Wang and
H. Luo, J. Alloys Compd., 2014, 585.
20 S. Som and S. K. Sharma, J. Phys. D: Appl. Phys., 2012, 45,
415102.
21 Y. K. Mishra, S. Kaps, A. Schuchardt, I. Paulowicz, X. Jin,
D. Gedamu, S. Freitag, M. Claus, S. Wille, A. Kovalev,
S. N. Gorb and R. Adelung, Part. Part. Syst. Charact., 2013,
30, 775.
22 R. L. Frost, Y. Xi, R. Scholz, F. M. Belotti and M. Beganovic,
Spectrochim. Acta, Part A, 2013, 110, 7.
23 V. Kumar, N. Singh, V. Kumar, L. P. Purohit, A. Kapoor,
O. M. Ntwaeaborwa and H. C. Swart, J. Appl. Phys., 2013,
114, 134506.
Acknowledgements
The authors are thankful to University Grants Commission,
New Delhi, Government of India for funding this work under
Special Assistance Programme (no. F. 530/1/DRS/2009 (SAP-I)).
The research is also supported by the South African Research
Chairs Initiative of the Department of Science and Technology
and National Research Foundation of South Africa. The finan-
cial support from the Cluster program of the University of the
Free State is highly recognized.
24 S. Jebril, H. Kuhlmann, S. Muller, C. Ronning, L. Kienle,
V. Duppel, Y. K. Mishra and R. Adelung, Cryst. Growth Des.,
2010, 28, 42.
25 S. Som, S. K. Sharma and S. P. Lochab, Phys. Status Solidi A,
2013, 210, 1624.
References
1 R. Martínez-Martínez, S. Rivera, E. Yescas-Mendoza, 26 X. Jin, J. Strueben, L. Heepe, A. Kovalev, Y. K. Mishra,
E. Álvarez, C. Falcony and U. Caldiño, Opt. Mater., 2011, 33,
1320.
R. Adelung, S. N. Gorb and Anne Staubitz, Adv. Mater.,
2012, 24, 5676.
2 M. Jiao, N. Guo, W. Lu, Y. Jia, W. Lv, Q. Zhao, B. Shao and 27 G. Q. Zhang and X. W. D. Lou, Sci. Rep., 2013, 3, 1470.
H. You, Inorg. Chem., 2013, 52, 10340.
3 S. Som, S. Dutta, V. Kumar, V. Kumar, H. C. Swart and
S. K. Sharma, J. Lumin., 2014, 146, 162.
28 M. Ye, H. Zhong, W. Zheng, R. Li and Y. Li, Langmuir, 2007,
23, 9064.
29 R. Li, X. Tao and X. Li, J. Mater. Chem., 2009, 19, 983.
4 H. Li, H. M. Noh, B. K. Moon, B. C. Choi, J. H. Jeong, 30 S. Som, S. K. Sharma and S. P. Lochab, Mater. Res. Bull.,
K. Jang, H. S. Lee and S. S. Yi, Inorg. Chem., 2013, 52,
11210.
5 X. Jin, M. Götz, S. Wille, Y. K. Mishra, R. Adelung and
C. Zollfrank, Adv. Mater., 2013, 25, 1342.
6 S. Dutta, S. Som and S. K. Sharma, Dalton Trans., 2013, 42,
9654.
2013, 48, 844.
31 A. E. Morales, E. S. Mora and U. Pal, Rev. Mex. Fis. S, 2007,
53(5), 18.
32 S. Dutta, S. Som, J. Priya and S. K. Sharma, Solid State Sci.,
2013, 18, 114.
33 J. Tauc and A. Menth, J. Non-Cryst. Solids, 1972, 8–10, 569.
7 N. Guo, H. You, Y. Song, M. Yang, K. Liu, Y. Zheng, 34 C. Guo, L. Luan, C. Chen, D. Huang and Q. Su, Mater. Lett.,
Y. Huanga and H. Zhang, J. Mater. Chem., 2010, 20, 9061.
2008, 62, 600.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans.

View Article Online
Paper
Dalton Transactions
35 S. Som, A. Choubey and S. K. Sharma, Phys. B, 2012, 407, 44 G. Blasse, Philips. Res. Rep., 1969, 24, 131.
3515.
36 W.-J. Yang, L. Luo, T.-M. Chen and N.-S. Wang, Chem. 46 D. L. Dexter and J. A. Schulman, J. Chem. Phys., 1954, 22,
Mater., 2005, 17, 3883. 1063.
37 X.-Y. Sun, M. Gu, S.-M. Huang, X.-L. Liu, B. Liu and C. Ni, 47 Y. Zhang, G. Li, D. Geng, M. Shang, C. Peng and J. Lin,
45 D. L. Dexter, J. Chem. Phys., 1953, 21, 836.
Phys. B, 2009, 404, 111.
Inorg. Chem., 2012, 51, 11655.
38 C.-H. Huang and T.-M. Chen, Opt. Express, 2010, 18, 5089.
39 N. Ruelle, M. Pham-Thi and C. Fouassier, Jpn. J. Appl. Phys.,
1992, 31, 2786.
40 C. Jia-li, L. Ruo-ying, Z. Chong-jian, T. Shao-long, X. Wan
and S. Jun-ying, Opt. Mater., 2012, 34, 1112.
48 I. M. Batyaev and S. M. Shilov, J. Appl. Spectrosc., 1984, 41,
815.
49 R. Martinez-Martinez, A. C. Lira, A. Speghini, C. Falcony
and U. Caldino, J. Alloys Compd., 2011, 509, 3160.
50 C. S. McCamy, Color Res. Appl., 1992, 17, 142.
41 P. I. Paulose, G. Jose, V. Thomas, N. V. Unnikrishnan and 51 C. Lorbeer and A. V. Mudring, J. Phys. Chem. C, 2013, 117,
M. K. R. Warrier, J. Phys. Chem. Solids, 2003, 64, 841. 12229.
42 G. Blasse and B. C. Grabmarier, Luminescent Materials, 52 http://www.hunterlab.com/appnotes/an05_05.pdf.
Springer-Verlag, Berlin, Germany, 1994, p. 96.
43 R. Reisfeld and N. Lieblich-sofer, J. Solid State Chem., 1979,
28, 391.
53 S. H. Lee, J. H. Park, S. M. Son and J. S. Kim, Appl. Phys.
Lett., 2006, 89, 221916.
54 T. W. Murphy Jr., J. Appl. Phys., 2012, 111, 1049091.
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