Narrow-band red-emitting phosphor with negligible concentration quenching for hybrid white LEDs and plant growth applications

Article abstract of DOI:10.1039/d1dt00449b

Narrow-band red-emitters are the key to solving problems encountered by the current white LED technology. In this context, a series of new red-emitting Li₃BaSrLa₃(MoO₄)₈:Eu³⁺phosphors were synthesized

Full text of DOI:10.1039/d1dt00449b

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Narrow-band red-emitting phosphor with
negligible concentration quenching for hybrid
white LEDs and plant growth applications†
Cite this: Dalton Trans., 2021, 50,
4986
Kasturi Singh,
Marikumar Rajendran, Rachna Devi and
Sivakumar Vaidyanathan
*
Narrow-band red-emitters are the key to solving problems encountered by the current white LED
technology. In this context, a series of new red-emitting Li3BaSrLa3(MoO4)8:Eu3+ phosphors were syn-
thesized and characterized through various spectroscopic methods. All phosphor compositions were
crystallized in the monoclinic phase, with space group C2/c. A broad charge transfer (O2− → Mo6+
)
extended up to the blue region along with strong 7F0 5L6, 5D3 absorption, making them looked-for
→
materials for warm white LED applications. The concentration quenching study reveals that there was no
concentration-quenching occuring and the quantum yield of this non-concentration-quenching
Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor reaches 92.6%. The Li3BaSrLa0.3Eu2.7(MoO4)8 retain >80% of its emis-
sion intensity at 150 °C. The best red-emitting composition was integrated with near UV LED and obtained
bright red emission with CIE x = 0.6647, y = 0.3357. White LED was fabricated by integrating the blue LED
with yellow dye + red phosphor and white LED showed bright white light with CCT (5546 K), CIE (0.331,
0.385), and CRI (81%). In addition, the red LED spectrum is well-matched with the phytochrome (Pr)
absorption spectrum and is useful for plant growth applications.
Received 9th February 2021,
Accepted 10th March 2021
DOI: 10.1039/d1dt00449b
rsc.li/dalton
warm white LEDs (for the purpose of general lightings) with
improved color rendering index (CRI) and/or the luminous
Introduction
Energy (efficient) saving lighting technologies play a critical efficacy of radiation (LER) can be obtained by combining a
role in the global energy crisis and are expected to dominate broadband of yellow or green phosphor with narrow-band
energy-saving technology. Since high-performance white LEDs (narrow full-width at half maximum (FWHM)) red phosphor
have begun to replace the conventional lighting sources and a blue InGaN LED/near UV LED.3 Even though red phos-
(expected to save half of the electricity used for lightings).1 phors (spectral line) contributes only one-third to the whole
However, the highly efficient warm white LEDs still rely on white spectrum, the change in red spectral lines will obviously
high-performance phosphor materials (an integral part in the affect the warm white LED (<4000 K of CCT) performance
creation of white light in solid-state lighting). The commercial rather than the cool one (>5000 K of color correlation tempera-
white LED technology is based on yellow emitting phosphor ture (CCT)).4 It is also worth noting that the oxide-based red
conjugated with a blue LED chip. However, this method is not phosphor is still a bottleneck for high-performance white
sufficient to surpass conventional lightings (low color render- LEDs (since alternative nitride (including oxy-nitride) and
ing index and high color temperature), due to the lack of red sulfide-based phosphors suffer from the chemical stability as
component in the white LED spectrum. In order to solve this well as harsh synthetic conditions).5–7 Among all lanthanides,
mystery, researchers have endeavoured in pursuit of new red the trivalent europium (Eu3+) ion offers special spectral charac-
phosphors that are suitable for excitation by blue teristics (both excitation and emission). It gives a narrow band
(460–480 nm) or near UV LED (370–410 nm) chips (white LEDs red-emission (λmax = 610–620 nm), owing to the 5D0 → 7F2 elec-
based on near UV LED + trichromatic phosphor technology tric dipole (ED) transition with a small full width at half
also need good red emitters).2 It is well understood that the maximum (FWHM) (highly essential for getting for better
luminous output) and the excitation lines appearing at 395 nm
(7F0 → 5L6) and 465 nm (7F0 → 5D2), where the near UV or blue
Department of Chemistry, National Institute of Technology, Rourkela, Odisha –
LED emission occurs.8 However, due to the forbidden nature
769008, India. E-mail: vsiva@nitrkl.ac.in; Tel: +91-661-2462654
of 4f–4f electronic transition of Eu3+ ion (near UV and blue
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
region), the oscillator (absorption) strength is very deprived.
d1dt00449b
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This phosphor also encountered the most problematic task amounts of Li2CO3 (99.9%, Merck), BaCO3 (99.0% Merck),
(light conversion energy loss) and it is imperative to decrease SrCO3 (Extra pure 99.0%, HIMEDIA), La2O3 (minimum 99.9%,
the loss of near UV excitation and converting it into red light. Sigma Aldrich), Eu2O3 (99.9% REO, Alfa Aesar) and MoO3 (ACS
Host lattices (expected to absorb the incident photons from Reagent >99.5%, Sigma Aldrich) were taken and ground with
the LED and convert them into red emission) play a vital role the help of an agate mortar and pestle, followed by transfer-
in enhancing the absorption strength and overall lumine- ring the same into an alumina crucible. The sample mixtures
scence efficiency of the Eu3+ based phosphor.
were heated at 500 °C for 6 h and reground, treated at 850 °C
The approach could be twofold: one is to find a suitable for 12 h. After the reaction hour, the reaction mixture was
lattice having enormous absorption in the near UV to the blue cooled to room temperature, ground again into a fine powder,
region (charge transfer band) so that it can contribute to the and restored in vials.
energy transfer progression, the second way is to find a lattice
that can host rich Eu3+ ion concentration with zero concen-
tration quenching (100% activation of activator ion in the
lattice). Simultaneous achievement of good charge transfer
Characterization
band and zero concentration quenching in Eu3+ substituted The phase formation of the phosphors was checked using a
oxide-based host lattice with non-layered structure scarcely powder X-ray diffractometer (Rigaku ULTIMA IV, Japan), which
reported.8 The former can be realized by having a host lattice utilizes CuKα1 radiation (λ = 1.5418 Å) and a graphite mono-
with the MO6/MO4 group (M = Mo, W, good absorption in the chromator and from 10 to 70°, resolution 2° min−1 and step
near UV region due to metal–oxygen charge transfer (CT) size 0.02°. The field emission transmission electron micro-
band, whereas the latter can be an achieved lattice with a scope (FE-TEM, Jeol JEM2100-F, Japan) operating at 200 kV
layered crystal structure. The scheelite-based crystal structure was used for the morphological studies of synthesized phos-
has shown promise for Eu3+ activation and potential red phos- phors. The UV-visible spectrofluorometer (PerkinElmer,
phor for white LEDs. Our group has comprehensively explored UV-5400) was used to record the diffuse reflectance spectra
host lattices with MO4/MO6 where M = W/Mo with different (DRS). The Fourier transform infrared spectroscopy (FT-IR)
structures like scheelites, perovskites, etc.,4,8–10 Recently, we was performed in the range of 400–4000 cm−1 by using a
have explored trivalent europium based red phosphor with Spectrum Two IR Spectrometer, PerkinElmer. The photo-
stratified scheelite as a potential red emitter for white luminescence excitation and emission spectra were recorded
LEDs.11,12 Through continuous research efforts on trivalent using a JOBIN YVON spectrofluorometer; it uses a xenon lamp
europium-based narrow band emitters, we have identified the as the excitation source. In addition, the luminescence lifetime
Li3BaSrLa3−xEux(MoO4)8 [x = 0.3–3, in steps of 0.3] with a and absolute quantum yield measurements were taken by
layered structure that showed zero concentration quenching using Edinburg Spectrofluorometer FS-5 instruments by
(highly essential for high-performance white LEDs). It is worth attaching SC-30 integrating sphere module. A pulsed xenon
noting that the distance between Eu–Eu is higher in different lamp was used as the excitation source and the signals were
layers, which favours negligible concentration quenching in detected with a photomultiplier. The CIE color coordinates
the lattice. Lastly, the red LED was fabricated by conjugating were calculated from the obtained spectral emission data of
the synthesized selected red phosphor compositions with a the phosphors by using MATLAB software. The Judd–Ofelt (J–
near UV LED chip. These results revealed that the obtained O) intensity parameters were calculated to support the spectral
Li3BaSrLa0.3Eu2.7(MoO4)8 phosphors can be a potential red emission profile, regarding the inverse symmetry of Eu3+ ion
component in the white LEDs. Efforts have also been made to using classical J–O theory. All the measurements were per-
fabricate hybrid white LED with the amalgamation of blue formed at room temperature (RT). The LED (red or white LED)
LED
Li3BaSrLa0.3Eu2.7(MoO4)8 phosphors. These combinations in the PMMA matrix and coated on the LED chip (either near
showed greater color rendering and commission UV or blue LED).
+
yellow
organic
dye
+
Red
emitters were fabricated by mixing an appropriate amount of phosphor
Internationale d’eclairage (CIE) value as compare to that of
without red phosphor hybrid white LED. The currently syn-
thesized phosphor showed outstanding quantum efficiency
and reasonably favorable thermal stability and the hybrid
white LEDs showed superior color quality and efficiency.
Results and discussion
Crystal structure and X-ray diffraction (XRD) study
The purity and phase formation of the synthesized phosphors
were checked by using the powder X-ray diffraction method.
The diffraction patterns for Li3BaSrLa3−xEux(MoO4)8, where x =
0–3 are depicted in Fig. 1a. The XRD patterns confirm the for-
Experimental
Intense red-emitting phosphors Li3BaSrLa3(MoO4)8:Eu3+, mation of a single monoclinic phase without any additional
where x = 0–3 (in steps of 0.3) were synthesized through a impurity peak with space group C2/c. The XRD patterns match
high-temperature solid-state reaction. Highly pure precursors well with the similar crystal structure of Li3Ba2Gd3(MoO4)8
were taken for the synthesis of the phosphors. The apposite (Joint committee for powder diffraction system (JCPDS) card
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Fig. 1 (a) Powder X-ray diffraction pattern of Li3BaSrLa3−xEux(MoO4)8, where x = 0–3, (b) extended plot of highest intensity with 2θ change.
number 77-0830). However, the shift was observed in the main the change of different lattice parameters, a plot of Eu3+ con-
diffraction peak shifted towards a high angle (Fig. 1a). The centration against the lattice parameters is given in Fig. 3c for
physical performance of the diffraction peaks was monitored all compositions.
along with increasing Eu3+ concentration in the host lattice.
The plot of the highest intensity peak against the activator con-
Morphological studies
centration is shown in Fig. 2b. It clearly shows the shifting of The field emission scanning electron microscopy (FE-SEM)
the peaks towards a higher 2θ angle, which may be due to the was used for topological studies of the presently synthesized
dissimilar ionic sizes of both the RE cations (Eu3+ and La3+).
phosphors. The SEM micrographs of Li3BaSrLa0.3Eu2.7(MoO4)8
The affinity of a smaller Eu3+ ion to replace a larger La3+ ion phosphors are shown in Fig. 3. The images reveal irregular
within the lattice causes a slight shrink of the parent host shape particles with aggregation. The average particle size was
lattice. The unit cell structure of the host lattice along with the found to be 3 to 5 μm. The particles are well separated from
co-ordination of the Ba and La atom with O-atom is depicted each other, implying a proper growth of the crystal. The pres-
in Fig. 2a. The crystal structure of Li3BaSrLa3(MoO4)8 belongs ence of constituted phosphor elements (including europium
to the layered structure, La sites are separated by LiO6 dis- ions) could be identified by using elemental mapping and
torted polyhedral and BaO10 polyhedra, which is shown in EDX.
Fig. 2b. The layered structure favours the high concentration
accommodation of Eu3+ ion or non-concentration quenching
Fourier-transform infra-red (FT-IR) spectroscopy
in the lattice. The lattice parameters are a = 5.2816 Å, b = The FT-IR spectroscopy was performed to observe the molyb-
12.8192 Å, c = 19.9946 Å, β = 92.8875 and V = 1291.305 Å3, date stretching frequency. The FT-IR spectra of
respectively. The lattice parameters for all other compositions Li3BaSrLa3(MoO4)8
host
as
well
as
doped
were also calculated and are tabulated in Table ST1.† The Li3BaSrLa0.3Eu2.7(MoO4)8 are given in Fig. 4. The strong and
value of lattice parameters ‘a’ and ‘b’ constantly decreases with intense peaks at 419 cm−1 and 461 cm−1 are attributed to the
a higher concentration of Eu3+ ions in the lattice. Similarly, ‘c’ stretching vibrations of O–Mo–O of the MoO4 group present in
also follows the same trend, whereas the angle ‘β’ and volume the host lattice.12 In general, the stretching vibrations of the
of the lattice decreases constantly but slight increases for a MoO4 group are observed below 500 cm−1. Not much change
higher amount of Eu3+ (x = 3). To have a brief idea regarding was observed in the IR spectra upon the insertion of Eu3+ ions.
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Fig. 2 (a) The unit cell structure of Li3BaSrLa3(MoO4)8, coordination of Ba and La with O – atom. (b) Unit cell shows the separation of La site by
LiO6 distorted polyhedral and BaO10 polyhedra (c) plot of Li3BaSrLa3−xEux(MoO4)8 unit cell parameters as a function of Eu3+ concentration.
However, in the FT-IR spectrum of the Eu3+ activated phos- culated using Kubelka–Munk equation. The relationship
phor, some extra vibration peaks were obtained at around between the diffuse reflectance of the sample (R∞), the absorp-
3000 cm−1. The small stretching frequency around 1650 cm−1 tion coefficient (K) and the scattering coefficient (S) is given by
is attributed to O–H stretching frequency, which arises due to the Kubelka–Munk function F(R∞):
the presence of H2O molecules in the air atmosphere.
2
ð1 ꢀ R Þ
K
S
1
1
FðR Þ ¼
¼
ð1Þ
1
2R
Diffuse reflectance spectra (DRS) studies
The relationship between linear absorption coefficient (α)
and band gap (Eg) is given by Tauc13
The DRS spectra of Li3BaSrLa3−xEux(MoO4)8 phosphors are
depicted in Fig. 5. The spectra are a combination of broad
absorption as well as line-like spectra. The broad absorption
starting from 200 nm extended up to 340 nm is attributed to
ligand (O2−) to metal (Eu3+/Mo6+) charge transfer (CT). O2− has
a tendency to transfer one of its non-bonding 2p electrons to
the vacant 4d orbital of the Mo6+ ion. The intra-line 4f–4f tran-
sitions contribute to the line-like emission in the higher wave-
n
2
ꢀ
ꢁ
αhϑ ¼ C1 hϑ ꢀ Eg
ð2Þ
The relation between the absorption coefficient (α) and the
incident photon energy (hν) can be written as:
n
αhϑ ¼ Aðhϑ ꢀ EgÞ
ð3Þ
length region (350 to 600 nm). The two-major lines appearing where A is constant, α is the absorption coefficient and n
at 465 and 534 nm are corresponding to the 7F0 5D2 and depends on the type of transition involved. The value can be 1/
7F0
→
→
5D3 transition of Eu3+ ion, respectively. All the phos- 2, 2, 3 and 3/2, the former set assigned to allow direct and
phors activated with Eu3+satisfie the strong absorption from indirect, while the latter one corresponding to forbidden direct
the near UV to the blue region (240 to 450 nm) required for or indirect transitions respectively.14 The band gap was deter-
the LED application. The band gap of the phosphors was cal- mined from DRS spectra, extending a straight line to the x-axis
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Fig. 3 SEM micrographs and elemental mapping (EDX) of the Li3BaSrLa0.3Eu2.7(MoO4)8.
Fig. 4 The
FTIR
spectra
of
Li3BaSrLa3(MoO4)8and
Li3BaSrLa0.3Eu2.7(MoO4)8.
Fig. 5 DRS spectra of Li3BaSrLa3−xEux(MoO4)8 where x = 0–3.
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(hν) given in Fig. S1.† With the successful replacement of La3+ Eu3+ion are electric dipole (ED) 5D0 → 7F2 and magnetic dipole
5
7
by Eu3+ in the lattice, the band gap of the samples decreases (MD) D0 → F1 transitions. In an inorganic host lattice, in a
(3.75 to 3.29 eV). However, for the fully substituted (Eu-rich) distinct site with a center of inversion (Ci), the ED transition is
composition, the band gap slightly increases (3.48 eV). The strictly forbidden whereas, without Ci, MD transitions are
band gap (Eg) values for all the compositions are tabulated in almost disappeared. Nevertheless, partial allowance of the
Table ST2.†
selection rule of the ED transition is granted and is dominated
in the host lattice, as they are hypersensitive. By monitoring
the 5D0 → 7F2 transition at λem = 615 nm, the excitation spectra
were recorded for all phosphors and are shown in Fig. 7a.
Without any argument, the spectra contain the typical absorp-
tion line expected from the trivalent Eu ion. The lower part of
the spectra is covered by a broad absorption ranging from 200
to 360 nm, while the higher part consists of several sharp spec-
tral lines. The wide band is the result of CT occurring within
the host lattice from either Eu3+ → O2− or O2− → Mo6+. The
strength and edge of the CT band were found to enhance with
an increase in the activator concentration. The Eu-rich phos-
phor yields a strong (broad) CT band when all La3+ ions resid-
ing in the lattice are completely replaced by the incoming Eu3+
ion. The broadening of the CT band is a result of decreasing
size of Eu3+ sites, the shorter Eu–O bonds always facilitate the
charge transfer process. The strongest line among the spectral
lines centered at 394 nm is ascribed to the 7F0 → 5L6 transition
of the Eu3+ ion. The other lines appearing at 411, 466, 527, 587
PL excitation and emission spectrum of
Li3BaSrLa0.3Eu2.7(MoO4)8
The photoluminescence excitation and emission spectra of
Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor is shown in Fig. 6. DRS, as
well as the excitation spectrum of the sample, consist of two
different spectral regions. The broad band was ascribed to
overlapping of the MoO42− group and O2− → Eu3+ charge trans-
fer band. The other part, which covers sharp peaks from 350
to 600 nm is related to 4f–4f electronic transitions of the triva-
lent Eu ion. The energy transfer process is efficient in nature,
2−
the energy absorbed by the MoO4 moiety is successfully
transferred to the Eu3+ ion resulting in a brightened and
intense red emission.
PL excitation and emission studies of
Li3BaSrLa3−xEux(MoO4)8, where x = 0.3–3
PLE and PL emission spectra of the selected compositions
Li3BaSrLa3−xEux(MoO4)8, where x = 0.3–3, in steps of 0.3 are
shown in Fig. 7a–d (all the compositions of PL and PLE
spectra are given in S2–S5†). The Eu3+ ion having 6s2 5d1 4f6 is
well shielded from the ligand environment and has a special
are attributed to 7F0
→
5D3,2,1,0 transitions, respectively. A
modest red shift was observed for the present samples with
increasing Eu content along with a slight broadening of the
CT band. The presence of the strongest spectral lines at 394
and 465 nm is providing new heights for the phosphor and
can be used for blue and near UV excited white LEDs.
The PL spectra of Li3BaSrLa3−xEux(MoO4)8 excited with CT
band, 394 and 465 nm are represented in Fig. 7b, c, and d,
respectively. The spectra consist of several sharp lines like
emission, which is clearly being a note in the emission spec-
trum. The bands originating at different wavelengths like 577,
choice towards site symmetry. The 5D0
→
7F2intra state tran-
sitions of Eu3+ ion are hypersensitive in nature, so the emis-
sion intensity is strongly prejudiced by the ligand ions in the
host lattice.15 Hence, the use of Eu3+ ions as a structural probe
in the host lattices is well-known. The pre-established selection
rule does not allow the transition of Eu3+ ions as they are for-
bidden in nature. The two-major electronic transitions of
Fig. 6 (a) Diffuse reflectance and excitation spectrum of Li3BaSrLa0.3Eu2.7(MoO4)8, inset energy level diagram of Eu3+ ion, (b) emission spectrum of
Li3BaSrLa0.3Eu2.7 (MoO4)8 at λexc = 394 nm (inset CIE color coordinates for the phosphor, digital images under daylight/365 nm UV lamp and La
coordination).
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Fig. 7 PL excitation (under a. λem = 615 nm) and emission (b. at λexc = CT band c. at λexc = 394 nm and d. at λexc = 465 nm) spectra of
Li3BaSrLa3−xEux(MoO4)8 where x = 0–3 for selected compositions.
5
7
589, 613, 652 and 700 nm are attributed to D0 → 7F0, 7F1, F2, 7F0 states are non-degenerate in nature. So, transitions that
7F3 and 7F4 intra 4f–4f electronic transitions of Eu3+ ion. are originating from both the non-degenerate levels will
Among all the spectral lines, the ED transition (5D0 → 7F2) was describe the number of emitting species in the host
found to be predominated in the emission spectra of xEu3+: lattice.16 Here, the 5D0–7F0 transition is a singlet, which
Li3BaSrLa3(MoO4)8 corresponding to red emission and non – implies the presence of single emitting species for the pre-
centrosymmetric residence of the activator ion at an excitation sently synthesized phosphors. Several possible explanations
wavelength (Fig. 7b–d). It was apparent that with increasing arising from these transitions do exist. One of them is
activator concentration, the intensity of the phosphor was when the electronic environment is extremely low and it
increasing gradually. It is worth noting that among entire exci- attains lower symmetry like Cs, C1, C2, C3, C4, etc. as per
tation wavelengths (CT band, 394 and 465 nm), the CT band selection rule-governed for electronic transitions. Another
excited emission spectrum gives signified emission intensity. one reported by Fujishiro et al.,17,18 this Judd–Ofelt as well
This clearly indicates that the energy transfers from the host as selection rule forbidden transition has a tendency to
(CT band) to Eu3+ are efficient enough. However, the concen- acquire some intensity from the 5D0
tration of Eu3+ does not influence the profile and position of sition and J → J0 mixing is also equally responsible for
the emission spectra, and both remain intact. A possible expla- this transition to be appearing. The 5D0 7F0, 5D0 7F1
7F2 transitions are the most sensitive transitions
→
7F2 and 7F1 tran-
→
→
nation for this behaviour can be isolated in an electronic and 5D0
→
environment (5s and 5p), hence 4f–4f electronic transitions are of the Eu3+ ion and taken into consideration while deciding
least affected by ligand ions in the host lattice. The remarkable the symmetry, whereas the remaining two transitions
contrast in the intensity of both ED and MD transition can be 5D0
→ →
7F3 and 5D0 7F4 are not falling into the category
noticed, the intervening ED transition elucidating the sym- of either ED or MD transition, though it has some
metry around the Eu3+ is low and the same deviates from the contribution to the emission spectra and its properties. The
inversion centre.
The interesting observation was due to the presence of be regulated by crystal field splitting of 5D0
5D0 7F0 spectral transition around 577 nm. The 5D0 and transitions.
site symmetry of the Eu3+ ion within the host lattice can
→
7FJ
→
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Asymmetric ratio and full width at half maximum (FWHM)
calculations
La3+ are fully replaced by Eu3+, which is when x = 3 critical con-
centration and N = 6. The calculated Rc value is 5.64 Å. The
energy transfer process can be explained further by identifying
the type of interaction involved in the energy transfer process.
Blasse et al. have already reported that the energy transfer
process is directly correlated with the mechanism of exchange
interaction.23 The emission from the 5D0 level of Eu3+ is
quenched due to the energy transfer process as well as the
exchange interaction. Huang has previously established the
relationship between the emission intensity and doping con-
centration of Eu3+ and has already developed a theoretical
expression. To recognize the type of interaction involved in
Li3BaSrLa3(MoO4)8:Eu3+ phosphors, we calculated these
values. The relationship between emission intensity (I) and
Eu3+ concentration, per Huang, is given by:
The intensity ratio of the ED (5D0
→ →
7F2) to MD (5D0 7F1)
transition is known to be the asymmetric ratio (R) and used as
an important spectroscopic probe to support the site symmetry
of Eu3+ ions within the host lattice.19 The calculated values for
the presently synthesized phosphors have been tabulated in
Table 3. The value state that the asymmetric ratio decreases
with increasing Eu3+ content, hence the symmetry around the
Eu3+ ion is decreasing. The high value of R indicates lowering
the site symmetry and favours the red emission. The results
are in good agreement with the emission spectra. The FWHM
values were also calculated and are shown in Table 1. The
phosphor to be used in white LEDs should possess FWHM <
30 nm and is considered to be the ideal phosphor. The experi-
mentally calculated value does suggest that all the samples
have an FWHM value < 10 nm and intense red emission with
appropriate color purity. It is well known that the narrow emit-
ting band red-emitting phosphor significantly enhances the
overall performance of white LEDs.20,21
ꢄ
ꢅ
s
s
d
1ꢀ
ð
dÞ
I / a
Γ 1 þ
ð5Þ
ð6Þ
a ¼ CΓð1 ꢀ d=sÞ½X0ð1 þ A=γÞꢁd=s
where A and X0 are constants, and Γ(1 + s/d) is a Γ function, γ
is the intrinsic transition probability of sensitizer and s is the
index for electric multipole, for electric dipole–dipole, electric
dipole-quadrupole, and electric quadrupole-quadrupole inter-
action; when s = 6, 8, 10, correspondingly. By deriving the eqn
(1) and (2), they can be rewritten as
Effect of activator concentration on luminescence profile of
Li3BaSrLa3−xEux(MoO4)8
The behaviour of an ideal lattice is to retain the emission pro-
perties even at higher concentrations of the activator. The
change in concentration profile is directly related to the emis-
sion intensity. Hence, the Eu3+ concentration versus emission
intensity profile is shown in Fig. 8a. The plot shows that the
Eu3+ concentration is directly proportional to the emission
intensity. In other words, the emission intensity increases with
increasing activator ion concentration in the lattice and
reaches and a maximum when all La3+ ions are substituted by
Eu3+ ions. It also states that the energy absorbed by the host
lattice has been efficiently transferred to the Eu3+ ion in the
centre. The critical distance between two adjacent Eu3+ will
help further to understand phenomena. Hence, a pre-estab-
lished relation by Blasse22 was used to calculate the critical
distance. The critical distance is when the rate of non – radia-
tive transfer rate equals the internal decay rate (radiative rate).
The equation is given by:
I
C
s
d
¼ ꢀ log C þ log f
ð7Þ
where c is the concentration of the activator ion and f is inde-
pendent of doping concentration. Using eqn (3), log c and log
(I/c) values for the 5D0
→
7F2 transition were calculated and
plotted together, depicted in Fig. 8b. The values obtained were
fitted using the linear fitting method and the slope was
−0.7451 for –s/d, and the value is close to 1. Hence, the s value
obtained ∼ equals to 3. Thus, we concluded that the exchange
interaction mechanism is dominated by the process of energy
transfer process involved between two adjacent Eu3+ ions in
Li3BaSrLa3(MoO4)8:Eu3+ phosphors.
The observed zero or negligible concentration quenching
can be understood by looking at the crystal structure of the
host lattice. The shortest distance between La3+ is 3.921 Å,
whereas the distance varies significantly in different layers i.e.,
the shortest distance between La3+ is 8.657 Å and they are iso-
ꢂ
ꢃ
1=3
3V
4πXcN
Rc ¼ 2
ð4Þ
where Rc is the critical distance, V is the volume of the lattice, lated by MoO4 groups (shown in Fig. 9). It is well acknowl-
N is the available site for dopants in the unit cell. In the edged that the exchange interaction is more effective between
present calculations, V is taken to 1266.653, it is mentioned the trivalent europium ion (activator) if the Eu–Eu distance is
earlier that the concentration quenching occurs only when all shorter than 5 Å. In the present investigation, the interaction
Table 1 Asymmetric ratio (AR) and FWHM of Li3BaSrLa3−xEux(MoO4)8 where x = 0.3–3
Composition (x) in steps of 0.3
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
AR = I2/I1
FWHM in nm
11.764
5.30
10.607
6.50
8.547
6.84
9.438
6.42
14.120
7.65
9.736
7.22
13.047
8.22
10.090
6.05
9.428
5.32
8.911
7.55
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Fig. 8 (a) Eu3+ concentration vs. emission intensity at λexc = 394, 465 and CT band nm for Li3BaSrLa3−xEux(MoO4)8 where x = 0–3. (b) The relation
of the concentration of Eu3+ ions (log C) and the log (I/C) for the 5D0
→
7F2 transition in Li3BaSrLa3(MoO4)8 :Eu3+phosphors λexc = 394 nm.
Judd–Ofelt (J–O) spectral intensity parameters
For the emission spectra of the europium–activated
Li3BaSrLa3(MoO4)8 phosphors, important spectral parameters
can be calculated like radiative and non-radiative lifetimes,
branching ratio and quantum efficiencies, etc. The most
important optical parameters that can be calculated by using
the spectral emission data are Judd–Ofelt (J–O) intensity para-
meters. The J–O parameters are based on the classical theory
developed by Judd–Ofelt.25,26 The detailed calculation for
deriving the Ω2 and Ω4 parameters was explained in our pre-
vious article.11 The Ω6 parameter has not been observed. The
obtained values are tabulated in Table 2. It can be observed
that the Ω2 parameter has a high value as compared to Ω4.
The greater Ω2 value provides necessary information regarding
the covalency among the metal and ligand bond (Eu–O) as well
as higher the value of Ω2 higher is the asymmetry around the
Eu3+ environment and greater is the emission intensity. The
5
Ω4 is independent of the ligand field as it is related to D0 →
7F4 transition, which is least affected by ligand environment
and magnetic dipole in nature. The parameters also support
Fig. 9 The unit cell of Li3BaSrLa3(MoO4)8 along an a-axis.
Table 2 Spectral parameters of Eu3+substituted Li3BaSrLa3(MoO4)8
phosphor
between the activator ions (Eu3+) in the Li3BaSrEu3(MO4)8
lattice is two-dimensional and the interaction is also very
much similar to that of the NaEuTiO4 lattice, in which the Eu–
Eu distance is 3.630 Å (interaction is also two-dimensional).24
However, the shortest Eu–Eu distance was found to be longer
in the present case than that in the NaEuTiO4 lattice.
Therefore, one can conclude that the exchange interaction
between the activator ions is weaker in the present case and
the Eu3+ ion can substitute completely in place of the La3+ site
in the host lattice without diminishing the emission intensity.
It is also worth noting that the La-containing polyhedra are
separated by the LiO6 distorted polyhedral and BaO10 polyhe-
dra, as shown in Fig S6.†
Judd–Ofelt parameters
Ω2
Ω4
A0–1
A0–2
A0–4
Concentration (10−20 cm2) (10−20 cm2) in S−1 in S−1
in S−1
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.8624
4.4951
4.3579
4.9609
5.1481
4.3834
5.2714
3.8812
5.0530
1.8879
0.8334
0.8148
0.8354
0.8258
0.8351
0.7554
0.9992
0.9048
0.9623
1.2903
50
50
50
50
50
50
50
50
50
50
366.12 33.50
374.44 32.91
427.54 33.65
413.25 33.35
363.01 33.87
321.74 30.51
440.29 40.31
322.76 36.44
419.65 38.77
159.07 51.98
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the information obtained from spectral emission data, the time because of the partial insufficiency of the theory to
electric dipole transition is predominated in the case of pre- predict the radiative properties of phosphor materials.27
sently synthesized phosphor materials. The larger Ω2 para-
meter is a good sign that Eu3+environmental sites are distorted
Temperature-dependent PL study
in the currently studied host lattice.
To use the phosphor in practical applications, it is indeed
necessary to check the thermal stability of the phosphor
material as it has a great influence on the light output and CRI
of white LEDs and it is considered one of the most vital techni-
cal parameters. The profile of emission intensity along with
temperature thermal quenching studies was executed in the
temperature range of 25–200 °C under 394 nm excitation and
the obtained data is shown in Fig. S7a.† Naturally, with
increasing temperature, the emission intensity slowly starts
degrading, but the loss is minimal. The phosphor
Li3BaSrLa2.7Eu0.3(MoO4)8 retains 60% of its emission intensity
at 150 °C. The activation energy (Eg) was calculated using the
Arrhenius equation. The estimated value for Eg for the present
phosphor was found to be 0.30 eV. A plot of ln[(I/I0) − 1] versus
1/KT is shown in Fig. S7b.† The value of Eg is comparatively
higher than that of the CaS:Eu2+ commercial phosphor28 and
much higher than the other Eu3+ activated phosphors such as
BaZrGe3O9:Eu3+ (ΔE = 0.175 eV),29 LiGd3(MoO4)5 (ΔE = 0.24
eV)30 and Gd2(MoO4)3:Eu3+ (ΔE = 0.24 eV) phosphors.31
Another advantage is that the position of peaks in the
emission spectra does not change with increasing tempera-
ture. Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor showed improved
thermal stability, as evidenced from the data in Fig. 10a and
the corresponding Eg value is 0.24 eV. A plot of ln[(I/I0) − 1]
versus 1/KT is shown in Fig. 10b. The red phosphor
Li3BaSrLa0.3Eu2.7(MoO4)8 retain >80% of its emission intensity
at 150 °C.
The Judd–Ofelt intensity parameters were utilised further to
calculate other vital optical properties like transition probabil-
ities (Arad, Anr), radiative lifetime (τrad), the branching ratio (β).
The total radiative transition probability can be written as
X
Arad
¼
A0ꢀJ
ð8Þ
J
where Arad, is the total radiative transition probabilities. The
radiative lifetime of the excited state of the Eu3+ ion and the
total radiative transition probabilities are interrelated and can
be expressed as:
1
Arad
τrad
¼
ð9Þ
The fluorescence-branching ratio (β) for the transitions
5
starting from the D0 level can be resulting from the following
equation:
Að0 ꢀ JÞ
β ¼
ð10Þ
Aradð0 ꢀ JÞ
The stimulated emission cross-section (σe) is also an impor-
tant parameter to predict the rate of energy extraction from the
material and their laser performances, which can be calculated
as per the equation given below:
ꢆ
ꢇ
λP4
8πcn2Δλeff
σeðλpÞ ¼
ðA0ꢀJ
Þ
ð11Þ
I0
ꢂ
ꢃ
IðTÞ ¼
ð12Þ
E
kT
where λp is the emission peak wavelength, c is the velocity of
light and Δλeff is the effective bandwidth of the emission. The
calculated values of transition probabilities, radiative lifetime,
branching ratios and emission cross-section are summarized
I þ A exp
PL decay analysis
in Table 3 for Li3BaSrLa3(MoO4)8:Eu3+ phosphors. The lifetime The luminescence decay lifetime (τ) of Eu3+ in
(τrad) obtained from the Judd–Ofelt parameters differ from the Li3BaSrLa3(MoO4)8 host was measured by monitoring 5D0
→
obtained experimental value. The possible explanation for this 7F2 transition at different excitation wavelengths (CT band, 394
can be the J–O theory, which overemphasizes the radiative life- and 465 nm). A plot of the intensity of the emission and time
Table 3 Spectral parameters of Eu3+substituted Li3BaSrLa3(MoO4)8 phosphor
Emission area of cross – section
Branching ratios
(10−3) cm2
Composition
Arad (s−1
)
τrad (ms)
β1
β2
β4
7.45
σ1
σ2
σ4
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
449.62
457.35
511.19
496.6
446.88
402.25
530.6
409.2
508.42
267.05
2.22
2.19
1.96
2.01
2.24
2.49
1.88
2.44
1.97
3.74
11.12
10.93
9.78
10.06
11.18
12.43
9.42
12.21
9.83
18.72
81.42
81.87
83.63
83.21
81.23
79.98
82.97
78.87
82.54
59.56
0.542
0.566
0.624
0.578
0.686
0.769
0.720
0.549
0.811
0.721
2.723
2.707
2.447
2.795
1.785
1.670
1.944
2.458
1.662
0.773
0.429
0.461
0.514
0.508
0.630
0.680
0.852
0.567
0.960
0.629
7.19
6.58
6.71
7.57
7.58
7.59
8.90
7.62
19.46
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Fig. 10 (a) Temperature dependence profile of emission intensity at λexc = 394 nm from 298 K to 473 K, (b) plot of ln(I/I0) − 1 versus 1/KT for
Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor.
at λexc of 394 nm is given in Fig. 11, while the other two are presence of a Eu3+ ion in an 8-coordinated site, with increasing
shown in Fig. S8 and S9.† The same values are tabulated in concentration of Eu ion in the lattice, no significant changes
Table ST3.† The decay curves observed are stripper for higher were noticed in the τ value. The results conclude that the Eu3+
Eu3+ concentration. All the spectral data were fitted using a ion is residing in the more distorted site (noncentrosymmetric)
5
7
single exponential decay function given by:
in the host lattice, as a result of this, an increased D0 → F2
transition was observed.
t
τ
IðtÞ ¼ I0eꢀ
ð13Þ
where I(t) is the intensity at a given time t, I0 is the initial CIE chromaticity coordinates
intensity, and τ is the lifetime. In general, a longer value of
The colorimetric technique is a good tool for commenting on
fluorescence lifetime (τ) is obtained when the environment
around Eu3+ is highly symmetric as a result of which the 4f–4f
transition becomes more forbidden in nature. Whereas, the
shorter value indicates the increase of disorders around Eu3+
and relaxation of transition selection rules.15 For the presently
studied phosphor, the unit cell structural data confirm the
the color purity of the phosphor samples. The Commission
International de ‘Eclairage (CIE) chromaticity for the
Li3BaSrLa3(MoO4)8:Eu3+ activated phosphors (selected compo-
sition) is shown in Fig. 12a. The calculation was performed
using the spectral emission data by using MATLAB software as
an imperative mathematical tool. The National Television
Standard Committee (NTSC) value for a red color is x = 0.67
and y = 0.32. In the case of the presently synthesized
phosphors, the chromaticity coordinates move to an intense
red region with increasing Eu3+ concentration. The
calculated values for all compositions with different excitation
wavelengths (394 and 465 nm), including CT band excitation
are being tabulated in Table ST4.† All the samples exhibit x
and y values close to the NTSC standard. The CIE x and y
values are also compared with commercial CaS:Eu and the
same given in Fig. 12b. Furthermore, the color purity of
Li3BaSrLa3−xEux(MoO4)8, where x = 0.3–3 phosphors were cal-
culated32 and are presented in Fig. 12c, S9 and S10.† The color
purity can be determined using the following equation:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
ðx ꢀ xiÞ þðy ꢀ yiÞ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Color purity ¼
ð14Þ
2
2
ðxd ꢀ xiÞ þ ðyd ꢀ yiÞ
In the above equation, (x, y) denotes the CIE chromaticity
coordinates of Li3BaSrLa3(MoO4)8:Eu3+ phosphor. (xi, yi) rep-
resents the CIE white light illumination (xi = 0.310, yi = 0.316,)
and the (xd, yd) refer to the chromaticity coordinates dominant
Fig. 11 PL lifetime for Li3BaSrLa2.7Eu0.3(WO4)8 phosphors with different
concentrations under 394 nm excitation.
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Fig. 12 (a) CIE color coordinates of selected compositions, inset the image of phosphor under day light and UV irradiation, (b) fragment of CIE 1931
color diagram with color points of Li3BaSrLa3−xEux(MoO4)8 where x = 0.3–3 phosphors as a function of Eu3+ concentration, (c) color purity percen-
tage of the Li3BaSrLa3−xEux(MoO4)8 phosphors (λex = 394 nm).
wavelength points (xd = 0.682, yd = 0.317). In the present study, Quantum yield measurement and comparison with Y2O2S and
the obtained CIE values for the Li3BaSrLa0.3Eu2.7(MoO4)8 phos- Y2O3:Eu3+
phor are x = 0.651, y = 0.348, and thus the color purity was
The absolute quantum yield (QY) of the selected composi-
tions was measured to support the efficiency of the
samples for versatile applications. The absolute QY of
Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor was found to be 92.6%
(Fig. S11†). The calculated value of QY for Y2O3:Eu3+ can be
90% under the excitation of ∼270 nm. However, QY is near
about ∼9.6% with excitation at 396 nm, which is previously
reported by Long et al.36 The emission intensity of the Y2O2S,
Y2O3:Eu3+ commercial red phosphor was compared with our
sample. The result (Fig. 13) shows that the currently syn-
thesized Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor shows more than
found to be 92.32%. Fig. 11c, S8 and S9† illustrate the
color purity of all the red phosphors, all of them exhibit color
purity
greater
than
92%.
The
presently-studied
Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor exhibited excellent color
purity than that shown by other reputed red phosphors (NaSrY
(MoO4)3:Eu3+ (93.6%),33 KBaGd(MoO4)3:Eu3+ (94%)34 and
SrMoO4:Eu3+ (85.8%)).35 This result suggests that the
Li3BaSrLa3(MoO4)8:Eu3+ red phosphor shows superior color
coordinates with greater color purity, which indicates that the
phosphor can perform as a suitable red component in the
white LEDs.
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order to increase the emission intensity of the phosphor, the
concentration of the red phosphor is increased in the poly-
methyl methacrylate (PMMA) matrix. The digital image of the
LED is given in the inset Fig. 14a. In addition, efforts have also
been made to fabricate white LED based on blue LED chips.
The white LED was fabricated by using yellow organic dye
(experimental details for the synthesis of yellow dye are given
in Scheme S1†)39 and red phosphor integrated with blue LED
chip and for comparison, the white LED spectrum integrated
with yellow dye with blue LED chip is shown in Fig. S13.† The
corresponding white LED spectrum is shown in Fig. 14b and
the color coordinate CIE, CCT, CRI and the digital image are
shown as insets. It is also worth noting that the recently
reported non-concentration red borate Ca3Eu2B4O12 phosphor-
based white LED showed yellowish-white (CIE x = 0.3823, y =
0.3683). However, the presently investigated red phosphor-
based hybrid white LED showed pure white light emission.40 It
is worth noting that the endorsed CRI values depend on the
application environment; though, CRI of >80 is recommended
for most applications. In addition, the luminous efficiency of
the radiation (LER) was also calculated for the hybrid white
LEDs, which describes how bright the radiation is perceived by
the average human eye in lumen per watt (lm per W).41
Luminous efficiency of the radiation (LER) for hybrid white
LED is ∼322 lm per W. The LER value for the red LED was also
calculated and the values are 184, 238, 271 lm per W for the
ratio (PMMA : Phosphor) 1 : 10, 1 : 20 at 1 : 30, respectively.
In addition, it is also worth noting that the presently syn-
thesized selected compositions based on the red LED emission
spectra have emerged with Pr absorption and both the near
UV LED emission as well as the electric dipole transition at
615 nm red LED emission is covering the Pr absorption
(Fig. 15). The results are promising and this LED could be
potential use in plant growth applications.
Fig. 13 Comparison of emission intensity with Y2O2S, Y2O3: Eu3+ with
Li3BaSrLa0.3Eu2.7(MoO4)8..
4 times higher emission intensity. The color purity and
high QY of the samples are favorable for white applications.
It is noteworthy that the quantum efficiency of red-
emitting phosphor Sr2Si5N8:Eu2+ is ∼64% under 450 nm
excitation.37,38
Fabrication of red and white LED (based on near UV/blue LED
chip)
The best composition Li3BaSrLa0.3Eu2.7(MoO4)8 red-emitting
phosphor is integrated with near UV LED (InGaN, 395 nm).
The emission spectrum of the fabricated LED shows a red
emission at 20 mA forward bias (Fig. 14a) and their CIE color
coordinates, as well as the diagram, are shown as an inset. In
Fig. 14 (a) Comparison of emission intensity with different ratios of Li3BaSrLa0.3Eu2.7(MoO4)8 phosphor with PMMA, inset corresponding CIE color
co-ordinates and a digital photograph. (b) White LED spectrum of fabricated white LED.
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References
1 C. R. Ronda, T. Justel and H. Nikol, Rare earth phosphors:
fundamentals and applications, J. Alloys Compd., 1998, 275,
669–676.
2 V. Sivakumar and S. Kasturi, Eu3+-Based Orange-Red-
Emitting Inorganic Color Convertors: An Overview, in
Phosphors: Synthesis and Applications, ed. S. J. Dhoble,
B. Deva Prasad Raju and V. Singh, 2018, ch. 8.
3 G. B. Nair, H. C. Swart and S. J. Dhoble, A review on the
advancements in phosphor-converted light emitting diodes
(pc-LEDs): Phosphor synthesis, device fabrication and
characterization,
100622.
Prog.
Mater.
Sci.,
2020,
109,
4 V. Sivakumar and U. V. Varadaraju, Intense red-emitting
phosphors for white light emitting diodes, J. Electrochem.
Soc., 2005, 152(10), H168–H171.
5 R. Devi and S. Vaidyanthan, Narrow band red emitting
europium complexes and their application in smart white
LEDs and vapoluminescent sensors, Dalton Trans., 2020,
49, 6205–6219.
Fig. 15 The spectral overlap between the red LED and Phytochrome
(Pr) absorption.
6 Y. Q. Li, J. E. J. van Steen, J. W. H. van Krevel, G. Botty,
A. C. A. Delsing, F. J. DiSalvo, G. de With and
H. T. Hintzen, Luminescence properties of red-emitting
M2Si5N8:Eu2+ (M = Ca, Sr, Ba) LED conversion phosphors,
J. Alloys Compd., 2006, 417, 273–279.
7 K. Kato and F. Okamoto, Preparation and cathodolumines-
cence of CaS:Eu and Ca1-xSrxS: Eu phosphors, Jpn. J. Appl.
Phys., 1983, 22, 76.
8 M. Rajendran and S. Vaidyanathan, Zero-concentration
quenching: a novel Eu3+ based red phosphor with non-
layered crystal structure for white LEDs and NaSrY(MoO4)3:
Sm3+ based deep-red LEDs for plant growth, Dalton Trans.,
2020, 49, 9239–9253.
9 V. Sivakumar and U. V. Varadaraju, A promising orange-red
phosphor under near UV excitation, Electrochem. Solid-State
Lett., 2006, 9(6), H35–H38.
Conclusions
Zero concentration quenching red Li3BaSrLa3−xEux(MoO4)8
phosphors were synthesised and their optical properties system-
atically investigated. All the compositions showed a narrow-
band red emission, due to electric dipole transitions and con-
centration quenching was not observed until x = 2.7 (clearly
indicates that the phosphor having zero concentration quench-
ing). The excitation and emission properties were similar for all
compositions, however, the increase in the emission intensity
was observed while moving from low to high concentration
(0.3–3) of the Eu3+ ions. The 5D0 → 7F2 ED transition dominated
all over the emission spectra. The chromaticity coordinates and
Judd–Ofelt parameter support the emission profile of the phos-
phors. The high value of emission intensity, color purity and
quantum yield (92.6%) makes the phosphor potential for white
LEDs. The fabricated LED with the best compositions show red
emission with CIE values are located in the deep-red region.
The phosphor can be efficiently excited by either blue rays or
near UV excitation, and are budding contenders for white LED
applications. White LED fabricated by using blue LED in conju-
gation with yellow organic dye + red phosphor and the LED is
shown as CCT (5546 K), CIE (0.331, 0.385), and CRI (81%). The
fabricated red LED spectral lines are well-matched with the phy-
tochrome (Pr) absorption and this clearly indicates that this red
LED could find prospective application in plant growth.
10 M. Rajendran and S. Vaidyanathan, New red emitting phos-
phors NaSrLa(MO4)3: Eu3+[M = Mo and W] for white LEDs:
synthesis, structural and optical study, J. Alloys Compd.,
2019, 789, 919–931.
11 S.
Kasturi
and
V.
Sivakumar,
Luminescence
properties of La2W2−xMoxO9 (x= 0–2): Eu3+ materials and
their Judd–Ofelt analysis: novel red line emitting phos-
phors for pcLEDs, Mater. Chem. Front., 2017, 1, 550–
561.
12 S. Kasturi and V. Sivakumar, Synthesis and Optical
Properties
of
New
Red
Emitting
Phosphor
Li3BaSrGd3-xEux(MO4)8 for White LEDs, ChemistrySelect,
2016, 1, 5448–5462.
13 N. M. Kozhevnikova and O. A. Kopylova, Synthesis and
X-ray diffraction and IR spectroscopy studies of ternary
molybdates Li3Ba2R3(MoO4)8 (R = La-Lu, Y), Russ. J. Inorg.
Chem., 2011, 56, 993–996.
14 J. Tauc and A. Menth, States in the gap, J. Non-Cryst. Solids,
1972, 569, 8–10.
Conflicts of interest
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 4986–5000 | 4999

View Article Online
Paper
Dalton Transactions
15 J. Tauc, R. Grigorovici and A. Vancu, Optical properties and 30 J. Liang, P. Du, H. Guo, L. Sun, B. Li and X. Huang, High-
electronic structure of amorphous germanium, Phys. Status
Solidi A, 1996, 15, 627–637.
16 K. Binnemans, Interpretation of europium(III) spectra,
Coord. Chem. Rev., 2015, 295, 1–45.
17 F. Du, R. Zhu, Y. L. Huang, Y. Tao and H. J. Seo,
Luminescence and microstructures of Eu 3+-doped
Ca9LiGd2/3(PO4)7, Dalton Trans., 2011, 40, 11433–11440.
18 F. Fujishiro, R. Sekimoto and T. Hashimoto,
efficiency and thermal-stable Ca3La (GaO)3(BO3)4:Eu3+ red
phosphors excited by near-UV light for white LEDs, Dyes
Pigm., 2018, 157, 40–46.
31 D. Lu, X. Gong, Y. Chen, J. Huang, Y. Lin, Z. Luo and
Y. Huang, Synthesis and photoluminescence characteristics
of the LiGd3(MoO4)5:Eu3+ red phosphor with high color
purity and brightness, Opt. Mater. Express, 2018, 8, 259–
269.
Photoluminescence properties of CuLa1− xLnxO2 (Ln: 32 F. Meng, X. Zhang, S. Il Kim, Y. M. Yu and H. J. Seo,
lanthanide)—intense and peculiar luminescence from Ln3+
at the site with inversion symmetry, J. Lumin., 2013, 133,
217–221.
Luminescence properties of Eu3+ in gadolinium molybdate
β′-Gd2Mo3O12 phosphors, Optik, 2014, 125, 3578–3582.
33 M. Song, L. Wang, Y. Feng, H. Wang, X. Wang and D. Li,
Opt. Mater., 2018, 84, 284–291.
19 F. Fujishiro, M. Murakami, T. Hashimoto and
M. Takahashi, Orange luminescence of Eu3+-doped CuLaO2 34 P. Du and J. S. Yu, RSC Adv., 2015, 5, 60121–60127.
delafossite oxide, J. Ceram. Soc. Jpn., 2010, 18, 1217–1220.
20 X. X. Luo and W. H. Cao, Upconversion luminescence pro-
perties of Li+-doped ZnWO4:Yb,Er, J. Mater. Res., 2005, 20,
2078–2083.
35 R. Devi, R. Boddula, K. Singh, S. Kumar and
S. Vaidyanathan, New europium complexes and their use in
red light-emitting diodes and vapoluminescent sensors,
J. Inf. Disp., 2021, DOI: 10.1080/15980316.2021.1879960.
21 T. Jüstel, H. Nikol and C. Ronda, New developments in the 36 S. Long, J. Hou, G. Zhang, F. Huang and Y. Zeng, High
field of luminescent materials for lighting and displays,
Angew. Chem., Int. Ed., 1998, 37, 3084–3103.
22 V. Sivakumar and U. V. Varadaraju, An orange-red phos-
quantum efficiency red-emission tungstate based phosphor
Sr(La1−xEux)2Mg2W2O12 for WLEDs application, Ceram. Int.,
2013, 39, 6013–6017.
phor under near-UV excitation for white light emitting 37 R. J. Xie, N. Hirosaki, T. Suehiro, F. F. Xu and
diodes, J. Electrochem. Soc., 2007, 154(1), J28–J31.
23 G. Blasse, Energy transfer in oxidic phosphors, Philips Res.
Rep., 1969, 24, 131–144.
M. A. Mitomo, A Simple, Efficient Synthetic Route to
Sr2Si5N8:Eu2+-Based Red Phosphors for White Light-
Emitting Diodes, Chem. Mater., 2006, 18, 5578–
5583.
24 P. A. M. Berdowski and G. Blasse, The critical concen-
tration for quenching of the Eu3+ luminescence in some 38 R. J. Xie, N. Hirosaki, Y. Li and T. Takeda, Rare-earth acti-
Gd3+: Eu3+systems, J. Solid State Chem., 1986, 1, 86–88.
25 P. A. M. Berdowski and G. Blasse, J. Lumin., 1984, 29, 243–
260.
vated nitride phosphors: synthesis, luminescence and
applications, Materials, 2010, 3, 3777–3793.
39 A. B Kajjam, S. Giri and V. Sivakumar, Triphenylamine-
based donor–π–acceptor organic phosphors: synthesis,
characterization and theoretical study, Mater. Chem. Front.,
2017, 1(3), 512–520.
40 G.-H. Li, N. Yang, J. Zhang, J.-Y. Si, Z.-L. Wang, G.-M. Cai
and X.-J. Wang, The Non-Concentration-Quenching
Phosphor Ca3Eu2B4O12 for WLED Application, Inorg.
Chem., 2020, 59(6), 3894–3904.
26 G. S. Oflet, Intensities of crystal spectra of rare-earth ions,
J. Chem. Phys., 1962, 37, 511–520.
27 B. R. Judd, Optical absorption intensities of rare-earth
ions, Phys. Rev., 1962, 127, 750–761.
28 S. Dutta, S. Som and S. K. Sharma, Excitation spectra and
luminescence decay analysis of K+ compensated Dy3+
doped CaMoO4 phosphors, RSC Adv., 2015, 5, 7380–7387.
29 S. Kasturi and V. Sivakumar, Eu2+ luminescence in 41 R. Boddula, J. Tagare, K. Singh and S. Vaidyanathan, White
Ca3Si2O7 and spectral widening and tuning of Eu2+ emis-
sion color (orangish-red to green) by crystal chemical sub-
stitution, RSC Adv., 2016, 6(101), 98652–98662.
light emissive Europium complex and their versatile appli-
cations, Mater. Chem. Front., 2021, DOI: 10.1039/
D1QM00083G.
5000 | Dalton Trans., 2021, 50, 4986–5000
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