Enhanced red emission from Bi3+ sensitized CaWO4:Eu3+ as red component for near UV/blue LED pumped white light emission

Article abstract of DOI:10.1016/j.jallcom.2020.156022

Bi³⁺ (y = 0–15 at.percent) sensitized CaWO₄:Eu³⁺ (5 at.percent) red emitting phosphors were successfully prepared by hydrothermal method. All the samples generated intense red emission under the excitation at 393 and 464 nm. The incorporation of Bi³⁺ in CaWO₄ does not induce any interference among the optical properties of WO₄^2? and Bi³⁺. The sensitization with Bi³⁺ in CaWO₄:Eu³⁺ leads to the enhancement of ～393 nm (⁷F₀ → ⁵L₆) and ～464 nm (⁷F₀ → ⁵D₂) absorption peak of Eu³⁺. At optimal concentration of Bi³⁺, the enhancement is 5 and 10 for ～393 nm (⁷F₀ → ⁵L₆) and ～464 nm (⁷F₀ → ⁵D₂), respectively. A further enhancement in emission at optimized Bi³⁺ concentration by a factor of ～5.8 and ～22 under the respective excitation of 393 and 464 nm is observed after annealing the samples at 500 °C. The energy transfer efficiency from Bi³⁺ to Eu³⁺ was improved from 75percent to 95percent post annealing. The energy transfer process is observed to be occurred mainly due to dipole-dipole interactions. The CIE chromaticity coordinates reveal the nearly 100percent saturation of red color emission.

Full text of DOI:10.1016/j.jallcom.2020.156022

Journal Pre-proof
3+
3+
Enhanced red emission from Bi sensitized CaWO :Eu as red component for
4
near UV/blue LED pumped white light emission
Takhe Yaba, Ranjoy Wangkhem, N. Shanta Singh
PII:
S0925-8388(20)32386-0
DOI:
https://doi.org/10.1016/j.jallcom.2020.156022
JALCOM 156022
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 5 May 2020
Revised Date: 29 May 2020
Accepted Date: 9 June 2020
3+
Please cite this article as: T. Yaba, R. Wangkhem, N.S. Singh, Enhanced red emission from Bi
3+
sensitized CaWO :Eu as red component for near UV/blue LED pumped white light emission, Journal
4
of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.156022.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.

CRediT authorship contribution statement
Takhe Yaba: Writing – Original Draft, Investigation, Formal analysis. Ranjoy
Wangkhem: Writing – Review & Editing. Naorem Shanta Singh: Supervision, Writing –
Original Draft, Writing – Review & Editing.

Photographs of (a) as prepared CaWO4:Eu3+(5 at.%)/Bi3+(11 at.%) and (b) 500 oC annealed
CaWO4:Eu3+(5 at.%)/Bi3+(5 at.%) sample under the exposure of (i) 268 nm, (ii) 290 nm, (iv)
393 nm and (v) 464 nm wavelength light sources.

Enhanced red emission from Bi3+ sensitized CaWO4:Eu3+ as red component for near
UV/blue LED pumped white light emission
Takhe Yaba, Ranjoy Wangkhem and N. Shanta Singh*
Department of Physics, Nagaland University, Lumami-798627, Nagaland, India
Abstract
Bi3+ (y = 0 – 15 at.%) sensitized CaWO4:Eu3+ (5 at.%) red emitting phosphors were
successfully prepared by hydrothermal method. All the samples generated intense red
emission under the excitation at 393 and 464 nm. The incorporation of Bi3+ in CaWO4 does
not induce any interference among the optical properties of WO42- and Bi3+. The sensitization
with Bi3+ in CaWO4:Eu3+ leads to the enhancement of ~393 nm (7F0 → 5L6) and ~464 nm (7F0
5
→ D2) absorption peak of Eu3+. At optimal concentration of Bi3+, the enhancement is 5 and
10 for ~393 nm (7F0 → 5L6) and ~464 nm (7F0 → 5D2), respectively. A further enhancement in
emission at optimized Bi3+ concentration by a factor of ~5.8 and ~22 under the respective
excitation of 393 and 464 nm is observed after annealing the samples at 500 °C. The energy
transfer efficiency from Bi3+ to Eu3+ was improved from 75% to 95% post annealing. The
energy transfer process is observed to be occurred mainly due to dipole-dipole interactions.
The CIE chromaticity coordinates reveal the nearly 100% saturation of red color emission.
Keywords: Photoluminescence; energy transfer; red emitting phosphor; n-UV/blue light
*Corresponding author
E-mail: ssnaorem@nagalanduniversity.ac.in (N.S. Singh), +91-9862032690
1

1. Introduction
Lately, white light emitting diodes (w-LEDs) have attracted much interest in lighting
industries due to their significant long working lifetime, higher efficiency, low energy
consumption, eco-friendly and less thermal radiations [1-8]. White LEDs, therefore, have the
advantage over widely used incandescent and fluorescent lamps [9-11]. Mostly, w-LEDs are
achieved by a combination of LED chip with down-converting phosphors and they are known
as phosphor converted white LEDs (pc-LEDs). The commercially available w-LEDs are
produced by a combination of yellow phosphor (e. g. Y2Al2O12:Ce3+) with InGaN blue LED
chip working in the wavelength range of 450–470 nm [12-16]. However, this approach has
some drawbacks in the quest of (i) high color rendering index (CRI ˃ 90), (ii) lower
correlated color temperature (2700–4000 K) primarily due to the lack of efficient red emitting
component [17-20]. The alternative ways to generate white light with better CRI are (i) by
coating a near UV (n-UV) LED chip with a mixture of blue, green and red emitting
phosphors and (ii) blue LED chip combined with green and red phosphor [21,22]. In these
approaches, an efficient red phosphor is required. Therefore, the demand for potential and
stable red emitting phosphor, which can be efficiently excited either by n-UV or blue light,
dramatically increases. In this aspect, recent reports are available on red emitting phosphors
activated with Eu2+ or Eu3+. Generally, these red phosphors include thermally stable
nitride/oxynitride/garnet phosphors activated with Eu2+, e.g. Sr2Si5N8:Eu2+ [23],
Sr[LiAl3N4]:Eu2+ [24], M2Si5N8:Eu2+ (M = Eu, Sr, Ba) [25], Sr3Y2Ge3O12:Eu2+ [26], etc. They
have shown promising broad absorption in ~400–480 nm which is well matched with the blue
emission from InGaN based LEDs. However, their emission band is very broad, extending
beyond human eye sensitivity limit (~700 nm) and, hence, they have lower luminescence
efficacy [27]. Moreover, such phosphors require the synthesis conditions of reduced
atmosphere and high temperature leading to higher investment. On the other hand, the very
efficient Eu3+ activated phosphors with high color saturation such as Gd/YVO4:Eu3+ [28,29],
Gd2ZnTiO6:Eu3+ [30], CaMoO4:Eu3+ [31], CaWO4:Eu3+ [32,33], etc. are normally excited
through the charge/energy transfer from the host matrix excited in the UV region. This limits
their use as red component in w-LEDs with n-UV/blue LED chips as a pumping source.
However, Eu3+ emission has gain interest lately thanks to the new ways: (i)
5
enhancement of f-f absorptions of Eu3+ mainly ~394 nm (7F0 → L6) and ~464 nm (7F0 →
5D2) through sensitization, (ii) developing new type of composite host matrix etc. which are
suitable for n-UV/blue LED chip excitation. With regard to later means, Du and Yu reported
2

the shift in the charge transfer band at 379 nm with enhanced absorption when the tungstate
and molybdate are formed as a composite viz., La2MoO6-La2WO6 structure activated with
5
Eu3+ [34]. We have also recently reported the enhanced ~393 nm (7F0 → L6) and ~464 nm
5
(7F0 → D2) absorption peaks of Eu3+ in Bi3+ sensitized CaMoO4:Eu3+ red nanophosphor
which possibly is suitable for n-UV/blue LED pumped w-LEDs [35]. One such promising
candidate is CaWO4:Eu3+ which has excellent thermal and chemical stability and good optical
properties [32,33,36]. In this front, P. Du et al. recently reported the enhancement of ~464
nm (7F0 → 5D2) absorption peak of Eu3+ in Bi3+ sensitized CaWO4:Eu3+ which can be suitable
for blue LED pumped red emission [37]. There are also reports on the enhanced energy
transfer from the Bi3+ (sensitizer) to the Eu3+ (activator) ions in CaWO4:Eu3+ [38,39]. Xia et
al., reported the enhanced luminescence excited by ~392 nm (7F0 → 5L6) due to lower crystal
symmetry in Mg2+ and Sr2+ co-doped CaWO4:Eu3+ phosphor [40].
In this study, we report on hydrothermally synthesized Bi3+/Eu3+ co-doped CaWO4
nanophosphor and the influence of Bi3+ sensitization. The effect of Bi3+ sensitization and
annealing on the optical properties of CaWO4:Eu3+ is thoroughly discussed. We also report
the mechanism of the energy transfer from sensitizer to activator in CaWO4:Eu3+
nanocrystals. The details of photoluminescence (PL), PL decay lifetime, quantum yield and
coordinates in Commission Internationale de l´Eclairage (CIE) chromaticity diagram are
discussed.
2. Materials and methods
2.1 Sample preparation
All the syntheses were done via hydrothermal method. The raw materials calcium
nitrate tetra hydrate (Ca(NO3)2·4H2O, ≥ 99%), sodium tungstate dihydrate (Na2WO4·2H2O, ≥
99%), europium nitrate hexahydrate (Eu(NO3)3·6H2O, 99.9%), bismuth nitrate pentahydrate
(Bi(NO3)3·5H2O, ≥ 98%), poly(acrylic)acid (PAA) and sodium hydroxide (NaOH, ≥ 97%)
were purchased from Sigma-Aldrich and used without further purification. PAA was used as
a capping agent. Typically, to synthesize Bi3+ (1 at.%) sensitized Eu3+ (5 at.%) activated
CaWO4 nanocrystals, 250 mg of Ca(NO3)2·4H2O, 19 mg of Eu(NO3)3·6H2O, and 20 mg of
PAA were dissolved in 20 mL of de-ionized (DI) water under continuous stirring. Then 5.5
mg of Bi(NO3)3·5H2O already dispersed in 5 mL of DI water was added to this solution. As a
next step, 372 mg of Na2WO4·2H2O was added and stirred well until a homogeneous solution
3

was formed. The pH of the solution was maintained at ~7 by addition of NaOH (1 M)
solution. The obtained solution was then transferred into a 100 mL Teflon lined stainless steel
autoclave and heated in an oven at 180 C for 3 h. The precipitate so formed was separated
with centrifugation and washed with deionized water and ethanol five times. The obtained
precipitate was dried in the oven at 60 C and grounded. All other samples were synthesized
following the same procedure. The as-prepared samples were heated in a furnace at 500
for 3 h in the air.
C
2.2 Characterization
The phase purity and structural analysis of the samples were performed by using a
Bruker, D8 Advance X-ray diffractometer with a Cu Κα (λ = 1.5406 ). The Fourier
transform infrared (FT-IR) spectroscopy studies were carried out using a Shimadzu (model
8400S) spectrometer. The transmission electron microscopy (TEM) and the high-resolution
transmission electron microscopy (HR-TEM) images of the samples were recorded using an
JEOL 2000FX. All the steady state luminescence spectra and decay lifetime measurements
were carried out using a Horiba FluoroMax-4CP Spectrofluorometer equipped with 150 W
xenon lamp and 25 W μsec flash lamp as excitation light sources. For quantum yield
measurement, a K-sphere ‘petite’ integrating sphere (Reflectivity: 94-97% (380 – 500 nm),
96-98% (500 – 1500 nm) (Photon Technology International (PTI)) was used. Before
measurement, the instrument was calibrated with standard Rhodamine B dissolved in water
(standard QY ~31%) with observed quantum yield (29±2%). All the measurements were
performed at room temperature.
3. Results and Discussion
3.1 X-ray diffraction (XRD), FT-IR and TEM studies
The XRD patterns of (a) as-prepared and (b) 500 C annealed Bi3+ (y = 0, 5, 7 and 9
at.%) sensitized 5 at.% Eu3+ codoped CaWO4 (thereafter termed as CaWO4:Eu3+ (5 at.%)/Bi3+
(y at.%)) nanocrystals are respectively displayed in Fig. S1 of the supplementary material. In
all the patterns, the presence of tetragonal phase of CaWO4 (ICDD card no. 41-1431) crystal
system with scheelite structure having space group I41/a is observed [32,33]. It can be
ascertained that the substitution of Eu3+ and Bi3+ ions occurs Ca2+ sites. This is due to the
nearly equivalent ionic radii of Eu3+ (r = 1.07 Å), Bi3+ (r = 1.17 Å) and Ca2+ (r = 1.12 Å) for
the coordination number of CN = 8. However, the substitution in the W6+ (r = 0.41 Å) lattice
4

site may be ignored due to large difference in ionic radii as well as larger charge difference.
Therefore, in case of as prepared samples, co-doping of Bi3+ (~up to 9 at.%) and Eu3+ (5
at.%) ions do not induce (i) any deviation from the tetragonal phase of CaWO4 crystal
structure and (ii) formation of any other possible phases such as Bi2O3, Eu2O3 etc. Thus, as-
prepared samples has solid solution formation up to 9 at.% of Bi3+. Earlier, we reported the
limit of 9 at.% of Bi3+ for the solid solution formation in CaMoO4:Eu3+ [35]. However,
CaWO4:Eu3+, after annealing at 500 °C, such limit is found to be ≥ 7 at.% of Bi3+. This is
confirmed from the observance of Bi2O3 peak in CaWO4:Eu3+ (5 at.%)/Bi3+ (9 at.%)
(indicated by the symbol ‘●’) of the Fig. S1b (ICDD card no. 01-074-1375) [41]. The average
crystallite sizes (d) are calculated using Scherrer equation, [42]
d = kλ/βcosθ
(1)
where d is the average crystallite size, k = 0.9 is the Scherrer constant, λ = 1.5406 is the X-
ray wavelength, β is the full width half maximum (FWHM) and θ is the diffraction angle. The
average crystallite sizes for as-prepared and 500 °C annealed samples are observed to be 15–
18 and 20–32 nm, respectively. The calculated average crystallite size values are listed in
Table S1.
The IR spectrum of as-prepared CaWO4: Eu3+ (5 at.%)/Bi3+ (1 at.%) sample is shown
in Fig. S2. The broad absorption band ranging from 3000–4000 cm-1 cantered at 3398 cm-1
and the band at 1635 cm-1 are assigned to O-H stretching and H-O-H bending vibrations.
These two bands correspond to the hydrogen-bridging of the water molecules adsorbed on the
sample surface which are totally different from the coordinated water complexes [43-45]. The
absorption bands at 2943 and 1415 cm-1 are assigned as sp3 C-H stretching and CH2 bending
modes. The –C-O and –COO- asymmetric stretching modes are assigned to 1455 and 1552
cm-1, respectively [46,47]. The presence of above absorption bands confirms the surface
functionalization of PAA. The absorption bands at 834 and 437 cm-1 originate from the anti-
symmetric stretching vibration and bending vibration in tetrahedral WO4 group [48-50].
The TEM images obtained for CaWO4: Eu3+ (5 at.%)/Bi3+ (5 at.%) sample annealed at
500 C are displayed in Fig. S3 of the supplementary material. The formation of
nanoparticles with irregular shapes and sizes was observed in Fig. S3a. The particle sizes
calculated from the TEM image are in the range of 30-40 nm, as shown in the Fig. S3b. The
inter-plane spacing calculated from HR-TEM image (Fig. S3c) are found to be 0.33 and 0.19
5

nm and they correspond to the (112) and (213) planes of the tetragonal CaWO4. The selected
area electron diffraction (SAED) pattern is shown in Fig. S3d. The indexed diffraction planes
correspond to (101), (112), (213), (215) and (402) of tetragonal structure of CaWO4.
3.2 Photoluminescence (PL) studies
The photoluminescence excitation (PLE) spectra of the host CaWO4 sample
monitored at different emission wavelengths from 400 to 500 nm (10 nm increment) are
shown in Fig. S4a of the supplementary material. The absorption band appeared in the range
230–275 nm is observed in the excitation spectra. This is due to the charge transfer from O2-
2-
ligands to the central W6+ ions of tetrahedrally coordinated WO4 and, this transition is
designated as 1A1→1T1 which is spin allowed [33]. The emission spectra are observed under
the excitation at different wavelengths from 220 to 290 nm (10 nm increment) (Fig. S4b of
the supplementary material). The emission band peaking at 420 nm (~350–540 nm) is
attributed to W6+→ O2- electronic transition of CaWO4 and, this is assigned as 1A1→3T1 [51].
However, no luminescence has been observed under the excitation of 220, 230, 280 and 290
nm. Meanwhile, the PLE spectra (emission wavelength, λemi = 400 to 500 nm with 10 nm
increment) of 1 at.% Bi3+ doped CaWO4 nanophosphor are given in Fig. S5a of the
supplementary material. All the spectra are composed of two prominent peaks: (i) due to
WO42- absorption band in the range of 220–278 nm and (ii) a broad band in the range of 279–
308 nm centered at 290 nm is assigned to be the 6s2 → 6s6p electronic transition of Bi3+ ion.
2-
The presence of WO4 absorption is obvious as the range of λemi taken is well within the
emission range of WO42- (refer Fig. S4b). The emission spectra were recorded under different
excitation wavelengths of 10 nm increment from 240 to 290 nm (Fig. S5b of the
supplementary material). The emission band observed under the excitation of 240 to 270 nm
are only related to WO42- emission. While the emission bands recorded under the excitation at
3
280-300 nm are associated solely with Bi3+ due to P1→1S0 transition. This is in conjunction
with the nonappearance of emission under the excitation of 280 and 290 nm in CaWO4 (refer
Fig. S4b of the supplementary material).
The PLE spectra (normalized at 270 nm) of as-prepared samples of CaWO4:Eu3+ (5
at.%)/Bi3+ (y = 0, 1, 3, 5, 7, 9, 11, 13 and 15 at.%) monitored at 615 nm emission wavelength
are displayed in Fig. 1a. All the spectra show a broad band in the UV region (230 to 325 nm)
peaking at ~270 nm. This band is due to the overlapping of both O2-→W6+ ligand absorption
and charge transfer band (CTB) originating from electron transfer from oxygen 2p states to
6

the empty 4f states of Eu3+ (O2- → Eu3+) [37,52]. This can be ascertained from the broadening
of full width at half maximum (FWHM) from 18 nm of CaWO4 to 37 nm of CaWO4:Eu3+ (5
at.%) in the excitation band of 230–325 nm region (See Fig. S6 of the supplementary
material). In addition to this peak, we can also observe a peak at ~290 nm in the CaWO4:Eu3+
(5 at.%)/Bi3+ (y = 1, 3, 5, 7, 9, 13 and 15 at.%) samples which is related to the Bi3+ absorption
(discussed in the previous para). Moreover, the sharp lines in the longer wavelength region
are also observed. These peaks are originating from the intra configurational 4f-4f transitions
7
5
7
5
of Eu3+ ions at 362, 376, 381, 393, 416 and 464 nm corresponding to F0 → D4, F0 → G2,
5
7
5
7
5
7
5
7F0 → G3, F0 → L6, F0 → D3 and F0 → D2 transitions, respectively. Similarly, same
features of excitation can be observed in the 500 °C annealed samples (Fig. 1b). However,
one can clearly observe more pronounced Bi3+ absorption peak after annealing as compared
to as-prepared samples. This may be the indication of Bi3+ occupying the lattice sites in the
CaWO4 more perfectly upon annealing as compared to as-prepared samples. Interestingly, in
both the spectra, we can observe the enhancement in the f-f absorption intensities with the
sensitization of Bi3+, among which, there is a significant enhancement in the absorption lines
5
5
corresponding to 393 (7F0 → L6) and 464 (7F0 → D2) nm. In the case of as-prepared
samples, the maximum enhancement is observed in CaWO4:Eu3+ (5 at.%)/Bi3+ (11 at.%) with
7
5
7
5
~4.8 and ~7 fold increase in F0 → L6 (393 nm) and F0 → D2 (464 nm) absorptions,
respectively (inset of Fig. 1a). While, upon annealing, this enhancement is found to be more
by a factor of ~5 for 7F0 → 5L6 and ~10 for 7F0 → 5D2 in CaWO4:Eu3+ (5 at.%)/Bi3+ (5 at.%),
as illustrated in the inset of Fig. 1b. The reasons for such enhancement are related to the
distortion of crystal field around the Eu3+ due to difference in charge states of Ca2+ to Bi3+
and/or Eu3+ and lack of charge compensation that eventually creates vacancies at Ca2+
positions in the quest of maintaining the charge neutrality in the system [35,53].
The PL emission spectra of (a) as-prepared and (b) 500 °C annealed CaWO4:Eu3+ (5
at.%)/Bi3+ (y = 0, 1, 3, 5, 7, 9, 11, 13 and 15 at.%) samples under the 393 nm excitation are
displayed in Fig. 2. In the case of as-prepared samples, the emission intensity of Eu3+
increases with the increase of Bi3+ concentration. The maximum increase is found at ~9–11
at.% of Bi3+ thereafter, it decreases. On the other hand, this emission increase for annealed
samples rises up to ~5 at.% of Bi3+ and, then, it decreases with the further increase of Bi3+
concentration. The emission intensity decreases with further increase of Bi3+ concentration
beyond the mentioned concentration i.e. 11 and 5 at.%, respectively, for as-prepared and
annealed samples is understandable, as the Bi3+ concentration increases, there is a chance of
7

clustering among the Bi3+ ions leading to the quenching of luminescence. Similar behavior in
CaMoO4 has been reported earlier by us [35]. The identical pattern of emission spectra in
both the sets of samples i.e. (a) as-prepared and (b) 500 °C annealed are also observed under
the excitation of 464 nm (Fig. 3). The emission bands observed in all the spectra of both
figures are related to f-f transitions of Eu3+ [54-56]. The emission peaks at 591 nm (5D0→7F1)
and 655 nm (5D0→7F3) are related to parity allowed magnetic dipole transitions. While the
strongest peak at 615 nm (5D0→7F2) is due to hypersensitive electric dipole transition
whereas peak at 702 nm (5D0→7F4) is originated from electric quadrupole transitions. In all
the spectra shown in both the figures, the electric dipole transition (615 nm) is the strongest
among all other observed transitions. Such dominant peak compared to remaining peaks is
well explained on the basis of symmetry around the Eu3+ environment. In CaWO4, the Ca2+
sites are coordinated with 8 (eight) oxygen atoms with S4 point symmetry without inversion
center. This leads to the hypersensitive electric dipole transitions of Eu3+ whenever Eu3+
replaces Ca2+ ions in the CaWO4 crystal. As reported in the previous studies [35,52,57], the
possibility of Eu3+ and/or Bi3+ substitution in the Ca2+ lattice sites for charge compensation is
as (i) 3Ca2+ → 2Eu3+ + Ca2+ vacancy and/or (ii) 3Ca2+ → 2Bi3+ + Ca2+ vacancy. The creation
of such vacancies during the crystal formation while doping Eu3+ and/or Bi3+ also likely to
add more asymmetric environment around the Eu3+ leading to hypersensitive electric dipole
transitions.
As compared to unsensitized CaWO4:Eu3+ (5 at.%) in the case of as-prepared
samples, the luminescence intensity is substantially increased when CaWO4:Eu3+ (5 at.%) is
sensitized with 11 at.% of Bi3+. The enhancement is approximately by a factor of ~3.5 and
~4.5 under the excitation at 393 and 464 nm respectively (Figs. 4a and 4b). While, in the case
of 500 °C annealed samples, the enhancement is found to be higher as compared to as-
prepared samples with the factor of ~5.8 and ~22 under the excitation at 393 and 464 nm
respectively, when 5 at.% Bi3+ is used in CaWO4:Eu3+ (5 at.%) (Figs. 4c and 4d). This
enhancement in emission was expected. This agrees with the enhanced absorption intensities
of the 393 and 464 nm transitions observed in the excitation spectra (Fig. 1). It is worth to
5
mention that the strictly forbidden D0→7F0 transition at 579 nm, with the note that this
transition is allowed in Cs, Cn and Cnv site symmetries, is more pronounced in all the Bi3+
sensitized emission spectra, as compared to unsensitized samples (insets of Fig. 4). This
further corroborates the (i) occupation of Eu3+ ions in single equivalent site in the absence of
inversion symmetry with more distorted lattice [52,57,58] (ii) indication of more asymmetric
8

environment due to Bi3+ sensitization leading to more enhanced luminescence intensity in
both excitation and emission. In Fig. 5, the photographic evidence is shown for red color
emission under the exposure to different light sources of 268, 290, 393 and 464 nm of as-
prepared CaWO4:Eu3+ (5 at.%)/Bi3+ (11 at.%) and 500 °C annealed CaWO4:Eu3+ (5
at.%)/Bi3+ (5 at.%) samples.
3.3 Mechanism of energy transfer from Bi3+ to Eu3+
To investigate the energy transfer mechanism, we have selected the energy transfer
from Bi3+ over the O2-→W6+ and/or O2-→Eu3+ to excited states of Eu3+ simply because of
two reasons: (i) more intense absorption of Bi3+ over the other excitation spectra (Fig. 1) in
both as-prepared and annealed samples and (ii) not many reports are available on the detail
investigation of energy transfer from Bi3+ to Eu3+ ions in CaWO4. For this, a series of
samples were prepared with different Eu3+ concentrations and fixed Bi3+ concentration (say
11 at%). The excitation wavelength was used at 290 nm for all the cases. This wavelength is
related to Bi3+ absorption and its subsequent emission, as discussed earlier (Fig. S4 and S5 in
the supplementary material). The PL (normalized at 590 nm) emission spectra of as-prepared
and 500 C annealed CaWO4:Eu3+ (x = 0, 1, 3, 5, 7, 9, 11, 13 and 15 at.%)/Bi3+ (11 at.%)
under the 290 nm excitation are shown in Figs. 6 and 7, respectively. The inset of the graph
shows the enlarged Gaussian fitted broad emission band over 325–565 nm. From the spectra,
it is clearly observed that the emission intensity related to Bi3+ gradually decreases with the
increase of Eu3+ concentration. This may be understood as the increase of non-radiative
energy transfer from the Bi3+ to the excited states of Eu3+ ions randomly distributed in the
crystal lattice. Such energy transfer is possible because of the overlapping of emission band
of Bi3+ with the excitation bands of Eu3+ (Fig. S7 in the supplementary material). This type of
energy transfer is commonly referred as resonance energy transfer. To quantify the extent of
energy transfer with the increase of Eu3+ concentration, the energy transfer efficiency (ꢀꢁ
)
from the sensitizer Bi3+ to the activator Eu3+ was calculated by using following formula: [30]
ꢂ
ꢃ
ꢀꢁ = 1 − ꢂ
(2)
ꢃ
ꢄ
where ꢀꢁ is the energy transfer efficiency, ꢅꢆ and ꢅꢆ are the emission intensities of the
ꢄ
sensitizer Bi3+ in the presence and absence of the activator (i.e. Eu3+ in the present case),
respectively. The energy transfer efficiency of the as-prepared and 500 C annealed samples
is shown in Fig. 8. The energy transfer efficiency (ꢀꢁ) for both as-prepared and 500
C
9

annealed samples gradually increases with increasing Eu3+ concentration with the maximum
energy transfer efficiency reaching up to 75% and 95%, respectively.
Typically, a resonance type of energy transfer mechanism can be of two types: (i)
exchange interaction and (ii) multipolar interaction. Both the interactions are fully dependent
on the distance between the sensitizer and activator ions. The critical distance, Rc for the
energy transfer from Bi3+ to Eu3+ can be calculated by the following formula suggested by
Blasse, [59]
ꢇꢈ = 2 ꢉꢌꢍꢎ ꢐꢑꢒ/ꢊ
(3)
ꢊꢋ
ꢏ
where, ꢇꢈ is the critical distance, V is the volume of the unit cell, xc is the critical
concentration and N is the number of host cations in the unit cell. Here, for CaWO4 host, V is
312.63 3, N = 4. The critical concentrations are xc = 0.2 and 0.22, respectively, for annealed
and as-prepared samples. Therefore, the critical distance Rc value for the energy transfer of
CaWO4:Eu3+ (x at.%)/Bi3+ (11 at.%) was determined to be ~9 Å. It is well known that the
critical distance for energy transfer by exchange interaction is restricted by 5 Å. Here, the
calculated Rc value exceeds the range of exchange interaction mechanism. Thus, the energy
transfer between the Bi3+ and Eu3+ ions can be considered mainly due to the multipolar
interactions. Such multipolar interactions can be identified on the basis of Dexter’s energy
transfer equation and Reisfeld’s approximation, [60]
ꢂ
ꢃ
ꢛ/ꢊ
ꢔꢕ ꢘꢙꢚ
ꢄ
∝ ꢓ
(4)
ꢖꢗ
ꢖꢗ
ꢂ
ꢃ
where C is the total concentration of Bi3+ and Eu3+ ions in the sample and n = 6, 8 and 10
corresponding to dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions.
The plots between ꢅꢆ /ꢅꢆ and ꢓꢛ/ꢊ
for different n values are depicted in Fig. 9 for as-
ꢖꢗ
ꢖꢗ
ꢄ
ꢔꢕ ꢘꢙꢚ
prepared (a-c) and 500 C (d-f) annealed samples. Among different values of n, the best fit
of linear plot is observed at n = 6 in both the cases with a goodness of R2 = 0.9403 and
0.9753, respectively. This indicates that the energy transfer from Bi3+ to Eu3+ in CaWO4 is
mainly through dipole-dipole interactions.
3.4 Decay lifetime and quantum yield study
The luminescence decay lifetime is an important parameter to understand the
mechanism of luminescence in phosphors. The photoluminescence decay of 5D0 level of Eu3+
10

(615 nm emission) of as-prepared samples of CaWO4:Eu3+ sensitized with various Bi3+
concentrations are shown in Figs. 10a and 10c, while for 500 °C annealed samples are given
in Figs. 10b and 10d. In all the cases, the excitation wavelengths were 393 and 464 nm. All
the luminescence decay can be well fitted by mono exponential function given below,
ꢅ = ꢅꢜ exp ꢝ− ꢟꢞꢠ
(5)
where, I is the luminescence intensity at time t, I0 is the initial luminescence intensity at time
t = 0 and ꢡ is the luminescence decay lifetime. This suggests the location of Eu3+ ions in
uniform cation sites, as mentioned in the previous subsection of steady state luminescence.
This behavior is also expected as excitation is direct. The typical linear fitting in the
logarithmic scale of PL decay profile is presented in Fig. S8 of the supplementary material.
The PL decay lifetime values of 5D0 level of Eu3+ in Bi3+ sensitized CaWO4:Eu3+ as-prepared
and 500 °C annealed samples are summarized in Table S2 of the supplementary material. The
PL decay lifetime draws two understandings: (i) the lifetime increases with the Bi3+
5
sensitization may be ascertained to the reduction in the non-radiative relaxation of D0 level
to the ground states. This may be induced by relaxing the forbidden transitions due to higher
distortion of crystal lattice upon addition of Bi3+ and (ii) the lifetime increase after annealing
the samples when compared to as-prepared samples may be attributed to the removal of non-
radiative pathways by various organic groups of capping agent present on the particle surface.
On the other hand, the annealing also leads to larger particle size with the formation of more
perfect crystals.
3.5 Quantum yield and CIE chromaticity
The quantum yield (φ) is given by the ratio of number of emitted photons to number
of photons absorbed by the fluorophore. Generally, it is defined by the measure of ratio of
total radiative transition rate (kr) to the total radiative (kr) and non-radiative (knr) transition
rates as: [51]
ꢣ
ꢤ
ꢢ =
(6)
ꢣ ꢘꢣ
ꢤ
ꢥꢤ
However, experimentally the ‘φ’ is calculated as absolute measure using integrating sphere
(mentioned in the characterization section) with the following relation,
ꢂ
ꢦꢧꢨꢃꢃꢨꢩꢥ
φ =
(7)
ꢂ
ꢯꢂ
ꢃꢪꢧꢫꢬꢦ ꢭꢩꢬꢮꢦꢤ ꢃꢪꢧꢫꢬꢦ
11

where Iemission is the luminescence emission intensity of sample, Isample holder is the intensity of
light used to excite pure sample holder (Quartz slide mounted on Teflon in the present case),
and Isample is the intensity of light used for exciting sample in sample holder. Therefore,
difference between areas of Isample holder and Isample gives the photons absorbed by the sample.
The quantum yield measurements were done for optimized samples i.e. as-prepared,
CaWO4:Eu3+ (5 at.%)/Bi3+ (11 at.%) and annealed, CaWO4:Eu3+ (5 at.%)/Bi3+ (5 at.%) under
the excitation wavelengths of 393 and 464 nm. A typical measurement snapshot along with
CIE coordinate output of CaWO4:Eu3+ (5 at.%)/Bi3+ (5 at.%) under the 464 nm excitation is
shown in the supplementary material, Fig. S9. The values measured for as-prepared are 12
and 8%. While for annealed samples, the values are found to be 14 and 17%. The respective
excitation wavelengths in both cases are 393 and 464 nm.
The Commission Internationale de l´Eclairage (CIE) chromaticity diagram coordinate
calculation allows to predict the color trueness of the light emission theoretically. The
calculated CIE coordinates of the as-prepared CaWO4:Eu3+ (5 at.%)/Bi3+ (y = 0, 1, 3, 5, 7, 9,
11, 13 and 15) samples excited at 393 and 464 nm are listed in Table S3 of the supplementary
material. The positions of related points are presented in Fig. S10 of the supplementary
material. Similarly, the characteristics of 500 C annealed CaWO4:Eu3+ (5 at.%)/Bi3+ (y = 0,
1, 3, 5, 7, 9, 11, 13 and 15) samples excited at 393 and 464 nm are shown in Table S4 and
Fig. S11 of the supplementary material. From these results, it can be confirmed that all the
samples show the red color emission under these excitation wavelengths. The emitted color
from these samples is nearly 100% saturated. A typical calculation for color saturation value
for 500 °C annealed CaWO4:Eu3+ (5 at.%)/Bi3+ (5 at.%) under the 464 nm excitation is
presented in Fig. S12. The calculated color saturation values of as-prepared CaWO4:Eu3+ (5
at.%)/Bi3+ (11 at.%) and 500 °C annealed CaWO4:Eu3+ (5 at.%)/Bi3+ (5 at.%) under 393 and
464 nm are tabulated in Table S5 of the supplementary material.
Conclusions
The hydrothermally synthesized Bi3+ activated CaWO4:Eu3+ nanocrystals were
thoroughly characterized by XRD, FT-IR and TEM techniques. We have extensively
investigated the luminescence properties and energy transfer mechanism of these samples. An
enhancement in the intensity of absorption peaks of Eu3+ viz ~393 nm (7F0 → 5L6) and ~464
nm (7F0 → 5D2) is observed upon Bi3+ sensitization. The Eu3+ emission at 615 nm (5D0 → 7F2)
was improved by a factor of ~5.8 and ~22 under the excitation of 393 and 464 nm,
12

respectively, when 5 at.% Bi3+ was used as sensitizer in CaWO4:Eu3+ (5 at.%) after annealing
at 500 °C. Reduction in the non-radiative transitions pathways is observed upon annealing the
as-prepared samples. A resonance type of energy transfer occurs from Bi3+ to the excited
states of Eu3+ mainly through dipole-dipole interaction. At the optimal concentration of Bi3+,
the quantum yield values are found to be (i) 12 and 8% for as-prepared and (ii) 14 and 17%
for annealed samples under the excitation at 393 and 464 nm. CIE chromaticity coordinates
reveal the nearly 100% saturation of red color emission for optimized samples. These
nanophosphors can be promising candidate as red emitter for both n-UV and blue LED
pumped w-LEDs for solid state lighting applications.
CRediT authorship contribution statement
Takhe Yaba: Writing – Original Draft, Investigation, Formal analysis. Ranjoy
Wangkhem: Writing – Review & Editing. Naorem Shanta Singh: Supervision, Writing –
Original Draft, Writing – Review & Editing.
Acknowledgement
NS Singh thanks the Science & Engineering Research Board (DST), New Delhi, for
financial support (CRG Project No. CRG/2018/003154). Takhe Yaba thanks the Ministry of
Tribal Affairs, New Delhi for National fellowship for higher education (NFHE) for ST
students and Ranjoy Wangkhem thanks the Council of Scientific and Industrial Research
(CSIR), New Delhi for Senior Research Fellowship. Authors also thank SAIF, NEHU
Shillong and NIT Manipur for TEM and XRD facilities.
Appendix A. Supplementary material
XRD pattern, FTIR spectrum, TEM images, additional PL spectra, Lifetime
tabulation, Quantum yield measurement, CIE color coordinates and Color purity calculation.
13

References
[1] C.H. Huang, T.M. Chen, A novel single-composition trichromatic white-light
Ca3Y(GaO)3(BO3)4:Ce3+,Mn2+,Tb3+ phosphor for UV-light emitting diodes, J. Phys.
Chem. C 115 (2011) 2349–2355.
[2] N. Guo, Y.J. Huang, H.P. You, M. Yang, Y.H. Song, K. Liu, Y.H. Zheng,
Ca9Lu(PO4)7:Eu2+,Mn2+: A potential single-phased white-light-emitting phosphor
suitable for white-light-emitting diodes, Inorg. Chem. 49 (2010) 10907–10913.
[3] N.S. Singh, R. Wangkhem, T. Yaba, S. Devi, M. N. Luwang, N. Yaiphaba, H. S. Devi,
T.D. Singh, Multicolour and nearly white light emission in YP0.8V0.2O4:Sm3+ nanorods:
controlled energy transfer, J. Alloys Compd. 726 (2017) 1161-1167.
[4] C.H. Huang, T.M. Chen, W.R. Liu, Y.C. Chiu, Y.T. Yeh, S. M. Jang, A single-phase
emission-tunable phosphor Ca9Y(PO4)7:Eu2+,Mn2+ with efficient energy transfer for
white-light-emitting diodes, ACS Appl. Mater. Interfaces 2 (2010) 259–264.
[5] H. Zhou, Y. Jin, M. Jiang, Q. Wanga, X. Jiang, A single-phased tunable emission
phosphor MgY2Si3O10: Eu3+, Bi3+ with efficient energy transfer for white LEDs, Dalton
Trans. 44 (2015) 1102-1109.
[6] Z. Xia, Y. Zhang, M.S. Molokeev, V.V. Atuchin, Y. Luo, Linear structural evolution
induced tunable photoluminescence in clinopyroxene solid-solution phosphors, Sci.
Reports 3 (2013) 3310.
[7] H. Ji, Z. Huang, Z. Xia, M.S. Molokeev, V.V. Atuchin, M. Fang, S. Huang, New yellow-
emitting Whitlockite-type structure Sr1.75Ca1.25(PO4)2:Eu2+ phosphor for near-UV
pumped white light-emitting devices, Inorg. Chem. 53 (2014) 5129−5135.
[8] J. L. Leaño, S-Y. Lin, A. Lazarowska, S. Mahlik, M. Grinberg, C. Liang, W. Zhou, M.S.
Molokeev, V.V. Atuchin, Y-T. Tsai, C.C. Lin, H-S. Sheu, R-S. Liu, Green light-
excitable Ce-doped nitridomagnesoaluminate Sr[Mg2Al2N4] phosphor for white light-
emitting diodes, Chem. Mater. 28(19) (2016) 6822-6825.
[9] X. Zhang, M. Gong, A new red-emitting Ce3+, Mn2+-doped barium lithium silicate
phosphor for NUV LED application, Mater. Lett. 65 (2011) 1756-1758.
[10] Y. Linghong, Z. Liya, W. Zhengliang, S. Jianhua, G. Fuzhong, W. Weiping, W. Wei,
KGd(MoO4)2:Eu3+ as a promising red phosphor for light-emitting diode application,
Curr. Appl. Phys. 10 (2010) 208-213.
14

[11] S. Nakamura, G. Fasol, The Blue Laser Diode, first ed., Springer, Berlin, 1997.
[12] X. Meng, Y. Wang, Hydrothermal synthesis and luminescent properties of LixEuxSr1-
2xMoO4 red phosphor, J. Rare Earth 29 (2011) 1040–1044.
[13] J.K. Park, M.A. Lim, C.H. Kim, H.D. Park, J.T. Park, S.Y. Choi, White light-emitting
diodes of GaN-based Sr 2 SiO 4 : Eu and the luminescent properties, Appl. Phys. Lett. 82
(2003) 683-685.
[14] V.V. Atuchin, N.F. Beisel, E.N. Galashov, E.M. Mandrik, M.S. Molokeev, A.P.
Yelisseyev, A.A. Yusuf, Z. Xia, Pressure-stimulated synthesis and luminescence
properties of microcrystalline (Lu,Y)3Al5O12:Ce3+ garnet phosphors, ACS Appl. Mater.
Interfaces 7(47) (2015) 26235-26243.
[15] H. Ji, L. Wang, M.S. Molokeev, N. Hirosaki, R. Xie, Z. Huang, Z. Xia, O.M. Kate, L.
Liu,
V.V.
Atuchin,
Structure
evolution
and
photoluminescence
of
Lu3(Al,Mg)2(Al,Si)3O12:Ce3+ phosphors: new yellow-color converters for blue LED-
driven solid state lighting, J. Mater. Chem. C 4 (2016) 6855-6863.
[16] E.N. Galashov, V.V. Atuchin, T.A. Gavrilova, I.V. Korolko, Y.M. Mandri, A.P.
Yelisseyev, Z. Xia, Synthesis of Y3Al5O12:Ce3+ phosphor in the Y2O3–Al metal–CeO2
ternary system, J. Mater. Sci. 52 (2017) 13033-13039.
[17] Y. Zhang, W. Gong, J. Yu, Y. Lin, G. Ning, Tunable white-light emission via energy
transfer in single-phase LiGd(WO4)2:Re3+(Re=Tm, Tb, Dy, Eu) phosphors for UV-
excited WLEDs, RSC Adv. 5 (2015) 96272-96280.
[18] Z.B. Tang, C.L. Xu, X.R. Wei, X.G. Zhang, Y.B. Chen, Improved photoluminescence
intensity and thermal stability brought by increasing Eu3+ content in KBaY1-xEuxSi2O7
solid-solution phosphors, J. Alloys Compd. 695 (2017) 2745-2750.
[19] C.F. Guo, J. Yu, X. Ding, M. Li, Z.Y. Ren, J.T. Bai, A dual-emission phosphor
LiCaBO3:Ce3+,Mn2+ with energy transfer for near-UV LEDs, J. Electrochem. Soc. 158
(2011) J42–J46.
[20] M.M. Shang, G.G. Li, X.J. Kang, D.M. Yang, D.L. Geng, J. Lin, Tunable luminescence
and energy transfer properties of Sr3AlO4F:RE3+ (RE = Tm/Tb, Eu, Ce) phosphors, ACS
Appl. Mater. Interfaces 3 (2011) 2738–2746.
[21] G. Li, Y. Tian, Y Zhao, J. Lin, Recent progress in luminescence tuning of Ce3+ and Eu2+-
activated phosphors for pc-WLEDs, Chem. Soc. Rev. 44 (2015) 8688-8713.
15

[22] D.Y. Wang, Y.C. Wu, T.M. Chen, Synthesis, crystal structure, and photoluminescence
of a novel blue-green emitting phosphor: BaHfSi3O9:Eu2+ J. Mater. Chem. 21 (2011)
18262–18265.
[23] R.J. Xie, N. Hirosaki, T. Suehiro, F.F. Xu, M. Mitomo, A simple, efficient synthetic
route to Sr2Si5N8:Eu2+-based red phosphors for white light-emitting diodes, Chem.
Mater. 18 (2006) 5578-5583.
[24] P. Pust, V. Weiler, C. Hecht, A. Tücks, A.S. Wochnik, A.K. Henß, D. Wiechert, C.
Scheu, P.J. Schmidt, W. Schnick, Narrow-band red-emitting Sr[LiAl3N4]:Eu2+ as a next-
generation LED-phosphor material, Nature Mater. 13 (2014) 891-896.
[25] M. Zeuner, F. Hintze, W. Schnick, Low temperature precursor route for highly efficient
spherically shaped LED-phosphors M2Si5N8:Eu2+ (M = Eu, Sr, Ba), Chem. Mater. 21
(2009) 336-342.
[26] S.K. Hussain, J.S. Yu, Broad red-emission of Sr3Y2Ge3O12:Eu2+ garnet phosphors under
blue excitation for warm WLED applications, RSC Adv. 7 (2017) 13281-13288.
[27] X. Huang, Red phosphor converts white LEDs, Nat. Photonics (2014) 748-749.
[28] N.S. Singh, R.S. Ningthoujm, L.R. Devi, N. Yaiphaba, V. Sudarsan, S.D. Singh, R.K.
Vatsa, R. Tewari, Luminescence study of Eu3+ doped GdVO4 nanoparticles:
Concentration, particle size, and core/shell effects, J. Appl. Phys. 104 (2008) 104307.
[29] N.S. Singh, R.S. Ningthoujam, M.N. Luwang, S.D. Singh, R.K. Vatsa, Luminescence,
lifetime and quantum yield studies of YVO4:Ln3+ (Ln3+ = Dy3+, Eu3+) nanoparticles:
Concentration and annealing effects, Chem. Phys. Lett. 480 (2009) 237-242.
[30] S.H. Lee, Y. Cha, H. Kim, S. Lee, J.S. Yu, Luminescent properties of Eu3+-activated
Gd2ZnTiO6 double perovskite red-emitting phosphors for white light-emitting diodes and
field emission displays, RSC Adv. 8 (2018) 11207-11215.
[31] A.K. Parchur, R.S. Ningthoujam, S.B. Rai, G.S. Okram, R.A. Singh, M. Tyagi, S.C.
Gadkari, R. Tewari, R.K. Vasta, Luminescence properties of Eu3+ doped CaMoO4
nanoparticles, Dalton Trans. 40 (2011) 7595-7601.
[32] W.J. Zhang, W.L. Feng, Y.M. Nie, Photoluminescence properties of red europium doped
calcium tungstate phosphors for blue-pumped light-emitting diodes, Optik. 126 (2015)
1341–1343.
[33] Y. Su, L. Li, G. Li, Synthesis and optimum luminescence of CaWO4-based red
phosphors with codoping of Eu3+ and Na+, Chem. Mater. 20 (2008) 6060–6067.
16

[34] P. Du, J.S. Yu, Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors with ultrabroad
excitation band for white light-emitting diodes, Sci. Reports 7 (2017) 11953.
[35] R. Wangkhem, T. Yaba, N.S. Singh, R.S. Ningthoujam, Red emission enhancement
from CaMoO4:Eu3+ by co-doping of Bi3+ for near UV/blue LED pumped white pcLEDs:
Energy transfer studies, J. Appl. Phys. 123 (2018) 124303.
[36] T. Yaba, R. Wangkhem, N.S. Singh, Photoluminescence properties of Dy3+ activated
CaWO4 nanophosphors: a potential single phase near white light emitter, J. Fluoresce. 29
(2019) 435-443.
[37] P. Du, L.K. Bharat, J.S. Yu, Strong red emission in Eu3+/Bi3+ ions codoped CaWO4
phosphors for white light-emitting diode and field-emission display applications, J.
Alloys Compd. 633 (2015) 37–41.
[38] R.B. Pode, S.J. Dhoble, Photoluminescence in CaWO4:Bi3+,Eu3+ material, Phys. Stat.
Sol. (b) 203 (1997) 571-577.
[39] B. Liu, L. Peng, Photoluminescent properties of Na+, Bi3+ co-doped CaWO4: Eu3+
phosphor for PDPs, Appl. Mech. Mater. 341-342 (2013) 229-232.
[40] M. Xia, Z.Ju, H. Yang, Z. Wang, X. Gao, F. Pan, X. Liu, Red-emitting enhancement by
inducing lower crystal field symmetry of Eu3+ site in CaWO4:Eu3+ phosphor for n-UV w-
LEDs, J. Alloys Compd. 739 (2018) 439-446.
[41] G. Liu, S. Li, Y. Lu, J. Zhang, Z. Feng, C. Li, Controllable synthesis of α-Bi2O3 and γ-
Bi2O3 with high photocatalytic activity by α-Bi2O3→γ-Bi2O3→α-Bi2O3 transformation in
a facile precipitation method, J. Alloys Compd. 689 (2016) 787-799.
[42] A.L. Patterson, The scherrer formula for X-ray particle size determination, Phys Rev. 56
(1939) 978–982.
[43] N.N. Golovnev, M.S. Molokeev, S.N. Vereshchagin, V.V. Atuchin, M.Y. Sidorenko,
M.S. Dmitrushkov, Crystal structure and properties of the precursor
[Ni(H2O)6](HTBA)2∙2H2O and the complexes M(HTBA)2(H2O)2 (M = Ni, Co, Fe),
Polyhedron 70 (2014) 71–76.
[44] N.N. Golovnev, M.S. Molokeev, S.N. Vereshchagin, V.V. Atuchin, Synthesis and
thermal
transformation
of
a
neodymium(III)
complex
[Nd(HTBA)2(C2H3O2)(H2O)2]·2H2O to non-centrosymmetric oxosulfate Nd2O2SO4, J.
Coord. Chem. 68(11) (2015) 1865-1877.
17

[45] N.N. Golovnev, M.S. Molokeev, A.S. Samoilo, V.V. Atuchin, Influence of alkyl
substituents in 1,3-diethyl-2-thiobarbituric acid on the coordination environment in
M(H2O)2(1,3-diethyl-2- thiobarbiturate)2 M = Ca2+, Sr2+, J. Coord. Chem. 69(6) (2016)
957-965.
[46] C.W. Liew, H.M. Ng, A. Numan, S. Ramesh, Poly(acrylic acid)–based hybrid
inorganic–organic electrolytes membrane for electrical double layer capacitors
application, Polymers 8 (2016) 179.
[47] J.S. Shaikh, R.C. Pawar, N.L. Tarwal, D.S. Patil, P.S. Patil, Supercapacitor behavior of
CuO–PAA hybrid films: Effect of PAA concentration, J. Alloys Compd. 509 (2011)
7168–7174.
[48] K.G. Sharma, N.S. Singh, Y.R. Devi, N.R. Singh, S.D. Singh, Effects of annealing on
luminescence of CaWO4:Eu3+ nanoparticles and its thermoluminescence study, J. Alloys
Compd. 556 (2013) 94–101.
[49] C.S. Lim, A. Aleksandrovsky, M. Molokeev, A. Oreshonkov, V. Atuchin, Microwave
sol–gel
synthesis
and
upconversion
photoluminescence
properties
of
CaGd2(WO4)4:Er3+/Yb3+ phosphors with incommensurately modulated structure, J. Solid
State Chem. 228 (2015) 160–166.
[50] C.S. Lim, V.V. Atuchin, A.S. Aleksandrovsky, Y.G. Denisenko, M.S. Molokeev, A.S.
Oreshonkov, Fabrication of microcrystalline NaPbLa(WO4)3:Yb3+/Ho3+ phosphors and
their upconversion photoluminescent characteristics, Korean J. Mater. Res. 29(12)
(2019) 741-746.
[51] W.M. Yen, S. Shionoya, H. Yamamoto (Eds.), Phosphor Handbook, second ed., CRC
Press, Taylor & Francis Group, Florida, 2007.
[52] Y. Su, L. Li, G. Li, Synthesis and optimum luminescence of CaWO4-Based Red
phosphors with codoping of Eu3+ and Na+, Chem. Mater. 20 (2008) 6060-6067.
[53] S. Yan, J. Zhang, X. Zhang, S. Lu, X. Ren, Z. Nie, X. Wang, Enhanced Red Emission in
CaMoO4:Bi3+,Eu3+, J. Phys. Chem. C 111 (2007) 13256-13260.
[54] H. Ji, Z. Huang, Z. Xia, M. S. Molokeev, V.V. Atuchin, M. Fang, Y. Liu, Discovery of
new solid solution phosphors via cation substitution-dependent phase transition in
M3(PO4)2:Eu2+ (M = Ca/Sr/Ba) quasi-binary sets, J. Phys. Chem. C 119(4) (2015)
2038−2045.
18

[55] H. Ji, Z. Huang, Z. Xia, M.S. Molokeev, X. Jiang, Z. Lin and V.V. Atuchin,
Comparative investigations of the crystal structure and photoluminescence property of
eulytite-type Ba3Eu(PO4)3 and Sr3Eu(PO4)3, Dalton Trans. 44(16) (2015) 7679-7686.
[56] V.V. Atuchin, A.K. Subanakov, A.S. Aleksandrovsky, B.G. Bazarov, J.G. Bazarova,
T.A. Gavrilova, A.S. Krylov, M.S. Molokeev, A.S. Oreshonkov, S.Yu. Stefanovich,
Structural and spectroscopic properties of new noncentrosymmetric self-activated borate
Rb3EuB6O12 with B5O10 units, Mater. Des. 140 (2018) 488-494.
[57] W. Wang, P. Yang, Z. Cheng, Z. Hou, C. Li, J. Lin, Patterning of red, green, and blue
luminescent films based on CaWO4:Eu3+, CaWO4:Tb3+, and CaWO4 phosphors via
microcontact printing route, ACS Appl. Mater. Interfaces 3 (2011) 3921–3928.
[58] Y. Huang, H. Lin, H.J. Seo, Luminescence properties and local structures of Eu3+ in
EuBaB9O16 crystal, J. Electrochem. Soc. 157(12) (2010) J405-J409.
[59] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A 28 (1968) 444–445.
[60] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953)
836–850.
19

Fig. 1. Excitation spectra of (a) as-prepared and (b) 500 °C annealed CaWO4:Eu3+ (5
at.%)/Bi3+ (y = 0, 1, 3, 5, 7, 9, 11, 13 and 15 at.%) samples monitored at 615 nm emission
wavelength.
1

Fig. 2. Emission spectra of (a) as-prepared and (b) 500 °C annealed CaWO4:Eu3+ (5
at.%)/Bi3+ (y = 0, 1, 3, 5, 7, 9, 11, 13 and 15 at.%) samples under 393 nm excitation.
2

Fig. 3. Emission spectra of (a) as-prepared and (b) 500 °C annealed CaWO4:Eu3+ (5
at.%)/Bi3+ (y = 0, 1, 3, 5, 7, 9, 11, 13 and 15 at.%) samples under 464 nm excitation.
3

Fig. 4. Comparison of PL emission intensities of (a and b) as-prepared CaWO4:Eu3+ (5
at.%)/Bi3+ (y = 0 and 11 at.%) and (c and d) 500 °C annealed CaWO4:Eu3+ (5 at.%)/Bi3+ (y =
0 and 5 at.%) samples under the 393 and 464 nm excitations.
4

o
Fig. 5. Photographs of (a) as-prepared CaWO4:Eu3+ (5 at.%)/Bi3+ (11 at.%) and (b) 500 C
annealed CaWO4:Eu3+ (5 at.%)/Bi3+ (5 at.%) sample under the exposure to different light
sources of (i) 268, (ii) 290, (iv) 393 and (v) 464 nm.
5

Fig. 6. Normalized (at 590 nm) emission spectra of as-prepared CaWO4:Eu3+ (x = 0, 1, 3, 5,
7, 9, 11, 13 and 15 at.%)/Bi3+ (11 at.%) under 290 nm excitation. The inset is the enlarged
Gaussian fitted broad emission band between 325–565 nm.
6

Fig. 7. Normalized (at 590 nm) emission spectra of 500 °C annealed CaWO4:Eu3+ (x = 0, 1,
3, 5, 7, 9, 11, 13 and 15 at.%)/Bi3+ (11 at.%) under 290 nm excitation. The inset is the
enlarged Gaussian fitted broad emission band between 325–565 nm.
7

Fig. 8. Energy transfer efficiency ꢀꢁꢂꢃ of different concentration of Eu3+ (x = 0, 1, 3, 5, 7, 9,
11, 13 and 15 at.%) in Bi3+ (11 at.%) sensitized CaWO4 of as-prepared (black solid line with
⁰
∆ symbol) and 500 C annealed (red dashed line with ● symbol) samples. Excitation
wavelength for both cases was 290 nm.
8