Site-Bi3+ and Eu3+ dual emissions in color-tunable Ca2Y8(SiO4)6O2:Bi3+, Eu3+ phosphors prepared via sol-gel synthesis for potentially ratiometric temperature sensing

Article abstract of DOI:10.1016/j.jallcom.2019.02.087

A series of Ca₂Y₈(SiO₄)₆O₂ (CYSO):Bi³⁺, Eu³⁺ phosphors were prepared via a Pechini-type sol-gel reaction method. The refinement results for CYSO:Bi³⁺, Eu³⁺ phosphors implied that they had a pure phase. The blue-green emission ascribed to Bi^{3+ 3}P₁→¹S₀ transition was generated upon UV excitation in Bi³⁺ singly-doped CYSO samples. Spectral analysis indicated that two main emission bands around 414 and 494 nm correspond to two kinds of Bi³⁺ occupying the crystal lattices of 4f and 6 h available for Y³⁺ in CYSO, denoted as Bi³⁺(2) and Bi³⁺(1), respectively. A broad spectral overlap between Bi³⁺ emission and Eu³⁺ excitation spectra implied the existence of energy transfer from Bi³⁺ to Eu³⁺ ions in CYSO:Bi³⁺,Eu³⁺, which resulted in the tunable emission color from blue-green to red. The energy transfer mechanism from Bi³⁺ to Eu³⁺ ions was determined to be a dipole-quadrupole interaction. Moreover, the quite different luminescence thermal quenching behaviors between Bi³⁺(2) and Eu³⁺ showed good temperature sensing properties with a temperature range of 298–523 K by analyzing the temperature sensitivity of the fluorescent intensity ratio [Bi³⁺(2)/Eu³⁺(612)]. The maximum absolute and relative sensitivities reached as high as 0.07174 K^-1 (523 K) and 0.958% K^?1 (423 K), which can be compared to the highest values of 0.015 K^-1 and 1.1%K^?1 in reported optical thermometric materials before, respectively, based on the thermally coupled level (TCLs) of Er³⁺. Meanwhile, the luminescence thermal quenching mechanism in this system was investigated in detail. Results inspire that a feasible method based on site-Bi³⁺ and Eu³⁺ emissions is potential as one of candidate strategies for developing novel ratiometric optical thermometry materials.

Full text of DOI:10.1016/j.jallcom.2019.02.087

Journal of Alloys and Compounds 787 (2019) 86e95
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Site-Bi3þ and Eu3þ dual emissions in color-tunable
Ca2Y8(SiO4)6O2:Bi3þ, Eu3þ phosphors prepared via sol-gel synthesis
for potentially ratiometric temperature sensing
Kai Li*, Rik Van Deun
L3eLuminescent Lanthanide Lab, Department of Chemistry, Ghent University, Krijgslaan 281-S3, B-9000 Ghent, Belgium
a r t i c l e i n f o
a b s t r a c t
A series of Ca2Y8(SiO4)6O2 (CYSO):Bi3þ, Eu3þ phosphors were prepared via a Pechini-type sol-gel reaction
method. The refinement results for CYSO:Bi3þ, Eu3þ phosphors implied that they had a pure phase. The
blue-green emission ascribed to Bi3þ 3P1/1S0 transition was generated upon UV excitation in Bi3þ
singly-doped CYSO samples. Spectral analysis indicated that two main emission bands around 414 and
494 nm correspond to two kinds of Bi3þ occupying the crystal lattices of 4f and 6 h available for Y3þ in
CYSO, denoted as Bi3þ(2) and Bi3þ(1), respectively. A broad spectral overlap between Bi3þ emission and
Article history:
Received 21 November 2018
Received in revised form
5 February 2019
Accepted 7 February 2019
Available online 8 February 2019
Eu3þ excitation spectra implied the existence of energy transfer from Bi3þ to Eu3þ ions in CYSO:Bi3þ,Eu3þ
,
Keywords:
which resulted in the tunable emission color from blue-green to red. The energy transfer mechanism
from Bi3þ to Eu3þ ions was determined to be a dipole-quadrupole interaction. Moreover, the quite
different luminescence thermal quenching behaviors between Bi3þ(2) and Eu3þ showed good temper-
ature sensing properties with a temperature range of 298e523 K by analyzing the temperature sensi-
tivity of the fluorescent intensity ratio [Bi3þ(2)/Eu3þ(612)]. The maximum absolute and relative
sensitivities reached as high as 0.07174 K-1 (523 K) and 0.958% Kꢀ1 (423 K), which can be compared to the
highest values of 0.015 K-1 and 1.1%Kꢀ1 in reported optical thermometric materials before, respectively,
based on the thermally coupled level (TCLs) of Er3þ. Meanwhile, the luminescence thermal quenching
mechanism in this system was investigated in detail. Results inspire that a feasible method based on site-
Bi3þ and Eu3þ emissions is potential as one of candidate strategies for developing novel ratiometric
optical thermometry materials.
Energy transfer
Silicate
Luminescence
Sol-gel
Ratiometric optical thermometry
© 2019 Published by Elsevier B.V.
1. Introduction
temperature sensitivity, most conventional FIR materials are
concentrated on lanthanide doped phosphors, whose emissions
As one of the most important parameters in research require-
ment and actual application for devices, temperature should be
accurately measured. Recently, a large amount of investigations
have been done on developing novel optical thermometers based
on fluorescent intensity ratio (FIR) technique because of its superior
merits of non-contact, high sensitivity and good accuracy etc. [1e3],
in which the temperature can be calibrated by analyzing the vari-
ation of emission intensity with the varying temperatures of sam-
ples. Moreover, it can get rid of the influence from environmental
factors such as measuring condition and resolution, and can be
applied in fast-moving objects [4,5]. To acquire the high
originate from their abundant energy levels, especially the ther-
mally coupled level (TCL) pairs [6,7], are utilized as the temperature
label since the population in the upper and lower levels would
change oppositely to induce the variation of their emission in-
tensities with the changing temperature. On the basis of special 4f
energy level structure, with the excitation of 980 nm laser, Er3þ
doped phosphors are recently selected as possible candidates of
temperature sensors materials, which illustrate
a maximum
sensitivity of 83.29 ꢁ 10ꢀ4 Kꢀ1 at 583 K (323e593 K) in Yb3þ/Er3þ
co-doped BaMgF4 with [8], 45.8 ꢁ 10ꢀ4 Kꢀ1 at 573 K (298e573 K) in
Yb3þ/Er3þ co-doped Ba3Y4O9 [9], 0.82% at 423 K (303e423 K) in
Yb3þ/Er3þ doped LuVO4 [10], 0.00413 Kꢀ1 at 573 K (313e573 K)
in Yb3þ/Er3þ doped BiPO4 [11], 0.65% Kꢀ1 at 498 K (313e573 K) in
Yb3þ/Er3þ doped Ba2In2O5 phosphors [7], 54.09 ꢁ 10ꢀ4 Kꢀ1 at 531 K
(298e568 K) in Er3þ/Yb3þ co-doped fluorotellurite glasses [12].
* Corresponding author.
E-mail address: kailiciac@126.com (K. Li).
https://doi.org/10.1016/j.jallcom.2019.02.087
0925-8388/© 2019 Published by Elsevier B.V.

K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
87
However, the narrow energy gap with the TCL pairs (e.g., 2H11/2 and
4S3/2 levels for Er3þ) in these materials would induce overlap of the
two monitored emission peaks, resulting in an inferior signal dis-
criminability to limit the temperature sensitivity of materials for
actual application. Therefore, it is required to look for some other
temperature sensing materials systems with high sensitivity.
Recently, some novel temperature sensing materials based on
the phonon assisted energy transfer between lanthanide ion pairs
with that of NaY9(SiO4)6O2. The incorporation of Bi3þ into such
silicate CYSO here with two different crystallographic sites pro-
duces two different emission bands in blue and green areas,
respectively, which overlap the excitation spectrum of Eu3þ doped
CYSO, resulting in the sensitization of Bi3þ for Eu3þ ions in Bi3þ and
Eu3þ co-doped CYSO phosphors. In order to explore its potential
application in ratiometric optical thermometry, the temperature
sensitivity has been investigated in detailed for the first time in Bi3þ
and Eu3þ co-doped systems.
(e.g. Eu3þ/Tb3þ Dy3þ/Tb3þ Ce3þ/Tb3þ and Nd3þ/Yb3þ pairs)
, , ,
[13e16] have been demonstrated as the candidate temperature
sensing materials. Moreover, a novel candidate strategy based on
the different temperature sensing properties for two kinds of ions
in dual-phase nanoparticles dispersed in glass ceramic was pro-
posed recently, which showed good results [17e19]. In addition,
another alternative strategy combining the transitional metal (TM)
ions and rare earth ions (Ln3þ) in phosphors have been demon-
strated to be feasible as the luminescent thermometers [20,21],
inspiring that other kinds of luminescent ions except for transi-
tional metal ions, can also be utilized to combine with Ln3þ in a
matrix as the temperature sensing materials. As a different ion from
TM, Bi3þ can emit a blue-green even red band around 400e650 nm
attributed to its 6s6p/6s2 transition upon UV excitation [22,23]. It
is well accepted that Eu3þ can often mainly have a red emission line
due to its characteristic 5D0/7F2 electric dipole transition [24,25].
In addition, the nature of this transition line strongly relies on its
local environment and can be adopted to probe the site symmetry.
Many characteristic excitation lines locating at 300e500 nm are
originated from its intra-configurational 4f-4f transition. As re-
ported before, the energy transfer would like to take place based on
the spectral overlap of Eu3þ excitation and Bi3þ emission bands
[26,27]. Therefore, tunable emission can be acquired when they are
co-doped into a suitable host via the energy transfer from ions Bi3þ
to Eu3þ upon the same excitation environment, which is usually
considered to be potentially applied in UV w-LEDs [28,29]. How-
ever, there is rare report on Bi3þ-Eu3þ co-doped systems as the
temperature sensing materials, which is based on taking advantage
of the quite different thermal quenching properties between Bi3þ
and Eu3þ ions.
2. Experimental section
2.1. Materials and preparation
A series of samples Ca2Y8-x-y(SiO4)6O2:xBi3þ, yEu3þ (CYSO:xBi3þ
,
yEu3þ) (x ¼ 0e0.48, y ¼ 0e0.64) were synthesized by a Pechini-
type sol-gel reaction process. Typically, first, Y2O3 (99.99%) and
Eu2O3 (99.99%) were dissolved in dilute nitric acid (HNO3) under
continuous stirring and heated to form a colorless solution of
Y(NO3)3 and Eu(NO3)3, respectively. Stoichiometric amounts of
Y(NO3)3, Eu(NO3)3, Ca(NO3)2$4H2O and Bi(NO3)3$5H2O were dis-
solved in deionized water, with 10 min stirring, after which the
citric acid with the fixed stoichiometric value [2:1 (mol/mol) citric
acid/metal ion] was added. Then, the pH of the solution was
adjusted to ~2 with HNO3 followed by the addition of a stoichio-
metric amount of tetraethyl orthosilicate [Si(OC2H5)4] dissolved in
appropriate ethanol after 20 min stirring. Finally, a certain amount
of poly(ethylene glycol) (PEG; molecular weight ¼ 20000, analyt-
ical reagent grade) was appended as
a cross-linking agent
(CPEG ¼ 0.005 mol/L) after stirring for about 15 min. The ultimate
mixtures were stirred for 2 h and then heated at 85 ꢂC with a water
bath to form homogeneous gels. After that, the obtained gels were
prefired at 600 ꢂC for 4 h in air followed by thorough grinding and
calcination at 1250 ꢂC for 6 h in a muffle furnace to generate the
final powder materials.
2.2. Measurement and characterization
Silicate have been broadly studied as the hosts for rare earth
(Ln3þ) ions introductions due to their versatile crystal structures,
and good chemical and physical stabilities [30,31]. As a large family
of silicates, familiar apatite-related silicate compounds have been
investigated as the hosts for Ln3þ ions incorporations to show good
photoluminescence (PL) properties for solid state lighting [32e34].
There are two cationic sites in oxy-apatite host lattices, that is,
seven-fold coordinated 6 h sites with Cs point symmetry and the
nine-fold coordinated 4f sites with C3 point symmetry. Both of
them have been verified to be appropriately and easily accommo-
dated various Ln3þ ions [35]. As one of the apatite-related silicate,
the Ca2Y8(SiO4)6O2 (CYSO) has been demonstrated to accommodate
various rare earth and mercury-like ions [32,35e37], among which
Eu3þ presents good luminescence properties. Therefore, co-doping
Bi3þ and Eu3þ ions into this host may generate a tunable emission
via the efficient energy transfer from Bi3þ to Eu3þ ions. In this work,
Bi3þ and Eu3þ doped CYSO was prepared using a Pechini sol-gel
method since it is a useful and convenient way to get inorganic
phosphors materials, together with its merits such as homogenous
mixing of raw materials at molecular level, low synthetic temper-
ature, short calcination time, uniform morphology etc., compared
with solid-state reaction method. In addition, we take the crystal
structure of oxy-apatite NaY9(SiO4)6O2 (which can be considered
the transmission of two Ca replaced by Na þ Y) reported by R. P.
GUNAWARDANE etc. as the original mode to do the refinements for
related CYSO samples because of absence of its definite crystal
structure [38], which indicate the crystal of CYSO is isostructural
All the measurements were done using the finely ground
powder. D8 Focus diffractometer at a scanning rate of 10ꢂminꢀ1 in
the 2
Ka radiation (
q
range from 10ꢂ to 110ꢂ with graphite-monochromatized Cu
l
¼ 0.15405 nm) was used to identify the phase purity
of samples. The morphology of the samples were inspected using a
scanning electron microscope (SEM, S-4800, Hitachi). Selected-area
electron diffraction (SAED) pattern and high-resolution trans-
mission electron microscopy (HRTEM) was performed using FEI
Tecnai G2 S-Twin with a field emission gun operating at 200 kV.
Images were acquired digitally on a Gatan multiple CCD camera.
Infrared spectra were obtained on a VERTEX 70 Fourier transform
infrared (FT-IR) spectrometer (Bruker). The solid-state nuclear
magnetic resonance (NMR) was measured using a conventional
impulse spectrometer DSX advance (Bruker) operating with a
resonance frequency of 400 MHz for 1H (B ¼ 11.3 T). X-ray photo-
electron spectroscopy (XPS) data were obtained with a VG ESCALAB
250 using monochromatic Al Ka X-ray sources. The excitation
source from a fluorescence spectrophotometer equipped with a
450 W xenon lamp (Cary 4000) was used to carry out the photo-
luminescence (PL) measurements. The luminescence decay life-
times were measured and obtained from a Lecroy Wave Runner
6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse
width ¼ 4 ns, gate ¼ 50 ns) as the excitation (Continuum Sunlite
OPO) source. All the measurements above were performed at room
temperature (RT) condition. Moreover, the temperature-dependent
(298e523 K) PL spectra were recorded on Edinburgh Instruments
FLSP-920 with a temperature controller.

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K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
Table 1
3. Results and discussion
Crystallographic data and details in the data collection and refinement parameters
for the CYSO, CYSO:0.10Bi3þ and CYSO:0.10Bi3þ, 0.48Eu3þ samples.
The identifications of phase composition and purity have been
conducted by GASA Rietveld refinements at room temperature for
the representative CYSO host (Fig. 1a), CYSO:0.10Bi3þ (Fig. 1b) and
CYSO:0.10Bi3þ, 0.48Eu3þ (Fig. 1c) samples. As crystal structure of
CYSO has never been reported so far, the crystallographic data of
NaY9(SiO4)6O2 (ICSD 27191) were taken as the original structure
model for the refinements. As exhibited in Fig. 1a, b and c, the olive
solid line and black crosses represent the calculated and experi-
mental patterns, respectively. The red short vertical lines indicate
the Bragg reflection positions of the calculated pattern. The differ-
ences between the experimental and calculated results are plotted
by the dark cyan line at the bottom. All the atomic positions, frac-
tion factors, and thermal vibrational parameters were converged
and refined, which can be reflected with the residual factors and
lattice Table 1. The residual factors are Rwp ¼ 4.45% and Rp ¼ 3.40%,
Rwp ¼ 5.18% and Rp ¼ 3.86%, Rwp ¼ 4.54% and Rp ¼ 3.39% for CYSO
host, CYSO:0.10Bi3þ and CYSO:0.10Bi3þ, 0.48Eu3þ, respectively,
indicating that the refined results are reliable. The final refined
results illustrate that the structure of CYSO is consistent with that of
NaY9(SiO6)O2, which crystallizes in a hexagonal system with the
space group P63/m, as shown in Fig. 1d. There are two different sites
for Y3þ ions, that is, seven-fold coordinated 6 h sites (Y1) with Cs
point symmetry and the nine-fold coordinated 4f sites (Y2) with C3
point symmetry. It is found that all the lattice parameters a, b, c and
cell volume (V) in CYSO:0.10Bi3þ and CYSO:0.10Bi3þ, 0.48Eu3þ are
larger than those of CYSO host, indicating that Bi3þ and Eu3þ are
successfully introduced into the CYSO submatrix. It is acceptable
that both Bi3þ and Eu3þ are expected to substitute Y sites based on
their similar ionic radii {[Y3þ, 0.96 Å for coordination number
(CN ¼ 7) and 1.10 Å for CN ¼ 9], Bi3þ, 1.07 Å for CN ¼ 7 and 1.14 Å for
CN ¼ 9, Eu3þ, 1.03 Å for CN ¼ 7 and 1.14 Å for CN ¼ 9} and the same
Sample
CYSO
CYSO:0.10Bi3þ
CYSO:0.10Bi3þ, 0.48Eu3þ
Space group
P63/m
P63/m
P63/m
Symmetry
a, Å
b, Å
hexagonal
9.3470(1)
9.3470(1)
6.7876 (1)
513.56(1)
1
10e110
4.45
3.40
hexagonal
9.3479(1)
9.3479(1)
6.7885 (1)
513.73(2)
1
10e110
5.18
3.86
hexagonal
9.3516(2)
9.3516(2)
6.7940 (2)
514.55(3)
1
10e110
4.54
3.39
c, Å
V, Å3
Z
2
q
-interval,ꢂ
Rwp
Rp
c2
2.595
3.634
2.627
valence. However, the larger Bi3þ and Eu3þ result in the increases of
lattice parameters, as mentioned above. Fig. 2 shows the XRD
patterns for as-prepared CYSO host and Bi3þ, Eu3þ-doped CYSO
phosphors, as well as standard patterns of NaY9(SiO4)6O2 (JCPDS
74-1404). It can be found that the profiles of as-prepared samples
are consistent with each other, indicating that all the patterns are
well indexed to NaY9(SiO4)6O2 phase, which infers the pure phase
of CYOS in as-prepared samples. Furthermore, the incorporations of
Bi3þ and Eu3þ ions into CYSO compound do not arouse any
impurities.
The morphology of selected samples CYSO:0.10Bi3þ and
CYSO:0.10Bi3þ, 0.48Eu3þ were detected using the SEM technology.
As presented in Fig. 3a and b, the morphology of these two samples
are similar to each other with some aggregated particles. It is
reasonable that the distribution of size of particles prepared by a
sol-gel reaction method locates at the range of 0.5e5
and d shows the low-resolution TEM image and the corresponding
mm. Fig. 3c
Fig. 1. Rietveld refinements for CYSO (a), CYSO:0.10Bi3þ (b) and CYSO:0.10Bi3þ, 0.48Eu3þ (c) samples, and the approximate crystal structure of CYSO compound (d).

K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
89
profile of CYSO:0.10Bi3þ, 0.40Eu3þ annealed at 600 ꢂC is quite
different from those annealed at 1250 ꢂC. The peaks at 1083, 1426,
and 1632 cmꢀ1 are attributed to the vibrations of CeO, C]C, C]O
bonds, respectively [39], indicating that the CYSO phase is not
formed at such low temperature. However, the profiles of FT-IR
spectra of CYSO host, CYSO:0.10Bi3þ, and CYSO:0.10Bi3þ, 0.48Eu3þ
samples annealed at 1250 ꢂC are similar to each other, as shown in
Fig. S1b, which illustrates that the incorporations of Bi3þ and Eu3þ
into this host have made little change to the structure of CYSO,
consistent with the XRD analysis mentioned above. Taking the
CYSO:0.10Bi3þ sample as an example to analyze the FT-IR spectrum,
the bond vibration position around 559 cmꢀ1 corresponds to the
bending mode of a SiO4 tetrahedron, and the peak at 940 cmꢀ1 is
assigned to the asymmetric SieO stretching modes of a SiO4 tet-
rahedron. Those results are in good agreement with the data [400-
600 cmꢀ1 refers to (SiO4) and 900-1100 cmꢀ1 refers to
d n(SiO4)]
reported in the previous paper [40,41]. The peak at 720 cmꢀ1 in
mid-frequency range of 600e800 cmꢀ1 can be attributed to the
symmetric OeSieO stretching vibrations in (SiO4) group. Another
peak of at 3448 cmꢀ1 is attributed to the OHꢀ stretching vibration
mode from water on the surface of this sample under air condition.
It is well known that solid-state NMR is sensitive to the local field,
and it thus can be conducted to investigate the connection envi-
ronment. Fig. S1c exhibits the 29Si MAS-NMR spectrum of the CYSO
host, in which an obvious resonance at around 99.4 ppm can be
observed, further certifying the existence of the SiO4 tetrahedron in
CYSO host, as illustrated in FT-IR spectra.
Fig. 2. XRD patterns of CYSO, CYSO:0.10Bi3þ, CYSO:0.48Eu3þ, CYSO:0.10Bi3þ, 0.48Eu3þ
as well as the standard reference of NaY9(SiO4)6O2 compound (JCPDS 74-1404).
,
Fig. S2 illustrates the XPS images of the representative
CYSO:0.10Bi3þ, 0.48Eu3þ phosphor. The binding energy of all exis-
tent elements can be identified in this sample, which are main Ca2p
(347.3 eV), Y3d (159.7 eV), Si2p (101.1 eV), 1s (530.3 eV), Bi4f
(156.7 eV) and Eu3d (1139.0 eV). The bonding energy intensities of
Ca, Y, Si and O elements are evident, while the intensities for doped
Bi and Eu are relatively weak on account of their low doping
concentrations.
It is well known the ground state of Bi3þ ions is 1S0 with a 6s2
configuration, whereas the excited state is triplet state (3P0, 3P1, 3P2)
originating from its 6s6p configuration and 1P1 singlet state with
rising excitation energy. Sometimes a charge transfer transition can
be found in some Bi3þ-doped hosts, however, its energy is much
higher than that of 1P1 level [42]. Moreover, both emission and
absorption spectra of Bi3þ are considered to show broad bands,
which strongly depend on its coordination environment, covalence,
bond volume polarizability and current charge, similar to Eu2þ and
Pb2þ [43,44]. Fig. 4a and b displays the PL excitation and emission
spectra of CYSO:0.10Bi3þ phosphor. Upon 343 nm excitation, a
broad emission band ranging from 350 to 650 nm can be found
with two main emission peaks at 494 and 414 nm, corresponding to
the 3P1/1S0 transition of Bi3þ in Fig. 4a and a minor emission band
around 375 nm corresponding to Bi3þ-O2- charge transfer transi-
tion. It is readily accepted that the two main emission peaks above
can be ascribed to the occupations of two kinds of Y3þ by Bi3þ. As is
well known [45], the Bi3þ d-band edge emission is proportion to
the host cations for Bi3þ substitution and number of anions around
“active” cation Bi3þ. As the coordination number of Y1 is smaller
than that of Y2, the Y(1)eO bond is much shorter than that of Y(2)
eO, therefore, the coordinated bond of Bi(1)eO is much stronger
than that of Bi(2)eO. Based on this, we can deduce that the band
around at 414 nm originates from Bi3þ 3P1/1S0 transition in Y(2)
sites with nine-coordination, defined as Bi3þ(2), while the other
one around 494 nm is ascribed to Bi3þ ions in Y(1) ions sites with
seven-coordination, defined as Bi3þ(1) [46]. Monitored at 494 nm,
the excitation spectrum mainly presents a peak at 331 nm and a
weaker one at 343 nm, originating from the 3P1 )1S0 transition of
Bi3þ(1), as shown in Fig. 4a. Fig. 4b illustrates the PL excitation
Fig. 3. SEM images of CYSO:0.10Bi3þ (a) and CYSO:0.10Bi3þ, 0.48Eu3þ samples (b), low-
resolution TEM image of one particle of (c) and HRTEM of the selected area in
CYSO:0.10Bi3þ, 0.48Eu3þ sample (d), inset is the corresponding SAED pattern.
HRTEM image of the selected area of a CYSO:0.10Bi3þ, 0.48Eu3þ
particle, respectively. The result reveals that the sample well crys-
tallizes in the hexagonal system. Moreover, the selected-area
electron diffraction (SAED) image of the sample is presented in
inset of Fig. 3d. One set of the regular hexagonal diffraction pattern
is observed, further indicating that the sample has a hexagonal
crystal structure. The interplanar distance d ¼ 0.522 nm marked in
Fig. 3d can be assigned to the (101) plane.
The FT-IR spectra for the precursor of CYSO:0.10Bi3þ, 0.48Eu3þ
sample annealed at 600 ꢂC and the CYSO host, CYSO:0.10Bi3þ, and
CYSO:0.10Bi3þ, 0.48Eu3þ samples annealed at 1250 ꢂC are pre-
sented in Figs. S1a and b, respectively. We can observe that the

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K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
Fig. 4. PL excitation (monitored at 494 nm) and emission (lex ¼ 343 nm) spectra (a), and PL excitation (monitored at 414 nm) and emission (lex ¼ 374 nm) spectra (b) for
CYSO:0.10Bi3þ. (c) Fluorescent decay curves monitored at 414 and 494 nm (lex ¼ 343 nm) for CYSO:0.10Bi3þ
.
spectrum detected at 414 nm and emission spectrum excited at
374 nm UV. It can be observed only one band around 414 nm from
the emission spectrum appears. This emission band is in accor-
dance with that in Fig. 4a, therefore, it is confirmed to be the
3P1/1S0 transition of Bi3þ(2). The two primary excitation bands
around 340 and 374 nm can be attributed to the 3P0)1S0 transition
of Bi3þ(2). In order to further identify the existence of two kinds of
Bi3þ emission centers in CYSO:0.10Bi3þ, the fluorescent decay
curves monitored at 414 and 494 nm have been measured and
depicted in Fig. 4c. The decay times can be approximately calcu-
lated using the following equation [47]:
excitation band from 200 to 350 nm peaking at 266 nm and several
sharp lines peaking at 363, 384, 396, 414, 467 and 535 nm from 350
to 550 nm, which can be attributed to the charge transfer transition
of Eu3þ-O2- and Eu3þ 5D4)7F0, 5G2)7F0, 5L6)7F0, 5D3)7F0,
5D2)7F0, and 5D1)7F0 characteristic transitions [21], respectively.
The emission position of charge transfer band varies relying on the
various hosts. Upon 266, 396 and 467 nm excitation, the profiles of
emission spectra are similar to each other. They all consist of many
emission peaks at 540, 582, 588 and 597, 612, 657, 706 nm in the
range of 500e750 nm, which are assigned to the Eu3þ characteristic
transitions 5D1/7F1 and 5D0/7FJ(J¼0,1,2,3,4)
, respectively. The
5D0/7F1 transition corresponding to the 588 and 597 nm emission
peaks is a magnetic-dipole transition, which is considered to be
insensitive to site symmetry. While the red emission 5D0/7F2
electric-dipole transition with the 612 nm emission peak is gener-
ated due to the lack of inversion symmetry at the Eu3þ site and can
be much stronger than that to the 7F1 level. This 5D0-7F2 dominates
their emission spectrum when Eu3þ ions locate in a low local
symmetry environment. Considering the similar ionic radius and
same charge, Eu3þ ions intend to occupy Y3þ sites in CYSO, making
5D0-7F2 emission of Eu3þ at 612 nm the prominent peak in the
emission spectrum, indicating that Eu3þ ions occupy the sites
without inversion symmetry. We can observe that significant
spectral overlap between Bi3þ emission and Eu3þ excitation in
Fig. 5a, which gives us a perspective that energy transfer from Bi3þ
∞
ð
t
¼
IðtÞdt
(1)
0
where
t represents calculated lifetime value, and I(t) is the
normalized intensity of decay curves. The lifetimes are evaluated to
be 240.0 and 473.4 ns corresponding to 414 and 494 nm, respec-
tively. It is obvious that these two decay times are much different
from each other, indicating two kinds of Bi3þ emission centers exist
in CYSO:0.10Bi3þ phosphor.
Fig. 5a exhibits the PL properties of CYSO:0.48Eu3þ and emission
spectrum of CYSO:0.10Bi3þ excited at 343 nm. The PL excitation
spectrum monitored at 612 nm of CYSO:0.48Eu3þ contains a broad
Fig. 5. (a) PL emission spectra of CYSO:0.40Eu3þ excited at 266, 396 and 467 nm, and overlap of emission spectrum of CYSO:0.10Bi3þ excited at 343 nm with the excitation spectrum
of CYSO:0.40Eu3þ monitored at 612 nm. (b) PL excitation spectra monitored at 414, 494 and 612 nm, and emission spectrum upon 343 nm excitation in CYSO:0.10Bi3þ, 0.40Eu3þ
.

K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
91
to Eu3þ ions would like to take place when they are co-doped into
the CYSO host. This can be demonstrated by the PL emission and
excitation spectra in CYSO:0.10Bi3þ, 0.40Eu3þ sample, as illustrated
in Fig. 5b. It can be observed the emission spectrum involves not
only blue-green area with 414 and 494 nm bands from Bi3þ but also
some sharp emission lines at 575e675 nm from Eu3þ upon a
343 nm UV excitation. The excitation spectra monitored at 414 and
494 nm are well consistent with those obtained in Bi3þ single
doped CYSO sample in Fig. 4a and b. Monitored at 612 nm, the
excitation spectrum consists of many emission bands and several
sharp peaks in the range of 200e550 nm, which can be assigned to
O2--Eu3þ charge transfer band, host absorption and Eu3þ charac-
teristic excitation lines. In addition, the obvious bands around 331
and 343, and 374 nm from Bi3þ confirm the energy transfer from
beyond y ¼ 0.56 is attributed to the general concentration
quenching effect. The variation of PL emission spectra result in the
change of CIE chromaticity coordinate from cyan [y ¼ 0 (0.168,
0.263)] to red region [y ¼ 0.64 (0.494, 0.333)], as illustrated in CIE
chromaticity diagram of Fig. S3 and Table S1. These results illustrate
the energy transfer from Bi3þ to Eu3þ ions in CYSO:Bi3þ, Eu3þ
.
To better support the spectral analysis, the fluorescent decay
curves (lex ¼ 343 nm) detected at Bi3þ(2) and Bi3þ(1) emission
bands were measured and plotted in Fig. 7. The values of decay
times are determined to be 240.0, 207.7, 166.2, 154.6 and 126.7 ns
for Bi3þ(2) at 414 nm, and 473.4, 381.4, 320.4, 287.7 and 208.7 ns for
Bi3þ(1) at 494 nm corresponding to y ¼ 0, 0.10, 0.24, 0.40 and 0.56
according to Eqn (1), as presented in Fig. 7a and b, respectively. The
monotonous drops of decay times with increasing Eu3þ concen-
tration in CYSO:0.10Bi3þ, yEu3þ samples further certify the exis-
tence of energy transfer from Bi3þ to Eu3þ ions in CYSO:Bi3þ, Eu3þ
phosphors.
Bi3þ(1) and Bi3þ(2) to Eu3þ
PL emission spectra of
.
a
series of CYSO:0.10Bi3þ yEu3þ
,
(y ¼ 0e0.64) phosphors as a function of Eu3þ concentration, as
displayed in Fig. 6a, were measured to further investigate the en-
ergy transfer properties from Bi3þ to Eu3þ ions, all of which show
both Bi3þ and Eu3þ emission bands upon 343 nm UV excitation. It is
clear that the emission intensity of Bi3þ [Bi3þ(1) and Bi3þ(2)]
gradually decreases with increasing Eu3þ concentration, while the
Eu3þ emission intensity monotonously ascends to the maximum
until the Eu3þ concentration y ¼ 0.56, reflected from the variations
in Fig. 6b. The subsequent decrease of Eu3þ emission intensity
As there is a spectral overlap between Bi3þ emission and Eu3þ
excitation bands, the energy transfer from Bi3þ to Eu3þ ions would
like to take place via the non-radiative resonant type. It is well
accepted that critical distance (Rc) value is quite important for
assessing energy transfer mechanism from Bi3þ to Eu3þ in
CYSO:Bi3þ, Eu3þ, which can be calculated using the following Eqn
[48].
Fig. 6. (a) PL emission spectra of CYSO:0.10Bi3þ, yEu3þ upon 343 nm excitation with different concentrations of Eu3þ ion. (b) Variations of emission intensity of Bi3þ and Eu3þ ions as
a function of Eu3þ concentration y in CYSO:0.10Bi3þ, yEu3þ
.
Fig. 7. Fluorescent decay curves of CYSO:0.10Bi3þ, yEu3þ monitored at 414 (a) and 494 nm (b) (lex ¼ 343 nm), respectively.

92
K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
ð
l2S fq
R8tS
ꢀ
ꢁ
.
1=3
PSAðdqÞ ¼ 3:024 ꢁ 1012
fSðEÞfAðEÞ E4dE
(5)
3V
XcN
Rcz2
(2)
4
p
where fq represents the oscillator strength of the
dipoleequadrupole electric absorption transition of the activator,
and lS (4140 Å) and E stand for the wavelength with highest in-
tensity of Bi3þ and the energy involved in the transfer process (in
eV), respectively; !fS(E)fA(E)/E4 dE refers to the spectral overlap
between the normalized shapes of the sensitizer Bi3þ emission fS(E)
and the activator Eu3þ excitation fA(E), and in this case it is deter-
mined to be about 0.00374 eVꢀ1. Considering the energy transfer
probability equals to the radiative emission probability of the
sensitizer, i.e., the distance for which PSA(dq)tS ¼ 1, the Rc of energy
transfer from the sensitizer to the activator is defined, which can be
obtained through the spectral overlap way whose expression is
displayed as follows:
in which the different Xc value is the critical concentration of
dopant ions (total concentration of Bi3þ and Eu3þ, approximately
0.1/8 þ 0.40/8 ¼ 0.0625 in this case), at which the luminescence
intensity of Bi3þ is half of original one in CYSO:0.10Bi3þ. Accord-
ingly, the calculated Rc is 12.52 Å, ruling out the mechanism of
exchange interaction (the Rc is usually 5 Å). Thus, the electric-
multipole interactions would contribute to the energy transfer
from Bi3þ to Eu3þ ions in CYSO:Bi3þ, Eu3þ. According to Dexter's
and Reisfeld's theories for energy transfer formula of multipolar
interaction, the following relationship can be gained [49e51]:
hS0
hS
fCa
(3)
=
3
ð
.
where C is the total concentration of the Bi3þ and Eu3þ ions. hS and
hS0 stand for the luminescence quantum efficiencies of Bi3þ ions
R8c ¼ 3:024 ꢁ 1012lS2fq fSðEÞfAðEÞ E4dE
(6)
with and without Eu3þ ions, respectively. The values
a
¼ 6, 8 and 10
In current case, it is unfortunate that fq of Eu3þ has not been
reported so far. However, Verstegen et al. suggested that it can be
reasonable for fq/fd ¼ 10ꢀ3-10ꢀ2, in which fd ¼ 10ꢀ6 is the oscillator
strength for the electric dipole transition [53]. Consequently, the
value of Rc is determined to be 10.86e14.48 Å, which is in accor-
dance with that calculated by concentration quenching method
above (12.52 Å). This result can further confirm that the electric
dipole-quadrupole interaction for the energy transfer mechanism
from Bi3þ to Eu3þ ions in CYSO:Bi3þ, Eu3þ samples is believable.
The dependence of emission intensity on temperature is one of
the important parameters for phosphors in virtual application,
especially for luminescent thermometers. Therefore, deep under-
standing of the thermal quenching mechanism of phosphors is
necessary. Fig. 9a shows the variation of PL emission spectra
refer to different types of dipole-dipole, dipole-quadrupole, and
quadrupole-quadrupole interactions, respectively. Nevertheless,
the hS0/hS is often replaced by the straightforward IS0/IS, where IS0
and IS are the luminescence intensities of Bi3þ ions without and
with the existence of Eu3þ ions, respectively, consequently, the
above equation can be expressed as follows:
IS0
IS
fCa
(4)
=3
The relationship between IS0/IS and Ca/3 are plotted in Fig. 8. The
best linear behavior can be found when
a
¼ 8 by comparing the
values of linear fitting factor R2 in Fig. 8a, b, and c. As a result, the
dipole-quadrupole mechanism would like to be responsible for the
energy transfer effect from Bi3þ to Eu3þ ions.
(
lex ¼ 343 nm) as a function of temperature with the range of
298e523 K. Minor differences for emission profiles in Fig. 9a from
Fig. 6a can be found, which may be attributed to the different
measurement spectrophotometer. With the increase of tempera-
ture, the intensities of Bi3þ(1) emission band and Eu3þ emission
band of the CYSO:0.10Bi3þ, 0.56Eu3þ monotonously decline, while
the emission intensity of Bi3þ(2) band first increases and then de-
creases, as shown in the inset of Fig. 9a. Moreover, the emission
peak of Bi3þ(1) band gradually shift to short wavelength, which
may be ascribed to the thermally active phonon-assisted excitation
from the excited states of the lower-energy emission band to the
higher-energy emission band in the excited states. These phe-
nomena can be explained by the simple configurationally coordi-
nate diagram in Fig. 9b. The a1 and a2 curves represent the ground
states of Eu3þ (7F2) and Bi3þ (1S0), and the b1, b2 and b3 curves
stand for the excited states of Eu3þ (5D0), Bi3þ(2) (3P1) and Bi3þ(1)
(3P1), respectively. A, B and E are the lowest positions of b1, b2 and
b3, C, D and F are the crossing points of a1 and b1, a2 and b2, and a2
and b3, respectively. Q and P represent the crossing positions of b1
and a2 and b1 and b2, respectively. E1, E2, E3 and E4 are the energy
differences of A to C, B to D, E to F and B to P, respectively. Under the
excitation of UV light, the electrons are activated to excited states,
that is, from a1 to b1 and from a2 to b2 and b3, respectively. At
room temperature, most of the electrons go back to the ground
states along the red dotted line ➀ to arouse the red emission of Eu3þ
and the purple dotted lines ➁ and wine dotted lines ⑦ to display
the blue-green emission of Bi3þ. Except for processes ➀ and ➁, the
electrons of the Bi3þ(2) excited state are very likely to overcome
energy barrier E4 under electronephonon coupling, and transfer its
energy to Eu3þ along the process ➂, resulting in an enhancement of
Learning from Dexter's energy transfer theory [52], the energy
transfer via multipolar interactions often depends on the spectral
overlap between sensitizer emission and activator absorption. In
view of the dipole-quadrupole interaction, the energy transfer
probability from the sensitizer (here Bi3þ) to the activator (here
Eu3þ) can be assessed by following equation [52]:
Fig. 8. Dependence of (IS0/IS) of Bi3þ on (a) C6/3 (b) C8/3 and (c) C10/3 for CYSO:Bi3þ
,
Eu3þ samples.

K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
93
Fig. 9. (a) Dependence of emission spectra (lex ¼ 343 nm) for CYSO:0.10Bi3þ, 0.56Eu3þ on temperature, the inset is the enlarged profile with the emission wavelength from 350 to
575 nm and the emission intensity of Bi3þ(2) as a function of operating temperature. (b) A simple configurationally coordinate diagram of the ground states and excited states of
Bi3þ and Eu3þ. (c) The experimental data and fitting curve according to Eqn (7) of FIR [I (2)/I
] as a function of temperature. (d) Absolute sensitivity Sa and relative sensitivity Sr
3þ
3þ
Bi
Eu (612)
versus temperature.
the Eu3þ emission intensity. With increasing temperature, more
electrons can overcome energy barriers of E1, E2 and E3, and will be
back to the ground states along the red dotted line ④, purple dotted
line ⑤ and wine dotted line ⑩ from the crossing points C, D and F
via a non-radiative relaxing process, respectively, originating from
the stronger electronephonon coupling. Simultaneously, the pro-
cess ⑧ and ⑤ compete with each other for Bi3þ (2). With increasing
temperature, the process ⑧ first play the dominant role to enhance
the emission of Bi3þ (2) and the process ⑤ play the dominant role
resulting in the weakness of emission at higher temperature. In
addition, more electrons from the excited state b2 and b3 transfer
to the Eu3þ excited state b1 under the stronger phonon vibrations.
to express the relationship between temperature and PL intensity:
ꢀ
ꢂ
ꢃꢁꢀ1
IðTÞ
E
kT
¼
1 þ A exp
ꢀ
(7)
I0
in which I0 is the PL intensity at absolute temperature 0 K, A and k
are a pre-exponential constant and the Boltzmann constant,
respectively, T is the absolute temperature, and E is the quenching
activate energy, which refers to the distance from the bottom of the
excitation state to the intersection between the excitation and
ground states. Based on Eqn (7), the FIR therefore can be deduced as
follows [55]:
i
h
i
i
o
ꢅ
ꢆ
ꢅ
I Bi3þð2Þ
I Eu3þð612Þ
I0 Bi3þð2Þ
I0 Eu3þð612Þ
1 þ A Eu3þð612Þ expð ꢀ E=kTÞ
1 þ A Bi3þð2Þ expð ꢀ E=kTÞ
i
h
i
i
o
FIR ¼
¼
ꢅ
ꢆ
ꢅ
(8)
zB þ C expð ꢀ
D
E=kTÞ
Although the energy transfer process from Bi3þ to Eu3þ ions is
enhanced, the emission intensity of Eu3þ is not consolidated, which
may be ascribed to the lower position of crossing point Q than that
of crossing point C to supply a channel for electrons to go back to
the ground state along the olive dotted line ➅.
where B, C and △E are the parameters related to the constants
from these two components in current case. Accordingly, the ab-
solute and relative temperature sensitivities, denoted as Sa and Sr,
can be deduced to the following equation:
As Bi3þ(2) and Eu3þ emissions show quite different lumines-
cence thermal quenching rates with increasing temperature, which
could serve them as the two components in fluorescent intensity
ratio (FIR) technique for ratiometric thermometer. Herein, we
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
vFIR
vT
DE
Sa ¼
¼ C expð ꢀ
DE=kTÞ ꢁ
(9)
kT2
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
3þ
3þ
defined the FIR ¼ I3Bþi (2)/I
, in which I
is the integrated
Eu (612)
Bi (2)
1 vFIR
FIR vT
C expð ꢀ
DE=kTÞ
DE
emission intensity for Bi3þ(2) in the range of 352e450 nm, and
Sr ¼ 100% ꢁ
¼ 100% ꢁ
ꢁ
kT2
B þ C expð ꢀ
DE=kTÞ
3þ
I
is the emission intensity of Eu3þ at 612 nm. According to
Eu (612)
(10)
Struck and Fonger theory [54], the following equation can be used

94
K. Li, R. Van Deun / Journal of Alloys and Compounds 787 (2019) 86e95
[5] S. Sinha, M.K. Mahata, K. Kumar, Comparative thermometric properties of
As presented in Fig. 9c, the plot of calculated results of FIR on
Bifunctional Er3þ-Yb3þ doped rare earth (RE ¼ Y, Gd and La) molybdates,
temperature from the emission spectra of CYSO:0.10Bi3þ, 0.56Eu3þ
can be fitted well with Eqn (8) (R2 ¼ 0.99855). Then, the Sa and Sr,
presented in Fig. 9d, can be calculated according to the fitted results
in Fig. 9c. It is found that the maximum values for Sa and Sr are
0.0717 K-1 (523 K) and 0.958 %Kꢀ1 (423 K), respectively, in which
the former one is much than the highest value of 0.015 K-1 and the
later one can be compared to the highest value of 1.1%Kꢀ1 in pre-
viously reported optical thermometric materials, respectively,
based on the TCL of Er3þ [45,56]. The results suggest that this kind
of Bi3þ and Eu3þ co-doped phosphors can be as one of the poten-
tially ratiometric temperature sensing candidates.
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4. Conclusions
A series of Bi3þ and Eu3þ doped Ca2Y8(SiO4)6O2 (CYSO) phos-
phors were synthesized via a Pechini-type sol-gel reaction process.
Two kinds of Bi3þ sites were analyzed in Bi3þ singly-doped CYSO,
which result in the cyan emission upon a 343 nm UV excitation. The
spectral overlap between Bi3þ intense blue-green emission under
UV excitation and Eu3þ excitation monitored at red emission can
deduce the energy transfer from Bi3þ to Eu3þ ions, which is sub-
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Bi3þ and Eu3þ emissions, PL emission spectra, and fluorescent
decay times of Bi3þ under UV excitation in CYSO:Bi3þ, Eu3þ sam-
ples. The non-radiative resonant energy transfer mechanism is
demonstrated to be a dipole-quadrupole interaction. The consis-
tence of calculated critical distances of energy transfer from Bi3þ to
Eu3þ ions by concentration quenching (12.52 Å) and spectral
overlap routes (10.86e14.48 Å) can further support the dipole-
quadrupole interaction energy transfer mechanism. In addition,
the extremely different luminescence thermal quenching behaviors
between Bi3þ(2) and Eu3þ present good temperature sensing
properties in a temperature range of 298e523 K by analyzing the
temperature sensitivity of the fluorescent intensity ratio [Bi3þ(2)/
Eu3þ(612)]. The maximum absolute and relative sensitivities reach
as high as 0.07174 K-1 (523 K) and 0.958% Kꢀ1 (423 K), which can be
compared to the highest values 0.015 K-1 and 1.1%Kꢀ1 in previously
reported optical thermometric materials, respectively, based on the
TCL of Er3þ. Meanwhile, the luminescence thermal quenching
mechanism in this system was investigated in detail. Results
encourage that a feasible method based on site-Bi3þ and Eu3þ dual
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2019.02.087.
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