Energy transfer from Bi3+ to Ho3+ triggers brilliant single green light emission in LaNbTiO6:Ho3+, Bi 3+ phosphors

Article abstract of DOI:10.1039/c3ra47833e

Excitation of Ho³⁺ and Bi³⁺ co-doped LaNbTiO ₆ particles with 453 nm blue light gave an intense single green glow. All the phosphors were synthesized via a facile sol-gel and combustion approach, and the crystal structure, particle morphology, photoluminescence (PL) properties of the phosphors and energy transfer between Bi³⁺ and Ho³⁺ were also investigated. The large spectral overlap between the broad emission band of Bi³⁺ around 425-570 nm and the excitation band of Ho³⁺ supports the efficient energy transfer from Bi³⁺ to Ho³⁺, which enhances the PL intensity remarkably. When the PL intensity is considered, the best composition for producing green light is LaNbTiO₆:4 mol% Ho³⁺, 2 mol% Bi³⁺. The luminescence mechanisms of Ho³⁺ doped and Ho³⁺/Bi ³⁺ co-doped in the LaNbTiO₆ host were also discussed. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ra47833e

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Energy transfer from Bi3+ to Ho3+ triggers brilliant
single green light emission in LaNbTiO6:Ho3+, Bi3+
phosphors†
Cite this: RSC Adv., 2014, 4, 13680
a
a
Xingshuang Zhang, Guangjun Zhou, Juan Zhou,b Haifeng Zhou,a Peng Kong,a
*
Zhichao Yua and Jie Zhana
Excitation of Ho3+ and Bi3+ co-doped LaNbTiO6 particles with 453 nm blue light gave an intense single green
glow. All the phosphors were synthesized via a facile sol–gel and combustion approach, and the crystal
structure, particle morphology, photoluminescence (PL) properties of the phosphors and energy transfer
between Bi3+ and Ho3+ were also investigated. The large spectral overlap between the broad emission
band of Bi3+ around 425–570 nm and the excitation band of Ho3+ supports the efficient energy transfer
from Bi3+ to Ho3+, which enhances the PL intensity remarkably. When the PL intensity is considered, the
best composition for producing green light is LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+. The luminescence
mechanisms of Ho3+ doped and Ho3+/Bi3+ co-doped in the LaNbTiO6 host were also discussed.
Received 20th December 2013
Accepted 25th February 2014
DOI: 10.1039/c3ra47833e
www.rsc.org/advances
investigations of the optical spectra of Ho3+ have been carried
out for M3Al5O12:Ho3+ (M ¼ Y, Lu),12 MLiF4:Ho3+ (M ¼ Gd, Y,
Lu),13 MBi(XO4):Ho3+ (M ¼ Li, Na; X ¼ W, Mo).14 In addition,
Introduction
Over the past several decades, activation of rare earth (RE)
compounds by various lanthanide ions has been extensively
investigated owing to their fascinating optical characteristics
based on their unique intra-4f transitions that could result in
long-lived luminescent excited states and narrow emission bands.
At the same time, lanthanide ion involved intra-4f transitions are
barely affected by the external environment or crystal eld on
account of the shielding of the 4f orbitals by the lled outer 5s
and 5p orbitals.1–3 In view of the above mentioned facts, some
luminescent materials have been widely applied in lighting,
display elds and biological applications.4–7 As we all know, the
production of white light by means of combination of red, green,
and blue phosphors with a near-ultraviolet or ultraviolet diode is
highly favoured. In view of the huge potential market in home
lighting, many LED manufacturers globally want to develop them.
Therefore, faced with this trend, new-type highly efficient red,
green, and blue phosphors must be developed rapidly so as to
keep up with the advances made in diode emission wavelength.8,9
Ho3+ possesses a wealth of energy levels and, consequently,
has many luminescent states due to the complexity of the 4f10
energy level system.10 Malinowski et al. pointed out that Ho3+
systems have been investigated for application in infrared lasers
for remote sensing and medical purposes.11 Extensive
researchers have reported that Bi3+ usually acts as an excellent
sensitizer for RE ions in a variety of hosts such as rare earth
oxides, phosphates, molybdates, vanadates, tungstates and
nibobates, for instance, Y2O3:Bi3+, Ln3+ (Ln ¼ Sm, Eu, Dy, Er,
Ho),15 YPO4:Bi3+, Eu3+,16 CaMoO4:Bi3+, Eu3+,17 MVO4:Bi3+, Ln3+
(M ¼ Y, Gd; Ln ¼ Eu, Sm, Dy, Ho, Yb),18–22 ZnWO4:Eu3+, Bi3+,23,24
LnNbO4:Dy3+, Bi3+ (Ln ¼ La, Y, Gd).25 All these stem from the
fact that the UV efficiency of the phosphors can be remarkably
enhanced by the energy transfer (ET) from Bi3+ to RE ions under
the excitation of UV light. Compared with other rare earth
elements, La is more abundant in rare earth mineral resources
and lanthanum oxide is much cheaper than other rare earth
oxides. Nevertheless, the development of La-based materials is
still inadequate and lanthanum oxide is overstocked in the rare
earth industry. Therefore, it is fairly valuable to fundamentally
and practically study the La-based materials for the balanced-
utilization of the rare earth natural resources.26 Although many
lanthanum compounds materials with various morphologies
such as nanospheres, nanorods, nanowires, and nanoplates etc.
have been synthesized in the past few years, lanthanum tita-
noniobates doped with RE ions as well as their optical proper-
ties have rarely been investigated up to now. In addition, for
conventional powder phosphors, reduction in the particle size
is achieved by mechanically grinding techniques. While this
mechanically grinding method easily results in formation of
large amount of surface defects which provides non-radiative
recombination process, and ultimately decreases the lumines-
cent efficiency.27 Therefore, the direct preparation of lumines-
cent materials in nanoscale has become vital.
aState Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R.
China. E-mail: gjzhou@sdu.edu.cn; Fax: +86 531 88361206
bCenter for Disease Prevention and Control of Jinan Military Command, Jinan 250014,
P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47833e
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Herein, a facile combinatorial chemistry approach, charac- with the 2q range of 10–80ꢀ, step width of 0.02ꢀ and count time
terized by sol–gel and combustion was employed to synthesize of 0.2 s per step. Thermal analysis of the powder that was dried
the Ho3+, Bi3+ co-doped LaNbTiO6 powder phosphors with at 120 C for 16 h was carried out from 30 to 1000 C by ther-
aeschynite-type structure. During the course of spectroscopic mogravimetry-differential thermal analysis (TG-DTA) (Perkin
investigations on Ho3+ and Bi3+ co-doped LaNbTiO6, strong Elmer Corporation, Diamond TG-DTA) with a constant heating
green emitting originated from Ho3+ and Bi3+ was observed rate of 20 ꢀC minꢁ1. The microstructure and stoichiometry data
under 453 nm excitation. Besides, the mechanism of the energy were obtained by SEM (Hitachi, S-4800) and EDS (Horiba EMAX
ꢀ
ꢀ
transfer from Ho3+ to Bi3+ was also discussed.
Energy, EX-350), respectively. The PL property measurements
were recorded on a uorescence spectrophotometer (JEOL,
F-4500 and FLS920), and a 450 W Xe lamp serves as the exci-
tation source. The absolute quantum efficiencies of the phos-
phor were measured on an Edinburgh FLS920 uorescence
spectrometer and its quantum yield measurement system. In
addition, a uorescence microscope (Nikon Eclipse 80i) was
used for the luminescence observation of the samples. All the
measurements were taken at room temperature.
Experimental
Synthetic procedures
Pure and doped LaNbTiO6 samples were synthesized via a facile
sol–gel and combustion process. Lanthanum oxide, holmium
oxide, bismuth nitrate, niobium oxide, tetra-n-butyl titanate,
nitric acid, hydrouoric acid (HF, 40%), ammonium nitrate,
citric acid were used as starting materials to prepare the phos-
phor samples. All the reagents were used without further puri-
cation. Lanthanum nitrate and holmium nitrate solutions
were produced previously by dissolving the lanthanum oxide
and holmium oxide with excess diluted nitric acid, respectively.
The acquired lanthanum nitrate and holmium nitrate solutions
were heated at 120 ꢀC to evaporate distilled water and the excess
nitric acid. Then, the lanthanum nitrate and holmium nitrate
solid were made up to solutions of 0.5 mol Lꢁ1 and 0.05 mol
Lꢁ1, respectively. Citric acid was not only used as prominent
complexant for sol process in aqueous solution, but also facil-
itated the formation of gel. Meanwhile, citric acid and ammo-
nium nitrate with the mole ratio of 1 : 5 acted as fuels for the
combustion process.
Results and discussion
Structure and morphology
It is well known that the crystallinity, crystallite size and surface
morphology have a strong inuence on the PL properties of
phosphor materials. Our previous work have veried that the
best crystallinity of pure LaNbTiO6 crystals can be successfully
synthesized via a facile method aer being annealed at 1100 ꢀC
for 1 h, and the PL intensity of doped phosphors is optimal at
the same time. Consequently, all the samples were annealed at
ꢀ
1100 C for 1 h ultimately in this study.
The composition and phase purity of the as-prepared pure
and doped LaNbTiO6 products were rst measured by XRD.
Fig. 1(a) shows the XRD patterns of pure LaNbTiO6,
LaNbTiO6:4 mol% Ho3+ and LaNbTiO6:4 mol% Ho3+, 2 mol%
Bi3+ phosphors, and all the diffraction peaks of the as-synthe-
sized sample are in good agreement with aeschynite-type
LaNbTiO6 (Joint Committee on Powder Diffraction Standards
JCPDS File Card no. 73-1059). This can be explained that rare
earth ions have similar co-ordination structure and atomic
radius, as a result the crystal structure does not change
dramatically when the metal ions (La3+) are replaced by one or
more kinds of RE ions. The main peaks (200), (210), (111), (220),
(301), (121), (311), (002) and so on are well indexed in the stan-
dard pattern, respectively. From the XRD patterns, it was
conrmed that no separate Ho3+ or Bi3+ related phases were
detected at the current doped level, indicating that the pure
LaNbTiO6 crystals were successfully prepared via this facile
method. Furthermore, according to JCPDS no. 73-1059 data le,
LaNbTiO6 crystallizes as a orthorhombic structure with a space
group of Pnma (62), and lattice parameters were achieved with a
Firstly, 0.5 mmol Nb2O5 was dissolved with excess hydro-
ꢀ
uoric acid in a water bath at 90 C, and the pH of the NbF5
solution was regulated to 9.0 by adding ammonia aqueous
solution. Then, the white precipitate of niobic acid obtained
was ltered and washed with deionized water for several times
to make sure that the Fꢁ ions were completely removed. Aer-
ward, the precipitate of niobic acid was dissolved with tetra-n-
butyl titanate and citric acid with the mole ratio of 1 : 1 : 3
under heating at 80 ꢀC. Then, tetra-n-butyl titanate, lanthanum
nitrate, and ammonium nitrate with the mole ratio of 1 : 1 : 15
were added and mixed homogeneously under continuous stir-
ring and heating at 80 ꢀC for 4–5 h until the sol formed. Aer the
water evaporated, the transparent sol turned into gel with high
ꢀ
viscosity. The gel was dried at 120 C for 16 h to form yellow
xerogel. The obtained xerogel was then introduced into cruci-
bles, then directly transferred into a muffle furnace annealed at
ꢀ
1100 C for 1 h, respectively. Finally, all samples were ground
into powder for characterization. The synthesis processes of
Ho3+-doped, Bi3+-doped and Ho3+, Bi3+ co-doped LaNbTiO6
could refer to SI.
˚
˚
˚
¼ 10.934 A, b¼ 7.572 A, c¼ 5.446 A, with volume unit cell of
3
450.9 A and Z¼ 4. At the same time, the trivalent Ho3+ and Bi3+
˚
ions have been effectively incorporated into the LaNbTiO6 host
by substituting La3+ because of their similar ionic radius. The
ionic radii for eight-coordinated La3+, Ho3+, and Bi3+ are 1.172 A,
Characterization
˚
˚
˚
The phase composition and structure were characterized using 1.041 A, and 1.170 A, respectively. In addition, it can be observed
X-ray powder diffraction patterns (Germany Bruker Axs D8- from Fig. 1(b) that the diffraction peaks positions are slightly
Avance X-ray diffractometer with graphite monochromatized Cu shied to a greater degree when Ho3+ or Ho3+/Bi3+ ions were
˚
Ka irradiation (l ¼ 1.5418 A)), and all the data were collected doped into the LaNbTiO6 host, which can be attributed to the
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Fig. 2 (a) Three-dimensional space crystal structure of LaNbTiO6, and
(b) balls-sticks model of NbTi–O octahedron.
Fig. 3 TG-DTA curves of pure LaNbTiO6 precursor xerogel powder.
Fig. 1 (a) XRD patterns of the as-synthesized pure LaNbTiO6, LaNb-
TiO6:4 mol% Ho3+ and LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+ samples
annealed at 1100 ꢀC for 1 h successively as well as the JCPDS card
73-1059 of LaNbTiO6 for comparison. (b) Comparison of the samples
as-synthesized from 2q of 28.0–31.0ꢀ.
ꢀ
stage, weight loss is about 69.32% below 288 C. Moreover, in
the DTA curve, there are two weak exothermic peaks around 127
and 197 ꢀC owing to cross-link effect, and one exothermic peak
decrease of the interplanar spacing owing to the substitution of at 246 ꢀC on account of combustion of organic components
larger-size La3+ sites by the smaller-size Ho3+ and Bi3+, leading to such as citric acid or the remaining organic components from
the lattice distortion effect, and decrease of lattice parameters tetra-n-butyl titanate and decomposition of the complex
and volume accordingly. The XRD results demonstrating that compound. The second stage of weight loss is approximately
the structure of LaNbTiO6 host lattice and phase composition of 8.26%, mainly because of the further combustion of the citrate
phosphors in our experimental range were unchanged upon the and the organic residues, such as the oxidation and dehydrox-
doping of Ho3+ ions or the co-doping of Ho3+/Bi3+.
ylation of the decomposers, accompanied by one sharp
According to the lattice parameters and the related les exothermic peak in the DTA curve from 288 to 596 ꢀC at the
provided by American Mineralogist Crystal Structure Database, the same time. Weight loss of the third stage is about 8.06%,
three-dimensional structure diagram was described in Fig. 2. From accompanied by one obviously exothermic peak at 764 ꢀC which
the crystal structure of LaNbTiO6 in Fig. 2(a), the blue octahedrons can be ascribed to the process of monoclinic and orthorhombic
represent the NbTi–O groups, in the meantime, La3+ is in the phase formation, crystallization and transition.28 Once the
center of the eight NbTi–O octahedrons, and Ho3+/Bi3+ will combustion process reached an end, the sample underwent no
substitute La3+ to occupy this position in the Ho3+ doped or Ho3+/ further transformations and there is nearly no weight loss in the
Bi3+ co-doped phosphors. In addition, from the balls-sticks model TG curve when the temperature is above 950 ꢀC, which illus-
of NbTi–O groups in Fig. 2(b), it can be found that NbTi is in the trates that the sample has reached a relatively stable state in the
center of the octahedron which is composed of six oxygen atoms. process of phase transition.
Fig. 3 shows the TG-DTA curves of pure LaNbTiO6 precursor
Fig. 4 shows typical scanning electron microscope (SEM)
xerogel powder that is the yellow xerogel dried at 120 ꢀC for 16 h. images of pure and doped samples at 1100 ꢀC for 1 h. The
The TG curve displays three main weight loss stages. At the rst sample exhibits agglomeration phenomenon due to sintering at
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high temperature, resulting in irregular particle shapes. By
taking agglomeration effects into account, many particles of
LaNbTiO6 in Fig. 4(a) formed plates or blocks whose diameters
are up to 400–800 nm. From the micrograph of LaNbTiO6:4 mol
% Ho3+ in Fig. 4(b), it can be seen that morphology of the as-
synthesized samples is similar to that in Fig. 4(a) and there are
short rods and some irregular particles form from several adja-
cent particles connect and agglomerate with each other. Simi-
larly, Fig. 4(c) demonstrates the image of LaNbTiO6:4 mol%
Ho3+, 2 mol% Bi3+. These particles become smaller than that of
pure LaNbTiO6 and LaNbTiO6:4 mol% Ho3+ overall, almost
below 500 nm. That is to say, the agglomeration effect could be
weakened when doped with Ho3+ or Ho3+/Bi3+ co-doped in the
host. Further evidence concerning composition of the 4 mol%
Ho3+ and 2 mol% Bi3+ co-doped LaNbTiO6 sample was achieved
by energy dispersive X-ray spectrometer (EDS) spectrum which is
presented in Fig. 4(d). By means of multiple tests and calculating
on the subject of EDS spectra, the ratio of M(La, Ho, Bi)/Nb/Ti/O
is 1.06 : 1.09 : 1 : 6.06, very close to M/Nb/Ti/O ¼ 1 : 1 : 1 : 6,
indicating that the sample probably is composed of LaNbTiO6.
Furthermore, the ratio of Ho/M is 3.92% by calculating in the
EDS spectra, close to 4% as well, and the ratio of Bi/M is 2.09%,
which is close to 2%, demonstrating that corresponding Ho3+
and Bi3+ ions were incorporated into the host successfully.
Photoluminescence properties
Fig. 5 shows the excitation spectra, emission spectra and energy
level diagram of LaNbTiO6:x mol% Ho3+. Fig. 5(a) displays the
excitation spectra of the phosphors monitored with emission
wavelength at 545 nm, and all the excited peaks could be
ascribed to the typical Ho3+ intra-4f10 transition absorption.
According to the energy level diagram in the inset of Fig. 5(a)
and the labels over the excited peaks, it can be found that all the
excited peaks originate from the ground state 5I8 energy level of
Ho3+ transfers to the higher excited state energy levels. For
example, the sharpest excitation band around 453 nm is derived
from the transition from the ground state 5I8 energy level to 5F1
5
5
and G6. Similarly, excitation band at 521 nm (5I8 / F4) and
ve weak excitation bands located at 361 nm (5I8 / H6), 393
3
nm (5I8 / 5G4), 420 nm (5I8 / 5G5), 478 nm (5I8 / 5F2) and 490
nm (5I8 / 5F3), respectively.29,30 Therefore, it can be also found
that blue laser diodes and light emitting diodes may be efficient
pumping sources in obtaining Ho3+ emissions. Hereby, it can be
inferred that the excitation light at 453 nm can make the elec-
trons jump from excited state to ground state, and decline to
lower energy level through non-radiative transitions and relax-
ation effect between multi-photons. Fig. 5(b) presents the
emission spectra of the LaNbTiO6:x mol% Ho3+ (x ¼ 0.5–5)
Fig. 4 SEM micrographs of pure and doped samples obtained at 1100 ꢀC for 1 h: (a) LaNbTiO6; (b) LaNbTiO6:4 mol% Ho3+; (c) LaNbTiO6:4 mol%
Ho3+, 2 mol% Bi3+; and (d) EDS spectrum of LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+ sample and the inset shows the weight percent and atom
percent of all elements.
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measured.31–33 While there is few reports concerning single
green emission of Ho3+ doped phosphors materials so far.
Under blue light LDs and LEDs pumping, LaNbTiO6:Ho3+
phosphors could produce effective homogeneous green emis-
sion, and have potential application value in the eld of at
panel display and trichromatic phosphor.
Doping a certain amount of sensitized ions into phosphors
materials could affect the symmetry of the ligand eld, and
promote the process of the energy transfer which increases the
PL intensity of the RE ions doped phosphors effectively. In
addition, many reports conrmed that Bi3+ is a wonderful
candidate in many hosts.9,16,34 The electronic conguration of
1
Bi3+ is [Xe]4f145d106s2. The ground state is S0 with 6s2 cong-
uration and the excited states of the 6s6p conguration can be
split into the 3P0, 3P1, 3P2 and 1P1 levels in an increasing energy
1
order. The S0 / 3P0 transition is strongly spin forbidden, but
the 1S0 / 3P1 and 1S0 / 3P2 transitions become allowed due to
1
1
the spin-orbit coupling. For the S0 / P1 transition, it is an
allowed electric dipole transition.35 The 3P1 level of Bi3+ is split
into two and three sets of energy levels under S6 and C2
symmetry, respectively. The blue emissions is ascribed to the
3
1
transition from the splitting P1 levels to the ground state S0
level of Bi3+ (S6), and the green emission is assigned to the
transition from the bottom of 3P1 to S0 level of Bi3+ (C2).36,37
1
From the excitation spectrum and emission spectrum of
LaNbTiO6:Bi3+ in Fig. 6, it can be found that Bi3+ has a strong
absorption around the 328 nm and the broad blue-green
emission band around 425–570 nm largely overlaps with the
excitation band of Ho3+. Therefore, it could be inferred that the
incorporation of Bi3+ could enhance the PL intensity of LaNb-
TiO6:Ho3+, in addition, LaNbTiO6:Ho3+, Bi3+ phosphors were
prepared and the PL properties were also studied in the next
experiment.
Fig. 7 presents the emission spectra (lex ¼ 453 nm) of
LaNbTiO6:4 mol% Ho3+, y mol% Bi3+ (y ¼ 0–5) as a function of
the Bi3+ doped concentrations under excitation at 453 nm.
Comparison with LaNbTiO6:4 mol% Ho3+, the strongest emis-
Fig. 5 (a) Excitation spectrum of Ho3+-doped LaNbTiO6 sample
(lem ¼ 545 nm) and the inset is the energy level diagram of Ho3+ in
LaNbTiO6, (b) emission spectra of Ho3+-doped LaNbTiO6 samples
prepared with a function of doped concentrations (lex ¼ 453 nm).
under excitation at 453 nm. All these energy transitions are also sion peak position still located at 545 nm when Bi3+ was doped
ascribed to the characteristic f–f transitions of Ho3+. The green in the host, while the PL intensity at 545 nm increases
emission peak at 545 nm could be ascribed to the transition of
Ho3+ from excited state 5F4 and 5S2 to ground state 5I8. Similarly,
the other two weak emission peaks at 483 nm and 650 nm
5
5
5
5
derived from F2 / I8 transition and F5 / I8 transition of
Ho3+, respectively. These can be supported by the inset of
Fig. 5(a). In addition, from the emission intensities at 545 nm as
a function of the Ho3+ doped concentrations demonstrated in
Fig. 5(b), it can be seen that when the Ho3+ doped concentration
varies from 0.5 to 4 mol%, the PL intensity increased rst, and
beyond that gradually decreased. That is to say, when the Ho3+
doped concentration is up to 4 mol%, the PL intensity reaches
to the maximum. To sum all, Ho3+ acts as an activator in the
host and makes the phosphors present strong green light
through its characteristic emissions.
Although Ho3+ possesses fairly rich energy structure (4f10),
and there are multiple characteristic emission line corre-
sponding to 5F4 + 5S2 / 5I8 transitions (green light), 5F5 / 5I8
Fig. 6 Excitation spectrum (lem ¼ 509 nm) and emission spectrum
and 5F4
+
5S2
/
5I7 transitions (red light) could be (lex ¼ 328 nm) of LaNbTiO6:Bi3+ sample.
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dramatically that could be up to 1.8 times of LaNbTiO6:4 mol%
Ho3+. Generally speaking, there are two kinds of energy transfer
mechanism as to the phenomenon of sensitization. One is
radiation energy transfer that the luminescence of sensitizer was
absorbed by the activator, and the other is non-radiation energy
transfer produced by the multipole interaction between sensi-
tizer and activator, which is electric dipole and electric dipole
interaction, electric dipole and electric quadrupole interaction,
or electric quadrupole and electric quadrupole interaction.
While in the system of LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+, the
characteristic peaks of Ho3+ do not change, while no character-
istic peak of Bi3+ is detected, indicating that efficient energy
levels from Bi3+ to Ho3+ occurs. Under the excitation at 453 nm,
the host absorbs energy, and the charge transfer occurs from
NbTiO6 to Bi3+ and Ho3+. Moreover, P1 excited state of Bi3+
could transfer energy to Ho3+, promoting the energy transition of
Ho3+ and enhancing the PL intensity. The effective energy
transfer process Bi3+ / Ho3+ is mainly through the radiation
energy transfer of Bi3+ to increase the f–f transitions of Ho3+.
Accordingly, when Bi3+ was introduced into the host, Bi3+ can
efficiently transfer its absorption energy to Ho3+.
3ꢁ
3
Fig. 8 Schematic diagram of NbTiO63ꢁ, Bi3+, and Ho3+ energy levels,
excitation, emission, and energy transfer in LaNbTiO6.
Accordingly, the excitation and emission processes of
LaNbTiO6:Ho3+, Bi3+ luminescence can be summarized in
Fig. 8. Apart from that, in the host of LaNbTiO6, it is easy for Bi3+
to substitute La3+ (1.172 A) and occupy the lattice sites with a
˚
low symmetry for the radius of Bi3+ (1.170 A) is slightly smaller
˚
than that of RE3+. The doping of Bi3+ could increase the disor-
ganization of the ambient environment around Ho3+ which is
also advantageous to improve the PL intensity of Ho3+. From the
emission spectra of Fig. 7, it can be found that the optimum
doped concentration of Bi3+ is 2 mol%. Because of the radiation
energy transfer of Bi3+ / Ho3+, the emission intensity of the
phosphors increases with the doped concentration of Bi3+
within certain limits. However, once the doped concentration of
Bi3+ exceeds 2 mol%, non-radiation energy transfer of Bi3+
/
Bi3+ increases remarkably and energy transfer of Bi3+ / Ho3+ is
weakened, resulting in the decrease of PL intensity, which is
generally called concentration quenching.
Fig.
9 CIE chromaticity diagram and microscope fluorescence
images for (a) LaNbTiO6:4 mol% Ho3+ and (b) LaNbTiO6:4 mol% Ho3+
2 mol% Bi3+ phosphors.
,
The chromaticity coordination of LaNbTiO6:4 mol% Ho3+
and LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+ is converted to the x, y
CIE (Commission Internationale deI' Eclairage) 1931 chroma-
ticity diagram in Fig. 9. The CIE chromaticity coordinates of the
LaNbTiO6:4 mol% Ho3+ and LaNbTiO6:4 mol% Ho3+, 2 mol%
Bi3+ phosphors are (0.244, 0.550) and (0.256, 0.587), respec-
tively, both corresponding to green emission in a different
proportion. The absolute quantum efficiencies of the phos-
phors are 9% (LaNbTiO6:4 mol% Ho3+) and 16%
(LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+). Furthermore, the
Fig. 7 Emission spectra of Ho3+, Bi3+ co-doped LaNbTiO6 samples
prepared with a function of concentrations of Bi3+ under excitation at
453 nm.
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microscope uorescence images of the Ho3+ doped, Ho3+ and
Bi3+ co-doped LaNbTiO6 are presented in the inset of Fig. 9, and
both the two kinds of phosphors are under blue light excitation
being magnied 40 times. It is clear that the images present a
green color with high brightness and high homogeneity.
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´
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7 E. Tellez-Flores, R. Aceves, R. Perez-Salas, I. Camarillo and
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Conclusions
A single green light emitting phosphor LaNbTiO6:Ho3+, Bi3+ was 11 M. Malinowski, M. Kaczkan, A. Wnuk and M. Szuinska,
synthesized via a sol–gel and combustion approach. It has been
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Ho3+, Bi3+ co-doped LaNbTiO6 green phosphors also were inves-
Matter, 2005, 17, 7643.
tigated. With the activating of Ho3+, under excitation at 453 nm 14 A. Mendez-Blas, M. Rico, V. Volkov, C. Zaldo and C. Cascales,
´
blue light, emission spectra of the phosphors LaNbTiO6:Ho3+
Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 75, 174208.
exhibited the strongest green light glow at about 545 nm owing to 15 G. Ju, Y. Hu, L. Chen, X. Wang, Z. Mu, H. Wu and F. Kang,
5
5
5F4 + S2 / I8 transition of Ho3+ ions. In addition, it has been
found that the broad emission band originating from 3P1 / 1S0 16 M. Niraj Luwang, R. S. Ningthoujam, S. K. Srivastava and
transition of Bi3+ overlaps with excitation band of Ho3+ though
R. K. Vatsa, J. Am. Chem. Soc., 2011, 133, 2998.
researching the excitation spectrum and emission spectrum of 17 S. Yan, J. Zhang, X. Zhang, S. Lu, X. Ren, Z. Nie and X. Wang,
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TiO6, Bi3+ acts as an effective sensitizer of Ho3+, inducing the 18 Y. Chen, Y. Wu, D. Wang and T. Chen, J. Mater. Chem., 2012,
energy transfer of Bi3+ / Ho3+ and increasing the PL intensity
22, 7961.
remarkably. Considering the PL intensity, the best composition 19 D. Chen, Y. Yu, P. Huang, H. Lin, Z. Shan, L. Zeng, A. Yang
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for producing green light is LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+,
and Y. Wang, Phys. Chem. Chem. Phys., 2010, 12, 7775.
and the PL intensity of LaNbTiO6:4 mol% Ho3+, 2 mol% Bi3+ was 20 U. Rambabu and S. Han, Ceram. Int., 2013, 39, 1603.
enhanced by 1.8 times than that of LaNbTiO6:4 mol% Ho3+ at 545 21 U. Rambabu, N. R. Munirathnam, S. Chatterjee, B. Sudhakar
nm. Finally, the microscope uorescence images and CIE chro-
maticity picture visually exhibit the emitting colors of Ho3+ doped 22 C. V. Devi, G. Phaomei, N. Yaiphaba and N. R. Singh, J. Alloys
and Ho3+/Bi3+ co-doped in phosphors. From the above, prelimi-
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nary studies have indicated that the LaNbTiO6:Ho3+, Bi3+ may 23 L. Wang, Q. Wang, X. Xu, J. Li, L. Gao, W. Kang, J. Shi and
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and trichromatic phosphor, serving as a wonderful green light 24 L. Wang, Z. Lv, W. Kang, X. Shangguan, J. Shi and Z. Hao,
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Acknowledgements
This work was supported by projects from The Chinese PLA 27 G. Li, C. Li, Z. Xu, Z. Cheng and J. Lin, CrystEngComm, 2010,
Medical Science and Technique Foundation (CWS11J243); 12, 4208.
Independent Innovation Foundation of Shandong University, 28 Q. Ma, A. Zhang, M. Lu, Y. Zhou, Z. Qiu and G. Zhou, J. Phys.
IIFSDU (2011JC024); National Science Foundation of China
Chem. B, 2007, 111, 12693.
(51372138) and The Scientic Research Foundation for the 29 L. Sun, Y. Qiu, T. Liu, J. Z. Zhang, S. Dang, J. Feng, Z. Wang,
Returned Overseas Chinese Scholars, State Education Ministry.
H. Zhang and L. Shi, ACS Appl. Mater. Interfaces, 2013, 5, 9585.
30 K. Biswas, A. D. Sontakke, R. Sen and K. Annapurna,
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31 J. A. Capobianco, J. C. Boyer, F. Vetrone, A. Speghini and
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