Color-Tunable Luminescence of YNbO4:Ln3+ (Ln3+ = Dy3+ and/or Eu3+) Nanocrystalline Phosphors Prepared by a Sol-Gel Process

Article abstract of DOI:10.1002/ejic.201500699

A series of YNbO₄:Ln³⁺ (Ln³⁺ = Dy³⁺ and/or Eu³⁺) phosphors have been synthesized by a Pechini-type sol-gel method. X-ray diffraction, field-emission scanning electron microscopy, and transmission electron microscopy were utilized to characterize the prepared phosphors. The pure phase of YNbO₄ was obtained after annealing at 1000 C. The morphologies of the YNbO₄:Ln³⁺ (Ln³⁺ = Dy³⁺ and/or Eu³⁺) phosphors are approximately spherical grains with a diameter of around 40-60 nm. The luminescence properties of the YNbO₄:Ln³⁺ (Ln³⁺ = Dy³⁺ and/or Eu³⁺) phosphors were investigated by photo- and cathodoluminescence methods. Under the excitation of UV light and a low-voltage electron beam, the YNbO₄:Ln³⁺ (Ln³⁺ = Dy³⁺ and/or Eu³⁺) phosphors obtained show characteristic blue wide-band emission from the NbO₄ group, whitish light emission from Dy³⁺, and red emission from Eu³⁺. Energy is transferred from the YNbO₄ host lattice to the Dy³⁺ and Eu³⁺ ions. Single-phase white-light-emision was observed in YNbO₄:Dy³⁺ phosphors under the excitation of UV light and a low-voltage electron beam. The color rendering index of the YNbO₄:Dy³⁺ phosphor can be further improved by co-doping YNbO₄:Dy³⁺ with the Eu³⁺ ion. The emission color of the YNbO₄:Dy³⁺,Eu³⁺ phosphors can be simply tuned by changing the relative ratio of the Dy³⁺/Eu³⁺ ions. The prepared YNbO₄:Dy³⁺ and YNbO₄:Dy³⁺,Eu³⁺ phosphors have potential applications in UV white-light-emitting diodes and other optoelectronics display devices.

Full text of DOI:10.1002/ejic.201500699

FULL PAPER
DOI:10.1002/ejic.201500699
Color-Tunable Luminescence of YNbO4:Ln3+ (Ln3+ =
Dy3+ and/or Eu3+) Nanocrystalline Phosphors Prepared by
a Sol–Gel Process
Ying Lü,[a][‡] Chen Chen,[a][‡] Shuailong Li,[a] Xiaoming Liu,*[a]
Liushui Yan,[a] Yuhua Dai,[a] Aiqin Zhang,[a] Yu Xie,[a] and
Xinghua Tang*[a]
Keywords: Luminescence / Doping / Niobates / Lanthanides / Phosphors
A series of YNbO4:Ln3+ (Ln3+ = Dy3+ and/or Eu3+) phosphors
have been synthesized by a Pechini-type sol–gel method. X-
ray diffraction, field-emission scanning electron microscopy,
and transmission electron microscopy were utilized to char-
acterize the prepared phosphors. The pure phase of YNbO4
was obtained after annealing at 1000 °C. The morphologies
of the YNbO4:Ln3+ (Ln3+ = Dy3+ and/or Eu3+) phosphors are
approximately spherical grains with a diameter of around 40–
60 nm. The luminescence properties of the YNbO4:Ln3+ (Ln3+
= Dy3+ and/or Eu3+) phosphors were investigated by photo-
and cathodoluminescence methods. Under the excitation of
UV light and a low-voltage electron beam, the YNbO4:Ln3+
(Ln3+ = Dy3+ and/or Eu3+) phosphors obtained show charac-
teristic blue wide-band emission from the NbO4 group, whit-
ish light emission from Dy3+, and red emission from Eu3+
.
Energy is transferred from the YNbO4 host lattice to the Dy3+
and Eu3+ ions. Single-phase white-light-emision was ob-
served in YNbO4:Dy3+ phosphors under the excitation of UV
light and a low-voltage electron beam. The color rendering
index of the YNbO4:Dy3+ phosphor can be further improved
by co-doping YNbO4:Dy3+ with the Eu3+ ion. The emission
color of the YNbO4:Dy3+,Eu3+ phosphors can be simply tuned
by changing the relative ratio of the Dy3+/Eu3+ ions. The pre-
pared YNbO4:Dy3+ and YNbO4:Dy3+,Eu3+ phosphors have
potential applications in UV white-light-emitting diodes and
other optoelectronics display devices.
light. The former approach usually leads to a poor render-
ing index and high color temperature, because the red com-
ponent is lacking.[12] In the latter approach, the LED device
often gives low luminescence efficiency owing to re-absorp-
tion and a large Stokes shift.[13] However, most of these
problems can be avoided by single-phase white-emitting
phosphors with an excellent color index and higher lumi-
nous efficiency.[14] For these reasons, considerable attention
has been devoted to the development of new single-compo-
sition white-emitting phosphors that can be excited ef-
ficiently by UV or near-ultraviolet (NUV) light for solid-
state lighting.[14,15]
Introduction
Phosphors have attracted considerable interest for their
many potential applications as well as being essential in illu-
mination, displays and imaging fields such as fluorescent
tubes, light-emitting diodes (LEDs), field-emission displays
(FEDs), cathode ray tube displays (CRTs), and X-ray image
converter devices.[1–9] In addition, white-light-emitting di-
odes (w-LEDs) are rapidly replacing traditional incandes-
cent, fluorescent lamps, and they are regarded as the next-
generation lighting source on account of their extraordinary
luminous efficiency, low power consumption, long lifetime,
and environmental friendliness.[10,11] More importantly,
LEDs with phosphors to generate white light are an impor-
tant technology in the lighting industry. Many approaches,
such as yellow phosphors combined with a blue LED chip
and tricolor phosphors combined with a UV LED chip, can
be employed to fabricate w-LEDs through the mixing of
Trivalent rare-earth (RE) ions, which are widely used as
activators in inorganic phosphors, generally show narrow
lines with high color purity, because their 4f electrons are
well shielded by the outer 5s and 5p electrons. The emission
spectrum of Dy3+ often shows two principal emissions in
4
the blue and yellow regions corresponding to F9/2Ǟ6H15/2
and 4F9/2Ǟ6H13/2 transitions, respectively, of the 4f9 config-
uration of Dy3+. White-light emission can be obtained by
adjusting the relative emission intensity of yellow to blue
(Y/B ratio).[15] Unfortunately, Dy3+ cannot be efficiently ex-
cited by UV light due to its low oscillator strength and the
narrow line width of the 4f–4f absorption. Furthermore, the
charge-transfer band (CTB) of Dy3+ is often located below
200 nm.[16] Accordingly, many host lattices are doped with
[a] School of Environment and Chemical Engineering,
Nanchang Hangkong University,
Nanchang 330063, P. R. China
E-mail: xmliu2014@163.com
txh38006@nchu.edu.cn
http://huanhua.nchu.edu.cn/index.asp
[‡] These authors contributed equally to this paper
ORCID(s) from the author(s) for this article is/are available on
the WWW under http://dx.doi.org/10.1002/ejic.201500699
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Dy3+ to obtain high-efficiency luminescence materials
through a host sensitization process, such as in YVO4:Dy3+
,
GdNbO4:Dy3+, Gd2(MoO4)3:Dy3+, Gd3Al5O12:Dy3+, and
Mg2Al4Si5O18:Dy3+ phosphors.[17–21] However, the white
light from Dy3+ commonly shows a low color rendering
index (CRI) with poor color quality owing to its emission
spectrum lacking a red component. On the other hand,
Eu3+ usually exhibits red-light emission corresponding to
5D0Ǟ7FJ (J = 0, 1, 2, 3) transitions.[22] Therefore, it is rea-
sonable that co-doping Eu3+ and Dy3+ ions in a host matrix
leads to high-quality, single-composition, white-light phos-
phors, such as in NaGdF4:Dy3+,Eu3+, Sr3AlO4F:Dy3+,Eu3+,
Figure 1. Representative XRD patterns of (a) YNbO4,
Ca2Gd8Si6O26:Dy3+,Eu3+
Gd2(MoO4)3:Dy3+,Eu3+
Y2O2S:Dy3+,Eu3+ phosphors.[23–29]
,
LiInW2O8:Dy3+,Eu3+
,
(b) YNbO4:0.03Dy3+
,
and (c) YNbO4:0.06Dy3+ annealed at
,
BaLa2WO7:Dy3+,Eu3+
,
and
1000 °C. The standard XRD pattern for YNbO4 (JCPDS No. 23-
1486) is used as a reference.
Lanthanide niobates with fergusonite structure have been
widely investigated by many researchers due to their special
physical and chemical properties.[30] YNbO4 is known as an
excellent host material as a result of its appropriate phonon
energy and stability,[30–32] and shows strong blue emission
under excitation with UV light at 254 nm.[32] Unfortunately,
only limited information is available on RE3+-doped
YNbO4 phosphors.[30,32–35] Furthermore, Dy3+-doped and
Dy3+,Eu3+-co-doped YNbO4 phosphors have not been de-
scribed. Accordingly, in this manuscript, we report the syn-
thesis of Dy3+ singly doped and Dy3+,Eu3+-co-doped
YNbO4 phosphors by a Pechini-type sol–gel method and
investigate their photoluminescence (PL) and cathodolumi-
nescence (CL) properties in more detail.
Our research has shown that there is efficient energy
transfer from the YNbO4 host lattice to the Dy3+ and Eu3+
ions in YNbO4:Ln3+ (Ln3+ = Dy3+ and/or Eu3+) phos-
phors. White PL and CL with high color quality have been
realized in a single YNbO4 host lattice by co-doping Dy3+
and Eu3+ ions. The YNbO4:Dy3+ and YNbO4:Dy3+,Eu3+
phosphors obtained have potential applications in the area
of UV w-LEDs and other optoelectronic devices.
Results and Discussion
Phase Purity and Crystal Morphology
The crystallization behavior and morphologies of the
studied samples were investigated by XRD, FESEM, and
HRTEM.
The
results
for
YNbO4:Eu3+
and
YNbO4:Dy3+,Eu3+ are similar to those of YNbO4:Dy3+
and will not be described here.
The pure phase of YNbO4 prepared by a Pechini-type
process was obtained after 6 h at 1000 °C, which is lower
than the temperature of 1400 °C employed in its prepara-
tion by the solid-state reaction (SSR) method.[36–39] Fig-
ure 1 shows the representative XRD patterns of
YNbO4:xDy3+ samples annealed at 1000 °C for 6 h. It can
clearly be seen that all of the diffraction peaks of the sam-
ples obtained can be indexed to the C2/c space group of
the monoclinic YNbO4 phase (JCPDS No. 23-1486). No
obvious shifting of peaks or a second phase (such as traces
Figure 2. (a) FESEM and (b) HRTEM images of the
of residual Y2O3 or Nb2O5 and other impurity phases) were YNbO4:0.03Dy3+ sample annealed at 1000 °C.
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observed, even at high doping concentration (Dy3+ 6 at-% broad excitation band is ascribed to the charge-transfer
of Y3+), which indicates that Dy3+ is completely dissolved (CT) transition from O2– (2p6) to the empty 3d0 state of the
in the YNbO4 host lattice by substituting for Y3+. This may Nb5+ ion. The narrow lines beyond 350 nm are characteris-
be due to their similar ionic radii (Dy3+, eight-coordinate, tic of fǞf transitions of Dy3+ within its 4f9 configuration
r = 0.912 Å; Y3+, eight-coordinate, r = 0.908 Å).[40] Figure 2 (the corresponding energy levels of Dy3+ are shown in Fig-
shows the FESEM and HRTEM images of the ure 12), that is, 6H15/2Ǟ6P5/2 (353 nm), 6H15/2Ǟ6P7/2
6
6
YNbO4:0.03Dy3+ sample. The FESEM image (Figure 2a) (368 nm), H15/2Ǟ4F7/2 (389 nm), H15/2Ǟ4G11/2 (430 nm),
indicates that the phosphor consists of aggregated particles 6H15/2Ǟ4I15/2 (453 nm), and H15/2Ǟ4F9/2, (480 nm).[18]
6
with approximate spherical morphologies with sizes ranging
Upon exciting the host lattice at 255 nm, the
from 40 to 60 nm. The HRTEM image (Figure 2b) of a part YNbO4:0.03Dy3+ shows a blue-white emission, and the
of the YNbO4:0.03Dy3+ particle shows several lattice planes CIE (Commission Internationale de l’Eclairage) chromatic-
with perfect crystallinity. The lattice fringe is characteristic ity coordinates of YNbO4:0.03Dy3+ were determined to be
of the YNbO4:0.03Dy3+ crystal. The distances of 0.314 and x = 0.2672 and y = 0.2385. The emission spectrum (Fig-
0.266 nm between adjacent lattice fringes correspond to in- ure 3c) shows not only the blue wide-band emission of the
terplanar distances (d) of the 021 and 200 planes, respec- NbO4 group from 300 to 550 nm peaking at 410 nm, but
tively, which are in good agreement with the values of d also the characteristic emission of Dy3+. These characteris-
described in the literature (0.312 nm for the 021 plane, tic emission lines of Dy3+ are dominated by two main
0.266 nm for the 200 plane, JCPDS No. 23-1486).
groups, one in the blue region peaking at 483 nm and the
other in the yellow-orange region peaking at 580 nm, which
correspond to the Dy3+ transitions 4F9/2Ǟ6H15/2, and
4F9/2Ǟ6H13/2, respectively. Monitoring the NbO4 group
emission at 410 nm, the excitation spectrum obtained also
shows a broad band peaking at 255 nm (Figure 3b). Com-
paring the two excitation spectra of YNbO4:0.03Dy3+ (Fig-
ure 3a,b) with that of bare YNbO4 (Figure 3d), it can be
seen that the broad excitation bands have similar shapes,
albeit with different intensities. Therefore, the broad exci-
tation band from 200 to 290 nm peaking at around 255 nm
can be ascribed to the charge-transfer (CT) transitions in
the YNbO4 host lattice. This means that there is energy
transfer from the YNbO4 host lattice to the Dy3+ ion in
YNbO4:Dy3+. Accordingly, the characteristic emission of
the Dy3+ ion can be ascribed to the sensitization of the
YNbO4 host lattice in YNbO4:Dy3+. A similar phenome-
non can also be found in other phosphors, such as
Photoluminescence Properties
Photoluminescence Properties of Dy3+ Singly Doped
YNbO4
The PL excitation and emission spectra of the
YNbO4:0.03Dy3+ sample annealed at 1000 °C are shown in
Figure 3a–c. For the purpose of comparison, the excitation
and emission spectra of the bare YNbO4 prepared under
the same conditions are also shown (Figure 3d,e). The bare
YNbO4 sample shows a strong blue emission under exci-
tation with UV light at 255 nm. Its excitation spectrum
(Figure 3d) shows a broad band from 200 to 290 nm peak-
ing at 255 nm, and the corresponding emission spectrum
(Figure 3e) shows a broad band from 300 to 550 nm peak-
ing at 410 nm. These bands have been ascribed to the tran-
sition of a self-activated optical center corresponding to
octahedral coordinated NbO4 groups.[41]
GdNbO4:Dy3+, β-Ga2O3:Dy3+, and LaGaO3:Dy3+ [16,18,42]
.
The presence of
a strong blue NbO4 emission in
YNbO4:Dy3+ (Figure 3c) indicates that the energy transfer
from NbO4 to Dy3+ ion is far from complete.
The luminescence color of YNbO4:Dy3+ can be tuned to
some extent by changing the doping concentration of the
Dy3+ ion in YNbO4. The corresponding emission spectra
of Y(1–x)NbO4:xDy3+ phosphors with different doping con-
centration of Dy3+ ions (x = 0.001–0.06) are shown in Fig-
ure 4a. It can clearly be seen that the luminescence color of
Y
(1–x)NbO4:xDy3+ can be tuned from blue, through white-
blue, to white by increasing the doping concentration of the
Dy3+ ion. For pure YNbO4, only a blue wide-band emis-
sion of the NbO4 group with high color purity is observed
(the corresponding CIE chromaticity coordinates are x =
0.1542 and y = 0.0463). At a low doping concentration of
Dy3+ in Y(1–x)NbO4:xDy3+ (x Ͻ 0.007), only limited energy
is transferred from the YNbO4 host lattice to Dy3+, the
broad band of the NbO4 group (Nb5+ – O2–, 400 nm) being
Figure 3. Photoluminescence excitation (a,b) and emission (c) spec-
tra of the YNbO4:0.03Dy3+ sample annealed at 1000 °C and the
photoluminescence excitation (d) and emission (e) spectra of the
YNbO4 prepared under the same conditions.
The excitation spectrum of Dy3+ in YNbO4 (Figure 3a) dominant in the emission spectra. Therefore, the
monitored at 580 nm (4F9/2Ǟ6H13/2 transition of Dy3+) ex- YNbO4:Dy3+ obtained gives blue luminescence (e.g., the
hibits a broad excitation band from 200 to 290 nm peaking CIE chromaticity coordinates for YNbO4:0.007Dy3+ are x
at 255 nm and some narrow weak lines beyond 350 nm. The = 0.2181 and y = 0.1539). At a moderate doping concentra-
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tion [0.007 Ͻ x Ͻ 0.03 in Y(1–x)NbO4:xDy3+], under exci-
tation with UV light, the characteristic emission of Dy3+
has an intensity comparable to the blue broad-band emis-
sion of the YNbO4 host lattice, resulting in a blue-white
emission. At high doping levels [x Ն 0.03 in Y(1–x)
NbO4:xDy3+], due to most of the excitation energy being
transferred from the YNbO4 host lattice to Dy3+, the emis-
sion is dominated by the characteristic white emission of
Dy3+. Therefore, the phosphors obtained show a strong
white emission (e.g., the CIE chromaticity coordinates for
YNbO4:0.04Dy3+ were determined to be x = 0.2735 and y
= 0.2486). Thus, the luminescence color can be tuned from
blue, through blue-white, to white by increasing the doping
concentration of Dy3+ ions in Y(1–x)NbO4:xDy3+ phos-
phors. The corresponding CIE chromaticity diagram of
Y
(1–x)NbO4:xDy3+ phosphors with doping concentrations
of Dy3+ varying from 0.001 to 0.06 is shown in Figure 5a.
The cross-points indicate the CIE chromaticity coordinate
positions. The CIE chromaticity coordinates change from x
= 0.1691 and y = 0.0724 (blue) to x = 0.2816 and y = 0.2615
(white) by increasing the doping concentration of Dy3+
from x = 0.001 to x = 0.06 in the Y(1–x)NbO4:xDy3+ phos-
phors.
Figure 5. Corresponding CIE chromaticity diagram for
(a) YNbO4:xDy3+, (b) YNbO4:xEu3+, (c) YNbO4:0.02Dy3+,yEu3+
,
and (d) YNbO4:0.035Eu3+,xDy3+
.
tively.[43,44] Figure 4b shows the efficiency of energy transfer
from the NbO4 group to Dy3+ (red line and dots) and the
emission intensity of Dy3+ (green line and dots) as a func-
tion of doping concentration (x) of Dy3+ in Y(1–x)
NbO4:xDy3+. It can be seen that, first, the efficiency of en-
ergy transfer in Y(1–x)NbO4:xDy3+ increases quickly from
0.1 to 0.46 with increasing doping concentration of Dy3+
from x = 0.01 to 0.03. Then it increases slowly as the dop-
ing concentration of Dy3+ increases from 0.03 to 0.04, after
which it remains almost constant, even with further in-
creases in doping concentration of Dy3+, which might be
due to a concentration quenching effect of Dy3+. For the
NbO4 group, it can be seen from Figure 4a that the emis-
sion intensity of the NbO4 group decreases monotonically
with increasing doping concentration of Dy3+, which means
that after excitation of the host lattice, the energy transfer
ratio from the NbO4 group to Dy3+ increases with increas-
ing doping concentration of Dy3+. However, for Dy3+, its
emission intensity first increased with increasing doping
concentration (x), and then, after reaching a maximum
value at x = 0.007, its emission intensity decreased with
increasing x as a result of a concentration quenching effect
(Figure 4b). Therefore, the optimum doping concentration
Figure 4. (a) Emission spectra of Y(1–x)NbO4:xDy3+ with different
Dy3+ ions concentration and (b) the corresponding energy transfer
efficiencies of Y(1–x)NbO4:xDy3+ and the PL emission intensity of
Dy3+ as
NbO4:xDy3+
a function of doping concentration (x) in Y(1–x)
.
In general, the efficiency of energy transfer from the host for Dy3+ is about 0.007 of Y3+ in YNbO4. For Dy3+ in
lattice/donor (YNbO4) to the luminescence center/acceptor YNbO4, the concentration quenching is mainly caused by
(Dy3+) in Y(1–x)NbO4:xDy3+ phosphors can be calculated cross-relaxation, that is, energy transfer between Dy3+ ions
according to ηET = 1 – Id/Id0, in which Id and Id0 are the through the transitions 4F9/2 (Dy3+) + 6H15/2 (Dy3+) Ǟ 6F3/2
6
corresponding luminescence intensities of the host lattice/ (Dy3+) + F11/2 (Dy3+).[18]
donor (NbO4 group) in the presence and absence of the
The PL color of the YNbO4:Eu3+ phosphors can be
luminescence center/acceptor (activators, Dy3+), respec- tuned to some extent from blue, through pink-red, to red
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by changing the doping concentration of Eu3+ in the Y(1–x)
-
phosphor, the Eu3+ ion was co-doped with the Dy3+ ion in
NbO4:xEu3+ phosphors.[37] Similarly to YNbO4:Dy3+, en- the YNbO4 matrix to achieve a high quality of white light.
ergy transfer occurs from the YNbO4 host lattice to the Figure 7 shows the typical excitation and emission spectra
Eu3+ ion in the YNbO4:Eu3+ phosphor. Under excitation of the YNbO4:0.02Dy3+,0.04Eu3+ phosphor annealed at
at 255 nm, the emission spectra of the YNbO4:xEu3+ (0.01 1000 °C. The excitation spectrum (Figure 7a, monitoring
Յ x Յ 0.10) phosphors show a broad emission band from the 5D0Ǟ4F2 transition of Eu3+ at 616 nm) of
YNbO4 and some narrow characteristic line emissions orig- YNbO4:0.02Dy3+,0.04Eu3+ shows a broad band from 200
inating from Eu3+ (Figure 6a). As a result of energy trans- to 290 nm with a maximum at 255 nm and a number of
fer, the emission intensities of YNbO4 decrease sharply, and weak line excitation peaks (300–470 nm) assigned to the
the emission intensities of Eu3+ clearly increase with in- fǞf transitions of Eu3+ within the 4f6 configuration. When
4
creasing concentration of Eu3+ in YNbO4:xEu3+ phos- monitoring the F9/2Ǟ6H13/2 transition of Dy3+ at 576 nm,
phors. Owing to concentration quenching of Eu3+ in the excitation spectrum obtained also shows a broad band
YNbO4:xEu3+ phosphors, the optimized doping concentra- peaking at 255 nm (Figure 7b) as well as some weak lines
tion of Eu3+ in YNbO4:xEu3+ phosphors was determined characteristic of the emission of Dy3+ at 353, 368, and
to be 0.07 of Y3+. The emission color of Y(1–x)NbO4:xEu3+ 389 nm. Monitoring the NbO4 group emission at 411 nm,
can be tuned from blue, through pink, and finally to red as the excitation spectrum obtained also shows a broad band
the content of Eu3+ increases from 0.01 to 0.10 (Figure 6b), peaking at 255 nm (Figure 7c). Comparing these excitation
and the corresponding CIE chromaticity coordinates spectra with that of pure YNbO4 (Figure 3d), it is clear
(shown in Figure 5) change from x = 0.1883 and y = 0.0792 that, except for their intensity, the broad excitation bands
(blue) to x = 0.5985 and y = 0.3116 (red) accordingly.
are almost identical. This means that, similarly to the
YNbO4:Dy3+ phosphor, energy transfer occurs from the
NbO4 group to Dy3+ and Eu3+. Under excitation at 255 nm,
the YNbO4:0.02Dy3+,0.04Eu3+ phosphor shows the blue
wide-band emission from 300 to 550 nm of the host lattice
YNbO4 with a maximum at 411 nm and the characteristic
emissions of Dy3+, that is, 4F9/2Ǟ6H15/2 (483 nm) and
4F9/2Ǟ6H13/2 (580 nm), as well as the characteristic emis-
sions of Eu3+, that is, 5D3Ǟ7F1 (420 nm), 5D2Ǟ7F3
(450 nm), 5D1Ǟ7F1 (540 nm), 5D1Ǟ7F2 (556 nm),
5D0Ǟ7F1 (596 nm), and 5D0Ǟ7F2 (616 nm). These emission
bands and lines cover the blue, green, yellow, and red region
with reasonable comparative intensity, resulting in a strong
white-light emission. The corresponding CIE chromaticity
coordinates of YNbO4:0.02Dy3+,0.04Eu3+ were determined
to be x = 0.3445 and y = 0.2592, which are in the white-
light zone in the CIE chromaticity diagram, as shown in
Figure 5. It is clear from Figure 7 that the Eu3+ and Dy3+
ions are isolated luminescent centers. As a result of the mis-
match of the energy levels of the Eu3+ and Dy3+ ions in the
YNbO4 host lattice, there is no energy exchange between
the Eu3+ and Dy3+ ions in the YNbO4:0.02Dy3+,0.04Eu3+
phosphor, although both are sensitized by the YNbO4 host
Figure 6. (a) Photoluminescence excitation and emission spectra of
the YNbO4:0.03Eu3+ sample annealed at 1000 °C and (b) emission
spectra of Y(1–x)NbO4:xEu3+ at different doping concentrations of
Eu3+
.
Because the luminescence quenching is caused by energy
transfer between the same rare-earth ions, as suggested by
Blasse and Bril, the critical distance (Rc) can be estimated
from Rc = 2(3V/4πNXc)1/3, in which V is the volume of the
unit cell, Xc is the critical concentration, and N is the
number of available crystallographic sites occupied by the
activator ions in the unit cell.[14,30] The values of V and N
for crystalline YNbO4 are 292.52 Å3 and 4, respectively,
and the optimized doping concentration is considered to be
the critical concentration Xc. On this basis, the values of Rc
for Dy3+ and Eu3+ in the YNbO4 nanocrystalline host were
determined to be 27.12 and 12.59 Å, respectively.
lattice in YNbO4:Dy3+,Eu3+
.
Photoluminescence Properties of Eu3+- and Dy3+-Co-Doped
YNbO4
Although the YNbO4:Dy3+ phosphor emits white light,
it has a poor CRI (Ͻ 80), because it lacks a red component.
Figure 7. Photoluminescence excitation and emission spectra of the
To improve the CRI of the white light of the YNbO4:Dy3+ YNbO4:0.02Dy3+,0.04Eu3+ sample annealed at 1000 °C.
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To achieve high-quality white light with excellent color reported that the valence and conduction bands (VB and
purity, a series of orthogonal experiments were carried out CB) of YNbO4 consist mainly of the O 2p orbital and the
to optimize the relative contents of Dy3+ and Eu3+ in Nb 4d orbital, respectively.[30,41,45] As suggested by Silver
Y
(1–x–y)NbO4:xDy3+,yEu3+. When the doping concentra- and co-workers,[38] the Y 4d orbital will also contribute to
tion of Dy3+ was fixed at x = 0.02, the emission color of the CB to some extent; therefore, Y3+ will have some impact
Y
(0.98–y)NbO4:0.02Dy3+,yEu3+ could be tuned from blue- on the position of the CB of the O2–ǞNb5+ charge-transfer
white to white by increasing the Eu3+ doping concentration band in YNbO4. The different 4f electronic configurations
(y) from 0.01 to 0.05. The corresponding emission spectra (4f9 for Dy3+ and 4f6 for Eu3+, there is no 4f orbital for
and chromaticity coordinates are shown in Figures 8a and Y3+) and ionic radii (0.0908 nm for Dy3+, 0.095 nm for
5c, respectively. Similarly, when the doping concentration Eu3+, and 0.088 nm for Y3+, CN = 8)[40] of the three ions
of Eu3+ was fixed at 0.035, the emission color of Y(0.965–x)
-
will perturb the crystal structure, crystal field, and CB posi-
NbO4:xDy3+,0.035Eu3+ could be changed from purple to tion of YNbO4 to some extent, which might lead to the
white by increasing the doping concentration of Dy3+ (x) blueshift of the absorption band of YNbO4:Eu3+
from 0.01 to 0.06. The corresponding emission spectra and YNbO4:Dy3+, and YNbO4:Eu3+,Dy3+ accordingly.[46]
chromaticity coordinates are shown in Figures 8b and 5d,
,
respectively.
Figure 9. UV/Vis diffuse reflectance spectra of undoped, singly
doped, and co-doped YNbO4 samples.
Figure 10 shows the luminescence decay curves of
YNbO4:0.02Eu3+,0.04Dy3+, obtained by monitoring the
O2–ǞNb5+ charge-transfer transition of the YNbO4 host
lattice at 410 nm, the 4F9/2Ǟ6H13/2 transition of Dy3+ at
580 nm, and the 5D0Ǟ7F2 transition of Eu3+ at 616 nm. As
shown in Figure 10a, the decay curve for the YNbO4 host
emission can be well fitted to the single-exponential func-
tion I = I0exp(–t/τ), in which τ is the decay time. The life-
time of the YNbO4 host emission was determined to be
3.13 μs, which is in good agreement with the values reported
previously.[30] The luminescence decay curve of Eu3+ (moni-
toring the 5D0Ǟ7F2 transition at 616 nm) in YNbO4
(Figure 10b) can also be fitted to a single-exponential func-
tion, and its decay lifetime was estimated to be 0.68 ms.
However, the decay curve for Dy3+ in YNbO4 can be fitted
Figure 8. Emission spectra of (a) YNbO4:0.02Dy3+,yEu3+ with dif-
ferent Eu3+ ion contents and (b) YNbO4:0.035Eu3+,xDy3+ with
different Dy3+ ion contents.
Figure 9 shows the UV/Vis diffuse reflectance spectra of to the double-exponential function I = A1exp(–t/τ1) +
YNbO4,
YNbO4:0.03Eu3+
,
YNbO4:0.04Dy3+
,
and A2exp(–t/τ2), in which τ1 and τ2 are the rapid and slow life-
YNbO4:0.02Eu3+,0.04Dy3+. YNbO4 shows wide-band ab- times of the exponential components, and A1 and A2 are
sorption from 230 to 600 nm and peaking at 281 nm the fitting parameters; the results of the fitting are shown
with a shoulder at around 350 nm. The absorption in Figure 10c. Two lifetimes, a short one, τ1 = 0.026 ms,
spectra of YNbO4:0.03Eu3+
,
YNbO4:0.04Dy3+
,
and and a long one, τ2 = 0.173 ms, were obtained for the
YNbO4:0.02Eu3+,0.04Dy3+ have a shape similar to that of 4F9/2Ǟ6H13/2 transition of Dy3+. The average lifetime for
2
YNbO4, but clearly show a blueshift of around 13 nm peak- Dy3+ can be determined from the formula τ = (A1τ1
+
2
ing at 268 nm. These absorption bands can be ascribed to A2τ2 )/(A1τ1 + A2τ2),[47] and the average lifetime for Dy3+
the O2–ǞNb5+ charge-transfer band transition. It has been in YNbO4 was determined to be 0.158 ms, which is of the
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same order of magnitude as reported previously.[16] By com- the YNbO4 host to Eu3+. Therefore, the decay lifetime of
paring the decay lifetimes of the YNbO4 host emission and Eu3+ in YNbO4 should fit a double-exponential decay be-
Eu3+ and Dy3+ in YNbO4, it is clear that the lifetimes of havior. The reason why the decay lifetime of Eu3+ in
Dy3+ and Eu3+ are much longer than that of the YNbO4 YNbO4 can be fitted by a single- rather than a double-ex-
host emission. These results show that the characteristic ponential function might be because the short lifetime
emission spectra of Dy3+ and Eu3+ can appear in the (from the YNbO4 host as sensitizer) is too short (in the
YNbO4 host independently.
order of microseconds, that is, 3.13 μs for the YNbO4 host)
5
to be detected when monitoring the D0Ǟ7F2 transition of
Eu3+ (in the order of milliseconds, i.e., 0.68 ms for Eu3+).
A similar situation has been found in the YVO4:Eu3+ and
LaPO4:Ce3+,Tb3+ phosphor systems, in which the lumines-
cence decay of Eu3+ and Tb3+ can be fitted by a single- and
not a double-exponential function, even though they are
sensitized by the YVO4 host lattice and the Ce3+ ion.[49,50]
Three primary nonradiative energy transfer mechanisms,
termed as exchange interaction, radiation reabsorption, and
electric multipolar interaction, by which the excited energy
is transferred from the donor (sensitizer) to the acceptor
(activator) are usually proposed.[51] The exchange interac-
tion mechanism only happens when Rc is smaller than 5 Å.
As discussed above, Rc for Dy3+ and Eu3+ in the YNbO4
host were estimated to be 27.12 and 12.59 Å, respectively,
which eliminates the possibility of the exchange interaction
in Dy3+ (Eu3+) doped YNbO4 phosphors. The radiation re-
absorption effect is only efficacious when the emission spec-
trum of the donor (sensitizer) and the excitation spectrum
of the acceptor (activator) widely overlap, which is also not
observed in the Dy3+ (Eu3+) doped YNbO4 phosphors.
Thus, electric multipolar interactions may be responsible for
the energy transfer from the YNbO4 host lattice to the acti-
vators.
According to Dexter’s energy transfer formula of
multipolar interactions and Reisfeld’s approximation, rela-
tion (1) can be obtained:[52,53]
(1)
in which C is the activator concentration of Dy3+ (Eu3+
)
and IS0 and IS are the luminescence intensities of the sensi-
tizer (host) in the absence and presence of the acceptor (ac-
tivators), respectively. The plots of IS0/IS versus Cn/3 with n
= 6, 8, or 10, which correspond to dipole–dipole, dipole–
quadrupole, or quadrupole–quadrupole interactions,
respectively, for YNbO4:Dy3+ and YNbO4:Eu3+ are shown
in Figure 11a and b, respectively. It can clearly be seen that
both can be fitted by straight lines for each Cn/3. The best
Figure 10. Luminescence decay curves for YNbO4:0.02Dy3+,0.04Eu3+
monitored at (a) 410, (b) 616, and (c) 580 nm, respectively.
Double-exponential decay behavior of an activator is fre- linear behavior was observed when n = 6 in Figure 11a and
quently observed when the excitation energy is transferred b, which indicates that the energy transfer from the YNbO4
from the matrix (sensitizer or donor) to the activator.[18,47] host to the Dy3+ and Eu3+ ions takes place by dipole–dipole
Similar situations have been observed in the Ga2O3:Dy3+
YVO4:Dy3+, and Y(P0.8V0.2)O4:Dy3+ systems, in which ef-
,
mechanisms.
A simple model illustrating the blue emission process in
ficient energy transfer occurs from the host (sensitizer) to YNbO4 and the energy transfer from NbO4 (O2–ǞNb5+
, .
Dy3+ as in YNbO4:Dy3+,Eu3+ [16,37,48] For Eu3+ in charge-transfer band) to Eu3+ and Dy3+ in YNbO4 is
YNbO4:Dy3+,Eu3+, it is clear from the luminescent (exci- shown in Figure 12. YNbO4 (NbO4 group) shows self-acti-
tation and emission, Figure 7) and UV/Vis diffuse reflec- vating luminescence upon 255 nm UV excitation, and the
tance spectra (Figure 10) that there is energy transfer from excited electrons jump from the VB (O2– ligand, donor) to
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Cathodoluminescence Properties
Typical CL spectra of the YNbO4:Dy3+ and
YNbO4:Dy3+,Eu3+ phosphors are shown in Figure 13. For
the purpose of comparison, representative CL spectra of
the YNbO4 and YNbO4:Eu3+ phosphors are also shown.
Under low-voltage electron beam excitation, YNbO4 shows
blue emission with high color purity (shown in Figure 13a,
blue line). Its emission spectrum shows a wide band from
370 to 600 nm with a peak at 420 nm. Compared with its
PL spectrum, the whole CL spectrum shows a redshift of
around 10 nm. The YNbO4:Dy3+ (Figure 13a, black line),
YNbO4:Eu3+ (Figure 13a, red line), and YNbO4:Dy3+,Eu3+
phosphors (Figure 13b), similarly to their PL spectra, show
not only blue wide-band host emission, but also the charac-
teristic line emissions of Dy3+ and Eu3+ ions with compar-
able intensity of blue-white, red, and white light, respec-
tively, under the excitation of a low-voltage electron beam.
The CL properties of YNbO4:Dy3+, YNbO4:Eu3+, and
YNbO4:Dy3+,Eu3+ are in good agreement with their PL
properties. For example, the CL color of YNbO4:Dy3+ can
be tuned from blue, through blue-white to white. The CRI
of YNbO4:Dy3+ can be further improved by co-doping the
Eu3+ in YNbO4:Dy3+ and tuning the relative ratio of Dy3+
/
Eu3+ in YNbO4:Dy3+,Eu3+. As a result of their excellent
low-voltage CL properties and high stability, the
YNbO4:Dy3+ and YNbO4:Dy3+,Eu3+ phosphors have po-
tential applications in display devices such as FEDs and
VFDs.
Figure 11. Dependence of IS0/IS of host emission on C6/3, C8/3, and
C10/3 in (a) YNbO4:Dy3+ and (b) YNbO4:Eu3+
.
the CB (Nb5+, acceptor). The possible excited states of
1
1
3
3
Nb5+ are T1, T2, T1, and T2, and the ground state is
1A1.[38,41,45] After excitation, transitions from the 3T1 ex-
cited level to the ground state A1 occur and give its blue
1
emission at around 411 nm.[30,32,41] When Dy3+ (and/or
Eu3+) is present in the YNbO4 host lattice, upon 255 nm
UV radiation, the excited energy is transferred from the
NbO4 group (O2–ǞNb5+ charge-transfer band) to the ex-
cited states of the Dy3+ (and/or Eu3+) ions, resulting in their
characteristic emission, as shown in Figure 12.[37]
Figure 12. Simple model illustrating the blue emission from the
NbO4 group, the energy-transfer process from the NbO4 group to
Eu3+ and Dy3+, and the characteristic emission of Eu3+ and Dy3+
in the YNbO4 host.
Figure 13. Representative CL spectra of (a) YNbO4, YNbO4:0.02Dy3+
,
and YNbO4:0.04Eu3+ and (b) YNbO4:0.02Dy3+,0.04Eu3+ under the
excitation of a low-voltage electron beam.
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source (Continuum Sunlite optical parametric oscillator). All the
measurements were performed at room temperature.
Conclusions
YNbO4:Dy3+
, ,
YNbO4:Eu3+ and YNbO4:Dy3+,Eu3+
phosphors have been prepared by a Pechini-type sol–gel
process. Under excitation with UV light and a low-voltage
Acknowledgments
electron beam, the YNbO4:Dy3+
,
YNbO4:Eu3+ and
The authors would like to thank Prof. Jun Lin (Changchun Insti-
tute of Applied Chemistry, Chinese Academy of Sciences) for his
fruitful comments. This project was financially supported by the
National Natural Science Foundation of China (NSFC) (21161015,
21165013), the Natural Science Foundation of the Jiangxi Province
of China (20152ACB20011, 2009GZH0082), the Natural Science
Foundation of the Jiangxi Higher Education Institutions of China
(GJJ09180, GJJ14513), the open fund of the Key Laboratory of
Jiangxi Province for Persistant Pollutant Control and Resources
Recycle, and the Nanchang Hangkong University Doctoral Foun-
dation.
YNbO4:Dy3+,Eu3+ phosphors exhibit not only the charac-
teristic emission of the YNbO4 host lattice, but also the
characteristic emissions of Dy3+ and Eu3+ with appropriate
intensities. Their emission color can be tuned from blue,
through blue-white to white by changing the doping con-
centration and relative ratio of Dy3+/Eu3+ in the YNbO4
host lattice. A single-composition white-light emission with
high CRI and color purity in response to UV and low-volt-
age electron beam excitation has been achieved with the
YNbO4:Dy3+,Eu3+ phosphor. As a result of its good mor-
phology, excellent PL and CL properties, CRI, and high
color purity, the YNbO4:Dy3+ and YNbO4:Eu3+,Dy3+
phosphors obtained have potential applications in fluores-
cent lamps, UV w-LEDs, VFDs, and FEDs.
[1] C. Feldmann, T. Jüstel, C. R. Ronda, P. J. Schmidt, Adv. Funct.
Mater. 2003, 13, 511.
[2] H. A. Höppe, Angew. Chem. Int. Ed. 2009, 48, 3572; Angew.
Chem. 2009, 121, 3626.
[3] C. C. Lin, R.-S. Liu, J. Phys. Chem. Lett. 2011, 2, 1268.
[4] N. C. George, K. A. Denault, R. Seshadri, Annu. Rev. Mater.
Res. 2013, 43, 481.
[5] P. Pust, V. Weiler, C. Hecht, A. Tücks, A. S. Wochnik, A.-K.
Henß, D. Wiechert, C. Scheu, P. J. Schmidt, W. Schnick, Nat.
Mater. 2014, 13, 891.
[6] X. Liu, J. Lin, J. Mater. Chem. 2008, 18, 221.
[7] A. U. Pawar, A. P. Jadhav, C. W. Kim, H. G. Cha, U. Pal, Y. S.
Kang, J. Lumin. 2015, 157, 131.
[8] A. P. Jadhav, A. U. Pawar, U. Pal, B. K. Kim, Y. S. Kang, Sci.
Adv. Mater. 2012, 4, 597.
[9] A. U. Pawar, A. P. Jadhav, U. Pal, B. K. Kim, Y. S. Kang, J.
Lumin. 2012, 132, 659.
[10] G. Li, J. Lin, Chem. Soc. Rev. 2014, 43, 7099.
[11] L. Sudheendra, G. K. Das, C. Li, D. Stark, J. Cena, S. Cherry,
I. M. Kennedy, Chem. Mater. 2014, 26, 1881.
[12] L. Zhang, J. Zhang, X. Zhang, Z. Hao, H. Zhao, Y. Luo, ACS
Appl. Mater. Interfaces 2013, 5, 12839.
[13] J.-C. Zhang, Y.-Z. Long, H.-D. Zhang, B. Sun, W.-P. Han,
X.-Y. Sun, J. Mater. Chem. C 2014, 2, 312.
[14] W.-R. Liu, C.-H. Huang, C.-W. Yeh, J.-C. Tsai, Y.-C. Chiu, Y.-
T. Yeh, R.-S. Liu, Inorg. Chem. 2012, 51, 9636.
[15] M. Shang, C. Li, J. Lin, Chem. Soc. Rev. 2014, 43, 1372.
[16] X. Liu, C. Yu, C. Li, J. Lin, J. Electrochem. Soc. 2007, 154,
P86.
Experimental Section
The YNbO4:Dy3+ and YNbO4:Dy3+,Eu3+ samples were all pre-
pared by a Pechini-type sol–gel process.[36,37] The doping concen-
trations of Dy3+ and Eu3+ were 0.1–10 at-% of Y3+ in YNbO4.
Stoichiometric amounts of Y2O3, Dy2O3, and Eu2O3 (99.99%,
Shanghai Yuelong Non-Ferrous Metals Limited, China) were dis-
solved in dilute HNO3 [analytical reagent (A.R.), Beijing Fine
Chemical Company, China] with stirring and heating to give solu-
tions of Y(NO3)3, Dy(NO3)3, and Eu(NO3)3. NbCl5 was dissolved
in absolute ethanol to give a solution of niobium alcoholate.
Stoichiometric amounts of solutions of Y(NO3)3, Dy(NO3)3,
Eu(NO3)3, and niobium alcoholate were mixed with stirring. The
final pH of the resulting solutions was tuned to 1 by addition of
dilute HNO3. Then citric acid and polyethylene glycol (PEG, mo-
lecular weight 10000) were added to the above solution (CPEG
=
0.01 m, citric acid/metal ion = 2:1). The resultant mixtures were
stirred for 2 h and condensed at 75 °C in a water bath to give dry
gels. After drying in an oven at 110 °C for 10 h, the gels were
ground and pre-fired at 450 °C in air for 4 h. Then the samples
were fully ground and sintered at the desired temperatures for 6 h
to produce the final samples. The crystalline phases of the prepared
YNbO4:Dy3+, YNbO4:Eu3+, and YNbO4:Dy3+,Eu3+ samples were
investigated by X-ray diffraction (XRD, Bruker D8 ADVANCE
[17] F. Wang, X. Xue, X. Liu, Angew. Chem. Int. Ed. 2008, 47, 906;
Angew. Chem. 2008, 120, 920.
[18] Y. Lü, X. Tang, L. Yan, K. Li, X. Liu, M. Shang, C. Li, J. Lin,
J. Phys. Chem. C 2013, 117, 21972.
[19] W. Xia, X. Wang, Z. Fu, S. Zhou, S. Zhang, J. H. Jeong, Mater.
Res. Bull. 2012, 47, 2535.
[20] J. Li, J.-G. Li, S. Liu, X. Li, X. Sun, Y. Sakka, J. Mater. Chem.
C 2013, 1, 7614.
diffractometer) using graphite-monochromatized Cu-Kα (λ
0.15406 nm) radiation. The morphologies of the samples were ex-
amined by field-emission scanning electronic microscopy
=
a
(FESEM, JSF-6700) and high resolution transmission electron mi-
croscopy (HRTEM, JEOL 2010). The PL excitation and emission
spectra were recorded by using a Hitachi F-7000 spectrophotome-
ter equipped with a 150 W xenon light source under a working
voltage of 40 V. The excitation and emission slits were both set at
5.0 nm, and the scan speed was 1200 nm/min. The CL measure-
ments were conducted in an ultrahigh-vacuum chamber
(Ͻ 10–8 Torr), in which the phosphors were excited by an electron
beam in the voltage range 1.0–3.0 kV at different filament currents;
the emission spectra were recorded by using an F-7000 spectropho-
tometer. Time-resolved emission decay behavior was investigated
with a Lecroy Wave Runner 6100 Digital Oscilloscope (1GHZ)
using a tunable laser (pulse width 4 ns, gate 50 ns) as the excitation
[21] Z·Ci, Q. Sun, S. Qin, M. Sun, X. Jiang, X. Zhang, Y. Wang,
Phys. Chem. Chem. Phys. 2014, 16, 11597.
[22] A. P. Jadhav, A. U. Pawar, U. Pal, Y. S. Kang, J. Mater. Chem.
C 2014, 2, 496.
[23] H. Guan, G. Liu, J. Wang, X. Dong, W. Yu, New J. Chem.
2014, 38, 4901.
[24] V. R. Bandi, B. K. Grandhe, H.-J. Woo, K. Jang, D.-S. Shin,
S.-S. Yi, J.-H. Jeong, J. Alloys Compd. 2012, 538, 85.
[25] E. Pavitra, G. Seeta Rama Raju, J. S. Yu, Ceram. Int. 2013, 39,
6319.
[26] S. Asiri Naidu, S. Boudin, U. V. Varadaraju, B. Raveau, J. Solid
State Chem. 2011, 184, 2566.
[27] J. Wan, L. Cheng, J. Sun, H. Zhong, X. Li, W. Lu, Y. Tian, H.
Lin, B. Chen, J. Alloys Compd. 2010, 496, 331.
Eur. J. Inorg. Chem. 2015, 5262–5271
5270
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
[28] Y. Deng, S. Yi, J. Huang, J. Xian, W. Zhao, Mater. Res. Bull.
2014, 57, 85.
[29] S. Som, P. Mitra, V. Kumar, V. Kumar, J. J. Terblans, H. C.
Swart, S. K. Sharma, Dalton Trans. 2014, 43, 9860.
[30] G. Blasse, A. Bril, J. Lumin. 1970, 3, 109.
[42] X. Liu, J. Lin, J. Appl. Phys. 2006, 100, 124306.
[43] Y. Chen, Y. Li, J. Wang, M. Wu, C. Wang, J. Phys. Chem. C
2014, 118, 12494.
[44] W. Lv, M. Jiao, Q. Zhao, B. Shao, W. Lu, H. You, Inorg. Chem.
2014, 53, 11007.
[31] S. H. Shin, D. Y. Jeon, K. S. Suh, J. Appl. Phys. 2001, 90, 5986. [45] M. Wiegel, W. Middel, G. Blasse, J. Mater. Chem. 1995, 5, 981.
[32] Y. Zhou, Q. Ma, M. Lü, Z. Qiu, A. Zhang, J. Phys. Chem. C [46] Q. Liu, H. Li, B. Li, W. Wang, Q. Liu, Y. Zhang, J. Dai, Euro-
2008, 112, 19901.
phys. Lett. 2014, 108, 37003.
[47] Z. Hou, R. Chai, M. Zhang, C. Zhang, P. Chong, Z. Xu, G.
Li, J. Lin, Langmuir 2009, 25, 12340.
[48] M. Yu, J. Lin, J. Fang, Chem. Mater. 2005, 17, 1783.
[49] N. S. Singh, N. K. Sahu, D. Bahadur, J. Mater. Chem. C 2014,
2, 548.
[50] R. Chai, H. Lian, P. Yang, Y. Fan, Z. Hou, X. Kang, J. Lin, J.
Colloid Interface Sci. 2009, 336, 46.
[51] K. Li, X. Liu, Y. Zhang, X. Li, H. Lian, J. Lin, Inorg. Chem.
2015, 54, 323.
[52] D. L. Dexter, J. Chem. Phys. 1953, 21, 836.
[53] C. H. Huang, T. M. Chen, W. R. Liu, Y. C. Chiu, Y. T. Yeh,
S. M. Jang, ACS Appl. Mater. Interfaces 2010, 2, 259.
Received: June 24, 2015
[33] Lj. R. Dacanin, M. D. Dramicanin, S. R. Lukic-Petrovic,
D. M. Petrovic, M. G. Nikolic, T. B. Ivetic, I. O. Guth, Ceram.
Int. 2014, 40, 8281.
[34] E. Y. Lee, Y. J. Kim, Thin Solid Films 2010, 518, e72.
[35] X. Z. Xiao, B. Yan, J. Alloys Compd. 2006, 421, 252.
[36] X. Liu, J. Lin, Appl. Phys. Lett. 2007, 90, 184108.
[37] X. Liu, Y. Lü, C. Chen, S. Luo, Y. Zeng, X. Zhang, M. Shang,
C. Li, J. Lin, J. Phys. Chem. C 2014, 118, 27516.
[38] X. Jing, C. Gibbons, D. Nicholas, J. Silver, A. Vecht, C. S.
Frampton, J. Mater. Chem. 1999, 9, 2913.
[39] X. Liu, J. Lin, J. Lumin. 2007, 122–123, 700.
[40] R. Shannon, Acta Crystallogr., Sect. A 1976, 32, 751.
[41] S. K. Lee, H. Chang, C.-H. Han, H.-J. Kim, H. G. Jang, H. D.
Park, J. Solid State Chem. 2001, 156, 267.
Published Online: October 16, 2015
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