Soft-chemical synthesis and tunable luminescence of Tb3+, Tm3+/Dy3+-doped SrY2O4 phosphors for field emission displays

Article abstract of DOI:10.1039/c2dt32592f

Tb³⁺, Tm³⁺, and Dy³⁺-activated SrY ₂O₄ phosphors have been prepared via Pechini-type sol-gel method. X-Ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL) and lifetimes, as well as cathodoluminescence (CL) spectra were used to characterize the samples. Under low-voltage electron beam excitation, the Tb ³⁺-doped samples show a green luminescence, with a better CIE coordinates and higher emission intensity than the commercial product ZnO: Zn. Blue and yellow emissions could be obtained by doping with Tm³⁺ and Dy³⁺, respectively. A color-tunable emission in SrY₂O ₄ phosphors can be realized by co-doping with Tm³⁺ and Dy³⁺. White cathodoluminescence (CL) has been realized in a single-phase SrY₂O₄ host by co-doping with Tm³⁺ and Dy³⁺ for the first time with CIE (0.315, 0.333). Furthermore, the cathodoluminescence (CL) properties of SrY₂O₄: Tb ³⁺/Tm³⁺/Dy³⁺ phosphors including the dependence of CL intensity on accelerating voltage and filament current, the decay behaviour of CL intensity under electron bombardment, and the stability of CIE chromaticity coordinate have been investigated in detail. The as-prepared phosphors might be promising for use in field-emission display (FED) devices.

Full text of DOI:10.1039/c2dt32592f

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Transactions
PAPER
Soft-chemical synthesis and tunable luminescence of
Tb3+, Tm3+/Dy3+-doped SrY2O4 phosphors for field
emission displays†
Cite this: Dalton Trans., 2013, 42, 4799
Yang Zhang,a,b Dongling Geng,a,b Mengmeng Shang,a,b Xiao Zhang,a,b Xuejiao Li,a,b
Ziyong Cheng,a Hongzhou Liana and Jun Lin*a
Tb3+, Tm3+, and Dy3+-activated SrY2O4 phosphors have been prepared via Pechini-type sol–gel method.
X-Ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron
microscopy (TEM), photoluminescence (PL) and lifetimes, as well as cathodoluminescence (CL) spectra
were used to characterize the samples. Under low-voltage electron beam excitation, the Tb3+-doped
samples show a green luminescence, with a better CIE coordinates and higher emission intensity than
the commercial product ZnO: Zn. Blue and yellow emissions could be obtained by doping with Tm3+ and
Dy3+, respectively. A color-tunable emission in SrY2O4 phosphors can be realized by co-doping with Tm3+
and Dy3+. White cathodoluminescence (CL) has been realized in a single-phase SrY2O4 host by co-doping
with Tm3+ and Dy3+ for the first time with CIE (0.315, 0.333). Furthermore, the cathodoluminescence (CL)
properties of SrY2O4: Tb3+/Tm3+/Dy3+ phosphors including the dependence of CL intensity on accelerat-
ing voltage and filament current, the decay behaviour of CL intensity under electron bombardment, and
the stability of CIE chromaticity coordinate have been investigated in detail. The as-prepared phosphors
might be promising for use in field-emission display (FED) devices.
Received 30th October 2012,
Accepted 6th December 2012
DOI: 10.1039/c2dt32592f
www.rsc.org/dalton
current saturation, good chemical durability.7–9 While many
commercial sulfide-based phosphors such as Y2O2S: Eu3+,
1. Introduction
Nowadays, field-emission displays (FEDs) are a developing Gd2O2S: Tb3+, SrGa2S4: Ce3+, and ZnS: Ag, Cl, etc., have been
technology which has the potential to provide displays with a explored as possible low-voltage phosphors, the stability of
high brightness, high contrast ratio, wide viewing, self-emis- these compounds has prohibited their wide use in FEDs.9–11
sion, quick response time, high brightness, light weight, and On the one hand, owing to the poor stability, sulfide-based
low-power consumption.1–3 Thus, it has been considered as phosphors often degrade under electron bombardment, which
the next generation flat panel display. Especially, phosphors contaminates emission tips and shortens device lifetime.11–15
have been playing an important role for developing advanced On the other hand, sulfide-based phosphors are not friendly
lighting and display technology.4–6 For FEDs, phosphors must to the environment.16 Therefore, to improve the performance
be operated at significantly lower excitation voltages (≤5 kV) of FED devices, it is necessary to find novel phosphors with
and higher current densities (10–100 μA cm−2) than CRTs, so high luminescence efficiency, stability, and reasonable con-
it is urgent for us to develop some highly efficient phosphors ductivity.1 Oxide-based phosphors are more stable and
with a higher efficiency at low voltages, higher resistance to environmentally friendly in comparison with sulfides.7 So,
oxide-based phosphors for FEDs have been of great interest
due to their excellent light output, color rendering properties
aState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
and superior stability under electron bombardment.17
Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China.
Due to the abundant emission colors based on their 4f →
E-mail: jlin@ciac.jl.cn; Fax: +86-431-85698041; Tel: +86-431-85262031
bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China
4f or 5d → 4f transitions, rare-earth (RE) ions have been
playing an irreplaceable role in modern lighting and display
fields.18–20 The use of rare-earth element-based phosphors,
based on “line-type” f–f transitions, can narrow the emissions
to the visible range, resulting in high efficiency and a high
lumen equivalent.16 Generally, the Tb3+ ions are used as an
†Electronic supplementary information (ESI) available: Typical photolumine-
scence emission spectra of SrY2O4: xTb3+ with different Tb3+ ion concentrations,
the photoluminescence (PL) intensity of Tm3+ and Dy3+ ions as a function of
concentration (y/z) in SrY2O4: Ln3+, respectively, the comparison of full width at
half maxima (FWHM) of cathodoluminescence (CL) spectra of Y2SiO5: Ce3+
(black line) and SrY2O4: Tm3+ (blue line). See DOI: 10.1039/c2dt32592f
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 4799–4808 | 4799

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Dalton Transactions
activator in green phosphors, whose emission is mainly due
2. Experimental
2.1 Materials
to transitions of 5D3
→ →
7FJ in the blue region and 5D4 7FJ
in the green region (J = 6, 5, 4, 3, 2) depending on its doping
concentration. As the Tb3+ concentration increases, the cross- The initial rare-earth oxides, including Y2O3 (99.999%), Tb4O7
relaxation of 5D3
→
5D4 occurs owing to the interaction (99.999%), Tm2O3 (99.999%) and Dy2O3 (99.999%) were
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between Tb3+ ions, which enhances the transitions of 5D4
→
purchased from Science and TechnoVloiegwyJoPuranrael n| VtieCwoIsmsupe any of
7FJ with a dominant green emission.21 Tm3+-doped phosphors Changchun Institute of Applied Chemistry, all other chemicals
have attracted substantial attention in recent years because were purchased from Beijing Chemical Company. All chemi-
the major emission peak of the Tm3+ ion was at about 455 nm cals were analytical grade reagents and were used directly
(1D2
→
3F4), which could provide blue luminescence without further purification.
with appropriate lifetimes, color-rendering properties. Thus
phosphors doped with Tm3+ have been found potential appli-
cations in cathode ray tube screens, field-emission displays,
2.2 Preparation
plasma display panels, and electroluminescent devices such as SrY2O4: Tb3+/Tm3+/Dy3+ samples were prepared by Pechini-type
LaOCl: Tm3+, Y3Al5O12: Tm3+, SrHfO3: Tm3+, Y3GaO6: Tm3+, sol–gel method.29 The doping concentrations of Tb3+, Tm3+
LaGaO3: Tm3+, and LaAlGe2O7: Tm3+, etc.9,16,17 The lumine- and Dy3+ are 0.1–4%, 0.1–2% and 0.1–2% of Y3+ in SrY2O4,
scence of Dy3+ mainly consists of narrow lines in the blue respectively. The Y2O3, Tb4O7, Tm2O3 and Dy2O3 with purities
4
6
(460–500 nm, F9/2 → H15/2) and yellow-orange (550–600 nm, higher than 99.99% were dissolved in dilute HNO3 under stir-
4F9/2
→
6H13/2
)
regions. At
a
suitable yellow-to-blue ring and heating, resulting in the formation of a colorless solu-
intensity ratio, Dy3+ will emit white light.22 In summary, it tion of Ln(NO3)3 (Ln = Y, Tb, Tm or Dy). Stoichiometric
is very meaningful for us to explore some RE3+-doped oxide- amounts of Ln(NO3)3, Sr(NO3)2, were mixed in deionized water
based phosphors for FEDs.
under stirring. The citric acid was dissolved in the above solu-
Binary rare earth oxides AR2O4 (for A = Ca, Sr, Ba and R = tion (citric acid–metal ion = 2 : 1 in moles). After agitation for
rare earth) within the CaFe2O4 structure have many potential 15 min, polyethylene glycol (PEG, molecular weight = 20 000)
applications due to their special properties as a result of their was added into the above solution (CPEG = 0.01 mol L−1). The
luminescent and magnetic properties.23–25 In 2001, Xu et al. resultant mixtures were stirred for 3 h and heated at 80 °C in a
reported that SrY2O4: Eu3+ is one of the promising red phos- water bath until homogeneous gels formed. After being dried
phors for FED applications, which was prepared by conven- in an oven at 100 °C for 10 h, the gels were ground and pre-
tional solid-state reaction (SSR). Later, this phosphor has fired at 500 °C for 4 h in air. Then the samples were fully
aroused much attention, especially dependence of the Eu3+ ground and calcined at the desired temperature (1300 °C) for
luminescence on the crystal structure.26–28 Compared with the 3 h to produce the final samples. The Tb-doped powders were
conventional SSR method, soft chemical methods have the then reduced at 500 °C for 2 h under a 10% H2/90% N2 atmos-
advantage of short calcination time, small particle size and phere to produce the final samples.
narrow particle size distribution. The Pechini sol–gel (PSG)
method is a useful and attractive technique for the preparation
of oxide-based materials, especially complex metal oxides.29
2.3 Characterization
This method includes first dissolving corresponding hydrous X-ray diffraction (XRD) was performed on a D8 Focus diffracto-
oxides or alkoxides of elements with a chelating agent such meter at a scanning rate of 10° min−1 in the 2θ range from 10
as citric acid (CA) to form stable metal complexes, and their to 80° with graphite-monochromated Cu Kα radiation (λ =
polyesterification with a polyhydroxy alcohol such as ethylene 0.15405 nm). The morphology of the samples was inspected
glycol (EG) or poly(ethylene glycol) (PEG) forms a polymeric using a field-emission scanning electron microscope (FE-SEM,
resin. Immobilization of the metal complexes in such rigid S-4800, Hitachi). Transmission electron microscopy (TEM)
organic polymer networks reduces segregation of particular images were performed using a FEI Tecnai G2 S-Twin with a
metal ions, ensuring compositional homogeneity. Subsequent field-emission gun operating at 200 kV. The photolumine-
thermal reactions are performed at relatively low temperature scence (PL) measurements were recorded with a Hitachi F-7000
and
a
highly homogeneous powder can be obtained.30 spectrophotometer equipped with a 150 W xenon lamp as the
However, there are few reports about the SrY2O4: Ln3+ excitation source. The cathodoluminescence (CL) measure-
synthesized by PSG process, and the corresponding photo- ments were conducted in an ultra-high-vacuum chamber
luminescence (PL) and cathodoluminescence (CL) proper- (<10−8 Torr), where the phosphors were excited by an electron
ties.24,26,27,31,32 Thus, in the present work, we reported the beam at a voltage range of 2–6 kV with different filament cur-
synthesis of Tb3+, Tm3+/Dy3+ doped SrY2O4 phosphors using a rents 80–96 mA, and the spectra were recorded using an
Pechini-type sol–gel process (soft chemical synthesis), and a F-7000 spectrophotometer. The luminescence decay curves
5
5
systematic study on the cross-relaxation of D3 → D4 of Tb3+ were obtained from a Lecroy Wave Runner 6100 Digital Oscillo-
and the tunable PL and CL of Tm3+/Dy3+ doped phosphors scope (1 GHz) using a tunable laser (pulse width = 4 ns, gate =
has been carried out in detail. It is found that they are very 50 ns) as the excitation (Contimuum Sunlite OPO). All the
promising for applications in FEDs.
measurements were performed at room temperature (RT).
4800 | Dalton Trans., 2013, 42, 4799–4808
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Paper
Table 1 Structure parameters of SrY2O4 and ionic radii (Å) for given coordi-
nation number (CN) of Y3+, Tb3+, Tm3+ and Dy3+ ions
Ionic
Coordination
number
radius (IR)
Symmetry (Å)
Ion
Y3+
Space group
Orthorhombic
6
Cs
0.900
0.923
0.880
0.912
Tb3+ Pnam (62)
Tm3+
Dy3+
Fig. 2 (a) SEM image, (b) TEM and HRTEM image, (c) FFT pattern of the
SrY2O4: Tb3+ sample.
Fig. 1 (a) The XRD patterns of Tb3+/Tm3+/Dy3+ doped SrY2O4 samples
annealed at 1300 °C for 3 h and the standard data of SrY2O4 (JCPDS no. 32-
1272). (b) Crystal structure of SrY2O4 along the c-axis direction.
the basis of the similar effective ionic radius and valence of
cations (as shown in Table 1), we suggest that Tb3+/Tm3+/Dy3+
ions prefer to occupy Y3+ sites.33
The SEM and TEM images of the SrY2O4: Tb3+ sample are
shown in Fig. 2. As shown in Fig. 2a, one can find that the
sample is composed of particles with sizes ranging from 100
to 500 nm, which is consistent with the result of the TEM
image (Fig. 2b). The aggregated crystallites are attributed to
the high temperature annealing process. The results for other
samples are similar to those of the SrY2O4: Tb3+ sample and
will not be shown here. The fine structure of SrY2O4: Tb3+
was also studied by the HRTEM technique (inset of Fig. 2b), in
which the lattice fringes with a d spacing of 0.227 nm corre-
spond to the distance of the (231) plane of SrY2O4, which is
consistent with the corresponding fast Fourier transformation
(FFT) (Fig. 2c). The result further confirms the presence of
a highly crystalline SrY2O4 phase after being annealed at
1300 °C, agreeing well with the XRD results.
3. Results and discussion
3.1 Crystallization behavior and morphology
The composition and phase purity of the as-prepared powder
samples were first examined by XRD. The XRD patterns of
SrY2O4: Tb3+/Tm3+/Dy3+ together with the Joint Committee on
Powder Diffraction Standards (JCPDS) card no. 32-1272 are
shown in Fig. 1a. All the diffraction peaks of these samples
can be exactly assigned to pure orthorhombic phase of SrY2O4
[space group: Pnam (62)], indicating that the obtained samples
are single phase and the doped Tb3+, Tm3+ and Dy3+ ions do
not cause any obvious change in the host structure. The
SrY2O4 compound belongs to CaFe2O4 type structure, which
has cell parameters of a = 10.07 Å, b = 11.91 Å, c = 3.41 Å, V =
409.37 Å3, and Z = 4. The crystal structure of the SrY2O4 is
shown in Fig. 1b in a view along the crystallographic c axis. Y3+
ions have two non-equivalent sites, which both are coordi-
nated by six oxygen atoms and both have Cs symmetry. The Y1
site is nearly octahedral while the Y2 site is in a more distorted
environment. The Sr2+ ions fill the honeycomb cavities in the
network of edge and corner shared YO6 octahedra, which
are coordinated by eight atoms and have Cs symmetry.23 On
3.2 Photoluminescence properties of SrY2O4: Tb3+/Tm3+/Dy3+
SrY2O4: Tb3+. Owing to the high degeneracy of the Tb3+
levels involved in several transitions, it is difficult to derive
information from the Tb3+ spectra on the nature of the Tb3+
centre.30 Generally, the Tb3+ ions are used as an activator in
green phosphors, whose emission is mainly due to transitions
of 5D3
→ →
7FJ in the blue region and 5D4 7FJ in the green
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demonstrates that the spectral-energy distribution of the Tb3+
emission depends strongly on the Tb3+ concentration. For the
Tb3+ ion, the 5D3
→
5D4 energy gap (5915 cm−1) is close to
7
7
the F6 → F0 energy gap (6000 cm−1).21 As revealed in Fig. 3b,
the emission intensity of 5D3 decreases with increasing Tb3+
concentration, accompanied by the enhancement of the emis-
5
sion from the D4 level. This results from cross-relaxation via
the resonant energy transfer process: Tb3+ (5D3) + Tb3+ (7F6) →
Tb3+ (5D4) + Tb3+ (7F0).30 As the concentration of Tb3+ is
increased, the cross-relaxation effect becomes stronger, which
enhances the intensity of green emission at 545 nm.
The significance of the energy transfer (cross-relaxation)
from 5D3 to 5D4 depends on the Tb–Tb distance. Exchange
interactions and multipolar interactions are main two aspects
responsible for the resonant energy-transfer mechanism.34 It
is known that if energy transfer results from the exchange
interaction, the critical distance between the sensitizer and
activator should be shorter than 4 Å.35 The critical distance RC
between Tb3+ ions can be estimated using the concentration
quenching method according to the relation given by Blasse:36
ꢀ
ꢁ
1=3
3V
4πXCN
RC ꢀ 2
ð1Þ
where N is the number of available sites for the dopant in the
unit cell, XC is the critical concentration of Tb3+, and V is the
volume of the unit cell. In this work, N = 8 and V = 409.37 Å3
and XC = 0.01. Therefore, the critical distance (RC) of energy
transfer was calculated to be about 21.38 Å. This value is much
longer than 4 Å, indicating little possibility of cross-relaxation
via the exchange interaction mechanism. Thus, the cross-relax-
ation between the Tb3+ ions mainly takes place via electric
multipolar interactions. On the basis of Dexter’s energy-trans-
fer expressions of multipolar interaction and Reisfeld’s
approximation, the following relation can be given:37
Fig. 3 (a) Excitation and emission spectra of SrY2O4: 0.005Tb3+ sample. (b) The
variation of 5D3 and 5D4 energy of Tb3+ in the SrY2O4: xTb3+ system with chan-
ging Tb3+ doping concentration.
region (J = 6, 5, 4, 3, 2) depending on its doping concen-
tration.21 Fig. 3a presents the PLE and PL spectra for SrY2O4:
0.005Tb3+ at room temperature. The excitation spectrum
shows a strong broad band (250–350 nm, λmax = 304 nm) with
a shoulder at about 214 nm, monitoring the wavelength at
545 nm. According to ref. 28, the band gap of SrY2O4 is about
6.1 eV (204 nm), thus the weak shoulder between 200 and
220 nm should be assigned to the host absorption due to
electronic transition from the valence to conduction band. On
the other hand, a larger crystal-field splitting is anticipated for
the double octahedral type coordination of Y2O42− and a lower
coordination number of six for Y3+, which results in the red
shift of the f–d transition of Tb3+. Consequently, the strong
excitation band (250–350 nm, λmax = 304 nm) should be classi-
fied to the f → d transition of Tb3+.27 Under 304 nm UV exci-
η0
/ Cn=3
ð2Þ
η
where η0 and η are the luminescence quantum efficiencies of
Tb3+ (5D3) at different concentrations; C is the content of Tb3+;
n = 6, 8, and 10 corresponding to dipole–dipole, dipole–
quadrupole, and quadrupole–quadrupole interactions, respect-
ively. The value of η0/η can be approximately calculated by the
ratio of related luminescence intensities as:
I0
Ix
/ Cn=3
ð3Þ
where I0 is the luminescence intensity of Tb3+ (5D3) at lowest
doping concentration and Ix is the emission intensity of 5D3
for a given doping concentration (x). The I0/Ix − Cn/3 plots
are shown in Fig. 4, and the relationships are observed when
n = 6, 8 and 10. A linear relationship was only observed
when n = 6. This clearly indicates that the cross-relaxation
tation, the emission spectra of the as-prepared SrY2O4:
5
0.005Tb3+ consist of the D3,4
→
7FJ transitions of Tb3+, as
shown in Fig. 3a. In order to investigate the concentration
dependent luminescence properties of Tb3+ ions in SrY2O4, a
series of SrY2O4: xTb3+ have been prepared and their PL
emission spectra are given in Fig. S1.† It is obvious that the
emission spectra show different ratios between the 5D3 and
the 5D4 emissions with changing Tb3+ concentrations, which
5
5
mechanism of Tb3+ ions via energy transfer from D3 to D4 is
a dipole–dipole interaction type, which is consistent with the
previous report.21
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Fig. 4 Dependence of I0/Ix of 5D3 (Tb3+) on (a) CTb6/3; (b) CTb8/3; (c) CTb
.
10/3
Fig. 5 The relation of ηCR versus Tb3+ ion concentration.
After the 5D3 level of a Tb3+ ion is populated, the cross-relax-
ation energy transfer between the excited Tb3+ and another
SrY2O4: Tm3+/Dy3+. Fig. 7 shows the photoluminescence
excitation and emission spectra of SrY2O4: 0.005Tm3+, SrY2O4:
0.003Dy3+ and SrY2O4: 0.005Tm3+, 0.003Dy3+ samples. The
excitation spectra of SrY2O4: Tm3+, SrY2O4: Dy3+ and SrY2O4:
Tm3+, Dy3+ samples all show a broad band from 200 to 220 nm
with a maximum at 217, 214 and 217 nm, respectively, and
some weak narrow lines around 360 nm, which should be
assigned to the host absorption due to electronic transition
from valence to conduction band and the typical 4fn → 4fn
intraconfiguration forbidden transitions of Tm3+/Dy3+,
respectively.9 Upon excitation with 217 nm irradiation, the
SrY2O4: 0.005Tm3+ phosphor shows a strong blue emission,
and the obtained emission spectrum consists of the f–f tran-
sition lines within the Tm3+ 4f (ref. 9) electron configuration,
7
Tb3+ in the ground state may occur, as described by (5D3, F6)
7
5
→ (5D4, F0), which results the quenching of the D3 level and
the increase of the 5D4 level.38 For a dipole–dipole energy
transfer process, the transfer probability is given by Inokuti
and Hirayama:39
ꢂ
ꢃ
1
Is0
PSA
¼
ꢁ 1
ð4Þ
τs Is
where Is0 is the intensity of the sensitizer (5D3 energy level) in
the absence of the activator and Is the intensity of the sensi-
tizer in the presence of the activator (5D4 energy level). In
addition, the cross-relaxation efficiency (ηCR) of Tb3+ (5D3
→
1
3
1
3
1
3
5D4) can be expressed as:40
i.e., D2 → H6 (363 nm), D2 → F4 (455 nm) and G4 → H6
(477 nm), respectively, as shown in Fig. 7a. The major emis-
sion peak of SrY2O4: 0.003Dy3+ was at 574 nm and a weak peak
was observed at 491 nm (Fig. 7b), corresponding to the tran-
Ix
ηCR ¼ 1 ꢁ
I0
ð5Þ
4
4
sitions F9/2
→ →
6H13/2 and F9/2 6H15/2, respectively, under
5
where I0 is the emission intensity of D3 at the lowest doping
concentration of Tb3+ and Ix is the emission intensity of 5D3
for a given doping concentration (x). ηCR, the cross-relaxation
excitation with 214 nm irradiation. The transition 4F9/2 → 6H13/2
belongs to the hypersensitive transition with ΔJ = 2, which is
strongly influenced by the outside environment of Dy3+.41
5
efficiency of Tb3+ (5D3 → D4) in SrY2O4 phosphors calculated
Besides the (5D0
→ →
7F2)/(5D0 7F1) emission ratio of Eu3+,
as a function of x under 304 nm UV excitation, is shown in
Fig. 5. The cross-relaxation efficiency was found to increase
gradually from x = 0 to 0.04 with the increase in Tb3+ concen-
tration. The maximum cross-relaxation efficiency can reach
74% when using 304 nm UV as the monitoring wavelength.
To further elucidate the cross-relaxation process, we
measured the Tb3+ decay curves and then calculated the life-
times. Generally, the decay time curves can be influenced
by energy transfer in the host material. When there is no inter-
action between rare-earth ions, the decay curve is usually
a single exponential function.21 However, the decay curves
of Tb3+ can be well fitted by a double exponential function as
I = A1exp(−t/τ1) + A2exp(−t/τ2), with a lifetime of 4.43 ms
(Fig. 6a), which is on the same order of magnitude as those
reported previously, indicating interactions between Tb3+
ions.1,17
(4F9/2 → 6H13/2)/(4F9/2 → 6H15/2) the emission ratio (Y/B) of Dy3+
could also be used as a probe to detect the local symmetry of
the activator ions.41–45 Dy3+ (Y3+) ions have two non-equivalent
sites, both sites are coordinated by six oxygen atoms and both
have Cs symmetry, which has an asymmetrical chemical
4
environment.23 Therefore, the yellow emission F9/2
→
6H13/2
4
(electric dipole transition) dominates the blue emission F9/2
→ 6H15/2 (magnetic dipole transition) in the emission spectra.
The PL intensities of Tm3+ and Dy3+ as a function of their
doping concentration in SrY2O4: Tm3+/Dy3+ samples were
shown in Fig. S2.† At first, the PL emission intensity of Tm3+
and Dy3+ increases with the increase in the concentrations y
and z, reaching maximum values at y = 0.005 and z = 0.007,
respectively, then decreases with further increasing concen-
tration (y and z) due to the concentration quenching effect.
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Fig. 7 Photoluminescence excitation and emission spectra of (a) SrY2O4:
0.005Tm3+; (b) SrY2O4: 0.003 Dy3+; (c) SrY2O4: 0.005Tm3+, 0.003Dy3+ samples.
quenching site, accompanied by the decrease of PL intensity.
Especially, for the concentration quenching of Dy3+, lumines-
cence is mainly caused by cross relaxation, i.e., energy transfer
among Dy3+ ions, for example, Dy3+ (4F9/2) + Dy3+ (6H15/2) →
Dy3+ (6F3/2) + Dy3+ (6F11/2).1 It is well known that blue and
yellow emission mixed in a suitable proportions will show a
white light, which is also confirmed by our experiments.46
Upon excitation with 217 nm irradiation, the emission spec-
trum of SrY2O4: 0.005Tm3+, 0.003Dy3+ (Fig. 7c) consists of the
1
3
1
characteristic f–f transitions from D2 → H6 (363 nm), D2 →
3F4 (455 nm) of Tm3+ and F9/2
→ →
6H15/2 (491 nm), F9/2
4
4
6H13/2 (574 nm) of Dy3+. Moreover, the calculated CIE chroma-
ticity coordinates of SrY2O4: 0.005Tm3+, 0.003Dy3+ from PL
emission spectra are x = 0.292 and y = 0.331, which are located
in the white light region.
The photoluminescence decay curves of SrY2O4: 0.005Tm3+
and SrY2O4: 0.003Dy3+ samples were shown in Fig. 6b and
3
6c. Both of the decay curves of Tm3+ (1D2 → F4, 455 nm) and
Dy3+ (4F9/2
→
6H13/2, 574 nm) could be well fitted to single
exponential functions as I = I0exp(−t/τ) (I0 is the initial inten-
sity at t = 0, τ is the 1/e lifetime), from which the lifetime of
Tm3+ (1D2) and Dy3+ (4F9/2) were calculated to be 14.83 and
0.52 μs respectively, which are consistent with the previous
reports.17,22
It is well-known that phosphors doped with Ce3+ always
show blue color.32 However, the synthesized SrY2O4: Ce3+
sample does not show efficient photoluminescence, which is
due to the thermal quenching of Ce3+ at room temperature.47
This result is consistent with previous reports.32,47,48
Fig. 6 Photoluminescence decay curves for (a) SrY2O4: 0.01Tb3+
,
λem
=
=
545 nm; (b) SrY2O4: 0.005Tm3+, λem = 455 nm; (c) SrY2O4: 0.003Dy3+, λem
574 nm. Black circles and red solid lines represent the experimental data and
fitting results, respectively.
The optimal doping concentration of Tm3+ (0.005) and Dy3+
(0.007) are quite low compared with other systems, which
resulted from the short Y–Y distance due to the double octa-
3.3 Cathodoluminescence properties of SrY2O4: Tb3+/Tm3+/
Dy3+
2−
hedral of Y2O4 in SrY2O4 compound. Generally, the concen-
In order to explore the potential application of the as prepared
SrY2O4: Ln3+ phosphors in field emission displays, their
CL properties are investigated in detail. As shown in Fig. 8,
under low-voltage electron-beam excitation (accelerating
tration quenching of luminescence is due to the energy
migration among the activator ions at the high concentrations,
during which the excitation energy will be lost at a killer or
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Fig. 8 Cathodoluminescence spectra of SrY2O4: 0.003Tb3+ (green line) and
ZnO: Zn (black line), under low-voltage electron-beam excitation (accelerating
voltage = 3 kV, filament current = 90 mA).
voltage = 3 kV, filament current = 90 mA), the CL spectra of
SrY2O4: 0.003Tb3+ samples is similar to their PL spectra, which
give the characteristic transitions of Tb3+ with a maximum
emission peak at 545 nm, producing a bright green emission.
The reduced form of ZnO, usually denoted as ZnO: Zn, exhibits
green luminescence and has been proved to be a good candi-
date for an FED phosphor with low-voltage excitation.30 For
comparison, the emission spectra of commercial phosphor
ZnO: Zn is also shown in Fig. 8 (black line), which shows
broad band emissions with maximum at about 500 nm. The
CIE coordinates of SrY2O4: 0.003Tb3+ and ZnO: Zn are deter-
mined to be (0.278, 0.531) and (0.204, 0.441), and the corre-
sponding luminescent photographs are also shown in Fig. 9
insets 1 and 2. Consequently, the CIE coordinates (emission
color) of SrY2O4: 0.003Tb3+ are more saturated than ZnO: Zn.
For CL, the efficiency of a luminescent material includes the
radiant efficiency (η) and the luminous efficiency (L, bright-
ness), which can be compared roughly by their emission peak
areas (integrated luminescence intensity) and the CL intensity
(in height), respectively.9,49 It can be seen from Fig. 8 that the
emission peak area of the SrY2O4: 0.003Tb3+ is smaller than
that of ZnO: Zn. However, the CL intensity of the SrY2O4:
0.003Tb3+ is higher than that of the ZnO: Zn under the same
excitation condition. So η of ZnO: Zn might be higher than
SrY2O4: 0.003Tb3+, but L of SrY2O4: 0.003Tb3+ is higher
than ZnO: Zn. So, the SrY2O4: Tb3+ may be a promising green
phosphor for practical applications in FEDs.
Fig. 9 The CIE chromaticity diagram for the (1) SrY2O4: 0.003Tb3+ (2) ZnO: Zn
(3) SrY2O4: 0.009Tm3+ (4) SrY2O4: 0.003Dy3+ (5) SrY2O4: 0.005Tm3+, 0.02Dy3+
(6) SrY2O4: 0.01Tm3+, 0.005Dy3+ (7) SrY2O4: 0.009Tm3+, 0.02Dy3+ (8) SrY2O4:
0.009Tm3+, 0.003Dy3+ (9) SrY2O4: 0.003Tm3+, 0.003Dy3+ samples under low
voltage electron beam excitation, and the selected digital cathodoluminescence
photographs (accelerating voltage = 3 kV, filament current = 90 mA).
Fig. 10 Cathodoluminescence spectra of (a) SrY2O4: 0.009Tm3+; (b) SrY2O4:
0.003Dy3+; (c) SrY2O4: 0.009Tm3+, 0.003Dy3+ samples (accelerating voltage =
3 kV, filament current = 90 mA).
Under low-voltage electron-beam excitation, the CL spectra
of SrY2O4: Tm3+, SrY2O4: Dy3+, and SrY2O4: Tm3+, Dy3+ are
similar to their PL spectra (Fig. 10). The SrY2O4: 0.009Tm3+
phosphor shows a strong blue emission (the CIE chromaticity
coordinates are x = 0.148 and y = 0.059), with emission peaks
color purity and chromaticity coordinate.9,16 The emission
color of the SrY2O4: 0.009Tm3+ phosphor is obviously blue to
the naked eye, as shown in inset 3 of Fig. 9. It is known that
the emission spectrum of Dy3+ can be separated into two
groups, the blue emission from 460 to 500 nm and the yellow
emission from 550 to 600 nm accompanied by a group of weak
at 376 nm (1D2
→
3H6), 398 nm (3P0
→
3F4), 455 nm (1D2
→
3
3F4) and 483 nm (1G4 → H6). Due to the forbidden transition
feature of the 1D2
→
3F4 transition of Tm3+, the emission
spectrum is quite narrow compared with Y2SiO5: Ce3+, as
shown in Fig. S3† which is quite beneficial to improving the
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Fig. 11 The cathodoluminescence (CL) intensity of Tb3+, Tm3+ and Dy3+ ions as
a function of concentration (x/y/z) in SrY2O4: Ln3+, respectively.
lines in the red region from 650 to 700 nm.9 These emissions
4
6
correspond to the transitions from F9/2 to H15/2,
6H13/2 and
6H11/2 of Dy3+, respectively. Under low-voltage electron-beam
excitation, the obtained CL spectrum of SrY2O4: 0.003Dy3+ con-
sists of only the f–f transition lines within the Dy3+ 4f9 electron
4
6
configuration, i.e., F9/2 → H15/2 (491 nm) in the blue region,
4
as well as F9/2
→
6H13/2 (581 nm) in the yellow region, as
shown in Fig. 10b. Compared with the PL of SrY2O4:
0.003Dy3+, we could find there was some difference about Y/B
ratio of Dy3+, which was due to the different excitation source.
Fig. 12 The cathodoluminescence intensities of SrY2O4: 0.009Tm3+, SrY2O4:
0.003Tb3+, SrY2O4: 0.003Dy3+ and SrY2O4: 0.009Tm3+, 0.003Dy3+ samples as a
function of (a) filament current and (b) accelerating voltage.
In CL, the blue (4F9/2
→ →
6H15/2, 491 nm) and yellow (4F9/2
6H13/2, 581 nm) emissions have comparable intensity, which
results in the light yellow emission with CIE (x = 0.363, y =
0.395) (Fig. 9, inset 4). Fig. 11 shows the CL intensity of Tb3+
,
zDy3+ from blue to yellow through controlling the relative con-
centrations of Tm3+/Dy3+, which was also confirmed by their
CIE coordinates shown in Fig. 9. In summary, the excellent
white CL of SrY2O4: Tm3+, Dy3+ phosphors means that they
may have potential applications as backlights in display
devices.
Tm3+ and Dy3+ as a function of their doping concentrations in
SrY2O4: Ln3+ samples, respectively. At first, the CL emission
intensity of Tb3+, Tm3+ and Dy3+ increases with the increase of
its concentration x, y and z, reaching maximum values at x =
0.003, y = 0.003 and z = 0.009, respectively, then decreases with
further increasing concentration (x, y and z) due to the concen-
tration quenching effect.
The CL emission intensities of SrY2O4: 0.009Tm3+, SrY2O4:
0.003Tb3+, SrY2O4: 0.003Dy3+
,
and SrY2O4: 0.009Tm3+,
As stated above, the SrY2O4: Tm3+ can give a blue emission
and SrY2O4: Dy3+ shows a light-yellow emission, due to the
dual transitions of Tm3+ and Dy3+ when excited under low-
voltage electron beam, it is possible to obtain white light emis-
sion in the SrY2O4 host lattice by co-doping with Tm3+ and
Dy3+ and appropriately adjusting their concentrations. As
shown in Fig. 10c, the CL spectra of the SrY2O4: 0.009Tm3+,
0.003Dy3+ shows not only the characteristic emission of Tm3+
0.003Dy3+ samples have been investigated as a function of the
filament current and the accelerating voltage, as shown in
Fig. 12. When the accelerating voltage is fixed at 4 kV, the CL
intensity increases upon raising the filament current from 82
to 92 mA (Fig. 12a). Similarly, under a 92 mA filament current,
the CL intensity also increases upon increasing the accelerat-
ing voltage from 2 to 6 kV (Fig. 12b). As revealed in Fig. 12,
there is no obvious saturation effect for the CL intensity of
these samples with the increase of filament current and accel-
erating voltage. The increase in CL brightness with an increase
in electron energy and filament current is attributed to the
deeper penetration of the electrons into the phosphor body
and the larger electron-beam current density. The electron
penetration depth can be estimated using the empirical
formula:9,49
(1D2
→
3F4, 455 nm), but also the characteristic emission of
4
4
Dy3+, i.e., F9/2
→
6H15/2 (491 nm), F9/2 6H13/2 (574 nm).
→
These emission lines of Tm3+ and Dy3+ cover the blue and
yellow regions with comparable intensity, resulting in a white
emission with CIE coordinates (x = 0.315, y = 0.333). The emis-
sion color is white to the naked eye, as seen clearly from the
luminescence photograph in inset 8 of Fig. 9. In addition, we
could also precisely tune the emission color of SrY2O4: yTm3+,
4806 | Dalton Trans., 2013, 42, 4799–4808
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of the SrY2O4: 0.009Tm3+, 0.003Dy3+ sample are nearly invari-
able under a continuous electron radiation for 2 h, as pre-
sented in Fig. 13. Consequently, the short time experiment
(2 h) indicates that the stability of the CL intensity and CIE
color coordinate of the as-prepared SrY2O4: 0.009Tm3+,
0.003Dy3+ samples is good, which shows potential for appli-
cations in FEDs.
4. Conclusions
This study reports the synthesis of a novel single-phase color
tunable-emitting phosphor SrY2O4 using a Pechini-type sol–gel
process. Under low voltage electron beam excitation, the
SrY2O4: Tb3+ shows a bright green emission with better
chromaticity purity than those of commercial product ZnO:
Zn. The SrY2O4: Tm3+ and SrY2O4: Dy3+ phosphors exhibit blue
and yellow luminescence with good chromaticity coordinates,
color purity and appropriate intensity. White CL has been rea-
lized in a single-phase SrY2O4 host by co-doping with Tm3+
and Dy3+ for the first time. It is noteworthy that the SrY2O4:
Tm3+, Dy3+ phosphors have superior stability under electron
Fig. 13 Dependence of relative CL intensity and CIE color coordinates of a
representative SrY2O4: 0.009Tm3+, 0.003Dy3+ sample on the radiation time
under the accelerating voltage = 5.0 kV, filament current = 88 mA electron
beam excitation.
ꢂ ꢃꢂ
ꢃ
n
A
ρ
E
1:2
˚
pffiffiffi
L ½Aꢂ ¼ 250
;
n ¼
ð6Þ
1 ꢁ 0:29 Ig Z
Z
where A is the atomic or molecular weight of the material, ρ is bombardment. Because of their strong CL intensity, good CIE
the bulk density, Z is the atomic number or the number of chromaticity, stability and low raw-materials cost, these phos-
electrons per molecule in the compounds, and E is the acceler- phors have potential applications in FEDs.
ating voltage (kV). For example, for SrY2O4: 0.009Tm3+,
0.003Dy3+, Z = 148, A = 329.43, ρ = 5.91 g cm−3, the estimated
electron penetration depth at 4, 5 and 6 kV are about 37.93,
78.15 and 141.09 nm, respectively. For CL of the above
Acknowledgements
samples, the Tm3+ and Dy3+ ions are excited by the plasma This project is financially supported by National Basic
produced by the incident electrons. The deeper the electron Research Program of China (Grants 2010CB327704) and the
penetration depth, the more plasma will be produced, which National Natural Science Foundation of China (Grants NSFC
results in more activator ions being excited, and thus the CL 60977013, 51172227, 21221061).
intensity increases.
For FED application, the degradation property of the phos-
phor is also an important factor apart from emission color and
luminescence intensity. Thus we also investigated the degra-
Notes and references
dation behavior of SrY2O4: 0.009Tm3+, 0.003Dy3+ samples
under low voltage electron beam excitation (accelerating
voltage = 5.0 kV, filament current = 88 mA). The CL peak posi-
tions are almost the same as those before electron bombard-
ment. However, the CL intensity of the studied sample
monotonously decreases with prolonging of the electron bom-
bardment time, as shown in Fig. 13 (green line). After the con-
tinuous electron radiation for 2 h, the CL intensities of the
SrY2O4: 0.009Tm3+, 0.003Dy3+ sample still have the 92% of the
initial value, which is due to the excellent chemical stability of
the oxide materials. It is well known that the accretion of gra-
phitic carbon during electron-beam exposure at high current
densities will prevent low-energy electrons from reaching the
phosphor grains and also exacerbate surface charging, and
thus lower the CL efficiency.50 In addition, after stopping bom-
bardment for a while the CL intensity could not restore to the
initial value, indicating permanent damage to the phosphor
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