Enhanced luminescence of rare-earth complexes Tb1-xEux(m-NBA)3Phen in ZnS

Article abstract of DOI:10.1016/j.saa.2007.07.041

Rare-earth ternary complexes Tb_1-xEu_x(m-NBA)₃Phen (X = 1, 0.25, 0.5, 0.75, 1.0) were synthesized and characterized by IR, DTA-TG, UV, fluorescent spectra and elemental analysis. It was found that luminescence of Eu^3+<

Full text of DOI:10.1016/j.saa.2007.07.041

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Spectrochimica Acta Part A 70 (2008) 253–257
Enhanced luminescence of rare-earth complexes
Tb1−xEux(m-NBA)3Phen in ZnS
Yuguang Lva,c, Jingchang Zhanga,∗, Weiliang Caoa, Lin Songb, Zheng Xub
a Institute of Modern Catalysis, Beijing University of Chemical Technology, State Key Laboratory
of Chemical Resource Engineering, Beijing 100029, China
b Institute of Optoelectronic Technology, Laboratory of Materials for Information Storage and Displays,
Northern Jiaotong University, Beijing 100044, China
c College of Chemistry and Pharmacy, Jiamusi University, Jiamusi 154007, China
Received 30 December 2006; received in revised form 14 July 2007; accepted 24 July 2007
Abstract
Rare-earth ternary complexes Tb1−xEux(m-NBA)3Phen (X = 1, 0.25, 0.5, 0.75, 1.0) were synthesized and characterized by IR, DTA–TG, UV,
fluorescent spectra and elemental analysis. It was found that luminescence of Eu3+ complex was enhanced by doped with Tb3+. It is proved by
TG curve that the complexes are stable, ranging from ambient temperature to 360 ◦C in air. The organic–inorganic combined structural device was
fabricated, and the electroluminescence intensity of the combined structural device was improved compared with the device of the purely organic
components.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ternary complexes; Synthesis; Characteristic; Electroluminescence
1. Introduction
organic–inorganic combined structural device (ITO/PVK: Eu
complexes/ZnS/Al) was fabricated. It may be an effective
method to improve the electroluminescence intensity of lan-
thanide complex by using inorganic semiconductor materials.
Rare-earth metal complexes are suitable to be used as the
emission materials because of their special characteristics, such
as extremely narrow emission bands and high internal quantum
efficiencies [1–4]. And considerable studies have been focused
on the design and assembly of lanthanide complexes with
organic ligands such as ␤-diketone, cryptands, calixarenes, etc.
Therefore, many rare-earth complexes have been synthesized
and used as photoluminescence emitters and electrolumines-
cence devices [5–8]. It has been reported that introduction of
Tb3+ into europium complex can ensure brighter red light under
particular conditions [9,10]. Because the energy transfer from
Tb3+ to Eu3+ can be effective, introduction of Tb3+ can enhance
europium emission. However, little attention has been paid to
rare-earth ternary complexes with Eu3+ (or Tb3+), m-NBA (m-
nitrobenzoic acid) and 1,10-phen (1,10-phenanthroline).
2. Experiments details
2.1. Sample preparation
One millimole mixture of EuCl3·6H2O and TbCl3·6H2O
(mole ratios of the Eu3+ to Tb3+ are 1.0:0, 0.75:0.25, 0.5:0.5,
0.25:0.75 and 0:1.0, respectively), and 3 mmol of m-NBA were
dissolved in ethanol. The pH value of the mixture was adjusted
to 6–7 by adding 3 mmol ammonia. Then 1,10-phenanthroline
in ethanol solution was added to the reaction mixture, the molar
ratio of 1,10-phenanthroline to RE3+ ion was 1:1. The precipi-
tate was filtered, washed with water and ethanol, dried at room
temperature, and then stored in a silica-gel drier. The structure
of ternary complexes is shown in Fig. 1.
In this paper, rare-earth (Eu3+ and Eu3+/Tb3+) ternary com-
plexes with m-NBA and 1,10-phen, Tb1−xEux(m-NBA)3Phen
(X = 0, 0.25, 0.5, 0.75, 1.0) were synthesized, and the
2.2. Electroluminescence device preparation
∗
In order to improve the performance of ternary complexes
Corresponding author. Tel.: +86 1 64434904; fax: +86 1 64434898.
E-mail address: zhangjc1@mail.buct.edu.cn (J. Zhang).
thin film, Tb1−xEux(m-NBA)3Phen was doped into PVK at dif-
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.07.041

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Y. Lv et al. / Spectrochimica Acta Part A 70 (2008) 253–257
Fig. 2. DTA–TG plots of complex Tb0.5Eu0.5(m-NBA)3Phen.
Fig. 1. Chemical structure of ternary complexes.
3. Results and discussion
ferent weight ratio. ZnS thin film was prepared by electron beam
˚
evaporation at the growth rate of 0.5 A/S under high vacuum of
3.1. DTA–TG analysis
2 × 10−6 Torr. The top Al electrode was prepared by thermal
evaporation about 100 nm. Two kinds of structural devices were
fabricated and signed symbols A and B, respectively [11–15]:
DTA–TG plots of Tb0.5Eu0.5(m-NBA)3Phen are shown in
Fig. 2. Different content of doped ion showed similar curve pat-
terns. For DTA plots of Tb0.5Eu0.5(m-NBA)3Phen, exothermic
peak at 435.8 and 497.3 ◦C, the gross weight loss is 67.5%.
Tb1−xEux(m-NBA)3Phen decompose in two steps, ranging from
360 to 600 ◦C, TG curves prove that the complexes are stable.
Table 1 shows the complexes contents (RE2O3%) are not sig-
nificantly different from the results of calculation, according to
the formula.
• Device A: ITO/PVK: Tb1−xEux(m-NBA)3Phen (60 nm)/
ZnS(30 nm)/Al
• Device B: ITO/PVK: Tb1−xEux(m-NBA)3Phen(60 nm)/BCP
(10 nm)/Alq3(8 nm)/Al
2.3. Measurements
DTA–TG curves were obtained with a TGA-DTA 1700-
Perkin-Elmer. Infrared spectra were recorded in the range of
4000–400 cm−1 by a prostige-21IR spectrophotometer in KBr
flake. Elemental analyses were carried out by the Perkin-Elmer
240 ◦C analytical instrument. UV–vis spectra were performed
on a UV-2501PCS double spectrophotometer. The excitation
and emission spectra measurements were performed on a Shi-
madzu 5301 spectrofluorophotometer equipped with a 150 W
xenon lamp as the excitation source. Lifetimes were measured
with a Spex 1934D phosphorimeter using a 450-W flash lamp as
the excitation source (pulse width = 3 ␮s). The EL spectra were
measured by SPEX Fluorolog-3 spectrometer at room temper-
ature. The luminance was measured by PR-650 spectra-scan
spectrometer. The current–voltage characteristics of the devices
were analyzed using Keithley Source Meter 2410.
3.2. IR spectra analysis and composition of the complexes
FTIR absorption spectra of ligands and complexes are
shown in Fig. 3. It is noted that similar results are obtained
from Eu3+ and Eu3+/Tb3+ complexes. The band in the spec-
tra of complexes at about 525 cm−1 which cannot be observed
in the ligands reveals the presence of O → RE. The band
at wavenumber 1560 cm−1 is due to stretching vibration of
N C, while at 721 cm−1 is corresponding to rC H vibration
of 1,10-phenanthroline. In addition, typical asymmetric vibra-
tion of VC O group bands is detected at about 1608 and
1531 cm−1. The IR spectra of doped complexes are similar to
that of the pure complexes [16,17]. The compositions of the
complexes were confirmed by elemental analysis. Elemental
analysis data of europium (terbium) complex and Tb1−xEux(m-
Table 1
DTA–TG peaks of complexes
Sample
Decompose temperature
Gross
RE2O3%
Tm1 (◦C) weight loss (%)
Tm2 (◦C) weight loss (%)
Weight loss (%)
Tb(m-NBA)3Phen
Eu(m-NBA)3Phen
Tb0.5 Eu0.5 (m-NBA)3Phen
366.6
383.7
435.8
19.6
21.2
23.3
518.1
510.3
497.3
42.3
43.5
44.2
61.9
64.7
67.5
21.4 (21.9)
20.7 (20.8)
20.6 (20.9)

Y. Lv et al. / Spectrochimica Acta Part A 70 (2008) 253–257
255
Fig. 4. UV spectra of ternary complexes: (a) Tb(m-NBA)3Phen, (b) Eu(m-
NBA)3Phen and (c) Tb0.5Eu0.5(m-NBA)3Phen.
Fig. 3. (a) Infrared spectra of ligands and complexes in the range of
4000–400 cm−1. (b) Infrared spectra of ligands and complexes in the range
of 2300–400 cm−1
.
Fig. 5. Typical excitation spectrum of complex Tb0.5Eu0.5(m-NBA)3Phen.
NBA)3Phen (X = 0.5, which is selected randomly) were listed in
Table 2.
Excited state manifold diagram of lanthanide ternary com-
plexes is shown in Figs. 5 and 6. The excitation and emission
spectra of all complexes with different content of doped ion
showed the similar profile. For Tb0.5Eu0.5(m-NBA)3Phen, the
excitation spectrum consists of a broad band, ranging from 230
to 320 nm (λmax = 298 nm). Fig. 6 shows the emission spectra of
Tb0.5Eu0.5(m-NBA)3Phen, the spectra are typical of Eu3+(III)
centered transitions from the 5D0 levels to the lower 7F0–4 levels.
3.3. UV absorption spectra and fluorescence properties
The UV absorption spectra of the complexes are shown in
Fig. 4, it can be seen that the absorption bands of Tb1−xEux(m-
NBA)3Phen do not show significant difference in comparison
with the pure complexes except that the absorption peaks shifted
slightly to a longer wavelength. They all exhibit domain absorp-
tion peaks in the ultraviolet region (200–400 nm), and the
maximum absorption peaks are located at 292 and 272 nm,
respectively.
7
As expected, the main emissions occurred to be the 5D0 → F2
transition, the ligands (and Tb3+) absorb the energy and the
energy was transferred to Eu(III) ions, then the Eu(III) emits
Table 2
Elemental analysis
Complex
Eu (Tb) (%)
Analytically found (calculated) (%)
C
H
N
Eu(m-NBA)3Phen
Tb(m-NBA)3Phen
Tb0.5Eu0.5(m-NBA)3Phen
18.11 (18.28)
18.63 (18.96)
18.32 (18.58)
47.70 (47.63)
46.98 (47.23)
47.12 (47.31)
2.26 (2.41)
2.13 (2.38)
2.20 (2.39)
8.65 (8.42)
8.42 (8.35)
8.28 (8.36)

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Y. Lv et al. / Spectrochimica Acta Part A 70 (2008) 253–257
Fig. 7. EL spectra of complex Tb0.5Eu0.5(m-NBA)3Phen in different devices A
and B (at a driving voltage of 12 V).
Fig. 6. Typical emission spectrum of complex Tb0.5Eu0.5(m-NBA)3Phen.
typically red luminescence. Fluorescence emission intensity of
the complexes is enhanced with an addition of Tb3+ ion. For
Tb1−xEux(m-NBA)3Phen complex, different mole fraction of
Tb3+ have different influences on the emissions intensity of
doped complexes. The optimum mole fraction of doped Tb3+
is 0.5, which exhibited optimal photoluminescence emission at
614 and 699 nm. However, at higher Tb3+ content, the emis-
sion intensity was reduced. A very weak emission from Tb3+
(in complex Tb(m-NBA)3Phen) is observed at 545 nm. And the
complex of Tb0.5Eu0.5(m-NBA)3Phen shows the longest life-
time in Table 3 (about 1026 ␮s).
3.4. Electroluminescence properties
Fig. 7 shows EL emission spectra of Tb0.5Eu0.5(m-
NBA)3Phen at a driving voltage of 12 V. In general, BCP thin
film acts as a hole blocking layer and Alq3 thin film acted as
an electron-transporting layer for the device B. In the device A,
semiconductor ZnS thin film acts as electron function (transport-
ing, acceleration) layer and hole blocking layer according to its
electronic characteristics and energy level instead of BCP and
Alq3 thinfilmsinthedeviceB, theintensityoftheredemissionat
614 nm in device A is stronger than device B. In both structural
devices, the characteristic emissions of europium ions at 594,
614, 655 and 690 nm are obtained. These emission peaks are
ascribed to four energy level transitions of 5D0–7FJ (J = 1–4) of
europium ion (Eu3+), respectively. The luminescence intensity
of 5D0–7F2 is the strongest.
Fig. 8. Current–voltage curves for the two types of structure devices.
Thereisanewroutefortheexcitationoflanthanideionsbased
on the organic–inorganic combined structural device: electrons
directly impact excitation the lanthanide ions through resonant
energy transfer. It is found that the combined structural device
may be an effective way to improve the electroluminescence
intensity of lanthanide ions. As shown in Fig. 8, the electron
current and hole current for two types of structural devices are
studied, the electron current in the combined device is much
larger than in the pure organic structural device due to the dras-
tically different electron mobility. Another important reason is
the influence of the electric field strength in the PVK layer on
Table 3
Emission peak positions and relative intensity of the complexes
Eu:Tb Sample (Tb1−xEux(m-NBA)3Phen)
λex (nm)
λem (nm)
Relative intensities (a.u.)
Lifetimes (␮s)
0.0:1.0
0.25:0.75
0.5:0.5
0.75:0.25
1.0:0.0
351
352
352
352
352
545
613
613
613
613
360
760
1898
1350
850
372
845
1026
965
665

Y. Lv et al. / Spectrochimica Acta Part A 70 (2008) 253–257
257
hole current. The electric field strength of the PVK layer in the
combined structural device is larger than that in the pure organic
structural device under the same driving voltage. So, the hole
current in the combined structural device is larger than that in
the pure organic structural device.
412), and Education Commission of Hei Long Jiang Province
(11521288).
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4. Conclusion
Following conclusions can be drawn from this work:
The complexes of Tb1−xEux(m-NBA)3Phen show good lumi-
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Eu3+ ions. It is proved by TG curve that the complexes are stable,
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Acknowledgements
This work was supported by Institute of Optoelectronic Tech-
nology Beijing Jiao Tong University, the Doctoral Program of
Higher Education (No. 20050010014), the National Develop-
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