Materials Sciences and Applications, 2010, 1, 25-31
doi:10.4236/msa.2010.11005 Published Online April 2010 (http://www.SciRP.org/journal/msa)
Copyright © 2010 SciRes. MSA
Blue-Shift and Enhanced Photoluminescence in
BaMgAl10O17:Eu2+ Nanophosphor under VUV
Excitation for PDPs Application
Raghvendra Singh Yadav1,2, Shiv Kumar Pandey1,3, Avinash Chandra Pandey1,2
1Nanophosphor Application Centre, University of Allahabad, Allahabad, India; 2Physics Department,University of Allahabad, Alla-
habad, India; 3Motilal Nehru National Institute of Technology, Allahabad, India.
Email: raghvendra_nac@yahoo.co.in
Received January 18th, 2010; revised January 28th, 2010; accepted February 9th, 2010.
ABSTRACT
In this report, five systems of varied diameters viz. 62 nm, 85 nm, 115 nm, 160 nm and 450 nm of BaMgAl10O17:Eu2+
nanorods are prepared by solution combustion approach and with annealing at different temperature in reduced at-
mosphere (N2 + H2). An intense broad blue photoluminescence (PL), corresponding to the electronic transition of Eu2+
from the 4f 6 5d excited state to the 4f 7 ground state, is observed. The blue-shift and enhanced photoluminescence is
also observed, and found to be highly dependent on the size of diameter of BaMgAl10O17:Eu2+ nanostructures. The
change in decay time and color-coordinates with change in size of diameter of BaMgAl10O17:Eu2+
nanostructure have
been analysed and thoroughly discussed.
Keywords: Quantum Confinement Effect, Nanophosphor, PDPs
1. Introduction
Recently, plasma display panels (PDPs) have been used
for large screen television at home and for a public in-
formation display. However, luminescent properties such
as intensity and efficiency of the resulting PDPs are still
inferior to those of traditional cathode-ray tube display
[1-4]. To overcome these drawbacks, optimization of ph-
osphors used in PDPs becomes necessary. Red, blue and
green phosphors used in PDPs are key materials to im-
prove the performance of PDPs such as brightness,
cost-effectiveness and stability. BaMgAl10O17:Eu2+
(BAM) is an important blue-emitting phosphor for
plasma display panel (PDP) because it can efficiently
absorb the vacuum ultraviolet (VUV) light coming from
the resonance radiation line of Xe atoms (147 nm) and
from the excited state of molecular Xe (172 nm).
BaMgAl10O17:Eu2+ is an insulating host material with a
wide band gap of 6.4 eV. Recently, nanocrystals com-
posed of wide band gap insulator have attracted a lot of
attention, since several interesting results have been ob-
served in semiconductor nanocrystal [5-8]. Therefore, it
is interesting to study whether a similar behaviour can be
observed for wide band gap insulators. It is well known
that size-dependent quantum confinement has significant
effects on both radiative and non-radiative electronic
transitions in nanocrystals. The principal consequences
of quantum confinement are an increase in the band gap
energy, and an associated increased probability of radia-
tive transitions. Confinement of carriers in real space
causes their wavefunctions to spread out in momentum
space, increasing the probability of radiative processes
due to greater wavefunction overlap [9-11]. B. Mercier et
al. [12] observed the quantum confinement effect on the
wide band gap of a material such as Gd2O3 (5.4 eV),
however the observed effect was quite weak. Recently,
Guiling Ning et al. [13] succeeded in reducing the size of
BaMgAl10O17:Eu2+ phosphor (particle size ~ 400 nm) and
observed the increase in the intensity due to quantum size
effect of nano-scale phosphor, but they failed to study
optical properties with further reduction of particle size.
In this paper, a novel solution combustion approach to
synthesize BAM nanophosphors has been reported. Our
primary aim is to explore and study the photolumines-
cence property of BaMgAl10O17:Eu2+ nano particles with
reduction of particle size under quantum confinement ef-
fect. The synthesized BaMgAl10O17:Eu2+ nanoparticles
have enhanced photoluminescence property which has a
technological importance in plasma display panels
(PDPs).
Blue-Shift and Enhanced Photoluminescence in BaMgAlO:Eu2+ Nanophosphor under
26 10 17
VUV Excitation for PDPs Application
2. Experimental Details
A novel solution combustion approach has been adapted
for the synthesis of BaMgAl10O17:Eu2+ nanoparticles
[14]. The typical of preparation is given below. Eu2+
doped BaMgAl10O17 nanophosphor powders with the
composition of Ba1-xMgAl10O17:xEu2+ (x = 0.35) were
prepared from the starting precursor Ba(NO3)2 (99.9%),
Mg(NO3)26H2O (99.9%), Al(NO3)39H2O (99.9%) and
Eu(NO3)3 (99.99%). A stoichiometric amount of these
above nitrates were dissolved in deionized water, then
the required amount of urea as combustible fuel was ad-
ded and stirred well to obtain homogeneously mixed so-
lution. The container containing the above prepared solu-
tion was introduced into a muffle furnace maintained at
400 ± 5. After 15 min, the solution boils and under-
goes dehydration followed by decomposition with liber-
ating a large amount of gases such as CO2, NH3, etc., th-
en the mixture swells and burning instantaneously with
bright flame. At last, white colored product was obtained
within 2-5 min. To investigate the effect of variation of
size on the optical properties, the BaMgAl10O17:Eu2+
phosphor obtained by solution combustion approach
were further annealed at 600, 800, 1000 and
1200 for 4 h in reducing (N2 + H2) atmosphere. As
prepared BaMgAl10O17:Eu2+ powder by solution com-
bustion approach and annealed in reducing atmosphere at
600, 800, 1000 and 1200 were henceforth
termed as BAM-1, BAM-2, BAM-3, BAM-4 and BAM-
5, respectively.
The crystal structure of BaMgAl10O17:Eu2+ nanocrys-
tals was characterized by X-ray diffraction (XRD, Ri-
gaku D/Max -2200 H/PC, Cu K radiation). The VUV
photoluminescence study was carried out on McPherson
spectrometer. The excitation spectra study was carried
out on a Perkin Elmer LS 55 spectrometer. The field em-
ission-scanning electron microscopy (FE-SEM) images
were taken on Quanta 200 FEG (FEI Company, Eindhov-
en, The Netherlands). Quanta 200 FEG fitted with an en-
ergy dispersive x-ray spectroscopy (EDS; Genesis 2000,
EDAX), was used for elemental analysis.
3. Result and Discussion
The crystallographic phase purity of BaMgAl10O17:Eu2+
nanophosphor synthesized by solution combustion app-
roach was confirmed by X-ray diffraction (XRD). Figure
1 shows the XRD pattern of BAM nanophosphor synthe-
sized by solution combustion method. Barium magne-
sium aluminate BaMgAl10O17 (BAM) is classified into β-
alumina structure with a space group P 63/mmc [15]. The
BaMgAl10O17 phase obtained by solution combustion
method shows good agreement with Joint Committee on
Powder Diffraction Standards card No. 26-0163 [16].
Figure 1. XRD pattern of BaMgAl10O17:Eu2+ (BAM) blue
PDP nanophosphor synthesized by solution combustion
method
Moreover, the broadening of major diffraction peaks ind-
icates nanocrystalline nature of particles present in pow-
der. No additional diffraction peaks are observed in XRD
pattern, this indicate that obtained BaMgAl10O17:Eu2+
powder synthesized by combustion approach at 400
has pure phase. No change in phase purity of crystal
structure of BaMgAl10O17:Eu2+ nanoparticles annealed at
different temperature have been also observed.
Figure 2 shows the field emission-scanning electron
microscopy (FE-SEM) images of BaMgAl10O17:Eu2+ ph-
osphor powder synthesized by solution combustion me-
thod: (a) as prepared (BAM-1), (b) annealed at 600
(BAM-2), (c) annealed at 800 (BAM-3), (d) annea-
led at 1000 (BAM-4), and (e) annealed at 1200
(BAM-5). From Figure 2(a), it is clear that as prepared
BaMgAl10O17:Eu2+ has nonorods in structure with diam-
eter ~ 62 nm and length ~ 300-600 nm. The change in di-
ameter of BaMgAl10O17 nanorods with annealing tem-
perature, can be seen in Figure 2 and Table 1.
The spectrum obtained during Energy Dispersive X-
ray Analysis (EDX) study is used to know the presence
of europium in BaMgAl10O17. Figure 3 shows the EDX
spectrum of BaMgAl10O17 nanophosphor particles syn-
thesized by solution combustion method. The EDX spec-
trum of BaMgAl10O17:Eu2+ nanostructure shows the
presence of barium, magnesium, aluminium, oxygen and
europium. The EDX spectrum confirms the doping of
europium in BaMgAl10O17 nanostructures.
Figure 4 shows the photoluminescence spectra of
BaMgAl10O17:Eu2+ nanorods of different diameter D, viz.
62 nm, 85 nm, 115 nm, 160 nm and 450 nm under VUV
(147 nm) excitation. The emission spectra shows the blue
Copyright © 2010 SciRes. MSA
Blue-Shift and Enhanced Photoluminescence in BaMgAl10O17:Eu2+ Nanophosphor under
VUV Excitation for PDPs Application
Copyright © 2010 SciRes. MSA
27
a
b
c
d
e
Figure 2. FE-SEM image of BaMgAl10O17:Eu2+ (BAM) blue PDP nanophosphor synthesized by solution combustion method:
(a) as prepared (BAM-1); (b) annealed at 600 (BAM-2); (c) annealed at 800 (BAM-3); (d) annealed at 1000 (BAM-4);
(e) annealed at 1200 (BAM-5)
241
193
145
96
48
0
O
K
EuM
AlK
BaL
EuL
MgK BaL
EuL
Energy - kev
1.00 2.00 3.00 4.00 5.00 6.00 7.00
Figure 3. EDX spectrum of BaMgAl10O17:Eu2+ nanostructure synthesized by solution combustion method
Blue-Shift and Enhanced Photoluminescence in BaMgAlO:Eu2+ Nanophosphor under
28 10 17
VUV Excitation for PDPs Application
Table 1. The mean grain size (diameter) D (estimated by FE-SEM, the standard deviation is given in brackets), emission en-
ergy (eV), decay time τ ( μs ), color-coordinates and relative intensity (%)
Samples D ( nm ) Emission Peak Decay time ( μ s ) Color-coordinate Relative
Position (eV ) τ1 τ2 x y Intensity (%)
BAM-1 62 ( ±4) 2.775 1.07 7.18 0.1387 0.0784 100
BAM-2 85 (±6) 2.744 2.91 8.89 0.1377 0.0866 87
BAM-3 115 (±8) 2.719 4.33 10.27 0.1356 0.0905 73
BAM-4 160 (±20) 2.702 4.75 10.69 0.1351 0.0936 69
BAM-5 450 (±26) 2.670 2.46 8.51 0.1289 0.1088 96
emission in the region 400-510 nm with strongest peak at
~ 450 nm, corresponding to the electronic transition of
Eu2+ from the 4f 65d excited state to the 4f 7 ground state
[17]. It can also be clearly seen from the Figure 4 that
with the reduction in size of diameter of BaMg Al10O17:
Eu2+ nanostructure there is blue-shift in emission peak
position as well as change in intensity of luminescence
under 147 nm excitation. There are many reports on the
blue-shift effect in the luminescence spectra of semicon-
ductor nanocrystals [18-19]. However, few reports are
available on nano-sized rare earth doped insulating lu-
minescent material. The mechanism of luminescence in
the semiconductor is the recombination of electron in the
conduction band and holes in the valence band. A larger
band gap is necessary for quantum confinement to cause
the blue-shift in the luminescence. However, the lumi-
nescence in BaMgAl10O17:Eu2+ is due to the transition
between energy levels of Eu2+ atoms as the luminescence
center. Therefore, a different interpretation is required to
explain the blue-shift in the luminescence spectra of the
BaMgAl10O17:Eu2+ nanorods.
Figure 5 shows the excitation spectra of BaMgAl10O17:
Eu2+ nanostructures at 450 nm emission peak. The exci-
tation spectrum consists of a broad band with a maxi-
Figure 4. Photoluminescence spectra of BaMgAl10O17:Eu2+
(BAM) blue PDP nanophosphor under 147 nm excitation
mum ~ 328 nm with two shoulders at 270 nm and 380
nm, respectively, which are due to the transitions from
the ground state 8S7/2 of Eu2+ with 4f 7 configuration to
the different crystal field splitting components of the
Eu2+ with (4f 6)5d configuration in the excited states [20].
Recently, S. Zhang et al. [21] reported that when the
BaMgAl10O17:Eu2+ phosphor is excited at wavelength
147 nm, the excitation energy is first absorbed by the
host and then transferred to Eu2+ ions. In this case, the
emission efficiency of Eu2+ ions therefore depends stro-
ngly on the host. The change in the particle size will sig-
nificantly affect the emission intensity of Eu2+ ions in the
indirect excitation.
From excitation spectra, it is clear that there is also
blue-shift in the broad band corresponding to 4f 651 4f7
transition of Eu2+ ions in BaMgAl10O17:Eu2+ nanostruc-
ture host with decrease in size. Here, it is noticeable that
the BaMgAl10O17:Eu2+ nanostructure with diameter 450
nm has little greater intensity as compare to nanostruc-
ture with diameter size 160 nm, and also deviates from
the trend of decrease in PL intensity with increase of size,
while the trends of decrease of PLE intensity with in-
crease in size is maintained in PLE spectra. This can be
explained as: the BaMgAl10O17:Eu2+ nanostructure with
Figure 5. Excitation spectra of BaMgAl10O17:Eu2+ (BAM)
blue PDP nanophosphor
Copyright © 2010 SciRes. MSA
Blue-Shift and Enhanced Photoluminescence in BaMgAlO:Eu2+ Nanophosphor under 29
10 17
VUV Excitation for PDPs Application
diameter 450 nm have low PLE intensity and also due to
larger size it has low surface defects as compare to sma-
ller diameter sized nanostructure. The surface defects in
nanostructures play an important role in influencing the
luminescence properties. The surface defects, mainly res-
ponsible for non-radiative transitions, quenchs the PL
intensity. Here, in case of BaMgAl10O17:Eu2+ nanostruc-
ture with diameter size 450 nm, the abrupt increase in PL
intensity with lower PLE intensity is mainly due to decr-
ease in surface states which are responsible for non-ra-
diative transition. It is also noticeable that the deviation
in PL intensity does not support the degradation in PL
intensity due to annealing temperature. Therefore the
observed change in PL and PLE intensity is mainly due
to change in size of diameter of BaMgAl10O17:Eu2+
nanostructures. The observed blue-shift in PLE and con-
sequently PL is related to change in position of energy
levels of 4f 65d1 4f 7
transition of Eu2+ ions in
BaMgAl10O17:Eu2+ nanostructure host. It is already ob-
served that the energy states associated with the lumi-
nescent center are influenced by the host lattice material.
The degree to which they are influenced depends also on
the size and shape of nanostructures [22,23]. Recently, Y.
Lin et al. [24] reported preparation of the ultrafine
SrAl2O4:Eu,Dy needle-like phosphor and its optical
properties. This group observed that the optical absorp-
tion edge shifts to the blue as the phosphor particle size
decreases. They explained that it may be associated with
the quantum size effect of the nanometer phosphor,
which increased the kinetic energy of the electrons and
resulted in a larger band gap, and thus required higher
energy to excite the luminescent powders. Simultane-
ously they observed that the emission maximum shifted
to shorter wavelength, they explained that it may be
caused by the prepared technology, which resulted in
some changes of the crystal field around Eu2+. Although
the 4f electrons of Eu2+ are not sensitive to lattice envi-
ronment due to the shielding function of the electrons in
the inner shell, the 5d electron may couple strongly to the
lattice. Consequently, the mixed states of 4f and 5d will
be split by the crystal field, which may lead to the
blue-shift of its emission peak. In our case, the blue-shift
in the emission band may be attributed to the changes of
the crystal field around Eu2+ arising from the nanosized
particles. Since the excited 4f 65d
1 configuration of Eu2+
ion is extremely sensitive to the change in the lattice en-
vironment in contrast to the shielded 4f 7 ground con-
figurations, the 5d electron may couple strongly with the
lattice. Therefore, the mixed states of 4f and 5d will be
influenced strongly by the crystal field. On the other
hand, the particles with nanometer size make the surface
energy increase significantly, which causes the change of
the crystal field around the local environment of Eu2+.
These reasons lead to the blue-shift of PLE and PL emis-
sion peak in BaMgAl10O17:Eu2+ nanophosphor with de-
crease in particle size. Table 1 contains information
about change in emission peak position, decay time,
color coordinate and relative intensity with change in
diameter of BaMgAl10O17:Eu2+ nanostructure.
The colorimetric coordinate (x, y) for BAM:Eu2+
nanophosphor were calculated using equidistant wave-
length method [25]. Table 1 summarizes the comparison
of CIE colorimetric coordinates for BaMgAl10O17 nanos-
tructure with variation of diameter size of nanostructure.
To study the decay behaviours of Eu2+ luminescence in
BaMgAl10O17 nanostructures with varied diameter size,
fluorescence decay curve for the 4f 65d1 4f 7 transition
of the Eu2+ were studied [26]. Figure 6 shows the fluo-
rescence decay curve of BaMgAl10O17 nanostructure of
diameter size 62 nm. The decay curve can be well fitted
by double exponential equation: I (t) = I0
+ A exp (-t/τ1) +
B exp (-t/τ2), where I and I0 is the luminescence intensity,
A and B are constants, t is the time, τ1 and τ2 are the de-
cay time for the exponential component, respectively. It
is found that the life time of BaMgAl10O17 nanostructure
is varied as a function of diameter size of nanostructure.
Table 1 summarizes the change in decay time of
BaMgAl10O17:Eu2+ nanostructure with variation in diam-
eter size. From table I, it is clear that the life time varies
with the diameter size of BaMgAl10O17:Eu2+ nanorods
and decay rate decreases with increase in diameter size.
The PL intensity of the 4f
65d 1 4f 7 transition is str-
ongly related to the decay time of a particular transition.
The decrease in decay rate trend is continue to diameter
size 62 nm, 85 nm, 115 nm and then 160 nm, however
the trend of decrease of decay rate with increase of size
deviates in case of BaMgAl10O17:Eu2+ nanostructure with
diameter size 450 nm. The increment in decay rate at
diameter size 450 nm in case of BaMgAl10O17:Eu2+
nanorods is due to decrease in surface states, responsible
for non-radiative transition, with increase of size. In gen-
eral, the photoluminescence (PL) decay rate is a sum of
Figure 6. Fluorescence-decay curve of 62 nm diameter sized
BaMgAl10O17:Eu2+ nanostructure
Copyright © 2010 SciRes. MSA
Blue-Shift and Enhanced Photoluminescence in BaMgAlO:Eu2+ Nanophosphor under
30 10 17
VUV Excitation for PDPs Application
the radiative and non-radiative decay rates:
111
P
LRNR


where τPL, τR, and τNR are the photoluminescence, rad-
iative and non-radiative decay time constants, respecti-
vely. Therefore, the origion of the decrease in PL decay
rate versus size may be radiative and non-radiative. The
possibility may be invoked for the non-radiative process
in origion is surface states, responsible for non-radiative
decay, which changes as surface-to-volume ratio varies
with size. Other possibility for radiative in origion, may
be considered for the size dependence as quantum confi-
nement effect may lead to size dependent oscillator str-
ength. The quantum confinement effect in nanoparticles
predicts a decreased radiative decay rate as the size in-
creases [27]. As PL and PLE spectra are dependent on
the size of diameter of BAM nanostructures indicating
that the quantum confinement effect as well as surface
effect both is responsible for change in decay rate. These
finding provide glimpse for detailed study of quantum
confinement effect for rare earth doped wide band gap
insulating material for enhancement of emission intensity,
emission energy, improvement in color-coordinates and
decay time etc. for better results of phosphor in LEDs
and PDPs.
4. Conclusions
In summary, we have successfully synthesized BaMg
Al10O17: Eu2+ nanorods with variation in size of diameter
of nanostructure by solution combustion approach and
annealing at different temperature under reducing atom-
sphere (H2 + N2). Enhanced blue photoluminescence
emission and blue-shift is observed under 147 nm excita-
tion, and found to be highly dependent on the size of
diameter of BaMgAl10O17:Eu2+ nanostructures. Decre-
ased radiative decay rate with increase in the diameter
size of BaMgAl10O17:Eu2+ is also observed.
5. Acknowledgements
The authors gratefully acknowledge the financial support
of the Council of Scientific and Industrial Research
(CSIR), India under New Millennium Indian Technology
Leadership Initiative (NMITLI) project and Dr. D. Hara-
nath, National Physical Laboratory, India for discussions
and annealing of nanophosphor at different temperature
in reducing atmosphere.
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