Indian J. Phys. 83 (4) 423-428 (2009)
© 2009 IACS
*Corresponding Author
Intense blue-emitting Ca
5
Al
8
O
14
: Eu phosphor for mercury
free lamp
Abhay D Deshmukh
1
*, S J Dhoble
1
, S V Godbole
2
, M K Bhide
2
and D R Peshwe
3
1
Kamla Nehru College, Sakkardara Square, Nagpur-440 009, Maharashtra, India
2
Spectroscopy Section, Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India
3
Department of Metallurgical and Materials Engineering, VNIT, Nagpur-440 011, Maharashtra, India
E-mail : abhay_d1984@yahoo.co.in
Abstract : The calcium aluminates doped with Eu ions, Ca
5
Al
8
O
14
: Eu, phosphors are prepared by the
combustion method. The formation of crystalline aluminates was confirmed by X-ray diffraction pattern. The
prepared phosphors were characterized by SEM, TGA, DTA, particle size analyzer and Photoluminescence (PL)
techniques. From the UV-excited luminescence spectra it was found that the Eu ions acts as a luminescent
centre with luminescence at the blue (λ
max
= 470 nm) region due to 4f
6
5d
1
→ 4f
7
transition. The excitation
spectra show the broad band at 355 nm wavelength (λ
em
= 470 nm). The excitation 355 nm is a mercury free
excitation and therefore Ca
5
Al
8
O
14
: Eu may be useful for the solid state lighting phosphor in lamp industry.
Keywords : Ca
5
Al
8
O
14
, PL, phosphor, noncentrosymmetric.
PACS Nos. : 61.66.Fn, 61.72.Ww, 61.50.Ah
1. Introduction
The fabrication of a blue-emitting GaN light emitting diode (LED) in 1993 [1] brought
a significant revolution in lighting technology and then, the first commercial white
emitting LED solid-state lighting was developed using this blue-emitting LED in 1997
[2]. It is the combination of a 460 nm blue GaN-LED and the yttrium aluminum garnet
(YAG) : Ce
3+
phosphor, which shows a bright yellow emission under excitation with
blue radiation (460 nm) [3]. However, the spectral composition of the light produced by
this approach is different from that of natural white light due to the poor colour
rendering property caused by the lack of red component. Presently, the emission bands
of LEDs are shifted to near UV range around 400 nm, which can offer a higher
efficiency solid-state lighting [4]. Alternatively mixing of blue, green and red phosphors
with one LED is explored [5]. The current phosphors for near UV GaN-based LEDs are
Abhay D Deshmukh, S J Dhoble, S V Godbole, M K Bhide and D R Peshwe" "
BaMgAl
10
O
17
: Eu
2+
for blue, ZnS : (Cu
+
, Al
3+
) for green and Y
2
O
2
S : Eu
3+
for red [6].
However, the efficiency of the Y
2
O
2
S : Eu
3+
red phosphor is about eight times less
than that of the blue and green phosphors, the lifetime of Y
2
O
2
S : Eu
3+
is inadequate
under UV irradiation, and these sulfide-based phosphors are chemically unstable. The
search for stable, inorganic rare-earth-based red phosphors with high absorption in the
near UV/blue spectral region is therefore an attractive research task.
Noncentrosymmetric oxides represent an interesting class of materials owing to
their technologically important properties; such as ferroelectricity a second order
nonlinear optical (NLO) behaviour are the numerous applications [7]. Among them
Ca
5
Al
8
O
14
is a noncentrosymmetric oxide with properties such as optical activity;
pyroelectricity and piezoelectricity [7,8]. In this study, we concentrate on the polar
crystal class of oxide Ca
5
Al
8
O
14
doped with Eu, is synthesized by combustion
technique. The prepared phosphors are further characterized by X-ray diffraction pattern;
thermogravimetric analysis, differential thermal analysis, Scanning electron microscopy
(SEM) and photoluminescence techniques.
2. Experimental
The Ca
5
Al
8
O
14
phosphors were prepared by the combustion technique. All the reagents
were of analytical purity and used without further purification. The starting materials
were taken as aluminium nitrate [Al(NO
3
)
3
.9H
2
O], calcium nitrate [Ca(NO
3
)
2
.6H
2
O] with
high purity [99.9%]. Europium oxide [Eu
2
O
3
] was used as europium nitrate by
dissolving europium into nitric acid. The correct amount of excess urea [CO(NH
2
)
2
] was
then injected into the precursor solution. Each precursor solution was vigorously stirred
for 10 min until it became gelatinous. The precursor solution was then introduced into
a muffle furnace maintained at 500°C. Initially, the solution underwent dehydration; then
spontaneous ignition occurred followed by smoldering combustion with enormous
swelling. This process produced foamy and voluminous products followed by large
amount of gases.
The prepared phosphors were characterized by XRD, TG, DTA, SEM, particle
size analyzer and photoluminescence techniques.
3. Results
In order to determine the crystal structure and establish chemical nature of the
combustible product, XRD study was carried out. The small amount of doped rare earth
ions has virtually no effect on phase structures. The result shown in Figure 1 indicates
that the obtained products are of high purity and crystallinity when prepared by the
combustion process.
Photoluminescence behaviour :
Eu
2+
doped phosphors usually show intense broad band PL with a short decay time
Intense blue-emitting Ca
5
Al
8
O
14
: Eu phosphor for mercury free lamp " #
of the order of nanoseconds. The emission of Eu
2+
is very strongly dependent on the
host lattice and can occur from the ultraviolet to red region of the electromagnetic
spectrum. This is because the 5d → 4f transition is associated with the change in
electric dipole and the 5d excited state is affected by crystal field effect. The Eu
2+
emission from many phosphor hosts is intense enough to find important applications,
e.g. in fluorescent lamps, LED and plasma display. An important class of phosphor is
based on Eu
2+
doped aluminate of alkaline earth metals.
The incorporation and stabilization of Eu ions in the sample were confirmed by
luminescence investigations. The intense excitations observed in the UV region (254 nm
and 355 nm) are associated with the 4f → 5d electronic transition. The broad band
excitation at 355nm is due to the 4f
7
(8S
7/2
) → 4f
6
5d transition as shown in Figure
2. It may be noted that the PL spectra of Ca
5
Al
8
O
14
shows intense broad Eu
2+
emission bands peaking at 472 nm for the 355 nm excitation. This is as expected from
the allowed nature of emission transition of Eu
2+
ion between 4f
6
5d
1
→ 4f
7
levels
(Figure 3). The highest intensity for Eu
2+
ion is observed for the 9-mol% concentration
in Ca
5
Al
8
O
14
phosphor by 355 nm excitation. At 11-mol% concentration the PL
intensity of phosphor is reduced due to the concentration quenching effect.
Figure 1. XRD analysis of Ca
5
Al
8
O
14
.
1500
1000
500
0
20 30 40 50 60 70
Position (2 meb)
Ca
5
Al
8
O
14
: Eu
Figure 3. PL emission spectra of Ca
5
Al
8
O
14
:
Eu (λ
ex
= 355 nm).
200.000
100.000
0.000
Intensity (a.u)
355.0 550
Wavelength (nm)
9%
11%
7%
5%
1%
0.1%
Figure 2. PL excitation spectra of Ca
5
Al
8
O
14
: Eu.
Intensity (a.u)
355 nm
9%
11%
5%
1%
230.8 367.3 503
Wavelength (nm)
Abhay D Deshmukh, S J Dhoble, S V Godbole, M K Bhide and D R Peshwe" $
Scanning electron microscopy :
In order to study the morphology structure of phosphor prepared by combustion
synthesis, scanning electron microscopy has been carried out. The SEM micrographs
in Figure 4 shows the microstructure and the surface of the Ca
5
Al
8
O
14
foam. Irregularly
Figure 4. SEM study of Ca
5
Al
8
O
14
: Eu.
shaped particles with sharp corners are seen in Figure 4. A wide range of distribution
of particles ranging from 0.5 to 10 µm is observed. The foamy structure of phosphor
reflects the inherent nature of the reaction. In case of combustion synthesis, instantaneous
and in situ very high temperature combined with release of large volumes of volatiles
from liquid mixture is likely to result in the production of particles in fluffy form.
Reaction in the presence of urea allows the growth of faceted crystals. The energy-
dispersive X-ray spectroscopy (EDS) of the micron phosphor shown in Figure 5
confirms that the nano-phosphors are composed of aluminium, calcium, oxygen, and
europium elements (C came from the carbon tape used to support the sample), in
which the platinum originates from the platinum sputtered sample for SEM measurement.
Figure 6 gives the particle size distributions of sample Ca
5
Al
8
O
14
. The average particle
size observed by particle size analyzer is about 4.3 µm.
Thermal analysis :
The TGA and DTA curves presented in Figure 7 indicate a significant weight loss up
to 200°C, which is attributed to the moisture loss, absorbed after the thermal treatment
of gel at 500°C. After 200°C the weight loss curve appears to be linear, which shows
Figure 5. EDS spectrum of Ca
5
Al
8
O
14
: Eu.
1500
1350
1200
1050
900
750
600
450
300
150
0
Counts
OKa
EuMz
EuMa
EuMr
AlKa
CaKa
CaKl
EuLl
EuLa
EuLb
EuLb2
EuLr
EuLr3
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
Figure 6. Particle size analysis.
100
80
60
40
20
0
0.1 1.0 10.0 100
Size in microns
Percentage
Intense blue-emitting Ca
5
Al
8
O
14
: Eu phosphor for mercury free lamp " %
that the phosphor is stable and there is no weight loss up to 900°C. The heat flow
curve shows that there is an endothermic and exothermic peak at 600°C and 800°C,
which may be due to the increase in crystallinity of the sample and due to
disappearance of impurity phases. According to TG curve, the total weight loss of the
sample is nearly 5%, which is negligible. Hence it can be stated that the phosphor
prepared by the combustion technique is thermally stable and does not decompose
over a usable temperature range.
4. Conclusion
The Ca
5
Al
8
O
14
: Eu, phosphors are prepared by the combustion method. The formation
of crystalline aluminates is confirmed by X-ray diffraction pattern. The prepared
phosphor is characterized by SEM, TGA, DTA and photoluminescence (PL) techniques.
SEM micrograph shows the faceted 5–10 µm crystals. From the UV-excited luminescence
spectra it is found that the Eu ions act as luminescent centres with luminescence at
the blue (λ
max
= 470 nm) region due to 4f
5
5d
1
→ 4f
7
transition. The excitation spectra
show a broad band at 355 nm wavelength (λ
em
= 470 nm). The excitation at 355 nm
is a mercury free excitation and therefore Ca
5
Al
8
O
14
: Eu may be useful for mercury
free lamp phosphor in lamp industry.
Acknowledgment
One of us (SJD) is thankful to BRNS (No. 2005/37/19/BRNS/1745), Department of
Atomic Energy, Government of India, for the financial support.
References
[1] S Nakamura, M Senoh and T Mukai Appl. Phys. Lett. 62 2390 (1993)
[2] S Nakamura and G Fasol The Blue Laser Diode : GaN Based Light Emitters and Laser (Berlin : Springer-
Verlag) p216 (1997)
[3] R Muller-Mach and G O Mueller IEEE J. Sel. Top. Quantum Electron. 8 339 (2002)
[4] D A Steigerwald, J C Bhat, D Collins, R M Fletcher, M O Holcomb, M J Ludowise, P S Martin and S L Rudaz
IEEE J. Sel. Top. Quantum Electron. 8 310 (2002)
Figure 7. TGA/DTA analysis of Ca
5
Al
8
O
14
phosphor.
0.1646
0.1
0.0
–0.1
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–0.8
–0.9283
Derivative Microvolt (mv/min) –
·
– ––––
28.33 200 400 600 851.1
Temperature (°C)
2.132e–3
0.000
–2.000e–3
–4.000e–3
–6.000e–3
–8.000e–3
–0.010
–0.012
–0.014
–0.016
–0.018
–0.02044
Derivative Weight (mg/min) –
·
·
–
