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JALCOM-17427; No.of Pages4
Journal of Alloys and Compounds xxx (2008) xxx“xxx
Mg
1.96-1.96x
Zn
1.96x
GeO
4
:Mn
0.04
phosphors for electroluminescent
display applications
G. Anoop, K. Mini Krishna, M.K. Jayaraj
*
Optoelectronic Devices Laboratory, Department of Physics, Cochin University of Science and Technology, Kochi 682022, Kerala, India
Received 27 June 2007; received in revised form 7 January 2008; accepted 9 January 2008
Abstract
Orthorhombic magnesium germenate (Mg
2
GeO
4
) doped with manganese was synthesized by solid-state reaction technique. A broad red emission
with a peak at 653 nm was observed independent of excitation wavelength. The effect of Zn incorporation on structural and optical properties is
investigated, keeping the concentration of Mn fixed at 2 at.%. XRD and DRS analysis of the samples reveal the formation of a solid solution up to
x = 0.10 beyond which phase segregation occurs. Formation of a sub-band gap is observed for Mn doped samples which decreased with Zn doping.
Both green and red emission is observed for Zn doped samples above x = 0.10. Red emission is attributed to the
4
T
1
?
6
A
1
transition of Mn
2+
at
Mg
2+
site in Mg
2
GeO
4
and green emission is from
4
T
1
?
6
A
1
transition of Mn
2+
at Zn
2+
site of Zn
2
GeO
4
. PLE was found to be red-shifted with
Zn doping.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Mg
2
GeO
4
; Luminescence; Phosphor
1. Introduction
In the past few years, oxide phosphors have been widely
investigated as potential candidates for electroluminescent dis-
play applications [1“4]. They exhibit extreme stability in
vacuum and emit fewer harmful gases under irradiation of
electrons in comparison to conventional sulphide phosphors.
Several oxide phosphor hosts like Y
2
O
3
[5], ZnGa
2
O
4
[6“14],
Zn
2
GeO
4
[15“18], etc. have been investigated for full colour
electroluminescent devices. One of the necessary criteria for
phosphor hosts is its wide band gap. The band gap should be
greater than 3 eV so that visible radiation emitted by the activa-
tor/impurity is not absorbed by the host. Therefore, wide band
gap oxides are of considerable interest. But there is a limit
to band gap for electroluminescent display phosphors, above
which no electroluminescence is observed, since in most cases
luminescence arises due to resonant energy transfer from the
host to activator. Manganese is an excellent activator for yellow
(in ZnS) [19], green (ZnGa
2
O
4
, Zn
2
GeO
4
) and red (ZnMgS)
*
Corresponding author. Tel.: +91 484 2577404; fax: +91 484 2577595.
E-mail address: mkj@cusat.ac.in (M.K. Jayaraj).
[20], Mg
2
GeO
4
[21]) emissions. Eu and Cr doped Mg
2
GeO
4
also shows red emission, serves as a phosphor for plasma dis-
play panels [22,23]. In orthorhombic Mg
2
GeO
4
(a = 10.29A,
b = 6.023A, c = 4.905A) Mg
2+
ions occupy tetrahedral sites
while Ge
3+
ions occupy octahedral sites in crystal lattice. The
wide optical band gap of Mg
2
GeO
4
makes it as a suitable can-
didate for wide band gap oxide phosphor. However, efficient
high field electroluminescence from Mg
2
GeO
4
host material
is not observed due to its high band gap and inefficient trans-
fer of energy from the host to activator. Also Mg
2
GeO
4
: Mn
phosphor has not been widely studied for luminescent appli-
cations. Co-doping has been proved as an excellent technique
for engineering the band gap [18,24“26]. Among the various
ions that can replace Mg, Zn is better choice due to its sim-
ilarity in ionic radii and valency. More over the band gap of
ZnO has been engineered by alloying it with Mg for many opto-
electronic applications [25]. Alloying up to x = 0.33 is observed
in Zn
1-x
Mg
x
O thin films [26]. Due to abundance and non-
toxicity, compared to other ions like Cd, Zn is more appropriate
co-dopant for engineering the band gap of Mg
2
GeO
4
. In the
present work, the effect of zinc co-doping on the crystal struc-
ture, band gap and photoluminescence of Mg
2
GeO
4
:Mn is
studied.
0925-8388/$ “ see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.01.043Page 2

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G. Anoop et al. / Journal of Alloys and Compounds xxx (2008) xxx“xxx
2. Experiment
The samples were synthesised by conventional high temperature solid-state
reaction of constituent oxides, namely MgO, ZnO, GeO
2
. Manganese was added
in the form of manganous acetate [Mn(CH
3
COO)
2
]. The stoichiometric powders
were mixed in ethanol medium and calcined in air at 1200
?
C for 12 h in a
tubular furnace to obtain Mg
1.96-1.96x
Zn
1.96x
GeO
4
:Mn
0.04
(x was varied from
0 to 0.5). The concentration of Mn was fixed at 2 at.% in all the samples. The
concentration of Zn was varied from 0 to 50 at.% of Mg. The crystal structure
of the powder phosphors were analyzed using X-ray powder diffraction method
using Cu K radiation (Rigaku, Japan). The diffuse reflectance spectra were
recorded to analyze the band gap using Jasco V-570 spectrophotometer with
an integrating sphere attachment. BaSO
4
was used as the reference. The room
temperature photoluminescent emission and excitation spectra were recorded
using Spex Fluoromax-3 Spectro-fluorimeter in the range 200“800 nm.
3. Results and discussion
Fig. 1 shows XRD patterns of pure Mg
2
GeO
4
:Mn, pure
Zn
2
GeO
4
:Mn and zinc doped Mg
1.96-1.96x
GeO
4
:Mn
0.04
at dif-
ferent concentrations (x = 0.1, 0.15, 0.20, 0.25, 0.5) of zinc.
The pattern clearly shows the formation of solid solution up
to x = 0.10. But above x = 0.10, additional peaks are observed
indicating phase segregation. These additional peaks are iden-
tified as that of Zn
2
GeO
4
. No traces of constituent oxides like
ZnO and MgO, were found in the XRD pattern as the pattern
matches well with the JCPDS data of Zn
2
GeO
4
and Mg
2
GeO
4
.
Normally Magnesium germenate crystallizes in orthorhombic
structure and zinc germenate crystallizes in rhombohedral struc-
ture, thereby limiting simple incorporation of Zn in to Mg
2
GeO
4
lattice. Also the ionic radii of Zn and Mg differ; substituting Mg
with Zn will not be possible for all concentrations and at a par-
ticular concentration phase will start to segregate. The phase
segregation occurs due to inbuilt strain due to the difference
in ionic radii and strain reaches a maximum at particular Zn
concentration. However, in Zn
1.96-1.96x
Mg
1.96x
GeO
4
Mg forms
solid solution up to x = 0.3 clearly indicating that rhombohedral
Zn
2
GeO
4
have more stability than Mg
2
GeO
4
naturally favour-
ingtheformationofZn
2
GeO
4
[18].Therefore,asmoreandmore
ZnaddstotheMg
2
GeO
4
:Mnsystem,formationofZn
2
GeO
4
:Mn
Fig. 1. XRD patterns of Mg
1.96-1.96x
Zn
1.96x
GeO
4
:Mn
0.04
(0 = x = 1).
Fig. 2. Variation of Cell volume and band gap with Zn concentration. Straight
(dashed) line shows apparent linear fit to data.
is favoured and when x = 0.5, Zn
2
GeO
4
phase got enhanced
thereby forming Zn
1.96-1.96x
Mg
1.96x
GeO
4
:Mn. The cell volume
[27] is found to increase with x which is expected as the cell
volume of Zn
2
GeO
4
is larger than that of Mg
2
GeO
4
(Fig. 2).
The band gap of Mg
2
GeO
4
:Mn
2+
and Zn
2
GeO
4
:Mn
2+
, cal-
culated from the spectra are 5 eV and 3.45 eV, respectively. In
both cases strong sub-band absorption is observed. This sub-
band state lie nearly 1 eV below the conduction band and formed
due to intrinsic oxygen vacancies during the compound forma-
tion. The schematic energy level scheme is shown in Fig. 3. The
variation of band gap of Mg
2
GeO
4
:Mn with Zn concentration
(x) is shown in Fig. 4. As x increases band gap is found to be
red-shifted. Fig. 4 clearly indicates mixed phase for doped sam-
ples (x = 0.15, 0.2, 0.25 and 0.5). So from the DRS spectra it is
concluded that formation of solid solution is favoured for the
zinc doping up to x = 0.10 and beyond that phase segregation
occurs due to inbuilt strain in the lattice. The shift in band gap
can be attributed to difference in ionic radii of Zn
2+
and Mg
2+
ions. Moreover, when Zn is co-doped in to Mg
2
GeO
4
system,
Zn
2+
will create states below the sub-band levels created due to
oxygen vacancy. So as more and more Zn gets added in to the
Mg
2
GeO
4
matrix, the band gap reduces.
Fig. 5 shows PL emission spectra of Mg
1.96-1.96x
Zn
1.96x
GeO
4
:Mn
0.04
(0 = x = 0.25) samples. All samples show emis-
sion in the red region and has intensity greater than Zn free
Fig. 3. Energy level scheme describing the excitation and emission mechanism
of Mg
2
GeO
4
:Mn phosphor.Page 3

Please cite this article in press as: G. Anoop, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.01.043
ARTICLE IN PRESS
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JALCOM-17427; No.of Pages4
G. Anoop et al. / Journal of Alloys and Compounds xxx (2008) xxx“xxx
3
Fig. 4. The band gap of Mg
1.96-1.96x
Zn
1.96x
GeO
4
:Mn
0.04
(0 = x = 0.5).
sample. But the peak emission wavelength changes as x varies.
In Mg
2
GeO
4
:Mn the emission is observed from tetrahedrally
coordinated Mn
2+
at Mg sites [21]. The 3d
5
electrons of Mn
2+
is
highly influenced by crystal field environment. The crystal field
environment determines the colour or the emission wavelength
of the activator. In Mg
2
GeO
4
, since we are getting red emis-
sion, crystal field has considerable effect on 3d
5
electrons. Also
energy transfer mechanism is not so efficient as in the case of
Zn
2
GeO
4
:Mn. In Zn
2
GeO
4
the mechanism of PL is identified as
resonant energy transfer from a sub-band gap level, due to intrin-
sic defects, to Mn
2+
levels [18,28].InMg
2
GeO
4
also a sub-band
gap level is observed as the Mn
2+
emission is triggered from a
level which is nearly 1 eV low from its conduction band. But this
level is not in the vicinity of excited levels of Mn
2+
for efficient
resonant energy transfer to take place. But as more Zn is added
to the system, transfer will take place to Mn
2+
levels through
Zn
2+
levels formed near Mn
2+
levels. This is also observed in
DRS measurements as reduction in band gap is observed as
more Zn adds to the system. However, no red emission can be
detected from x = 0.5 sample indicating the complete replace-
ment of Mn
2+
at Zn
2+
sites in Zn
2
GeO
4
. Therefore, similar to the
Fig. 5. PL emission spectra (red) of Mg
1.96-1.96x
Zn
1.96x
GeO
4
(0 = x = 0.25),
?
exc
= 300 nm.
Fig. 6. Room temperature Photoluminescent emission spectra (green) of
Mg
1.96-1.96x
GeO
4
:Mn
0.04
(0 = x = 0.5), ?
exc
= 300 nm.
formation of Zn
2
GeO
4
, which is more favourable, Mn
2+
is more
likely to replace Zn
2+
rather than Mg
2+
. Green emission (Fig. 6)
is also detected from the samples at and above x = 0.15 clearly
showing the limit of solid solubility of Zn in Mg
2
GeO
4
:Mn. But
in Zn
2
GeO
4
:Mn
2+
, Mg is found to be soluble up to x = 0.3 [18].
This is due to greater stability of rhombohedral Zn
2
GeO
4
struc-
ture compared to orthorhombic Mg
2
GeO
4
. The green emission
at 535 nm can be attributed to
4
T
1
?
6
A
1
transition of Mn
2+
in
Zn
2
GeO
4
and gets enhanced when Zn concentration increases.
The photoluminescence excitation (PLE) spectra for red
emission (653 nm) are shown in Fig. 7. A broad excitation peak-
ing at 268 nm is obtained for Mg
2
GeO
4
:Mn which is found to
be red-shifted as more Zn
2+
replaces Mg
2+
. In Zn
2
GeO
4
:Mn
PLE at 332 nm is obtained for emission at 535 nm showing
the formation of sub-band gap [18]. Since increased Zn con-
centration results in phase segregation, the excitation for red
emission shifts to higher wavelength region indicating the pres-
Fig. 7. PLE spectra of Mg
1.96
GeO
4
:Mn
0.04
and inset shows its variation with
Zn concentration, ?
em
= 653 nm.Page 4

Please cite this article in press as: G. Anoop, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.01.043
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G. Anoop et al. / Journal of Alloys and Compounds xxx (2008) xxx“xxx
enceofZn
2
GeO
4
.Butwhenexcitedforgreenemission(535 nm)
the PLE spectra show a peak at 300 nm, for x = 0.50 sample,
indicating the formation of Zn
1.96-1.96x
Mg
1.96x
GeO
4
, Mg being
substituted for Zn, there by emphasizing the fact that formation
of Zn
2
GeO
4
is more favored rather than Mg
2
GeO
4
.
4. Conclusion
Manganese doped Mg
2
GeO
4
is synthesized by solid-state
reaction. The phosphor shows emission in red region. The effect
of Zn doping on structural and optical properties have been
explored in detail. XRD patterns show solid solution forma-
tion up to 10 at% Zn doping and beyond that phase segregation
occurs. When Zn is co-doped in to Mg
2
GeO
4
:Mn, PL emis-
sion intensity increases compared to Zn free sample and PL
peak excitation wavelength red-shifted with Zn addition. Both
green and red emission is observed for Zn concentrations above
10 at% indicating the presence of Mn
2+
ions replacing Mg in
Mg
2
GeO
4
and Zn in Zn
2
GeO
4
. The phosphor can be used as an
active layer in alternating current thin film electroluminescent
(ACTFEL) devices.
Acknowledgements
This work has been supported by Department of science and
Technology. One of the authors, KMK, wishes to thank CSIR
for the grant of fellowship. The authors wish to thank Dr. S.
Jayalekshmi and Rajive Tomy for their help during this work.
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