111
Chapter IV
Electro-Optical Studies of
Barium and strontium Tartrate Crystals
IV.1 Introduction
Most of the tartrate compounds are insoluble in water and
decomposes before melting. Hence crystals of such type of compounds
cannot be grown by either slow evapouration or melt techniques. In
this situation gel method is the appropriate one for the growth of
barium tartrate crystals. Crystal growth in gels offers itself as a simple
and inexpensive method for useful experiments on the growth of
tartrate crystals for scientific investigations. Henisch [1] and Arora [2]
carried out surveys of crystal growth in gels. Compounds of tartaric
acid find several practical applications in Science and Technology [3].
Tartrate single crystals show many interesting physical properties such
as ferroelectric, piezoelectric, dielectric, optical and other pertinent
characteristics. They are used for non-linear optical devices based on
their optical Second Harmonic Generation and optical transmission
characteristics. They are also used for fabrication of crystal oscillators,
resonators and controlled laser emission [4-9].
Barium tartrate, (BT), (BaC4H4O6) and strontium tartrate, (ST),
(SrC4H4O6) crystals crystallize in orthorhombic system in the space
group P212121 (C2V) containing four molecules/unit cell. Their unit cell
dimensions are a = 7.59000nm, b = 23.78000nm and c = 7.53600nm
112
(JCPDS – CAS: 66904-98-1) and a = 0.948nm, b = 1.096nm and c =
0.946nm [10] respectively. The detailed analysis of the FTIR vibrational
spectra of strontium and rubidium tartrate has been reported [11].
Thermal behaviour of gel grown tartrates of yitrium and samarium
[12], mixed rare earth [13], Iron [14] and strontium [15] was also
reported. The dielectric behaviour of various tartrate compounds such
as calcium tartrate [16], sodium-ammonium tartrate [17] and zinc
tartrate [18] was also observed. Strontium tartrate is identified for
pyroelectric
and
ferroelectric
transitions
[19].
The
non-linear
properties of strontium doped calcium tartrate crystals were also
reported [20].
In this chapter, we present the structural, optical and electrooptical properties of BT and ST crystals and the effect of Mn doping
on these properties. The electro-optical phenomena, which are mainly
considered here, are (i) photoconductivity (PC), (ii) photovoltaic (PV)
effect, (iii) photoluminescence (PL), and (iv). electroluminescence
(EL). In a quantum system photoelectronic phenomena can occur
through the absorption or emission of photons [21].
113
IV.2
Results and Discussion
IV.2.1 Structural Studies
IV.2.1.1 X-Ray Diffraction
The structure of the tartrate ion has been determined by many
authors using X-ray diffraction [22–27]. The ion consists of two planar
halves, each containing a carboxyl group, a tetrahedral carbon atom
and hydroxyl oxygen atoms. The chain of carbon atoms is almost
planar. The planes of two CO2 groups are inclined nearly at 60˚ to the
plane. Hence C2V symmetry is given for the tartrate ions. Structure of
the BT and ST crystals are studied by taking XRD using Cu Kα (λ =
0.154056nm) radiation.
5M
L intensity (A.U)
4M
3M
2M
0
10
20
30
1M
0.5M
d = 2.497
(230)d = 3.423
(013)
(040) d = 5.94
(121) d = 4.88
(131) d = 4.43
1.5M
undoped
40
50
o
2θ
60
70
80
90
Fig.4.1. XRD Pattern of Mn doped BT crystal.
114
The XRD patterns of BT and ST crystals (undoped and doped at
different concentrations of Mn) are given in Fig.4.1 and Fig.4.2
respectively.
The XRD analysis of BT crystals shows crystalline nature
along (013) plane at 2θ =36.192o. The presence of other orientations such
as (040), (121), (131) and (230) was also detected at 2θ = 14.808o,
18.287o, 19.872o and 26.015o respectively with lower intensities. The
calculated‘d’ values, (hkl) and peak intensities of BT crystals were
compared with those of Barium tartrate for orthorhombic crystals
reported in JCPDS data files.
On Mn doping few extra peaks were detected with lower intensities
but the intensity of the prominent peak is decreased and that of certain other
peaks were increased for both crystals. On increasing the concentration of
the dopant, the intensity of the prominent peak further decreased and others
increased due to the decrease and increase in the atomic density of Mn in
these planes respectively. The decrease in peak intensities is due to the
replacement of Ba2+ / Sr2+ ions with Mn2+ ions in the lattice of BT and ST
crystals. Increase in dopant concentration leads to the movement of Mn2+
ions to the interstitial sites and also an increase in disorders.
115
L intensity (A.U)
5M
4M
3M
2M
1.5M
1M
0.5M
undoped
0
10
20
30
40
50
60
70
80
90
o
2θ
Fig.4.2. XRD Pattern of Mn doped ST crystal.
IV.2.1.2 Thermal Studies
The thermal decomposition behaviour of the samples is studied by
Thermogravimetry (TG). In TG analysis weight of the BT sample taken is
12.494 mg and that of ST sample is 11.695 mg. Heating rate was
maintained at 10 oC/min in N2 atmosphere. Fig.4.3 shows the TG curve
recorded for BT crystal. Here the decomposition starts at 280oC. The first
stage of decomposition continues up to 300 oC, leading to the
116
13
11
10
9
8
0
200
400
600
800
0
Temperature ( C)
Fig.4.3. TG curve of BT crystal.
12
11
10
Weight (mg)
Weight (mg)
12
9
8
7
6
5
0
200
400
600
0
Temperature ( C)
Fig.4.4. TG curve of ST crystal.
800
117
formation of barium tartrate monohydrate. The second stage of
decomposition starts at 310oC resulting in the formation of barium oxalate
monohydrate (BaC2O4. H2O). In the third stage between 345oC and 380oC
Barium oxalate monohydrate eliminates water molecules and decomposes
into Barium carbonate (BaCO3).
Fig.4.4 shows the TG curve recorded for ST crystal. Here the
decomposition starts at 140 oC. The first stage of decomposition continues
up to 220 oC, leading to the formation of strontium tartrate monohydrate.
The second stage of decomposition starts at 250oC resulting in the
formation of strontium oxalate monohydrate (SrC2O4. H2O). In the third
stage between 340oC and 400oC strontium oxalate monohydrate
eliminates water molecules and decomposes into strontium carbonate
(SrCO3). The results of both BT and ST crystals are quite similar to those
reported [15, 28and29].
IV.2.1.3 FTIR Studies
The FTIR spectra are recorded in the frequency range
corresponding to 100-4000 cm –1. The FTIR spectra of BT and ST crystals
are shown in Fig.4.5 and Fig.4.6 and their observed vibrational data and
corresponding assignments are given in Tables 4.1and4.2 respectively.
The results are found to be in good agreement with that of tartrates
reported [11].
118
100
Transmission (%)
80
60
40
20
0
500
1000
1500
2000
2500
3000
W a v e N u m b e r (c m
-1
3500
4000
)
Fig.4.5. The FTIR spectrum of BT crystal.
100
Transmission (%)
80
60
40
20
0
0
500
1000
1500
2000
2500
3000
W ave N u m b er (cm
3500
-1
)
Fig.4.6. The FTIR spectrum of ST crystal.
4000
4500
Table. 4.1. The observed
vibrational data and their
assignments of BT crystal.
Wave number
(cm –1)
Assignments
512.02
δ (CO2)
538.08
δ(CH)
617.51
δ(CH)
691.94
δ(CH)
754.08
δ (CO2)
799.51
δ (CO2)
837.70
ν (CC)
873.78
ν (CC)
921.05
ρr (CH)
1006.52
ν (CC)
1080.08
ν (CO)
1136.42
ν (CO)
1218.54
δ(OH)
1256.57
ν(CH)
1301.62
δ(CH)
1345.07
δ(CH)
1392.81
δ(OH)
1599.68
νas (CC)
2676.05
ν(CH)
2736.27
νs (CH)
2819.07
νas (CH)
2876.28
νs (OH)
3105.31
νas (OH)
Table. 4.2. The observed
vibrational data and their
assignments of ST crystal.
Wave number
(cm –1)
478.20
559.36
635.95
706.76
844.95
934.48
995.04
1079.33
1129.64
1233.07
1267.56
1280.34
1314.05
1370.58
1425.65
1478.77
1601.20
2262.82
2700.57
2761.86
2917.14
3171.98
3386.19
3535.10
Assignments
δ (CO2)
δ (CO2 + ρt OH)
ρr (CH)
ρr (CH)
ν (CO)
ν (CO)
ν (CO)
νs (CO)
ν as( CO)
ρr (CC)
νs (CC)
ν(CC)
δ(OH)
δ(CH)
δ(CO2)
νs (CO2)
ν as (OCO)
νs (C≡C)
δ(CH)
δ(CH)
νs(OH)
ν as(OH)
νs(OH)
ν as(OH)
[δ-Bending mode, ν- Stretching mode,
ρr - Rocking mode, ρt - Twisting mode,
νas- Asymmetric stretching mode and
νs -Symmetric stretching mode.].
120
IV.2.2 Diffused Reflectance Spectroscopy
Optical band gap (Eg) is measured from Diffused Reflectance
Spectroscopy (DRS). Diffused Reflectance Spectrogram for BT and ST
crystals with percentage of reflectivity R versus wavelength λ [30] are
given in Fig.4.7 (a) and Fig.4.8 (a). Eg is found by extrapolating the
straight line graph of {(k/s)hυ}2 versus hυ in Fig.4.7 (b) and Fig.4.8 (b)
at k = 0 and is found to be 5.49 eV for BT crystal and 5.46 eV for ST
crystal (where k is the absorption coefficient and s is the scattering
coefficient). Wide band gap compounds are especially promising for light
emitting devices in the short wavelength region of visible light.
60
90
80
50
(a)
70
60
%R
[(k/s) hν]2
40
30
50
40
30
20
20
0
500
1000
1500
2000
2500
λ (nm)
10
(b)
0
0
1
2
3
4
5
6
7
hν (eV)
Fig.4.7. (a). Reflectivity R versus wavelength λ, (b). {(k/s)hυ}2 versus hυ
of BT crystal.
121
100
90
80
70
80
(a)
[(k/s) hν]2
%R
60
60
50
40
30
40
20
10
0
500
1000
1500
2000
2500
λ (nm)
20
(b)
0
0
1
2
3
4
5
6
7
hν (eV)
Fig.4.8. (a). Reflectivity R versus wavelength λ, (b). {(k/s)hυ}2 versus hυ
of ST crystal.
IV.2.3 Electro-Optical Studies
IV.2.3.1 Photoconductivity Studies
Photoconductivity (PC) is the increase in the electrical
conductivity of the material because of additional carriers created due to
optical absorption. The important processes involved in PC are
absorption, photo generation and recombination. The study of PC of semi
conducting materials has gained industrial importance owing to its use in
PV devices [31]. The samples are annealed to various temperatures such
as 50 oC, 100oC, 150 oC and 200oC. As the PC cell is exposed to excitation
source, the photocurrent increases with time and reaches saturated value.
(Saturated value of photocurrent of sample is measured for each case
separately). A representative plot of the time dependence of Photocurrent
122
(primary photocurrent) of both tartrate crystal samples annealed at 100oC
is shown in Fig.4.9.
2400
ST
Photo Current (nA)
2000
1600
1200
BT
800
400
0
0
5
10
15
20
Time (mins)
Fig.4.9. Variation of photocurrent of BT and ST crystals with respect to
time.
Saturated photocurrent increases as annealing temperature of the
samples is increased and obtained maximum for B T and S T crystals
annealed at 100oC and then starts decreasing as annealing temperature
increases which is shown in Fig.4.10. The main purpose of annealing is to
increase the crystallanity of the crystals, which produces an increment in
photocurrent [32]. Increase in saturated photo current with annealing
temperature up to 100oC can be explained on the basis of defects or
impurity effect in the material. Imperfections having high probability for
capturing the minority carriers with subsequent small probability for
capturing a majority carrier increase the photosensitivity [33].
123
2400
ST
Photo Current (nA)
2000
1600
1200
BT
800
400
0
50
100
150
200
o
Temperature ( C)
Fig.4.10. Variation of saturated photocurrent of BT and ST ccrryyssttaallss
with annealing temperature.
The decrease in saturated photo current when annealed beyond 100oC
is due to the increase in the disorders [32]. Also imperfections acting as
efficient recombination centers, decrease the photosensitivity [33].
It is found that saturated value of photocurrent increases with
increasing intensity of excitation (Fig.4.11) and also with increasing
applied voltage (Fig.4.12) for both crystals. When the intensity of light
and the applied voltage increases, more and more charge carriers reach at
the respective electrodes and the photocurrent increases [33]. The nonlinearity in Figs. (4.11 and 4.12) represents the dependence of saturated
photo current (secondary photocurrent) on the intensity of excitation and
applied voltage. The variation in saturated photo current can be expressed
in terms of a two-centre model. According to this model when the hole
demarcation level is lowered through the level of sensitizing centers by
increasing the light intensity at a constant temperature, the photo sensitivity
124
increases as the electron life time increases, giving rise to super-linear or sublinear photoconductivity [33].
3000
Photo Current (nA)
ST
2500
2000
1500
BT
1000
500
40
60
80
100
120
140
160
2
Intensity (mW/cm )
Fig.4.11. Variation of saturated photocurrent of BT and ST crystals with
intensity of light.
Fig.4.13 depicts the variation of the saturated value of photocurrent
with concentration of the dopant Mn, for BT crystal and that of ST crystal
at room temperature. As the concentration increases from 0.5M to 5M, the
charge carriers also increase and hence photocurrent increases. It is
observed that photocurrent reaches maximum at 1.5M concentration of
Mn. At this concentration the charge concentration seems to be optimum
for better PC [33]. As the concentration of Mn increased from 1.5M,
photocurrent is decreased due to concentration quenching.
125
3600
ST
Photo Current (nA)
3000
2400
1800
BT
1200
600
0.2
0.3
0.4
0.5
0.6
0.7
0.8
V (volts)
Fig.4.12. Variation of saturated photocurrent of BT and ST crystals with
applied voltage.
350
Photo Current (nA)
300
250
200
ST
150
100
BT
50
0
1
2
3
4
5
Concentration (molar)
Fig.4.13. Variation of saturated photocurrent of BT and ST crystals at
different concentrations of the dopant (Mn).
126
IV.2.3.2 Photovoltaic Studies
Photovoltaic (PV) effect is involved, when absorption of radiation
by a material causes the appearance of a p.d. between the two portions of
the material. The PV effect is essentially a minority carrier phenomenon
[34]. The photovoltage increases with time and reaches saturated value as
the PV cell is exposed to excitation source. (Saturated value of
photovoltage of samples is measured for each case separately). A
representative plot of the time dependence of photovoltage of BT and ST
crystals annealed at 100oC is shown in Fig.4.14.
350
ST
Photo Voltage (mV)
300
250
200
BT
150
100
50
0
-2
0
2
4
6
8
10
12
14
16
Time (mins)
Fig.4.14. Variation of photovoltage of BT and ST crystals annealed at
100oC with respect to time.
As the annealing temperature increases the saturated photovoltage
increases and reaches maximum, for both the samples annealed at 100 oC
(Fig.4.15) and then start decreasing as annealing temperature increased
beyond 100 oC. This increase in photovoltage with annealing temperature
up to 100oC and the decrease in PV beyond 100oC of both samples can be
explained in a way similar to that of its PC variation (section IV.2.3.1).
127
350
Photo Voltage (mV)
300
ST
250
200
150
BT
100
50
20
40
60
80
100
120
140
160
180
200
220
o
Temperature ( C)
Fig.4.15. Variation of saturated photovoltage of BT and ST crystals at
different temperature.
Saturated value of the photovoltage of BT and ST crystals also
increases with increasing intensity of excitation (Fig.4.16). With the
increase in intensity, more and more charge carriers are separated at the
respective electrodes and hence the photovoltage increases. Fig.4.17
depicts the variation of the saturated photovoltage of BT and ST crystals
with concentration of the dopant Mn at room temperature and it is
observed that saturated photovoltage is maximum at 1.5M concentration
of Mn. PV increases on increasing the concentration of Mn and reaches
the maximum at 1.5M concentration and decreases further on increasing
the concentration. The increase in photovoltage with concentration of Mn
is due to enhancement of charge carrier concentration and decrease is due
to the concentration quenching and increase of amorphous phase.
128
ST
350
Photo Voltage (mV)
300
250
200
BT
150
100
40
60
80
100
120
140
160
2
Intensity (mW/cm )
Fig.4.16. Variation of saturated photovoltage of BT and ST crystals at
different intensities of light.
220
200
Photo Voltage (mV)
180
160
140
ST
120
100
BT
80
60
40
0
1
2
3
4
5
Concentration (molar)
Fig.4.17. Variation of saturated photovoltage of BT and ST crystals at
different concentrations of the dopant (Mn).
IV.2.3.3 Electroluminescence Studies
Electroluminescence (EL) is the phenomenon of the emission of
electro magnetic radiation from condensed matter, subjected to an external
electric field. In order to convert the electrical energy from an applied
129
potential difference in a crystal at ordinary temperature into visible or near
visible radiation, three distinct steps are involved. (i) Excitation of crystals by
applied electric field to an energy state at least few electron volts above the
ground state. The excited state can be an ordinary conduction state of the
crystal, an excited state of an impurity system or a state of high kinetic
energy within the conduction or valence band. The excitation process may be
by means of injection of minority carriers, valence electrons from field
ionization of host atoms or impurities, or acceleration of charge carriers
within a band to optical energies. (ii) Transfer of the excitation energy
through the crystals to a region where de-excitation can occur with large
probability for radiative transition. The energy transport can be by charge
carriers, excitation migration or by resonance transfer. (iii) Radiative
de-excitation thus involves localized states of impurity systems; inter band
transition or intraband transitions [35]. Various combination of these
excitation, energy transport and emission processes can occur in actual
crystals [36].
The experimental AC/DC powder EL cells used for the present
investigation is essentially a parallel plate condenser with the powdered
crystal between a transparent conducting SnO2 film and a copper plate. No
binder is used for enhancing the dielectric strength. The light emitted from
the EL cell on application of suitable voltage was detected by the PMT and
the corresponding current is measured with the help of a digital
nanoammeter.
130
IV.2.3.3.1 AC Electroluminescence
In the AC EL process, the electrons and holes are injected into the host
material from both the electrodes and also from the defects on the application
of the field. The electrons from the donor centers or from the intrinsic electron
traps are freed by the electric field and they enter the conduction band, where
they are accelerated by the positive half cycle of the field, until they gain
sufficient kinetic energy to ionize or excite activator centers by the elastic
impact. In the reverse half cycle of the field, these electrons recombine with the
luminescence centers with emission of characteristic radiation. This
mechanism corresponds to the impact ionization of the activator centers [3740]. If the next pulse is of the same polarity as the preceding one, the system is
already partially polarized, so that the light out put is necessarily reduced, due
to the lower internal electric field and the fewer electrons trapped at the
defects. Luminescent centers formed at the cracks, voids, dislocations and
stacking faults and provide sites for the precipitation of the excess carriers if it
is produced.
The integrated light intensity, (brightness) B, produced during each
cycle of the field is accurately given by the expression (equation 1.1)
B= A exp (-C/V1/2)
→ (4.1)
where V is the applied voltage, A and C are constants independent of the
voltage. The constant A describes the source of charge carriers
responsible for the excitation process while C is a complex quantity,
which relates to the local field intensity. This expresses that the
mechanism of excitation is acceleration- collision type. Log (B) versus
V-1/2, of BT electroluminor is given in Fig.4.18 and that of ST
electroluminor is given in Fig.4.19. Linearity of the plots holds the above
131
relation. The photoelectronic phenomena are much influenced by
presence of defects in the original lattice [33].
1.0
0.5
Log (B) (Arb. Units)
0.0
-0.5
-1.0
-1.5
-2.0
0.036
0.038
0.040
0.042
0.044
1/2
0.046
0.048
0.050
0.052
-1
1/V (volts )
Fig.4.18. Plot of Log (B) Vs. V-1/2 of BT crystal.
1.0
0.8
Log (B) (Arb. Units)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0.036
0.038
0.040
0.042
1/2
0.044
0.046
0.048
0.050
0.052
-1
1/V (volts )
Fig.4.19. Plot of Log (B) Vs. V-1/2 of
ST crystal.
EL Intensity (Arb.units)
15
2M
1.5M
1M
3M
4M
0.5M
5M
undoped
10
5
0
350
400
450
500
550
600
650
700
750
800
850
AC Voltage
Fig.4.20. AC EL of BT crystal at different concentrations of Mn.
40
1.5M
EL Intensity (Arb.units)
32
2M
24
1M
3M
4M
5M
0.5M
undoped
16
8
0
350
400
450
500
550
600
650
700
750
800
850
AC Voltage
Fig.4.21. AC EL of ST crystal at different concentrations of Mn.
AC EL Brightness as a function of applied a.c voltage of both
electroluminors at different concentrations of the dopant (Mn) is given in
Fig.4.20 and Fig.4.21 respectively. EL intensity increases with increasing
Mn concentration and reaches the maximum at 2 M then decreases on
increasing concentration due to concentration quenching.
133
EL Intensity (Arb.units)
16
12
100 C
0
50 C
8
150 C
0
0
Room Temp
4
0
200 C
0
400
450
500
550
600
650
700
750
800
850
AC Voltage
Fig.4.22. AC EL of T crystal at different annealing temperature.
0
100 C
12
0
EL Intensity (Arb.units)
50 C
0
150 C
8
RoomTemp
0
200 C
4
0
400
450
500
550
600
650
700
750
800
850
AC Voltage
Fig.4.23. AC EL of ST crystal at different annealing temperature.
AC EL intensity of the two electroluminors at different annealing
temperatures is given in Fig. 4.22 and Fig.4.23. EL intensity increases
with increasing annealing temperature (crystallanity increases) and
reaches the maximum at 1000C then decreases on increasing annealing
temperature due to the increase in the amorphous phase and disorders.
The overall AC EL emission of both electroluminors is bluish.
134
Fig.4.24. AC EL emission photograph of BT crystal.
Fig.4.25. AC EL emission photograph of ST crystal.
AC EL emission photographs of the two electroluminors taken
with a digital camera are given in the figures, Fig. 4.24 for BT crystal and
Fig. 4. 25 for ST crystal.
IV.2.3.3.2 DC Electroluminescence
On the initial application of a DC voltage, a high current flows and
no light is observed. This is because the high conductivity of the phosphor
surface and the interparticles themselves. At a critical power density, the
current falls and light emission appears at the positive electrode. An
increase in applied voltage produces a temporary rise in current density
but this falls rapidly and the brightness increases. This is termed as
forming process. The forming process does not occur simultaneously over
the whole of the panel area but usually begins where the shortest current
path that exists between the electrodes. This represents the region of
135
higher density. When forming takes place, the resistance of the phosphor
increases and the current density falls. This increases the current density in
adjacent unformed areas and forming will take place at this region. Thus the
formed region will spread across the panel area until a complete plane of high
resistance phosphor develops between the electrodes. These devices, except
when pulsed, are invariably operated at a final forming voltage. On continuing
the increase of the voltage, it leads to a further increase in resistance of the
panel. Practically all the applied voltage is then dropped across the formed
region and EL emission is obtained from the formed layer.
3.0
2M
EL Intensity (Arb.units)
2.5
1.5M
1M
3M
4M
0.5M
5M
undoped
2.0
1.5
1.0
0.5
0.0
450
500
550
600
650
700
750
800
DC Voltage
EL Intensity (Arb.units)
Fig.4.26. DC EL of BT crystal at different concentrations of the dopant (Mn).
5
1.5M
4
2M
3
1M
3M
4M
5M
0.5M
undoped
2
1
0
350
400
450
500
550
600
650
700
750
800
850
DC Voltage
Fig.4.27. DC EL of ST crystal at different concentrations of the dopant (Mn).
136
DC EL Brightness as a function of applied d.c voltage of B T
electroluminor at different concentrations of the dopant (Mn) is given in
Fig.4.26 and that of S T electroluminor is given in Fig.4.27. EL intensity
increases with increasing Mn concentration and reaches the maximum at
2 M then decreases on increasing concentration due to concentration
quenching.
3
0
EL Intensity (Arb.units)
100 C
2
0
50 C
0
150 C
Room Temp
1
0
200 C
0
500
550
600
650
700
750
800
DC Voltage
Fig.4.28. DC EL of BT crystal at different annealing temperature.
0
2.0
100 C
1.5
50 C
0
150 C
EL Intensity (Arb.units)
0
Room Temp
1.0
0
200 C
0.5
0.0
400
450
500
550
600
650
700
750
800
850
DC Voltage
Fig.4.29. DC EL of ST crystal at different annealing temperature.
137
DC EL intensities of both electroluminors at different annealing
temperatures are given in Fig.4.28 and Fig.4.29. EL intensity increases
with increasing annealing temperature and reaches the maximum at
1000C, then decreases on increasing annealing temperature due to the
increase in the amorphous phase and disorders. The overall DC EL
emission of the electruluminors is also a bluish one. DC EL emission
phtographs of the two electroluminors taken with a digital camera are
given in the following figures (Fig 4.30 for BT and Fig 4.31 for ST
crystals).
Fig.4.30. DC EL emission photograph of BT crystal.
Fig.4.31. DC EL emission photograph of ST crystal.
IV.2.3.4 Photoluminescence Studies
Photoluminescence (PL) is the spontaneous emission of light from
a material under optical excitation. PL excitation and emission spectra are
recorded in the wavelength range of 400-650 nm by using Flourimeter.
Emission peaks of undoped BT crystals are observed around emission
wavelength λem = 409 nm (see Fig.4.32), 508 nm (see Fig.4.34), and 610
nm (see Fig.4.36) with excitation wavelength λexc = 372 nm. Emission
peaks of undoped ST crystals are observed around λem = 417 nm (see
138
Fig.4.38), 513 nm (see Fig.4.40), and 620 nm (see Fig. 4.42), with
excitation wavelength λex = 379 nm.
Excitation and emission spectra were obtained by changing the
excitation wavelength under a fixed emission wavelength and vice versa.
PL Excitation spectra are taken for B T crystals with their λem‘s and they
show a band at 372 nm as shown in Fig.4.32. PL Excitation spectra are
taken for ST crystals with their λem’s and they show a broad band at 379
nm as shown in Fig.4.38.
The highest resolution used is 0.1nm for
excitation and 0.3nm for the emission. The normal luminescent bands are
1200000
4M
5M
undoped
600000
360
emission
1800000
excitation
PL Intensity (Counts/sec)
2400000
370
380
390
400
410
420
λ (nm)
Fig.4.32. PL emission at λem = 409 nm with λexc =372 nm of BT crystal at different
concentrations of Mn.
attributed to interaction between emission centers and the host crystal
lattice [41]. Thus the luminescent emission depends upon the nature of the
activator and its concentration in the host lattice. At higher concentration
the activator atoms destroy the matrix, which results in quenching of
emission [42]. The presence of traps also influences the PL emission. The
emission may be delayed due to their presence [43].
139
PL Intensity (Arb.units)
15000000
14400000
13800000
13200000
1.5M
2M
3M
1M
4M
12600000
12000000
400
420
λ (nm)
Fig.4.33. PL emission at λem = 409 nm spectra of BT crystal at different
concentrations of Mn.
PL Intensity (Arb.units)
1000000
800000
600000
5M
400000
0.5M
undoped
500
520
λ (nm)
Fig.4.34. PL emission at λem = 507 nm spectra of BT crystal at different
concentrations of Mn.
PL emission spectra of B T photoluminor doped with Mn are
given in Figs. (4.32 - 4.37). Two separate figures are given to both the
photoluminors for each emission wavelength is due to large difference
in peak intensity on doping. The peak at 409 nm corresponds to
2
D 5/2 → 2 P 0
3/2
transitions (6d - 6p) of Ba [44] and the peak at 610
140
nm correspond to 2P0
3/2→
2
D
5/2
transition (6p - 5d) of Ba [44]. The
luminescent emission at 510 nm is originated because of the presence of
some defects in the host lattice, which produces certain impurity site or
centres during the preparation [33].
10200000
PL Intensity (Arb.units)
9600000
9000000
8400000
1.5M
2M
3M
1M
7800000
7200000
4M
6600000
500
520
λ (nm)
Fig.4.35. PL emission at λem = 507 nm spectra of BT crystal at different
concentrations of Mn.
PL Intensity (Arb.units)
700000
600000
500000
400000
5M
300000
0.5M
undoped
200000
600
620
λ (nm)
Fig.4.36. PL emission at λem = 610 nm spectra of BT crystal at different
concentrations of Mn.
141
PL Intensity (Arb.units)
8400000
7800000
7200000
6600000
1.5M
2M
3M
1M
4M
6000000
5400000
600
620
λ (nm)
Fig.4.37. PL emission at λem = 610 nm spectra of BT crystal at different
concentrations of Mn.
4M
emission
14000000
excitation
PL Intensity (Arb.units)
12000000
10000000
8000000
5M
6000000
4000000
undoped
2000000
0
370
380
390
400
410
420
430
440
450
λ (nm)
Fig.4.38. PL emission at 417 nm and 440 nm with λexc = 379 nm of ST crystal at
different concentrations of Mn.
142
19000000
1.5M
1M
PL Intensity (Arb.units)
18000000
17000000
2M
16000000
3M
15000000
0.5M
14000000
380
400
420
440
460
480
500
λ (nm)
Fig.4.39. PL emission at λem = 417 nm and 440 nm spectra of ST crystal at
different concentrations of Mn.
7000000
PL Intensity (Arb.units)
6000000
5000000
4M
5M
4000000
undoped
3000000
2000000
480
490
500
510
520
530
540
550
560
λ (nm)
Fig.4.40. PL emission at λem = 513 nm spectra of ST crystal at different
concentrations of Mn.
143
14000000
PL Intensity (Arb.units)
12000000
10000000
1.5M
1M
8000000
2M
6000000
3M
0.5M
4000000
480
490
500
510
520
530
540
550
560
λ (nm)
Fig.4.41. PL emission at λem = 513 nm spectra of ST crystal at different
concentrations of Mn.
PL emission spectra of S T photoluminor doped with Mn are given
in Figs. (4.38 - 4.43). The peak at 417 nm corresponds to
2 0
P
1/2→
2
S 1/2
transition (4d - 5s) of ionized Sr [44]. A new broad peak is observed
around 440 nm on doping which is due to the fact that the energy levels of
host lattice is affected by that of the dopant Mn. This wavelength
corresponds to the a4 D
7/2→
a6 S
5/2
transition (3d6(5d) 4s - 3d54s2) of
Mn [44]. The other two luminescent emissions are originated because of
the presence of some defects in the host lattice as mentioned in the above
paragraph.
The peak positions are not much affected on doping for both the
photoluminors. PL intensity increases with increasing Mn concentration
for both photoluminors. The increase in intensity of the peaks on doping
is due to transfer of energy by the dopant through the levels of the host
crystals. PL intensity reaches the maximum at 1.5 M concentration and
144
then decreases on increasing concentration due to concentration
quenching as reported [45].
PL Intensity (Arb.units)
6000000
5500000
5000000
4500000
4000000
600
4M
5M
undoped
605
610
615
620
625
630
635
640
λ (nm)
Fig.4.42. PL emission at λem = 620 nm spectra of ST crystal at different
concentrations of Mn.
11000000
PL Intensity (Arb.units)
10000000
9000000
1.5M
1M
8000000
2M
3M
0.5M
7000000
6000000
600
605
610
615
620
625
630
635
640
λ (nm)
Fig.4.43. PL emission at λem = 620 nm spectra of ST crystal at different
concentrations of Mn.
145
1400000
PL Intensity (Counts/sec)
1300000
1200000
1100000
1000000
0
100 C
0
50 C
0
150 C
0
200 C
Room Temp
900000
800000
700000
400
420
λ (nm)
Fig.4.44. PL emission at λem = 409 nm spectra of BT crystal at different
annealing temperatures.
PL emission spectra of B T photoluminor at different annealing
temperatures are given in Figs. (4.44 - 4.46) and that of Strontium
Tartrate photoluminor are given in
Figs. (4.47 - 4.49). PL intensity
increases with increasing annealing temperature and reaches maximum at
1000C, then decreases on increasing annealing temperature for both the
photoluminors.
PL Intensity (Counts/sec)
5000000
4000000
3000000
0
100 C
0
50 C
0
150 C
0
200 C
2000000
1000000
Room Temp
0
500
520
λ (nm)
Fig.4.45. PL emission at λem = 507 nm spectra of BT crystal at different
annealing temperatures.
146
PL Intensity (Counts/sec)
4000000
3000000
0
100 C
2000000
0
50 C
0
150 C
1000000
0
200 C
Room Temp
0
600
620
λ (nm)
Fig.4.46. PL emission at λem = 610 nm spectra of BT crystal at
different annealing temperatures.
14000000
PL Intensity (Arb.units)
12000000
10000000
8000000
0
50 C
0
100 C
0
150 C
0
200 C
6000000
4000000
2000000
Room Temp
400
410
420
430
440
450
λ (nm)
Fig.4.47.
PL emission at λem = 417 nm spectra of ST crystal at different
annealing temperatures.
147
8000000
PL Intensity (Arb.units)
7000000
6000000
0
50 C
0
100 C
0
150 C
0
200 C
5000000
4000000
Room Temp
3000000
490
500
510
520
530
540
λ (nm)
Fig.4.48. PL emission at λem = 513 nm spectra of ST crystal at
different annealing temperatures.
8000000
PL Intensity (Arb.units)
7200000
0
6400000
50 C
0
100 C
5600000
150 C
0
200 C
0
4800000
4000000
600
Room Temp
610
620
630
640
λ (nm)
Fig.4.49. PL emission at λem = 620 nm spectra of ST crystal at
different annealing temperatures.
Increase in PL intensity with temperature up to 100oC can be
explained on the basis of impurity effect in the material. Photosensitivity will
be decreased if imperfections act as efficient recombination centers. But
photosensitivity will be increased if imperfections are having higher
probability for capturing the minority carriers than the majority carrier. The
decrease in PL intensity beyond 100oC is due to the increase in the expected
amorphous nature and disorders on increasing temperature.
IV.3 Conclusion
Conditions for the growth of BT and ST crystals by controlled
ionic diffusion of reactants through silica hydro gel have been optimized.
The material confirmation was done by carrying out XRD, TGA and
FTIR spectroscopy. In this chapter we have presented the electro-optical
properties of the Mn doped BT and ST crystals. The XRD analysis shows
crystalline nature of the BT and ST crystals. On Mn doping a few extra
peaks were detected with lower intensities but the intensity of the
prominent peak is decreased and that of certain other peaks were
increased. This is due to the change in the atomic density of Mn in these
planes.
The results of TGA and FTIR studies of BT and ST crystals were
quite similar to those reported for tartrates. Band gap (Eg) was measured
from Diffused Reflectance Spectroscopy (DRS) and is found to be
5.49 eV for BT crystals and 5.46 eV for ST crystals.
PC/PV of the crystals were studied as a function of intensity of
light, annealing temperature and concentration of the dopant. PC variation
as function of applied voltage is also studied. It is found that saturated
value of photocurrent and photovoltage increases with increasing intensity
149
of excitation for BT and S T crystals. Photocurrent also increases with
increase in applied voltage. It is found that both the materials are more
photosensitive at an annealing temperature of 100oC. On Mn doping PC/PV
increases and shows maximum value at 1.5 M concentration. S T crystal is
found to be more photoconducting and shows more photovoltaic effect than
B T crystal.
AC/DC Electroluminescence Brightness increases with the applied
electric field. The brightness of a powder EL cell increases non-linearly as
excitation voltage is increased. AC/DC EL study on Brightness as a
function of applied AC/DC voltage for both crystals at different
concentrations of the dopant (Mn) and at different annealing temperatures
is carried out. EL intensity increases with increasing Mn concentration and
reaches the maximum at 2 M concentration, then decreases on increasing
concentration. EL intensity increases with increasing annealing temperature
and reaches the maximum at 1000C, then decreases on increasing annealing
temperature. The EL phenomenon is very much influenced by presence of
defects in the original crystal lattice. The overall AC/DC EL emissions of
both electroluminors are bluish one. The AC/DC EL Brightness intensity of
S T crystal reveals that it is a better electroluminor than B T crystal.
The Photoluminescence emission is observed for BT crystal around
λem = 409 nm, 508 nm, and 610 nm with λexc = 372 nm and that for ST
crystal is observed at λem = 417 nm, 440 nm, 513 nm and 620 nm with λex
= 379 nm. The peak of BT crystal at 409 nm corresponds to 2D 5/2 → 2P0 3/2
transitions of Ba and the peak at 610 nm correspond to 2P0
transition of Ba.
3/2→
2
D
5/2
150
The peak of ST crystal at 417 nm corresponds to 2P0
1/2→
2
S
1/2
transition of Sr. A new broad peak is observed around 440 nm on doping
which is due to the fact that the energy levels of host lattice is affected by
that of the dopant Mn. This wavelength corresponds to the a4 D 7/2→ a6S 5/2
transition of manganese. But in BT system Mn doping creates no peak
corresponding to it. The luminescent emission at 510 nm of BT crystal and
the emissions at 513 nm and 620 nm of ST crystal are originated because of
the presence of defects in the host lattice. The emission peak intensity
reveals that both the materials could be used as a scintillator phosphor. PL
intensity increases for both crystals with increasing Mn concentration and
annealing temperature and reaches the maximum at 1.5 M and 1000C
respectively. The PL intensity peaks reveals that ST crystal is a better
photoluminor than B T crystal.
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