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Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013
Thermoluminescence, Optical Absorption and
XRD Studies on Potassium Iodide Doped Zinc
Nitrate Hexa Hydrate Single Crystals
Dr.S. Bangaru; 2Mrs.P.Revathi; 3Dr. G. Muralidharan
1*
Department of Physics, Arignar Anna Government Arts College, Namakkal-637002, India;
1*
Department of Physics, Vivekanandha College for Women, Namakkal-637002, India;
2
Department of Physics, Gandhigram Rural University, Gandhigram – 624 302, India;
3
[email protected]; [email protected]; [email protected]
1*
Abstract
The
optical
absorption,
Photoluminescence
(PL),
Thermoluminescence (TL) characterization of Zn (NO3)
26H2O
doped KI single crystals prepared by slow
evaporation solution growth process is reported. The PL
results show the typical emission bands centered at 424 and
497 nm excited with light of 320 nm. The TL signal obtained
after exposure to β and γ irradiation, indicates a high TL
sensitivity of the samples and that the presence of the
dopant ions induces changes in the trapping process and the
charge recombination efficiency. The functional groups
present in the crystal confirms that using FT-IR Technique
Optical absorbance shows meagre absorption from the entire
visible region. The high efficiency of the PL and TL suggests
a good potential of this crystal as β and γ irradiation
dosimetry as well as for photonics application.
Keywords
Photoluminescence; Thermoluminescence; KI and Zn (NO3)2 6H2O
Introduction
Thermoluminescence (TL) dosimetry materials based
on alkali halides have been studied for the past two
decades. Of the various materials studied so far, TL
and optical properties are modified in alkali halide
crystals when doped with divalent impurities which
give rise to new optical absorption bands, on the
energy level of excited states of such ions is strongly
affected by the kind of host matrix and the crystalfield strength of crystalline surrounding them. While
most of the impurities undergo a valence change on
irradiation, the divalent alkaline earth impurity ions
Ca, Ba and Sr (impurities having low second
ionization potential) are normally immune to it but
known only to perturb the F center configuration,
giving rise to a new series of electron excess centers
called Z centers. Recently, Z centers in alkali halide
40
crystals are known to offer several attractive features
for uses as media in holographic information storage
[Ruuskanen, 1981] and mode locking of high power
lasers [Demchuk, 1984]. So it will be very useful to
study in detail the nature of these centers indifferent
host matrices [Rodriguez et al., 2004, Sastry, 1985]. The
other is the improvement of our basic understanding
of the TL phenomena by an analysis of the TL glow
curves, PL emission spectra and the lattice parameters
for the alkali halides have a wide range, varying from
7.34Å for RbI to 4.03Å for LiF. This gives the
possibility of incorporating impurity ions of different
sizes in their lattices. Such analysis of TL glow peaks
with respect to their parameter like trap depth,
frequency factors, order of kinetics etc., are also
discussed [Bangaru and Muralidharan, 2009, Bangaru
and Muralidharan, 2010, Chandra et al., 1980]. Here
we report the results of some experiments on the
optical absorption, Photoluminescence (PL), TL glow
and XRD of γ-irradiated potassium iodide doped
Zn(NO3)26H2O singly crystals.
Experimental Methods
Single crystals of Zn (NO3)26H2O doped KI were grown
by slow evaporation solution growth technique. The
starting materials were synthesis by taking potassium
iodide and Zn (NO3)2 6H2O were dissolved. Distilled
water in the molar ratio 1:1 the starting material
started well for about 1 hour, and the starting solution
was filtered out and covered with perforated
polyethylene sheet and kept for evaporation, further,
the evaporation of solvent yielded the good quality
crystals for a period of 20 days. The grown crystals
were kept in an air tight container since it is reported
as air sensitive. The phase of the powder crystals was
Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013
confirmed by X-ray diffraction techniques using Cu –
Kα radiation.
TL measurements were done using Nucleonix make
TLD reader at linear heating rate of - 50C/S for dose
rate of γ–irradiation 250 Gy – Co60 and for β–
irradiation at 120 Gy – Sr90 – Υ90. The PL spectra were
recorded using Perkin Elmer – LS55 spectro
fluorometer. The absorption spectra were recorded
using Perkin Elmer Lambda 35UV – vis spectro photo
meter in the region 190 – 1100 nm.
Optical Absorption
The optical absorption spectra of: Zn (NO3)2 6H2O
doped KI single crystals before and after γ–irradiation
at room temperature are shown in fig(1). The
unirradiated crystals exhibited a sharp absorption at
194 nm, 220 nm, 237 nm and a small peak at 307 nm as
shown in fig.(1a) and 1 hour of γ-irradiated crystals
resulted in the formation of a characteristic F-band
centered at 600 nm in fig.(1b). The bands observed at
235, 258, 320 and 566 nm have been noted on irradiation
while the 234 and 258 nm band got reduced in intensity.
The 194 nm peak is due to the exciton absorption of
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the host matrix [Phol, 1938]. The Zn (NO3)2 6H2O ion
absorbs light in the UV region [Joshi et al., 1973] and is
confirmed by the 320 nm absorption band as shown in
fig.(1a) which is attributable to the absorption of the
Zn (NO3)2 6H2O ions. The absorption in other regions
is discussed along with a discussion on the
photoluminescence studies of the crystals. On Fbleaching, absorption at the F band shows a
broadening on the longer wave length side. The
broadening is indicative of the formation of new
species. In order to identify the species that have been
formed, the absorption in the F band region has been
normalized, taking the maximum of absorption as
unity and rescaling the other areas under the curve.
On γ–irradiation, there is no reduction in the intensity
in 320 nm absorption band but the 234 nm peaks
totally disappears on F-bleaching absorption around
the F band, showing a broadening on the lower wave
length side as shown in fig.(1).
This new absorption is attributable to the electron
centers perturbed by the Zn (NO3)2 6H2O ions. There
are other kinds of F centers that are formed when
another alkali ion or alio valent cation is introduced
into the crystal [Radhakrishna and Chowdari, 1972].
FIG (1). OPTICAL ABSORPTION OF ZN (NO3)2 6H2O DOPED KI (a) BEFORE Γ-IRRADIATION (b) AFTER 1
HOUR Γ-IRRADIATION (c) F-BLEACHING 2 MIN.
The well known perturbed F centers are the FA and
F2(z) centers. FA centers are formed when one of the six
surrounding alkali ions is replaced by a different alkali
ion of smaller size compared to the host cation [Pick,
1972]. Z centers are another type of perturbed F
centers. Here the perturbation is due to a divalent
cationic impurity and its associated cation vacancy (IV) complex. After suitable optical and/or thermal
treatment, as many as five types of (Z1 to Z5) perturbed
centers are reported to be formed [Radhakrishna and
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Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013
Chowdari, 1972]. Considering the fact that the new
absorption appears on F bleaching, it appears more
appropriate to assign the new absorption to an
electron excess center. The divalent ion perturbed F
centers, namely the Z center has been observed on the
longer wave length side of the F band. However, as
well in the present work, the new absorption is on the
higher wavelength side of the F band. In their work
[Vijayan et al., 1989] on Gd2+ doped KCl have reported
the formation of Z1 centers observed on the higher
wavelength side of the F band. Considering this, it
seems appropriate to assign the new absorption in Zn
(NO3)2 6H2O: KI to Z1 bands. The activation energies of
the two glow peaks of Zn (NO3)2 6H2O doped KI
crystals (Table 1) are close enough, suggesting
interpretation in terms of isoelectronic color centers in
several different local environments, for example, for F
centers with substitution cation impurities in the
neighborhood (Z centers of F centers).
Photoluminencence
The excitation spectra of Zn (NO3)2 6H2O doped KI
samples show in the spectral range studied a band
with maxima located at 356 nm and a shoulder at 375
nm as it can be seen in fig(2a). Excitation at these
maxima yielded similar emission spectra which peak
at 497 nm and a broad band at 424 nm. On exciting the
unirradiated crystals with 320 nm light, emission band
at 497 nm and a broad band at 424 nm were observed
in fig (2b). The emission spectrum for excitation at 320
nm yielded two bands at 424 and 497 nm (Fig 2b)
though no distinct absorption could be observed in
the optical absorption (the absorption in the region
below 320 nm showed a strong but broad absorption)
of Zn (NO3)2 6H2O doped KI crystals, and the
appearance of the excitation peak at 320nm indicated an
absorbing species. These excitations have confirmed the
presence of Zn (NO3)2 6H2O in its valent state.
FIG (2). PHOTOLUMINESCENCE OF ZN (NO3)2 6H2O DOPED KI (A). EXCITATION SPECTRUM
EMISSION AT 425 NM (B) EMISSION SPECTRUM EXCITATION AT 320 NM
Band shift and fluorescence energy have been observed
only in samples of Zn (NO3)2 6H2O: KI both excitation
and emission maxima shift about 5 nm towards longer
wave lengths. The separation between the maxima in
excitation spectrum and the splitting in the emission
spectrum were more pronounced in the doped samples
[Li. et al.,1996, Blasse and Grabmaier, 1994].
Tl Glow Curve
The TL glow curves of the Zn (NO3)2 6H2O doped KI
42
crystals recorded after 1 hour of β-irradiation is shown
in fig(3a). It is observed that Zn (NO3)2 6H2O doped KI
have a single peak around 2500C and a shoulder
around 1400C. The TL glow curves of the Zn (NO3)2
6H2O doped KI crystals recorded after 1 hour of γ irradiation is shown in fig(3b). It is observed that the
TL glow curves have a single peak around 250Oc and a
shoulder around 150 oC as shown in fig (3b). The TL
glow curves of the crystal were recorded after γ–
irradiation along with that for TLD 600 irradiated to
the same β–dose. It is noted that in Zn (NO3)2 6H2O
Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013
doped KI samples; there is only one TL peak around
2500C and a shoulder around 1250C. The high
temperature shoulder observed in the case of γirradiation, TL intensity was slightly increased. The
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low temperature shoulder observed in the case of γ–
irradiation and TL intensity decreased. This may be
because of the different interaction mechanism of Beta
and Gamma radiation in the crystals.
600
a
TL INTENSITY (arb.U)
500
b
400
300
200
100
0
0
50
100
150
200
250
300
350
400
o
TEMPERATURE( C)
FIG (3). TL GLOW CURVES OF ZN(NO3)2 6H2O DOPED KI (a). Β-IRRADIATED 1 HOUR (b)
AFTER 1 HOUR Γ-IRRADIATION
FIG (4). TL GLOW CURVES ZN (NO3)2 6H2O DOPED KI Β-IRRADIATED 1 HOUR
DECONVOLUTED
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Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013
Thermoluminescence
The glow curve for β–irradiated Zn (NO3)2 6H2O
doped KI and γ–irradiated Zn (NO3)2 6H2O doped KI
crystals are given in fig(4) and fig(5) respectively
(recorded at a heating rate 5oC/s of β-120Gy – Sr90 –
Y90/min and γ–ray 250 Gy-Co60). From the fig(4), it is
clear that for 5 min of β–irradiation the curve exhibits
a broad shoulder centered around (1430C) there by
making the 2630C peak sharper and dominant as
shown in fig.(4). The integrated light intensity is two
orders magnitude higher as compared to the samples
for similar doses of irradiation and heating rates as
shown in table(1). KI: ZnNo3 6H2O crystals under
similar (γ-irradiated) irradiation show glow peak at
127oC and 2580C and these have been attributed to the
decay of F centers. Fig.(4) could be analyzed in two
glow peaks with maxima at 1430C and 2630C. Table(1)
gives other relevant TL parameters for β– and γ–
irradiated have trap depths 0. 32, 0.11, 0.34 and 0.08 eV
respectively. This is the common observation in the
sense that a high temperature glow peak shows a
higher trap depth value. This behavior is similar to
that observed in the RbCl: Eu2+ Systems [Sastry et al.,
1981] overlapping glow peaks (main peak) can lead to
the broadening of the glow peak and consequently
appear to lower the value of trap depth and lift the
value of frequency factor [Bangaru, 2011, Sastry et al.,
1981c, Sastry, 1985].
Comparing the results of optical absorption, the low
temperatures peak in fig.(4) 1270C is attributed to Z1
centers. The same is confirmed by the reappearance
of the first glow peak on F-light bleaching
subsequent to partial thermal cleaning upto the first
glow peak. On γ–irradiated fig(5), trap parameters
have been calculated using Chen’s method [Chen,
1969].
FIG (5). TL GLOW CURVE ZN(NO3)2 6H2O: KI Γ-IRRADIATED 1 HOUR DECONVOLUTED
Modifying the method proposed by Halperin and
Braner[Halperin and Braner, 1960], Chen proposed an
easy and non-iterative way to calculate the trap
parameters like activation energy and frequency factor
[Chen, 1969]. The method consists of the
measurements of the total half-width[W], low
temperature-side half-width(ξ) and high temperatureside half-width(δ) of the glow peaks. The equation
deduced by Chen is Eα = (CαKT2g/α) – (bα(2KTg)),
where Eα activation energy and α can be either W= T2T1 (where T1-T2 are low and high maximum intensity
temperature).
ξ = T2-Tg (where Tg is the glow peak maximum
44
temperature) or δ = T2-Tg, K is Boltzmann’s constant.
The values of C and b are constant and given by Chen.
The glow peaks are deconvoluted using the software
“Peak fit” from which the values of T1, T2 and Tg are
determined and the trap parameters are calculated
and tabulated in Table(1). The values of the trap
parameters are comparable with the reports given by
[Chen, 1969, Demchuk, 1984]. The kinetic order
indicated that the peaks can be categorized as first
order or second order by a shape factor µg i.e. µg = δ/ω,
where δ is the high temperature half-width, ω is the
total half-width of glow peak. The glow shape factor is
µ = 0.42 for the first order and µ = 0.52 for the second
Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013
order glow peaks. The glow peaks have been
deconvoluted using the software “peak it” and results
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of TL parameters have been tabulated in the table (1) a
& b.
TABLE–1
(a) TL Parameter of Zn(NO3)2 6H2O: KI β– ray irradiated for 50C/Sec. heating rate of 120ºC/min
S.No
1
2
Peak Temp. in C
143
263
Intensity
98
372
Trap depth eV
0. 32
0.11
Frequency factor S (s-1)
3.32 × 106
2.22 × 108
order of kinetics s-1
I
I
(b) TL parameters of Zn(NO3)2 6H2O: KI γ–ray irradiated of 50C/Sec. heating rate of 250ºC/m
S.No
1.
2.
Peak Temp. in C
127
258
Intensity
175
451
Trap depth eV
0. 34
0.08
Xrd Spectrum
Fig.(6) shows the XRD patterns for combustion
synthesized Zn (NO3) 26H2O doped KI powders, both
as synthesized and annealed at 500oC for 30 min with
a commercial standard shown as a reference. The only
The determined unit cell parameters are in good
agreement with the standard values. The main peak of
the Tetragonal crystal system is centered at 30.2o and
corresponds to the crystalline plane with miller indices
of [114]. The XRD studies have been made to find the
crystalline structure of crystals and miller indices. The
pattern is found to match with the standard data
except for variations in intensities.
Frequency factor S (s-1)
2.56 × 104
5.87 × 105
order of kinetics s-1
I
I
identifiable phases present in the synthesized powders
were the [110, 112, 114, 202, 211, 213, 215, 222, 302] and
the lattice parameters a = 5.94 Å, b=c=18.13 Å were
found. The XRD peaks have been explained in terms
of the garnet structure according to the standard
JCPDS, CAS#7758-02-4.
Ft-Ir Analysis
The middle IR spectrum Zn(NO3)2 6H2O doped KI
Crystals are shown in fig. (7). In the high energy
region, there is a broad band between 3952 and 2408
cm-1 including N-H stretches at 463 and 1383 cm-1 and
aliphatic C-H stretch at 1763 cm-1. There are fine
structures at 1383 and 1763 cm-1 due to hydrogen
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Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013
cm-1 a band is seen due to stretching of CO group. Due
to stretching of CNH group, there are bands at 463 and
at 711 cm-1. The intense peaks at 3726 and 3840 cm-1 are
assigned to the asymmetrical and symmetrical
stretching modes of COO- respectively. This
observation clearly indicates the protonation of amine
nitrogen lone pair rather than the carboxylate group
by Zn(NO3)2 6H2O .
Transmittance(%)
bonding of NH3+ and =NH2+ groupings. There is a
combination band at 2070 cm-1, which may be assigned
to the combination of asymmetric NH2+ bending
vibration at 2408 cm-1 and torsional oscillation of the
NH2+ at 463 cm-1. The peaks at 1383 and 835 cm-1 are
assigned to C=O stretching vibrations of the
carboxylate group,. due to symmetrical plain bending
of CH3 bands appearing at 3095 and 3200 cm-1. At 2754
Wavenumber(cm-1)
FIG (7). FTIR SPECTRUM OF ZN(NO3)2 6H2O DOPED KI CRYSTALS
Conclusions
ACKNOWLEDGEMENTS
The Zn(NO3)26H2O ion displays its characteristic
absorption. Doped KI samples have been obtained by
precipitation method. XRD spectra showed a welldefined crystalline structure at 200oC. No other phase
has been observed upto 400oC. The measurement of
the FT-IR spectra of Zn(NO3)2 6H2O doped KI Crystals
provided a complete set of vibrational data that can be
used to determine and monitor the structural
vibrations. The PL results showed that the emission
spectra of Zn(NO3)26H2O ions were similar to those
obtained in bulk samples reported by others. The
presence of the dopant impurities modified the lattice
factor and in consequence the TL efficiency and the
calculated glow curve kinetic parameters. The low
temperature peak is attributed to thermal mobilization
of Z1 centers and the high temperature peak to the
thermal mobilization of F-centers.
The corresponding author *Dr.S.Bangaru gratefully
acknowledges the Radiation Safty Division, Indra
Gandhi Centre for Atomic Research, Kalpakkam, India
for providing experimental support.
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