Performance of crystal violet doped triglycine sulfate single crystals for optical and communication applications

Article abstract of DOI:10.1039/c5ce00703h

Single crystals of 0.01 and 0.02 mol% crystal violet dye doped triglycine sulfate (NH₂CH₂COOH)₃·H₂SO₄ (TGS) have been grown by a slow evaporation technique. Enhanced dielectric, optical, thermal and mechanical properties have been achieved by dye doping. Monoclinic structures showing particular coloring patterns and morphology change with dye concentration were obtained. Various functional groups and dye incorporation in the grown crystals were confirmed qualitatively by FTIR and Raman analysis. A significant increase in Curie temperature from 50 to 55 °C with a decrease in maximum permittivity has been observed. The UV-vis spectra showed an increased transmittance window and an increased optical band gap from 5.61 to 6.11 eV as a result of doping. An increase in the sharpness and intensity of the photoluminescence peak with a blue shift has been observed in doped crystals. The thermal stability and the decomposition temperature were found to increase by about 10 °C in 0.01 mol% dye doped TGS. The mechanical strength of the grown crystals was estimated by the Vickers microhardness test and was found to be high for dye doped TGS. A high piezoelectric charge coefficient d₃₃ of 16 pC N^-1 was observed for pure TGS which decreased as a result of the dye effect. The fabrication of a patch antenna was carried out after simulating its resonant frequency, making it suitable for telecommunication applications.

Full text of DOI:10.1039/c5ce00703h

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DOI: 10.1039/C5CE00703H.
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Title Page
Performance of crystal violet doped triglycine sulfate single crystals for
optical and communication applications
Nidhi Sinhaa,b, Sonia Bhandaria, Harsh Yadava, Geeta Raya, Sanjay Godaraa, Nidhi
Tyagia, Jyoti Dalala, Sonu Kumara and Binay Kumara*
aCrystal Lab, Department of Physics and Astrophysics, University of Delhi, Delhiꢀ110007, India
a,bDepartment of Electronics, SGTB Khalsa College, University of Delhi, Delhiꢀ110007, India
*Corresponding author: Dr. Binay Kumar
Email: b3kumar69@yahoo.co.in Fax: +91ꢀ011ꢀ27667061, Mobile: +91ꢀ9818168001
ABSTRACT
Single crystals of 0.01 and 0.02 mol% crystalꢀviolet dye doped triglycine sulfate
(NH2CH2COOH)3H2SO4 (TGS) have been grown by slow evaporation technique. Enhanced
dielectric, optical, thermal and mechanical behavior has been achieved by dye doping. Monoclinic
structures showing particular coloring pattern and morphology change with dye concentration were
obtained. Various functional groups and dye incorporation in grown crystals were confirmed
qualitatively by FTIR and Raman analysis. A significant increase in Curie temperature from 50 to
55 °C with decrease in maximum permittivity has been observed. UVꢀVis spectra showed an
increased transmittance window and increased optical band gap from 5.61 to 6.11 eV as a result of
doping. Increase in the sharpness and intensity of photoluminescence peak with a blue shift have
been observed in doped crystals. The thermal stability and the decomposition temperature were
found to increase by about 10 °C in 0.01mol% dye doped TGS. The mechanical strength of the
grown crystals was estimated by Vickers microhardness test and was found high for dye doped
TGS. High piezoelectric charge coefficient d33 of 16 pC/N was observed for pure TGS which
decreased as a result of dyeꢀeffect. The fabrication of patch antenna was carried out after
simulating its resonant frequency that makes it suitable for telecommunication applications.
1

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DOI: 10.1039/C5CE00703H
Performance of crystal violet doped triglycine sulfate single crystals for
optical and communication applications
Nidhi Sinhaa,b, Sonia Bhandaria, Harsh Yadava, Geeta Raya, Sanjay Godaraa, Nidhi
Tyagia, Jyoti Dalala, Sonu Kumara and Binay Kumara*
aCrystal Lab, Department of Physics and Astrophysics, University of Delhi, Delhiꢀ110007, India
a,bDepartment of Electronics, SGTB Khalsa College, University of Delhi, Delhiꢀ110007, India
*Corresponding author: Dr. Binay Kumar
Email: b3kumar69@yahoo.co.in Fax: +91ꢀ011ꢀ27667061, Mobile: +91ꢀ9818168001
ABSTRACT
Single crystals of 0.01 and 0.02 mol% crystalꢀviolet dye doped triglycine sulfate
(NH2CH2COOH)3H2SO4 (TGS) have been grown by slow evaporation technique. Enhanced
dielectric, optical, thermal and mechanical behavior has been achieved by dye doping. Monoclinic
structures showing particular coloring pattern and morphology change with dye concentration were
obtained. Various functional groups and dye incorporation in grown crystals were confirmed
qualitatively by FTIR and Raman analysis. A significant increase in Curie temperature from 50 to
55 °C with decrease in maximum permittivity has been observed. UVꢀVis spectra showed an
increased transmittance window and increased optical band gap from 5.61 to 6.11 eV as a result of
doping. Increase in the sharpness and intensity of photoluminescence peak with a blue shift have
been observed in doped crystals. The thermal stability and the decomposition temperature were
found to increase by about 10 °C in 0.01mol% dye doped TGS. The mechanical strength of the
grown crystals was estimated by Vickers microhardness test and was found high for dye doped
TGS. High piezoelectric charge coefficient d33 of 16 pC/N was observed for pure TGS which
decreased as a result of dyeꢀeffect. The fabrication of patch antenna was carried out after
simulating its resonant frequency that makes it suitable for telecommunication applications.
2

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1. Introduction
Triglycine sulfate (TGS) is an important material in the fabrication of IRꢀdetectors as the
crystal offers both pyroelectric and ferroelectric properties at room temperature.1ꢀ3 The monoclinic
structure for TGS single crystal was reported by Hoshino et al.4 who reported its Curie temperature
to be 47 °C, where because of second order ferroelectric phase transition its space group changes
from P21 to P21/m. Later, many studies performed on TGS crystals showed its Curie temperature
5,6
to be around 49ꢀ50 °C.
The structure of TGS is complex in the form of a grid composed of
glycine molecules (N+H3CH2COOꢀ) and SO4 tetrahedron linked together via hydrogen bonding.7
However, one of the major drawbacks in TGS crystals is the depolarization with time leading to a
decrease in its device efficiency.8 The ferroelectric and pyroelectric properties of TGS with various
dopants are extensively studied. H.V. Alexandru et al. studied temperature dependence of
pyroelectric coefficients for Lꢀ and Dꢀalanine doped TGS crystals.9 K. Meera et al. showed from
PꢀE hysteresis studies an increase in coercive field values for Lꢀcystine doped TGS.10
In addition, ferroelectric crystals are well known for their outstanding optical properties as
the occurrence of ferroelectricity provides a major contribution to the local electric field that
results in enhanced optical properties. Dyeꢀhost interaction is a subject of high interest because of
its potential applications in display and electroꢀoptic devices. A small amount of dye can alter
optical nonlinearity of ferroelectric host material drastically.11 When optical field excites the dye
molecules, a change in dyeꢀhost interaction leads to molecular reorientation within the host lattice.
This effective modification of molecular reorientation results in a corresponding change of
material properties. The affinity of the dye molecule for different planes of the crystal is different
because the values of electrostatic potential between dye molecules and the various crystal planes
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are different. The value of electrostatic potential depends on the nature of the charge molecules
appeared on the crystal plane and different planes have exposed the different topology of the
molecules which leads to the variation in the potential on the surface of the crystal plane.12
However, only a few reports are available on the potential of dye sensitization of these
ferroelectric crystals and making them useful for optical applications.13 Optical properties are
influenced due to organicꢀinorganic hybrids and play a key role in material properties.14,15 A dye is
classified as an organic aromatic chemical group that is water soluble and strongly emits or
absorbs visible light effecting the optical, thermal, electrical and mechanical properties of a
material that are important for device fabrication.16 There are many classes of dyes according to
their chemical constitution and chromophore group, out of which azo dyes and aryl group dyes are
extensively utilized for industrial applications.17,18 Crystalꢀviolet (CV) belongs to the class of
triarylmethane dyes containing three aryl groups attached to a central carbon atom. Crystal violet
having molecular formula C25H30N3Cl was a material of choice in the present work because of its
numerous advantages in medical and scientific community. CV is used as a pH indicator.12 In the
medical community, it is employed as a bacteriostatic agent and an active ingredient in Gram’s
stain. The dye is used as an external skin disinfectant in humans and animals. It is widely used as a
purple dye for textiles such as cotton and silk, and in paints and ink printing.16,19 With dye
inclusions, crystals offer rich stereochemistry and thus are promising candidates for solid state
lasers, spectroscopic and photonic applications.20
Inclusion of dye in salt crystals is found to show a variety of photoꢀchemical/photoꢀ
physical phenomena and affects various other properties of salt crystals.13,19 Further, the
incorporation of the dye in the TGS host matrix is expected to give rise to a considerable local
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strains and the associated electric polarization in the crystal, thus making it possible to affect
dielectric and optical properties. The objective of this study is to investigate the effect of CV dye
doping on the growth, morphology and properties of TGS single crystal. This paper presents the
optical, thermal, dielectric and mechanical properties of 0.01 and 0.02 mol% CV doped TGS (T1C
and T2C, respectively) in comparison to pure TGS single crystals. Because of the very significant
increase in the surface area of the grown crystals, with enhanced photoluminescence, dielectric,
thermal and mechanical properties, these CV doped TGS crystals proved beneficial for use as laser
gain media for solid state lasers, light emitting device applications, telecommunication applications
and other optoelectronic systems.
2. Experimental procedure
2.1 Crystal growth
Single crystals of pure triglycine sulfate were grown by slow evaporation technique taking
high purity glycine (Sigma Aldrich) and sulfuric acid in the molar ratio 1: 3 with distilled water as
solvent. The summary reaction for the complex formation is as follows:
3 (NH2CH2COOH) + H2SO4
(NH2CH2COOH)3.H2SO4
(1)
The solution was maintained at a temperature of 45˚C during stirring to vary the solubility
conditions of the solute in distilled water. The prepared homogeneous solution was filtered and
kept for evaporation in a petridish. Transparent colorless crystals were obtained in 20 days. Dye
doped single crystals were obtained by adding 0.01, 0.02 and 0.03 mol% of CV dye to the above
solution keeping all the growth parameters alike. The dye doped crystals were obtained in a lesser
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time of about 15 days. Similar growth promoting effect was also observed in other dye doped
single crystals.21 Shape and size of the grown crystals changed significantly with addition of dye
and is shown in Figure 1aꢀc. A typical coloring pattern for 0.01 mol% dye doped crystal (T1C) was
observed giving evidence of different affinities of various facets for CV dye which markedly differ
for 0.02 mol% crystals (T2C) producing morphological alteration and making them appear
translucent. This morphological change is a result of rate of adsorption onto the crystal surface that
affects solution properties by modifying interfacial surface tension leading to changes in its
anisotropic growth rate.22 Also, with high concentration of dye dopant (~ 0.03 mol %), the
morphology varied to a great extent giving crystals of irregular shapes or in the form of clusters.
Hence, these crystals with higher doping were not considered for characterization purpose.
2.2 Characterization techniques
The grown crystals were subjected to powder XꢀRay diffraction analysis using Rigaku
Ultima IV Xꢀray diffractometer with Cu Kα radiation (λ = 1.54056 Å) using a tube voltage and
current of 40 kV and 40 mA, respectively. To analyze the dye incorporation in the crystals hence
obtained energy dispersive Xꢀray analysis (EDAX) was performed using a JEOL JSM 6610LV
scanning electron microscope.For functional group confirmation, Fourier transforms infrared
spectra using Perkin Elmer FTIR spectrometer in the range 500 – 4000 cmꢀ1 and Raman spectra
using Renishaw Laser Raman spectrometer with argon ion laser at 514.5 nm in the range 110ꢀ1700
cmꢀ1 were recorded. Using Agilent E4980A LCR meter with a programmable temperature
controller, the dielectric constant and losses were measured at a heating rate of 0.25 °C minꢀ1. For
optical studies, the absorption spectra were recorded in the range 190ꢀ900 nm using Perkin lambda
35 spectrophotometer. The photoluminescence spectrum of the crystals have been recorded using
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FluoroLog 3ꢀ22 Spectrofluorometer with ozone free 450 W Xenon lamp as excitation source at
room temperature with excitation wavelength of 240 nm. Thermal analysis were carried out using
Diamond Perkin Elmer system in the temperature range 30 – 700 °C in nitrogen atmosphere at a
heating rate of 20 °C minꢀ1 . The variation in the piezoelectric charge coefficient d33 with field of 2
kV/mm was measured using a piezometer (PM 300, Piezotest, UK).The hardness measurements
were made on the smooth surface of the crystals of 2 mm thickness using a Vickers hardness tester
fitted with a Vickers diamond pyramidal indenter. Patch antenna characteristics were measured
using Agilent 8722ES vector network analyzer.
3. Results and Discussion
3.1 Powder XRD and elemental analysis
The typical morphology of grown crystal with corresponding faces is shown in Figure 1d
and was found to change as a result of increasing dye concentration. Pure and CV dye doped
samples were subjected to powder XꢀRay diffraction analysis to identify the unit cell parameters
for the as grown crystals. The samples were scanned over the range 5 – 45° at the rate of 2° minꢀ1
and the diffraction pattern obtained are shown in Figure 1e. The observed values are in good
agreement with the JCPDS XRD data (00ꢀ015ꢀ0947) of TGS. Marked variation in peak intensity
of doped samples in comparison to pure TGS was observed confirming CV incorporation into TGS
lattice. Also the spectra showed slight changes in the peak position which can be attributed to
strains in the lattice, resulted because of incorporation of heavy CV molecules causing anisotropic
effects in the lattice changing the positions and the shapes of the Xꢀray diffraction peaks.23
Particular coloring pattern were obtained for dye doping in these crystals. The lattice parameters
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were determined from checkcell refinement software and are listed in Table 1. In order to establish
the incorporation of crystal violet dye in TGS lattice, which cannot be predicted by mere
coloration of the crystals, EDAX analysis was performed. The EDAX spectra of TGS and T1C
crystals are shown in Figure 2. Although the presence of dye doping in crystals was confirmed
using EDAX analysis, but it is difficult to be quantified due to small amount of crystal violet dye
and analysis limitation to measure hydrogen composition. But presence of chlorine in T1C
confirms dyeꢀdoping in the TGS crystal lattice.
Figure 1: Photographs of the asꢀgrown single crystals (a) Pure TGS (b) 0.01 mol% CV doped TGS
(T1C) (c) 0.02 mol% CV doped TGS (T2C) (d) Growth morphology of grown crystal (e) Xꢀray
powder diffraction pattern for the asꢀgrown single crystals.
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Crystal
TGS
a (Å)
b (Å)
c (Å)
β
Volume (Å3)
643.916
9.4350
9.4045
9.3774
12.6671
12.6358
12.6229
5.7457
5.7331
5.7173
110.33
110.38
110.15
T1C
638.638
T2C
635.335
Table 1: Lattice parameters for TGS single crystals doped with crystalꢀviolet (CV) dye
Figure 2: EDAX analysis of TGS and T1C single crystals to confirm dye incorporation.
3.2 Functional group analysis
The presence of crystal violet dye in the doped samples was qualitatively estimated by
FTIR and Raman analysis. The infrared spectra were taken using KBr pellet technique and are
shown in Figure 3a. For better understanding of the low frequency spectrum, Raman analyses were
performed complementary to FTIR studies and are shown in Figure 3b. The –COOH group of TGS
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+
causes protonation of NH2 group, giving rise to COOꢀ and NH3 group. The FTIR shows a broad
spectrum between 3350 – 2800 cmꢀ1 which arises because of the asymmetric and symmetric NꢀH
stretching of NH3 group, CꢀH stretching of the CH2 group and OꢀH stretching of the hydrogen
bonded carboxyl group.24 The absorption around 1706 cmꢀ1 is assigned to the stretching of C=O
bond. while in case of CV dye doped crystals, it was found to be very slightly shifted at 1708 cmꢀ1
which still shows the stretching of C=O bond. Similarly, in Raman spectra the peak positions in
pure TGS crystal at 1649 and 1675 cmꢀ1 are slightly shifted in case of dye doped crystals. The
bands at 1623 and 1567 cmꢀ1 in FTIR and corresponding Raman bands around 1609 cmꢀ1 are
assigned to antisymmetric and symmetric stretching modes of COOꢀ carboxylate ion group.
Bending vibrations of NH3 at 1538 and 1501 cmꢀ1 appear to conflate as a result of CV doping in
+
FTIR spectra and found shifted to around 1484 cmꢀ1 in Raman spectra. The strong absorption in the
2ꢀ
range 1127ꢀ977 cmꢀ1 is because of SO4 ion of the sulfate group that become less sharp with the
increasing content of dye dopant confirming CV incorporation in the lattice devoid of sulfate
group. Similar changes were also observed in Raman spectra in the wavenumber range between
1115 and 903 cmꢀ1 respectively. The peaks due to NH3 oscillations are evident in the range 615 to
+
501 cmꢀ1 showing variation in absorption intensity with doping concentration. A corresponding
broadening of bands along with minor changes in peak positions confirms CV incorporation in to
TGS lattice.
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Figure 3: (a) FTIR spectra of pure and dye doped TGS single crystals (b) Raman spectra of pure
and dye doped TGS single crystals.
3.3 Dielectric studies
Dielectric studies were carried out to find the transition temperature and corresponding
lattice dynamics as a result of dye doping. The crystals were polished and their opposite faces were
coated with high grade silver paste to obtain a good conductive surface layer. The magnitude of
dielectric constant depends on the displacement of polarized charges and was found to be higher in
case of pure TGS crystals. In all the samples, the dielectric constant decreases with increasing
frequency and attains saturation at higher frequency. The variation of dielectric constant with
temperature at 10 kHz is shown in Figure 4a depicting an increase in phase transition temperature
with increasing dye concentration. The transition temperatures were found to be 50 °C for pure
TGS, 53 °C for T1C and 55 °C for T2C, respectively. Also, the incorporation of dye in TGS lattice
has resulted in a decrease of dielectric constant value thereby tuning the dielectric polarization
mechanism in the grown crystals. However, the dielectric losses as a function of temperature were
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found to increase with increasing dye concentration and are shown in Figure 4b. But the values
obtained are still lower in comparison to other TGSꢀdoped crystals.22,25 In case of dye doped
crystals, a partial substitution of glycine molecules with dye dopant occurs and is expected to
prevent glycine group displacements, thereby decreasing the height of phase transition maxima.
The low values of dielectric constant and losses reveal the good optical quality of the grown
crystals making them desirable for use in various optical and communication devices.
Figure 4: (a) Temperature dependence of phase transition in pure and dye doped TGS at 10kHz.
(b) Dielectric losses in pure and dye doped TGS at 10kHz.
3.4 UVꢀVis analysis
The main optical properties of any material are its absorption coefficient (α), refractive
index (n) and extinction coefficient (k). These optical properties are influenced as a result of dye
doping in crystals because of modifications in their dependence on the electromagnetic radiation.
Moreover, the evaluation of optical constants is an important factor for the design and analysis of
optoelectronic devices and laser frequency conversion applications in industries. Knowledge of
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refractive indices and extinction coefficient at energies other than the fundamental absorption edge
guides in understanding the electronic bandꢀstructures of the materials. Also, a preferred lower cutꢀ
off in the transmittance analysis in the range between 200 and 300 nm is considered effective for
optical applications.
Figure 5a shows the UVꢀVis transmittance spectra for pure and CV dye doped TGS
recorded in the wavelength range 190ꢀ900 nm. Figure 5a inset shows transmittance data of CV dye
depicting four characteristic absorption peaks with highest intensity peak at 590 nm. The cutꢀoff
wavelength for pure TGS was observed to be 246 nm but with dye inclusion it shifted to 240 nm in
both the samples T1C and T2C. A broad but weak absorption peak at 594 nm was observed for
T2C but is obscured in case of T1C. The induced lowering in cutꢀoff as a result of dye doping
suggests the usefulness of these crystals in several optoelectronic applications.
3.4.1 Optical absorption coefficient
The optical absorption coefficient (α) is a function of incident photon energy that increases
exponentially and then saturates, a behavior typically governed by the density of states in the
conduction band. It can be calculated using transmittance data from the following relation: 21, 26
ꢈ
ꢁ.ꢂꢃꢂ ꢄꢅꢆꢇ ꢊ
ꢉ
ꢀ =
(2)
ꢋ
Where d is crystal thickness and T is the transmittance. According to the Tauc relation, the
absorption coefficient is related to photon energy (hν) in the grown crystals as:
ꢌ
ꢎꢁ
ꢀℎꢍ = ꢏꢌℎꢍ − ꢐꢑꢎ
(3)
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where Eg is the optical band gap and A is a constant, indicating the allowed direct interband
transition. Figure 5b shows the plot of (αhν)2 vs photon energy for pure and dye doped TGS where
the energy band gap was evaluated by extrapolating the linear portion of the curve to (αhν)2 = 0.0
and was found to increase with dye doping in TGS crystals.
3.4.2 Optical properties and constants
The reflectance (R) in terms of absorption coefficient can be written as:27
ꢌ
ꢎ
ꢓꢌꢔꢕꢖꢗꢘ ꢕꢙꢋ ꢚꢖꢗꢘꢌꢙꢋꢎ
ꢒ = 1 ∓
(4)
ꢌ
ꢎ
ꢔꢚꢖꢗꢘ ꢕꢙꢋ
The reflectance for the dye doped crystals nearly reduced by half as compared to pure TGS in the
high photon energy region (Figure 5c), suggesting high transmittance in case of dye doped
crystals.
The optical constants, refractive index (n) and extinction coefficient (k) were determined from the
reflectance data and in terms of absorption coefficient as:
ꢝ
ꢌ
ꢎ
ꢛ = [ꢕ{ ꢜꢚꢔ ∓ꢓꢂꢜ ꢚꢔꢃꢜꢕꢂ}]
(5a)
(5b)
ꢁ ꢌꢜꢕꢔꢎ
k = λα/4π
where λ is the wavelength and are shown in Figure 5c and 5d. The high value of n along with
lower k values for dye doped crystals in comparison to pure TGS makes them a promising
candidate for use in photonic device application such as light emitting diode (LED) and image
sensors.
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The optical conductivity (σ) of the grown crystals was determined using the following equation:28
ꢙꢟꢠ
ꢞ =
(6)
ꢡꢢ
where c is the velocity of light and spectra shown in Figure 5d (inset). From the spectra it was
observed that the optical conductivity remains a constant till 5 eV photon energy, beyond which it
increases linearly. As a consequence of its increased band gap and optical constant studies, the dye
doped crystals can be of potential interest in optical applications.
Figure 5: (a) UVꢀVis spectra of pure TGS and dye doped TGS. (inset) Transmittance spectrum of
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CV dye. (b) Estimation of optical band gap energy, Eg (c) Variation of reflectance (R) and
refractive index (n) as a function of photon energy (hν) (d) Variation of extinction coefficient (k)
and optical conductivity as a function of photon energy (hν).
3.5 Photoluminescence properties
Most arylmethane dyes give low emission in fluid solvents as fluorescing particles interact
with solution molecules influencing their properties. Hence PL was performed for the solid
samples. PL emission spectra for all the samples were recorded under an excitation wavelength of
270 nm and are shown in Figure 6a and 6b. Changes were observed in the PL spectra of pure and
dye doped TGS samples. A single broad peak was obtained in pure TGS which splits into two with
dye doping. For pure TGS, blue green emissions centered at 450 nm were observed. For CV
doped samples, a shift in blue green emissions centered with two peaks at 416 and 435 ꢀ 438 nm
was observed.
This shift arises because of lattice defects, created in the form interstitials because of CV
dye which is devoid of sulfur.29 Moreover, it is because with dye doping, the effective band gap
increases which resulted in the emitted photon to have comparatively higher energy values. Hence
these high energy photons give photoluminescence peaks at shorter wavelength as evident from PL
spectra. A red emission peak centered at 644 ꢀ 655 nm observed in addition to the blue green
emissions which are a characteristic of CV dye. The intensity of this red emission increases with
the increased dye doping in TGS and showing a corresponding decrease in blue green emissions.
The emission of red fluorescence confirms the incorporation of CV in the TGS lattice. The
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decreased luminescence broadness of dye doped crystals with increasing concentration indicates
usefulness of these crystals for developing coherent sources in applications that require violetꢀblue
light.
Figure 6: (a) Photoluminescence spectra of pure and dye doped TGS single crystals. (b) Emission
of red fluorescence confirming crystal violet incorporation in the TGS lattice.
3.6 Thermal Analysis
Figure 7a – 7c represents the thermo gravimetric and differential thermal analysis (TGAꢀ
DTA) curves of pure and dye doped TGS crystals in the temperature range between 40 and 700 °C
at the heating rate of 10 °C minꢀ1 in nitrogen atmosphere. There is no weight loss in the pure and
dye doped TGS samples in the temperature range between 40 and 200 °C, establishing their high
temperature thermal stability. The first phase of weight loss begins at 212 °C in pure TGS and can
be attributed to the decomposition of glycine.22 With dye doping of 0.01 mol%, this weight loss
starts at 220 °C making them thermally more stable than pure TGS. The TGA curves show that
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maximum weight loss (~ 55 %) for the pure and dye doped TGS occur in the temperature range of
210 ꢀ 260 °C because of glycine getting decomposed into CO2 and NH3. But the associated weight
loss percentage in case of 0.02 mol % dye doped TGS decrease to 45 % in this temperature range,
evidently establishing a reduction in the decomposition rate of glycine with dye doping.
The endothermic peak at 229 °C in the DTA curve represents the decomposition of pure
TGS. With dye doping, an increase in this endothermic peak up to 238 oC and thermal stability up
o
to 220 C was achieved for T1C crystal. On further increasing temperature, the gases CO2 and O2
might have evolved leaving behind sulfur in residual form. The well defined endothermic peaks in
pure and dye doped crystals establish their good crystalline quality.
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Figure 7: TGꢀDTA curve of (a) Pure TGS single crystal (b) 0.01 mol% CV doped TGS (T1C)
single crystal (c) 0.02 mol% CV doped TGS (T2C) single crystal.
3.7 Piezoelectric Response
The interaction between mechanics and electrical fields of a structure can be described in
terms of piezoꢀresponse of the material. It can be determined using stressꢀcharge form with the
help of a piezometer measuring the piezoelectric charge coefficient (d33). For this measurement, a
poling process was performed at an applied field of 2kV mmꢀ1 for an easy alignment of domains.
In response to the applied electric field, the piezoelectric charge coefficient value d33 was
found to decrease with increase in dye concentration. With electric field optimization at a tapping
force and frequency of 0.25 N and 110 Hz, pure TGS showed a d33 value of 16 pC Nꢀ1. The d33
response decreased to about 9 pC Nꢀ1 for T1C crystals which further decreased to only 1 pC Nꢀ1 in
case of T2C single crystals. The reduction in piezoresponse of the grown crystals can be attributed
to their modified domain structure showing lack of polarization orientation as also evident from
dielectric study.
3.8 Hardness Testing
The mechanical properties of the grown crystals were evaluated by Vicker’s microhardness
tester and shown in Figure 8a. The Vicker’s microhardness number (Hv) was calculated using the
equation: 30
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ꢣꢤ = ꢔ.ꢥꢦꢡꢡ ꢧ
(7a)
ꢝ
ꢋ
where P is the applied load (in g) and d is the diagonal length of the indentation (in ꢁm).
Microhardness number (Hv) as evaluated is used to examine the ability of a material to resist
indenter penetration. This is significantly affected by the presence of cracks, pores, and inclusions
and hence gives important information regarding tensile strength and yield strength of the grown
crystals. The indentation marks were made on the surface of grown crystals by applying loads of 5,
10, 20 and 30 g with a constant indentation time of 15 s. As microcracks appear at higher loads,
the maximum load value was restricted to 30g only.
Meyer’s index (n) was evaluated for crystals in order to analyze the nature of grown
crystals. According to Meyer’s power law:
P = Adn
(7b)
where A is a constant for a given material. The value of n is evaluated from the slope of
log d vs. log P plot and is shown in Figure 8b. The values of Meyer’s index for crystal TGS, T1C
and T2C came out to be 2.57, 1.68 and 1.71, respectively. This value of n lies in the range 1.0 ≤ n
≤ 1.6 for hard materials and n > 1.6 for soft materials.31 In present case, all the samples having n >
1.6 suggests soft nature of the grown crystal material. However, smaller n value in the case of
doped crystal suggests increasing hardness as a result of dye doping.
In pure TGS the value of hardness number increases from 5 g to 10 g, after which it becomes
constant. It is due to the softer nature of the crystal which helps adjusting energy easily by sliding
the parallel planes of the crystal and hence no cracks appeared. As the contribution of the dye is
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increased in the pure TGS crystal, its voids are filled by the dye molecules which help to prevent
the sliding of parallel planes. The value of Meyer’s index is decreased by the incorporation of dye
molecule in pure TGS which is also used as an experimental evidence for increase in the hardness
of the doped crystal. Moreover, due to increase in hardness of doped crystal and problem in the
sliding of planes, cracks are observed beyond 10 g load in doped crystal.
Figure 8: (a) plot of the Vicker’s microhardness profile vs. load for grown single crystals. (b) Plot
of log d vs. log P for Meyer’s index evaluation.
3.9 Patch antenna simulation and fabrication
The application of piezoelectric materials reaches at the very far level in technological
devices. It has contributed in many aspects from energy harvesting to sensors level.32 To fulfill the
demand of the various technical aspects of communication, where various constraints (size, weight
and performance) matters, patch antenna are required.33 We have fabricated a microstrip patch
antenna based on piezoelectric substrate, which behaves both in wireless transmission as well as
energy harvesting device. We have chosen 0.01 % CV doped TGS single crystal as a substrate for
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fabricating GHz patch antenna due to its low dielectric constant and defects free transparent
geometry.
The design of patch antenna was formulated by transmissionꢀline model and dimensions of
TGS crystal (εr = 16.1 and h = 1.98 mm) based patch antenna was computed for an operating
frequency fr = 6.5 GHz as follows:34
Geometrical width of the patch antenna has been calculated as,
ꢨ
ꢁ
ꢫε ꢚꢔ
ꢪ
W = ꢁꢩ
= 7.89mm
(8)
(9)
ꢪ
The effective dielectric constant of the patch was computed as,
ε ꢚꢔ
ꢪ
ε ꢕꢔ
ꢔ
ꢪ
εꢖꢩꢩ
=
+
= 12.32
ꢁ
ꢁ
ꢈꢝꢬ
ꢭ
ꢫ
ꢔꢚ
The actual geometrical length was found by using following realtions
ꢱ
ꢲ
ꢎ
∆ꢮ = 0.412 × ℎ ꢌꢌ
= 0.76 mm
(10)
ε
ꢚꢃ.ꢂ ꢌ ꢚꢃ.ꢁꢳꢡꢎ
ꢯꢰꢰ
ꢯꢰꢰ
ꢱ
ꢲ
ꢎ
ε
ꢕꢃ.ꢁꢦꢥ ꢌ ꢚꢃ.ꢥꢎ
ꢠ
ꢮ = ꢁꢩ
− 2 × ∆ꢮ = 5.15 mm
(11)
ε
ꢓ
ꢪ
ꢯꢰꢰ
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Figure 9: (a) Simulated S11 parameter versus frequency and 3D farꢀfield radiation pattern
(in inset) at the resonance frequency of 6.5 GHz. (b) Experimentally measured S11 parameter
versus frequency in which a resonance peak at 7.0 GHz has been obtained. Photograph of
fabricated rectangular patch antenna (in inset) by using 0.01 % CV doped TGS (T1C) single
crystal as a dielectric substrate (‘εr’ = 5.1).
CST microwave studio was used for the simulation of patch antenna at 6.5 GHz resonant
frequency. The inset feed was given to patch antenna by 50 Ω microstrip line. Optimization of
impedence matching between patch and microstrip line was performed in the CST environment by
cutting the two rectangulat slots in the patch. Copper plate of thickness of thickness 1.04 mm and
standard SMA connector were used to fabricate the patch antenna. Figure 9a shows the simulated
S11 parameter of the patch antenna at 6.5 GHz resonance frequency with a return loss of ꢀ19.6 db.
The inset of Figure 8a shows the 3D farꢀfield radiation pattern of the patch antenna. Due to the
radiation blockage from ground, intensity of the radiation is directive from the top of patch. The
inset of Figure 9b shows the fabricated patch antenna on the piezoelectric crystal substrate. From
experimental analysis, operating frequency of the patch antenna was found at 7.0 GHz (Figure 9b)
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with a return loss of ꢀ17 db. Thus our fabricated patch antenna exhibited very good resemblance to
the simulated values of operating frequency and return loss. The minor change in operating
frequency (0.5 GHz) and return loss (2.6 dB) was observed in theoretical and experimental values.
This may be due to an error in the measurement of dielectric constant, dimensions of used crystal
substrate and copper strip.
CONCLUSION
Pure and CV dye doped TGS single crystals have been grown by solution method. Grown
crystals were observed to be transparent up to 0.01 mol% CV with characteristic dye coloring
pattern. Dye inclusion showed anisotropic growth promoting effect altering their morphology. CV
dye incorporation confirmed from XRD, FTIR and Raman analysis. Dielectric studies reveal an
increase in phase transition temperature from 50 to 55 oC with corresponding decrease in dielectric
constant value making them useful for optoelectronic applications. The crystals were found to
show good transparency in the UVꢀVisible region with increased band gap and enhanced optical
constants making them useful for photonic device applications. The decreased luminescence
broadness and increased coherence makes them a promising photoluminescent material for use in
light emitting applications that require violetꢀblue light. Thermal studies demonstrate a reduction
in the decomposition rate of glycine along with an increase in thermal stability and decomposition
temperature with dye doping. The crystals showed a decrease in piezoelectric charge coefficient
value with increase in dye concentration. CV doped crystals were found to be mechanically more
stable than pure TGS. A patch antenna on the 0.01 mol% CV doped TGS substrate was simulated
and fabricated for telecommunication applications. A good resemblance in the experimentally
observed values of resonance frequency and return loss visꢀàꢀvis their simulated values were
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achieved. Combined with piezoelectric and dielectric properties, this set up can also be used for
pressure sensor and energy harvesting applications.
ACKNOWLEDGEMENT
We are thankful for financial support received from the DST project (sanction no.
SR/S2/CMPꢀ0068/2010) and DU R&D Grant (Sanction No. RC/2014/6820). We would also like
to thank Prof. R.P.Tandon (University of Delhi), to help us perform patch antenna study on
network analyser and Prof. Vinay Gupta (University of Delhi), to help us perform UVꢀVis
analyses. Sonia Bhandari is thankful to DST for providing the Senior Research Fellowship (SRF).
Harsh Yadav is thankful to UGC for Meritorious Scholarship. Geeta Ray and Jyoti Dalal are
thankful to CSIR while Sanjay Godara, Nidhi Tyagi and Sonu Kumar are thankful to UGC for
Research Fellowship.
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