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Phenol red dyed Bis thiourea Zinc acetate crystal growth and characterization for electro-optic applications
Optik 158 (2018) 997–1005
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Phenol red dyed Bis thiourea Zinc acetate crystal growth and characterization for electro-optic applications V. Ganesh a,c , Mohd. Shkir a,c,∗ , I.S. Yahia a , Jafar M. Parakkandy b , S. AlFaify a,c,∗ a Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia b Department of Physics, College of Science and Humanity studies, Prince Sattam bin Abdulaziz University, Alkharj, Kingdom of Saudi Arabia c Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha-61413, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 13 November 2017 Accepted 19 December 2017 Keywords: Phenol red dye Crystal growth FT-IR spectroscopy Optical properties Thermal properties
a b s t r a c t In current work, the growth of dyed Bis thiourea Zinc acetate single crystals of size ∼30 mm × 7 mm × 5 mm is achieved by solution technique. X-ray diffraction and FT-IR spectroscopy are used to verify crystal structure and functional groups. Intensity of diffraction peaks confirms good crystalline nature of dyed Bis thiourea Zinc acetate crystals than pure. Two absorption bands at ∼387 and 558 nm were observed in diffused reflectance spectra of dyed crystals. The energy gap was calculated for both crystals and establishes to be ∼4.87 eV for pure and ∼4.93 eV for dye crystals. Photoluminescence spectra were recorded and a green emission band was observed in dyed Bis thiourea Zinc acetate crystal at ∼513 nm was observed in dyed crystal. The grown crystals are found to be thermally stable up to 195 ◦ C confirmed by differential scanning calorimetry. The value of dielectric permittivity is found to be reduced in dyed crystal. Microhardness of dyed crystals is much better than pure crystal. Surface study shows that the dyed crystals contain lesser defects than pure. Hence, the improved properties of dyed Bis thiourea Zinc acetate make it more useful than pure in electro-optic devices. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Semi-organic crystals have been studied extensively for many years due to its promising applications in opto-electronic devices [1,2]. The beauty of these crystals is that they can easily interact with many dopants like organic, inorganic, metalorganic and dyes [3,4]. Among all the dopants, dye interaction with semi-organic crystals has been focused in lime light due to its potential applications in display and modern optical devices [5–7]. In addition to this, a small amount of dye in to semi-organic material can alter huge second and third harmonic properties of host material. Among all these semi-organic materials bis thiourea zinc acetate is consider a potential candidate to interact with various dopants due to its simple nature. Semi-organic crystals contain dye have been attracting many researchers as an alternative of polymers and glasses due to their huge applications in the field of photonic devices [3,8–10]. In past few years dying of crystal is in a lime light due to their usage in solid state micro lasers [6,7]. Dying of crystal with matrix of semi-organic materials increases intermolecular band strength and provide highly stable physical parameters like thermal conductivity, less dislocations, low scattering with
∗ Corresponding authors. E-mail addresses: [email protected] (Mohd. Shkir), [email protected] (S. AlFaify). https://doi.org/10.1016/j.ijleo.2017.12.097 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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Fig. 1. As grown single crystals of pure and dyed BTZA.
interaction of light, high stability and high photoconductivity [11]. In literature many workers have been studied the effect of dye especially in solution growth crystals by their sophisticated experimental techniques and concluded that the interaction of dye enhances the growth sectors of pure crystals and modify the various growth parameters [12–15]. Hence, in view of this, here we have chosen efficient semi-organic Bis thiourea Zinc acetate (BTZA) as a host material for the interaction of Phenol red dye. There is sufficient reports are already available on host material viz., growth, structural, optical, spectral and nonlinear optical properties [16–18]. But surprisingly there is no reports are found on dying of such important crystal. Hence, in present study, we report a systematic study on dying effect of BTZA crystals on various physical parameters and their results are discussed in their respective sections. Hence, in view of this, here we have chosen efficient semi-organic Bis thiourea Zinc acetate (BTZA) as a host material for the interaction of Phenol red dye. Therefore, in present study, we report a systematic study on dying of BTZA crystals on various physical parameters and their results are discussed in their respective sections.
2. Experimental 2.1. Crystal growth and solubility Commercially available thiourea and zinc acetate of AR grade have been purchased from Merck and these materials are dissolved in Millipore double-distilled deionized water in 1:2 ratio. In case of pure material the dissolved salts are allowed for slow evaporation in a container covered with a polythene sheet with small pores and the BTZA was acquired via a slow evaporation process. In case of dye addition, a supersaturated solution of repeatedly recrystallized BTZA salt was used, and a quantity of ∼ 3 mol% of Phenol red dye was added to pure solution. The prepared solutions of pure and dyed were continuously stirred for 3days to get homogeneous solution. The solutions were later filtered and kept into a temperature bath for slow evaporation process. Further, the pure and dyed BTZA crystals of size 30 × 6 × 8 mm3 and 38 × 7 × 8 mm3 respectively were obtained in a period of 15 days (Fig. 1).
2.2. Characterization techniques To identify the structural parameters of pure and dyed BTZA crystals were analyzed by powder X-ray diffraction on a Shimadzu X-600 Japan powder X-ray diffractometer. The effect of dyeing was determined by scanning electron microscopy/EDX studies using JEOL JSM 6360 LA, Japan. The functional groups were determined using thermo scientific DXR FT-IR spectrometer by KBr pallet method. The diffuse reflectance study was carried in the range 190–1200 nm using Shimadzu UV–vis-NIR spectrophotometer (model UV-3600). To identify the thermal stability a Differential scanning calorimetric (SETARAM DCS) was used in the temperature range from 30 to 300 ◦ C. The optical device property of grown crystals was also measured by Photoluminescence study using a Thermo Fisher Scientific Lumina fluorescence spectrometer in the wavelength range of 300–550 nm at 300 K. Further, the dielectric and mechanical studies were carried out.
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Fig. 2. (a) XRD patterns and (b) EDX spectra for pure and dyed BTZA crystals.
3. Results and discussion 3.1. Powder X-ray diffraction and EDX studies To analyze the effect of dye on the structural properties of BTZA crystal a powder X-ray diffraction patterns were recorded as shown in Fig. 2(a). The sharp diffraction peaks in both patterns confirms that the grown crystals are of good crystallinity and the crystallinity of pure BTZA is not much affected by dye. Moreover, the lattice parameters were determined using the recorded data of X-ray diffraction for both samples through POWDEX software. The calculated lattice parameters are found to be: For pure BTZA: a = 6.93747 Å, b = 17.67306 Å, c = 11.78813 Å, V = 1334.35438 Å3 , ␣ = ␥ = 90.000◦ ,  = 112.596◦ . and for PRBTZA, a = 6.93676 Å, b = 17.67377 Å, c = 11.78957 Å, V = 1334.44764 Å3 , ␣ = ␥ = 90.000◦ ,  = 112.594◦ . The calculated parameters suggests that no structural modification is observed for dyed BTZA crystals apart from a minute disparity in their values and are in harmony with earlier data [16]. The close view of diffraction patterns in two different ranges has been also shown in Fig. 2(a) and which confirms that there is a minute variation in the peaks position towards higher angle due to dye interaction with BTZA matrix.
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Fig. 3. FT-IR spectra for (a) pure and (b) dyed BTZA crystals.
Furthermore, we have also recorded EDX spectra for pure and dyed crystals as shown in Fig. 2(b). EDX spectra confirm the synthesis of BTZA crystal, however a small variation in the elemental ratio was observed. 3.2. Vibrational analysis To identify the signature of the functional groups in pure and dyed BTZA samples, the FT-IR spectrum was recorded using a thermo scientific DXR FT-IR spectrometer by KBr pallet method. From Fig. 3 the absorption peaks observed at 3848 cm−1 , 3736 cm−1 , 3621 cm−1 and 3650 cm−1 are related to characteristic O H bond stretching vibrations are shifted two lower frequency side indicates the interaction of dye with pure material. The N H absorption peaks at observed at 3100–3400 cm−1 in pure material are completely missing in dyed samples and shifted to lower frequency side suggest that the adsorption of phenol red on the surface BTZA crystal. The symmetric and asymmetric C S stretching vibrations at 770 and 1404 cm−1 are completely broadened in case of dyed samples phenol red is bound to the pure crystal matrix. Further all the lower wavelength region groups are also shifted to lower wavelength region indicates the coordination of dye molecules with BTZA crystal. The identified functional groups provide evidence of the successful incorporation of dye in the BTZA crystal. 3.3. Optical studies 3.3.1. Diffused reflectance analysis To see the effect of dye on the optical properties of BTZA crystals the diffused reflectance measurement was carried in solid state by preparing the powders of the grown crystals as shown in Fig. 4(a). From figure it can be observe that there is a variation in cut-off wavelength in dyed crystals compare to pure. As a signature of dye we observed some new absorption bands in dyed crystals at wavelength about 387 and 558 nm which are absent in pure BTZA crystals and confirms the interaction of dye with BTZA matrix [8,9,12,13,15,19]. Moreover, for calculating the energy gap (Eg ) we have used Kubelka-Munk method (1−R)2 , where R is absolute 2R F (R) , here t the thickness of the t
and Kubelka-Munk function [F(R)] was calculated using Kubelka-Munk relation [20]: F (R) = reflectance and the absorption coefficient (˛) can be written in terms of F(R): ˛ =
absorbance t
circular sample holder (viz. ∼2 mm). The Eg can be determined using the equation: (˛h)
1 n
= =
F(R)h s
1n
A h − Eg , here all
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Fig. 4. (a) DR spectra and (b) Tauc’s plot for pure and dyed BTZA crystals.
Fig. 5. PL spectra for pure and dyed BTZA crystals.
2
symbols have common sense and for direct allowed transition the values of n= 1/2 has been used. The Tauc’s plot [(˛h) vs h plot] has been presented in Fig. 4(b). The energy band gap from the plot are found to be ∼4.87 eV and 4.93 eV for pure and dyed BTZA crystals. The obtained values shows that the energy gap has been enriched due to dye, however these are very high values and hence can be useful in optoelectronic applications [21–23]. It may be mentioned here that a new energy gap is observed which is ∼2.11 eV and assigned to presence of dye in BTZA crystals.
3.3.2. Photoluminescence (PL) analysis To know the effect of dye on luminescence properties of BTZA crystals we have measured the PL as shown in Fig. 5 using the excitation wavelength, exc = 320 nm at 300 K. PL spectra show two emission bands at 358 nm and 547 nm in pure BTZA crystal and at 357 and 513 nm in dyed crystals which can be assigned to UV-A emission and green emission bands respectively. There is remarkable shift in the strong intensity emission band was observed at higher wavelength due to interaction of dye with BTZA. The luminescence intensity is also found to be remarkably enhanced in dyed crystals compared to pure which may be predicted as defects created by dye that acts as color centers.
3.4. DSC analysis The thermal study of pure and dyed BTZA crystals are carried out using Differential calorimetric technique in a homogeneous nitrogen atmosphere at a heating rate of 10 ◦ C min−1 . Fig. 6. Show DSC micrographs of the present studied samples. From this figure it is clearly understood that there is sharp endothermic peak is observed in pure and dyed crystals, which indicates that there is no decomposition of samples occurred before melting point. The endothermic peak at 194 ◦ C and 196 ◦ C for pure and dyed samples shows good thermal stability of material and the increase in melting point indicates the effect of dyeing in pure samples. The observed values in case of pure BTZA crystals are near to the previously reported data [18,24], whereas in case of dyed crystals these values are quite high, Which might be due to high interaction of dye molecules with pure material.
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Fig. 6. DSC spectra for (a) pure and (b) dyed BTZA crystals.
Fig. 7. (a) Dielectric constant (b) dielectric loss and (c) total electrical conductivity plots for pure and dyed BTZA crystals.
3.5. Dielectric studies The dielectric measurement was carried using a KEITHLEY 4200-SCS over a wide range of frequency range of 1 kHz–10 MHz at 290 K. Dielectric constant (1 ), dielectric loss (2 ) and total alternating current conductivity (ac ) samples were calculated by measuring the capacitance (C), Impedance (Z) and loss tangent (tan␦), with a parallel plate capacitor method using the following relations [25,26]: ε1 = εCdA , and ε2 = tanı × ε1 , where εo is free space permittivity (εo = o
8.854 × 10−12 F m−1 ), d and A are the thickness and area of crystal. The dielectric constant of the pure and dyed BTZA sample was measured by varying the frequency as shown in Fig. 7(a). From figure it is clearly observed that the dielectric constant is high at lower and higher frequency regions. However in both the frequency regions, the pure material is showing high dielectric constant compare to the dyed sample, indicating that high interaction of dye showing less dipole response to the applied alternating electric field and suggest that these materials useful in various devices [27]. From Fig. 7(b) suggests that the dielectric loss in both the crystals is decreasing with the increasing the frequency and it is less in dye doped crystals compared to pure crystal. The reason for the low dielectric loss in dyed crystal could be
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Fig. 8. Plots of (a) load variation (b) lnp vs lnd and (c) p vs d2 of pure and dyed BTZA crystals.
explained due to interaction of dye molecules with pure material that reduced the defects and increase the quality of the d crystal. Further, the alternating current conductivity (ac ) was calculated using the following equation: tot. ac = ZA , and s tot. ac = dc + Bω , here, dc direct current conductivity, B is constant, angular frequency and s is frequency exponent. Fig. 7(c) shows the alternating current conductivity of the pure and dyed samples. From figure it is clearly understood that the ac of both the crystals obeys the universal frequency power law behavior. The value of s was obtained by the slopes of the curves and the values are ∼1.004 for pure and 1.001 for dyed crystals with standard error 0.055 and 0.057, respectively, this indicates that the hopping motion in grown crystals involves a translation motion with a sudden hopping.
3.6. Mechanical studies The study of the strength parameters of any material gives the information about various physical parameters like intermolecular distance; lattice energy, lattice perfection and about stability [28–30]. Among the different strength parameter testing methods the Vicker’s microhardness, is simple and non-destructive technique, to identify hardness of the soft and hard materials. In the present study, the microhardness of the Pure and dyed BTZA crystals were examined at different loads of ranging 10–100 g for a constant indentation time of 15 s. All the indentations of present crystals were made on (200) face and corresponding diagonal lengths were measured for hardness calculation. The hardness number (Hv ) was calculated using the relation, HV = 1854.4 P/d2 , Where P is the applied load in Kg and d is the diagonal length in m [31]. Fig. 8(a) shows the variation of Vicker’s hardness with applied load, from figure, it is clear that the hardness of the crystals increases with an increase in the applied load and attain the saturation. The reason for the increasing hardness of the pure and dyed samples is discussed in our previous work on various types of crystal [8,10]. However, in present study the hardness of the dyed samples are showing higher hardness value compare to the pure material indicating that interaction of dye makes pure BTZA as harder substance. The work hardening coefficient (n) of the grown crystals was determined using Meyer’s relation of P = A dn , the slope of the present relation gives n value by drawing the plots of ln P versus log d shown in Fig. 8(b). From this plots the n is found to be 2.12 and 1.57 suggest that dyed BTZA crystals are harder than pure material [32]. Further, the corrected hardness and minimum load for plastic deformation is also calculated by using the well-known Hay’s–Kendal approach [33–35], by using P = W + A1 d2 relation. Where W is the minimum load to initiate plastic deformation and A1 is a load independent constant. The values of W and A1 have been estimated from the plots P vs d2 [Fig. 8(c)]. From these plots the estimate value of W represents the exact behavior of sample for total applied load region. All the estimated values of pure dyed crystals are depicted in Table 1.
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Table 1 Hardness parameters of BTZA and PRBTZA crystals. sample
Hv (kg/mm2 )
n
W
BTZA PRBTZA
95 116
2.12 1.57
−8.57 −6.72
4. Conclusion The bulk growth of dyed BTZA single crystal of size ∼30 mm × 7 mm × 5 mm was attained through solution technique. The crystal structure and vibrational modes were confirmed through X-ray diffraction and FT-IR spectroscopy analysis. The high intensity of diffraction peaks confirms the good crystalline nature of both BTZA crystals. The recorded diffused reflectance spectra for both crystals shows that there are two absorption bands at ∼387 and 558 nm in dyed crystals which are absent in pure. The enhancement in energy band gap was observed in dyed crystals which from 4.87 to 4.93 eV. In PL emission spectra there are two emission bands named UV-A and green emission at around 357 and 513 nm in dyed crystals and also the luminescence intensity is enriched. DSC study confirm that the grown crystals are stable up to 195 ◦ C. The reduction in the value of dielectric constant was observed in dyed crystals indicating that high interaction of dye showing less dipole response to the applied alternating electric field. The mechanical parameters are found to be enriched in dyed crystals. The improvement in the properties of dyed BTZA crystals make them applicable in electro-optic devices. Conflict of interest There is no conflict of interest exists in the current article. Acknowledgement The authors would like to express their gratitude to Research Center for Advanced Materials Science – King Khalid University, Saudi Arabia. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Phenol red dyed Bis thiourea Zinc acetate crystal growth and characterization for electro-optic applications V. Ganesh a,c , Mohd. Shkir a,c,∗ , I.S. Yahia a , Jafar M. Parakkandy b , S. AlFaify a,c,∗ a Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia b Department of Physics, College of Science and Humanity studies, Prince Sattam bin Abdulaziz University, Alkharj, Kingdom of Saudi Arabia c Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha-61413, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 13 November 2017 Accepted 19 December 2017 Keywords: Phenol red dye Crystal growth FT-IR spectroscopy Optical properties Thermal properties
a b s t r a c t In current work, the growth of dyed Bis thiourea Zinc acetate single crystals of size ∼30 mm × 7 mm × 5 mm is achieved by solution technique. X-ray diffraction and FT-IR spectroscopy are used to verify crystal structure and functional groups. Intensity of diffraction peaks confirms good crystalline nature of dyed Bis thiourea Zinc acetate crystals than pure. Two absorption bands at ∼387 and 558 nm were observed in diffused reflectance spectra of dyed crystals. The energy gap was calculated for both crystals and establishes to be ∼4.87 eV for pure and ∼4.93 eV for dye crystals. Photoluminescence spectra were recorded and a green emission band was observed in dyed Bis thiourea Zinc acetate crystal at ∼513 nm was observed in dyed crystal. The grown crystals are found to be thermally stable up to 195 ◦ C confirmed by differential scanning calorimetry. The value of dielectric permittivity is found to be reduced in dyed crystal. Microhardness of dyed crystals is much better than pure crystal. Surface study shows that the dyed crystals contain lesser defects than pure. Hence, the improved properties of dyed Bis thiourea Zinc acetate make it more useful than pure in electro-optic devices. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Semi-organic crystals have been studied extensively for many years due to its promising applications in opto-electronic devices [1,2]. The beauty of these crystals is that they can easily interact with many dopants like organic, inorganic, metalorganic and dyes [3,4]. Among all the dopants, dye interaction with semi-organic crystals has been focused in lime light due to its potential applications in display and modern optical devices [5–7]. In addition to this, a small amount of dye in to semi-organic material can alter huge second and third harmonic properties of host material. Among all these semi-organic materials bis thiourea zinc acetate is consider a potential candidate to interact with various dopants due to its simple nature. Semi-organic crystals contain dye have been attracting many researchers as an alternative of polymers and glasses due to their huge applications in the field of photonic devices [3,8–10]. In past few years dying of crystal is in a lime light due to their usage in solid state micro lasers [6,7]. Dying of crystal with matrix of semi-organic materials increases intermolecular band strength and provide highly stable physical parameters like thermal conductivity, less dislocations, low scattering with
∗ Corresponding authors. E-mail addresses: [email protected] (Mohd. Shkir), [email protected] (S. AlFaify). https://doi.org/10.1016/j.ijleo.2017.12.097 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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Fig. 1. As grown single crystals of pure and dyed BTZA.
interaction of light, high stability and high photoconductivity [11]. In literature many workers have been studied the effect of dye especially in solution growth crystals by their sophisticated experimental techniques and concluded that the interaction of dye enhances the growth sectors of pure crystals and modify the various growth parameters [12–15]. Hence, in view of this, here we have chosen efficient semi-organic Bis thiourea Zinc acetate (BTZA) as a host material for the interaction of Phenol red dye. There is sufficient reports are already available on host material viz., growth, structural, optical, spectral and nonlinear optical properties [16–18]. But surprisingly there is no reports are found on dying of such important crystal. Hence, in present study, we report a systematic study on dying effect of BTZA crystals on various physical parameters and their results are discussed in their respective sections. Hence, in view of this, here we have chosen efficient semi-organic Bis thiourea Zinc acetate (BTZA) as a host material for the interaction of Phenol red dye. Therefore, in present study, we report a systematic study on dying of BTZA crystals on various physical parameters and their results are discussed in their respective sections.
2. Experimental 2.1. Crystal growth and solubility Commercially available thiourea and zinc acetate of AR grade have been purchased from Merck and these materials are dissolved in Millipore double-distilled deionized water in 1:2 ratio. In case of pure material the dissolved salts are allowed for slow evaporation in a container covered with a polythene sheet with small pores and the BTZA was acquired via a slow evaporation process. In case of dye addition, a supersaturated solution of repeatedly recrystallized BTZA salt was used, and a quantity of ∼ 3 mol% of Phenol red dye was added to pure solution. The prepared solutions of pure and dyed were continuously stirred for 3days to get homogeneous solution. The solutions were later filtered and kept into a temperature bath for slow evaporation process. Further, the pure and dyed BTZA crystals of size 30 × 6 × 8 mm3 and 38 × 7 × 8 mm3 respectively were obtained in a period of 15 days (Fig. 1).
2.2. Characterization techniques To identify the structural parameters of pure and dyed BTZA crystals were analyzed by powder X-ray diffraction on a Shimadzu X-600 Japan powder X-ray diffractometer. The effect of dyeing was determined by scanning electron microscopy/EDX studies using JEOL JSM 6360 LA, Japan. The functional groups were determined using thermo scientific DXR FT-IR spectrometer by KBr pallet method. The diffuse reflectance study was carried in the range 190–1200 nm using Shimadzu UV–vis-NIR spectrophotometer (model UV-3600). To identify the thermal stability a Differential scanning calorimetric (SETARAM DCS) was used in the temperature range from 30 to 300 ◦ C. The optical device property of grown crystals was also measured by Photoluminescence study using a Thermo Fisher Scientific Lumina fluorescence spectrometer in the wavelength range of 300–550 nm at 300 K. Further, the dielectric and mechanical studies were carried out.
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Fig. 2. (a) XRD patterns and (b) EDX spectra for pure and dyed BTZA crystals.
3. Results and discussion 3.1. Powder X-ray diffraction and EDX studies To analyze the effect of dye on the structural properties of BTZA crystal a powder X-ray diffraction patterns were recorded as shown in Fig. 2(a). The sharp diffraction peaks in both patterns confirms that the grown crystals are of good crystallinity and the crystallinity of pure BTZA is not much affected by dye. Moreover, the lattice parameters were determined using the recorded data of X-ray diffraction for both samples through POWDEX software. The calculated lattice parameters are found to be: For pure BTZA: a = 6.93747 Å, b = 17.67306 Å, c = 11.78813 Å, V = 1334.35438 Å3 , ␣ = ␥ = 90.000◦ ,  = 112.596◦ . and for PRBTZA, a = 6.93676 Å, b = 17.67377 Å, c = 11.78957 Å, V = 1334.44764 Å3 , ␣ = ␥ = 90.000◦ ,  = 112.594◦ . The calculated parameters suggests that no structural modification is observed for dyed BTZA crystals apart from a minute disparity in their values and are in harmony with earlier data [16]. The close view of diffraction patterns in two different ranges has been also shown in Fig. 2(a) and which confirms that there is a minute variation in the peaks position towards higher angle due to dye interaction with BTZA matrix.
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Fig. 3. FT-IR spectra for (a) pure and (b) dyed BTZA crystals.
Furthermore, we have also recorded EDX spectra for pure and dyed crystals as shown in Fig. 2(b). EDX spectra confirm the synthesis of BTZA crystal, however a small variation in the elemental ratio was observed. 3.2. Vibrational analysis To identify the signature of the functional groups in pure and dyed BTZA samples, the FT-IR spectrum was recorded using a thermo scientific DXR FT-IR spectrometer by KBr pallet method. From Fig. 3 the absorption peaks observed at 3848 cm−1 , 3736 cm−1 , 3621 cm−1 and 3650 cm−1 are related to characteristic O H bond stretching vibrations are shifted two lower frequency side indicates the interaction of dye with pure material. The N H absorption peaks at observed at 3100–3400 cm−1 in pure material are completely missing in dyed samples and shifted to lower frequency side suggest that the adsorption of phenol red on the surface BTZA crystal. The symmetric and asymmetric C S stretching vibrations at 770 and 1404 cm−1 are completely broadened in case of dyed samples phenol red is bound to the pure crystal matrix. Further all the lower wavelength region groups are also shifted to lower wavelength region indicates the coordination of dye molecules with BTZA crystal. The identified functional groups provide evidence of the successful incorporation of dye in the BTZA crystal. 3.3. Optical studies 3.3.1. Diffused reflectance analysis To see the effect of dye on the optical properties of BTZA crystals the diffused reflectance measurement was carried in solid state by preparing the powders of the grown crystals as shown in Fig. 4(a). From figure it can be observe that there is a variation in cut-off wavelength in dyed crystals compare to pure. As a signature of dye we observed some new absorption bands in dyed crystals at wavelength about 387 and 558 nm which are absent in pure BTZA crystals and confirms the interaction of dye with BTZA matrix [8,9,12,13,15,19]. Moreover, for calculating the energy gap (Eg ) we have used Kubelka-Munk method (1−R)2 , where R is absolute 2R F (R) , here t the thickness of the t
and Kubelka-Munk function [F(R)] was calculated using Kubelka-Munk relation [20]: F (R) = reflectance and the absorption coefficient (˛) can be written in terms of F(R): ˛ =
absorbance t
circular sample holder (viz. ∼2 mm). The Eg can be determined using the equation: (˛h)
1 n
= =
F(R)h s
1n
A h − Eg , here all
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Fig. 4. (a) DR spectra and (b) Tauc’s plot for pure and dyed BTZA crystals.
Fig. 5. PL spectra for pure and dyed BTZA crystals.
2
symbols have common sense and for direct allowed transition the values of n= 1/2 has been used. The Tauc’s plot [(˛h) vs h plot] has been presented in Fig. 4(b). The energy band gap from the plot are found to be ∼4.87 eV and 4.93 eV for pure and dyed BTZA crystals. The obtained values shows that the energy gap has been enriched due to dye, however these are very high values and hence can be useful in optoelectronic applications [21–23]. It may be mentioned here that a new energy gap is observed which is ∼2.11 eV and assigned to presence of dye in BTZA crystals.
3.3.2. Photoluminescence (PL) analysis To know the effect of dye on luminescence properties of BTZA crystals we have measured the PL as shown in Fig. 5 using the excitation wavelength, exc = 320 nm at 300 K. PL spectra show two emission bands at 358 nm and 547 nm in pure BTZA crystal and at 357 and 513 nm in dyed crystals which can be assigned to UV-A emission and green emission bands respectively. There is remarkable shift in the strong intensity emission band was observed at higher wavelength due to interaction of dye with BTZA. The luminescence intensity is also found to be remarkably enhanced in dyed crystals compared to pure which may be predicted as defects created by dye that acts as color centers.
3.4. DSC analysis The thermal study of pure and dyed BTZA crystals are carried out using Differential calorimetric technique in a homogeneous nitrogen atmosphere at a heating rate of 10 ◦ C min−1 . Fig. 6. Show DSC micrographs of the present studied samples. From this figure it is clearly understood that there is sharp endothermic peak is observed in pure and dyed crystals, which indicates that there is no decomposition of samples occurred before melting point. The endothermic peak at 194 ◦ C and 196 ◦ C for pure and dyed samples shows good thermal stability of material and the increase in melting point indicates the effect of dyeing in pure samples. The observed values in case of pure BTZA crystals are near to the previously reported data [18,24], whereas in case of dyed crystals these values are quite high, Which might be due to high interaction of dye molecules with pure material.
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Fig. 6. DSC spectra for (a) pure and (b) dyed BTZA crystals.
Fig. 7. (a) Dielectric constant (b) dielectric loss and (c) total electrical conductivity plots for pure and dyed BTZA crystals.
3.5. Dielectric studies The dielectric measurement was carried using a KEITHLEY 4200-SCS over a wide range of frequency range of 1 kHz–10 MHz at 290 K. Dielectric constant (1 ), dielectric loss (2 ) and total alternating current conductivity (ac ) samples were calculated by measuring the capacitance (C), Impedance (Z) and loss tangent (tan␦), with a parallel plate capacitor method using the following relations [25,26]: ε1 = εCdA , and ε2 = tanı × ε1 , where εo is free space permittivity (εo = o
8.854 × 10−12 F m−1 ), d and A are the thickness and area of crystal. The dielectric constant of the pure and dyed BTZA sample was measured by varying the frequency as shown in Fig. 7(a). From figure it is clearly observed that the dielectric constant is high at lower and higher frequency regions. However in both the frequency regions, the pure material is showing high dielectric constant compare to the dyed sample, indicating that high interaction of dye showing less dipole response to the applied alternating electric field and suggest that these materials useful in various devices [27]. From Fig. 7(b) suggests that the dielectric loss in both the crystals is decreasing with the increasing the frequency and it is less in dye doped crystals compared to pure crystal. The reason for the low dielectric loss in dyed crystal could be
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Fig. 8. Plots of (a) load variation (b) lnp vs lnd and (c) p vs d2 of pure and dyed BTZA crystals.
explained due to interaction of dye molecules with pure material that reduced the defects and increase the quality of the d crystal. Further, the alternating current conductivity (ac ) was calculated using the following equation: tot. ac = ZA , and s tot. ac = dc + Bω , here, dc direct current conductivity, B is constant, angular frequency and s is frequency exponent. Fig. 7(c) shows the alternating current conductivity of the pure and dyed samples. From figure it is clearly understood that the ac of both the crystals obeys the universal frequency power law behavior. The value of s was obtained by the slopes of the curves and the values are ∼1.004 for pure and 1.001 for dyed crystals with standard error 0.055 and 0.057, respectively, this indicates that the hopping motion in grown crystals involves a translation motion with a sudden hopping.
3.6. Mechanical studies The study of the strength parameters of any material gives the information about various physical parameters like intermolecular distance; lattice energy, lattice perfection and about stability [28–30]. Among the different strength parameter testing methods the Vicker’s microhardness, is simple and non-destructive technique, to identify hardness of the soft and hard materials. In the present study, the microhardness of the Pure and dyed BTZA crystals were examined at different loads of ranging 10–100 g for a constant indentation time of 15 s. All the indentations of present crystals were made on (200) face and corresponding diagonal lengths were measured for hardness calculation. The hardness number (Hv ) was calculated using the relation, HV = 1854.4 P/d2 , Where P is the applied load in Kg and d is the diagonal length in m [31]. Fig. 8(a) shows the variation of Vicker’s hardness with applied load, from figure, it is clear that the hardness of the crystals increases with an increase in the applied load and attain the saturation. The reason for the increasing hardness of the pure and dyed samples is discussed in our previous work on various types of crystal [8,10]. However, in present study the hardness of the dyed samples are showing higher hardness value compare to the pure material indicating that interaction of dye makes pure BTZA as harder substance. The work hardening coefficient (n) of the grown crystals was determined using Meyer’s relation of P = A dn , the slope of the present relation gives n value by drawing the plots of ln P versus log d shown in Fig. 8(b). From this plots the n is found to be 2.12 and 1.57 suggest that dyed BTZA crystals are harder than pure material [32]. Further, the corrected hardness and minimum load for plastic deformation is also calculated by using the well-known Hay’s–Kendal approach [33–35], by using P = W + A1 d2 relation. Where W is the minimum load to initiate plastic deformation and A1 is a load independent constant. The values of W and A1 have been estimated from the plots P vs d2 [Fig. 8(c)]. From these plots the estimate value of W represents the exact behavior of sample for total applied load region. All the estimated values of pure dyed crystals are depicted in Table 1.
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Table 1 Hardness parameters of BTZA and PRBTZA crystals. sample
Hv (kg/mm2 )
n
W
BTZA PRBTZA
95 116
2.12 1.57
−8.57 −6.72
4. Conclusion The bulk growth of dyed BTZA single crystal of size ∼30 mm × 7 mm × 5 mm was attained through solution technique. The crystal structure and vibrational modes were confirmed through X-ray diffraction and FT-IR spectroscopy analysis. The high intensity of diffraction peaks confirms the good crystalline nature of both BTZA crystals. The recorded diffused reflectance spectra for both crystals shows that there are two absorption bands at ∼387 and 558 nm in dyed crystals which are absent in pure. The enhancement in energy band gap was observed in dyed crystals which from 4.87 to 4.93 eV. In PL emission spectra there are two emission bands named UV-A and green emission at around 357 and 513 nm in dyed crystals and also the luminescence intensity is enriched. DSC study confirm that the grown crystals are stable up to 195 ◦ C. The reduction in the value of dielectric constant was observed in dyed crystals indicating that high interaction of dye showing less dipole response to the applied alternating electric field. The mechanical parameters are found to be enriched in dyed crystals. The improvement in the properties of dyed BTZA crystals make them applicable in electro-optic devices. Conflict of interest There is no conflict of interest exists in the current article. Acknowledgement The authors would like to express their gratitude to Research Center for Advanced Materials Science – King Khalid University, Saudi Arabia. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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