Enhancement of Optical, Piezoelectric, and Mechanical Properties in

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Enhancement of optical, piezoelectric and mechanical properties in crystal violet dye doped benzophenone crystal grown by Czochralski technique Binay Kumar, Harsh Yadav, Nidhi Sinha, and Nidhi Tyagi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00792 • Publication Date (Web): 19 Aug 2015 Downloaded from http://pubs.acs.org on August 24, 2015

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Crystal Growth & Design

Cover Page

Enhancement of optical, piezoelectric and mechanical properties in crystal violet dye doped benzophenone crystal grown by Czochralski technique Harsh Yadav a, Nidhi Sinha a,b, Nidhi Tyagi a, Binay Kumar a,* a

Crystal Lab, Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India

a,b

Department of Physics & Electronics, SGTB Khalsa College, University of Delhi, Delhi

110007, India *Corresponding author: Dr. Binay Kumar; M: 9818168001; [email protected]

Abstract Pure and crystal violet dye doped (0.05 M%) benzophenone single crystals have been grown by Czochralski technique. Morphology of the grown crystal was analyzed by geometrical and bond energy consideration. Structural characterizations have been performed by powder X-ray diffraction analysis. Crystalline perfections of the grown crystals were examined by high resolution X-ray diffractometry (HRXRD). From UV-Vis measurements, increased transmittance with characteristic absorption at 590 nm for CV dye was observed in doped crystal. The photoluminescence intensity has increased by the effect of dye. Dielectric study was performed with temperature at various frequency ranges. In conductivity study, the activation energy in a temperature range of 30 – 44 °C for pure and doped crystals were found to be 0.24 eV and 0.22 eV at 5 KHz. The piezoelectric charge coefficient (d33) for pure and dye doped crystal was improved from 1 to 3 pC N-1 along [001]. The non-linear optical characterization has shown the

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increased SHG efficiency of dye doped crystal. Mechanical properties of the grown crystals have been studied experimentally by Vicker’s microhardness technique and variability of the measured data was statistically analyzed by Weibull distribution. No structural changes were observed after indentation in Raman studies confirming the durability of crystal.


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Title Page

Enhancement of optical, piezoelectric and mechanical properties in crystal violet dye doped benzophenone crystal grown by Czochralski technique Harsh Yadav a, Nidhi Sinha a,b, Nidhi Tyagi a, Binay Kumar a,* a

Crystal Lab, Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India a,b

Department of Physics & Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India

*Corresponding author: Dr. Binay Kumar; M: 9818168001; [email protected]



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Abstract Pure and crystal violet dye doped (0.05 M%) benzophenone single crystals have been grown by Czochralski technique. Morphology of the grown crystal was analyzed by geometrical and bond energy consideration. Structural characterizations have been performed by powder X-ray diffraction analysis. Crystalline perfections of the grown crystals were examined by high resolution X-ray diffractometry (HRXRD). From UV-Vis measurements, increased transmittance with characteristic absorption at 590 nm for CV dye was observed in doped crystal. The photoluminescence intensity has increased by the effect of dye. Dielectric study was performed with temperature at various frequency ranges. In conductivity study, the activation energy in a temperature range of 30 – 44 °C for pure and doped crystals were found to be 0.24 eV and 0.22 eV at 5 KHz. The piezoelectric charge coefficient (d33) for pure and dye doped crystal was improved from 1 to 3 pC N-1 along [001]. The non-linear optical characterization has shown the increased SHG efficiency of dye doped crystal. Mechanical properties of the grown crystals have been studied experimentally by Vicker’s microhardness technique and variability of the measured data was statistically analyzed by Weibull distribution. No structural changes were observed after indentation in Raman studies confirming the durability of crystal.

1. Introduction Nonlinear optics (NLO) has emerged in a vast manner by the advancement of material science in the growth of promising organic and inorganic single crystals. Organic crystal has considerable potential in photonic and potential second harmonic efficiency (SHG) efficiency as compared to its counterpart inorganic crystal [1-2]. Owing to their large second-order nonlinear optical susceptibilities and high potential for optical frequency conversion, organic crystals generally 4 ACS Paragon Plus Environment

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cover a wide spectrum of the optical frequency-mixing field [3]. Organic materials have also shown a promising response in the optical data storage, communication application, optical switching and THz wave generation and detection [4]. The optical (linear and non-linear) and piezoelectric properties of the crystal are highly anisotropic in nature [5-6]. The piezoelectric response of the organic crystal with low dielectric constant makes it applicable for microwave electronic devices used in communication and sensor [7]. The optical and piezoelectric properties of the organic crystal have easily tailored by solid solutions or mixed crystal and impurities in the solid [8]. SHG efficiency of organic crystals has improved by introducing the NLO active chromophores in its lattice [9]. However, it is very challenging to grow active NLO chromophores into large size single crystal with high crystallinity, which compromises various device applications. To overcome this problem we have doped the crystal violet (CV) dye molecules in the organic host material. CV is a considerable NLO active cationic chromophore with high values of hyperpolarizability [10]. CV dye molecules are difficult to crystallize into large single crystal, but in the organic lattice matrix it can easily crystallize to show promising NLO behavior with improved crystallinity for practical applications. Dye absorption on the crystal plane depends on the various factors like electrostatic potential, degree of protonation, crystal surface conditions and steric exclusion of the dye molecules [11]. Benzophenone is a well-known NLO active material in the family of organic crystals. It has a great interest to the scientific community due to its attractive optical and piezoelectric properties [12-13].

Benzophenone

single

crystals

were

grown

by

various

techniques

like

Sankaranarayanan-Ramasamy (SR) and Czochralski (CZ) techniques [14-15]. Unidirectional SR grown bulk benzophenone single crystal has shown the plastic deformation and strain in the 5 ACS Paragon Plus Environment


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crystal by the confined growth inside the ampoule [16-17]. The growth rates of the different crystal faces were observed in the melt growth of the benzophenone crystal [18]. Growth rates of the crystal faces are decided by the intermolecular binding energy. The growth rate of (001) plane was higher than other faces. In microtube-Czochralski technique, the morphology of benzophenone crystal was changed by varying the rotation rate during the initial period of the nucleation [19]. The laser damage thresholds of pure benzophenone crystal at the wavelength of 1064 nm and 532 nm are 17.6 GW/cm2 and 39.5 GW/cm2, respectively [20]. Benzophenone crystals belong to orthorhombic lattice with non-centrosymmetric space group P212121, which is the prerequisite for NLO activity [21]. M. Tachibana et al. was reported the various possible dislocations introduced in the benzophenone crystal grown by CZ method [21]. The burger vector of the dislocation is proportional to the shortest lattice parameter, which is along [001] in the case of CZ grown benzophenone crystal due to the shortest value of c = 7.88 Å. In the present work, crystal morphology of the grown crystal was analyzed by both geometrical and interaction energy between crystallizing units [22-23]. Geometrical consideration was explained by Bravais-Friedel-Donnay-Harker (BFDH) law and in consideration of bond energies were analyzed by Hartman-Perdok theory. Piezoelectric properties of the organic crystals are highly influenced by the π-π stacking arrangement in the crystal structure, which plays an important role to screen the H bond polarization in the applied electric field [24]. Microhardness of the material is one of the key parameter to determine its mechanical response for its utility in device formation. The Vicker’s microhardness technique was used to characterize the hardness of the grown crystal [25]. Statistical analysis of the indentation parameter has been discussed by Weibull model, which shows the reliability and variability in the measured data [26-27]. In this work, we have reported the piezoelectric properties of benzophenone crystal and examined its


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dependence on CV doping which is not covered in the literature. The structural (powder XRD and HRXRD), optical (UV-Vis, PL, SHG, etc.), dielectric and mechanical properties have been investigated as a result of CV doping. 2. Material synthesis and crystal growth 2.1. Crystal morphology The morphology of crystals depends on the internal and external factors. Crystal morphology has a significant impact on the bioactivity of pharmaceutical, dense packing and separation of the materials etc [28]. According to the geometrical consideration, BFDH law is used to predict the crystal morphology. BFDH law states that the growth rate of a particular plane of the crystal is inversely proportional to its interplanar distance. We have solved the morphology of benzophenone crystal using algorithm of BFDH law with the help of WinX-morph Software (Figure 1a) [29].



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Figure 1. (a) Morphology of benzophenone crystal based on BFDH law and (b) Hartman-Perdok theory.

Besides the geometrical explanation, Hartman-Perdok theory [30] gives the more vision to explain the discrepancy in the BFDH law. Crystal structure has formed by the various kinds of bonds present inside to maintain the translation symmetry. The characteristic of the bonds is defined in many ways, strong (ionic and covalent) and weak (van der Waals) forces, respectively. Hartman-Perdok theory connects the relation between crystal morphology and bonds present in the crystal structure. By considering above intuition, crystals have grown in the


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direction of strong bonds and these bonds must form an uninterrupted periodic chain in the crystal system. These periodic chains are known as a periodic bound chain vector (P.B.C. vector). Based on the position of P.B.C. vectors, crystal faces are classified into three classes; flat faces (F), stepped faces (S) and kinked faces (K). F faces are parallel to the two or more P.B.C. vectors, whereas S faces are coplanar to one P.B.C. vector and finally K faces do not contain any single P.B.C vector [31-32]. The rate of growth of P.B.C. vectors in the direction of F faces are much higher than S and K faces, whereas the rate of growth of S faces is larger than K. The order of probability associated with the crystal faces appearance is following as, F > S > K. We have modeled the P.B.C. vectors in the crystal by the analyses of intermolecular interactions. In order to understand the intermolecular interactions in the crystal system we have used the Hirshfeld surfaces [33]. To visualize the both inner and outer interaction on Hirshfeld surfaces were mapped by introducing dnorm. The dnorm is defined as [33], d =

d − r r

+

d − r r

where de is the distance from the surface to the nearest nucleus outside the surface and di is the distance from the surface to the nearest nucleus inside the surface. The radius parameters r and r are Van der Waals (vdW) radii of the appropriate atom internal and external to the molecular surface. For the sake of convenience, color code informations are used to interpret the different interactions on its isosurfaces: red for van der Waals (vdW) < dnorm, blue for vdW > dnorm and white for dnorm equals to zero. P.B.C. vectors are uniformly repeated in the whole crystal system. By using the Hirshfeld surface analysis, we can easily find the direction of strong hydrogen bond and weak π···π stacking (C···C and H···H) in the crystal structure. This intermolecular interaction plays a vital role to predict the crystal morphology. On the basis of 9 ACS Paragon Plus Environment


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Hirshfeld surfaces of the benzophenone molecule (Figure 1b), the (110) crystal plane has two P.B.C. vectors, which are shown in orange and purple color. Apart from this, the projections of the blue and purple color P.B.C. vectors are larger along (101) and (011) and has weaker π···π stacking along these planes. Weak π···π stacking makes a significant effect on the crystal morphology due to its low attachment strength as compared with strong hydrogen bond. Based on the position of P.B.C. vectors, (110) plane belongs to the F faces, (101) and (011) planes correspond to the S faces and the rest of other planes belong to the K faces. Moreover, (110) crystal face has higher morphological importance than (101) and (011), which is also predicted by BDFH law. 2.2. Synthesis and crystal growth Single crystals of pure and dye doped of benzophenone were successfully grown by CZ technique. The crystals were grown in the glass crucible of dimension 70 mm height and 50 mm in diameter. To maintain the growth of the crystal at constant temperature in CZ method, the crucible was placed in silicon oil for thermal stability. The vapour pressure of benzophenone is 1.57 Pa at 323.02 K [34]. The melting point of the pure and CV doped benzophenone crystals are found to be 321 and 321.5 K, respectively. Therefore, the melt charge of benzophenone was kept stable at 323 K, which is the essential requirement for CZ growth of the crystal. Seed crystals were prepared from the slow evaporation method. By using the seed of (001) orientation, pure benzophenone crystal was grown from the melted charge at 323 K with constant pulling and rotation rates were kept at 1.5 mm/h and 10 rpm. Crystal violet dye in 0.05 mol % was mixed in the melted charge of benzophenone at 323 K. For the growth of high quality crystal, the homogenous solution was prepared by stirring the dye doped melted charge for 10 h at constant temperature (323 K). Finally, (001) oriented single crystal was grown from CZ technique at the 10 ACS Paragon Plus Environment

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same previous conditions. The diameter and length of the grown crystals were maintained at 10 mm and 15 mm, respectively, by controlling temperature, rotational and pulling speed, etc., for both pure and dye doped benzophenone (Figure 2). The inset of Figure 2 shows the polished crystals of pure and CV doped benzophenone crystal.

Figure 2. As-grown pure and CV doped benzophenone single crystals by CZ technique and (inset) polished crystals.

2.3. Characterization techniques Powder XRD of the specimen was carried out by using a Rigaku diffractometer (Ultima IV, CuKα X-ray source λ = 1.5408 Å). The detailed analysis of the crystalline perfection was



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characterized using HRXRD by employing a multicrystal X-ray diffractrometer [15]. The rocking curves were recorded by using ω scan wherein the detector was kept at the same angular position of 2ωB with a wide opening of its slit. The well polished surfaces (002) of the grown crystals were used to record the diffraction curves. Liquid chromatography-high resolution mass spectrometry (LC-HRMS) was carried out by Accurate-mass 6530 Q-TOF LC/MS of Agilent Technology system in positive ion electrospray mode with selected ion monitoring of [M+H]+ peaks. The transmittance spectrum was recorded in the UV-vis range 200-1100 nm using a SHIMADZU UV-2501PC. PL excitation and emission spectra were measured by using a Horiba Jobin Yvon Spectrophotometer. Piezoelectric measurement was carried out using a piezometer (PM 300 Piezotest) at a tapping frequency of 110 Hz with tapping force 0.25 N. Dielectric analysis was carried out from 30 – 44 °C by an impedance analyzer (Agilent E 4980A) in the frequency range 1 kHz to 2 MHz. Raman spectra was recorded by using a Jobin-Yvon T64000 spectrophotometer in the range of 200-1800 cm-1. The Kurtz powder technique was used to investigate the SHG efficiency by using Nd:YAG laser source of wavelength 1054 nm. The Vicker's microhardness technique was used to measure the hardness of the material for a fixed dwell time of 10 second.

3. Results and discussion 3.1. Powder XRD analysis Phase, purity and crystallinity of the crystal were studied by using powder XRD method. Powder sample of the crushed crystals was used for XRD analysis in the range of 2θ from 10 - 60°. Xray diffraction patterns of the pure and CV doped benzophenone crystal were shown in Figure


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3a, b. Both the crystals belong to the monoclinic lattice with non-centrosymmetric space group P212121. The remarkable increase in the intensities of (002), (121), and (111) planes in CV doped crystal are observed. Dye molecules are not randomly distributed in the entire crystal structure; they are generally occupied the growth sector of the particular plane, which depends on the electrostatic potential and crystal surface condition etc. [35]. In the grown crystal, the projection of the π stacking intermolecular interactions present in the benzophenone molecule is higher along [001], which attracts the dye molecules to grow along with it. Therefore, this may be the reason for an increase in the intensity of (002) and other planes. As such, there is no significant shift in the peak position, which suggests that the crystal structure remains unchanged due to dye doping. The Le Bail method has been used to refine the crystal parameters by Expo2014 program [36] for pure and CV doped benzophenone crystal. The values of refined lattice parameters of the both crystals were depicted as Table 1.



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Figure 3. (a) X-ray powder diffraction patterns of the pure and (b) CV doped benzophenone single crystals.


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Table 1 Refined powder XRD data of pure and CV doped benzophenone crystal Parameters

Pure benzophenone

CV doped benzophenone

Crystal System

Monoclinic

Monoclinic

Cell length, a (Å)

10.30

10.32

Cell length, b (Å)

12.14

12.13

Cell length, c (Å)

7.99

7.99

Cell angle

90

90

Volume, (Å3)

999.08

1000.20

Rp, Rwp,

12.638, 17.269,

12.191, 15.764,

R-Bragg & R-F

1.508 & 1.409

3.176 & 3.071

α = β= γ (°)

3.2. High-resolution X-ray diffractometry analysis Figure 4a, b represents the high-resolution X-ray diffraction curves for pure and CV doped benzophenone single crystals using (002) diffracting planes. Rocking curves were recorded in the symmetrical Bragg geometry by employing a multicrystal X-ray diffractometer. On deconvolution of the diffraction curve of pure benzophenone crystal, it was observed that curve contains two additional peaks on the lower-angle side at an angular separation of 109'' and 190'' from the main peak. The internal structural low angle (tilt angle > 1' but less than a degree) and very low angle (tilt angle ≤ 1') boundaries are responsible for these additional peaks [37]. The tilt angles in these grain boundaries of pure benzophenone crystal are found to be 109'' and 81'' from



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there adjacent boundaries. The full width at half maxima (FWHM) of main peak and the low angle boundaries are 104'', 55'' and 157'' respectively. In the case of CV doped benzophonone crystal, the diffraction curve contains one additional peak which is 19'' away from the main peak on the lower-angle side. The FWHM of the main peak and the low angle boundary are 16'' and 19''. It is observed that the number of low grain boundary and its FWHM are decreased in CV doped benzophenone crystal. In the presence of appropriate dopants, the crystallinity of the material was improved [38-39]. CV dye makes a complex bonding structure with the benzophenone molecules which helps to remove the low angle grain boundaries present in the pure benzophenone single crystal. Therefore, the incorporation of CV molecule increases the crystalline perfection of benzophenone crystal.


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Figure 4. High-resolution X-ray diffractogram curves for (a) pure and (b) CV doped benzophenone single crystals.

3.3. LC-HRMS Figure 5a, b displays the LC-HRMS/MS spectra of CV doped benzophenone crystal. The main fragment results at m/z 183.0820 and empirical formula C13H11O correspond to the benzophenone molecule. The formation of the ion at m/z 408.2186 and empirical formula



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C25N3H31Cl belongs to the CV dye molecule. Therefore, the presence of CV dye molecules in doped benzophenone crystal was confirmed by LC-HRMS analysis.

Figure 5. (a) and (b) LC-HRMS/MS spectra of CV doped benzophenone crystal.


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3.4. UV-Vis analysis In UV-Vis region, the bonding electron has enough energy to excite into higher energy orbitals. Moreover, in the organic system the delocalization of π-electrons have a significant effect on the decrease in the band gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [40]. The optical transmission spectra of pure and CV doped benzophenone crystal is shown in Figure 6. An enhancement in the transmission spectra of CV doped crystal was observed, which reveals that the inclusion of dye reduces the structural grain boundaries. A small absorption peak at 590 nm is observed in the transmission spectra of CV doped benzophenone crystal. Various types of chromophores are responsible for the UV-Vis absorption in organic molecules [41]. The above-mentioned absorbance comes out due to the characteristic of CV dye present in the crystal system [42].

Figure 6. UV-Vis transmission spectra of pure and CV doped crystal. Absorbance peak at 590 nm is due to the effect of CV doping. 19 ACS Paragon Plus Environment


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The direct and indirect characteristic of band gap is an important aspect for photoluminescence and photoconductivity. Both radiative and non-radiative processes are equally significant for the efficiency of various applications. The topological aspect of stacking pattern in the molecules predicts the types of band gaps [43]. When the molecules are stacked in such a way that the intermolecular interaction occurs in between the same site of atoms, then an indirect band gap appeared. If the intermolecular interaction belongs to the atoms of different site, then the direct band gap is anticipated. In the case of benzophenone crystal, topological aspect of π-π stacking pattern is used to predict the band types. Figure 7 represents the stacking of different sites of carbon atoms in the benzophenone molecule by the π intermolecular interaction. Therefore, the topological pattern in benzophenone crystal leads to the formation of direct band gap.

Figure 7. Topology of the π-π stacking between carbon atoms of different sites in the benzophenone single crystal are represented by blue color dashed lines.


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Optical band gap of the materials was estimated on the basis Tauc relation: (αhν)n = A(hν - Eg), where n = 2 for direct transition and n = ½ for indirect transition [44]. Absorption coefficient ‘α’ is deduced using the relation: α = ln(1/T)/t, where ‘t’ is the thickness of sample and ‘T’ is the transmittance. The band gap of pure and CV doped benzophenone crystal were computed to be 3.33 eV and 3.35 eV respectively, which are shown in the plot of (αhν)2 vs. hν (Figure 8 a, b). The band gap of the CV doped crystal was not significantly changed. Therefore, optical properties of the doped crystal were not deteriorated. The enhancement in the transmission region is because of the decrease in the grain boundaries in CV doped crystal which makes the material more promising for optical applications.

Figure 8. (a) Plot of (αhν)2 vs. photon energy with evaluation of band gap of pure and (b) CV doped benzophenone crystal.



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3.5. Photoluminescence analysis Luminescence is very sensitive to changes in the local atomic configuration of the crystal structure. The variation in crystal momentum for allowing transition of the electron is followed by ∆K = Kelectron – Khole, where ∆K is equal to the momentum of UV-Vis or infrared wavelength, which is neglected (∆K = Khv = 0) as compared with the unit cell parameters of the solids. In direct band gap materials, conservation of crystal momentum must be satisfied for radiative recombination of electron and hole. Whereas, the competition between phonon emission and electron-hole recombination in indirect band gap material are responsible for the radiative emission. Photoluminescence (PL) spectrum of the pure and CV doped benzophenone crystal is shown in Figure 9. The excitation spectra of pure and doped crystal have showed a maximum absorption at 360 nm. The PL emission peaks of both crystals were found at 430 nm, which are recorded at the maximum absorption wavelength. The strong blue emissions with a significant difference in the PL intensities are observed in both crystals. Intermolecular interactions in the organic crystal play a vital role to determine its optical properties, which is very sensitive to small deformation in the crystal structure [45]. Deformation in the crystal system causes translational and rotational displacement in the molecule. An increase in the PL intensity of doped crystal is explained by the Rashba effect in which the dipole strength of the local exciton increases due to the increase in localization radius of the excitons by mixing complex impurity excited states and exciton band states [46]. Doping generally creates the defect levels in the crystal system, which causes to increase in the PL yield. It makes the CV doped benzophenone crystal more promising for laser and photonic applications.


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Figure 9. PL excitation and emission spectra of pure and CV doped benzophenone crystal.

3.6. Dielectric and conductivity analysis Dielectric behavior is directly related to the polarization of the material, which are further categorized into two parts, displacement and relaxation polarization. Displacement polarization is associated with the elastic displacement of the charged particles [47]. Relaxation polarization depends on the thermal orientation of the permanent dipole and weakly bound ions. In noncentrosymmetric space group materials, orientational polarization occurrs in the absence of an electric field, because the centers of negative and positive charge do not coincide. The plots of dielectric variation vs. temperature at different frequency range are shown in Figure 10a and



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Figure 10c for pure and CV doped benzophenone crystal. The dielectric constant is increased in the case of doped crystal in the frequency range of 5 KHz - 1.5 KHz and remains constant with temperature variation from 32 to 44 °C. CV dye has an ionic characteristic, which helps to make a complex-bonding network in the crystal lattice. Therefore, the net polarization of crystal is increased, which increases its dielectric constant. Figure 10b and Figure 10d have shown the dielectric loss of the pure and CV doped grown crystal. In the case of dye doped crystal, the value of dielectric loss is low due to the lesser defect concentration and increase in the crystalline perfection.

Figure 10. Evolution of dielectric constant and dielectric loss for pure (a, b) and CV doped (c, d) benzophenone crystal with frequencies over temperature range 30 - 44 °C. 24 ACS Paragon Plus Environment

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In highly pure state, organic crystals with conjugated π-electron system are generally insulator in the range of low electric field at RT or below [48]. The conductivity of this system is modified by the effect of defects and dopants. Further, by introducing these defects, thermally ionized charge carriers are generated resulting in an increased in its conductivity. At the same time, defects can also trap excess charge carrier and reduce its conductivity. The topology of the organic molecule also influences its conductivity. In the case of benzophenone crystal, the conductivity is low since the planer molecules stacks are not parallel and equidistant to one another. Figure 11 shows the temperature dependence (30 – 44 °C) of ac conductivity measurement of pure and CV doped benzophenone crystal at 5 KHz. The value of ac conductivity is decreased in CV doped crystal as the charge carriers are trapped in the defect levels.

The

activation

energies

are

calculated

using

the

Arrhenius

equation:

σac = σo exp(-Ea/kBT), where σo is the pre-exponential factor and kB is the Boltzmann constant. From the linear plot of ln σac vs. 1000/KT, the activation energies of pure and CV doped grown crystal were found to be 0.24 eV and 0.22 eV. In the case of CV doped crystal, the value of activation energy is decreased, because of the thermally generated charge carrier are trapped in the energy levels of the dye molecules.



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Figure 11. Plot of lnσac vs. 1000/KT at 5 KHz for pure and CV doped benzophenone crystal.

3.7. Piezoelectricity Piezoelectricity is defined as the linear interaction between mechanical and electrical parameters in non-centrosymmetric space group originated crystal structures [49]. Piezoelectricity depends on the coupled tensor relations between elastic variables (stress and strain) and dielectric variables (electric field and electric charge density). The direct piezoelectric effect is characterized by the piezoelectric charge coefficient dij, which is the ratio of electric displacement vector in the i direction and mechanical stress in the j direction. We measured d33 value, in which the developed charge is measured in the direction of applied force which is the ‘c-axis’ (represented by ‘3’) of the crystal. A well polished crystals with silver coated on the


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opposite planes for electrical contacts were chosen for piezoelectric measurement. The measured d33 values of the pure and the CV doped benzophenone crystal was found to be 1 pC/N and 3 pC/N along [001]. Piezoelectric response in the doped crystal is increased due to the effective polar orientation of the ionic dye molecules in the CV-doped crystal and it helps to generate more asymmetry of charge distribution on the application of pressure. Organic compounds are generally linked with polar hydrogen bonding and π⋯π stacking and the piezoelectric properties are highly influenced by the direction of these forces [50]. Due to the effects of stress on the crystal, the net polarization of hydrogen bonds is increased and it is suppressed by the shielding of the π⋯π stacking. Therefore, the resultant of hydrogen bonds and π⋯π stacking determines the net piezoelectric effect in a particular direction of the crystal plane.

3.8. SHG Second harmonic generation (SHG) efficiency in the grown crystals was studied by Kurtz and Perry powder technique [51]. The induce polarization is no longer a linear relation with electric field in the crystal due to the large optical field. SHG is the specific case in which frequency doubling takes place. It is essential for the NLO activity; the material should belong to noncentrosymmetric space group because polarization depends on the second order non-linear susceptibility tensor. Organic molecules are rich in delocalized π conjugate electron system, which is responsible for the higher NLO activity. Intermolecular interaction plays a prominent role for the delocalization of the wave function. For the SHG measurement, uniform particle size of 63 µm of the crystalline powder was filled in a microcapillary tube of 1.5 mm diameter. Nd:YAG laser of wavelength 1064 nm and energy 0.68 J/pulse was incident normally on the powdered sample. The experimentally observed SHG coefficient of the pure and CV doped 27 ACS Paragon Plus Environment


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benzophenone crystal were found to be 9.1 mJ and 12.4 mJ, respectively, while that for reference material KDP of identical particle size, it was 3.4 mJ. In the pure benzophenone crystal, the delocalization of the wave function is weak as compared to CV doped crystal. However, the effects of ionic CV molecules increase the delocalization of π electrons, which makes the CV doped crystal more efficient for laser applications. 3.9. Hardness The mechanical stability of the material is of great interest for its application in device fabrication. Indentation hardness of the material is characterized by Vicker’s microhardness technique in which the pyramid shape diamond with apex angle 136° was used. The three most important phenomena as observed in microhardness process are: anisotropy effect, indentation size effect (ISE) and reverse indentation size effect (RISE) [52]. ISE is referred as a decrease in hardness with increasing applied load, whereas in the RISE, hardness increases with increasing applied load. The hardness depends on the degree of lattice order of crystalline material and it is highly influenced by defects and intermolecular forces [53]. The Vicker’s microhardness number (VHN) Hv was computed using formula: Hv = KP/d2, where K = 2 sin (136°/2), P is the intender load and d is the average of diagonals [54]. Figure 12 shows the variation of microhardness number with applied load for both pure and CV doped benzophenone crystals along [001]. The microhardness values of the CV doped crystal are higher as compared to pure benzophenone crystal, which is due to the filling of the void space by dye molecules. Moreover, the ionic nature of CV dye helps to make the packing more compact by complex characteristic bond formation in the crystal. In both the crystals, RISE was observed up to 20 g, after that, Hv decreased drastically due to increases in the pile-up effect and energy was released in the form of cracks. The slip of the crystal planes is used to understand the plastic deformation in the single crystal. 28 ACS Paragon Plus Environment

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In respect of slip behavior it is known that (1) the direction of slip of the plane is along the densely atomic packed region, (2) slip generally occurs on the most closely packed plane, and (3) from the specified set of slip planes and directions only those planes and directions are activated for which the resolved shear stress is maximum [55]. CV doped crystal has a densely packed structure as compared to pure crystal. Therefore, in case of CV doped crystal the number of slip planes is increased which improves the hardness of the grown crystal.

Figure 12. Plot of Vicker’s microhardness number vs. indentation load for (001) planes of pure and CV doped benzophenone single crystal.

The Weibull distribution is widely used for modeling experimental data in modern statistics. It describes the reliability of the product over its life cycle under standard environment condition [56]. It covers a wide range of application of the life span testing for the mechanical strength of the brittle materials. Weibull distribution describes the failure probability of the varied events in 29 ACS Paragon Plus Environment


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a fixed sample space. The Weibull distribution function of two-parameter form is expressed as [57],

Fx = 1 − exp − ,

(1)

where ‘x’ is the hardness parameter, ‘x ’ is the characteristic value of the hardness below which 63.2% of the data lie and ‘m’ is the shape parameter or Weibull modulus, which represents the brittleness, reliability, repeatability of brittle materials and homogeneity of the testing data. The low value of Weibull modulus attributes to the high variability of material strength and viceversa [58]. Figure 13 shows the Weibull plot by taking the natural logarithms twice of the equation (1), i.e.: !

ln ln !"#$ = m&lnx − ln x '.

(2)

The mean value of F(x) is computed by arranging the data point in ascending series and considering

Fx = )! ,

(3)

where ‘i’ is the ith order of the ascending sequence of the data series and ‘n’ is the total number of data points. Weibull plot of hardness has been measured on the pure and CV doped benzophenone crystal surface with different loads (Figure 13a, b). Vickers microhardness numbers closely follow the Weibull distribution. Least square method was used to determine the Weibull parameters. Table 2 summarizes the statistical analysis results of hardness values of both pure and CV doped crystals. The smallest value of the correlation coefficient (R) between the measured data points


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and the fitted line was 0.917. The measured correlation coefficient values were found to be much larger than the critical correlation coefficient value (0.641) of the regressed line under 0.99 confidence level for the sample size of 15 data points. The value of Weibull modulus shows the significant change in pattern from pure to CV doped crystal. Moreover, it reflects the variability of the hardness measurement in pure and CV doped crystal.

Table 2. The statistical analysis results for pure and CV doped benzophenone crystal measured on [001]. Average hardness, Indenter load (g) 5

-2

Hv (Kg mm )

Weibull

Correlation

modulus, m

coefficient, R

Pure CV doped Pure CV doped Pure CV doped benzophenone benzophenone benzphenone benzophenone benzophenone benzophenone 0.73 0.74 45.03 16.50 0.989 0.949

10

1.15

1.41

17.61

29.15

0.948

0.983

20

1.64

1.64

62.65

15.71

0.977

0.948

.



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Figure 13. (a) Weibull plot of Vicker’s microhardness of pure and (b) CV doped benzophenone crystal


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3.10. Structral studies by Raman spectroscopy Indentation affects the structure of the pure and CV doped benzophenone crystals. The densification of the material caused by the indentation is directly observed in the Raman spectra [59]. Moreover, this non-destructive spectroscopy is used for the analysis of compressioninduced phase transformation in single crystals and ceramics [60]. Figure 14a, b shows the Raman spectra in the various marked regions on the plane (001) of pure and CV doped benzophenone crystal. Point A and B are located at the center and the rim of an indentation spot of load 20 g for dwell time 20 second, whereas point C is located outside the indentation region of the pure benzophenone crystal. A', B' and C' are the corresponding points in CV doped benzophenone crystal. The size of the indentation region (125 µm) is much larger than the spot size (0.7 µm) of laser. In Raman studies, the sharp peaks at the 1648 cm-1 and 561 cm-1 are assigned to the C=O stretching and bending modes. The band appears at 1598 cm-1 attribute to the C=C oscillations. The mode, involving stretching motions of the bridge carbon atoms was observed at 1148 cm-1. The variation of the Raman intensities of these three points is due to the densification of the material. Intensity gradually increases from center to rim and finally attains a maximum value for points ‘C’. It is easily concluded from the Raman peak intensities that the effect of densification is higher in the center of the indentation point and gradually decreases from center to surface of the crystal. In the case of CV doped benzophenone crystal, Raman peak intensities follow the same pattern, but the variation of the peak intensities are slightly decreased due to the increase in hardness of the crystal. The reason for the increase in the hardness of the crystal is that the doped dye molecules are fitted into the crystal voids and increases its rigidity. Moreover, indentation does not induce any structural phase transformation in the materials.



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Figure 14. Raman spectra of (a) pure and (b) CV doped benzophenone crystal at the various region of the indentation by 10 g for the fixed dwell time 10 s. The marked region (A, A'), (B, B') and (C, C') shows the spectra measured at central, rim and surface of the pure and doped crystal.

Conclusion In summary, unidirectional [001] bulk single crystals of pure and CV doped benzophenone have been successfully grown by the CZ method. Powder XRD confirms that both the crystals belong to the monoclinic lattice with non-centrosymmetric space group P212121. Theoretical modeling for the morphology of grown crystal was studied by BFDH law and which is also confirmed by the Hartman-Perdok theory. HRXRD analysis reveals that the structural grain boundaries are decreased as result of CV doping in benzophenone crystal. CV doped crystal has shown the good optical transparency in UV-Vis region. High PL yield in blue region has been observed in the dye doped crystal. SHG efficiency has improved by the effect of CV doping in the grown crystal. The piezoelectric coefficient was measured along [001] and its value was found to be 1 and 3 pC 34 ACS Paragon Plus Environment

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N-1 for pure and doped crystal. As a result of dye doping, Weibull modulus shows the high variability of the hardness number in case of CV doped crystal. Raman spectroscopy of the indented areas reveals that no new phase formation has been developed after the indentation. CV doped benzophenone crystal has shown the promising optical and piezoelectric properties which makes it useful for various optoelectronic, laser and patch antenna for wireless communication applications.

Acknowledgements The authors are grateful to DRDO for the financial support received in the project (Sanction No.ARMREB/MAA/2015/163) and DU R&D Grant (Sanction No. RC/2014/6820). We are thankful to Prof. Vinay Gupta, Physics Dept. DU and Dr. K. K. Maurya, NPL, New Delhi for UV-Vis and HRXRD measurements, respectively. Authors express their thanks to Dr. B. K. Singh, University of Aveiro, Portugal, for help in installing the CZ growth apparatus. Harsh Yadav is thankful to UGC for Merit Scholarship. Nidhi Tyagi is thankful to UGC for providing the Senior Research Fellowship.

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For Table of Contents Use Only Enhancement of optical, piezoelectric and mechanical properties in crystal violet dye doped benzophenone crystal grown by Czochralski technique Harsh Yadav, Nidhi Sinha, Nidhi Tyagi, Binay Kumar

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Hartman-Perdok morphology of photoluminescent and piezoelectric, pure and crystal violet dye doped benzophenone single crystals grown by CZ technique. UV-Vis and photoluminescence studies revels the better transmission and higher PL yield for CV doped crystal. SHG efficiency of the CV dye doped crystal is increased.



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Hartman-Perdok morphology of photoluminescent and piezoelectric, pure and crystal violet dye doped benzophenone single crystals grown by CZ technique. UV-Vis and photoluminescence studies revels the better transmission and higher PL yield for CV doped crystal. SHG efficiency of the CV dye doped crystal is increased. 74x47mm (300 x 300 DPI)

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Enhancement of Optical, Piezoelectric, and Mechanical Properties in

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