Piperazine based Zirconium Oxy-Chloride (PzZrOCl) single crystal: A Third Order Nonlinear Optical material for Opto-electronic Device Applications

Single crystals of Piperazine Zirconium Oxy-Chloride (PzZrOCl) are grown successfully by slow evaporation technique for Opto-electronic device applications. Suitability of the material is estimated and reported as follows. Crystal habits, purity, crystallite size, microstructure and bulk crystals structures are essential parameters for device compatibility hence they were examined with basic characterization techniques and studied to estimate their effects on optoelectronic properties. The outcomes from structural aspects are reasonably good. The functional groups of a molecule, their bond vibration frequencies and mode of alignments have been examined by Fourier Transform Infrared spectroscopy (FT-IR). The transmittance (98%) in the entire visible range with lowest cut-off wavelength (215 nm) and green emission (545 nm) is another evidence of suitability. The thermal and mechanical strength via hardness, Mayer index, yield strength and elastic stiffness constant is evaluated; it is thermally stable up to 162°C, mechanically strong and belongs to soft category. Laser induced damage is estimated by using Nd:YAG laser of wavelength 1064 nm and the observed values is three times higher than KDP, 1.2 times higher than LAPP. The optical nonlinear nature and its efficiency are examined by using Z-scan technique. The analysis discussed in detailed and the results strongly recommends PzZrOCl single crystals are suitable for optical and electronic device applications.


Introduction
Non-linear optics (NLO) explains the nonlinear response of frequency, phase and path difference of the incident light, polarization density of non-linear media as a function of higher orders electric field of the light. Up on the interaction of incident light with non-linear media, generation of light with a doubled, tripled frequency with respect to the incident light termed as Second harmonic generation (SHG), Third harmonic generation (THG). The non-linear processes are non-invasive, non-destructive and highly sensitive towards atomic structure and bonding status, these are promising techniques for interfacial study, light manipulation in nanoscale and in highly sensitive bio-molecular detection [1]. THG and odd-order processes are more convenient as they require less intense beam since the symmetry rules are relaxed in most of the optical materials. It is believed that THG has high conversion efficiency at the crystal-air interface [2] with a normal incidence but the detailed relationship excitation depth, incident angle and THG intensity yet to be mentioned. THG includes the potential applications like production of high power laser sources, high power conversion efficiency, in telecommunication for optical switching and signal processing, in transimition based devices, materials are required with high nonlinear refractive with minimal absorption losses. The other applications include 3D fluorescence imaging; 3D optical data storage [3][4][5][6][7][8][9] allows the focused research on the third order nonlinear optical material due to their efficient nonlinear phenomena. In the initial stage for the nonlinear applications inorganic materials are highly explored due to their physical properties, easy to synthesize due to ionic bonding nature. Some of the most useful crystals are like quartz, potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), barium metaborate, potassium niobium oxide and lithium niobium oxide single crystals etc.
These inorganic materials are widely employed in optical devices consequently they have dominated optical technology over the last 40 years. But due to inherent ionic nature and absence of extended electrons makes the available nonlinearities were limited. Later on organic nonlinear optical materials are focused due to their potentially high nonlinear susceptibility( ), hyperpolarisability( ), modification through standard synthetic methods, and relative ease of device fabrication, high damage resistance and rapid response to electro-optic effect compared to inorganic NLO materials due to delocalized bonds. Many new organic crystals have been examined by their molecular engineering approach and potential applications in telecommunication and optical information process [10]. Instead of screening the known materials for NLO applications, the researchers begin to design new materials with better nonlinearities. But most of the materials are unsuitable for device due to the limitations in mechanical strength, low dimensional properties, poor 3D-bonding and linear optical properties of organic materials. Circumstances forced the researchers to search new materials with combined favorable effects of both inorganic and organic materials for the rapid development in photonic technologies. Since photons are capable of processing the information with the speed of the light. The combined effect gives the research on semiorganics which includes the development in the materials with 1) High nonlinear coefficients, 2) High damage threshold, 3) low angular sensitivity, 4) Low UV cut-off; 5) Crystals can be grown in bulk with optimized conditions and 6) Good mechanical properties. It is a new approach for high efficiency where a polarizable organic NLO material is stoichiometrically bonded with an inorganic host material.
Semi-Organic materials possesses the combined properties of organic and inorganic which have more appropriate for device fabrication [11,12]. In this consequence we chose Piperazine as an organic NLO material since it plays an important role as a unit present in the more complex molecules studied in several field [13][14][15]. Piperazine in crystals has a chair conformation with N-H bonds in the equatorial positions and resides at the crystallographic inversion center [16].
The piperazine ring is used as a building block for pharmaceuticals [17,18] and found to be a biologically active compound due to its privileged structure in drug discovery. It is an achiral organic weak base with excellent hydrogen bond acceptor nature due to this reason it is capable of yielding both second and third NLO properties. Donor-acceptor groups and large πdelocalization present in the benzene derivatives are responsible for high molecular nonlinearity [19]. It has been found that some polycationic ligand, including Piperidine and Piperazine rings, exhibit a substantial degree of selective RNA building [20].
The other inorganic material is Zirconium oxychloride it is basically a strong acid when reacts with piperazine gives weak coordination bonds. The Zirconium (Zr) based material has high thermal, chemically stable, tunable refractive index [21] corrosion protection, good Young's modulus and high ionic conducting nature. Zirconium finds extensive use as a ceramic material it also has important applications in catalysts [22]. It is also observed that Zr based crystal complexes enhances the mechanical strength by reducing the plastic deformation [23].
Zirconium based materials are generally possess high transimition percentage, high band gap, high dielectric constant with high refractive index [24]. In the present investigation Piperazinium with Zirconium Oxy-chloride provides a chance to develop NLO materials for their practical applications. The NL phenomenon has been reported with widely used Z-scan technique. The Zscan technique is a popular method to measure the optical nonlinearity of a given material by using contrast generating mechanism. It has the advantage of high sensitivity and simplicity is used to measure the third order nonlinear optical properties [25,26].

Material synthesis and crystal growth
In the current work, base compound PzZrOCl has been synthesized from piperazine (Merch India, 99%) and zirconium oxychloride (Merch India, 99%) in equal molar (1:1) ratio. The solvent plays crucial role to grow bulk and transparent single crystals through its way of interaction with faces of crystal and can change the shape of the crystals. Sometimes the solvent effects stoichiometry of the composition and can produce different morphology. Hence in this work we chosen deionized water as a solvent and stoichiometrically weighted piperazine is dissolved in 100 ml solvent and stirred well for 1h at room temperature. To the above solution further add small amounts of zirconium oxy-chloride followed by stirring to obtain the homogeneous mixing. The homogeneous mixing allows the degree of bulk mass transfer for growth and influence the crystals shape. Well stirred solution was filtered with filter paper and allows the filtered solution to evaporate at the room temperature. The final solution was covered with polythene sheet and placed in a dark and dust free room for a period of 20 days. We observed colorless crystalline salt and it is recrystallized continuously to get purified salt. The purified salt is well dissolved again in deionized water and by following above procedure for a period of 15 days to get considerable size single crystals. The good quality crystals are crucial for assessing the best nonlinear properties. The reaction scheme of the title compound is shown in Fig. 1. The PzZrOCl single crystals were grown by slow evaporation solution technique. After the growth period of 35 days, a good quality bulk single crystal of dimension up to 10 × 10 × 7 3 has been harvested. Fig. 2 shows crystal pictures and their crystal habits, it is an external appearance of the crystals. It is observed that crystals of different shapes with various faces may be due to different saturation levels, it is a key factor for the growth kinetics. The well defined and several main habits of PzZrOCl are solved by WinX-morph software. The interfacial angles are estimated and established first by using a contact goniometer followed by optical goniometry.
All the face sets were indexed and found they are all well, equally developed. It is further seen that the growth rates along a, b and c axis are almost equal and acquired crystals are non-

Powder X-ray diffraction analysis
It is most widely used rapid and nondestructive technique to identify the unknown crystalline materials through their unique diffraction peaks originating from unique structure.
Hence the crystalline structure of PzZrOCl is examined by taking its finely grounded, (1) Where, is Scherrer constant (0.9), is X-ray wavelength, is the broadening at half maximum intensity (FWHM) and is Bragg's angle. Since crystallite phase and particle size is effective parameters in determining the Band gap. The observed crystallite size for the planes (321) and (401) is 0.30 and 0.34 respectively.

Single crystal X-ray diffraction (XRD) analysis
Visibly transparent, defect free (cracks and inclusions) and well shaped single crystal of PzZrOCl were selected for single crystals X-ray diffraction studies at the room temperature.
The obtained lattice parameter values are = 6.46 Å, = 12.81 Å, = 12.85 Å, = 89.95°, = 89.96° and = 90.03° and the volume of the unit cell found to be V=1063 Å 3 . The single crystal XRD studies reveal that the PzZrOCl crystal belongs to the tetragonal system with space group P. The unit cell volume is increased 4.3 times to that of Piperazine monoclinic, [27]. The increase in cell volume may be due to the development of internal strain caused by the monoclinic, ( 2 1 / ) distortion in to tetragonal ( ). It confirms the reaction between zirconium oxychloride with piperazine. The change in crystal system confirms the zirconium oxy chloride doped in host lattice.

FT-IR analysis
The advantage of FT-IR spectroscopy is the structural identification of functional groups.
In this analysis all the functional groups of molecule absorbs the energy in definite frequency range and causes bond vibrations in stretch or bend with respect to neighboring molecules. The observed vibrational frequencies were confirmed quantitatively and their assignments are recorded in FTIR spectrum which is shown in Fig.3. The heteroaromatics N-H group shows its N-H stretching absorption at 3240 cm -1 [28]. The symmetric and asymmetric stretching of C-H is observed at 2821 cm -1 and 2757 cm -1 . The presence of peak at 2620 cm -1 confirms N-H--O vibration of the amine with zirconium oxy chloride. The presence of water is confirmed by a shoulder peak at 1666 cm -1 . The peak at 1558 cm -1 is assigned to aromatic ring vibrations. The aliphatic CH2 groups are present at 1441 and 1381 cm -1 respectively. The peak appeared at 1381 cm -1 is due to CH2 bending mode. The evident of Zr-O stretches are illustrated at 1319 cm -1 [29].
The peaks at 1198 and 1063 are due to asymmetric and symmetric stretching vibrations of C-N group. FTIR absorption peak at about 888 cm -1 region is due to Zr-O vibration [30,31]. The peak at 761 cm -1 is due to N-H bending mode. The CCN deformation is assigned at 579 cm -1 . All these observations represent the presence of zirconium oxy-chloride in the piperazine crystal lattice.

UV-vis-NIR spectral analysis
The optical absorption and transmittance spectrum of the grown crystal was recorded and the resultant spectrum is shown in Fig.4. Generally, the spectrum in UV visible is due to the transition from one electronic sate to other electronic state hence the absorption depends on the electronic structure of the molecule. From the spectrum, the transimition is quite low below the band gap region; it may be due to presence of defects or free charge carriers. Further, the transmittance suddenly decreases in the short wavelength region due to the band gap absorption.
Moreover, from absorption spectrum it is clear that the absorption is decreasing with increasing wavelength and becomes very small in visible to IR region ( > 300 ) hence the crystal is transparent in the entire visible to IR region. The UV cut off wavelength is found to be 215 nm below which the material itself absorbs all the light. The UV cut off is due to electronic transition from top of the valence band to bottom of the conduction band i.e. → * or ligand to metal.
Cut off wavelength is an essential parameter for deciding the candidature of a material. The observed cutoff wavelength is shorter than the reported cutoff wavelength 260 nm of piperazine.
Such a lower cut off is due to lack of delocalized charge distributions. Generally, the drop in the cutoff wavelength is due to incorporation of some unknown impurities [32], but in present case indicates the incorporation of zirconium oxy-chloride in piperazine lattice. In present case the transmission level. There is no considerable absorption of light to any appreciable extent in the entire visible region of electromagnetic spectrum, which is the intrinsic property of all amino acids such as absence of strong conjugated bonds and also filled 10 shells of the metal ion in the complex [33]. These results infer PzZrOCl is suitable candidate for frequency doubling.

Photoluminescence analysis
Photoluminescence (PL) is a type of electromagnetic spectroscopy that investigates fluorescence from a given material. The structural perfection, crystalline quality and luminescence property of the grown crystal are identified using the fluorescence study. It has wide application in the branches of medical, chemical and biochemical research fields. PL studies are most preferable compare to optical absorption to detect the lower concentration of defects. The emission spectrum was recorded and shown in Fig. 5. The sample was excited at 244 nm and the emission peak was observed at 545 nm (2.27eV), which is Green emission corresponds to the n -π* transition. The strong emission in the observed band gap region may be due to the presence of intrinsic defects in the forbidden band gap region. From this the grown PzZrOCl is suitable for optoelectronic applications.

Morphological studies
The analysis on the microstructure of grown single crystals is an essential parameter for device applications because only few facets are usable. Some crystals during the growth do exhibit microstructures on their habit faces. The surface morphology of such a grown crystal has been shown in Fig.6. It indicates layered like growth of the crystals, further the grown layers spreading from the nucleus. The observed microstructures suggesting the crystal growth by twodimensional spreading and filing up of layers indicates growth at high supersaturation and moreover the surface brighter and darker uneven areas which is most commonly observed in solution grown crystals.

Thermal analysis
The thermal stability of PzZrOCl crystal was carried out, the respective Thermogravimetry (TG), Differential thermal analysis (DTA) is shown in Fig.7 fourth endothermic peak at 292°C may be due to the removal of the hydroxyl and amino groups expelled from the compound respectively [34,35]. The peak sharpness of the peaks indicates the purity of the material.

Mechanical measurements
Mechanical strength is direct consequence of structure (molecular bonding) and composition of the material and it was demonstrated by using hardness measurement. It is a nondestructive technique, plays a vital role in device fabrication [36]. Under this investigation by studying the resistance offered by the crystal lattice to the local deformation, one can analyze the mechanical properties; such as fracture behavior, yield strength, brittleness index and temperature of cracking. According to Gong [37], in the process of indentation, the external  increasing the load on the crystal and < 2, for RISE behavior > 2, where the hardness increases with increasing load. It is true in our system and PzZrOCl comes under soft category [39].

Laser damage threshold studies
An NLO material to be a good candidate for laser applications, the main and essential criteria is its Laser damage threshold (LDT). It is the maximum limit of fluency (energy/area) beyond which an optical material will be damaged permanently. This lens helps us to set the beam spot on the sample to a desired value (beam diameter ′ ).
During the process of irradiation, the damage on the surface can be estimated by visual formation of the damage and input energy density of laser beam where the crystal gets damaged is recorded by power meter. The damage of the crystals caused by the power density ( ) is calculated by the following relations.
Where is input pulse energy in milli joule (38.7) of the laser beam where the damage has occurred, the pulse width, is thickness of the sample and is the radius of the circular beam spot on the sample ( ′ /2), it can be calculated as follows.
The value of ′ is obtained as 0.057 mm. The calculated laser damage threshold value for PzZrOCl is 0.6 GW/cm 2 . The observed value is three times higher than KDP (0.2 GW/cm 2 ) [40] and 1.2 times higher than LAPP (0.50 GW/cm 2 ) [41]. Thus PzZrOCl has good optical damage tolerance, it is suitable high power laser applications. Aside from thermal effects, multiphoton absorption the mechanical hardness of the material also affects the laser induced damage.

Non-linear properties: Z-scan technique
The third order non-linear optical properties can be evaluated from a standard Z-scan technique. It is an appropriate technique to determine the nonlinear refractive index( 2 ), effective nonlinear absorption( ) and optical susceptibility ( (3) ) etc, of the medium with high accuracy. This can be done by analyzing the distortions in spatial, spectral and temporal parameters of the high power laser radiation. In order to represent experimental data of the nonlinear indices and nonlinear absorption coefficient widely used Gaussian decomposition method was adopted and its description was given in [42,43].
Where Δ is the peak value on the open aperture -scan curve. The calculated value for is 5.40 × 10 −6 ( ) ⁄ and it is lesser than 0, a positive value. Hence the non-linear absorption is due RSA in particular two photon absorption (TPA). The positive nonlinear absorption may be due to electron excitation via interband transition. < 0 is due to positive value of (3) .
Such a low value of compared to the materials in literature [47,48]  The real and imaginary parts of the third order nonlinear optical susceptibility (3) were estimated from experimental determination of 2 and according to the following relations.  and (3) , it is observed that

Conclusion
Single crystals of PzZrOCl were synthesized and crystallized using piperazine and zirconium oxy chloride as a starting material by slow evaporation method at room temperature. Crystals are transparent colorless with 10 × 10 × 7 3 dimensions. Crystalline phase, purity and cell parameters were confirmed with powder X-ray diffraction and single crystals X-ray diffraction techniques. The crystalline quality is reasonably good and crystal belongs to tetragonal system with space group P. Structural identification of functional groups is examined by FT-IR spectroscopy. The suitability of the crystals for the optical application was tested by UV-Vis-NIR and fluorescence spectroscopy. The lower cutoff wavelength and high transparency in the entire visible region and green emission from PL proves PzZrOCl is a potential candidate for optical device applications. FESEM analysis on the grown single crystals reveals layered like growth under 2D growth mechanism. Further thermal stability analyzed by TG/DTA technique confirms the title compound thermally stable up to 169°C. Mechanical strength of the PzZrOCl is tested with microhardness measurement. The crystal exhibits reverse indentation size effect and there was not a significant crack till 100 g load. The work hardening index ( ) is estimated to be 3.31 by linear fitting and PzZrOCl comes under soft category. Laser induced damage threshold value is 0.6 GW/cm 2 . The observed value is three times higher than KDP (0.2 GW/cm 2 ) and 1.2 times higher than LAPP (0.50 GW/cm 2 ). The Z-scan technique on the present material with relative third order nonlinear refractive index ( 2 ), effective nonlinear absorption ( ) and optical susceptibility ( (3) ) indicates it can also capable of generating third harmonics. It is concluded that PzZrOCl is a suitable candidate for optoelectronic applications.