On the search of new NLO materials with better mechanical properties, many researchers have focused on the small organic molecules having a large dipole moment and a chiral structure. These molecules are usually linked through the hydrogen bond 1-2. Nonlinear optical (NLO) single crystals establish a variety of applications to perform functions like electro-optic switching, optical memory storage, frequency conversion, second harmonic generation and high energy lasers for inertial con?nement fusion research 3–7. Because of the large nonlinearities and optical threshold of organic materials, a wide range of such materials has been found by many researchers 8-10. In general, most of the organic molecules designed for nonlinear optical applications are the derivatives of an aromatic system substituted with donor and acceptor substituent 11-13. In recent years there have been extensive researches in the investigation of nonlinear optical crystals because of their potential applications in fabrication of optoelectronic devices 11–14.
The organic crystals have large nonlinearity, but they have poor mechanical and thermal stability and are susceptible to damage during processing. Moreover, the growth of large size single crystal is difficult to grow for the fabrication of devices. Inorganic crystals have excellent mechanical and thermal properties, but possess relatively modest optical nonlinearity because of the lack of extended pi-electron delocalization. Due to the above reasons, investigations have been made with semi-organic crystals which have combined properties of both organic and inorganic crystals and it is more suitable for device fabrication 15-19.
Sulphanilic acid (SAA) is a very interesting compound due to a number of medical, biological, NLO, irradiation and radiation dosimeter properties. SAA is virtually tissue equivalent, which enables its use in radiation therapy. EPR signal intensity shows noticeable stability of transfer dosimeter 20-24. SAA containing two functional groups like sulfonic acid (-So3H) and amine groups, thereby it acts as a base which makes a compound through nitrogen (amino) atom. In recent times, the growth and NLO properties of sulphanilic acid derivatives have been reported.
In this research article, the growth of transition metal incorporated novel SAA crystal grown from an aqueous solution involving electron transfer from donor to acceptor followed by hydrogen bond formed from the acceptor is presented. Also the characterization studies, like FT-IR, UV-Vis-NIR, X-ray diffraction, NMR, thermal, hardness, etching, EDAX analysis, nonlinear optical and dielectric properties were carried out and the obtained results are reported in the present work and discussed. These results are not yet reported in the literature to our knowledge.
2. EXPERIMENTAL PROCEDURE
2.1 Materials and spectral measurements
The compound Sulpanilic acid and Zirconium oxychloride are purchased from Sigma-Aldrich and Merck Chemicals Company. These chemicals were used without purification. The grown crystals have been subjected to single crystal X- ray diffraction studies using an ENRAF NONIUS CAD4 diffractometer with MoK? radiation (?=0.71073 A?) to determine the unit cell dimensions with space group. The powder samples have been analyzed by using BRUCKER, Germany (model D8 Advance) X-ray diffractometer with CuK (wavelength=1.5405A0) radiation. The powder sample was scanned over the range 10– 80oC at a scan rate of 1oC/ min. Infrared spectrum was recorded using the Alpha Bruker ATR technique with a resolution of 2 cm-1 at room temperature. 1H & 13C NMR spectra of the crystals were run on a Bruker FT-NMR spectrophotometer operated at 400 MHz at room temperature using dimethyl methoxide (DMSO) as a solvent and tetra methylsilane (TMS) as an internal reference. The optical transmission spectrum was recorded using DOUBLE BEAM UV-Vis-NIR Spectrophotometer (Model:2202) in the region 200-1200nm. Thermal stability of the crystals was done by thermogravimetric analysis (TGA) method using Dupont 951 thermogravimertic analyzers. The work was performed from 30 to 800°C at the heating rate of 200C min-1 in a nitrogen atmosphere with a gas flow rate of 100ml min-1. The Vicker’s hardness test was carried out on the grown crystal using SHIMADZU HMV microhardness tester fitted with a diamond pyramidal indenture. The surface morphology and particle sizes of the samples were determined by field emission scanning electron microscopy (FESEM; Hitachi S4800; Japan).
2.2 Synthesis of SAOZR crystals
The SAOZR has been synthesized by taking the chemicals in equimolar ratio and dissolved in aqueous solution. The solution was stirred well using a magnetically stirred at room temperature. The solution was kept in undisturbed condition and after 45 days transparent crystals of SAOZR in pyramidal shape were collected. These crystals are in yellow-reddish color with an average size of 8x7x15 mm3. The purity of the synthesized crystal was improved by successive re-crystallization process. The synthesis route is shown in figure 1.
2.3 Crystal Growth of SAOZR
The bulk growth of SAOZR single crystal was carried out from slow evaporation solution growth method and good quality single crystals were obtained. Figure 2 shows photographs of the as grown crystal. The optimized growth conditions of SAOZR single crystal are given in table 1. All the crystals have good compositional stability. Samples were stored at room temperature and at 90% relative humidity showed no degradation after several months. Fig. 2. The photograph of the as grown crystal of SAOZR.
3. RESULTS AND DISCUSSION
3.1. Single Crystal X-Ray Diffraction (SXRD) Analysis
The single crystal X-ray diffraction studies of pure SAA and SAOZR crystal revealed that both pure and SAOZR single crystal crystallize in the orthorhombic system with space group belong to P212121. The lattice parameters are a= 7.31 Å, b = 7.51 Å, c = 13.92Å and volume =765 Å3 and slight variations in these values were observed in SAZOR crystal when compared with pure SAA. This type of variations may be recognized to the incorporation of zirconium oxychloride in the SAA crystal lattices. This result is presented in table 2.
3.2. Powder X-ray diffraction (PXRD) analysis
In powder XRD pattern, a well defined Bragg’s peak is obtained at specific 2? angles for SAOZR crystal. The crystallinity of SAOZR was confirmed by PXRD analysis and diffraction sharp peaks are indexed from crystal structure parameters shown in figure 3. This reveals that the grown crystal has good quality and possesses high crystalline nature. The variations of intensity of peaks compare with pure SAA crystal 24. It is clearly indicated that the doping (zirconium oxychloride) could be incorporated into the pure SAA crystal lattice.
3.3. FT-IR Spectral Analysis
The FT-IR spectral analysis of as grown crystal SAOZR was carried out between 4000 and 500 cm-1. The observed spectrum is shown in figure 4. The peak at 2648 cm-1 is assigned to hydrogen bonded N-H—O vibration of amine with zirconium oxychloride. The asymmetric bending vibration of NH3 of sulphanilic acid occurs at 1631 cm-1. The benzene ring resonance vibration produces peaks at 1598, 1549, 1578 and 1423 cm-1. The corresponding asymmetric vibration of SO3- vibration gives the peaks at 1318, 1246, 1158 and 1162 cm-1. The peak at 564 cm-1 is due to torsional oscillation of NH3+. The broad and intense peak due to C-H stretching, vibration appeared as a strong absorption band in the region 2881 and 3065 cm-1. The -NH2+ bending at 1598 cm-1 is shifted to a higher frequency region as 1631 cm-1 due to the presence of resonance structure (–NH3+ and -NH2+). The peak observed at 1318 cm-1 reveals C=S bend. Symmetric C=S stretching vibrations at 831 cm-1 is shifted to the low frequency region at 685 cm-1.
3.4. 1H and `13C NMR Spectra
Figures 5a and 5b represent the proton NMR and carbon NMR spectrum of SAOZR respectively. The presence of NH3+ protons in the synthesized SAOZR crystal appears at ?=4.3ppm in the 1H NMR spectrum. The aromatic protons are appearing around 6.8 and 7.1 ppm. The synthesized crystal SAOZR contains only aromatic carbon atom which appears in 13C NMR at ?=127ppm. Fig.5. 1H and `13C NMR spectrum of SAOZR crystal
3.5. Optical transmission studies
The optical transmission spectrum of SAOZR crystal is shown in Figure 6. The transmission is maximum in the entire visible region and infrared region. In the grown SAOZR crystal, the UV transparency cutoff wavelength lies at 247nm and the percentage of transmission is high in the entire visible region from 247nm to 1200nm. The absence of absorption in the entire visible region makes the SAOZR crystal as a potential candidate for second harmonic generation and optical applications
3.6. SHG efficiency studies
The Powder SHG test offers the possibility of assessing the non-linearity of new materials. Kurtz-Perry powder second harmonic generation (SHG) measurements were carried out using a spectrum-physics Q-switched Nd:YAG laser with the first harmonic input at 1064nm and a pulse width of 10ns at a repetition rate of 10Hz. This SHG efficiency diagram was shown in figure 7. The second harmonic signal generated by the compound was confirmed by emission of green radiation and the powder SHG efficiency of SAOZR was found to be comparable to that of Potassium Dihydrogen Phosphate (KDP). The SHG behavior was confirmed from the emission of bright green radiation (532nm). So it is a good NLO material for several applications.
3.7. Photoluminescence study of SAOZR crystal
The PL study finds wide applications in the field of medical, biochemical and chemical research for analyzing compounds. Photoluminescence in solids is the phenomenon in which electronic states of solids are excited by light of particular energy and the excitation energy is released as light. The photon energies reflect the variety of energy states that are present in the material. Figure 8 shows a PL emission spectrum recorded in the range of 300–800nm with an excitation wavelength of 341nm. The highest emission peaks in the spectrum were observed at 341nm and 667nm which indicates the emission of blue and red light. Other observed peaks are due to anionic and cationic nature of the sample.
3.8. Thermal analysis of SAOZR crystal
The thermogram and differential thermogravimetric traces are shown in Figure 9. It is observed from DTA curve, the material exhibits single sharp weight loss at 366.5 °C. It is observed that there is no weight loss from ambient temperature to 366.5 °C which indicates the grown SAOZR crystal is totally devoid of any inclusion of solvent and also indicating that the SAOZR crystal is stable up to 383.9 °C. At this decomposition stage, 86.8 % weight loss was observed from the thermogravimetric (TG) analysis.
3.9. Mechanical study of SAOZR crystal
The fastest and simplest type of mechanical testing is the Vickers microhardness measurement. Among the different testing methods, the Vicker’s hardness test method is more commonly used. Microhardness measurements were done on SAOZR for the applied load (p) varying from 25 to 100g for a constant indentation time 10s.Several indentations were made for each load and the diagonal length (d) of the indentation was measured.
The Vickers hardness number was determined using the formula H? = 1.8544 P/d2 (Kg/mm2). A graph was plotted between H? and load (p) shown in Figure 10. For an indentation load of 100 g, crack was initiated on the crystal surface, around the indented. This is due to the release of internal stress locally initiated by indentation. The work hardening coefficient (n) has been calculated from the slope of a straight line between log p and log d from Figure 11 and it is found to be 2.9 which indicates moderately soft nature of material (25-27).
3.10. Dielectric Studies of SAOZR crystal
The dielectric constant and the dielectric loss of the SAOZR sample were measured using HIOKI 3532-50 LCR HITESTER. Dielectric constant and dielectric loss of the sample has been measured for different frequencies (100 Hz to 5 MHz) at different temperatures (308 to 368 K). Figures 12 and 13 shows the variations of dielectric constant and dielectric loss, respectively, as a function of frequency at different temperatures. It is observed from Figure 12 that the dielectric constant decreases with increase in frequency from 50 Hz to 5 MHz and then attains almost constant. The same trend is observed for other temperatures too. It is also observed that the value of dielectric constant increases with temperature. Such variations in higher temperature may be attributed to the blocking of charge carriers at the electrodes. The decrease of dielectric constant at low frequency region may be due to space charge polarization. Figure 13 indicates that as the frequency increases, the dielectric loss decreases exponentially and then attains constant. The low value of dielectric loss confirms that the sample possesses lesser defects.
3.11. Etching Analysis of SAOZR crystal
The etching study was demonstrated for 5 s and 10 s, and the observed etch patterns of as grown SAOZR crystals are shown in Figure 14a and 14b. From the Figure 14a, it is observed that there is a smooth surface and rectangular shape etch pits observed on the surface of the sample when the etch pattern was taken within 5s. In the etch pattern recorded for 10s, in addition to rectangle shape etch pits, the dark spot is also observed. These etch pits are due to the chemical impurities and crystal undergoes selective dissolution during growth.
3.12. EDAX Analysis of SAOZR crystal
The elemental analysis was done using the Oxford INCA Energy Dispersive Atomic X-ray Fluorescence Spectrometer (EDAX). From the analysis, it is noticed that the equal mole percentage of Zirconium Oxychloride sulphanillic acid has been incorporated into the as grown crystal of SAOZR. The element chloride was traced in EDAX analysis and shown in the figure 15. The chemical composition is also calculated theoretically as C, 25.79; H, 2.16; N, 5.01; O, 22.90; S, 11.48; Zr, 32.65 and these values agree with EDAX analysis.
The surface morphology and the particle size of the crystal were evaluated through FESEM analysis and the results are displayed in Figure 16. From the figure, it is interpreted as Zirconium particle agglomerated on the surface of the crystal (17, 28).
Well developed good quality transparent crystal of (((4-sulfonatophenyl)ammonio)oxy)zirconium (SAOZR) was grown successfully by slow evaporation technique. A single crystal XRD study has been carried out to identify the lattice parameters and the grown crystal belongs to the orthorhombic crystal system. Powder XRD shows good crystallinity of the grown crystal. The UV cutoff wavelength of SAOZR crystal is found to be around 247nm, which reveals grown crystal is a potential candidate for NLO applications. TGA and DTA analysis were carried out to characterize the melting behavior and stability crystal. The emission of intense green light from SAOZR crystal confirms the nonlinear optical properties. Photoluminescence study shows in wide blue color light emission. The dielectric behavior of the sample has been analyzed with various frequencies at different temperatures. The mechanical strength of growing crystal was analyzed by the Vickers micro hardness tester. The elemental analysis was done by EDAX. The dielectric response of the these crystals was studied in the frequency range 50 Hz to 5MHz at various temperatures and the results are discussed. Finally, it is concluded that SAOZR crystal is suitable for industrial applications as it possesses good thermal stability, moderate SHG efficiency and soft nature.