4.5. Photoluminescence studies Photoluminescence is a contactless, non-destructive method of probing the electronic structure of materials. Fluorescence may be expected generally in molecules that are aromatic or contain multiple conjugated double bond switch a high degree of resonance stability. The emission spectra of DMAPAB were recorded using a FP-6500 spectrofluorometer. The sample was excited at 300 nm and the emission spectrum was measured in the range of 300″650 nm. From the recorded photoluminescence spectrum shown in Fig. 8, it was observed that the material exhibits a high intense emission peak at 350 nm.
Hence, the photoluminescence analysis concludes that DMAPAB crystal exhibits violet fluorescence. The maximum intensity which appears at 350 nm is attributed to n-* transition with N-H and O-H functional group molecules. The broadening of emission band is due to intermolecular interactions of the DMAPAB crystal system. Fig.10. Photoluminescence spectrum of DMAPABcrystal4.6. Thermal analysis The thermal analysis of DMAPAB was carried out in the temperature ranging from 30 to 500oC at a heating rate of 10oC/min in nitrogen atmosphere.
The thermogram and the differential thermo gravimetric trace of DMAPAB crystal shown in Fig. 11. In TGA, there is a weight loss at 123oC due presence of water molecules in synthesized salt. It is seen that the major weight loss above at 214oC and it continues up to 290oC it indicates a larger weight loss is due to the decomposition of functional group molecules like NH, CH3 evaporates for it’s before melting point. The small residue mass is at end of the thermogram,s due to some other small molecules present in compound, which is left out in the crucible may be carbon mass present after all the decomposition processes. The nature of weight loss indicates the decomposition point of the material. The synthesized compounds of DMAPAB have a thermal stability up to 214oC. In the DTA, a small bend in the curve occurred is due to the water molecules, also there is a sharp endothermic peak observed at 180oC due to decomposition or melting point of compound of DMAPAB crystal. The second sharp endothermic peak observed at 275oC is assigned as the decomposition point. At this stage heavy weight loss in TGA has been noticed and it indicates that it exactly coincides with the decomposition in TG analysis. Sharpness of the endothermic peaks observed in DTA indicates good degree of crystallinity of the sample. Fig.9. TG/DTA Curves of DMAPAB compound4.7. Dielectric studies The dielectric behaviors of the material are important tools for study about the lattice mobility within the crystal lattice. Dielectric studies have been useful to provide important information such as electro-optical molecular responses, molecular dynamics and molecular anisotropy [18-21]. The capacitance of the crystal and their and dielectric loss were measured at various temperatures ranging from 313 to 343 K with four different frequencies. The cut and polished transparent crystal of size 6x 5x 5 was used for dielectric measurements. The opposite faces were polished and silver paste coated with good conductive surface layer. The experiment was carried out using HIOKI 3532-50 LCR HITESTER meter. The dielectric constant and dielectric loss of the crystal have been calculated using the relations, (7)and (8)Where C is the capacitance, A is the area of the sample, d is the thickness, µois the absolute permittivity of the free space and tan ґ is dielectric loss. The variation of dielectric constant and dielectric loss as a function of frequency is studied and shown in Fig. 12(a) and 12(b) respectively. From the graphs it is clear that the dielectric constant and dielectric loss of DMAPAB crystal decreases as the frequency increases, at all temperatures. The very high value of dielectric constant at low frequencies may be due to the presence of all the four polarizations namely, space charge, orientation, electronic and ionic polarizations. The space charge polarization is more dependent on the higher purity and defect free crystalline nature of a crystal, concurrently which actively influences at higher temperature due to temperature variation of polarizability. The electronic polarization and ionic polarizations are due to the displacement of electrons and ions respectively under an applied electric field and are temperature independent. The orientational polarization occurs due to the alignment of permanent dipoles which are otherwise randomly oriented, under the action of electric field. Dependence of dielectric loss on frequency also can lead to polarization mechanisms. The low dielectric loss provides that the grown of DMAPAB crystal is found to be good optical quality and this parameter is of vital importance for nonlinear optical elements in their device applications . Fig.11(a) Plot of dielectric constant vs. log frequency of DMAPAB crystal Fig. 11(b) Plot of dielectric loss vs. log frequency of DMAPAB crystal4.8. Z Scan Measurement The Z-scan technique study is needed to examine for optical nonlinear behaviour which is used to finds third-order nonlinearity and determines the changes in the nonlinear refractive index and variation in absorption . The value of the nonlinear absorption coefficient (І) and nonlinear refractive index (n2) of the DMAPAB crystals were calculated from the Z-scan data [24,25]. In this method, the sample is translated in the Z-direction along the axis of a focused Gaussian beam from the He-Ne laser at 632.8 nm and the far field intensity is measured as a function of the sample position. The normalized transmittance for the positioned DMAPAB crystal was measured at different positions and was used to calculate third-order nonlinear optical property of the material. The Z-scan curves in open and closed aperture modes are shown in Figs.13 and 14 respectively. From the Z-scan data curves, the difference between the valley and peak transmittances (€Tv-p) is written in terms of the on axis phase shift at the focus as, (9)Where S is the linear transmittance aperture and it is calculated using the relation, (10)Where ra’ is the radius of aperture and ‰a’ is the beam radius at the aperture. The nonlinear refractive index (n2) was calculated using closed aperture Z-scan data and it is given by [ 26, 27], (11)where k’ is the wave number ( k = 2/”), I0 is the intensity of the laser beam at the focus (Z = 0) and Leff = [1- exp (-±L)]/± is the effective thickness of the sample, ± is the linear absorption and L is the thickness of the sample. The nonlinear absorption coefficient (І) can be estimated from the open aperture Z-scan data. (12)where €T is one of the peak values at the open aperture curve. The real and imaginary parts of the third order nonlinear optical susceptibility ((3)) were estimated using [ 28,29] (cm2/W) (13)and (cm/W) (14)whereµo’is the vacuum permittivity and c’ is the velocity of light in vacuum. no’is the refractive index of the sample, “‘ is the wavelength of the He-Ne laser beam. The third order nonlinear optical susceptibility was calculated using the relation, (15)The third order nonlinear optical susceptibility was given in Table 4.The Z-scan method confirmed the DMAPAB crystal can be a promising NLO material for optical device applications such as optical modulators, optical limiters. Fig.12. Open curvature for DMAPAB crystal Fig.13.Closed curvature for DMAPAB crystalTable 4Parameters measured in Z-scan experiment for DMAPABLaser beam wavelength (l) 632.8 nmLens focal length (f) 18.5 cmOptical path distance (Z) 115 cmSpot-size diameter in front of the aperture (‰a) 1 cmAperture radius (ra) 4mmEffective thickness (Leff) 0.9974 mmNonlinear refractive index (n2) -0.9584 —10€’10cm2/WNonlinear absorption coefficient (b) 2.4283 —10€’3 cm/W Third-order nonlinear optical susceptibility (c(3)) 1.8594 —10€’10esu4.9. Micro hardness Test Measuring the hardness properties of the crystal is one of the important parameters in determining the applicability of the specific device to its performance, it is vital to carry out hardness studies for the grown crystals. In the present study, the micro-hardness of the DMAPAB crystal was evaluated by using Vickers’s microhardness measurement. The indentation stiffness is measured as the ratio of the applied load to the spread over area of indentation. The applied load (P) on the assisted surface of the crystal was varied from 10, 25, 50 and 100g loads for indentation with time of 10 s. The Vickers’s microhardness were calculated using the relation,H_V=1.8544P/d^2 (kg/mm^2 ) (16)Where Hv’ is the hardness number, P is the applied load in kg and d is the diagonal length of the indenter impression in mm. The typical Vickers hardness values of the grown crystal for various loads on the prominent plane were plotted in Fig.15,its illustrate that the hardness number increases with increasing load. This phenomenon is known as the reverse indentation size effect (RISE) which indicates the crystal having good mechanical strength. A plot of Fig.16 obtained between log (p)’ and log (d)’ gives more or less a straight line. The relation connecting the applied load and diagonal lengthd’ of the indenter is given by Meyer’s relation P=K1dn, Where n’ is Meyer’s index or work hardening coefficient. This indicates the hardness of the material is found to increase with increase in load and it confirms the prediction of onistsch . When the load was increased by100 g cracks developed on the smooth surface of the crystal due to release of internal fatigue generated during indentation. According to Onistsch and Hanneman, n’ should lie between 1 and 1.6 for moderately hard materials and above 1.6 for softer material category.From the plot of Meyer’s relation the n’ of DMAPAB crystal value was found to be 1.10. Thus the DMAPAB crystal belongs to the hard material category. Fig. 14.Plot of hardness number (Hv) vs. Load (P) of DMAPABcrystal 5. Conclusion The novel Organic single crystal of DMAPAB was grown by the slow evaporation solution growth technique. The crystal structure DMAPAB salt was determined by single XRD, it is crystallized in triclinic space group P1 …. From the UV”Vis spectral analysis, it is found that the DMAPAB crystal is transparent in the entire visible region and its wavelength is found to be 318nm. The band gap energy was found to be 3.63eV. The photoluminescence analysis concludes that DMAPAB crystal exhibits violet fluorescence, it is very useful for photonic application. The Vickers’s microhardness test reveals that DMAPAB belongs to hard category nonlinear optical crystal. From the TG”DTA curve, it is clearly revealed that the material is stable up to 214o C.