Single crystals of BGHB were grown by slow evaporation technique. The unit cell dimensions and space group of the grown crystals were confirmed by single crystal X-ray diffraction. The modes of vibration of the molecules and the presence of functional groups were identified using FTIR technique. The microhardness study shows that the Vickers hardness number of the crystal increases with the increase in applied load. The optical properties of the crystals were determined using UV-Visible spectroscopy. The thermal properties of the grown crystal were also determined. The refractive index was determined as 1.396 using Brewster’s angle method. The emission of green light on passing the Nd: YAG laser light confirmed the second harmonic generation property of the crystals and the SHG efficiency of the crystals was found to be higher than that of KDP. The dielectric constant and dielectric loss measurements were carried out for different temperatures and frequencies. The ac conductivity study of the crystals was also discussed. The photoconductivity studies confirm that the grown crystal has negative photoconductivity nature. The etching studies were carried out to study the formation of etch pits.
Nonlinear optical (NLO) applications demand good quality single crystals, which inherit large NLO coefficient, coupled with improved physical parameters. One potentially attractive system, where there is a potential for realizing very large second order nonlinear coefficient is based on organic crystals. Organic materials have been of particular interest because the nonlinear optical responses in this broad class of materials is microscopic in origin, offering an opportunity to use theoretical modeling coupled with synthetic flexibility to design and produce novel materials [
Bisglycine hydrobromide salt was synthesized by dissolving glycine and hydrobromic acid in stoichiometric ratio (3 : 1) in double distilled water. The solution was stirred continuously using a magnetic stirrer. The obtained saturated solution was further purified and allowed to evaporate at higher temperature which yields powder form of the synthesized bisglycine hydrobromide. Synthesized material was purified by repeated recrystallization process. Tiny seed crystals with good transparency were obtained due to spontaneous nucleation. Among them, a defect free seed crystal was selected and suspended in the mother solution, which was allowed to evaporate at room temperature. Large size single crystals were obtained due to the collection of monomers at the seed crystal sites from the mother solution, after the nucleation and growth processes were completed. After a period of 24 days, colourless and transparent crystals were obtained with dimensions (17 × 7 × 6 mm3). Figure
Photograph of grown BGHB single crystal.
Single-crystal X-ray diffraction is an analytical technique to determine the actual arrangement of atoms within a crystalline specimen. Single-crystal X-ray diffraction (XRD) is a nondestructive tool to analyze crystal structure of compounds, which can be grown as single crystals. XRD is employed for finding unit cell parameters, space groups, and three-dimensional coordinates of atoms in the unit cell. The single-crystal X-ray diffraction analysis of the grown crystals was carried out to identify the cell parameters using an ENRAF NONIUS CAD4 automatic X-ray diffractometer. The lattice parameters are estimated to be
The powder X-ray diffraction analysis was carried out to confirm the crystallinity and also to ascertain the purity of the grown BGHB crystal. Powder XRD pattern was recorded by scanning the sample over the range 0–90° at a scan speed of 0.02°/min. The recorded XRD pattern of BGHB is shown in Figure
Powder X-ray diffraction pattern of BGHB single crystal.
The FTIR spectrum of bisglycine hydrobromide was recorded in the region 4000 cm−1–400 cm−1 employing BRUKER IFS 66V spectrometer. In the FTIR spectrum shown in Figure
FTIR spectrum of BGHB single crystal.
This observation confirms that one glycine is exists in zwitter-ionic form and the peak at 672 cm−1 is due to N–H out-of-plane bending vibrations [
The optical transmission spectrum of BGHB single crystal was recorded in the wavelength region 200–2000 nm and it is shown in Figure
UV-Vis transmission spectrum of BGHB.
Plot of
From the transmission spectrum, the lower cut-off wavelength is found to be 230 nm and the lower percentage absorption indicates that the crystal readily allows the transmission of the laser beam in the range between 230 nm and 2000 nm. It shows that the grown crystal has a good transparency in UV, visible, and near IR region indicating that it can be used for NLO applications. Using Tauc’s plot, the energy gap
Brewster’s angle method was used to determine the refractive index of the crystal. A He-Ne laser of wavelength 632.8 nm was used as the source. The polished crystal was kept on the rotating mount at an angle varying from 0 to 90 degrees. The angular reading, when the crystal was perfectly perpendicular to beam, was noted. The crystal was rotated along the beam for maximum power output. Brewster’s angle
The most widely used technique for confirming the SHG efficiency of NLO materials to identify the materials with noncentrosymmetric crystal structures is the Kurtz Powder technique [
The thermogravimetric analysis (TGA) was carried out by using TAQ-500 analyzer at a heating rate of 25°C/min. for temperature range 25–800°C as shown in Figure
6 TG/DTA of BGHB.
The microhardness measurements were carried out with a load range from 25 to 100 g using Vickers hardness tester (LEITZ WETZLER) fitted with a diamond pyramidal indenter and attached to an incident light microscope. The Vickers microhardness number was calculated using the relation
Variation of
Plot of
The dielectric studies of the grown crystal were carried out by using the instrument, HIOCKI 3532-50 LCR HITESTER. A sample of dimension 1 × 0.5 × 0.1 cm3 having silver coating on the opposite faces was placed between the two copper electrodes and a parallel plate capacitor was thus formed. The capacitance of the sample was measured by varying the frequency from 100 Hz to 5 MHz and the dielectric constant versus logarithmic frequency is plotted (Figure
Variation of the dielectric constant against
Variation of the dielectric loss with
The ac conductivity study of the BGHB crystal was carried out. In the high temperature (Intrinsic) region, the effect of impurity on electrical conduction has not made any appreciable change whereas in the low temperature (extrinsic) region, the presence of impurity in the crystal has an impact and particularly increases its conductivity. The electrical conduction in dielectrics is mainly a defect controlled process in the low temperature region (Figure
Variation of ac conductivity with 1000/
Plot of
The photoconductivity measurements were carried out by using Keithley 485 picoammeter. The dark current was recorded by keeping the sample unexposed to any radiation. Figure
Field dependent photoconductivity of grown single crystal.
The crystals with defects may destroy the mechanical and electrical properties, which affect the usefulness of the crystals. The nonlinear optical properties such as SHG efficiency, damage threshold, depend on the crystalline perfection. Etching is one of the selective tools to identify the defects in the grown crystals. The (100) plane of the BGHB crystal has been completely immersed for 60 s in the water etchant and the sample was wiped out with dry filter paper. From Figure
Etching photograph of BGHB.
A single crystal of bisglycine hydrobromide (BGHB) was grown by slow evaporation technique. The unit cell dimensions of the grown crystals were confirmed by single-crystal X-ray diffraction. The functional groups present in the material have been confirmed by FTIR analysis. UV-Visible absorption spectrum shows excellent transmission in the entire visible region. The band gap energy for the grown crystal is found to be 3.70 eV. NLO study reveals that the grown crystal has SHG efficiency equal to that of KDP crystal. From thermal analysis, it is found that the crystal can retain its stability up to 230°C. The mechanical studies were carried out for the grown crystal. The microhardness studies indicate that the Vickers hardness number of the crystal increases with the increase in applied load. The dielectric constant and dielectric loss measurements were carried out at different temperatures and frequencies. The activation energy was determined from the plot of ac conductivity. The photoconductivity studies confirm that this material has negative photoconductivity nature. The etching studies were carried to study the formation of etch pits in water solvent. This material exhibits NLO behaviour remarkably due to its better optical and dielectric properties.
The authors declare that there is no conflict of interests regarding the publication of this paper.