Spectroscopic and thermal studies of γ-glycine crystal grown from potassium bromide for optoelectronic applications

3.1 Single crystal X-ray diffraction

Single crystal X-ray diffraction data recorded using ENRAF-NONIUS MACH3 X-ray diffractometer with Mo Kα radiation (λ = 0.71073 Å) to reveal the crystal structure of the grown crystal. The structure was solved by direct method and refined by the full matrix least square technique using SHELXL program. The determined unit cell parameters are tabulated in Table 1 and these values are in good agreement with the reported values (Anbuchezhiyan et al., 2010).

3.2 Powder X-ray diffraction analysis

A sample in powdered form was subjected to XRD analysis using SIEFERT 3003 TT powder X-ray diffractometer with a characteristic Cu Kα (λ = 1.540598 Å) radiation from 10° to 60° at a scan rate of 2°/min. Powder XRD pattern for the grown crystal is shown in Fig. 2. Appearance of sharp and strong peaks confirms the good crystallinity of the grown sample. The observed values are in good agreement with the data available in JCPDS Card No: 06-0230. Slight shift in the sharp peak position at 25.3° in lower angle side may be due to doping of KBr in the γ-glycine crystal, which is also confirmed by the slight variation observed in lattice parameters of the grown crystal. Table 2 represents the lattice parameters of γ-glycine with various solvents.

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Figure 2

Powder XRD pattern of γ-glycine crystal.

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3.3 Vibrational analysis

The FTIR spectrum of γ-glycine was recorded in the range of 4000–450 cm⁻¹ using Perkin Elmer spectrometer by KBr pellet technique. The resulting spectrum is shown in Fig. 3. The observed frequencies and their assignment of the γ-glycine crystals are given in Table 3.

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Figure 3

FTIR spectrum of γ-glycine crystal.

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The transition peaks due to the carboxylate group (COO⁻) of free glycine is normally observed at 607, 1414 and 1605 cm⁻¹, where as in the case of γ-glycine these peaks were shifted to 607, 1391 and 1596 cm⁻¹_. Similarly, the transmission peaks due to the ammonium group $({\text{NH}}_3^{+})$ of free glycine were observed in the region 1133, 1507 and 2614 cm⁻¹, but in the present case these are shifted to 1127, 1497 and 2602 cm⁻¹, respectively. This observation is strongly evident that the glycine molecule in crystalline state exists in zwitterionic form in which the carboxylic acid group is present as a carboxylate ion and the amino group is present as ammonium group. The peaks observed at 890 and 1436 cm⁻¹ are attributed to CCN and CH₂ stretching groups, respectively. The presence of various functional groups of the grown crystal is in very good agreement with those reported in the literature (Dhas and Natarajan, 2007).

3.4 UV–vis–NIR spectral analysis

The optical absorption spectrum of good quality γ-glycine crystal was recorded in the wavelength range of 200–800 nm using Perkin Elmer Lamda 935 UV–vis–NIR spectrometer. The obtained absorption spectrum is shown in Fig. 4 in which the lower cut off region is obtained at 238 nm. The UV spectra shows the presence of a wide transparency window lying between 238 and 800 nm with λ_max = 238 nm. From the relation E_g = 1.243 × 10³/λ_max the forbidden energy gap was estimated and is found to be 5.23 eV, which is the typical value of dielectric materials (Khandpekar and Pati, 2010). Hence, from the analysis of absorption spectrum, it is evident that the grown crystal is transparent in the entire visible region without any absorption peak, which is the key requirement for any nonlinear optical crystal having applications in second harmonic generation, parametric oscillations, etc.

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Figure 4

Absorption spectrum of γ-glycine crystal.

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3.5 Thermal analysis

Thermo gravimetric and differential thermal analysis of the grown γ-glycine was carried out from 20 to 800 °C in the nitrogen atmosphere at a heating rate of 20 °C/min using SDT Q600 thermal analyzer. Crystal weighing about 3.157 mg was used for this study. The obtained profile is shown in Fig. 5. The TGA curve shows no change in weight up to 213.03 °C, which eliminates the possibility of hydrate or solvent formation of the crystal. The major weight loss of 35.32% in TGA curve occurring between 213.03 and 262.92 °C suggests that the sample could be sublimation and decomposition of γ-glycine. From the DTA thermogram, it is observed that an endothermic event begins at 192.89 °C and then a sharp peak appears at 259.65 °C which corresponds to decomposition point of the sample. Thus, in the present work, γ-glycine crystal grown from a mixture of glycine and potassium bromide is structurally stable up to 213.03 °C. Hence, it can be useful for making the NLO devices below its decomposition temperature (Selvaraju et al., 2006; Parimaladevi and Sekar, 2010).

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Figure 5

TG/DTA profile.

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3.6 Nonlinear optical studies

The second harmonic generation efficiency of the powdered material was tested using the Kurtz and Perry method (Kurtz and Perry, 1968). A Q-switched Nd:YAG laser beam of wavelength 1064 nm and 10 ns pulse width with an input rate of 10 Hz was used to test the NLO property of the sample. The grown γ-glycine SHG signal of 64 mV was obtained while the KDP gave an SHG signal of 25 mV for the input beam energy of 7 mJ/Pulse. Hence, the SHG efficiency of γ-glycine crystal grown in the medium of potassium bromide is 2.56 times higher than that of the KDP crystal. The SHG conversion efficiency of the crystals grown from different solvents is given in Table 4 for comparison. Hence, it is used for applications in nonlinear optical devices.