CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 11 2441-2445
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Growth and Characterization of Nonlinear Optical Amino Acid Single Crystal: L-Alanine N. Vijayan,† S. Rajasekaran,# G. Bhagavannarayana,† R. Ramesh Babu,#,‡ R. Gopalakrishnan,*,# M. Palanichamy,| and P. Ramasamy‡ Materials Characterization DiVision, National Physical Laboratory, New Delhi - 110 012, India, Department of Physics and Crystal Growth Centre and Department of Chemistry, Anna UniVersity, Chennai - 600 025, India ReceiVed December 1, 2004; ReVised Manuscript ReceiVed June 13, 2005
ABSTRACT: Amino acid family crystals exhibit excellent nonlinear optical and electrooptical properties. L-Alanine single crystal belongs to the amino acid group and has been grown by the slow evaporation solution growth technique at room temperature. The grown crystals were characterized by high-resolution X-ray diffractometry (HRXRD), nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy, UV-Vis, Raman spectroscopy, mass spectra analyses and density measurements. Its laser damage threshold was measured and its nonlinear optical response was tested by using a Q-switched Nd:YAG laser, and the value of laser damage threshold is greater than that of potassium dihydrogen phosphate (KDP). Introduction In the modern world, the development of science in many areas has been achieved through the growth of single crystals. Nonlinear optical (NLO) materials are expected to play a major role in the technology of photonics including optical information processing.1-4 Some organic compounds exhibit large NLO response, in many cases, orders of magnitude larger than widely known inorganic materials. They also offer the flexibility of molecular design and the promise of virtually an unlimited number of crystalline structures. Traditionally, crystals of organic materials have been grown from the melt5,6 or from vapor7 or solution.8,9 The title compound is the smallest molecule among the amino acids. Although its second harmonic generation (SHG) efficiency is about one-third that of potassium dihydrogen phosphate (KDP), the knowledge of studying the properties is very important since L-alanine can be considered as the fundamental building block of more complex amino acids.10 In this paper, we report the bulk growth and characterization studies of L-alanine single crystal. L-Alanine is an efficient organic NLO compound under the amino acid category. The title compound was first crystallized by Bernl11 and later by Simpson et al.12 It belongs to the orthorhombic crystal system (space group P212121) with a molecular weight of 89.09 and * Corresponding author: Dr. R. Gopalakrishnan. Department of Physics, Anna University, Chennai - 600 025, India; E-mail: krgkrishnan@ annauniv.edu;
[email protected]. Tel. +91-44-2220 3374. Fax: +9144-22203374. † Materials Characterization Division, National Physical Laboratory. # Department of Physics, Anna University. ‡ Crystal Growth Centre, Anna University. | Department of Chemistry, Anna University.
has a melting point of 297 °C. The solubility of L-alanine was estimated at different temperatures using deionized water as a solvent. The grown single crystals were characterized by various methods. The NLO response and laser damage threshold were tested by using an Nd:YAG laser. The refractive indices of L-alanine as a function of wavelength at 20 °C were measured by the minimum deviation method.10 Experimental Section Solubility Measurements. To grow bulk crystals from solution by the slow evaporation technique, it is desirable to select a solvent in which the molecule is moderately soluble. The size of a crystal depends on the amount of material available in the solution, which in turn is decided by the solubility of the material in that solvent. Hence, we determined the solubility of L-alanine in deionized water. The solubility of L-alanine in the above solvent was determined by adding a known quantity of solute in the solvent which was maintained at a constant temperature until it was completely dissolved. Using this technique, we evaluated the magnitude of the solubility of L-alanine for various temperatures viz. RT, 40, 45, and 50 °C. The temperature dependence of solubility of L-alanine is shown in Figure 1. From the graphs, it is found that the solubility of L-alanine increases with an increase of temperature. Hence, we attempted to grow bulk crystals of L-alanine using deionized water as a solvent. Growth. To obtain organic single crystals of high quality, purification of starting material was found to be an important step. The commercially available material (Thomas and Baker make) was further purified by repeated recrystallization processes, and the recrystallized material was used for growth. The concentrated solution was prepared with the help of the solubility diagram. According to the data of the solubility diagram, L-alanine of 15.5 g was dissolved in 100 mL of deionized water at an ambient temperature. A saturated solution of 150 mL was taken, and the solution was filtered using a Schleicher &
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Vijayan et al.
Figure 3. Powder X-ray diffractogram of L-alanine.
Figure 1. Solubility diagram of L-alanine.
Figure 2. Solution grown L-alanine single crystal using deionized water as a solvent.
Schuell filter paper. The filtered solution was taken in a beaker, which was optimally closed with a thick filter paper so that the rate of evaporation could be minimized. Good transparent single crystals (size 1.7 × 0.8 cm2) were obtained after two weeks (Figure 2).
Results and Discussion Characterization. The grown single crystal has been subjected to powder X-ray diffraction analysis. From the powder X-ray diffraction studies, it is confirmed that the grown crystal belongs to the orthorhombic crystal system. The crystalline perfection was evaluated by high-resolution X-ray diffraction analysis (HRXRD). The functional groups of the grown crystal have been confirmed by FTIR and Raman spectra analyses. The number of protons present in the compound was confirmed by FT-NMR analysis. The optical behavior was analyzed by UVVis measurement. The total molecular weight was confirmed by mass spectra analysis. The density was determined by the flotation technique. The laser damage threshold and NLO behavior were analyzed by a Q switched Nd:YAG laser. Powder X-ray Diffraction Analysis. The grown single crystal of L-alanine was subjected to powder X-ray diffraction. The powder form of the above-mentioned crystal was taken for analysis using a Rich Seifert powder X-ray diffractometer with a scan speed of 1°/min. The resulting powder X-ray diffraction pattern is shown in Figure 3. The obtained two-theta values were used for indexing by using the PROSZKI software package. It was confirmed that the crystal belongs to the orthorhombic crystal system with the lattice parameters a ) 6.032 Å, b ) 12.343 Å, c ) 5.784 Å. The obtained lattice parameter values are in good agreement with the reported literature values.10,13
High-Resolution X-ray Diffractometry (HRXRD). To study the crystalline perfection, high-resolution X-ray diffraction curves (DCs) (rocking curves) were recorded in symmetrical Bragg geometry using a multicrystal X-ray diffractometer developed at National Physical Laboratory (NPL), New Delhi.14 The diffractometer has (+, -, -, +) configuration using dispersive setting (+, -, -) for the three monochromators, and the specimen takes the fourth crystal stage. The surfaces of the as-grown bulk crystal were lapped with fine alumina powder and chemically etched with a solution of water and acetone (1:1 ratio) to remove the residual damage. The DCs were recorded using MoKR1 radiation and found to contain multipeaks. The multipeaks indicate that the crystal contains a large number of grain boundaries, which may be originated from the thread which was used to hang the seed crystal to grow the bulk crystal. For a better understanding of the effect of nylon thread in the generation of grain boundaries, a systematic study was made by preparing different specimens from the same bulk crystal: (a) very close to the interface between the seed and bulk crystal, (b) 4 mm away from the interface, (c) at the surface (i.e., ∼8 mm away from the interface), and (d) the original seed crystal. Figure 4 shows the DC recorded for (020) diffracting planes under identical conditions for all four specimens mentioned above. As seen in the Figure 4a, the DC recorded for the specimen prepared close to the interface is quite broad extending over an angular span of 2500 arc sec. Using a Gaussian distribution function, the curve (solid line) was simulated for the best fit. The best fit was obtained by convolution of four peaks (dotted line). The half widths of these four peaks are 670, 90, 244, and 550 arc sec. These are much higher than that of the theoretically expected values from the dynamical theory of X-ray diffraction for any ideally perfect crystal.15 The tilt angles between adjacent peaks are 247, 130, and 291 arc sec, which indicate that the crystalline perfection at the interface is very poor and this portion of the bulk crystal consists of low angle boundaries. Figure 4b shows the DC of the specimen (b) (prepared 4 mm away from the interface). In comparison to that of Figure 4a, the curve is less broad extending over an angular range of 500 arc sec which is one-fifth of that of spectrum (a). The best Gaussian fit was obtained for three peaks having half widths of 103, 48 and 115 arc sec. The tilt angles between the adjacent peaks are 50 and 72 arc sec. The lower half widths of the deconvoluted peaks and lower tilt angles indicate that the crystalline perfection is better than that of specimen (a). Figure 4c shows the DC recorded for specimen (c) (prepared from the top portion of the specimen). As seen from the curve, there is only one additional peak to the main peak at a tilt angle of 22 arc sec. The very low angle of 22 arc sec indicates that the grain boundary is of Very low angle type16 (tilt angle e 1 arc min). The lower half widths of both the peaks (main and satellite peaks) and very low tilt angle of the boundary indicate that
Characterization of Single Crystal L-Alanine
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Figure 5. FT-NMR spectrum of L-alanine.
Figure 6. FTIR spectral pattern of L-alanine.
Figure 4. Diffraction curves (DCs) recorded for (020) diffracting planes of L-alanine specimens prepared from a bulk crystal at (a) the interface, (b) 4 mm away from the interface, and (c) the surface (∼8 mm from the interface). (d) DC for a seed crystal.
this portion of the specimen is much better than that of specimens (a) and (b). Figure 4d shows the DC recorded for the seed crystal having a very sharp and single peak. This indicates that the original seed crystal does not contain any grain boundaries, and its crystalline perfection is very good. The above diffractometry study on L-alanine specimens prepared from bulk crystals suggests that thread used to hang the seed crystal has a lot of influence in determining the quality of the crystal.
However, the specimen prepared away from the interface or thread had good crystalline perfection. FT-NMR Analysis. 1H-NMR spectrum of L-alanine is shown in Figure 5. The powder sample of L-alanine was dissolved in deuterated water, and the spectrum was recorded using a JEOL GSX 400 spectrometer at 23 °C. The chemical shift values of the protons are plotted on the X-axis, and the intensity is plotted on the Y-axis. There are three peaks in the spectrum illustrating three types of protons. The peak in the upfield region 1.7 ppm is assigned to methyl protons. The peak at 3.8 ppm is due to the methane proton (CH). The NH3+ proton shows an intense signal at 4.7 ppm. Since the spectrum carries no other peaks the crystal is recorded as pure. Fourier Transform Infrared (FTIR) Analysis. The FTIR spectral analysis of L-alanine was carried out in the middle infrared region extending from 450 to 4000 cm-1 using a Bruker 66V FT-IR spectrometer. The spectrum is shown in Figure 6. The sample was prepared by mixing it with KBr. In the higher energy region, there is a broad intense band due to the N-H stretch of NH3+. There is a fine structure in the lower energy region of the band due to hydrogen bonding of NH3+ with COO- in the crystal lattice. The bands due to CH stretching modes appear just below 3000 cm-1. They overlap in the lower energy position of the intense band. The sharp intense peak at 2112.44 cm-1 is due to the combination of NH3+ asymmetrical stretching (1621.67 cm-1) and its torsional oscillation. The symmetrical NH3+ stretch is observed to give a less intense peak at 1518.92 cm-1. The CdO stretching of COO- gets overlapped with the NH3+ asymmetric stretching mode. The CH2 bends appear as well-resolved sharp peaks at 1361.97 and 1455.45 cm-1 as they do not contribute to crystal-packing forces. The C-COO- vibrations produce peaks at 1306.52, 1236.39,
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Figure 7. FT-Raman spectrum of L-alanine: (a) crystal form and (b) powder form.
1151.84, and 1113.95 cm-1. From this study, it is evident that the hydrogen bonding due to NH3+ and COO- moieties is the additional major force in the crystal lattice. FT-Raman Spectral Analysis. FT-Raman spectrum was recorded for L-alanine single crystal using a Bruker FRA 106 spectrometer, and it is shown in Figure 7a. The NH and CH vibrations produce sharp peaks just above 3000 cm-1. The peaks just below 3000 cm-1 are due to CH vibrations. The NH vibration, which generally appears around 3300 cm-1, is shifted to lower energy, just above 3000 cm-1, due to hydrogen bonding. The CdO stretch of COO- vibration appears just above 1500 cm-1. The NH3+ bends and CH2 bends are positioned between 150 to 1500 cm-1. The COO- vibration produces a peak at about 1000 cm-1. The peak at above 500 cm-1 is due to NH3+ torsional oscillation. The FT-Raman powder spectrum of L-alanine is shown in Figure 7b. The spectrum carries similar characteristic peaks as observed in the single crystal. But close observation of the intensities of the peaks of both the spectra reveals higher intensity for the peaks in the spectrum of the powder. This is attributed to freedom of different groups on the surface of each crystal. Mass Spectral Analysis. The mass spectrum of L-alanine was recorded using a Finnigan MAT 8230MS analyzer (spectrum not shown). The molecular ion peak produces a signal at 89 m/z. The M + 1 is also observed. The loss of CH3 from the molecular ion gives an intense peak at 74 m/z. It is the base peak in the spectrum. The different fragments from the ion of mass 74 give peaks at lower m/z values. Melting Point and Density Measurements. The finely powdered material of the L-alanine single crystal was inserted into the capillary tube, and the capillary was placed in the melting point apparatus. The temperature was increased at a rate of 2 °C/min, and the melting of material at 297 °C was noted. It is in good agreement with the reported value. The error in the measurements is (1 °C. The measurement of density is one of the important methods to study the purity of crystals. The most sensitive method to determine the density of the crystal, namely, the flotation technique,17 was used to analyze the crystal. In the present study
Vijayan et al.
Figure 8. Optical spectra of L-alanine: (a) UV-Vis spectrum and (b) DRS-UV-Vis spectrum.
also, the density measurement was carried out by the flotation technique. In this analysis, toluene and CCl4 were used for the measurements. The experimentally determined value is in good agreement with the theoretically found value using the formula F ) (MZ)/(NV); where M is the molecular weight, Z is the number of molecules per unit cell, N is the Avogadro number, and V is the volume of the unit cell. The theoretical value is obtained from the crystallographic data. The density values are given below:
experimental: 1.35 g/cm3 theoretical: 1.37 g/cm3 Optical Examination. The UV spectrum of L-alanine in liquid form (dissolved in deionized water) was recorded between 300 to 190 nm using a Shimadzu UV-1061 UV-Vis spectrophotometer. The spectrum is shown in Figure 8a. As the powder is colorless, the analysis was carried in the near UV region. When the absorbance was monitored from 300 to 190 nm, the absorption was evident below 240 nm. It is assigned to electronic excitation in the COO- group of L-alanine. DRS-UV-Vis Spectral Analysis. The DRS-UV-Vis spectral analysis was carried out between 400 and 2000 nm. The reflection spectrum is shown in Figure 8b. The absorbance is minimum in the visible region; it is an important requirement for NLO materials having nonlinear optical applications.18 The spectrum shows maximum absorption below 750 nm due to overtones/combination bands. Measurement of Laser Damage Threshold and SHG Test. The laser damage threshold measurement was made on L-alanine single crystals using a Q-switched Nd:YAG laser for 20 ns laser pulses operating in (TEM00) mode at a wavelength of 1064 nm. The laser beam divergence was 2.5 mrad. The output intensity of the laser was controlled with a variable attenuator and delivered to the test sample located at the near focus of the converging lens. The lens with a focal length of 5 cm was used, which was useful in setting the spot size to the desired value. The sample was mounted on the goniometer, which was used to position the different sites in the beam. During laser radiation, the power meter records the energy density of the input laser
Characterization of Single Crystal L-Alanine
beam for which the crystal gets damaged. Single and multiple laser damage measurements were made on the polished face of the grown crystal. The energy density was calculated by using the following formula:
energy density ) E/A (GW/cm2) where E is the input energy in millijoules, and A is the area of the circular spot size. In the present study, the laser damage threshold energy density was found to be 14.51 GW/cm2. The laser damage threshold value is lower when the crystal is subjected to multiple shots. It reveals the fact that the single shot damage threshold is higher than the multiple shot damage. Apart from the thermal effect, multiphoton ionization is an important cause of laser-induced damage. For the pulse widths of several nanoseconds, the thermal effects are unavoidable, while for the picosecond pulse widths, the thermal effects are negligible. This is because the thermal effects take several nanoseconds to buildup and could take several milliseconds to decay. The observed damage threshold value is greater than that of standard KDP and other known organic single crystals.19-21 The second harmonic generation behavior was tested by the Kurtz powder technique using Nd:YAG laser as a source. The sample was prepared by sandwiching the graded crystalline powder between two glass slides. The powder sample of L-alanine was illuminated by the laser source (λ ) 1064 nm). The second harmonic signal generated in the sample was collected by the lens and detected by the monochromator, which is coupled with the photomultiplier tube. The bright green emission was observed from the output of the powder form of the L-alanine. KDP sample was used for the reference material, and the output power intensity of L-alanine was comparable with the output power of KDP, and it agrees well with the reported literature.10,13 Conclusions The bulk size single crystals of L-alanine were successfully grown by the slow evaporation solution growth method at room temperature. The grown crystals have been subjected to various characterization studies. The crystalline perfection was evaluated by high-resolution X-ray diffraction (HRXRD) analysis. The functional groups of the grown compound have been identified by FTIR and Raman analyses. From these studies, it is evident that the hydrogen bonding due to NH3+ and COO- moieties is the additional major force in the crystal lattice. The total molecular weight was confirmed by mass spectra analysis. The
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minimum absorption in the visible region is observed from the UV-Vis measurement. It is an important requirement for the materials having NLO properties. The laser damage threshold was measured and compared with that of standard KDP and other known organic single crystals. The SHG behavior was tested by a Q switched Nd:YAG laser. Acknowledgment. The authors thank the Director NPL and Head, Materials Characterization Division (MCD), for their scientific support. The authors acknowledge the help rendered by Dr. Suranjan Pal, Dr. S. Datta, Mr. Anurag Kushawa, and Mr. Anil Raj Singh of LASTECH, New Delhi, for laser damage threshold measurements. References (1) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (2) Marder, S. R.; Sohn, J. E.; Stucky, G. D., Ed.; Materials for NonLinear Optics; American Chemical Society: Washington, DC, 1991. (3) Saleh, B. E. A.; Teich, M. C. Fundamentals of Photonics; Wiley: New York, 1991. (4) Penn, B. G.; Cardelino, B. H.; Moore, C. E.; Shields, A. W.; Frazier, D. O. Prog. Cryst. Growth Charact. 1991, 22, 19-51. (5) McArdle, B. J.; Sherwood, J. N.; Damask, A. C. J. Cryst. Growth 1974, 22, 193-200. (6) Arivanandhan, M.; Sankaranarayanan, K.; Ramamoorthy, K.; Sanjeeviraja, C.; Ramasamy, P. Thin Solid Films 2005, 477, 2-6. (7) Ayers, S.; Faktor, M. M.; Marr, D.; Stevensons, J. L. J. Mater. Sci. 1972, 7, 31. (8) Aggarwal, M. D.; Lal, R. B. ReV. Sci. Instrum. 1983, 54, 772-773. (9) Hampton, E. M.; Shah, B. S.; Sherwood, J. N. J. Cryst. Growth 1974, 22, 22-28. (10) Misoguti, L.; Varela, A. T.; Nunes, F. D.; Bagnato, V. S.; Melo, F. E. A.; Mendes Filho, J.; Zilio, S. C. Opt. Mater. 1996, 6, 147-152. (11) Bernal, J. D. Z. Kristallogr. 1931, 78, 363. (12) Simpson, H. J., Jr.; Marsh, R. E. Acta Crystallogr. 1966, 8, 550555. (13) Razzetti, C.; Ardoino, M.; Zanotti, L.; Zha, M.; Paorici, C. Cryst. Res. Technol. 2002, 37, 456-465. (14) Lal, K.; Bhagavannarayana, G. J. Appl. Crystallogr. 1989, 22, 209215. (15) Batterman, B. W.; Cole, H. ReV. Mod. Phys. 1964, 36, 681-717. (16) Choubey, A.; Bhagavannarayana G.; Shubin, Yu. V.; Chakraborty B. R.; Lal, K. Z. Crystallogr. 2002, 217, 515-521. (17) Ioffe, A. F. Phys. Status Solidi 1989, 116, 457. (18) Vijayan, N.; Ramesh Babu, R.; Gopalakrishnan, R.; Ramasamy, P.; Harrison, W. T. A. J. Cryst. Growth 2004, 262, 490-498. (19) Bhat, H. L. Bull. Mater. Sci. 1994, 17, 1233-1250. (20) Boomadevi, S.; Dhanasekaran, R. J. Cryst. Growth 2004, 261, 7076. (21) Boomadevi, S.; Mittal, H. P.; Dhanasekaran, R. J. Cryst. Growth 2004, 261, 55-62.
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