Synthesis, Growth, and Characterization of a New Semiorganic

Synthesis, Growth, and Characterization of a New Semiorganic Nonlinear Optical Crystal: L-Alanine Sodium Nitrate (LASN) K. Sethuraman,† R. Ramesh Babu,‡ R. Gopalakrishnan,*,† and P. Ramasamy§ Department of Physics, Anna UniVersity, Chennai-600 025, India, School of Physics, Bharathidasan UniVersity, Tiruchirappalli-620 024, India, and SSN College of Engineering, KalaVakam-603110, Tamil Nadu, India

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1863–1869

ReceiVed October 4, 2007; ReVised Manuscript ReceiVed February 9, 2008; Accepted February 11, 2008

ABSTRACT: A new semiorganic material of L-alanine sodium nitrate (LASN) has been synthesized and successfully grown by the slow evaporation solution growth method. The grown crystals have been subjected to various characterization techniques such as single crystal X-ray diffraction studies, FT-IR and laser Raman analyses to determine the crystal structure and functional groups, respectively. Crystalline perfection of the grown crystal was analyzed by the high resolution X-ray diffraction technique (HRXRD). Mechanical behavior of the grown LASN crystal was analyzed by Vicker’s microhardness test. Thermogravimetric and differential thermal analyses of the grown crystal revealed that there is no phase transformation in LASN, and its thermal stability was found to be good. The dielectric measurement was made as function of frequency (range 0.2–200 kHz). The growth feature of the LASN crystal was studied by wet chemical etching studies. Lower cutoff wavelength and optical transmission window of LASN crystal was found by UV–vis-NIR studies. Laser damage threshold value was determined to be 15.6 and 11.4 GW/cm2, respectively, for single shot and multiple shots, by using 5 ns laser pulses at a 10 Hz repetition rate from a Q-switched Nd:YAG laser with a wavelength of 1064 nm. Second harmonic generation efficiency was determined and is about two times that of KDP crystal.

1. Introduction Nonlinear optical (NLO) crystal for the UV–visible region is extremely important for laser spectroscopy and laser processing. Hence, it is important to search for new NLO material, which possesses large NLO coefficient, shorter cutoff wavelength, transparency in the UV region, and higher laser damage threshold.1–3 In recent years, efforts have been made on amino acid with organic and inorganic complexes for potential NLO applications. Semiorganic materials possess several attractive properties such as high NLO coefficient, high laser damage threshold and wide transparency range, high mechanical strength and thermal stability, which make the materials suitable for second harmonic generation (SHG) and other NLO applications. A series of studies on semiorganic amino acid compounds such as L-arginine phosphate (LAP),4 5 L-arginine hydrobromide (L-AHBr), L-histidine tetrafluoroborate 6 (L-HFB), L-arginine hydrochloride (L-AHCl),7 L-alanine acetate (L-AA),8 and glycine sodium nitrate (GSN)9 as potential NLO crystals have been reported. Amino acids are interesting materials for NLO applications as they have proton donor carboxyl acid groups (COO-) and proton acceptor amino groups (NH2-). In this context, L-alanine sodium nitrate (LASN) has been chosen for study as a potential crystal for nonlinear optics. L-Alanine is an amino acid, and it forms a number of complexes on reaction with inorganic acid and salts to produce an outstanding material for NLO applications. LASN is grown for the first time as a bulk single crystal in the literature. Good optical quality single crystals of LASN have been grown by a slow evaporation solution growth method after estimating the solubility in water. The grown crystals have been subjected to various characterization studies. * Corresponding author. E-mail: [email protected]; krgkrishnan@ annauniv.edu. Tel.: +91-44-2220 3374. Fax: +91-44-2220 3160. † Anna University. ‡ Bharathidasan University. § SSN College of Engineering.

Figure 1. Solubility curve of L-alanine sodium nitrate.

2. Experimental Procedures 2.1. Synthesis, Solubility, and Growth of L-Alanine Sodium Nitrate (LASN). The commercially available reagent L-alanine was purified by repeated crystallization using deionized water as the solvent. LASN was synthesized by dissolving an equimolar ratio of recrystallized L-alanine and sodium nitrate (99.9%) in double distilled water and stirring well for 3 h using a motorized magnetic stirrer, and the mixture was kept at 45 °C. The synthesized salt of LASN was purified by a repeated crystallization process. The key factor for bulk crystal growth is the solubility of the material. Solubility of LASN was determined by using water, ethanol, and methanol as the solvents at temperatures ranging from 30 to 50 °C (Figure 1). The solubility of LASN at 35 °C is 21.5 g/100 mL of water. As the solubility of LASN in ethanol and methanol solvents are poor, water has been chosen as the solvent for growth. LASN exhibits a positive solubility temperature gradient in aqueous solution. Thus, large size single crystals of LASN can be grown by both slow evaporation and slow cooling techniques. Water is found to be the best solvent for amino acid hybrid materials.4–8 Good optical quality seed crystals were obtained from spontaneous nucleation, and these seed crystals were used for bulk crystal growth. For bulk crystal growth, saturated solution of LASN was kept at 35 °C (pH value of 4.5) in a constant temperature bath with a control accuracy of (0.01 °C. Evaporation of the LASN solution was controlled to avoid

10.1021/cg700965d CCC: $40.75  2008 American Chemical Society Published on Web 05/20/2008

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Figure 4. High resolution X-ray diffraction spectrum of L-alanine sodium nitrate. Figure 2. As grown L-alanine sodium nitrate (LASN) single crystal.

Figure 3. Powder X-ray diffraction pattern of L-alanine sodium nitrate.

secondary nucleation during the crystal growth process. Finally, a transparent bulk single crystal of size (6 × 5 × 6) mm3 was obtained (Figure 2) with a growth period of 30 days.

3. Characterization 3.1. X-ray Diffraction Analysis. Single crystal X-ray diffraction data of LASN crystal were collected using ENRAF NONIUS CAD-4 X-diffractometer with Cu KR radiation. It is observed that the crystal belongs to the orthorhombic system with noncentrosymmetric space group P212121. The lattice parameters are a ) 6.127 Å, b ) 12.394 Å, c ) 5.797 Å, and the cell volume is V ) 440.21 Å.3 In addition, we have recorded the powder X-ray diffraction pattern using RICH SEIFERT powder X-ray diffractometer with Cu KR radiation (λ ) 1.5428 Å). The sample was scanned in the 2θ values ranging from 10 to 70° at the rate of 3°/min (Figure 3). The obtained peaks were indexed by using the PROZSKI software package.

3.2. High Resolution X-ray Diffraction (HRXRD) Analysis. The idea of studying the defect distribution in the crystal is of significant importance for device fabrication. The defects created during growth have severe consequences on the physical properties of the crystal. The degree of crystalline perfection can be obtained by recording high resolution X-ray diffraction curves. A multicrystal X-ray diffractometer (MCD) developed at National Physical Laboratory (NPL), New Delhi,10 was used for high resolution X-Ray diffraction (HRXRD) to study the real structure of the solution grown LASN single crystal. The well collimated and monochromated Mo KR1 beam obtained from the three monochromator crystals set in dispersive (+,-,-) configuration has been used as the exploring X-ray beam. The specimen crystal is aligned in the (+,-,-,+) configuration. Because of the dispersive configuration, though the lattice constants of the monochromator crystal(s) and the specimen are different, the unwanted dispersion broadening in the diffraction curve of the specimen crystal due to monochromator crystals is insignificant. The sample was prepared for diffraction analysis by lapping followed by nonpreferential chemical etching using acetone and water mixture in a 1:1 ratio. The prepared specimen surface was parallel to (011) planes. Figure 4 shows the high resolution X-ray diffraction curve recorded for the (011) diffracting plane with multicrystal X-ray diffractometer using Mo KR1 radiation in symmetrical Bragg geometry. The square points are experimental points. The curve with solid line is obtained by convolution of the main peak and the two additional peaks, with adjacent angles 240 and 695 arc sec away from the main peak. These two additional peaks correspond to two low angle boundaries, whose tilt angles are 240 and 695 arc sec. The peaks are not sharp, and the halfwidths of the main peak and the low angle boundaries are 100, 365, and 415 arc sec. This result indicates the incorporation of some impurities and solvent in the crystal during growth. Some of them are segregated due to which the low angle boundaries were formed. However, the occurrence of such boundaries is not strange as observed in other solution grown crystals.11 3.3. Fourier Transform Infrared (FTIR) and Laser Raman Spectral Analyses. FT-IR analysis was carried out on the grown LASN single crystal in the region 4000 cm-1 to 400 cm-1 using FT-IR spectrometry (model BRUKER IFS 66V FTIR), and the spectrum is shown in Figure 5. The symmetric and asymmetric NH3+ stretching vibrations appear at frequencies 3083 and 2937 cm-1, respectively. The C-H and N-H bending

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Figure 5. FT-IR spectrum of L-alanine sodium nitrate. Table 1. FT-IR Functional Group Assignments of the Grown L-Alanine Sodium Nitrate Single Crystal wave number (cm-1)

assignment

3083 2988 2937 2248 1619 1594 1508 1455 1412 1362 1306 1236 1151 1113 1014 918 849 772 648 539 487

asymmetric NH3+ stretching symmetric CH3+ stretching symmetric NH3+ stretching CH3 stretching NH3+ bending NH3+ bending NH3+ bending asymmetric CH3+ bending symmetric stretching of C-COONO3 stretching C-H and N-H bending NH3+ rocking NH3+ rocking NO3 stretching overtone of torsional oscillation of NH3+ overtone of torsional oscillation of NH3+ NO3 stretching NO3 stretching COO- in plane deformation torsional oscillation of NH3+ NH3+ in plane rocking

frequency is observed at 1306 cm-1. The absorption peaks at 1619, 1594, and 1508 cm-1 confirm the presence of NH3 bending.12 The observed wavenumbers and the proposed assignment of the spectrum are shown in Table 1. The presence of nitro groups in the spectrum confirms the grown LASN compound. The laser Raman spectrum of L-alanine sodium nitrate has been recorded between 100 cm-1 and 2000 cm-1 by using a JASCO NR-1100 instrument. The powder form of the L-alanine sodium nitrate was taken for this study. The spectrum shows (Figure 6a) the more intense peak around 890 cm-1 is due to COO- stretching mode of vibrations. The less intense COH bending mode vibrations are observed around 580 cm-1 and 400 cm-1. The peaks at 1350 cm-1, 1125 cm-1, and 1055 cm-1 are assigned to NO3 stretching. The CH and NH bending

vibrations are observed at 1300 cm-1 as a sharp peak. The spectrum shows asymmetric CH3 bending at 1460 cm-1 and O-H bending is around 950 cm-1. Rocking of O-H vibrations observed at 1410 cm-1. The laser Raman spectrum of the single crystal sample of LASN is shown in Figure 6b. In a crystalline powder groupings at the end of the faces of each crystal are free to change their polarizability, and hence peaks due to various groupings are more intense. Conversely, in a single crystal they are fewer numbers of groupings that have free exposure, and there is only one crystal that interacts with light. Unlike the crystalline powder, in which multitudes of tiny grains (crystallites) are exposed, the intensities of different vibrations in single crystals are less intense than that of the powder sample. 3.4. Microhardness Studies. The microhardness studies were carried out to determine the mechanical strength of the grown crystal using Leitz Weitzler hardness tester fitted with a diamond pyramidal indentor. The well polished LASN crystal of size (6 × 5 × 6) mm3 was placed on the Vicker’s microhardness tester, and the indentations were made for the loads 2, 4, 6, 8, and 10 g, and the diagonal lengths were measured for each load. The indentation time was kept at 3 s for all the loads. The Vicker’s microhardness Hv was calculated using the following equation.

Hv )

(1.8544P) kg/mm2 2 d

(1)

where P is the applied load in kg, d is the diagonal length of the indentation impression in micrometers, and 1.8544 is a constant of the geometrical factor for the diamond pyramid. Figure 7 shows the variation of Vicker’s hardness number with load for LASN. The graph clearly indicates that the hardness increases with the increase of load. The cracks were observed beyond the 20 g load around the indentor mark. The work hardening coefficient (n) of the material was calculated by the relation P ) adn, where a is the arbitrary constant for a given material, and n is the Meyer’s index or work hardening exponent. The plot of log P with log d is a straight line. In this

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Figure 6. Laser Raman spectrum of L-alanine sodium nitrate (a) powder sample and (b) crystal sample.

Figure 7. Variation of Vicker’s hardness number with load for L-alanine sodium nitrate.

study, the work hardening exponent n is found to be 2.55 by the least-squares fitting method. This supports the concept of Onitsch13 that, if n > 2, the microhardness increases with the increase of load. 3.5. TG/ DTA and Differential Scanning Calorimetry (DSC) Analysis. The thermogravimetric analysis of LASN was carried out in the temperature ranging from 40 to 400 °C at a heating of 10 °C/min in the nitrogen atmosphere. The thermogram and its differential thermogravimetric trace are shown in Figure 8a. There is no weight loss between 100 and 200 °C. This indicates that there is no inclusion of water in the crystal lattice, which was used as the solvent for crystallization. The TG spectrum reveals that the major weight loss (around 95%) starts at 295.2 °C, and it continues up to 365 °C. The nature of weight loss indicates the decomposition point of the material. However, below this temperature no weight loss is observed. In the DTA spectrum (Figure 8b) an irreversible exothermic

peak observed around 300 °C corresponds to the decomposition temperature of the material. The DSC analysis of the grown LASN was carried out between 40 to 500 °C in the nitrogen atmosphere at a heating rate of 10 °C/min (Figure 8c). An irreversible exothermic peak is observed around 300 °C, which is assigned to intense weight loss of the material, and it also coincides with the DTA trace shown in Figure 8b. Hence, this compound has decomposition before melting. There is no phase transition before the decomposition point 300 °C. Hence, this material has very good thermal stability up to 300 °C. 3.6. Dielectric Studies. LASN samples with dimensions of 5 × 5 × 6 mm3 were used for dielectric studies. The plates of LASN crystal were polished, and the side faces were coated with air drying silver paste, which act as an electrode. A HP 4275 Multifrequency LCR meter was used to measure capacitance (C) and dissipation factor (D) of the sample as a function of frequency (0.2-200 kHz) and temperature (35-100 °C). The dielectric constant (εr) and dielectric loss (ε′′) were calculated from the values of C and D using the relations

εr ) Cd/ε0A and ε′′ ) Dεr

(2)

where d is the thickness of the sample, ε0 is the permittivity of free space, and A is the area of cross section of the sample. Figure 9a gives the plot of variation of dielectric constant with frequency at different temperatures. From the graph it is found that the material has a dielectric constant value of 39 at 1 MHz. It is observed that the dielectric constant of crystal at higher frequencies is almost constant. The magnitude of εr depends on the degree of polarization of charge displacement in the crystals. The dielectric constant of the materials is due to the contribution of electronic, ionic, dipolar, and space charge polarizations, which depends on the frequency of the ac voltage applied across the material.14 All the four polarizations might be active at low frequencies. The space charge polarization will depend on the purity and perfection of the material,

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Figure 8. (a) TG, (b) DTA, and (c) DSC spectrum of L-alanine sodium nitrate.

Figure 9. (a) Dielectric constant vs frequency; (b) dielectric loss vs frequency for LASN crystal.

and its influence is large at high temperatures. The values of dielectric constant (εr) (Figure 9a) and dielectric loss (ε′′) (Figure 9b) decrease with increasing frequency and almost remain constant in the higher frequency region. The decrease in

dielectric loss with the increasing frequency indicates that the grown LASN single crystal possesses low defects. 3.7. Etching Studies. It is essential to study the microstructural imperfections of the grown LASN crystal. Chemical etching is an important tool for the identification of crystal defects, which is able to develop some of the features such as growth hillocks, etch spirals, rectangular etch pits, etc. on the crystal surface. In the present case water has been used as etchant. The surface of the as-grown LASN crystal (Figure 10a) was polished before etching studies. Etching was carried out at room temperature, ranging from a few seconds to a few minutes. The etched surface was dried by gently pressing them between the filter papers, and surface micromorphology was photographed using an optical microscope (Leitz wetzler) in reflection mode. Close spaced growth striations were observed for the etching time of 1 s (Figure 10b). The striations indicate the controlled growth rates of the faces. Increasing the etching time to 5 and 15 s, well-defined rectangular etch pits were observed as shown in Figure 10, c and d, respectively, for water etchant. The size of the etch pits was found to increase with an increase in time of etching. However, Sangwal15 points out that they are produced when the supersaturation at some points on the growing surface of a crystal is higher than at other parts. Thus, these points act as centers of repeated two-dimensional nucleation for growth fronts, which spread and pile up on the growing surfaces. Further, he pointed out that the number and location of the growth centers probably depend on temperature, supersaturation, crystal face, etc. From the etching studies, it is concluded that the crystals of LASN grow in a two-dimensional nucleation mechanism. 3.8. Transmission Spectra. Transmission spectral studies are very important for any NLO material because NLO material must have a wide transparency window for optical applications. The optical transmission spectrum for the wavelengths between 200 and 2000 nm was recorded using a Shimadzu UV-1061 UV–visible spectrophotometer with an optically polished LASN single crystal of thickness 2 mm. The recorded UV–vis-NIR transmittance spectrum is shown in Figure 11. The transmittance is found to be maximum in the entire UV–visible region, and the UV cutoff wavelength of LASN is below 300 nm. There is no absorption of light in the UV–visible range of the electromagnetic spectrum, and it can be used as a potential material for SHG or other applications in the blue and violet regions.

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Figure 10. (a) As-grown LASN surface; (b, c, d) etch patterns of LASN crystal with different etching times (water as an etchant).

Figure 11. UV–visible transmittance spectrum of L-alanine sodium nitrate.

3.9. Second Harmonic Generation and Laser Induced Damage Threshold Studies. Kurtz-Perry powder technique was used to study the SHG properties of the grown crystal.16 The powdered material of the LASN was densely packed between two transparent glass slides. A Q-switched Nd:YAG laser beam of wavelength 1064 nm was made to fall on the sample cell (pulse width of 10 ns), and the SHG behavior was analyzed by measuring output light. The SHG efficiency of the grown crystal was obtained relative to KDP crystal, that is, deff ) 2.16deff KDP. Laser damage threshold value of solution grown LASN crystal was measured by using Q-switched Nd:YAG laser operating in transverse mode (TM00). Pulse width of 5 ns with a repetition rate of 10 Hz and fundamental wavelength of 1064 nm was

used for this study. Laser induced breakdown of materials caused by various physical mechanism. For transparent materials, the damage is due to avalanche and multiphoton ionizations. The damage threshold in the case of strong absorbing materials is due to the temperature rise, which creates strain-induced fracture.17 To investigate the laser damage threshold of LASN crystal, the 4 mm diameter laser beam was focused to a target by using 100 mm focal length lens. An attenuator was used to vary the energy of the laser pulses with a polarizer and a halfwave plate. The pulse energy of each shot was measured by using combination of phototube and oscilloscope. A He-Ne laser beam arrangement was made parallel with an Nd:YAG laser beam to pass through the Nd:YAG laser point on the crystal. A copropagating 500 nm beam from a He-Ne laser was focused by a 200 nm focal length of cylindrical lens through the surface of the crystals and illumination in any damage points. If small damage occurred, we can observe the scattered He-Ne laser beam and also decrease in intensity of the transmitted He-Ne laser beam. A power meter was placed behind the crystal to measure the transmitted He-Ne laser. The measured single shot and multiple shot laser damage threshold values are 15.6 GW/cm2 and 11.4 GW/cm2, respectively, for 1064 nm Nd: YAG laser radiation. Single shot laser damage needs higher energy to induce damage than do the multiple shots. Figure 12 shows the variation of threshold intensity with the number of shots for LASN crystal. In the graph, the number of shots is plotted in logarithmic scale, and hence, we cannot see the saturation in damage threshold intensity with increasing shots. This required energy reduces with increasing number of shots and is saturated at a certain value. The cumulative thermal effect is the reason for decreasing energy with increasing laser shots. LASN crystals have high damage threshold values like other solution grown semiorganic NLO crystals.

4. Conclusions Good quality single crystals of LASN were successfully grown by the slow evaporation solution growth method from aqueous solution and reported for the first time in the literature. The crystal exhibits pyramidal morphology. The crystal system

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References

Figure 12. Dependence of laser damage threshold at 1.064 µm with the number of shots irradiating on the LASN crystal.

was identified by the single crystal XRD studies and found to be orthorhombic. The presence of functional groups of LASN was confirmed by FT-IR and laser Raman analyses. The thermal behavior of the LASN crystal was analyzed by TGA and DSC studies, and the thermal stability of the material was determined. Optical assessment shows that it has a large transmission window, and it may be used for frequency doubling and other NLO applications. The powder SHG efficiency of LASN single crystals is 2 times greater than that of KDP. LASN crystals have high damage threshold values. Acknowledgment. Authors are grateful to the Department of Science and Technology (DST), Government of India, for funding the project (Sanction order No. SP/S2/LOP-3/2002 dated 13.05.2003). The authors thank Dr. G. Bhagavannarayana, National Physical Laboratory, New Delhi, for HRXRD analysis.

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