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Synthesis, growth and characterization of a nonlinear optical crystal: Bis l -proline hydrogen nitrate
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 537–543
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Synthesis, growth and characterization of a nonlinear optical crystal: Bis L-proline hydrogen nitrate K. Selvaraju ⇑, K. Kirubavathi Post Graduate and Research Department of Physics, Government Arts College, Ariyalur 621 713, Tamilnadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The NLO crystal of BLPHN has been
synthesized and grown from its aqueous solution. Crystal of BLPHN is transparent over the entire UV–vis–NIR region. Thermal analyses gives, the compound is stable up to 235 °C. The SHG efficiency of this crystal has higher than KDP, first time investigated.
a r t i c l e
i n f o
Article history: Received 22 February 2013 Received in revised form 15 May 2013 Accepted 13 June 2013 Available online 28 June 2013 Keywords: Solution growth Infrared analysis Second harmonic generation Nonlinear optical materials
a b s t r a c t The single crystals of bis L-proline hydrogen nitrate (BLPHN) belonging to non-centrosymmetric space group were successfully grown by the slow evaporation solution growth technique. The BLPHN crystals of size 10 7 3 mm3 were obtained in 35 days. Initially, the solubility tests were carried out for two solvents such as deionized water and mixed of deionized water–acetone. Among the two solvents, the solubility of BLPHN was found to be the highest in deionized water, so crystallization of BLPHN was done from its aqueous solution. As grown, crystals were characterized by single crystal X-ray diffraction studies and optical transmission spectral studies. Infrared spectroscopy, thermo gravimetric analysis and differential thermal analysis measurements were performed to study the molecular vibration and thermal behavior of the grown BLPHN crystals. Nonlinear optical (NLO) behavior of BLPHN crystal was studied by Kurtz and Perry powder method. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Nonlinear optics is given increasing attention due to its wide application in the area of laser technology, optical communication and data storage technology [1]. The NLO materials will be the key elements for future photonic technologies based on the fact that photons are capable of processing information with the speed of light [2,3]. The search for new and efficient NLO materials in which to carry out nonlinear optical processes has been very active since
⇑ Corresponding author. Tel.: +91 4329 222050; fax: +91 4329 221260. E-mail address: [email protected] (K. Selvaraju). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.06.061
second harmonic generation (SHG) was first observed in single crystal quartz by Franken and co-workers in 1961. In the beginning, studies were concentrated on inorganic materials such as quartz, potassium dihydrogen phosphate (KDP), lithium niobate (LiNbO3), and its analogues, potassium titanyl phosphate (KTP) and its analogues, Beta Barium Borate [4] and semiconductors such as cadmium sulfide, selenium, and tellurium. However, inorganic crystals face a ‘trade-off’ problem between response time and magnitude of optical nonlinearity. Recently organic materials with delocalized conjugated pi-electrons have gained much attention because of their large NLO properties and quick response [5–8]. Organic second order NLO materials have the ability to double the frequency of incident light
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and have important commercial applications. Typical organic NLO molecules must have a dipole and be polarizable. The organic crystals exhibit higher nonlinear second order coefficients [9–12], but their transparency domain is reduced. Also, they suffer from problems such as low thermal stability, mechanical weakness, etc. Presently, inorganic and organic materials are being replaced by semi-organics. They share the properties of both organic and inorganic materials. Recent interest is concentrated on metal complexes of organic compounds owing to their large non-linearity [13,14]. The approach of combining the high nonlinear optical coefficients of the organic molecules with the excellent physical properties of the inorganics has been found to be overwhelmingly successful in the recent past. Hence, recent search is concentrated on semi-organic materials due to their large nonlinearity, high resistance to laser induced damage, low angular sensitivity and good mechanical stability [15]. Recently complexes of amino acids have been explored as interesting materials for NLO applications in the semi-organics [16,17]. The importance of amino acids in NLO applications is due to the fact that all the amino acids have chiral symmetry and crystallize in noncentrosymmetric space groups [18]. Complexes of amino acids with inorganic acids and salts are promising materials for optical second harmonic generation (SHG), as they tend to combine the advantages of the organic amino acid with that of the inorganic acid [19]. Many number of natural amino acids are individually exhibiting the nonlinear optical properties because they are characterized by chiral carbons, a proton-donating carboxyl (ACOOH) group and the proton accepting amino (ANH2) group. The crystal structures of amino acids and their complexes have provided a wealth of interesting information to the patterns of their aggregation and the effect of other molecules and ions on their interactions and molecular properties [20]. Thus, Glycine [21], L-arginine [22], L-alanine [23], L-histidine [24], L-valine [25], L-tyrosine [26], L-cystine [27], L-lysine [28], L-glutamic acid [29], L-phenylalanine [30], and L-Proline [31–33] have been exploited for the formation of salts with different organic/inorganic acids. The L-proline is an efficient NLO material under the amino acid family. Several new complexes incorporating the amino acid L-proline has been recently crystallized and their structural, optical, thermal and nonlinear optical property has been investigated [34–40]. The crystal structure of the title compound has already been reported [41]. The spectral studies such as the FTIR and FT-Raman studies are also discussed [42]. To the best of our knowledge much report are not available on the growth, characterization and NLO properties of the this bis L-proline hydrogen nitrate compound. The present study includes the growth of bis L-proline hydrogen nitrate ½2C5 H9 NO2 Hþ NO 3 by slow evaporation method using the solvent of deionised water and its characterization were studied by single crystal XRD, FTIR, FTRaman, UV–vis–NIR analyses. The thermal studies were also carried out on the grown crystals. The NLO property of the title compound was confirmed by Kurtz Perry technique also have been discussed.
The calculated amount of nitric acid was dissolved in solvent of deionised water and then L-proline was added to the solution slowly by stirring. The mother solution was thoroughly stirred using magnetic stirrer to yield a homogenous mixture of solution. The prepared mother solution was filtered using fine porosity of filter paper. The filtered solution was transferred to crystal growth vessels and allowed to dry at room temperature and the salts were obtained by slow evaporation technique. The purity of the synthesized salt was further improved by successive recrystallization process. Solubility measurements Solubility data of a material is governed by the amount of material which will be available for the growth, thereby also depends on the total size of the crystal. Solubility must be moderate and should have positive temperature gradient in a selected solvent. The solvent selection is a very important factor for the growth of good quality of single crystals [43]. In order to identify the suitable solvent, solubility of BLPHN in different solvents such as deionied water and mixed solvent of acetone and deionied water was determined. The solubility of the above solvents 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 BLPHN for various temperatures viz. 30, 40, 45 and 50 °C. The temperature dependence of solubility is BLPHN as shown in Fig. 1. From the graph, it is observed that the solubility of BLPHN in deionized water is high when compared to mixed solvent at the same temperature. Also observed that the solubility of BLPHN in water increases as the temperature increases and hence the title compound has positive temperature coefficient. Hence deionized water has been chosen for bulk growth of BLPHN crystals. Growth The impurity content of the synthesized salt was minimized by purifying the growth solution by repeated recrystallization processes. The solvent evaporation technique was employed for the growth of BLPHN crystal at room temperature. The recrystallized salt was taken as the raw material for growth. The solvent of deionised water was taken in a beaker and the purified material was added gradually in order to get the saturation. The saturated solution was further purified by filtering through the whatman filter paper provided with fine pores of size 1 micrometer porosity. The filtered solution was tightly closed with perforated sheets so
Experimental section Material synthesis The commercially available AR-grade of L-proline (C5H9NO2) and nitric acid (HNO3) (E-Merck) in 2:1 M ratio were used to synthesize bis L-proline hydrogen nitrate (BLPHN). The required amount of starting materials for the synthesis of BLPHN was calculated according to the following chemical reaction:
2C5 H9 NO2 þ HNO3 ! 2C5 H9 NO2 Hþ NO3 L-proline
Nitric Acid
bis L-proline hydrogen nitrate
Fig. 1. Solubility of BLPHN.
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were recorded in the range 400–4000 cm1 employing a PerkinElmer Fourier transform infrared spectrometer by the KBr pellet method to study the functional groups in the grown crystal. Linear optical properties of the crystals were studied using a PerkinElmer Lambda 35 UV–vis spectrometer in the region 200–1100 nm. The thermal stability was identified by thermo gravimetric (TG) and differential thermal analyses (DTA). The thermal analyses were carried out using STA1500 thermal analyzer at a heating rate of 10 °C/min in nitrogen atmosphere. Microhardness measurements for BLPHN crystal were carried out using Vickers microhardness tester. To confirm the nonlinear optical property, the Kurtz and Perry powder second harmonic generation (SHG) test was performed on the grown crystals using a Q-switched Nd:YAG laser as source. Results and discussion
Fig. 2. As grown BLPHN single crystal.
Single crystal X-ray diffraction analysis Table 1 Lattice parameter values of BLPHN. Lattice parameters
Present work
Reported work [10]
a b c Volume System Space group Z
7.2209 Å 7.6989 Å 24.0854 Å 1338.97 Å3 Orthorhombic – –
7.20006 Å 7.711 Å 24.060 Å 1335.9 Å3 Orthorhombic P212121 4
that the rate of evaporation could be minimized and kept in dust free environment. Well developed optical single quality crystals of dimension 10 7 3 mm3 were obtained after 35 days. The grown crystal is shown in Fig. 2.
Characterization The grown single crystals of BLPHN crystals were subjected to single-crystal XRD studies using an ENRAF NONIUS CAD4 diffractometer with MoKa radiation. The FTIR spectra of BLPHN crystals
Single crystal X-ray diffraction studies have been carried out using ENRAF NONIUS CAD4 diffractometer. From this study, the crystallinity nature and the lattice parameter values of the grown BLPHN crystal are calculated. Using the orthorhombic crystallographic equation, the lattice parameter values of BLPHN crystals are calculated and compared with the literature values [19]. The calculated value has a very good agreement with the literature values. The calculated lattice parameter values are presented in Table 1. This confirms that the grown BLPHN single crystal retain its own crystal system. BLPHN belongs to orthorhombic system, space group P212121 [19] which is recognized as noncentrosymmetric, thus satisfying one the basic and essential material requirements for the SHG activity of the crystal [44]. FTIR studies and FT-Raman spectral analysis To identify the elements and the functional groups present in the grown crystal qualitatively, the Fourier transform infrared (FTIR) and FT-Raman spectrum was obtained using Perkin Elmer spectrometer in the 450 cm1 to 4000 cm1 and BRUKER FT-Raman Spectrometer in the range 50–5000 cm1 as shown in Figs. 3 and 4 respectively.
Fig. 3. FTIR spectrum of BLPHN crystal.
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Fig. 4. FT-Raman of BLPHN crystal.
In this present study, the absorption band at 2960 cm1 in IR and 2988 cm1 in Raman are assigned to the CH2 asymmetric stretching mode of vibrations. The carbonyl (C@O) stretching modes of vibration are observed in strong IR band at 1732 cm1 and a weak peak at 1720 cm1 for Raman are observed to confirm the presence of carbonyl group in the compound. The carboxylic acid group, attached to the carbon atom of the aliphatic compounds, are usually present nearly at 555–520 cm1 are attributed to the CAC@O in-plane vibration mode. The medium intensity IR band occurs in 573 cm1 and a weak Raman band at 552 cm1 in both are attributed to the CAC@O in-plane vibration mode. The carboxylate ion group COO usually absorbs strongly near 1600–1570 cm1 and more weakly near 1440 cm1 because of its antisymmetric and symmetric stretching modes respectively [45]. The peak observed at 1678 cm1 in IR and a weak Raman band at 1653 cm1 are assigned to the antisymmetric stretching mode of COO group. A strong band at 1596 cm1, 1165 cm1 and 904 cm1 in IR reflects the NH2+ scissoring, wagging and CH2 rocking mode of vibrations respectively. These bands confirm the occurrence of protonation in the imino group. The strong band occurring near 2988 cm1 for Raman, 2960 cm1 for IR are attributed to the CH2 antisymmetric stretching mode of vibration. The band observed at 1461 cm1 in Raman spectra for CH2 scissoring mode of vibration. The normal modes of vibrations belong to the nitrate anion ðNO 3 Þ symmetric stretching occurs at 1049 cm1, symmetric deformation at 830 cm1, antisymmetric stretching and antisymmetric deformation at 1355 cm1 and 690 cm1 respectively. In the present case, the Raman active symmetric stretching mode is observed at 1039 cm1 as a very strong band, also IR inactive mode is found at 1039 cm1 as a strong peak. The IR active symmetric deformation mode is observed at 825 cm1. The band at 1330 cm1 in IR and 1329 cm1 in Raman are assigned to the antisymmetric stretching mode of the nitrate anion. This may be confirming the presence of nitrate anion ðNO 3 Þ in solid crystalline compound of BLPHN. The important and prominent wave numbers of vibrations and their assignments have been presented and it will be compared to the reported work summarized in Table 2. Melting point and density measurements The melting point of the grown crystal is evaluated using the TEMPO-2300 melting point apparatus. The finely powdered
Table 2 Wave numbers (cm1) of FTIR absorption peaks and FT-Raman lines in the spectra of BLPHN crystal samples and their assignments. BLPHN (reported) [42]
BLPHN (present work)
IR
Raman
IR
Raman
3430 2960 2757 2470 1732 1622 1597 1452 1385 1326 1235 1180 1040 990 825 759 673 493
– 2985 – – 1725 1650 – 1463 – 1320 1233 1179 1042 995 841 757 694 502
3412 2960 2757 2472 1732 1678 1596 – 1384 1330 1230 1165 1039 946 825 759 673 493
– 2988 – – 1720 1653 – 1451 – 1329 1238 1172 1039 997 862 762 707 508
Assignments
H2O trace CH2 asymmetric stretching Overtone and combination bands C@O stretching C@O stretching COO asymmetric stretching NH2 scissoring CH2 scissoring NHþ 2 twist NO 3 symmetric stretching CH2 wagging NH2 wagging NO3 symmetric stretching CACAN stretching CH2 rocking Skeletal deformations COO scissoring COO torsion
material of the BLPHN single crystal was inserted into the capillary tube, and it was placed in the melting point apparatus. The temperature was increased at a rate of 3 °C/min, and the melting point of the BLPHN crystals is observed at 235 °C. It is a good agreement with the TG/DTA values. The measurement of density is one of the important methods to study the purity of the grown crystals. The theoretical density of BLPHN crystal was calculated by using the ratio of the cell mass to cell volume, the equation to calculate the density is qt = (MZ)/(NV) where M is chemical formula weight of BLPHN, Z is the number of molecules per unit cell, N is the Avogadro number, and V is the volume of the unit cell. The experimental density of BLPHN crystal is measured by the most sensitive method of flotation technique [46]. The density of an as-grown crystal was measured by the flotation method at room temperature, in a mixture of carbon tetrachloride and toluene resulted in 1.453 g/cm3. This agrees well with the theoretical and reported value [19] of the grown BPLHN crystals.
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harmonic lights (k = 354.6 nm) (k = 1064 nm) [47].
from
the
Nd:YAG
laser
Thermal analyses
Fig. 5. Optical spectrum of BLPHN crystal.
Density of BLPHN crystal (g/cm3) Reported value [19]
Experimental value (qe)
Theoretical value (qt)
1.458
1.453
1.456
The thermal properties of the BLPHN crystal are studied using thermo gravimetric analysis (TGA)/and differential thermal analysis (DTA). Powdered sample is analyzed in N2 atmosphere by using STA 1500 thermal analyzer equipment. The analysis is carried out simultaneously at a heating rate of 10 °C/min for a temperature range of 25 °C to 500 °C. The TGA/DTA curve is shown in Fig. 6. Quite interesting and important point to be noticed is the good thermal stability of the material up to 235 °C. The absence of water of crystallization in the molecular structure is indicated by the absence of weight loss around 100 °C. Further there is no decomposition up to the melting point. This ensures the suitability of the material for possible applications in lasers, where the crystals are required to withstand high temperature ranges. From the TGA curve, it is observed that the compound starts to lose single molecule of amino group at about 235 °C and continuous up to 275 °C. In the DTA curve, the endothermic peak at 260 °C corresponds to melting point of the substance, which is associated with weight loss as observed from the TGA curve. Second harmonic generation efficiency measurement
UV–vis spectral analysis The UV–vis spectrum gives useful information about the structure of the molecules, that is the promotion of electrons in r and p orbits from the ground state to a higher energy state. The UV–vis transmission spectrum was recorded with a Shimadzu UV-2502 spectrophotometer in the range of 200–1200 nm. The crystal shows a good transmittance of more than 75% in the visible region. The transmission spectrum (Fig. 5) shows there is no significant absorption in the range of 200–1100 nm and also it is evident that crystal has lower cut off wavelength at 249 nm. This is an advantage of the use of amino acids, where the absence of strongly conjugated bonds gives to wide transparency ranges in the visible regions. This transmittance window (300–1100 nm) is suitable for the generation of second harmonic (k = 532 nm) as well as third
The second harmonic generation efficiency was measured for the grown crystal using the standard Kurtz and Perry powder technique [48]. Kurtz and Perry method is extremely valuable tool for initial screening of NLO materials. A quantitative measurement of the conversion efficiency of BLPHN was determined by the modified version of powder technique developed by Kurtz and Perry. Using this technique Kurtz surveyed a very large number of compounds of potential interest [48]. The schematic diagram of powder SHG measurement is shown in Fig. 7. A Q-switched Nd:YAG laser beam of wavelength of 1064 nm was allowed to strikeout the powdered samples. The powdered samples were filled air-tight in separate micro-capillary tubes of uniform bore of about 1.5 mm diameter. The SHG radiations of 532 nm green light was emitted and collected by a photomultiplier tube (PMT – Philips Photonics – model 8563) after being monochromated (monochromator – model Traix – 550) to collect only the 532 nm radiation. The optical signal incident on the photomultiplier tube was converted into voltage output at the
Fig. 6. TG/DTA curve of BLPHN crystal.
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Fig. 7. Schematic of SHG setup.
oscilloscope (Tektronix – TDS 3052B). The fundamental beam of a Q-switched Nd:YAG laser with a wavelength of 1064 nm, input energy of 3.2 mJ, pulse duration of 8 ns, a repetition rate of 10 Hz and a spot size of 1 mm diameter was used. The grown single crystal was crushed to fine powder and then packed in a micro-capillary of uniform bore and exposed to laser radiation. The powder sample of BLPHN was illuminated by the laser source. The second harmonic generated signal was collected by the lens and detected by the monochromator, which is coupled with the photomultiplier tube. A bright green emission was observed from the output of the powder form of the BLPHN. The potassium dihydrogen phosphate (KDP) crystal was used for the reference materials. KDP crystal was powdered to the identical size and was used as reference materials in the SHG measurement. The same procedure was repeated with KDP sample, and the output power intensity of BLPHN was compared with the output power of KDP. The results obtained by this method shows that the SHG efficiency of BLPHN sample is nearly 2.8 times more than that of KDP.
60 g cracks are developed. Several trials were performed for the same load and the average diagonal length was taken for each load. The indentation time was 10 sec. The Vickers Hardness Number (VHN) of BLPHN was calculated using the expression Hv = 1.8544 P/d2 kg/mm2 where Hv is the Vickers hardness number in kg/mm2, P is the applied load in kg, and d is the average diagonal length of the indentation in mm. A graph (Fig. 8) has been plotted between the hardness number Hv and the applied load P. From the graph it is observed that the hardness value decreases and then cracks are developed with the increase in the applied load. In the above 60 g of load, the cracks started developing due to the release of internal stress generated locally by indentation. It is noted that the hardness decreases as the load is increased, which is in agreement with the normal Indentation Size Effect (ISE) observed for other NLO crystals[50]. The hardness number of the compound is found to be 68 Kg/mm2.
Mechanical studies
A potential new semi-organic second order nonlinear optical material of bis L-proline hydrogen nitrate was grown by slow evaporation technique at room temperature. From the structural study, it is found that the bis L-proline hydrogen nitrate crystallizes under non-centrosymmetric orthorhombic system. The lattice parameters evaluated from the single crystal X-ray diffraction are in good agreement with the reported crystal structure. The grown crystal had good optical transmittance in UV–vis–NIR region, and the lower cutoff wavelength was 249 nm, which is one of the mandatory requirements for NLO properties. The vibrational frequencies were assigned from FTIR and FT-Raman spectral analysis, which confirm the presence of various functional groups are present in the grown crystal of bis L-proline hydrogen nitrate. The thermal studies confirm that the crystal structure is stable up to 235 °C and indicate its suitability for application in lasers field. The microhardness was determined in order to understand mechanical stability of the grown crystal. The NLO behavior of the bis L-proline hydrogen nitrate crystal was observed by Kurtz and Perry powder method by the emission of green radiation. From this method the SHG efficiency of bis L-proline hydrogen nitrate crystal is nearly 2.8 times more than that of KDP.
The microhardness testing is one of the best methods for understanding the mechanical properties of crystals [49]. The microharndess studies have been carried out on BLPHN crystals using Vickers hardness tester fitted with a Vickers diamond pyramidal indenter. The indentations are made for loads 10 g–70 g. Beyond
Conclusions
Acknowledgements
Fig. 8. Load vs. hardness number of BLPHN crystal.
The authors are grateful to Sophisticated Analytical Instrument Facilities, Indian Institute of Technology-Madras, Chennai 600 036,
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis, growth and characterization of a nonlinear optical crystal: Bis L-proline hydrogen nitrate K. Selvaraju ⇑, K. Kirubavathi Post Graduate and Research Department of Physics, Government Arts College, Ariyalur 621 713, Tamilnadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The NLO crystal of BLPHN has been
synthesized and grown from its aqueous solution. Crystal of BLPHN is transparent over the entire UV–vis–NIR region. Thermal analyses gives, the compound is stable up to 235 °C. The SHG efficiency of this crystal has higher than KDP, first time investigated.
a r t i c l e
i n f o
Article history: Received 22 February 2013 Received in revised form 15 May 2013 Accepted 13 June 2013 Available online 28 June 2013 Keywords: Solution growth Infrared analysis Second harmonic generation Nonlinear optical materials
a b s t r a c t The single crystals of bis L-proline hydrogen nitrate (BLPHN) belonging to non-centrosymmetric space group were successfully grown by the slow evaporation solution growth technique. The BLPHN crystals of size 10 7 3 mm3 were obtained in 35 days. Initially, the solubility tests were carried out for two solvents such as deionized water and mixed of deionized water–acetone. Among the two solvents, the solubility of BLPHN was found to be the highest in deionized water, so crystallization of BLPHN was done from its aqueous solution. As grown, crystals were characterized by single crystal X-ray diffraction studies and optical transmission spectral studies. Infrared spectroscopy, thermo gravimetric analysis and differential thermal analysis measurements were performed to study the molecular vibration and thermal behavior of the grown BLPHN crystals. Nonlinear optical (NLO) behavior of BLPHN crystal was studied by Kurtz and Perry powder method. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Nonlinear optics is given increasing attention due to its wide application in the area of laser technology, optical communication and data storage technology [1]. The NLO materials will be the key elements for future photonic technologies based on the fact that photons are capable of processing information with the speed of light [2,3]. The search for new and efficient NLO materials in which to carry out nonlinear optical processes has been very active since
⇑ Corresponding author. Tel.: +91 4329 222050; fax: +91 4329 221260. E-mail address: [email protected] (K. Selvaraju). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.06.061
second harmonic generation (SHG) was first observed in single crystal quartz by Franken and co-workers in 1961. In the beginning, studies were concentrated on inorganic materials such as quartz, potassium dihydrogen phosphate (KDP), lithium niobate (LiNbO3), and its analogues, potassium titanyl phosphate (KTP) and its analogues, Beta Barium Borate [4] and semiconductors such as cadmium sulfide, selenium, and tellurium. However, inorganic crystals face a ‘trade-off’ problem between response time and magnitude of optical nonlinearity. Recently organic materials with delocalized conjugated pi-electrons have gained much attention because of their large NLO properties and quick response [5–8]. Organic second order NLO materials have the ability to double the frequency of incident light
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and have important commercial applications. Typical organic NLO molecules must have a dipole and be polarizable. The organic crystals exhibit higher nonlinear second order coefficients [9–12], but their transparency domain is reduced. Also, they suffer from problems such as low thermal stability, mechanical weakness, etc. Presently, inorganic and organic materials are being replaced by semi-organics. They share the properties of both organic and inorganic materials. Recent interest is concentrated on metal complexes of organic compounds owing to their large non-linearity [13,14]. The approach of combining the high nonlinear optical coefficients of the organic molecules with the excellent physical properties of the inorganics has been found to be overwhelmingly successful in the recent past. Hence, recent search is concentrated on semi-organic materials due to their large nonlinearity, high resistance to laser induced damage, low angular sensitivity and good mechanical stability [15]. Recently complexes of amino acids have been explored as interesting materials for NLO applications in the semi-organics [16,17]. The importance of amino acids in NLO applications is due to the fact that all the amino acids have chiral symmetry and crystallize in noncentrosymmetric space groups [18]. Complexes of amino acids with inorganic acids and salts are promising materials for optical second harmonic generation (SHG), as they tend to combine the advantages of the organic amino acid with that of the inorganic acid [19]. Many number of natural amino acids are individually exhibiting the nonlinear optical properties because they are characterized by chiral carbons, a proton-donating carboxyl (ACOOH) group and the proton accepting amino (ANH2) group. The crystal structures of amino acids and their complexes have provided a wealth of interesting information to the patterns of their aggregation and the effect of other molecules and ions on their interactions and molecular properties [20]. Thus, Glycine [21], L-arginine [22], L-alanine [23], L-histidine [24], L-valine [25], L-tyrosine [26], L-cystine [27], L-lysine [28], L-glutamic acid [29], L-phenylalanine [30], and L-Proline [31–33] have been exploited for the formation of salts with different organic/inorganic acids. The L-proline is an efficient NLO material under the amino acid family. Several new complexes incorporating the amino acid L-proline has been recently crystallized and their structural, optical, thermal and nonlinear optical property has been investigated [34–40]. The crystal structure of the title compound has already been reported [41]. The spectral studies such as the FTIR and FT-Raman studies are also discussed [42]. To the best of our knowledge much report are not available on the growth, characterization and NLO properties of the this bis L-proline hydrogen nitrate compound. The present study includes the growth of bis L-proline hydrogen nitrate ½2C5 H9 NO2 Hþ NO 3 by slow evaporation method using the solvent of deionised water and its characterization were studied by single crystal XRD, FTIR, FTRaman, UV–vis–NIR analyses. The thermal studies were also carried out on the grown crystals. The NLO property of the title compound was confirmed by Kurtz Perry technique also have been discussed.
The calculated amount of nitric acid was dissolved in solvent of deionised water and then L-proline was added to the solution slowly by stirring. The mother solution was thoroughly stirred using magnetic stirrer to yield a homogenous mixture of solution. The prepared mother solution was filtered using fine porosity of filter paper. The filtered solution was transferred to crystal growth vessels and allowed to dry at room temperature and the salts were obtained by slow evaporation technique. The purity of the synthesized salt was further improved by successive recrystallization process. Solubility measurements Solubility data of a material is governed by the amount of material which will be available for the growth, thereby also depends on the total size of the crystal. Solubility must be moderate and should have positive temperature gradient in a selected solvent. The solvent selection is a very important factor for the growth of good quality of single crystals [43]. In order to identify the suitable solvent, solubility of BLPHN in different solvents such as deionied water and mixed solvent of acetone and deionied water was determined. The solubility of the above solvents 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 BLPHN for various temperatures viz. 30, 40, 45 and 50 °C. The temperature dependence of solubility is BLPHN as shown in Fig. 1. From the graph, it is observed that the solubility of BLPHN in deionized water is high when compared to mixed solvent at the same temperature. Also observed that the solubility of BLPHN in water increases as the temperature increases and hence the title compound has positive temperature coefficient. Hence deionized water has been chosen for bulk growth of BLPHN crystals. Growth The impurity content of the synthesized salt was minimized by purifying the growth solution by repeated recrystallization processes. The solvent evaporation technique was employed for the growth of BLPHN crystal at room temperature. The recrystallized salt was taken as the raw material for growth. The solvent of deionised water was taken in a beaker and the purified material was added gradually in order to get the saturation. The saturated solution was further purified by filtering through the whatman filter paper provided with fine pores of size 1 micrometer porosity. The filtered solution was tightly closed with perforated sheets so
Experimental section Material synthesis The commercially available AR-grade of L-proline (C5H9NO2) and nitric acid (HNO3) (E-Merck) in 2:1 M ratio were used to synthesize bis L-proline hydrogen nitrate (BLPHN). The required amount of starting materials for the synthesis of BLPHN was calculated according to the following chemical reaction:
2C5 H9 NO2 þ HNO3 ! 2C5 H9 NO2 Hþ NO3 L-proline
Nitric Acid
bis L-proline hydrogen nitrate
Fig. 1. Solubility of BLPHN.
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were recorded in the range 400–4000 cm1 employing a PerkinElmer Fourier transform infrared spectrometer by the KBr pellet method to study the functional groups in the grown crystal. Linear optical properties of the crystals were studied using a PerkinElmer Lambda 35 UV–vis spectrometer in the region 200–1100 nm. The thermal stability was identified by thermo gravimetric (TG) and differential thermal analyses (DTA). The thermal analyses were carried out using STA1500 thermal analyzer at a heating rate of 10 °C/min in nitrogen atmosphere. Microhardness measurements for BLPHN crystal were carried out using Vickers microhardness tester. To confirm the nonlinear optical property, the Kurtz and Perry powder second harmonic generation (SHG) test was performed on the grown crystals using a Q-switched Nd:YAG laser as source. Results and discussion
Fig. 2. As grown BLPHN single crystal.
Single crystal X-ray diffraction analysis Table 1 Lattice parameter values of BLPHN. Lattice parameters
Present work
Reported work [10]
a b c Volume System Space group Z
7.2209 Å 7.6989 Å 24.0854 Å 1338.97 Å3 Orthorhombic – –
7.20006 Å 7.711 Å 24.060 Å 1335.9 Å3 Orthorhombic P212121 4
that the rate of evaporation could be minimized and kept in dust free environment. Well developed optical single quality crystals of dimension 10 7 3 mm3 were obtained after 35 days. The grown crystal is shown in Fig. 2.
Characterization The grown single crystals of BLPHN crystals were subjected to single-crystal XRD studies using an ENRAF NONIUS CAD4 diffractometer with MoKa radiation. The FTIR spectra of BLPHN crystals
Single crystal X-ray diffraction studies have been carried out using ENRAF NONIUS CAD4 diffractometer. From this study, the crystallinity nature and the lattice parameter values of the grown BLPHN crystal are calculated. Using the orthorhombic crystallographic equation, the lattice parameter values of BLPHN crystals are calculated and compared with the literature values [19]. The calculated value has a very good agreement with the literature values. The calculated lattice parameter values are presented in Table 1. This confirms that the grown BLPHN single crystal retain its own crystal system. BLPHN belongs to orthorhombic system, space group P212121 [19] which is recognized as noncentrosymmetric, thus satisfying one the basic and essential material requirements for the SHG activity of the crystal [44]. FTIR studies and FT-Raman spectral analysis To identify the elements and the functional groups present in the grown crystal qualitatively, the Fourier transform infrared (FTIR) and FT-Raman spectrum was obtained using Perkin Elmer spectrometer in the 450 cm1 to 4000 cm1 and BRUKER FT-Raman Spectrometer in the range 50–5000 cm1 as shown in Figs. 3 and 4 respectively.
Fig. 3. FTIR spectrum of BLPHN crystal.
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Fig. 4. FT-Raman of BLPHN crystal.
In this present study, the absorption band at 2960 cm1 in IR and 2988 cm1 in Raman are assigned to the CH2 asymmetric stretching mode of vibrations. The carbonyl (C@O) stretching modes of vibration are observed in strong IR band at 1732 cm1 and a weak peak at 1720 cm1 for Raman are observed to confirm the presence of carbonyl group in the compound. The carboxylic acid group, attached to the carbon atom of the aliphatic compounds, are usually present nearly at 555–520 cm1 are attributed to the CAC@O in-plane vibration mode. The medium intensity IR band occurs in 573 cm1 and a weak Raman band at 552 cm1 in both are attributed to the CAC@O in-plane vibration mode. The carboxylate ion group COO usually absorbs strongly near 1600–1570 cm1 and more weakly near 1440 cm1 because of its antisymmetric and symmetric stretching modes respectively [45]. The peak observed at 1678 cm1 in IR and a weak Raman band at 1653 cm1 are assigned to the antisymmetric stretching mode of COO group. A strong band at 1596 cm1, 1165 cm1 and 904 cm1 in IR reflects the NH2+ scissoring, wagging and CH2 rocking mode of vibrations respectively. These bands confirm the occurrence of protonation in the imino group. The strong band occurring near 2988 cm1 for Raman, 2960 cm1 for IR are attributed to the CH2 antisymmetric stretching mode of vibration. The band observed at 1461 cm1 in Raman spectra for CH2 scissoring mode of vibration. The normal modes of vibrations belong to the nitrate anion ðNO 3 Þ symmetric stretching occurs at 1049 cm1, symmetric deformation at 830 cm1, antisymmetric stretching and antisymmetric deformation at 1355 cm1 and 690 cm1 respectively. In the present case, the Raman active symmetric stretching mode is observed at 1039 cm1 as a very strong band, also IR inactive mode is found at 1039 cm1 as a strong peak. The IR active symmetric deformation mode is observed at 825 cm1. The band at 1330 cm1 in IR and 1329 cm1 in Raman are assigned to the antisymmetric stretching mode of the nitrate anion. This may be confirming the presence of nitrate anion ðNO 3 Þ in solid crystalline compound of BLPHN. The important and prominent wave numbers of vibrations and their assignments have been presented and it will be compared to the reported work summarized in Table 2. Melting point and density measurements The melting point of the grown crystal is evaluated using the TEMPO-2300 melting point apparatus. The finely powdered
Table 2 Wave numbers (cm1) of FTIR absorption peaks and FT-Raman lines in the spectra of BLPHN crystal samples and their assignments. BLPHN (reported) [42]
BLPHN (present work)
IR
Raman
IR
Raman
3430 2960 2757 2470 1732 1622 1597 1452 1385 1326 1235 1180 1040 990 825 759 673 493
– 2985 – – 1725 1650 – 1463 – 1320 1233 1179 1042 995 841 757 694 502
3412 2960 2757 2472 1732 1678 1596 – 1384 1330 1230 1165 1039 946 825 759 673 493
– 2988 – – 1720 1653 – 1451 – 1329 1238 1172 1039 997 862 762 707 508
Assignments
H2O trace CH2 asymmetric stretching Overtone and combination bands C@O stretching C@O stretching COO asymmetric stretching NH2 scissoring CH2 scissoring NHþ 2 twist NO 3 symmetric stretching CH2 wagging NH2 wagging NO3 symmetric stretching CACAN stretching CH2 rocking Skeletal deformations COO scissoring COO torsion
material of the BLPHN single crystal was inserted into the capillary tube, and it was placed in the melting point apparatus. The temperature was increased at a rate of 3 °C/min, and the melting point of the BLPHN crystals is observed at 235 °C. It is a good agreement with the TG/DTA values. The measurement of density is one of the important methods to study the purity of the grown crystals. The theoretical density of BLPHN crystal was calculated by using the ratio of the cell mass to cell volume, the equation to calculate the density is qt = (MZ)/(NV) where M is chemical formula weight of BLPHN, Z is the number of molecules per unit cell, N is the Avogadro number, and V is the volume of the unit cell. The experimental density of BLPHN crystal is measured by the most sensitive method of flotation technique [46]. The density of an as-grown crystal was measured by the flotation method at room temperature, in a mixture of carbon tetrachloride and toluene resulted in 1.453 g/cm3. This agrees well with the theoretical and reported value [19] of the grown BPLHN crystals.
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harmonic lights (k = 354.6 nm) (k = 1064 nm) [47].
from
the
Nd:YAG
laser
Thermal analyses
Fig. 5. Optical spectrum of BLPHN crystal.
Density of BLPHN crystal (g/cm3) Reported value [19]
Experimental value (qe)
Theoretical value (qt)
1.458
1.453
1.456
The thermal properties of the BLPHN crystal are studied using thermo gravimetric analysis (TGA)/and differential thermal analysis (DTA). Powdered sample is analyzed in N2 atmosphere by using STA 1500 thermal analyzer equipment. The analysis is carried out simultaneously at a heating rate of 10 °C/min for a temperature range of 25 °C to 500 °C. The TGA/DTA curve is shown in Fig. 6. Quite interesting and important point to be noticed is the good thermal stability of the material up to 235 °C. The absence of water of crystallization in the molecular structure is indicated by the absence of weight loss around 100 °C. Further there is no decomposition up to the melting point. This ensures the suitability of the material for possible applications in lasers, where the crystals are required to withstand high temperature ranges. From the TGA curve, it is observed that the compound starts to lose single molecule of amino group at about 235 °C and continuous up to 275 °C. In the DTA curve, the endothermic peak at 260 °C corresponds to melting point of the substance, which is associated with weight loss as observed from the TGA curve. Second harmonic generation efficiency measurement
UV–vis spectral analysis The UV–vis spectrum gives useful information about the structure of the molecules, that is the promotion of electrons in r and p orbits from the ground state to a higher energy state. The UV–vis transmission spectrum was recorded with a Shimadzu UV-2502 spectrophotometer in the range of 200–1200 nm. The crystal shows a good transmittance of more than 75% in the visible region. The transmission spectrum (Fig. 5) shows there is no significant absorption in the range of 200–1100 nm and also it is evident that crystal has lower cut off wavelength at 249 nm. This is an advantage of the use of amino acids, where the absence of strongly conjugated bonds gives to wide transparency ranges in the visible regions. This transmittance window (300–1100 nm) is suitable for the generation of second harmonic (k = 532 nm) as well as third
The second harmonic generation efficiency was measured for the grown crystal using the standard Kurtz and Perry powder technique [48]. Kurtz and Perry method is extremely valuable tool for initial screening of NLO materials. A quantitative measurement of the conversion efficiency of BLPHN was determined by the modified version of powder technique developed by Kurtz and Perry. Using this technique Kurtz surveyed a very large number of compounds of potential interest [48]. The schematic diagram of powder SHG measurement is shown in Fig. 7. A Q-switched Nd:YAG laser beam of wavelength of 1064 nm was allowed to strikeout the powdered samples. The powdered samples were filled air-tight in separate micro-capillary tubes of uniform bore of about 1.5 mm diameter. The SHG radiations of 532 nm green light was emitted and collected by a photomultiplier tube (PMT – Philips Photonics – model 8563) after being monochromated (monochromator – model Traix – 550) to collect only the 532 nm radiation. The optical signal incident on the photomultiplier tube was converted into voltage output at the
Fig. 6. TG/DTA curve of BLPHN crystal.
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Fig. 7. Schematic of SHG setup.
oscilloscope (Tektronix – TDS 3052B). The fundamental beam of a Q-switched Nd:YAG laser with a wavelength of 1064 nm, input energy of 3.2 mJ, pulse duration of 8 ns, a repetition rate of 10 Hz and a spot size of 1 mm diameter was used. The grown single crystal was crushed to fine powder and then packed in a micro-capillary of uniform bore and exposed to laser radiation. The powder sample of BLPHN was illuminated by the laser source. The second harmonic generated signal was collected by the lens and detected by the monochromator, which is coupled with the photomultiplier tube. A bright green emission was observed from the output of the powder form of the BLPHN. The potassium dihydrogen phosphate (KDP) crystal was used for the reference materials. KDP crystal was powdered to the identical size and was used as reference materials in the SHG measurement. The same procedure was repeated with KDP sample, and the output power intensity of BLPHN was compared with the output power of KDP. The results obtained by this method shows that the SHG efficiency of BLPHN sample is nearly 2.8 times more than that of KDP.
60 g cracks are developed. Several trials were performed for the same load and the average diagonal length was taken for each load. The indentation time was 10 sec. The Vickers Hardness Number (VHN) of BLPHN was calculated using the expression Hv = 1.8544 P/d2 kg/mm2 where Hv is the Vickers hardness number in kg/mm2, P is the applied load in kg, and d is the average diagonal length of the indentation in mm. A graph (Fig. 8) has been plotted between the hardness number Hv and the applied load P. From the graph it is observed that the hardness value decreases and then cracks are developed with the increase in the applied load. In the above 60 g of load, the cracks started developing due to the release of internal stress generated locally by indentation. It is noted that the hardness decreases as the load is increased, which is in agreement with the normal Indentation Size Effect (ISE) observed for other NLO crystals[50]. The hardness number of the compound is found to be 68 Kg/mm2.
Mechanical studies
A potential new semi-organic second order nonlinear optical material of bis L-proline hydrogen nitrate was grown by slow evaporation technique at room temperature. From the structural study, it is found that the bis L-proline hydrogen nitrate crystallizes under non-centrosymmetric orthorhombic system. The lattice parameters evaluated from the single crystal X-ray diffraction are in good agreement with the reported crystal structure. The grown crystal had good optical transmittance in UV–vis–NIR region, and the lower cutoff wavelength was 249 nm, which is one of the mandatory requirements for NLO properties. The vibrational frequencies were assigned from FTIR and FT-Raman spectral analysis, which confirm the presence of various functional groups are present in the grown crystal of bis L-proline hydrogen nitrate. The thermal studies confirm that the crystal structure is stable up to 235 °C and indicate its suitability for application in lasers field. The microhardness was determined in order to understand mechanical stability of the grown crystal. The NLO behavior of the bis L-proline hydrogen nitrate crystal was observed by Kurtz and Perry powder method by the emission of green radiation. From this method the SHG efficiency of bis L-proline hydrogen nitrate crystal is nearly 2.8 times more than that of KDP.
The microhardness testing is one of the best methods for understanding the mechanical properties of crystals [49]. The microharndess studies have been carried out on BLPHN crystals using Vickers hardness tester fitted with a Vickers diamond pyramidal indenter. The indentations are made for loads 10 g–70 g. Beyond
Conclusions
Acknowledgements
Fig. 8. Load vs. hardness number of BLPHN crystal.
The authors are grateful to Sophisticated Analytical Instrument Facilities, Indian Institute of Technology-Madras, Chennai 600 036,
K. Selvaraju, K. Kirubavathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 537–543
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