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Growth and characterization of gel grown single crystals of bis-glycine hydrogen chloride (BGHC)
Optical Materials 29 (2007) 657–662 www.elsevier.com/locate/optmat
Growth and characterization of gel grown single crystals of bis-glycine hydrogen chloride (BGHC) K. Ambujam a, K. Rajarajan c, S. Selvakumar c, J. Madhavan c, Gulam Mohamed b, P. Sagayaraj c,* a
Department of Physics, Queen Mary’s College, Chennai 600004, India b Department of Physics, The New College, Chennai 600014, India c Department of Physics, Loyola College, Chennai 600034, India Received 30 June 2005; accepted 26 November 2005 Available online 5 January 2006
Abstract Single crystals of bis-glycine hydrogen chloride, a semiorganic nonlinear optical material (NLO) were grown by silica gel method. Good optical quality single crystals with dimensions up to 15 · 3 · 4 mm3 were obtained. The grown crystals were characterized by single crystal XRD, FTIR and optical transmission spectrum. The second harmonic generation (SHG) efficiency of gel grown BGHC was found to be five times that of KDP. The thermal studies indicate that the gel grown BGHC is stable up to 188.5 C. The dielectric response of the crystal with varying frequencies was studied. The photoconductivity studies confirmed the negative photoconductive nature of gel grown BGHC crystal. 2005 Elsevier B.V. All rights reserved. Keywords: Gel growth; Semiorganic; NLO; Negative photoconductivity; Dielectric constant
1. Introduction Complexes of amino acids with inorganic acids and salts are promising materials for optical second harmonic generation, as they tend to combine the advantages of the organic amino acid with that of the inorganic acid/salt. The salts of amino acids like L-arginine [1] and L-histidine [2] are reported to have high second harmonic conversion efficiency. Glycine is the simplest amino acid. Unlike other amino acids, it has no asymmetric carbon atom and is optically inactive. It has three polymorphic crystalline forms a, b and c. Glycine and its methylated analogues form complexes with mineral acids exhibiting interesting physical properties like ferroelastic, ferroelectric or antiferroelectric behaviour. According to the structural analysis of ferro-
*
Corresponding author. Tel.: +91 (0)44 28175660; +91 (0)44 28175566. E-mail address: [email protected] (P. Sagayaraj).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.11.008
electric triglycine sulphate [3] there are two kinds of glycine groups, glycinium ions and zwitter ions. Such configurations of glycine ions interconnected by short O–H O hydrogen bonds are regarded as particularly important for the ferroelectric behaviour of this crystal. In the ferroelectric and paraelectric structure of (glycine)2HNO3, one of the glycine molecules has the zwitterionic configuration and the other is monoprotonated [4,5]. A similar structure has been found in diglycine hydrochloride [6], hydrobromide [7], hydroiodide [8] and disarcosine hydrobromide [9]. In all the above mentioned structures the molecules are held together by a network of N–H X, N–H O and O–H O hydrogen bonds. In the present work, we have attempted to grow single crystals of bis-glycine hydrogen chloride (BGHC) in silica gel at room temperature. The paper mainly focuses on the optimized growth conditions needed for growing relatively big size crystals of BGHC. The morphology of BGHC crystals, the effect of gel density and the pH of the solution on the growth of BGHC crystals have been
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investigated and discussed. A thorough scan on literature reveals that no systematic work has been done to grow BGHC crystals. There are reports on the crystal structure of BGHC [10]. To the best of our knowledge, we are reporting on the optical, thermal, dielectric and photoconductivity properties of gel grown BGHC crystals for the first time. 2. Growth of single crystals of BGHC The growth of single crystals in gels at ambient temperatures is found to be very effective for many materials such as ADP and KDP. Substances, which have high solubility in water, can be grown by solubility reduction method. The supersaturation is provided by reduction of water content using a water-absorbing agent. As BGHC is highly water soluble, single crystals of BGHC are conveniently grown by solubility reduction using methanol. Single crystals of BGHC are grown in silica gel using straight tubes of length 15 cm and diameter 2.5 cm. Freshly prepared stock solution of sodium metasilicate having density 1.04–1.06 g/cc is used for the experiments. The other solution is prepared by mixing 25 cc of 14 M glycine and 25 cc of 2 M hydrochloric acid [11]. This solution is added to 50 cc of sodium metasilicate stock solution. This mixture of solutions is taken in test tubes. After the gel sets, equal volume of methanol is added slowly above the gel and the test tubes are sealed with rubber corks to prevent evaporation of methanol. Diffusion of methanol into the gel is followed by formation of nuclei, which then grow in size. Transparent good quality inclusion free single crystals of BGHC are obtained in a period of 40 days. The typical size of the grown crystal is 15 · 3 · 4 mm3. The gel grown single crystals of BGHC are shown in Fig. 1. Crystals are formed near the diffusion interface in the gel and also within the gel. The crystals produced near the diffusion interface are numerous and smaller in size whereas crystals formed in the central portion of the gel column is larger in size. Very few crystals were formed in the lower region of the gel.
To study the effect of pH of the gel on the grown crystals, different pH values ranging from 3 to 4 were tried. When the pH of the gel is increased, the setting time was found to decrease. Besides, the number of BGHC nuclei also was found to increase. The optimum pH yielding maximum size of transparent BGHC crystal was found to be 3. In gel growth, the rate of diffusion is governed by the width of the micropores of the gel medium. The width of the micropores depends upon the density of the gel solution. In the present work, the density of the gel solution is changed from 1.04 to 1.08 g/cc. The concentration of the reactants is kept a constant. At higher gel densities the crystallization is restricted appreciably and the number of well defined crystals reduced considerably. The optimum value of gel density for growing well-defined crystals was found to be 1.04–1.06 g/cc. A greater gel density implies smaller pore size and poor communication among the pores, thus decreasing the nucleation density [12]. 3. Characterization 3.1. Single crystal XRD The X-ray data were collected using an ENRAF NONIUS CAD4 automatic X-ray diffractometer. The structure was solved by the direct method and refined by the full matrix least-square technique using the SHELXL program. The calculated lattice parameter values are ˚ a ¼ 5:3292 A;
˚ and b ¼ 8:1282 A
˚ c ¼ 18:0625 A;
˚ 3. Volume of the unit cell ¼ 782:4097 A The XRD data prove that the crystal is orthorhombic in structure with the space group of P212121. The XRD results are in good agreement with the reported values and thus confirm the grown crystal [10]. The crystal data is given in Table 1. The morphology of the gel grown BGHC single crystal is shown in Fig. 2. It is seen that the growth rate along a is always greater than that along other directions. ð0 0 1Þ face is predominant. The study indicates that the crystal has five faces with a axis along the length of the crystal. There are well developed faces (0 1 3), ð0 1 2Þ and ð0 0 1Þ. The end faces are (1 0 0) and ð1 0 0Þ. The width of the crystal is in the c direction. Table 1 Crystal data of BGHC
Fig. 1. Photograph of as grown crystals of BGHC.
Crystal system Space group a b c a b c Volume Z
Orthorhombic P212121 ˚ 5.3292 A ˚ 8.1282 A ˚ 18.0625 A 90 90 90 ˚3 782.4097 A 4
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Table 2 IR spectral assignments of BGHC
Fig. 2. Morphology of BGHC single crystal.
3.2. FTIR analysis The FTIR analysis of BGHC was carried out between 4000 and 450 cm1 using BRUKER IFS 66 V spectrometer. The FTIR spectrum is shown in Fig. 3. In the high energy region, there is a broad band between 2100 and 3500 cm1. The intense sharp peaks in this band at 3129, 3067 and 2903 cm1 may be assigned to N–H stretching vibrations. The peaks at 2845 and 2704 cm1 are attributed to the C–H stretching mode vibrations. The overtone region between 1900 and 2100 cm1 contains two prominent bands at 2027 and 1943 cm1. The C@O stretch of –COOH seems to have intense peaks at 1752, 1720 and 1028.7 cm1. The asymmetric and symmetric stretching modes of COO group are revealed by peaks at 1591, 1501, 1448 and 1409 cm1. This observation confirms that one glycine exists in zwitterionic form. The involvement of NHþ 3 in hydrogen bonding is evident by the fine structure of the band in lower energy region. The peaks at 1328 and 1253 cm1 are due to COOH group. The peak at 673 cm1 is due to N–H out-of plane bending
Fig. 3. FTIR spectrum of BGHC single crystal.
Frequency in wavenumber (cm1)
Assignment of vibrations
1591 1501 1129, 1112 1720 1448 1253 1618 1386 673 581, 515 504 1409 1336, 1328 915 1028 887, 874
NHþ 3 asymmetric bend NHþ 3 symmetric bend NHþ 3 rocking C@O stretch of –COOH C–O stretch of –COOH C@OH in plane bend of –COOH Asymmetric stretch of –COO Symmetric stretch of –COO –COO bend –COOwag –COO rock –CH2 scissoring –CH2 wag –CH2 rock CN stretch C–C stretch
vibration. The intense sharp peak at 515 cm1 reveals the torsional oscillation of NHþ 3 . Particularly the peak at 1752 cm1 is characteristic of a-amino acid hydrochlorides [13]. All these observations clearly demonstrate the existence of glycine in its salt form with hydrochloric acid. Frequency assignment of the absorption peaks are presented in Table 2, which is in good agreement with reported work [14]. 3.3. Optical transmission spectrum The optical transmission spectrum was recorded in the range 200–2000 nm using Varian Cary 5E Spectrophotometer. As the crystal is colourless, its transmission is very high in the entire UV–VIS–NIR region (Fig. 4). This is
Fig. 4. UV–VIS–NIR spectrum of BGHC single crystal.
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the most desirable property of the crystals used for NLO applications. The cut-off frequency is around 300 nm. 3.4. NLO studies A Q-switched, mode locked Nd: YAG laser was used to generate about 6 mJ/pulse at the 1064 nm fundamental radiation. In the present investigation, the single shot mode of 8 ns laser pulse with a spot radius of 1 mm was employed. This experimental setup used a mirror and a 50/50 beam splitter (BS) to generate a beam with pulse energy about 6 mJ. The input laser beam was passed through an IR reflector and then directed on the microcrystalline powdered sample packed in a capillary tube of diameter 0.154 mm. The assembly of an oscilloscope and photodiode detector is employed to measure the light emitted by the sample. The grown BGHC crystal was crushed to a fine powder and microcrystalline form of KDP was taken as the reference. The two samples were subjected to the SHG test. It is observed that the frequency doubling efficiency of the title compound is 5 times that of KDP. Thus, the SHG efficiency of BGHC is higher than KDP, c glycine and BTCC, comparable to urea and less than LiB3O5. A comparison of the SHG efficiency of various NLO crystals is presented in Table 3 [15–17]. 3.5. Thermogravimetric analysis The thermogravimetric analysis of BHGC was carried out between room temperature (28 C) and 900 C at a heating rate of 10 K/min, in nitrogen atmosphere. The resulting TGA trace is shown in Fig. 5. Although the TGA trace appears nearly straight up to 200 C, a careful examination of DTA thermogram reveals a minor exothermic peak around 100 C, which could be due to physically adsorbed water. But at 218.6 C onwards, a steady decrease in weight is observed (61.58%) up to 400 C which may be due to the decomposition of the sample. At higher temperature, above 400 C, the residue undergoes final stage of decomposition, accompanied by a weight loss equal to 25.69%.
Fig. 5. Thermogravimetric curve of BGHC single crystal.
3.6. Differential thermal analysis The DTA of BGHC was carried out between 28 and 900 C in nitrogen atmosphere using NETZSCH STA 409C at a heating rate of 10 K/min. The resulting DTA trace is shown in Fig. 6. There is a weak endotherm at about 146.8 C which may be due to isomorphic transformation as there is no corresponding weight loss in TGA trace. This is followed by a sharp endotherm at about 188 C due to its melting. This is followed by mild endotherms at 213, 240 and 299 C accompanied by heavy weight loss in TGA due to decomposition of the sample in stages. Hence from this study it can be said that the crystal is thermally stable up to 188.5 C. However, the isomorphic transformation at 146.8 C restricts its application up to 146.8 C only.
Table 3 Comparison of NLO property of various NLO crystals Compound
NLO efficiency
KDP c glycine BGHCa Urea LiB3O5 BTCC BBO LAP KTP
1.0 1.5 5 6.1–10.6 8 0.73 times urea 28 40 215
a
Present work.
Fig. 6. DTA trace of BGHC single crystal.
K. Ambujam et al. / Optical Materials 29 (2007) 657–662
3.7. Dielectric study The dielectric study on BGHC single crystal was carried out on the ð0 0 1Þ face using the instrument, HIOKI 353250 LCR HITESTER. A sample of dimension 10 · 4 · 1 mm3 having silver coating on the opposite faces was placed between the two copper electrodes and thus a parallel plate capacitor was formed. The capacitance of the sample was measured by varying the frequency from 100 Hz to 5 MHz. Fig. 7 shows the plot of dielectric constant (e 0 ) versus applied frequency. The dielectric constant has high values in the lower frequency region and then it decreases with the applied frequency. The dielectric constant has a high value of 504 at 100 Hz and decreases to 113 at 5 MHz.
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The very high value of e 0 at low frequencies may be due to the presence of all the four polarizations namely; space charge, orientational, electronic and ionic polarization and its low value at higher frequencies may be due to the loss of significance of these polarizations gradually. The dielectric loss is also studied as a function of frequency at room temperature and is shown in Fig. 8. These curves suggest that the dielectric loss is strongly dependent on the frequency of the applied field, similar to that of dielectric constant. This behaviour is common in the case of ionic systems [18,19]. 3.8. Photoconductivity Photoconductivity measurements are carried out on a polished sample of the grown single crystal by fixing it onto a microscope slide. The sample is connected in series with a DC power supply and KEITHLEY 485 picoammeter. The sample is covered with a black cloth and the voltage applied is increased from 0 to 300 V in steps of 20 V. The dark current is recorded. The sample is illuminated by the radiation from 100 W halogen lamp containing iodine vapour and tungsten filament. The photocurrent is recorded for the same values of the applied voltage. The photocurrent is found to be less than the dark current at every applied electric field. Fig. 9 shows the field dependent conductivity of the sample. This phenomenon is known as negative photoconductivity. Generally, this may be attributed to the loss of water molecules in the crystal [20]. However, in the present case the contribution of water molecules to negative photoconductivity is ruled out, as the crystal, in this case does not possess any water in its structure and further its decomposition starts only at
Fig. 7. Variation of dielectric constant with frequency of the electric field.
Fig. 8. Variation of dielectric loss with frequency of the electric field.
Fig. 9. Field-dependent conductivity of BGHC single crystal.
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188.5 C. However, this may be due to the reduction in the number of charge carriers or their lifetime in the presence of radiation [21,22]. Decrease in lifetime with illumination, could be due to the trapping process and increase in carrier velocity according to the relation: s ¼ ðvsNÞ1 ; where v is the thermal velocity of the carriers, s is the capture cross section of the recombination centers and N is the carrier concentration. As intense light falls on the sample, the lifetime decreases [23]. In Stockman model, a two level scheme is proposed to explain negative photoconductivity in semiconductors and the details are reported elsewhere [24]. Negative photoconductivity has been reported in a number of materials during the recent years and the mechanism proposed to account for the observation has varied for the different conditions. An alternative model to explain the negative photoconductivity in p-type semiconductors has been advanced [24]. Very recently, extremely high negative photoconductivity was reported by Hopfel in a p-type modulation doped GaAs–AlGaAs quantum well structure at low temperature. This suggests that negative photoconductivity in quantum well structures has different types of origins. 4. Conclusion Optically good quality single crystals of BGHC were conveniently grown using silica gel technique in period of 40 days. Optical transmission studies confirm that BGHC is transparent in the entire visible region. The SHG efficiency of BGHC is found to be five times more than that of KDP. TGA reveals that the sample is thermally stable up to 188.5 C without decomposition. The variation of dielectric constant and dielectric loss were studied with varying frequency at room temperature. Photoconductivity investigations reveal negative photoconductivity of the sample. Thus, the optical, NLO and thermal properties of the crystal indicate the suitability of this crystal for photonics device fabrications. Further work on SHG efficiency and other physico-chemical properties is under progress.
Acknowledgements One of the authors (K.A.) is grateful to the University Grants Commission, Government of India, for the sanction of Research Grant. We thank Dr. S. Ganesan (Department of Physics, Anna University), Dr. Babu Varghese, SAIF, IIT, Chennai and Dr. A. Ramanand, Head, Department of Physics, Loyola College for their help, support and encouragement. The authors thank the UGC for funding this research project. References [1] D. Eimerl, S. Velsko, L. Davis, F. Wang, G. Loiaceono, G. Kennedy, IEEE J. Quantum Electron. 25 (1989) 179. [2] M.D. Aggarwal, J. Choi, W.S. Wang, K. Bhat, R.B. Lal, A.D. Shield, B.G. Penn, D.O. Frazier, J. Cryst. Growth 204 (1999) 179. [3] S. Hoshino, Y. Okaya, R. Pepinsky, Phys. Rev. 115 (1959) 323. [4] S. Sato, J. Phys. Soc. Jpn. 25 (1968) 185. [5] J. Baran, A.J. Barnes, H. Ratajczak, Spectrochim. Acta A 51 (1995) 197. [6] S. Natarajan, C. Muthukrishnan, S.A. Bahadur, R.K. Rajaram, S.S. Rajan, Z. Kristallogr. 198 (1992) 265. [7] M.J. Buerger, R. Barney, T. Hahn, Z. Kristallogr. 108 (1956) 130. [8] P. Piret, J. Meunier-Piret, J. Verbist, M. Van Meerssche, Bull. Soc. Chim. Belges 81 (1972) 539. [9] S.C. Bhattacharyya, N.N. Saha, J. Cryst. Mol. Struct. 8 (1978) 209. [10] T. Hahn, Z. Kristallogr. 113 (1960) 403. [11] G.M. Patel, J. Cryst. Growth 54 (1981) 602. [12] C.C. Desai, M.S.V. Ramana, J. Cryst. Growth 102 (1990) 191. [13] R. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, John Wiley & Sons, London, 1981. [14] R.K. Khanna, M. Horak, E.R. Lippincott, Spectrochim. Acta 22 (1966) 1759. [15] B. Narayana Moolya, A. Jayarama, M.R. Sureshkumar, S.M. Dharmaprakash, J. Cryst. Growth 280 (2005) 581. [16] A. Pricilla Jeyakumari, J. Ramajothi, S. Dhanuskodi, J. Cryst. Growth 269 (2004) 558. [17] S. Radhakrishna, B.C. Tan, Laser Spectroscopy and Nonlinear Optics of Solids, Narosa Publishing House, India, 1990. [18] K.V. Rao, A. Smakula, J. Appl. Phys. 36 (1965) 2031. [19] K.V. Rao, A. Smakula, J. Appl. Phys. 37 (1966) 319. [20] D. Pathinettam Padiyan, S. John Ethilton, K. Paulraj, Cryst. Res. Technol. 35 (2000) 87. [21] R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1981. [22] S. Pandi, D. Jayaraman, Mater. Chem. Phys. 71 (2001) 14. [23] I.M. Ashraf, H.A. Elshaik, A.M. Badr, Cryst. Res. Technol. 39 (2004) 63. [24] V.N. Joshi, Photoconductivity, Marcel Dekker, New York, 1990.
Growth and characterization of gel grown single crystals of bis-glycine hydrogen chloride (BGHC) K. Ambujam a, K. Rajarajan c, S. Selvakumar c, J. Madhavan c, Gulam Mohamed b, P. Sagayaraj c,* a
Department of Physics, Queen Mary’s College, Chennai 600004, India b Department of Physics, The New College, Chennai 600014, India c Department of Physics, Loyola College, Chennai 600034, India Received 30 June 2005; accepted 26 November 2005 Available online 5 January 2006
Abstract Single crystals of bis-glycine hydrogen chloride, a semiorganic nonlinear optical material (NLO) were grown by silica gel method. Good optical quality single crystals with dimensions up to 15 · 3 · 4 mm3 were obtained. The grown crystals were characterized by single crystal XRD, FTIR and optical transmission spectrum. The second harmonic generation (SHG) efficiency of gel grown BGHC was found to be five times that of KDP. The thermal studies indicate that the gel grown BGHC is stable up to 188.5 C. The dielectric response of the crystal with varying frequencies was studied. The photoconductivity studies confirmed the negative photoconductive nature of gel grown BGHC crystal. 2005 Elsevier B.V. All rights reserved. Keywords: Gel growth; Semiorganic; NLO; Negative photoconductivity; Dielectric constant
1. Introduction Complexes of amino acids with inorganic acids and salts are promising materials for optical second harmonic generation, as they tend to combine the advantages of the organic amino acid with that of the inorganic acid/salt. The salts of amino acids like L-arginine [1] and L-histidine [2] are reported to have high second harmonic conversion efficiency. Glycine is the simplest amino acid. Unlike other amino acids, it has no asymmetric carbon atom and is optically inactive. It has three polymorphic crystalline forms a, b and c. Glycine and its methylated analogues form complexes with mineral acids exhibiting interesting physical properties like ferroelastic, ferroelectric or antiferroelectric behaviour. According to the structural analysis of ferro-
*
Corresponding author. Tel.: +91 (0)44 28175660; +91 (0)44 28175566. E-mail address: [email protected] (P. Sagayaraj).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.11.008
electric triglycine sulphate [3] there are two kinds of glycine groups, glycinium ions and zwitter ions. Such configurations of glycine ions interconnected by short O–H O hydrogen bonds are regarded as particularly important for the ferroelectric behaviour of this crystal. In the ferroelectric and paraelectric structure of (glycine)2HNO3, one of the glycine molecules has the zwitterionic configuration and the other is monoprotonated [4,5]. A similar structure has been found in diglycine hydrochloride [6], hydrobromide [7], hydroiodide [8] and disarcosine hydrobromide [9]. In all the above mentioned structures the molecules are held together by a network of N–H X, N–H O and O–H O hydrogen bonds. In the present work, we have attempted to grow single crystals of bis-glycine hydrogen chloride (BGHC) in silica gel at room temperature. The paper mainly focuses on the optimized growth conditions needed for growing relatively big size crystals of BGHC. The morphology of BGHC crystals, the effect of gel density and the pH of the solution on the growth of BGHC crystals have been
658
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investigated and discussed. A thorough scan on literature reveals that no systematic work has been done to grow BGHC crystals. There are reports on the crystal structure of BGHC [10]. To the best of our knowledge, we are reporting on the optical, thermal, dielectric and photoconductivity properties of gel grown BGHC crystals for the first time. 2. Growth of single crystals of BGHC The growth of single crystals in gels at ambient temperatures is found to be very effective for many materials such as ADP and KDP. Substances, which have high solubility in water, can be grown by solubility reduction method. The supersaturation is provided by reduction of water content using a water-absorbing agent. As BGHC is highly water soluble, single crystals of BGHC are conveniently grown by solubility reduction using methanol. Single crystals of BGHC are grown in silica gel using straight tubes of length 15 cm and diameter 2.5 cm. Freshly prepared stock solution of sodium metasilicate having density 1.04–1.06 g/cc is used for the experiments. The other solution is prepared by mixing 25 cc of 14 M glycine and 25 cc of 2 M hydrochloric acid [11]. This solution is added to 50 cc of sodium metasilicate stock solution. This mixture of solutions is taken in test tubes. After the gel sets, equal volume of methanol is added slowly above the gel and the test tubes are sealed with rubber corks to prevent evaporation of methanol. Diffusion of methanol into the gel is followed by formation of nuclei, which then grow in size. Transparent good quality inclusion free single crystals of BGHC are obtained in a period of 40 days. The typical size of the grown crystal is 15 · 3 · 4 mm3. The gel grown single crystals of BGHC are shown in Fig. 1. Crystals are formed near the diffusion interface in the gel and also within the gel. The crystals produced near the diffusion interface are numerous and smaller in size whereas crystals formed in the central portion of the gel column is larger in size. Very few crystals were formed in the lower region of the gel.
To study the effect of pH of the gel on the grown crystals, different pH values ranging from 3 to 4 were tried. When the pH of the gel is increased, the setting time was found to decrease. Besides, the number of BGHC nuclei also was found to increase. The optimum pH yielding maximum size of transparent BGHC crystal was found to be 3. In gel growth, the rate of diffusion is governed by the width of the micropores of the gel medium. The width of the micropores depends upon the density of the gel solution. In the present work, the density of the gel solution is changed from 1.04 to 1.08 g/cc. The concentration of the reactants is kept a constant. At higher gel densities the crystallization is restricted appreciably and the number of well defined crystals reduced considerably. The optimum value of gel density for growing well-defined crystals was found to be 1.04–1.06 g/cc. A greater gel density implies smaller pore size and poor communication among the pores, thus decreasing the nucleation density [12]. 3. Characterization 3.1. Single crystal XRD The X-ray data were collected using an ENRAF NONIUS CAD4 automatic X-ray diffractometer. The structure was solved by the direct method and refined by the full matrix least-square technique using the SHELXL program. The calculated lattice parameter values are ˚ a ¼ 5:3292 A;
˚ and b ¼ 8:1282 A
˚ c ¼ 18:0625 A;
˚ 3. Volume of the unit cell ¼ 782:4097 A The XRD data prove that the crystal is orthorhombic in structure with the space group of P212121. The XRD results are in good agreement with the reported values and thus confirm the grown crystal [10]. The crystal data is given in Table 1. The morphology of the gel grown BGHC single crystal is shown in Fig. 2. It is seen that the growth rate along a is always greater than that along other directions. ð0 0 1Þ face is predominant. The study indicates that the crystal has five faces with a axis along the length of the crystal. There are well developed faces (0 1 3), ð0 1 2Þ and ð0 0 1Þ. The end faces are (1 0 0) and ð1 0 0Þ. The width of the crystal is in the c direction. Table 1 Crystal data of BGHC
Fig. 1. Photograph of as grown crystals of BGHC.
Crystal system Space group a b c a b c Volume Z
Orthorhombic P212121 ˚ 5.3292 A ˚ 8.1282 A ˚ 18.0625 A 90 90 90 ˚3 782.4097 A 4
K. Ambujam et al. / Optical Materials 29 (2007) 657–662
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Table 2 IR spectral assignments of BGHC
Fig. 2. Morphology of BGHC single crystal.
3.2. FTIR analysis The FTIR analysis of BGHC was carried out between 4000 and 450 cm1 using BRUKER IFS 66 V spectrometer. The FTIR spectrum is shown in Fig. 3. In the high energy region, there is a broad band between 2100 and 3500 cm1. The intense sharp peaks in this band at 3129, 3067 and 2903 cm1 may be assigned to N–H stretching vibrations. The peaks at 2845 and 2704 cm1 are attributed to the C–H stretching mode vibrations. The overtone region between 1900 and 2100 cm1 contains two prominent bands at 2027 and 1943 cm1. The C@O stretch of –COOH seems to have intense peaks at 1752, 1720 and 1028.7 cm1. The asymmetric and symmetric stretching modes of COO group are revealed by peaks at 1591, 1501, 1448 and 1409 cm1. This observation confirms that one glycine exists in zwitterionic form. The involvement of NHþ 3 in hydrogen bonding is evident by the fine structure of the band in lower energy region. The peaks at 1328 and 1253 cm1 are due to COOH group. The peak at 673 cm1 is due to N–H out-of plane bending
Fig. 3. FTIR spectrum of BGHC single crystal.
Frequency in wavenumber (cm1)
Assignment of vibrations
1591 1501 1129, 1112 1720 1448 1253 1618 1386 673 581, 515 504 1409 1336, 1328 915 1028 887, 874
NHþ 3 asymmetric bend NHþ 3 symmetric bend NHþ 3 rocking C@O stretch of –COOH C–O stretch of –COOH C@OH in plane bend of –COOH Asymmetric stretch of –COO Symmetric stretch of –COO –COO bend –COOwag –COO rock –CH2 scissoring –CH2 wag –CH2 rock CN stretch C–C stretch
vibration. The intense sharp peak at 515 cm1 reveals the torsional oscillation of NHþ 3 . Particularly the peak at 1752 cm1 is characteristic of a-amino acid hydrochlorides [13]. All these observations clearly demonstrate the existence of glycine in its salt form with hydrochloric acid. Frequency assignment of the absorption peaks are presented in Table 2, which is in good agreement with reported work [14]. 3.3. Optical transmission spectrum The optical transmission spectrum was recorded in the range 200–2000 nm using Varian Cary 5E Spectrophotometer. As the crystal is colourless, its transmission is very high in the entire UV–VIS–NIR region (Fig. 4). This is
Fig. 4. UV–VIS–NIR spectrum of BGHC single crystal.
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the most desirable property of the crystals used for NLO applications. The cut-off frequency is around 300 nm. 3.4. NLO studies A Q-switched, mode locked Nd: YAG laser was used to generate about 6 mJ/pulse at the 1064 nm fundamental radiation. In the present investigation, the single shot mode of 8 ns laser pulse with a spot radius of 1 mm was employed. This experimental setup used a mirror and a 50/50 beam splitter (BS) to generate a beam with pulse energy about 6 mJ. The input laser beam was passed through an IR reflector and then directed on the microcrystalline powdered sample packed in a capillary tube of diameter 0.154 mm. The assembly of an oscilloscope and photodiode detector is employed to measure the light emitted by the sample. The grown BGHC crystal was crushed to a fine powder and microcrystalline form of KDP was taken as the reference. The two samples were subjected to the SHG test. It is observed that the frequency doubling efficiency of the title compound is 5 times that of KDP. Thus, the SHG efficiency of BGHC is higher than KDP, c glycine and BTCC, comparable to urea and less than LiB3O5. A comparison of the SHG efficiency of various NLO crystals is presented in Table 3 [15–17]. 3.5. Thermogravimetric analysis The thermogravimetric analysis of BHGC was carried out between room temperature (28 C) and 900 C at a heating rate of 10 K/min, in nitrogen atmosphere. The resulting TGA trace is shown in Fig. 5. Although the TGA trace appears nearly straight up to 200 C, a careful examination of DTA thermogram reveals a minor exothermic peak around 100 C, which could be due to physically adsorbed water. But at 218.6 C onwards, a steady decrease in weight is observed (61.58%) up to 400 C which may be due to the decomposition of the sample. At higher temperature, above 400 C, the residue undergoes final stage of decomposition, accompanied by a weight loss equal to 25.69%.
Fig. 5. Thermogravimetric curve of BGHC single crystal.
3.6. Differential thermal analysis The DTA of BGHC was carried out between 28 and 900 C in nitrogen atmosphere using NETZSCH STA 409C at a heating rate of 10 K/min. The resulting DTA trace is shown in Fig. 6. There is a weak endotherm at about 146.8 C which may be due to isomorphic transformation as there is no corresponding weight loss in TGA trace. This is followed by a sharp endotherm at about 188 C due to its melting. This is followed by mild endotherms at 213, 240 and 299 C accompanied by heavy weight loss in TGA due to decomposition of the sample in stages. Hence from this study it can be said that the crystal is thermally stable up to 188.5 C. However, the isomorphic transformation at 146.8 C restricts its application up to 146.8 C only.
Table 3 Comparison of NLO property of various NLO crystals Compound
NLO efficiency
KDP c glycine BGHCa Urea LiB3O5 BTCC BBO LAP KTP
1.0 1.5 5 6.1–10.6 8 0.73 times urea 28 40 215
a
Present work.
Fig. 6. DTA trace of BGHC single crystal.
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3.7. Dielectric study The dielectric study on BGHC single crystal was carried out on the ð0 0 1Þ face using the instrument, HIOKI 353250 LCR HITESTER. A sample of dimension 10 · 4 · 1 mm3 having silver coating on the opposite faces was placed between the two copper electrodes and thus a parallel plate capacitor was formed. The capacitance of the sample was measured by varying the frequency from 100 Hz to 5 MHz. Fig. 7 shows the plot of dielectric constant (e 0 ) versus applied frequency. The dielectric constant has high values in the lower frequency region and then it decreases with the applied frequency. The dielectric constant has a high value of 504 at 100 Hz and decreases to 113 at 5 MHz.
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The very high value of e 0 at low frequencies may be due to the presence of all the four polarizations namely; space charge, orientational, electronic and ionic polarization and its low value at higher frequencies may be due to the loss of significance of these polarizations gradually. The dielectric loss is also studied as a function of frequency at room temperature and is shown in Fig. 8. These curves suggest that the dielectric loss is strongly dependent on the frequency of the applied field, similar to that of dielectric constant. This behaviour is common in the case of ionic systems [18,19]. 3.8. Photoconductivity Photoconductivity measurements are carried out on a polished sample of the grown single crystal by fixing it onto a microscope slide. The sample is connected in series with a DC power supply and KEITHLEY 485 picoammeter. The sample is covered with a black cloth and the voltage applied is increased from 0 to 300 V in steps of 20 V. The dark current is recorded. The sample is illuminated by the radiation from 100 W halogen lamp containing iodine vapour and tungsten filament. The photocurrent is recorded for the same values of the applied voltage. The photocurrent is found to be less than the dark current at every applied electric field. Fig. 9 shows the field dependent conductivity of the sample. This phenomenon is known as negative photoconductivity. Generally, this may be attributed to the loss of water molecules in the crystal [20]. However, in the present case the contribution of water molecules to negative photoconductivity is ruled out, as the crystal, in this case does not possess any water in its structure and further its decomposition starts only at
Fig. 7. Variation of dielectric constant with frequency of the electric field.
Fig. 8. Variation of dielectric loss with frequency of the electric field.
Fig. 9. Field-dependent conductivity of BGHC single crystal.
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188.5 C. However, this may be due to the reduction in the number of charge carriers or their lifetime in the presence of radiation [21,22]. Decrease in lifetime with illumination, could be due to the trapping process and increase in carrier velocity according to the relation: s ¼ ðvsNÞ1 ; where v is the thermal velocity of the carriers, s is the capture cross section of the recombination centers and N is the carrier concentration. As intense light falls on the sample, the lifetime decreases [23]. In Stockman model, a two level scheme is proposed to explain negative photoconductivity in semiconductors and the details are reported elsewhere [24]. Negative photoconductivity has been reported in a number of materials during the recent years and the mechanism proposed to account for the observation has varied for the different conditions. An alternative model to explain the negative photoconductivity in p-type semiconductors has been advanced [24]. Very recently, extremely high negative photoconductivity was reported by Hopfel in a p-type modulation doped GaAs–AlGaAs quantum well structure at low temperature. This suggests that negative photoconductivity in quantum well structures has different types of origins. 4. Conclusion Optically good quality single crystals of BGHC were conveniently grown using silica gel technique in period of 40 days. Optical transmission studies confirm that BGHC is transparent in the entire visible region. The SHG efficiency of BGHC is found to be five times more than that of KDP. TGA reveals that the sample is thermally stable up to 188.5 C without decomposition. The variation of dielectric constant and dielectric loss were studied with varying frequency at room temperature. Photoconductivity investigations reveal negative photoconductivity of the sample. Thus, the optical, NLO and thermal properties of the crystal indicate the suitability of this crystal for photonics device fabrications. Further work on SHG efficiency and other physico-chemical properties is under progress.
Acknowledgements One of the authors (K.A.) is grateful to the University Grants Commission, Government of India, for the sanction of Research Grant. We thank Dr. S. Ganesan (Department of Physics, Anna University), Dr. Babu Varghese, SAIF, IIT, Chennai and Dr. A. Ramanand, Head, Department of Physics, Loyola College for their help, support and encouragement. The authors thank the UGC for funding this research project. References [1] D. Eimerl, S. Velsko, L. Davis, F. Wang, G. Loiaceono, G. Kennedy, IEEE J. Quantum Electron. 25 (1989) 179. [2] M.D. Aggarwal, J. Choi, W.S. Wang, K. Bhat, R.B. Lal, A.D. Shield, B.G. Penn, D.O. Frazier, J. Cryst. Growth 204 (1999) 179. [3] S. Hoshino, Y. Okaya, R. Pepinsky, Phys. Rev. 115 (1959) 323. [4] S. Sato, J. Phys. Soc. Jpn. 25 (1968) 185. [5] J. Baran, A.J. Barnes, H. Ratajczak, Spectrochim. Acta A 51 (1995) 197. [6] S. Natarajan, C. Muthukrishnan, S.A. Bahadur, R.K. Rajaram, S.S. Rajan, Z. Kristallogr. 198 (1992) 265. [7] M.J. Buerger, R. Barney, T. Hahn, Z. Kristallogr. 108 (1956) 130. [8] P. Piret, J. Meunier-Piret, J. Verbist, M. Van Meerssche, Bull. Soc. Chim. Belges 81 (1972) 539. [9] S.C. Bhattacharyya, N.N. Saha, J. Cryst. Mol. Struct. 8 (1978) 209. [10] T. Hahn, Z. Kristallogr. 113 (1960) 403. [11] G.M. Patel, J. Cryst. Growth 54 (1981) 602. [12] C.C. Desai, M.S.V. Ramana, J. Cryst. Growth 102 (1990) 191. [13] R. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, John Wiley & Sons, London, 1981. [14] R.K. Khanna, M. Horak, E.R. Lippincott, Spectrochim. Acta 22 (1966) 1759. [15] B. Narayana Moolya, A. Jayarama, M.R. Sureshkumar, S.M. Dharmaprakash, J. Cryst. Growth 280 (2005) 581. [16] A. Pricilla Jeyakumari, J. Ramajothi, S. Dhanuskodi, J. Cryst. Growth 269 (2004) 558. [17] S. Radhakrishna, B.C. Tan, Laser Spectroscopy and Nonlinear Optics of Solids, Narosa Publishing House, India, 1990. [18] K.V. Rao, A. Smakula, J. Appl. Phys. 36 (1965) 2031. [19] K.V. Rao, A. Smakula, J. Appl. Phys. 37 (1966) 319. [20] D. Pathinettam Padiyan, S. John Ethilton, K. Paulraj, Cryst. Res. Technol. 35 (2000) 87. [21] R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1981. [22] S. Pandi, D. Jayaraman, Mater. Chem. Phys. 71 (2001) 14. [23] I.M. Ashraf, H.A. Elshaik, A.M. Badr, Cryst. Res. Technol. 39 (2004) 63. [24] V.N. Joshi, Photoconductivity, Marcel Dekker, New York, 1990.