Metastable Zone Width, Nucleation Parameters and other Studies of L-alanine Hydrogen Chloride (LAHC) Crystals Grown by Slow Cooling Technique

Transcription

1 Advances in Optoelectronic Materials (AOM) Volume 1 Issue 4, November 2013 Metastable Zone Width, Nucleation Parameters and other Studies of L-alanine Hydrogen Chloride (LAHC) Crystals Grown by Slow Cooling Technique R. Jothi mani 1, P. Selvarajan 1*, H. Alex Devadoss 2, D. Shanthi 1 1 Department of Physics, Aditanar College of Arts and Science, Tiruchendur , Tamilnadu, India 2 Department of Physics, St.John s College, Tirunelveli, India *pselvarajanphy@yahoo.co.in Abstract The NLO property is highly pronounced when L-alanine is mixed with hydrogen chloride to form L-alanine hydrogen chloride (LAHC) sample. Solubility and metastable zone width were measured at different temperatures. Induction period measurements were carried out and it was noticed that induction period decreases with supersaturation ratio and the critical nucleation parameters for LAHC sample were evaluated. Single crystals of LAHC salt was grown by slow cooling technique. EDAX studies were carried out to identify the elements present in the grown crystal. Evaluation of values of lattice parameters, absorption coefficient, optical band gap, yield strength, stiffness constant and SHG efficiency for LAHC crystals was carried out. The band gap value for the grown LAHC crystal was found to be 5.21 ev and the obtained results were reported. Keywords L-alanine Complex; Nucleation Kinetics; Metastable Zone Width; Cooling Method; XRD; Microhardness; SHG; Stiffness Constant; Yield Strength Introduction Nonlinear optical (NLO) materials play a vital role in the field of optoelectronics and the development of NLO materials are suitable for application in frequency conversion, optical telecommunication, image processing, optical computing, and data storage. Advances have been accounted recently in the field of nonlinear optics in the area of materials engineering and the associated optoelectronic device technologies [1, 2]. NLO materials are classified into organic, inorganic and semiorganic and the advantage of organic amino acid materials over the inorganic materials is that they have high polarizability and the organic crystals with large NLO effects make them attractive for applications in frequency conversion and optical processing. L-alanine is an organic NLO crystal that contains an acid functional group and an amine functional group on adjacent carbon atoms. If both the amino and carboxyl groups are attached to the same carbon atom, it is called as α-amino acid and L-alanine is an α-amino acid. The second harmonic generation (SHG) efficiency of L-alanine is about one third of that of KDP [3-5]. In this work, L-alanine and hydrochloric acid were mixed to form L-alanine Hydrogen Chloride (LAHC) crystal, a semiorganic nonlinear optical crystal. A survey of literature showed that the X-ray crystal structure of LAHC was reported by Yameda et al [6] and the synthesis, growth and spectroscopic studies of L-alanine hydrogen chloride (LAHC) crystals were reported by Lucia Rose [7]. The nucleation kinetic studies were carried out for the first time and growth of LAHC crystals was carried out by slow cooling technique. Growth of bulk crystal of LAHC was carried out using the optimized growth parameters obtained from nucleation kinetic studies. The aim of this paper is to report the growth of LAHC crystals by slow cooling technique and the results from various characterization studies of the harvested crystals of LAHC. Materials and Methods Solubility and Metastable Zone Width L-alanine Hydrogen Chloride (LAHC) salt was synthesized by taking L-alanine (99% purity) and analar grade HCl (98% purity) at the molar ratio of 1:1 in double distilled water. The dissolved solution was heated at 60 o C for the synthesis of LAHC salt. The purity of the synthesized salt was further increased by 67

2 Advances in Optoelectronic Materials (AOM) Volume 1 Issue 4, November 2013 repeated re-crystallization. Initially, the temperature was maintained at 30 o C in the constant temperature bath. The re-crystallized sample of LAHC was added step by step to 50 ml of double distilled water in an air-tight container kept in the constant temperature bath and stirring was continued till a small precipitate was formed. This gave confirmation of supersaturated condition of the solution. Then 5 ml of the solution was pipetted out and taken in a petri dish and it was warmed up at 40 o C till the solvent was evaporated out. By measuring the amount of salt present in the petri dish, the solubility (in g/100 ml) of LAHC in double distilled water was determined and this method of measuring solubility is known as gravimetrical method [8]. FIG. 1 SOLUBILITY AND NUCLEATION CURVES FOR LAHC SAMPLE Metastable zone width (MSZW) is a basic and an important parameter in terms of temperature for growing a crystal by solution growth technique. In the present work, the metastable zone width of LAHC was measured by means of the conventional polythermal method [9]. Saturated solution of LAHC has been prepared in accordance with the solubility data. The studies were carried out in a constant temperature bath controlled to an accuracy of o C provided with a cryostat for cooling below room temperatures. A constant volume of 10 ml of solution was used in this experiment. The solution was preheated to 5 o C above the saturated temperature for homogenization and left at the superheated temperature for 1 h before cooling. The equilibrium saturated solution was cooled from the overheated temperature and the temperature at which the first visible crystal nucleus in the solution was noted and this is the nucleation temperature. This experiment was carried out for the solution saturated at 30, 40 and 50 o C. Repeated trials were performed to ascertain the correctness of the observed results. The difference between the saturation temperature and the nucleation temperature gives the metastable zone width. From the solubility curve and the nucleation curve (Fig.1), the metastable zone width is determined at any temperature in the range of o C. Critical Nucleation Parameters To attain the crystal growth, nucleation is an important phenomenon. The principle of nucleation theory states that the nucleation is formed on the homogeneous condition. Crystallization starts with nucleation and control of nucleation is crucial for the control of the number, size, perfection, polymorphism and other characteristics of crystalline materials. When few atoms, ions or molecules join together in a supersaturated solution, a cluster or nucleus is formed and the overall excess free energy change ( G) between the nucleus and solute in the supersaturated solution is the Gibbs free energy change. Once the nucleation occurs in the supersaturated solution, the nucleus grows quickly and a bright sparkling particle is seen. The time interval in which the observation of the first sparkling particle in the undisturbed supersaturated solution is called the induction period (τ). The expression for the induction period in terms of Gibbs free energy is given by ln τ =-B + G / kt where B is a constant, k is the Boltzmann s constant and T is the absolute temperature. The Gibbs free energy will be maximum for a certain value of radius (r * ) of nucleus, which is known as critical radius. Supersaturation ratio S is given by S = C/Co where C is the supersaturated concentration and Co is the saturated concentration. The induction period was measured at different supersaturation ratios viz. 1.2, 1.25, 1.3, 1.35 and 1.4 by isothermal method [10]. A plot of 1/ (ln S) 2 against ln τ forms a straight line and the slope (m) is calculated. After finding the slope (m), the value of interfacial tension (σ) is calculated using the equation σ = (RT/N) [3m/16πv 2 ] 1/3 where R is the universal gas constant, v is the volume of a molecule (v = volume of unit cell/number of molecules per unit cell) and N is the Avogadro s number. The size of the critical nucleus (r * ) and critical Gibbs free energy change ( G * ) are given by r*= 2 σv N / RT ln S and G* = mrt / [N (ln S) 2 ] respectively and the number of molecules in a critical nucleus is found using equation n = (4/3) (/v) r* 3. The number of crystals produced in the supersaturated solution is expressed as nucleation rate i.e. the number of crystals produced per unit 68

3 Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013 volume per unit time. The nucleation rate (J) can be calculated using the equation J = A exp[- G*/(kT)] where A is the pre-exponential factor and the derivations for equations of critical nucleation parameters such as r *, σ, G * and n are given in the literature [10]. In the obtained results, it is noticed that the plot of ln τ against 1/(ln S) 2 is approximately linear (Fig.2) and this explains the classical theory of homogeneous nucleation. By linear fit method, the slope (m) is obtained from the figure 2 and using the relevant above equations, the critical nucleation parameters were determined. The variations of Gibbs free energy change, radius of critical nucleus and number of molecules in the critical nucleus (n), critical radius and nucleation rate with the supersaturation ratio (S) for LAHC sample are presented in the figures 3, 4, 5 and 6. FIG.4 VARIATION OF CRITICAL RADIUS WITH SUPERSATURATION RATIO FOR LAHC SAMPLE FIG.5 VARIATION OF NUMBER OF MOLECULES IN THE CRITICAL NUCLEUS WITH SUPERSATURATION RATIO FOR LAHC SAMPLE FIG.2 VARIATION OF LN Τ VERSUS 1/(LN S) 2 FOR LAHC SAMPLE FIG.6 VARIATION OF NUCLEATION RATE WITH SUPERSATURATION RATIO FOR LAHC SAMPLE FIG.3 VARIATION OF GIBBS FREE ENERGY CHANGE WITH SUPERSATURATION RATIO FOR LAHC SAMPLE In the present study, various critical nucleation parameters were determined based on classical theory of homogeneous nucleation assuming the shape of critical nucleus as spherical. It is noticed from the results that the values of G*, r* and n are found to be decreasing with increase of supersaturation ratio. The 69

4 Advances in Optoelectronic Materials (AOM) Volume 1 Issue 4, November 2013 crucial parameter involved in between a growing crystal and the surrounding mother liquor is the interfacial tension or interfacial energy and it is an important parameter in the theories of nucleation and crystal growth. The interfacial energy value obtained from the experimental results is 1.13 x 10-3 J/m 2 for LAHC crystal. From the figure 6, it is observed that the nucleation rate increases with supersaturation ratio. Studies on nucleation kinetics of LAHC are carried out in order to have the controlled nucleation rate. Based on the data obtained from the nucleation kinetic studies, the optimized conditions are obtained and these conditions can be used for the growth of good quality crystals of LAHC [11-13]. Crystal Growth FIG.7 A HARVESTED CRYSTAL OF LAHC The saturated solution of the re-crystallized salt of LAHC was prepared in accordance with the solubility and the optimized nucleation kinetic data. The calculated amounts of the reactants were thoroughly dissolved in double distilled water and stirred well for about 3 h using a magnetic stirrer to ensure homogeneous temperature and concentration over entire volume of the solution. The solution was filtered and transferred to crystal growth vessels and crystallization was allowed to take place in a constant temperature bath (accuracy o C) by slow cooling method. Initially, the temperature was set at 40 o C in the CTB and the temperature of the solution was reduced at the rate of 0.5 o C per day. Some seeds obtained from spontaneous nucleation were placed in the growth vessels to obtain big-sized crystals. The harvested crystal of LAHC is shown in the figure 7. It is observed that the grown crystal is transparent, colourless and it is well formed with sharp edges. The dimensions of the crystal are noticed to be 19 x 13 x 7 mm 3. Characterization after growth of crystals EDAX Studies Energy Dispersive Analysis by X-rays (EDAX) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). This technique detects X-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the sample. The data generated by EDAX analysis consist of spectra showing peaks corresponding to the elements making up the true composition of the sample being analyzed. When the sample is bombarded by the SEM's electron beam, electrons are ejected from the atoms comprising the sample's surface. The resulting electron vacancies are filled by electrons from a higher state and an X-ray is emitted to balance the energy difference between the two electrons states. The X-ray energy is characteristic of the element from which it was emitted. The spectrum of X-ray energy versus counts is evaluated to determine the elemental composition of the sample. The sample X-ray energy values from the EDS spectrum are compared with known characteristic X-ray energy values to determine the presence of an element in the sample. In the present study, EDAX studies were performed using the EDAX detector (Hitachi model S-3000H scanning electron microscope). The EDAX spectrum for the grown LAHC crystal is presented in the figure 8. Using this spectrum, the elements such as carbon, chlorine, nitrogen and oxygen present in the sample are identified. XRD Studies FIG.8 EDAX SPECTRUM FOR LAHC CRYSTAL The grown LAHC crystal was analyzed by single crystal XRD studies to confirm the crystallinity and also to estimate the lattice parameters by employing Bruker-Nonious MACH3/CAD4 single X-ray diffractometer. From single crystal X-ray diffraction data, it is observed that the LAHC crystal belongs to orthorhombic in structure with the lattice parameters 70

5 Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013 a=6.197(4) Å, b=9.929(2) Å, c=11.751(1) Å, α=β=γ=90 and V =723.04(3) Å 3. The obtained lattice parameters for LAHC crystal in this work are found to be in agreement with the data reported in the literature [6]. Microhardness, Yield Strength and Stiffness Constant Microhardness studies have found wide applications in the study of material properties of solids. Hardness testing has been widely used to study the strength and deformation in materials. Hardness is defined as the ratio of the load applied to the surface area of the indentation. The method of measuring hardness is not dependent on a single physical property but may involve both the elastic and plastic deformation characteristics such as work hardening coefficient, tensile strength, stiffness constant and elastic modulus etc. Vickers microhardness test is found to be the most suitable (among various types of hardness measurements available) for the measurement of microhardness of crystals. Hardness measurement is to be performed on a limited area with small damage to the area measured and must yield extremely reliable results. A hardness tester fitted with a diamond pyramidal indenter attached to an incident light microscope isused for this study. Microhardness analysis was carried out using Vickers microhardness tester fitted with a diamond indenter. A good quality LAHC crystal was placed on the platform of the Vickers microhardness tester and loads of different magnitudes were applied for a fixed interval of 10 seconds. The hardness number was calculated using the equation Hv P/d 2 kg /mm 2 where P is the applied load in kg and d is the diagonal length of the indentation impression in millimeter [14]. The plot between hardness number (Hv) and load (P) LAHC crystals are shown in Figure 4. It is observed that Vickers hardness number of all the crystals increases gradually with increase in load and above 100 g cracks develop on the smooth surface of the crystal due to release of the internal stresses generated locally by indentation. A plot obtained between log P against log d gives a straight line which is derived from the Mayer s law, the relation connecting the applied load is given by P = a d n. Here, n is the Mayer s index or work hardening coefficient that has been calculated from the slope of the straight line from the figure 10. The value of n for LAHC crystal is found to be According to Onitsch [15], if n is greater than 1.6, the material belongs to the category of soft materials. In the present study, the work hardening coefficient of grown LAHC crystal is greater than 1.6 and hence it is a soft material. The microhardness is related to yield strength (σy) by the equation Hv = 3σy and the first order elastic stiffness constant (C11) is related to the microhardness by the equation log C11 = (7/4) log Hv [16, 17]. The load dependence of yield strength and stiffness constant are given in the figures 11 and 12 and it is observed that the values of yield strength and stiffness constant increase with the applied load on the crystal. FIG.9 DEPENDENCE OF MICROHARDNESS WITH LOAD FOR LAHC CRYSTAL FIG.10 PLOT OF LOG P VERSUS LOG D FOR LAHC CRYSTAL FIG.11 VARIATION OF YIELD STRENGTH WITH LOAD FOR LAHC CRYSTAL 71

6 Advances in Optoelectronic Materials (AOM) Volume 1 Issue 4, November 2013 FIG.12 VARIATION OF STIFFNESS CONSTANT WITH LOAD FOR LAHC CRYSTAL FIG.13 UV-VISIBLE SPECTRUM FOR LAHC CRYSTAL Linear Optical Studies UV-visible spectral study may be assisted in understanding the electronic structure of the optical band gap of the crystal and the study of the absorption edge is vital in connection with the theory of electronic structure. UV-visible transmittance spectrum for the grown LAHC crystal was recorded in the region nm using a Varian Cary 5E UV-Vis-NIR spectrophotometer and it is presented in the figure 13. The optical absorption coefficient (α) was calculated from transmittance using the relation d) log (1/T) where T is the transmittance and d is the thickness of the crystal. The variation of absorption coefficient with wavelength is depicted in the figure 14. The absorption coefficient (α) obeys the following relation for photon energy (hν) (h r hg) where Eg is optical band gap of the crystal and A is a constant. The value of the exponent r denotes the nature of the transition; for example, r = ½ for indirect transitions and r=2 for direct transitions [18,19]. The plot drawn using this relation is known as the Tauc s plot and the extrapolating the linear region to the abscissa yields the energy of the optical band gap of the material. From the functional dependence obtained for the absorption coefficient on photon energy for the sample of this work, the value of r is found to be close to 2 and this suggests that the optical transition in the sample is a direct transition. As a direct band gap material, the plot of (αhν) 2 versus hν is drawn and shown in figure 15 and the value of optical band gap is obtained to be 5.21 ev. Nonlinear Optical Studies Second harmonic generation (SHG) is a nonlinear FIG.14 VARIATION OF ABSORPTION COEFFICIENT WITH WAVELENGTH FOR LAHC CRYSTAL FIG.15 TAUC S PLOT FOR LAHC CRYSTAL optical process that results in the conversion of an input optical wave into an output wave of twice the input frequency. The light propagated through a crystalline solid, which lacks a center of symmetry, generates light at second and higher harmonics of the applied frequency. Such frequency doubling processes are commonly used to produce green light (532 nm) 72

7 Advances in Optoelectronic Materials (AOM) Volume 1 Issue 3, August 2013 from, for example, a Nd:YAG (Yttrium-Aluminium- Garnet) laser operating at 1064 nm. The Kurtz powder technique is a convenient method to screen large number of powdered materials for the second order NLO activity without the need to grow large single crystals. A laser is directed onto a powdered sample and the emitted light is collected, filtered and detected with a photo multiplier tube. The technique is crude in the sense that it detects a convolution of all the tensor components of second order susceptibility (χ (2 ) and makes little attempt to account for the propagation characteristic of the beams. Since the results are particle-size dependent, great care must be taken in applying this technique quantitatively [20,21]. A Q- switched, flash lamp pumped Nd:YAG laser of power 72 mj was used for this study. The generation of second harmonics was confirmed as the LAHC crystal is found to produce green light of halved wavelength (532 nm) when it is irradiated by fundamental wavelength (1064 nm) of Nd:YAG laser. It is observed from this study that the relative SHG efficiency for LAHC crystal grown by slow cooling technique is 0.87 times than that of the reference KDP crystal. Conclusion L-alanine hydrogen chloride (LAHC) salt was synthesized and metastable zone width and nucleation kinetic studies were carried out and the single crystals of LAHC were grown by slow cooling technique for the first time. The critical nucleation parameters such as Gibbs free energy change, radius of critical nucleus, interfacial tension and nucleation rate for the sample were evaluated. EDAX and XRD studies were performed for the grown LAHC crystal to confirm the elements present and crystal structure. Microhardness studies were carried out to evaluate work hardening coefficient, yield strength and stiffness constant for sample of this work. Optical band gap was evaluated using the Tauc s plot and the obtained value was 5.21 ev. Nonlinear optical activity for LAHC crystal was checked by Kurtz-Perry technique. ACKNOWLEDGEMENT The authors are grateful to Department of Science and technology (DST), Government of India for the financial support to carry out this work. Also the authors are thankful to the management of Aditanar College of Arts and Science, Tiruchendur and St.John s College, Tirunelveli for the encouragement given to us to carry out the research work. REFERENCES A.S.J. Lucia Rose, P. Selvarajan, S. Perumal, Mater. Chem.Phys. 30 (2011) B. Helina, P. Selvarajan and A S J Lucia Rose, Physica Scripta 85 (2012) C. Razzetti, M.Ardoido, L.Zanotti, M.Zha, C.Parorici, Cryst. Res. Technol. 37(2002)456. D. Jayalakshmi,R. Sankar, R. Jayavel, J. Kumar, J. Crystal Growth 276 (2005) D. L. Wood, J. Tauc, Weak absorption tails in amorphous semiconductors,phys.rev. 5 (1972) D. Shanthi, P. Selvarajan, K.K. HemaDurga, S. Lincy Mary Ponmani, Spectrochimica Acta Part A 110 (2013) 1 6. E. M. Onitsch, Mikroskopie 2(1947)131. J. Glorium Arul Raj, P. Selvarajan, S. Perumal, and N. Murali Krishnan,Mater. Manufact. Process. 26 (2011) J. Zyss, Molecular Nonlinear Optics: Materials, Physics and Devices, Academic Press, New York, K. K. Hema Durga,P. Selvarajan, D. Shanthi, Int. J. Curr. Res. Rev. 4 (2012) M. Diem, P.L. Polavarapu, M. Oboodi, L.A. Nafie, J. Am. Chem. Soc. 104(1982) P.M. Ushasree, R. Muralidharan, R. Jayavel, P. Ramasamy, J. Crystal Growth 210 (2000) P. Santhanaraghavan, P. Ramasamy, Crystal Growth- Processes and Methods, KRU Publications, Kumbakonam, India (2003). P. Selvarajan, J. Glorium Arulraj, S.Peruml,J. Crystal Growth 311 (2009) R. W. Boyd, Nonlinear Optics, 2 nd Edn, Academic, Inc., San Diego (2003). R. Wytt, Metal Ceramics & Polymers, Cambridge University Press, London (1974). S. B. Monaco, L. E. Devis, S. P. Velsko, F. T. Wang, D. Eimerl, J. Crystal Growth 85 (1987) 252. S. Krishnan, C. Justin Raj, S. Dhinakaran, S. Jerome Das, Cryst. Res. Technol. 43 (2008) 670. Thenneti Raghavalu, G. Ramesh Kumar, S. Gokul Raj, V. Mathivanan, R. Mohan, J. Crystal Growth 307 (2007) 112. W. A. Wooster, Widersande and Science Anwendung, Rep. Prog. Phys. 1(1953) 62. Yamada K., Sato A., Shimizu T., Yamazaki T. and Yokoyama S., Acta Cryst. E64 (2008) o