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Growth of Cu2+ and Mg2+ doped nonlinear optical LATF crystals and their characterization
Materials Science and Engineering B 166 (2010) 203–208
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Growth of Cu2+ and Mg2+ doped nonlinear optical LATF crystals and their characterization X.J. Liu a,∗ , D. Xu b , X.Q. Wei a , M.J. Ren a , G.H. Zhang b a b
School of Science, University of Jinan, Jiwei Road 106#, Jinan 250022, Shandong, China State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 3 August 2009 Received in revised form 10 November 2009 Accepted 10 November 2009 Keywords: Characterization Growth from solutions Analog of LAP Nonlinear optical materials
a b s t r a c t Single crystals of pure, Cu2+ and Mg2+ doped l-arginine trifluoroacetate (LATF) have been grown by the temperature lowering method. The presence of Cu2+ and Mg2+ was determined by atomic absorption spectroscopy (AAS). Single crystal X-ray diffraction studies were performed to calculate the lattice parameters of the pure and doped crystals. Absorption of these crystals was analyzed and the result confirms that they possess low absorption in the range 230–1100 nm. Thermal analysis (TGA, DTA) and Fourier transform infrared (FTIR) spectroscopy were carried out to investigate the thermal behavior and molecular vibrations of these crystals, respectively. The second harmonic generation (SHG) measurement reveals the NLO properties of pure and doped crystal. Surface morphologies of these crystals were also observed and studied in detail by atomic force microscopy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Amino acid family crystals are the potential candidates for optical second harmonic generation (SHG) because all the amino acids except glycine contain chiral carbon atom and crystallize in noncentrosymmetric space groups [1]. Salts of l-arginine have become promising NLO materials after l-arginine phosphate (LAP) was discovered as an alternate to potassium dihydrogen phosphate (KDP) [2–4]. l-Arginine trifluoroacetate (LATF) is a promising semiorganic NLO material, which possesses excellent optical, thermal, mechanical properties, especially it has high optical nonlinearity and large optical damage threshold (1064, 19 and 3.5 GW/cm2 for pump laser pulse widths of 20 ps, 1 and 10 ns, respectively) [5]. Previously, LATF crystal has been synthesized and bulk single crystals are successfully grown by our research group for the first time [6,7]. Furthermore, surface morphologies and detailed characterization of its properties were also described [8–10]. Pure LATF crystallizes in the monoclinic system with space group P21 . The reported cell parameter values are a = 10.581 Å, b = 5.7100 Å, c = 10.861 Å, ˇ = 106.81◦ . Morphological analysis of the LATF crystal shows that its growth has a polyhedron with nine developed facets, and the (1¯ 0 1) facet is the most prominent one [11]. For practical applications of nonlinear crystals, there is a need to modify the chemical and physical parameters as well as to find new and better materials. Recent experiments reveal that nonlinear performance
∗ Corresponding author. Tel.: +86 531 8836 2776; fax: +86 531 8857 4135. E-mail address: [email protected] (X.J. Liu). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.11.027
like optical, mechanical, and chemical stability can be improved by bringing out the compositional modification and metallic substitutions [12–15]. A series of metal-organic compounds such as lanthanum-doped calcium titanates, Zn2+ doped BTCA and Cu2+ (Mg2+ ) doped LAP, LBO, KB5 have been reported with moderately high mechanical, optical and chemical stability [16–21]. With the help of the earlier experience in investigation of pure LATF crystals, attempts have been made in the present study to report the effects of Cu2+ and Mg2+ on the growth, structural, physical properties and surface morphologies of LATF single crystals. 2. Experimental procedure 2.1. Crystal growth Pure LATF was synthesized from the starting materials of l-arginine and trifluoroacetic acid taken in equimolar ratio as delineated before [7]. The synthesized salts of pure and doped LATF were purified by successive recrystallization process in deionized water. The solubility studies were carried out for five different temperatures (30 ◦ C, 35 ◦ C, 40 ◦ C, 45 ◦ C, 50 ◦ C) and the curves are drawn in Fig. 1. Three thermostatically controlled vessels (50 ml) are filled with pure and doped solution, with some undissolved solutes and stirred for 24 h, respectively. On the next day, a certain amount of the solution is pipetted out, and its composition is determined gravimetrically, respectively. It has been found that the solubility of doped LATF is less than that of pure LATF. As to grow single crystals of LATF: Cu2+ and LATF: Mg2+ , 2 mol % of CuCl2 ·2H2 O and MgCl2 were added to the saturated solution of LATF, respectively. Trans-
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Fig. 1. Solubility curve for pure, Cu2+ and Mg2+ doped LATF crystals.
parent seeds of pure and doped crystals obtained by spontaneous nucleation were selected for growth, respectively. The saturated solution was filtered twice to remove the suspended impurities. Before starting the crystal growth process, the solution temperature was kept 3–6 ◦ C above the saturation point for 10 h. Then the solution temperature was decreased to the saturation temperature to start growth. A cooling rate of 0.1 ◦ C/day was used in the initial and final stages of the growth process, and the seed was rotated at a rate of 30 rpm. Fig. 2 shows the photograph of optical quality bulk crystals of pure and doped LATF. 2.2. Characterization The presence of metals in the crystal lattice of grown crystals was determined by HP 3510 atomic absorption spectrophotometer. The single crystal X-ray diffraction studies of pure and doped LATF single crystals were carried out using Rigaku D/Max-␥A diffractometer. The transmittance spectra were taken on flat polished crystal samples of about 4 mm thickness employing a Hitachi model UV340 recording spectrophotometer in a wide wavelength range 200–2600 nm at room temperature. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were carried out on a SDT Q600 V8.0 Build 95 instrument with a heating rate of 10 ◦ C min−1 from 30 ◦ C to 600 ◦ C. The FTIR spectra recorded on pure and doped crystals were obtained from KBr pellets on a NEXUS 670 FTIR spectrometer in the region 4000–400 cm−1 . The powder technique of Kurtz and Perry [22] was used for the comprehensive analysis of second-order nonlinearity, which is regarded as the simplest method to assess the nonlinearities. A Nd:YAG laser with fundamental radiation of 1064 nm was used as the optical source and directed onto these powder samples. The images of surface morphologies obtained from these crystal samples were collected ex situ in contact mode under ambient conditions at room temperature employing a Nanoscope IIIa MultiMode atomic force microscopy (AFM) instrument (Digital Instruments).
Fig. 2. Photograph of the grown pure LATF, LATF: Cu2+ and LATF: Mg2+ single crystals.
the crystal lattice may be explained by the smaller ionic radius of Mg2+ (0.65 Å) when compared to Cu2+ (0.72 Å).
3. Results and discussion 3.2. Single crystal XRD analysis 3.1. Atomic absorption spectroscopy (AAS) analysis 0.1 g of fine powder of Cu2+ and Mg2+ doped LATF were dissolved in 25 ml of triple distilled water and the prepared solution was subjected to AAS analysis. From the obtained results, it has been confirmed that only 0.06% of Cu2+ and 0.15% of Mg2+ are present in respective solutions. The reason for more Mg2+ having entered into
The calculated parameter values are tabulated in Table 1. It is observed that both the pure and the doped LATF crystallize into monoclinic system and belong to the P21 space group. However, there are slight variations in the lattice parameters as well as in the cell volume values. It is believed that the incorporation of metal ions in LATF lattice account for these variations.
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Table 1 XRD data of pure and doped LATF crystals. Sample
Pure LATF LATF: Cu2+ LATF: Mg2+
Crystal parameters a (Å)
b (Å)
c (Å)
ˇ (◦ )
Z
Crystal system
Volume (Å)3
10.581 10.572 10.590
5.710 5.690 5.721
10.861 10.866 10.862
106.81 101.20 103.38
2 2 2
Monoclinic Monoclinic Monoclinic
628.20 641.19 640.22
3.3. UV–vis–NIR analysis It is evident from the spectrum (Fig. 3) that the percentage of transmission is very high for both the pure and the doped LATF crystals, which is a desirable property for the crystals used for NLO applications. In addition, the light absorbing properties of doped ones are less than the parent LATF. This gives an idea that the fundamental energy gap may be more for the doped crystals compared with the pure LATF, the doped materials can have enhanced NLO properties. Similar results were obtained in other Cu2+ and Mg2+ ion-doped amino acid systems [18,23,24]. As is well known, by doping with a variety of metal ions it is easy to generate suitable defects in a crystal structure in order to increase a range for NLO applications [25–29]. To our knowledge, the transmission spectrum changes brought about by doping with dopants can be associated with the atom position in the lattice, atomic and electronic parameters, as well as its close surroundings. 3.4. Thermal studies The characteristic curves obtained from these analyses are shown in Fig. 4. For pure LATF crystal, the TGA curve shows that there is a weight loss of about 69.2% in the temperature range 217–382 ◦ C due to the liberation of volatile substances, such as CO, CO2 and H2 O molecules. The DTA curve indicates that the crystal undergoes an irreversible endothermic transition around 217 ◦ C followed by another sharp endothermic peak at 241 ◦ C. For LATF: Cu2+ and LATF: Mg2+ crystals, the first fully decompositioned temperatures are observed at 213 ◦ C and 215 ◦ C, respectively. Recently, interests in the influence of dopants on the thermal behavior of optical materials have attracted many researchers to study the mechanisms between them. For example, Sagayaraj and coworkers [20] studied the NLO optical crystal KB5 doped with Mg+ , Ca+ and Cu+ ions. They reported that, as a consequence of the decreased bond energy due to the doping, the decomposition temperature of doped crystals is lower than that of the pure ones.
Fig. 3. Optical transmittance spectrum of pure LATF, LATF: Cu2+ and LATF: Mg2+ crystals. The sample thickness is about 4 mm.
Fig. 4. TGA and DTA curves for (a) pure LATF, (b) LATF: Cu2+ and (c) LATF: Mg2+ crystals.
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Fig. 5. FTIR spectra of (a) pure LATF, (b) LATF: Cu2+ and (c) LATF: Mg2+ crystals.
3.5. Fourier transform infrared (FTIR) spectroscopy analysis
3.6. NLO property studies
As seen in Fig. 5, there is no significant change in these spectra, thus it can be deduced that the loading level of Cu2+ and Mg2+ is too tiny to engender any characteristic change in the FTIR spectra. In the spectra of pure LATF (Fig. 5a), the absorption bands in the high-frequency 3500–2800 cm−1 regions are related to stretching vibrations of N–H bonds and C–H bonds. The peak with the highest frequency at 3305 cm−1 belongs to the stretching vibrations of N–H bonds. The strong absorption band in the region 1700–1500 cm−1 with peaks at 1694 cm−1 , 1641 cm−1 and 1587 cm−1 is characteristic for arginine salts. These absorption peaks may be assigned to the asymmetric stretching vibrations of carboxylate group (COO− ) and the deformation vibrations of NH3 + (ıas , s ) group. Absorption band observed at 1443 cm−1 is occurred due to asymmetric bending of CH2 groups. Furthermore, a medium intensity due to symmetric bending of CH2 groups is observed at 1364 cm−1 . Symmetric bending of COO− group is observed at 1419 cm−1 . A very strong band at 1201 cm−1 may be assigned to C–F stretching. The next strong band at 1134 cm−1 corresponds to asymmetric stretching of C–C–H bond. Absorption peak at 845 cm−1 is caused by rocking of COO− group. In-plane deformation due to COO− group gives rise to a band of strong intensity at 654 cm−1 .
The NLO property of the crystal was confirmed by the Kurtz Perry powder technique. The crystals are ground to powder and packed between two transparent glass slides. The first harmonic output of 1064 nm from a Nd:YAG laser was made to fall normally passed through the pure and doped LATF powder sample after reflection from an IR reflector. The SHG behavior in these crystals was confirmed from the emission of intense green radiation ( = 532 nm) by the sample. Intensity of the green radiation was found to increase for LATF: Cu2+ (1.2 times) and LATF: Mg2+ (1.5 times) with reference to pure LATF. Therefore, owing to the presence of metal dopants, there is an increase in polarizability of the molecule which tends to increase the SHG efficiency. 3.7. Surface morphologies Prior to the observations, the crystals were gently placed on the AFM sample holder using a two-sided tape, keeping {1 0 1} faces parallel to the surface of the holder. Fig. 6(a) presents an AFM image of the pure LATF crystal surface, where the facet plane exhibits a highly regular stepped morphology. Most of the steps are elementary steps possessing monolayer height (0.6 nm). Upon metallic
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Fig. 6. Typical examples of step morphologies observed on pure and doped LATF crystals (a–c). Scanning areas are 3.2 m × 3.2 m, 1.9 m × 1.9 m, 5 m × 5 m for images (a–c), respectively.
ions (Cu2+ and Mg2+ ) doping, the regular step trains disappeared and a remarkable change was detected as shown in Fig. 6(b) and (c), respectively. The striking feature of these images is the sinuous character of the stepped structure, which suggests that the metallic ions absorbed on the terrace or step-edges disturb the step growth and influence the advancement of steps. To our best knowledge, the dopant (Cu2+ and Mg2+ ) may serve as impurities which will lead to the morphological changes by altering the properties of solution (speciation or solubility) in a variety of ways, for example, by altering the characteristic of the adsorption layer at the crystal–solution interface and affecting the integration of the growth units [30]. In addition, the sinuous macrosteps (height: 1.5–6 nm) often coalesce into larger steps or break into smaller steps, which subsequently give rise to the resulting macroscopic roughness of surface and even the formation of defects [31]. Further studies are in progress. 4. Conclusions Good quality single crystals of pure, Cu2+ and Mg2+ doped LATF were grown successfully by temperature lowering method. The AAS result reveals that the amount of dopant incorporated into the crystal lattice is less than the concentration of the dopant in the corresponding solution. Single crystal X-ray diffraction studies show that the small variation in lattice parameter values which due to the contribution of metals dopant in the interstitial sites. The pure and doped LATF crystals have low absorption in the range 230–1100 nm and the doped crystals possess decreased absorption. From the TGA and DTA curves, it is seen that the decomposition temperature of the doped LATF crystals decrease when compared to pure LATF. The FTIR analysis confirms that the presence of various functional groups such as NH, NH3 + and COO− , which indicate that the l-arginine molecule might be protonated so as to give an improvement of the dipolar strength of the crystals. NLO studies proved that the metal dopants have increased the efficiency of pure LATF. Surface morphology studies indicate that the metal dopants will give rise to dissimilar morphological features, in particular macrostep
formation, i.e. the spacing between succeeding surface steps transform in a slowly varying manner. Once the macrosteps lose their stability, defects will be induced. Acknowledgement This work is supported by the grants (No. 60608010) of National Natural Science Foundation of China (NNSFC) and the Doctoral Foundation of University of Jinan (XBS0920). References [1] D.S. Chemla, J. Zyss, Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987. [2] S.B. Monaco, L.E. Davis, S.P. Velsko, F.T. Wang, D. Eimerl, J. Cryst. Growth 85 (1987) 252. [3] D. Eimerl, S. Velsko, L. Davis, F. Wang, G. Loiacono, G. Kennedy, IEEE J. Quant. Electron. QE-25 (1989) 179. [4] A. Yokotani, T. Sasaki, K. Yoshida, S. Nakai, Appl. Phys. Lett. 55 (1989) 2692. [5] D. Xu, X.Q. Wang, W.T. Yu, S.X. Xu, G.H. Zhang, J. Cryst. Growth 253 (2003) 481. [6] X.J. Liu, Z.Y. Wang, G.H. Zhang, X.Q. Wang, A.D. Duan, et al., J. Cryst. Growth 308 (2007) 130. [7] X.J. Liu, Z.Y. Wang, A.D. Duan, G.H. Zhang, X.Q. Wang, et al., J. Cryst. Growth 310 (2008) 2590. [8] X.J. Liu, Z.Y. Wang, D. Xu, X.Q. Wang, Y.Y. Song, et al., J. Alloy Compd. 441 (2007) 323. [9] X.J. Liu, Z.Y. Wang, G.H. Zhang, G.W. Yu, X.Q. Wang, et al., J. Phys. Chem. C 111 (2007) 14165. [10] X.J. Liu, Z.Y. Wang, G.H. Zhang, G.W. Yu, X.Q. Wang, D. Xu, Mater. Lett. 62 (2008) 1986. [11] X.J. Liu, Z.Y. Wang, X.Q. Wang, G.H. Zhang, S.X. Xu, et al., Cryst. Growth Des. 8 (2008) 2270. [12] R. Mohan Kumar, Ph.D. Thesis, Anna University, Chennai, India, 2001. [13] V.I. Burkov, A.V. Egorysheva, Y.F. Kargin, A.A. Marin, Inorg. Mater. 34 (1998) 962. [14] S. Riehemann, F. Rickermann, V.V. Volkov, A.V. Egorysheva, G. Von Bally, J. Nonlinear Opt. Phys. Mater. 6 (1997) 235. [15] D. Eimerl, S. Velsko, L. Davis, F. Wang, G. Loiacoo, G. Kennedy, IEEE J. Quantum Electron. 25 (1989) 179. [16] V. Vashook, L. Vasylechko, M. Knapp, H. Ullmann, U. Guth, J. Alloy Compd. 354 (2003) 13. [17] S. Selvakumar, S.M. Ravi Kumar, Ginson P. Joseph, K. Rajarajan, et al., Mater. Chem. Phys. 103 (2007) 153.
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[18] A.S. Haja Hameed, G. Ravi, M.D.M. Hossain, P. Ramasamy, J. Cryst. Growth 204 (1999) 333. [19] J.K. Kang-Soo Parka, D.J. Ahna, H.K. Kima, Y.H. Kima, Hwanga, et al., J. Cryst. Growth 249 (2003) 483. [20] S.A. Rajasekar, K. Thamizharasan, A. Joseph Arul Pragasam, J. Packiam Julius, P. Sagayaraj, J. Cryst. Growth 247 (2003) 199. [21] S.A. Rajasekar, K. Thamizharasan, A. Joseph Arul Pragasam, J. Packiam Julius, P. Sagayaraj, Mater. Chem. Phys. 84 (2004) 157. [22] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798. [23] P. Praveen Kumar, V. Manivannan, S. Tamilselvan, S. Senthil, et al., Opt. Commun. 281 (2008) 2989. [24] R. Calvo, S.B. Oseroff, H.C. Abache, J. Chem. Phys. 72 (1980) 760.
[25] D. Syamala, V. Rajendran, R.K. Natarajan, S. Moorthy Babu, Cryst. Growth Des. 7 (2007) 1695. [26] L.Y. Ren, L.R. Liu, D. Liu, B.L. Yao, Opt. Commun. 238 (2004) 363. [27] F. Mersch, K. Buse, W. Sauf, H. Hesse, E. Kratzig, Phys. Stat. Sol. A 140 (1993) 273. [28] M. Ishiia, Y. Kuwano, T. Asai, N. Senguttuvan, T. Hayashi, M. Kobayashi, et al., J. Cryst. Growth 257 (2003) 169. [29] N. Senguttuvana, M. Ishiia, M. Shimoyama, M. Kobayashi, N. Tsutsui, M. Nikl, et al., Nucl. Instrum. Meth. A 486 (2002) 264. [30] J. Mullin, Crystallization, Butterworth Heinmann, London, 1993, p. 238. [31] W.W. Mullins, R.F. Sekerka, J. Appl. Phys. 34 (1963) 323.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Growth of Cu2+ and Mg2+ doped nonlinear optical LATF crystals and their characterization X.J. Liu a,∗ , D. Xu b , X.Q. Wei a , M.J. Ren a , G.H. Zhang b a b
School of Science, University of Jinan, Jiwei Road 106#, Jinan 250022, Shandong, China State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 3 August 2009 Received in revised form 10 November 2009 Accepted 10 November 2009 Keywords: Characterization Growth from solutions Analog of LAP Nonlinear optical materials
a b s t r a c t Single crystals of pure, Cu2+ and Mg2+ doped l-arginine trifluoroacetate (LATF) have been grown by the temperature lowering method. The presence of Cu2+ and Mg2+ was determined by atomic absorption spectroscopy (AAS). Single crystal X-ray diffraction studies were performed to calculate the lattice parameters of the pure and doped crystals. Absorption of these crystals was analyzed and the result confirms that they possess low absorption in the range 230–1100 nm. Thermal analysis (TGA, DTA) and Fourier transform infrared (FTIR) spectroscopy were carried out to investigate the thermal behavior and molecular vibrations of these crystals, respectively. The second harmonic generation (SHG) measurement reveals the NLO properties of pure and doped crystal. Surface morphologies of these crystals were also observed and studied in detail by atomic force microscopy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Amino acid family crystals are the potential candidates for optical second harmonic generation (SHG) because all the amino acids except glycine contain chiral carbon atom and crystallize in noncentrosymmetric space groups [1]. Salts of l-arginine have become promising NLO materials after l-arginine phosphate (LAP) was discovered as an alternate to potassium dihydrogen phosphate (KDP) [2–4]. l-Arginine trifluoroacetate (LATF) is a promising semiorganic NLO material, which possesses excellent optical, thermal, mechanical properties, especially it has high optical nonlinearity and large optical damage threshold (1064, 19 and 3.5 GW/cm2 for pump laser pulse widths of 20 ps, 1 and 10 ns, respectively) [5]. Previously, LATF crystal has been synthesized and bulk single crystals are successfully grown by our research group for the first time [6,7]. Furthermore, surface morphologies and detailed characterization of its properties were also described [8–10]. Pure LATF crystallizes in the monoclinic system with space group P21 . The reported cell parameter values are a = 10.581 Å, b = 5.7100 Å, c = 10.861 Å, ˇ = 106.81◦ . Morphological analysis of the LATF crystal shows that its growth has a polyhedron with nine developed facets, and the (1¯ 0 1) facet is the most prominent one [11]. For practical applications of nonlinear crystals, there is a need to modify the chemical and physical parameters as well as to find new and better materials. Recent experiments reveal that nonlinear performance
∗ Corresponding author. Tel.: +86 531 8836 2776; fax: +86 531 8857 4135. E-mail address: [email protected] (X.J. Liu). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.11.027
like optical, mechanical, and chemical stability can be improved by bringing out the compositional modification and metallic substitutions [12–15]. A series of metal-organic compounds such as lanthanum-doped calcium titanates, Zn2+ doped BTCA and Cu2+ (Mg2+ ) doped LAP, LBO, KB5 have been reported with moderately high mechanical, optical and chemical stability [16–21]. With the help of the earlier experience in investigation of pure LATF crystals, attempts have been made in the present study to report the effects of Cu2+ and Mg2+ on the growth, structural, physical properties and surface morphologies of LATF single crystals. 2. Experimental procedure 2.1. Crystal growth Pure LATF was synthesized from the starting materials of l-arginine and trifluoroacetic acid taken in equimolar ratio as delineated before [7]. The synthesized salts of pure and doped LATF were purified by successive recrystallization process in deionized water. The solubility studies were carried out for five different temperatures (30 ◦ C, 35 ◦ C, 40 ◦ C, 45 ◦ C, 50 ◦ C) and the curves are drawn in Fig. 1. Three thermostatically controlled vessels (50 ml) are filled with pure and doped solution, with some undissolved solutes and stirred for 24 h, respectively. On the next day, a certain amount of the solution is pipetted out, and its composition is determined gravimetrically, respectively. It has been found that the solubility of doped LATF is less than that of pure LATF. As to grow single crystals of LATF: Cu2+ and LATF: Mg2+ , 2 mol % of CuCl2 ·2H2 O and MgCl2 were added to the saturated solution of LATF, respectively. Trans-
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Fig. 1. Solubility curve for pure, Cu2+ and Mg2+ doped LATF crystals.
parent seeds of pure and doped crystals obtained by spontaneous nucleation were selected for growth, respectively. The saturated solution was filtered twice to remove the suspended impurities. Before starting the crystal growth process, the solution temperature was kept 3–6 ◦ C above the saturation point for 10 h. Then the solution temperature was decreased to the saturation temperature to start growth. A cooling rate of 0.1 ◦ C/day was used in the initial and final stages of the growth process, and the seed was rotated at a rate of 30 rpm. Fig. 2 shows the photograph of optical quality bulk crystals of pure and doped LATF. 2.2. Characterization The presence of metals in the crystal lattice of grown crystals was determined by HP 3510 atomic absorption spectrophotometer. The single crystal X-ray diffraction studies of pure and doped LATF single crystals were carried out using Rigaku D/Max-␥A diffractometer. The transmittance spectra were taken on flat polished crystal samples of about 4 mm thickness employing a Hitachi model UV340 recording spectrophotometer in a wide wavelength range 200–2600 nm at room temperature. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were carried out on a SDT Q600 V8.0 Build 95 instrument with a heating rate of 10 ◦ C min−1 from 30 ◦ C to 600 ◦ C. The FTIR spectra recorded on pure and doped crystals were obtained from KBr pellets on a NEXUS 670 FTIR spectrometer in the region 4000–400 cm−1 . The powder technique of Kurtz and Perry [22] was used for the comprehensive analysis of second-order nonlinearity, which is regarded as the simplest method to assess the nonlinearities. A Nd:YAG laser with fundamental radiation of 1064 nm was used as the optical source and directed onto these powder samples. The images of surface morphologies obtained from these crystal samples were collected ex situ in contact mode under ambient conditions at room temperature employing a Nanoscope IIIa MultiMode atomic force microscopy (AFM) instrument (Digital Instruments).
Fig. 2. Photograph of the grown pure LATF, LATF: Cu2+ and LATF: Mg2+ single crystals.
the crystal lattice may be explained by the smaller ionic radius of Mg2+ (0.65 Å) when compared to Cu2+ (0.72 Å).
3. Results and discussion 3.2. Single crystal XRD analysis 3.1. Atomic absorption spectroscopy (AAS) analysis 0.1 g of fine powder of Cu2+ and Mg2+ doped LATF were dissolved in 25 ml of triple distilled water and the prepared solution was subjected to AAS analysis. From the obtained results, it has been confirmed that only 0.06% of Cu2+ and 0.15% of Mg2+ are present in respective solutions. The reason for more Mg2+ having entered into
The calculated parameter values are tabulated in Table 1. It is observed that both the pure and the doped LATF crystallize into monoclinic system and belong to the P21 space group. However, there are slight variations in the lattice parameters as well as in the cell volume values. It is believed that the incorporation of metal ions in LATF lattice account for these variations.
X.J. Liu et al. / Materials Science and Engineering B 166 (2010) 203–208
205
Table 1 XRD data of pure and doped LATF crystals. Sample
Pure LATF LATF: Cu2+ LATF: Mg2+
Crystal parameters a (Å)
b (Å)
c (Å)
ˇ (◦ )
Z
Crystal system
Volume (Å)3
10.581 10.572 10.590
5.710 5.690 5.721
10.861 10.866 10.862
106.81 101.20 103.38
2 2 2
Monoclinic Monoclinic Monoclinic
628.20 641.19 640.22
3.3. UV–vis–NIR analysis It is evident from the spectrum (Fig. 3) that the percentage of transmission is very high for both the pure and the doped LATF crystals, which is a desirable property for the crystals used for NLO applications. In addition, the light absorbing properties of doped ones are less than the parent LATF. This gives an idea that the fundamental energy gap may be more for the doped crystals compared with the pure LATF, the doped materials can have enhanced NLO properties. Similar results were obtained in other Cu2+ and Mg2+ ion-doped amino acid systems [18,23,24]. As is well known, by doping with a variety of metal ions it is easy to generate suitable defects in a crystal structure in order to increase a range for NLO applications [25–29]. To our knowledge, the transmission spectrum changes brought about by doping with dopants can be associated with the atom position in the lattice, atomic and electronic parameters, as well as its close surroundings. 3.4. Thermal studies The characteristic curves obtained from these analyses are shown in Fig. 4. For pure LATF crystal, the TGA curve shows that there is a weight loss of about 69.2% in the temperature range 217–382 ◦ C due to the liberation of volatile substances, such as CO, CO2 and H2 O molecules. The DTA curve indicates that the crystal undergoes an irreversible endothermic transition around 217 ◦ C followed by another sharp endothermic peak at 241 ◦ C. For LATF: Cu2+ and LATF: Mg2+ crystals, the first fully decompositioned temperatures are observed at 213 ◦ C and 215 ◦ C, respectively. Recently, interests in the influence of dopants on the thermal behavior of optical materials have attracted many researchers to study the mechanisms between them. For example, Sagayaraj and coworkers [20] studied the NLO optical crystal KB5 doped with Mg+ , Ca+ and Cu+ ions. They reported that, as a consequence of the decreased bond energy due to the doping, the decomposition temperature of doped crystals is lower than that of the pure ones.
Fig. 3. Optical transmittance spectrum of pure LATF, LATF: Cu2+ and LATF: Mg2+ crystals. The sample thickness is about 4 mm.
Fig. 4. TGA and DTA curves for (a) pure LATF, (b) LATF: Cu2+ and (c) LATF: Mg2+ crystals.
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Fig. 5. FTIR spectra of (a) pure LATF, (b) LATF: Cu2+ and (c) LATF: Mg2+ crystals.
3.5. Fourier transform infrared (FTIR) spectroscopy analysis
3.6. NLO property studies
As seen in Fig. 5, there is no significant change in these spectra, thus it can be deduced that the loading level of Cu2+ and Mg2+ is too tiny to engender any characteristic change in the FTIR spectra. In the spectra of pure LATF (Fig. 5a), the absorption bands in the high-frequency 3500–2800 cm−1 regions are related to stretching vibrations of N–H bonds and C–H bonds. The peak with the highest frequency at 3305 cm−1 belongs to the stretching vibrations of N–H bonds. The strong absorption band in the region 1700–1500 cm−1 with peaks at 1694 cm−1 , 1641 cm−1 and 1587 cm−1 is characteristic for arginine salts. These absorption peaks may be assigned to the asymmetric stretching vibrations of carboxylate group (COO− ) and the deformation vibrations of NH3 + (ıas , s ) group. Absorption band observed at 1443 cm−1 is occurred due to asymmetric bending of CH2 groups. Furthermore, a medium intensity due to symmetric bending of CH2 groups is observed at 1364 cm−1 . Symmetric bending of COO− group is observed at 1419 cm−1 . A very strong band at 1201 cm−1 may be assigned to C–F stretching. The next strong band at 1134 cm−1 corresponds to asymmetric stretching of C–C–H bond. Absorption peak at 845 cm−1 is caused by rocking of COO− group. In-plane deformation due to COO− group gives rise to a band of strong intensity at 654 cm−1 .
The NLO property of the crystal was confirmed by the Kurtz Perry powder technique. The crystals are ground to powder and packed between two transparent glass slides. The first harmonic output of 1064 nm from a Nd:YAG laser was made to fall normally passed through the pure and doped LATF powder sample after reflection from an IR reflector. The SHG behavior in these crystals was confirmed from the emission of intense green radiation ( = 532 nm) by the sample. Intensity of the green radiation was found to increase for LATF: Cu2+ (1.2 times) and LATF: Mg2+ (1.5 times) with reference to pure LATF. Therefore, owing to the presence of metal dopants, there is an increase in polarizability of the molecule which tends to increase the SHG efficiency. 3.7. Surface morphologies Prior to the observations, the crystals were gently placed on the AFM sample holder using a two-sided tape, keeping {1 0 1} faces parallel to the surface of the holder. Fig. 6(a) presents an AFM image of the pure LATF crystal surface, where the facet plane exhibits a highly regular stepped morphology. Most of the steps are elementary steps possessing monolayer height (0.6 nm). Upon metallic
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Fig. 6. Typical examples of step morphologies observed on pure and doped LATF crystals (a–c). Scanning areas are 3.2 m × 3.2 m, 1.9 m × 1.9 m, 5 m × 5 m for images (a–c), respectively.
ions (Cu2+ and Mg2+ ) doping, the regular step trains disappeared and a remarkable change was detected as shown in Fig. 6(b) and (c), respectively. The striking feature of these images is the sinuous character of the stepped structure, which suggests that the metallic ions absorbed on the terrace or step-edges disturb the step growth and influence the advancement of steps. To our best knowledge, the dopant (Cu2+ and Mg2+ ) may serve as impurities which will lead to the morphological changes by altering the properties of solution (speciation or solubility) in a variety of ways, for example, by altering the characteristic of the adsorption layer at the crystal–solution interface and affecting the integration of the growth units [30]. In addition, the sinuous macrosteps (height: 1.5–6 nm) often coalesce into larger steps or break into smaller steps, which subsequently give rise to the resulting macroscopic roughness of surface and even the formation of defects [31]. Further studies are in progress. 4. Conclusions Good quality single crystals of pure, Cu2+ and Mg2+ doped LATF were grown successfully by temperature lowering method. The AAS result reveals that the amount of dopant incorporated into the crystal lattice is less than the concentration of the dopant in the corresponding solution. Single crystal X-ray diffraction studies show that the small variation in lattice parameter values which due to the contribution of metals dopant in the interstitial sites. The pure and doped LATF crystals have low absorption in the range 230–1100 nm and the doped crystals possess decreased absorption. From the TGA and DTA curves, it is seen that the decomposition temperature of the doped LATF crystals decrease when compared to pure LATF. The FTIR analysis confirms that the presence of various functional groups such as NH, NH3 + and COO− , which indicate that the l-arginine molecule might be protonated so as to give an improvement of the dipolar strength of the crystals. NLO studies proved that the metal dopants have increased the efficiency of pure LATF. Surface morphology studies indicate that the metal dopants will give rise to dissimilar morphological features, in particular macrostep
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