Effect of Co2+ on the growth, optical properties and laser damage threshold of potential nonlinear optical l -alanine single crystal

Optik 125 (2014) 5069–5074

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Effect of Co2+ on the growth, optical properties and laser damage threshold of potential nonlinear optical l-alanine single crystal M. Lydia Caroline a,∗ , G. Mani a , S. Usha b a b

PG & Research Department of Physics, Arignar Anna Govt. Arts College, Cheyyar 604 407, Tamil Nadu, India Bharath University, Selaiyur, Chennai 600 073, India

a r t i c l e

i n f o

Article history: Received 6 October 2013 Accepted 10 April 2014 PACS: 42.70.Mp 81.10.Dn 61.05.Cp 81.70.Pg Keywords: Nonlinear optical material Growth from solutions X-ray diffraction Thermal analysis Optical properties

a b s t r a c t Good transparent bulk single crystals of pure l-alanine (LA) and cobalt doped LA crystals have been synthesized and successfully grown by slow-cooling method from their aqueous solutions. The concentration of metal dopants in the mother solution with 0.5 mol% for cobalt was carried out individually and crystals were obtained with well defined morphology. The as grown metal doped and pure single crystals were characterized by single crystal XRD studies which confirm that the incorporation of metallic dopants has not changed the basic structure of the parent crystal. The absorption of these crystals was analyzed and the result confirms that they possess low absorption in the range 230–1100 nm. Fourier transform infrared (FTIR) spectroscopy was carried out to investigate the molecular vibrations of these crystals and to confirm the incorporation of the dopants. The thermal properties have been studied by TGA/DTA curves. The EDAX measurement and surface morphology were studied for pure and metal doped LA crystals. The second harmonic generation (SHG) signals were observed using Nd: YAG laser with fundamental wavelength of 1064 nm in pure and metal doped crystals. The laser damage threshold was measured for pure and metal doped LA crystals and also tested by using a Q-switched Nd: YAG laser showed enhanced LDT value for metal (Co2+ ) doped LA crystal compared to pure LA crystal due to the metallic substitutions thus proving their useful candidature for nonlinear optical applications. © 2014 Elsevier GmbH. All rights reserved.

1. Introduction Amino acid family has over the years been subjected to extensive investigations due to their non-linear optical properties for their possible applications in various fields like telecommunication, optical computing, optical data storage and optical information processing. Complexes of amino acids with inorganic salts exhibit the advantage of the organic amino acids and the inorganic salts. The generation of coherent blue light through second harmonic generation (SHG) from near infrared (NIR) laser sources is an important technological problem that has attracted much attention in the last few years. Potential applications lie in the fields of highdensity data storage, high-resolution printing and spectroscopy [1–3]. Organic crystals have large nonlinear susceptibilities compared to inorganic crystals. Among organic crystals for nonlinear optics (NLO) applications, amino acids display specific features

∗ Corresponding author. Tel.: +91 04182 225313/44 22281622; fax: +91 04182 222286; mobile: +91 9841720216/9443099026. E-mail addresses: [email protected] (M. Lydia Caroline), [email protected] (G. Mani). http://dx.doi.org/10.1016/j.ijleo.2014.06.055 0030-4026/© 2014 Elsevier GmbH. All rights reserved.

of interest, such as (i) molecular chirality, which secures acentric crystallographic structures; (ii) absence of strongly conjugated bonds, leading to wide transparency ranges in the visible and UV spectral regions; (iii) zwitterionic nature of the molecule, which favors crystal hardness [4]. Further to that, ␣-aminoacids can be used (iv) as chiral auxiliaries for nitro-aromatics and other donoracceptor molecules with large hyperpolarizabilities [5] and (v) as a basis for synthesizing organic-inorganic compounds like l-arginine phosphate and derivatives [6]. l-Alanine (LA) is the smallest, naturally occurring chiral amino acid with a non-reactive hydrophobic methyl group ( CH3 ) as a side chain. LA has the zwitterionic form (+ NH3 -C2 H4 -COO− ) both in crystal and in aqueous solution over a large range of pH. The crystalline state is well defined structurally and can be successfully used for a detailed examination of a broad range of molecular properties. l-Alanine (LA) is the smallest, naturally occurring chiral amino acid with a non-reactive hydrophobic methyl group ( CH3 ) as a side chain. l-Alanine (CH3 CHNH2 COOH), the simplest acentric member of the family, was first crystallized by Bernal [7] and later by Simpson et al. [8] and Destro et al. ˚ b = 12.343 A, ˚ c = 5.784 A, ˚ [9], who refined the structure (a = 6.032 A, ˛ = ˇ =  = 90◦ ) and assigned it the P21 21 21 space group. The growth and characterization of l-alanine crystals were reported by many

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Fig. 1. (a) and (b) Photograph of as grown l-alanine bulk single crystal & morphology of as grown l-alanine single crystal.

authors earlier [10–14]. l-Alanine is an efficient organic NLO compound under the amino acid category [15]. Already growth and characterization of l-alanine crystal was reported by Caroline et al. [13]. Amino acids mixed with inorganic acid and salts produce outstanding material for NLO applications [16–19]. In this paper, attempts were made to grow bulk size crystals of l-alanine with metal dopants with good transparency to find more suitable for optical characterization. Our present work deals with growth of LA crystals with metallic dopants and their characterization by single crystal XRD, FTIR, thermal, optical and NLO studies. The laser damage threshold measurement for pure l-alanine and Co2+ doped l-alanine crystals is reported in this present work.

and essential material requirements for the SHG activity of the crystal [14]. The observed lattice parameters are consistent with reported values [10,11,13]. The crystallographic parameters were ˚ b = 12.245 A, ˚ c = 5.831 A, ˚ V = 430.68 A˚ 3 ; for determined as a = 6.032 A, 2+ cobalt doped LA. The Co doped l-alanine crystals show a slight change in unit cell parameters retaining the same crystal system. Thus the variations in lattice parameters and increase in cell volume clearly indicates that cobalt could be incorporated into the pure LA crystal lattice.

2. Experimental

3.2. FTIR spectral analysis

2.1. Crystal growth

In the Fourier Transform Infrared Spectroscopy recorded in the KBr phase in the frequency region 450–4000 cm−1 using a BRUKER 66V FT-IT spectrometer is shown in Fig. 3 cobalt doped LA single crystals. In the present study, the characteristic vibrational spectra for the various functional groups present in the pure LA crystal (Fig. 2a) agree well with the FTIR spectra in the reported literature [13]. The assignments of the fundamental vibrational modes due to COO− , NH3 + , CH2 , CH groups were made. The carboxylic group is found to exit as the COO− , in the crystal. It is well known that an ionized carboxylic group (COO− ) has characteristics wavenumbers in the regions 1680–1540 cm−1 (strong asymmetric stretching), 1410 cm−1 (weak symmetric stretching) and 660 cm−1 (symmetric deformation). The peak at 1412 cm−1 is assigned to the symmetric stretching mode. The O C O bending mode at 772 cm−1 has been identified and assigned. The COO− scissoring modes appears at 649 cm−1 . Also peak at 486 cm−1 is assigned to COO− rocking mode. Thus the ionization of the carboxyl group is confirmed from the absorption bands observed at 539 cm−1 and at 1412 cm−1 .

l-Alanine has good solubility of 19.0 g/100 ml in water at 45.0 ◦ C according to solubility data [12]. Pure LA crystals and Co2+ doped crystals were grown using good quality seed crystals from aqueous solution by slow evaporation method. The seed crystals were obtained by slow evaporation technique of the respective solutions in aqueous solution after a growth period of 6–10 days which were used for the growth of bulk crystals. Growth of bulk size pure and metal doped crystals were carried out from saturated aqueous solution of 200 ml of l-alanine and metal-doped LA solutions each of them taken in different crystallizing vessels with perforated covers and placed in a constant temperature bath controlled to an accuracy of ±0.01 ◦ C. The temperature of the bath was lowered at the rate of 0.1 ◦ C day−1 . The dopants cobalt (Co2+ ) was added to the mother solution in the concentration range of 5 wt%. Bulk transparent single crystals of l-alanine and cobalt doped LA with dimensions up to 2.4 cm × 1.2 cm × 1.6 cm and 2.0 cm × 1.0 cm × 1.8 cm with welldefined morphology were harvested after a typical growth period of 2 weeks. One of the grown bulk transparent single crystals pure LA, cobalt doped LA is shown in (Figs. 1a and 2a). Morphology of the l-alanine crystal (Fig. 1b) shows that the growth rate along b-axis is large compared to the other two crystallographic directions, hence l-alanine is elongated along b-axis. Among the well-developed faces in l-alanine, the (2 0 −2) face is larger in area.

Fig. 2. (a) Photograph of cobalt doped LA single crystal.

3. Results and discussion 3.1. Single crystal X-ray diffraction The single crystal X-ray diffraction of pure l-alanine (LA) and cobalt doped LA single crystals were carried out using an ˚ The Enraf Norius CAD4 diffractometer with MoK␣ ( = 0.7170 A). pure l-alanine crystal is orthorhombic with unit cell parame˚ b = 12.323(8) A, ˚ c = 5.779(2) A, ˚ V = 428.9(3) A˚ 3 ; ters a = 6.022(8) A, ◦ ˛ = ˇ =  = 90 and assigned to P21 21 21 space group which is recognized as noncentrosymmetric, thus satisfying one of the basic

Fig. 3. FTIR spectrum of cobalt doped LA single crystal.

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Fig. 4. TG-DTG of LA single crystals.

In amino acids containing NH3 + group, the stretching and bending vibrational wavenumbers are expected [20,21] in the regions 3150–3000, 1660–1610 and 1550–1480 cm−1 . The observation of IR bands at 3082, 1620 and 1519 cm−1 is indicative of the presence of NH3 + group in the crystal. Thus it is confirmed that each of the three hydrogen atoms in the NH3 + moiety participates in hydrogen bonding, which weakens the N H bond. The band around 1152 cm−1 in IR spectrum is also an indicative rocking modes of NH3 + . The asymmetric mode of NH3 + group (1620 cm−1 ) and its torsional oscillations (i.e. at 486 cm−1 ) appear as sharp peak at 2113 cm−1 . The presence of asymmetric and symmetric stretching mode of the methylene group has been identified with its deformation modes. For the material under study the asymmetric stretching mode of the methylene group is observed at around 2960 cm−1 in the IR spectrum. The strong peak at 1362 cm−1 and 1236 cm−1 indicates the region of deformation stretching of CH3 and CH2 wagging modes in the crystal lattice. The stronger CH2 stretching vibrations overlap the CH stretching vibrations below 3000 cm−1 . The vibrational bands of the C C N have been identified and assigned. The absorption bands arising from C N and C C stretching vibrations are observed in the wavenumber region 850–1150 cm−1 . IR band corresponding to the observation at 918 and 849 cm−1 are assigned to C C N symmetric stretching vibration. Thus it is concluded that in the zwitterionic structure of amino acids, the NH3 + group is a good hydrogen bond donor and carboxylic group is an excellent acceptor and strong hydrogen bonds are formed between these groups in the crystal lattice of l-alanine. The influence and incorporation of the metal dopants is usually ascertained from the wavelength assignments at low frequency regions [22]. The observed shift in vibrational wavenumbers pure l-alanine (LA), cobalt doped LA confirms the incorporation of metal dopants in the pure l-alanine crystal structure which also has support from the EDAX analysis. 3.3. Thermal analysis Thermogravimetry (TG) and differential thermogravimetric (DTA) analysis were carried out for l-alanine powder sample using a SDT Q600 V8.0 thermal analyzer in the temperature range from 28 ◦ C to 1200 ◦ C at the heating rate of 27 ◦ C/min in nitrogen atmosphere. The TGA trace of pure l-alanine (LA) with the DTG (Differential thermogravimetric analysis) shown in the same figure (Fig. 4) and cobalt doped LA is shown in Fig. 5a and b. In the thermogram for pure and doped crystals, there is a single major weight loss starting at about 295 ◦ C (LA), 240 ◦ C (Co2+ doped LA). But below these temperatures, there is no weight loss; hence, the crystals are completely free of any entrapped water. The sharp endothermic peak at 297 ◦ C is due to the melting of the sample agrees well with reported value [12,13] for l-alanine, is desired by its DTA analysis, showing a sharp endotherm starting at about

Fig. 5. (a) and (b) TG-DTG and DTA of cobalt doped LA single crystals.

297 ◦ C (LA). Also it is seen that a sharp peak at 247.6 ◦ C (Co2+ doped LA), which coincides with the decomposition in TG analysis shows the crystal of metal doped LA is stable up to their melting points with good degree of crystallinity and purity of the crystals [23]. There is a gradual and significant weight-loss as the temperature is increased above the melting point. It is seen that at different stages various gaseous fractions like CO, CO2 , NH2 , etc., are liberated, leading to bulk decomposition of the pure and doped compounds before 600 ◦ C. Based on this analysis it is to be said that these materials can be exploited to NLO applications up to its melting point. 3.4. Optical transmittance and nonlinear optical studies The UV–VIS–NIR spectrum gives information about the structure of the molecule because the absorption of UV and visible light involves the promotion of electron in ␴ and ␲ orbitals from the ground state to higher energy states. The optical transmission range, transparency cutoff and the absorbance band are the most important optical parameters for laser frequency conversion applications. To find the transmission range of the as grown crystals, optical transmission spectrum was recorded in the range 200–1200 nm. Transmission spectra are very important for any NLO material because a nonlinear optical material can be of practical use only if it has a wide transparency window [24]. A good transmittance in the entire UV region and lower cut off wavelength at 245 nm and at 240 nm for LA and doped LA crystals makes it valuable for those applications requiring blue/green light (Fig. 6a and b). By adding cobalt-ions to l-alanine crystal, the transmittance and the value of cut off wavelengths did not get affected. The crystals has strong absorption band in the UV-region, due to n–␲* transition and is attributable to the presence of aromatic ring

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M. Lydia Caroline et al. / Optik 125 (2014) 5069–5074 4.5 4.0 3.5

Absorbance

3.0 2.5 2.0 1.5 1.0 0.5

Fig. 7. Laser damage measurement experimental set up. 0.0 200

400

600

800

1000

1200

present study, an actively Q-switched diode array side pumped Nd: YAG laser was used for the Laser Damage Threshold (LDT) studies on the pure LA and metal doped LA crystals. The experiment set up used for laser damage studies is shown in Fig. 7. The beam energy of 6.4 mJ/s with pulse width 8 ns and the repetition rate 10 Hz respectively, at 1064 nm radiation was employed to study the LDT of LA and metal doped LA. For this measurement 8 mm diameter beam was focused onto the sample with 42 cm focal length lens. The beam spot size on the sample was 8 mm. The energy density was calculated using the formula, viz.,

Wavelength (nm)

b 4.0

Absorbance (a.u)

3.5

3.0

2.5

E (GW/cm2 ) A

2.0

energy density =

1.5

where E is the input energy measured in milli-joules and A, the area of the circular spot. The laser damage threshold was found to be 21.1 MW/cm2 for LA and 39.43 MW/cm2 for cobalt doped LA crystals respectively. The inclusion of dopants in the parent crystal could be the reason for enhanced LDT value and also their usefulness in the field of NLO applications [26]. Apart from thermal effect, multiphoton ionization is the important cause for laser-induced damage.

200

400

600

800

1000

1200

Wavelength (nm) Fig. 6. (a) and (b) UV–vis spectrum single crystal of l-alanine and cobalt doped LA single crystals.

and C O group. The absence of the absorption in the visible region can be exploited for NLO applications in the room temperature [25]. The NLO property of the crystal was confirmed by the Kurtz and Perry technique [25] to identify the materials with noncentrosymmetric crystal structures. The fundamental beam of 1064 nm from Q switched Nd: YAG laser is used to test the Second Harmonic Generation (SHG) property of LA and metal-doped LA crystals. Pulse energy of 3.0 ± 0.2 mJ/pulse and pulse width of 8 ns and repetition rate of 10 Hz is used. 90◦ geometry was employed. The output-doubled frequency SHG signal was separated from fundamental ones by an IR filter. A photomultifier tube (Hamasu R2059) was used as detector. The second harmonic wave of 532 nm generated from the sample was detected by a photomultiplier tube (Hamamatsu-R 2059) and converted into electrical signal. The converted electrical signal was displayed on an oscilloscope (Tektronix-TDS 3000B). The signal amplitude in volts indicates the SHG efficiency of the sample which was confirmed by emission of green radiation from the respective samples. KDP crystals grounded into identical size were used as the reference material. The powder SHG efficiency of l-alanine single crystal was found to be 0.33 times that of KDP and metal-doped LA crystal is comparable to that of parent crystal.

3.6. Edax analysis Energy dispersive X-ray analysis (EDAX) used in conjunction with all types of electron microscope has become an important tool for characterizing the elements present in the crystals. In the present study, the crystal was analyzed by KV: 30.00 AMPT: 102.4 DETECTOR SUTW-SAPPHIRE RESOLUTION: 129.35 energy dispersive micro analyzer equipped with LED steroscan scanning electron microscope. The result obtained in EDAX of doped crystals is shown in Fig. 8 and this confirms the presence of cobalt in the doped samples. The presence of cobalt-ions as the doped specimen has been confirmed by EDAX spectrum and the concentration of

3.5. Laser Damage Threshold (LDT) studies One of the decisive criteria for a NLO crystal to perform as a device is its resistance to laser damage, since high optical intensities are involved in nonlinear processes. If the material has a low damage threshold, it severely limits its applications, even though it has many excellent properties like high optical transmittance, high decomposition temperatures and high SHG efficiency. In the

Fig. 8. The EDAX spectrum of cobalt in pure l-alanine single crystals.

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4. Conclusion

Fig. 9. (a)–(c) The surface morphology of l-alanine.

the incorporated dopants into the LA crystalline matrix can be clearly seen in same figure.

3.7. SEM analysis HR–SEM analysis has been carried out for the grown crystals to study the nature and surface morphology. The surface morphology of the LA and doped LA crystals was studied using JEOL JSM-6360 scanning electron microscope. SEM acceleration voltage was 20 kV and the sample was kept in high vacuum. The transparent regions of the crystals were cut into few mm for examining the surface morphology. The SEM micrographs of pure and doped crystals taken in the same magnification are presented in Figs. 9a–c and 10a–c respectively. The micrograph of pure l-alanine (Fig. 9a–c) shows that the surface is smooth and continuous. The surface is free from an inclusions and it shows the purity of the compound. But the micrograph of the metal doped (Co2+ ) shows the presence of some visible inclusions deposited on the surface of the crystals (Fig. 10a–c). This may be due to the presence of metal doped in the crystalline matrix and temperature fluctuations during the growth process. To our best knowledge, the dopant (Co2+ ) may serve as impurities which will lead to the morphological changes by altering the properties of the solution in variety of ways [27].

Fig. 10. (a)–(c) The surface morphology of cobalt doped l-alanine.

Transparent single crystals of l-alanine and Co2+ doped lalanine crystals have been conveniently grown from by slow evaporation technique at room temperature and by slow cooling technique. Its structural and spectral studies were carried out using single crystal X-ray diffraction and FTIR analysis to confirm the functional group present in the crystal. The presence of dopants has marginally altered the lattice parameters without altering the basic structure. The transmittance spectra of these crystals show a very low absorption in the visible region, which confirms the suitability of these materials for NLO applications. The thermal study of pure and metal doped crystals proved their suitability to withstand the high temperature encountered in laser experiments. Also the NLO behavior of the pure and metal doped LA crystals has been observed by Kurtz-Perry powder technique. The laser damage threshold measurement on pure and Co2+ doped crystals clearly reveals that the doped LA crystals show enhanced LDT value. The presence of cobalt as the doped specimen has been confirmed by EDAX spectrum and the concentration of the incorporated dopants into the LA crystalline matrix. From the SEM analysis, we observe from the pure and metal doped crystals, that in the micro level the crystal surface is very smooth which shows that it can add more molecules to grow in to a large crystal. Analysis of the surface at different sites reveals that the incorporation of cobalt-ions is non-uniform over the whole crystal surface. Acknowledgements We are thankful for the scientific supports extended by SAIF, IITM. We thank Prof. P.K. Das, ICPC, IISc, Bengaluru, for support in SHG and laser damage threshold measurements. References [1] X.Q. Wang, D. Xu, D.R. Yuan, Y.P. Tian, W.T. Yu, S.Y. Sun, Z.H. Yang, Q. Fang, M.K. Lu, Y.X. Yan, F.Q. Meng, S.Y. Guo, G.H. Zhang, M.H. Jiang, Synthesis, structure and properties of a new nonlinear optical material: zinc cadmium tetrathiocyanate, Mater. Res. Bull. 34 (1999) 2003–2011. [2] Y.J. Ding, X. Mu, X. Gu, Growth, optical, thermal and di-electric studies of an amino acid organic nonlinear optical material: l-alanine, J. Nonlinear Opt. Phys. Mater. 9 (2000) 21. [3] M.S. Wong, C. Bosshard, F. Pan, P. Gunter, Non-classical donor–acceptor chromophores for second order nonlinear optics, Adv. Mater. 8 (1996) 677–680. [4] J.F. Nicoud, R.J. Twieg, in: D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, London, 1987, pp. 227–296. [5] M. Delfino, A comprehensive optical second harmonic generation study of the non-centrosymmetric character of biological structures, Mol. Cryst. Liq. Cryst. 52 (1979) 271–284. [6] D. Eimerl, S. Velsko, L. Davis, F. Wang, Progress in nonlinear optical materials for high power lasers, Prog. Cryst. Growth Charact. 20 (1990) 59–113. [7] J.D. Bernal, The crystal structure of the natural amino acids and related compounds, Z. Kristallogr. 78 (1931) 363–369. [8] H.J. Simpson Jr., R.E. Marsh, The crystal structure of l-alanine, Acta Cryst. 8 (1966) 550–555. [9] R. Destro, R.E. Marsh, R. Bianchi, A low-temperature (23 K) study of l-alanine, J. Phys. Chem. 92 (1988) 966–973. [10] (a) L. Misoguti, A.T. Varela, F.D. Nunes, V.S. Bagnato, F.E.A. Melo, J. Mendes Filho, S.C. Zilio, Optical properties of l-alanine organic crystals, Opt. Mater. 6 (1996) 147–152; (b) C. Razzetti, M. Ardoido, L. Zanotti, M. Zha, C. Parorici, Solution growth and characterization of l-alanine single crystals, Cryst. Res. Technol. 37 (2002) 456–465. [11] V. Bisker-Leib, M.F. Doherty, Modeling crystal shape of polar organic materials: applications to amino acids, Cryst. Growth Des. 3 (2003) 221–237. [12] N. Vijayan, S. Rajasekaran, G. Bhagavannarayana, R. Ramesh Babu, R. Gopalakrishnan, M. Palanichamy, Growth and characterization of nonlinear optical amino acid single crystal: l-alanine, Cryst. Growth Des. 6 (2006) 2441–2445. [13] M. Lydia Caroline, R. Sankar, R.M. Indirani, S. Vasudevan, Growth, optical, thermal and dielectric studies of an amino acid organic nonlinear optical material: l-alanine, Mater. Chem. Phys. 114 (2009) 490–494. [14] A.M. Petrosyan, R.P. Sukiasyan, H.A. Karapetyan, S.S. Terzyan, R.S. Feigelson, Growth and investigation of new nonlinear optical crystals of LAP family, J. Cryst. Growth 213 (2000) 103–111.

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