Preparation of novel optical-grade metanitroaniline and polymethylmethacrylate-coated single crystals and their optical properties

Materials Letters 58 (2004) 3929 – 3932 www.elsevier.com/locate/matlet

Preparation of novel optical-grade metanitroaniline and polymethylmethacrylate-coated single crystals and their optical properties Y.S. Negia,*, P.V. Adhyapaka, S.R. Damkalea, R.K. Goyala, M. Islamb, R.C. Aiyerb a

Polymer and Materials R&D, Centre for Materials for Electronics Technology [C-MET], Department of Information Technology, Government of India, Panchwati, off Pashan Road, Pune-411 008, India b Department of Physics, University of Pune, Pune-411 007, India Received 16 June 2004; accepted 10 August 2004 Available online 15 September 2004

Abstract Preparation and optical properties of metanitroaniline (m-NA) and polymethylmethacrylate (PMMA)-coated m-NA single crystals were investigated. In this report, we developed relatively large-sized m-NA single crystals in methyl ethyl ketone (MEK) using solution growth techniques. The average size of the single crystals achieved is about 1055 mm. The developed crystals were further coated with laboratory-developed optical-grade PMMA to study the effect of polymer coating on the second harmonic generation (SHG) properties of the single crystals. m-NA single crystals were characterized using DSC, FTIR and XRD measurement techniques. Optical characterization was done using UV-visible spectroscopy. Morphology was studied using scanning electron microscopy (SEM). Improved nonlinear optical (NLO) properties corresponding to SHG intensity values were also recorded for these coated and uncoated single crystals. PMMA-coated mNA single crystal surfaces showed high SHG values. D 2004 Elsevier B.V. All rights reserved. Keywords: Metanitroaniline (m-NA); Organic crystal; Second harmonic generation; Polymer-coated single crystals

1. Introduction Optical-grade organic single crystals of substituted benzene derivatives with high optical nonlinearities and low melting temperatures are promising materials for optoelectronics and nonlinear optical (NLO) applications. Organic crystals with large second-order nonlinear susceptibilities are of great interest because of their potential use of an optical parametric amplifier and an optical parametric oscillator in an infrared (IR) region [1,2]. In recent years, nonlinear second-order cascading effects in organic crystals draw much attention in the context of not only basic understanding of the nonlinear optical processes that occur in organic crystals but also in the context of potential

* Corresponding author. Tel.: +91 020 25899273; fax: +91 020 2589180. E-mail address: [email protected] (Y.S. Negi). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.08.021

applications for optical devices [3,4]. Metanitroaniline (mNA) is an important material in the research field of nonlinear optics. Its monocrystal has a unique polar axis c; thus, the nonlinear optical effect is very strong. Organic single crystals are known to exhibit comparable or even better NLO properties than known inorganic materials, especially second harmonic generation (SHG). Metanitroaniline is one of the organic single crystals extensively studied due to its high nonlinear and electrooptic effects. The effect of second harmonic generation properties of mNA is higher by one order of magnitude as compared with that of LiNbO3 [5]. It serves as a multifunctional material. However, it is difficult to grow large-sized organic single crystals. The knowledge of intrinsic properties achievable in very pure and perfect crystals is important. Therefore, we have carried out a program to grow single crystals of high purity and crystalline perfection. In order to preserve the single crystals for repeated use, the single crystals require a polymer coating for a prolonged life and protection of the

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surface for reproducible results. Therefore, an attempt was made to coat the single crystal surface with polymethylmethacrylate (PMMA) thin surface by dip coating followed by drying. Development of organic single crystals is one of the important emerging areas for optoelectronics and molecular engineering for advanced high-tech devices. Metanitroaniline is one of the organic single crystals that show high nonlinear and electrooptic effects [6,7]. An m-NA single crystal is grown either by chemical vapor deposition or by the Bridgemann method [8]. Huang et. al. reported [9] growth of an m-NA single crystal using solution growth techniques. However, these methods produce only smallersized crystals. In the recent past, attempts have been focused on simple methods such as solution growth techniques to produce organic single crystals. This paper reports the single crystals of m-NA using solution growth techniques. Neat crystals and polymer coated crystals were characterized by different techniques.

2. Experimental setup Commercial m-NA powder was recrystallized several times in acetone for better purity. The recrystallized powder was again purified with utmost care by vacuum sublimation technique before use. Single crystals of m-NA were developed by solution growth techniques in methyl ethyl ketone (MEK). In making the crystals, the saturated solution of m-NA in MEK was kept for a few days to evaporate the solvent slowly (about 3–4 weeks). Transparent crystals were obtained by evaporating the solvent at room temperature. To speed up the crystal formation process, seed crystals of m-NA were introduced in the solution. To minimize the rate of evaporation, the beaker was kept tightly closed. These single crystals were further coated with PMMA (10% PMMA in toluene). Coating of PMMA was done uniformly by a dip-coating method. After coating, the crystals were dried in an oven at 80 8C for 24 h. The single crystal was studied with a differential scanning calorimeter (DSC) (Mettler Toledo 851). The

Fig. 1. Melting endotherm of an m-NA single crystal.

Fig. 2. FTIR spectrum of an m-NA single crystal.

experiment was carried out at 7 8C min 1 ramp rate from 25 to 300 8C in nitrogen atmosphere. The infrared spectrum of m-NA single crystal in the transmission mode was recorded on a Perkin Elmer Spectrum 2000 FTIR spectrophotometer. The spectrum was collected in the range 4000–400 cm 1. X-ray diffractogram of m-NA was recorded on a Philips PW 1710 diffractometer using CuKa (k=1.541) radiation and Ni filter. Scanning electron microscopy (SEM) image was obtained by means of a Philips 30 XL instrument, which was operated at 10 kv in the secondary electron imaging mode.

3. Results and discussion In the present study, we have tried to grow m-NA single crystals in MEK with the use of solution growth techniques. We used solution growth techniques to grow single crystals of m-NA because they yielded moderately large single crystals with relative ease. The rate of evaporation of solvent plays an important role in deciding the size and nature of single crystals. Slow-evaporating solvents such as MEK take several days to grow single crystals. Light yellow-colored single crystals with an average size of 1055 mm were grown by slow evaporation of solvent at room temperature [10,11].

Fig. 3. Powdered X-ray diffraction pattern of an m-NA single crystal.

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Table 1 X-ray diffraction results 2h

hkl

d values Calculated

Observed

18.3 19.4 33.5 34.2

(040) (130) (221) (080)

4.79 4.54 2.67 2.39

4.84 4.57 2.63 2.42

The use of m-NA is prominently required as a reference sample as NLO active organic materials and their incorporation with PMMA to prepare the NLO active polymer system and finally with SHG properties will be compared with synthetically designed NLO polymers. A solution growth technique is one of the techniques widely used to grow organic single crystal molecules. MEK is used as a solvent to grow the single crystals. Rates of solvent evaporation certainly affect the growth of single crystals. As MEK is a slow-evaporating solvent, the crystals obtained are of uniform size and shape. DSC results (Fig. 1) show that the melting temperature of crystal is around 115 8C, which is better than the reported value (112.3 8C, [12]), is the indication of an optically pure material. FTIR analysis further supports the purity of material. The IR obtained is shown in Fig. 2. The peaks obtained at 3328 and 3430 cm 1 correspond to symmetric and asymmetric stretching modes of the free NH2 group. The peaks at 3200, 3090 and 2920 cm 1 are due to weakly bonded N–H stretching modes. NO2 stretching modes are observed at 1520 and 1348 cm 1. The dipole interaction between neighboring molecules is expected to be the dominating molecular force in molecular packing of crystal lattices. The UV-visible spectrum taken on a Hitachi U-3210 model in wavelength range 400–900 shows the wavelength cutoff at around 510 nm and k max at around 378 nm, while some studies with doped polymer films with single crystals show the shift in a k max value. The powdered X-ray diffraction spectrum is shown in Fig. 3. The reported [13] hkl and d values of m-NA are tabulated in Table 1. The single-crystal XRD data of m-NA show orthorhombic unit cells, and the obtained lattice parameter values of an m-NA single crystal are tabulated in Table 2. Morphology of single crystals was also studied by scanning electron microscopy. An SEM picture (Fig. 4a) shows minor defects in the crystal structure. These minor defects may be developed at the time of crystal develop-

Fig. 4. Scanning electron microscopic images of an (a) m-NA single crystal and a (b) PMMA-coated m-NA single crystal.

Table 2 Lattice parameter values of an m-NA single crystal Lattice parameters

Reported (2)

Experimental (2)

a b c

6.501 19.330 5.082

6.507 19.529 5.087

Fig. 5. Second harmonic generation studies of an (a) m-NA single crystal and a (b) PMMA-coated m-NA single crystal.

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ment. After polymer coating (Fig. 4b), moreover, the surface of a crystal becomes smooth and may reduce the scattering of laser light. This reflects the increase in the SHG intensity. Second harmonic generation study using Nd: YAG laser (1 pps), 40 ns pulse width; results of m-NA single crystal and PMMA-doped single crystal are shown in Fig. 5a and b. Both the coated and uncoated crystals generate the second harmonics. In the case of PMMA-coated crystals, the intensity of emitted light is higher than that of uncoated single crystals. This may be attributed to the synergistic effect of the polymer layer and m-NA crystal lattice packing. Moreover, due to polymer coating, the surface of the crystal becomes smooth and ultimately reduces the loss in laser radiation, and due to which there is an increase in SHG intensity values.

4. Conclusions In this report, single crystals of m-NA were grown, and the conditions were optimised using solution growth techniques. The solvent used was MEK. It is possible to grow a single crystal with a 1055-mm size. The optical characterization of these single crystals shows second harmonic generation. Further polymer coating of these crystals shows an increase in SHG intensity values. The polymer-coated crystals are a new type of crystals in which the polymer forms a protective layer on a crystal and improves the optical property of a crystal.

Acknowledgement Authors would like to thank the Executive Director of CMET (Pune) for giving permission to carry out this research. Authors would also like to thank Dr. Mohan Bhadbhade (NCL) for single-crystal XRD.

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