Synthesis, growth and characterisation of a semiorganic nonlinear optical material: l-threonine cadmium chloride single crystals

1 Introduction

Nonlinear optics (NLO) is a forefront of current research because of its importance in providing the key functions of frequency conversion, light modulation, and optical memory storage for the emerging technologies in areas such as telecommunications, signal processing and optical interconnections. In recent years, there has been extensive research on the growth of nonlinear optical materials because of their wide applications in optoelectronics. Most of the organic crystals have inadequate transparency, poor optical quality and a low laser damage threshold (Jiang and Fong, 1999; Prasad and Williams, 1991; Chemla and Zyss, 1987). Moreover, growth of a bulky large sized single crystal is difficult for device applications. Inorganic crystals have excellent mechanical and thermal properties but they possess relatively modest nonlinearity. Due to the above reasons, a lot of research has been carried out on semiorganic materials which have combined properties of both organic and inorganic materials (Franken et al., 1961). The semiorganic materials are more suitable for device fabrication due to the wide transparency window and high second harmonic generation efficiency with mechanical and chemical stability. Hence, the researcher has preferred to focus on semiorganic compounds due to their large nonlinearity, high resistance to laser damage threshold, low angular sensitivity and good mechanical hardness (Redrothu and Kalainathan, 2012). In the semiorganic coordination complexes the organic ligand shows higher nonlinear optical property (Masilamani et al., 2012). The metallic part focus is on group II B metals (Zn, Cd and Hg). The compounds usually have high transparency in the UV region.

Now-a-days, amino acids are more suitable organic materials for nonlinear optical applications (Ilayabarathi and Chandrasekaran, 2012; Dhanuskodi et al., 2007; Anbuchezhiyan et al., 2010; Manij et al., 2011), because they are of dipolar nature due to the presence of a protonated amino group (NH₃⁺) and deprotonated carboxylic group (COO⁻). The potential semiorganic nonlinear optical materials like l-alanine cadmium chloride, l-asparagine cadmium chloride, etc. are some proven examples for this approach. In the present work l-threonine cadmium chloride (LTCC) was grown from aqueous solution by the slow evaporation method. The material was characterised by single crystal and powder X-ray diffraction, UV–Vis, Fourier transform infrared analysis (FTIR), mechanical hardness and nonlinear optical studies were discussed in detail.

2 Experimental procedure

2.1

2.1 Material synthesis and crystal growth

l-threonine cadmium chloride (LTCC) crystal was synthesised from l-threonine and cadmium chloride taken in 1:1 equimolar ratio. The required quantity of l-threonine and cadmium chloride was thoroughly dissolved by adding double distilled water according to their solubility and stirred well for about four hours using a magnetic stirrer to obtain a homogenous mixture. The solution was filtered to remove insoluble impurities using Whatman filter paper of pore size ten micrometres. Then, the saturated solution of l-threonine cadmium chloride (LTCC) was taken in a beaker with a perforated lid in order to control the evaporation rate and kept at room temperature for crystallisation. Finally, a well-defined single crystal was obtained after 35 days by the slow evaporation method. The photograph of the grown crystal of l-threonine cadmium chloride (LTCC) is shown in Plate 1.

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Plate 1 The photograph of the LTCC crystal.

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3 Results and discussion

3.1

3.1 X-ray diffraction analysis

The crystalline nature of the grown crystal was checked by taking the X-ray diffraction pattern of powder samples of l-threonine cadmium chloride (LTCC) crystal with Cu kα (λ = 1.5406 Å) radiation. The sample was scanned in the range of 10–80° at the rate of 2°/min. The recorded powder XRD pattern is shown in Fig. 1. The presence of a sharp and well defined peak confirms the good crystalline nature of the l-threonine cadmium chloride (LTCC) crystal (Masilamani et al., 2012). The unit cell parameters of the grown l-threonine cadmium chloride (LTCC) crystal were obtained by using a single crystal X-ray diffractometer. It was found to be a = 15.23 Å, b = 5.82 Å and c = 7.36 Å and cell volume V = 652.3 A³. It reveals that l-threonine cadmium chloride (LTCC) crystallised in an orthorhombic crystal system with a space group of P2₁2₁2₁ which is recognised as non centrosymmetric, thus satisfying one of the basic and essential requirements for NLO material.

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Figure 1 Powder XRD pattern of the LTCC crystal.

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3.2

3.2 UV–Visible spectral analysis

The UV–Visible transmission spectrum is very important for optical material because of its wide transmittance window (Bright and Freeda, 2010). The suitability of l-threonine cadmium chloride (LTCC) single crystal for optical applications was known from optical transmission spectra. The crystal is dissolved in methanol (HPLC grade). UV–Visible transmission spectra are recorded between 190 and 1100 nm and shown in Fig. 2. The high transparency was confirmed from the recorded spectra and it was observed that there was no significant absorption in the range 332–1100 nm. There is an advantage in the use of amino acids, where the absence of strongly conjugated bonds leads to a wide transparency range in the visible and UV spectral regions. The lower cut-off wave length was found to be around 332 nm which combined with good transparency attests to the usefulness of l-threonine cadmium chloride (LTCC) material for opto-electronic applications.

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Figure 2 UV–Vis spectrum of the LTCC crystal.

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3.3

3.3 FTIR analysis

The recorded FTIR spectra of l-threonine cadmium chloride (LTCC) crystal are shown in Fig. 3 and the tentative vibrational assignments are given in the Table 1 (Gargaro et al., 1993; Clark and Hester, 1984; Griffiths and Haseth, 1986;George Socrates, 2001). The broad absorption of medium intensity peak at 3168 cm⁻¹ was assigned due to NH₃⁺ asymmetric stretching vibration. The medium absorption peak at 2711 cm⁻¹ and a weak absorption peak at 2049 cm⁻¹ were due to NH₃⁺ symmetric stretching vibrations. The peak observed at 2516 cm⁻¹ indicates the CH stretching vibration. The asymmetric NH₃⁺ deformation vibration occurs at 1629 cm⁻¹. The absorption peak at 1480 cm⁻¹ was due to symmetric NH₃⁺ deformation vibration. The peak at 1417 cm⁻¹ reveals the COO⁻ symmetric and asymmetric stretching vibrations. The NH₃⁺ rocking vibration was observed at 1247 and 1114 cm⁻¹. The C–C–N deformation vibration was observed at 559 cm⁻¹. Hence, the presence of various functional groups was confirmed from the above tentative assignment.

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Figure 3 FT-IR spectrum of the LTCC crystal.

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Table 1 Tentative vibrational assignments of the LTCC crystal.

Wave number (cm−1)	Tentative vibrational assignments
3168	NH3+ asymmetric stretching
2711, 2049	NH3+ symmetric stretching
2516	C–H stretching
1629	NH3+ asymmetric deformation
1480	NH3+ symmetric deformation
1417	COO− symmetric stretching
1247, 1114	NH3+ rocking
559	C–C–N deformation

3.5

3.5 Microhardness study

The hardness of a material is a measure of its resistance to local deformation (Mott, 1956). It is correlated with other mechanical properties like elastic constants, yield strength, brittleness index and temperature of cracking. Microhardness studies have been carried out on l-threonine cadmium chloride (LTCC) single crystals using the microhardness tester fitted with a Vickers diamond pyramid indenter. Vicker’s microhardness number was calculated using the relation Hv = 1.8544P/d² kg/mm², where P is the applied load in kg and d is the average diagonal length of the indentation. A graph was plotted between the hardness number (Hv) and the applied load (P) as shown in Fig. 4. From the graph, it is observed that the hardness value increases as the load increases and attains a maximum value at 100 g. At the above load of 100 g, multiple cracks were initiated on the crystal surface around the indenter. This was due to the release of internal stress generated locally by indentation. Hence, the l-threonine cadmium chloride (LTCC) semiorganic crystal was proven to be a suitable material for optoelectronic device fabrication.

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Figure 4 Hardness vs. load graph of the LTCC crystal.

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4 Conclusions

A Single crystal of l-threonine cadmium chloride (LTCC) was grown from an aqueous solution containing l-threonine and cadmium chloride by the slow evaporation method. The orthorhombic structure was confirmed by single crystal XRD analysis and good crystalline nature was confirmed by powder XRD analysis. The optical transmission study reveals that l-threonine cadmium chloride (LTCC) crystal had good transparency in the entire visible region and there was no absorption in this range. The presence of various functional groups in the crystal was identified by FTIR spectra. The carbon, hydrogen, and nitrogen percentages were determined by elemental analysis. The mechanical stability was calculated by using a Vicker’s microhardness tester. The second harmonic generation efficiency was found to be 0.7 times higher than that of the potassium dihydrogen phosphate (KDP) crystal. Hence, l-threonine cadmium chloride (LTCC) single crystal is proven to be a more suitable material for optoelectronic devices.