Growth, Spectroscopic, Dielectric and Non Linear Optical Studies of Strontium Metal Ion Doped Novel Semi-Organic L-Alanine Cadmium Chloride

Growth, Spectroscopic, Dielectric and Non Linear Optical Studies of Strontium Metal Ion Doped Novel Semi-Organic L-Alanine Cadmium Chloride

T. Retnakumar, R. Ilangovan*

Department of Nanoscience and Technology, Alagappa University, Karaikudi 630 004, India.

K. Sankaranarayan

School of Physics, Alagappa University Karaikudi 630 004, India

K. C. Bright

Department of Physics, St. Johns College, Anchal, Kollam 691 306, India.

T. H. Freeda

Crystal Research Centre, S.T. Hindu College, Nagercoil 629 00 , India.

S. Vinu

Department of Physics, Kamarajar Government Arts College, Surandai, Tirunelveli 627 859, India

.

Abstract The single crystals of pure and strontium metal ion doped L-alanine cadmium chloride, a semi-organic compound was grown by slow evaporation solution growth technique at room temperature. The grown crystals were subjected to various characterization techniques such as single crystal and powder XRD, scanning electron microscopy (SEM), energy dispersive X- ray spectrum (EDX), FT-IR and UV- Visible spectroscopy. The presence of various functional groups has been identified by FT-IR spectral analysis ranging between 4000 and 400 cm1 and the UVvisNIR spectroscopic study revealed that the grown crystal has good optical transmittance. Its nonlinear optical property has been studied by Kurtz powder method. Electrical parameters such as dielectric constant and dielectric loss have also been studied from 100 to 1 MHz. The low dielectric constant and dielectric loss suggested that this material is a good candidate for opto-electronic applications.

Keywords: Sr doped L-alanine, Single crystal XRD, Dielectric constant, NLO.

1. INTRODUCTION

   Semi-organic crystals attracted the scientific community very much due to their large nonlinear optical coefficients, high resistance to laser induced damage, low angular sensitivity and excellent mechanical hardness. Also advanced opto-electronic technology requires nonlinear optical (NLO) materials for frequency conversion, optical modulation and optical switching [1]. The recent development in optical fiber communication has motivated the scientists to search for new NLO materials that are capable of fast and efficient processing of optical signals and hence the researchers are always in search for new NLO materials [2]. Recently, amino-acid based materials have been proved to be attractive materials for NLO applications,

   because they contain a proton donating carboxyl group as well as proton accepting amino group in them. Owing to the excellent results of amino acid based semi-organic materials such as l-alanine cadmium chloride [3], l-asparagine thiourea monohydrate [4] and l-valine cadmium chloride [5], they are considered as best materials for NLO applications. Inspired by these results, in this present work Sr2+ ion doped LACC crystals were grown and subjected to different characterization in order to know its suitability for NLO applications.

2. EXPERIMENT

   The strontium ion doped LACC was synthesized using L-alanine, cadmium chloride and one mole percent of strontium chloride. At first the parent LACC compound was synthesized from the equimolar ratio of L-alanine and cadmium chloride, according to the following reaction:

   CH3CHNH2COOH + CdCl2 CH3CHNH2COOH.CdCl2

   Along with the compound solution, one mole percentage of strontium chloride was added to get the Sr2+ ion doped LACC crystal. The calculated amount of the reactants were thoroughly dissolved in double distilled water and stirred well for 2 hours using a magnetic stirrer to obtain a homogeneous mixture. Then the mixture was evaporated to dry the material. The temperature was kept to below an optimum temperature of 60 oC, to prevent the possible decomposition. It was then dissolved thoroughly in double distilled water to form a saturated solution. The solution was filtered well to remove the suspended impurities and allowed to crystallize by slow evaporation of solvent at room

   temperature of about 30 oC for four weeks. Well defined single crystals (size up to 20 x 15 x 4 mm3) with good transparency were collected in four weeks. The size of the pure crystal is about 12 x 7 x 2 mm3 [Figure 1].

   Figure 1: The single crystals of undoped and Strontium ion doped L-alanine cadmium chloride single crystal.

   The LACC crystal was subjected to single crystal XRD using a computer controlled Enraf NoniusCAD4 single crystal diffractometer with Mo-K radiation ( = 0.71073 Ã…) to determine the unit cell dimensions and morphology.

   Phase analysis of the grown crystals has been done using powder XRD (Philips Xpert Pro) equipped with an Xcelerator and a mono-chromator at the diffracted beam side. The X-ray scans were made in 2 and at 0.01 degree/step for all the samples. Scans were done using CuK radiation with a tube voltage of 40 kV and current 30 mA by exposing a constant area of the sample. The phase identification of the samples was performed using Xpert High score software in support with the ICDD-PDF-2 data base.

   The surface morphology of the samples have been observed using a scanning electron microscope (SEM) JEOL- JSM 5600 LV model, in back scattered mode and is operated at 20 kV. The elemental analyses of the samples were carried out using JEOL JSM 5600 LV equipped with EDAX.

   The FTIR spectrum of the sample was recorded with a Fourier transform infrared spectrometer in the range of 400 4000 cm-1 by KBr pellet technique. The optical transmittance range determine the suitability of strontium ion doped LACC single crystals for optical applications, the UV-VisNIR spectrum was recorded with a Varian Cary 100 Bio UV-Vis spectrophotometer in the range 200-2500 nm. The powder SHG measurements were performed using the Kurtz and Perry powder technique. The fine powdered sample was densely packed between two transparent glass slides. A fundamental laser beam of 1064 nm wavelength (8 ns pulse width with 10 Hz pulse rate) from an Nd-YAG LASER was made to fall normally on the sample cell and the incident beam was measured using a power meter. The input laser energy incident on the capillary tube was 5.3 mJ per pulse, an energy level optimized not to cause any chemical decomposition in the sample. The KDP crystal was used as the reference material in the SHG measurement. The well grown crystals were polished in proper size and the graphite coating was applied on the opposite faces of the samples for good electrical contact. Agilent 4284 A, Precision LCR meter was used for the measurement of the capacitance and dielectric loss in the frequency range of 100 Hz -1MHz at the temperature range of 30 110 oC along the b-axis of the crystal.

3. RESULTS AND DISCUSSION

There are lot of reports on growth and characterization of LACC single crystals with some dopants. The statues of dopants are still dispute [6]. In order to reveal the dopants, the single crystal XRD was carried out. The unit cell dimensions of pure LACC obtained from single crystal X-ray diffraction analysis are a = 16.33, b = 7.31, c = 8.00 Å, cell volume V = 854 Å3, = = 90° and =116.44°. Similarly for strontium doped LACC crystals, the unit cell parameters obtained from the single crystal X-ray diffraction analysis are a = 16.352 (± 0.011) Å, b = 7.303 (± 0.005) Å, c = 7.978 (±

.006) Å, cell volume V = 851.7(± 1.7) Å3, = = 90° and

=116.62 (± 0.03)°. The powder XRD pattern of pure and strontium doped LACC crystals are shown in Figure 2 (a) and (b), which consists of peaks, corresponds to LACC, and no other peaks of other phase or element are etected within the detection limit of the instrument. The unit cell dimensions calculated from the powder XRD data for pure LACC are a

=16.275 (± 0.002), b = 7.265 (± 0.002) and c = 7.97 (± 0.002)

Å, cell volume V = 843.786 (± 0.002) Å3, = = 90° and

=116.44°. For strontium doped LACC, a = 16.325 (± 0.004), b = 7.331 (± 0.002) and c = 7.947 (± .005) Å, cell volume V

= 850.26 (± .05) Å3. It agrees with the reported values [7]. The grown crystal belongs to the monoclinic system with space group C2 and the number of molecules per unit cell (Z) is 4. It shows that the dopant Sr2+ ion does not alter the crystal structure, but it enters into the crystal lattice of pure LACC.

Intensity in arb. unit

(011)

Figure 3 shows the SEM images of pure and strontium chloride doped LACC crystals. From the SEM images it was observed that due to strontium chloride doping there is a slight improvement in the alignment of grains and it may be due to the bunching of small grains around the impurity atom. Figure 4 shows the EDX images of pure and strontium chloride doped LACC crystals. From the EDX analysis it was observed that the Sr2+ ions are successfully doped into the crystal lattice of LACC.

Pure LACC

(201)

(201)

(401)

(112)

(312)

(202)

(510)

(212)

(202)

(313)

(613)

12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60

2 (degree)

Figure 2: (a) The powder X-ray diffraction pattern of pure

PURE LACC

1990.55

100

% Transmittance

2613.98

2500.34

1343.14

1299.48 1208.41

1108.85

1001.99

923.13

841.03

762.28

622.57

536.86

419.84

75

3501.48

3424.99

3044.37

50

1616.51

1495.96

1412.88

25

0

Figure 2(b): The powder X-ray diffraction pattern of Strontium ion doped L- alanine cadmium chloride.

Figure 3: The SEM image of pure and one mole percentage strontium ion doped L-alanine cadmium chloride single crystal.

Figure 4: The EDS image of pure and one mole percentage strontium ion doped L-alanine cadmium chloride crystal.

Figure 5 (a) and (b) shows the FT-IR spectrum of pure and strontium ion doped LACC single crystals. The NH2 group of L-alanine is protonated by the COOH group, giving rise to NH3+ and COO- groups. The broad envelope in the higher energy region 30443250 and 34253500 cm-1 in pure and 30763228 and 34003525 in Strontium doped LACC is due to NH3+ symmetric and antisymmetric stretching vibrations, and the absorption peak at 1616 cm-1 in pure and 1595 cm-1 in Strontium doped LACC is assigned to NH3+ bending degenerate mode [8]. The region of absorption bands from 3044 to about 2500 cm-1 in pure and 30762599 cm-1 for Strontium doped LACC single crystals is due to multiple combinations of overtone bands.

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

Figure 5: (a) The FTIR spectrum of pure L-alanine cadmium chloride crystal.

Figure 5: (b) The FTIR spectrum of strontium ion doped L-alanine cadmium chloride crystal.

The strong absorption at 1412 cm-1 in pure and 1417 cm-1 in doped LACC single crystals corresponds to COO- symmetric stretch [9]. The COO- bending and rocking frequencies occur in the normal positions at 762, 622 and 536 cm-1 in pure and for Sr doped LACC single crystals, which is at 766, 631 and 544 cm-1. Also the absorption at 1343 and 1001 cm-1 in pure and 1347 and 1012 cm-1 in doped LACC single crystals are due to CH3 symmetric bending and rocking mode. The absorption peaks at 923 and 841 cm-1 in pure and 923 and 848 cm-1 in Sr doped LACC are assigned to CCN symmetric stretching vibrations. These vibrations prove the presence of expected functional groups in the compound.

The UV-VisNIR spectrum of pure and LACC single crystals doped with one mole percentage of strontium chloride is shown in figure 6. From the figure it was noticed that there was no significant absorption after 200 nm. The lower cut at 200 nm along with good optical transparency enhances the usefulness of these crystals for optoelectronic applications and it is an essential parameter for NLO applications. The NLO property of the crystal was confirmed by the Kurtz and Perry powder technique. The transmitted fundamental wave was passed over a monochromator which separates 532 nm (second harmonic signal) from 1064 nm and absorbed by a CuSO4 solution (which removes the 1064 nm light). The green light was detected by a photomultiplier tube and displayed on a storage oscilloscope. The powder SHG efficiency of the crystals are compared with KDP and it was found that the pure and LACC doped with one mole percentage of strontium chloride was 0.57 and 0.82 times that of KDP. Thus it was observed that the SHG efficiency of

strontium chloride doped crystal increases with respect to pure LACC single crystals. The results are given in Table 1.

the increase of polarisability of the dipoles caused as a result of the improvement in grain alignment.

Pure LACC

(a)

AT 353 K

AT 363 K

AT 373 K

AT 383 K

PURE LACC:

AT 313 K

AT 323 K

AT 333 K

AT 343 K

7

absorbance (arb. unit)

Dielectric constant

6

5

4

3

200 400 600 800

Wavelength (nm)

Dielectric constant

Figure 6: (a) The UV – Vis – NIR spectrum of pure L-alanine cadmium chloride

(b)

2

AT 353 K

AT 363 K

AT 373 K

AT 383 K

AT 313 K

AT 323 K

AT 333 K

AT 343 K

2

LACC + 1 Mole SrCl :

12

10

8

6

4

2

100 1000 10000 100000 1000000

Frequency(Hz)

Figure 6: (b) The UV – Vis NIR spectrum of one mole percentage strontium ion doped L-alanine cadmium chloride

Table 1: Comparision of SHG signals energy output.

Figure 7 (a) and (b) shows the variation of dielectric constant with respect to frequencies at different temperature for undoped and strontium ion doped LACC single crystals respectively. The dielectric constant increases with decrease of frequency and increase of temperature. The variations of dielectric constant with frequency suggest the presence of higher space charge polarization of the material decreases with increase of frequency, which leads to the reduction of dielectric constant [10]. Also the increase of dielectric constant with increase of temperature was due to the presence of space charge polarization near the grain boundary interfaces which depends on the purity of the sample. Again as compared with pure LACC single crystals, a slight increase of dielectric constant in the doped crystals is due to

Figure 7: (a) Variation of dielectric constant with frequencies for different temperature of pure and (b) Strontium ion doped L-alanine cadmium chloride.

Figure 8 (a) and (b) shows the variation of dielectric loss with respect to frequencies at different temperature for undoped and one mole percentage strontium chloride doped LACC single crystals respectively. The nature of the curves show that the dielectric loss decreases with increase of frequency and decrease of temperature. Generally dielectric crystal can be represented by the circuit analog of resistance in parallel with capacitor. At higher frequency the capacitor offers low reactance to the sinusoidal signal, which minimizes the conduction losses in the resistor. Hence the value of dielectric loss decreases at higher frequencies. Again the material with low dielectric constant will have less number of dipoles per unit volume. As a resut it will have minimum loss as compared to the material with high dielectric constant [11]. It suggests that the dielectric loss strongly depends on the frequency of the applied field which is similar to the dielectric loss in the ionic system. Again, figure 8 (a) and (b) shows that the dielectric loss of doped crystal increases slightly at lower frequencies. It may be due to higher relaxation time of the dipoles at lower frequencies and hence consume much power than that at higher frequencies.

(a)

Dielectric loss

0.2

0.1

0.0

(b)

PURE LACC:

AT 313 K AT 353 K

AT 323 K AT 363 K

AT 333 K AT 373 K

AT 343 K AT 383 K

2

LACC + 1 Mole SrCl :

ACKNOWLEDGMENT

The authors acknowledge STIC Cochin for SEM, EDAX, UV-Vis, TGA-DTA analyses , Dr. P.K. Das IISc Bangalore for SHG measurement and SAIF Chennai for single crystal XRD measurements.

REFERENCES

1. H.O. Marcy, L. F. Warren, M. S. Webb, C. A. Ebbers, S. P. Velsko, G. C. Catella, Second harmonic generation in zinc tris(thiourea) sulfate, Appl. Opt. 31(1992) 5051-5060.

2. H. Nalwa, S. Miyata, Nonlinear Optics of Organic Molecules and Polymers, CRC Press, New York, 1997.

3. S. Dhanuskodi, K. Vasantha, Mary, P. A. Angeli, Structural and thermal characterization of a semiorganic NLO material: l-alanine cadmium chloride, Spectrochim. Acta Mol. Biomol. Spectrosc 66

   0.3

   AT 313 K AT 353 K

   AT 323 K AT 363 K

   AT 333 K AT 373 K

   Dielectric loss

   AT 343 K AT 383 K

   0.2

   0.1

   0.0

   100 1000 10000 100000 1000000

   Frequency (Hz)

   Figure 8: (a) Variation of dielectric loss with frequencies for different temperature of pure and (b) one mole percentage strontium ion doped L- alanine cadmium chloride.

   CONCLUSIONS

   The single crystals of pure and strontium chloride doped L-alanine cadmium chloride were grown by slow evaporation technique and characterized by both single crystal and powder X-ray diffraction studies, which showed the presence of Sr in the LACC single crystals. SEM study shows that the alignments of grains are improved in strontium doped single crystals and the presences of strontium ions in the crystal lattice of LACC crystals are also confirmed by EDS analysis. The FT-IR spectral analysis confirms the presence of functional groups in the compound. The material has good optical transparency in the entire visible region with a lower cut of wave length at 200 nm. The higher space charge polarization present in lower frequency region increases the dielectric constant at lower frequency and it is further increased due to the incorporation of Sr2+ ion into the lattice of pure LACC. The NLO property is confirmed by SHG measurement and its value is 0.82 times the value of KDP where as for pure LACC single crystals it is only 0.57 times. The second harmonic efficiency is remarkably enhanced by the Sr 2+ ions and hence the strontium ion doped LACC crystals are used along with laser devices for frequency conversion and also the low values of dielectric constant and dielectric loss at higher frequency suggests that this material is a good candidate for opto electronic applications.

   (2007) 637-642.

4. Prakash, J. Thomas Joseph, S. Kumararaman, Growth and characterization of l-proline cadmium chloride monohydrate single crystals, Mater. Lett. 62 (2008) 4097-4099.

5. P. Maadeswaran, J. Chandrasekaran, Synthesis, growth and characterization of l-valine cadmium chloride monohydrate-A novel semiorganic nonlinear optical crystal, Optik 122 (2011) 1128-1131.

6. A. M. Petrosyan, V. V. Ghazaryan, M. Fleck, On the existence of l-threonine formate, l-alanine lithium chloride and bis l- alanine lithium chloride crystals, Spectrochim. Acta Mol. Biomol. Spectrosc 105 (2013) 623-625.

7. I. Kathleen. Schaffers, A. Douglas. Keszler, Acta Cryst. C 49 (1993) 1156 – 1158.

8. K. Nagamoto, IR Spectra of Organic and Coordination Compounds, 2nd ed., Wiley Interscience, New York, (1978).

9. R. Silverstein, G. Bassler, T. C. Morril, Spectrometric Identification of Organic Compounds, John Wiley & Sons, New York, (1981).

10. S. K. Aroa, V. Patel, B. Amin, A. Kothari, Dielectric behavior of strontium tartrate single crystals, Bull. Mater. Sci. 27 (2004) 141- 147.

11. S. Sohma, H. Takashashi, T. Taniuchi, H. Ito, Organic nonlinear optical crystal DAST growth and its device applications, Chem. Phys. 245 (1999) 359-364.