Optical reflectance, optical refractive index and optical conductivity measurements of nonlinear optics for l -aspartic acid nickel chloride single crystal

Optics Communications 291 (2013) 304–308

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Optical reflectance, optical refractive index and optical conductivity measurements of nonlinear optics for L-aspartic acid nickel chloride single crystal G. Anbazhagan a, P.S. Joseph b, G. Shankar c,n a

PG and Research Department of Physics, H.H. Rajah’s College, Pudukottai-622 001, India PG and Research Department of Physics, Thanthai Hans Roever College, Perambalur-621 212, India c Department of Physics, Government College of Engineering, Salem-636 011, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 September 2012 Received in revised form 22 October 2012 Accepted 25 October 2012 Available online 21 November 2012

Single crystals of L-aspartic acid nickel chloride (LANC) were grown by the slow evaporation technique at room temperature. The grown crystals were subjected to Powder X-ray diffraction studies to confirm the crystal structure. The modes of vibration of different molecular groups present in LANC have been identified by FTIR spectral analysis. Optical transferency of the grown crystal was investigated by UVVis-NIR spectrum. The lower optical cut off wavelength for this crystal is observed at 240 nm and energy band gap 5.179 eV. The optical reflectance and optical refractive index studies have been carried out in this crystal. Finally, the optical conductivity and electrical conductivity studies have been carried out on LANC single crystal. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.

Keywords: L-Aspartic acid nickel chloride Optical band gap Optical reflectance Optical refractive index Optical conductivity

1. Introduction Nonlinear optical materials have attracted much attention because of the potential applications in emerging optoelectronic technologies [1–3]. Many investigations are being done to synthesis new organic materials with large second order optical nonlinearities in order to satisfy day to day technological requirements. Due to unique properties, the nonlinear optical NLO single crystals have promising applications in the area of photonic such as high speed information processing, frequency conversion, optical communication, optical computing and high optical disk data storage etc [4–9]. The organic materials with aromatic rings having high nonlinear optical coefficient, fast response with tailor- made flexibility, low mobility and large band gap find wide applications [10–13]. Organic nonlinear optical NLO materials have attracted great attention as they provide the key functions of optical frequency doubling, optical modulation, optical switching and optical memory for emerging technologies in areas such as optical communication, signal processing and optical information storage devices [14–15]. In the present study, LANC was synthesized and the crystals were grown from saturated water and KOH solution by slow evaporation technique, which has been widely used for the

growth of organic and inorganic materials [16]. But we present here, powder X-ray diffraction analysis, Fourier transforms infrared spectroscopic analysis, Ultra Violet spectral analysis, optical band gap, optical reflectance, optical refractive index, optical conductivity and electrical conductivity measurements, which are reported in this article.

2. Materials and methods 2.1. Synthesis The starting material was prepared by dissolving, KOH and acid in water in 1:1 ratio with one part of nickel chloride at room temperature. The pH value of the solution is 8. All the materials are commercially available. The saturated solution of LANC crystal was stirred well to enable homogenization of the solution. The seed crystals are prepared by slow evaporation technique. Good quality seed was chosen and kept suspended in to the super saturated solution. The product was purified by repeated recrystallization before it was used for growth of LANC crystal. The optical transparent crystals were obtained in 30 days.

L-aspartic

2.2. Characterization analysis n

Corresponding author. E-mail addresses: [email protected] (G. Anbazhagan), [email protected], [email protected] (G. Shankar).

The single crystal of LANC is subjected to different methods such as power X-ray diffraction, FTIR spectrum, UV spectral analysis,

0030-4018/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.10.073

G. Anbazhagan et al. / Optics Communications 291 (2013) 304–308

optical band gap, optical reflectance, optical refractive index, optical conductivity and electrical conductivity measurements.

3. Results and discussion 3.1. Powder X-ray diffraction studies The recorded power X-ray diffraction pattern for LANC is shown in Fig. 1. The sharp nature of the diffraction peaks indicates well crystalline of grown crystal. The recorded data was used to calculate lattice parameter using JCPDS from which it was confirmed that the grown crystal belongs to monoclinic crystal system with space group P21/a. The calculated lattice parameters ˚ b¼8.67 A˚ and c ¼13.50 A. ˚ are a¼13.91 A, 3.2. Fourier transforms infrared spectroscopic studies The FTIR spectrum was recorded using the Perkin Elamer spectrophotometer in the wavelength range 400–4000 cm  1 by KBr pellet technique. The FTIR spectrum is shown in Fig. 2. Explanation of the spectrum for high to low wavelength reveals

800

600 311

3.3. Optical studies of LANC crystal

400

121

115

061

200

136 225

142

Intensity (arb units)

that the OH stretching appears as a strong and very broad band in the region 2500–3300 cm  1. In the higher energy region, the peak at 3338 cm  1 is assigned to NH asymmetric stretching appears strong at 1618 cm  1 signifying the C ¼C stretching modes. The weakly resolved peak which appears at about 1404 cm  1, shows the presence of C ¼O deformation. The CH2 wagging vibration produces a sharp intense peak of 1248 cm  1. The peaks seen at around 1166 cm  1 and 1099 cm  1 are associated to C–C stretching. The C–N stretching vibration produces its characteristics peak at 1055 cm  1 which appears as a sharp absorption peak. Thus, the bands at about 1055 cm  1 and at 716 cm  1 establish the presence of benzene ring. The band at 860 cm  1 and 610 cm  1 illustrates the presence of substituted ring 1, 4 distributions. The NH3þ and CH2 rocking vibrations of LANC give it characteristic vibrations at 229 cm  1 and 716 cm  1, respectively. The presence of NO3 is clearly illustrated by the peaks at about 1310 cm  1 and 829 cm  1 in the title compound. Also the peak at 1248 cm  1 is assigned to COO vibrations. This spectrum appears largely different from that of L-phenylalanine reported in the literature [17].

3.3.1. Optical absorption studies The optical absorption spectrum of LANC crystal was recorded in the wavelength region 190–1100 nm and the structure obtained is shown in Fig. 3. For opto-electronic devices fabrication, the crystal should be highly transparent in the considerable region of wavelength [18–19]. Examination of the spectrum from lower to higher wavelength reveals low absorption in the entire UV and visible region. This is important for materials possessing NLO properties. The UV cut off wavelength of the grown crystal was found to be at 240 nm.

222

1000

305

0 0

10

20

30

40

50

60

70

80

90

2θ (degrees) Fig. 1. X-ray diffraction of LANC crystal.

3.3.2. Optical transmittance studies Optical transmission spectrum of LANC single crystal Fig. 4 was recorded in the wavelength region form 190–1100 nm. The good transmissions of the crystal in the entire visible region suggest its suitability for second harmonic generation devices [20–23]. The dependence of optical absorption coefficient with the photon energy helps to study the band structure and the type of transition of electrons [24].

4.0 3.5 3.0 2.5 2.0 %T

1.5 1.0 0.5 0.0 -0.5 -1.0 4000

3500

3000

2500

2000

1500

Wavenumbers (cm-1) Fig. 2. FTIR spectrum of LANC crystal.

1000

500

306

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6

8.00E+011

6.00E+011 (αhυ)2 eV2 Cm-2

Absorption (arb.units)

5 4 3 2 1

4.00E+011

2.00E+011

0.00E+000

0

1

2

3

4

5

6

7

Photo energy (eV)

200

400

600

800

1000

1200

Fig. 5. Photon energy verses (ahn)2 of LANC crystal.

Wavelength (nm) Fig. 3. Optical absorption spectrum of LANC crystal.

the following relation [27]. T¼

100

1R2 expð2adÞ

ð3Þ

where d is the thickness and a is related to extinction coefficient (K) by

80

K¼

al 4p

ð4Þ

The refractive index can be determined from the reflectance (R) data using the relation [28–29].

40

R¼

%T

60

ðn1Þ2 ðn þ1Þ2

ð5Þ

The reflectance in terms of the optical absorption coefficient can be derived from the above equations. Hence, the equation pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 17 1expðadÞ þ expðadÞ ð6Þ R¼ 1 þ expðadÞ

20

0 200

400

600 800 Wavelength (nm)

1000

1200

Fig. 4. Optical transmission spectrum of LANC crystal.

The optical absorption coefficient (a) was calculated from the transmittance using the following equation,   2:3036log 1=T a¼ d

ð1Þ

where T is the transmittance and d is the thickness of the crystal. As a direct band gap material, the crystal under study has an optical absorption coefficient (a) obeying the following equation for high photon energies (hn).

a¼

ð1RÞ2 expðadÞ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A hnEg hn

ð2Þ

where Eg is optical band gap of the crystal and A is a constant. The plot of variation of (ahn)2 verses photon energy (eV) is shown in Fig. 5. Eg is evaluated by the extrapolation of the linear part [25]. The band gap is found to be 5.179 eV. As a consequence of wide band gap, the grown crystal has large transmittance in the visible region [26]. The optical constants (n, K) are determined from the transmittance (T) and reflectance (R) spectrum based on

where d is the thickness of the crystal, n is the optical refractive index and a is the optical absorption coefficient of the crystal. The optical refractive index (n) can be determined from the optical reflectance data using, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðR þ1Þ 7 3R2 þ10R3 n¼ ð7Þ 2ðR1Þ 3.3.3. Optical reflectance Fig. 6 shows the variation of optical reflectance (R) with optical absorption coefficient (a) for the LANC crystal. When optical absorption coefficient increases, optical reflectance also increases slowly. From Fig. 6 it is noted that the optical reflectance value is 40 at the optical absorption coefficient value of 75000. The value of optical absorption coefficient after 75,000 optical reflectance resulted in a narrow straight line. Fig. 7 represents the dependence of optical reflectance on optical extinction coefficient in LANC crystal. When the optical extinction coefficient increases, the optical reflectance also increases slowly. Optical extinction coefficient 0.00125–0.00225 increases resulting in a narrow straight line for optical reflectance. Fig. 8 shows the variations of optical reflectance as a function of optical refractive index of LANC crystal. It increases up to a certain refractive index and decreases and finally becomes zero. It is noted from the figure that lower optical refractive index suddenly drops down to nearly zero and higher optical refractive index becomes zero and is independent of optical refractive index

5

5

4

4

3

0

0

-20000

0

20000 40000 60000 80000 100000 120000 140000 Optical absorption coefficient (cm-1)

Fig. 6. Optical absorption coefficient verses optical reflectance of LANC crystal.

0.00E+000

1

-5.00E-027

1

-1.00E-027

2

-2.00E-027

2

307

-3.00E-027

3

-4.00E-027

Reflectance (%)

Reflectance (%)

G. Anbazhagan et al. / Optics Communications 291 (2013) 304–308

Electrical polarization Fig. 9. Electrical polarization verses optical reflectance of LANC crystal.

5

[30]. The plot optical refractive index verses electrical polarization is shown in Fig. 9. It is seen that the optical reflectance decreases with increasing electric polarization.

Reflectance (%)

4

3

3.3.4. Optical refractive index measurement Fig. 10 represents the variation of optical refractive index against wavelength. From Fig. 10 it is obvious that energy dependence on optical refractive index occurs in the wavelength range of 190–1100 nm. This wavelength region shows sharp upper peaks. This observation is in agreement with that in (2-aminopyridinium maleate, 4-amino benzophenone and dichlorobis (L-proline) zinc(II) reported in the literature [31–33]

2

1

0 0.0000

0.0005 0.0010 0.0015 0.0020 Optical extinction coefficient

0.0025

Fig. 7. Optical extinction coefficient verses optical reflectance of LANC crystal.

Reflectance (%)

5

3.3.5. Conductivity measurements 3.3.5.1. Optical conductivity measurement. The variation of conductivity remains at zero up to photon energy of 5.95 eV, where the optical conductivity drops down to the lowest value. Beyond that the optical conductivity is constant in Fig. 11. Incident photon energy is given by E ¼ hn

ð8Þ

4

where it depends on frequency; the optical conductivity is directly proportional to optical extinction coefficient. In the optical conductivity from the Shankar and Joseph equation [34].

3

sop ¼

Knc

l

ð9Þ

2

where K is the optical extinction coefficient, n is the optical refractive index, c is the velocity of light and l is the wavelength of light.

1

3.3.5.2. Electrical conductivity measurement. The electrical conductivity measurement uses the Shankar and Joseph equation

0

sec ¼ -500

0

500

1000 1500 2000 2500 3000 3500

Optical refractive index Fig. 8. Optical refractive index verses optical reflectance of LANC crystal.

2Kcn

a

ð10Þ

where K is the optical extinction coefficient, n is the refractive index, c is the velocity of light and a is the optical absorption coefficient. Fig. 12 shows that the electrical conductivity affects photon energy. The photon energy increases up to 2.50 eV and the

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G. Anbazhagan et al. / Optics Communications 291 (2013) 304–308

4. Conclusion

4000

Optical refractive index

3000

2000

1000

0

-1000 200

400

600 800 Wavelength (nm)

1000

1200

Fig. 10. Wavelength verses optical refractive index of LANC crystal.

References

Optical conductivity (S-1)

0.00E+000 -5.00E+014 -1.00E+015 -1.50E+015 -2.00E+015 -2.50E+015 1

2

3 4 5 Photon energy (eV)

6

7

Fig. 11. Optical conductivity verses photon energy of LANC crystal.

Electrical conductivity (s/cm)

80000

60000

40000

20000

0

-20000 1

2

3

4

5

6

Optical, single crystals of LANC have been grown successfully by continuous slow evaporation technique. Powder crystal X-ray diffraction studies confirm that the grown crystal belongs to monoclinic structure with the space group P21/a. The optical absorption studies infer that the crystals possess very low absorption in the entire visible and NIR region. The presence of functional groups is confirmed by FTIR spectral analysis. The optical transmittances of the crystal confirm the transferency of the crystal, by tailoring the optical absorption coefficient and tuning the optical band gap of the materials. The optical constants such as optical absorption coefficient, optical extinction coefficient, optical reflectance and optical refractive index are calculated and reported. Finally, the optical and electrical conductivity for the crystals suggest that the LANC crystals are suitable for fabrication of optoelectronic, electro optic and photonic devices.

7

Photon energy (eV) Fig. 12. Electrical conductivity verses photon energy of LANC crystal.

electrical conductivity suddenly shoots up. Beyond that the electrical conductivity slowly decreases, and finally reaches zero and negative values. The electrical conductivity is inversely proportional to the optical absorption coefficient.

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