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Growth, linear and nonlinear optical studies on guanidinium 4-nitrobenzoate (GuNB): An organic NLO material
Optik 123 (2012) 1153–1156
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Growth, linear and nonlinear optical studies on guanidinium 4-nitrobenzoate (GuNB): An organic NLO material T. Arumanayagam, P. Murugakoothan ∗ PG and Research Department of Physics, Pachaiyappa’s College, Chennai 600030, India
a r t i c l e
i n f o
Article history: Received 4 February 2011 Accepted 15 May 2011
Keywords: Nonlinear optic materials Growth from solution Optical and dielectric constants
a b s t r a c t Single crystals of guanidinium 4-nitrobenzoate (GuNB) were grown using solvent evaporation technique by mixing aqueous solutions of guanidine carbonate and 4-nitrobenzoic acid at ambient temperature. X-ray diffraction analysis characterized the unit cell parameters of the grown crystal and the crystal belongs to monoclinic system. The optical properties of the grown crystal have been studied by means of transmission measurements in the wavelength region between 200 and 1200 nm. The optical constants such as refractive index (n) and extinction coefficient (k) have been determined from the transmittance data. The optical band gap (Eg ), the real and imaginary parts of the dielectric constant of the grown crystal was determined. Second harmonic generation (SHG) efficiency of the grown crystal has been studied using Nd:YAG laser and was measured as 3.2 times that of KDP. The low dielectric constant suggests the suitability of this compound material for NLO applications. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction The development and focus on new materials possessing high nonlinear optical (NLO) properties have received much attention due to their optoelectronic applications such as high speed information processing, optical communications and optical data storage devices. Organic NLO materials offer the potential of relatively low-power laser-driven nonlinear optical system due to their high optical nonlinearities and other optical properties such as optical response time, non-resonant susceptibility and second harmonic generation (SHG) efficiency compared to inorganic materials. Another advantage of organic materials over inorganic materials is that the properties of organic compound can be easily tailored using molecular engineering and chemical synthesis [1]. Organic molecules with delocalized electron systems are of particular interest in NLO research because of their large nonlinear response [2,3]. Guanidine is a strong base and readily reacts with most of the organic acids to give product with good crystallinity, because of the presence of six potential donor sites for hydrogen bonding interaction. Thus researchers are very much interested in searching guanidine based materials with good nonlinear properties such as aminoguanidinium (1+) hydrogen ltartrate monohydrate [4], guanidinium tartrate hydrate [5]. Such a work has been carried out for zinc guanidinium sulfate [6] and reported recently from our laboratory. In this paper, we are present-
∗ Corresponding author. Tel.: +91 04426 507 586. E-mail address: [email protected] (P. Murugakoothan). 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.05.037
ing a report on the growth and optical properties of organic single crystal guanidinium 4-nitrobenzoate (GuNB) by slow evaporation technique. 2. Experimental 2.1. Synthesis and growth The starting material was synthesized by mixing aqueous solutions of high purity guanidine carbonate and 4nitrobenzoic acid in the molar ratio 0.5:1.0. The required amount of starting materials for synthesis of guanidinium 4nitrobenzoate salt was calculated according to the following reaction:C(NH2 )3 + + C6 H4 NO2 COO− → C(NH2 )3 + C6 H4 NO2 COO− The pH of the working solution was adjusted to 8.2 by the addition of few drops of standard potassium hydroxide and the pH remained constant at least for 3 h, indicating the stability of the solution. The filtered solution was allowed to dry at room temperature and the salt was obtained by slow evaporation technique. The purity of the synthesized salt was further improved by successive recrystallization in water–ethanol (1:1) solvent. Fig. 1(a) shows the optically transparent pale yellow coloured crystal obtained from solvent evaporation method after the period of 32 days. 3. Characterization studies The grown crystal of guanidinium 4-nitrobenzoate was confirmed by single crystal X-ray diffraction analysis using ENRAF NONIUS CAD-4 single crystal X-ray diffractometer. The UV–vis–NIR
T. Arumanayagam, P. Murugakoothan / Optik 123 (2012) 1153–1156
transmittance spectrum for GuNB crystal has been recorded in the wavelength range from 200 to 1200 nm to determine its optical range using Shimadzu Model 1601 spectro-photometer. The second harmonic generation efficiency of GuNB is measured by Kurtz and Perry powder technique.
Extinction coefficient, k
0.008
4. Results and discussion 4.1. Single crystal and powder X-ray diffraction studies
36
n 30
k
0.006
24 0.004 18 12
0.002
k
0
Single crystal X-ray diffraction study reveals that the GuNB crystal belongs to the monoclinic crystallographic system with non-centrosymmetric space group P21 . The unit cell parameters ˚ b = 6.64 (2) A, ˚ c = 6.94 (2) A, ˚ ˇ = 107.97◦ , obtained are a = 11.13 (2) A, and v = 512.9 (3) A˚ 3 . The obtained lattice parameter values are in good agreement with the reported literature values [7].
The optical transmittance of grown GuNB crystal was carried out between the wavelength range 200 nm and 1200 nm and the spectrum is presented in Fig. 1(b). From the spectrum it is clear that the grown crystal has high transparency (about 75%) in the entire visible region. From the transmittance data, the optical absorption coefficient (˛) was calculated using the formula, ˛=
2.303 log(1/T ) t
(1)
where T is the transmittance and t is the thickness of the sample (cm). The optical constants, reflectance (R), refractive index (n) and extinction coefficient (k) have calculated from the transmittance data. The relation between transmittance and reflectance can be written as, T=
(1 − R)2 e−˛t 1 − R2 e−2˛t
(2)
The refractive index (n) was calculated directly from the following well known Eqs. [8] and [9]; R=
(n − 1)2 + k2 (n + 1)2 + k2
(3)
where k is the extinction coefficient, which is related to the absorption coefficient by k = ˛/4. Fig. 2 shows the plot of refractive index and extinction coefficient against wavelength. It could be noticed that n and k decreases abruptly as the wavelength increases and saturates beyond the wavelength of 400 nm. The refractive index of
6
n
0.000
4.2. Linear and non linear optical studies
Refractive index, n
1154
400
600
800
1000
1200
Wavelength (nm) Fig. 2. Extinction coefficient and refractive index of GuNB crystal.
grown GuNB crystal for longer wavelength (visible region) was calculated to be 2.82. The high refractive index at lower wavelength is due to the absorption of the photon energy by the crystal. Fig. 3(a) shows the variation of absorption coefficient (˛) as a function of photon energy at room temperature. From this graph, it is evident that the absorption coefficient varies from 1.8 to 23.8 cm−1 with increasing of photon energy of 3.0–3.4 eV. In this high photon energy region, the energy dependence of absorption coefficient suggests the occurrence of direct band gap of the crystal [10]. The energy dependence of optical absorption coefficient on photon energy is in the high absorption region to obtain the detailed information about the energy band gap. The absorption coefficient (˛) and photon energy (h) can be related by [11]: ˛=
ˇ(hv − Eg ) hv
(4)
where ˇ is a constant, Eg is the optical band gap and h is the energy of incident photon. The is an index that characterizes the type of optical transition and it is theoretically equal to 2 and 1/2 for indirect and direct allowed transitions, respectively. The type of transition and the value of can be obtained from the plot ln (˛h) versus ln(h − Eg ). The value of for GuNB was calculated as 1/2 which shows the optical transition is direct one and also it is clear that optical transition is an electronic transition from the valence band to conduction band and strong electronic absorption. The graph between (˛h)2 and photon energy (h) has been plotted and shown in Fig. 3(b). Ideally the graph should be linear; the deviation from linearity at low incident photon energy can be attributed to the presence of irregularities and imperfection of the crystals. The intercept obtained by the extrapolation of the linear portion
25 120
20 -1
α (cm )
100 80
15 60 40
10
20
5
0
3.05 eV 1.0
1.5
2.0
2.5
3.0
3.5
hν (eV)
0 2.5
3.0
3.5
4.0
4.5
hν (eV)
Fig. 1. (a) As grown GuNB crystal. (b) Transmittance spectrum of GuNB crystal.
Fig. 3. (a) Plot of variation of absorption coefficient as a function of photon energy of GuNB crystal. (b) Plot of (˛h)2 versus h for GuNB crystal.
T. Arumanayagam, P. Murugakoothan / Optik 123 (2012) 1153–1156
0.012
1.105 1.100
εr
1.095
0.008
1.090
0.004
1.085 1.080
0.000 2
3
4
8
Optical conductivity (σ) x10
Real dielectric function, εr
εi
1.110
35
Imaginary dielectric function, εi
0.016
1.115
1155
5
30 25 20 15 10 5 0
Photon energy (eV)
0.5
1.0
1.5
2.0
2.5
Absorption coefficient (α)
3.0
3.5
Fig. 4. Plot of εr and εi as a function of photon energy of GuNB crystal. Fig. 5. Optical conductivity against absorption coefficient of GuNB crystal.
of the plot gives the band gap energy of the crystal. The band gap energy of the GuNB crystal was estimated as 3.05 eV. 4.3. Dielectric study The complex dielectric constant (ε = εr + εi ) characterizes the optical properties of the crystals. The real and imaginary parts of dielectrics were also determined using the following expressions [12], εr = n2 − k2
(5)
and εi = 2nk
(6)
The real and imaginary parts of the dielectric constant of the grown crystal were determined and their variation with respect to the photon energy is shown in Fig. 4. From this graph, it is clear that a both real and imaginary part of the dielectric constants increases with increase of photon energy. The value of real part of the dielectric constant is higher than the imaginary part. The low value of dielectric constant with wide band gap of GuNB crystal suggests the suitability of optoelectronic devices. The frequency dependence of dielectric reflects the fact that a material’s polarization does not respond instantaneously to an applied field. For this reason, dielectric constant is often treated as complex function of the frequency of the applied field. A perfect dielectric is a material that has no conductivity. However the grown crystals associated with low dielectric loss inhibit the propagation of electromagnetic energy which aided conductivity. The optical conductivity () of the crystal was calculated using the following relations [13,14]: ω Im(ε) = 4
(7)
where the value of Im (ε) is given by, Im(ε) =
C2 ω2 ( r )
(k˛)
(8)
where r is the relative permeability. For most crystalline materials r is very close to 1 at optical frequencies. On putting the value of Im (ε) in Eq. (7) =
˛nc 4
(9)
where c is the velocity of light. The plot between the optical conductivity against absorption coefficient was depicted in Fig. 5. The spectrum indicates that the optical conductance increases with increase of absorption coefficient.
4.4. Nonlinear optical study The second harmonic generation efficiency of GuNB was measured by Kurtz and Perry powder technique [15]. A fundamental beam of wavelength 1064 nm with a pulse duration of 10 ns and frequency repetition of 10 Hz from Q-switched Nd:YAG laser was used as the source and passed through the powder sample. The SHG behavior was confirmed from the output of the laser beam which had bright green emission ( = 532 nm) from the powder sample. The optical signal incident on photo multiplier tube was converted in to voltage output. The second harmonic signal of 35 mV was obtained for an input energy of 31 mJ/pulse, while the standard KDP crystal gives a SHG signal of 11 mV for the same input energy. It shows that the SHG efficiency of GuNB is 3.2 times that of standard NLO material, KDP. 5. Conclusion Good quality single crystal of guanidinium 4-nitrobenzoate was synthesized and those crystals were subjected to optical studies. The basic optical properties and optical constants investigated by means of transmittance spectrum of the grown crystal. The optical absorption spectra shows that the absorption process is a direct transition and band gap of the GuNB crystal were determined. The complex dielectric functions of the grown crystal were studied. The SHG efficiency of the grown crystal is 3.2 times that of KDP. These optical studies suggest that the GuNB crystal could be used in optoelectronic devices. References [1] Bosshard Ch, M. Bosch, I. Liakatas, M. Jager, P. Gunter, Springer Series in Optical Science, vol. 72, Berline, Heidelberg, New York, 2000, pp. 163–263. [2] J. Zyss, J.F. Nicoud, M. Coquillay, Chirality and hydrogen bonding in molecular crystals for phase-matched second harmonic generation: N-(4-nitrophenyl)–(l)-prolinol (NPP), J. Chem. Phys. 81 (1984) 4160– 4168. [3] M. Ravi, P. Gangopadhyay, D.N. Rao, S. Cohen, I. Aganat, T.P. Radhakrishnan, Dual influence of H-bonding on the solid-state second-harmonic generation of a chiral quinonoid compound, Chem. Mater. 10 (1998) 2371–2377. [4] Zorka Machova, Ivan Nemec, Karel Teubner, Petr Nemec, Premysl Vanek, Zdenk Micka, The crystal structure, vibrational spectra, thermal behaviour and second harmonic generation of aminoguanidinium (1+) hydrogen l-tartrate monohydrate, J. Mol. Struct. 832 (2007) 101–107. [5] W. Krumbe, S. Haussühl, R. Fröhlich, Crystal structure and physical properties of monoclinic guanidinium tartrate hydrate, Zeitschrift fur Kristallographie 187 (1989) 309–318. [6] V. Siva shanker, R. Siddeswaran, T. Bharthasarathi, P. Murugakoothan, Growth and characterization of new semiorganic nonlinear optical zinc guanidinium sulfate single crystal, J. Cryst. Growth 311 (2009) 2709–2713. [7] J.M. Adams, R.G. Pritchard, Crystal data for some guanidinium salts, J. Appl. Cryst. 8 (1975) 392–395.
1156
T. Arumanayagam, P. Murugakoothan / Optik 123 (2012) 1153–1156
[8] K. Goksen, N.M. Gasanly, H. Ozkan, Optical absorption and reflection studies of Tl4 InGa3 S8. layered single crystals, Acta Phys. Pol. A 112 (2007) 93–100. [9] R. Tintu, V.P.N. Nampoori, P. Radhakrishnan, Sheenu Thomas, Photoinduced changes in optical properties of Ga-Sb-Ge-Se glasses, Opt. Commun. 284 (2011) 222–225. [10] B.K. Periyasamy, R.S. Jebas, N. Gobalakrishnan, T. Balasubramanian, Development of NLO tunable band gap organic devices fpr optoelectronics applications, Mater. Lett. 61 (2007) 4246–4249. [11] S.H. Chaki, Seebeck coefficient and optical studies of cadmium doped CuInS2 single crystal, Acta Phys. Pol. A 116 (2009) 221–225.
[12] Fahrettin Yakuphanoglu, Hilmi Erten, Refractive index dispersion and analysis of the optical constants of an ionomer thin film, Opt. Appl. 35 (2005) 969–976. [13] A. Lucarelli, S. Lupi, P. Calvani, P. Maselli, G. De Marzi, P. Roy, N.L. Saini, A. Bianconi, T. Ito, K. Oka, Optical conductivity of the nonsuperconducting cuprate La8 Àx Srx Cu8 O20 , Phys. Rev. B 65 (2002) 054511–54521. [14] E.I. Ugwu, A.S. Olayinka, F.I. Olabode, Analysis of wave propagation in a homogeneous dielectric crystal, J. Eng. Appl. Sci. 4 (2009) 126–131. [15] S.K. Kurtz, T.T. Perry, A powder technique for theevaluation of nonlinear optical materials, J. Appl. Phys. 39 (1968) 3798–3813.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Growth, linear and nonlinear optical studies on guanidinium 4-nitrobenzoate (GuNB): An organic NLO material T. Arumanayagam, P. Murugakoothan ∗ PG and Research Department of Physics, Pachaiyappa’s College, Chennai 600030, India
a r t i c l e
i n f o
Article history: Received 4 February 2011 Accepted 15 May 2011
Keywords: Nonlinear optic materials Growth from solution Optical and dielectric constants
a b s t r a c t Single crystals of guanidinium 4-nitrobenzoate (GuNB) were grown using solvent evaporation technique by mixing aqueous solutions of guanidine carbonate and 4-nitrobenzoic acid at ambient temperature. X-ray diffraction analysis characterized the unit cell parameters of the grown crystal and the crystal belongs to monoclinic system. The optical properties of the grown crystal have been studied by means of transmission measurements in the wavelength region between 200 and 1200 nm. The optical constants such as refractive index (n) and extinction coefficient (k) have been determined from the transmittance data. The optical band gap (Eg ), the real and imaginary parts of the dielectric constant of the grown crystal was determined. Second harmonic generation (SHG) efficiency of the grown crystal has been studied using Nd:YAG laser and was measured as 3.2 times that of KDP. The low dielectric constant suggests the suitability of this compound material for NLO applications. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction The development and focus on new materials possessing high nonlinear optical (NLO) properties have received much attention due to their optoelectronic applications such as high speed information processing, optical communications and optical data storage devices. Organic NLO materials offer the potential of relatively low-power laser-driven nonlinear optical system due to their high optical nonlinearities and other optical properties such as optical response time, non-resonant susceptibility and second harmonic generation (SHG) efficiency compared to inorganic materials. Another advantage of organic materials over inorganic materials is that the properties of organic compound can be easily tailored using molecular engineering and chemical synthesis [1]. Organic molecules with delocalized electron systems are of particular interest in NLO research because of their large nonlinear response [2,3]. Guanidine is a strong base and readily reacts with most of the organic acids to give product with good crystallinity, because of the presence of six potential donor sites for hydrogen bonding interaction. Thus researchers are very much interested in searching guanidine based materials with good nonlinear properties such as aminoguanidinium (1+) hydrogen ltartrate monohydrate [4], guanidinium tartrate hydrate [5]. Such a work has been carried out for zinc guanidinium sulfate [6] and reported recently from our laboratory. In this paper, we are present-
∗ Corresponding author. Tel.: +91 04426 507 586. E-mail address: [email protected] (P. Murugakoothan). 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.05.037
ing a report on the growth and optical properties of organic single crystal guanidinium 4-nitrobenzoate (GuNB) by slow evaporation technique. 2. Experimental 2.1. Synthesis and growth The starting material was synthesized by mixing aqueous solutions of high purity guanidine carbonate and 4nitrobenzoic acid in the molar ratio 0.5:1.0. The required amount of starting materials for synthesis of guanidinium 4nitrobenzoate salt was calculated according to the following reaction:C(NH2 )3 + + C6 H4 NO2 COO− → C(NH2 )3 + C6 H4 NO2 COO− The pH of the working solution was adjusted to 8.2 by the addition of few drops of standard potassium hydroxide and the pH remained constant at least for 3 h, indicating the stability of the solution. The filtered solution was allowed to dry at room temperature and the salt was obtained by slow evaporation technique. The purity of the synthesized salt was further improved by successive recrystallization in water–ethanol (1:1) solvent. Fig. 1(a) shows the optically transparent pale yellow coloured crystal obtained from solvent evaporation method after the period of 32 days. 3. Characterization studies The grown crystal of guanidinium 4-nitrobenzoate was confirmed by single crystal X-ray diffraction analysis using ENRAF NONIUS CAD-4 single crystal X-ray diffractometer. The UV–vis–NIR
T. Arumanayagam, P. Murugakoothan / Optik 123 (2012) 1153–1156
transmittance spectrum for GuNB crystal has been recorded in the wavelength range from 200 to 1200 nm to determine its optical range using Shimadzu Model 1601 spectro-photometer. The second harmonic generation efficiency of GuNB is measured by Kurtz and Perry powder technique.
Extinction coefficient, k
0.008
4. Results and discussion 4.1. Single crystal and powder X-ray diffraction studies
36
n 30
k
0.006
24 0.004 18 12
0.002
k
0
Single crystal X-ray diffraction study reveals that the GuNB crystal belongs to the monoclinic crystallographic system with non-centrosymmetric space group P21 . The unit cell parameters ˚ b = 6.64 (2) A, ˚ c = 6.94 (2) A, ˚ ˇ = 107.97◦ , obtained are a = 11.13 (2) A, and v = 512.9 (3) A˚ 3 . The obtained lattice parameter values are in good agreement with the reported literature values [7].
The optical transmittance of grown GuNB crystal was carried out between the wavelength range 200 nm and 1200 nm and the spectrum is presented in Fig. 1(b). From the spectrum it is clear that the grown crystal has high transparency (about 75%) in the entire visible region. From the transmittance data, the optical absorption coefficient (˛) was calculated using the formula, ˛=
2.303 log(1/T ) t
(1)
where T is the transmittance and t is the thickness of the sample (cm). The optical constants, reflectance (R), refractive index (n) and extinction coefficient (k) have calculated from the transmittance data. The relation between transmittance and reflectance can be written as, T=
(1 − R)2 e−˛t 1 − R2 e−2˛t
(2)
The refractive index (n) was calculated directly from the following well known Eqs. [8] and [9]; R=
(n − 1)2 + k2 (n + 1)2 + k2
(3)
where k is the extinction coefficient, which is related to the absorption coefficient by k = ˛/4. Fig. 2 shows the plot of refractive index and extinction coefficient against wavelength. It could be noticed that n and k decreases abruptly as the wavelength increases and saturates beyond the wavelength of 400 nm. The refractive index of
6
n
0.000
4.2. Linear and non linear optical studies
Refractive index, n
1154
400
600
800
1000
1200
Wavelength (nm) Fig. 2. Extinction coefficient and refractive index of GuNB crystal.
grown GuNB crystal for longer wavelength (visible region) was calculated to be 2.82. The high refractive index at lower wavelength is due to the absorption of the photon energy by the crystal. Fig. 3(a) shows the variation of absorption coefficient (˛) as a function of photon energy at room temperature. From this graph, it is evident that the absorption coefficient varies from 1.8 to 23.8 cm−1 with increasing of photon energy of 3.0–3.4 eV. In this high photon energy region, the energy dependence of absorption coefficient suggests the occurrence of direct band gap of the crystal [10]. The energy dependence of optical absorption coefficient on photon energy is in the high absorption region to obtain the detailed information about the energy band gap. The absorption coefficient (˛) and photon energy (h) can be related by [11]: ˛=
ˇ(hv − Eg ) hv
(4)
where ˇ is a constant, Eg is the optical band gap and h is the energy of incident photon. The is an index that characterizes the type of optical transition and it is theoretically equal to 2 and 1/2 for indirect and direct allowed transitions, respectively. The type of transition and the value of can be obtained from the plot ln (˛h) versus ln(h − Eg ). The value of for GuNB was calculated as 1/2 which shows the optical transition is direct one and also it is clear that optical transition is an electronic transition from the valence band to conduction band and strong electronic absorption. The graph between (˛h)2 and photon energy (h) has been plotted and shown in Fig. 3(b). Ideally the graph should be linear; the deviation from linearity at low incident photon energy can be attributed to the presence of irregularities and imperfection of the crystals. The intercept obtained by the extrapolation of the linear portion
25 120
20 -1
α (cm )
100 80
15 60 40
10
20
5
0
3.05 eV 1.0
1.5
2.0
2.5
3.0
3.5
hν (eV)
0 2.5
3.0
3.5
4.0
4.5
hν (eV)
Fig. 1. (a) As grown GuNB crystal. (b) Transmittance spectrum of GuNB crystal.
Fig. 3. (a) Plot of variation of absorption coefficient as a function of photon energy of GuNB crystal. (b) Plot of (˛h)2 versus h for GuNB crystal.
T. Arumanayagam, P. Murugakoothan / Optik 123 (2012) 1153–1156
0.012
1.105 1.100
εr
1.095
0.008
1.090
0.004
1.085 1.080
0.000 2
3
4
8
Optical conductivity (σ) x10
Real dielectric function, εr
εi
1.110
35
Imaginary dielectric function, εi
0.016
1.115
1155
5
30 25 20 15 10 5 0
Photon energy (eV)
0.5
1.0
1.5
2.0
2.5
Absorption coefficient (α)
3.0
3.5
Fig. 4. Plot of εr and εi as a function of photon energy of GuNB crystal. Fig. 5. Optical conductivity against absorption coefficient of GuNB crystal.
of the plot gives the band gap energy of the crystal. The band gap energy of the GuNB crystal was estimated as 3.05 eV. 4.3. Dielectric study The complex dielectric constant (ε = εr + εi ) characterizes the optical properties of the crystals. The real and imaginary parts of dielectrics were also determined using the following expressions [12], εr = n2 − k2
(5)
and εi = 2nk
(6)
The real and imaginary parts of the dielectric constant of the grown crystal were determined and their variation with respect to the photon energy is shown in Fig. 4. From this graph, it is clear that a both real and imaginary part of the dielectric constants increases with increase of photon energy. The value of real part of the dielectric constant is higher than the imaginary part. The low value of dielectric constant with wide band gap of GuNB crystal suggests the suitability of optoelectronic devices. The frequency dependence of dielectric reflects the fact that a material’s polarization does not respond instantaneously to an applied field. For this reason, dielectric constant is often treated as complex function of the frequency of the applied field. A perfect dielectric is a material that has no conductivity. However the grown crystals associated with low dielectric loss inhibit the propagation of electromagnetic energy which aided conductivity. The optical conductivity () of the crystal was calculated using the following relations [13,14]: ω Im(ε) = 4
(7)
where the value of Im (ε) is given by, Im(ε) =
C2 ω2 ( r )
(k˛)
(8)
where r is the relative permeability. For most crystalline materials r is very close to 1 at optical frequencies. On putting the value of Im (ε) in Eq. (7) =
˛nc 4
(9)
where c is the velocity of light. The plot between the optical conductivity against absorption coefficient was depicted in Fig. 5. The spectrum indicates that the optical conductance increases with increase of absorption coefficient.
4.4. Nonlinear optical study The second harmonic generation efficiency of GuNB was measured by Kurtz and Perry powder technique [15]. A fundamental beam of wavelength 1064 nm with a pulse duration of 10 ns and frequency repetition of 10 Hz from Q-switched Nd:YAG laser was used as the source and passed through the powder sample. The SHG behavior was confirmed from the output of the laser beam which had bright green emission ( = 532 nm) from the powder sample. The optical signal incident on photo multiplier tube was converted in to voltage output. The second harmonic signal of 35 mV was obtained for an input energy of 31 mJ/pulse, while the standard KDP crystal gives a SHG signal of 11 mV for the same input energy. It shows that the SHG efficiency of GuNB is 3.2 times that of standard NLO material, KDP. 5. Conclusion Good quality single crystal of guanidinium 4-nitrobenzoate was synthesized and those crystals were subjected to optical studies. The basic optical properties and optical constants investigated by means of transmittance spectrum of the grown crystal. The optical absorption spectra shows that the absorption process is a direct transition and band gap of the GuNB crystal were determined. The complex dielectric functions of the grown crystal were studied. The SHG efficiency of the grown crystal is 3.2 times that of KDP. These optical studies suggest that the GuNB crystal could be used in optoelectronic devices. References [1] Bosshard Ch, M. Bosch, I. Liakatas, M. Jager, P. Gunter, Springer Series in Optical Science, vol. 72, Berline, Heidelberg, New York, 2000, pp. 163–263. [2] J. Zyss, J.F. Nicoud, M. Coquillay, Chirality and hydrogen bonding in molecular crystals for phase-matched second harmonic generation: N-(4-nitrophenyl)–(l)-prolinol (NPP), J. Chem. Phys. 81 (1984) 4160– 4168. [3] M. Ravi, P. Gangopadhyay, D.N. Rao, S. Cohen, I. Aganat, T.P. Radhakrishnan, Dual influence of H-bonding on the solid-state second-harmonic generation of a chiral quinonoid compound, Chem. Mater. 10 (1998) 2371–2377. [4] Zorka Machova, Ivan Nemec, Karel Teubner, Petr Nemec, Premysl Vanek, Zdenk Micka, The crystal structure, vibrational spectra, thermal behaviour and second harmonic generation of aminoguanidinium (1+) hydrogen l-tartrate monohydrate, J. Mol. Struct. 832 (2007) 101–107. [5] W. Krumbe, S. Haussühl, R. Fröhlich, Crystal structure and physical properties of monoclinic guanidinium tartrate hydrate, Zeitschrift fur Kristallographie 187 (1989) 309–318. [6] V. Siva shanker, R. Siddeswaran, T. Bharthasarathi, P. Murugakoothan, Growth and characterization of new semiorganic nonlinear optical zinc guanidinium sulfate single crystal, J. Cryst. Growth 311 (2009) 2709–2713. [7] J.M. Adams, R.G. Pritchard, Crystal data for some guanidinium salts, J. Appl. Cryst. 8 (1975) 392–395.
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