Third order nonlinear optical, luminescence and electrical properties of bis glycine hydrobromide single crystals

Optical Materials 36 (2014) 945–949

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Third order nonlinear optical, luminescence and electrical properties of bis glycine hydrobromide single crystals R. Surekha a,⇑, P. Sagayaraj b, K. Ambujam c a

Department of Physics, Prathyusha Institute of Technology and Management, Tiruvallur Dist. 602 025, India Department of Physics, Loyola College, Chennai 600 034, India c Queen Mary’s College, Chennai 600 004, India b

a r t i c l e

i n f o

Article history: Received 11 June 2013 Received in revised form 20 December 2013 Accepted 26 December 2013 Available online 20 January 2014 Keywords: Crystal growth Electrical properties Luminescence Negative photoconductivity Z-scan

a b s t r a c t Optical quality bis glycine hydrobromide (BGHB) single crystal was grown by slow evaporation technique. The third order nonlinear refractive index and nonlinear absorption coefficient of the grown crystal were measured by Z-scan studies. The third order nonlinear susceptibility was found to be 9.612  104 esu which is fairly higher than the other glycine compounds. The Photoluminescence spectra reveal the emission bands for BGHB crystals. The band gap energy was calculated to be 3.1 eV. The Photoconductivity studies were employed to determine the dependence of photocurrent on the applied electric field. Negative photoconductivity was exhibited by the sample. The d.c. conductivity of the grown crystal was measured by the complex impedance analysis wherein the obtained plot in the form of semicircle finds application in Debye relaxation for materials having large dc conductivity. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, there has been considerable interest on organic materials due to their potential applications such as frequency conversion, optical signal processing, light modulation, optical switching and logic gates. Though organic crystals are highly advantageous due to their structural flexibility and large NLO coefficients, still they possess some intrinsic weaknesses like physicochemical strength, narrow transparency window, small size and low mechanical strength. To overcome these difficulties, semi-organic crystals have been focused which combine the advantages of both organic and inorganic complexes. The incorporation of inorganic salts or acids in organic compounds forms semi-organic complexes whose mechanical stability and thermal properties get enhanced. Basically, the semi-organic group of crystals involving organic–inorganic salts [1] and metal–organic coordination complexes [2] were extensively researched for the past two decades. Combining the high optical nonlinearity and chemical flexibility of organics with temporal and thermal stability and excellent transmittance of inorganics, semi organic materials have been proposed and are attracting a great deal of attention in the nonlinear optical field [3]. Among the semi organic crystals, the crystal structures of amino acids and their complexes have provided a wealth of interesting information on the patterns of their aggregation ⇑ Corresponding author. Tel.: +91 9840979940; fax: +91 44 37673767. E-mail address: [email protected] (R. Surekha). 0925-3467/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.12.042

and the effect of other molecules and ions on their interactions and molecular properties [4]. Of all the existing amino acid organic crystals, glycine plays a remarkable role. Glycine is hydrogen bonded simplest amino acid, optically inactive, dipolar exhibiting high melting point. It is the most extensively studied amino acid as it is known to form innumerable complexes with metals, inorganic salts and inorganic acids. It has secured vast attention in the field of crystal growth due to its interesting optical and electrical properties, structural phase transition and easy crystallization. Reports say that using glycine as a dopant, even the second harmonic generation of a semi-organic material was enhanced [5]. When glycine reacts with HBr, either glycine hydrobromide (GHB) or bis glycine hydrobromide (BGHB) may be formed depending on the ratio of the reactants taken and its growth environment. The crystal structures and their bulk crystal growth are already reported in the literature [6–9]. Recently in one of the papers, the BGHB growth process, crystal parameters, FTIR, UV–Vis spectral studies, dielectric, thermal and mechanical properties were thoroughly discussed [10]. However, to the best of author’s knowledge, there is no report available on the third order nonlinear properties of the BGHB crystal which is presented here for the first time. This paper brings out photoluminescence, photoconductive, complex impedance analysis of the crystal along with its z-scan studies that reveals the high-speed switching operations which are ideally faster than even the conventional electronics.

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2. Materials processing 2.1. Synthesis The BGHB crystals were synthesized and recrystallized as per the method reported in literature [10]. The analytical grade glycine and hydrobromic acid were taken in the ratio of 2:1 and dissolved in the mixed solvent of water: acetone (in the ratio 1:1). The resulting solution was completely stirred for about 6 h to obtain a homogenous mixture after which it was filtered using Whatmann’s filter paper and poured into a beaker with perforated lid. To initiate slow evaporation, the beaker along with the contents was suspended in a Constant Temperature Bath at a temperature of 40 °C. After a period of 50–55 days, non-hygroscopic bulk transparent crystals were harvested. The yielded crystal was confirmed to be BGHB by X-ray diffraction studies [8]. It crystallizes in the non-centrosymmetric orthorhombic crystal system having space group P212121. The composition of the synthesized material is established as C4H11N2O4Br. From the single crystal structural study, the structural formula of the compound is affirmed as:

According to this structure, it can be implied that molecule II  occurs as a zwitter ion of the type NHþ 3 CH2 COO , while molecule I has the normal glycine configuration. This molecule formally receives an additional H+ ion from HBr and these H+ acts as an ionic connection between molecule I and a pair of Br ions, thus making the formula structure of the molecule II as given below:

The carboxyl acid group is a proton donor and the amino group is a proton acceptor. In a glycine molecule, the carboxylic acid group donates its proton to the amino group to form molecule II  namely (NHþ 3 CH2 COO ). This arrangement fulfils the rule of maximum hydrogen bonding, since all six hydrogens belonging to the two glycine molecules are engaged in hydrogen bond formation. The last hydrogen, which is formally part of HBr, connects one molecule with two bromines but not as a hydrogen bond in the usual meaning. The two-carboxyl groups behave quite differently. In molecule I, it exists as a carboxylic acid group, whereas it is present as a carboxylate ion in molecule II.

3. Results and discussions 3.1. Z-scan analysis The z-scan technique is a versatile standard tool in the study of nonlinear optics which evaluates the third order nonlinearity and determines the changes in nonlinear refractive index and variation in absorption. The third order optical nonlinearity was investigated by the Z-scan technique which enables simultaneous measurement of magnitude and sign of the nonlinear refractive index (n2) and the nonlinear absorption coefficient (b) of the sample leading

to different properties like acoustically induced optical Kerr effect, self-defocusing effect, etc. The study of acoustically induced optical Kerr effect in NLO materials leads to design of variety of acoustically-operated quantum electronic devices [11]. The z-scan is a single beam technique developed by Sheik Bahae to measure the magnitude of nonlinear absorption as well as the sign and magnitude of nonlinear refraction [12]. Basically, the method consists of translating a sample through the focus of a Gaussian beam and monitoring the changes in the far field intensity pattern as shown in Fig 1. When the intensity of the incident laser beam is sufficient to induce nonlinearity in the sample, it either converges (self-focusing) or diverges (self-defocusing) the beam, depending on the nature of that nonlinearity. By moving the sample through the focus, the intensity dependent absorption is measured as a change of transmittance through the sample (open aperture). The nonlinear refraction is determined by the intensity variation at the plane of a finite aperture placed in front of the detector (closed aperture), because the sample itself acts as a thin lens with varying focal length as it moves through the focal plane. The Z-scan experiments were performed using a 532 nm diode pumped Nd:YAG laser beam (Coherent Compass TM 215M-50), which was focused by 3.5 cm focal length lens as depicted in Fig 1. The distance between the lens and the laser was 13 cm. The BGHB crystal is translated across the focal region along the axial direction that is the direction of the propagation of laser beam. The transmission of the beam through an aperture placed in the far field was measured using photo detector fed to the digital power meter (Field master GS-coherent). For an open aperture Z-scan, a lens to collect the entire laser beam transmitted through the sample replaced the aperture. The Z-scan theory is of vital significance in the field of nonlinear optics as it is an optimum tool for separately determining the nonlinear changes in index and absorption. A spatial distribution of the temperature in the crystal surface is produced due to the localized absorption of a tightly focused beam propagating through the absorbing sample. Hence a spatial variation of the refractive index is produced which acts as a thermal lens resulting in the phase distortion of the propagating beam [13]. The difference between the peak and valley transmission (DTpv) is written in terms of the on axis phase shift at the focus as,

DT pv ¼ 0:406ð1  SÞ0:25 jDUj;

ð1Þ

where S is the aperture linear transmittance and is calculated using the relation,

S ¼ 1  expð2r2a =x2a Þ;

ð2Þ

where ra is the aperture and xa is the beam radius at the aperture. The nonlinear refractive index is given by,

n2 ¼ DU=KI0 Leff ;

ð3Þ

where k = 2p/k (k is the laser wavelength), I0 is the intensity of the laser beam at the focus (Z = 0), Leff = [1exp(aL)]/a is the effective thickness of the sample, a is the absorption and L is the thickness of the sample. From the open aperture Z-scan data, the nonlinear absorption coefficient is estimated as,

pffiffiffi b ¼ 2 2DT=I0 Leff ;

ð4Þ

where DT is the one valley value at the open aperture Z-scan curve. The value of b will be negative for saturable absorption and positive for two photon absorption. The real and imaginary parts of the third order nonlinear optical susceptibility v(3) are defined as,

Re vð3Þ ðesuÞ ¼ 104 ðe0 C 2 n20 n2 Þ=p ðcm2 =WÞ;

ð5Þ

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Fig. 1. Z-scan experimental technique.

Im vð3Þ ðesuÞ ¼ 102 ðe0 C 2 n20 kbÞ=4p2

ðcm2 =WÞ;

2.2

where e0 is the vacuum permittivity, n0 is the linear refractive index of the sample and c is the velocity of light in vacuum. This analysis was carried out by a Z-scan setup with 532 nm laser pulses operating at a 1 kHz repetition rate using a sample thickness of 2 mm as shown in Fig. 1.

2.1

3.1.1. Open aperture z-scan The open aperture z-scan for the grown crystal is measured as shown in the Fig. 2. The open aperture pattern of the curve shows that the nonlinear absorption is multi-photon absorption. For 532 nm resonant absorption, both excited state absorption and two-photon absorption can be responsible for the observed NLO effects. Multi-photon absorption suppresses the peak and enhances the valley, while saturation produces the opposite effect [14,15]. The nonlinear absorption coefficient b is calculated to be 1.356  103 cm/W which may be due to the large number of delocalized p electrons resulting from the protonation of amino group in the crystal lattice [16]. This delocalization enhances the polarizability and the nonlinear susceptibility and leads to large third order NLO properties [17]. 3.1.2. Closed aperture z-scan The Closed Aperture z-scan for the grown crystal reveals the peak to valley configuration of the curve as in Fig. 3. This characteristic pattern suggests that the refractive index change is negative implying the self-defocusing effect of the sample [18]. This may be an advantage for the application in production of optical sensors such as night vision devices [19]. The measured z-scan parameters are enumerated as shown in Table 1 which suggests that the compound may be used for the

Normalized Transmittance

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 -10

-5

0

5

10

Z (mm) Fig. 2. Open aperture z-scan curve for the grown BGHB crystal.

Normalized Transmittance

ð6Þ

2.0 1.9 1.8 1.7 1.6 1.5 1.4

-8

-6

-4

-2

0

2

4

6

Z (mm) Fig. 3. Closed aperture z-scan curve for BGHB crystal.

optical device applications such as optical limiters, optical modulators. Table 2 depicts the comparison of z-scan parameters with other glycine related compounds.

3.2. Photoluminescence The Photoluminescence analysis is an indigenous non-destructive technique for probing the electronic structure of materials and in assessing the transparency of the grown crystal. The photoluminescence spectrum was recorded for the as-grown inclusion free single crystal of BGHB at room temperature with an excitation wavelength of 316 nm using Bruker S4 Pioneer X-ray Fluorescence Spectrophotometer. The Photoluminescence spectra not only explains the transition mechanisms of ions but also it is used to study the crystal defects as reported in the paper [20] where PL spectra reveals features related to anion vacancy and cation antisite defect in a nonlinear optical crystal LiGaS2. The PL spectrum as shown in the Fig. 4 shows a broad peak centered at 400 nm with intensity comparable to that of the conducting polymer composites. This may be due to the protonation of amino group to the carboxyl group. The absorption of a photon leads to the formation of an exciton. There are four absorption peaks found in the region 320 nm to 370 nm which leads to the emission of K-violet at 400 nm. There is also another sharp absorption peak observed at 410 nm which may be due to the affinity be tween NHþ 3 and Br ions present in the grown crystal. The reason for enhanced PL emission in the case of BGHB would be the pres ence of donor NHþ 3 and the electron withdrawing group COO that enhances the mobility of electrons. The maximum intensity peak at 400 nm due to the lowering of PL intensity at higher wavelength

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Table 1 Measurement details and results of the Z-scan technique. Parameter

Measured values for BGHB crystal

Laser beam k Lens focal length (f) Optical path distance Spot size dia in front of the aperture Aperture radius Incident intensity at the focus (z = 0) Linear absorption coefficient Nonlinear refractive index (n2) Nonlinear absorption coefficient (b) Real part of the third order susceptibility (Re (v3)) Imaginary part of the third order susceptibility (Im (v3)) Third order nonlinear optical susceptibility (v3)

532.9 nm 10 cm 12.5 cm 1 cm 4 mm 508.95 W/cm2 0.010004 cm/W 1.969  1012cm2/W 1.356  103 cm/W 9.226  1010 esu 2.697  107 esu 9.612  104 esu

Table 2 Comparison of z-scan parameters for BGHB and other glycine related compounds. Z-scan parameters n2 b

v(3)

a-Glycine [24]

BGHBr 12

1.969  10 cm2/W 1.356  103 cm2/W 9.612  104 esu

8

2.14435  10 cm2/W 2.4763  105 cm2/W 8.018  108 esu

c-Glycine [25] 6

4.44  10 cm2/W 0.0527 cm2/W 2.863  104 esu

Fig. 4. PL spectra of the grown BGHB crystal.

to the different applied voltage. Then sample was illuminated by the radiation from 100 W halogen lamp containing iodine vapour and tungsten filament and the corresponding photocurrent is recorded for the same values of the applied voltage. The field dependent photoconductivity of the crystal is shown in Fig. 5. The photocurrent is found to be less than that of the dark current for all ranges of applied field indicating negative photoconductivity. This arises due to the decrease of concentration of majority electrons making the conductivity lower than original dark conductivity. The light source used excites only the electrons to the next higher level creating free holes to recombine with free electrons. This is why photoconductivity under such light excitation is reduced and called negative photoconductivity Torres et al. have evaluated the contribution of photoconductivity and the absorptive response for optical excitations with 50 ps pulse duration and could control the manifestation of these different effects in his research paper [22]. 3.4. Complex impedance analysis The ac conductivity studies using complex impedance spectroscopy is performed to calculate the bulk resistance of the crystalline sample. The alternative to dc conductivity measurements is to use ac methods and make measurements over a wide range of frequencies. The dc conductivity values can usually be extracted from the ac conductivity data and it is possible to obtain information about electrode capacitance, grain boundary resistances and capacitances and the amount of electronic conductivity present within the specimen. Since the grown crystal is nonlinear optical natured, the electrical conductivity alone can be determined from Cole–Cole plot which establishes the electrical property of the grown material. The dc conductivity of the grown crystal was experimented by grinding the specimen thoroughly and pressed together with electrodes on both the faces under a pelletizing pressure of 5000 kg cm2 to make circular pellets of 8 mm diameter. The electrodes consisting of metallic silver powder mixed with the sample in the weight ratio 2:1 have been used. Complex impedance analysis has been carried out on all the pellet specimens using a Hewlett Packard model HP4284A Precision LCR Meter in the frequency range 20 Hz1 MHz by having the pellets held in between two silver electrodes. A Chromel–Alumel thermocouple has been employed to record the sample temperature. The bulk conductivity (r) of the given sample may be expressed as

r ¼ t=A  Rb ;

region may be attributed to a relatively low barrier for rotation of the carboxyl group around the central C–C bond. Moreover absence of visible emission bands indicates the crystalline purity of BGHB crystals [21]. The calculated band gap energy from PL spectra was found to be 3.105 eV. 3.3. Photoconductivity Photoconductivity forms the basis of photo resistors, vidicons, memories and other devices. Photo conducting materials should be capable of transforming the low energy content of sub picosecond laser pulses to electric pulses of relatively high amplitude. This study was carried out by employing a polished portion of the grown crystal fixed onto a microscopic slide. Two electrodes of thin copper wire of 0.3 mm thickness were fixed using silver paint and the sample was connected in series with a DC power supply and KEITHLEY 485 picoammeter. The sample was covered with a black cloth and the dark current of the crystal was recorded with respect

Fig. 5. Plot of applied voltage dependent photoconductivity of the BGHB crystal.

R. Surekha et al. / Optical Materials 36 (2014) 945–949

associated with photoconductivity. The negative photoconductivity contributes to the absorptive nonlinearity of the sample. The d.c conductivity of the sample is calculated from the Cole–Cole plot which is appreciably fair when compared to other amino acid crystals.

250000 200000

Z''

949

150000

Acknowledgements 100000 50000 0 0

100000 200000 300000 400000 500000 600000 700000

The author is thankful to Dr. M. Basheer Ahmed, Prof & Head, Dept. of Physics, B.S. Abdur Rahman University, Chennai for the z-scan studies in his lab. Further the authors are grateful to the Sophisticated Analytical Instrumentation Facility Center, IIT Madras, for the single crystal X-ray diffraction and photoluminescence studies in their laboratory.

Z' Fig. 6. Impedance spectrum of the grown BGHB crystal.

where t is the thickness of the pellet, A is the area of cross section and Rb is the resistance of the sample. The value of Rb was found from the Cole–Cole plots shown in the Fig. 6. Here Z00 represents Z sin h (imaginary part of impedance) and Z0 as Z cos h (real part of impedance). The DC electrical conductivity was found to be 2.7072  106 mho/m from the Cole–Cole plot. The low value of electrical conductivity in this crystal is due to the decrease in mobility of the charge carriers by ion size which brings prominent changes in the electronic band structure [23]. The obtained plot in the form of semicircle finds application in Debye relaxation for materials having large dc conductivity. The magnitude of relaxation time of charge carriers can also be evaluated. If the radius of the semicircle is larger, greater will be the relaxation time. 4. Conclusion Optically transparent BGHB crystals were grown by slow evaporation technique. Third order nonlinear optical studies showed that the BGHB crystal has defocusing nature with nonlinear refractive index and nonlinear absorption coefficient. The nonlinear absorption coefficient b is positive which implies the two photon absorption and the value is negative for saturable absorption. The open and closed aperture z-scan traces yield the pure nonlinear refraction curve, with peak-valley configuration showing negative nonlinear refractive index. The Photoluminescence characteristic provides information of different energy states available between the valence band and the conduction band that is responsible for radiative recombination. The nonlinear absorptive effect is

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