Bulk Crystal Growth, Optical, Electrical, Thermal, and Third Order NLO

Sep 21, 2017 - *(S.K.) E-mail: [email protected]. ... Bulk organic nonlinear optical (NLO) single crystals of DEBM ... Citation data is made avail...
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Bulk Crystal Growth, Optical, Electrical, Thermal and Third Order NLO Properties of 2-[4(Diethylamino)benzylidene]malononitrile (DEBM) Single Crystal Priyadharshini Asokan, and Sivaperuman Kalainathan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07805 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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Bulk Crystal Growth, Optical, Electrical, Thermal and Third Order NLO Properties of 2-[4-(Diethylamino)benzylidene]malononitrile (DEBM) Single Crystal Priyadharshini Asokana, Sivaperuman Kalainathan a* a, a*

Centre for Crystal Growth, VIT University, Vellore –632 014, India. Corresponding author: E-mail Id: [email protected]

ABSTRACT Bulk

organic

nonlinear

optical

(NLO)

single

crystal

of

DEBM

(2-[4-(Diethylamino)benzylidene]malononitrile), with the size up to 22×5×12 mm3 has been grown in ethyl acetate solution by slow evaporation method. The lattice parameters and crystalline purity of the titled crystal were measured by single crystal X-ray diffraction and powder XRD, respectively. The morphology of the grown crystal was portrayed by WinXMorph program. Fourier transform infrared (FT-IR) spectral studies have been performed to identify the functional groups. The molecular structure was confirmed through 1H-NMR. UV-Vis-NIR spectral analysis signifies that the DEBM crystal is transparent in entire visible and near infra-red region, which has a lower cut-off wavelength around 476.8 nm. The optical band gap (Eg) was estimated from the Tauc plot and was found to be about 2.68 eV. The low value of the dielectric constant (εr) of DEBM suggests that it has a possible strategy towards the microelectronics industry. Thermal stability and melting point (137.8 ºC) were studied with TGA-DSC analysis. The laser-induced damage threshold (LDT) experiments showed that the grown DEBM bulk crystal possess an excellent resistance to laser radiation with a high threshold upto 1.95 GW cm-2, it is still better compared to other organic and inorganic NLO materials. The third - order optical nonlinearities of the DEBM crystal was confirmed by single beam Z-scan

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technique. All these results demonstrate that the organic crystal DEBM is promising for NLO applications. 1. INTRODUCTION The nonlinear optical (NLO) materials have been continuously attracting all over the world due to their relative nonlinearity properties, and wide spread use in a telecommunications system, optical computing, optical information processes, optical switching, optical signal processing, optoelectronics, photonics, laser science and technology, etc.

1–8

Among all NLO

crystals, the organic crystals have aroused much interest because of their higher molecular nonlinearities, color display, ultra rapid optical response, higher optical damage thresholds and structural diversity, rather than the inorganic materials.9 More specifically, organic charge transfer (OCT) materials have paid highly reasonable scientific insight in nonlinear optics,10 light-emitting diodes (LED),11 fluorescent dyes,12 and optoelectronic device (OED) fabrications.13 At present, design and synthesis of donor–π–acceptor (D–π–A) chromophore type of organic molecules are the subjects of focused attention, mainly because of the high optical nonlinearities, due to the delocalized π- electronic cloud.14–16 Notably, benzylidene malononitriles derivatives fulfil this circumstance. 17–20 In this series, π-conjugated molecules act as a bridge mediator positioned at the centre, and the corresponding electron donor moiety (D) and an electron acceptor moiety (A) are located in both sides. It is the interplay between the donor and the acceptor groups create a chromophore based on the intramolecular charge transfer (ICT) in the crystal system, which significantly enhanced NLO responses.21 Based on this criteria

we

choose

the

new

classical

type

of

chromophore

material

2-[4-(Diethylamino)benzylidene]malononitrile (DEBM). It represents a group of disubstituted

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benzenes of the type D-π-A. However, as it has been seen in the position and type of the electron donors and acceptors are the key features for the fruitful development of the NLO response.22 In this respect, DEBM consists of a benzene ring, in which a donor diethylamino group is substituted with one para position and an acceptor nitrile group is substituted directly across the ring in the other para position and forms a centric crystal (centrosymmetric crystal, space group P21/n). This strategy formally adopts the Third Harmonic Generation (THG) signal to express its NLO response. It is the class of organic material with NLO chromophore centrosymmetric system which enhances the large second-order hyperpolarizability (γ).23,24 considering the above factor DEBM material is likely to be a good candidate for NLO materials. In

this

paper,

we

report

the

bulk

single

crystal

growth

of

DEBM

(2-[4-(Diethylamino)benzylidene]malononitrile) by slow evaporation method. According to the keen literature survey, no reports have been found related to bulk crystal growth, optical, electrical, thermal and third-order nonlinear optical (TONLO) properties of this material to till date; however, only the synthesis and structure of DEBM has been reported earlier.25 The grown single crystal was structurally characterized by 1H -NMR and Mass spectral analysis. 2. EXPERIMENTAL SECTIONS 2.1 Reagents 4-(diethylamino) benzaldehyde, malononitrile, ethanol, piperidine, diethyl ether, and ethyl acetate were purchased from commercial sources and used without further purification. 2.2 Material synthesis and crystal growth The title material of 2-[4-(Diethylamino)benzylidene]malononitrile (DEBM) was synthesized by Knoevenagel condensation reaction method by using 3 ACS Paragon Plus Environment

4-(diethylamino)

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benzaldehyde and malononitrile stoichiometric ratio (1:1) and dissolved in ethanol (25 ml). The above mixture solution was taken in the 100 ml round bottom flask (RBF), and 5 drops piperidine was added as a catalyst. The resulting reaction mixture was refluxed at 65 ºC for 2h. The reaction mixture was cooled to ambient temperature. Reddish salt was collected by filtration, washed with diethyl ether several times and then dried in an oven. The chemical reaction is shown in Scheme 1.

Scheme 1. Synthesis of DEBM. The dried product of DEBM was dissolved in ethyl acetate at 35 ºC to form a saturated solution for 12 h. The resulting solution was filtered into a 150 ml beaker and placed in a constant temperature water bath (CTB accuracy of ± 0.01 ºC) at 35 ºC for slow evaporation. After that, good quality single crystals were harvested with approximate dimensions 22×5×12 mm3 after 28 days. The grown DEBM crystal is shown in Figure 1 (a). It is worthy to mention that as-grown DEBM single crystal is non–hygroscopic and chemically stable in the environment. The typical morphology of a grown crystal with its corresponding faces along the axis shown in Figure 1(b).

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Figure 1. (a) A photograph of the bulk DEBM crystal. (b) Morphology of the grown crystal obtained using the WinXMorph software. 2.3 Instrumentation In the present study, a suitable size DEBM single crystal was selected for the X-ray diffraction analysis by using Enraf-Nonius CAD-4 diffractometer with MoKα (λ = 0.71073 Å) radiation to determine the cell parameter. Powder XRD analysis was made on the finely crushed powder of DEBM using BRUKER X-ray diffractometer with CuKα (λ=1.5406 Å) radiation, the step size of 0.02°S-1 over a 2θ range 10° to 40° and the identical diffraction peak are indexed with the help of Powder- X software package. The morphology of the grown crystal with the plane was solved by WinXMorph software. The FT-IR spectrum of the DEBM sample was scanned at the middle infrared region of 400-4000 cm-1 using a SHIMADZU IRAFFINITY spectrometer. Furthermore, the formation of the molecular structure of DEBM was identified with a BRUKER-400 MHz FT-NMR spectrometer using DMSO-d6 as a solvent. The mass spectrum of DEBM was recorded using a JEOL GCMATE II GC–MS double focusing mass spectrometer. For the optical studies, the absorption spectrum was recorded on a Varian Carry ELICO SL 218 double beam UV–Vis–NIR spectrometer in the spectral region of 200–1000 nm. 5 ACS Paragon Plus Environment

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The photoluminescence emission spectrum of the DEBM was recorded in the spectral region of 460–660 nm using F-7000 FL spectrophotometer instrument. The dielectric measurement of DEBM crystal have been carried out in the temperature range of 308 -338 K as a frequency (50 Hz – 5 MHz) using a HIOKI 3532-50 LCR HITESTER meter. Thermal analysis (TG-DSC) of DEBM were carried out using a SDT Q600 V20.9 Build 20 instrument. The laser-induced damage threshold of DEBM crystal was recorded using Nd : YAG laser radiation (λ = 1064 nm) as a source. The microstructure feature of DEBM crystal was analyzed through the etching study using the Carl Zeiss optical microscope (50× magnification). The third order NLO properties have been investigated by Z-scan measurement employing the continuous He - Ne laser operated at 632.8 nm wavelength. 3. Results and discussions 3.1 Single crystal and powder X-ray diffraction analysis From the single crystal XRD measurement, we found that the grown DEBM crystal belongs to a monoclinic system and has a centrosymmetric nature with the space group of P21/n. The calculated lattice parameters are a = 9.358 (5) Å, b = 9.371(6) Å, c = 14.671 (8) Å, α = γ = 90 °, β = 98.98 (7) °, and the unit cell volume V = 1298 (2) Å3. The observed values are consistent with the reported values (CCDC-828698).25 In order to check the phase purity of these compounds, the powder X-ray diffraction (PXRD) patterns of DEBM compound were checked at room temperature. The simulated and experimental PXRD patterns of DEBM are shown in Figure 2. The peaks of the measured diffraction position are in good agreement with those of the simulated diffraction position generated from single crystal XRD data. The high intensity well defined sharp peaks at specific 2θ angle reveal the high crystalline perfection and phase purity of the DEBM crystal. The 6 ACS Paragon Plus Environment

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patterns are shown that there is no solvent incorporation in ethyl acetate-grown DEBM single crystal.

Figure 2. Powder X-ray diffraction patterns of DEBM crystal. 3.2 FT-IR spectral analysis Fourier transform infrared spectroscopy is prime importance factor for understanding the role of multifarious functional groups, chemical bonding and molecular structure of the titular crystal. The FT-IR spectrum was taken for the powdered sample of the DEBM is presented in Figure 3. The peak observed at 3082 cm-1 is due to the aromatic C-H stretching vibrations. The peak at 2978 cm-1 belongs to the alkyl C-H stretch. The well-defined sharp peaks at 2206 cm-1 pertaining to the C≡N stretch modes of nitrile group. The peak at 1512, 1419 and 1345 cm-1 are

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due to aromatic C=C stretching vibrations. The aromatic C-N stretching vibration brings forth a sharp peak at 1276 cm−1. The aromatic C–H in-plane bending modes were observed at 1072 and 1008 cm−1. Moreover, the absence of aldehyde band at 2720 cm-1 confirms that there is the formation of a new product without aldehyde group. All observed spectrum confirm the formation of DEBM single crystal.

Figure 3. FT-IR spectrum of DEBM 3.3 1H-NMR and Mass spectral analysis NMR spectroscopy plays a key role for the confirmation of structure of the DEBM material and it provides the paramount information about the number of signals in the spectrum, and also it traces the way and position of the hydrogen atoms in the molecule. The recorded 1H-NMR spectrum of the grown crystal is shown in Figure 4. There is an intense triplet

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peak observed at 1.151 ppm owes to the methyl protons (CH3)2 and the sharp quartet peak observed at 3.494 ppm are strongly correspond to the ethyl protons (CH2)2 of the 2-[4-(Diethylamino) benzylidene] moiety. The doublet occurring at 6.842 and 7.825 ppm confirm the presence of the aromatic protons. Singlet peak observed at 7.996 ppm is strongly assignable to olefin-hydrogen bond (C–H). In short, there is no confirmative peak between 10 ppm and 12 ppm, due to this the absence of the aldehyde clearly supports the formation of a new compound. The mass spectrum of the title crystal was measured, as shown in Figure 5. The crushed sample of DEBM has been used for this analysis. The mass to charge ratio (m/z) of the sample was scanned, and the observed molecular weight 225.14 amu is consistent with theoretically calculated value 225.29 amu. It should be noted that there is no significant deviation of the traced molecular weight of the crystalline compound, which impact the absence of fractional impurities in the DEBM crystal. The spectrum showed the molecular ion peak at m/z = 225.14 (C14H15N3), and the other peaks may represent charged fragments of this ion.

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/

Figure 4. 1H NMR spectrum of DEBM.

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Figure 5. Mass spectrum of DEBM.

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3.4 UV-Vis-NIR spectral analysis The absorption and transmittance studies help to identify the potential NLO material. In fact, a crystal which has limited absorption and large optical transparency in the possible electromagnetic region are highly useful for all optical related application. Figure 6(a) shows the recorded UV-Visible absorption spectrum for the 1.72 mm thickness of the DEBM crystal. The lower cut-off wavelength in the case of DEBM sample is around 476.82 nm. The transmittance spectrum (Figure 6 (b)) reveals that DEBM crystal has a large transmission window from 490–1000 nm and a transmittance ratio (up to 66 %). There are no characteristic absorptions observed in the region between 490 and 1000 nm which justifies that the DEBM crystal is suitable for photonic and optical applications. Moreover, the crystallinity of the grown crystal was easily concluded from the tail26 of the Figure 6 (b). Further, the optical band gap (Eg) of the grown crystal was calculated according toTauc relation.27 The inset of Figure 6 (c) shows the plot of (αhν)2 vs. photon energy (hυ) and calculated value of the band gap is 2.68 eV.

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Figure 6. (a) Room temperature optical absorption spectrum of DEBM crystal and its corresponding lower cut-off wavelength (λ) is 476.82 nm (b) Optical transmission spectrum of DEBM single crystal contribute the 66 % of transmittance in the Vis-NIR region (c) Plot of (αhυ)2 versus photon energy (hυ). 3.5 Dielectric measurement The dielectric measurement offers an opportunity to understand the electric properties of the grown crystal. Despite the dielectric behaviour has an interesting feature in almost every field such as optics, solid state, electronics and photonic device fabrication.28 As per the concern over the electric and magnetic energy storage and dissipation of any material under the influence of dielectric behaviour is a necessary task. Ideally, the dielectric properties are directly interconnected with the electric field distribution with in the solid materials. For doing the 14 ACS Paragon Plus Environment

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dielectric measurement, we selected defect free DEBM single crystal (2 mm thickness), and the high-grade silver paste applied to its either side surfaces, for to get a good conductive on the surface layers and to make parallel electrodes. The dielectric constant (εr) of the DEBM crystal was calculated from widely used ideal relation, εr = Ct/Aεo, C is the capacitance (in Farad), t is the thickness of the crystal (in millimeter), A is the area of the cross section of the crystal (in cm) and εo is absolute dielectric permittivity (in Farad/meter), The dielectric constant of the material is not only related to the polarization, but also it directly associated with the applied electric field and the responses of the different sites within the molecule, such as an atoms, ions and its bonding formation. Figure 7 depicted the frequency dependence dielectric constant of the grown crystal with respect to various temperatures. From the plot, it is clearly observed that, dielectric constant increases with increasing temperature due to survive of the space charge polarization (SCP) which depends on the purity and perfection of the sample.29 Moreover, it also notices that the dielectric constant gradually decreases with increasing frequency region and becomes saturation at higher frequencies. At lower frequencies, accumulation of the charge carrier follow the applied electric field, which gains the higher εr value, whilest in higher frequencies dipoles are unable to follow, which result in the reduction of the εr value. Thus the low value of the dielectric constant in the grown crystal infer that it is still better and to be efficiently useful for microwave electronic device fabrications. Figure 8 shows the plot of frequency dependence dielectric loss (tan δ) of the DEBM crystal. Which also exhibit the same trend of the dielectric constant, The low value of the tan δ suggests that DEBM crystal has lesser defects and it is firmly suitable for optical and communication devices.30 However, it is worth to mention that the dielectric characteristics

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reported here for DEBM single crystal identify it to be a promising material having potential application in microelectronics technology and electro – optic device fabrications.

Figure 7. Frequency dependance dielectric constant of DEBM crystal at different temperatures.

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Figure 8. The frequency response of dielectric loss (tan δ) of DEBM crystal at different temperature. 3.6 Solid state parameter of DEBM crystal Note, the value of dielectric constant in giant frequency position (MHz) has been employed to evaluate the Plasma energy, Penn gap, Fermi energy and electrical polarisability of the grown crystal.31,32 In order to determine these values, it is necessary to take the known values like Mass (M= 225.29 g/mol) and density (ρ= 1.187 Mg/m3) from single crystal XRD data.33 Likewise, the effective number of valance electrons Z = 82 and the high-frequency dielectric constant εr (for 1 MHz) at 40 ºC of the DEBM crystal found to be 39.67, which helps

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us to evaluate aforementioned parameters. Consequently, the correlation between the valance electron plasma energy (ħωρ), penn gap (Ep) and Fermi energy (EF) could be described based on the following formulas. 34,35 1/2

Z  hω p = 28.8  ρ  M Ep =

(1)

hωp

( ε r -1)

(2)

E F = 0.2948(hω p ) 4/3

(3)

where ωp is plasma angular frequency α is electronic polarizability of the grown crystal which can be calculated from the following equation

 ( hω )2 S  p o  × M × 0.396×10-24 cm 3 α= 2  ( hω ) S + 3E 2  ρ p o P  

(4)

where So is a constant and its value could be found by using following equation,  E  1 E  So = 1-  P  +  P   4E F  3  4E F 

2

(5)

The value of electric polarizability thus obtained is in good agreement with the value calculated from the Clausius–Mossotti equation and is given by,

α=

3M  ε r -1    4πN a ρ  ε r - 2 

(6)

where Na is the Avogadro’s number (N=6.023×1023) and the optical band gap (Eg) of the grown crystal being known (From Figure 6c), then the electronic polarizability (α) was determined as 36

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 Eg  M  × × 0.396×10-24 cm 3 α = 1 4.06  ρ

(7)

All these assorted calculated solid state data of DEBM crystal are compared with the KDP crystal37 and tabulated in Table 1. From the table, it seems to be justifying that calculated values of DEBM are superior to KDP.

Table 1 Comparison of solid state parameter of the DEBM crystal with KDP crystal Parameters

DEBM crystal

KDP crystal

Plasma energy (eV)

19.38

12.27

Penn gap (eV)

3.11

2.07

Fermi energy (eV)

15.35

12.07

Electronic polarizability (α) (i) With respect to Penn analysis

6.94 × 10-23 cm3

4.13 × 10-23 cm

(ii) With respect to Clausius–Mossotti

6.98 × 10-23 cm3

4.16 × 10-23 cm3

4.48 × 10-23 cm3

4.12 × 10-23 cm3

equation (iii) With respect to optical band gap

3.7 Thermal analysis Apart from nonlinear optical properties, it is fundamental to know thermal parameters likes, thermal stability, decomposition, the melting point of the compound for application usage. The grown DEBM crystal has been crushed into fine powder, and the powder form of the weighted (4.015 mg) sample is added into Al2O3 crucible and heated from room temperature (RT) to 450 º C at a heating rate of 20 ºC/min in an inert nitrogen atmosphere. Figure 9 shows the TG/ DSC profile of DEBM. In the TG trace, it was comply that DEBM was stable up to 123.63° C and there was no appropriate mass loss detected in DEBM sample below 123.63 °C, 19 ACS Paragon Plus Environment

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and this specifies that absence of water and solvent molecules in the grown crystal. The first weight loss of 3.59 % occurred between 123.63 °C and 222.27 °C. There was a major weight loss of 94.35 % starting at about 222.27 °C and ending at 319.44 °C, which was due to the decomposition and volatilization of the compound. The final residue left at 430 °C is 2.06 %. In DSC curve, the first endothermic peak at 137.84 °C corresponding to the melting point of DEBM crystal and which merely coincides with the TG curve. The spiky endothermic peak confirmed the good crystallinity of DEBM sample. Furthermore, a complete decomposition or volatilization was also observed at 319.63 °C with the appearance of the second endothermic peak. From this results, it can be concluded that the grown DEBM crystal propabaly used for optoelectronic device applications up to 123°C. Moreover, the melting point of DEBM crystal is still better than reported organic NLO crystals like, CMOBA (125 ºC),38 BMP (107 ºC),39 DBA (119 ºC),40 DMMC (93.98 ºC).41

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Figure 9. TG and DSC profile of DEBM crystal. DEBM was stable up to 123° C and its corresponding melting point 137.84 °C

3.8 Photoluminescence The choice of photoluminescence (PL) is the fundamental study to identify the quality of the material. As for the origin of luminescence behaviour is based on, the electronic states of solids are excited by absorption of the light energy, later this energy can be released in the form of photons with respect to the wavelength and which is officially related to the electronic structure of the target material. The recorded PL spectrum of DEBM crystal is shown in

Figure 10. A strong and broad emission peak appeared at 535 nm, which implies the presence of

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green emission spectrum. However, the maximum intensity occurs in compounds due to highly conjugated double-bond (–C=C–) structure with low energy π–π* transition levels. It also points out herein, the absence of the additional emission peak in the visible region confirms that grown DEBM crystal has good crystalline nature and structure perfection. This result authorized that DEBM single crystal having more possibility to be used as a new green-light emitting material42 and also it may be useful for luminescence dosimetry applications, X-ray storage, etc.43,44

Figure 10. The photoluminescence spectrum of the DEBM single crystal

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3.9 Measurement of laser-induced damage threshold (LDT) Here it is a better measurement to the selection of NLO materials based on its optical damage tolerance. In general look, the nonlinear optical materials harmonic conversion is directly proportional to power transferred per unit area. Here, Q-switched Nd: YAG (1064 nm) laser with a pulse width of 10 ns, a frequency of 10 Hz and a 30 cm focal length was used to measure the laser damage threshold of a DEBM single crystal. However, the output intensity of the laser was controlled via variable attenuator. During laser radiation, a laser beam spot (diameter of 1mm) ran towards target sample, to get irradiate on the crystal surface, which gain onset damage on a specimen surface. Meanwhile, the affording energy density of the input laser beam was recorded with the help of the energy meter (model no. EPM 2000). Multi-shot laser damage measurement was made on the highly polished surface of the grown crystal. The surface damage threshold of DEBM crystal was appraise using the following expression45 Energy density (I) = E/τA (GW/cm2)

(8)

where E is the input energy required to promote the damage (mill joules), τ is the pulse width (nanoseconds), A is the area of the damaged spot (millimetre). The multi-shot and magnified damage profile of the DEBM crystal is presented in Figure 11: this image reveals the nature of the damage and its possible origin. The measured surface laser damage threshold of DEBM was found to be 1.95 GW/cm2. The LDT value of DEBM is quite better compared with other familiar NLO crystals, which are shown in Table 2. Hence, the DEBM crystal is used for NLO and high power laser applications because the threshold value is superior.

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Figure 11. Magnified damage profile of the DEBM crystal.

Table 2 Laser damage threshold of DEBM crystal probably compared with other NLO crystals. Crystals Name

Multi-shot LDT value in GW/cm2

KDP46

0.20

BBO47

0.35

Urea47

1.32

Benzamidazole46

1.71

DEBM Present work

1.95

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3.10 Chemical etching analysis Etching is a significant tool to identify the defects in the grown crystal. Generally defect free optical crystals are substantial for NLO device fabrications. The NLO efficiency (SHG and THG), electrical, mechanical strength, thermal stability and laser damage threshold, etc,. mainly depend on the quality and structural perfection of the materials. Moreover, chemical etching has an influence to develop the growth features on the crystal surface.48,49 The patterns of etch pits was qualified with three factors: 1) type of solvent (etchant), 2) etching time 3) crystal faces. The chemical etching study of DEBM crystal was characterized by using ethyl acetate as an etchant for different intervals 15 s and 30 s. After that, the etched surface of the crystal was gently wiped with the help of tissue paper, which can contribute their microstructures via reflection mode of an optical microscope. Figure 12 (a) shows the surface features of the DEBM single crystal before etching, and the rectangle type building block etches patterns observed for 15 s

(Figure 12 (b)). As the time of etching has been increased 30 s (Figure 12 (c)), well-defined triangle shape etched pattern are observed in the grown DEBM crystal. The average dislocation etch pit density (EPD) of the DEBM crystal is found to be 1.201 ×103 cm- 2, and comparable to other reported crystals.50–52 The low value of EPD ensures that the grown DEBM crystal has good crystalline perfection with a lesser defect. Therefore it can be favourably suitable for NLO applications.

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Figure 12. Recorded micrographic image of the DEBM crystal ;(a) surface of the grown crystal before etching; (b) etching after 15 sec; (c) etching after 30 sec.

3.11 Third -order NLO properties According to single crystal XRD data, it can be concluded that grown DEBM crystal belongs to monoclinic crystal system with centrosymmetric space group of P21/n. Since centrosymmetric space group crystals possess the inversion symmetry thus its second order susceptibility (χ(2)) becomes zero.53 Based on this phenomenon, the centrosymmetric nature of the DEBM crystal prominently obeys the third- order nonlinear optical properties (TONLO), which was confirmed by single beam Z-scan technique.54

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Z-scan technique is a phenomenological pathway to accurately measure the optical nonlinearities of the title material, based on the four fundamental factors: (i) Absorption coefficient (β); (ii) nonlinear refraction index (n2); (iii) third order nonlinear optical susceptibility (χ(3)); (iv) hyperpolarizability (γ). Here, the Rayleigh length (ZR= πωo/λ) of Gaussion beam was calculated about 0.72 mm, higher than the thickness of the sample (0.58mm), which was an essential requirement (i.e., ZR >> L, where L is sample thickness) for the validity of the Z-scan theory.55 The crystal has been mounted on a linear translational stage, and which was allowed to move step by step (with the help of computer program controller), likes in the direction of negative Z (-15) to positive Z (+15), with respect to the focal point (Z=0), and its corresponding normalized transmittance was recorded via digital power meter. However, here it is possible to access the two different series of measurements simultaneously and which gives the two valid informations: The cloesd aperture (CA) and the open aperture (OA) Z-scan measurement was done by with and without aperture in front of the detector. The closed aperture Z-scan data gives the information about sign and magnitude of the nonlinear refractive index (n2), whereas open aperture Z-scan is valid for the measurement of nonlinear absorption coefficient (β). The CA Z-scan profile of DEBM crystal is shown in Figure 13. As it can be noticed in the peak to valley configuration signifies that DEBM crystal obey self-defocusing behavior (SD) which appreciated the negative refraction nonlinearity.56 Without any doubt, DEBM material with negative nonlinear refraction is suitable for protection of night vision sensors.57 The measurable quantity ∆TP-V, the change in normalized transmittance between peak and valley with respect to on axis phase shift |∆ɸo|, which can be expressed as,

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∆TP-V = 0.406(1-S)0.25 ∆Φo

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(9)

Here S is the linear transmittance of the aperture, which can be determined by following relationship,58

 -2r 2  S = 1- exp  a   ωa  (10) In the equation (10), ra denotes as the radius of the circular aperture (2 mm) and ωa is the beam radius of the aperture (3.3 mm). The value of third-order nonlinear refractive index (n2) of the grown crystal can be evaluated from the CA data, which can be described by,

n2 =

∆Φ o KIo L eff

(11)

In this equation, the wave number K is simply 2π/λ, (where λ is the wavelength of the He-Ne laser) Io is the on axis irradiation at the focus (Io = 26.50 MW /m2) and the effective sample length L

eff

= [1- exp(-αL)]/α, where α and L are the linear absorption coefficient and

thickness of the DEBM crystal respectively. A valley in the OA curve, as seen in Figure 14 confirms the occurrence of strong RSA with positive nonlinear absorption coefficient (β) in the DEBM crystal. As can be seen, in the DEBM crystal, the absorption of the molecule in excited state is relatively higher than its ground state, which ensure the strong RSA effect.59–63 With this respect, its exhibit exquisitely behavior in optical limiting, photonic and medical applications.64,65

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The following relation helps to calculate the nonlinear absorption coefficient (β).66

β=

2 2∆T Io L eff

(12)

where ∆T is the valley value at the focus (Z=0), traced from the OA Z-scan data. Furthermore, in order to determine the real [Re (χ(3))] and imaginary [Im (χ(3))] parts of third order nonlinear optical susceptibilities (χ(3)) of the sample in the esu, the following formulas were used.67–69

Re χ (3) (esu) =

10-4 (ε o c 2 n o2 n 2 ) cm 2 W -1 π

10-2 (ε o c 2 n o λβ) Im χ (esu) = cm 2 W -1 2 4π

(13)

(3)

(14)

where εo is the vacuum permittivity (8.8518 X 10-12 Fm-1), c is the velocity of light, no is the linear refractive index of the sample. The effective value of third-order NLO susceptibility (χ(3)), and the second-order molecular hyperpolarizability (γ) of the DEBM crystal could be estimated by the following equations, 2 2 χ ( 3) = ( Re(χ (3) ) ) + ( Im(χ (3) ) )   

Re [ γ ] =

(15)

Re  χ (3)  Nf 4

(16)

Here, N is number of molecule per unit volume (N = 3.173×1027 m-3), and f is the localfield correction factor calculated from f = (݊௢ଶ + 2)/3. The calculated values of n2, β, (χ(3)) and γ are listed in Table 3 at the input wavelength of 632.8 nm. Considering the remarkable value of 29 ACS Paragon Plus Environment

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(χ(3)) and (γ) are due to the highly delocalized π-electronic configuration, and more effective intra-molecular charge transfer in the DEBM molecular system, which may be responsible for the third-order optical nonlinearities.70 Apart from this, here we introduced the quite interesting nomination factors, namely figure of merit (FOM) W = n2I/αλ and, T = βλ/n2 pinpoints the importance and suitability of the material in all-optical switching devices application. Currently, our best material yield the values of W is 10.62 and T factor is 0.64, which fulfils the basic strategies W>>1>>T. With this respect, the grown DEBM crystal can be considered as a fruitful candidate for optical switching device application.71 Thus, large value (χ(3)) DEBM crystal is numerically compared with typical KDP crystal, and other NLO materials tabulated in Table 4. Hence, overall results signature that, the grown DEBM crystal contains better NLO response, and it might be considered as a favourable candidate for optical data processing, optical logic gate, optical limiting and all-optical switching device applications.

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Figure 13. Self-defocusing closed aperture Z-scan profile of DEBM crystal. The peak to valley pathway on the z-axis gains the negative nonlinearity.

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Figure 14. Open aperture normalized transmittance spectrum of DEBM crystal.

Table 3 Calculated optical nonlinearities of DEBM crystal. Calculated parameters Obtained values

β (mW-1)

n2 (m2 W-1)

χ(3) (esu)

γ (esu)

2.071 × 10-5

2.023 × 10-11

7.661 × 10-5

6.785×10-34

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Table 4 Comparison of third-order nonlinear susceptibility (χ(3)) of DEBM crystal with other NLO crystals. Name of

DEBM

the

Present work

BCBA72

4BPTS73

DMAPHB53 4Br4MSP74

4.853×10−7

4.162 ×10-8

1.016×10-13

KDP75

crystals χ(3) (esu)

7.661×10-5

1.65 × 10-14 4× 10-14

4. Conclusion Good quality single crystals of 2-[4-(Diethylamino)benzylidene]malononitrile (DEBM) were grown by the slow evaporation solution growth method. The single crystal X-ray diffraction analysis shows that DEBM crystallizes in monoclinic system. The FT- IR analysis confirms the presence of functional groups constituting DEBM. The Uv-Vis-NIR spectrum reveals that the crystal is transparent between 490 nm and 1100 nm and the band gap energy was determined as 2.68 eV. The low value of dielectric constant (εr) and dielectric loss (tan δ) at high frequencies ensure that the DEBM crystal has better optical prominence with fewer defects. The TG-DSC studies elucidated the decomposition and confirmed the thermal stability of DEBM and suggested that it could be used in optical applications below its melting point (137.84 ◦C). The laser -induced surface damage threshold of grown crystal was found to be 1.95 GW/cm2 for 1064 nm Nd: YAG laser radiation. According to Z-scan measurement DEBM crystal exhibit self – defocusing behaviour (SDF) and reverse saturation absorption (RSA). All these factors point out that DEBM crystal considers as a potential NLO material in all optical applications.

Acknowledgement The authors want to thank VIT management for providing major financial support and excellent research facilities. 33 ACS Paragon Plus Environment

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