Bulk crystal growth and nonlinear optical characterization of a stilbazolium derivative crystal: 4-[2-(3,4-dimethoxyphenyl)ethenyl]-l methylpyridinium tetraphenylborate (DSTPB) for NLO device fabrication

Abstract

The search for organic crystals with highly efficient nonlinear optical (NLO) properties with a high surface laser damage threshold (LDT) has become more of a demand in the frontier areas of optical switching and communications applications. A single crystal of 4-[2-(3,4-dimethoxyphenyl)ethenyl]-l methylpyridinium tetraphenylborate (DSTPB), which is an organic material has been successfully synthesized in the pure phase. Bulk single crystals were grown with dimensions of 28 × 11 × 6 mm³ using a slow evaporation method and reported for the first time. The structure of the title crystal and its lattice parameter were confirmed by single-crystal X-ray diffraction studies and it was found that it crystallizes in the non-centrosymmetric space group Cc with a monoclinic crystal system. The calculated HOMO and LUMO energies show that charge transfer takes place within the molecular structure, and they also indicate the NLO activity of the title crystal. A Kurtz powder test showed large second harmonic generation (SHG) about 4.53 times that of KDP crystal and make it a suitable candidate for electro-optic applications. It also exhibits good transparency (371 nm to 1100 mm) in the visible and near infra-red spectral (NIR) ranges and its thermal stability was found to be up to 260 °C. It shows a high laser-induced damage threshold of up to 4.3 GW cm⁻², which is greater than that of some known organic and inorganic NLO materials. Third-order nonlinear optical properties of the title crystal were studied by the single beam Z-scan technique at 632.8 nm using a He–Ne laser. It was found that it exhibits saturable absorption (SA) and a self-focusing nature with large second-order hyperpolarizability (γ) 3.15 × 10⁻³³ esu, which are mainly associated with electronic processes. The results indicate that the crystals exhibit large third-order optical susceptibility compared with some reported NLO crystals. The enhanced second-order and third-order optical nonlinearity of DSTPB makes it a promising candidate in the field of nonlinear optical devices.

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1. Introduction

Second-order nonlinear optical (NLO) materials containing organic chromophores play a key role as constructing blocks for organic electro-optic OEO materials.^1,2 Research of nonlinear optical materials containing pyridinium units has proved them to be good candidates for the optoelectronic and photonics industries. In recent years, the design and synthesis of organic π-conjugated donor/acceptor materials have attracted tremendous attention because of their interesting applications in optical communication, optical computing, second harmonic devices, data storage systems, optical limiting, modern information technology, and so on.^3–7 Among the organic and inorganic crystals studied, organic stilbazolium derivatives have attracted considerable attention due to their large second-order optical nonlinearity, modulation of laser reading, terahertz (THz) wave applications including THz detection and generation.^8–12 The original reason for this is due to the presence of active hydrogen bonds with a π-conjugated system and Coulomb interactions between a stilbazolium cation and counter anion.^13–17 The experimental results also showed a combination of large NLO response with better mechanical properties, low dielectric constant, photochemical, and thermal stability. It is easier to modify the structure for the construction of integrated optical devices than for inorganic materials.^18–20 In the recent past, many stilbazolium derivatives have been widely investigated by different research groups using the same stilbazolium cation with counterion variations (aryl sulfonate anions) in order to create new molecules with a large NLO response which is based on Coulomb interactions. For example, DAST (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate) with para-toluenesulfonate anion, DSNS (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2-naphthalenesulfonate) with 2-naphthalenesulfonate anion have been reported with extremely large nonlinearity.^14,21 Over the last few decades, most of the research articles have focused mostly on second harmonic generation (SHG) of crystals.

However, there has been a large need for molecules with large third-order susceptibilities, which have been established to play a major role in all-optical switching/limiting devices, photonics and optoelectronics device applications.^22,23 It has been recently reported that among the organic compounds stilbazolium derivatives yield high third-order NLO efficiency mainly due to the presence of a strong intramolecular charge transfer (ICT) transition.²⁴ For the first time, we present here investigations on the bulk growth of DSTPB, spectral analysis, HOMO–LUMO analysis, thermal stability, Vickers microhardness, dielectric properties, surface analysis, and photoconductivity studies. The structural relationship, second-order [SHG] and third-order [THG] NLO properties of DSTPB have been investigated to realize the nonlinear optical properties by HOMO–LUMO analysis. The results indicate that the NLO activity is far larger than for the well-known KDP crystal.

2. Experimental procedures

2.1. Material synthesis and bulk crystal growth

Analytical pure grade iodomethane, 4-methylpyridine, 3,4-dimethoxybenzaldehyde, and sodium tetraphenylborate were purchased from Alfa Aesar and Sigma-Aldrich and used for the synthesis of the title material without further purification.

The title compound was synthesized by a previously reported method (Scheme 1).²⁵ In the present synthesis process, the title crystal was prepared by a two-step synthesis route. In the first step, 4-[2-(3,4-dimethoxy-phenyl)-vinyl]-1-methyl-pyridinium iodide 1 was synthesized by adding a 1 : 1 molar ratio of 1,4-dimethyl-pyridinium iodide, 3,4-dimethoxybenzaldehyde, and piperidine (few drops) in hot methanol (20 ml). The resulting reaction mixture was refluxed for 8 h. The resultant pale yellow precipitate which formed was filtered off and washed with diethyl ether and purified by repeated recrystallization from methanol. In the second step, the resultant purified compound 1 (electron attracting group-cation) was dissolved in deionized water (40 ml) and mixed with a saturated aqueous solution of sodium tetraphenylborate (electron donating group-anion). Further, it was refluxed for 1 h at 80 °C. The mixture yielded a pale yellow solid of 4-[2-(3,4-dimethoxyphenyl)ethenyl]-l-methylpyridinium tetraphenylborate (DSTPB) 2 when the solution was slowly cooled to room temperature. The resulting DSTPB was washed with deionized water, and the purity was improved by successive recrystallization from methanol several times.

Scheme 1

The purified salt of DSTPB was dissolved in DMF at 40 °C to form a saturated solution, and the solution was stirred using a motorized magnetic stirring device for 2 h to obtain a homogeneous solution. The resultant solution was filtered into a 150 ml beaker using high-quality Whatman filter paper. The solvent was allowed to slowly evaporate at 40 °C in a constant temperature water bath (accuracy of ±0.01 °C). Optical quality DSTPB crystals were harvested with dimensions 28 × 11 × 6 mm³ after a period of 45 days by macro-defect-free seed addition to the saturated solution as shown in Fig. 1a.

Fig. 1 (a) Photograph of as-grown DSTPB and (b) morphology of DSTPB.

3. Results and discussions

3.1. Single crystal and powder X-ray diffraction analysis

A suitably sized grown single crystal was selected and subjected to single crystal XRD analysis by using a Bruker Kappa APEX II diffractometer (MoKα λ = 0.71073 Å radiation). It reveals that the grown crystal has crystallized in the monoclinic crystallographic system with non-centrosymmetric space group Cc. The cell parameters of the DSTPB crystal are a = 11.2979(2) Å, b = 16.988(2) Å, c = 17.2547(2) Å and β = 100.232(10) Å. The volume of the crystal system is 3254 (Å)³. The obtained cell parameter values are similar to the reported values (CCDC-137966).²⁵ and thus confirm the grown title crystal. The crystal structure and packing fraction of DSTPB are shown in Fig. S1 and S2,† respectively.

The grown crystals were finely powdered and subjected to powder X-ray diffraction analysis using a BRUKER X-Ray diffractometer with Cu Kα radiation (λ = 1.540598 Å). The XRD profile reveals (Fig. S3†) well-defined sharp Bragg’s peaks at specific 2θ angles without detectable impurities. The obtained diffraction peaks were indexed by using the Powder-X software package. The experimentally observed XRD peaks are closely matched with that of the single crystal XRD pattern and shown in Fig. S3.† Thus, the XRD analysis confirms the good crystallinity and purity of the title compound.

Information about the growth morphology of DSTPB was generated and indexed by using single crystal XRD data (CIF format) as input to the WinXMorph software program.²⁶ The indexed morphology of DSTPB is shown in Fig. 1b. It was found that the (0 0 1) is the most prominent plane, and a faster growth rate has elongated the crystallographic a and b direction compared to the c-axis.

3.2. HOMO and LUMO studies

In order to understand the relationship between the frontier molecular orbital energy gap (HOMO and LUMO) and nonlinear response of the DSTPB crystal, the frontier molecular orbital energy gap was calculated using the Spartan’10 V1.0.1 program.²⁷ The frontier molecular orbital energy gap (HOMO and LUMO) is very important to characterize the optical polarizability, electrical property, nature of reactivity, kinetic stability of the molecule as well as in quantum chemistry.²⁸ The HOMO energy represents the electron donating nature, and the LUMO energy characterizes the ability to obtain an electron. A molecule with a small energy gap is more polarizable, which encourages more active NLO properties of the system and also defines the soft nature of the molecule.²⁹ As seen from Fig. S4,† the HOMO orbitals are only localized on two of the phenyl rings in the tetraphenylborate (counter anion) while the LUMO orbitals are located over the stilbazolium cation except in the methyl groups and the counter anion. The calculated energy gap (energy gap = HOMO–LUMO) and dipole moment values are 1.39 eV and 19.3 D respectively. The low value of the energy gap and the higher value of the dipole moment implies that molecular charge transfer is taking place within the molecule. This causes more polarizability of the structure and is directly related to the NLO efficiency of the DSTPB structure.

3.3. Molecular electrostatic potential surface

The molecular electrostatic potential (MEP) surface diagram is used to understand the reactive behaviour, electron density distributions and nucleophilic and electrophilic attack of the molecule in terms of color grading.^30–32 The negative region is for nucleophilic sites whereas the positive region is for potential electrophilic centres as shown in Fig. 2. The color line of the MEP is found to be in the range between 243.5 a.u. (deepest red) and −244.2 a.u. (deepest blue) for the title compound. It can be clearly seen that (Fig. 2) the positive region is located on one of the phenyl groups of the counter anion and the methoxy groups in the stilbazolium cation. The positive region covers only the pyridinium ring in the stilbazolium cation whereas the predominance of the green region (zero potential) in the MEP surfaces explains the potential halfway between the counter anion and stilbazolium cation. Thus, the MEP confirms the existence of intramolecular interactions.

Fig. 2 Molecular electrostatic potential diagram of DSTPB.

3.4. FT-IR spectral studies

In order to confirm the presence of various vibrational motions of the functional groups in the grown crystal, FT-IR transmittance spectra have been recorded in the wavenumber range 4000–400 cm⁻¹ using a SHIMADZU IRAFFINITY spectrometer. A powder of the title crystal (3 mg) was mixed thoroughly with dried KBr (300 mg) and made into pellets, which were subjected to FTIR analysis. Fig. 3 shows the FT-IR spectra of the title crystal. The absorption peak at 3443 cm⁻¹ corresponds to the H–O bond stretches in H₂O, which may be moisture absorption from the atmosphere. The aromatic ring exhibits multiple bands in the region 3100–3000 cm⁻¹ due to the C–H in-plane and C–H out of plane bending vibrations.³³ The band at 2831 cm⁻¹ is assigned to the methoxy C–H stretching mode vibrations.³⁴ The peak seen at 2964 cm⁻¹ is assigned to C–H stretching vibrations of the methyl group (N–CH₃) in the pyridine ring. The aromatic C=C stretching vibrations and in-plane stretching frequency of π-conjugated stilbazolium chromophore C–C=C–C (DSTPB structure) were established by the presence of peaks appearing at about 1647 cm⁻¹ and 1579 cm⁻¹. This is responsible for an electron delocalization between the strong electron donor and electron acceptor of DSTPB. The peak observed at 1595 cm⁻¹ corresponds to the olefinic C–C stretching vibrations. The asymmetric bending vibrations of the methyl group (O–CH₃) are usually, found in the region 1465–1440 cm⁻¹.³⁵ The observed peaks in the region 1150–850 cm⁻¹ can be attributed to the skeletal vibrations of C–C and C–N bonds. Thus, all the necessary strong NLO active vibrational mode functional groups of the DSTPB crystal were confirmed from the characterization results.

Fig. 3 FTIR spectra of DSTPB.

3.5. UV-Vis-NIR spectral analysis

Optical transmittance ranges of single crystals are important factors for nonlinear optical applications because an optical material can be of practical device use only if it has a lesser absorption of light in the Vis-NIR region.³⁶ To find the transmission window, the UV-Vis-NIR absorption spectrum (Fig. 4a) of DSTPB was measured with an ELICO SL 218 double beam UV-Vis-NIR spectrometer in the wavelength range between190 and1100 nm. It is clear from Fig. 4a that the title crystal possesses wider transparency in the Vis-NIR region. The optical absorption was found to be at 258 and 396 nm. The lower cutoff of the wavelength at 258 nm is due to the n–π electronic transitions, and another major peak at 396 nm corresponds to the π–π* electronic transitions through an extended conjugated system (stilbazolium chromophore). Hence, it can be a potential candidate for optoelectronic and NLO applications.³⁷

Fig. 4 (a) UV-Vis-NIR spectra of DSTPB and (b) emission spectra of DSTPB.

3.6. Photoluminescence studies

Fig. 4b shows the photoluminescence (PL) spectrum of DSTPB recorded from 340–650 nm using an F-7000 FL spectrophotometer at room temperature. The spectrum exhibits a stable, strong emission peak at 471 nm when excited by UV-Vis light at 396 nm. The maximum intensity at 471 nm (2.63 eV) is attributed to the π–π* electronic transition between the donor (C₁₆H₁₈NO₂⁺) and acceptor groups (C₂₄H₂₀B⁻) through the stilbazolium chromophore. This caused the luminescence characteristic nature of DSTPB. The absence of any other emission peak in the measured region confirms the good degree of crystallinity and structural perfection of the DSTPB.³⁸ Hence, the resultant emission peak shows that the material has blue emission, which makes it more suitable for tunable laser system and optoelectronic devices.^39,40

3.7. Thermo-gravimetric and differential thermal analysis

TGA and DTA analysis are important for NLO materials to shed light on the thermal behaviour of the substance.⁴¹ To analyze the melting point, thermal behaviour, and the decomposition point of the DSTPB crystal an NETZSCH STA 409 F3 thermal analyzer was used and the results shown in Fig. S5.† The TG–DTA curves for DSTPB were recorded over a temperature range of 30 to 550 °C at a heating rate of 10 °C min⁻¹ under flowing N₂ gas. An initial pure sample of weight 3.196 mg was used for the analysis. The TGA trace shows the first weight loss is 3.46% over a temperature range 35 to 245 °C, which illustrates the loss of physically adsorbed moisture and volatile solvents during the crystallization. This is followed by two major weight losses occurring in the TGA. The first step is associated with a major weight loss of about 63.99% between 245 and 315 °C, and another weight loss corresponds to 26.92% up to 550 °C. These weight losses indicate decomposition of the DSTPB and gradual volatilization. Finally, TGA shows that the final residual mass of weight is only 5.63% after heating to 550 °C. In the DTA trace, one endothermic peak was found at 260 °C, which is assigned to the melting point of DSTPB. It is followed by a stepwise broad decomposition peaking at 315 °C over a range between 245 and 550 °C. These endotherms are associated with decomposition of the DSTPB crystal structure. A similar observation was seen with the TGA trace. Hence, on the basis of this analysis we conclude that the material is suitable for NLO applications up to its melting point because there are no exothermic or endothermic peaks up to the melting point.

3.8. Microhardness studies

The microhardness of a crystalline solid mainly depends on the crystal structure, the number of bonds per unit volume and the composition of the crystalline solid.⁴² Microhardness of a crystal is strongly influenced by various parameters such as interatomic spacing lattice energy, Debye temperature and heat of formation.^43,44 The calculation of mechanical properties of materials such as resistance, fracture behavior, brittleness index, yield strength, elastic constants, and temperature of cracking is very important in device fabrication.⁴⁵ The well-polished prominent plane (1 0 0) surface of the DSTPB crystal was subjected to microhardness studies at room temperature using a MH-112 Vicker’s hardness tester (Mututoyo MH 112, Japan) fitted with a diamond pyramidal indenter. In the present investigation, the applied load P was varied between 10 and 100 g and the indentation time is set at 10 s for all indentations. The average of two diagonal lengths (d) of the indentation mark was measured for each load using a calibrated micrometer attached to a metallographic microscope. The microhardness number (H_v) value was calculated from the following formula.⁴⁶where P is the applied load in kg and d is the average diagonal length of indentation impression in μm, and 1.8544 is the constant of the geometrical factor for the diamond pyramid indenter. Fig. S6† shows the variation of H_v with applied load. From this graph, it is very clear that the grown DSTPB crystal exhibits a reverse indentation size effect (ISE). This means that H_v increases with increasing applied load (P) in the low load region.^47–49 The increase of H_v is purely due to the work hardening of the surface layers, and the ISE may be attributed to the generation of cracks around the indentation.^47,50 The value of the Meyer index or the work hardening coefficient (n) was calculated from Fig. S7a,† using the least-squares fitting method.⁵¹ In this study, the Meyer index number is estimated to be 2.5. This is in good agreement with the Onitsch concept that if n is >2 the H_v value should increase with an increase in load P and reverse ISE behavior.^52,53 Thus, the calculated value of the Meyer index number suggests that DSTPB belongs to the softer organic materials category. From the hardness value, the yield strengths of the title crystal can be calculated using the following relationship

Fig. S7b† shows the dependent yield strength as a function of applied loads of 10 to 100 g. The stiffness constant gives an idea about the nature of bonding between neighboring atoms.⁵⁴ The elastic stiffness constant (C₁₁) can be calculated for loads from 10 to 100 g (Fig. S7c†) using Wooster’s empirical expression as

C₁₁ = (H_v)^7/4 (3)

Table 1 presents the calculated yield strength and stiffness constant for different loads.

Table 1 Microhardness, yield strength and elastic stiffness constant values of DSTPB

Load P (g)	Hv (kg mm^−2)	σy (GN m^−2)	C11 × 10^14 Pa
10	8.21	5.01	68.41
25	14.34	8.74	1.81
50	19.72	12.02	3.17
100	27.18	16.57	5.55

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3.8.1. Analysis of Hays–Kendall approach. According to the Hays–Kendall approach⁵⁵ there is a minimum load W to initiate plastic deformation and the load dependence of hardness of the grown crystal has been calculated using the relationship P = W + A₁d², where W and A₁ are the minimum loads to initiate plastic deformation and load-independent constant and the exponent n = 2. The values of W and A₁ (slope value) can be estimated by plotting the load (load range 10 to 100 g) vs. d² (Fig. S7d†). The value of plastic deformation ‘W’ is the intercept value along the load axis (Y-axis). It is defined as the resistance pressure for the duration of the indentation process, and it should be smaller than the applied load P.⁵⁶ The corrected hardness H_o (Table S1†) for the title crystal can be estimated using the formula H_o = 1854 × A₁. Thus, the DSTPB crystal has a hardness value of 27 kg mm⁻² at 100 g, which indicates that the grown DSTPB is a good candidate for NLO applications. It is significantly higher than some other NLO crystals such as urea (6.5–11 kg mm⁻²) and N-methylurea (12–19 kg mm⁻²).⁵⁷ The good mechanical property defines the strong intramolecular interactions of the title crystal.

3.9. Chemical etching studies

The nonlinear efficiency of devices mainly depends on the quality of the grown crystals because imperfections which occur during growth result in the distortion of the optical beam. For good performance of optical devices, crystals free from defects and light scattering are required.⁵⁸ The utility of a NLO crystal depends on its surface quality because the laser damage threshold of the NLO crystals decreases with increasing defects in crystals.⁵⁹ Etching studies were carried out on the as-grown crystal with crack-free surfaces completely immersed in N,N-dimethylformamide as the etchant for an etching time of 20–40 s. Then the etched surfaces were cleaned using tissue paper, and their microstructures were examined using an optical microscope (Carl Zeiss optical microscope with a 50× magnification). Fig. S8a† shows the surface micrograph of the as-grown crystal. The microstructure of the etched crystal surface for an etching time 20–40 s is shown in Fig. S8b and c†. It is observed that the size of etch pits decreases with the increase of etching time. These results suggest that the grown crystal has rectangular type growth mechanism with less dislocation revealing good crystalline perfection.

3.10. Laser-induced damage threshold studies

The operation of NLO devices depends not only on the linear and nonlinear optical (NLO) properties but also on the ability to withstand high-power laser intensity sources. The second harmonic conversion efficiency is proportional to the density of incident beam intensity.^60,61 Hence, newly discovered NLO materials with high optical surface damage tolerance (by high-power lasers) become extremely important in the performance of nonlinear optical (NLO) and optoelectronic device applications.^62,63 For this measurement, a Q-switched Nd:YAG (1064 nm radiation) pulsed laser was used, and the pulse width is 10 ns with10 Hz repetition rate operating in transverse TM00 mode. During laser irradiation, damage of the surface can be determined by the visual formation damage and the input laser energy density was recorded by a power meter (model no: EPM 2000). The damaged spot was measured using a Carl Zeiss optical microscope with 50× magnification. The laser damage threshold of the grown crystal was evaluated by the following relationship.⁶⁴where I is the energy density required to cause damage, E is the input energy measured in mJ, τ is the pulse width, A is the area of the laser spot. Thus, the estimated surface damage threshold value of the DSTPB crystal is found to be 4.3 GW cm⁻². It is higher than that of the well-known KDP crystal (0.2 GW cm⁻²) and urea (1.5 GW cm⁻²).⁶⁵

3.11. Dielectric studies

3.11.1. Dielectric constant and dielectric loss measurements. Dielectric properties are highly related to the quality of materials.⁶⁶ The selected grown crystals were cut and polished to obtain about 2 mm thickness. They were subjected to dielectric studies at room temperature using a HIOKI 3532-50 LCR HITESTER meter in the frequency region 50 Hz to 5 MHz. The opposite surface of the crystal was coated with high-grade silver paste and was placed between two copper electrodes, and thus uniform electrical contact was formed on the crystal surface. The dielectric constant (ε_r) and dielectric loss (tan δ) were calculated using the standard relationships.⁶⁷ From the results (Fig. 5a and b), the dielectric constant and dielectric loss were found to have high values in the low frequencies region. Further, it decreased with an increase of frequency and becomes almost constant at the higher frequency region. Hence, it increases the potential of the material for the fabrication of NLO devices and electro-optic applications.^68–70 The large value of both dielectric constant and dielectric loss in the low-frequency region can be explained on the basis of the contribution of space charge polarization.⁷¹ In general, the magnitude of dielectric constant and dielectric loss purely depends on the chemical structure (purity), electric field, the perfection of the crystal and also the temperature.⁷²

Fig. 5 (a) Dielectric constant vs. log F and (b) dielectric loss vs. log F.

3.11.2. DSTPB crystal based on single crystal XRD. Solid state parameters are important to estimate the electronic polarizability of a material and to evaluate its SHG efficiency. The SHG efficiency depends upon the electronic polarizability of the medium. To interpret SHG efficiency, the high frequency dielectric constant value of DSTPB is used as input to estimate the electronic properties like valence electron plasma energy, Penn gap, Fermi energy and electronic polarizability. From the single XRD studies, the molecular weight of DSTPB is M = 575.5 g mol⁻¹ and density ρ = 1.172 g cm⁻³. The value of dielectric constant at 1 MHz is calculated to be ε_r = 185. The total number of valence electrons of DSTPB is Z = 218. From this data, the valence electron plasma energy ћω_p can be calculated using standard relationship.⁷³

According to the Penn model,⁷⁴ the average Penn gap (E_P) and Fermi energy (E_F)⁷⁵ for DSTPB are calculated using the relationship

E_F = 0.2948(ħω_p)^4/3 (7)

Then, we obtained the electronic polarizability (α) of the title crystal using the relationship⁷⁶

where S_o is a constant which can be obtained by

The obtained value of polarizability α agrees well with that of the Clausius–Mossotti equation which is given by

where N_A is Avogadro number. These solid state parameter values are compared with those of standard material KDP and are listed in Table S2.† From the table, the polarizability of DSTPB is found to be more than that of KDP.⁷⁷ This result is in good agreement with the Kurtz and Perry powder technique. Thus, from this result it is concluded that SHG efficiency depends on the polarizability of the medium.⁷⁸

3.12. Second harmonic generation efficiency measurements

It is worthy studying the SHG efficiency DSTPB crystal by the Kurtz and Perry powder technique⁷⁹ and the effectiveness of the title compound was compared with the well-known SHG microcrystalline powder of KDP. In the present investigation, the crystal (DSTPB and KDP) was finely powdered with uniform particle size and subjected to powder XRD studies. The particle size was determined by using the Scherer equation and was found to be 125–150 μm. A fundamental laser beam of 1064 nm from a Q-switched Nd:YAG laser with 8 ns pulse width and 10 Hz repetition rate was normally exposed on the pre-packed microcapillary tube. The frequency conversion was confirmed by the emission of green radiation (second harmonic signal (SHG) λ = 532 nm) with an output pulse are 8.8 mV and 39.9 mV for DSTPB and KDP respectively. The SHG responses mainly depend on the particle size, field-gain coefficient, the power of the fundamental beam and minimum beam waist.⁸⁰ Comparing the SHG conversion efficiency, the title compound was found to be about 4.53 times that of the KDP crystal. Thus, the good SHG efficiency suggests that the title compound can be a potential candidate for frequency conversion applications.

3.13. Z-scan measurements

The third-order nonlinear optical properties of DSTPB were investigated by the standard Z-scan technique.⁸¹ It is widely used to evaluate both the nonlinear absorption and nonlinear index (n₂) of a material along with the sign of nonlinearity. This experiment was performed with a He–Ne laser at 632.8 nm as the excitation source with a beam diameter 0.5 mm. The output of the laser beam was focused with a Gaussian filter to get the Gaussian intensity profile, which was focused by a 30 mm focal length lens. The beam waist ω_o of the Gaussian beam at the focal point is measured to be 12.25 μm. It is known that the thickness of the sample is very important to minimize the phase transition in a Z-scan experiment. For this technique a sample thickness of 0.65 mm was used. This is less than the Rayleigh diffraction length (L < K = Z_R, where L is the thickness of crystal, and Z_R corresponds to the Rayleigh diffraction length of the Gaussian beam). The Rayleigh length was calculated to be about 0.72 mm using the formula Z_R = πω_o2/λ (λ is the wavelength).⁸² Hence, the thickness condition could be satisfied with a thin medium for this measurement. The crystal is translated in the direction of negative −Z to positive +Z axis (laser beam direction) under a computer-controlled translation stage. The amplitude of the phase shift was monitored by the change in the transmitted intensity through a small aperture with respect to the sample position (closed aperture method) using a digital power meter. The variations of transmitted intensity completely depend on the aperture size since a large aperture size will reduce the variations in transmittance intensity. For an open aperture method, intensity dependent absorptions were collected directly in the detector by placing a lens in front of the detector and without placing an aperture at the detector in order to resolve the nonlinear refraction (NLR) and nonlinear absorption (NLA). As the crystalline sample is exposed to the focal plane the intensity of the laser beam decreases or increases directly depending on the material refractive index and its absorption nature. Fig. 6a and b depict the closed and open aperture Z-scan curves of title crystal. The open aperture scan method indicates the absence of reverse saturation absorption (RSA) with the enhanced transmission towards the focal point, which demonstrates the strong saturation of absorption (SA) process in DSTPB. It is a necessary parameter of a material to find application in the laser applications such as laser pulse compression, laser pulse narrowing, and optical switching applications.^83–85 This type of saturation process (SA) at the focal point is known as ground state absorption rather than the absorption of the excited state. In the closed aperture data, the valley and peak configuration clearly suggest that the title crystal has a positive sign of third-order nonlinear refractive index, which reveals the self-focusing effect. The complete experimental setup for Z-scan measurement can be found in our earlier work.^86,87

Fig. 6 (a) Open aperture mode Z-scan plot of DSTPB and (b) self-focusing (closed aperture) Z-scan plot of DSTPB.

From the closed aperture Z-scan data, the difference between the normalized valley and peak transmittances (ΔT_p–v), as seen in Fig. 6b can be obtained using the relationship^81,88,89

ΔT_p–v = 0.406(1 − S)^0.25|ΔΦ₀| (11)

where S is the linear transmittance of the aperture in the absence of a sample which is obtained using the following relationship⁸⁵where r_a is the radius of the aperture and ω_a is the beam radius at the aperture. The third-order nonlinear refractive index (n₂) of the grown crystal was calculated using closed aperture data, and it is given bywhere K is the wave vector (K = 9.924 × 10⁶ m⁻¹), λ is the wavelength of the laser and I₀ (I₀ = 26.50 MW m⁻²) is the intensity of the laser beam at the focal point (Z = 0). The effective thickness of the grown crystal can be estimated as L_eff = [1 − exp(−αL)]/α, where L is the thickness of the crystal and α is the linear absorption coefficient. The nonlinear absorption coefficient (β) can be estimated from the open aperture Z-scan data. The third-order nonlinear absorption coefficient (β) can be determined using open aperture data by the following formula⁸¹where ΔT is the peak value of the open aperture Z-scan curve. The obtained third-order nonlinear refractive index n₂ and nonlinear absorption coefficient are 1.56 × 10⁻¹¹ m² W⁻¹ and 3.83 × 10⁻⁴ m W⁻¹, respectively. This can be used to determine the real and imaginary parts of the third-order nonlinear optical susceptibility by the following formula^90,91where and ε_o is the vacuum permittivity (8.8518 × 10⁻¹² F m⁻¹), n₀ is the linear refractive index of the crystal. The effective value of the third-order nonlinear optical susceptibility χ⁽³⁾ and the molecular second hyperpolarizability of the crystal can be calculated through the following expressionswhere f is the local-field correction factor, and N is the number of molecules per unit volume in cm⁻³

In order to know the suitability of a grown crystal for all-optical switching device applications, the two figures of merit W = n₂I/αλ, T = βλ/n₂ were estimated for the title crystal based on the obtained third-order NLO parameter.⁹² For all-optical switching applications, a value of W ≫ 1 and T ≪ 1 is needed. The calculated value of W is 1.81and T is 15.53, which is way short for the requirement of all-optical switching device applications. However, this is a basic requirement in laser Q-switching, laser mode-locking, and optical bistability field because of the strong saturation absorption (SA) properties.⁹³ The calculated third-order nonlinear optical parameters such as nonlinear absorption coefficient β, nonlinear refractive index n₂, third-order nonlinear susceptibility χ⁽³⁾, second -order hyperpolarizabilities of the title crystal are tabulated in Table 2. The large value of χ⁽³⁾ can be attributed to the electron density transfer (donor to acceptor) within the molecular system.⁹⁴ Therefore, the polarization of π-conjugated electrons will be high in the molecular system and will contribute to the large value of χ⁽³⁾ and γ for the DSTPB crystal. The large third-order nonlinear property of DSTPB is compared with those of some organic NLO materials tabulated in Table 3. Thus, the obtained results imply that the title crystal can be a good candidate for third-order NLO applications.

Table 2 Obtained non-linear optical parameters from open- and closed-aperture Z-scan measurement data for DSTPB

Laser beam wavelength (λ)	632.8 nm
Lens focal length (f)	30 mm
Optical bath length	85 cm
Beam radius of the aperture (wa)	3.5 mm
Aperture radius (ra)	1.5 mm
Sample thickness (L)	0.55 mm
Effective thickness (Leff)	6.95 × 10^−4 m
Nonlinear refractive index (n2)	1.56 × 10^−11 m^2 W^−1
Nonlinear absorption coefficient (β)	3.83 × 10^−4 m W^−1
Third-order nonlinear optical susceptibility (χ^(3))	5.25 × 10^−4 esu
Second-order molecular hyperpolarizability (γ)	3.15 × 10^−33 esu
Number of molecules per unit volume	1.22 × 10^27 m^−3

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Table 3 Comparison of χ⁽³⁾ values with some organic NLO materials

Crystal	Third-order nonlinear optical susceptibility χ^(3)	Ref.
DSTPB	5.25 × 10^−4 esu	Present work
VSNS	6.56 × 10^−5 esu	86
MMST	2.29 × 10^−7 esu	95
4Br4MSP	1.65 × 10^−14 esu	96

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3.14. Photoconductivity study

Photoconductivity measurements were studied on the polished surface of the grown crystal using a Keithley 485 picoammeter at room temperature. Two thin copper wires were fixed at a spacing of about 0.3 cm using silver paint in order to give a good electrical contact. Then the sample was connected in series with a DC power supply and a picoammeter. For measuring the dark current, a DC supply was applied from 0 to 30 V in steps of 2 V, and the corresponding dark current was noted in the dark atmosphere. The same sample was exposed to radiation with the help of a convex lens to measure the photocurrent using a halogen lamp (100 W) containing a tungsten filament and iodine vapour. Then the corresponding photocurrent was noted for the same range of the applied field. Fig. S9† shows an increase in both the dark and photocurrent of the title crystal linearly with applied voltage. However, the dark current was seen to increase less than the photocurrent, which defines the positive photoconductive property of the title crystal. This phenomenon is due to the absorption and excitation of charge carriers on illumination.^97–99 This characteristic of photo current actually plays a key role in guided weapons, photodetection and ultraviolet (UV) and infrared (IR) detector applications.^100,101

4. Conclusion

A bulk single crystal of DSTPB has been synthesized and grown by the slow evaporation method and its large NLO property characterized. The lattice parameter was confirmed by single crystal XRD analysis. Various vibrational modes of functional groups present in the crystal were established through FT-IR analysis. The grown crystal has a wide transparency nature in the Vis-NIR spectral range, which makes these crystals a potentially active material for non-linear optical device applications. Luminescence assessment indicates strong emission of blue radiation around 471, and it may be used for the light emitting diode (LED) applications. The TG/DTA study shows that the crystal possesses good thermal stability up to 260 °C. Mechanical studies reveal a reverse indentation size effect (RISE) and crack development for loads above 100 g. The work hardening coefficient was found to be 2.5, and yield strength (σ_y) and elastic stiffness constant (C₁₁) were calculated. The dielectric constant of the DSTPB signified that the crystal possessed excellent optical quality with defect-free nature. The grown crystal has a superior laser damage threshold (4.3 GW cm⁻²) at 1064 nm wavelength of Nd:YAG laser compared with some NLO materials. An etching study of DSTPB crystals reveals a layer type etch pattern. The powder SHG efficiency was found to be 4.53 times that of KDP crystal. A third-order nonlinear optical study of the DSTPB crystal reveals that a strong saturation absorption and positive sign of nonlinear refractive index (n₂). These results imply that the title crystal can be a used for third-order NLO applications. All the above results suggest that the grown crystal can considered for device fabrication of optical limiting and photonic devices in the future.

Acknowledgements

The authors thank the Defence Research and Development Organisation (DRDO), Government of India, for financial assistance to execute this research work and VIT University management for their help, constant support and scientific research facilities.

References

1. F. Liu, Y. Yang, S. Cong, H. Wang, M. Zhang, S. Bo, J. Liu, Z. Zhen, X. Liua and L. Qiu, RSC Adv., 2014, 4, 52991.
2. Q. Q. Li, C. G. Lu, J. Zhu, E. Fu, C. Zhong, S. Y. Li, Y. P. Cui, J. G. Qin and Z. Li, J. Phys. Chem. B, 2008, 112, 4545–4551.
3. S. R. Marder, B. Kippelen, A. K.-Y. Jen and N. Peyghambarian, Nature, 1997, 388, 845.
4. H. Ma, A. K.-Y. Jen and L. R. Dalton, Adv. Mater., 2002, 14, 1339.
5. N. B. Singh, T. Henningsen, E. P. A. Metz, R. Hamacher, E. Cumberledge and R. H. Hopkins, Mater. Lett., 1991, 12, 270–275.
6. W. Wu, R. Tang, Q. Li and Z. Li, Chem. Soc. Rev., 2015, 44, 3997.
7. Z. Li, Q. Li and J. Qin, Polym. Chem., 2011, 2, 2723.
8. G. Zhuo, X. Wang, D. Wang, C. Wang, Z. Shao, Q. Fang and M. Jiang, Opt. Commun., 2001, 190, 345.
9. N. Nakanishi, H. Matsuda, S. Okada and M. Kato, Proc. MRS Int. Mtg. Adv. Mater., 1989, 1, 97.
10. Z. Yang, L. Mutter, M. Stillhart, B. Ruiz, S. Aravazhi, M. Jazbinsek, A. Schneider, V. Gramlich and P. Gunter, Adv. Funct. Mater., 2007, 17, 2018.
11. T. Matsukawa, Y. Takahashi, R. Miyabara, H. Koga, H. Umezawa, I. Kawayama, M. Yoshimura, S. Okada, M. Tonouchi, Y. Kitaoka, Y. Mori and T. Sasaki, J. Cryst. Growth, 2009, 311, 568–571.
12. T. Matsukawa, T. Notake, K. Nawata, S. Inada, S. Okada and H. Minamide, Opt. Mater., 2014, 36, 1995–1999.
13. X. M. Duan, H. Konami, S. Okada, H. Oikawa, H. Matsuda and H. Nakanishi, J. Phys. Chem., 1996, 100, 17780.
14. S. R. Marder, J. W. Perry and W. P. Schaefer, Science, 1989, 245, 626.
15. P. J. Kim, J. H. Jeong, M. Jazbinsek, S. J. Kwon, H. Yun, J. T. Kim, Y. S. Lee, L. H. Baek, F. Rotermund, P. Günter and O. P. Kwon, CrystEngComm, 2011, 13, 444.
16. F. Pan, M. S. Wong, Ch. Bosshard and P. Gunter, Adv. Mater., 1996, 8, 592.
17. J. Ogawa, S. Okada, Z. Glavcheva and H. Nakanishi, J. Cryst. Growth, 2008, 310, 836.
18. H. Adachi, Y. Takahashi, J. Yabuzaki, Y. Mori and T. Sasaki, J. Cryst. Growth, 1999, 98, 568.
19. X. M. Duan, H. Konami, S. Okada, H. Oikawa, H. Matsuda and H. Nakanishi, J. Phys. Chem., 1996, 100, 17780.
20. B. J. Coe, J. A. Harris, A. K. Clays, G. Olbrechts, A. Persoons, J. T. Hupp, R. C. Johnson, S. J. Coles, M. B. Hursthouse and K. Nakatani, Adv. Funct. Mater., 2002, 12, 110.
21. B. Ruiz, Y. Yang, V. Gramlich, M. Jazbinsek and P. Gunter, J. Mater. Chem., 2006, 16, 2839.
22. J. H. Klein-Wiele, M. A. Bader, L. I. Bauer, S. Soria, P. Simon and G. Marowsky, Synth. Met., 2002, 127, 53–57.
23. D. Y. Wang, C. L. Zhan, Y. Cheng, Y. J. Li, Z. Z. Lu and Y. X. Nie, Chem. Phys. Lett., 2003, 369, 621.
24. C. Zhan, Y. Li, D. Li, D. Wang and Y. Nie, Opt. Mater., 2006, 28, 289.
25. D. C. Zhang, L. Q. Ge, T. Z. Zhang, Y. Q. Zhang and K. B. Yu, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1999, 55, 1938–1939.
26. W. J. Kaminsky, J. Appl. Crystallogr., 2007, 40, 382.
27. I. Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley, London, 1976.
28. T. Karakurt, M. Dinçer, A. Çetin and M. Sekerci, Spectrochim. Acta, Part A, 2010, 77, 189–198.
29. E. Selvakumar, G. Anandhababu, P. Ramasamy, Rajnikant, T. Uma Devi, R. Meenakshi and A. Chandramohan, Spectrochim. Acta, Part A, 2014, 125, 114–119.
30. E. Scrocco and J. Tomasi, Adv. Quantum Chem., 1978, 103, 115–121.
31. N. Okulik and A. H. Jubert, Internet Electron. J. Mol. Des., 2005, 4, 17–21.
32. F. J. Luque, J. M. Lopez and M. Orozco, Theor. Chem. Acc., 2000, 103, 343–345.
33. G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic Press, New York, 1969.
34. D. L. Vein, N. B. Colthup, W. G. Fateley and J. G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, NewYork, 1991.
35. M. Silverstein, G. C. Basseler and C. Morill, Spectrometric Identification of organic compounds, Wiley, New York, 1981.
36. V. Krishnakumar and R. J. Xavier, Spectrochim. Acta, Part A, 2005, 61, 1799–1809.
37. P. N. Prasad, Springer Wave Phenom., 1989, 9, 305–327.
38. S. Sudhahar, M. Krishna Kumar, B. N. Sornamurthy and R. Mohan Kumar, Spectrochim. Acta, Part A, 2014, 118, 929–937.
39. M. A. F. M. Da Silva, I. C. S. Carvalho, N. Cella, H. N. Bordallo and L. P. Sosman, Opt. Mater., 2013, 543–546.
40. J. X. Wang, S. S. Xie, H. J. Yuan, X. Q. Yan, D. F. Liu, Y. Gao, Z. P. Zhou, L. Song, L. F. Liu, X. W. Zhao, X. Y. Dou, W. Y. Zhou and G. Wang, Solid State Commun., 2004, 131, 435–440.
41. R. Sankar, C. M. Raghavan and R. Jayavel, Cryst. Growth Des., 2007, 7, 501–505.
42. K. K. Rao and D. B. Sirdeshmukh, Bull. Mater. Sci., 1983, 5, 449–452.
43. J. Jiaghong Gong, J. Mater. Sci. Lett., 2000, 19, 515.
44. R. Uthrakumar, C. Vesta, C. Justin Raj, S. Krishnan and S. Jerome Das, Curr. Appl. Phys., 2010, 10, 548–552.
45. Micro Indenation Hardness Testing, ed. B. Mott, Butterworth, London, 1956.
46. B. W. Mott, Micro Indentation Hardness Testing, Butterworths, London, 1957.
47. T. S. Shyju, S. Anandhi and R. Gopalakrishnan, CrystEngComm, 2012, 14, 1387–1396.
48. M. Shakir, V. Ganesh, M. A. Wahab, G. Bhagavannarayana and K. K. Rao, Mater. Sci. Eng., B, 2010, 172, 9–14.
49. P. Mythili, T. Kanagasekaran and R. Gopalakrishnan, Cryst. Res. Technol., 2007, 42, 791.
50. J. Hernandez-Paredes, O. Daniel Glossman-Mitnik, H. Hernandez-Negrete, M. E. Esparza-Ponce, R. Alvarez, R. Rodrıguez Mijangos and A. Duarte-Moller, J. Phys. Chem. Solids, 2008, 69, 1974–1979.
51. S. Kalainathan and K. Jagannathan, J. Cryst. Growth, 2008, 310, 2043–2049.
52. E. M. Onitsch, Mikroskopie, 1947, 2, 131.
53. K. Sangwal and A. Klos, Cryst. Res. Technol., 2005, 40, 429–438.
54. J. Rani ChristhuDhas, C. Bennet and F. D. Gnanam, J. Cryst. Growth, 1994, 137, 295.
55. C. Hays and E. G. Kendall, Metallography, 1973, 6, 275.
56. B. Lal, K. K. Bamzai, P. N. Kotru and B. M. Wanklyn, Mater. Chem. Phys., 2004, 85, 353–365.
57. E. E. A. Shepherd, J. N. Sherwood and G. S. Simpson, J. Cryst. Growth, 1996, 167, 709.
58. M. Senthil Pandian, P. Ramasamy and B. Kumar, Mater. Res. Bull., 2012, 47, 1587–1597.
59. R. Ramesh Babu, S. Kumaresan, N. Vijayan, M. Gunasekaran, R. Gopalakrishnan, P. Kannan and P. Ramasamy, J. Cryst. Growth, 2003, 256, 387–392.
60. G. C. Bhar, A. K. Chaudhary and P. Kumbhakar, Appl. Surf. Sci., 2000, 161, 155–162.
61. Z. Sun, G. Zhang, X. Wang, Z. Gao, X. Cheng, S. Zhang and D. Xu, Cryst. Growth Des., 2009, 9, 3251–3259.
62. N. L. Boling, M. D. Crisp and G. Dube, Appl. Opt., 1973, 12, 650–660.
63. G. H. Sun, G. H. Zhang, Z. H. Sun, X. Q. Wang and D. Xu, Mater. Chem. Phys., 2011, 127, 265–270.
64. Z. Sun, G. Zhang, X. Wang, Z. Gao, X. Cheng, S. Zhang and D. Xu, Cryst. Growth Des., 2009, 9, 3251.
65. K. Ambujam, K. Rajarajan, S. Selvakumar, I. Vetha Potheher, G. P. Joseph and P. Sagayaraj, J. Cryst. Growth, 2006, 286, 440–444.
66. C. Balarew and R. Duhlew, J. Solid State Chem., 1984, 55, 1–6.
67. K. Sethuraman, R. Ramesh Babu, R. Gopalakrishnan and P. Ramasamy, Cryst. Growth Des., 2008, 8, 1863–1869.
68. P. Rajesh, P. Ramasamy and G. Bhagavannarayana, J. Cryst. Growth, 2009, 311, 4069.
69. M. Vimalana, T. Rajesh Kumar, S. Tamilselvan, P. Sagayaraj and C. K. Mahadevan, Phys. B, 2010, 405, 3907–3913.
70. C. Balarew and R. J. Duhlew, Solid State Chem., 1984, 55, 1–6.
71. S. I. Bhat, P. M. Rao, A. P. G. Bhat and D. K. Avasthi, Surf. Coat. Technol., 2002, 158, 725–728.
72. J. S. Pan and X. W. Zhang, Acta Mater., 2006, 54, 1343–1348.
73. J. D. Jackson, Classical Electrodynamics, Wiley, Eastern, 1978, p. 321.
74. D. R. Penn, Phys. Rev., 1962, 128, 2093–2097.
75. N. M. Ravindra, R. P. Bhardwaj, K. Sunil Kumar and V. K. Srivastava, Infrared Phys., 1981, 21, 369–381.
76. N. M. Ravindra and V. K. Srivastava, Infrared Phys., 1980, 20, 67–69.
77. P. Vasudevan, S. Sankar and D. Jayaraman, Bull. Korean Chem. Soc., 2013, 34, 128–132.
78. P. Vasudevan, S. Sankar and S. GokulRaj, Optik, 2013, 124, 4155–4158.
79. S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968, 44, 455.
80. P. C. RajeshKumar, V. Ravindrachary, K. Janardhana and B. Poojary, J. Cryst. Growth, 2012, 354, 182–187.
81. M. Sheik-Bahae, A. A. Said and E. W. van Stryland, Opt. Lett., 1989, 14, 955.
82. T. Jia, T. He, P. Li, Y. Mo and Y. Cui, Opt. Laser Technol., 2008, 40, 936.
83. X.-B. Sun, Y.-L. Wang, Q. Ren, Fu-jun Zhang, Y. Gao, H.-L. Yang, L. Feng, X.-Q. Wang and D. Xu, Opt. Mater., 2007, 29, 1305–1309.
84. L. Irimpan, A. Deepthy, B. Krishnan, L. M. Kukreja, V. P. N. Nampoori and P. Radhakrishnan, Opt. Commun., 2008, 281, 2938–2943.
85. V. P. N. Nampoori and P. Radhakrishnan, Opt. Commun., 2008, 281, 2938–2943.
86. K. Senthil, S. Kalainathan, A. Ruban Kumar and P. G. Aravindan, RSC Adv., 2014, 4, 56112.
87. K. Senthil, S. Kalainathan, F. Hamada, M. Yamada and P. G. Aravindan, Opt. Mater., 2015, 46, 565–577.
88. E. W. van Stryland and M. Sheik-Bahae, Z-scan technique for nonlinear materials characterization, in Characterization Techniques and Tabulations for Organic Nonlinear Materials, ed. M. G. Kuzyk and C. W. Dirk, Marcel Dekker, New York, 1998, pp. 655–692.
89. T. Kanagasekaran, P. Mythili, P. Srinivasan, A. Y. Nooraldeen, P. K. Palanisamy and R. Gopalakrishanan, Cryst. Growth Des., 2008, 8, 2335–2339.
90. C. Gayathri and A. Ramalingam, Spectrochim. Acta, Part A, 2007, 68, 578–582.
91. T. Cassano, R. Tommasi, M. Ferrara, F. Babudri, G. M. Farinola and F. Naso, Chem. Phys., 2001, 272, 111–118.
92. J. Sun, W. F. Guo, X. Q. Wang, G. H. Zhang, X. B. Sun, L. Y. Zhu, Q. Ren and D. Xu, Opt. Commun., 2007, 280, 183–187.
93. H.-L. Fan, Q. Ren, X.-Q. Wang, T.-B. Li, J. Sun, G.-H. Zhang, D. Xu, G. Yu and Z.-H. Sun, Nat. Sci., 2009, 1, 136–141.
94. Y. S. Zhou, E. B. Wang, J. Peng, J. Liu, C. W. Hu, R. D. Huang and X. You, Polyhedron, 1999, 18, 1419–1423.
95. M. Krishna Kumar, S. Sudhahar, A. Silambarasan, B. M. Sornamurthy and R. Mohan Kumar, Optik, 2014, 125, 751–755.
96. E. D. D’silva, G. Krishna Podagatlapalli, S. Venugopal Rao and S. M. Dharmaprakash, Mater. Res. Bull., 2012, 47, 3552–3557.
97. R. H. Bube, Photoconductivity of solids, Wiley Interscience, New York, 1981.
98. N. V. Joshi, Photoconductivity, Marcel Dekker, New York, 1990.
99. R. H. Bube, Photoconductivity of Solids, Wiley Interscience, New York, 1960.
100. R. N. Jayaprakash and P. Kumaradass, Orient. J. Chem., 2013, 29, 1409–1414.
101. B. Milton Boaz, R. Samuel Selvaraj, K. Senthil Kumar and S. Jerome Das, Indian J. Phys., 2009, 83, 1647–1657.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14186a

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