Crystal growth, perfection, linear and nonlinear optical, photoconductivity, dielectric, thermal and laser damage threshold properties of 4-methylimidazolium picrate: an interesting organic crystal for photonic and optoelectronic devices

A 4-MIP single crystal was grown by a slow solvent evaporation technique using methanol as a solvent. The starting materials of 4-methylimidazole (98% of purity) and picric acid (99% of purity) were taken in an equimolar ratio. The reagents were dissolved in 95% of methanol solvent separately. Then the mixture was stirred for more than 2 h at room temperature and then filtered. The filtered pale yellow solution was kept in a beaker and allowed to evaporate in an air atmosphere for 2 weeks. Needle-shaped yellow seed crystals of 4-MIP were harvested. The seed crystals were carefully separated from the parental solution and purified by a successive recrystallization process. For further characterization, the bulk crystals were obtained after a period of 21 days and are shown in figure 1.

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Single and powder XRD studies were carried out to confirm the crystal structure of the 4-MIP single crystal. Single crystal XRD studies showed that the lattice parameter values of the grown crystal were a = 9.305 Å, b = 9.431 Å and c = 15.189 Å and it was crystallized in a monoclinic crystal system with the centrosymmetric space group P2₁/c. These values are in good agreement with the reported values [8]. A powder XRD pattern of 4-MIP crystal was recorded over the range 10°–70° at a scan rate of 1°/min. The powder XRD pattern of the crystal is shown in figure 2. The PXRD peaks indexed and the cell parameter of the crystal were calculated. The cell parameter values of the grown 4-MIP crystal found from the single crystal XRD study are compared with the reported values and are shown in table 1.

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The strain calculations of the crystal in the grain boundaries can be made from the powder XRD values by using the Hall–Williamson equation [9] given below:

$$\begin{eqnarray*}&&\beta \,\cos \,\theta ={K}\lambda /\tau +\eta \,\sin \,\theta ,\end{eqnarray*}$$

where, β, θ and K are the full width at half maxima (FWHM) of the diffraction peak, Bragg diffraction angle and Scherrer constant, λ, τ and η are the wavelength of the x-rays, crystalline size and slope value respectively. The strain value was calculated from the slope value in the plot between β cos θ and sin θ, as shown in figure 3. The strain of the crystal was found to be 0.004 32 and the actual values are slightly different from the linear fit, which suggests that the crystal carries a minimum compressive strain.

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The crystalline quality and perfection of the 4-MIP crystal was studied by high resolution XRD (HRXRD). The recorded diffraction curve of the 4-MIP crystal (figure 4) contains a sharp single peak without any satellite peaks which results in the crystal being completely free from any structural grain boundaries [10]. However, the parental solution was purified by the reflux method. Water molecules were distilled from the solution which yielded the presence of a single peak and the absence of impurities at the macroscopic level. This shows that the grown crystal has a good crystalline quality. The FWHM of the curve is 31 arc sec which is very close to an ideally perfect crystal [11]. The scattered intensity is high in the positive direction when compared to the negative direction. This demonstrates that the 4-MIP crystal contains more interstitial defects than vacancy defects [12]. The compressive stress and lattice parameter decrease due to these interstitial defects which leads to a high intensity and Bragg angle [13].

The linear optical properties of the 4-MIP crystal were studied, giving useful information about UV and visible light. The transmission graph in figure 5(a) reveals that the grown crystal exhibits low transmittance in the UV region and high transmittance in the entire visible region. This property is one of the most highly advantageous in the field of opto-electronics for device fabrication [14]. The lower cut off wavelength is 209 nm and this kind of minimum lower cutoff wavelength is a desirable requirement for frequency doubling application using a solid state laser and diode [15].

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The optical band gap energy (E_g) of the crystal was calculated from the absorption coefficient and it is found to be 4.37 eV, which is in good agreement with the value obtained from Tauc's plot in figure 5(b). One can easily find the E_g value by extrapolating the linear part in the Tauc's plot [16].

The PL technique is one of the best and accurate methods for determining the optical quality as well as its excitation fine structure [17]. The PL emission spectrum was recorded using a Perkin Elmer PL unit at room temperature with a slit width of 10 nm in the wavelength range of 200–500 nm. The observed PL emission spectrum of the 4-MIP crystal is shown in figure 6. The spectrum contain an excitation peak at a wavelength of λ_ex = 296 nm. A single high intense emission band and only one excitation state has been observed due to intermolecular interactions of lattice vibrations in the 4-MIP crystal. This shows the better optical and crystalline quality of the crystal.

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Low power laser devices are widely used in various applications, such as optical switches and modulators [18]. The linear optical absorption coefficient value of the material was examined in an air atmosphere at room temperature by the OL method. The room temperature was nearly 25 °C.

The low intense laser light from the source was focused on the sample and the output power was measured using a power meter detector. This process was repeated for various input power values and the corresponding output values obtained from the power meter were recorded. A similar procedure was followed to record the output of the laser without the incident of any material. A graph was drawn between the input and output power and is shown in figure 7.

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The linear absorption α can be measured by using the following relation:

$$\begin{eqnarray*}&&\alpha =\displaystyle \frac{1}{L}L{\rm{n}}\displaystyle \frac{P}{{P}_{{\rm{o}}}}\end{eqnarray*}$$

where, P is the output power of the laser after hitting the sample, P_o is the output power for the direct laser source without hitting the sample and L is the focal length. The linear absorption of the 4-MIP crystal was found to be 2.97 J cm⁻¹.

A birefringence study is another method to find the optical perfection in the crystal. The homogeneity and defects in the grown crystals can be analyzed by a birefringence study [19]. The modified channel spectrum method is used for quantative assessment for the optical homogeneity of the grown crystal. When highly intense light is passed through the crystal, the ordinary and extraordinary waves are perpendicular to each other. The phase difference is 180° between these two polarized components of wavelength. The birefringence values have been calculated by finding the absolute fringe orders using the relation:

$$\begin{eqnarray*}&&{\rm{\Delta }}{\rm{n}}=\displaystyle \frac{{K}\lambda }{t}\end{eqnarray*}$$

where, λ is the wavelength, t is the thickness of the crystal and K is the order of the fringe. Figure 8 shows the variation of the birefringence with the wavelength. From the graph, it can be seen that the value of the birefringence lies in between 0.032–0.057 in the wavelength range 320 nm–610 nm. The minimum variation in the birefringence over a wide range of wavelength indicates that the crystal is suitable material for harmonic generation device fabrication [20]. The obtained values were found to be positive integer and increased with increasing wavelength, which illustrates that the grown 4-MIP crystal possesses a positive dispersion of birefringence [21]. The plot shows that some distortion from the linearity is present in the plot. A slight dispersion evident in the birefringence can be very helpful in the frequency conversion process, such as second and third harmonic generations [22].

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The NLO property of the grown crystal is characterized by the Kurtz–Perry powder technique with the use of a Nd:YAG laser light [23]. The standard SHG crystals of KDP and urea have been used as the reference materials for this study. The output SHG signal of 43.9 mV for the 4-MIP crystal was obtained for an input energy of 5 mJ/pulse, whereas the KDP and urea crystals gave an output of 18.4 mV and 25.9 mV respectively for the same input signal . This confirms that the SHG efficiency of the 4-MIP crystal is 2.38 times that of the KDP crystal and 1.70 times that of urea. This is the main advantage of this crystal and it is suitable for device fabrication. Comparison of the SHG value of the title compound along with KDP and urea are shown in table 2.

Third harmonic generation is one of the best methods to prove the higher order NLO property. This will fulfill the requirements in technological applications of optical information processing and broadband communications in optical amplification, switching and image processing [24].

Another advantageous technique for measuring the third order harmonic nonlinear efficiency is the single beam z-scan technique. It is a simple, highly sensitive and accurate method to determine the nonlinear refractive index n₂ (real part) and non-linear absorption coefficient β (imaginary part) of the third order generation [25].

A Q-switched Nd:YAG laser of 1064 nm wavelength is used for this experiment. The obtained results of the z-scan curves in open and closed aperture modes are shown in figures 9 and 10.

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The nonlinear refractive index can be obtained by the relation given by,

$$\begin{eqnarray*}&&{n}\,\,=\,| {\rm{\Delta }}{\rm{\Phi }}{\rm{0}}| /{{KI}}_{0}{{L}}_{{\rm{eff}}}\end{eqnarray*}$$

where K is the wave number (K = 2π/λ) and I₀ is the intensity of the laser beam at the focus (Z = 0), L_eff is the effective thickness of the sample, L is the thickness of the sample and α is the linear absorption. From the open aperture curve line, the nonlinear absorption coefficient (β) was estimated by,

$$\begin{eqnarray*}&&\beta =[2\sqrt{2}{\rm{\Delta }}{\rm{T}}/{{I}}_{0}{{L}}_{{\rm{eff}}}]\end{eqnarray*}$$

where ΔT is the peak value at the open aperture data. The absolute value of the optical susceptibility (χ³) was calculated by using the equation,

$$\begin{eqnarray*}&&| {\chi }^{3}| \,=[{{\rm{R}}}_{{\rm{e}}}{[({\chi }^{3})]}^{2}]+[{{\rm{I}}}_{{\rm{m}}}{[({\chi }^{3})]}^{2}]\end{eqnarray*}$$

where R_e and I_m are the real part and imaginary part of the optical susceptibility. From the z-scan data, the calculated value of (χ³) is 2.353 × 10⁻³ esu and the nonlinear refractive index (n₂) is 4.898 × 10⁻⁷ and it is due to the π electron cloud movement from the donor to the acceptor which makes the molecules highly polarized [26].