Third-order nonlinear optical properties of thermally treated and untreated zeolitic imidazolate framework-8

Zeolitic Imidazolate Framework-8 (ZIF-8) is a hybrid material with organic and inorganic moieties. The synergetic effect of thermal treatment and solvent polarity unveil significant nonlinear optical (NLO) properties of ZIF-8. Material with good porosity, high surface area, better charge transfer, charge bonding and displaying good NLO behaviour is the key requirements for photonic applications. In this regard for the first time, we report the synthesis and studies on the third-order nonlinear susceptibility χ⁽³⁾ of thermally treated (calcined) and untreated zeolitic imidazolate framework-8 (ZIF-8) under continuous wave (CW) laser. ZIF-8 powder at room temperature (RT) was synthesized using the solvothermal method, and then it was calcined at different temperatures (450 °C and 650 °C). To study the effect of optical nonlinearity, the thermally treated and untreated ZIF-8 samples (RT, 450 °C and 650 °C) were dispersed in various polar solvents, viz., distilled water, dimethylformamide, ethanol and methanol. The samples were subjected to structural, morphological, and optical characterizations. X-ray diffraction patterns reveal a good crystalline nature for all samples, and the restructuring from ZIF-8 into ZnO due to calcination was evident. Field emission scanning electron microscopic images depict a rhombic dodecahedron shape for ZIF-8 (RT) and a cubic shape for ZIF-8 (650 °C), and the particle size varies with an increase in the calcination temperature. The images also confirm the formation of porous ZnO structure due to the calcination of ZIF-8. Thermogravimetric and Brunauer-Emmett-Teller analysis disclose good thermal stability (435 °C) and a high BET surface area of 1001 m²/g for ZIF-8 (RT) and with further increase in calcination temperature, the surface area decreases. The nonlinear optical studies show reverse saturable absorption and self-defocusing nature for all the samples. High third-order nonlinear susceptibility χ⁽³⁾ and a good optical clamping of ∼3.2 mW were displayed by ZIF-8 (RT) dispersed in methanol.