Synthesis of thermochromic CdHgI4 thin film: Insight into the thickness-dependent structural, morphological, linear, and nonlinear spectroscopic properties

Abstract

This study introduces an in-depth exploration of structural, morphological, linear, and nonlinear optical properties of CdHgI4 thin films deposited by the thermal evaporation technique. In this study, cadmium tetra iodomercurate (CdHgI4) nanopowder was synthesized by fusion method from which thin films of various thicknesses (150 to 450 nm) were deposited on glass substrates by thermal evaporation technique. The Structural investigation conducted by X-ray diffractometer (XRD) established that both CdHgI4 nanopowder and thin film formed in a single phase with a tetragonal crystallographic system. However, the CdHgI4 nanopowder and films display a different preferred orientation as corroborated by texture coefficient determination (Tc). The microstructural features obtained by High-resolution transmission electron microscopy (HRTEM) revealed that the powder particles were agglomerated; however, the particles of thin film were well-defined with high compactness and uniform size. The purity as well as stoichiometry were judged by energy dispersive X-ray analysis confirming the absence of any impurities and samples adopted near stoichiometry features. The surface morphology done by field emission scanning electron microscope revealed that films constituted of nanostructured grains with noticeable growth particles with increasing thickness. The CdHgI4 nanopowder had a direct optical transition of band gap of about 1.41 eV evaluated from diffuse reflectance spectroscopy. The band gap of thin films determined from specular optical spectroscopy varied from 2.20 to 1.58 eV, as the thickness changed from 150 to 450 nm. The thickness dependence of other linear optical parameters, such as Urbach energy (Eu), refractive index (n), optical dielectric constants (ε1, ε2), the surface and volume energy loss (SELF, VELF), were elaborately discussed. The nonlinear optical properties including first-order linear susceptibility, third-order susceptibility (χ(1), χ(3)), NL refractive index (n2), and two-photon absorption coefficients (βc) were greatly dependent on the film thickness.