Optimizing nanostructure and optical properties of CdO thin films via molarity-controlled SILAR deposition

Abstract

The characteristics of cadmium oxide (CdO) thin films are closely tied to the synthesis parameters employed during their preparation. Among these, the precursor solution molarity significantly affects the structural, surface, and optical properties of the deposited films. Despite its importance, the specific impact of molarity has not been adequately addressed in the literature, posing a challenge for optimizing these films for applications such as gas sensors and optoelectronic devices. In the present investigation, CdO films were deposited via the successive ionic layer adsorption and reaction (SILAR) method using precursor concentrations of 0.10, 0.15, and 0.20 M. X-ray diffraction studies confirmed the formation of a polycrystalline cubic phase with a dominant (111) orientation. An upward trend in crystallite size was observed with increasing molarity, registering approximate values of 16.27 nm, 16.91 nm, and 18.91 nm for the respective concentrations. Concurrently, the lattice strain decreased from 21.35 to 18.82, and the dislocation density decreased from 37.84 to 29.47 with increasing molarity. FE-SEM and particle size analysis of CdO films show denser, more compact surfaces with increasing molarity. At 0.10 M, the surface is smoother with clear grain boundaries, whereas higher concentrations yield larger, clustered particles. Particle size increased from 31.81 to 35.24 nm, reflecting enhanced growth at higher molarity. Optical characterization revealed a transmittance decrease and an absorbance increase with the increasing molarity. In addition, the optical parameters of absorption coefficient, extinction coefficient, and refractive index also decreased with the molarity. Specifically, the optical gap value dropped from 2.83 eV at 0.10 M to 2.70 eV at 0.20 M. The morphological optical property dependence of increasing molarity was attributed to the precursor concentration effect on the structure formation of CdO thin film. The dense, structurally homogeneous nanofilms offer a promising perspective for applications ranging from sensing to enhanced surface area and structural stability.