Synthesis and characterization of ultra-stable Bi-based thin films: Tailoring multifunctionality in Bi2-xCoxO3

This study presents the synthesis and comprehensive characterization of ultra-stable cobalt-doped bismuth oxide thin films with the nominal composition Bi_2-xCo_xO₃, prepared via spin coating and subsequent thermal treatments. The incorporation of Co into the Bi₂O₃ matrix significantly modified the structural, optical, and dielectric properties of the films. XRD analyses confirmed the construction of a stable tetragonal phase with developed crystallinity and strengthened crystallite size (from 21.05 to 37.37 nm) upon increasing cobalt doping. Structural strain and dislocation density were found to decrease with increasing Co concentration, indicating enhanced structural integrity. UV–Vis spectroscopy revealed a tunable bandgap, decreasing initially from 3.12 eV to 2.97 eV, then rising to 3.58 eV at higher doping levels, highlighting quantum confinement and saturation effects. A marked variation in optical constants, for instance, the extinction coefficient, refractive index, and Urbach energy, suggested improved electronic transitions and reduced material disorder. Dielectric studies further indicated enhanced real and imaginary permittivity, while the loss tangent (tan δ) decreased with doping, signifying reduced energy dissipation. Energy loss functions (VELF and SELF) increased with Co content, suggesting potential utility in applications requiring controlled energy dissipation. These findings establish Bi_1.95Co_0.05O₃ (BOC-0.05) as a highly stable and multifunctional material, ideal for optoelectronic, catalytic, and high-temperature electronic applications.