<p>Cadmium sulfide (CdS) thin films were synthesized using the powder vapor transport (PVT) technique and irradiated with 700&#xa0;keV Cu²⁺ ions at fluence of 1 × 10¹³ and 1 × 10¹⁴ ions/cm². The structural, morphological, and optical properties of the films were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet–visible (UV–Vis) spectroscopy. XRD analysis indicated that the as-grown film was polycrystalline, exhibiting a stable hexagonal structure with preferred orientation along the (002) plane after ion irradiation. The peak intensity decreased with higher fluence, suggesting a gradual disorder in crystallinity. SEM images revealed that the initial growth of the film produced 1&#xa0;μm particles with nonuniform grain sizes and irregular shapes. Heavy ion irradiation led to atomic displacement, lattice distortion, and surface modifications, inducing grain coalescence and increased roughness. The Cu²⁺ ion-irradiated films exhibited a gradual decrease in bandgap from 2.7&#xa0;eV to 1.2&#xa0;eV, reduced optical transmission, and increased refractive indices with higher irradiation fluence. The results suggest that Cu-ion irradiation significantly alters the electronic structure, which may be useful for tailoring the optical properties of CdS-based optoelectronic devices.</p>