Understanding how composition controls photoluminescence is essential for improving semiconductor nanomaterials used in optical sensing and light-emitting technologies. In this work, cadmium sulfide (CdS) nanostructures were synthesized using a sonochemical method with different Cd:S molar ratios (1:0.1, 1:0.25, 1:0.5, 1:0.75, and 1:1), and their structural and emission properties were systematically investigated. Ultrasound-driven nucleation produced nanocrystals ranging from irregular Cd-rich aggregates to well-defined, uniform particles at stoichiometric compositions. XRD analysis revealed a controlled phase transition from purely hexagonal CdS to mixed hexagonal–cubic structures, with crystallite sizes between ~6 and 35 nm.

Photoluminescence measurements demonstrated two characteristic emission regions: near-band-edge (≈420 nm) and deep-level defect-related bands (≈540–560 nm). Emission intensity and spectral position were strongly dependent on precursor stoichiometry. The 1:1 sample showed the highest PL intensity with a dominant emission around 547 nm, indicating enhanced defect-assisted radiative recombination. In contrast, intermediate compositions, particularly 1:0.5, exhibited broadened PL bands and suppression of higher-order Raman modes, consistent with phonon confinement and increased surface defect activity. UV–Vis absorption confirmed composition-dependent band gap modulation, reflecting quantum confinement and defect state evolution.

These results show that tuning the Cd:S ratio provides a simple and effective strategy for controlling crystal phase, defect density, and photoluminescent behavior in CdS nanomaterials. This approach is promising for light-emitting devices, UV photodetectors, and photochemistry-driven optoelectronic applications.