Bio-inspired synthesis of Eu-doped CdS thin films using Pongamia pinnata extract: A green pathway for tunable optoelectronic materials

A natural bio-inspired approach for tailoring the optical properties of cadmium sulfide (CdS) thin films through europium (Eu) doping was reported in this paper, wherein Pongamia pinnata leaf extract was employed as a sustainable medium for chemical bath deposition. The phytochemicals present in the extract were utilized as effective capping agents, by which the controlled growth of high-quality nanocrystalline films was achieved. X-ray diffraction confirmed the hexagonal phase of CdS; and systematic decrease of crystallite size, 41.9 nm (pristine) to 6.1 nm (0.1 M Eu) was seen when Eu was incorporated, with notable micro strain within the lattice. This structural modification was found to facilitate precise tuning of the optoelectronic properties. A substantial bandgap reduction from 3.30 eV (pristine CdS) to a tunable range of 2.25–2.70 eV (Eu-doped films) was revealed through optical analysis. Characteristic sharp emissions corresponding to Eu³ ⁺ transitions (⁵D₀ → ⁷F₂), along with a defect-related band, were exhibited in photoluminescence spectra. Surface states were effectively passivated by the phytochemical environment, resulting in enhanced optical purity and emission intensity of Eu³ ⁺. To the best of our knowledge, the synthesis of Eu-doped CdS thin films using Pongamia pinnata extract has been reported here for the first time. The demonstrated combination of significant bandgap tunability and enhanced luminescence purity highlights the potential of this green synthesis route for the development of wavelength-tunable light-emitting diodes (LEDs), thin-film photovoltaic devices, and advanced photonic sensors.