Pongamia pinnata-mediated green synthesis of Er-doped CdS thin films: enhanced photocatalysis and high-efficiency optoelectronics

This study presents a green synthesis route for the fabrication of pristine and erbium (Er)-doped cadmium sulfide (CdS) thin films using Pongamia pinnata (karanj) leaf extract as a natural reducing and capping agent. The bioactive phytochemicals in the extract enabled the controlled synthesis of highly crystalline CdS nanostructures via a chemical bath deposition process. X-ray diffraction confirmed the hexagonal wurtzite phase across all samples, with a reduction in crystallite size from 19.34 nm (pristine CdS) to 6.72 nm (0.05 M Er-doped CdS), attributed to the inhibition of grain growth by phytochemical interactions. Optical analysis using UV–Vis spectroscopy revealed a tunable bandgap from 3.2 eV to 2.05 eV, modulated by Er³⁺-induced defect states and quantum confinement effects. Photoluminescence measurements demonstrated enhanced visible emission in the red region (678–681 nm) due to Er³⁺ intra-4f transitions. Density Functional Theory (DFT) simulations further supported the experimental findings by confirming changes in band structure and density of states upon Er incorporation. The combined experimental and theoretical approach demonstrates the potential of green-synthesized Er-doped CdS films for photocatalytic and optoelectronic applications.