Energy Level Modification in Lead Sulfide Quantum Dot Thin Films Through Ligand Exchange

We report the influence of changing surface ligands of synthesized CdSe nanocrystals on its structural, optical, and electrical properties. TOPO-capped CdSe nanocrystals of 2 to 7 nm sizes have been synthesized by hot injection method, and the surface capping molecule, TOPO was subsequently replaced by tributylamine. It was found that the exchange of surface ligands significantly altered the physical properties of nanocrystals. The shifting toward the shorter wavelength (higher energy) in the visible range of optical absorption band edge was observed by UV-VIS spectroscopy, and a blue-shift of photoluminescence peaks was observed with luminescent quenching. A slight reduction in nanocrystal's size during the ligand exchange procedure was confirmed by transmission electron microscopy, indicating the exchange in quantum confinement effect. The surface ligand exchange was also found to cause the increase in the nanocrystal's surface energy, resulting in the improvement of charge carrier separation.