Surface Effects on Photoluminescence and Optical Nonlinearity of CdS Quantum Dots Stabilized by Sulfonated Polystyrene in Water

Abstract

A simple aqueous method has been used for the direct synthesis of water-soluble sulfonated polystyrene (SPS) encapsulated CdS quantum dots (QDs). The aqueous solution of the QDs, stabilized by two SPSs of different sulfonation level, shows a blue shift in ultraviolet–visible (UV–vis) absorption and band edge emission. High resolution transmission electron microscope (HR-TEM) establishes the presence of quantum confined CdS particles. The sulfonation level of SPS influences the size of QDs and strongly affects the photoluminescence (PL). The CdS QDs passivated by SPS with less sulfonation level exhibit only band edge emission (463 nm), while QDs in aqueous solution of SPS with high sulfonic acid content emits predominantly defect state emission (610 nm) against the band edge emission (463 nm). The PL of the SPS-CdS nanocomposites demonstrates an interesting variation in emission spectra and quantum yield via the change in sulfonation level, the change in pH and on keeping in aqueous solution. In addition, the z-scan measurements show negative nonlinear refraction coefficient, indicating self-defocusing phenomena. The absolute value of the third order nonlinear refractive index and nonlinear absorption are decided by the CdS QDs surface chemistry.