Photoluminescence and optical nonlinearity of CdS quantum dots synthesized in a functional copolymer hydrogel template

Abstract

A hydrogel template assisted synthesis was employed to prepare CdS quantum dots (QDs) in poly(methyl methacrylate-co-acrylic acid), p(MMA-co-AAc), for the ultimate formation of a nanocomposite. The copolymer hydrogelates in an aqueous solution of precursor reagents and acts as a template for the absorption of Cd²⁺ ions, which is followed by CdS QD formation. The CdS QDs are positioned inside the core of the micelles in the copolymer hydrogel. The ultraviolet-visible (UV-vis) absorption and band edge emission of the composite solution are blue shifted relative to bulk CdS. The composition of acrylic acid (AAc) in the copolymer has an influence on the size as well as on the surface effect of the CdS QDs, which is reflected in the photoluminescence (PL) of the composite. The copolymer is capable of passivating CdS QDs for intense band edge emission with weak defect state emission. The z-scan technique gives an exhibition of the nonlinear refraction and absorption of the composite, which are determined by the AAc content and thus the size of the CdS QDs present in the composite. The composites show self defocusing phenomena for their negative nonlinear refraction. The magnitude of nonlinear refractive index increases with the CdS QD size. The copolymer with a large number of AAc units forms a CdS QD composite with reverse saturable absorption (RSA) near the focus and saturable absorption (SA) at the focus. The RSA behavior of the composites shows that they can be used in optical limiting devices.