Optical Response of Dendrimer-Encapsulated CdS Quantum Dots—Regulation of Inter-Particle Electronic Coupling

Abstract

Optical properties of metal and semiconductor nanoparticles are sensitively dependent upon electronic coupling between the individual particles. In another work, we have demonstrated the ability to regulate inter-particle electronic coupling between metal nanoparticles based on the solution-phase properties of the dendrimers used to encapsulate and stabilize the particles. Here, we attempt to apply the knowledge derived in those studies of metal nanoparticles to understand the optical response observed in dendrimer-stabilized CdS quantum dots. Absorption and photoluminescence measurements have been combined with TEM to determine if dendrimerrelated aggregation phenomena affect optical response in these systems. The results show that dendrimer-stabilized CdS nanoparticles exhibit quite different aggregation behavior from the analogous metal nanoparticles. In direct contrast to the metal systems, the dendrimer-stabilized CdS nanoparticles tend to flocculate at low pH and are most stable at high pH. The degree of protection offered at high pH is dependent on the terminal functional groups of the dendrimer. Carboxylated dendrimers give higher protection than aminated dendimers and yield significantly higher luminescence efficiencies. The results challenge our prior conclusions regarding aggregation mechanisms in dendrimer nanocomposites and will provide important insight for future efforts to incorporate these nanoparticles into photovoltaic or luminescent devices.