Investigation of the surface states in quantum-confined semiconductor clusters

Abstract

Photoluminescence measurements have been shown to be a means of elucidating the chemical nature of surface defects in both bulk and Q-size cadmium sulfide (Q-CdS), a direct bandgap material. In this work, surface modification of Q-CdS clusters with various photoluminescence (PL)-modifying Lewis acids/bases was examined. Lanthanide $\beta$-diketonate complexes (Lewis acids) and amines (Lewis bases) were shown to enhance PL in inverse micelle stabilized Q-CdS. The effect of the addition of amino- and carboxylic acid-substituted ferrocene derivatives on the PL of inverse micelle stabilized Q-CdS was also investigated. Due to problems inherent in the use of inverse micelle stabilizers, a novel Q-cluster stabilizer was developed. The synthesis of Q-CdS stabilized by aminocalixarene molecules was shown to produce three distinct types of Q-CdS clusters whose properties depend on the ratio of aminocalixarene to CdS. Aminocalixarene-QCdS clusters were shown to possess a cationic stabilizer-solution interface, in stark contrast to other stabilizers commonly employed in Q-cluster synthesis. Such stabilizers typically possess an anionic stabilizer-solution interface. The differences in stabilizer electrostatics were investigated through a comparison of the stabilizer-mediated quenching of inverse micelle stabilized, aminocalixarene stabilized, and thiophenol capped Q-CdS. The study of the influence of surface chemistry on Q-cluster photophysics was extended to include an indirect bandgap material: Crystallites derived from porous silicon. The quenching effect of amines on photoluminescence from a solution of these Si crystallites was compared to the known quenching behavior of PL from porous Si wafers. The ability of amines to reversibly adsorb onto porous Si wafers and crystallites from porous Si is described via a 'bottle neck' mechanism. An intriguing 'antiquenching' effect of alcohols and acidified ethylene glycol on Si crystallite luminescence was also demonstrated.