Effect of benzene derivatives bearing electron-releasing and/or electron-withdrawing groups on the fluorescence of CdS-Q clusters

The several surface traps in the semiconductor clusters give origin to luminescence bands with different efficiency and lifetime and are also surface sites which can have different affinity for the quencher. We have mentioned in a previous work [J. Photochem. Photobiol. A: Chem. 99 (1996) 177] that the excitonic band originates in the radiative recombination of electrons in the conduction band, after some interval of time residing in shallow surface traps, which are in thermal equilibrium with the conduction band. Broad band and trapped band originate, in turn, in the direct radiative recombination of electrons in deep surface traps, which are not in thermal equilibrium with the conduction band. We propose in this paper a mechanistic scheme, which fits our experimental results, based in the consideration of multiple active surface sites.