The effect of tetraalkylammonium and alkyl sulfate salts on the fluorescence bands of quantum-sized CdS

The effect of alkyl sulfates and tetraalkylammonium salts on fluorescence quantum yield and lifetime of several colloidal Q-CdS particles was studied. The Q-CdS clusters were prepared to emit an isolated fluorescence band, with a maximum fluorescence wavelength at 460 nm, 481 nm or 559 nm. Quaternary ammonium salts and alkyl sulfates bearing alkyl groups with several chain lengths were used: R,NCI, with R=C(2)H(5), C(4)H(9) or C(6)H(13) and NaRSO(4), with R=CH(3), C(8)H(17) and as a reference, Na(2)SO(4) and/or NaCl. A partial quenching effect of the salts on the Q-CdS clusters fluorescence was observed. For a given alkyl group, the quenching effect increases with the maximum wavelength of the fluorescence band. The influence of the alkyl groups on the quenching of a given fluorescence band was found to be different for the two kinds of salts. We believe that the quenching by tetraalkylammonium salts is due to the adsorption of the cations on the clusters surface. This could lead to ionic pair formation by R(4)N(+) with negative either phosphate or surface sites on the Q-CdS clusters, stabilizing electrons in surface traps. The quenching effect increased with the length of the alkyl chain. This may be due to a hydrophobic effect acting to give this trend. In the case of quenching by sodium alkylsulfate salts, the values of K(sv) are similar for the different substrates and for a given band. This could be explained if the quenching effect is mainly due to Na(+). However, the values of K(sv) are different for the different bands. Since Cd(+2) is used to activate the fluorescence when the clusters are prepared, we believe that the quenching process consists in a displacement of Cd(+2) by Na(+) on the surface of the clusters. Since the several clusters have different surface properties, the effect varies for each of the clusters. (C) 1998 Elsevier Science S.A. All rights reserved.