FLUORESCENCE ACTIVATION AND SURFACE-STATE REACTIONS OF SIZE-QUANTIZED CADMIUM-SULFIDE PARTICLES IN LANGMUIR-BLODGETT-FILMS

Monolayers formed from dioctadecyldimethylammonium bromide (DODAB) on aqueous sodium hexametaphosphate (Na-HMP)-stabilized, 40- and 60-Angstrom-diameter CdS sols were characterized by surface pressure us surface area isotherms and Brewster angle microscopy. The quantitative transfer of ''DODAB-HMP-stabilized CdS clusters-DODAB'' sandwich units onto quartz substrates, by Langmuir-Blodgett. (LB) techniques, was demonstrated by absorption and fluorescence spectroscopy. The ''DODAB-HMP-stabilized CdS clusters-DODAB'' sandwich units remained stable for weeks in the solid state and could be immersed into aqueous solutions without any structural alteration. Penetration of small ions between the LB films permitted the investigation of surface-state reactions by fluorescence spectroscopy and fluorescence decay measurements. The fluorescence spectrum of HMP-stabilized sols in aqueous solutions bad a narrow band at 495 nm (excitonic fluorescence) and a broad band with a maximum ca. 670 nm (due to the decay of trapped holes). Excitonic fluorescence, predominating in the dry ''DODAB-HMP-stablized CdS clusters-solutions of cadmium, strontium, and silver ions, and triethylamine. Fluorescence decay times (tau) and beta values, calculated from I(t) = I-0 exp-[-(t/tau)(beta)], of aqueous CdS sols changed from tau 10.03 +/- 0.31 ns and beta = 0.64 +/- 0.22 to tau = 14.99 +/- 0.33 ns and beta = 0.63 +/- 0.015 upon insertion into dry DODAB LB films and to tau = 12.00 +/- 0.21 ns and beta = 0.65 +/- 0.1 upon immersion of the ''DODAB-HMP-stabilized CdS clusters-DODAB'' sandwich unit into water.