Langmuir-Blodgett film formation from fluorescence-activated, surfactant-capped, size-selected CdS nanoparticles spread on water surfaces

Fluorescence-activated sodium hexametaphosphate stabilized CdS particles have been transferred from water into apolar organic solvents (chloroform and hexane) by dioctadecyldimethylammonium bromide (DODAB) and by cetyltrimethylammonium chloride (CTAC) and spread on water surfaces in a Langmuir film balance. Surface pressure vs surface area isotherm, absorption spectroscopic, emission spectroscopic, Brewster-angle microscopic, reflectivity, and scanning probe microscopic determinations have established the formation of a monoparticulate layer of CdS which contained islands of DODAB monolayers. The surfactant-coated CdS monoparticulate layers have been transferred, layer-by-layer by the Langmuir-Blodgett technique, to solid supports. The fluorescence spectrum of films, built from DODAB-capped CdS nanoparticulate layers, was characterized by a sharp excitonic emission bands at 478 nm, whose intensity dramatically increased upon exposure to water. Fluorescence measurements of DODAB-capped CdS nanoparticulate films, in the 12-140 K range, indicated the predominance of shallow surface traps in the semiconductors. Scanning tunneling spectroscopy of individual DODAB-capped CdS nanoparticles, in the dark and under illuminations, have established them to be n-type semiconductors.