Enhanced photoluminescence of CdS nanocrystallites in LB multilayers on aqueous treatment

Abstract

CdS nanocrystallites were formed within the LB matrix of cadmium arachidate–arachidic acid composite multilayer by post deposition H 2S exposure of precursor cadmium arachidate multilayer. The CdS containing multilayers were subjected to post deposition heat treatment in air in the temperature range of 100–300 °C, either directly after the sulphidation process or after being introduced into an aqueous bath. FT-IR spectroscopy was used to study the chemical changes in composite multilayers at different stages of processing. Optical properties of the CdS nanocrystallites were studied by UV–VIS absorption and photoluminescence measurements. A strong enhancement of excitonic emission and suppression of surface states emission from CdS nanocrystallites was observed after heat treatment at 100–150 °C in the case of aqueous treated composite multilayer. X ray reflection and atomic force microscopy show that the aqueous treatment results in expulsion of arachidic acid to the surface of the multilayer resulting in its facile removal during heat treatment. These results suggest that the response to heat treatment of the composite multilayer containing CdS nanocrystallites depends critically on the proximity and access of the unreacted cadmium arachidate to CdS nanocrystallites. This in turn, provides Cd ions for the passivation of Cd vacancies on the surface of nanocrystallites resulting in a strong enhancement of excitonic emission. Heat treatment at higher temperatures results in drastic reduction of photoluminescence, due to particle growth as well as removal of organic moieties.