Size Tunablity of CdTe Crystallites in Dendrimer Nanocomposites and Temperature Dependent Focusing of Size Distribution

Abstract

CdTe/Dendrimer nanocomposites have been synthesized for the first time in aqueous and nonaqueous media using PAMAM dendrimer (Generation 5.0). The average size of the as-prepared nanocomposites, as determined from dynamic light scattering (DLS) measurements, was found to be typically 182 nm and 23 nm in water and methanol, respectively under identical conditions of temperature (5 degrees C) and reagent ratio (Cd2+:Te2-, 1:0.5). The size of CdTe NPs within the nanocomposites, was found to be 3.1 and 2.8 nm for the aforementioned samples determined from optical absorption spectra using tight binding approximation. The NPs possess good degree of cystallinity as discernible from the lattice fringes in high-resolution transmission electron microscopic (HRTEM). Transmission electron microscopic (TEM) image and the cubic crystal phase was ascertained from the small area electron diffraction (SAED) pattern. Analysis of FTIR data suggests that CdTe NPs are bound to the surface amine groups as well as -NHCO- moieties lying in the interior of dendrimer structure. The present work demonstrates how the quality of the CdTe NPs formed within the dendrimer matrix can be nicely tuned by varying the parameters, namely, temperature, molar ratio of Cd2+: Te2- and pH. Changing of Cd2+: Te2- ratio of 1:1 to 1:0.5, decreased the average particle size from 5.0 nm to 3.4 nm with concomitant narrowing of size distribution by approximately 35% at 10 degrees C. On lowering down the synthesis temperature (25 degrees C-->5 degrees C), the average particle size remained unaffected while the size distribution became sharply focused. However, the extent of focusing was found to be more in methanol (40%) than that in water (30%).