Effect of Tellurium Concentration of the Solution on the Structural, Optical and Luminescence Properties of Cadmium Telluride Nanoparticles

Water-soluble cadmium telluride (CdTe) nanoparticles (NPs) were prepared using L-cystine as a capping agent. The reaction was carried out under basic conditions (at pH=11) and refluxed at 100ºC for 1 h. In this work, different tellurium (Te) concentrations in the solution of L-cystine functionalized CdTe NPs were prepared at low temperature of 100ºC and discussion of its effect was done in detail. The CdTe NPs were analyzed using X-ray diffraction (XRD), ultraviolet-vis and photoluminescence (PL) spectroscopy. Structural parameters estimated by XRD revealed cubic phase of CdTe at lower Te concentrations and hexagonal phase of Te emerging at higher Te concentration. XRD results showed that the NPs consisted of small CdTe nanocrystallites, 3.1-4.5 nm in size. Scanning electron microscopy was used to analyze the surface morphology of the samples and was found that it varied with different Te concentration in the aqueous solution. They consisted of spherical, rod-like and blade-like type of morphologies. The absorption thresholds of the CdTe NPs were red shifted when Te concentration was increased, due to the quantum confinement effects displayed by nanocrystalline CdTe NPs. Optical band gap energy estimated showed a decrease with an increase in the amounts of Te present while the band emissions observed in the PL spectra of CdTe NPs red shifted from 536-559 nm upon an increase in Te ratio. The PL full width at half maximum increased from 49-60 nm with increase in Cd:Te from 1:0.1 to 1:1 displaying narrow size distribution of the NPs. Highest PL intensity was realized for a 1:0.4 Cd:Te molar ratio which is an indication of improved crystallinity.