Impact of size and shape on trap state controlled luminescence properties of trioctylphosphine-capped cadmium selenide quantum dots

Color tunability using different synthesis approaches and to intentionally change the shape of cadmium selenide (CdSe) quantum dots (QDs) using different synthesis processes have been widely investigated. Even in a single synthesis procedure, there is a probability of shape change with QD growth time. In this work, we have analyzed the size- and shape-dependent effects on the quantum yield (QY) and average lifetime with the growth of trioctylphosphine (TOP)-capped CdSe nanoparticles synthesized using a standard synthesis approach. The decrease in the average fluorescence lifetime and QY with the increase in particle size of the QDs has been reported. Transmission electron microscopy (TEM) analysis shows that as the size of QDs increases, the shape changes from spherical to approximately ovoid; thus, the surface area increases at a higher rate. From x-ray photoelectron spectroscopy, we have found that selenium content is increasing at a higher rate with shape change, and TOP has the capability of capping cationic facets; hence, there is an increase in the weakly bonded anionic facets per unit area, and there are atomic dislocations during the fabrication process. So, these two factors create a large number of trap states, resulting in the increment of nonradiative recombination centers. Effects of these trap states on radiative lifetime, nonradiative lifetime, average lifetime, QY, radiative recombination, and nonradiative recombination rates have been analyzed.