Bimodal Photoluminescence during the Growth of PbS Quantum Dots

We examine the effect of reaction time, temperature, molar ratio of precursors, capping ligands, and aging on the growth of PbS quantum dots in a “nonviscous”, solventless, mild-constant-temperature system with PbCl2 and S as precursors. It is found in our system that Ostwald ripening leads to bimodal photoluminescence corresponding to the bimodal size distribution of PbS quantum dots, as evidenced by transmission electron microscopy, in contrast to commonly reported single-peak broadening due to Ostwald ripening. The evolution of photoluminescence spectra mediated by Ostwald ripening is strongly affected by reaction parameters. Under certain conditions, the photoluminescence band exhibits some initial broadening upon the action of Ostwald ripening and then gradually transforms into a bimodal structure. In other cases, bimodal peaks rapidly appear as early as 1 min after the reaction starts. Moreover, it is found that the addition of an additional ligand trioctylphosphine immediately initiates the bimodal photoluminescence, which, however, on the basis of photoluminescence spectra characteristics is not thought to be associated with Ostwald ripening. For the bimodal emission structure, aging can greatly enhance the emission peak at shorter wavelength, but it does not affect the emission peak at longer wavelength. Using our simple system, we are able to synthesize PbS quantum dots with a narrow size distribution evidenced by a full width at half-maximum of the photoluminescence peak as low as 58 meV.