Ligand and precursor effects on the synthesis and optical properties of PbS quantum dots

We study how the as-received chemical reagents of a commonly used ligand oleylamine (C18-amine) and precursor PbCl2, each at two different purity statuses, affect the growth of PbS quantum dots in a solventless, relatively green, constant reaction-temperature synthesis system. It is found that the growth behavior of PbS quantum dots reflected from their absorption and photoluminescence spectra is quite sensitive to the purity status of the ligand and precursor under certain circumstances, while the lifetime and quantum yield of quantum dots exhibiting a monomodal or nearly monomodal photoluminescence band are not considerably affected. For instance, the effect of the ligand purity status is particularly evident when a higher PbCl2/S ratio is applied. The use of lower purity C18-amine leads to the growth showing much stronger temperature dependence and also facilitates the earlier entry of Ostwald process highlighted by a bimodal photoluminescence structure. Consistently, a 2 wt% increase in the PbCl2 purity from 98 wt% to 100 wt% (or the absence of 2 wt% of impurities) largely postpones the start of Ostwald process and thus significantly improves both absorption and photoluminescence spectra. These results imply that in order to produce PbS quantum dots with narrow absorption and photoluminescence peaks, one needs to optimize reaction parameters as well as select chemicals of appropriate purities. Moreover, the unintentional involvement of chemicals of different purity status may partially account for the irreproducibility problem often encountered in quantum dot synthesis.