Controllable Electrolysis Reveals the Microscopic Composition of Lead Sulfide Quantum Dots

The size and composition of colloidal lead sulfide (PbS) quantum dots (QDs) are closely related to their optoelectronic properties, such as the band structure, carrier transportation, resistivity toward surface oxidation, etc., and hence would greatly affect the device performance. In this work, we developed a simple electrochemical methodology for probing the microscopic composition of the PbS QDs capped with oleic acid (OA). Cathodic and anodic voltammetry of PbS-OA QDs dropcast onto an electrode surface corresponding to the reduction of lead(II) and the oxidation of sulfide was separately investigated to quantify the amount of lead and sulfur. The submonolayer QDs underwent complete electrolysis at low potential scan rates, while under high scan rates, the larger reduction to oxidation charge ratio reflects that probably, only the Pb(II)-rich surface layer was electrolyzed. Based on the controllable electrolysis, not only the surface composition was revealed, but also, a depth profile of the elemental distribution for PbS-OA QDs was proposed. In addition, the size-dependent composition of PbS-OA QDs was also successfully determined, demonstrating it as a powerful tool for quantifying the composition of QDs.