Lead sulfide nanocrystals suitable for solar cells have a nearly one-to-one ratio of lead to sulfur atoms, but MIT researchers discovered that to make uniformly sized quantum dots, a higher ratio of lead to sulfur precursors – 24 to 1 – is better. MIT chemical engineering graduate student Mark C. Weidman developed the synthetic recipe in the lab of William A. Tisdale, the Charles and Hilda Roddey Career Development Professor in Chemical Engineering at MIT, with colleagues Ferry Prins, Rachel S. Hoffman and 2013 Summer Scholar Megan Beck. Uniformity of size can promote long exciton diffusion lengths in lead sulfide (PbS) quantum-dot films, Weidman says.
Usually quantum dots are synthesized as a colloid, with particles suspended in a liquid. If the quantum dots are all of the same size, they can self-assemble into an ordered lattice. “If they are monodisperse enough, it’s the thermodynamically favored state,” Weidman explains. Read on at MIT News. Photo by Denis Paiste.