The size of a quantum-confined nanocrystal determines the energies of its excitonic transitions. Previous work has correlated the diameters of PbS nanocrystals to their excitonic absorption; however, we observe that PbS quantum dots synthesized in saturated dispersions of PbCl2 can deviate from the previous 1Sh-1Se energy vs diameter curve by 0.8 nm. In addition, their surface differs chemically from that of PbS quantum dots produced via other syntheses. We find that these nanocrystals are coated in a shell that is measurable in transmission electron micrographs and contains lead and chlorine, beyond the monatomic chlorine termination previously proposed. This finding has implications for understanding the growth mechanism of this reaction, the line width of these quantum dots’ photoluminescence, and electronic transport within films of these nanocrystals. Such fundamental knowledge is critical to applications of PbS quantum dots such as single-photon sources, photodetectors, solar cells, light-emitting diodes, lasers, and biological labels.

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Films of nanocrystalline PbSe were fabricated with a set of structurally varied short-chain dicarboxylic acids. Oxidation rates were studied via NIR spectroscopy to determine the effect of the structure of the diacid ligands on film stability under ambient conditions. Ligands favoring a non-bridging bonding mode were found to provide the best protection against oxidation, while among ligands expected to bridge between adjacent nanocrystals in the films, those with shorter chain lengths conferred better oxidative stability. Electronic coupling was observed as a red shift in the optical data of the ground excitonic peak of the PbSe films and found to be strongly influenced by the structure of the ligand. Transport measurements were made in air using thin-film transistors that were treated with a thin Al 2 O 3 coating via remote plasma ALD. Films prepared using fumaric, maleic, and oxalic acids yielded mobility numbers of 2.5 × 10 −5 , 3.7 × 10 −5 , and 1.6 × 10 −3 cm 2 / V•s, respectively. Results suggest that the internanocrystal distance is the major contributor to electron mobility through the nanocrystalline films, while the electronic coupling is heavily influenced by multiple factors related to the structure of the surface ligands in addition to the internanocrystal distance.

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