Many technologically relevant semiconductors contain toxic, highly regulated elements (Cd, Pb, As), or relatively scarce and poorly distributed elements (In, Li). In contrast, alkali-based APnE2 semiconductors are made of biocompatible or relatively abundant elements (A = Na, K; Pn = Sb, Bi; E = S, Se). These ternary semiconductors adopt a rock salt structure, where the chalcogenide (E) forms an fcc lattice, and a mixture of monovalent/trivalent (A/Pn) cations occupies the octahedral holes. Along with the presence of polarizable lone s2 electrons on Pn, this leads to efficient phonon scattering, which is ideal for thermoelectrics. APnE2 semiconductors are also predicted to be defect tolerant photovoltaics, much like halide perovskites, where point defects are unlikely to result in mid gap or trap states. We have successfully synthesized colloidal NaBiS2nanocrystals from commercially available precursors (J. Am. Chem. Soc. 2018, 140, 3736–3742). Just as the formation mechanism and phase evolution of ternary and more complex nanocrystals differ substantially from those of unary (elemental) or binary nanocrystals, the organic-inorganic interfacial chemistry of multinary nanocrystals is inherently more complex and unique. We use IR, NMR, and XPS to identify and quantify the native ligands present on the surface of colloidal NaPnE2 nanocrystals. These include negatively charged X-type (carboxylate) and neutral L-type (primary amine) ligands, bound ion pairs or neutral acceptor Z-type ligands (NaX, BiX3), and additional neutral L-type ligands such as thioamides, amidines, and thioethers formed during synthesis. Finally, we observe that smaller NaBiS2nanocrystals are much more stable to oxidation. The surface of these rock salt materials becomes more cation rich with decreasing particle size. Because both X and L type ligands preferentially binds cations, it makes sense that smaller particles containing mostly cation-only (111) facets are better passivated, and thus are more stable against surface oxidation. We also explore ligand exchange of the native ligands with dithiols (DHLA) and trithiols, under thermal and photochemical conditions, in order to produce water soluble NaPnE2 nanocrystals. We hope this work will help unlock a new class of low cost and environmentally friendly ternary semiconductor nanocrystals for energy conversion.

Unveiling the surface chemistry of colloidal NaPnE2 nanocrystals (Pn = Sb, Bi; E = S, Se)

2019 Spring National Meeting

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