Alkaline-earth sulfides doped with rare earth ions are versatile optical materials. However, relatively little is known about the controlling the dimensionality, the surface chemistry, and the inherent optical properties of undoped versions of alkaline-earth mono- and poly-chalcogenides. We describe the colloidal synthesis of alkaline-earth chalcogenide nanocrystals from readily available molecular precursors. Systematic exploration of synthetic phase space allows us to tune particle sizes over a wide range. Solid-state NMR spectroscopy confirms the phase purity of the selenide compositions. Surface characterization reveals that bridging carboxylates preferentially terminate the surface of the nanocrystals. While these materials are colloidally stable in the mother solution, the selenides are susceptible to oxidation over time, eventually degrading to selenium metal through polyselenide intermediates. As part of these investigations, we developed the first colloidal syntheses of barium di- and tri-selenides, two of the few reported nanocrystalline alkaline-earth polychalcogenides. Density of states and band structure calculations reveal that both materials are indirect band gap semiconductors, in agreement with experiment.

Alkaline-earth chalcogenide nanocrystals: Solution-phase synthesis and surface chemistry

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