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Heavy pnictogen chalcohalides are often termed lead-free, perovskite-inspired materials. Despite theoretical predictions, incontrovertible experimental demonstrations of heavy pnictogen chalcohalides adopting a perovskite structure are lacking. Here we report our attempts to prepare CsBiSCl 2 adopting a perovskite structure as colloidal nanocrystals. Synthesis of nanoscale materials can indeed rely on fast, nonequilibrium reactions and on large, eventually thermodynamically favorable surface energies, leading to the possibility of stabilizing kinetically trapped or metastable phases. However, we obtained no CsBiSCl 2 , but a mixture of nanocrystals of secondary phases, namely Cs 3 BiCl 6 submicrometric polyhedra, Bi 2 S 3 nanoscopic rods, and Cs 3 Bi 2 Cl 9 nanoscopic dots, whose low polydispersity enabled an effective separation via size/shape selective precipitation. This work confirms that heavy pnictogen chalcohalides are hardly prone to adopting a perovskite structure. Nevertheless, chemistry at the nanoscale offers multiple possibilities for overcoming phase segregation and pursuing the synthesis of prospective mixed anion compound semiconductors.

Here we present a colloidal approach to synthesize bismuth chalcohalide nanocrystals (BiEX NCs, in which E=S, Se and X=Cl, Br, I). Our method yields orthorhombic elongated BiEX NCs, with BiSCl crystallizing in a previously unknown polymorph. The BiEX NCs display a composition-dependent band gap spanning the visible spectral range and absorption coefficients exceeding 10 5  cm -1 . The BiEX NCs show chemical stability at standard laboratory conditions and form colloidal inks in different solvents. These features enable the solution processing of the NCs into robust solid films yielding stable photoelectrochemical current densities under solar-simulated irradiation. Overall, our versatile synthetic protocol may prove valuable in accessing colloidal metal chalcohalide nanomaterials at large and contributes to establish metal chalcohalides as a promising complement to metal chalcogenides and halides for applied nanotechnology., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Ruling over the surface chemistry of metal halide perovskite nanocrystals (NCs) is crucial to access reliable luminophores. Here, we provide an atomic-level description of the surface of colloidal CsPbBr 3 NCs, achieving an effective passivation strategy that leads to near-unity photoluminescence quantum yield. To this end, we used two different types of CsPbBr 3 NCs, which had been synthesized with an outer shell of either oleylammonium bromide ion pairs or Cs-oleate complexes. We perturbed the dynamic equilibria at the NCs' surface with ligands from a comprehensive library, including amines (and their conjugated acids) with different basicities, chain lengths, and steric encumbrances. We demonstrate that control of both ligand binding affinity and ligand-to-NC molar ratio is essential to attain thermodynamically stable coordination of the NC surface. We thus present a reliable protocol for managing the surface chemistry of colloidal CsPbBr 3 NCs and for selectively addressing their ligand-induced morphological (and structural) transformations.