Colloidal Synthesis of Shape-Controlled Cs₂NaBiX₆ (X = Cl, Br) Double Perovskite Nanocrystals: Discrete Optical Transition by Non-Bonding Characters and Energy Transfer to Mn Dopants

Abstract

As an alternative to lead halide perovskites, various types of lead-free perovskites have been recently studied for optoelectronic materials. This paper reports the synthesis of highly phase-pure Cs2NaBiX6 (X = Cl, Br) double perovskite (DP) nanocrystals (NCs). They show characteristic absorption features of sharp and discrete peaks mostly originating from the Bi3+ s–p transition (6s2 → 6s1p1) in [BiX6]3– units within the crystal lattice of the elpasolite structure. Such unique optical properties are attributed to the non-bonding character of electropositive sodium and electronically isolated [BiX6]3– units in crystals. The shape of Cs2NaBiX6 NCs could be quantitatively controlled by adjusting the reaction temperature. Reaction temperatures above 180 °C favor development of a cuboctahedral (CO) shape, whereas development of a cuboidal (CB) shape is favored below 170 °C. CB NCs can be subsequently converted to CO NCs by heating to 200 °C. The CO NCs promote the growth of heterostructure adducts on the (111) facets; these adducts could be posteliminated by etching. Mn-doped Cs2NaBiCl6 NCs are synthesized; they show efficient energy transfer from the NC host to the dopants. The synthesis and shape control of Cs2NaBiX6 NCs and Mn-doped Cs2NaBiCl6 NCs could expand a new class of lead-free DP NCs applicable to optoelectronic applications.