Picoperovskites: The Smallest Conceivable Isolated Halide Perovskite Structures Formed within Carbon Nanotubes

Halide perovskite structures are revolutionizing the design of optoelectronic materials, including solar cells, light-emitting diodes, and photovoltaics when formed at the quantum scale. Four isolated sub-nanometer, or picoscale, halide perovskite structures formed inside approximate to 1.2-1.6 nm single-walled carbon nanotubes (SWCNTs) by melt insertion from CsPbBr3 and lead-free CsSnI3 are reported. Three directly relate to the ABX(3) perovskite archetype while a fourth is a perovskite-like lamellar structure with alternating Cs-4 and polyhedral Sn4Ix layers. In approximate to 1.4 nm-diameter SWCNTs, CsPbBr3 forms (Cs3PbBr5)-Br-II nanowires, one ABX(3) unit cell in cross section with the Pb2+ oxidation state maintained by ordered Cs+ vacancies. Within approximate to 1.2 nm-diameter SWCNTs, CsPbBr3 and CsSnI3 form inorganic-polymer-like bilayer structures, one-fourth of an ABX(3) unit cell in cross section with systematically reproduced ABX(3) stoichiometry. Producing these smallest halide perovskite structures at their absolute synthetic cross-sectional limit enables quantum confinement effects with first-principles calculations demonstrating bandgap widening compared to corresponding bulk structural forms.