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 ≈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 ABX₃ perovskite archetype while a fourth is a perovskite-like lamellar structure with alternating Cs₄ and polyhedral Sn₄ I_x layers. In ≈1.4 nm-diameter SWCNTs, CsPbBr₃ forms Cs₃ Pb^II Br₅ nanowires, one ABX₃ unit cell in cross section with the Pb²⁺ oxidation state maintained by ordered Cs⁺ vacancies. Within ≈1.2 nm-diameter SWCNTs, CsPbBr₃ and CsSnI₃ form inorganic-polymer-like bilayer structures, one-fourth of an ABX₃ unit cell in cross section with systematically reproduced ABX₃ 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.