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

Abstract 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 Sn₄I_x layers. In ≈1.4 nm‐diameter SWCNTs, CsPbBr₃ forms Cs₃Pb^IIBr₅ 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.