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 CsPbBr 3 and lead‐free CsSnI 3 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 Sn 4 I x layers. In ≈1.4 nm‐diameter SWCNTs, CsPbBr 3 forms Cs 3 Pb II Br 5 nanowires, one ABX 3 unit cell in cross section with the Pb 2+ oxidation state maintained by ordered Cs + vacancies. Within ≈1.2 nm‐diameter SWCNTs, CsPbBr 3 and CsSnI 3 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.