| Summary: | <p>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<sub>3</sub> and lead-free CsSnI<sub>3</sub> are reported. Three directly relate to the ABX<sub>3</sub> perovskite archetype while a fourth is a perovskite-like lamellar structure with alternating Cs<sub>4</sub> and polyhedral Sn<sub>4</sub>I<sub><em>x</em></sub> layers. In ≈1.4 nm-diameter SWCNTs, CsPbBr<sub>3</sub> forms Cs<sub>3</sub>Pb<sup>II</sup>Br<sub>5</sub> nanowires, one ABX<sub>3</sub> unit cell in cross section with the Pb<sup>2+</sup> oxidation state maintained by ordered Cs<sup>+</sup> vacancies. Within ≈1.2 nm-diameter SWCNTs, CsPbBr<sub>3</sub> and CsSnI<sub>3</sub> form inorganic-polymer-like bilayer structures, one-fourth of an ABX<sub>3</sub> unit cell in cross section with systematically reproduced ABX<sub>3</sub> 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.</p>
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