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.