Picoperovskites: the Smallest Conceivable Isolated Halide Perovskite Structures Formed Within Carbon Nanotubes

Halide perovskite structures are revolutionising the design of optoelectronic materials, including solar cells, light emitting diodes and photovoltaics when formed at the quantum scale. We report four isolated sub-nm, or pico-scale, halide perovskite structures formed inside ∼1.2-1.6 nm single walled carbon nanotubes (SWCNTs) by melt insertion from CsPbBr3 and lead-free CsSnI3. Three directly relate to the ABX3 perovskite archetype while a fourth is a perovskite-like lamellar structure with alternating Cs4 and polyhedral Sn4Ix layers. In ∼1.4 nm diameter SWCNTs, CsPbBr3 forms Cs3PbIIBr5 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 1/4 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 band gap widening compared to corresponding bulk structural forms.