Precision Synthesis of a Single Chain Polymorph of a 2D Solid within Single-Walled Carbon Nanotubes

The discovery and synthesis of atomically precise low-dimensional inorganic materials have led to numerous unusual structural motifs and nascent physical properties. However, access to low-dimensional van der Waals (vdW)-bound analogs of bulk crystals is often limited by chemical considerations arising from structural factors like atomic radii, bonding or coordination, and electronegativity. Using single-walled carbon nanotubes (SWCNTs) as confinement templates, we demonstrate the synthesis of a short-wave infrared-absorbing quasi-1D (q-1D) chain polymorph of Sb₂Te₃ ([Sb₄Te₆]_n) that is structurally and electronically distinct from its 2D counterpart. It is found that the q-1D chain polymorph has both three- and five-coordinate Sb atoms covalently bonded to Te and is thermodynamically stabilized by the electrostatic interaction between the encapsulated chain and the model SWCNT. The complementary experimental and computational results demonstrate the synthetic advantage of vdW nanotube confinement in the discovery of low-dimensional polytypes with drastically altered physical properties and potential applications in energy conversion processes.