Extreme Tellurium nanowires encapsulated within narrow-diameter single-walled carbon nanotubes: Theory and experiments.

Extreme nanowires are the ultimate class of crystalline materials: They are the smallest possible periodic materials. With atom-wide motifs repeated along one single ~dimension, they offer a unique perspective into the Physics and Chemistry of low-dimensional systems. The interior of narrow single-walled carbon nanotubes (NSWCNTs), on the other hand, ~provides an ideal environment for the creation of such materials. We report the observation of extreme Te nanowires grown inside NSWCNTs. We start by discussing how implicit single-walled carbon nanotubes SWCNTs can be introduced to speed up the structural searches on SWCNT-encapsulated structures. Then, using ~high- precision, high-throughput \textit{ab initio} calculations, along with state-of-the-art imaging techniques, we unambiguously determine the phase evolution of encapsulated Te as a function of the diameters of the encapsulating NSWCNTs. From 1-atom-wide linear chains -- the ultimate extreme nanowires, elemental Te evolves into zigzag chains and, still within very narrow SWCNTs, forms helical structures that are the one-dimensional analogues of bulk Tellurium.