Extreme Te nanowires encapsulated within ultra-narrow single-walled carbon nanotubes

Extreme nanowires (ENs) represent the ultimate class of crystalline materials: They are the smallest possible periodic materials. With atom-wide motifs repeated in 1D, they offer a unique perspective into the Physics and Chemistry of low-dimensional systems. Single-walled carbon nanotubes (SWCNTs) provide ideal environments for the creation of such materials. Here we report the observation of Te ENs grown inside ultra-narrow SWCNTs with diameters between 0.7nm and 1.1nm. Through state-of-the-art imaging techniques and high-precision, high-throughput ab initio calculations, we unambiguously determine the competing structures of encapsulated Te as a function of the encapsulating diameters. From 1-atom-wide Peierls-distorted linear chains -- the ultimate ENs, Te morphs into zigzag chains and then gives rise to helical structures that are the 1D analogues of bulk Te. The pitch of the encapsulated Te coils varies non-monotonically with the diameter of the encapsulating SWCNTs.