Linear and helical cesium iodide atomic chains in ultranarrow single-walled carbon nanotubes : impact on optical properties

One-dimensional (1D) atomic chains of CsI were previously reported in double-walled carbon nanotubes (DWCNTs) with ~0.8 nm inner diameters. Here, we demonstrate that while 1D CsI chains form within narrow ~0.73 nm diameter single-walled carbon nanotubes (SWCNTs), wider tubules (~0.8-1.1 nm) promote formation of helical chains of CsI two atoms across and one atom thick. Topologically formed 2 x 1 CsI helices create complimentary oval distortions in varying diameter SWCNTs with highly strained helices formed from highly strained Cs2I2 parallelogram units in narrow tubules to lower strain variants in wider tubules. The observed structural changes and charge distribution were analysed by density functional theory and Bader Analysis. Ion size effects relating to differences in K+ vs. Cs+ Raman tuning into resonance were also noted. CsI chains also produce conformation-selective changes to the electronic structure of the encapsulating tubules measurable by photoluminescence spectroscopy, demonstrating new pathways to optoelectronic tuning