Non-covalent inter-chain interactions dictate single chain encapsulation behavior of pnictogen chalcogenides within van der Waals nanotubes

A fully developed understanding of the chemistry that governs interactions in low-dimensional materials is essential to realizing control over their structure, nanoscale morphology, and distinct physical properties. Yet, precisely achieving this in extended solids comprised of highly polarizable main group elements has remained a grand challenge especially in one-dimensional materials that approach the atomic scale. This is owing to the propensity of these heavy main group elements to drive the unusual localization or delocalization behavior of electrons in the system and the formation of competing metastable polytypes due to lone pair effects. In this report, we establish the composition-dependent encapsulation behavior of single chains derived from the low-dimensional Pn2Ch3 pnictogen chalcogenide class (Pn = Sb, Bi; Ch = S, Se, Te) and their strong dependence on the strength of inter-chain non-covalent interactions. Using Sb2S3 and Bi2S3 as model phases, we find that the degree of pnictogen (Pn) bonding is a significant factor in dictating the packing of the constituent quasi-1D chains in the bulk and their growth behavior as single chains upon encapsulation within single-walled carbon nanotubes (SWCNTs). Specifically, chains with weaker inter-chain interactions like Sb2S3 preferentially crystallize under slow-cooled growth parameters that resemble more thermodynamically preferred conditions while chains with stronger inter-chain interactions like Bi2S3 prefer to crystallize and isolate as bulk and only form the elusive encapsulated clustered and crystalline chain domains under rapidly quenched conditions. Systematic Sb/Bi alloying of the precursors revealed the pnictogen-dependence of the chain crystallization where Sb-rich alloys tend to more preferentially crystallize within SWCNTs. In these alloys, Bi preferentially occupies the peripheral three-coordinate site to maximize inter-chain interactions in the bulk, while Sb preferentially occupies the same site when encapsulated within SWCNT and devoid of chains adjacent to it. These results demonstrate that non-covalent bonding concepts in molecular main group systems track in low-dimensional solids that approach the atomic regime when encapsulated within nanotubes, marking a significant step forward in the rational and predictable encapsulation of all-inorganic chains within confined spaces.