Directed Crystallization of a Nanoscale Quasi-1D van der Waals Topological Insulator

Anisotropy often yields unexpected structures and properties in the solid state. In van der Waals (vdW) solids comprised of 1D or quasi-1D (q-1D) chains, anisotropy in both intra- and inter-chain directions results in an abundance of packing motifs and altered physical states. Q-1D vdW solids with topologically protected states are most sought after due to their potential as building blocks for quantum and spintronic devices. Yet, access to such facet- and edge-specific states is limited by the stochastic nature of micromechanical exfoliation. Here, we demonstrate that Bi4I4, a q-1D vdW topological insulator, can be crystallized from the vapor phase either into nanowires or quasi-2D nanosheets. We find that gold nanoparticles (Au NPs) on the growth substrate, in conjunction with the anisotropic structure of Bi4I4, direct the dimensionality of Bi4I4 nanostructures. Systematic variation of Au NP diameters, Bi:I precursor ratios, and growth-deposition temperatures reveal that Au NPs generally act as nucleation sites for vapor-solid growth of nanowires. Post-synthesis analyses of 20 nm Au NPs show that the 1:1 ratio of Bi to I within the Au NPs uniquely triggers the vapor-liquid-solid growth of [001]-oriented nanosheets from laterally-ordered [Bi4I4]n chains along the [100] direction. We rationalize the bimodal growth pathways and the morphologically distinct nanostructures based on crystallization habits of the nanostructures, Bi:I ratios in the Au NPs post-synthesis, and orientation of stereochemically active Bi lone pairs between adjacent chains. We anticipate that these pathways are adaptable to other halide- and chalcogen-based 1D vdW nanocrystals with diverse physical and quantum properties.