Polymer-like Conformation and Growth Kinetics of Bi2S3 Nanowires

One-dimensional inorganic crystals (i.e., crystalline nanowires) are one of the most intensely investigated classes of materials of the past two decades. Despite this intense effort, an important question has yet to be answered: do nanowires display some of the unique characteristics of polymers as their diameter is progressively decreased? This work addresses this question with three remarkable findings on I the growth and form of ultrathin Bi2S3 nanowires. (i) Their crystallization in solution is quantitatively describable as a form of living step-growth polymerization: an apparently exclusive combination of addition of "monomer" to the ends of the nanowires and coupling of fully formed nanowires "end-to-end", with negligible termination and initiation. (ii) The rate constants of these two main processes are comparable to those of analogous processes found in polymerization. (iii) The conformation of these nanowires is quantitatively described as a worm-like conformation analytically analogous to that of semiflexible polymers and characterized by a persistence length of 17.5 nm (shorter than that of double-stranded DNA) and contour lengths of hundreds of micrometers (longer than those of most synthetic polymers). These findings do not prove a chemical analogy between crystals and polymers (it is unclear if the monomer is a molecular entity tout court) but demonstrate a physical analogy between crystallization and polymerization. Specifically, they (i) show that the crystallization of ensembles of nanoscale inorganic crystals can be conceptually analogous to polymerization and can be described quantitatively with the same experimental and mathematical tools, (ii) demonstrate that one-dimensional nanocrystals can display topological characteristics of polymers (e.g., worm-like conformation in solution), (iii) establish a unique experimental model system for the investigation of polymer-like topological properties in inorganic crystals, and (iv) provide new heuristic guidelines for the synthesis of polymer-like nanowires.