Atomistic modeling of morphology and electronic properties of colloidal ultrathin Bi2S3 nanowires

Ultrathin crystalline Bi<inf>2</inf>S<inf>3</inf> nanostructures are studied by first-principles atomistic modeling and supported by experiments. Consistent with previous findings, the theoretical analysis shows that nanowires with lateral sizes as small as a few nanometers are energetically possible. Also, we were able to synthesize ultrathin nanowires by means of a low-cost, nontoxic colloidal route. Transmission electron microscopy data reveal a coherence length of the nanowires exceeding 30 nm. The simulations show that surfaces affect the electronic structure of the material inducing peculiar 1D-like electronic states on the nanowire edges that are located 300 meV above the valence band. Sulfur vacancies are instead responsible for localized states a few hundred millielectronvolts below the conduction band. The possibility of eliminating the surface-induced intragap states is theoretically investigated by passivating the surfaces of the nanowires with carboxylic and amine groups that are commonly employed in colloidal synthesis. Small molecules methylamine and acetic acid are expected to fully passivate the surfaces of the nanowires, removing the edge states and restoring a clean band gap. Present results suggest a possible route for improving optoelectronic properties of Bi<inf>2</inf>S<inf>3</inf> nanostructures by tuning the size of the ligand molecules.