Crystalline α-Sm₂S₃ nanowires: Structure and optical properties of an unusual intrinsically degenerate semiconductor

The lanthanide sulfides have long been a promising class of semiconductors because of their infrared-to-visible range band gaps and excellent thermoelectric properties. However, their applications have been limited due to their time consuming conventional synthetic processes and the lack of sufficient understanding of their electronic properties. To address these shortcomings, here we report a rapid, chemical vapor deposition route which results in thin films of crystalline α-phase samarium sesquisulfide (α-Sm ₂S₃) nanowires within a few hours, rather than the typical 4-7 days required in previous synthetic processes. In addition, density functional theory was, for the first time, utilized to calculate the electronic band structure of α-Sm₂S₃ in order to shed insight into the interpretation of their UV-Vis absorption spectrum. We found that the theoretical direct gap in the band states of α-Sm₂S₃ is 1.7 eV. Computation results suggest that this gap can be tuned to a solar optimal ∼1.3 eV via systematic sulfur vacancy sites engineered into the crystal structure. Most significantly, the degenerate semiconductor-like behavior long observed in lanthanide sulfide samples have been shown to be present even in the ideal α-Sm₂S₃ structure, suggesting that the observed heavily p-type behavior is an unusual intrinsic property of the material resulting from the Fermi level being located significantly below the optically active 1.7 eV band edge.