Electronic properties of Cu_2-xSe nanocrystal thin films treated with short ligand (S^2-, SCN^-, and Cl^-) solutions

To exploit interesting electronic properties of colloidal semiconductor nanocrystals (NCs) in thin film devices, replacement of the original bulky ligands attached on the NC surface to short ones is essential. Here, we investigate the electronic properties of thin films of Cu_2-xSe NCs treated chemically with short sulfide (S^2-), thiocyanate (SCN^-), and chloride (Cl^-) ligands that are known to yield superior physical properties compared to the first-generation short ligand systems including amines and thioles. Specifically, the study focuses on the impact of ligand treatment on their direct/indirect bandgap and NIR-localized surface plasmon resonance (LSPR) in the near-IR regime as well as their electrical conductivity and thermoelectric properties. While the application of S^2- solution resulted in exchange of the original oleylamine (OAm) on NC surface with S^2- ligands, use of SCN^- and Cl^- solutions only removed the original ligands. The different ligands consistently led a red-shift of the direct and indirect bandgap. The LSPR was also red-shifted after applying solutions with SCN^- and Cl^- but was blue-shifted after applying solutions with S^2-, which we attribute to the formation of sulfur shell on NC surface. Conductivity as high as 442 S/cm and Seebeck coefficient of 13 μV/K could be obtained from the NC films with Cl^- and SCN^- ligands, respectively. We believe that the understanding on Cu_2-xSe NCs will expand the materials library for electronic applications of copper chalcogenide NCs.