Highly Conductive Copper Selenide Nanocrystal Thin Films for Advanced Electronics

Investigation of the influence of the nanocrystal (NC) surface chemistry on the (opto)­electronic properties of NC-based thin films is of paramount importance for their further application in various devices. In this work, macroscopic superlattices of copper selenide (Cu_2–xSe) NCs in the form of thin films were prepared by self-assembly at the liquid/air interface accompanied by simultaneous ligand exchange with the inorganic S^2– as well as organic 1,2-ethanedithiol, 1,4-butanedithiol, 1,6-hexanedithiol, and 1,8-octanedithiol ligands. By using X-ray photoelectron spectroscopy and optical absorption spectroscopy, we revealed that the newly introduced ligands largely replaced the native surfactants on the Cu_2–xSe NC surface and acted as cross-linkers between neighboring particles. Transport measurements, specifically focusing on the interparticle distance, indicated 5–9 orders of magnitude increase in electrical conductance with decreasing the size of the ligands from octanedithiol to inorganic sulfide ions, suggesting that the newly introduced species provide efficient and controllable electronic coupling for adjacent Cu_2–xSe NCs. Finally, the NC arrays were employed as chemiresistors to show the potential of these macroscopic superlattices in sensing applications.