Removing Cadmium Impurities from Cation-Exchange-Derived CuInSe2/CuInS2 Nanorods for Enhanced Infrared Emission and Photodetection

Abstract

Metal chalcogenide nanocrystals with variable composition and shape can conveniently be produced using cation exchange synthesis; however, the presence of cation-containing ligands inherited from the starting material often results in contamination of the final product. To address this issue, a two-step ligand replacement strategy is developed to fabricate CuInSe<inf>2</inf>/CuInS<inf>2</inf> nanorods from CdSe/CdS nanorods via removal of Cd-phosphonates from an intermediate Cu<inf>2-x</inf>Se/Cu<inf>2-x</inf>S phase used in the cation exchange conversion. This synthetic approach furnishes CuInSe<inf>2</inf>/CuInS<inf>2</inf> nanorods with cadmium content below 1 at.%, and high photoluminescence quantum yields reaching 40% in the near-infrared spectral range. Transient absorption studies reveal that the band alignment in the CuInSe<inf>2</inf>/CuInS<inf>2</inf> heterostructure features a quasi-type II character, with an electron localized in the core and a hole wavefunction spread over the entire nanorod. The efficient passivation of the core and the reduced Cd content leads to excitonic emission with full width at half maximum down to 110 meV, superimposed with a broad emission band from copper-induced defects. Field-effect transistors based on cadmium-free CuInSe<inf>2</inf>/CuInS<inf>2</inf> nanorods show two orders of magnitude lower noise current density compared with the cadmium-rich devices. The responsivity and specific detectivity of these devices reach 230 mA W⁻¹ and 10⁸ Jones, respectively, under near-infrared excitation at room temperature.