Luminescence Enhancement in CuInS2 Nanoparticles through the Selective Passivation of Nonradiative Recombination Sites by Phosphine Ligands

Surface modification of semiconductor nanoparticles (NPs) is the key factor in improving fluorescence properties. Encapsulation of the NP cores within an inorganic shell and/or tailoring the surfactant ligands is a standard procedure to passivate carrier trapping sites on the surface of NPs, manipulate carrier recombination processes, and improve fluorescence. Type I–III–VI2 NPs are strong candidates for visible fluorophores composed of nontoxic elements substituting CdSe NPs. The defect emission due to the recombination of excited carriers trapped at intragap levels is essentially the origin of their luminescence. However, the band-edge emission characteristic of radiation with high monochromaticity has not been realized until recent observation in AgInS2 NPs with trioctylphosphine (TOP) ligands. Nevertheless, the band-edge emission still has not been observed in most other I–III–VI2 NPs. Here, we examined luminescence modifications caused by the postsynthesis treatment of CuInS2 NPs using TOP. Although the band-edge emission was not generated, the TOP treatment significantly improved the luminescence quantum yield. This is because the defect emission mechanisms and carrier trapping centers are different between AgInS2 and CuInS2 NPs. The TOP and/or TOP oxide ligands could selectively coordinate the surface sites, acting as the nonradiative recombination center originally formed, and could successfully passivate them.