Absorption and emission in the visible range by ultra-small PbS quantum dots in the strong quantum confinement regime with S-terminated surfaces capped with diphenylphosphine

Abstract

The synthesis and characterization of PbS QDs absorbing and emitting in the visible range is a very challenging task, since it requires the QDs to be within the strong quantum confinement regime (QD size<2.5 nm). It implies not only having “small” QDs, but also stable and monodisperse; characteristics that have been elusive to achieve for many researchers. In the current work, ultra-small PbS QDs (size ~2 nm) were synthesized based on a modification of the Hines method, controlling the reaction time, and adding diphenylphosphine (DPP) which serves as a catalyst and a protective agent in the reaction synthesis. Novel ultra-small PbS QDs with S-terminated surfaces were obtained, which formed at the early stages of the synthesis reaction and are stabilized by the DPP; as it was suggested by the TEM, FTIR and Raman results. The ultra-small PbS QDs display a maximum peak of optical absorption at ~532 nm, with a corresponding optical band gap of 1.82 eV; a maximum peak of emission at ~679 nm, which results in a Stokes shift of 119 nm, smaller than the Stokes shift observed in “larger” PbS QDs. These ultra-small QDs displayed an average size of ~2 nm, with a standard deviation of ~0.3 nm, which was the smallest among the synthesized samples, based on TEM measurements. Finally, the LUMO and HOMO levels were measured by means of cyclic voltammetry and optical absorption spectroscopy. The values of the optical band gap and the energies measured for the LUMO and HOMO levels of these ultra-small PbS QDs were affected by their atomistic surface arrangement and the capping ligand interacting with their surface. Producing variations in their values that doesn’t follow the trends established for quantum confinement effects related to size variation only. Thorough physical and chemical characterization of such ultra-small PbS QDs are crucial in understanding the origin of their optoelectronic properties, which will contribute to better delineate possible future applications.