Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Here we present a conceptually novel approach to design light-harvesting nanomaterials based on colloidal quantum dots (QDs). Indeed, a drastic broadband enhancement of QD optical absorption, along with preservation of good long-term colloidal stability, is obtained upon quantitative replacement of the bulky electrically-insulating ligands at the QD surface coming from the synthetic procedure with suitable short conjugated organic molecules. The rational design of the pendant and anchoring moieties that constitute the replacing ligand framework provides tunable broadband increase of the optical absorption above 300 % for colloidal PbS QDs also at high energies (> 3.1 eV), which could not be predicted nor described by using formalisms derived from Maxwell-Garnett effective medium theory. We attribute such a drastic absorbance increase to ground-state ligand/core orbital mixing, as inferred by density functional theory calculations; in addition, our findings suggest that the optical bandgap reduction commonly observed for PbS QD solids treated with thiol-terminating ligands can be prevalently ascribed to 3p orbitals localized on anchoring sulfur atoms, which mix with the highest occupied states of the PbS core.
More broadly, we provide evidence that organic ligands and inorganic cores are inherently electronically coupled materials thus yielding peculiar chemical species (the colloidal QDs themselves), which display arising (opto)electronic properties that cannot be merely described as the sum of those of the ligand and core components.
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