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“Darker-than-Black” PbS Quantum Dots: Enhancing Optical Absorption of Colloidal Semiconductor Nanocrystals via Short Conjugated Ligands
journal contribution
posted on 2015-02-11, 00:00 authored by Carlo Giansante, Ivan Infante, Eduardo Fabiano, Roberto Grisorio, Gian Paolo Suranna, Giuseppe GigliColloidal
quantum dots (QDs) stand among the most attractive light-harvesting
materials to be exploited for solution-processed optoelectronic applications.
To this aim, quantitative replacement of the bulky electrically insulating
ligands at the QD surface coming from the synthetic procedure is mandatory.
Here we present a conceptually novel approach to design light-harvesting
nanomaterials demonstrating that QD surface modification with suitable
short conjugated organic molecules permits us to drastically enhance
light absorption of QDs, while preserving good long-term colloidal
stability. Indeed, rational design of the pendant and anchoring moieties,
which constitute the replacing ligand framework leads to a broadband
increase of the optical absorbance larger than 300% for colloidal
PbS QDs also at high energies (>3.1 eV), which could not be predicted
by using formalisms derived from effective medium theory. We attribute
such a drastic absorbance increase to ground-state ligand/QD orbital
mixing, as inferred by density functional theory calculations; in
addition, our findings suggest that the optical band gap 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 QDs.
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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Short Conjugated LigandsColloidal quantum dots3 p orbitalscore componentsQD surfacePbS QD solidslight absorptiontheory calculationsColloidal Semiconductor NanocrystalsEnhancing Optical Absorptionmedium theorychemical speciesband gap reductionligand frameworkPbS QDsbroadband increasenovel approachQD surface modificationsulfur atomsabsorbance increase
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