title: "Darker-than-black" PbS quantum dots: enhancing optical absorption of colloidal semiconductor nanocrystals via short conjugated ligands.
abstract: Colloidal 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.
authors.affiliation: Center for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia , via Barsanti 1, 73010 Arnesano, Lecce, Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Giansante
authors.forename: Ivan
authors.initials: I
authors.lastname: Infante
authors.forename: Eduardo
authors.initials: E
authors.lastname: Fabiano
authors.forename: Roberto
authors.initials: R
authors.lastname: Grisorio
authors.forename: Gian Paolo
authors.initials: GP
authors.lastname: Suranna
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Gigli
mesh_terms: D065638:Absorption, Radiation; D003102:Colloids; D007854:Lead / Q000737:chemistry*; D008024:Ligands; D008027:Light; D008958:Models, Molecular; D008968:Molecular Conformation; D055620:Optical Phenomena; D045663:Quantum Dots / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D003102:Colloids; D008024:Ligands; D013440:Sulfides; C018391:lead sulfide; D007854:Lead