title: Quantum-Confined and Enhanced Optical Absorption of Colloidal PbS Quantum Dots at Wavelengths with Expected Bulk Behavior.
abstract: Nowadays it is well-accepted to attribute bulk-like optical absorption properties to colloidal PbS quantum dots (QDs) at wavelengths above 400 nm. This assumption permits to describe PbS QD light absorption by using bulk optical constants and to determine QD concentration in colloidal solutions from simple spectrophotometric measurements. Here we demonstrate that PbS QDs experience the quantum confinement regime across the entire near UV-vis-NIR spectral range, therefore also between 350 and 400 nm already proposed to be sufficiently far above the band gap to suppress quantum confinement. This effect is particularly relevant for small PbS QDs (with diameter of ≤4 nm) leading to absorption coefficients that largely differ from bulk values (up to ∼40% less). As a result of the broadband quantum confinement and of the high surface-to-volume ratio peculiar of nanocrystals, suitable surface chemical modification of PbS QDs is exploited to achieve a marked, size-dependent enhancement of the absorption coefficients compared to bulk values (up to ∼250%). We provide empirical relations to determine the absorption coefficients at 400 nm of as-synthesized and ligand-exchanged PbS QDs, accounting for the broadband quantum confinement and suggesting a heuristic approach to qualitatively predict the ligand effects on the optical absorption properties of PbS QDs. Our findings go beyond formalisms derived from Maxwell Garnett effective medium theory to describe QD optical properties and permit to spectrophotometrically calculate the concentration of PbS QD solutions avoiding underestimation due to deviations from the bulk. In perspective, we envisage the use of extended π-conjugated ligands bearing electronically active substituents to enhance light-harvesting in QD solids and suggest the inadequacy of the representation of ligands at the QD surface as mere electric dipoles.
authors.affiliation: Dipartimento di Matematica e Fisica 'E. De Giorgi', Università del Salento , via per Arnesano, 73100 Lecce, Italy.
authors.forename: Doriana
authors.initials: D
authors.lastname: Debellis
authors.affiliation: Dipartimento di Matematica e Fisica 'E. De Giorgi', Università del Salento , via per Arnesano, 73100 Lecce, Italy.|NANOTEC-CNR Istituto di Nanotecnologia , via per Arnesano, 73100 Lecce, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Gigli
authors.affiliation: Department of Theoretical Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam , De Boelelaan 1083, 1081 HV, Amsterdam, The Netherlands.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Ten Brinck
authors.affiliation: Department of Theoretical Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam , De Boelelaan 1083, 1081 HV, Amsterdam, The Netherlands.
authors.forename: Ivan
authors.identifier: 0000-0003-3467-9376
authors.initials: I
authors.lastname: Infante
authors.affiliation: Dipartimento di Matematica e Fisica 'E. De Giorgi', Università del Salento , via per Arnesano, 73100 Lecce, Italy.|NANOTEC-CNR Istituto di Nanotecnologia , via per Arnesano, 73100 Lecce, Italy.
authors.forename: Carlo
authors.identifier: 0000-0003-4558-5367
authors.initials: C
authors.lastname: Giansante
mesh_terms: D007854:Lead / Q000737:chemistry*; D049449:Luminescence; D008956:Models, Chemical; D036103:Nanotechnology; D010316:Particle Size; D045663:Quantum Dots / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D013440:Sulfides; C018391:lead sulfide; D007854:Lead
keywords: Colloidal quantum dots; absorption coefficients; broadband quantum confinement; conjugated ligands; optical absorption enhancement; surface chemistry