Surface Chemistry Control of Colloidal Quantum Dot Band Gap

Surface chemistry modification of as-synthesized colloidal inorganic semiconductor nanocrystals (QDs), commonly referred to as ligand exchange, is mandatory toward effective QD-based optoelectronic and photocatalytic applications. The widespread recourse to ligand exchange procedures on metal chalcogenide QDs often narrows the optical band gap, although little consensus exists on explanation of this experimental evidence. This work attempts at providing a comprehensive description of such a phenomenon by exploiting rationally designed thiol ligands at the surface of colloidal PbS QDs, as archetype of material in the strong quantum confinement regime: the thiol(ate)-induced QD optical band gap reduction almost linearly scales with the inorganic core surface-to-volume ratio and mainly depends on the sulfur binding atom, which is here suggested to contribute occupied 3p orbitals to the valence band edge of the QDs. As opposed to QD models based on the analogy with core/shell heterostructures, the indecomposable character of ligand/core adducts (the colloidal QDs themselves) arises.