There's Plenty of Surface at the Bottom: Surface Chemistry Enhances Light Absorption by Colloidal Semiconductor Nanocrystals

Abstract

The paraphrasis of Feynman's statement is used to highlight the inherent relevance of surfaces -- and interfaces -- at the nanoscale. Here, the marked impact of the surface chemistry on the light absorption by colloidal inorganic semiconductor nanocrystals is discussed and demonstrated as general. Chemical species at the surface (ligands) of colloidal metal chalcogenide quantum dots (QDs) are shown to induce broadband absorption enhancement and band gap reduction. A comprehensive library of chalcogenol(ate) ligands is exploited to infer the role of surface chemistry on the QD optical absorption properties: ligand chalcogen binding atoms mainly determine band gap reduction, related to np occupied orbital contribution to the valence band edge, and mediates broadband absorption enhancement, fostered by the conjugation of the ligand pendant moiety, with further contribution from electron donor substituents. These findings point to a description of colloidal QDs that may conceive ligands as part of the overall QD electronic structure, beyond common models derived from analogies with core/shell heterostructures depicting ligands as mere perturbation to the core properties.