Colloidal-ALD-Grown Metal Oxide Shells Enable the Synthesis of Photoactive Ligand/Nanocrystal Composite Materials

Colloidal nanocrystals (NCs) are ideal materials for a variety of applications and devices, which span from catalysis and optoelectronics to biological imaging. Organic chromophores are often combined with NCs as photoactive ligands to expand the functionality of NCs or to achieve optimal device performance. The most common methodology to introduce these chromophores involves ligand exchange procedures. Despite their ubiquitous nature, ligand exchanges suffer from a few limitations, which include reversible binding, restricted access to binding sites, and the need for purification of the samples, which can result in loss of colloidal stability. Herein, we propose a methodology to bypass these inherent issues of ligand exchange through the growth of an amorphous alumina shell by colloidal atomic layer deposition (c-ALD). We demonstrate that c-ALD creates colloidally stable composite materials, which comprise NCs and organic chromophores as photoactive ligands, by trapping the chromophores around the NC core. As representative examples, we functionalize semiconductor NCs, which include PbS, CsPbBr₃, CuInS₂, Cu_2-X, and lanthanide-based upconverting NCs, with polyaromatic hydrocarbons (PAH) ligands. Finally, we prove that triplet energy transfer occurs through the shell and we realize the assembly of a triplet exciton funnel structure, which cannot be obtained via conventional ligand exchange procedures. The formation of these organic/inorganic hybrid shells promises to synergistically boost catalytic and multiexcitonic processes while endowing enhanced stability to the NC core.