Influence of solvation and the persistence of adsorbed linkers on the attachment of CdSe quantum dots to TiO ₂ via linker-assisted assembly

CdSe quantum dots (QDs) were attached to the surfaces of nanocrystalline TiO ₂ films functionalized with 16-mercaptohexadecanoic acid (MHDA). The solvent from which MHDA was adsorbed to TiO ₂ determined the amount of adsorbed MHDA, the extent of ordering within monolayers, and the lability of MHDA-TiO ₂ interactions, which in turn dictated the quality of QD-functionalized TiO ₂ films. When MHDA was adsorbed to TiO ₂ from dichloromethane, toluene, or heptane, it desorbed into toluene dispersions of CdSe QDs, causing the flocculation of QDs and the formation of opaque, nonuniform QD-functionalized TiO ₂ films overcoated with thick (0.1-0.5 μm) layers of agglomerated CdSe. When MHDA was adsorbed to TiO ₂ from tetrahydrofuran, it persisted upon exposure to toluene dispersions of QDs. The resulting QD-functionalized TiO ₂ films were transparent with a uniform loading of QDs and without an agglomerated overlayer. Control experiments revealed that flocculation and the formation of low-quality films were correlated with the presence of MHDA in dispersions of QDs; we propose that MHDA adsorbed to CdSe QDs and decreased their dispersibility in nonpolar solvents. The susceptibility of QDs to MHDA-induced flocculation and agglomeration increased with postsynthesis purification. Our results highlight the influence of solvation on the persistence of linker-substrate interactions, which is of central importance in optimizing the linker-assisted assembly of interfaces.