Influence of solvation and the persistence of adsorbed linkers on the attachment of CdSe quantum dots to TiO2 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.