Exceptionally mild reactive stripping of native ligands from nanocrystal surfaces by using Meerwein's salt.

Native coordinating ligands acquired during the chemical synthesis of colloidal nanocrystals are optimized primarily for their ability to exert control over nanocrystal size, composition, morphology, and dispersibility, and not necessarily for their final application. In general, they are hydrophobic and highly insulating, and constitute a significant barrier for charge or ion transport in devices configured therefrom. Bare nanocrystal surfaces, while desirable for many applications, can be difficult to obtain reliably and without undesirable consequences. For example, removal of native ligands from nanocrystal dispersions usually results in aggregation or etching, while in thin films their chemical displacement (e.g., by hydrazine or formic acid) often gives inefficient removal of surface ligands. Thermal treatments inevitably leave behind an undesirable residue, require lengthy annealing times, or result in particle sintering. Nevertheless, these approaches have demonstrated that near-bare nanocrystal surfaces are useful in a broad spectrum of advanced energy applications, from light-emitting diodes to field-effect transistors and photovoltaics. Dispersions of bare nanocrystals would also be useful as nanoinks and for facilitating their transfer into polar media for biomedical applications and catalysis. In pursuit of a universal reagent for producing stable dispersions or thin films of ligand-stripped nanocrystals with retention of their physical properties, we show here that this can be uniquely achieved by using Meerwein s and other trialkyl oxonium salts. Recently, NOBF4 was proposed as a general reagent for stripping of oleate ligands from metal oxides, metals, dielectrics, and some semiconductors. While this technique is quite effective for certain nanocrystals, it is not applicable to materials with limited chemical stability, which include most metal chalcogenides and some important oxides, such as both doped and undoped ZnO, presumably due to the oxidative ability and Lewis acidity of the nitrosonium cation. Acids such as HBF4 and HPF6 have also been explored recently [12]