Tuning Electron-Conduction and Spin Transport in Magnetic Iron Oxide Nanoparticle Assemblies via Tetrathiafulvalene-Fused Ligands

We report a strategy to coat Fe3O4 nanoparticles (NPs) with tetrathiafulvalene-fused carboxylic ligands (TTF-COO-) and to control electron conduction and magnetoresistance (MR) within the NP assemblies. The TTF-COO-Fe3O4 NPs were prepared by replacing oleylamine (OA) from OA-coated 5.7 nm Fe3O4 NPs. In the TTF-COO-Fe3O4 NPs, the ligand binding density was controlled by the ligand size, and spin polarization on the Fe3O4 NPs was greatly improved. As a result, the interparticle spacing within the TTF-COO-Fe3O4 NP assemblies are readily controlled by the geometric length of TTF-based ligand. The shorter the distance and the better the conjugation between the TTF's HOMO and LUMO, the higher the conductivity and MR of the assembly. The TTF-coating further stabilized the Fe3O4 NPs against deep oxidation and allowed I2-doping to increase electron conduction, making it possible to measure MR of the NP assembly at low temperature (<100 K). The TTF-COO-coating provides a viable way for producing stable magnetic Fe3O4 NP assemblies with controlled electron transport and MR for spintronics applications.