Solution-phase synthesis and magnetic properties of single-crystal iron germanide nanostructures

Iron-germanium (Fe-Ge) is a complex alloy system that includes several structurally and compositionally diverse phases that exhibit a range of interesting magnetic properties that can change significantly when reduced to nanoscale dimensions. Fe-Ge nanostructures have been synthesized using chemical and physical deposition methods but have not previously been accessible as solution-synthesized colloidal materials. Here, we show that colloidal Fe-Ge nanostructures can be synthesized via the hot injection of an oleylamine solution of Fe(CO)₅ into a solution containing GeI₄, oleylamine, oleic acid, and hexamethyldisilazane. This approach effectively merges recent advances in the synthesis of colloidal Ge nanocrystals with methods routinely used to synthesize metal and alloy nanoparticles. At 260 C, spherical nanocrystals of Ni₂In-type Fe₃Ge₂ form. Heating the solution at 300 C transforms the spherical Fe ₃Ge₂ nanocrystals into CoGe-type FeGe nanowires. The Fe₃Ge₂ nanocrystals are ferromagnetic with T_c ≈ 265 K, whereas the FeGe nanowires are only weakly magnetic.