Composition and size tailored synthesis of iron selenide nanoflakes

Regular square FeSe_x nanoflakes with tetragonal PbO-type phase and selenium deficiency have been grown from an inorganic iron source, i.e. ferrous chloride and selenium trioctylphosphine through a simple solution-based route in the co-solvent of oleylamine and oleic acid. The phase and structure are identified by powder X-ray diffraction and electron microscopy. The square nanoflakes are revealed to be bound by two larger {001} planes and four equivalent smaller {100} side surfaces. The appearance of the larger {001} planes and flaky morphology inherently results from the greatest atom density on {001} lattice planes and the largest d₀₀₁ spacing. The addition of the organic acid, i.e. oleic acid, at a proper stage during heating of the starting materials, is of great importance to the yield of the tetragonal PbO-type phase. Interestingly, the chemical composition and the edge length of the single crystalline FeSe_x nanoflakes can be tailored readily by using an organic diol which is suggested to serve as a reducing agent and chemical ligand. Thus, the deficiency of selenium can be highly reduced and the top-bottom surface area be enlarged.