Phase-pure FeSe(x) (x = 1, 2) nanoparticles with one- and two-photon luminescence

Iron chalcogenides hold considerable promise for energy conversion and biomedical applications. Realization of this promise has been hindered by the lack of control over the crystallinity and nanoscale organization of iron chalcogenide films. High-quality nanoparticles (NPs) from these semiconductors will afford further studies of photophysical processes in them. Phase-pure NPs from these semiconductors can also serve as building blocks for mesoscale iron chalcogenide assemblies. Herein we report a synthetic method for FeSe(x) (x = 1, 2) NPs with a diameter of ca. 30 nm that satisfy these needs. The high crystallinity of the individual NPs was confirmed by transmission electron microscopy (TEM) and energy-dispersive X-ray analysis. TEM tomography images suggest pucklike NP shapes that can be rationalized by bond relaxation at the NP edges, as demonstrated in large-scale atomic models. The prepared FeSe(x) NPs display strong photoluminescence with a quantum yield of 20%, which was previously unattainable for iron chalcogenides. Moreover, they also show strong off-resonant luminescence due to two-photon absorption, which should be valuable for biological applications.