Investigation of the oxide shell forming on ɛ-Co nanocrystals

This paper reports on the TEM characterization of the surface oxide layer forming on Co nanocrystals (NCs) prepared using a standard method [Puntes, V.F., Krishnan, K.M., Alivisatos, P., 2001. Synthesis, self-assembly, and magnetic behavior of a two-dimensional superlattice of single-crystal epsilon-Co nanoparticles. Appl. Phys. Lett. 78 (15), 2187-2189]. Complementary transmission electron microscopy (TEM)-related techniques presented direct evidence of a 1.5-3 nm CoO shell forming on ɛ-Co NCs. The crystalline structure of the Co NCs was confirmed by selected area diffraction study while the nature of the shell was probed by energy-loss near-edge spectroscopy and energy-filtered TEM. Based on these results, we comment on the detection of nanoscale feature with energy-filtered imaging.