title: Nanocrystallinity and the ordering of nanoparticles in two-dimensional superlattices: controlled formation of either core/shell (Co/CoO) or hollow CoO nanocrystals.
abstract: Here it is demonstrated that the diffusion process of oxygen in Co nanoparticles is controlled by their 2D ordering and crystallinity. The crystallinity of isolated Co nanoparticles deposited on a substrate does not play any role in the oxide formation. When they are self-assembled in 2D superlattices, the oxidation process is slowed and produces either core/shell (Co/CoO) nanoparticles or hollow CoO nanocrystals. This is attributed to the decrease in the oxygen diffusion rate when the nanoparticles are interdigitated. Initially, polycrystalline nanoparticles form core/shell (Co/CoO) structures, while for single-domain hexagonal close-packed Co nanocrystals, the outward diffusion of Co ions is favored over the inward diffusion of oxygen, producing hollow CoO single-domain nanocrystals.
authors: Laboratoire des Matériaux Mésoscopiques et Nanométriques, UMR CNRS 7070,  Université P. et M. Curie  Bât F, 4 Place Jussieu, 75005 Paris, France.
authors: Zhijie
authors: Z
authors: Yang
authors: Isabelle
authors: I
authors: Lisiecki
authors: Michael
authors: M
authors: Walls
authors: Marie-Paule
authors: MP
authors: Pileni