In Situ Structural Evolution of Self-Assembled Oxide Nanocrystals

Self-assembling of organic passivated nanocrystals has attracted a lot of interest recently. In this paper, the structural evolution of tetrahedral CoO nanocrystals is studied in situ using transmission electron microscopy. The as-prepared superlattices are Na(AOT)-passivated CoO nanocrystals, packed into monolayer and multilayer arrays on an amorphous carbon film. As the specimen temperature increased from 170 to 200 °C, the passivation layer is gradually evaporated/decomposed. For temperatures higher than 200 °C, the CoO nanocrystals experience a solid-state reaction, while the monolayer packing configuration is still preserved, but not the multilayers. The replacement reaction finishes at ∼600 °C, and the final product is identified as mixed carbide nanocrystals of Co2C and Co3C.