Lawrence Berkeley National Laboratory

Surfactant-coated cobalt nanocrystals can be prepared with a reasonable degree of control over particle size and shape using a thermolytic route. The small crystallite size, enhanced reactivity and tunable interparticle interactions enable use of this material as starting material for demonstration of achievement of novel structures using extremely simple solution-based approaches. In particular, formation of hollow cobalt sulfide nanocrystals upon chemical modification and emergence of long-range orientational order upon drying-mediated assembly of cobalt nanocrystals is reported here.Colloidal preparation of Co nanocrystals has been well-studied. Here, we emphasize general principles and crystallographic/morphological characterization of disk-shaped hcp-Co nanocrystals. Use of surfactant molecules enables achievement of multiple morphologies in one synthetic system.Formation of hollow structures upon in-solution sulfidation of Co nanocrystals is presented and discussed. A Kirkendall-type effect, involving dominant outward mass transport during formation of the ionic shell material explains the results naturally. It is expected that this phenomenon will generalize extensively to formation of hollow structures of an enormous variety of compositions. Detailed study of particle morphology as a function of reaction conditions suggest phenomena likely to be generally relevant to use of this approach. A short report of crystallographic co-alignment into vortex-like structures is also provided. Our current best picture of this process involves an interplay of packing and magnetic interactions between faceted particles.