AVS 49th International Symposium
    Magnetic Interfaces and Nanostructures Monday Sessions
       Session MI+NS-MoA

Invited Paper MI+NS-MoA3
Surfactant-mediated Shape Control, Magnetism and Self-assembly of Cobalt Nanocrystals

Monday, November 4, 2002, 2:40 pm, Room C-205

Session: Self-Assembly and Nanomagnetism
Presenter: K.M. Krishnan, University of Washington
Authors: K.M. Krishnan, University of Washington
Y.P. Bao, University of Washington
M. Beerman, University of Washington
Correspondent: Click to Email
Magnetic nanocrystals, monodisperse in size, shape and surface structure are now routinely synthesized by the rapid injection of an organometallic precursor into a hot coordinating solvent containing appropriate surfactants. The success of this method depends on temporally separating the nucleation and growth of the nanocrystals in solution and to control the precursor concentration to achieve size distribution focusing. In addition, careful choice of surfactant pairs that preferentially bond to different crystallographic planes of the growing nanocrystal can also be used to control the shape of the nanocrystals, including the preparation of high-aspect ratio particles or nanorods. For practical applications these nanocrystals must be organized into ordered arrays with well-defined inter-particle distances. In such nanocrystals, with a weak (van der Waals) attractive interaction between the metallic cores and a weak steric repulsion arising from the surfactant chains, entropy-driven self-assembly processes dominate. If particles of two different sizes are allowed to self-assemble, entropy driven wetting arising from depletion forces can be used to control the final organization. Use of solvent-nonsolvent pairs can be used to gently precipitate the nanocrystals into large scale ordered arrays. Details of the growth of Co nanocrystals with controlled size and shape, their subsequent self-assembly into ordered arrays, their characterization by a range of electron microscopy measurements and their isolated and collective magnetic behavior will be discussed.