Magnetic-dipolar-interaction-induced self-assembly affords wires of hollow nanocrystals of cobalt selenide.

Self-assembly, an attractive and practical methodology, allows the formation of a wide range of nanostructures for promising applications—for example, nanoparticle arrays for new optical band-gap materials or high-density magnetic recording media, self-assembled monolayers (SAM) for nanometerthick films on a variety of substrates, and nanofibers of selforganized small molecules or oligopeptides for drug delivery and tissue engineering. Hydrogen bonding and/or van der Waals interactions are usually the driving forces that result in self-assembly at the nanoscale. Despite extensive investigation, it is still difficult to control and predict the nanostructures resulting from these two types of forces. Although several recent reports have shown that magnetic dipolar interactions induce rather predictable nanoassemblies, methods for maintaining these assemblies when the magnetic force decreases or vanishes have been less thoroughly investigated. Therefore, we decided to use the magnetic dipoles inherently associated with magnetic nanoparticles to form 1D assemblies of hollow nanocrystals of semiconductors. We chose to make wires of hollow nanocrystals of CoSe2 to demonstrate the concept, because 1) the isolation of a nanonecklace of cobalt nanocrystals on a substrate from a dispersion after removal of the solvent indicated that magnetic dipolar interactions are sufficient to maintain 1D assemblies of nanocrystals in solution; 2) the recently reported formation of hollow CoSe nanocrystals from cobalt nanocrystals through the Kirkendall effect suggested that a similar process could be used to generate hollow CoSe2 nanocrystals from 1D assemblies of cobalt nanocrystals; 3) hollow nanostructures have received considerable attention because they exhibit properties that differ from their solid counterparts; and 4) nanowires of hollow nanocrystals of CoSe2 are unknown. Herein, we show that when their sizes reach approximately 20 nm cobalt nanocrystals self-assemble into wires in solution at temperatures as high as 455 K. Through the nanoscale Kirkendall effect, wires of hollow CoSe2 nanocrystals form without loss of the preassembled nanostructure induced by the magnetic dipolar interactions of the cobalt nanocrystals. In control experiments, when the size of the cobalt nanocrystals is 6 nm, or when 20-nm cobalt nanocrystals are in an alternating magnetic field, no wires of hollow CoSe2 nanocrystals form. These observations confirm that the magnetic interaction plays a key role in the formation of the nanowires of hollow CoSe2 crystals. Several types of ferromagnetic metallic nanocrystals (iron, cobalt, nickel, FePt, or CoPt) can be prepared readily, and their selfassembly depends on the magnetic dipolar interactions of particles of a certain size. Therefore, the method shown herein should be a general route to 1D assemblies of hollow nanocrystals. The synthesis of wires of hollow CoSe2 nanocrystals is easy and straightforward. Cobalt nanocrystals of approximately 20 nm in diameter were prepared according to a modified literature procedure and then dispersed in a solvent (for example, 1,2-dichlorobenzene) by using trioctylphosphine oxide (TOPO) as a surfactant. The injection of a dichlorobenzene solution of selenium into the dispersion of cobalt nanocrystals under reflux at 455 K resulted in a black dispersion of CoSe2 nanocrystals after approximately 30 min. Transmission electron microscope (TEM) images reveal that the CoSe2 nanocrystals form wires of intercon[*] J. Gao, Prof. B. Xu Department of Chemistry The Hong Kong University of Science & Technology Clear Water Bay, Hong Kong (China) Fax: (+852)2358-1594 E-mail: chbingxu@ust.hk