Morphological control in nanostructures has become a key issue in the preparation of electronic and mechanical nano-devices. Among all anisotropic shapes, ferromagnetic nanowires have attracted a most vivid interest for magnetic information storage devices and advanced materials. At present, the complex and often multi-step preparation of nanowires by structure directing templates (polymer templates, anodized alumina templates) has motivated for investigations into template free methods.

The present contribution investigates the preparation of ferromagnetic nanowires at large production rates in a one-step gas phase process. Applying an external magnetic field on a metallic cobalt nanoparticle-loaded gas stream resulted in the formation of self aligned metallic cobalt nanowires of 20-50 nm diameter (Figure 1(a)). This template-free, continuous and rapid production method afforded nanowires with an aspect ratio of over 1000 (length / diameter, Figure 1(b)) at a production rate of over 30 g h^-1. At higher magnetic fields the metallic nanowires formed bundles with distinctive parallel orientation (Figure 1(c)). Since the anisotropic agglomeration of the metal nanoparticles takes place at elevated temperature, the as formed magnetic nanowire is consolidated by sintering of the metal particles.

Fig. 1: Transmission electron micrograph of air stable metallic nanowires (a). An overview (SEM image, (b)), corroborated the exclusive formation of large amounts of template free nanofibers. The application of a stronger magnetic field resulted in thicker self aligned bundles of nanowires (c).

References:

E.K. Athanassiou, P. Grossmann, R.N. Grass and W.J. Stark, Nanotechnology, 2007, 18(16), 165606.

R.N. Grass, M. Dietiker, C. Solenthaler, R. Spolenak, W.J. Stark, Nanotechnology, 2007, 18(3), 035703.