Carbon nanostraws: nanotubes filled with superparamagnetic nanoparticles.

A two-step magnetically assisted capillary action method is demonstrated as a facile technique to produce hollow carbon nanotubes filled with uniformly dispersed Fe3O4 nanoparticles (NPs). Template-assisted chemical vapor deposition (CVD) grown CNTs with average diameter 200-300 nm and length 5-6 microm were effectively used as 'nanostraws' to suck in chemically synthesized Fe3O4 nanoparticles (mean size approximately 6 nm) in a ferrofluid suspension. Temperature and magnetic field-dependent DC magnetization measurements indicate that these functionalized nanotubes are superparamagnetic at room temperature with enhanced interparticle interactions due to the close packing of the nanoparticles within the tubes. Magnetic relaxation phenomena in these filled nanotubes are probed using frequency-dependent AC susceptibility. The reasonably large saturation magnetization (M(s) = 65 emu g(-1)) attained in these nanostructures makes them very promising for a diverse set of applications that utilize both the magnetic and dielectric functionalities of these composite nanotube materials.