DOI: 10.1002/cvde.200506441 Full Paper Synthesis, Properties, and Applications of Ferromagnetic-Filled Carbon Nanotubes By Albrecht Leonhardt,* Silke Hampel, Christian Müller, Ingolf Mönch, Radinka Koseva, Manfred Ritschel, Dieter Elefant, Kati Biedermann, and Bernd Büchner Ferromagnetic-filled carbon nanotubes are new nanostructured materials with many possible applications. They can be synthesized using the thermal decomposition of metallocenes of the iron triad. Two different methods (solid and liquid source CVD) are suitable for producing, at very high filling rates, filled nanotubes on precoated Si substrates. The diameters of de- posited filled nanotubes are particularly dependent on the size of catalyst particles on the substrate, while the lengths depend more on the sublimation and decomposition rate of metallocene. The growth mechanism of filled carbon nanotubes is based on the root growth mode. Multiwalled carbon nanotubes, filled with body-centered cubic Fe, show unusual magnetic proper- ties. Aligned-growth nanotube ensembles can reach coercivities up to 130 mT (bulk iron 0.09 mT). Ferromagnetic-filled carbon nanotubes can be successfully used both as cantilever tips in magnetic force microscopy and as a nanocontainer for new therapies in medicine. Keywords: Carbon nanotubes, Ferromagnetism, CVD of filled carbon nanotubes 1. Introduction In recent years, filled multiwalled carbon nanotubes have become a promising subject of research. In particular, car- bon nanotubes filled with ferromagnetic materials such as iron, cobalt, or nickel are considered to have potential ap- plication in various areas. The structures of these nanotubes may thought of as met- al nanowires or nanomagnets inside the carbon nanotube. The nanowires are well protected by the encapsulation against oxidation and other chemical reactions and influ- ences. Hence, such nanowires possess long-term stability, and this is an enormous advantage compared to pure, un- coated nanowires, and also opens new application fields. Ferromagnetic-filled carbon nanotubes may find applica- tions ranging from magnetic data-storage devices, [1] to im- plementation of individual filled tubes in a sensor system for magnetic force microscopy. [2] Another possible use of filled carbon nanotubes is seen in biomedicine as ferromag- netic nanocontainers initiating a new antitumor therapeutic concept in the treatment of cancer. [3] These filled carbon nanotubes represent a suitable material for magnetically- guided hyperthermia and, functionalized inside and outside the tube, a unique drug delivery/carrier system. [4] The study of ferromagnetic-filled carbon nanotubes is not only interesting because of the potential applications, but also from a scientific point of view. Compared to bulk material, the encapsulated metal nanowires often exhibit new structural and magnetic properties, which originate from their low dimensionality and large geometric aspect ratio. In fact, the internal cavity of carbon nanotubes may readily serve as an ideal nanoscale crucible for performing metallurgical operations at the nanoscale with iron or other metals. The range of interests has varied between the influence of the effects of carbon nanotubes confinement on the structure of metal phases, on phase distribution and phase transformation (in the case of iron, the stability of body- centered cubic (bcc)-Fe, face-centered cubic (fcc)-Fe, and Fe 3 C – or other higher carbides – can change), and finally on the changing of physical (magnetic) properties. 2. Synthesis of Filled Multiwalled Carbon Nanotubes A well-known method for the synthesis of single- and multiwalled carbon nanotubes is the so-called catalytic (c)CVD. By using this method, the transition metal catalyst plays a key role. In general, there are two possible methods of loading the catalyst into the synthesis process. Firstly, Sen et al., [5] and later Cheng et al., [6] reported a method that employs benzene as the carbon feedstock, hydrogen as the carrier gas, and ferrocene as the catalyst precursor. In this method, ferrocene is vaporized and carried into 380 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2006, 12, 380–387 – [*] Dr. A. Leonhardt, Dr. S. Hampel, C. Müller, Dr. I. Mönch, R. Koseva, Dr. M. Ritschel, Dr. D. Elefant, K. Biedermann, Prof. B. Büchner Institute of Solid State and Materials Research Dresden Helmholtzstrasse 20, 01069 Dresden (Germany) E-mail: A.Leonhardt@ifw-dresden.de