DOI: 10.1002/cvde.200506441 Full Paper Synthesis, Properties, and Applications of Ferromagnetic-Filled Carbon Nanotubes By Albrecht Leonhardt,* Silke Hampel, Christian M��ller, Ingolf M��nch, Radinka Koseva, Manfred Ritschel, Dieter Elefant, Kati Biedermann, and Bernd B��chner 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 Fe3C ��� 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. M��ller, Dr. I. M��nch, R. Koseva, Dr. M. Ritschel, Dr. D. Elefant, K. Biedermann, Prof. B. B��chner Institute of Solid State and Materials Research Dresden Helmholtzstrasse 20, 01069 Dresden (Germany) E-mail: A.Leonhardt@ifw-dresden.de

Full Paper a quartz tube reactor by benzene and hydrogen gas. The vaporized ferrocene decomposes at temperatures of 1100���1200 ��C and forms iron particles that can make possi- ble the growth of unfilled single- and/or multiwalled carbon nanotubes. Other methods employ a solid catalyst particle as nucleus or starting point for the catalytic growth of the nanotubes. For example, Li et al.[7] synthesized aligned carbon nano- tubes by using a method based on Fe particles embedded in mesoporous silica, acetylene as the carbon feedstock, and nitrogen as the carrier gas. Ivanov[8] and Yacaman[9] also used graphite, or silica covered with transition metal nanoparticles, as the substrates and acetylene as the carbon source. By using such procedures, only unfilled carbon nanotubes could grow with the catalyst particle at the base or tip of the nanotubes. cCVD is also suitable for the synthesis of filled car- bon nanotubes, or, in other words, the synthesis of transi- tion metal nanowires encapsulated in carbon nanotubes. Filled carbon nanotubes, with a relatively high filling rate, can already be achieved by a simple pyrolysis of a metal- locene (an organometallic complex family, e.g., ferrocene) in a carrier gas, hydrogen or argon, and in the absence of a hydrocarbon. This method has the advantage that the growth and filling of the nanotubes take place simulta- neously. The metallocene family has a ���sandwich structure��� of two parallel cyclopentadienyl rings with a metal in the cen- ter between these rings (M(C5H5)2 with M = Fe, Co, Ni), is solid at room temperature, and dissolvable in various organ- ic solvents, but also shows suitable decomposition behavior in the temperature range 600���1150 ��C. The pyrolysis of these (Fe, Co, Ni) metallocenes was analyzed by Dyagileva et al.[10] and they showed that the decomposition is a com- plex process of a series of consecutive, parallel, and catalytic reactions of a homogeneous/heterogeneous nature. A further increase in the filling rate can be expected if, additionally, the substrate used is covered with catalytically active material (Fe, Co, Ni). These (few nanometers thick) catalyst layers, synthesized by sputter techniques or elec- tron (e)-beam evaporation, will promote the filling of car- bon nanotubes. The first experimental results on the pyrolysis of metallo- cenes were reported by Sen et al.[5] They synthesized par- tially filled tubes in a quartz-tube reactor. The nonoriented material was deposited on the reactor walls. Aligned struc- tures of iron- and invar-filled carbon nanotubes were pro- duced by Grobert,[11] using a mixture of ferrocene and C60 as the precursor. The transport of ferrocene into the reaction zone was accomplished using a different method. Sen[5] and Gro- bert[11] used the direct sublimation of this compound in an Ar/hydrogen flow at 200���250 ��C, and later the aerosol tech- nique[12] where a benzene solution containing ferrocene is nebulized by an aerosol generator in an argon flow and di- rectly injected into the reactor through a nozzle. These and other authors have synthesized iron- and co- balt-filled carbon nanotubes in a two-zone furnace reactor by direct sublimation of metallocenes (ferro- and/or cobal- tocene) followed by direct deposition on the reactor wall,[13,14] or on oxidized[15,16] Si substrates pre-coated with iron,[17,18] cobalt,[17] or nickel[13] (so-called solid-source (SS)CVD). Figure 1 shows the two-zone furnace reactor used. In recent publications,[19] the growth of aligned, filled carbon nanotubes on various substrates and their structural and magnetic properties were systematically studied in re- lation to their dependence on the main synthesis parame- ters, such as deposition time and reaction temperature. Recently, we have constructed new CVD equipment for a continuous deposition process of filled carbon nanotubes. This set-up is equipped with a liquid-source system for loading of the metallocene in the reactor (so-called liquid source (LS)CVD), which consists of a continuously acting band evaporator. The ferrocene precursor, dissolved in cy- clopentane, drops on the band, the solvent evaporates in the lower-temperature region, and the ferrocene is com- pletely sublimated in the higher temperature region, then injected into the reactor with no additional solvent frac- tions. The reactor itself is equipped with a mobile tape on which the substrates are fixed (see Fig. 2). 3. Results and Discussion By using ferrocene as a precursor, with a decomposition temperature of 750���950 ��C, both methods (SSCVD and LSCVD) deliver Fe-filled, multiwalled carbon nanotubes with a relatively high filling rate ( 50 % of the internal hol- low volume, estimated by scanning electron microscopy (SEM) images). Figure 3 shows aligned-grown filled carbon nanotubes on a Fe precoated oxidized Si substrate. The filling, which can be clearly seen, covers well over 50 % of the internal vol- ume. In whatever range of application these nanotubes are to be used, their dimensions and the rate and composition of Chem. Vap. Deposition 2006, 12, 380���387 �� 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cvd-journal.de 381 Fig. 1. Two-zone furnace for the pyrolysis of metallocenes (SSCVD) and the structure of substrates used (metal: Fe, Co, Ni).