Appl. Phys. A 67, 101–105 (1998) Applied Physics A Materials Science & Processing  Springer-Verlag 1998 Filling carbon nanotubes D. Ugarte 1 , T. Stöckli 2 , J.M. Bonard 2 , A. Châtelain 2 , W.A. de Heer 3 1 Laborat´ orio Nacional de Luz S´ ıncrotron (CNPq/MCT), Caixa Postal 6192, 13083-970 Campinas SP, Brazil 2 Institut de Physique Exp´ erimentale, Dept. Physique, Ecole Polytechnique F´ ed´ erale de Lausanne, 1015 Lausanne, Switzerland 3 School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA Received: 2 March 1998 Abstract. Filling hollow carbon nanotubes with chosen ma- terials opens new possibilities of generating nearly one- dimensional nanostrutures. One simple approach to fill nano- tubes is to use capillarity forces. Here, we have studied the wetting and capillarity by metal salts. First, nanotubes were opened by oxidation in air; subsequently, nanotubes were im- mersed in molten salts. We have observed a size-dependence filling indicating a lowering of the cavity–salt interface en- ergy with decreasing diameter. By expressing the wetting conditions as a function of polarizabilities, it is possible to predict the threshold diameter for capillary filling of different materials. PACS: 61.46+w; 68.45.Gd; 61.16.Bg Fullerenes and other related graphitic structures have many fascinating electronic, chemical, and mechanical properties. They are particularly interesting for the nanomaterials re- search in order to develop substances with predefined proper- ties oriented towards specific technological applications. Among the large variety of nanostructures in the fuller- ene family, recently one particular member has become the focus of a great deal of scientific and technological atten- tion: the nanotube [1]. The basic structural unit of a nanotube is a graphitic sheet rolled into a cylinder, while the tube tip is closed by hemispherical or polyhedral graphitic domes. These tubes present impressive aspect ratios from 100 to 1000, with diameters as small as 1 nm and lengths ranging from μ m to mm in some cases. In practice, we can roughly divide the nanotubes (NTs) into two different classes, either by considering their struc- ture or synthesis method, these are single-walled nanotubes (SWNTs) [2, 3] and multiwalled nanotubes (MWNTs) [1]. The first class includes cylinders formed by a single graphitic layer where the typical diameter is 1–2 nm. They have cur- rently been synthesized by an electric arc operated in an inert- gas atmosphere using carbon electrodes containing a few per- cent of a transition metal element (Co, Fe, Ni, etc.); SWNTs are recovered from the fluffy carbon powder which coats the vacuum chamber [2, 3]. Recently, it has been shown that they can also be efficiently synthesized using laser evaporation [4]. The second class of tube includes structures formed by the coaxial arrangement of several (2–50) SWNTs; their external diameter is of the order of 10–20 nm. They are synthesized using a pure carbon arc run in an inert-gas atmosphere and are found in the deposit formed on the cathode [5]. Aside from their nanoscopic dimensions and their remark- able thermal and mechanical robustness, their main attractive feature relates to the predicted electronic properties: theoret- ical calculations indicate that nanotubes may be insulators, semiconductors, or metals depending on their radius and he- licity [6]. If the tube radius increases, the properties of the tubes approach those of macroscopic planar graphite. Another fascinating aspect of fullerene-related materials is their cavities, which can be used to incorporate atoms or molecules in order to generate novel compounds or nanos- tructured materials. These nanocavities may even display cat- alytic activity. Furthermore, the very long cavities of the NTs may potentially be used as templates for elongated nanostruc- tures. Some succesful attemps to form long nanometric fila- ments were based on the electric-arc method, using composite electrodes impregnated with the filling material [7]. But al- though these experiments led to the observation of a great variety of interesting filled graphitic structures, the efficiency and control of the process was very low hindering further de- velopment. But, since the synthesis methods of NTs are rather efficient [5], it is feasible to separate the production and fill- ing procedures. A promising approach exploits the capillary properties of the nanotubes [8, 9]. The procedure to fill nanotubes may be classified in two groups: (a) the chemical method, using wet chemistry [10]; and (b) the physical method, where capillarity forces in- duce the filling of a molten material [9, 11, 12]. In the wet- chemistry method, the nanotubes are refluxed in a nitric acid bath in order to open their tips. When a metal salt is simul- taneously used in the bath, it is possible to obtain oxide or pure metal particles by a subsequent annealing [10]. Also, the metal–salt solvent bath can used on previously opened tubes. In the physical method no solvent is used in the pro- cess and opened tubes are directly immersed in the molten material whereby the liquid is driven into the tube by capil-