Synthesis of carbon nanotubes and nano-necklaces by thermal plasma process Hanako Okuno a , Eusebiu Grivei b , Fr ed eric Fabry b , Thomas M. Gruenberger c , Jos  e Gonzalez-Aguilar c , Andre € ı Palnichenko a , Laurent Fulcheri c , Nicolas Probst b , Jean-Christophe Charlier a,d, * a Unit e de Physico-Chimie et de Physique des Mat eriaux, Universit e Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la Neuve, Belgium b Timcal Belgium S.A., Av. Louise 534, Bte 1, B-1050 Brussels, Belgium c Ecole des Mines de Paris, Rue C. Daunesse, F 06904 Sophia-Antipolis Cedex, France d Research Center for Micro- and Nano-materials and Electronic Devices, Universit e Catholique de Louvain, B-1348 Louvain-la Neuve, Belgium Received 27 August 2003; accepted 24 May 2004 Available online 2 July 2004 Abstract High-yield syntheses of peculiar carbon nanotubes and carbon ‘necklace’-like morphologies have been obtained using a sophisticated thermal plasma technology. This method is based on a thermal plasma, which vapourizes the carbonaceous precursor in the presence of metal catalyst. Electron microscopy analyses provide evidences for a ‘stacked-cup’ structure for the carbon nanotubes. Carbon nano-‘necklaces’ are constituted by the repetition of multi-wall carbon spheres, connected along one direction in a ‘bell’-like structure, and containing frequently an encapsulated metal particle. Microscopic growth mechanisms are also proposed to interpret both syntheses. Due to their intriguing topology, these new carbon nanotubes and necklaces may find important applications in nano-technology. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes, Carbon beads; B. Heat treatment, Plasma reactions; C. Electron microscopy 1. Introduction Carbon nanomaterials reveal a rich polymorphism including multi- and single-wall carbon nanotubes (CNTs) [1], fullerene molecules [2], graphitic onions [3], toroids [4], boxes [5], cones [6], etc. Each step in the discovery of a new carbon material morphology leads to new potential application fields. In particular, it is ex- pected that the original nanometric morphologies of CNTs could have an important potential impact in applicative domains such as molecular electronics or high-strength composite materials [7]. Carbon nanotubes are mainly produced by three conventional techniques: arc discharge [8,9] or laser ablation methods [10], which are two high-temperature processes (T  4000 K), and chemical vapour deposition (CVD) methods [11], which operate at medium tem- perature (T  1500 K). In the arc-discharge and laser ablation techniques, the carbon and the catalyst are vapourized simultaneously. In the CVD methods, the carbon source is obtained from the decomposition of a gas phase (CO, methane, ethylene, etc.) in the presence of small metallic particles covering a substrate. In the present letter, a new method, based on a 3-phase alter- native current (AC) plasma technology, is used to syn- thesize carbon nanostructures. 2. Experimental Our plasma technology, initially designed for the production of carbon black by cracking of liquid or * Corresponding author. Tel.: +32-10-473359; fax: +32-10-473452. E-mail address: charlier@pcpm.ucl.ac.be (J.-C. Charlier). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.05.037 Carbon 42 (2004) 2543–2549 www.elsevier.com/locate/carbon