Observation and formation mechanism of individual graphene foils inside multi-walled carbon nanotubes G. Alonso-Nu ´n ˜ez a, *, M. Avalos-Borja a , J. Lara-Romero b , G. Berhault c , A. Marquez-Lucero d a Universidad Nacional Auto ´noma de Me ´xico, Centro de Nanociencias y Nanotecnologı´a, KM 107 Carretera Tijuana-Ensenada, Ensenada, B.C., C. P. 22860, Mexico b Facultad de Ingenierı´a Quı´mica, UMSNH, Morelia, Mich., Mexico c Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256 CNRS – Universite ´ de Lyon, 2 avenue Albert Einstein, 69626 Villeurbanne cedex, France d Centro de Investigacio ´n en Materiales Avanzados, Miguel de Cervantes #120, Complejo Industrial Chihuahua, Chihuahua, Chih., 31109, Mexico 1. Introduction Graphene has attracted a huge scientific interest since it was first discovered in 2004 [1–5]. It is a single layer of atomic carbon atoms densely packed in a honeycomb crystal lattice. Graphene is also one of few two-dimensional materials that are stable [1] and its sheets can be freely suspended for example on a microfabricated scaffold in vacuum or air [2]. The high crystal quality of its two-dimensional structure and the mimicking of its charge carriers of massless relativistic particles provide unique electron transport properties. Its special band structure and other unusual physical properties [3] permit to envisage very important applications for this material like the formation of switching functions for atomic-scale electronic devices through the control of the band gap, as nanoscale resonators or through the use of its very high breaking strength [6–12]. In the present article, we will show the presence of individual graphene foils (GF) at the center of a multi-walled carbon nanotube (CNT). These foils may be formed by detachment and necking from internal CNT walls and are attached to a catalyst nanoparticle placed inside the nanotube during the process of the CNT growth. This structure is interesting because it opens the possibility to study the physical properties of graphene foils encapsulated inside carbon nanotubes. 2. Experimental The carbon nanotube (CNT) synthesis was carried out by spray pyrolysis in a Vycor tubing connected to a pneumatic system employed as solution atomizer. The overall tube dimensions had a 9.0 mm internal diameter and a 23.0 cm length. The tube was heated by a cylindrical furnace (Thermolyne 1200) with a high precision temperature controller (Æ1 8C). The solution feed time was kept constant during 15 min for all experiments. 25 ml of toluene (Aldrich, 99.8%) and 1.0 g of ferrocene (Aldrich, 98.0%) as catalyst were mixed with argon (99.99%, Praxair) which was employed as the carrier gas. The flow rate was regulated by a mass flow controller at 83.3 cm 3 /s. The argon/toluene/ferrocene mixture was fed into the Vycor tubing after the furnace temperature was set at 900 8C. As a result, a black film of multi-walled CNTs, formed at the inner surface of the Vycor tubing, was mechanically removed and was analyzed by HRTEM. Transmission electron micrographs were obtained on a JEOL 2010F transmission electron microscope (TEM) operating at 200 kV. TEM specimens were prepared by dispersing the as-formed CNTs in acetone before placing them into an ultrasonic bath for 2 min. A drop of suspension was put onto a holey carbon-coated Cu grid and was allowed to dry. Raman spectroscopy was performed using a micro- Raman system (Dilor LabRam) equipped with a 20 mW He–Ne laser Materials Research Bulletin 46 (2011) 658–661 ARTICLE INFO Article history: Received 25 March 2010 Received in revised form 19 January 2011 Accepted 31 January 2011 Available online 25 February 2011 Keywords: Raman spectroscopy Electron diffraction Nanostructures Multilayers Epitaxial growth ABSTRACT The formation of individual graphene foils (GF) has been evidenced in the present study inside multi- walled carbon nanotubes. The graphene foils are attached on one side to a catalyst nanoparticle and on the other side to the internal walls of the nanotube. Moreover, results suggest that a necking process occurs in which internal carbon walls are deformed until formation of the graphene foil. A possible mechanism for the GF formation is then proposed. ß 2011 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +52 646 1744602; fax: +52 646 1744603. E-mail address: galonso@cnyn.unam.mx (G. Alonso-Nu ´n ˜ ez). Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.01.024