Catalyst rotation, twisting, and bending during multiwall carbon nanotube growth Michael J. Behr, K. Andre Mkhoyan, Eray S. Aydil * Department of Chemical Engineering & Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Avenue SE, Minneapolis, MN 55455, USA ARTICLE INFO Article history: Received 23 April 2010 Accepted 18 June 2010 Available online 25 June 2010 ABSTRACT Internal crystalline structure of cementite catalyst particles located inside the base of mul- tiwall carbon nanotubes was studied using nanoprobe convergent-beam electron diffrac- tion. The catalyst particles are single crystalline but exhibit combinations of small-angle (1°–3°) rotations, twists, and bends along their axial length between adjacent locations. Distortions are most severe away from the base up into the nanotube where the number of walls is large. Near the attachment point to the substrate, however, where few, if any graphene walls exist, particles remain undistorted. This suggests that the stresses gener- ated by the surrounding nanotube distort the catalyst particle during growth. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Precise structural control of carbon nanotubes (CNTs) re- mains one of the key challenges to realizing their techno- logical potential. Plasma-enhanced chemical vapor deposition (PECVD) using catalytic metal nanoparticles en- ables large-scale growth of CNT films, however, much is still unknown about what happens to the catalyst particle dur- ing growth [1–3]. Numerous studies with electron micros- copy have captured the dynamics of in situ CNT growth from crystalline catalyst particles [4–9]. In these studies, and in others, conducted ex situ, close inspection of bright-field (BF) transmission electron microscope (TEM) images reveals significant contrast variations along the length of the catalyst particles [10,11]. Sources of contrast variation in BF–TEM images include changes in crystal ori- entation relative to incident electron beam, strain, or thick- ness, and can indicate a distorted crystalline structure [12]. We examined, in detail, the crystalline structure of iron car- bide catalyst particles found inside the base of multiwall carbon nanotubes using a nano-scale electron probe in the TEM. 2. Experimental Multiwall carbon nanotubes (MWCNTs) with diameters rang- ing from 20 to 70 nm were grown using an inductively-cou- pled plasma and iron catalyst through catalytic PECVD. The iron catalyst was deposited on a native-oxide coated silicon substrate as a 10 nm-thick film using electron-beam evapora- tion. Before nanotube growth, the iron catalyst film was ex- posed to a plasma maintained in a mixture of H 2 and Ar (50 sccm H 2 and 5 sccm Ar) at 200 W and 700 °C for 15 min. This hydrogen plasma pretreatment breaks apart the iron film to form nanometer-sized metal islands, and reduces iron oxides present in the catalyst film. Nanotubes were grown at 800 °C and 10 Torr using a 13.56 MHz radio-frequency plasma maintained with 200 W power in a CH 4 (5 sccm), H 2 (5 sccm), and Ar (68 sccm) gas mixture. A dense film of multiwall car- bon nanotubes grew to an average length of four microns after 30 min of plasma deposition. Nanotubes were removed from the Si/SiO 2 substrate by sonication in ethanol for 30 s, and then transferred to a copper TEM grid coated with a lacey carbon support film. In some experiments the metal catalyst was deposited directly on SiO 2 TEM grids and carbon nano- 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.06.049 * Corresponding author: Fax: +1 612 626 7246. E-mail address: aydil@umn.edu (E.S. Aydil). CARBON 48 (2010) 3840 – 3845 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon