Carbon nanotube growth on AlN support: Comparison between Ni and Fe chemical states and morphology Baran Eren a,b , Laurent Marot a,⇑ , Roland Steiner a , Teresa de los Arcos c , Marcel Düggelin d , Daniel Mathys d , Kenneth N. Goldie e , Vesna Olivieri d , Ernst Meyer a a Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland b Material Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States c Institute of Experimental Physics II, Research Department Plasma, Ruhr-Universität Bochum, 44780 Bochum, Germany d Center of Microscopy, University of Basel, Klingelbergstrasse 50/70, CH-4056 Basel, Switzerland e Center for Cellular Imaging and Nano Analytics, Biozentrum, University of Basel, Klingelbergstrasse 50/70, CH-4056 Basel, Switzerland article info Article history: Received 1 April 2014 In final form 23 June 2014 Available online 1 July 2014 abstract In this work, carbon nanotubes (CNTs) are grown from Ni and Fe nanoparticles supported on a rough AlN surface. Although, identical experimental parameters are used during dewetting (island formation) via thermal treatment, Ni particles appear metallic and larger, whereas Fe particles are smaller and slightly oxidized. This difference in the nanoparticle chemical state and morphology reflects to CNTs during catalytic chemical vapor deposition in terms of their CNT growth mode and size: tip-growth mode for Ni catalyst with CNT diameters of up to 40 nm, whereas base-growth mode for Fe with CNT diameters typically less than 10 nm are observed. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Carbon nanotubes (CNTs) are well known to grow by catalytic chemical vapor deposition (CCVD) from the underlying metal nanoislands in a hydrocarbon ambient at elevated temperatures. The most common catalysts used for this process are Fe, Co, and Ni nanoparticles (NPs) due to their high catalytic activities and high solubilities of carbon at high temperatures. CCVD growth of CNTs can be briefly described in four stages [1]: (1) Adsorption and dissociation of hydrocarbon precursors, (2) carbon diffusion in or on the catalyst particle, (3) CNT nucleation, and (4) incorpo- ration of carbon into the growing structure. However, CCVD growth of CNTs depends on a plethora of experimental parameters, which has led to a wide range of different observations so far in the literature. A typical example, especially for Fe, is the chemical state of the NPs, i.e. whether they are metallic, carbidic or oxidic during the growth process. In 2008, Yoshida et al. advocated Fe 3 C phase with the electron diffraction analysis [2], whereas in 2009 Wirth et al. claimed pure metallic catalysts from the nucleation till the growth termination with in situ X-ray photoelectron spectroscopy (XPS) and electron microscopy measurements [3]. In 2011, He et al. showed that both chemical states could co-exist, with the ratio of the a-Fe phase to the carbide phase increasing with increasing growth temperature [4]. Similarly, by performing XPS measure- ments during the growth, Wirth et al. claimed in 2012 that the metallic Fe is the main active phase for c-rich particles, whereas Fe 3 C formation is dominant for a-Fe particles under the eutectoid temperature [5]. Other points which are disputed are the diffusion pathway of carbon (bulk diffusion or surface diffusion) during the second stage and the growth mode (tip-growth or base-growth) during the fourth stage of the CCVD process [6]. The diffusion path- way depends on the size of metal NPs [7]: surface diffusion is the dominant mass transport mechanism for particles with a size less than 20 nm due to the large surface to area ratio [1,8], whereas bulk diffusion dominates for particles with a size larger than 100 nm [9]. If NPs are carbidic or oxidic, the mass transport dom- inantly takes place on the surface due to the low carbon diffusivity of the bulk [6]. The growth mode depends on the adhesion of the catalyst to its support [9]: if NPs adhere strongly to their support, CNTs will grow on top of the catalyst [7,10]. On the contrary, low adhesion of NPs to their support will lead to CNT walls extruding out from the bottom of the catalyst particles as NPs arise with the growing CNTs [11]. One critical parameter is the choice of the support layer, since the chemical and physical nature of the support-catalyst interac- tions play a substantial role in the evolution of chemistry and mor- phology of the catalyst particles, which eventually determine the characteristics of CNTs. This is especially important when the NP synthesis is performed by thermal treatment of thin films which http://dx.doi.org/10.1016/j.cplett.2014.06.042 0009-2614/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: laurent.marot@unibas.ch (L. Marot). Chemical Physics Letters 609 (2014) 82–87 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett