Letters to the Editor Growth and surface engineering of vertically-aligned low-wall-number carbon nanotubes Maoshuai He a, * , Sami Vasala b , Hua Jiang c , Maarit Karppinen b , Esko I. Kauppinen c , Marita Niemela ¨ a , Juha Lehtonen a a Department of Biotechnology and Chemical Technology, School of Chemical Technology, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland b Department of Chemistry, School of Chemical Technology, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland c Department of Applied Physics and Center for New Materials, School of Science, Aalto University, P.O. Box 15100, FI-00076 Aalto, Finland ARTICLE INFO Article history: Received 20 February 2012 Accepted 14 February 2012 Available online 19 May 2012 ABSTRACT A novel catalyst of cobalt supported by single crystal MgO was prepared by atomic layer deposition and used for carbon nanotube growth. With CO as carbon source, vertically- aligned carbon nanotubes with predominant double-walled (82%) were produced at 700 °C. Similar carbon nanotube array with a majority of single-walled tubes (62%) was pro- duced at 900 °C using methane as the carbon source. Due to their high flexibility compared with multi-walled carbon nanotubes, the low-wall-number carbon nanotube array could form a 3-dimensional honeycomb-like network when being spread with acetone. Ó 2012 Elsevier Ltd. All rights reserved. Initiated by Iijima’s landmark work [1], carbon nanotubes (CNTs) have generated a great deal of interest because of their remarkable properties and potential applications. Compared with multi-walled carbon nanotubes (MWCNTs), low-wall- number carbon nanotubes (LWNCNTs), defined as CNTs with only 1–4 walls, have superior electrical and mechanical prop- erties. Although tremendous progress has been achieved in growing vertically-aligned MWCNTs in the past decade, grow- ing vertically-aligned LWNCNTs was not possible until recent development of the Fe/Al 2 O 3 catalyst [2,3]. As suggested by both experimental results [4] and theoretical predictions [5], CNTs grow on the catalyst nanocrystals epitaxially. Therefore, the chirality of a CNT is associated to the structure of the ac- tive nanocatalyst and the support on which the particle situ- ates. In order to modify the chirality distribution of CNTs, it is necessary to develop other catalyst systems. Besides the growth, surface engineering of CNT array is also of great importance for their potential applications in nanofiltration devices and anti-wetting materials [6,7]. Some early studies have demonstrated the self-assembly of aligned MWCNTs into micro-patterns of relative large size, it is still challenging to acquire CNT patterns with sizes down to several microme- ters. In this work, we develop a novel Co–MgO catalyst for the synthesis of vertically-aligned LWNCNTs under different growth conditions. In addition, the capillarity-driven assem- bly of LWNCNT array was demonstrated, showing the poten- tial of forming small size patterns. MgO (100) single crystal (Heifei Kejing Materials Technol- ogy Co., China) was used as the substrate for the deposition of Co by atomic layer deposition (ALD). The catalyst deposi- tion process is similar to that for preparing porous SiO 2 sup- ported Co catalyst [8]. Prior to deposition, the MgO single crystal substrate was preheated in the ALD reactor (F120) at 400 °C for 5 h under a nitrogen atmosphere. The precursor (cobalt (III) acetylacetonate, Aldrich, 98%) was evaporated and deposited onto the MgO substrates at 190 °C. After 6 h 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.05.028 * Corresponding author. E-mail address: maoshuai.he@aalto.fi (M. He). CARBON 50 (2012) 4750 – 4764 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon