Metal–nanotube interactions – binding energies and wetting properties Amitesh Maiti a, * , Alessandra Ricca b a Materials Science and Nanotechnology, Accelrys Inc., 9685 Scranton Road, San Diego, CA 92121, USA b Center for Nanotechnology, NASA Ames Research Center, Moffett Field, CA 94035, USA Received 18 May 2004; in final form 18 May 2004 Available online 4 August 2004 Abstract Recent experiments indicate wide variation of the quality of metal–nanotube contacts with the choice of metal. With Au, Pt and Pd as examples, we present DFT results for the interaction of a metal atom, monolayer, multilayer and cluster with a graphene sur- face, a representative of wide-diameter carbon nanotubes. We also study nanotubes placed on flat metal surfaces. We discover inter- esting effects including, strong variations of binding energy as a function of metal configuration, and cross-section-deformation of nanotubes placed on Pt and Pd surfaces driven by sp 2 ! sp 3 transition of metal-adjacent C-atoms. The results provide an explana- tion for poorer Pt-contacts as compared to Pd. Ó 2004 Elsevier B.V. All rights reserved. Carbon nanotubes interacting with metal nanoparti- cles are gaining considerable interest as sensing materials [1–4], catalysts [5–8], in the synthesis of metallic nano- wires [9,10], as well as in nanoelectronics applications as field-effect-transistor (FET) devices [11]. A systematic study of electron-beam-evaporation-coating of sus- pended single-walled nanotube SWNT with various met- als reveals that the nature of the coating can vary significantly depending upon the metal [12]. Thus, Ti, Ni and Pd form continuous and quasi-continuous coat- ing, while Au, Al and Fe form only discrete particles on the SWNT surface. In fact, Pd is a unique metal in that it consistently yields ohmic contacts to metallic nanotubes [13], and zero or even negative Schottky bar- rier at junctions [14] with semiconducting SWNTs for FET applications. The Schottky barrier (for p-channel conductance) could, in principle, be made even lower if a higher work function metal, e.g., Pt is used [14]. Unfor- tunately, Pt appears to form non-ohmic contacts to both metallic [13] and semiconducting SWNTs [14] with lower p-channel conductance than Pd-contacted junctions. The computed interaction energy of a single metal atom on a SWNT [15] follows the trend E b (Ti)  E b (Pt) > E b (Pd) > E b (Au). These trends would suggest that Ti sticks the best to the SWNT and Au the worst, in good agreement with experiment. However, it does not explain why Pt consistently makes worse contacts than Pd, and why Ti, in spite of its good wetting of a CNT surface, yields Ohmic contacts only rarely [13].A detailed investigation of the metal–SWNT contact at full atomistic detail is a significant undertaking, and is likely beyond the realm of todayÕs first-principles Quan- tum Mechanics codes. Nevertheless, as a first attempt, it is instructive to look into the interactions of SWNTs with metallic entities beyond single atoms. In this Letter, we use density functional theory (DFT) to investigate the interaction of isolated atoms, mono- layers, multilayers and clusters of metals with a gra- phene sheet (representative of large-diameter SWNTs). For concreteness, we have chosen Au, Pd and Pt for the present study. In addition we have investigated the interaction of two semiconducting SWNTs on flat metal 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.024 * Corresponding author. Fax: 1 858 7995100. E-mail address: amaiti@accelrys.com (A. Maiti). www.elsevier.com/locate/cplett Chemical Physics Letters 395 (2004) 7–11