Self-assembled CNT circuits with ohmic contacts using Pd hexadecanethiolate as in situ solder† Thiruvelu Bhuvana,‡ a Kyle C. Smith, b Timothy S. Fisher * b and Giridhar U. Kulkarni * a Received 23rd April 2009, Accepted 14th August 2009 First published as an Advance Article on the web 16th September 2009 DOI: 10.1039/b9nr00035f An easy and elegant method of CNT nanocircuit fabrication using a metal organic precursor of Pd, namely, Pd hexadecanethiolate, is presented. This precursor directs the self-assembly of individual CNTs spanning a gap between Au electrodes. This is achieved by first patterning the precursor along the edges of the gap electrodes, as it enables direct patterning by e beam. Further, thermal activation of the precursor at 250  C leads to metallization and the ohmic electrical contact between the CNTs and the electrodes beneath. A resistive fuse action of the soldered CNTs is observed as well. 1. Introduction Carbon nanotubes (CNTs), because of their unique electrical, mechanical and other interesting properties, have been projected as prototype building blocks of nanoscale architectures. 1 While applications involving random collections of CNTs are relatively straightforward to develop and study, the exploitation of the properties of individual CNTs has remained a daunting task, given that CNTs are too large for chemical manipulation and rather small for mechanical grippers! Accordingly much research activity has focused on CNT manipulation. 2 An important aspect of this activity is the fabrication of electrical circuits with CNTs as active elements mechanically coupled to electrodes. There are at least two pertinent questions raised in this context, namely addressing of a CNT and the nature of coupling with its electrode. This article describes a method by which CNTs are made to self- assemble across electrodes through passive interaction with a patterned Pd-containing precursor that is subsequently con- verted, by simple thermolysis, to an interconnected dense collection of Pd nanoparticles. The latter are shown to provide ohmic-like contact between the electrode surface and the CNTs. Among the investigated methods for contacting CNTs, one may differentiate those in which individual or a set of CNTs are addressed from a random or an orderly collection spread over a substrate, from those wherein CNTs are deposited only onto desired locations. The former methodology is time intensive as it involves multiple steps: locating a CNT using a microscopy tool, marking its location and finally depositing electrodes. This is usually achieved by performing electron (or ion) beam induced metal deposition (EBID or FIBID) facilitated by a scanning electron microscope or by shadow masking prior to physical vapor deposition. 3–6 Instead of metal deposition, there are a few reports where movable microcantilevers served as contacting electrodes. 7,8 Langford et al. 9 have discussed the various methods for electrical contacts on CNTs, and Yaglioglu et al. 10 have studied means of characterising sheet and contact resistance. Several methods have been reported in the literature for the deposition of CNTs at desired locations, using approaches such as chemical functionalisation and self-assembly. For instance, Lewenstein et al. 5 fabricated circuits by amino-functionalising the metal electrodes whereas LeMieux et al. 11 functionalised the gap between the electrode. Klinke et al. 12 used functionalised CNTs which were placed in designated areas, and electrodes were built subsequently over each adsorbed CNT. Using AFM for manipulation, Gao et al. 13 placed an individual CNT between two opposing metal electrodes and realised a four-point arrangement using two additional CNTs. Li et al. 14 made use of a PDMS microchannel mold and directed SWNTs into channels under gas pressure to transfer them between a pair of gap elec- trodes. Using a catalyst bed beneath an anodised alumina membrane, Maschmann et al. 15 successfully grew vertical SWNTs through the pores and established top contact by elec- troplating Pd nanocubes. Another commonly employed method is dielectrophoresis, in which the electric dipole of a CNT exposed to an inhomogeneous electric field is made to guide a CNT to a specific electrode location. 16,17 Despite these innovations to make CNT circuits, contact resistance reduction at the CNT-electrode interface remains an active area of research. Bachtold et al. 18 exposed a prefabricated CNT-Au electrode system to the electron beam in a SEM chamber to improve the contact resistance between a CNT and Au electrode. The contact resistance was reduced by several orders of magnitude due to exposure to the e-beam. EBID 19,20 has also been employed to create metal deposits at CNT- electrode junctions effectively to serve as solder material. Though such deposits are not truly metallic, 21 they do improve contact. In another study, Au nanoparticle sol was used as an ink to write as a ‘fountain pen’ at contacts. 22 Contacts between CNTs and metal electrodes can also be improved by rapid thermal annealing. 23 The present work is aimed at building single CNT circuits, primarily through a self-assembly process in contrast to those a Chemistry and Physics of Materials Unit and DST Unit on Nanoscience, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P O, Bangalore, 560 064, India. E-mail: kulkarni@jncasr.ac.in b Department of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN, 47907-2057, USA. E-mail: tsfisher@purdue.edu † Electronic supplementary information (ESI) available: Details of the thermolysis of Pd hexadecanethiolate and preliminary results on carbon fibre as active element between the gap electrodes. See DOI: 10.1039/b9nr00035f ‡ Present address: Purdue University, West Lafayette, IN 47907-2057, USA. This journal is ª The Royal Society of Chemistry 2009 Nanoscale, 2009, 1, 271–275 | 271 PAPER www.rsc.org/nanoscale | Nanoscale