IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 20 (2009) 235307 (10pp) doi:10.1088/0957-4484/20/23/235307 Directional growth of metallic and polymeric nanowires Prem S Thapa 1,3 , Bruce J Ackerson 1 , Daniel R Grischkowsky 2 and Bret N Flanders 1,3,4 1 Department of Physics, Oklahoma State University, Stillwater, OK 74078-3072, USA 2 School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, OK 74078, USA E-mail: bret.flanders@phys.ksu.edu Received 3 February 2009, in final form 26 March 2009 Published 18 May 2009 Online at stacks.iop.org/Nano/20/235307 Abstract This work delineates the mechanism by which directional nanowire growth occurs in the directed electrochemical nanowire assembly (DENA) technique for growing nanowires on micro-electrode arrays. Indium, polythiophene, and polypyrrole nanowires are the subjects of this study. This technique allows the user to specify the growth path without the use of a mechanical template. Nanowire growth from a user-selected electrode to within ±3 μm of the straight line path to a second electrode lying within a ∼140 ◦ angular range and a ∼100 μm radius of the selected electrode is demonstrated. Theory for one-dimensional electrochemical diffusion in the inter-electrode region reveals that screening of the applied voltage is incomplete, allowing a long range voltage component to extend from the biased to the grounded electrode. Numerical analysis of two-dimensional multi-electrode arrays shows that a linear ridge of electric field maxima bridges the gap between selected electrodes but decays in all other directions. The presence of this anisotropic, long range voltage defines the wire growth path and suppresses the inherent tip splitting tendency of amorphous polymeric materials. This technology allows polythiophene and polypyrrole to be grown as wires rather than fractal aggregates or films, establishing DENA as an on-chip approach to both crystalline metallic and amorphous polymeric nanowire growth. 1. Introduction Precise nanowire growth techniques are vital to nanoelectron- ics-development. One seeks control over the wire composition, dimensions, and growth direction in a single approach. This has been an elusive goal. Templated growth is in broad-use as the wire-compositions can be metallic [1], semiconductor, or polymeric [2, 3]. The wire-shapes are reproducible [4], and the output is scalable. However, prefabrication of mechanical growth channels and post-growth release of the wires are typically required. Methods for circumventing these laborious steps are, therefore, sought. Dielectrophoretic assembly is a template-free approach that uses a voltage to chain metallic or semiconductor particles into wires in the gaps between electrodes [5–9], but these nanoparticulate materials suffer 3 Present address: Department of Physics, Kansas State University, Manhattan, KS 66506-2601, USA. 4 Author to whom any correspondence should be addressed. from resistivities several orders of magnitude in excess of bulk metals [6, 9]. The vapor–liquid–solid (VLS) technique does not require growth channels and produces single crystalline wires in high yield [10, 11]. VLS is especially useful for semiconductor–nanowire assembly. However, this approach can only fabricate crystalline materials. Conducting polymeric nanowires are amorphous materials that are needed for basic transport studies [12, 13] and sensor-applications [14, 15] and are also promising electrophysiological materials [16, 17]. The wire-lengths that are attainable in most approaches to polymeric wire growth are limited to 10 μm or less [14, 18]. Dip-pen lithography relaxes the wire-length constraint [19], but is restricted to applications where the use of a scanning probe is feasible. In response to the need for precise nanowire growth techniques, the present work delineates methodology for the directional growth of both crystalline metallic wires and amorphous polymeric wires between user-selected sites in on- chip circuitry. A letter reporting the basic capabilities of this methodology has been published elsewhere [20]. 0957-4484/09/235307+10$30.00 © 2009 IOP Publishing Ltd Printed in the UK 1