ASSAD ET AL . VOL. 6 ’ NO. 6 ’ 4702–4712 ’ 2012 www.acsnano.org 4702 May 03, 2012 C 2012 American Chemical Society Spray-Coating Route for Highly Aligned and Large-Scale Arrays of Nanowires Ossama Assad, † Alexander M. Leshansky, † Bin Wang, † Thomas Stelzner, ‡ Silke Christiansen, ‡,§ and Hossam Haick †, * † The Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, TechnionÀIsrael Institute of Technology, Haifa 32000, Israel, ‡ Institute of Photonic Technology e.V., Albert-Einstein-Straße 9, 07745 Jena, Germany, and § Max-Planck-Institute for the Science of Light, Günther-Scharowsky-Straße 1, 91058 Erlangen, Germany T he ability of individual nanowires (NWs) to carry electrical current makes them promising building blocks in various nanoelectronic device concepts. Individual NWs have been used to assemble a range of nanoscale devices, including pÀn diodes, 1À3 bipolar junction transistors, 4 field effect transistors, 3À9 electrochemical devices, 10À12 biochemical sensors, 5,6,13À24 optoelectronic devices, 25À30 and more. Nevertheless, there are still many challenges in creating such devices: (i) difficulties in connecting the individual NW to the macroscopic world, although significant progress has been made in this field; (ii) fluctuations in the position of NWs between adjacent metallic electrodes; (iii) understanding the mode of power dissipation when NWs are used as devices or in devices; and (iv) main- taining high signal-to-noise ratio in complex nanodevices. For technological applications, the ease and effectiveness with which NWs are assembled and integrated into large- scale devices are of critical importance. An alternative approach to the individual NW-based devices 31À44 is the application of a platform in which NW arrays are used to exploit their specific properties (e.g., large surface-to-volume ratio), but which do not require highly sophisticated fabrication techniques. 31À44 This approach is based on controlling deposition properties of NWs, yielding synergetic combinations of the nano- scale and the macroscale worlds. These NW arrays circumvent the requirement of posi- tion and structural control because the de- vices display the average properties of many distributed NWs. The NW arrays could be processed into devices of varying sizes using conventional microfabrication tech- nology, characterized by high stability, re- peatability, and better error tolerance. 31À44 Arrays of NWs can be achieved either by top- down or bottom-up fabrication techniques. The top-down approach depends heavily on multistep (e-beam) lithography and lift-off techniques, which are serial, time- consuming, and expensive. The bottom-up approach relies on the transfer of NWs from the growth chip to a suitable substrate. This includes, but is not confined to, the following: • Flow-assisted alignment: this approach incorporates microfluidic channels to enable directional flow and deposition of a NW suspension on a solid-state surface. 24,25 • Deposition by chemical interactions: this approach relies on the interactions be- tween the NWs and the receiver substrate. The deposition sites strongly depend on the surface chemical functionality. 44 * Address correspondence to hhossam@technion.ac.il. Received for review November 20, 2011 and accepted May 3, 2012. Published online 10.1021/nn204513y ABSTRACT Technological implementa- tion of nanowires (NWs) requires these com- ponents to be organized with controlled orientation and density on various substrates. Here, we report on a simple and efficient route for the deposition of highly ordered and highly aligned NW arrays on a wide range of receiver substrates, including silicon, glass, metals, and flexible plastics with controlled density. The deposition approach is based on spray-coating of a NW suspension under controlled conditions of the nozzle flow rate, droplet size of the sprayed NWs suspension, spray angle, and the temperature of the receiver substrate. The dynamics of droplet generation is understood by a combined action of shear forces and capillary forces. Provided that the size of the generated droplet is comparable to the length of the single NW, the shear-driven elongation of the droplets results presumably in the alignment of the confined NW in the spraying direction. Flattening the droplets upon their impact with the substrate yields fast immobilization of the spray-aligned NWs on the surface due to van der Waals attraction. The availability of the spray-coating technique in the current microelectronics technology would ensure immediate implementation in production lines, with minimal changes in the fabrication design and/or auxiliary tools used for this purpose. KEYWORDS: silicon . silver . GaN . nanowire . array . spray-coating . deposition ARTICLE