This procedure requires two steps, which is contrary to Joule heating recipes presented previously. A welding procedure is prepared and tested for reliable welding of silver nanowires, which destroys intermediate layers and welds the two nanowires. Using a common probe station, current is driven through the electrodes and, hence, the nanowires for the purpose of welding the two nanowires together. In this paper, photolithography is used to initiate contact to silver nanowire junctions using gold electrodes. Welding of these polyol-synthesized silver nanowires on an individual scale is of great interest and has not previously been reported. Because of the adaptability of this process to large-scale industrial synthesis, this type of nanowire is already being used in commercial devices such as transparent electrodes. This method allows one to synthesize crystalline silver nanowires cheaply and quickly. The nanowires used in this project were synthesized in solution using a variation of the polyol method. The combination of photolithography to deposit electrodes, followed by Joule heating to weld individual nanowires together, has not yet been implemented to the best of our knowledge. Photolithography is cost effective and widely available, and thousands of electrodes can be deposited in a single run. For the purposes of this project then, common photolithography was used to contact the nanostructures. However, these techniques require costly equipment, can induce damage in the nanowires, are slow, and can be difficult to execute. Procedures such as e-beam lithography, focused ion beam chemical vapor deposition, and micro-actuators have previously been used to contact nanowires. For these reasons, Joule heating is chosen here as the method of welding.įor Joule heat welding, electrical contacts to the nanowires must be created. Joule heating can also be applied regardless of contact direction and is concentrated at the junction, whereas bombardment techniques can cause damage to the nanowire away from the contact point and introduce impurities. Joule heat welding is more versatile and only requires the materials to be capable of conducting current. However, because the wavelength required for plasmonic welding is material dependent, a sophisticated light source is required, and there can be difficulty in welding dissimilar materials. This trait is only shared with certain types of plasmonic welding, where the localized plasmonic excitations decrease once welding is completed. Using Joule heating, one can construct a self-executing process whereby the welding mechanism autonomously stops when the weld is complete. This is in contrast to the other methods which require equipment such as ion or electron guns, lasers, advanced microscopy, and vacuum technology which make them costly and complex. Electrical contact to small structures is already well understood, and the equipment required for this process is readily available and inexpensive. Of these, Joule heat welding has several advantages. Methods used to weld nanowires include electron beam bombardment, ion beam bombardment, plasmonic welding, soldering, cold fusion, pulsed laser, and Joule heating. Connecting nanowires with electrodes and the formation of nanowire interconnects also require ohmic welding. Welding can be used to construct complex structures from the bottom-up out of simple constituents. Silver nanowires have been used in devices such as plasmon carriers, sensors, and nano-electromechanical systems (NEMS) packaging interconnections. Silver nanowires in particular are of interest since silver has the highest electrical and thermal conductivity of any metal. Their high aspect ratio and surface area lead to unique properties, which are exploited in applications as diverse as solar cells, integrated circuits, and bio-sensing. Nanowires (NWs) are elongated structures with two nanosized dimensions and a third dimension which is typically several hundreds of nanometers or more.
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