Enabling DC Field-Directed Chaining of Nanowires for Microelectronic Applications

Date

2024-01-03

Authors

Arafat, Md Yeashir

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Abstract

Solar cells, light-emitting diodes, small-scale sensors, and large-area displays are examples of devices that benefit from the use of transparent conductive electrodes (TCEs). Indium tin oxide (ITO) is the most widely used transparent electrode material, exhibiting both high transparency and conductivity. However, the low concentration of indium in its ores makes it an expensive material to process. Indium price fluctuations lead to unsteadiness in manufacturing costs. Moreover, the fragile nature of ITO limits its usefulness in the fabrication of flexible electrodes. To address these issues, transparent conductive oxides and polymers, carbon nanotubes, graphene, and metal nanowires are being explored as potential candidates to replace ITO as the primary transparent conductor. Nanowire (NW) networks offer several advantages over ITO in terms of low cost, ease of fabrication, and flexibility. Large area coverage with ordered NW chains is challenging as it is difficult to control an electric field and its gradient in large electrode gaps. Electric field-directed chaining in a nanowire (NW) suspension was previously demonstrated as a simple and cost-effective process for large area coverage, with high conductivity and transparency. However, generating an effective dielectrophoretic (DEP) force for the desired NW assembly requires a high frequency to overcome the charge screening effect due to the polarity of water or alcohol, commonly used as suspension media. This requirement is a major limitation. High frequency can also generate harmful electromagnetic radiation as well as power loss in wiring. Moreover, the magnitude of the electric field and DEP force decreases sharply in the region away from the electrodes. Therefore, more NWs are bunched in the vicinity of electrodes, while at distant locations NWs are observed to form curls and branches, producing poorly aligned chains. Here we present the use of squalane (C30H62), a non-polar, non-toxic, unreactive, viscous organic liquid, for the suspension of NWs in an electric field-directed assembly. Our theoretical analysis suggested that squalane could reduce voltage drop at the electrode, enabling adequate DEP force for chaining. Moreover, this could be done at a lower frequency because of the low electrical conductivity and dielectric constant of squalane. Additionally, we may expect that the high viscosity of squalane will suppress the electroosmotic flow of the medium and Brownian motion of NWs, thereby facilitating the chaining process. Experiments have been performed with both polar and non-polar suspension media to observe their effectiveness in DEP-assisted NW chaining. Our experiments confirmed that squalane does generate NW chains at low-frequency AC (and down to DC) fields, whereas conventional polar suspension media require substantially higher frequency. Finally, a magneto-electro-kinetic model has been developed to explore how combining an external magnetic field with the electric field may enable better control of the NW alignment far from the electrodes.

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Keywords

Nanowire chaining, AC/DC Dielectrophoresis, Polar/non-polar suspension medium, Viscosity, Transparent conductive electrodes

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