One-step patterning of aligned nanowire arrays by programmed dip coating.

A fundamental step in the construction of nanowire devices is transfer of the nanowires from their stock to the substrate on top of which the device will be built. Therefore, alignment and controlled positioning of the nanowires are highly desirable, especially for the large-scale (e.g., on a 4-inch (10-cm) wafer) fabrication of parallel device arrays. Nanowires are normally synthesized and processed in solution. Therefore, any subsequent patterning technique would inevitably involve a dewetting step. Herein we show that nanowires can be aligned and selectively deposited at the edge of a drying droplet as a result of evaporation-induced capillary flow. This contact-line deposition can be regulated with a simple dip-coating setup to create massive nanowire arrays with predefined spacing and tunable wire density. It also enables the selective placement of the nanowire arrays directly onto prefabricated electrode arrays, thus providing a facile and inexpensive method for the large-scale fabrication of nanowire-based devices. Although it is possible to integrate nanowires directly into some specific device platforms during synthesis, it is more common to make nanowire devices starting from a nanowire suspension with a subsequent patterning process. A few methods have been developed to make aligned nanowire arrays on a substrate, for example, Langmuir–Blodgett, microfluidic, 7] electric-field-assisted assembly, and optical trapping. For all these patterning processes, nanowires are either made or processed within a solvent. As a result, any such technique would eventually encounter solvent evaporation before the final dried nanowire pattern is obtained. Therefore, it should be of great interest and technological importance to explore the full extent of the dewetting process for assembling nanostructures. 12] Herein we report our observation of contact-line deposition and alignment of nanowires in an evaporating droplet. These findings are then employed and tailored for large-scale patterning of nanowires by dip coating. Ring-shaped stains are often observed when a droplet of colloidal solution (e.g., coffee) is dried. An outward capillary flow of the solvent is necessary to compensate the loss of solvent at the perimeter, which also carries the dispersed materials to the solvent–substrate contact line and leads to highly selective deposition along the perimeter of the droplet. We have discovered that in a drying droplet containing nanowires, this capillary flow also sorts the nanowires along the radial direction (Figure 1a), especially when the solvent is volatile (e.g., methylene chloride or chloroform). Figure 1b shows a typical ring-shaped stain obtained from drying a droplet of nanowire dispersion in methylene