Crafting ordered structures of nanomaterials via flow-enabled self-assembly (FESA) and controlled evaporative self-assembly (CESA)
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The use of spontaneous self-assembly as a lithography free means to construct well-ordered, often intriguing structures has received much attention for its ease of producing complex, centimeter-scale structures with small feature sizes. These self-organized structures promise new opportunities for developing miniaturized optical, electronic, optoelectronic, and magnetic devices. One extremely simple route to intriguing structures is the evaporative self-assembly of nonvolatile solutes from a sessile droplet on a solid substrate. However, flow instabilities during the evaporation process often result in non-equilibrium and irregular dissipative structures (e.g., randomly organized convection patterns, stochastically distributed multi-rings, etc.). Therefore, in order to fully control the evaporative self-assembly of solutes, two strategies, namely, controlled evaporative self-assembly (CESA) and flow-enabled evaporative-induced self-assembly (FESA) were developed to create ordered structures of various nanomaterials. First, hierarchical assemblies of amphiphilic diblock copolymer (i.e., polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP)) micelles were crafted by FESA. The periodic threads comprising a monolayer or a bilayer of PS-b-P4VP micelles were precisely positioned and patterned over large areas. Second, highly aligned parallel DNA nanowires in the forms of nanostructured spokes over a macroscopic area were created via evaporative self-assembly (CESA) by subjecting DNA aqueous solution to evaporate in a curve-on-flat geometry composed of a spherical on a flat substrate. Third, large-scale aligned metallic nanowires templated by highly oriented DNA were produced by flow-enabled self-assembly (FESA). A simple yet robust swelling-induced transfer printing (SIT-Printing) technique was developed to transfer ultralong DNA nanowires onto the desirable substrate. Subsequently, the resulting DNA nanowires were exploited as templates to form metallic nanowires by exposing DNA nanowires preloaded with metal salts under oxygen plasma. Moreover, DNA nanowires were also employed as scaffold for aligning metal nanoparticles and nanorods. Fourth, colloidal microchannels (i.e., cracks) on a large scale were yielded by fully controlling the drying process of colloidal suspensions via flow-enabled self-assembly (FESA). The influence of chemically patterned substrate (i.e., hydrophobic stripes on a hydrophilic substrate) on the formation of colloidal microchannels was explored. In addition, such colloidal microchannels with tunable center-to-center distance between the adjacent cracks, λ_(c-c) was exploited as template for aligning inorganic nanoparticles. Importantly, theoretical study of the formation mechanism of parallel stripes of solutes by FESA was conducted. The relationship between the characteristic spacing of adjacent stripes λ_(c-c) and other experimental parameters such as the stripe width, the stop time and the moving speed of lower substrate were scrutinized. Such theoretical modeling would provide guidance for the precise design and crafting of ordered structures composed of nanomaterials by FESA in the future study. Interestingly, during the preparation of Au nanorods, the formation of ultrathin gold nanowires were unexpectedly observed. Based on conventional synthetic route to Au nanorods using CTAB as soft-templates, we discovered that the addition of a small amount of hydrophobic solvent (e.g., toluene or chloroform) to the Au growth solution entailed the formation of ultrathin Au nanowire, rather than Au nanorods. The growth mechanism of such intriguing water-soluble ultrathin Au nanowires, differed from those formed by using oleylamine (i.e., non-water-soluble Au nanowires), was explored. In general, the ability to craft ordered structures comprising nanomaterials by FESA and CESA provides new opportunities for organizing nanomaterials for use in electronics, optics, optoelectronics, sensors, nanotechnology and biotechnology.