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Posted: Sep 30, 2009

Nanotechnology fabrication with 'coffee rings'

(Nanowerk Spotlight) To date, a number of nanotechnology fabrication studies have focused on creating hierarchically ordered nanostructures using lithographic techniques. However, lithographic methods involve high processing and maintenance costs, and require an iterative, multi-step procedure that makes the structure formation process more complex and less reliable. By contrast, a novel nanofabrication method is fast and cost-effective, dispensing with the need for multistage lithography and externally applied fields. This new technique needs only a drop of diblock polymer solution, a curved upper surface and a flat silicon substrate, and a selective solvent.
This is the first study of creating hierarchically ordered nanostructures composed of block copolymers with unprecedented regularity by controlled evaporative self-assembly. Basically, a drop that contains nanotechnology materials such as quantum dots or carbon nanotubes is left to dry and in the process these nonvolatile solutes contained in the drop assemble into concentric rings and other intriguing structures. The trick is to precisely control the evaporation process so that it results in the desired, highly ordered structures.
"The core of our finding is that we developed a simple and robust method for producing intriguing hierarchically ordered nanomaterials in a precisely controllable manner," Zhiqun Lin, an assistant professor in the Materials Science and Engineering Department at Iowa State University, tells Nanowerk. "Our approach utilizes two consecutive self-assembly processes to precisely organize unique nanomaterials into spatially ordered structures that can serve as functional materials for potential applications in optical, electronic, and photonic devices, biosensor arrays, templates for complex structures and pattern transfer, scaffolds for inorganic hierarchical structures, among other areas."
evaporative self-assembly of nanostructures
Figure 1: a) Schematic illustration of a drop of PS-b-PMMA diblock copolymer toluene solution constrained in a microscopic gap formed by placing a spherical lens in contact with a silicon substrate (i.e., sphere-on-silicon geometry), resulting in a capillary-held PS-b-PMMA solution. b) Cross section of concentric PS-b-PMMA coffee rings produced by controlled, repetitive pinning–depinning cycles of a three-phase contact line from the capillary edge as the solvent evaporates (Figure a). The sphere/silicon contact area is marked 'contact center'. The distance of the rings from the contact center is Xn (n=1–3; X1, X2, and X3 correspond to the outer, intermediate, and inner region, respectively, where the rings were formed). (Image: Dr. Lin, Iowa State University)
The team have reported their findings in the September 24, 2009 online edition of Angewandte Chemie International Edition ("Evolution of Ordered Block Copolymer Serpentines into a Macroscopic, Hierarchically Ordered Web").
Lin's team confined cylinder-forming diblock copolymer solutions in a restricted geometry comprised of a spherical lens placed upon a flat substrate (see Figure 1 above). At the micrometer scale, the synergy of the controlled evaporative self-assembly of a polymer solution and controlled fingering instabilities mediated by the interaction between the polymer and substrate yields intriguing concentric serpentine microstructures of diblock copolymer over large areas.
Evolution of regular PS-b-PMMA serpentines to hierarchically woven mesh arrays via acetone vapor annealing
Figure 2: Evolution of regular PS-b-PMMA serpentines to hierarchically woven mesh arrays via acetone vapor annealing. Upper panels: AFM height images: 0 hr (left), 5 hr (center), 12 hr (right). Scan size = 80 x 80 µm2. Lower panels: Close-up AFM phase images from surface of PS-b-PMMA patterns at each solvent annealing stage shown in upper panels. Scan size = 2 x 2 µm2. (Image: Dr. Lin, Iowa State University)
Selective solvent vapor annealing then transforms these microstructures into a macroscopic web-like pattern composed of regularly arranged microporous mesh arrays, at the same time forming domains of closely packed, nanoscopic hexagonal cylinders of diblock copolymer that are vertically oriented to the surface of the web at the nanoscale (see Figure 2 above).
Evolution of surface morphology of asymmetric PS-b-PMMA block copolymer upon exposure to acetone vapor, a selective solvent for the PMMA block, monitored in-situ by optical microscope. The team notes that real-time optical micrographs fluctuated due to (i) deformation of the plastic pedestal used to support the PS-b-PMMA patterns on the silicon substrate (part of the pedestal was immersed in an acetone bath) and (ii) possible air flow in the lab.
Lin explains that normally, in the process of drying, due to convection and a lack of control over the evaporation process of the drop, irregular and stochastically organized structures are often formed. "The challenge therefore remains to use evaporative self-assembly rationally to 'synthesize' these dissipative structures of high regularity and fidelity for use in microelectronics, data storage devices, and biotechnology applications."
Rather than allowing the drop to evaporate over the entire substrate surface, as previous studies on evaporative self-assembly have done, Lin's team restricted evaporation in the sphere-on-silicon geometry to the edge of the drop.
"Upon pinning of the drop, fingering instabilities set in owing to unfavorable interfacial interaction between the polystyrene block and the silicon substrate, thus resulting in the formation of a 'coffee ring with a serpentine appearance in the outer region," Lin describes the process. "The drop then depinned and formed a new ring with fingers, contracting the drop interface inward. Finally, the drop reached the sphere/silicon contact center and the solution evaporated, locking in the patterns."
Consequently, the repetitive pinning and depinning (i.e., stick–slip motion) of the contact line, together with the concurrent fingering instabilities of the rings, produced a lateral surface pattern consisting of hundreds of concentric, highly ordered serpentines.
The result is a simple, scalable, low-cost, lithography-free method for creating nanostructured materials that exhibit unique and highly regular structural hierarchies over large areas. These structures exhibit two independent characteristic dimensions, i.e., a macroscopic web composed of regularly arranged micropore arrays, and self-assembled, hexagonal nanocylinders of diblock copolymer vertically oriented to the surface of the web.
Lin's team is now working to create an entire family of of complex, hierarchically assembled surface patterns of varying architectures (see also: "Robust Self-Assembly of Highly Ordered Complex Structures by Controlled Evaporation of Confined Microfluids"). They will do this by a) choosing other shapes than spheres for the upper surface and b) experimenting with different morphologies (spheres, cylinders, or lamellae) and chemical structures (glassy, semicrystalline, or rubbery blocks) of nanoscale block copolymers.
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