Using Copolymer Films as Scaffolds: A Novel Way to Self-Assemble Organic-Inorganic Hybrid Structures

Self-assembly is emerging as elegant, “bottom-up” method to fabricate nanostructured materials. It becomes particularly powerful when the ease and control offered by the self-assembly of organic components is combined with the special electronic, magnetic or photonic properties of inorganic components. Here we demonstrate a versatile hierarchical approach for the assembly of organic-inorganic, copolymer-metal nanostructures in which one level of self-assembly guides the next. In a first step, ultrathin diblock copolymer films form a regular scaffold of highly anisotropic, stripe-like nanodomains. During a second assembly step, differential wetting guides diffusing metal atoms to selectively aggregate along the scaffold, producing exquisitely organized metal nanostructures. We find that metal on copolymer scaffolds produces the most highly ordered configurations far from equilibrium. We delineate two distinct assembly modes of the metal component, “nanochains” of separate nanoparticles, and continuous “nanowires”, each characterized by different ordering kinetics and strikingly different current-voltage characteristics. These results provide new possibilities for guided, large-scale assembly of laterally nanostructured systems. Read more ...a nugget on this work. Plus: an article in the UofC Chronicle. Also: a colorized TEM image from this work ended up as the promotional cover of Nature Nanotechnology.

Polymers Orient in Electric Fields: First Demonstration of Lateral Domain Alignment in Ultrathin Copolymer Films

Highly controlled alignment of diblock-copolymer ultrathin films is demonstrated through the use of external electric fields. Ultrathin films of Polystyrene-Polymethylmethacrylate (PS-PMMA) diblock-copolymers were spin-cast onto silicon nitride membrane substrates with prefabricated in-plane electrodes, forming cylindrical PMMA domains. The films were annealed with an applied electric field (E=3.7V/µm) at 250°C for 24h under an argon atmosphere. Alignment of copolymer cylindrical axes along the electric field was observed directly by TEM. A quantitative measure for the degree of alignment was obtained by correlating the local field strength, E, and direction with the observed cylinder orientation. The alignment was found to saturate above E≈3V/µm, and to decrease rapidly as E falls below this value. Read more...a nugget on this work with more explanations of copolymers.

Discovery of a New Ultra-Thin Film Morphology that Exhibits Lateral Domain Patterns Along the Free Surface

We have investigated the morphology of lamellar polystyrene-polymethylmetha- crylate diblock copolymers in the ultrathin-film limit, spin-cast and annealed on silicon nitride substrates. Our experiments show evidence for a morphology change from lamellar domains parallel to the substrate to perpendicular domains that appears to be unique to a thickness of one lamellar repeat spacing and low annealing temperatures. We find that in these ultrathin films phase separation occurs well before thickness quantization, suggesting effective confinement as a possible mechanism. A model for the free energy of this system is developed which takes into account termination (“capping”) of the perpendicular lamellae inside the film and at its top surface.

First Direct Defect Tracking in Copolymer Thin Films Using AFM