Jessica Gwyther

Templated self-assembly of square symmetry arrays from an ABC triblock terpolymer.

Self-assembly provides the ability to create well-controlled nanostructures with electronic or chemical functionality and enables the synthesis of a wide range of useful devices. Diblock copolymers self-assemble into periodic arrays of microdomains with feature sizes of typically 10-50 nm, and have been used to make a wide range of devices such as silicon capacitors and transistors, photonic crystals, and patterned magnetic media(1-3). However, the cylindrical or spherical microdomains in diblock copolymers generally form close-packed structures with hexagonal symmetry, limiting their device applications. Here we demonstrate self-assembly of square-symmetry patterns from a triblock terpolymer in which one organometallic block imparts high etch selectivity and etch resistance. Long-range order is imposed on the microdomain arrays by self-assembly on topographical substrates, and the orientation of both square lattices and in-plane cylinders is controlled by the substrate chemistry. Pattern transfer is demonstrated by making an array of square-packed 30 nm tall, 20 nm diameter silica pillars. Templated self-assembly of triblock terpolymers can generate nanostructures with geometries that are unattainable from diblock copolymers, significantly enhancing the capabilities of block copolymer lithography.

Uniform, High Aspect Ratio Fiber-like Micelles and Block Co-micelles with a Crystalline π-Conjugated Polythiophene Core by Self-Seeding

Monodisperse fiber-like micelles with a crystalline π-conjugated polythiophene core with lengths up to ca. 700 nm were successfully prepared from the diblock copolymer poly(3-hexylthiophene)-block-polystyrene using a one-dimensional self-seeding technique. Addition of a polythiophene block copolymer with a different corona-forming block to the resulting nanofibers led to the formation of segmented B-A-B triblock co-micelles by crystallization-driven seeded growth. The key to these advances appears to be the formation of a relatively defect-free crystalline micelle core under the self-seeding conditions.

Dimensional Control of Block Copolymer Nanofibers with a π-Conjugated Core: Crystallization-Driven Solution Self-Assembly of Amphiphilic Poly(3-hexylthiophene)-b-poly(2-vinylpyridine)

With the aim of accessing colloidally stable, fiberlike, π-conjugated nanostructures of controlled length, we have studied the solution self-assembly of two asymmetric crystalline-coil, regioregular poly(3-hexylthiophene)-b-poly(2-vinylpyridine) (P3HT-b-P2VP) diblock copolymers, P3HT23-b-P2VP115 (block ratio=1:5) and P3HT44-b-P2VP115 (block ratio=ca. 1:3). The self-assembly studies were performed under a variety of solvent conditions that were selective for the P2VP block. The block copolymers were prepared by using Cu-catalyzed azide-alkyne cycloaddition reactions of azide-terminated P2VP and alkyne end-functionalized P3HT homopolymers. When the block copolymers were self-assembled in a solution of a 50% (v/v) mixture of THF (a good solvent for both blocks) and an alcohol (a selective solvent for the P2VP block) by means of the slow evaporation of the common solvent; fiberlike micelles with a P3HT core and a P2VP corona were observed by transmission electron microscopy (TEM). The average lengths of the micelles were found to increase as the length of the hydrocarbon chain increased in the P2VP-selective alcoholic solvent (MeOH3 μm) fiberlike micelles were prepared by the dialysis of solutions of the block copolymers in THF against iPrOH. Furthermore the widths of the fibers were dependent on the degree of polymerization of the chain-extended P3HT blocks. The crystallinity and π-conjugated nature of the P3HT core in the fiberlike micelles was confirmed by a combination of UV/Vis spectroscopy, photoluminescence (PL) measurements, and wide-angle X-ray scattering (WAXS). Intense sonication (iPrOH, 1 h, 0 °C) of the fiberlike micelles formed by P3HT23-b-P2VP115 resulted in small (ca. 25 nm long) stublike fragments that were subsequently used as initiators in seeded growth experiments. Addition of P3HT23-b-P2VP115 unimers to the seeds allowed the preparation of fiberlike micelles with narrow length distributions (L(w)/L(n) < 1.11) and lengths from about 100-300 nm, that were dependent on the unimer-to-seed micelle ratio.

Highly ordered square arrays from a templated ABC triblock terpolymer.

Square-symmetry patterns are of interest in nanolithography but are not easily obtained from self-assembly of a diblock copolymer. Instead, we demonstrate highly ordered 44 nm period square patterns formed in a thin film of polyisoprene-block-polystyrene-block-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer blended with 15% PS homopolymer by controlling the film thickness, solvent anneal conditions, the surface chemistry and topography of the substrates. The square patterns consist of PFS pillars that remained after removal of the PI and PS with an oxygen plasma. On an unpatterned smooth substrate, the average grain size of the square pattern was increased dramatically to several micrometers by the use of brush layers and specific solvent anneal conditions. Templated self-assembly of well-ordered square patterns was demonstrated on substrates containing nanoscale topographical sidewalls and posts, written by electron beam lithography, in which the sidewalls and base of the substrate were independently chemically functionalized.

Large-Area Nanosquare Arrays from Shear-Aligned Block Copolymer Thin Films

While block copolymer lithography has been broadly applied as a bottom-up patterning technique, only a few nanopattern symmetries, such as hexagonally packed dots or parallel stripes, can be produced by spontaneous self-assembly of simple diblock copolymers; even a simple square packing has heretofore required more intricate macromolecular architectures or nanoscale substrate prepatterning. In this study, we demonstrate that square, rectangular, and rhombic arrays can be created via shear-alignment of distinct layers of cylinder-forming block copolymers, coupled with cross-linking of the layers using ultraviolet light. Furthermore, these block copolymer arrays can in turn be used as templates to fabricate dense, substrate-supported arrays of nanostructures comprising a wide variety of elements: deep (>50 nm) nanowells, nanoposts, and thin metal nanodots (3 nm thick, 35 nm pitch) are all demonstrated.

Metal-Containing Block Copolymer Thin Films Yield Wire Grid Polarizers with High Aspect Ratio

Highly selective etch masks are formed by thin films of a polystyrene-b-poly(ferrocenylisopropylmethylsilane) diblock copolymer, PS-PFiPMS, containing hemicylindrical domains of PFiPMS. These domains, with a period of 35 nm, are readily aligned through mechanical shear. Aligned PS-PFiPMS templates are employed to fabricate high-aspect-ratio nanowire grids from amorphous silicon, which can polarize deep ultraviolet radiation, including 193 nm, at >90% efficiency.

Scalable and uniform 1D nanoparticles by synchronous polymerization, crystallization and self-assembly

The preparation of well-defined nanoparticles based on soft matter, using solution-processing techniques on a commercially viable scale, is a major challenge of widespread importance. Self-assembly of block copolymers in solvents that selectively solvate one of the segments provides a promising route to core-corona nanoparticles (micelles) with a wide range of potential uses. Nevertheless, significant limitations to this approach also exist. For example, the solution processing of block copolymers generally follows a separate synthesis step and is normally performed at high dilution. Moreover, non-spherical micelles-which are promising for many applications-are generally difficult to access, samples are polydisperse and precise dimensional control is not possible. Here we demonstrate the formation of platelet and cylindrical micelles at concentrations up to 25% solids via a one-pot approach-starting from monomers-that combines polymerization-induced and crystallization-driven self-assembly. We also show that performing the procedure in the presence of small seed micelles allows the scalable formation of low dispersity samples of cylindrical micelles of controlled length up to three micrometres.

Hierarchical templating of a BiFeO3-CoFe2O4 multiferroic nanocomposite by a triblock terpolymer film.

A process route to fabricate templated BiFeO3/CoFe2O4 (BFO/CFO) vertical nanocomposites is presented in which the self-assembly of the BFO/CFO is guided using a self-assembled triblock terpolymer. A linear triblock terpolymer was selected instead of a diblock copolymer in order to produce a square-symmetry template, which had a period of 44 nm. The triblock terpolymer pattern was transferred to a (001) Nb:SrTiO3 substrate to produce pits that formed preferential sites for the nucleation of CFO crystals, in contrast to the BFO, which wetted the flat regions of the substrate. The crystallographic orientation and magnetic properties of the templated BFO/CFO were characterized.

Magnetic properties of ceramics from the pyrolysis of metallocene-based polymers doped with palladium

Solution processing is a facile method to generate magnetic thin films. Polyferrocenylethylmethylsilane (PFEMS) was doped with palladium (II) acetylacetonate using two methods: sublimation of Pd(acac)2 to form Pd nanoparticles in the PFEMS films and direct mixing of Pd with the PFEMS polymer precursor prior to film deposition. These polymer composites all exhibit paramagnetic behavior, with increasing magnetic susceptibility for increasing Pd content. Pyrolysis of the precursors yields ferromagnetic ceramics at room temperature. The effect of the pyrolysis temperature and atmosphere on the magnetic properties, chemical composition, and crystalline structure of the ceramics was explored. For ceramics containing Pd, FePd alloys are observed to form pyrolyzed under argon at 1000 °C. The formation of these alloys results in enhanced coercivity, remanent magnetization, and saturation magnetization of the ceramics.

Enabling Heterogeneous Gold Catalysis with Patchy Micelles

Patch works! Patchy block copolymer micelles with a corona consisting of two chemically different patches have been designed for the selective binding of catalytically active nanoparticles. The fabrication of nonwoven supports by electrospinning was combined with crystallization-driven self-assembly for precise control over micelle formation to prepare a new recyclable catalyst platform.