Heesook Cho

High-Performance Organic Optoelectronic Devices Enhanced by Surface Plasmon Resonance

The surface plasmon effect on polymer solar cells and polymer light-emitting diodes is demonstrated by using metal nanoparticles prepared from block copolymer templates. Light absorption of the polymer thin layer is increased with the incorporation of metallic nanostructures, resulting in a significant surface plasmon effect in the optoelectronic devices.

Precise placements of metal nanoparticles from reversible block copolymer nanostructures

We demonstrate the fabrication of reversible block copolymer (BCP) templates that can be used to control the spatial location of metal precursors and nanoparticles. A highly ordered polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) micellar array was obtained by solvent annealing. Subsequent immersion of the films in a preferential solvent for the minor component block caused a reorganization of the film to generate a porous structure upon drying. When such reconstructed films were subjected to external stimuli, like solvent vapor or heat, the initial morphology was recovered. These reversible BCP templates were used to control the placement such that, subsequently, oxygen plasma treatment led to the precise placement of metal nanoparticles in a film. By controlling the concentration of the metal precursor solutions, the sizes of the nanoparticles could be tuned.

Development of various PS-b-P4VP micellar morphologies: Fabrication of inorganic nanostructures from micellar templates

We demonstrate a simple route for preparing various micellar nanostructures, like spheres, cylinders, and vesicles, by spin-coating or drop-casting process of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymer solutions in pure tetrahydrofuran (THF), THF/water, and THF/ethanol mixture. Upon drying, a solvent selectivity plays an important role in determining micellar nanostructures in thin films. In solution, micellar sizes and shapes of these PS-b-P4VP copolymers were investigated by dynamic and static light scattering. Immediately after spin-coating the polymer solutions, surface and internal morphologies of the films were observed by atomic force microscopy and transmission electron microscopy. As the polymer concentration in THF or the amounts of water or ethanol added in THF solutions was varied, a remarkable difference in the PS-b-P4VP micellar morphologies was observed, from which spherical or cylindrical or vesicular micelles were developed. These micellar films were used as scaffolds or templates for fabricating metal nanodots or nanowires arrays.

Fabrication of gold dot, ring, and corpuscle arrays from block copolymer templates via a simple modification of surface energy

We demonstrate a simple method for tuning the morphologies of as-spun micellar thin films by modifying the surface energy of silicon substrates. When a polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymer dissolved in o-xylene was spin-coated onto a PS-modified surface, a dimple-type structure consisting of a thick PS shell and P2VP core was obtained. Subsequently, when the films were immersed in metal precursor solutions at certain periods of time and followed by plasma treatment, metal individual dots in a ring-shaped structure, metal nanoring, and metal corpuscle arrays were fabricated, depending on the loading amount of metal precursors. In contrast, when PS-b-P2VP films cast onto silicon substrates with a native oxide were used as templates, only metal dotted arrays were obtained. The combination of micellar thin film and surface energy modification offers an effective way to fabricate various nanostructured metal or metal oxide films.

Ordering evolution of block copolymer thin films upon solvent-annealing process.

Morphologies of polystyrene-block-poly(2-vinylpyridine) copolymer (S2VP) thin films, which are forming poly(2-vinylpyridine) cylinders in bulk phase, were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM) to account for their ordering behavior induced by solvent annealing. Initially, when the copolymer was dissolved in toluene, which is selective solvent for majority polystyrene (PS) blocks, and was spin-coated on Si substrates, dimple-type micellar structures of S2VP were formed. After the film was placed in a solvent-annealing chamber covered with a lid under the existence of chloroform, surface morphologies of S2VP were measured as a function of annealing time. In this study, it was found that the morphologies of S2VP thin film repeated the cycle of the creation and extinction of various morphologies on ordering process. Namely, S2VP exhibited the various transformations between different morphologies, including highly disordered state, cylinders normal to the plane, and cylinders parallel to the plane. Each of the morphologies observed here was employed as a template to synthesize gold (Au) nanoparticles or nanowires. The arrays of Au nano-objects were used to tune a surface plasmon resonance.

Gold Double-Ring Structures Synthesized from Block Copolymer Corpuscle Templates

Organic and inorganic nanorings have recently attracted considerable attention from theoretical, experimental, and devices viewpoints, since the quantum size effects and large surface-to-volume ratio of the nanometer-sized rings contribute to their unique properties, like magnetic, optical, electrical, or chemical sensing. Many efficient approaches have been developed to make nanorings using lithography, template-based methods, colloidal lithography, capillary force lithography, and self-assembly of block copolymers. The self-organization of block copolymers (BCPs) that can be used as templates to fabricate well-defined nanostructures is an attractive strategy among these methods, due to the tunability in size and shape, the ease of processing without introducing disruptive technologies, and the use of functionalities. In particular, BCP micelles have been used to make a rich variety of metal and metal oxide nanostructures. Among them, spherical micelles consisting of an insoluble core and a soluble corona, in which only one of the blocks is dissolved in a preferential solvent, are the most common shape and can be transferred onto a solid substrate with a kinetically frozen dimple-type structure. Selective binding between the metal precursors and one of the blocks takes place by electrostatic interactions, metal coordination reactions with a polar functional group, and p complexation with olefin groups. Subsequent chemical or plasma treatment leads to the formation of periodic metal nanoring arrays. For example, Wang et al. demonstrated the fabrication process of gold (Au) nanoring arrays by immersing a reverse polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) micellar thin film into a solution mixture of Au precursor (HAuCl4) and hydrogen chloride, with subsequent reduction of the Au salts by plasma treatment. Zu et al. reported a more general approach to make metal or metal oxide nanorings using a salt precipitation reaction. Selective swelling of minor blocks in a metal-incorporated micellar film led to the formation of metal or metal oxide nanoring structures. As a direct approach, toroidal micelles in solution or ascasted films were used as effective templates for metal nanorings. After polyisoprene-block-poly(2-vinylpyridine) was dissolved in a good solvent for both blocks, the gradual addition of a preferential solvent for the P2VP minor component resulted in the formation of toroidal micelles. Subsequent loading of Au precursors onto the micellar templates and the reduction of Au salts provides an efficient way to make uniform-sized Au nanorings. Recently, our group reported fabrication of gold ring arrays by controlling the loading time of gold precursors within block copolymer templates on a surface-modified substrate. However, the examples mentioned above showed metal or metal oxide single-ring structures due to the morphologies of copolymer templates. Herein, we demonstrate a novel method for making Au double-ring structures using corpuscle-like PS-b-P2VP templates spin-coated onto a surface-modified silicon substrate. The fabrication processes are illustrated in Scheme 1 and

Ag2Se micropatterns via viscoelastic flow-driven phase separation

A novel approach to prepare micropatterns of metal chalcogenides is proposed by employing viscoelastic flow-driven patterning. A consecutive process involving deposition of the Se precursor on a pattern of a crystalline polymer, chemical reduction of the precursor into amorphous Se (a-Se), and short-time thermal annealing above the melting temperature of the patterned polymer generated regular patterns of a-Se. This work demonstrates patterns of periodic lines and circles which is driven by the viscoelastic polymer flow and the phase separation of Se from the polymer. Additional thermal annealing facilitated the lateral growth of trigonal-Se (t-Se) nanowires from the Se patterns. The growing t-Se nanowires eventually meet each other to produce a 2D network structure. Chemical transformation of the Se into Ag2Se generated metal chalcogenide network structures.

Fabrication of gold nanoparticles and silicon oxide corpuscles from block copolymers

We have prepared silicon oxide corpuscles, gold dots inside a silicon oxide matrix, and silicon oxide dots from poly(2-vinylpyridine)-block-poly(dimethyl siloxane) block copolymer micelles, which were made using the preferential solvent for one of the block copolymer components. Three different spatial distributions of silicon oxide and gold nanoparticles can be obtained via the incorporation of the metal precursor into vinylpyridyl units and/or conversion of PDMS to silicon oxide. Controlling spatial distributions of inorganic nanostructured materials enables us to tune the surface plasmon bands.