Du Yeol Ryu

Directed Assembly of High Molecular Weight Block Copolymers: Highly Ordered Line Patterns of Perpendicularly Oriented Lamellae with Large Periods

The directed assembly of block copolymer nanostructures with large periods exceeding 100 nm remains challenging because the translational ordering of long-chained block copolymer is hindered by its very low chain mobility. Using a solvent-vapor annealing process with a neutral solvent, which was sequentially combined with a thermal annealing process, we demonstrate the rapid evolution of a perpendicularly oriented lamellar morphology in high molecular weight block copolymer films on neutral substrate. The synergy with the topographically patterned substrate facilitated unidirectionally structural development of ultrahigh molecular weight block copolymer thin films-even for the structures with a large period of 200 nm-leading to perfectly guided, parallel, and highly ordered line-arrays of perpendicularly oriented lamellae in the trenched confinement. This breakthrough strategy, which is applicable to nanolithographic pattern transfer to target substrates, can be a simple and efficient route to satisfy the demand for block copolymer assemblies with larger feature sizes on hundreds of nanometers scale.

On the synergistic coupling properties of composite CdS/TiO2 nanoparticle arrays confined in nanopatterned hybrid thin films

Two-dimensional (2D) arrays of hybrid CdS/TiO2 composite nanodots were fabricated on solid substrates using amphiphilic poly(styrene-block-ethylene oxide) diblock copolymer (PS-b-PEO) micelles as templates loaded with CdS nanoparticles (NPs) and TiO2 sol–gel precursors. The inorganic precursors were selectively incorporated into PEO domains of PS-b-PEO due to the specific interactions. The addition of CdS quantum dots (QDs) into thin films of the PS-b-PEO/TiO2 sol–gel mixture led to the morphological changes from mixed wire/hexagonal dot to well-defined, quasi-hexagonal dot arrays. The PS-b-PEO/TiO2/CdS system showed an enhanced absorption along with red shift behavior in the UV-visible spectral range compared with the PS-b-PEO/TiO2 films. Photoluminescence (PL) studies showed a quenching of CdS emission in the presence of TiO2. An enhanced photocatalytic degradation of methylene blue (MB) was observed in the hybrid PS-b-PEO/TiO2/CdS thin film.

Preparation of TiO2 spheres with hierarchical pores via grafting polymerization and sol–gel process for dye-sensitized solar cells

Titania (TiO2) nanoparticles were surface-modified via atom transfer radical polymerization (ATRP) with hydrophilic poly(oxyethylene) methacrylate (POEM), which can coordinate to the TiO2 precursor, titanium(IV) isopropoxide (TTIP). Following application of a sol–gel process and calcination at 450 °C, TiO2 nanospheres with hierarchical pores were generated, as confirmed by the shifting of conduction bands in TiO2 using UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS). The particle size and morphology of TiO2 were characterized using wide angle X-ray scattering (WAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Brunauer–Emmett–Teller (BET) analysis revealed bimodal distribution of TiO2 pore sizes with peaks at 6 nm and 50 nm to afford better penetration of polymer electrolyte, as confirmed by electrochemical impedance spectroscopy (EIS). Dye-sensitized solar cells (DSSC) made from TiO2 nanospheres with hierarchical pores exhibited improved photovoltaic efficiency (3.3% for low molecular weight (Mw) and 2.5% for high Mw polymer electrolytes), as compared to those from neat TiO2 nanoparticles (2.4% for low Mw and 1.3% for high Mw) at 100 mW/cm2, owing to the increased surface areas and light scattering.

Directed self-assembly of block copolymers in the extreme: guiding microdomains from the small to the large

The self-assembly of block copolymers (BCPs) is emerging as a promising route for numerous applications to generate templates and scaffolds for the fabrication of nanostructured materials. Here, we present an overview of recent progress in the directed self-assembly of BCPs with a focus on guiding the assemblies of extreme (small and large) features over large areas. We introduce inorganic-containing hybrid BCPs that enable access to ultra-small feature sizes. To achieve the desired orientation and lateral ordering of the BCP microdomains, we discuss routes that combine top-down (lithographic) processes with the bottom-up self-assembly of BCPs. On the extreme large size scale, high-molecular-weight BCPs and brush-type copolymers are discussed, covering the other extreme of this promising strategy.

Three-Dimensional Multilayered Nanostructures with Controlled Orientation of Microdomains from Cross-Linkable Block Copolymers

Three-dimensional (3D) nanostructures were obtained by the directed formation of multilayer block copolymer (BCP) thin films. The initial step in this strategy involves the assembly and cross-linking of cylinder-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) BCP, in which 1.5 mol % of reactive azido (-N(3)) groups were randomly incorporated along the styrene backbone. Significantly, assembly of thin films of lamellar-forming BCPs on top of the underlying cross-linked cylindrical layer exhibited perpendicular orientations of microdomains between lamellae and cylinder layers. From the theoretical calculation of free energy in the multilayers, it was found that the nematic interactions between polymer chains at the interface play a critical role in the perpendicular orientation of lamellae on the cross-linked cylinder layers. Removal of the PMMA domains then affords nonsymmetrical nanostructures which illustrate the promise of this strategy for the design of well-defined 3D nanotemplates. It was also demonstrated that this structure can be effectively used to enhance the light extraction efficiency of GaN light-emitting diodes. Furthermore, we anticipate that such 3D nanotemplates can be applied to various areas, including advanced BCP nanolithography and responsive surface coating.

Efficiency improvement of dye-sensitized solar cells using graft copolymer-templated mesoporous TiO2 films as an interfacial layer

Organized mesoporous TiO2 films with high porosity and good connectivity were synthesized via sol–gel by templating an amphiphilic graft copolymer consisting of poly(vinyl chloride) backbone and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM. The randomly microphase-separated graft copolymer was self-reorganized to exhibit a well-ordered micellar morphology upon controlling polymer–solvent interactions, as confirmed by atomic force microscope (AFM) and glazing incidence small-angle X-ray scattering (GISAXS). These organized mesoporous TiO2 films, 550 nm in thickness, were used an an interfacial layer between a nanocrystalline TiO2 thick layer and a conducting glass in dye-sensitized solar cells (DSSC). Introduction of the organized mesoporous TiO2 layer resulted in the increased transmittance of visible light, decreased interfacial resistance and enhanced electron lifetime. As a result, an energy conversion efficiency of DSSC employing polymer electrolyte was significantly improved from 3.5% to 5.0% at 100 mW cm−2.

Synthesis and photocatalytic properties of hierarchical metal nanoparticles/ZnO thin films hetero nanostructures assisted by diblock copolymer inverse micellar nanotemplates

Metal dot-on-ZnO type hierarchical nanostructures composed of ordered arrays of noble metal nanoparticles with controlled areal density, prepared from poly(styrene-block-vinyl pyridine) diblock copolymer inverse micelles loaded with metal precursors, on sol-gel process based approximately 48 nm thick ZnO thin films exhibit enhanced photocatalytic activities compared with pure ZnO thin films in terms of photodegradation of methylene blue.

Selective growth of Ge islands on nanometer-scale patterned SiO2∕Si substrate by molecular beam epitaxy

The authors studied the selective growth of Ge islands by molecular beam epitaxy on Si(001) covered with nanometer-scale patterned SiO2 mask generated using self-assembled diblock copolymer. Selective growth is made possible by Ge adatoms desorbing from the SiO2 surface as well as diffusing into the exposed Si area. For the Ge coverage of 2nm, multiple islands are observed along the periphery of individual exposed Si areas. At 3.5nm coverage, the coalescence of small islands with significant strain relaxation becomes evident. The ramifications of the multiple islands morphology and their coalescence on potential device applications are discussed.

A polymer brush organic interlayer improves the overlying pentacene nanostructure and organic field-effect transistor performance

We investigated the crystalline nanostructures and film morphologies of pentacene films deposited onto a polymer brush organic interlayer in high performance organic field-effect transistors (OFETs). Polymer brushes were grafted onto the oxide substrates by spin-coating and thermal annealing. Pentacene FETs fabricated on top of the polymer brushes showed excellent device performance, with a field-effect mobility of 0.82 cm2 V−1s−1 and an on/off current ratio of 107. These properties were superior to those of devices using typical surface modification techniques, such as octadecyltrichlorosilane (ODTS) and hexamethyldisilazane (HMDS). The improvements in OFET performance appeared to be due to the pentacene layers crystalline nanostructure and grain interconnectivity, which formed during the submonolayer stage of film growth. This stage of growth is strongly correlated with the surface energy, morphology, and viscoelastic properties of the resulting gate dielectrics. The inclusion of a polymer brush dielectric surface modification is a significant step toward optimizing the nanostructures of organic semiconductors, which are directly linked to device performance enhancement, by engineering the interfaces in OFETs.

Nanoporous block copolymer membranes for ultrafiltration: a simple approach to size tunability.

Nanoporous structures were obtained by the self-assembly of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymers (BCP) where, in thick films, cylindrical microdomains were oriented normal to the substrate and air interfaces, and in the interior of the films, the microdomains were randomly oriented. Continuous nanopores that penetrated through the film were readily produced by a simple preferential swelling of the PMMA microdomains. The confined swelling and rapid contraction of PMMA microdomains generated well-defined uniform pores with diameters to 17.5 nm. The size selectivity and rejection of Au nanoparticles (NPs) for these ultrafiltration (UF) membranes were demonstrated, suggesting an efficient route to tunable, noncomponent-degradative UF membranes.