Seung Keun Cha

Au–Ag Core–Shell Nanoparticle Array by Block Copolymer Lithography for Synergistic Broadband Plasmonic Properties

Localized surface plasmon resonance of metallic nanostructures receives noticeable attention in photonics, electronics, catalysis, and so on. Core-shell nanostructures are particularly attractive due to the versatile tunability of plasmonic properties along with the independent control of core size, shell thickness, and corresponding chemical composition, but they commonly suffer from difficult synthetic procedures. We present a reliable and controllable route to a highly ordered uniform Au@Ag core-shell nanoparticle array via block copolymer lithography and subsequent seeded-shell growth. Size-tunable monodisperse Au nanodot arrays are generated by block copolymer self-assembly and are used as seed layers to grow Ag shells with variable thickness. The resultant Au@Ag core-shell nanoparticle arrays exhibit widely tunable broadband enhancement of plasmonic resonance, greatly surpassing single-element nanoparticle or homogeneous alloy nanoparticle arrays. Surface-enhanced Raman scattering of the core-shell nanoparticle arrays showed an enhancement factor greater than 270 from Au nanoparticle arrays.

Laser Writing Block Copolymer Self-Assembly on Graphene Light-Absorbing Layer

Recent advance of high-power laser processing allows for rapid, continuous, area-selective material fabrication, typically represented by laser crystallization of silicon or oxides for display applications. Two-dimensional materials such as graphene exhibit remarkable physical properties and are under intensive development for the manufacture of flexible devices. Here we demonstrate an area-selective ultrafast nanofabrication method using low intensity infrared or visible laser irradiation to direct the self-assembly of block copolymer films into highly ordered manufacturing-relevant architectures at the scale below 12 nm. The fundamental principles underlying this light-induced nanofabrication mechanism include the self-assembly of block copolymers to proceed across the disorder-order transition under large thermal gradients, and the use of chemically modified graphene films as a flexible and conformal light-absorbing layers for transparent, nonplanar, and mechanically flexible surfaces.

Monodisperse pattern nanoalloying for synergistic intermetallic catalysis.

Nanoscale alloys attract enormous research attentions in catalysis, magnetics, plasmonics and so on. Along with multicomponent synergy, quantum confinement and extreme large surface area of nanoalloys offer novel material properties, precisely and broadly tunable with chemical composition and nanoscale dimension. Despite substantial progress of nanoalloy synthesis, the randomized positional arrangement and dimensional/compositional inhomogeneity of nanoalloys remain significant technological challenges for advanced applications. Here we present a generalized route to synthesize single-crystalline intermetallic nanoalloy arrays with dimensional and compositional uniformity via self-assembly. Specific electrostatic association of multiple ionic metal complexes within self-assembled nanodomains of block copolymers generated patterned monodisperse bimetallic/trimetallic nanoalloy arrays consisting of various elements, including Au, Co, Fe, Pd, and Pt. The precise controllability of size, composition, and intermetallic crystalline structure of nanoalloys facilitated tailored synergistic properties, such as accelerated catalytic growth of vertical carbon nanotubes from Fe-Co nanoalloy arrays.

Flash Light Millisecond Self-Assembly of High χ Block Copolymers for Wafer-Scale Sub-10 nm Nanopatterning

One of the fundamental challenges encountered in successful incorporation of directed self-assembly in sub-10 nm scale practical nanolithography is the process compatibility of block copolymers with a high Flory-Huggins interaction parameter (χ). Herein, reliable, fab-compatible, and ultrafast directed self-assembly of high-χ block copolymers is achieved with intense flash light. The instantaneous heating/quenching process over an extremely high temperature (over 600 °C) by flash light irradiation enables large grain growth of sub-10 nm scale self-assembled nanopatterns without thermal degradation or dewetting in a millisecond time scale. A rapid self-assembly mechanism for a highly ordered morphology is identified based on the kinetics and thermodynamics of the block copolymers with strong segregation. Furthermore, this novel self-assembly mechanism is combined with graphoepitaxy to demonstrate the feasibility of ultrafast directed self-assembly of sub-10 nm nanopatterns over a large area. A chemically modified graphene film is used as a flexible and conformal light-absorbing layer. Subsequently, transparent and mechanically flexible nanolithography with a millisecond photothermal process is achieved leading the way for roll-to-roll processability.

Nanodomain swelling block copolymer lithography for morphology tunable metal nanopatterning.

Ordered metal nanopatterns are crucial requirements for electronics, magnetics, catalysts, photonics, and so on. Despite considerable progress in the synthetic route to metal nanostructures, highly ordered metal nanopatterning over a large-area is still challenging. Nanodomain swelling block copolymer lithography is presented as a general route to the systematic morphology tuning of metal nanopatterns from amphiphilic diblock copolymer self-assembly. Selective swelling of hydrophilic nanocylinder domains in amphiphilic block copolymer films during metal precursor loading and subsequent oxygen based etching generates diverse shapes of metal nanopatterns, including hexagonal nanoring array and hexagonal nanomesh and double line array in addition to common nanodot and nanowire arrays. Solvent annealing condition of block copolymer templates, selective swelling of hydrophilic cylinder nanodomains, block copolymer template thickness, and oxygen based etching methods are the decisive parameters for systematic morphology evolution. The plasmonic properties of ordered Au nanopatterns are characterized and analyzed with finite differential time domain calculation. This approach offers unprecedented opportunity for diverse metal nanopatterns from commonly used diblock copolymer self-assembly.

Controlled Segmentation of Metal Nanowire Array by Block Copolymer Lithography and Reversible Ion Loading

Spatial arrangement of 1D nanomaterials may offer enormous opportunities for advanced electronics and photonics. Moreover, morphological complexity and chemical diversity in the nanoscale components may lead to unique properties that are hardly anticipated in randomly distributed homogeneous nanostructures. Here, controlled chemical segmentation of metal nanowire arrays using block copolymer lithography and subsequent reversible metal ion loading are demonstrated. To impose chemical heterogeneity in the nanowires generated by block copolymer lithography, reversible ion loading method highly specific for one particular polymer block is introduced. Reversibility of the metal ion loading enables area-selective localized replacement of metal ions in the self-assembled patterns and creates segmented metal nanowire arrays with different metallic components. Further integration of this method with shear aligning process produces high aligned segmented metal nanowire array with desired local chemical compositions.

Electric field directed self-assembly of block copolymers for rapid formation of large-area complex nanopatterns

We investigate the unusually rapid directed self-assembly of block copolymer (BCP) thin films by imposing an in-plane electric field. Blending short chain BCPs dramatically enhances the generically retarded chain diffusivity for BCP self-assembly in thin films. In addition, photopatterned, few-layer solution-cast chemically modified graphene is introduced as a disposable and versatile electrode for electric field applications. A variety of laterally ordered self-assembled lamellar BCP nanopatterns are attained within a 10 min time scale, including an arbitrary large-area unidirectionally oriented lamellar array.