Kahyun Hur

A 3D Optical Metamaterial Made by Self-Assembly

Optical metamaterials have unusual optical characteristics that arise from their periodic nanostructure. Their manufacture requires the assembly of 3D architectures with structure control on the 10-nm length scale. Such a 3D optical metamaterial, based on the replication of a self-assembled block copolymer into gold, is demonstrated. The resulting gold replica has a feature size that is two orders of magnitude smaller than the wavelength of visible light. Its optical signature reveals an archetypal Pendry wire metamaterial with linear and circular dichroism.

Hierarchical Porous Polymer Scaffolds from Block Copolymers

A Complicated Scaffold, Simply Materials with tailored pore structures can be useful as catalysis supports and for lightweight materials. When preparing medical scaffolds, restrictive preparation conditions have to be met, which can prohibit multistep preparation procedures. Sai et al. (p. 530) describe a method for making porous polymers containing both relatively large (several microns) interconnecting pores and a second population of ∼ tens of nanometer pores. The process exploits spinodal decomposition of a block copolymer blended with small-molecule additives and requires a simple washing step with water, methanol, or ethanol. Spinodal decomposition of block copolymers and oligomeric additives produces three-dimensional hierarchical porous polymers. Hierarchical porous polymer materials are of increasing importance because of their potential application in catalysis, separation technology, or bioengineering. Examples for their synthesis exist, but there is a need for a facile yet versatile conceptual approach to such hierarchical scaffolds and quantitative characterization of their nonperiodic pore systems. Here, we introduce a synthesis method combining well-established concepts of macroscale spinodal decomposition and nanoscale block copolymer self-assembly with porosity formation on both length scales via rinsing with protic solvents. We used scanning electron microscopy, small-angle x-ray scattering, transmission electron tomography, and nanoscale x-ray computed tomography for quantitative pore-structure characterization. The method was demonstrated for AB- and ABC-type block copolymers, and resulting materials were used as scaffolds for calcite crystal growth.

Three‐Dimensionally Isotropic Negative Refractive Index Materials from Block Copolymer Self‐Assembled Chiral Gyroid Networks

In 1999, Pendry et al. predicted that specifically engineered artificial materials, that is, metamaterials, would have unusual magnetic responses, for example, negative permeability. Following this work, much effort has been devoted to the design and fabrication of metamaterials with negative refractive index. 3] Such negative index metamaterials have the potential, for example, in the form of superlenses, to overcome the diffraction limit in imaging or to enable novel optical effects, including cloaking. Today most metamaterial fabrication relies on top-down approaches such as lithography techniques, making efficient access to three-dimensionally (3D) isotropic metamaterials challenging thus hindering their practical applications. Recent progress in bottom-up type self-assembly offers promise to overcome some of these limitations. In particular block copolymer (BCP) selfassembly has emerged as a useful designer tool to create nanostructures including 3D continuous morphologies of disparate materials like ceramics and metals. The present paper makes clear theoretical predictions for how to design 3D isotropic materials with negative refraction and circularly polarized light propagation from a class of block copolymer based self-assembled materials not yet rigorously studied in the context of metamaterials. Through theoretical understanding and guidance on materials choices, characteristic length and frequency scales, which are determined by calculations and described in detail here, a “recipe” is provided for the synthesis, fabrication and characterization of these materials. We present calculations of the photonic properties of 3D periodic metallic nanomaterials with co-continuous cubic morphologies as illustrated in Figure 1. Such structures are experimentally accessible through self-assembly of AB diblock copolymers and ABC triblock terpolymers and are referred to as double gyroid (D-GYR) and alternating gyroid (A-GYR). Both of these structures have two 3D continuous cubic and interwoven minority networks separated by a matrix majority network. In the A-GYR the two minority networks are distinguishable leading to chirality while in the D-GYR they are not. We predict for the resulting metallic nanomaterials that the coupled surface plasmon resonance of the two minority networks of the D-GYR induces low frequency light propagation with a negative index of refraction. Due to their cubic symmetry, these materials are 3D isotropic (see Figure 1e). They also show circularly polarized light propagation originating from the chirality of the gyroid networks. We further predict that by tailoring BCP synthesis one can design materials with varying refractive index and frequency at which negative refraction occurs. Finally, in contrast to D-GYR metallic nanomaterials, chiral A-GYR metallic nanomaterials are expected to exhibit a surprising metallic band gap despite their smaller metallic fraction. We Figure 1. Schematic routes to 3-dimensionally co-continuous metamaterials with cubic symmetry and expected optical behavior. a) D-GYR; b) hollow D-GYR; and c) A-GYR metamaterials. For clarity of presentation, specific blocks are represented to be transparent. d) D-GYR metamaterial formed from many unit cells. The two chiral gyroid struts are depicted in different color for clarity. e) Projected images of a DGYR metamaterial unit cell with unit cell length a onto three orthogonal axes. Two struts are cut in different planes for showing full loops. Surface plasmon polaritons f) oscillate on the closed loop of gyroid networks and g) on a 1-dimensional metal/insulator/metal wave-guide.

Linking experiment and theory for three-dimensional networked binary metal nanoparticle–triblock terpolymer superstructures

Controlling superstructure of binary nanoparticle mixtures in three dimensions from self-assembly opens enormous opportunities for the design of materials with unique properties. Here we report on how the intimate coupling of synthesis, in-depth electron tomographic characterization and theory enables exquisite control of superstructure in highly ordered porous three-dimensional continuous networks from single and binary mixtures of metal nanoparticles with a triblock terpolymer. Poly(isoprene-block-styrene-block-(N,N-dimethylamino)ethyl methacrylate) is synthesized and used as structure-directing agent for ligand-stabilized platinum and gold nanoparticles. Quantitative analysis provides insights into short- and long-range nanoparticle-nanoparticle correlations, and local and global contributions to structural chirality in the networks. Results provide synthesis criteria for next-generation mesoporous network superstructures from binary nanoparticle mixtures for potential applications in areas including catalysis.

Design and Applications of Multiscale Organic–Inorganic Hybrid Materials Derived from Block Copolymer Self-Assembly

Block copolymer (BCP) self-assembly (SA) is a useful tool for designing materials with tunable nanostructure as well as controllable multiscale, hierarchical structure. A combination of BCP SA with inorganic materials results in functional hybrid materials with ordered structures down to the nanoscale, thereby exploiting both the advantageous features of structure tunability from BCP SA and functionality from inorganic materials. Rather than a comprehensive review of the entire field of hybrid materials, this overview summarizes a variety of BCP-derived synthetic approaches developed over the last 10–15 years, with emphasis on work by the Wiesner group at Cornell University on hybrid materials with structural characteristics on multiple length scales. This encompasses hybrids with thermodynamic equilibrium-type BCP nanostructures, controlled nonequilibrium-type structure formation processes leading to structural asymmetries, as well as formation of hierarchical BCP materials with control over nanoscale and macroscale structures. Besides the development of wet-chemical methodologies for their synthesis, this overview also features some promising first applications of such materials. Results suggest that BCP SA directed synthetic approaches may provide routes to cost-effective and large-scale materials fabrication potentially useful for both, new materials discovery and study of fundamental structure – property correlations as well as exploration of the materials in a number of today’s most pressing applications including water filtration and energy conversion and storage.

Soft self-assembly of Weyl materials for light and sound

Significance Soft materials self-assemble into complex structures that can replicate on a larger scale the symmetric arrangements of atomic crystals. Harnessing soft self-assembly for material synthesis requires controlling the symmetries of such structures, a job tailored for group-theoretical methods traditionally used to describe crystal symmetries and their breaking. We illustrate this approach by providing a blueprint for self-assembled materials with an exotic feature of wave propagation known as Weyl points. The presence of such topological objects means that, at certain wavelengths, light or sound can only propagate inside the material in a handful of directions. Most crucially, sound and light waves can propagate on the material surface without backscattering from imperfections. Soft materials can self-assemble into highly structured phases that replicate at the mesoscopic scale the symmetry of atomic crystals. As such, they offer an unparalleled platform to design mesostructured materials for light and sound. Here, we present a bottom-up approach based on self-assembly to engineer 3D photonic and phononic crystals with topologically protected Weyl points. In addition to angular and frequency selectivity of their bulk optical response, Weyl materials are endowed with topological surface states, which allow for the existence of one-way channels, even in the presence of time-reversal invariance. Using a combination of group-theoretical methods and numerical simulations, we identify the general symmetry constraints that a self-assembled structure has to satisfy to host Weyl points and describe how to achieve such constraints using a symmetry-driven pipeline for self-assembled material design and discovery. We illustrate our general approach using block copolymer self-assembly as a model system.

A Generalized Crystallographic Description of All Tellurium Nanostructures

Despite tellurium being less abundant in the Earths crust than gold, platinum, or rare-earth elements, the number of industrial applications of tellurium has rapidly increased in recent years. However, to date, many properties of tellurium and its associated compounds remain unknown. For example, formation mechanisms of many tellurium nanostructures synthesized so far have not yet been verified, and it is unclear why tellurium can readily transform to other compounds like silver telluride by simply mixing with solutions containing silver ions. This uncertainty appears to be due to previous misunderstandings about the tellurium structure. Here, a new approach to the tellurium structure via synthesized structures is proposed. It is found that the proposed approach applies not only to these structures but to all other tellurium nanostructures. Moreover, some unique tellurium nanostructures whose formation mechanism are, until now, unconfirmed can be explained.