Changui Ahn

Three-dimensional nanonetworks for giant stretchability in dielectrics and conductors

The realization of levels of stretchability that extend beyond intrinsic limits of bulk materials is of great importance to stretchable electronics. Here we report large-area, three-dimensional nano-architectures that achieve this outcome in materials that offer both insulating and conductive properties. For the elastomer poly(dimethylsiloxane), such geometries enhance the stretchability and fracture strain by ~62% and ~225% over the bulk, unstructured case. The underlying physics involves local rotations of narrow structural elements in the three-dimensional network, as identified by mechanical modelling. To demonstrate the applications of three-dimensional poly(dimethylsiloxane), we create a stretchable conductor obtained by filling the interstitial regions with liquid metal. This stretchable composite shows extremely high electrical conductivity (~24,100 S cm(-1)) even at strains >200%, with good cyclic properties and with current-carrying capacities that are sufficient for interconnects in light-emitting diode systems. Collectively, these concepts provide new design opportunities for stretchable electronics.

Conformal phase masks made of polyurethane acrylate with optimized elastic modulus for 3D nanopatterning

The pattern resolution of soft lithographic techniques is critically determined by the elastic modulus of the soft mold that can support fine and high-aspect-ratio features with conformal adhesion to target substrates. We present a strategy to fine-tune the elastic modulus of conformal molds made of polyurethane acrylate by optimizing the chemical structures and the composition of prepolymer and modulator. Trimethylolpropane ethoxylated (15) triacrylate plays a key role as a delicate modulator for increasing the elastic modulus of soft aliphatic urethane diacrylate oligomer with its low cross-linking density. The optimized molds have sufficiently high elastic modulus (>23 MPa) for defect-free replication of dense, high-aspect-ratio (>2) nanopillars and nanotrench structures while still preserving their conformality. The conformal mold with good mechanical and optical properties can serve as a semi-permanently usable optical phase mask with a wide range of phase modulations for generating three-dimensional (3D) nanostructures with high precision.

Soft Elastomeric Nanopillar Stamps for Enhancing Absorption in Organic Thin‐Film Solar Cells

An elastomeric poly(dimethylsiloxane) (PDMS) block engraved with periodically arrayed nanopillars serves as a transferable light-trapping stamp for encapsulated organic thin-film solar cells. Diffracted light rays from the stamp interfere with one another and self-focus onto the active layer of the solar cell, generating enhanced absorption, as indicated in the current density-voltage measurements.

Amplification of hot electron flow by the surface plasmon effect on metal-insulator-metal nanodiodes.

Au-TiO2-Ti nanodiodes with a metal-insulator-metal structure were used to probe hot electron flows generated upon photon absorption. Hot electrons, generated when light is absorbed in the Au electrode of the nanodiode, can travel across the TiO2, leading to a photocurrent. Here, we demonstrate amplification of the hot electron flow by (1) localized surface plasmon resonance on plasmonic nanostructures fabricated by annealing the Au-TiO2-Ti nanodiodes, and (2) reducing the thickness of the TiO2. We show a correlation between changes in the morphology of the Au electrodes caused by annealing and amplification of the photocurrent. Based on the exponential dependence of the photocurrent on TiO2 thickness, the transport mechanism for the hot electrons across the nanodiodes is proposed.

Controlled three-dimensional interconnected capillary structures for liquid repellency engineering

In this study, we investigated the wetting properties of solvents on highly periodic, porous substrates which have five different layer thicknesses (1, 3, 5, 9 and 13 layers) of three-dimensional (3D) nanoshell structured TiO2 with interconnected capillary spaces. After phosphonic acid (HDF-PA) self-assembly, contact angles of 3D nanoshell structured TiO2 enhanced with increasing number of layers but saturated from 9 layers and up. The omniphobic properties with different types of liquids—deionized water, dextrose, saline, and amino acid injection—were demonstrated by using the 3D-nanoshell-structured TiO2 with 9 layers.

Multifunctional Polymer Nanocomposites Reinforced by 3D Continuous Ceramic Nanofillers.

Polymer nanocomposites with inclusion of ceramic nanofillers have relatively high yield strength, elastic moduli, and toughness that therefore are widely used as functional coating and films for optoelectronic applications. Although the mechanical properties are enhanced with increasing the fraction of nanofiller inclusion, there generally is an upper limit on the amount of nanofiller inclusion because the aggregation of the fillers in the polymer matrix, which typically occurs, degrades the mechanical and/or optical performances above 5 vol % of inclusions. Here, we demonstrate an unconventional polymer nanocomposite composed of a uniformly distributed three-dimensional (3D) continuous ceramic nanofillers, which allows for extremely high loading (∼19 vol %) in the polymer matrix without any concern of aggregation and loss in transparency. The fabrication strategy involves conformal deposition of Al2O3 nanolayer with a precise control in thickness that ranges from 12 to 84 nm on a 3D nanostructured porous polymer matrix followed by filling the pores with the same type of polymer. The 3D continuous Al2O3 nanolayers embedded in the matrix with extremely high filler rate of 19.17 vol % improve compressive strength by 142% compared to the pure epoxy without Al2O3 filler, and this value is in agreement with theoretically predicted strength through the rule of mixture. These 3D nanocomposites show superb transparency in the visible (>85% at 600 nm) and near-IR (>90% at 1 μm) regions and improved heat dissipation beyond that of conventional Al2O3 dispersed nanocomposites with similar filler loading of 15.11 vol % due to the existence of a continuous thermal conduction path through the oxide network.

Emergence of New Density-Strength Scaling Law in 3D Hollow Ceramic Nano-Architectures

Density-strength tradeoff appears to be an inherent limitation for most materials and therefore design of cell topology that mitigates strength decrease with density reduction has been a long-lasting engineering pursue for porous materials. Continuum-mechanics-based analyses of mechanical responses of conventional porous materials with bending-dominated structures often give the density-strength scaling law following the power-law relationship with an exponent of 1.5 or higher, which consequentially determines the upper bound of the specific strength for a material to reach. In this work, a new design criterion capable of significantly abating strength degradation in lightweight materials is presented, by successfully combining the size-induced strengthening effect in nanomaterials with the architectural design of cellular porous materials. Hollow-tube-based 3D ceramic nanoarchitectures satisfying such criterion are fabricated in large area using proximity field nano-patterning and atomic layer deposition. Experimental data from micropillar compression confirm that the strengths of these nanoarchitectural materials scale with relative densities with a power-law exponent of 0.93, a hardly observable value in conventional bending-dominated porous materials. This discovery of a new density-strength scaling law in nanoarchitectured materials will contribute to creating new lightweight structural materials attaining unprecedented specific strengths overcoming the conventional limit.