Wearable and implantable devices require conductive, stretchable and biocompatible materials. However, obtaining composites that simultaneously fulfil these requirements is challenging due to a trade-off between conductivity and stretchability. Here, we report on Ag-Au nanocomposites composed of ultralong gold-coated silver nanowires in an elastomeric block-copolymer matrix. Owing to the high aspect ratio and percolation network of the Ag-Au nanowires, the nanocomposites exhibit an optimized conductivity of 41,850 S cm (maximum of 72,600 S cm). Phase separation in the Ag-Au nanocomposite during the solvent-drying process generates a microstructure that yields an optimized stretchability of 266% (maximum of 840%). The thick gold sheath deposited on the silver nanowire surface prevents oxidation and silver ion leaching, making the composite biocompatible and highly conductive. Using the nanocomposite, we successfully fabricate wearable and implantable soft bioelectronic devices that can be conformally integrated with human skin and swine heart for continuous electrophysiological recording, and electrical and thermal stimulation.

Inorganic-organic hybrid perovskite thin films have attracted significant attention as an alternative to silicon in photon-absorbing devices mainly because of their superb optoelectronic properties. However, high-definition patterning of perovskite thin films, which is important for fabrication of the image sensor array, is hardly accomplished owing to their extreme instability in general photolithographic solvents. Here, a novel patterning process for perovskite thin films is described: the high-resolution spin-on-patterning (SoP) process. This fast and facile process is compatible with a variety of spin-coated perovskite materials and perovskite deposition techniques. The SoP process is successfully applied to develop a high-performance, ultrathin, and deformable perovskite-on-silicon multiplexed image sensor array, paving the road toward next-generation image sensor arrays.

We examined the packing structure of polystyrene-coated gold nanoparticles (Au@PS) as a function of grafting density. A series of Au@PS nanoparticles with grafting densities in the range of 0.51−1.94 chains nm −2 were prepared by a ligand exchange process using thiol-terminated PS and then selfassembled at a liquid−air interface. We observed a transition from disordered to bodycentered cubic (bcc) to face-centered cubic (fcc) arrangements with increasing grafting density, even though the ligand length-to-core radius ratio (λ) was as high as 3.0, a condition that typically favors nonclose-packed bcc symmetry in the self-assembly of hard nanoparticles. To explain this phenomenon, we define λ eff to include the concentrated polymer brush regime as part of the "hard core", which predicts that the softness of Au@PS nanoparticles is reduced from 1.53 to 0.14 in a theta solvent as the grafting density increases from 0.51 to 1.94 chains nm −2 . This new definition of λ can also predict the effective radii of nanoparticles using the established optimal packing model. The experimental findings are supported by a combination of coarse-grained molecular dynamics simulation and adaptive common neighbor analysis, which show that changes in grafting density can drive the observed transitions in nanoparticle packing. These studies provide new insights for controlling the selfassembled symmetries of polymer-coated nanocrystals using a simple ligand exchange process to tune particle softness.

Icosahedral colloidal clusters are a new class of spherical colloidal crystals. This cluster allows for potentially superior optical properties in comparison to conventional onion-like colloidal supraballs because of the quasi-crystal structure. However, the characterization of the cluster as an optical material has until now not been achieved. Here we successfully produce icosahedral clusters by assembling silica particles using bulk water-in-oil emulsion droplets and systematically characterize their optical properties. We exploit a water-saturated oil phase to control droplet drying, thereby preparing clusters at room temperature. In comparison to conventional onion-like supraballs with a similar size, the icosahedral clusters exhibit relatively strong structural colors with weak nonresonant scattering. Simulations prove that the crystalline array inside the icosahedral cluster strengthens the collective specular diffraction. To further improve color saturation, the silica particles constituting the cluster are coated with a thin-film carbon shell. The carbon shell acts as a broad-band absorber and reduces incoherent scattering with long optical paths, resulting in vibrant blue, green, and red colors comparable to inorganic chemical pigments.

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