Advanced Functional Materials

As part of ongoing efforts to create metamaterials by engineered self-organization using solidification of eutectics, D. A. Pawlak et al. report on page 1116 the fabrication of a fractal material with split-tubes mimicking (in cross section) the famous split-ring resonators (SRRs). The latter, when made of metal, are the basic element of a structure showing negative refractive index. This article shows that metallo-dielectric structures can be fabricated. The method employed has high potential for manufacturing materials with various functionalities.

Over the last decade, direct laser writing via two-photon absorption turned from a laboratory curiosity into a versatile technique for the fabrication of 3D nano-and microstructures. Especially, in the field of nanophotonics novel structures could be fabricated and tested for agreement with theory. G. von Freymann and co-workers review recent progress in the field on page 1038. The cover image displays a potpourri of complex 3D nanostructures for photonics. Cover artwork by M. S. Rill.

Direct laser writing of 3D nanostructures for photonics has reached a level of maturity at which it can be considered the 3D analogue of planar electron-beam lithography. The recent progress in this field is reviewed, and examples for silicon-based woodpile photonic crystals, novel optical resonator designs within these structures, 3D chiral photonic crystals, and 3D icosahedral photonic quasicrystals are demonstrated.

Molecular mimetic self-assembly of colloidal particles—to translate the rules of molecular and supramolecular chemistry to supervise particle self-assembly—can not only give in-depth insight to the association processes occurring at the atomic and molecular level, but also lead to unprecedented materials with collective properties programmed at the mesoscopic level. Recent developments in this emerging field are outlined, and the challenges associated therein are underlined.

1D ZnO rod structures as an optical wave-guiding medium are proposed as a means of improving the light extraction efficiency of InGaN/GaN multiple quantum well blue LEDs. Different shapes of ZnO rod structures are produced using a simple non-catalytic wet-chemical growth method at low temperature on an ITO contact layer. Confocal scanning electroluminescence microscopy and micro-electroluminescence spectra confirmed the wave-guiding phenomenon within the ZnO sub-microrods.

Silk protein chemically modified with carboxylic acid functional groups shows a spectral colorimetric response to pH changes. The material exists in the liquid state as well as in thin film form and is functional and stable under a wide pH range. Functionalized silk in the solid form is incorporated into a hybrid microfluidic system to function as a bioactive optofluidic pH sensor.

Hybrid membranes composed of a -SO₃H-functional silica network, poly(VDF-co-HFP) and terpolymer exhibit a proton conductivity of 43 mS cm⁻¹ at 65 °C under 100% relative humidity and enhanced stability over standard hybrid -SO₃H-functional silica/poly(VDF-co-HFP) membranes. These enhanced properties are attributed to the occurrence of a better interface between the silica and the poly(VDF-co-HFP) copolymer network in the presence of the terpolymer.

Solar water-splitting with Fe₂O₃ (hematite) photoanodes is promising, but limited by poor material characteristics. Here, a SiO_x buffer-layer between the substrate and hematite drastically improves the performance of ultrathin hematite films. Scrutinizing the effects of this layer gives further insight to the important parameters for the performance of hematite as a photocatalyst for solar hydrogen production.

A universal behavior in rare-earth-substituted BiFeO₃ has been discovered. The property-enhancing morphotropic phase boundary can be achieved by controlling the average A-site ionic radius. An electric-field-induced transformation from a paraelectric orthorhombic phase to the polar rhombohedral phase is proposed as the origin of the observed double hysteresis loop and the concomitant enhancement in the piezoelectric coefficient at the boundary.

Using directional solidification of eutectics, the first experimental realization of self-organized particles with a split-ring resonator-like cross section is demonstrated. This unusual morphology/microstructure of the eutectic composite has a fractal character. The materials obtained present very high anisotropy (split tubes) and can be obtained in large pieces. Metallodielectric structures can be created by etching and filling the space with metal.

bottom-up approach for building up copper diffusion barriers using self-assembled monolayers (SAM_S) is reported. Based on a multi-characterization study, NH₂SAM appears to be a suitable candidate for BEOL integration in advanced interconnects environment.

A systematic set of mesoporous organosilica materials composed of important organic ligands and their properties regarding the coordination of transition metal cations is reported. Interesting cooperative magnetic interactions of the paramagnetic metal centers (Co²⁺, Mn²⁺, etc.) located at the surfaces of the pores are found.

In novel designs for sensing explosives, poly(4-vinylpyridine) (P4VP) is characterized as a nitroaromatic selective layer. P4VP thin films swell by over 30% upon exposure to nitroaromatic vapors. A nanoscale design is described, where swelling of films inside microfabricated trenches create switchlike responses in electrical resistivity. Fabricated devices successfully detect nitrobenzene vapors.

Novel bioinorganic nanocomposites of Au nanoparticles and bacteria cellulose (BC) nanofibers with controlled Au shell thickness are prepared via a facile one-step biotemplated method using poly(ethyl eneimine) (PEI) as the reducing and linking agent. The obtained Au–BC nanocomposites are used as a biocompatible support for horseradish peroxidase immobilization and then as a sensitive biosensor for the detection of H₂O₂.

A clear correlation between defect-related emissions and the magnetization of ZnO nanorods synthesized by a one-step aqueous chemical method is demonstrated. By increasing the size of the nanorods and the annealing temperature, the magnetization decreases with the reduction of the EPR signal and the defect-related band originating from singly charged oxygen vacancies. At an annealing temperature of 900 °C, the nanorods show diamagnetic behavior.

A multifunctional material (mesoporous, magnetic, and up-conversion luminescent) is prepared through a novel two-step sol–gel process. The materials show ordered mesopores, high magnetization values, and up-conversion luminescence. This product is used as a drug delivery system, which can be monitored or tracked based on their magnetic and up-conversion luminescent features.

A polybead tagged by CdTe quantum dots is produced for the preparation of a novel label for biomolecules by biotin–streptavidin binding, which has been used for electrochemical sub-femtomolar detection of DNA hybridization. Its excellent performance demonstrates that the QD-tagged polybeads represent a promising and versatile amplified platform for applications in biotechnology.

Striking morphological changes are observed in carbon-bridged and silicon-bridged dithiophene donor–acceptor polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. The present study shows that the higher photogeneration achieved in the films made with the silicon-bridged material correlates to the inactivation of a charge-transfer complex, and to a more favorable length scale for the donor–acceptor phase separation.

A water suspension of nanocomposite microcapsules with embedded ZnO nanoparticles in the capsule shell is reported. The microcapsule morphology is characterized before and after ultrasound treatment. A remarkably high capsule sensitivity to ultrasound is evidenced, which grows with increasing number of ZnO nanoparticle layers in the nanocomposite shell. This effect is correlated with the mechanical properties of microcapsules measured with AFM.