Advanced Materials

Jennifer A. Lewis, Jennifer T. Bernhard, and co-workers demonstrate on p. 1335 the conformal printing of electrically conductive meander lines onto hemispherical glass substrates. The bandwidth of these electrically small antennas approaches the fundamental limit for their size. This space-filling design offers nearly an order of magnitude improvement over rudimentary monopole antennas. Cover design by T. Malkowski and R. Durdle (Beckman Institute ITG), University of Illinois.

In microthermoforming, thin polymer films are heated to a softened, but still solid state and formed to thin-walled microdevices by three-dimensional stretching. This is in contrast to common polymer microreplication processes where the material is processed in a liquid state. In the present progress report, we review the still young state of the art of microthermoforming technology as well as its first applications.

A step-wise template process is developed for the fabrication of high-aspect-ratio, hierarchical TiO₂ nanotube arrays directly on transparent conductive oxides. Significantly enhanced dye loading capability and photoelectrochemical performances are demonstrated in dye-sensitized solar cells with the adoption of a hierarchical nanocrystalline shell.

Conformal printing of electrically small antennas onto the convex and concave surfaces of hemispherical glass substrates is demonstrated. Their bandwidth approaches the fundamental limit for their size, offering nearly an order of magnitude improvement over rudimentary monopole designs.

Two-component 3D polymer scaffolds with distinct mechanical and protein-binding properties are fabricated using direct laser writing. With this method, full control over the formation of cell adhesion sites and, consequently, cell shape in three dimensions can be achieved.

Micrometer-scale monolayer patterns of a phosphorus-containing molecular precursor are fabricated on nearly intrinsic Si(100) using nanoimprint lithography. The patterned sample is protected by a SiO₂capping layer applied by electron beam evaporation and subjected to rapid thermal annealing (RTA) to diffuse the phosphorus dopant atoms into the bulk silicon locally.

Non-volatile memory cells using Co-doped BaTiO₃ as an active layer exhibit high-performance unipolar resistive switching characteristics, with a resistance ratio over 10⁴, retention time longer than 7 × 10⁴ s, endurance over 10⁵ cycles, and switching speed less than 10 ns/70 ns for SET/RESET. Under high electric field and large Joule heating, the easily varied valence of Co ions, the pre-existing oxygen vacancies with sufficiently high density, and the local itinerant electrons introduced by the Co-doping all favor the local metal-insulator phase transition and therein the formation/rupture of conductive filaments, contributing to the stable resistive switching.

A significant size effect on the mechanical properties of GaAs nanowires (NWs) is reported. A remarkable elastic strain of ≈11% for NWs with diameters of 50–150 nm and obvious plastic deformation in NWs with diameters ≤25 nm are revealed. The Young’s modulus of the NWs can be more than double that of bulk GaAs.

Hybrid coatings are formed by dispersing inhibitor (benzotriazole) containing functionalized mesoporous silica nanoparticles throughout a polyester primer. They outperform the primer in corrosion tests both when intact and scratched. Adding the inhibitor directly (without encapsulation into nanoparticles) is found to be less effective. Given the afforded holistic protection, these hybrid coatings represent an excellent model for future coating design.

PbSe was expected to have a smaller bandgap and higher thermalconductivity than PbTe. Instead, these values are about the same at high temperature leading to comparable thermoelectric figure of merit, with zT> 1 achieved in heavily doped p-type PbSe.

A multifunctional organic spintronic device is demonstrated using tris(8-hydroxyquinolinato)aluminium (Alq₃)-based vertical spin valves with manganite and cobalt electrodes. The device shows a non-volatile electrical switching with dramatic effects on the spin transport behavior. The interplay of electrical and magnetic bistabilities enables the electrical control of the spin valve magnetoresistance. This multifunctionality is illustrated together with a phenomenological model that establishes a connection between the electrical and magnetic bistabilities.

Nanoporous membranes are used as templates for polyelectrolyte encapsu­lation of colloidal nanoparticles, leading to colloidally dispersible, multifunctional, and multicompartment microfibers. The protocol is simple and independent of the chemical nature of the nanoparticles and polyelectrolytes. Furthermore, the surface polyelectrolyte coating allows free transfer of the resulting microfibers into various aqueous and organic media.

A kind of optical modulators based on doped organic nanowire waveguides that output constant white light from the wire terminus has been demonstrated. When the waveguides are locally excited with a focused laser, bidirectional singlet and triplet energy transfers between the two compounds result in the fluctuations of the singlet and triplet excitons, which induce self-modulated white light outcoupling from each waveguide despite the doping contents.

A uniform nitrogen-doped carbon coating layer is formed on Li₄Ti₅O₁₂ particles by mixing porous Li₄Ti₅O₁₂ powder with an ionic liquid and then treating the mixture at moderate temperature. Uniformly coated Li₄Ti₅O₁₂ is shown to have significantly improved rate capability and cycling performance for Li-ion batteries. This relatively simple approach is versatile and can be extended to modify other electrode materials for electrochemical devices.

A photoinvertible chiral molecule allows for wide-range tuning encompassing the entirety of the optical range. UV light inverts the handedness of the cholesteric liquid-crystal (CLC) helix, which yields color tuning of the reflection notch. After the light is removed, the material quickly relaxes back to the original conformation. The optical properties of the CLC during tuning/inversion are correlated to the mesomorphic behavior of the mixture.

A memory effect has been demonstrated in magnetic nanowire arrays. The magnetic nanowire array has the ability to record the maximum magnetic field that the array has been exposed to after the field has been turned off. The origin of the memory effect is the strong magnetic dipole interaction among the nanowires. Switching field distributions among nanowires has been studied with a first-order reversal curve technique to elucidate the discrepancy between the experimental result and the theoretical explanation. Based on the memory effect, a novel and extremely low cost EMP detection scheme is proposed.

The three-dimensionally isolated morp­hology of secondary iron phosphide phases in a polycrystalline LiFePO₄ matrix is determined using a combination of electron tomography in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), and electron energy-loss spectroscopy (EELS). The present evidence shows that the enhanced bulk electronic conductivity in LiFePO₄ has no relation to secondary metallic phases at the grain boundaries.

High-quality, vertically aligned CdSe nanowires (figure, top left) are prepared by the vapor transport growth technique. Strong free-exciton absorption and emission are observed (top right) and, under optical excitation, near-infrared coherent random lasing is demonstrated for the first time from CdSe nanowires at room temperature (bottom). The results show that this material system holds promise for optoelectronic applications.

A new low bandgap polymer POD2T-DTBT has been synthesized and found to give excellent performances in organic thin-film transistor (OTFT) and organic photovoltaic (OPV) applications. OTFTs based on POD2T-DTBT have achieved good hole carrier mobilities of 0.2 cm²V⁻¹s⁻¹ and OPVs based on blends of POD2T-DTBT and PC₇₁BM have demonstrated promising power conversion efficiencies of 6.26%.