Articles in 2012

The control and manipulation of the magnetization of thin metallic films by means of an electric current is a promising strategy for ensuring that potential spintronic applications are energy efficient. It is now shown that large changes in the current-induced magnetic field can arise as a result of varying the thickness of the Ta layer in Ta|CoFeB|MgO heterostructures.

Graphene has attracted considerable interest for future electronics, but the absence of a bandgap limits its direct applicability in transistors and logic devices. It is now shown that vertical integration with MoS₂ and other layered materials enables the fabrication of vertical field-effect transistors with large on/off ratios and high current densities as well as complementary inverters with larger-than-unity voltage gain.

Non-trivial topological phases can allow for one-way spin-polarized transport along the interfaces of topological insulators but they are relatively uncommon in the condensed state of matter. By arranging judiciously designed metamaterials into two-dimensional superlattices, a photonic topological insulator has now been demonstrated theoretically, enabling unidirectional spin-polarized photon propagation without the application of external magnetic fields or breaking of time-reversal symmetry.

Gathering information on the evolution of small cracks in ceramic matrix composites used in hostile environments such as in gas turbines and hypersonic flights has been a challenge. It is now shown that sequences of microcrack damage in ceramic composites under load at temperatures up to 1,750 °C can be fully resolved with the use of in situ synchrotron X-ray computed microtomography.

Photocurrent generation in organic solar cells relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. Femtosecond spectroscopy and non-adiabatic simulations on the phthalocyanine–fullerene model system now reveal the relaxation dynamics of hot charge-transfer excitons in this process.

The layered iron pnictide superconductors are known for their unconventional electronic properties and high critical temperatures. Now, SmFeAs(O,F) is shown to undergo a transition from pinned Abrikosov-like to mobile Josephson-like vortices as the system is cooled below its critical temperature.

The properties of the insulating ground state from which the superconductivity of copper oxide materials emerges with chemical doping are a topic of extensive research. The observation that superconducting fluctuations are quenched by charge order at low temperatures now provides valuable information on the mechanism for the superconducting to insulator transition.

Nanoplasmonic structures that can detect trace analytes via surface-enhanced Raman spectroscopy typically require sophisticated nanofabrication techniques. Self-assembly of gold nanoparticles into close-packed arrays at liquid/liquid and liquid/air interfaces is now used for the detection of multi-analytes from aqueous, organic or air phases.

Photoconversion in organic photovoltaic cells, which relies on charge generation at donor/acceptor interfaces, is limited by short exciton-diffusion-lengths. Diluting an electron donor into a wide-energy-gap host material has now led to an ~50% increase in exciton diffusion length and enhanced power conversion efficiencies in planar heterojunction cells compared with optimized devices with an undiluted donor layer.

Hydrogen embrittlement in metals has proved problematic for designing strong and reliable structural materials. Direct molecular dynamics simulations now reveal a ductile-to-brittle transition caused by the suppression of dislocation emission at the crack tip due to the aggregation of hydrogen.

Implantable neural microelectrodes are critical to neuroscience research and emerging clinical applications including brain-controlled prostheses. A composite electrode consisting of a carbon fibre core, an insulating polymer coating and a polythiophene-based recording pad has now been developed that shows reduced chronic reactive tissue response in rats compared with existing architectures, owing to its smaller size and improved mechanical compliance with brain tissue.

Layered oxides are important as electrode materials for batteries and because of the strong electronic correlations resulting from their unique structure. Electrochemical investigations of the layered P2-Na_xVO₂ system in sodium batteries together with in situ X-ray diffraction experiments now result in the elucidation of the room-temperature phase diagram of this system.

Some of the most challenging issues in energy conversion are the insufficient activity of the catalysts for the oxygen-reduction reaction, catalyst degradation and carbon-support corrosion. A class of mesostructured carbon-free metallic catalysts based on thin films and with tunable near-surface composition, morphology and structure that lead to an improved affinity for the electrochemical reduction of oxygen are now reported.

Semiconductor photoelectrodes for solar hydrogen production by water photoelectrolysis require stable and abundant visible-light absorbers such as iron oxide. Although this material suffers from poor transport properties for efficient charge-carrier generation and collection, these drawbacks can now be addressed by using resonant light trapping in ultrathin films designed as optical cavities.

Enhancing and optimizing the performance and durability of nanocatalysts for the oxygen reduction reaction is crucial for fuel-cell applications. A class of Pt–Co nanocatalysts consisting of ordered Pt₃Co intermetallic cores with a 2–3 atomic-layer-thick platinum shell now exhibit a large increase in mass activity and specific activity when compared with disordered alloy nanoparticles.

Various artificial cells that can store molecules in cages are designed to generate mechanical motion by dissipating energy through chemical reactions or through the reorganization of molecules. A hybrid biomimetic motor system consisting of a metal–organic framework and diphenylanaline peptides is now designed to release guest molecules in the isotropic direction via a bond-breaking framework.

The thermodynamic properties of magnetocaloric materials show significant promise for energy-efficient cooling applications. The demonstration that large and reversible magnetocaloric effects can be created by means of strain suggests a new approach for inducing them in other magnetic materials.

It is now shown that, unlike most semiconductors, plasmonic metal nanostructures constructively couple the energy of photons and thermal energy, with the reaction rate positively responding to both stimuli. These unique characteristics suggest that these photocatalysts could prove useful for heterogeneous catalytic processes that cannot be activated using conventional thermal processes on metals or photocatalytic processes on semiconductors.

Although the search for new zeolites has traditionally been based on trial-and-error approaches, more rational methods are now available. Using the principle of inverse sigma transformation, the reactivity of framework germanium atoms in strong mineral acid has now been exploited to selectively remove germanium from a germanosilicate zeolite.

Stacked lipid bilayers usually display smectic order. It is now found that multicomponent stacked bilayers can also exhibit columnar order, which arises from the coupling of interlayer smectic order and intralayer phase-separated domains, and propagates across hundreds of layers.