Letters in 2013

The conversion efficiency of heat to electricity in thermoelectric materials depends on both their thermopower and electrical conductivity. It is now reported that, unlike their inorganic counterparts, organic thermoelectric materials show an improvement in both these parameters when the volume of dopant elements is minimized; furthermore, a high conversion efficiency is achieved in PEDOT:PSS blends.

The conversion of a spin current into an electric signal is known as the inverse spin Hall effect, and is expected to enable the full potential of spintronic devices to be realized. Although the effect has been extensively studied in inorganic metals and semiconductors, it is now shown also to occur in a solution-processed organic polymer placed in proximity to a magnetic insulator.

The development of metallic glasses is hindered by the lack of mechanistic understanding of why some alloys crystallize quickly and thus are poorer at forming glasses than those that crystallize slowly. A molecular dynamics study of the growth rate of a planar crystal surface in model metallic glasses now shows that their glass-forming ability is determined by the structure of the crystal/liquid interface.

Spillover of reactants from one active site to another is important in heterogeneous catalysis but is notoriously hard to detect or control, especially for hydrogen. The hydrogen spillover pathway on a Pd–Cu alloy can now be controlled by reversible adsorption of a spectator molecule. This effect observed during a surface catalysed reaction should prove useful for controlling uptake and release of hydrogen in a model storage system.

The silver chalcogenide semimetals are known for their appealing magnetoresistive properties. It is now shown that when copper silver selenide is doped with nickel, these properties are maintained, resulting in high electron mobilities and, in turn, a significant thermoelectric effect.

Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions. The kinetics of charge storage in T-Nb₂O₅ electrodes is now quantified and the mechanism of lithium intercalation pseudocapacitance should prove to be important in obtaining high-rate charge-storage devices.

Designing synthetic surfaces whose properties dynamically adapt in response to mechanical stimuli is challenging. Now, liquid-infused nanoporous elastic substrates that respond to stretching by continuously changing their transparency and wettability—a consequence of smooth variations in surface roughening as the liquid flows inside the pores—are demonstrated.

The optical and electronic performance of inorganic nanocrystal assemblies stabilized by organic ligands has been extensively investigated, whereas less attention has been paid to their thermal transport properties. It is now shown that the thermal conductivity of these composite systems is determined by the vibrational states of both inorganic and ligand regions, as well as by their relative volumes.

The dynamics of thin magnetic films revealed by ultrafast laser techniques cannot be explained by standard equilibrium descriptions. Diffraction experiments using an X-ray laser now allow the spin dynamics of the separate magnetic constituents of ferromagnetic GdFeCo alloys to be spatially resolved.

Despite recent progress in the production of bendable thin-film transistors, their development is limited by leakage currents and fragile inorganic oxides. Combining graphene and single-walled carbon nanotube electrodes with a geometrically wrinkled inorganic layer, highly stretchable and transparent field-effect transistors have now been demonstrated.

Artificially grown superlattices consisting of iron pnictide materials offer a strategy for tailoring their superconducting properties. The fabrication of compositionally modulated oxygen- and cobalt-doped BaFe₂As₂ heterostructures yields vertically aligned defects that introduce strong vortex pinning sites and enhance the materials’ critical current density.

Measuring and characterizing dynamic charge density waves in cuprate superconductors is a challenging task. By using a method based on ultrafast spectroscopy, the problem is overcome and detecting the presence and lifetimes of these fluctuations is made possible.

The tunnelling electroresistance effect that occurs at ferroelectric tunnel junctions could form the basis for a class of potential memory applications. Now, an enhanced effect is observed in a complex oxide interface as a result of a ferroelectrically induced phase transition.

The fabrication of microchips with vertically stacked circuits is challenging because they require arrays of electrical interconnections between the circuits, where accessibility is limited. An approach to generate conductive, mechanically stable plug-and-socket interconnections through three-dimensional actin-filament self-organization and selective metallization offers a potential solution to this problem.

The control and manipulation of domain walls in perpendicularly magnetized nanowires by means of an electric current has gained attention for possible device applications. Now, the depinning of domain walls in Pt/Co/Pt nanowires is shown to be driven by the spin Hall effect.

The crystallization of many minerals from solution has been shown to involve disordered precursors that agglomerate into an amorphous intermediate phase, a pathway that seems to contradict classical nucleation theory. It is now found that the crystallization of magnetite—a magnetic iron oxide with many bio- and nanotechnological applications—occurs classically from the accretion of precursors in the absence of amorphous intermediates.

How the shape of jammed particle packings influences their mechanical response is unknown except for specific cases. An algorithm that mutates the shapes of packings of bonded identical spheres to optimize the packing’s mechanical performance, and the experimental testing of the optimized shapes through three-dimensional printing, are now reported.

Metallic and ceramic surfaces can be rendered hydrophobic through a combination of multiscale surface structures and polymeric modifiers, but the imparted hydrophobicity is not robust to harsh environments. It is now shown that the lanthanide oxide series—a class of ceramics—is intrinsically hydrophobic as a result of their unique electronic structure, even after exposure to high temperatures and abrasive wear.

The expansion of a material in one or more directions under increasing hydrostatic pressure is a phenomenon known as negative linear compressibility. The demonstration that zinc dicyanoaurate exhibits an unusually large negative linear compressibility opens up possibilities for designing other materials with comparable properties.

It is shown that by controlling the relaxation of graphene adhered on a biaxially pre-stretched polymer substrate, graphene films can be reversibly crumpled and unfolded to form tailored hierarchical structures with tunable wettability and transmittance, and that the crumpled graphene–polymer laminates can be used as actuators.