Articles in 2013

A strategy for assessing blood microcirculation and tissue hydration relies on monitoring the temperature and thermal conductivity of skin, respectively. It is now shown that arrays of micrometre-sized sensors and heaters can be integrated on stretchable substrates that conformably adhere to the skin; these devices allow spatially resolved heating and real-time temperature mapping in patients without limiting their motion.

Although rechargeable lithium–air batteries are receiving significant attention because of their high theoretical specific energy, carbon cathodes that are currently used decompose during oxidation and promote electrolyte decomposition on cycling. A titanium carbide-based cathode is now shown to reduce side-reactions, and exhibits enhanced reversible formation and decomposition of Li₂O₂.

Low-temperature redox reactions in solids resulting in no thermomechanical degradation can be used to enhance the performance and lifetime of energy devices. Rapid and reversible redox activity has now been demonstrated at temperatures as low as 200 °C in both epitaxially stabilized oxygen-vacancy-ordered SrCoO_2.5 and thermodynamically unfavourable perovskite SrCoO_3−δ single-crystalline thin films.

Although quantitative understanding of nanocrystal phase transformations is important for efficient energy conversion and catalysis, difficulties in directly monitoring nanoscale systems in reactive environments remain. Direct quantification of hydriding transformations in palladium nanocrystals now clearly reveals that the transformation rates are governed by nanocrystal dimensions.

The recent demonstration that highly disordered polymer films can transport charges as effectively as polycrystalline semiconductors has called into question the relationship between structural order and mobility in organic materials. It is now shown that, in high-molecular-weight polymers, efficient charge transport is allowed due to a network of interconnected aggregates that are characterized by short-range order.

Polyampholyte hydrogels synthesized from the random polymerization of oppositely charged ionic monomers are shown to be mechanically tough and highly viscoelastic. Strong ionic bonds within the gel act as permanent crosslinks and weaker ionic bonds reversibly break and re-form, enhancing the fracture resistance, shock absorbance and self-healing properties of the materials.

The relative displacement of conducting molecules influences their electronic coupling and therefore the charge-transport properties of organic thin films. Electron diffraction patterns now reveal the dominant lattice vibrational modes in organic semiconductors with subnanometre precision and help predict the electronic behaviour of these materials.

The catalytic activity of highly dispersed platinum nanoparticles is not yet well understood. Now, a unique approach that allows precise control of both the size and coverage of platinum nanoclusters reveals that particle proximity influences the oxygen reduction rate of these size-selected clusters, especially in terms of mass normalized activity.

Contact-angle and spectroscopy experiments on clean supported graphene and graphite show that these surfaces become more hydrophobic as they adsorb airborne hydrocarbons. Furthermore, the water contact angle on these graphitic surfaces decreases if these contaminants are partially removed by both thermal annealing and controlled ultraviolet–ozone treatments, suggesting that graphitic surfaces are more hydrophilic than previously believed.

Heat is a form of energy that is transported from a hot to a cold region, but it is not a notion that is associated with the microscopic measurement of electronic properties. It is now shown that local thermoelectric measurements can be used for imaging structural disorder in graphene, with high sensitivity, on the atomic and nanometre scales, uncovering soliton-like domain-wall line-patterns separating different graphene regions.

Electronic devices usually rely on the charge or spin of electrons to encode information. A less exploited route is to manipulate the valley quantum number of electrons. It is now shown that the generation, macroscopic transport and detection of valley-polarized electrons in bulk diamond can be achieved with a relaxation time of 300 ns at 77 K, forming a basis for valleytronic devices.

Cathodes for Li-ion batteries operate mainly via an insertion–deinsertion redox process involving cationic species but this mechanism does not account for the high capacities displayed by Li-rich layered oxides. The reactivity of high-capacity Li₂Ru_1−ySn_yO₃ materials is now shown to be associated with a reversible redox process related to a reductive coupling mechanism.

Efficient evolution of hydrogen via electrocatalysis at low overpotentials is promising for clean energy production. Monolayered nanosheets of chemically exfoliated WS₂ are shown to be efficient catalysts for hydrogen evolution at very low overpotentials. The enhanced catalytic performance is associated with the high concentration of the strained metallic octahedral phase in the exfoliated nanosheets.

Assessing the effect of nanometre-scale structure on charge transport across micrometre-scale distances remains a fundamental challenge for many energy-conversion technologies. By correlating the structure and the charge transport with nanometre resolution across micrometre-scale distances, nanoparticle aggregates responsible for the high photoelectrochemical water-splitting activity of α-Fe₂O₃ electrodes are identified.

Solid-state catalysts do not participate efficiently in the reduction of N₂ to NH₃ because they tend not to form strong bonds with nitrogen molecules. It is now shown that, under ultraviolet radiation, hydrogen-terminated diamond can eject electrons directly in a liquid solution, thus allowing nitrogen reduction without requiring its preliminary adsorption on a solid surface.

Materials displaying colossal permittivity are promising for a range of energy-storage and microelectronics applications. A strategy for achieving temperature- and frequency-independent colossal permittivity using defect-generated giant dipoles is now demonstrated in (Nb+In) co-doped TiO₂ rutile.

Field-effect transistors based on molybdenum disulphide have latterly garnered significant interest. Their electrical transport characteristics are now studied for different dielectric configurations, and as a function of temperature.

Although the collective cellular motion involved in, for example, wound healing and tumour invasion is suspected to be driven by mechanical stresses within the advancing cell monolayer, how motion and stress relate has remained elusive. Now, stress-microscopy observations of an epithelial cell sheet advancing towards a region where cells cannot adhere reveal that the cells located nearby such a region exert forces that pull them towards the unfilled space, regardless of whether the cells approach or recede from it.

Although site-dependent metal surface segregation in bimetallic nanoalloys affects catalytic activity and stability, segregation on shaped nanocatalysts and their atomic-scale evolution is largely unexplored. Pt_xNi_1−x alloy nanoparticle electrocatalysts with unique activity for oxygen reduction reactions exhibit an unexpected compositional segregation structure across the {111} facets.

Domain walls forming within magnetic nanowires offer a valuable degree of freedom with which to explore possible future information storage and processing architectures. By taking advantage of the piezoelectric characteristics of perpendicularly magnetized GaMnAsP/GaAs nanowires, large variations in the current-induced domain wall mobilities are obtained.