Volume 12 Issue 9, September 2013

The challenge to link understanding and manipulation at the microscale to functional behaviour at the macroscale defines the frontiers of mesoscale science.

Semiconductor devices that convert light of arbitrary polarization into a flow of electron spin have now been demonstrated using an approach that is applicable to any semiconductor material.

Photoexcited diamond can inject highly energetic electrons in solution and promote the catalysis of a broad range of chemical reactions.

The search for materials with colossal permittivity for use in capacitors has been met with limited success. A newly discovered co-doped titanium oxide material has an extremely high permittivity and negligible dielectric losses, and is likely to enable further scaling in electronic and energy-storage devices.

Cells at the edges of migrating epithelial sheets pull themselves towards unfilled space regardless of their direction of motion.

Quantum wells based on mercury telluride are an experimental realization of a two-dimensional topological insulator. By using a scanning superconducting quantum interference device (SQUID) technique, the magnetic fields flowing through HgTe/CdTe heterostructures are imaged both in the quantum spin Hall and the trivial regimes, revealing the edge states associated with the quantum spin Hall state.

The epitaxial growth of large-area single-domain graphene on hexagonal boron nitride by plasma-assisted deposition is now reported. New sets of Dirac points are produced as a result of a trigonal superlattice potential, while Dirac fermion physics near the original Dirac point remain unperturbed. This growth approach could enable band engineering in graphene through epitaxy on different substrates.

A convincing explanation of why mixed phases of anatase and rutile TiO₂ outperform individual polymorphs is lacking. An energetic band alignment of ~0.4 eV is now shown to exist between the two phases with anatase possessing the higher electron affinity. This observation explains the separation of photoexcited charge carriers between phases and could lead to improved photocatalysts.

Making colloidal nanoparticles with controlled composition and shape is challenging because at the nanoscale surface energy favours highly symmetric structures. Now, a fast, wafer-scale fabrication scheme that combines low-temperature shadow deposition with nanoscale patterning has been developed that produces anisotropic hybrid nanocolloids with designed composition and feature sizes down to 20 nm.

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.

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.

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.

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.

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.

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.

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.

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.