Letters in 2011

The frictional properties of a two-dimensional colloidal crystal reveal that excitations known as kinks and antikinks form when the crystal is dragged along a solid surface. This phenomenon, which was predicted previously but never observed, demonstrates the potential of using colloidal crystals to study frictional properties that are otherwise difficult to characterize.

Magnons are quanta of spin-wave excitations and are likely to play a major role in the physical mechanisms of combining spin and heat transport. Now, a new device that enables the properties of magnons to be measured independently of the thermoelectric contribution of electrons and phonons is shown, providing crucial information for understanding the physics of electron–magnon interactions, magnon dynamics and thermal spin transport.

Colloidal particles adsorbed at liquid interfaces are commonly assumed to be at equilibrium, but holographic microscopy experiments now reveal that microspheres bound to a water/oil interface may take months to equilibrate. The observed ageing dynamics agree with a model of thermally activated hopping of the particle/interface contact line over nanoscale surface defects, and have implications for understanding the interactions between adsorbed colloidal particles.

Vesicles can rupture as a result of an imbalance in osmotic pressure between their inside and the exterior. Such an ‘osmotic shock’ has now been multiplexed in a coordinated fashion within an ordered material in which a minor component swells and ruptures, thus leading to a porous bicontinuous structure. Such perforated ordered materials may find applications in photonics, optoelectronics and nanofiltration.

A local atom probe has been used to study the transport properties of graphene, revealing the different effects of surface steps and changes in layer thickness on substrates. Understanding the details of the defect-induced degradation of transport properties is essential for improving the efficiency of devices.

Highly monodisperse silver polyhedral nanocrystals passivated with polymers are shown to behave as quasi-hard particles that self-assemble by sedimentation into millimetre-sized supercrystals, which correspond to the particles' three-dimensional densest packings. Monte Carlo simulations confirm the observed self-assembled structures, including an exotic structure for octahedra that is stabilized by depletion forces induced by an excess of polymer in solution.

One of the interesting features of graphene is that its properties change with the number of layers. A procedure to create monolithic devices with elements made out of different numbers of graphene layers is now shown, and a practical demonstration of this method is given by realizing transistor arrays with chemical-sensing functionalities.

Molecular hydrogen is expected to display metallic properties under high pressures, but so far experiments performed at low temperatures ( 100 K) have showed that hydrogen remains insulating up to 300 GPa. A transformation of normal molecular hydrogen to a conductive and metallic state at room temperature is now observed above 220 GPa.

Metamaterials are widely studied for their optical properties offering applications such as perfect lenses or cloaking. As is now shown, the interaction between the individual elements of metamaterials can also be used to design magnetoelastic metamaterials, which are able to change their structure in response to light.

The possibility of controlling magnetization by spin-polarized current could lead to devices more energy-efficient than traditional ones using external magnetic fields. Now, an even more efficient method has been demonstrated by using electric-field pulses to switch the magnetization in FeCo magnetic cells.

Polymer-based bulk-heterojunction solar cells have shown some of the highest photoconversion efficiencies in organic photovoltaics, but polymer polydispersity impacts their performance. A small-molecule donor is now reported that enables the fabrication of bulk-heterojunction devices with low acceptor content and photoconversion efficiencies of up to 6.7%.

Artificial materials that show negative refraction can be used for devices such as perfect lenses. The demonstration of negative refraction in nanostructured metal films, using a nonlinear optical effect—four-wave mixing—therefore opens new possibilities for optical devices.

It is shown that an elastic film on a viscoelastic substrate under biaxial compressive stress forms a hierarchical network of folds generated by repetitive wrinkle-to-fold transitions. The morphology of the hierarchical patterns can be controlled by modifying the geometry and boundary conditions of the membrane.

Oxide nanoprecipitates with typical sizes of smaller than five nanometres have been known to considerably enhance the mechanical properties of steel. An atomic-scale characterization is now able to directly verify the crystal structure of these stable oxide nanoclusters.

Different mechanistic processes explaining the catalytic activities of supported gold catalysts have been proposed. Au–Pd colloidal nanoclusters are now shown to exhibit high catalytic activity owing to an abundance of negatively charged Au atoms on the surface.

Semiconductor nanocrystals have for many years attracted attention for their optical properties and their potential use as superior fluorescence emitters. It is now shown that nanoplatelets can be controllably synthesized and have even more attractive properties.

Secondary batteries using organic electrode-active materials promise to surpass present lithium-ion batteries in terms of safety and price. Organic molecules with degenerate molecular orbitals as electrode-active materials are now used in high-capacity organic batteries exceeding 300 A h kg⁻¹.

Conjugated polymers are applied widely in organic optoelectronic devices. The performance of these devices depends critically on polymer morphology, which can be modified by solvent vapour annealing. This process has now been controlled on mesoscopic length scales, bridging the gap between single-molecule and bulk studies, and revealing long-range energy transport in ordered polymer aggregates.

The electrical control of magnetic properties is a key requirement for the development of spintronic devices. The demonstration that the ferromagnetic phase transition in cobalt can be changed by applying an electric field at room temperature represents a significant step towards devices that can switch magnetism on and off electrically.

Monodisperse octapod-shaped inorganic nanocrystals suspended in suitable solvents are shown to self-assemble into chains of interlocked octapods, which in turn aggregate to form three-dimensional crystals. Such hierarchical self-assembly is supported by a simulation model of the octapods, which shows that the favourable interlocked configuration is encoded in the octapod’s shape.