Letters in 2009

Coherent X-ray diffraction spectroscopy has recently emerged as a powerful tool for imaging strain at the nanoscale. Developments in both fabrication and experimental techniques have now enabled all nine components of the strain tensor in a nanorod to be determined, demonstrating the ability of coherent X-ray diffraction spectroscopy to yield measurements of strain in three dimensions with a resolution of a few tens of nanometres.

One of the more promising uses of metamaterials is in imaging, where the capability to control the propagation of light could lead to new applications. In particular, the realization of a broadband metamaterial lens that has an almost complete hemispherical field of view that is focused on a flat plane represents a significant step towards such new uses.

The existence of topological conducting surfaces on insulators has been demonstrated by angular photoemission spectroscopy, but the number of transport experiments on these systems have so far been scarce. Transport evidence of topological surface states is now shown in Bi₂Se₃ nanoribbons through the observation of Aharonov–Bohm oscillations.

Surface plasmon polaritons allow the control of light on a scale much smaller than its wavelength, and thus are important for nanophotonic applications. The demonstration of an electrical source of surface plasmon polaritons compatible with silicon electronics takes a step towards such integrated plasmonic circuits.

Dye-sensitized solar cells are a promising technology for sustainable energy generation. Most dyes in these types of solar cell act as sensitizers for injecting electrons into n-type semiconductors. But the development of a sensitizer that can efficiently inject holes into p-type semiconductors makes possible the realization of tandem cells that could exploit the two approaches together.

Occasionally, organic crystalline materials contract when heated (negative thermal expansion), and the mechanisms responsible for this phenomenon are poorly understood. The arrangement of dumbbell-shaped molecules in an organic material is shown to give rise to its negative thermal expansion. The packing and intermolecular interactions facilitate a cooperative mechanical response to temperature causing a decrease in lattice dimensions.

Using a liquid gate has allowed electrically induced superconductivity in a solid specimen by means of carrier accumulation on the surface. But this phenomenon was limited to materials that became superconductors at low carrier density. It is now shown that superconductivity can be induced in a much wider range of materials by using an ionic liquid.

Liquid-crystal gel networks of neurofilament assemblies play a key part in the mechanical stability of neuronal processes, and disruptions in the networks are a hallmark of motor-neuron diseases. Under pressure, these networks are shown to undergo an abrupt transition from expanded to condensed states, with distinct mechanical properties, helping to explain possible disruption mechanisms.

According to Fourier theory, thermal transport is a diffusive process. However, this cannot be the case at length scales smaller than the mean free path of the energy carriers. The first experimental study of thermal transport at the nanoscale is now reported in the case of a point-like heat source, providing a quantitative description of the transition between the ballistic and diffusive regimes.

Heat-responsive polymers grafted onto gold nanocages serve as a nanoscale delivery system for biologically important compounds. Laser irradiation of the nanocages heats the polymers by means of the photothermal effect; the polymers then change conformation and compounds are released. The polymers return to their original configuration when the laser is switched off, stopping further release.

Like their optical counterparts, acoustic metamaterials are capable of manipulating sound waves in unusual ways. An acoustic hyperlens is now demonstrated that is capable of magnifying subwavelength acoustic waves, and could therefore find applications in medical imaging or underwater sonar.

Conventional electroanalytical and structure-analysis techniques provide limited information about ionic fluxes in electrochemical systems. A quartz crystal microbalance is now used as a gravimetric probe of the concentration and compositional changes in microporous activated carbon.

Plasmonic biosensors are either based on freely propagating surface plasmons or plasmons localized at nanostructures. Despite advantages such as quantitative detection, localized surface-plasmon sensors have shown lower overall sensitivities. A nanorod metamaterial supporting new plasmonic modes is now shown to considerably outperform earlier plasmonic biosensors by combining and expanding their respective advantages.

The mechanical properties and corrosion behaviour of glassy metals are attractive for biodegradable implants. Magnesium-based glasses are particularly promising but they suffer from hydrogen evolution during corrosion. A distinct reduction in hydrogen evolution is now observed in zinc-rich magnesium glasses showing good tissue compatibility.

A new type of scanning electron microscope with aberration correction allows a resolution of 0.1 nm. The instrument also allows for simultaneous imaging of atoms on the surface and in the bulk of a sample, which represents a real breakthrough in the field.

Although much effort has been directed towards the separation of single-walled carbon nanotube mixtures, chiral-selective growth is required for scalable production and applications. The chiral distribution of carbon nanotubes can now be altered by varying the composition of nickel–iron nanocatalysts.

The magnetic-field-induced strain in magnetic shape-memory alloys can be used in several types of application. However, the strain is high (10%) only in single-crystalline specimens, which are difficult and expensive to obtain. Polycrystalline samples with comparable strain have now been fabricated by introducing pores of similar size to the grains.

When a tip slides on a carbon nanotube, the friction along the transverse direction is much larger than in the parallel direction. It is shown that this behaviour is due to hindered rolling of the tube, and a frictional dissipation that is negligible for a tip sliding along the axis.

High pressures have been very useful in stabilizing materials that cannot form at low pressure. However, often high-pressure phases are not stable when recovered in ambient conditions. Bombardment with high-energy heavy ions is now shown to stabilize a high-pressure phase of Gd₂Zr₂O₇ in ambient conditions.

Quantum cascade lasers are only one of several applications that could take advantage of the discrete nature of the energy levels in semiconductor quantum dots. It is now shown that the relaxation time between levels is highly sensitive to their energy separation. This knowledge will be essential for the design and optimization of actual devices.