Letters in 2011

Typically, the light-emission of semiconductors always occurs from thermalized electrons, as electrons excited above the bandgap energy relax quickly. In contrast, non-thermalized excitonic light emission has now been observed in nanowires using resonant plasmonic nanocavities. The much higher radiative light-emission rates of the hot excitons suggest their use for ultrafast nanophotonic devices.

The in vivo optical detection of bacterial infections requires highly specific imaging probes with small affinity to mammalian tissue. It is now shown that fluorescent dyes that are conjugated to maltohexaose can be internalized rapidly via the bacteria-specific maltodextrin transport pathway, enabling the in vivo imaging of Escherichia coli down to 10⁵ colony-forming units.

The low-temperature solution growth of ZnO nanostructures could enable the bottom-up fabrication of integrated electronic devices, but controlling their morphology has been challenging. It is now shown that the geometry of hydrothermally synthesised ZnO nanowires can be tuned precisely if the growth of selected crystal faces is inhibited by the competitive adsorption of non-zinc ions.

Spin current, that is, the flow of angular momentum without charge transfer, may be used in efficient spintronics devices. One problem is that spin current tends to decrease, owing to spin–orbit interaction. It is now shown that through interaction with spin waves it is possible to reverse this effect and enhance the spin current back.

Spin injection from a magnetic electrode into the non-magnetic active element of a spintronics device is seriously hampered by the impedance mismatch between the two materials. One common solution is to use high-quality tunnel barriers. An alternative strategy is now demonstrated through spin pumping based on dynamical spin exchange.

The electronic properties of inorganic devices such as memristors can be used to simulate neurological behaviour. In particular, ionic and electronic effects in a silver sulphide device are now shown to mimic short- and long-term synaptic functions.

In macroscopic spin valves, the current between two magnetic electrodes can be tuned by external magnetic fields. Here, a molecular-scale spin valve is demonstrated in which a single-molecule magnet, through its localized magnetic moment, modulates the conductance of a single-walled carbon nanotube quantum dot with magnetoresistance ratios reaching 300%.

Polymer electrolyte membranes selectively transport ions and polar molecules, and are of interest for applications such as polymeric batteries, fuel cells, mechanical actuators and water purification. Transport anisotropy is now shown to linearly depend on the degree of alignment, indicating that membrane stretching only causes domain reorientation without affecting channel dimensions or defect structure.

Nanowires have many applications across a number of disciplines. So far, their length has been largely limited to mesoscale dimensions. Through the adaption of an iterative fibre-drawing process it is now possible to fabricate millions of ordered nanowires and nanotubes of almost infinite length.

Although X-ray tomography has proven to be an efficient tool for three-dimensional imaging, its application to light materials has not been too successful. A new X-ray spectroscopy tomography method has now been developed that allows the mapping of chemical bonding in various types of structures, as well as the imaging of soft materials in three dimensions.

Materials with zero refractive index show unusual waveguiding properties and, for example, can squeeze light through narrow passages. It is now suggested that such properties can also be realized in a non-metallic photonic crystal. Furthermore, such photonic crystals can also show a Dirac point in the band structure—offering further possibilities, such as guiding waves unperturbed around bends and obstacles.

Although magnetic domain walls could one day be used for information storage, the current challenges to their use are the irreproducibility of their displacement and the limits to their maximum speed. It is now shown that the Rashba effect can be used to provide a solution to both these issues.

Organic materials are rarely considered for thermoelectric applications, because their low electrical conductivity limits the thermoelectric figure of merit (ZT). It is now shown that by optimizing the oxidation level in a polymer, ZT can reach 0.25, which approaches the values desirable for devices.

The production of fuels from sunlight is crucial to the development of a sustainable energy system. Although noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for large-scale use. Bioinspired molecular clusters based on molybdenum and sulphur are now shown to produce hydrogen at rates comparable to platinum.

Fourier-transform infrared (FTIR) spectroscopy is a widely used spectroscopic technique, particularly for infrared wavelengths. However, for imaging applications the spatial resolution of FTIR spectrometers is restricted by the diffraction limit. The use of an FTIR spectrometer to pick up the low signal from scanning near-field optical microscopy employing thermal radiation now enables infrared imaging with nanoscale resolution.

Chemical vapour deposition is one of the most promising strategies to grow high-quality graphene sheets on a large scale. It is now shown that this deposition technique can be extended to grow three-dimensional graphene networks with high conductivity and flexibility—both promising features for flexible electronics.

Plasmonic resonances are often associated with metals, but can also be realized in semiconductors. The observation of plasmon resonances at near-infrared wavelengths in semiconductor quantum dots in particular, offers the possibility to actively control plasmonic properties through quantum-size effects within the dots.

The controlled formation of micrometre-size drops is of importance for many technological applications such as microfluidics. A wetting-based destabilization mechanism of forced microfilaments on either hydrophilic or hydrophobic stripes leading to the periodic emission of droplets can now be used to control independently the drop size and emission period.

Microporous organic polymers (MOPs) are technologically important for low-dielectric materials, gas separation and gas-storage applications. A class of amorphous MOPs prepared by cycloaddition modification is shown to exhibit outstanding CO₂ separation performance and super-permeable characteristics

Bottom gates in epitaxial graphene structures can now be fabricated through a technique based on nitrogen implantation. This is an important achievement to increase both the versatility of the material for fundamental studies and the potential for its use in devices.