Letters in 2009

In conventional metals, strong interactions between electrons and the atomic lattice, so-called polarons, often render metals electrically insulating. In the rare case of polaronic metals, however, polaronic properties survive even in the metallic state. Neutron-scattering experiments now reveal that it is through quantum fluctuations that polarons manifest their presence in these enigmatic materials.

Biopatterning, which enables regulation of cell–material interaction, is usually achieved by techniques that rely on physical contact, which can seriously damage cells. A simple and efficient non-contact technique is now demonstrated using an aqueous two-phase polymer system.

Surfaces have an important role in solid–liquid phase transformations, but whereas melting is normally observed at surfaces, freezing usually originates in the bulk. Computational studies now predict surface-induced nucleation in supercooled liquid silicon and germanium, and the proposed nucleation mechanism could prove to be relevant for other tetrahedrally coordinated systems.

Fabricating defect-free three-dimensional photonic crystals over a large area is a challenge that has impeded advances in this field. The development of an etching process for creating such crystals from silicon may therefore allow a broader use of these photonic structures.

Luminescent materials are widely used for imaging and sensing because of their high sensitivity and rapid response. A strategy for modulating dual emission for radiometric sensing in a single component is now shown to enable tumour hypoxia imaging.

Many proteins have buried active sites in their folded states, which are only exposed when the protein is stretched. On mimicking this process with a combination of enzymes buried in polyelectrolyte layers on a silicone sheet, it is shown that enzymatic catalysis is possible only when the substrate is stretched to expose the enzymes, which enables reversible control of reaction progression.

The diffusion of atoms in a solid is essential to many of its properties. However, imaging atomic diffusion has been a difficult task. The development of a technique that allows direct time-resolved imaging of atomic diffusion with coherent X-rays may therefore allow a broader study of this process on the atomic scale.

An organic electronic device capable of precisely delivering neurotransmitters in vitro and in vivo is demonstrated. The device mimics the nerve synapse by converting electronic addressing in the delivery of neurotransmitters, thereby enabling exact dosage determination through electrochemical relationships. The system also ensures minimally disruptive delivery by avoiding fluid flow, and provides simple on–off switching.

Quantum confinement effects have an important role in photonic devices. However, rather than seeking perfect confinement of light, leaky-mode resonances are shown to be ideally suited for enhancing and spectrally engineering light absorption in nanoscale photonic structures.

Solar power is an important part of the strategy towards using more renewable energy. The development of low-cost photovoltaic nanopillar structures fabricated on thin aluminium substrates will contribute to this effort, as it promises new applications for flexible, mass-produced solar cells.

Hydrogels are hydrated polymer networks with applications in biotechnology and medicine. When created from alpha-helical peptides with engineered peptide sequences, their formation mechanisms can be controlled, leading to diverse properties. For instance, those with hydrogen-bonded networks melt on heating, but those formed through hydrophobic interactions strengthen when warmed.

The transport and mechanical properties of polymer electrolytes make them important materials for all-solid-state electrochemical devices such as batteries or electrochromic displays. Crystalline polymer electrolytes containing alkali metal salts are now found to exhibit ionic conductivity 1.5 orders of magnitude higher than the best conductor reported so far.

The electric control of magnetism in magnetic devices has remained problematic, particularly as energy losses due to current flow can be large. The demonstration of electric control of magnetization in a non-centrosymmetric insulating magnetic material therefore represents a new strategy for future applications.

Metamaterials allow the design of new functionality through the engineered control of light propagation, although broadband operation with these materials requires singularities in their refractive index. As a first example of a technique that uses a topological defect to achieve such behaviour in a real system, an omnidirectional metamaterial retroreflector is demonstrated.

Aberration-corrected microscopy can provide structural information with atomic precision. It is now shown that even single impurity atoms in a buried interface can be imaged, provided that a particular imaging mode is used. This result can lead to a much clearer understanding of advanced materials and devices that make use of the properties of interfaces.

‘Click’ chemistry has been broadly exploited, but the intrinsic toxicity of the reactions involved makes its translation to biological applications troublesome. Copper-free click chemistry avoids the problems of toxicity, enabling direct encapsulation of cells within click hydrogels. Tailoring of the gels with biological functionalities is also enabled in real time with micrometre-scale resolution.

A route connecting density functional theory and the numerical renormalization group method represents the first approach to studying atomic contacts—including magnetic elements—at an atomic level. When applied to the case of a nickel impurity in a gold nanowire, the strategy provides a clear connection between the geometry and the transport properties.

Designing and building molecular machines at the nanometre scale is a conceptual and synthetic challenge. Rotation of a single molecule has been observed but controlling the direction of the rotation has so far proved difficult. The step-by-step rotation of a molecular gear mounted on an atomic-scale axis is now controlled by a scanning tunnelling microscope.

Functionalizing colloidal particles with DNA is a powerful tool for guiding their assembly, using the complementary ‘sticky ends’ of the molecules. However, other attributes of DNA can be used to engineer interactions between particles more subtly. Temperature- or time-controlled formation of loops or hairpins in DNA provides switchable connections for novel materials from particle assemblies.

Superconductivity was recently observed in the binary iron-based compound, FeSe. It is now shown that under pressure, the transition temperature can rise above 36 K. In addition, no static magnetic ordering is observed for this system, contrary to FeAs superconductors.