Volume 13 Issue 8, August 2014

From its earliest days, crystallography has been viewed as a means to probe order in matter. J. D. Bernal's work on the structure of water reframed it as a means of examining the extent to which matter can be regarded as orderly.

Over the course of its long history, powder diffraction has provided countless insights into the properties of materials. It will continue to do so in the future, but with an emphasis on elucidating how materials respond to external stimuli.

Collective quantum phenomena such as magnetism, superfluidity and superconductivity have been pre-eminent themes of condensed-matter physics in the past century. Neutron scattering has provided unique insights into the microscopic origin of these phenomena.

Neutron science has been a remarkable success story for European research. For this to continue, scientists need to be prepared to forge new networks and technologies.

Standing spin-waves can be excited in artificial chains of magnetic atoms using inelastic electron tunnelling spectroscopy, thereby offering a route to speed up the switching of their magnetization.

Mastering the impact of surface chemistry on the electronic properties and stability of colloidal quantum dots enables the realization of architectures with enhanced photovoltaic performance and air stability.

Although promising, the use of organic semiconductors has not yet revolutionized consumer electronics. Synthesis of high-performance materials, enhanced control of morphology and smart exploitation of unique photophysical phenomena are the way forward to overcome the technological hurdles of this field.

The structural similarity of organic semiconductors to biological compounds suggests the use of these materials in biomedical applications, yet their implementation is not straightforward. Research in this area is growing fast, thanks to the combined efforts of the multidisciplinary bioelectronics community.

Transitions between stable quantum phases of matter typically involve going through an unstable quantum critical point, the unique properties of which have become a focus of research in the past decade or so. Extensive bulk measurements on the nickel oxypnictide system CeNiAsO uncover heavy-fermion behaviour, suggesting the family of oxipnictides may be ideal materials for examining quantum criticality more broadly.

The excitations that determine the low-temperature properties of ferromagnetic materials are called spin waves. Using a combination of inelastic electron tunnelling spectroscopy and numerical simulations, the spin waves occurring in a one-dimensional chain of iron atoms deposited on Cu₂N are now imaged, and their dynamics examined.

Trilayer graphene can be realized in two different stacking configurations, known as rhombohedral and Bernal stackings, which display different electronic characteristics. It is now shown that an applied perpendicular electric field can be used to switch between these two configurations.

The combination of photonic and spintronic devices offers significant promise for optoelectronic applications. In analogy to a photovoltaic cell, an optoelectronic device that spatially separates electrons with opposite spin orientations on absorption of circularly polarized light is now demonstrated.

Fabricating low-temperature solution-processed solar cells with good power-conversion efficiency and stability in ambient conditions has proved challenging. The use of ligands that protect colloidal quantum dots from degradation in air and tune their energy levels is now shown to be a viable approach for the realization of spin-coated solar cells with very high efficiency.

Palladium is of practical use as a hydrogen-storage metal and an effective catalyst for reactions related to hydrogen in a variety of industrial processes. Enhanced capacity and speed of hydrogen storage is now reported in Pd nanocrystals covered with a metal–organic framework.

Self-assembled nanoparticle superlattices, which consist of inorganic cores capped by organic ligands, can show emergent behaviour as a result of the coupling between their nanoscale components. The atom-level structure of a silver nanoparticle superlattice, deduced from X-ray imaging and simulations, is now reported as well as its response to hydrostatic compression, which involves anomalous pressure softening and correlated chiral rotation of the nanoparticles.

Variations in the internal conformational dynamics of supramolecular nanostructures may be important for their function, yet such dynamics have been difficult to probe experimentally. Now, the molecular motion through a nanofibre cross-section at subnanometre resolution has been quantified using site-directed spin labelling and electron paramagnetic resonance spectroscopy.

Liquid-crystalline elastomers combine rubber-like elasticity with the optical properties of liquid crystals, yet some of their properties depend on the particular liquid-crystal phase. Now, stretchable gels of the liquid-crystalline blue-phase I are reported. The blue-phase gels are electro-optically switchable under a moderate applied voltage, and their optical properties can be manipulated by an applied strain.

Although several techniques have been reported to obtain electron-rich colloidal quantum dots, these materials usually suffer from poor stability under air exposure. It is now shown that the use of strongly bound ligands and a careful ligands-exchange strategy lead to air-stable n-type quantum dots that can be used in solar cells and chemical sensors.

Peptide-based nanofibres with bioactive proteins attached can now be made such that the protein ligands are introduced in a controlled manner. This tailoring of the nanofibre’s composition enables the ratio of multiple different proteins to be highly tuned within the assemblies. By changing the protein content of the nanofibres, it is possible to adjust the antibody responses in mice to the different nanofibres.

Crystallographic techniques underpin many areas of materials science. To celebrate the International Year of Crystallography 2014, this focus issue highlights a selection of topics that demonstrates the depth and importance of this wide-ranging field.