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Stretchable conductors have many applications, from flexible electronics to medical implants; here polyurethane is filled with gold nanoparticles to give a composite with tunable viscoelastic properties arising from the dynamic self-organization of the nanoparticles under stress. Nanoparticle conductors with stretch Flexible electronics, neuroprosthetic and cardiostimulating implants, soft robotics and stretchable displays all require high stretchability and conductivity, properties that are difficult to combine. This paper reports polyurethane–gold nanoparticle composites that combine high conductivity and stretchability. Conventional stretchable conductors generally use nanotubes or nanowires as the conductive component, with high-aspect ratios, but these new materials achieve good performance despite the minimal aspect ratio of the nanoparticles. The properties of the composites derive from dynamic self-organization of the nanoparticles under stress, and they have the added advantage of electronically tunable viscoelastic properties. Research in stretchable conductors is fuelled by diverse technological needs. Flexible electronics, neuroprosthetic and cardiostimulating implants, soft robotics and other curvilinear systems require materials with high conductivity over a tensile strain of 100 per cent (refs 1 , 2 , 3 ). Furthermore, implantable devices or stretchable displays 4 need materials with conductivities a thousand times higher while retaining a strain of 100 per cent. However, the molecular mechanisms that operate during material deformation and stiffening make stretchability and conductivity fundamentally difficult properties to combine. The macroscale stretching of solids elongates chemical bonds, leading to the reduced overlap and delocalization of electronic orbitals 5 . This conductivity–stretchability dilemma can be exemplified by liquid metals, in which conduction pathways are retained on large deformation but weak interatomic bonds lead to compromised strength. The best-known stretchable conductors use polymer matrices containing percolated networks of high-aspect-ratio nanometre-scale tubes or nanowires to address this dilemma to some extent 6 , 7 , 8 , 9 , 10 , 11 . Further improvements have been achieved by using fillers (the conductive component) with increased aspect ratio, of all-metallic composition 12 , or with specific alignment (the way the fillers are arranged in the matrix) 13 , 14 . However, the synthesis and separation of high-aspect-ratio fillers is challenging, stiffness increases with the volume content of metallic filler, and anisotropy increases with alignment 15 . Pre-strained substrates 16 , 17 , buckled microwires 18 and three-dimensional microfluidic polymer networks 19 have also been explored. Here we demonstrate stretchable conductors of polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation. High conductivity and stretchability were observed in both composites despite the minimal aspect ratio of the nanoparticles. These materials also demonstrate the electronic tunability of mechanical properties, which arise from the dynamic self-organization of the nanoparticles under stress. A modified percolation theory incorporating the self-assembly behaviour of nanoparticles gave an excellent match with the experimental data.

Silver nanoparticles are susceptible to oxidation and have accordingly received less attention than gold nanoparticles; ultrastable silver nanoparticles are now reported, which can be produced in very large quantities as a single-sized molecular product, and the origins of their enhanced stability are elucidated using a single-crystal X-ray structure and first-principles calculations. Silver nanoparticles as good as gold Noble metals in nanoparticulate form find practical application as catalysts and in optoelectronics, energy conservation and many other fields. Gold nanoparticles, stable and easy to use, have proved much more useful and so have been studied more extensively than silver nanoparticles, which tend to be susceptible to oxidation. Anil Desireddy et al . describe a simple recipe for the large-scale production of single-sized silver nanoclusters, whose electronic structure gives them exceptional chemical stability. With the availability of stable silver nanoparticles, the metal's desirable electrical and physical properties, abundance and comparatively low cost could be harnessed in a wealth of new applications. Noble-metal nanoparticles have had a substantial impact across a diverse range of fields, including catalysis 1 , sensing 2 , photochemistry 3 , optoelectronics 4 , 5 , energy conversion 6 and medicine 7 . Although silver has very desirable physical properties, good relative abundance and low cost, gold nanoparticles have been widely favoured owing to their proved stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation (tarnishing), which has limited the development of important silver-based nanomaterials. Despite two decades of synthetic efforts, silver nanoparticles that are inert or have long-term stability remain unrealized. Here we report a simple synthetic protocol for producing ultrastable silver nanoparticles, yielding a single-sized molecular product in very large quantities with quantitative yield and without the need for size sorting. The stability, purity and yield are substantially better than those for other metal nanoparticles, including gold, owing to an effective stabilization mechanism. The particular size and stoichiometry of the product were found to be insensitive to variations in synthesis parameters. The chemical stability and structural, electronic and optical properties can be understood using first-principles electronic structure theory based on an experimental single-crystal X-ray structure. Although several structures have been determined for protected gold nanoclusters 8 , 9 , 10 , 11 , 12 , none has been reported so far for silver nanoparticles. The total structure of a thiolate-protected silver nanocluster reported here uncovers the unique structure of the silver thiolate protecting layer, consisting of Ag 2 S 5 capping structures. The outstanding stability of the nanoparticle is attributed to a closed-shell 18-electron configuration with a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, an ultrastable 32-silver-atom excavated-dodecahedral 13 core consisting of a hollow 12-silver-atom icosahedron encapsulated by a 20-silver-atom dodecahedron, and the choice of protective coordinating ligands. The straightforward synthesis of large quantities of pure molecular product promises to make this class of materials widely available for further research and technology development 14 , 15 , 16 , 17 , 18 .

Sodium under pressure It has recently been shown that, when high pressures are applied, crystals of lithium and sodium undergo a sequence of phase transitions — including (for sodium) a striking and as yet unexplained pressure-induced drop in the melting temperature. Jean-Yves Raty et al . have now identified the cause of this unusual melting behaviour: it emerges because liquid sodium undergoes a series of transitions similar to those seen in the solid state, but at much lower pressures. Intriguingly, one of these transitions is driven by the opening of a 'pseudogap' in the electronic density of states, the first time such an effect has been seen in a liquid metal. When high pressures are applied, crystals of lithium and sodium undergo a sequence of phase transitions, including a striking pressure-induced drop in the melting temperature. The cause of the unusual melting behaviour has now been identified: it emerges because liquid sodium undergoes a series of transitions similar to those seen in the solid state, but at much lower pressures. One of these transitions is driven by the opening of a 'pseudogap' in the electronic density of states. At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure 1 , 2 , 3 , 4 , 5 , 6 . It was predicted from theory 1 , 2 —and later confirmed by experiment 3 , 4 , 5 —that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve 6 have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states—an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well.

The search for electron spin qubits A point defect in diamond known as the nitrogen-vacancy (N-V) centre has generated a great deal of interest because it has a highly localized electronic spin state with quantum properties that can be easily accessed at room temperature. The search is on for similar defects in other semiconductors that are easier to grow and process into devices than diamond, or that offer alternative functionalities. Here Koehl et al . describe a new range of defect spin states in silicon carbide that can be optically addressed in the telecommunications wavelength range and coherently controlled up to room temperature. Their spin coherence properties are comparable to those of the diamond N-V centre, and silicon carbide is a material for which extensive microfabrication processes already exist in the semiconductor industry. These materials are therefore promising candidates for photonic, spintronic and quantum information applications. Electronic spins in semiconductors have been used extensively to explore the limits of external control over quantum mechanical phenomena 1 . A long-standing goal of this research has been to identify or develop robust quantum systems that can be easily manipulated, for future use in advanced information and communication technologies 2 . Recently, a point defect in diamond known as the nitrogen–vacancy centre has attracted a great deal of interest because it possesses an atomic-scale electronic spin state that can be used as an individually addressable, solid-state quantum bit (qubit), even at room temperature 3 . These exceptional quantum properties have motivated efforts to identify similar defects in other semiconductors, as they may offer an expanded range of functionality not available to the diamond nitrogen–vacancy centre 4 . Notably, several defects in silicon carbide (SiC) have been suggested as good candidates for exploration, owing to a combination of computational predictions and magnetic resonance data 4 , 5 , 6 , 7 , 8 , 9 , 10 . Here we demonstrate that several defect spin states in the 4H polytype of SiC (4H-SiC) can be optically addressed and coherently controlled in the time domain at temperatures ranging from 20 to 300 kelvin. Using optical and microwave techniques similar to those used with diamond nitrogen–vacancy qubits, we study the spin-1 ground state of each of four inequivalent forms of the neutral carbon–silicon divacancy, as well as a pair of defect spin states of unidentified origin. These defects are optically active near telecommunication wavelengths 11 , and are found in a host material for which there already exist industrial-scale crystal growth 12 and advanced microfabrication techniques 13 . In addition, they possess desirable spin coherence properties that are comparable to those of the diamond nitrogen–vacancy centre. This makes them promising candidates for various photonic, spintronic and quantum information applications that merge quantum degrees of freedom with classical electronic and optical technologies 2 , 14 , 15 , 16 , 17 .

A shining example of doping Many technological materials are intentionally 'doped' by the introduction of trace amounts of foreign elements to impart new and useful properties — a classic example is the doping of semiconductors. Feng Wang et al . describe a system in which lanthanide doping can be used to control the growth of NaYF 4 nanocrystals, making it possible to simultaneously tune the size, crystallographic phase and optical properties of the resulting materials. These findings increase our understanding of doping-induced structural transformations, and provide a straightforward route for the controlled synthesis of luminescent nanocrystals for many applications. Many technological materials are intentionally 'doped' with foreign elements to impart new and desirable properties, a classic example being the doping of semiconductors to tune their electronic behaviour. Here lanthanide doping is used to control the growth of nanocrystals, allowing for simultaneous tuning of the size, crystallographic phase and optical properties of the hybrid material. Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase 1 , modifying electronic properties 2 , 3 , 4 , modulating magnetism 5 as well as tuning emission properties 6 , 7 , 8 , 9 . Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF 4 nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion 10 , 11 , 12 emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF 4 upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels 12 to volumetric three-dimensional displays 13 .

The enhanced reversibility (stable transition temperature even at high strain under a solid-to-solid phase transition), low hysteresis and unusual riverine microstructure (ranging through thermal cycles) of the martensitic material Zn 45 Au 30 Cu 25 makes it attractive for applications from eco-friendly fridges to medical sensors. A boost for martensitic alloys Martensitic transformations are diffusionless, solid-to-solid phase transformations characterized by a change of crystal structure that can often be very useful. Applications include medical sensors, eco-friendly refrigerators and energy conversion devices. Repeated transformation cycles, however, can cause thermal hysteresis that modifies the material's properties and can cause permanent damage. Here Richard James and colleagues report the development of a martensitic alloy of zinc, gold and copper that maintains near-reproducible macroscopic properties despite drastic changes in its microstructure during each cycle. As well as providing a system that throws new light on the effects of hysteresis on reversible martensitic phase transformations, this work could help to extend applications for the materials in new areas — towards shape memory alloys for instance. Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators 1 , eco-friendly refrigerators 2 , 3 and energy conversion devices 4 . The ability to pass back and forth through the phase transformation many times without degradation of properties (termed ‘reversibility’) is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown 2 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling 6 , 9 , 12 , 15 . Recently, stronger compatibility conditions called the ‘cofactor conditions’ 5 , 15 have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn 45 Au 30 Cu 25 , that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles 9 , 16 . (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9 , 12 ). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.

The development of zeolite-like structures with extra-large pores (>12-membered rings, 12R) has been sporadic and is currently at 30R. In general, templating via molecules leads to crystalline frameworks, whereas the use of organized assemblies that permit much larger pores produces noncrystalline frameworks. Synthetic methods that generate crystallinity from both discrete templates and organized assemblies represent a viable design strategy for developing crystalline porous inorganic frameworks spanning the micro and meso regimes. We show that by integrating templating mechanisms for both zeolites and mesoporous silica in a single system, the channel size for gallium zincophosphites can be systematically tuned from 24R and 28R to 40R, 48R, 56R, 64R, and 72R. Although the materials have low thermal stability and retain their templating agents, single-activator doping of Mn 2+ can create white-light photoluminescence.

Graphene nanoribbons can exhibit either quasi-metallic or semiconducting behavior, depending on the atomic structure of their edges. Thus, it is important to control the morphology and crystallinity of these edges for practical purposes. We demonstrated an efficient edge-reconstruction process, at the atomic scale, for graphitic nanoribbons by Joule heating. During Joule heating and electron beam irradiation, carbon atoms are vaporized, and subsequently sharp edges and step-edge arrays are stabilized, mostly with either zigzag- or armchair-edge configurations. Model calculations show that the dominant annealing mechanisms involve point defect annealing and edge reconstruction.