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The phonon is the physical particle responsible for the transmission of sound and heat; controlling the properties of phonons in materials could trigger many advances, which are reviewed here. Prepare for the age of phononics In the emerging research area of phononics, control over the mechanical vibrations that transmit sound and heat — phonons — plays a central role. Like photons and electrons, phonons can be treated as particles for many purposes, so can be harnessed and manipulated for useful applications. The phonon spectrum covers a wide range of effects, from low- frequency acoustics, to ultrasound and to heat, so that phononic techniques could enable a wide range of applications such as in earth quake protection, acoustics and heat management. In this review, Martin Maldovan discusses several approaches to the control of phonons at different length scales, for example phononic crystals, metamaterials, thermoelectrics and optomechanical devices. Today's digital revolution is underpinned by the high degree of control that can be imposed over electrons in semiconductors; Maldovan argues that precise control over phonons could have similar surprising and exciting consequences. The phonon is the physical particle representing mechanical vibration and is responsible for the transmission of everyday sound and heat. Understanding and controlling the phononic properties of materials provides opportunities to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection. Here I review recent progress and the development of new ideas and devices that make use of phononic properties to control both sound and heat. Advances in sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics, and thermocrystals herald the next technological revolution in phononics.

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.

Graphene in its pristine form is one of the strongest materials tested, but defects influence its strength. Using atomistic calculations, we find that, counter to standard reasoning, graphene sheets with large-angle tilt boundaries that have a high density of defects are as strong as the pristine material and, unexpectedly, are much stronger than those with low-angle boundaries having fewer defects. We show that this trend is not explained by continuum fracture models but can be understood by considering the critical bonds in the strained seven-membered carbon rings that lead to failure; the large-angle boundaries are stronger because they are able to better accommodate these strained rings. Our results provide guidelines for designing growth methods to obtain sheets with strengths close to that of pristine graphene.

Fabrication techniques developed for graphene research allow the disassembly of many layered crystals (so-called van der Waals materials) into individual atomic planes and their reassembly into designer heterostructures, which reveal new properties and phenomena. Graphene research and beyond Andre Geim and Irina Grigorieva offer a forward-looking review of the potential of layering two-dimensional materials into novel heterostructures held together by weak van der Waals interactions. Dozens of these one-atom- or one-molecule-thick crystals are known. Graphene has already been well studied but others, such as monolayers of hexagonal boron nitride, MoS 2 , WSe 2 , graphane, fluorographene, mica and silicene are attracting increasing interest. There are many other monolayers yet to be examined of course, and the possibility of combining graphene with other crystals adds even further options, offering exciting new opportunities for scientific exploration and technological innovation. Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as ‘van der Waals’) have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

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 .

The control of heat conduction through the manipulation of phonons as coherent waves in solids is of fundamental interest and could also be exploited in applications, but coherent heat conduction has not been experimentally confirmed. We report the experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods. The measured thermal conductivity increased linearly with increasing total superlattice thickness over a temperature range from 30 to 150 kelvin, which is consistent with a coherent phonon heat conduction process. First-principles and Green's function-based simulations further support this coherent transport model. Accessing the coherent heat conduction regime opens a new venue for phonon engineering for an array of applications.

The reported thermal conductivity (κ) of suspended graphene, 3000 to 5000 watts per meter per kelvin, exceeds that of diamond and graphite. Thus, graphene can be useful in solving heat dissipation problems such as those in nanoelectronics. However, contact with a substrate could affect the thermal transport properties of graphene. Here, we show experimentally that κ of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper. It is lower than that of suspended graphene because of phonons leaking across the graphene-support interface and strong interface-scattering of flexural modes, which make a large contribution to κ in suspended graphene according to a theoretical calculation.

Quantum physics predicts that there is a fundamental maximum heat conductance across a single transport channel and that this thermal conductance quantum, G Q , is universal, independent of the type of particles carrying the heat. Such universality, combined with the relationship between heat and information, signals a general limit on information transfer. We report on the quantitative measurement of the quantum-limited heat flow for Fermi particles across a single electronic channel, using noise thermometry. The demonstrated agreement with the predicted G Q establishes experimentally this basic building block of quantum thermal transport. The achieved accuracy of below 10% opens access to many experiments involving the quantum manipulation of heat.

The hardness, toughness and chemical stability of the well-known superhard material cubic boron nitride have been improved by using a synthesis technique based on specially prepared ‘onion-like’ precursor materials. How to make superhard materials ultrahard Superhard polycrystalline cubic boron nitride, second only to diamond in hardness, is superior to diamond in terms of thermal and chemical stability and is used widely as an abrasive. The hardness of many materials can be improved by decreasing the grain size, and here Yongjun Tian and colleagues use this principle in a new synthesis technique — based on specially prepared 'onion-like' precursor materials — capable of increasing the hardness of cubic boron nitride. The structure of the resulting polycrystalline material is dominated by nanometre-scale twin domains, yielding a solid combining ultrahigh hardness (exceeding that of a synthetic diamond single crystal) with a high oxidization temperature and extreme fracture toughness. If nanotwins at similar scales can be reproduced in polycrystalline diamond, it may be possible to raise diamond itself to new levels of hardness and stability. Cubic boron nitride (cBN) is a well known superhard material that has a wide range of industrial applications. Nanostructuring of cBN is an effective way to improve its hardness by virtue of the Hall–Petch effect—the tendency for hardness to increase with decreasing grain size 1 , 2 . Polycrystalline cBN materials are often synthesized by using the martensitic transformation of a graphite-like BN precursor, in which high pressures and temperatures lead to puckering of the BN layers 3 . Such approaches have led to synthetic polycrystalline cBN having grain sizes as small as ∼14 nm (refs 1 , 2 , 4 , 5 ). Here we report the formation of cBN with a nanostructure dominated by fine twin domains of average thickness ∼3.8 nm. This nanotwinned cBN was synthesized from specially prepared BN precursor nanoparticles possessing onion-like nested structures with intrinsically puckered BN layers and numerous stacking faults. The resulting nanotwinned cBN bulk samples are optically transparent with a striking combination of physical properties: an extremely high Vickers hardness (exceeding 100 GPa, the optimal hardness of synthetic diamond), a high oxidization temperature (∼1,294 °C) and a large fracture toughness (>12 MPa m 1/2 , well beyond the toughness of commercial cemented tungsten carbide, ∼10 MPa m 1/2 ). We show that hardening of cBN is continuous with decreasing twin thickness down to the smallest sizes investigated, contrasting with the expected reverse Hall–Petch effect below a critical grain size or the twin thickness of ∼10–15 nm found in metals and alloys.

Coating is an essential step in adjusting the surface properties of materials. Superhydrophobic coatings with contact angles greater than 150° and roll-off angles below 10° for water have been developed, based on low-energy surfaces and roughness on the nano-and micrometer scales. However, these surfaces are still wetted by organic liquids such as surfactant-based solutions, alcohols, or alkanes. Coatings that are simultaneously superhydrophobic and superoleophobic are rare. We designed an easily fabricated, transparent, and oil-rebounding superamphiphobic coating. A porous deposit of candle soot was coated with a 25-nanometer-thick silica shell. The black coating became transparet after calcination at 600°C. After silanization, the coating was superamphiphobic and remained so even after its top layer was damaged by sand impingement.