Showing papers in "Nature Reviews Materials in 2019"

High-entropy alloys

Abstract: Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

Photovoltaic solar cell technologies: analysing the state of the art

Abstract: The remarkable development in photovoltaic (PV) technologies over the past 5 years calls for a renewed assessment of their performance and potential for future progress. Here, we analyse the progress in cells and modules based on single-crystalline GaAs, Si, GaInP and InP, multicrystalline Si as well as thin films of polycrystalline CdTe and CuInxGa1−xSe2. In addition, we analyse the PV developments of the more recently emerged lead halide perovskites together with notable improvements in sustainable chalcogenides, organic PVs and quantum dots technologies. In addition to power conversion efficiencies, we consider many of the factors that affect power output for each cell type and note improvements in control over the optoelectronic quality of PV-relevant materials and interfaces and the discovery of new material properties. By comparing PV cell parameters across technologies, we appraise how far each technology may progress in the near future. Although accurate or revolutionary developments cannot be predicted, cross-fertilization between technologies often occurs, making achievements in one cell type an indicator of evolutionary developments in others. This knowledge transfer is timely, as the development of metal halide perovskites is helping to unite previously disparate, technology-focused strands of PV research. Nearly all types of solar photovoltaic cells and technologies have developed dramatically, especially in the past 5 years. Here, we critically compare the different types of photovoltaic technologies, analyse the performance of the different cells and appraise possibilities for future technological progress.

Dimensional tailoring of hybrid perovskites for photovoltaics

Abstract: Hybrid perovskites are currently one of the most active fields of research owing to their enormous potential for photovoltaics. The performance of 3D hybrid organic–inorganic perovskite solar cells has increased at an incredible rate, reaching power conversion efficiencies comparable to those of many established technologies. However, the commercial application of 3D hybrid perovskites is inhibited by their poor stability. Relative to 3D hybrid perovskites, low-dimensional — that is, 2D — hybrid perovskites have demonstrated higher moisture stability, offering new approaches to stabilizing perovskite-based photovoltaic devices. Furthermore, 2D hybrid perovskites have versatile structures, enabling the fine-tuning of their optoelectronic properties through compositional engineering. In this Review, we discuss the state of the art in 2D perovskites, providing an overview of structural and materials engineering aspects and optical and photophysical properties. Moreover, we discuss recent developments along with the main limitations of 3D perovskites and assess the advantages of 2D perovskites over their 3D parent structures in terms of stability. Finally, we review recent achievements in combining 3D and 2D perovskites as an approach to simultaneously boost device efficiency and stability, paving the way for mixed-dimensional perovskite solar cells for commercial applications. Combining low-dimensional and 3D perovskites is a promising approach to achieve stable and efficient solar cells. In this Review, we discuss the structural, optical and photophysical properties of low-dimensional perovskites, compare the stability and efficiency of 2D and 3D perovskite devices, and consider 2D/3D composites as a strategy to increase the stability of perovskite solar cells.

Cancer nanomedicine for combination cancer immunotherapy

Abstract: Cancer immunotherapy is revolutionizing oncology. However, dose-limiting toxicities and low patient response rates remain major challenges in the clinic. Cancer nanomedicine in combination with immunotherapies offers the possibility to amplify antitumour immune responses and to sensitize tumours to immunotherapies in a safe and effective manner. In this Review, we discuss opportunities for combination immunotherapy based on nanoparticle platforms designed for chemotherapy, photothermal therapy, photodynamic therapy, radiotherapy and gene therapy. We highlight how nanoparticles can be used to reprogramme the immunosuppressive tumour microenvironment and to trigger systemic antitumour immunity, synergizing with immunotherapies against advanced cancer. Finally, we discuss strategies to improve tumour and immune cell targeting while minimizing toxicity and immune-related adverse events, and we explore the potential of theranostic nanoparticles for combination immunotherapy. Cancer nanomedicine in combination with immunotherapies offers the possibility to amplify antitumour immune responses and to sensitize tumours to immunotherapies. In this Review, the authors discuss combination immunotherapy based on nanoparticle platforms designed for chemotherapy, photothermal therapy, photodynamic therapy, radiotherapy and gene therapy.

Metal halide perovskite nanostructures for optoelectronic applications and the study of physical properties

Abstract: Nanostructures of inorganic semiconductors have revolutionized many areas of electronics, optoelectronics and photonics. The controlled synthesis of semiconductor nanostructures could lead to novel physical properties, improved optoelectronic device performance and new areas for exploration. Lead halide perovskites have recently excited the photovoltaic research community owing to their high solar-conversion efficiencies and ease of solution processing; they also hold great promise for optoelectronic applications, such as light-emitting diodes and lasers. In this Review, we summarize recent developments in the synthesis and characterization of metal halide perovskite nanostructures with controllable compositions, dimensionality, morphologies and orientations. We examine the advantageous optical properties, improved stability and potential optoelectronic applications of these 1D and 2D single-crystal perovskite nanostructures and compare them with those of bulk perovskites and nanostructures of conventional semiconductors. Studies in which perovskite nanostructures have been used to study the fundamental physical properties of perovskites are also highlighted. Finally, we discuss the challenges in realizing halide perovskite nanostructures for optoelectronic and photonic applications and offer our perspectives on future opportunities and research directions. Metal halide perovskite nanostructures are promising materials for optoelectronic applications. In this Review, we discuss the synthesis and properties of 1D and 2D single-crystal perovskite nanostructures, examine potential optoelectronic applications and highlight recent studies in which these nanostructures have been used to study the fundamental properties of perovskites.

Designing crystallization in phase-change materials for universal memory and neuro-inspired computing

Abstract: The global demand for data storage and processing has increased exponentially in recent decades. To respond to this demand, research efforts have been devoted to the development of non-volatile memory and neuro-inspired computing technologies. Chalcogenide phase-change materials (PCMs) are leading candidates for such applications, and they have become technologically mature with recently released competitive products. In this Review, we focus on the mechanisms of the crystallization dynamics of PCMs by discussing structural and kinetic experiments, as well as ab initio atomistic modelling and materials design. Based on the knowledge at the atomistic level, we depict routes to improve the parameters of phase-change devices for universal memory. Moreover, we discuss the role of crystallization in enabling neuro-inspired computing using PCMs. Finally, we present an outlook for future opportunities of PCMs, including all-photonic memories and processors, flexible displays with nanopixel resolution and nanoscale switches and controllers. Chalcogenide phase-change materials (PCMs) are leading candidates for non-volatile memory and neuro-inspired computing devices. This Review focuses on the crystallization mechanisms of PCMs as well as the design principles to achieve PCMs with high switching speeds and good data retention, which will aid the development of PCM-based universal memory and neuro-inspired devices.

Hierarchical 3D electrodes for electrochemical energy storage

Abstract: The discovery and development of electrode materials promise superior energy or power density. However, good performance is typically achieved only in ultrathin electrodes with low mass loadings (≤1 mg cm−2) and is difficult to realize in commercial electrodes with higher mass loadings (>10 mg cm−2). To realize the full potential of these electrode materials, new electrode architectures are required that can allow more efficient charge transport beyond the limits of traditional electrodes. In this Review, we summarize the design and synthesis of 3D electrodes to address charge transport limitations in thick electrodes. Specifically, we discuss the role of charge transport in electrochemical systems and focus on the design of 3D porous structures with a continuous conductive network for electron transport and a fully interconnected hierarchical porosity for ion transport. We also discuss the application of 3D porous architectures as conductive scaffolds for various electrode materials to enable composite electrodes with an unprecedented combination of energy and power densities and then conclude with a perspective on future opportunities and challenges. 3D electrodes with interconnected and interpenetrating pathways enable efficient electron and ion transport. In this Review, the design and synthesis of such 3D electrodes are discussed, along with their ability to address charge transport limitations at high areal mass loading and to enable composite electrodes with an unprecedented combination of energy and power densities in electrochemical energy storage devices.

Designing polymers for advanced battery chemistries

Abstract: Electrochemical energy storage devices are becoming increasingly important to our global society, and polymer materials are key components of these devices. As the demand for high-energy density devices increases, innovative new materials that build on the fundamental understanding of physical phenomena and structure–property relationships will be required to enable high-capacity next-generation battery chemistries. In this Review, we discuss core polymer science principles that are used to facilitate progress in battery materials development. Specifically, we discuss the design of polymeric materials for desired mechanical properties, increased ionic and electronic conductivity and specific chemical interactions. We also discuss how polymer materials have been designed to create stable artificial interfaces and improve battery safety. The focus is on these design principles applied to advanced silicon, lithium-metal and sulfur battery chemistries. Polymers are ubiquitous in batteries as binders, separators, electrolytes and electrode coatings. In this Review, we discuss the principles underlying the design of polymers with advanced functionalities to enable progress in battery engineering, with a specific focus on silicon, lithium-metal and sulfur battery chemistries.

Photonics with hexagonal boron nitride

Abstract: For more than seven decades, hexagonal boron nitride (hBN) has been employed as an inert, thermally stable engineering ceramic; since 2010, it has also been used as the optimal substrate for graphene in nanoelectronic and optoelectronic devices. Recent research has revealed that hBN exhibits a unique combination of optical properties that enable novel (nano)photonic functionalities. Specifically, hBN is a natural hyperbolic material in the mid-IR range, in which photonic material options are sparse. Furthermore, hBN hosts defects that can be engineered to obtain room-temperature, single-photon emission; exhibits strong second-order nonlinearities with broad implications for practical devices; and is a wide-bandgap semiconductor well suited for deep UV emitters and detectors. Inspired by these promising attributes, research on the properties of hBN and the development of large-area bulk and thin-film growth techniques has dramatically expanded. This Review offers a snapshot of current research exploring the properties underlying the use of hBN for future photonics functionalities and potential applications, and covers some of the remaining obstacles. Hexagonal boron nitride (hBN) is highly sought after for mid-IR nanophotonics, nonlinear and quantum optics, and as an efficient UV emitter. This Review surveys its fundamental physical properties, applications and synthesis.

Structure prediction drives materials discovery

Abstract: Progress in the discovery of new materials has been accelerated by the development of reliable quantum-mechanical approaches to crystal structure prediction. The properties of a material depend very sensitively on its structure; therefore, structure prediction is the key to computational materials discovery. Structure prediction was considered to be a formidable problem, but the development of new computational tools has allowed the structures of many new and increasingly complex materials to be anticipated. These widely applicable methods, based on global optimization and relying on little or no empirical knowledge, have been used to study crystalline structures, point defects, surfaces and interfaces. In this Review, we discuss structure prediction methods, examining their potential for the study of different materials systems, and present examples of computationally driven discoveries of new materials — including superhard materials, superconductors and organic materials — that will enable new technologies. Advances in first-principle structure predictions also lead to a better understanding of physical and chemical phenomena in materials. Recent breakthroughs in crystal structure prediction have enabled the discovery of new materials and of new physical and chemical phenomena. This Review surveys structure prediction methods and presents examples of results in different classes of materials.

Reprogrammable recovery and actuation behaviour of shape-memory polymers

Abstract: Shape memory is the capability of a material to be deformed and fixed into a temporary shape. Recovery of the original shape can then be triggered only by an external stimulus. Shape-memory polymers are highly deformable materials that can be programmed to recover a memorized shape in response to a variety of environmental and spatially localized stimuli as a one-way effect. The shape-memory function can also be generated as a reversible effect enabling actuation behaviour through macroscale deformation and processing, specifically by dictating the macromolecular orientation of actuation units and of the skeleton structure of geometry-determining units in the polymers. Shape-memory polymers can be programmed and reprogrammed into arbitrary shapes. Both recovery and actuation behaviour are reprogrammable. In this Review, we outline the common basis and key differences between the two shape-memory behaviours of polymers in terms of mechanism, fabrication schemes and characterization methods. We discuss which combination of macromolecular architecture and macroscale processing is necessary for coordinated, decentralized and responsive physical behaviour. The extraction of relevant thermomechanical information is described, and design criteria are shown for microscale and macroscale morphologies to gain high levels of recovered or actuation strains as well as on-demand 2D-to-3D shape transformations. Finally, real-world applications and key future challenges are highlighted. Shape-memory materials can generate programmable movements triggered by an external stimulus, such as an environmental change. In this Review, the authors discuss mechanisms, fabrication schemes, characterization methods and applications of the one-way shape-memory effect enabling shape recovery and of reversible shape-memory effects exhibiting actuation behaviour.

The role of the third component in ternary organic solar cells

Abstract: Ternary organic solar cells (TSCs) contain a single three-component photoactive layer with a wide absorption window, which is obtained without the need for multiple stacks. Subsequently, TSCs have attracted great interest in the photovoltaics field. Through careful selection of the three (or more) active components that form the photoactive layer, all photovoltaic parameters can be simultaneously enhanced within a TSC — a strategy that has resulted in record efficiencies for single-junction solar cells. In this Review, we outline key developments in TSCs, with a focus on the central role of the third component in achieving record efficiencies. We analyse the effects of the third component on the nanomorphology of the bulk heterojunction and the photovoltaic parameters of TSCs. Moreover, we discuss the charge-transfer and/or energy-transfer mechanisms and nanomorphology models that govern the operation of TSCs. We consider both polymer and small-molecule donors as well as fullerenes and recently developed non-fullerene acceptors. In addition, we summarize the recent success of TSCs in mitigating the stability issues of binary solar cells. Finally, we provide a perspective on the advantages of ternary blends and suggest design strategies for highly efficient and stable devices for commercial photovoltaics. Adding a third component into a binary blend is a promising strategy for simultaneously improving all photovoltaic parameters in organic solar cells. In this Review, we discuss the role of the third component in influencing the energetics, charge-carrier recombination and stability in ternary solar cells.

2D materials for quantum information science

Abstract: The transformation of digital computers from bulky machines to portable systems has been enabled by new materials and advanced processing technologies that allow ultrahigh integration of solid-state electronic switching devices. As this conventional scaling pathway has approached atomic-scale dimensions, the constituent nanomaterials (such as SiO2 gate dielectrics, poly-Si floating gates and Co–Cr–Pt ferromagnetic alloys) increasingly possess properties that are dominated by quantum physics. In parallel, quantum information science has emerged as an alternative to conventional transistor technology, promising new paradigms in computation, communication and sensing. The convergence between quantum materials properties and prototype quantum devices is especially apparent in the field of 2D materials, which offer a broad range of materials properties, high flexibility in fabrication pathways and the ability to form artificial states of quantum matter. In this Review, we discuss the quantum properties and potential of 2D materials as solid-state platforms for quantum-dot qubits, single-photon emitters, superconducting qubits and topological quantum computing elements. By focusing on the interplay between quantum physics and materials science, we identify key opportunities and challenges for the use of 2D materials in the field of quantum information science. 2D materials exhibit diverse properties and can be integrated in heterostructures: this makes them ideal platforms for quantum information science. This Review surveys recent progress and identifies future opportunities for 2D materials as quantum-dot qubits, single-photon emitters, superconducting qubits and topological quantum computing elements.

Nonlinear optical effects in epsilon-near-zero media

Abstract: Efficient nonlinear optical interactions are essential for many applications in modern photonics. However, they typically require intense laser sources and long interaction lengths, requirements that often render nonlinear optics incompatible with new nanophotonic architectures in integrated optics and metasurface devices. Obtaining materials with stronger nonlinear properties is a crucial step towards applications that require lower powers and smaller footprints. Recently, a new class of materials with a vanishing permittivity, known as epsilon-near-zero (ENZ) materials, has been reported to exhibit unprecedented ultrafast nonlinear efficiencies within sub-wavelength propagation lengths. In this Review, we survey the work that has been performed on ENZ materials and the related near-zero-index materials, focusing on the observation of various nonlinear phenomena (such as intensity-dependent refraction, four-wave mixing and harmonic generation), the identification of unique field-enhancement mechanisms and the study of non-equilibrium dynamics. Degenerately doped semiconductors (such as tin-doped indium oxide and aluminium-doped zinc oxide) are particularly promising candidates for ENZ-enhanced nonlinear optical applications. We conclude by pointing towards possible future research directions, such as the search for ENZ materials with low optical losses and the elucidation of the mechanisms underlying nonlinear enhancements. Materials with vanishingly small dielectric permittivity, known as epsilon-near-zero materials, enable strong ultrafast optical nonlinear responses within a sub-wavelength propagation length. This Review surveys the various observations of nonlinear phenomena in this class of materials.

Theory-guided design of catalytic materials using scaling relationships and reactivity descriptors

Abstract: The active sites of heterogeneous catalysts can be difficult to identify and understand, and, hence, the introduction of active sites into catalysts to tailor their function is challenging. During the past two decades, scaling relationships have been established for important heterogeneous catalytic reactions. More specifically, a physical or chemical property of the reaction system, termed as a reactivity descriptor, scales with another property often in a linear manner, which can describe and/or predict the catalytic performance. In this Review, we describe scaling relationships and reactivity descriptors for heterogeneous catalysis, including electronic descriptors represented by d-band theory, structural descriptors, which can be directly applied to catalyst design, and, ultimately, universal descriptors. The prediction of trends in catalytic performance using reactivity descriptors can enable the rational design of catalysts and the efficient screening of high-throughput catalysts. Finally, we outline methods to break scaling relationships and, hence, to break the constraint that active sites pose on the catalytic performance. Recently, scaling relationships have been established between certain physical or chemical properties of heterogeneous catalytic reactions. These properties, or reactivity descriptors, can describe and predict catalytic performance, and thus enable the rational design of new catalysts.

Material design for lymph node drug delivery.

Abstract: A significant fraction of the total immune cells in the body are located in several hundred lymph nodes, in which lymphocyte accumulation, activation and proliferation are organized. Therefore, targeting lymph nodes provides the possibility to directly deliver drugs to lymphocytes and lymph node-resident cells and thus to modify the adaptive immune response. However, owing to the structure and anatomy of lymph nodes, as well as the distinct localization and migration of the different cell types within the lymph node, it is difficult to access specific cell populations by delivering free drugs. Materials can be used as instructive delivery vehicles to achieve accumulation of drugs in the lymph nodes and to target specific lymph node-resident cell subtypes. In this Review, we describe the compartmental architecture of lymph nodes and the cell and fluid transport mechanisms to and from lymph nodes. We discuss the different entry routes into lymph nodes and how they can be explored for drug delivery, including the lymphatics, blood capillaries, high endothelial venules, cell-mediated pathways, homing of circulating lymphocytes and direct lymph node injection. We examine different nanoscale and microscale materials for the targeting of specific immune cells and highlight their potential for the treatment of immune dysfunction and for cancer immunotherapy. Finally, we give an outlook to the field, exploring how lymph node targeting can be improved by the use of materials.

Shaping and Structuring Supramolecular Gels

Abstract: Supramolecular gels assemble via non-covalent interactions between low-molecular-weight gelators (LMWGs). The gels form a solid-like nanoscale network spanning a liquid-like continuous phase, translating molecular-scale information into materials performance. However, gels based on LMWGs are often difficult to manipulate, easily destroyed and have poor rheological performance. The recurring image of newly discovered supramolecular gels is that of an inverted vial showing that the gel can support its own weight against gravity. Such images reflect the limitation that these gels simply fill the vessel in which they are made, with limited ability to be shaped. This property prevents supramolecular gels from having the same impact as polymer gels, despite greater synthetic tunability, reversibility and bio/environmental compatibility. In this Review, we evaluate strategies for imposing different shapes onto supramolecular gels and for patterning structures within them. We review fabrication methods including moulding, self-healing, 3D printing, photopatterning, diffusion and surface-mediated patterning. We discuss gelator chemistries amenable to each method, highlighting how a multicomponent approach can aid shaping and structuring. Supramolecular gels with defined shapes, or patterned structures with precisely controlled compositions, have the potential to intervene in applications, such as tissue engineering and nanoscale electronics, as well as opening up new technologies. Supramolecular gels comprise low-molecular weight gelators that assemble by non-covalent interactions. In this Review, a range of fabrication methods, as well as strategies for shaping, structuring and patterning supramolecular gels are discussed.

Design and applications of man-made biomimetic fibrillar hydrogels

Abstract: Many extracellular matrices (ECMs) have a filamentous architecture, which influences cell growth and phenotype and imparts tissues with specific properties. Man-made fibrillar hydrogels can function as biomimetic materials to reproduce the filamentous nature and properties of ECMs and to serve as scaffolds for 3D cell culture and tissue engineering. Different types of synthetic nanofibrillar hydrogels have been developed, with diverse mechanisms of assembly and a variety of physical properties and applications. In this Review, we explore the design and properties of biomimetic man-made nanofibrillar hydrogels. We discuss the assembly of peptides, block copolymer worm-like micelles and filamentous nanoparticles into fibrillar hydrogels and investigate the relationship between structure and physical as well as biochemical properties. Potential applications for 3D cell culture and tissue engineering are examined, and the properties and structure of natural and man-made fibrillar hydrogels are compared. Finally, we critically assess current challenges and future directions of the field. Man-made fibrillar hydrogels mimic the structure of filamentous extracellular matrices and can be used as biomaterials for 3D cell culture and tissue engineering. In this Review, the authors discuss the design and properties of fibrillar hydrogels and explore different building blocks, assembly mechanisms, properties and applications.

2D materials as an emerging platform for nanopore-based power generation

Abstract: Osmotic power generation, the extraction of power from mixing salt solutions of different concentrations, can provide an efficient power source for both nanoscale and industrial-level applications. Power is generated using ion-selective channels or pores of nanometric dimensions in synthetic membrane materials. 2D materials such as graphene and MoS2 provide energy extraction efficiencies that are several orders of magnitude higher than those of more established bulky membranes. In this Review, we survey the current state of the art in power generation with both 2D materials and solid-state devices. We discuss the current understanding of the processes underlying power generation in boron nitride nanotubes and 2D materials, as well as the available fabrication methods and their impact on power generation. Finally, we overview future directions of research, which include increasing efficiency, upscaling single pores to porous membranes and solving other issues related to the potential practical application of 2D materials for osmotic power generation. Synthetic nanopores in 2D materials are an emerging platform for power harvesting from the controlled mixing of fresh and salty water. This Review surveys their physics and materials properties and the progress in the design of new, high-density, ion-selective membrane materials.

Near-zero-index materials for photonics

Abstract: The discovery, design and development of materials are critically linked to advances in many areas of research, and optics is no exception. Recently, the spectral region in which the index of refraction of a material approaches zero has become a topic of interest owing to fascinating phenomena, such as static light, enhanced nonlinearities, light tunnelling and emission tailoring. As a result, such near-zero-index (NZI) materials bridge materials development and optical research. Here, we review recent advances in various classes of NZI platforms, with particular focus on homogeneous materials, including metals, semi-metals, doped semiconductors, phononic and interband materials, discussing the novel optical phenomena that they can produce. We also overview the developments in a key area for NZI materials, nonlinear optics, and survey some of the future goals in the field, such as the development of tailorable NZI materials in the visible range and the improvement of the theoretical description of the nonlinear enhancements in these materials. Materials with a near-zero optical refractive index have stimulated much interest, as they can be used to investigate fundamental light–matter interactions. This Review surveys the wide range of near-zero-index homogeneous materials that have been explored and highlights the key experimental advancements they have enabled.

Engineered materials for organoid systems

Abstract: Organoids are 3D cell culture systems that mimic some of the structural and functional characteristics of an organ. Organoid cultures provide the opportunity to study organ-level biology in models that mimic human physiology more closely than 2D cell culture systems or non-primate animal models. Many organoid cultures rely on decellularized extracellular matrices as scaffolds, which are often poorly chemically defined and allow only limited tunability and reproducibility. By contrast, the biochemical and biophysical properties of engineered matrices can be tuned and optimized to support the development and maturation of organoid cultures. In this Review, we highlight how key cell-matrix interactions guiding stem-cell decisions can inform the design of biomaterials for the reproducible generation and control of organoid cultures. We survey natural, synthetic and protein-engineered hydrogels for their applicability to different organoid systems and discuss biochemical and mechanical material properties relevant for organoid formation. Finally, dynamic and cell-responsive material systems are investigated for their future use in organoid research.

Charge-transfer electronic states in organic solar cells

Abstract: In organic solar cells, the charge-transfer (CT) electronic states that form at the interface between the electron-donor (D) and electron-acceptor (A) materials have a crucial role in exciton-dissociation, charge-separation and charge-recombination processes. Since the introduction of active layers consisting of D–A bulk heterojunctions, CT states have been the focus of extensive experimental and theoretical studies. In this Review, we assess the current understanding of CT states and describe how factors such as the geometry of the D–A interface, electronic polarization and the extent of electron delocalization affect their nature and influence the radiative and non-radiative decay processes. We focus on the description and application of fundamental concepts, which provides the framework to discuss the path to organic solar cells with efficiencies comparable to those in inorganic photovoltaic technologies. The charge-transfer electronic states that form at the interfaces between electron-donor and electron-acceptor components have a key role in the electronic processes in organic solar cells. This Review describes the current understanding of how these charge-transfer states affect device performance.

Interfaces in organic electronics

Abstract: Undoped, conjugated, organic molecules and polymers possess properties of semiconductors, including the electronic structure and charge transport, which can be readily tuned by chemical design. Mor ...

Heterogeneity at multiple length scales in halide perovskite semiconductors

Abstract: Materials with highly crystalline lattice structures and low defect concentrations have classically been considered essential for high-performance optoelectronic devices. However, the emergence of high-efficiency devices based on halide perovskites is provoking researchers to rethink this traditional picture, as the heterogeneity in several properties within these materials occurs on a series of length scales. Perovskites are typically fabricated crudely through simple processing techniques, which leads to large local fluctuations in defect density, lattice structure, chemistry and bandgap that appear on short length scales ( 10 μm). Despite these variable and complex non-uniformities, perovskites maintain exceptional device efficiencies and are, as of 2018, the best-performing polycrystalline thin-film solar cell material. In this Review, we highlight the multiple layers of heterogeneity ascertained using high-spatial-resolution methods that provide access to the relevant length scales. We discuss the impact that the optoelectronic variations have on halide perovskite devices, including the prospect that it is this very disorder that leads to their remarkable power-conversion efficiencies. Despite their excellent macroscopic operational parameters, halide perovskites exhibit heterogeneity in materials properties at all lateral and vertical length scales. In this Review, we discuss the nature of heterogeneity in halide perovskites and assess the impact of these non-uniformities on their optoelectronic properties and how the heterogeneity may even be beneficial for device properties.

Writing magnetic memory with ultrashort light pulses

Abstract: Laser pulses are the shortest stimulus known to control the magnetization of materials and to switch magnetic devices on the picosecond to femtosecond timescales. Femtosecond laser pulses have been able to trigger the fastest changes in the magnetic state of matter, and thus these pulses may lead to technologies with increased speed and energy efficiency of magnetic data storage and memory. In the past decade, materials enabling optical control of magnetism and concepts of devices employing such opto-magnetic phenomena have been shown. In this Review, we explore ultrafast all-optical switching (AOS) of magnetization as the least-dissipative and fastest method for magnetic writing. We outline the physical processes responsible for mechanisms of AOS, define the materials suitable for optical control of magnetism and test these mechanisms and materials against three important criteria of recording: speed, accompanying dissipations and scalability. In particular, we emphasize that switching magnetization with the help of light outperforms other methods in terms of the speed of the write–read magnetic recording event (less than 20 ps) and the unprecedentedly low heat load (<6 J cm−3). Finally, we outline the integration of AOS in spintronic devices and the perspective of large-scale integration towards magnetic random access memory and other memory applications with low-energy dissipations. Laser pulses can trigger fast changes in magnetic state, facilitating new magnetic data storage and memory devices. This Review outlines the mechanisms of all-optical switching and the materials suitable for the optical control of magnetism and tests these mechanisms and materials in terms of speed, accompanying dissipations and scalability. Finally, the large-scale integration of devices in memory applications with low-energy dissipations is discussed.

Kinetic stability of metal–organic frameworks for corrosive and coordinating gas capture

Abstract: Metal–organic frameworks (MOFs) have diverse applications involving the storage, separation and sensing of weakly interacting, high-purity gases. Exposure to impure gas streams and interactions with corrosive and coordinating gases raises the question of chemical robustness; however, the factors that determine the stability of MOFs are not fully understood. Framework materials have been previously categorized as either thermodynamically or kinetically stable, but recent work has elucidated an energetic penalty for porosity for all these materials with respect to a dense phase, which has implications for the design of materials for gas storage, heterogeneous catalysis and electronic applications. In this Review, we focus on two main strategies for stabilization of the porous phase — using inert metal ions or increasing the heterolytic metal–ligand bond strength. We review the progress in designing robust materials for the capture of coordinating and corrosive gases such as H2O vapour, NH3, H2S, SO2, nitrogen oxides (NOx) and elemental halogens. We envision that the pursuit of strategies for kinetic stabilization of MOFs will yield increasing numbers of robust frameworks suited to harsh conditions and that short-term stability towards these challenging gases will be predictive of long-term stability for applications in less demanding environments. Metal–organic frameworks, when evacuated, are metastable with respect to a dense phase of the same components. Here, we review methods for kinetic stabilization of the porous phase and discuss progress in designing stable frameworks for the capture of corrosive and coordinating gases.

Single-micelle-directed synthesis of mesoporous materials

Abstract: The work was supported by the National Natural Science Foundation of China (NSFC, 21733003, 21875043, 21701027), Key Basic Research Program of the Science and Technology Commission of Shanghai Municipality (17JC1400100), China National Key Basic Research Program (973 Project) (2018YFA0209400, 2017YFA0207303), Natural Science Foundation of Shanghai (18ZR1404600) and Shanghai Sailing Program (17YF1401000). This work was supported by Qatar University under High Impact-Fund Program Grant (QUHI-CAS-19/20-1).

Ferroelectric negative capacitance

Abstract: The capacitor is a key element of electronic devices and is characterized by positive capacitance. However, a negative capacitance (NC) behaviour may occur in certain cases and implies a local voltage drop opposed to the overall applied bias. Therefore, a local NC response results in voltage enhancement across the rest of the circuit. Within a suitably designed heterostructure, ferroelectrics display such an NC effect, and various ferroelectric-based microelectronic and nanoelectronic devices have been developed, showing improved performance attributed to NC. However, the exact physical nature of the NC response and direct experimental evidence remain elusive or controversial thus far. In this Review, we discuss the physical mechanisms responsible for ferroelectric NC, tackling static and transient NC responses. We examine ferroelectric responses to voltage and charge, as well as ferroelectric switching, and discuss proof-of-concept experiments and possibilities for device implementation. Finally, we highlight different approaches for the optimization of the intrinsic NC response to maximize voltage amplification. Ferroelectrics-based materials can display a negative capacitance (NC) effect, providing an opportunity to implement NC in electronic circuits to improve their performance. In this Review, the authors discuss static and transient NC responses in ferroelectrics and highlight proof-of-concept experiments and possibilities for device implementation.

Creating emergent phenomena in oxide superlattices

Abstract: Complex oxides are record holder materials for many phenomena, including ferroelectricity, piezoelectricity, superconductivity and multiferroicity. Complex oxides often have competing ground states with energies slightly higher than that of the true ground state. This competition is fortuitous because thermodynamic variables (for example, temperature, electric field, magnetic field, stress and chemical potentials) can access these metastable phases that are usually hidden but emerge as the energetic landscape is reshaped by adjusting the thermodynamic variables. Epitaxial superlattices are a platform for imposing thermodynamic boundary conditions to unleash the properties of hidden phases by altering the delicate balance between competing spin, charge, orbital and lattice degrees of freedom. Additionally, a feature of complex oxides with large responses (large property coefficients) is the coexistence of phases on the nanoscale. New phases can emerge at the heterointerfaces of oxide superlattices, and X-ray, electron, neutron and proximal probes as well as ab initio theoretical studies can provide insights into these emergent phenomena. Oxide superlattices reveal emergent phenomena if the balance between competing degrees of freedom is altered. In this Review, synthetic approaches to tap the properties of competing ground states are described, focusing on two examples — one example yielding a room-temperature multiferroic and a second producing polarization skyrmions.

Macroscale biomaterials strategies for local immunomodulation

Abstract: The immune system plays key roles in tissue homeostatic and disease processes, and manipulation of innate and adaptive immune responses is of great promise for a wide array of human afflictions, including tissue repair and regeneration, cancer, autoimmune syndromes and chronic infections. Systemic approaches to immunomodulation can correct both hypoactive and hyperactive immunity; however, they typically interfere with the homeostatic role of the immune system at nontarget sites, are associated with lifelong comorbidities and potentially fatal side effects. To overcome these issues, macroscale delivery devices can be placed at sites of interest in the body and engineered to locally control the pharmacokinetics of immunomodulatory agents, including small molecules, macromolecules and cells. In this Review, we outline important cellular targets of immunotherapies in tissue repair and cancer and discuss how macroscale delivery devices can be designed to modulate the release of molecular factors to impact immune cell behaviour, control the fate of delivered therapeutic cells or directly recruit, house and modulate host cells for immunotherapy applications. Macroscale delivery devices can be used to manipulate innate and adaptive immune responses. In this Review, the authors highlight important cellular targets of immunotherapies in tissue repair and cancer and discuss macroscale biomaterials strategies for therapeutic immunomodulation.