Perovskite solar cells are poised to be a game changer in photovoltaic technology with a current certified efficiency of 25.2%, already surpassing that for multicrystalline silicon solar cells. On the path to higher efficiencies and much needed higher stability, however, interfacial and bulk defects in the active material should be carefully engineered or passivated. Post-treatment techniques show great potential to address defect issues (e.g., by coarsening the perovskite grains or establishing an interfacial heterogeneous layer). In this article, we summarize current fundamental understanding of the major energy-loss routes in perovskite materials and devices, including bulk/interfacial defects mediated nonradiative recombination and band mismatch-induced recombination. This is followed by a survey of the important post-treatment techniques developed over the past few years to minimize energy loss in perovskite solar cells, including solvent annealing, amine halide solution dripping-induced Ostwald ripening, three-dimensional–two-dimensional interface layer from phenethylammonium iodide (PEAI) dripping, and wide bandgap interface layer engineering from n-hexyl trimethylammonium bromide washing. Finally, we provide a prospective view about further developments of post-treatment techniques.

As plasmonic materials transition from three-dimensional to two-dimensional (2D) form, unique optical and electronic phenomena arise that are unattainable in bulk materials and conventional thin films. Exceptional sensitivity to external perturbations, including electrical biasing and optical excitation as well as quantum effects, are expected to emerge, as has been demonstrated in graphene. Similarly, metallic or plasmonic films with thicknesses down to a few monolayers, called transdimensional materials (TDMs), are predicted to exhibit remarkably strong tunability of their optical response. The unique properties of 2D materials and TDMs have established them as promising platforms for dynamic nanophotonic devices. While novel 2D materials have been widely explored for nanophotonic devices, until recently, there have been minimal studies on the evolution of the optical properties of plasmonic TDMs. In this article, we highlight progress in exploring the thickness-dependent optical response of plasmonic materials as they approach the 2D regime. Experimental and theoretical investigations of the plasmonic properties of 2D materials, such as graphene and tungsten diselenide, TDMs, and plasmonic thin films, are discussed, with a focus on prospects for their utilization in dynamically tunable flat optical devices or metasurfaces.

Self-assembly, a process in which molecules, polymers, and particles are driven by local interactions to organize into patterns and functional structures, is being exploited in advancing silicon electronics and in emerging, unconventional electronics. Silicon electronics has relied on lithographic patterning of polymer resists at progressively smaller lengths to scale down device dimensions. Yet, this has become increasingly difficult and costly. Assembly of block copolymers and colloidal nanoparticles allows resolution enhancement and the definition of essential shapes to pattern circuits and memory devices. As we look to a future in which electronics are integrated at large numbers and in new forms for the Internet of Things and wearable and implantable technologies, we also explore a broader material set. Semiconductor nanoparticles and biomolecules are prized for their size-, shape-, and composition-dependent properties and for their solution-based assembly and integration into devices that are enabling unconventional manufacturing and new device functions.

The available x-ray brightness from accelerator-based light sources has been increasing at a rate faster than Moore’s Law. This is enabling new developments in x-ray tomography, using nanoscale x-ray beams on materials ranging in size from micrometers to centimeters. With scanning, one can record x-ray diffraction patterns to view nanoscale structure beyond the optic resolution, or fluorescence signals to study elemental distributions in three dimensions. These approaches are being used in various applications, for example, to study the redistribution of intrinsic metals in biological cells and tissues during biological processes, to compare as-fabricated versus as-designed integrated circuits, and to see the domain structure of soft ferromagnetic materials. While some applications face challenges, including radiation dose, new developments in sparse sampling and reconstruction algorithms, along with a new generation of diffraction-limited storage rings, offer exciting opportunities for materials research.

Protein materials are promising candidates as the building blocks for functional and high-performance bionanocomposites, owing to their unique and well-developed nanoscale structure, rich chemical functionality, excellent mechanical properties, biocompatibility, and biodegradability. Rational integration of protein materials with synthetic organic and inorganic nanomaterials through tailored interfacial interactions leads to synergistic enhancement in the properties compared to the individual components. In this article, we discuss the recent progress in protein-based nanocomposites, which aim to harness the unique structure and properties of proteins and synthetic nanomaterials for realizing advanced materials with greatly enhanced properties. Specifically, we highlight bionanocomposites based on two β-sheet rich proteins, silk fibroin and amyloid fibril, as representative examples as well as a few other protein materials such as keratin, elastin, and collagens. We describe the biotic–abiotic interfaces, processing methods, physical properties, and potential applications of these protein nanocomposites. Considering the additional value of renewability, abundancy, and ambient processability, such bionanocomposites are promising candidates for advanced and emerging applications, such as environmental remediation, biomedicine, biosensors, and photonics.

Hybrid organic–inorganic halide perovskites have been recently explored as memristive devices that can be programmed to two or more stable conductance states for analog computing. The wide variety and range of optoelectronic phenomena these materials portray offer immense potential to develop scaled-in neuromorphic devices and architectures with multibit memory storage and multimodal accessibility. This article provides a general summary of the structural and optoelectronic characteristics of this material class that could be utilized for neuromorphic computing, discusses insights into the underlying switching mechanisms, and reviews recent developments in the field of halide perovskite-based neuromorphic devices.

The efficiency of halide perovskite solar cells has progressed rapidly through a series of major breakthroughs. Currently, a certified efficiency of 25.2% has been achieved for a solar cell using a polycrystalline thin film. This is the result of having reached 75% of the Shockley–Queisser limit for single-junction solar cells. However, for further improvements, new breakthrough technologies are required. This article reviews the impact of previous breakthrough technologies on the efficiency of halide perovskite solar cells, based on certified efficiencies. We clarify the current status of halide perovskite solar cells and introduce photon recycling as the next technological innovation for higher efficiencies. Photon recycling keeps the photon concentration inside the light-harvesting layer high, and consequently, leads to open-circuit voltages close to the theoretical value. Although photon recycling has not yet been implemented in real halide perovskite solar cells, three key technologies for implementing it are examined.

Metasurfaces are thin-film electromagnetic devices with subwavelength-scale geometric structuring. They can be tailored to produce a broad range of optical functions due to the strong relationship between electromagnetic response and geometric shape. An open challenge has been understanding how to produce an ideal metasurface design when presented with a desired electromagnetic response. This article discusses the use of topology optimization as a design platform for high-performance, freeform metasurfaces. Two types of topology optimizers are covered—local gradient-based optimizers that leverage the adjoint variables method, and global population-based optimizers that reframe the optimization process as the training of a generative neural network. It is anticipated that these inverse design concepts will push metasurface performance to the physical limits of structured media and enable new functionalities in electromagnetic systems.

Dark-field x-ray microscopy is intended for the acquisition of three -dimensional (3D) movies of the nanostructure (grains, domains, and dislocations) and the associated local strain within bulk materials. It is analogous to dark-field electron microscopy in that an objective lens magnifies diffracting features of the sample. The use of high-energy synchrotron x-rays, however, means that these microstructural features can be large and deeply embedded. The spatial and angular resolution is on the order of 100 nm and 0.001°, respectively, and full maps can be recorded in seconds to minutes. Four applications of the technique are presented—domain switching in ferroelectrics, processing of metals, microstructural characterization of biominerals, and visualization of dislocations. The ability to directly characterize complex, multiscale phenomena in situ—and in 3D—is a key step toward formulating and validating multiscale models that account for the entire heterogeneity of materials.

This article addresses recent advances in liquid phase transmission electron microscopy (LPTEM) for studying nanoscale synthetic processes of carbon-based materials that are independent of the electron beam—those driven by nonradiolytic chemical or thermal reactions. In particular, we focus on chemical/physical formations and the assembly of nanostructures composed of organic monomers/polymers, peptides/DNA, and biominerals. The synthesis of carbon-based nanomaterials generally only occurs at specific conditions, which cannot be mimicked by aqueous solution radiolysis. Carbon-based structures themselves are also acutely sensitive to the damaging effects of the irradiating beam, which make studying their synthesis using LPTEM a unique challenge that is possible when beam effects can be quantified and mitigated. With new direct sensing, high frame-rate cameras, and advances in liquid cell holder designs, combined with a growing understanding of irradiation effects and proper experimental controls, microscopists have been able to make strides in observing traditionally problematic carbon-based materials under conditions where synthesis can be controlled, and imaged free from beam effects, or with beam effects quantified and accounted for. These materials systems and LPTEM experimental techniques are discussed, focusing on nonradiolytic chemical and physical transformations relevant to materials synthesis.

Electrochemical energy-storage systems such as supercapacitors and lithium-ion batteries require complex intertwined networks that provide fast transport pathways for ions and electrons without interfering with their energy density. Self-assembly of nanomaterials into hierarchical structures offers exciting possibilities to create such pathways. This article summarizes recent research achievements in self-assembled zero-dimensional, one-dimensional, and two-dimensional nanomaterials, ordered pore structure materials, and the interfaces between these. We analyze how self-assembly strategies can create storage architectures that improve device performance toward higher energy densities, longevity, rate capability, and device safety. At the end, the remaining challenges of scalable low-cost manufacturing and future opportunities such as self-healing are discussed.

Solidification processing offers the first opportunity to control microstructure, properties, and performance in metallic alloy components. Until recently, microstructural evaluations were limited to post-solidification characterization by destructive methods. We review the development of time-resolved, in situ imaging techniques capable of capturing solid–liquid interfacial evolution in metallic alloys with high spatial and temporal resolution under diverse solidification conditions relevant for applications ranging from conventional directional solidification, crystal growth, and casting, to welding and additive manufacturing. These experiments enable direct visualization of transient behaviors that would otherwise remain unknown, uniquely providing insights into the physics that impact microstructure and defect development, and strategies for microstructural control and defect mitigation. Understanding microstructural evolution and the characteristics that form under various solidification conditions is essential for the development of multiscale, experimentally informed predictive modeling. This is highlighted by solidification simulations that utilize in situ measurements of solidification dynamics from state-of-the-art experimental techniques.

Transient electronic systems represent an emerging class of technology defined by an ability to physically dissolve, sublime, chemically degrade, disintegrate, or transform in a controlled manner, either spontaneously or through a trigger event. Bioresorbable (or, equivalently, bioabsorbable) electronic devices, as a subset of transient technologies, are designed to undergo complete dissolution when immersed in biofluids. Applications include temporary implants and other medical devices that serve important purposes in diagnostics and therapies, but with finite lifetimes matched to those of natural biological processes such as wound healing. Here, transience by bioresorption eliminates the devices without a trace, thereby bypassing the costs, complications, and risks associated with secondary surgical procedures for device retrieval. Such systems demand complete sets of bioresorbable electronic materials, including semiconductors, dielectrics, and conductors, as the fundamental building blocks for functional components. The considerations are not only in electronic performance, but in degradation chemistry and biocompatibility of both the materials and the products of their reactions with biofluids. This article highlights recent progress in this area of materials science and describes some of the most sophisticated bioresorbable electronic systems that combine these materials with bioresorbable polymers, the biomedical applications of these devices, and some directions for future work.

Two-dimensional (2D) quantum materials offer a unique platform to explore mesoscopic phenomena driven by interfacial and topological effects. Their tunable electric properties and bidimensional nature enable their integration into sophisticated heterostructures with engineered properties, resulting in the emergence of new exotic phenomena not accessible in other platforms. This has fostered many studies on 2D ferromagnetism, proximity-induced effects, and quantum transport, demonstrating their relevance for fundamental research and future device applications. Here, we review ongoing progress in this lively research field with special emphasis on spin-related phenomena.

Nanoscale materials that contain metallic components can be designed to have excellent light-harvesting capabilities, and can also be used to direct the flow of energy from incident photons into small molecules at or near the surface of metal nanoparticles. One promising route for energy flow is through so-called hot charge carriers, which are optically excited on metal nanoparticles and subsequently transferred to molecules/materials that share an interface with the metal. This article provides an overview of the fundamentals of hot-carrier generation and transfer, discusses both theoretical and experimental means for interrogating these processes, and discusses several potential societally important applications of hot-carrier-driven chemistry to solar fuels and sustainable chemistry.

Hydrogen production from water electrolysis with renewable energy input has been the focus of tremendous attention, as hydrogen is widely advocated as a clean energy carrier. In order to realize large-scale hydrogen generation from water splitting, it is essential to develop competent and robust electrocatalysts that will substantially decrease the overpotential requirement and improve energy efficiency. Recent advances in electrocatalyst design reveal that interfacial engineering is an effective approach in tuning the adsorption–desorption abilities of key catalytic intermediates on active sites, accelerating electron transfer, and stabilizing the active sites for long-term operation. Consequently, a large number of hybrid electrocatalysts consisting of metal/compound interfaces have been demonstrated to exhibit superior performance for electrocatalytic hydrogen evolution from water. This article highlights examples of these hybrid electrocatalysts, including noble metal and non-noble metal candidates interfaced with a variety of compounds. Specific emphasis is placed on the synthetic methods, reaction mechanisms, and electrocatalytic activities, which are envisioned to inspire the design and development of further improved electrocatalysts for hydrogen evolution from water splitting on an industrial scale.

We show the design and simulation of organic neuromorphic circuits in a hybrid-computation approach that emulates Boolean and reversible logic gates based on multigate organic electrochemical transistors (OECTs). The organic neuromorphic circuits consist of input, hidden, and output layers that can carry out Boolean operations, including the Exclusive OR (XOR) function, with five or less OECTs. The multigate functionality of OECTs is harnessed to perform the summation function of the neurons. Connection weights of the networks are defined in an unconventional way that depends on the value of the drain-source current of the outputting neuron, which changes according to the input values of the circuit. The Boolean circuits can be cascaded together to build higher level circuits and are demonstrated to form a full adder circuit and the Double Feynman and Toffoli reversible logic gates. Using realistic experimental parameters, the energy per computation is estimated to be ~2.3 nJ for circuit designs with a bias voltage of 0.5 V, with ~230 fJ or less being achievable for lower bias voltages.

The development of optical elements has seen tremendous advances over the last few decades for a variety of applications, including displays, cameras, and microscopes. Various optical elements have been developed, ranging from refractive elements to diffractive elements. In recent years, metasurfaces have been in the spotlight to develop next-generation optical elements beyond existing refractive or diffractive optics. A metasurface is a two-dimensional metamaterial composed of subwavelength artificial structures and has been studied for the development of optical elements with the major advantage that the properties of light can be freely adjusted by a thin flat structure. Optical lenses using metasurfaces can be hundreds of times thinner than conventional lenses, while at the same time, provide excellent focusing performance. This suggests that they can be applied to mobile and high-performance imaging applications in the future. Here, we discuss developments of optical elements from refractive or diffractive optics to metasurface optics, including basic principles and properties, current issues, and future perspectives.

Metal-halide perovskite solar cells (PSCs) have become a promising candidate for photovoltaic applications. Current popular organic hole conductors for highly efficient PSCs bring cost and stability issues, which hinder the commercialization of the PSCs. Hole-conductor-free PSCs are attracting great interest because they eliminate the adverse effects of organic hole conductors by transporting holes in the perovskite itself. In this article, we summarize recent progress in conventional, inverted, and printable mesoscopic hole-conductor-free PSCs. Specifically, we emphasize the stunning stability and scale-up manufacturing of printable hole-conductor-free PSCs, discussing their potential from laboratory to market. The causes for hole-conductor-free PSCs’ current low efficiency are also discussed, and are primarily ascribed to energy-level alignment and interface recombination. We believe that the efficiencies of hole-conductor-free PSCs can be enhanced to be comparable with hole-conductor-containing PSCs by interface modification and material design.

Insights into the dynamics of electrochemical processes are critically needed to improve our fundamental understanding of electron, charge, and mass transfer mechanisms and reaction kinetics that influence a broad range of applications, from the functionality of electrical energy-storage and conversion devices (e.g., batteries, fuel cells, and supercapacitors), to materials degradation issues (e.g., corrosion and oxidation), and materials synthesis (e.g., electrodeposition). To unravel these processes, in situ electrochemical scanning/transmission electron microscopy (ec-S/TEM) was developed to permit detailed site-specific characterization of evolving electrochemical processes that occur at electrode–electrolyte interfaces in their native electrolyte environment, in real time and at high-spatial resolution. This approach utilizes “closed-form” microfabricated electrochemical cells that couple the capability for quantitative electrochemical measurements with high spatial and temporal resolution imaging, spectroscopy, and diffraction. In this article, we review the state-of-the-art instrumentation for in situ ec-S/TEM and how this approach has resulted in new observations of electrochemical processes.