Colloidal synthesis methods and ultrahigh-vacuum molecular beam epitaxy can tailor semiconductor-based nanoscale single emitters—quantum dots—as the building blocks for classical optoelectronic devices, such as lasers, light-emitting devices, and display technologies. These novel light sources have a basic resemblance of luminescent organic molecules, individually and in the aggregated forms. Highly ordered superstructures of quantum dots, obtained via scalable bottom-up self-assembly, exhibit diverse collective phenomena, such as band-like charge transport or superradiant emission. Superradiance emerges from coherent coupling of several emitters via a common radiation field resulting in a single giant dipole leading to short (sub-nanosecond) and intense (proportional to the squared number of coupled emitters) bursts of light. In this article, we review the basic principles and progress in the development of superradiant emitters with organic molecules and inorganic quantum dots, in view of their integration into classical and novel quantum light sources.

Emerging transient electronics capable of complete physical and chemical disintegration are derived from advanced materials and device design strategies. The area of exploring on-demand smart transient electronics has seen continuous development, allowing for the degradation process to be triggered or controlled through an instantaneous stimulus, thus offering significant potential in data security, undetectable spying, and bioresorbable electronics applications. In this article, we summarize recent progress in the design and strategies of on-demand smart transient electronics and emphasize the basic principles of selecting, processing, and integrating materials. After an introduction to the history and properties of triggered transient electronics, we discuss on-demand smart transient electronics based on their triggering stimuli, strategies for designing thermal, optical, or electrical triggers, and future development trends and challenges.

The presence of 6s2 (5s2) lone-pair electrons on the B-site Pb (Sn) in all-inorganic and hybrid halide ABX3 perovskites distinguishes these materials from the familiar tetrahedral semiconductors traditionally employed in optoelectronics and is key to many of their appealing properties. These electrons are stereochemically active, albeit often in a hidden fashion, resulting in unusual and highly anharmonic lattice dynamics that are linked to many of the special optoelectronic properties displayed by this material class. This article describes the connections between this atypical electronic configuration and the electronic structure and lattice dynamics of these compounds. We illustrate how the lone pair leads to favorable bandwidths and band alignments, mobile holes, large ionic dielectric response, large positive thermal expansion, and even possibly defect-tolerant electronic transport. Taken together, the evidence suggests that other high-performing semiconductors may be found among compounds with lone-pair-bearing cations in high symmetry environments and a high degree of connectivity between atoms.

It has been widely speculated that dominant motifs, such as short-range icosahedral order, can influence glass formation and the properties of glasses. Experimental data on both fragile and strong undercooled liquids show corresponding changes in their thermophysical properties consistent with increasing development of a network of interconnect motifs based on molecular dynamics. Describing these regions of local order, how they connect, and how they are related to property changes have been challenging issues, both computationally and experimentally. Yet the consensus is that metallic liquids develop interconnected medium-range order consisting of some regions with lower mobility with deeper undercooling. Less well understood is how these motifs (or “crystal genes”) in the liquid can inhibit nucleation in the deeply undercooled liquid or influence phase selection upon devitrification. These motifs tend to have local packing unlike stable compounds with icosahedral order tending to dominate the best glass formers. The underlying kinetic and thermodynamic forces that guide the formation of these motifs and how they interconnect during undercooling remain open questions.

Liquid phase electron microscopy is a new analytical method that has opened up a rapidly emerging field of research during the past decade. This article discusses this new microscopy modality within the context of imaging eukaryotic cells, bacteria, proteins, viruses, and biomineralization processes. The obtained resolution is typically not a function of the instrument, rather it is limited by the available electron dose within the limit of radiation damage. Therefore, different types of samples are best imaged with different electron microscopy (EM) modalities. The obtained information differs from that acquired with conventional EM as well as cryo-electron microscopy. This article gives an overview of achievements thus far in this area and the unique information that has been obtained. A discussion on potential future developments in the field, and technological advancements required to reach those goals conclude the article.

Fundamental materials properties are determined by electrons under the potential energy from the nuclei, the electron mass, and their mutual repulsion. The variable from material to material is the ion potential. The logical procedure of computing electronic properties is to go from the potential to the electron distribution. This enables practical computation of the material properties ranging from atoms and molecules to solids. This method has blossomed due to the effort of numerous people. The concept is analogous to changing prediction of human population distribution from the landscape of hills and dales to determination of the landscape from a population distribution. In atomic systems, quantum quirkiness allows this switch, but dictates that it is only one slice in the tomography of the quantum state. The author shares his experience in the development from this slice, but hews close to the powerful concept of switching the landscape with the population.

Hydrogen is a promising alternative fuel for efficient energy production and storage, with water splitting considered one of the cleanest, environmentally friendly, and sustainable approaches to generate hydrogen. Electrochemically catalyzed water splitting plays an important role in energy conversion for the development of hydrogen-based energy sources. Porphyrin and macrocycle derivatives are versatile and can electrochemically catalyze water splitting efficiently. Because of the significance of molecule activation of electrochemical water splitting, this article covers recent progress in hydrogen evolution and oxygen evolution reactions catalyzed by porphyrin and macrocycle derivatives.

Biological materials represent a major source of inspiration to engineer protein-based polymers that can replicate the properties of living systems. Combined with our ability to control the molecular structure of proteins at the single amino acid level, this results in a vast array of attractive possibilities for materials science, an interest that is undeniably related to simplified procedures in gene synthesis, cloning, and biotechnological production. In parallel, it has been increasingly appreciated that living organisms exploit liquid–liquid phase separation (LLPS) to fabricate extracellular structures. In this article, we discuss the central role of protein LLPS in the fabrication of selected biological structures, including biological adhesives and hard biomolecular composites, and how physicochemical lessons from these systems are being replicated in synthetic analogs. Recent translational applications of protein LLPS are highlighted, notably aqueous-resistant adhesives, stimuli-responsive therapeutics carriers, and matrix materials for green structural composites.

Modern nanomaterials contain complexity that spans all three dimensions—from multigate semiconductors to clean energy nanocatalysts to complex block copolymers. For nanoscale characterization, it has been a long-standing goal to observe and quantify the three-dimensional (3D) structure—not just surfaces, but the entire internal volume and the chemical arrangement. Electron tomography estimates the complete 3D structure of nanomaterials from a series of two-dimensional projections taken across many viewing angles. Since its first introduction in 1968, electron tomography has progressed substantially in resolution, dose, and chemical sensitivity. In particular, scanning transmission electron microscope tomography has greatly enhanced the study of 3D nanomaterials by providing quantifiable internal morphology and spectroscopic detection of elements. Combined with recent innovations in computational reconstruction algorithms and 3D visualization tools, scientists can interactively dissect volumetric representations and extract meaningful statistics of specimens. This article highlights the maturing field of electron tomography and the widening scientific applications that utilize 3D structural, chemical, and functional imaging at the nanometer and subnanometer length scales.

Photocatalytic hydrogen production from water is a facile and clean approach to convert rich solar energy into chemical fuel. Developing efficient and robust catalysts to accelerate water-splitting speed is key. Porphyrins exist widely in green plants and are a key photosensitizer to absorb and transfer light energy to other parts of the photosynthesis system of plants. They are considered an ideal model to construct artificial photocatalysts for hot-carrier-mediated hydrogen production. This article discusses recent achievements in constructing porphyrin-based photocatalysts for hydrogen production, including porphyrin molecules, self-assembled porphyrins, metal–organic frameworks, conjugated porphyrin polymers, and hybrid nanomaterial-based photocatalysts. The design and synthesis principles, structure–property relationships, as well as urgent issues to be solved in the future for every type of photocatalyst are also discussed.

The past decade has witnessed the advent of nanophotonics, where light–matter interaction is shaped, almost at will, with human-made designed nanostructures. However, the design process for these nanostructures has remained complex, often relying on the intuition and expertise of the designer, ultimately limiting the reach and penetration of this groundbreaking approach. Recently, there has been an increasing number of studies in applying machine learning techniques for the design of nanostructures. Most of these studies engage deep learning techniques, which entail training a deep neural network (DNN) to approximate the highly nonlinear function of the underlying physical process of the interaction between light and the nanostructures. At the end of the training, the DNN allows for on-demand design of nanostructures (i.e., the model can infer nanostructure geometries for desired light spectra). In this article, we review previous studies for designing nanostructures, including recent advances where a DNN is trained to generate a two-dimensional image of the designed nanostructure, which is not limited to a closed set of nanostructure shapes, and can be trained for the design of any geometry. This allows for better generalization, with higher applicability for real-world design problems.

An indirect exciton (IX), also known as an interlayer exciton, is a bound pair of an electron and a hole confined in spatially separated layers. Due to their long lifetimes, IXs can cool below the temperature of quantum degeneracy. This provides an opportunity to experimentally study cold composite bosons. This article overviews our studies of cold IXs, presenting spontaneous coherence and Bose–Einstein condensation of IXs and phenomena observed in the IX condensate, including the spatially ordered exciton state, commensurability effect of exciton density wave, spin textures, Pancharatnam–Berry phase, long-range coherent spin transport, and interference dislocations.

Functionally graded nanocomposite materials (FGNMs) have been known since the 1980s, although nanocomposite materials date back to the space race era of the 1960s. FGNMs are defined as materials in which the chemical and structural composition changes over their entire volume. Today, due to our current understanding, technology, and control over the nanostructure of materials, we can tune these properties at the nanoscale. Although FGNM applications have mostly focused on protective coatings, they have performed well in catalysis and hydrogen production applications. In this article, FGNMs are presented in a new light beyond their well-established applicability as protective coatings. This article focuses on the synergistic potential among mechanical/tribological properties and competitive catalytic performance, with special emphasis on energy and remediation applications. Also, ways by which the rational design and tailoring of catalytic properties can be achieved by means of FGNMs are described.

Liquid cell transmission electron microscopy (TEM) has become an essential tool for studying the structure and properties of both hard and soft condensed-matter samples, as well as liquids themselves. Liquid cell sample holders, often consisting of two thin window layers separating the liquid sample from the high vacuum of the microscope column, have been designed to control in situ conditions, including temperature, voltage/current, or flow through the window region. While high-resolution and time-resolved TEM imaging probes the structure, shape, and dynamics of liquid cell samples, information about the chemical composition and spatially resolved bonding is often difficult to obtain due to the liquid thickness, the window layers, the holder configuration, or beam-induced radiolysis. In this article, we review different approaches to quantitative liquid cell electron microscopy, including recent developments to perform energy-dispersive x-ray and electron energy-loss spectroscopy experiments on samples in a liquid environment or the liquid itself. We also cover graphene liquid cells and other ultrathin window layer holders.

Dielectrics are electrical insulator materials, polarizable by opposite displacement of positive and negative ionized atoms via electric fields across the material’s thickness. Dielectrics are used in energy-storage capacitors, as key components in modern micro-/nanoelectronics, high-frequency and mobile communication devices, and life-saving microchips and other devices such as defibrillators and pacemakers implantable in humans. A key dielectric parameter is the dielectric constant (k), which largely controls the capacitance in capacitors with nanoscale area and dielectric layer thickness. Extremely high dielectric constants (k ≥1000) were observed in oxides (e.g., La1.8Sr0.12NiO4) with relaxor/ferroelectric materials and in combined semiconducting bulk properties with highly resistive grain boundaries. Giant dielectric constant films have also been demonstrated, based on integrating relatively low-dielectric-constant oxides into nanolaminate structures (e.g., TiOx/Al2O3; TiO2/HfO2) with tailored sublayer thicknesses, interfaces, and oxygen atom distributions. This overview article addresses the science and technology of high-dielectric-constant oxide materials with different compositions and structures.

The technology around generating efficient and sustainable energy is rapidly evolving; hydrogen and fuel cells are versatile examples within a portfolio of options. This article provides an overview of the early-stage materials R&D in hydrogen and fuel cells at the US Department of Energy (DOE) Fuel Cell Technologies Office within the Office of Energy Efficiency & Renewable Energy. The article highlights technology status and progress toward achieving DOE targets, discusses R&D needs and challenges, and provides specific examples where advanced materials research is relevant to addressing those challenges. For broader context, materials R&D advances are discussed in the context of DOE’s H2@Scale initiative, which is enabling innovations to generate cost-competitive hydrogen as an energy carrier, enabling renewables, as well as nuclear, fossil fuels, and the grid, to enhance the economics of both baseload power plants and intermittent solar and wind, enhancing resiliency and avoiding curtailment.

Through diversity of composition, sequence, and interfacial structure, hybrid materials greatly expand the palette of materials available to access novel functionality. The NSF Division of Materials Research recently supported a workshop (October 17–18, 2019) aiming to (1) identify fundamental questions and potential solutions common to multiple disciplines within the hybrid materials community; (2) initiate interfield collaborations between hybrid materials researchers; and (3) raise awareness in the wider community about experimental toolsets, simulation capabilities, and shared facilities that can accelerate this research. This article reports on the outcomes of the workshop as a basis for cross-community discussion. The interdisciplinary challenges and opportunities are presented, and followed with a discussion of current areas of progress in subdisciplines including hybrid synthesis, functional surfaces, and functional interfaces.

The linear and nonlinear mechanical properties of recombinant protein polymer networks are reviewed, with particular emphasis on how to tune elastic and dissipative behavior through selection of cross-linking strategy. The design strategies used to produce modular recombinant protein polymer networks through chemical or physical cross-linking will be discussed. In particular, we will highlight how key parameters such as polymer concentration, molecular weight, architecture, cross-link density, and association strength influence mechanics of protein polymer networks. Tuning these parameters enables control of viscoelastic properties and formation of materials with applications in tissue engineering, drug delivery, and sustainable self-healing materials.

Gold nanoparticles (AuNPs) are one of the most versatile and accessible classes of nanomaterials. Their chemical stability, ease of colloidal synthesis, surface functionalization, and plasmonic resonance—tunable from the visible through the near-infrared—have made AuNPs the metal nanoparticle of choice for many applications. This article summarizes the chemical synthesis of AuNPs, particularly gold nanorods, with a focus on recent developments in shape control and surface modifications. Current applications using the photothermal properties of AuNPs, as well as AuNP connections to biology and the environmental sciences, will be discussed.

Self-assembly techniques are powerful and efficient methods for the synthesis of nanoscale materials. Using these techniques and their combination with other bottom-up fabrication processes, materials with hierarchical features can be produced with form and function in multiple length scales. We synthesize multifunctional nanoparticles through surfactant-assisted noncovalent interactions using nanoparticle building blocks. Self-assembly of these nano-building blocks results in functional materials that exhibit well-defined morphologies and hierarchical architectures for a wide range of applications. Hierarchically structured porphyrin nanocrystals can be synthesized through surfactant micelle-confined noncovalent interactions of photoactive porphyrins. We can amplify the intrinsic advantages of individual photoactive porphyrins by engineering them into well-defined active nanostructures. Through kinetic control, these nanocrystals exhibit precisely defined size, shape, and spatial arrangement of the individual porphyrins, which facilitates intermolecular mass and energy transfer. These self-assembly techniques provide remarkable flexibility to design morphologies and architectures that produce desirable properties for practical applications including photocatalysis, photodegradation, and phototherapy.