The domain of mesoscale science, where the discrete granularity of atoms and quantization of energy give way to apparently continuous and infinitely divisible matter and energy, presents a new frontier of scientific opportunity and yields new complex architectures, phenomena, and functionalities. In this article, we describe some hallmarks of mesoscale science and highlight research directions that are described in greater detail in subsequent articles in this issue of MRS Bulletin. The exciting progress of the past several years and the rich unexplored opportunities at the mesoscale offer extraordinary prospects for future advances.

In the present era of big data and the Internet of things (the interconnection of computing devices in the Internet infrastructure), the fabrication of mobile and other electronic devices by three-dimensional integrated circuits (3D ICs) is receiving wide attention. The concept of using 3D ICs to extend the limit of Moore’s Law of two-dimensional ICs, by combining chip technology and packaging technology, has existed for more than 10 years. However, we still do not mass produce 3D IC devices due to low yield and reliability, as well as high cost. Most problems are caused by materials selection and integration at the small scale. This issue offers a review of 3D ICs and emphasizes the materials challenges of this new technology.

Mechanical property measurement protocols have their origins in metallurgy as well as in mechanical and civil engineering—metals being the first materials used on a broad industrial scale. Recent decades have evidenced growing interest in applying these protocols to biological materials or materials mimicking or replacing biological tissue. However, the mechanical properties of biological materials have been found to be highly variable and seemingly hard to capture by traditional protocols. A slow, emerging thought is that perhaps the mechanical theories underlying the testing protocols emanating from the metals field might not be fully applicable to the highly complex, hierarchically organized biological materials and might need further development. The articles in this issue highlight different and complementary, yet also interdependent, approaches to the challenge of extending theoretical and applied mechanics to the level needed for satisfying and reliably capturing the properties of biological materials. This issue also encompasses corresponding, far-reaching consequences for measurement techniques and evaluation protocols aimed at the determination of mechanical protocols.

Aerospace applications have historically been a driver of advanced materials, from reinforced carbon–carbon thermal protection systems of space reentry vehicles to advanced metal alloy turbine blades. Although the industry now has to share the spotlight and attention of both material scientists and funding sources with potentially larger commercial market draws such as energy and health care, it still presents some unique challenges that can be met only by the application of engineered nanomaterials. This issue of MRS Bulletin reviews some of the more promising aerospace applications of nanomaterials with a focus on space rather than aeronautics, the challenges of integrating such materials into existing systems, and the challenges that remain for maturation and industry adoption.

In situ transmission electron microscopy (TEM) has become an increasingly important tool for materials characterization. It provides key information on the structural dynamics of a material during transformations and the ability to correlate a material’s structure and properties. With the recent advances in instrumentation, including aberration-corrected optics, sample environment control, the sample stage, and fast and sensitive data acquisition, in situ TEM characterization has become more powerful. In this article, a brief review of the current status and future opportunities of in situ TEM is provided. The article also introduces the six articles in this issue of MRS Bulletin exploring the frontiers of in situ electron microscopy, including liquid and gas environmental TEM, dynamic four-dimensional TEM, studies on nanomechanics and ferroelectric domain switching, and state-of-the-art atomic imaging of light elements (i.e., carbon atoms) and individual defects.

Greener technologies for more efficient power generation, distribution, and delivery in sectors ranging from transportation and generic energy supply to telecommunications are quickly expanding in response to the challenge of climate change. Power electronics is at the center of this fast development. As the efficiency and resiliency requirements for such technologies can no longer be met by silicon, the research, development, and industrial implementation of wide bandgap semiconductors such as gallium nitride (GaN) and silicon carbide (SiC) are progressing at an unprecedented pace. This issue of MRS Bulletin, although certainly not exhaustive, provides an overview of the pace and quality of research revolving around GaN and SiC power electronics, from the choice of substrates, film growth, devices, and circuits to examples of applications.

Biomineralization is the process by which living organisms orchestrate the synthesis and organization of minerals (biominerals), and it may be viewed as an ancient process for accumulation of metal ions in living systems. The structure and properties of biominerals have yet to be rivaled by any synthetic effort by scientists to date. Therefore, deciphering the assembly algorithms and the components that initiate and promote hierarchical deposition of cations has significant implications for the development of nanocomposites and nanotechnology as a whole. This issue of MRS Bulletin highlights some of the challenges in characterizing and replicating the biomineralization processes, and the role of non-collagenous proteins in the biomineralization process.

Emergent behavior can be achieved in composites by interfacing different materials at the nano- or mesoscales. Integrating different materials on a single platform or forming composite provides a new design paradigm to yield enhanced or novel functionalities that cannot be obtained in individual constituents. Nanocomposites, in particular, have been model systems for enhancing interface effects on physical properties because they provide reduced dimensionality or enlarged interfacial areas. To fabricate technologically relevant multifunctional materials, one needs to understand and control the interactions in different materials by manipulating interfaces at the nano- or mesoscales. This issue of MRS Bulletin focuses on nanocomposites, with an emphasis on approaches to the design and control of the functionalities of composite materials through controlled synthesis and advanced characterization in concert with simulation and modeling.

Perovskite solar cells based on methylammonium lead triiodide witnessed unprecedented progress after the seminal work of Miyasaka and co-workers in 2009, where they employed perovskite nanocrystals as sensitizers in a dye-sensitized solar-cell configuration. After key breakthroughs with solid-state perovskite photovoltaics in 2012, research efforts have grown exponentially, and several groups have demonstrated that the perovskite concomitantly acts as a light absorber and an electron and hole transporter in both mesoscopic networks and solid polycrystalline layers, where the perovskite layer can be deposited using a broad range of techniques. The methylammonium lead triiodide perovskite bandgap has been tuned by substituting various cations and anions. By optimizing the crystalline quality of the perovskite absorber and film formation by solvent engineering, a remarkable power-conversion efficiency of over 20% has been demonstrated, highlighting the exceptional photovoltaic properties of perovskite materials. The high efficiencies are due to a combination of long carrier lifetimes, substantial charge-carrier mobilities, and remarkably benign electronic defects. This issue highlights various deposition methods of the perovskite absorber, such as single-step, sequential, dual-source sublimation, and solution and sublimation processes, as well as hole-transporting-free and tandem perovskite solar cells.

Transition-metal dichalcogenides (TMDCs) are compounds consisting of a transition-metal M (Ti, Hf, Zr, V, Nb, Ta, Mo, W, Tc, Re, Pd, Pt) and chalcogen atoms X (S, Se, Te). There are approximately 60 compounds in the metal chalcogenide family, and two-thirds of them are in the form of layered structures where the in-plane bonds are strong (covalent), and the out-of-plane bonds are weak (van der Waals). This provides a means to mechanically or chemically thin (exfoliate) these materials down to a single atomic two-dimensional (2D) layer. While graphene, the 2D form of graphite, is metallic, the layered metal chalcogenides cover a wide range of electrical properties, from true metals (NbS2) and superconductors (TaS2) to semiconductors (MoS2) with a wide range of bandgaps and offsets. Multiple techniques are currently being developed to synthesize large-area monolayers, including alloys, and lateral and vertical heterostructures. The wide range of properties and the ability to tune them on an atomic scale has led to numerous applications in electronics, optoelectronics, sensors, and energy. This article provides an introduction to TMDCs, serving as a background for the articles in this issue of MRS Bulletin.

Three-dimensional (3D) printing represents the direct fabrication of parts layer-by-layer, guided by digital information from a computer-aided design file without any part-specific tooling. Over the past three decades, a variety of 3D printing technologies have evolved that have transformed the idea of direct printing of parts for numerous applications. Three-dimensional printing technology offers significant advantages for biomedical devices and tissue engineering due to its ability to manufacture low-volume or one-of-a-kind parts on-demand based on patient-specific needs, at no additional cost for different designs that can vary from patient to patient, while also offering flexibility in the starting materials. However, many concerns remain for widespread applications of 3D-printed biomaterials, including regulatory issues, a sterile environment for part fabrication, and the achievement of target material properties with the desired architecture. This article offers a broad overview of the field of 3D-printed biomaterials along with a few specific applications to assist the reader in obtaining an understanding of the current state of the art and to encourage future scientific and technical contributions toward expanding the frontiers of 3D-printed biomaterials.

The evolution of materials; their synthesis, shaping, and performance; and the engineering of artifacts and systems to meet societal demands are inextricably interwoven. In this article, we describe an evolving scenario of the relationship between materials and engineering that provides a framework for the articles that explore various facets of this theme in this special issue of MRS Bulletin.