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.

Superconductivity and magnetism at intermediate (“mesoscopic”) length scales between atomic and bulk have a long history of interesting science. New science emerges due to the presence of multiple length scales, especially when these become comparable to relevant geometric sizes. New phenomena may appear due to topological interactions, geometric confinement, proximity between dissimilar materials, dimensional crossover, and collective effects induced by periodicity. In this review, we select a few, recent highlights that illustrate the type of novel science that can be accomplished in superconducting and magnetic structures. These materials can serve as model systems and provide new ideas, which can be extended to other systems such as ferroelectrics and multiferroics. We also highlight general open questions and new directions in which the field may move.

Self-assembled epitaxial oxide composite films represent a new material form in which very high-quality mesoscopic structures can be created. The main focus has been on coupled electronic devices, but so far, only a very narrow range of compositions and structure types have been explored. Insufficient attention has been paid to the very wide window of possible materials combinations or to the novel materials properties that could be induced. Both of these aspects need to be addressed before we attain mesoscale devices with new properties. In this article, we review the unique materials properties of these epitaxially directed mesoscale composite structures, discussing their very high crystallinity, structural uniformity, and orientation. We also review how the structures can be size-tuned, from ∼2 nm up to ∼50 nm, and how they can be spatially ordered. We discuss how unusual strain states can be induced in the structures, and how epitaxial stabilization of the mesoscale surfaces within the films eliminates problems of surface degradation inherent to “free” nano/mesostructures. Several exemplar systems are given to show that composite films represent an unrivaled new approach to engineering new properties into mesoscale systems.

The ability to pattern materials in three dimensions is crucial for structural, optical, electronic, and energy applications. Three-dimensional printing allows one to design and rapidly fabricate materials in complex shapes without the need for expensive tooling, dies, or lithographic masks. A growing palette of printable materials, coupled with the ability to programmably control mesoscale architecture, open new avenues for creating designer materials with unprecedented performance.

Taking the mesoscale to mean length and time scales at which a material’s behavior is too complex to be understood by construction from the atomistic scale, we focus on three-dimensional characterization and modeling of mesoscale responses of polycrystals to thermal and mechanical loading. Both elastic and plastic internal structural responses are now accessible via high-energy x-ray probes. The combination of diffraction experiments and computed tomography, for example, is yielding new insights into how void formation correlates with microstructural features such as grain boundaries and higher-order junctions. The resulting large, combined data sets allow for validation of micromechanical and thermal simulations. As detectors improve in resolution, quantum efficiency, and speed of readout, data rates and data volumes present computational challenges. Spatial resolutions approach one micrometer, while data sets span a cubic millimeter. Examples are given of applications to tensile deformation of copper, grain growth in nickel and titanium, and fatigue cracks in superalloys.

Precise control of structural parameters through nanoscale engineering to continuously tailor optical and electronic properties of functional nanomaterials remains an outstanding challenge. Previous work focused largely on chemical or physical interactions that occur under ambient pressures. In this article, we introduce a new pressure-directed assembly and fabrication method that uses a mechanical compressive force applied to nanoparticles (NPs) to induce structural phase transitions and consolidate new nanomaterials with precisely controlled structures and tunable properties. By manipulating NP coupling through external pressure instead of through chemistry, a reversible change in assembly structure and properties can be demonstrated. In addition, over a certain threshold, the external pressure forces these NPs into contact, allowing the formation and consolidation of one- to three-dimensional nanostructures. Through stress-induced NP assembly, unusual materials engineering and synthesis, in which morphology and architecture can be readily tuned to produce desired optical and electrical properties, appear feasible.