Designing structural materials for tailored response at extreme conditions is a grand challenge in materials research. Such materials can be made using either “top-down” or “bottom-up” processes to create nanostructured metals and composites that contain atomically designed interfaces that not only block dislocation slip but also attract, absorb, and annihilate point and line defects. Such multifunctional material systems are not just high in strength but also tolerant of damage at extremes of irradiation, temperature, and mechanical stresses, and hence have applications as structural materials in nuclear power and other energy, transportation, and defense technologies. The exploration of these exceptional properties at extremes requires novel and unconventional methodologies, such as in situ experiments with high spatial and temporal resolution, complemented by simulation across multiple length and time scales.

Actinide solid-state and coordination chemistry has advanced through unexpected results that have further revealed the complex nature of the 5f elements. Nanoscale control of actinide materials is emerging, as shown by the creation of a considerable range of cluster and tubular topologies. Departures from established structural trends for actinyl ions are provided by cation-cation interactions in which an O atom of one actinyl ion is an equatorial ligand of a bipyramid of another actinyl ion. The solid-state structural complexity of actinide materials has been further demonstrated by open framework materials with interesting properties. The U(VI) tetraoxide core has been added to this cation's repertoire of coordination possibilities. The emergence of pentavalent uranium solid-state and coordination chemistry has resulted from the prudent selection of ligands. Finally, analogues of the uranyl ion have challenged our understanding of this normally unreactive functional group.

Understanding the mechanical properties of materials is crucial for their reliable application as bulk materials as well as in a miniaturized form. The deformation of materials is usually non-uniform and, hence, needs to be characterized on a local level. The following article focuses on the in-Situ determination of mechanical stresses in crystalline materials during deformation. This can be achieved by both diffraction as well as spectroscopical methods, where the elastic strain is the parameter measured, which is subsequently converted into stresses by the application of Hooke's law. As in Situ measurements require rapid data acquisition in conjunction with reasonable penetration depths, we will focus on x-rays. However, the different techniques described can be applied to any other diffraction probe as well. The description of diffraction techniques, which span the range from averaging techniques to 2D and 3D strain mapping, is complemented by a section on Raman spectroscopy as an alternative method for stress determination for non-metallic materials. Local stresses also can be correlated to local defect densities.

Oriented aggregation is a nonclassical crystal growth mechanism resulting in new secondary particles composed of crystallographically aligned primary crystallites. These secondary crystals often have unique and symmetry-defying morphologies, can be twinned, and can contain stacking faults and other significant defects. A wide range of materials, such as titanium dioxide, iron oxides, selenides and sulfides, and metal oxyhydroxides, are known to grow by oriented aggregation under certain conditions. Evidence for oriented aggregation also has been observed in natural materials. Over the last decade, reports of this crystal growth mechanism have appeared with increasing frequency in the scientific literature. The development of kinetic models aimed at improving our fundamental understanding as well as facilitating purposeful control over size, size distribution, and shape has ranged from simple dimer formation models to polymeric models and population balance models. These models have enabled detection and characterization of crystal growth by oriented aggregation using methods such as small-angle x-ray scattering, among others, in addition to transmission electron microscopy. As our fundamental understanding of oriented aggregation improves, novel and complex functional materials are expected to emerge. This article presents a summary of some recent results, methods, and models for characterizing crystal growth by oriented aggregation.

Van der Waals (vdW) interactions play a prominent role in the structure and function of organic/organic and organic/inorganic interfaces. Their accurate determination from first principles, however, is a notoriously difficult task. Recently, a surge of interest in modeling vdW interactions has led to promising theoretical developments. This article reviews the state-of-the-art of describing vdW interactions by density-functional theory with respect to accuracy and practicability. The performance of the different methods is demonstrated for simple systems, such as rare-gas dimers and small organic molecules. The nature of binding at organic/inorganic interfaces is then exemplified for the perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) molecule at surfaces of coinage metals. This fundamental system is the best-characterized organic molecule/metal interface in experiment and theory. We emphasize the crucial importance of a balanced description of both geometry and electronic structure in order to understand and model the properties of such systems. Finally, the relevance of vdW interactions to the function of actual devices based on interfaces is discussed.

Nanocellulose, such as that produced by the bacteria Gluconacetobacter xylinus (bacterial cellulose, BC), is an emerging biomaterial with great potential as a biological implant, wound and burn dressing material, and scaffolds for tissue regeneration. BC has remarkable mechanical properties despite the fact that it contains up to 99% water. The water-holding ability is the most probable reason why BC implants do not elicit any foreign body reaction. Moreover, the nanostructure and morphological similarities with collagen make BC attractive for cell immobilization and cell support. The architecture of BC materials can be engineered over length scales ranging from nano to macro by controlling the biofabrication process. This article describes current and future applications of BC in the biomedical field.

X-ray and neutron diffraction have been two key techniques for structural characterization of materials since their inception. If single crystals of the materials of interest cannot be synthesized, one has to resort to powder diffraction. This results in the loss of three-dimensional orientation information of the crystal, and one has to contend with the one-dimensional information that is inherent to powder diffraction, making it harder to analyze the data. The structural study of contemporary materials and their remarkable properties is a challenging problem, particularly when properties of interest result from interplay of multiple degrees of freedom. Very often these are associated with structural defects or relate to different length scales in a material. The signature of the defect-related phenomenon is visible as diffuse scattering in the diffraction pattern, and the signals associated with diffuse scattering are orders of magnitude smaller than Bragg scattering. Given these limitations, it is crucial to have high-resolution and high-intensity data along with the ability to carry out theoretical interpretation that goes beyond periodic lattice formalism of crystallography. Great advances have been achieved due to the advent of synchrotron and neutron sources, along with the availability of high-speed computational algorithms allowing materials scientists to work with a very small amount of sample (both single crystal and powder) and analyze vast amounts of data to unravel detailed structural descriptions that were not previously possible. This article presents some of these great advances in using scattering probes for materials characterization.

Recent discoveries of novel electronic states, including relatively high-temperature superconductivity, in the actinides point to exciting prospects for future discoveries at the bottom of the periodic table. A key ingredient in all of the successes discussed here is the role of high-quality synthesis in enabling advances. Results on PuCoGa51, NpPd5Al2, and single crystal uranium are discussed.

Advances in nanoscale directed assembly strategies have enabled researchers to analogize atomic assembly via chemical reactions and nanoparticle assembly, creating a new nanoscale “periodic table.” We are just beginning to realize the nanoparticle equivalents of molecules and extended materials and are currently developing the ground rules for creating programmable nanometer-scale coordination environments. The ability to create a diverse set of nanoscale architectures from one class of nanoparticle building blocks would allow for the synthesis of designer materials, wherein the physical properties of a material could be predicted and controlled a priori. Our group has taken the first steps toward this goal and developed a means of creating tailorable assembly environments using DNA-nanoparticle conjugates. These nanobioconjugates combine the discrete plasmon resonances of gold nanoparticles with the synthetically controllable and highly selective recognition properties of DNA. Herein, we elucidate the beneficial properties of these materials in diagnostic, therapeutic, and detection capabilities and project their potential use as nanoscale assembly agents to realize complex three-dimensional nanostructures.

In electronics and photonics, intrinsic properties of semiconducting materials play a dominant role in achieving high-performance devices and circuits. In this respect, carbon nanotubes are prime candidates because of their exceptionally high carrier mobility, low capacitance, and strong optical response (direct bandgap). Although these properties compare very favorably with those of crystalline silicon, several issues related to their synthesis, processing, and assembly have challenged efforts for making electronic and photonic devices. Tremendous progress, nevertheless, has been achieved over the years, and much has been learned from novel photonic devices and electronic circuits. We review some of the developments in nanotube transistor performance optimization, ac operation, nanotube circuits, self-assembly, thin-film devices, and nanotube optical devices such as light emitters and detectors. We also examine the issues and opportunities that still exist.

In the sophisticated architectures of various biominerals, a multilevel hierarchy of lengths from nanometers to millimeters is commonly observed. In this article, superstructures composed of inorganic crystals associated with organic molecules from carbonate-based biominerals were identified and categorized as mesocrystals, which are composed of crystallographically oriented crystals. For the production of hierarchically structured mesocrystals, two strategies using self-organized growth with organic agents in aqueous solution systems were proposed. Arranged structures of micrometric and nanometric units were induced with an insoluble gel matrix and soluble adsorbable organic molecules, respectively. Consequently, bioinspired hierarchical crystals were successfully achieved by combining the matrix and soluble species.

One of the major challenges in the field of regenerative medicine is how to optimize tissue regeneration in the body by therapeutically manipulating its natural ability to form scar at the time of injury or disease. It is often the balance between tissue regeneration, a process that is activated at the onset of disease, and scar formation, which develops as a result of the disease process that determines the ability of the tissue or organ to be functional. Using biomaterials as scaffolds often can provide a “bridge” for normal tissue edges to regenerate over small distances, usually up to 1 cm. Larger tissue defect gaps typically require both scaffolds and cells for normal tissue regeneration to occur without scar formation. Various strategies can help to modulate the scar response and can potentially enhance tissue regeneration. Understanding the mechanistic basis of such multivariate interactions as the scar microenvironment, the immune system, extracellular matrix, and inflammatory cytokines may enable the design of tissue engineering and wound healing strategies that directly modulate the healing response in a manner favorable to regeneration.

Plastic deformation can effectively produce nanostructured metals and alloys in bulk or surface-layer forms that are suitable for practical structural or functional applications. Such nanostructured materials are porosity-free and contamination-free, and therefore they are ideal for studying fundamental mechanisms and mechanical properties. In this article, we first give an overview of the principles of grain refinement by plastic deformation and an introduction to the reported processing techniques. Then the four most-developed and promising techniques will be described in detail: equal-channel angular pressing, high-pressure torsion, accumulative roll bonding for bulk nanostructured metals, and surface mechanical attrition treatment for nanostructured surface layers.

Organic light-emitting diodes bring a whole new level of image quality, power consumption, and very thin profiles to displays. In addition, with the appropriate choice of a flexible substrate, paper-like flexible displays that are lightweight, robust, and conformable can be produced. This will make it possible to roll or fold the displays for portability or incorporate them in clothing as wearable displays. Plastic substrates are considered prospective materials due to their inherent flexibility and optical qualities. However, one of the major drawbacks of plastics is the large thermal expansion. The thermal expansion of the substrate has to be compatible with those of the layers deposited on it, otherwise these layers will become strained and crack during the thermal cycling involved in the display manufacture. One of the proposed solutions to reduce the thermal expansion of plastics without appreciable loss in transparency is to reinforce them with nanofibers. These nanofibers are already available in enormous quantities in nature, in the form of cellulose, with the caveat that they have to be extracted properly. Here we present the methodologies required to obtain the cellulose nanofibers and to produce optically transparent composites for use in flexible displays.

In the last 40 years, the increased space activity created a new form of space environment of hypervelocity objects—space debris—that have no functional use. The space debris, together with naturally occurring ultrahigh velocity meteoroids, presents a significant hazard to spacecraft. Collision with space debris or meteoroids might result in disfunction of external units such as solar cells, affecting materials properties, contaminating optical devices, or destroying satellites. The collision normally results in the formation of additional debris, increasing the hazard for future missions. The hypervelocity debris effect is studied by retrieving materials from space or by using ground simulation facilities. Simulation facilities, which include the light gas gun and Laser Driven Flyer methods, are used for studying the materials degradation due to debris impact. The impact effect could be accelerated when occurring simultaneously with other space environment components, such as atomic oxygen, ultraviolet, or x-ray radiation. Understanding the degradation mechanism might help in developing materials that will withstand the increasing hazard from the space debris, allowing for longer space missions. The large increase in space debris population and the associated risk to space activity requires significant measures to mitigate this hazard. Most current efforts are being devoted to prevention of collisions by keeping track of the larger debris and avoiding formation of new debris.

Mechanical testing of micro- and nanoscale materials is challenging due to the intricate nature of specimen preparation and handling and the required load and displacement resolution. In addition, in Situ testing requires the entire experimental setup to be drastically miniaturized, because conventional high-resolution microscopes or analytical tools usually have very small chambers. These challenges are increasingly being addressed using microelectromechanical systems (MEMS)-based sensors and actuators. Because of their very small size, MEMS-based experimental setups are the natural choice for materials characterization under virtually all forms of in Situ electron, optical, and probe microscopy. The unique advantage of such in Situ studies is the simultaneous acquisition of qualitative (up to near atomic visualization of microstructures and deformation mechanisms) and quantitative (load, displacement, flaw size) information of fundamental materials behavior. In this article, we provide a state-of-the-art overview of design and fabrication of MEMS-based devices for nanomechanical testing. We also provide a few case studies on thin films, nanowires, and nanotubes, as well as adhesion-friction testing with a focus on in Situ microscopy. We conclude that MEMS devices offer superior choices in handling, actuation, and force and displacement resolutions. Particularly, their tight tolerances and small footprints are difficult to match by off-the-shelf techniques.

We present our understanding of the electronic energy landscape and dynamics of charge separation at organic donor/acceptor interfaces. The organic/organic interface serves as a valuable point of reference and plays an important role in emerging electronic and optoelectronic applications, particularly organic photovoltaics (OPVs). The key issue on electronic structure at organic donor/acceptor interfaces is the difference in the lowest unoccupied molecular orbitals or that in the highest occupied molecular orbitals. This difference represents an energy gain needed to overcome the exciton binding energy in a charge-separation process in OPV. A sufficiently large energy gain favors the formation of charge transfer (CT) states that are energetically close to the charge-separation state. At an organic donor/acceptor interface in an OPV device, these high-energy CT states, also called hot CT excitons, are necessary intermediates in a successful charge-separation process.