Annual Review of Materials Research - Volume 47, 2017

Metal oxides, when electronically doped with oxygen vacancies, aliovalent dopants, or interstitial dopants, can exhibit metallic behavior due to the stabilization of a substantial charge carrier concentration within the material. As a result, localized surface plasmon resonances (LSPRs) occur in nanocrystals of conducting metal oxides. Through deliberate choice of both the host material and the defect, these resonances can be tuned across the entirety of the near- and mid-infrared regions of the electromagnetic spectrum. Optical modeling has revealed that the defects present have profound impacts on charge carrier mobility and electronic structure, and in some cases, choosing one dopant over another is an important trade-off for optimizing plasmonic performance. These materials are distinct from classical metals in that one can tune their LSPR in energy and intensity through their elemental composition independently of any particular size or nanocrystal morphology. In addition, the LSPR in these materials is highly modulable through external stimuli over substantial spectral windows. As a result, these materials uniquely provide a responsive plasmonic material that can offer optimal nanocrystal arrangements and morphology without compromising the intended resonance frequency for light concentration at any infrared wavelength.

Directed crystallization of a large variety of nanoparticles, including proteins, via DNA hybridization kinetics has led to unique materials with a broad range of crystal symmetries. The nanoparticles are functionalized with DNA chains that link neighboring functionalized units. The shape of the nanoparticle, the DNA length, the sequence of the hybridizing DNA linker, and the grafting density determine the crystal symmetries and lattice spacing. By carefully selecting these parameters, one can, in principle, achieve all the symmetries found for both atomic and colloidal crystals of asymmetric shapes as well as new symmetries and can drive transitions between them. A scale-accurate coarse-grained model with explicit DNA chains provides the design parameters, including the degree of hybridization, to achieve specific crystal structures. The model also provides surface energy values to determine the shape of defect-free single crystals with macroscopic anisotropic properties, which has potential for the fabrication of materials with specific optical and mechanical properties.

Mechanical instabilities are traditionally regarded as a route toward failure. However, they can also be exploited to design architected cellular materials with tunable functionality. In this review, we focus on three examples and show that mechanical instabilities in architected cellular materials can be harnessed (a) to design auxetic materials, (b) to control the propagation of elastic waves, and (c) to realize reusable energy-absorbing materials. Together, these examples highlight a new strategy to design tunable systems across a wide range of length scales.

Poisson's ratio had long been considered to be an intrinsic material property, confined within a narrow domain and governed solely by the geometry of interatomic bonds. Materials with designed heterogeneity allow for control over the Poisson's ratio. Poisson's ratios of any value within the thermodynamically admissible domain may be attained, including negative Poisson's ratio (termed auxetic). In this article, we discuss the role of Poisson's ratio in elasticity, two-dimensional and three-dimensional materials, phase transformations, underlying causes in the microstructure, and other negative physical properties.

The recent advent of acoustic metamaterials has initiated a strong revival of interest on the subject of sound absorption. The present review is based on the physics perspective as the coherent basis of this diverse field. For conventional absorbers, viscous dissipation and heat conduction at the fluid-solid interface, when expressed through micro-geometric parameters, yield an effective medium description of porous media and micro-perforated panels as effectual sound absorbers. Local resonances and their geometric and symmetry constraints serve as the framework for surveying a variety of acoustic metamaterial absorbers that can realize previously unattainable absorption spectra with subwavelength-scale structures. These structures include decorated membrane resonators, degenerate resonators, hybrid resonators, and coiled Fabry-Pérot and Helmholtz resonators. As the acoustic response of any structure or material must obey the causality principle, the implied constraint—which relates the absorption spectrum of a sample to its required minimum thickness—is presented as a means to delineate what is ultimately possible for sound-absorbing structures. The review concludes by describing a recently reported strategy for realizing structures that can exhibit custom-designed absorption spectra, as well as its implementation in the form of a broadband absorber with a thickness that is close to the minimum value as dictated by causality.

Metamaterials provide a challenge for materials analysis, as large-scale 3D geometries confound traditional methods. X-ray optics that allow for novel beam geometries and for more efficient use of conventional X-ray sources can be important in defect and structure analysis to close the loop between design and performance. Fortunately, metamaterials have also provided a new variety of array and structured X-ray optics. Because X-rays barely interact with materials, their index of refraction in any material is only slightly different from unity, so it is very difficult to make the sort of refractive optics that are used for visible light. Instead, diffraction can be used to control the direction or wavelength of X-rays. Structured diffractive optics include transmission gratings, artificial multilayers, and arrays of curved crystals. X-rays also have very high reflectivity at grazing incidence. This phenomenon allows for reflective arrays such as multipore and polycapillary optics.

Most X-ray optics for use at synchrotron beamlines are structured to achieve a desired performance level. The fabrication steps needed to achieve a certain structure usually limit the final performance, such as for energy resolution and focus size. This review illustrates this point for monochromators, mirrors, multilayers, and zone plates, with a special emphasis on focusing optics because these provide some of the best examples of structuring. Elliptically shaped mirrors, Fresnel zone plates, and multilayer Laue lenses are reviewed.

We review recent advances in crystal growth techniques, focusing on the development of novel quantum materials. Recent progress in instrumentation design, opening new avenues in bulk crystal growth of oxide and intermetallic compounds, is highlighted. Specifically, we illustrate leading techniques that allow for the active control of crystal nucleation/growth and provide platforms for the realization of single crystals with ultrahigh purity and minimized defects. Advances in the postgrowth manipulation of crystals, as well as the impact of purification techniques on the stabilization of delicate quantum phases, are also discussed. Throughout, we highlight new scientific avenues opened by access to high-purity single-crystal samples.

Li ion batteries are important components of portable devices, electric vehicles, and smart grids owing to their high energy density, excellent cyclic performance, and safe operation. However, further development of electrode materials for these batteries is needed to satisfy continually increasing performance demands. Typically, both the charge/discharge kinetics and structural stability of these electrode materials depend on the transport and storage properties of the Li ions. High-spatial-resolution information on structural changes and on the strong interaction between electrons and ions is essential for a better understanding of the electrochemical performance of rechargeable batteries. In this article, we review the known atomic-scale structural changes of these electrode materials during the charge/discharge process, with special emphasis on ion/electron interactions.

Activated processes in materials are important for many of the properties that make them function. Batteries and catalysts are examples for which understanding how the component materials function on a timescale of milliseconds to seconds is critical to making improvements in a rational way. Modeling materials over these long timescales, relative to the timescale of atomic vibrations, is one of the grand challenges of the field. Transition state theory is central to bridging this timescale gap, and in the materials community, the harmonic approximation and the determination of saddle points to quantify reaction rates are ubiquitous. In this review, single- and double-ended methods for saddle point finding are discussed, as well as how finding saddle points can be used in the adaptive kinetic Monte Carlo method to model materials properties on the timescale of activated processes. Applications of surface diffusion and chemistry, phase boundary migration, and solid-solid phase transitions are presented.

We give a bird's-eye view of the plastic deformation of crystals aimed at the statistical physics community, as well as a broad introduction to the statistical theories of forced rigid systems aimed at the plasticity community. Memory effects in magnets, spin glasses, charge density waves, and dilute colloidal suspensions are discussed in relation to the onset of plastic yielding in crystals. Dislocation avalanches and complex dislocation tangles are discussed via a brief introduction to the renormalization group and scaling. Analogies to emergent scale invariance in fracture, jamming, coarsening, and a variety of depinning transitions are explored. Dislocation dynamics in crystals challenge nonequilibrium statistical physics. Statistical physics provides both cautionary tales of subtle memory effects in nonequilibrium systems and systematic tools designed to address complex scale-invariant behavior on multiple length scales and timescales.

Heusler compounds are a large family of binary, ternary, and quaternary compounds that exhibit a wide range of properties of both fundamental and potential technological interest. The extensive tunability of the Heusler compounds through chemical substitutions and structural motifs makes the family especially interesting. In this article we highlight recent major developments in the field of Heusler compounds and put these in the historical context. The evolution of the Heusler compounds can be described by four major periods of research. In the latest period, Heusler 4.0 has led to the observation of a variety of properties derived from topology that includes topological metals with Weyl and Dirac points; a variety of noncollinear spin textures, including the very recent observation of skyrmions at room temperature; and giant anomalous Hall effects in antiferromagnetic Heuslers with triangular magnetic structures. Here we give a comprehensive overview of these major achievements and set research into Heusler materials within the context of recent emerging trends in condensed matter physics.

We examine ice crystallization from liquid water and from water vapor, focusing on the underlying physical processes that determine growth rates and structure formation. Ice crystal growth is largely controlled by a combination of molecular attachment kinetics on faceted surfaces and large-scale diffusion processes, yielding a remarkably rich phenomenology of solidification behaviors under different conditions. Layer nucleation plays an especially important role, with nucleation rates determined primarily by step energies on faceted ice/water and ice/vapor interfaces. The measured step energies depend strongly on temperature and other factors, and it appears promising that molecular dynamics simulations could soon be used in conjunction with experiments to better understand the energetics of these terrace steps. On larger scales, computational techniques have recently demonstrated the ability to accurately model the diffusion-limited growth of complex structures that are both faceted and branched. Together with proper boundary conditions determined by surface attachment kinetics, this opens a path to fully reproducing the variety of complex structures that commonly arise during ice crystal growth.

Modern gas turbines rely on ceramic coatings to protect structural components along the hot gas path. These coatings are susceptible to accelerated degradation caused by silicate deposits formed when ingested environmental debris (dust, sand, ash) adheres to the coatings. This article reviews the current understanding of the deposit-induced failure mechanisms for zirconia-based thermal barrier coatings and silicate environmental barrier coatings. Details of the debris melting and crystallization behavior, the nature of the chemical reactions occurring between the deposits and coatings, and the implications for the thermocyclic durability of the coatings are described. Given the challenges posed in understanding how prospective coating materials and architectures will respond to a broad range of deposit compositions, it is proposed to develop an integrated framework linking thermochemical and thermomechanical models to predict coating durability. Initial progress toward developing this framework, and the requisite research needs, are discussed.

Historically, fibers are known to be relatively passive materials and are used primarily in textiles. Today, however, fibers with a range of functionalities such as electrical and thermal conductivity, superparamagnetic properties, temperature regulation, energy harvesting, and biomedical capability provide many possibilities. Most man-made fibers today are derived from petroleum, but there is increasing emphasis on making fibers biorenewable. Fibers are also the strongest structural materials available today. Different fiber fabrication technologies, available properties, and some near-term future prospects are discussed.

Synthetic two-dimensional polymers (2DPs) are an emerging subclass of 2D materials that are accessible by mild and rational synthesis procedures. Because of their structure, which is reminiscent of a molecular fisherman's net, 2DPs strongly differ from conventional linear polymers. They are expected to show applications in fields such as gas separation, nonlinear optics, and miniaturization of optical devices. The article sketches the historic development that led to the first fully proven representatives, describing each case in detail and comparing the current main access routes. Insights into some application aspects are also given. Alternative synthesis routes are also presented, together with the critical analytical issues. Structure analysis is perhaps the greatest challenge in this field, which arises with regard to proving the periodic structure of a 2DP. This article intends to stimulate the interest of an interdisciplinary community to join forces to develop this promising field even more quickly.

Transparent conducting oxides (TCOs) and transparent oxide semiconductors (TOSs) have become necessary materials for a variety of applications in the information and energy technologies, ranging from transparent electrodes to active electronics components. Perovskite barium stannate (BaSnO3), a new TCO or TOS system, is a potential platform for realizing optoelectronic devices and observing novel electronic quantum states due to its high electron mobility, excellent thermal stability, high transparency, structural versatility, and flexible doping controllability. This article reviews recent progress in the doped BaSnO3 system, discussing the wide range of physical properties, electron-scattering mechanism, and demonstration of key semiconducting devices such as pn diodes and field-effect transistors. Moreover, we discuss the pathways to achieving two-dimensional electron gases at the interface between BaSnO3 and other perovskite oxides and describe remaining challenges for observing novel quantum phenomena at the heterointerface.

We review current progress with respect to ultrafast, atomic-scale resolution X-ray scattering studies of materials. In the last decade, advances in sources and techniques have opened up new possibilities for probing the dynamics of how materials change in situ and in real time. These developments have enabled direct measurements of the first primary steps in how complex/functional materials transform and the flow of energy between different degrees of freedom. Unique insight into the mechanisms underlying how materials function is obtained through these studies. An outlook on future developments and new opportunities is also presented.

X-ray tomography has emerged as a powerful technique for studying lithium ion batteries, allowing nondestructive and often quantitative imaging of these complex systems, which contain solid components with length scales spanning orders of magnitude and which are in-filled with liquid electrolyte. Over the past decade, X-ray tomography has allowed interrogation of structure and material composition, providing quantitative or qualitative insight into battery operation and degradation. In this review, we first provide an overview of X-ray tomography and explore what types of experiments can yield insight into open questions in the lithium ion battery research field. In the second half of the review, we discuss the aspects a researcher must consider, and we summarize challenges and approaches to sample preparation, experimental setup, and data analysis. Finally, we describe both outstanding challenges and promise in using X-ray tomography for lithium ion battery research.