Multijunction solar cells have proven to be capable of extremely high efficiencies by combining multiple semiconductor materials with bandgaps tuned to the solar spectrum. Reaching the optimum set of semiconductors often requires combining high-quality materials with different lattice constants into a single device, a challenge particularly suited for metamorphic epitaxy. In this article, we describe different approaches to metamorphic multijunction solar cells, including traditional upright metamorphic, state-of-the-art inverted metamorphic, and forward-looking multijunction designs on silicon. We also describe the underlying materials science of graded buffers that enables metamorphic subcells with low dislocation densities. Following nearly two decades of research, recent efforts have demonstrated high-quality lattice-mismatched multijunction solar cells with very little performance loss related to the mismatch, enabling solar-to-electric conversion efficiencies over 45%.
Metal-organic frameworks (MOFs) are porous ordered arrays of inorganic clusters connected by organic linkers. The compositional diversity of the metal and ligand, combined with varied connectivity, has yielded more than 20,000 unique structures. Electronic structure theory can provide deep insights into the fundamental chemistry and physics of these hybrid compounds and identify avenues for the design of new multifunctional materials. In this article, a number of recent advances in materials modeling of MOFs are reviewed. We present the methodology for predicting the absolute band energies (ionization potentials) of porous solids as compared to those of standard semiconductors and electrical contacts. We discuss means of controlling the optical bandgaps by chemical modification of the organic and inorganic building blocks. Finally, we outline the principles for achieving electroactive MOFs and the key challenges to be addressed.
The US Materials Genome Initiative (MGI) has emphasized the need to accelerate the discovery and development of materials to maintain industry competitiveness in new and existing markets. While largely interpreted as an initiative arising from the materials community, it is important to address the coupling of materials with manufacturing and all other relevant aspects of product development in order to maximize its impact. The dual thrusts of Integrated Computational Materials Engineering and the MGI represent a long-term vision of industry, academic, and government stakeholders. The goal is to build a new kind of coupled experimental, computational, and data sciences infrastructure. The emphasis is on high-throughput methods to accelerate historical sequential processes of serendipitous materials discovery and largely empirical materials development by leveraging computation and modern data sciences and analytics. The notion of a materials innovation ecosystem is introduced as the framework in which to pursue acceleration of discovery and development of materials consisting of various elements of data sciences, design optimization, manufacturing scale-up and automation, multiscale modeling, and uncertainty quantification with verification and validation.
The epitaxial integration of III-V optoelectronic devices on silicon will be the enabling technology for full-scale deployment of silicon photonics and the key to improving communication systems. Silicon photonics also offer new opportunities for the realization of ultracompact and fully integrated sensing systems operating in the mid-infrared (MIR) regime of the spectrum. In this article, we review recent developments, through several approaches, in the direct metamorphic epitaxial growth of various III-V materials-based lasers on silicon substrates. We show that GaAs-based 1.3-mu m III-V quantum dot lasers and GaSb-based MIR quantum-well lasers grown on silicon substrates can operate with low threshold current density and high operating temperature, which hold promise for the future.
The well-known synthetic versatility of metal-organic frameworks (MOFs) is rooted in the ability to predict the metal-ion coordination geometry and the vast possibilities to use organic chemistry to modify the linker groups. However, the use of molecules occupying the pores as a component of framework design has been largely ignored. Recent reports show that the presence of these so-called "guests" can have dramatic effects, even when they are a seemingly innocuous species such as water or polar solvents. We term these guests "non-innocent" when their presence alters the MOF in such a way as to create a new material with properties different from the MOF without the guests. Advantages of using guest molecules to impart new properties to MOFs include the relative ease of introducing new functionalities, the ability to modify the material properties at will by removing the guest or inserting different ones, and avoidance of the difficulties associated with synthesizing new frameworks, which can be challenging even when the basic topology remains constant. In this article, we describe the "Guest@MOF" concept and provide examples illustrating its potential as a new MOF design element.
Metal-organic frameworks (MOFs), with their crystalline nanoporous three-dimensional structures, have emerged as unique multifunctional materials that combine high porosity with catalytic, photophysical, or other properties to reveal new fundamental science and applications. Because MOFs are composed of organic molecules linking metal centers in ways that are not usually conducive to the formation of free-charge carriers or low-energy charge-transport pathways, they are typically insulators. Accordingly, applications so far have harnessed the unique structural properties and porosity of MOFs, which depend only to a small extent on the ability to manipulate their electronic structure. An exciting new area has emerged due to the recent demonstration of MOFs with controlled electronic and optical properties, which is enabling new fundamental science and opens up the possibility of applications in electronics and photonics. This article presents an overview of the fundamental science issues related to controlling electronic and optical properties of MOFs, and how research groups worldwide have been exploring such properties for electronics, thermoelectrics, photophysics, and charge storage.
Colloidal systems offer ideal conditions to study the nucleation process, both from an experimental viewpoint, due to their relatively large size and long time scales, and from a modeling point of view, due to the tunability of their interactions. In this article, we review recent studies on the process of colloidal crystallization from a microscopic perspective. In particular, we focus on nonclassical pathways to nucleation, where the appearance of solid crystals involves fluctuations of two or more order parameters. Nonclassical behavior is interpreted as a decoupling of positional and orientational symmetry breaking. We then consider how the nucleation pathway determines which polymorph is selected upon nucleation from the melt. The study of nucleation pathways not only sheds new light on the microscopic mechanism of nucleation, but also provides important information regarding its avoidance, suggesting a deep link between crystallization and vitrification.
The goal of the Materials Genome Initiative is to substantially reduce the time and cost of materials design and deployment. Achieving this goal requires taking advantage of the recent advances in data and information sciences. This critical need has impelled the emergence of a new discipline, called materials data science and informatics. This emerging new discipline not only has to address the core scientific/technological challenges related to datafication of materials science and engineering, but also, a number of equally important challenges around data-driven transformation of the current culture, practices, and workflows employed for materials innovation. A comprehensive effort that addresses both of these aspects in a synergistic manner is likely to succeed in realizing the vision of scaled-up materials innovation. Key toolsets needed for the successful adoption of materials data science and informatics in materials innovation are identified and discussed in this article. Prototypical examples of emerging novel toolsets and their functionality are described along with select case studies.
The twinning-induced plasticity effect enables designing austenitic Fe-Mn-C-based steels with >70% elongation with an ultimate tensile strength >1 GPa. These steels are characterized by high strain hardening due to the formation of twins and complex dislocation substructures that dynamically reduce the dislocation mean free path. Both mechanisms are governed by the stacking-fault energy (SFE) that depends on composition. This connection between composition and substructure renders these steels ideal model materials for theory-based alloy design: Ab initio-guided composition adjustment is used to tune the SFE, and thus, the strain-hardening behavior for promoting the onset of twinning at intermediate deformation levels where the strain-hardening capacity provided by the dislocation substructure is exhausted. We present thermodynamic simulations and their use in constitutive models, as well as electron microscopy and combinatorial methods that enable validation of the strain-hardening mechanisms.
Hierarchical design down to the nanoscale has become possible in structural composite materials with the discovery of carbon nanomaterials such as carbon nanotubes (CNTs) and graphene. Composites that simultaneously combine microscopic continuous fibers and nanoscale reinforcements are known in the field as hierarchical or nanoengineered composites. The additional reinforcement at the nanoscale promises high-performance composites with unique combinations of mechanical properties and new functionalities. Here, we review advances in fiber-reinforced polymers modified with CNTs. Three routes for integration of CNTs in composites are discussed: deposition on fibers/plies, dispersion in the matrix, and assembly into fibers. We highlight opportunities and challenges focusing on mechanical performance and processing.
Research aimed at designing and optimizing open framework materials for commercial applications tend to focus on two critical objectives: identifying synthesis conditions that yield crystals with tailored physicochemical properties, and unlocking the untapped design space to achieve theoretical structures that far outnumber the list of synthetically realized materials. Accomplishing these goals requires detailed knowledge of nucleation in order to cultivate efficient, facile, and economical methods of controlling crystallization. The vast number of open framework materials that can be engineered through the judicious selection of inorganic or organic building units hold the promise for future discovery of materials with unique and superior properties compared to available porous materials. Herein, we review what is known about the nucleation of open framework crystals, highlighting the voids in our understanding of nucleation pathways, and we offer guidelines for advancing crystal engineering in this exciting area of research.
Our energy needs drive widespread materials research. Advances in materials characterization are critical to this research effort. Using new characterization tools that allow us to probe the atomic structure of energy materials in situ as they operate, we can identify how their structure is linked to their functional properties and performance. These fundamental insights serve as a roadmap to enhance performance in the next generation of advanced materials. In the last decade, developments in synchrotron instrumentation have made the pair distribution function (PDF) method and operando x-ray studies more readily accessible tools capable of providing valuable insights into complex materials systems. Here, the emergence of the PDF method as a versatile structure characterization tool and the further enhancement of this method through developments in operando capabilities and multivariate data analytics are described. These advances in materials characterization are demonstrated by several highlighted studies focused on energy storage in batteries.
This article summarizes recent research on laser-based processing of two-dimensional (2D) atomic layered materials, including graphene and transition-metal dichalcogenides (TMDCs). Ultrafast lasers offer unique processing routes that take advantage of distinct interaction mechanisms with 2D materials to enable extremely localized energy deposition. Experiments have shown that ablative direct patterning of graphene by ultrafast lasers can achieve resolutions of tens of nanometers, as well as single-step pattern transfer. Ultrafast lasers also induce non-thermal excitation mechanisms that are useful for the thinning of TMDCs to tune the 2D material bandgap. Laser-assisted site-specific doping was recently demonstrated and ultrafast laser radiation under ambient air environment could be used for the direct writing of high-quality graphene patterns on insulating substrates. This article concludes with an outlook on developing further advanced laser processing with scalability, in situ monitoring strategies, and potential applications.
Metal-organic framework (MOF) materials are well known as elegant gaseous energy-storage materials, but their potential for electrical energy storage has only recently been explored. Although numerous studies have focused on MOF-derived porous carbon or nanoscale metal oxide materials, less attention has been paid to the intrinsic properties achievable through the molecular design of MOFs. Indeed, the porous nature of MOF architectures is highly suitable for accommodating electrolyte ions in electrochemical processes, suggesting their potential as high-performance active materials for batteries. In this article, we consider recent examples employing MOF materials as battery electrode materials. Redox-active sites were incorporated on metal junctions, ligands, or both, in the MOF structures. In addition, we introduce novel electrochemical mechanisms observed in the electrochemical process of MOF electrode materials.
Frequently, fundamental scientific and technological issues are related to the chemical structure of a material at the nanometer or even atomistic length scales. This includes, but is not limited to, internal interfaces of complex topology as they appear, for example, in current energy-harvesting applications or advanced microelectronics. Scientific understanding of the underlying physics and chemistry requires advanced characterization tools that provide critical three-dimensional information at the subnanometer length scale. Atom probe tomography (APT) meets such requirements. Today, with remarkable progress in instrumentation and sample preparation, APT has become a very versatile tool to address fundamental questions of materials science. In this issue of MRS Bulletin, the APT technique is introduced, with a particular focus on recent developments and the broadening range of studied material classes and applications.
The recent decades have seen significant progress in linking the mechanical performance of materials to their underlying microstructure. This article presents an overview of some of these achievements, trends, and challenges. Attention is given to methods initially developed for micromechanics and their gradual evolution toward powerful multiscale methods. Various methods have been proposed for bridging scales in mechanics of materials, all aiming for efficiency and accuracy. Computational homogenization is one of these powerful approaches, now used systematically for the assessment of structure-property relations. Novel solution methods and model reduction techniques provide tools to speed up the structure-property analysis, whereby large-scale computations have been made possible. Truly fast analyses of microstructures may be expected in the near future.
Methods used in informatics require input data that are in a machine-readable, structured format. Materials data, in particular, can be exceedingly complex, so defining data formats to store any and all materials-related information is a daunting task. In this article, we discuss a hierarchical data structure used for storing materials data called the physical information file (PIF). The PIF is a flexible schema for storing the structure, processing history, and properties of materials, devices, and physical systems. In addition to a general discussion of the schema, we give examples of its use in representing complex materials systems. We also describe open-source tools that have been developed for building and reading files using the PIF schema.
A series of recent instrumental advances have facilitated the application of atom probe tomography (APT) to the characterization of an increasingly wide range of materials and devices. Whereas APT was previously mostly limited to the analysis of alloys, advances in areas such as laser pulsing and detectors have enabled characterization of semiconductors and brittle materials. Most recently, ultraviolet laser pulsing has facilitated the analysis of materials previously considered not viable for the atom probe, such as minerals and large bandgap insulator materials. The development of in situ gas reaction cells fully integrated in atom probe instruments has enabled the characterization of surface reactions of materials exposed to highly controlled environments. Finally, current work toward an integrated cryo-transfer system is anticipated to create new directions for APT research.
Surface textures with three-dimensional (3D) architectures demonstrate the ability to control interfacial, optical, chemical, and mechanical properties. Potential applications range from device-scale biomolecule sensing to meter-scale optical or nonwetting coatings. In recent years, capillary forming has become a versatile and scalable approach to creating complex geometries at the nano- and micron scales. Surface tension of a liquid can deform straight pillars and assemble them into 3D architectures with predetermined orientation, where short-range adhesion forces stabilize the final forms. A variety of techniques have been demonstrated for carbon nanotubes and polymer filamentary materials to fabricate useful devices and textures. We discuss these materials and processes as well as the underlying elasto-capillary physics. We indicate the need for new simulation tools to design and engineer elasto-capillary transformations and methods to increase their throughput toward scalable manufacturing.
A review of recent literature supports the notion that organisms may achieve nanoscale control over the hierarchical assembly of organic-inorganic materials by compartmentalizing reactions into small volumes containing specifically functionalized macromolecules. Such confinement may introduce a degree of determinism to the stochastic process of nucleation by greatly reducing the number of nucleation events, allowing an organism to control nucleation using "soft" organic substrates. In this way, the polymorph, orientation, shape, and size of a crystalline building block can be selected, and its assembly into a larger structure orchestrated by the organic matrix.