In a manner reminiscent of macroscale bending and folding techniques such as origami, the out-of-plane assembly of lithographically micro- and nanopatterned thin films, can be used to fabricate three-dimensional (3D) micro- and nanostructured devices. These 3D devices, including microelectronic circuits, sensors, antennas, metamaterials, robotic, and biomimetic constructs, enable new functionalities and are challenging to fabricate by other methods. In this article, we summarize important features of this set of techniques and the devices assembled thereof, with a focus on functional constructs that have been formed by bending, folding, or buckling. At small size scales, manipulation using manual or even wired probes face daunting practical challenges in terms of cost, scalability, and high-throughput manufacturability; hence we emphasize techniques that manipulate strain in thin films so that they can spontaneously assemble into programmed 3D geometries without the need for any wires or probes.
Universal access to abundant scientific data, and the software to analyze the data at scale, could fundamentally transform the field of materials science. Today, the materials community faces serious challenges to bringing about this data-accelerated research paradigm, including diversity of research areas within materials, lack of data standards, and missing incentives for sharing, among others. Nonetheless, the landscape is rapidly changing in ways that should benefit the entire materials research enterprise. We provide an overview of the current state of the materials data and informatics landscape, highlighting a few selected efforts that make more data freely available and useful to materials researchers.
The capabilities of metal additive manufacturing (AM) are evolving rapidly thanks to both increasing industry demand and improved scientific understanding of the process. This article provides an overview of AM of the Ti-6Al-4V alloy, which has essentially been used as a yardstick to gauge the capability of each metal AM process developed to date. It begins by summarizing the metal AM processes existing today. This is followed by a discussion of the macro-and microstructural characteristics, defects, and tensile and fatigue properties of AM Ti-6Al-4V by selective laser melting, laser metal deposition (both powder and wire), and selective electron-beam melting compared to non-AM Ti-6Al-4V. The tensile and fatigue properties of as-built AM Ti-6Al-4V (with machined or polished surfaces) can be made comparable, or even superior, to those of Ti-6Al-4V in the most commonly used millannealed condition. However, these properties can exhibit a large degree of scatter and are often anisotropic, affected by AM build orientations. Post-AM surface treatments or both the post-AM surface and heat treatments are necessary to ensure the minimum required properties and performance consistency. Future directions to further unlock the potential of AM of Ti-6Al-4V for superior and consistent mechanical properties are also discussed.
The development of metal-organic frameworks (MOFs) as microporous electronic conductors is an exciting research frontier that has the potential to revolutionize a wide range of technologically and industrially relevant fields, from catalysis to solid-state sensing and energy-storage devices, among others. After nearly two decades of intense research on MOFs, examples of intrinsically conducting MOFs remain relatively scarce; however, enormous strides have recently been made. This article briefly reviews the current status of the field, with a focus on experimental milestones that have shed light on crucial structure-property relationships that underpin future progress. Central to our discussion are a series of design considerations, including redox-matching, donor-acceptor interactions, mixed valency, and Pi-interactions. Transformational opportunities exist at both fundamental and applied levels, from improved measurement techniques and theoretical understanding of conduction mechanisms to device engineering. Taken together, these developments will herald a new era in advanced functional materials.
Biological surfaces display fascinating topographic patterns such as corrugated blood cells and wrinkled dog skin. These patterns have inspired an emerging technology in materials science and engineering to create self-organized surface patterns by harnessing mechanical instabilities. Compared with patterns generated by conventional lithography, surface instability patterns or so-called ruga patterns are low cost, are easy to fabricate, and can be dynamically controlled by tuning various physical stimuli-offering new opportunities in materials and device engineering across multiple length scales. This article provides a systematic review on the fundamental mechanisms and innovative functions of surface instability patterns by categorizing various modes of instabilities into a quantitatively defined thermodynamic phase diagram, and by highlighting their engineering and biological applications.
Short pulse laser irradiation has the ability to bring a material into a state of strong electronic, thermal, phase, and mechanical nonequilibrium and trigger a sequence of structural transformations leading to the generation of complex multiscale surface morphologies, unusual metastable phases, and microstructures that cannot be produced by any other means. In this article, we provide an overview of recent advancements and existing challenges in the understanding of the fundamental mechanisms of short pulse laser interaction with materials, including the material response to strong electronic excitation, ultrafast redistribution and partitioning of the deposited laser energy, the peculiarities of phase transformations occurring under conditions of strong superheating/undercooling, as well as laser-induced generation of crystal defects and modification of surface microstructure.
Hydrogen termination of diamond lowers its ionization energy, driving electron transfer from the valence band into an adsorbed water layer or to a strong molecular acceptor. This gives rise to p-type surface conductivity with holes confined to a subsurface layer of a few nanometers thickness. The transfer doping mechanism, the electronic behavior of the resulting hole accumulation layer, and the development of robust field-effect transistor (FED devices using this platform are reviewed. An alternative method of modulating the hole carrier density has been developed based upon an electrolyte-gate architecture. The operation of the resulting "solution-gated" FET architecture in two contemporary applications will be described: the charge state control of nitrogen-vacancy centers in diamond and biosensing. Despite 25 years of work in this area, our knowledge of surface conductivity of diamond continues to develop.
There is increasing interest in the use of additive manufacturing (AM) for Ni-based superalloys due to their various applications in the aerospace and power-generation sectors. Ni-based superalloys are known to have a complex chemistry, with over a dozen alloying elements in most alloys, enabling them to achieve outstanding high-temperature mechanical performance as well as oxidation resistance when processed using conventional routes (e.g., casting and forging). Nonetheless, this complex chemistry results in the formation of various phases that could affect their processability using AM, resulting in cracking. Furthermore, due to the directional solidification and rapid cooling associated with AM processes, the alloys experience significant anisotropy due to the epitaxially grown microstructure, as well as the residual stresses that can sometimes be difficult to mitigate using thermal postprocessing techniques. This article highlights the outstanding issues in Ni-based superalloys AM processing, with special emphasis on defect formation mechanisms, process optimization, and residual stress development.
Twins are domain crystals inside their parent crystals, where they share some of the same crystal lattice points in a symmetrical manner. The formation and growth of twins result in substantial evolution of microstructures and properties in a large variety of metallic materials. Twin boundaries that separate two crystals effectively strengthen the material by impeding mobile dislocations, and increase the ductility and work-hardening capability of metallic materials. The articles in this issue of MRS Bulletin overview the synthesis and mechanical behavior of nanotwinned metallic materials, as well as plasticity dominated by mechanical twinning.
Thermogelling polymers belong to a class of stimuli-responsive hydrogels that undergo a macroscopic sol-to-gel transition in response to temperature. Much of the ongoing research in this field is focused on hydrogels for biomedical applications as an injectable sustained drug-release matrix or scaffolds for tissue regeneration. Despite robust developments in biodegradable thermogelling polymers in recent decades, the field still faces challenges in the optimization of materials properties. Thorough investigation must be performed to understand the effectiveness of drug delivery using hydrogel-forming polymer carriers. A highlighted case study on OncoGel, an experimental drug delivery depot formulation, sheds some light on the shortcomings of biodegradable thermogelling polymers as drug delivery systems. In this article, we highlight developments in biodegradable thermoresponsive polymers for biomedical applications over the past three years, with a focus on materials/technical challenges and the approaches used to resolve these problems.
Aluminum alloys are in high demand for additive manufacturing (AM) processing. However, the physical properties of Al alloys are less favorable for the production of repeatable and reliable parts, with factors such as surface oxide scales, high thermal conductivity, and large solidification shrinkage. Despite these characteristics, processing strategies have been developed to overcome these hurdles. The objective of this article is to highlight the different microstructure-processing-properties characteristics for the three main families of aluminum alloys: pure, casting, and wrought chemistries. The article focuses on AM processes involving solidification, including powder bed and direct energy deposition for both powder and wire feedstock.
NiTi alloys are well known not only due to their exceptional shape-memory ability to recover their primary shape, but also because they show high ductility, excellent corrosion and wear resistance, and good biological compatibility. They have received significant attention especially in the field of laser additive manufacturing (AM). Among laser AM techniques, selective laser melting and laser metal deposition are utilized to exploit the unique properties of NiTi for fabricating complex shapes. This article reviews the properties of bulk and porous laser-made NiTi alloys as influenced by both process and material parameters. The effects of processing parameters on density, shape-memory response, microstructure, mechanical properties, surface corrosion, and biological properties are discussed. The article also describes potential opportunities where laser AM processes can be applied to fabricate dedicated NiTi components for medical applications.
Nanotwins require little energy to form in metals, but their impact on strength and ductility is dramatic. New mechanisms of strengthening, strain hardening, ductility, and strainrate sensitivity have been observed in nanowires, films, and bulk materials containing nanoscale twins as the twin-boundary spacing decreases. These mechanisms can act in concert to produce interface-dominated nanomaterials with extreme tensile strength and plastic deformation without breaking. This article reviews recent theoretical and experimental understanding of the physical mechanisms of plasticity in nanotwin-strengthened metals, with a particular focus on the fundamental roles of coherent, incoherent, and defective twin boundaries in plastic deformation of bulk and small-scale cubic systems, and discusses new experimental methods for controlling these deformation mechanisms in nanotwinned metals and alloys.
Three-dimensional (3D) printing of metallic materials involves the layerwise consolidation of feedstock materials in the form of powder, wire, or sheet using various energy sources to form complex shapes. The past two decades have witnessed significant advances in the field, in terms of both technologies and materials for metal 3D printing. This has led to widespread exploration and adoption of the technologies across industry, academia, and R&D organizations. This article presents an overview of the field of metal 3D printing. A brief history of metal 3D printing is followed by an overview of metal 3D printing methods and metallic material systems used in these methods. Microstructure and properties, and their relationship to process parameters are discussed next, followed by current challenges and qualification issues. The article concludes with future trends and a brief description of the invited articles included in this special issue.
The development of new hierarchical materials capable of efficient energy transfer along a predesigned pathway will boost various applications, ranging from organic photovoltaics to catalytic systems. Due to their exceptional tunability and structural diversity, metal-organic frameworks (MOFs) offer a unique platform to study and model directional energy-transfer processes and, thereby, an efficient path for energy utilization. This article summarizes the latest advances in MOF applications in the fields of optoelectronics, photoswitching, sensing, and photocatalysis, for which development is highly dependent on fundamental studies of MOF photophysics.
As a relatively new class of hierarchically structured materials, nanotwinned (NT) metals exhibit an exceptional combination of high strength, good ductility, large fracture toughness, remarkable fatigue resistance, and creep stability. This article reviews current studies on fracture, fatigue, and creep of NT metals, with an emphasis on the fundamental deformation and failure mechanisms. We focus on the complex interactions among cracks, dislocations, and twin boundaries, the influence of microstructure, twin size, and twinning/detwinning on damage evolution, and the contribution of nanoscale twins to fatigue and creep under indentation and irradiation conditions. The article also includes critical discussions on the effects of twin thickness and grain size on the fracture toughness, fatigue resistance, and creep stability of NT metals.
Materials that can expand and collapse, fold, and transform into a variety of shapes have attracted significant interest and have applications in the design of flexible electronics, color displays, smart windows, actuators, sensors, and both photonic and phononic devices. But how can we render a rigid device super-flexible so that it can wrap around a sphere without bending and stretching? How can flat surfaces be transformed into any desired three-dimensional (3D) structure without disruptive or catastrophic deformation? The key lies in cuts. Here, we review recent research progress in the design of super-conformable and foldable materials by employing fractal cutting and lattice-based kirigami elements that combine cutting and folding. By prescribing cuts with different motifs, identifying edges in the right geometry, and by programming the folding directions, we show that a single flat sheet can be transformed into a variety of targeted 2D and 3D structures-a pluripotent platform for new technologies.
Future electronics will be conformal, bendable, and wearable. Taking inspiration from the characteristics of human skin, we are developing a new generation of electronic materials to enable devices that are flexible, stretchable, biodegradable, and self-healable. We have developed various sensors and circuits, all of which are based on organic materials, polymers, and carbon nanomaterials. These materials will provide us with a long-term path toward adding various skin-inspired functions.
The remarkable properties of nanotwinned (NT) face-centered-cubic (fcc) metals arise directly from twin boundaries, the structures of which can be initially determined by growth twinning during the deposition process. Understanding the synthesis process and its relation to the resulting microstructure, and ultimately to material properties, is key to understanding and utilizing these materials. This article presents recent studies on electrodeposition and sputtering methods that produce a high density of nanoscale growth twins in fcc metals. Nanoscale growth twins tend to form spontaneously in monolithic and alloyed fcc metals with lower stacking-fault energies, while engineered approaches are necessary for fcc metals with higher stacking-fault energies. Growth defects and other microstructural features that influence nanotwin behavior and stability are introduced here, and future challenges in fabricating NT materials are highlighted.
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%.