This article provides a personal guided tour of multiferroic materials, from their early days as a theoretical curiosity, to their position today as a focus of worldwide research activity poised to impact technology. The article begins with the history of, and the answer to, the question of why so few magnetic ferroelectric multiferroics exist, then gives a survey of the mechanisms and materials that support such multiferroicity. After discussing the tremendous progress that has been made in the magnetoelectric control of magnetic properties using an electric field, some unusual applications of multiferroics in high-energy physics and cosmology are outlined. Finally, the most interesting open questions and future research directions are addressed.
Nitrogen-vacancy (NV) color centers in diamond are currently considered excellent solid-state magnetic field sensors. Their long coherence times at room temperature and their atomic size allow for achieving both high magnetic field sensitivity and nanoscale spatial resolution in ambient conditions. This article reviews recent progress in magnetic field imaging with NV centers. We focus on two topics: scanning probe techniques with single NV centers and their application in the imaging of nanoscale magnetic structures, as well as recent development of magnetometers with ensembles of NV centers, which image magnetic fields at micron-length scales with extremely high sensitivities.
The nanogap is possibly the single most important physical entity in surface-enhanced Raman scattering. Nanogaps between noble metal nanostructures deliver extremely high electric field-enhancement, resulting in an extraordinary amplification of both the excitation rate and the emission rate of Raman active molecules situated in the gap. In some cases, the resulting surface-enhancement in the gap can be so high that Raman spectra from single molecules can be measured. Here, we briefly review some important concepts and experimental results on nanoscale gaps for SERS applications.
Self-folding of thin films is a more deterministic form of self-assembly wherein structures curve or fold up either spontaneously on release from the substrate or in response to specific stimuli. From an intellectual standpoint, the study of the self-folding of thin films at small size scales is motivated by the observation that a large number of naturally occurring materials such as leaves and tissues show curved, wrinkled, or folded micro-and nanoscale geometries. From a technological standpoint, such a self-assembly methodology is important since it can be used to transform the precision of existing planar patterning methods, such as electron-beam lithography, to the third dimension. Also, the self-folding of graphene promises a means to create a variety of three-dimensional carbon-based micro-and nanostructures. Finally, stimuli responsive self-folding can be used to realize chemomechanical and tether-free actuation at small size scales. Here, we review theoretical and experimental aspects of the self-folding of metallic, semiconducting, and polymeric films.
Nano-sizing and scaffolding have emerged in the past decade as important strategies to control the kinetics, reversibility, and equilibrium pressure for hydrogen storage in light metal hydride systems. Reducing the size of metal hydrides to the nanometer range allows fast kinetics for both hydrogen release and subsequent uptake. Reversibility of the hydrogen release is impressively facilitated by nanoconfining the materials in a carbon or metal-organic framework scaffold, in particular for reactions involving multiple solid phases, such as the decomposition of LiBH4, NaBH4, and NaAlH4. More complex is the impact of nanoconfinement on phase equilibria. It is clear that equilibrium pressures, and even decomposition pathways, are changed. However, further experimental and computational studies are essential to understand the exact origins of these effects and to unravel the role of particle size, physical confinement, and interfaces. Nevertheless, it has become clear that nanoconfinement is a strong tool to change physicochemical properties of metal hydrides, which might not only be of relevance for hydrogen storage, but also for other applications such as rechargeable batteries.
This article reviews recent progress in understanding the resistive switching (RS) behavior and improvements in device performance of RS metal oxide (MO) thin-film systems and devices. The diverse RS MO materials are classified according to their switching mechanisms and characteristics. For each category, some representative materials are selected, and their characteristics are discussed. In addition, other factors such as the device structure, which also plays a crucial role in determining the device properties, are discussed as well. When applied in a real circuit (e.g., in a crossbar structure), there are device features/characteristics that need to be considered, including the bias polarity for switching, the current-voltage relationship, reliability, and scaling issues. Since nonvolatile RS in many MO materials is primarily associated with localized conduction channels, understanding the nature and the dynamic change of the current path structure is crucial and therefore is reviewed at length here. Guidelines for the choice of materials and access devices and their fabrication methods will also be provided. Finally, this review concludes with the outlook and challenges of MO-based resistance change devices for semiconductor memories.
Atomic layer deposition (ALD) uses self-limiting chemical reactions between gaseous precursors and a solid surface to deposit materials in a layer-by-layer fashion. This process results in a unique combination of attributes, including sub-nm precision, the capability to engineer surfaces and interfaces, and unparalleled conformality over high-aspect ratio and nanoporous structures. Given these capabilities, ALD could play a central role in achieving the technological advances necessary to redirect our economy from fossil-based energy to clean, renewable energy. This article will survey some of the recent work applying ALD to clean energy conversion, utilization, and storage, including research in solid oxide fuel cells, thin-film photovoltaics, lithium-ion batteries, and heterogenous catalysts. Throughout the manuscript, we will emphasize how the unique qualities of ALD can enhance device performance and enable radical new designs.
Emissive saturated colors are key components of new generations of lighting and display technologies. Quantum dots have evolved in the past two decades to fulfill many of the requirements of color purity, stability, and efficiency that are critical to transitioning these materials from the laboratory into these markets. A fundamental feature of quantum dots is the tunability of their emission color through precise control of their size and composition, giving access to UV, visible, and near-infrared wavelengths. Continuing improvements in engineering core-shell quantum dot structures, where a 1-10 nm binary, ternary, or alloyed semiconductor core particle is surrounded by a shell composed of one or more semiconductors of a wider bandgap, have resulted in materials with fluorescence quantum yields that approach unity, narrow symmetric spectral line shapes, and remarkable stabilities. In this article, we review progress in the development of highly luminescent core-shell quantum dots of different semiconductor families in view of their integration in light-emitting applications. CdSe-based quantum dots already fulfill many of the requirements of lighting and display applications in terms of fluorescence quantum yield, color purity, and stability.
Metal additive manufacturing (AM) processes, such as selective laser melting (SLM), enable powdered metals to be formed into arbitrary three-dimensional shapes. For aluminum alloys, which are desirable in many high-value applications for their low density and good mechanical performance, SLM is regarded as challenging due to the difficulties in laser melting aluminum powders. However, a number of recent studies have demonstrated successful aluminum processing, and have gone on to explore its potential for use in advanced AM componentry. In addition to enabling the fabrication of highly complex structures, SLM produces parts with characteristically fine microstructures that yield distinct mechanical properties. Research is rapidly progressing in this field, with promising results opening up a range of possible applications across scientific and industrial sectors. This article reports on recent developments in this area of research and highlights key topics that require further attention.
Carbon, with its variety of allotropes and forms, is the most versatile material, and virtually any combination of mechanical, optical, electrical, and chemical properties can be achieved with carbon by controlling its structure and surface chemistry. The goal of this article is to help readers appreciate the variety of carbon nanomaterials and to describe some engineering applications of the most important of these. Many different materials are needed to meet a variety of performance requirements, but they can all be built of carbon. Considering the example of supercapacitor electrodes, zero- and one-dimensional nanoparticles, such as carbon onions and nanotubes, respectively, deliver very high power because of fast ion sorption/desorption on their outer surfaces. Two-dimensional (2D) graphene offers higher charge/discharge rates than porous carbons and a high volumetric energy density. Three-dimensional porous activated, carbide-derived, and templated carbon networks, with high surface areas and porosities in the angstrom or nanometer range, can provide high energy densities if the pore size is matched with the electrolyte ion size. Finally, carbon-based nanostructures further expand the range of available nanomaterials: Recently discovered 2D transition-metal carbides (MXenes) have already grown into a family with close to 20 members in about four years and challenge graphene in some applications.
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.
Multiferroic magnetoelectric nanostructures with coupled magnetization and electric polarization across their interfaces have stimulated intense research activities over the past decade. Such interface-based magnetoelectric coupling can be exploited to significantly improve the performance of many devices such as memories, tunable radio-frequency/microwave devices, and magnetic sensors. In this article, we introduce a number of current or developing technologies and discuss their limitations. We describe how the use of magnetoelectric nanostructures can overcome these limitations to optimize device performance. We also present challenges that need to be addressed in pursuing practical applications of magnetoelectric devices.
The exceptional properties of graphene originate from its two-dimensional polymeric structure of sp(2)-bonded carbon. This feature also causes graphene to grow on metal substrates through mechanisms that are strikingly different from those of conventional heteroepitaxy. We provide here a brief review of graphene growth on metals, a subject with a rich history even before the recent explosion of interest in graphene. The current activities related to graphene growth on metals have been motivated by the need to develop low-cost, scalable processes for graphene synthesis and to understand how graphene-metal interfaces behave in devices. In this article, we examine the current state of the art, emphasizing the basic processes that distinguish graphene growth from normal crystal growth.
Three-dimensional (3D) printing represents the direct fabrication of parts layer-by-layer, guided by digital information from a computer-aided design file without any part-specific tooling. Over the past three decades, a variety of 3D printing technologies have evolved that have transformed the idea of direct printing of parts for numerous applications. Three-dimensional printing technology offers significant advantages for biomedical devices and tissue engineering due to its ability to manufacture low-volume or one-of-a-kind parts on-demand based on patient-specific needs, at no additional cost for different designs that can vary from patient to patient, while also offering flexibility in the starting materials. However, many concerns remain for widespread applications of 3D-printed biomaterials, including regulatory issues, a sterile environment for part fabrication, and the achievement of target material properties with the desired architecture. This article offers a broad overview of the field of 3D-printed biomaterials along with a few specific applications to assist the reader in obtaining an understanding of the current state of the art and to encourage future scientific and technical contributions toward expanding the frontiers of 3D-printed biomaterials.
Resistive memory devices based on organic materials that can be configured to two or more stable resistance states have been extensively explored as information storage media due to their advantages, which include simple device structures, low fabrication costs, and flexibility. Various organic-based materials such as small molecules, polymers, and composite materials have been observed to show bistability. This review provides a general summary about the materials, structures, characteristics, and mechanisms of organic resistive memory devices. Several critical strategies for device fabrication, performance enhancement, and integrated circuit architectures are also discussed.
Metallic materials are key for electrochemical energy conversion and storage when they are tailored into electrodes designed for rapid reaction kinetics, high electrical conductivities, and high stability. Nanoporous metals formed by dealloying could meet all of these requirements, as the dealloyed products beckon energy researchers with their fascinating structures and outstanding performance. In this article, we discuss the characteristics of dealloyed materials related to their functions in energy devices. We then review nanoporous metal electrodes for applications in fuel cells, supercapacitors, and batteries to provide insights into selection and design criteria for meeting the diverse needs of energy conversion and storage.
Diamond is a unique material that often exhibits extreme properties compared to other materials. Discovered about 30 years ago, the use of hydrogen in plasma-enhanced chemical vapor deposition (CVD) has enabled the growth and coating of diamond in film form on various substrate materials. CVD diamond research has been actively continued subsequently to develop new understanding and approaches for the growth and processing of this fascinating material. Currently, the study and development of diamond films has enabled a wide range of applications based on the combination of unique and extreme properties of diamond and the variety of film properties obtainable through tuning the microstructure, morphology, impurities, and surfaces. This issue of MRS Bulletin introduces the latest research, recent applications, and the challenges ahead for CVD diamond films.
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.
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.
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.