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Nanoporous metals obtained by dealloying have attracted significant attention for their unusual catalytic properties, and as model materials for fundamental studies of structure-property relationships in a variety of research areas. There has been a recent surge in the use of these metals for biomedical and bioanalytical applications, where many exciting opportunities exist. The goal of this article is to provide a review of recent progress in using nanoporous metals for biological applications, including as biosensors for detecting biomarkers of disease and multifunctional neural interfaces for monitoring and modulating the activity of neural tissue. The article emphasizes the unique properties of nanoporous gold and concludes by discussing its utility in addressing important challenges in biomedical devices.

Heat-assisted magnetic recording (HAMR) relies on careful management of heat flow at the nanoscale. This article describes the heat-transfer aspects of such a system that must be considered above and beyond standard Fourier’s Law-based heat conduction. A background on nanoscale heat transport is provided that discusses energy carriers and the role of interfaces and microstructure in nanoscale conduction. These heat-transport concepts are applied to the key components of the HAMR system—the head (principally, the near-field transducer [NFT]) and the magnetic medium. This analysis frames the central challenge of thermal engineering for a HAMR system—getting the medium hot enough while maintaining a NFT that it is cool enough to avoid degradation over time. Of particular note are discussions on the role of the interface thermal conductance in the NFT and the importance of thermal anisotropy in the medium due to its granular microstructure. [ABSTRACT FROM PUBLISHER]

Increasing the density of data storage is crucial to the future of inexpensive digital technology. The large majority of storage in the “Cloud” consists of magnetic hard-disk drives. The continued evolution of this USD$30 billion industry depends on the commercial introduction of heat-assisted magnetic recording (HAMR). This technology uses heat from a laser beam confined well below the diffraction limit, <50-nm wide, to write to media near 450°C with high magnetic anisotropy that would normally be unwriteable under available magnetic fields. This high anisotropy guarantees thermal stability even for grain sizes around 5-nm diameter, which are necessary for major increases in storage density. In this article and in the articles in this issue, we introduce HAMR requirements and discuss its numerous interdisciplinary materials challenges, including high-temperature/efficient plasmonic materials, low-loss optical materials, highly ordered/thermally anisotropic nanoscale magnetic grains, block copolymers for directed assembly below 10 nm, high-temperature nanometer-thick coatings/lubricants, materials/interfaces to control heat flow at nanometer length scales, and advanced spintronic materials. [ABSTRACT FROM PUBLISHER]

Heat-assisted magnetic recording (HAMR) is the next-generation technology that is required to deliver areal densities in excess of 2 terabit/in2 for high-capacity, low-cost hard drives.The recording process relies on spatially and temporally localized heating of the media surface to lower its coercivity during the magnetic writing process. This scheme drives substantial changes to the recording head write element architecture, combining the conventional electromagnet structure with integrated optical light delivery layers, focusing optics, and plasmonic nanostructures to generate subwavelength optical spots. Power losses associated with the strong optical fields required for heating the media can cause local temperatures in excess of 400°C at the recording head surface. Coupled with high pressures, an oxidative/corrosive air-bearing environment, and a sub-3 nm head-media spacing, this introduces new challenges for the functional materials in recording heads required to balance performance and long-term reliability demands. Here, we briefly review specific challenges associated with HAMR heads, highlighting the major requirements, failure modes, and needed innovations for the near-field transducer and optical-waveguide subsystems. [ABSTRACT FROM PUBLISHER]

Nanoporous metals made by dealloying are macroscopic network architectures that can contain ∼1015 nanoscale struts or ligaments per sample. Their mechanical performance is critical to their applications as functional or lightweight high-strength materials. Testing nanoporous metals at the macroscopic scale offers opportunities for unraveling the properties of nanoscale solids in general. The central questions in this area include whether the macroscopic strength and elastic modulus of nanoporous metals can be correlated with the properties of nanoscale ligaments by the classical Gibson–Ashby equations, whether the dealloying-made network structure differs from the conventional foam metals, how network connectivity influences mechanical response, and how ligament size and surface properties affect the elastic and plastic response of nanoscale solids and that of nanoporous metals, particularly the tension–compression asymmetry in strength. This article reviews the fundamental observations of the mechanical response of nanoporous metals with a focus on gold and the emerging understanding of the aforementioned issues. [ABSTRACT FROM AUTHOR]

Dealloying, the selective dissolution of less noble elements from an alloy, enables the preparation of monolithic macroscale bodies, which at the nanostructure level exhibit a network of “ligaments” with a well-defined characteristic size that can be tuned to between a few nanometers and several microns. These porous solids can be made with macroscale dimensions, and, prior to dealloying, can be shaped to form engineered components. Their surface-to-volume ratio is extremely large and their bicontinuous structure provides transport pathways to tune the surface state under control of an electric or chemical potential. These materials present new opportunities for exploring the impact of surfaces on material behaviors and for exploiting surface effects in novel materials design strategies. New experimental approaches unraveling surface effects involving small-scale plasticity and elasticity have been demonstrated. Approaches to new functional materials include electrochemical potential switching of strength, stiffness, fracture resistance, fluid sorption, actuation, and quasi-piezoelectric strain sensing. [ABSTRACT FROM AUTHOR]

Catalysts play essential roles in the chemical vapor deposition of single-wall carbon nanotubes (SWCNTs). In this article, we summarize studies on catalysts for the structure-controlled growth and mass production of SWCNTs, discussing the main progress and the remaining challenges. [ABSTRACT FROM AUTHOR]

When the size and spacing of catalyst nanoparticles are well controlled on a substrate, carbon nanotubes (CNTs) can grow and assemble into a unique, vertically aligned structure frequently called a “forest.” Long, aligned, and pure CNTs can easily be synthesized in simple or highly complex configurations. First reported in 1996, CNT forests have been shown to be unique and useful forms of CNTs, as they have spurred the development of novel processes and applications and in addition, served as test beds for investigations into CNT growth mechanisms. This article provides an overview of two decades of research in this area. [ABSTRACT FROM AUTHOR]

Nanocarbons, such as carbon nanotubes and graphene, have had a remarkable history and impact on current applications. We briefly review the genesis and development of nanocarbons over the last 50 years, referencing key articles, including the role of catalysts in their formation. This issue focuses on the formation mechanisms and controlled growth of carbon nanotubes and graphene on substrates through catalytic processes. The five contributions in this issue review the mechanisms and theory of catalytic growth of nanocarbons (carbon nanotube forests, superaligned arrays, single-wall carbon nanotubes), the growth of large-quantity, high-quality graphene on metal substrates, and the unique and excellent properties for current and potential commercial applications. [ABSTRACT FROM AUTHOR]

Helium ion microscopy (HIM), enabled by a gas field ion source (GFIS), is an emerging imaging and nanofabrication technique compatible with many applications in materials science. The scanning electron microscope (SEM) has become ubiquitous in materials science for high-resolution imaging of materials. However, due to the fundamental limitation in focusing of electron beams, ion-beam microscopy is now being developed (e.g., at 20 kV the SEM beam diameter ranges from 0.5 to 1 nm, whereas the HIM offers 0.35 nm). The key technological advantage of the HIM is in its multipurpose design that excels in a variety of disciplines. The HIM offers higher resolution than the best available SEMs as well as the traditional gallium liquid-metal ion source (LMISs) focused ion beams (FIBs), and is capable of imaging untreated biological or other insulating samples with unprecedented resolution, depth of field, materials contrast, and image quality. GFIS FIBs also offer a direct path to defect engineering via ion implantation, three-dimensional direct write using gaseous and liquid precursors, and chemical-imaging options with secondary ion mass spectrometry. HIM covers a wide range of tasks that otherwise would require multiple tools or specialized sample preparation. In this article, we describe the underlying technology, present materials, relevant applications, and offer an outlook for the potential of FIB technology in processing materials. [ABSTRACT FROM PUBLISHER]

The holy grail of regenerative technologies is to regrow a limb. This grand challenge requires us to develop truly innovative strategies and toolboxes across advanced materials science, stem cell science, physics, and developmental biology, and converge these advancements to address the limitations of wound healing and harness the regenerative capacity in humans. We discuss the convergence approach put forward by the field of regenerative engineering to develop translational strategies involving electrically conductive and novel polymers, stem cells, high-throughput technologies, and nanotechnology to regenerate the limb. [ABSTRACT FROM AUTHOR]

Among the most abundant biopolymers in the biosphere, lignin represents an untapped opportunity to create novel bioproducts. In this article, we discuss possibilities to synthesize nano- and microparticles by harnessing lignin’s inherent tendency to associate and to develop new material compositions and functions by controlling its capacity to assemble into supramolecular structures. Because lignin is biodegradable, antimicrobial, antioxidative, and carbon neutral, inexpensive industrial lignin streams could generate value-added particulate materials that preserve the structure, composition, and colloidal features inherent to this macromolecule. We present available routes for synthesis or isolation of lignin particles, including antisolvent and aerosol processing. Metallic and polymeric lignin particle hybrids for magnetic, antibacterial, catalytic, photonic, and other applications are also discussed. Overall, the facile formation of nano- and microparticles from lignins is expected to open new pathways toward future material development. [ABSTRACT FROM AUTHOR]

Agricultural soils are among the depositories of engineered nanomaterials (ENMs). Soil exposure to ENMs occurs through the intentional use of nano-agrochemicals, as well as through incidental contamination from industrial-waste release, irrigation with wastewater or gray water, amendment with ENMs-loaded sludge (soil conditioning to stimulate plant growth), or atmospheric fallouts. Concerns about ENM interactions with plants raise two questions. (1) Are ENMs taken up from soil by plants? (2) If they are taken up, do they remain in the nanoform within plant tissues? Experiments with crop plants have demonstrated that some ENMs such as TiO2 are taken up by roots and translocated to aboveground tissues, including fruits, without biotransformation. CeO2 ENM is also taken up by the roots; however, although most of it remains as ENM, it releases cerium ions that are incorporated into organic compounds. CeO2 ENM has been shown to be translocated from roots to seeds in soybean grown in soil amended with such ENM. On the other hand, ZnO ENM is transformed at the soil/root interface, leading to tissue Zn enrichment. Overall, most ENMs are taken up by plants with either low or no transformation, and accumulate in tissues. [ABSTRACT FROM AUTHOR]

Carbon materials exist in a large number of allotropic forms and exhibit a wide range of physical and chemical properties. From the perspective of fluidics, particularly within the confines of the nanoscale afforded by one-dimensional carbon nanotubes (CNTs) and two-dimensional graphene structures, many unique properties have been discovered. However, other questions, such as the link between electronic states and hydrodynamics and accurate model predictions of transport, remain unanswered. Theoretical studies, experiments in large-scale ensembles of CNTs and stacked graphene sheets, and precise measurements at the single-pore and single-molecule level have helped in our understanding. These activities have led to explosive growth in the field, now known as carbon nanofluidics. The ability to produce membranes and devices from fluid phases of graphene oxide, which retain these special properties in molecular-scale flow channels, promises realization of applications in the near term. [ABSTRACT FROM PUBLISHER]

Transport properties of fluids in nanopores are of both fundamental as well as practical interest. Water flow in carbon nanotubes (CNTs) has received significant attention since the early 2000s for technological applications of CNTs. In this article, we provide a brief overview of modeling the slip and flow enhancement of water in CNTs. A number of experimental and computational studies have found water to flow very fast in CNTs, but the measured flow rates, which are high compared to classical hydrodynamics predictions, are scattered over 2–5 orders of magnitude. Slip lengths of 1 to 500,000 nm, resulting in almost zero to 500,000 flow enhancement, are reported for water in CNTs with diameters of 0.8 to 10 nm. We highlight some challenges in modeling fluid flow in nanopores and outline a few research directions that may resolve the order of slip and flow enhancement of water in CNTs in computational studies. [ABSTRACT FROM PUBLISHER]

This issue of MRS Bulletin focuses on materials that enable nanofluidic systems with unusually high mass fluxes, termed “enhancement factor” or “slip flow.” There is now ample evidence of such flow enhancement in nanochannels, with sizes ranging from subnanometer to a few nanometers. Most of the studies to date, both experimental and modeling, have focused on carbon nanotubes and, more recently, on graphene. Different fabrication methods result in different structures, surface chemistries, and defects, with a significant effect on flow enhancement. As new one-dimensional and two-dimensional nanomaterials are synthesized, a deeper understanding of the nanoscale transport physics is needed, particularly in the relationship between material properties and flow behavior. Herein, authors at the forefront of experimental, modeling, and theoretical developments in nanofluidic flow describe the state of the art in materials development and characterization. [ABSTRACT FROM PUBLISHER]

This article discusses the modeling of liquid flow inside nanotube membranes. Applying known simplifications to the classical fluid model leads to the so-called Hagen–Poiseuille equation, which predicts no flow for diameters up to 1 nm, and very modest flows in nanochannels up to 100 nm. The main feature of classical fluid dynamics that negates the possibility of high flow is the assumption that fluid molecules closest to the channel wall stick to it, the no-slip boundary condition. In the past 10 years, a wealth of experimental evidence has, on the contrary, demonstrated significant water flow in nanotubes with diameters equal to or smaller than 1 nm, opening the possibility of nanotube membranes capable of high flows and fine separation. These high flows have also been observed in molecular dynamics simulations, particularly for water flowing through carbon nanotubes, showing the presence of strong water slip near the walls of the nanotubes. The term “flow enhancement” has been introduced to refer to the ratio of predicted (or measured) flows and the no-slip Hagen–Poiseuille equation. Both experimental and modeling results point to a strong effect on flow enhancement of the interaction between the fluid and the tube’s wall, particularly the wall surface chemistry and structure. [ABSTRACT FROM PUBLISHER]

Nanotubes (NTs) with diameters less than 2 nm have been proposed for next-generation reverse osmosis membranes. At this molecular scale, the NTs are narrow enough to block salt ions and other contaminants, but still wide enough to allow water to flow along the NTs at seemingly unprecedented rates. Simulations for design of NT membranes can be challenging. On the one hand, the standard equations for water flow through pipes are not applicable at sub-2-nm scales due to the dominance of non-continuum phenomena; on the other hand, full molecular simulations are computationally intractable for flows up to laboratory or prototype scales. This article describes recent multiscale approaches to simulating flows through aligned NT membranes of various materials. These multiscale techniques offer a unique and economical solution that can shed light on sometimes conflicting experimental results and point the way to future engineering design of nanostructured membranes. [ABSTRACT FROM PUBLISHER]

Technologies for detecting and analyzing a single molecule help us understand and engineer numerous phenomena observed in nature. Carbon nanotubes (CNTs) are highly efficient molecular conduits due to their atomically smooth surface. Because of their small diameters, comparable to the size of a single molecule, even a single blocking molecule can obstruct CNT fluidic channels. Analyzing these pore-blocking events in CNTs therefore enables single-molecule studies. The high-aspect ratios of CNT channels, which extend the time scale of transport, allow for studying molecular transport that is too fast to record in other systems. Both theoretical studies and ensemble experimental measurements have verified the enhanced flow of various ions and molecular species in CNTs. Experimental measurements of a single-CNT fluidic channel, however, have only recently begun, demonstrating the detection of individual DNA, polymer, and alkali-metal ions. This article reviews recent advances in single-nanotube fluidic channels with a focus on experimental measurements. [ABSTRACT FROM PUBLISHER]

This article highlights recent advances in analytical electron tomography (AET), the three-dimensional (3D) extension of conventional nanoanalytical techniques, in which electron energy loss, x-ray spectroscopy, and electron diffraction are combined with tomographic acquisition and reconstruction. Examples from the literature illustrate how new 3D information, gleaned from AET, provides insights into not just morphology and composition, but also the electronic, chemical, and optical properties of materials at the nanoscale. We describe how the “multidimensional” nature of AET leads to “big data” sets, how these can be analyzed optimally, and how AET may develop further. [ABSTRACT FROM PUBLISHER]

Block copolymers (BCPs) microphase-separate to form periodic patterns with periods of a few nanometers and above without the need for lithographic guidance. These self-assembled nanostructures have a variety of bulk geometries, including alternating lamellae, gyroids, arrays of cylinders or spheres, tiling patterns and core–shell structures, depending on the molecular architecture of the polymer and the volume fraction of its blocks. Non-bulk morphologies can be produced, the ordering of the microdomains can be improved, and their locations can be directed using various templates and processing strategies. The blocks themselves can constitute a functional material, such as a photonic crystal, or they can be used as a mask to pattern other functional materials, functionalized directly by various chemical approaches, or used as a scaffold to assemble nanoparticles or other nanostructures. BCPs offer tremendous flexibility in creating nanostructured materials with a range of applications in microelectronics, photovoltaics, filtration membranes, and other devices. [ABSTRACT FROM PUBLISHER]

Nanoparticles are ubiquitous in both natural and synthetic environments, providing much of the chemical reactivity for geochemical, planetary, environmental, and technological processes. However, this reactivity and differences between the bulk and nanoscale are thermodynamically, as well as kinetically, controlled. Energetic effects arising from differences in surface energies of different nanomaterials lead to changes in which phases are thermodynamically stable under given conditions. This results in crossovers in polymorphic stability as a function of particle size and substantial shifts in the positions of dehydration and redox equilibria. Examples of these phenomena in aluminum, cobalt, iron, and manganese oxides are presented, and implications for catalysts, battery materials, and other functional oxides are discussed. A hypothesis is presented that low surface energy and the resulting relatively weak water binding on the surface leads to better function when electrons or ions are transferred at the solid-solution interface. [ABSTRACT FROM PUBLISHER]

Aerospace applications have historically been a driver of advanced materials, from reinforced carbon–carbon thermal protection systems of space reentry vehicles to advanced metal alloy turbine blades. Although the industry now has to share the spotlight and attention of both material scientists and funding sources with potentially larger commercial market draws such as energy and health care, it still presents some unique challenges that can be met only by the application of engineered nanomaterials. This issue of MRS Bulletin reviews some of the more promising aerospace applications of nanomaterials with a focus on space rather than aeronautics, the challenges of integrating such materials into existing systems, and the challenges that remain for maturation and industry adoption. [ABSTRACT FROM PUBLISHER]

The ability to understand and control matter at the nanoscale has enabled the development of revolutionary new materials and devices. While nanotechnology is rapidly becoming pervasive in applications such as consumer electronics, health care, and cosmetics, there are only a limited number of examples in which nanotechnology has found its way into aerospace applications. This article discusses the potential of nanotechnology to impact future aerospace missions and vehicles, as well as the technical barriers to its wider use in aerospace. [ABSTRACT FROM PUBLISHER]

Space missions have unique requirements for payloads of electronics, sensors, instruments, and other components in terms of mass, footprint, power consumption, and resistance to various types of radiation. Nanomaterials offer the potential for future radiation-hardened or radiation-immune electronics. Gas-sensing needs in planetary exploration and crew-cabin air-quality monitoring are currently being met by bulky instruments. Routine health checkups of astronauts and testing of water in space habitats are being done on a delayed basis by bringing samples back to Earth. Instead, nanomaterials can be used to construct ultrasmall, postage-stamp-sized gas/vapor sensors with selective discrimination and also lab-on-a-chip biosensors for water-quality monitoring and crew health monitoring. [ABSTRACT FROM PUBLISHER]

In situ transmission electron microscopy (TEM) has become an increasingly important tool for materials characterization. It provides key information on the structural dynamics of a material during transformations and the ability to correlate a material’s structure and properties. With the recent advances in instrumentation, including aberration-corrected optics, sample environment control, the sample stage, and fast and sensitive data acquisition, in situ TEM characterization has become more powerful. In this article, a brief review of the current status and future opportunities of in situ TEM is provided. The article also introduces the six articles in this issue of MRS Bulletin exploring the frontiers of in situ electron microscopy, including liquid and gas environmental TEM, dynamic four-dimensional TEM, studies on nanomechanics and ferroelectric domain switching, and state-of-the-art atomic imaging of light elements (i.e., carbon atoms) and individual defects. [ABSTRACT FROM AUTHOR]

Catalytic nanomaterials play a major role in chemical conversions and energy transformations. Understanding how materials control and regulate surface reactions is a major objective for fundamental research on heterogeneous catalysts. In situ environmental transmission electron microscopy (ETEM) is a powerful technique for revealing the atomic structures of materials at elevated temperatures in the presence of reactive gases. This approach can allow the structure–reactivity relations underlying catalyst functionality to be investigated. Thus far, ETEM has been limited by the absence of in situ measurements of gas-phase catalytic products. To overcome this deficiency, operando TEM techniques are being developed that combine atomic characterization with the simultaneous measurement of catalytic products. This article provides a short review of the current status and major developments in the application of ETEM to gas-phase catalysis over the past 10 years. [ABSTRACT FROM AUTHOR]