Metals possess the highest conductivity among all room-temperature materials; however, ultrathin metal films demonstrate decent optical transparency but poor sheet conductance due to electron scattering from the surface and grain boundaries. This article discusses engineered metal nanostructures in the form of nanogrids, nanowires, or continuous nanofibers as efficient transparent and conductive electrodes. Metal nanogrids are discussed, as they represent an excellent platform for understanding the fundamental science. Progress toward low-cost, nano-ink-based printed silver nanowire electrodes, including silver nanowire synthesis, film fabrication, wire-wire junction resistance, optoelectronic properties, and stability, are also discussed. Another important factor for low-cost application is to use earth-abundant materials. Copper-based nanowires and nanofibers are discussed in this context. Examples of device integrations of these materials are also given. Such metal nanostructure-based transparent electrodes are particularly attractive for solar cell applications.
Ongoing technological advances in such disparate areas as consumer electronics, transportation, and energy generation and distribution are often hindered by the capabilities of current energy storage/conversion systems, thereby driving the search for high-performance power sources that are also economically viable, safe to operate, and have limited environmental impact. Electrochemical capacitors (ECs) are a class of energy-storage devices that fill the gap between the high specific energy of batteries and the high specific power of conventional electrostatic capacitors. The most widely available commercial EC, based on a symmetric configuration of two high-surface-area carbon electrodes and a nonaqueous electrolyte, delivers specific energies of up to similar to 6 Whkg(-1) with sub-second response times. Specific energy can be enhanced by moving to asymmetric configurations and selecting electrode materials (e.g., transition metal oxides) that store charge via rapid and reversible faradaic reactions. Asymmetric EC designs also circumvent the main limitation of aqueous electrolytes by extending their operating voltage window beyond the thermodynamic 1.2 V limit to operating voltages approaching similar to 2 V, resulting in high-performance ECs that will satisfy the challenging power and energy demands of emerging technologies and in a more economically and environmentally friendly form than conventional symmetric ECs and batteries.
Reducing our dependence on fossil fuels increases the demand for energy storage. Lithium-ion batteries have transformed portable electronics and will continue to be important but cannot deliver the step change in energy density required in the longer term in markets such as electric vehicles and the storage of electricity from renewables. There are a few alternatives. Here we describe two: Li-air and Li-sulfur batteries. We compare the energy densities of Li-ion, Li-air, and Li-S and discuss their differences and the challenges facing Li-air and Li-S, many of which are materials related.
Three-dimensional (3D) battery architectures have emerged as a new direction for powering microelectromechanical systems and other small autonomous devices. Although there are few examples to date of fully functioning 3D batteries, these power sources have the potential to achieve high power density and high energy density in a small footprint. This overview highlights the various architectures proposed for 3D batteries, the advances made in the fabrication of components designed for these devices, and the remaining technical challenges. Efforts directed at establishing design rules for 3D architectures and modeling are providing insight concerning the energy density and current uniformity achievable with these architectures. The significant progress made on the fabrication of electrodes and electrolytes designed for 3D batteries is an indication that a number of these battery architectures will be successfully demonstrated within the next few years.
Although photovoltaic cells have great potential for supplying carbon-free energy, they suffer from the lack of an efficient and cost-effective energy storage process that can supply energy for transportation and nighttime use. A direct way to convert solar energy into chemical fuels would solve this problem. Of several possible schemes, the photon-driven electrolysis of water to produce hydrogen and oxygen has been studied most. Photoelectrolysis of water can be achieved with either self-supported catalysts or with photoelectrochemical cells. This article will introduce the basic principles of solar water splitting and highlight recent developments with semiconductor light absorbers and co-catalysts. The role of combinatorial approaches in identifying new metal oxide visible light-absorbing semiconductors will be briefly described, and the potential of using nanomaterials for more efficient devices will be discussed. Separate articles in this special issue will focus on recent developments in water-splitting concepts.
This article reviews the physical and chemical constraints of environments on biofilm formation. We provide a perspective on how materials science and engineering can address fundamental questions and unmet technological challenges in this area of microbiology, such as biofilm prevention. Specifically, we discuss three factors that impact the development and organization of bacterial communities. (1) Physical properties of surfaces regulate cell attachment and physiology and affect early stages of biofilm formation. (2) Chemical properties influence the adhesion of cells to surfaces and their development into biofilms and communities. (3) Chemical communication between cells attenuates growth and influences the organization of communities. Mechanisms of spatial and temporal confinement control the dimensions of communities and the diffusion path length for chemical communication between biofilms, which, in turn, influences biofilm phenotypes. Armed with a detailed understanding of biofilm formation, researchers are applying the tools and techniques of materials science and engineering to revolutionize the study and control of bacterial communities growing at interfaces.
Advances in nanoscience and nanotechnology critically depend on the development of nanostructures whose properties are controlled during synthesis. We focus on this critical concept using semiconductor nanowires, which provide the capability through design and rational synthesis to realize unprecedented structural and functional complexity in building blocks as a platform material. First, a brief review of the synthesis of complex modulated nanowires in which rational design and synthesis can be used to precisely control composition, structure, and, most recently, structural topology is discussed. Second, the unique functional characteristics emerging from our exquisite control of nanowire materials are illustrated using several selected examples from nanoelectronics and nano-enabled energy. Finally, the remarkable power of nanowire building blocks is further highlighted through their capability to create unprecedented, active electronic interfaces with biological systems. Recent work pushing the limits of both multiplexed extracellular recording at the single-cell level and the first examples of intracellular recording is described, as well as the prospects for truly blurring the distinction between nonliving nanoelectronic and living biological systems.
Networks of nanoscale conductors such as carbon nanotubes, graphene, and metallic nanowires are promising candidates to replace metal oxides as transparent conductors. However, very few previous reports have described nanostructured thin films that reach the standards required by industry for high-performance transparent electrodes. In this review, we analyze the sheet resistance and transmittance data extracted from published literature for solution processed, nanostructured networks. In the majority of cases, as their thickness is reduced below a critical value, nanoconductor networks undergo a transition from bulk-like to percolative behavior. Such percolative behavior is characteristic of sparse networks with limited connectivity and few continuous conductive paths. This transition tends to occur for films with a transmittance between 50% and 90%, which means that the properties of highly transparent films are predominately limited by percolation. Consequently, to achieve low resistance coupled with high transparency, the networks must be much more conductive than would otherwise be the case. We show that highly conductive networks of metallic nanowires appear to be the most promising candidate to replace traditional transparent electrode materials from a technical standpoint. However, many other factors, including cost, manufacturability, and stability, will have to be addressed before commercialization of these materials.
Water splitting to produce H-2 using sunlight is a form of artificial photosynthesis in that light energy is converted to chemical energy. As such, water splitting using powdered photocatalysts has attracted attention in the framework of energy and environmental issues. This article reviews z-scheme photocatalyst systems for water splitting under visible light irradiation, especially focused on the systems consisting of SrTiO3:Rh of a H-2-evolving photocatalyst, and O-2-evolving photocatalysts with and without electron mediators. These photocatalyst systems showed activities for water splitting into H-2 and O-2 in a stoichiometric amount under visible light irradiation and even under sunlight irradiation. The photocatalytic activity was sensitive to pH. The optimum pH was 2.4 when iron ions were used as electron mediators. Co-catalysts also affected the activity. The photodeposited Ru co-catalyst gave an excellent performance. The best performance achieved by the pH adjustment and the selection of a co-catalyst was obtained mainly by suppression of back reactions to form H2O from evolved H-2 and O-2.
This article reviews and assesses recent progress in atomic layer deposition (ALD) and highlights how the field of ALD is expanding into new applications and inspiring new vapor-based chemical reaction methods. ALD is a unique chemical process that yields ultrathin film coatings with exceptional conformality on highly non-uniform and non-planar surfaces, often with subnanometer scale control of the coating thickness. While industry uses ALD for high-kappa dielectrics in the manufacturing of electronic devices, there is growing interest in low-temperature ALD and ALD-inspired processes for newer and more wide-ranging applications, including integration with biological and synthetic polymer structures. Moreover, the conformality and nanoscale control of ALD film thickness makes ALD ideal for encapsulation and nano-architectural engineering. Articles in this issue of MRS Bulletin present details of several growing interest areas, including the extension of ALD to new regions of the periodic table, and molecular layer deposition and vapor infiltration for synthesis of organic-based thin films. Articles also discuss ALD for nanostructure engineering and ALD for energy applications. A final article shows how the challenge of scaling ALD for high rate nanomanufacturing will push advances in plasma, roll-to-roll, and atmospheric pressure ALD.
Bacterial biofilms are interface-associated colonies of bacteria embedded in an extracellular matrix that is composed primarily of polymers and proteins. They can be viewed in the context of soft matter physics: the rigid bacteria are analogous to colloids, and the extracellular matrix is a cross-linked polymer gel. This perspective is beneficial for understanding the structure, mechanics, and dynamics of the biofilm. Bacteria regulate the water content of the biofilm by controlling the composition of the extracellular matrix, and thereby controlling the mechanical properties. The mechanics of well-defined soft materials can provide insight into the mechanics of biofilms and, in particular, the viscoelasticity. Furthermore, spatial heterogeneities in gene expression create heterogeneities in polymer and surfactant production. The resulting concentration gradients generate forces within the biofilm that are relevant for biofilm spreading and survival.
Energy storage is a critical hurdle to the success of many clean energy technologies. Batteries with high energy density, good safety, and low cost can enable more efficient vehicles with electrified drive trains, such as hybrid electric vehicles, electric vehicles, and plug-in hybrid electric vehicles. They can also provide energy storage for intermittent energy sources, such as wind and solar. Today, and for the foreseeable future, rechargeable lithium batteries deliver the highest energy per unit weight or volume at reasonable cost. Many of the important properties of battery materials can be calculated with first-principles methods, making lithium batteries fertile ground for computational materials design. In this article, we review the successes and opportunities in using first-principles computations in the battery field. We also highlight some technical challenges facing the accurate modeling of battery materials.
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.
Recent work indicates that materials with nanoscale architectures, such as nanolayered Cu-Nb composites and nanoscale oxide dispersion-strengthened steels, are both thermally stable and offer improved performance under irradiation. Current understanding of the atomic-level response of such materials to radiation yields insights into how controlling composition, morphology, and interface-defect interactions may further enable atomic-scale design of radiation-tolerant nanostructured composite materials. With greater understanding of irradiation-assisted degradation mechanisms, this bottom-up design approach may pave the way for creating the extreme environment—tolerant structural materials needed to meet the world's clean energy demand by expanding use of advanced fission and future fusion power.
We describe two ways in which pulsed lasers can be used to increase efficiency in photovoltaic devices. First, pulsed-laser hyperdoping can introduce dopants into a semiconductor at non-equilibrium concentrations, which creates an intermediate band in the bandgap of the material and modifies the absorption coefficient. Second, pulsed-laser irradiation can enhance geometric light trapping by increasing surface roughness. Hyperdoping in silicon enables absorption of photons to wavelengths of at least 2.5 mu m, while texturing enhances the absorptance to near unity at all absorbing wavelengths. This article reviews both effects and comments on outstanding questions and challenges in applying each to increasing the efficiency of photovoltaic devices.
In recent years, there has been rapid development in the field of nanoscale light trapping for solar cells. This has been driven by the decrease in thickness of solar cells in order to reduce materials costs, as well as advances in fabrication technology and computer power for simulating nanoscale structures. Nanoscale light trapping offers the possibility of enhancing absorption beyond the limits achievable with geometrical optics for certain structures. It also allows the optical design to be separated from the electrical design, as for example in plasmonic solar cells. Most importantly, thin-film cell designs will need to incorporate nanophotonic light trapping in order to reach their ultimate efficiency limits. In this article, we review the major types of nanophotonic light trapping, including plasmonic, diffraction gratings, and random scattering surfaces and describe the major advantages and disadvantages of each. In addition, we describe the most important related fabrication and characterization technologies and provide an outlook on future directions in this field.
Typically, materials with high electrical conductivity such as metals are opaque, and materials with high optical transparency such as glass are insulating. Finding materials that are both transparent to visible light and electrically conductive has proven to be a challenge. The need for such materials continues to grow, as many of today's popular devices such as liquid-crystal displays and organic light-emitting diodes in televisions, touch screens in phones or tablet computers, electrophoretic displays in e-readers, or solar cells on a roof require one or more layers to transmit visible light, while simultaneously applying a voltage or conducting a current. Today, the industry's need for such a material is serviced by various metal oxides, of which indium tin oxide (ITO) is by far the most common. The opto-electronic properties of ITO satisfy industry need for most devices; however, ITO has several drawbacks (e.g., brittle, expensive, and typically applied via costly sputtering techniques). To address these issues, recent advances in solution-processed nanomaterials have enabled several printable alternatives to sputtered ITO. These nanomaterials include conducting polymers, metallic nanostructures, ITO nanostructures, carbon nanotubes, and graphene. The ability to apply nanomaterials from the liquid phase opens the possibility to print these electronic materials roll-to-roll, greatly reducing cost and increasing yield and throughput, while the nanomaterial topology enables truly flexible devices.
Thin-film photovoltaic technologies have an enormous potential to reduce the cost of solar electricity. However, because thin photoactive layers are used, optical absorption is incomplete unless light-trapping strategies are employed. Since conventional light-trapping approaches based on geometric scattering are less effective in thin-film cells, coherent light-trapping approaches that exploit the wave nature of light are being explored to enhance optical absorption. In this article, we look at the various strategies for coherent light trapping in thin-film solar cells, including photonic crystals, metal nanostructures, and multilayer stacks. The suitability of a particular strategy depends on factors such as configuration of the solar cell, process compatibility, cost, desired angular response, and materials usage. We also discuss the physical limits of light trapping in thin films.
Climate change, diminishing reserves of fossil fuels, energy security, and consumer demand all depend on alternatives to our current course of energy usage and consumption. A broad consensus concurs that implementing energy efficiency and renewable energy technologies are necessities now rather than luxuries to be deferred to some distant future. Neither effort can effect serious change in our energy patterns without marked improvements in electrical energy storage, with electrochemical energy storage in batteries and electrochemical capacitors serving as key components of any plausible scenario. 1, 2 Consumer expectations of convenience and long-lived portable power further drive the need to push these old devices onto a new performance curve. This issue of MRS Bulletin addresses the significant advances occurring in research laboratories around the world as old electrode materials and designs are re-envisioned, and abandoned materials of the past are reinvigorated by arranging matter and function on the nanoscale to bring batteries and electrochemical capacitors into the 21st century.
This review focuses on recent developments in the study of hydrogen generation from water splitting using photoelectrochemical (PEC) cells based on metal oxide (MO) nanomaterials. The emphasis is on the unique properties of MO nanostructures and their advantages as well as limitations for PEC solar hydrogen generation. While abundant and stable, metal oxide nanomaterials tend to have weak visible light absorption that limits their use for solar energy conversion. In addition, MO nanomaterials tend to exhibit a high density of trap states or defect sites that limit their overall efficiency. Different strategies have been developed to enhance visible light absorption (e.g., doping, dye, or quantum dot sensitization and band structure engineering using composite structures) as well as to enhance transport by reducing the density of trap states via surface modification, improving crystallinity, or using 1D structures. In some cases, combining different strategies has led to strong synergistic effects. Recent studies point to the importance and promise of engineering electronic band structure for improving PEC performance of MO nanostructures for hydrogen generation and other potential applications.