The exotic features of quantum mechanics have the potential to revolutionize information technologies. Using superposition and entanglement, a quantum processor could efficiently tackle problems inaccessible to current-day computers. Nonlocal correlations may be exploited for intrinsically secure communication across the globe. Finding and controlling a physical system suitable for fulfi lling these promises is one of the greatest challenges of our time. The nitrogen-vacancy (NV) center in diamond has recently emerged as one of the leading candidates for such quantum information technologies thanks to its combination of atom-like properties and solid-state host environment. We review the remarkable progress made in the past years in controlling electrons, atomic nuclei, and light at the single-quantum level in diamond. We also discuss prospects and challenges for the use of NV centers in future quantum technologies.
Developing wireless nanodevices and nanosystems is critical for sensing, medical science, environmental/infrastructure monitoring, defense technology, and even personal electronics. It is highly desirable for wireless devices to be self-powered without using a battery. We have developed piezoelectric nanogenerators that can serve as self-sufficient power sources for micro-/nanosystems. For wurtzite structures that have non-central symmetry, such as ZnO, GaN, and InN, a piezoelectric potential (piezopotential) is created by applying a strain. The nanogenerator uses the piezopotential as the driving force, responding to dynamic straining of piezoelectric nanowires. A gentle strain can produce an output voltage of up to 20-40 V from an integrated nanogenerator. Furthermore, piezopotential in the wurtzite structure can serve as a "gate" voltage that can effectively tune/control charge transport across an interface/junction; electronics based on such a mechanism are referred to as piezotronics, with applications such as electronic devices that are triggered or controlled by force or pressure, sensors, logic units, and memory. By using the piezotronic effect, we show that optoelectronic devices fabricated using wurtzite materials can provide superior performance for solar cells, photon detectors, and light-emitting diodes. Piezotronic devices are likely to serve as "mediators" for directly interfacing biomechanical action with silicon-based technology. This article reviews our study of ZnO nanostructures over the last 12 years, with a focus on nanogenerators and piezotronics.
A method to cost-effectively upgrade the performance of an established small-bandgap solar technology is to deposit a large-bandgap polycrystalline semiconductor on top to make a tandem solar cell. Metal-halide perovskites have recently been demonstrated as large-bandgap semiconductors that perform well even as a defective and polycrystalline material. We review the initial experimental and modeling work performed on these tandems. We also discuss in-depth the challenges of perovskite-based tandems and the innovations needed from the solar research community to propel perovskite-based tandems into the high-efficiency (>25%) regime and reach commercial competitiveness.
We provide a review of the main aspects related to surface-enhanced Raman scattering (SERS) enhancement factors (EFs), from their origins to the important issue of their practical quantification. The discussion also focuses on correcting some long-held misconceptions regarding the EFs in SERS, which still persist through the literature. We explain the main topics in simple terms, aiming at clarification of basic concepts rather than an in-depth overview of the already existing literature.
Surfaces that display liquid contact angles greater than 150 degrees along with low contact angle hysteresis for liquids with both high and low surface tension values are known as superomniphobic surfaces. Such surfaces are of interest for a diverse array of applications, including self-cleaning surfaces, nonfouling surfaces, stain-free clothing, spill-resistant protective wear, drag reduction, and fingerprint-resistant surfaces. Recently, significant advances have been made in understanding the criteria required to design superomniphobic surfaces. In this article, we discuss the roles of surface energy, roughness, re-entrant texture, and hierarchical structure in fabricating superomniphobic surfaces. We also provide a review of different superomniphobic surfaces reported recently in the literature and emphasize the need for mechanical, chemical, and radiation durability of superomniphobic surfaces for practical applications. Finally, we conclude with a discussion of the unresolved challenges in developing durable superomniphobic surfaces that define the scope for further improvements in the field.
Plasmonics aims at combining features of photonics and electronics by coupling photons with a free-electron gas, whose subwavelength oscillations (surface plasmons) enable manipulation of light at the nanoscale and engender the exciting properties of optical metamaterials. Plasmonics is facing a grand challenge of overcoming metal losses impeding its progress. We reflect on the reasons why subwavelength confinement and loss are intimately intertwined and investigate the physics of loss in conductors beyond the conventional Drude model. We suggest that commonly used noble metals may not be the best materials for plasmonics and describe alternate materials such as transparent conducting oxides and transition metal nitrides. We consider the prospects of compensating the loss with gain materials and conclude that the so-far elusive solution to the loss obstacle lies in finding better materials with lower losses.
Materials play a critical enabling role in many energy technologies, but their development and commercialization often follow an unpredictable and circuitous path. In this article, we illustrate this concept with the history of lithium-ion (Li-ion) batteries, which have enabled unprecedented personalization of our lifestyles through portable information and communication technology. These remarkable batteries enable the widespread use of laptop and tablet computers, access to entertainment on portable devices such as hand-held music players and video game consoles, and enhanced communication and networking on personal devices such as cellular telephones and watches. A similar transformation of transportation to electric cars and of the electricity grid to widespread deployment of variable renewable solar and wind generation, effortless time-shifting of energy generation and demand, and a transition from central to distributed energy services requires next-generation energy storage that delivers much higher performance at lower cost. The path to these next-generation batteries is likely to be as circuitous and unpredictable as the path to today's Li-ion batteries. We analyze the performance and cost improvements needed to transform transportation and the electricity grid, and we evaluate the outlook for meeting these needs with next-generation beyond Li-ion batteries.
The phenomenon of electron tunneling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fields of physics, as well as creating sub-fields on its own. Spin-dependent tunneling in magnetic tunnel junctions has aroused considerable interest and development. In parallel with this endeavor, recent advances in thin-film ferroelectrics have demonstrated the possibility of achieving stable and switchable ferroelectric polarization in nanometer-thick films. This discovery opened the possibility of using thin-film ferroelectrics as barriers in magnetic tunnel junctions, thus merging the fields of magnetism, ferroelectricity, and spin-polarized transport into an exciting and promising area of novel research. Nowadays, this research has become an important constituent of a broader effort in multiferroic materials and heterostructures that involves rich fundamental science and offers a potential for applications in novel multifunctional devices. The purpose of this article is to review recent developments in ferroelectric and multiferroic tunnel junctions. Starting from the concept of electron tunneling, we first discuss the key properties of magnetic tunnel junctions and then assess key functional characteristics of ferroelectric and multiferroic tunnel junctions. We discuss the recent demonstrations of giant resistive switching observed in ferroelectric tunnel junctions and the new concept of electrically controlling the spin polarization in magnetic tunnel junctions with a ferroelectric tunnel barrier.
In the emerging field of soft machines, large deformation of soft materials is harnessed to provide functions such as regulating flow in microfluidics, shaping light in adaptive optics, harvesting energy from ocean waves, and stretching electronics to interface with living tissues. Soft materials, however, do not provide all of the requisite functions; rather, soft machines are mostly hybrids of soft and hard materials. In addition to requiring stretchable electronics, soft machines often use soft materials that can deform in response to stimuli other than mechanical forces. Dielectric elastomers deform under a voltage. Hydrogels swell in response to changes in humidity, pH, temperature, and salt concentration. How does mechanics meet geometry, chemistry, and electrostatics to generate large deformation? How do molecular processes affect the functions of transducers? How efficiently can materials convert energy from one form to another? These questions are stimulating intriguing and useful advances in mechanics. This review highlights the mechanics that enables the creation of soft machines.
Thermal-barrier coatings (TBCs) are complex, defected, thick films made of zirconia-based refractory ceramic oxides. Their widespread applicability has necessitated development of high throughput, low cost materials manufacturing technologies. Thermal plasmas and electron beams have been the primary energy sources for processing of such systems. Electron-beam physical vapor deposition (EBPVD) is a sophisticated TBC fabrication technology for rotating parts of aero engine components, while atmospheric plasma sprays (APS) span the range from rotating blades of large power generation turbines to afterburners in supersonic propulsion engines. This article presents a scientific description of both contemporary manufacturing processes (EBPVD, APS) and emerging TBC deposition technologies based on novel extensions to plasma technology (suspension spray, plasma spray-PVD) to facilitate novel compliant and low thermal conductivity coating architectures. TBCs are of vital importance to both performance and energy efficiency of modern turbines with concomitant needs in process control for both advanced design and reliable manufacturing.
Oxides hold great promise as new and improved materials for thermal-barrier coating applications. The rich variety of structures and compositions of the materials in this class, and the ease with which they can be doped, allow the exploration of various mechanisms for lowering thermal conductivity. In this article, we review recent progress in identifying specific oxides with low thermal conductivity from both theoretical and experimental perspectives. We explore the mechanisms of lowering thermal conductivity, such as introducing structural/chemical disorder, increasing material density, increasing the number of atoms in the primitive cell, and exploiting the structural anisotropy. We conclude that further systematic exploration of oxide crystal structures and chemistries are likely to result in even further improved thermal-barrier coatings.
Because of its fascinating electronic properties, graphene is expected to produce breakthroughs in many areas of nanoelectronics. For spintronics, its key advantage is the expected long spin lifetime, combined with its large electron velocity. In this article, we review recent theoretical and experimental results showing that graphene could be the long-awaited platform for spintronics. A critical parameter for both characterization and devices is the resistance of the contact between the electrodes and the graphene, which must be large enough to prevent quenching of the induced spin polarization but small enough to allow for the detection of this polarization. Spin diffusion lengths in the 100-mu m range, much longer than those in conventional metals and semiconductors, have been observed. This could be a unique advantage for several concepts of spintronic devices, particularly for the implementation of complex architectures or logic circuits in which information is coded by pure spin currents.
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.
Graphene is a material with outstanding properties that make it an excellent candidate for advanced applications in future electronics and photonics. The potential of graphene in high-speed analog electronics is currently being explored extensively because of its high carrier mobility, its high carrier saturation velocity, and the insensitivity of its electrical-transport behavior to temperature variations. Herein, we review some of the key material and carrier-transport physics of graphene, then focus on high-frequency graphene field-effect transistors, and finally discuss graphene monolithically integrated circuits (ICs). These high-frequency graphene transistors and ICs could become essential elements in the blossoming fields of wireless communications, sensing, and imaging. After discussing graphene electronics, we describe the impressive photonic properties of graphene. Graphene interacts strongly with light over a very wide spectral range from microwaves to ultraviolet radiation. Most importantly, the light-graphene interaction can be adjusted using an electric field or chemical dopant, making graphene-based photonic devices tunable. Single-particle interband transitions lead to a universal optical absorption of about 2% per layer, whereas intraband free-carrier transitions dominate in the microwave and terahertz wavelength range. The tunable plasmonic absorption of patterned graphene adds yet another dimension to graphene photonics. We show that these unique photonic properties of graphene over a broad wavelength range make it promising for many photonic applications such as fast photodetectors, optical modulators, far-infrared filters, polarizers, and electromagnetic wave shields. These graphene photonic devices could find various applications in optical communications, infrared imaging, and national security.
We have investigated protic ionic liquids (PILs) as proton conductors for non-humidified intermediate-temperature fuel cells. PILs exhibit proton conductivity and activity in fuel cell electrode reactions, as seen in acidic aqueous solutions and acidic polymer membranes. The wide molecular designability of PILs enabled the finding of a promising candidate, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), which exhibits favorable bulk properties and electrochemical activity. Solid thin films containing [dema][TfO] were fabricated using sulfonated polyimide as a matrix polymer. By using the composite membrane, non-humidifying fuel cell operation at 120 degrees C succeeded. The fuel cell performance can be further improved by the optimization of the catalyst layer and with further research on PILs.
The commercialization of lithium-ion batteries has intimately changed our lives and enabled portable electronic devices, which has revolutionized communications, entertainment, medicine, and more. After three decades of commercial development, researchers around the world are now pursuing major advances that would allow this technology to power the next generation of light-duty, electric, and hybrid-electric vehicles. If this goal is to be met, concerted advances in safety and cost, as well as cycle-life and energy densities, must be realized through advances in the properties of the highly correlated, but separate, components of lithium-ion energy-storage systems.
Stretchable and ultraflexible electronic devices have a broad range of potential uses, from robust devices for energy storage and conversion to biomedical devices that make conformal interfaces with the skin and internal organs. Organics have long been associated with mechanical compliance, which enables inexpensive manufacturing via roll-to-roll printing. This article provides an overview of the use of organic electronic materials, including -conjugated polymers and small molecules, in highly deformable devices. It begins with a comparison of devices based on organic devices to those based on inorganic composites. The thin-film nature of organic semiconductor devices has also led to the development of several techniques for metrology that can be applied specifically to brittle organic thin films. The article concludes with a brief discussion of the applications of stretchable and ultraflexible organic electronic devices and a prescriptive outlook for successful collaborative work in this exciting, interdisciplinary field.
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.
Colloidal semiconductor nanocrystals, also known as "quantum dots" (QDs), represent an example of a disruptive technology for display and lighting applications. The QDs' high luminescence efficiency and precisely tunable, narrow emission are nearly ideal for achieving saturated colors and enriching the display or TV color gamut. Quantum dot light-emitting diodes (QLEDs) can provide saturated emission colors and allow inexpensive solution-based device fabrication on almost any substrate. The first incorporation of QDs into the consumer market is using them as optical down-converters. Blue light from an efficient high energy light source (e. g., GaN blue LED) is absorbed and reemitted at any desired lower energy wavelength. Alternatively, electric current can be used for direct excitation of QDs. QLEDs are an exciting technical challenge and commercial opportunity for display and solid-state lighting applications. Recent developments in the field show that efficiency and brightness of QLEDs can match those of organic LEDs.
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.