Graphene is a two-dimensional (2D) material with over 100-fold anisotropy of heat flow between the in-plane and out-of-plane directions. High in-plane thermal conductivity is due to covalent sp(2) bonding between carbon atoms, whereas out-of-plane heat flow is limited by weak van der Waals coupling. Herein, we review the thermal properties of graphene, including its specific heat and thermal conductivity (from diffusive to ballistic limits) and the influence of substrates, defects, and other atomic modifications. We also highlight practical applications in which the thermal properties of graphene play a role. For instance, graphene transistors and interconnects benefit from the high in-plane thermal conductivity, up to a certain channel length. However, weak thermal coupling with substrates implies that interfaces and contacts remain significant dissipation bottlenecks. Heat flow in graphene or graphene composites could also be tunable through a variety of means, including phonon scattering by substrates, edges, or interfaces. Ultimately, the unusual thermal properties of graphene stem from its 2D nature, forming a rich playground for new discoveries of heat-flow physics and potentially leading to novel thermal management applications.
Surface-enhanced Raman spectroscopy (SERS) is highly dependent upon the substrate, where excitation of the localized metal surface plasmon resonance enhances the vibrational scattering signal of proximate analyte molecules. This article reviews recent progress in the fabrication of SERS substrates and the requirements for characterization of plasmonic materials as SERS platforms. We discuss bottom-up fabrication of SERS substrates and illustrate the advantages of rational control of metallic nanoparticle synthesis and assembly for hot spot creation. We also detail top-down methods, including nanosphere lithography for the preparation of tunable, highly sensitive, and robust substrates, as well as the unique benefits of tip-enhanced Raman spectroscopy for simultaneous acquisition of molecular vibrational information and high spatial resolution imaging. Finally, we discuss future prospects and challenges in SERS, including the development of surface-enhanced femtosecond stimulated Raman spectroscopy, microfluidics with SERS, creating highly reproducible substrates, and the need for reliable characterization of substrates.
Condensation is a phase change phenomenon often encountered in nature, as well as used in industry for applications including power generation, thermal management, desalination, and environmental control. For the past eight decades, researchers have focused on creating surfaces allowing condensed droplets to be easily removed by gravity for enhanced heat transfer performance. Recent advancements in nanofabrication have enabled increased control of surface structuring for the development of superhydrophobic surfaces with even higher droplet mobility and, in some cases, coalescence-induced droplet jumping. Here, we provide a review of new insights gained to tailor superhydrophobic surfaces for enhanced condensation heat transfer considering the role of surface structure, nucleation density, droplet morphology, and droplet dynamics. Furthermore, we identify challenges and new opportunities to advance these surfaces for broad implementation in thermofluidic systems.
Cellulose is one of the most abundant organic materials on earth, and cellulose paper is ubiquitous in our daily life. Re-engineering cellulose fibers at the nanoscale will allow this renewable material to be applied to advanced energy storage systems and optoelectronic devices. In this article, we examine the recent development of nanofibrillated cellulose and discuss how the integration of other nanomaterials leads to a wide range of applications. The unique properties of nanofibrillated cellulose enable multi-scale structuring of the functional composites, which can be tailored to develop new concepts of energy and electronic devices. Tapping into the nanostructured materials offered by nature can offer many opportunities that will take nanotechnology research to a new level.
Microfluidic paper-based analytical devices (mu PADs) use the passive capillary-driven flow of aqueous solutions through patterned paper channels to transport a sample fluid into distinct detection zones that contain the reagents for a chemical assay. These devices are simple, affordable, portable, and disposable; they are, thus, well suited for diagnostic applications in resource-limited environments. Adding screen-printed electrodes to the detection zones of a mu PAD yields a device capable of performing electrochemical assays (an E mu PAD). Electrochemical detection has the advantage over colorimetric detection that it is not affected by interference from the color of the sample and can be quantified with simple electronics. The accessibility of E mu PADs, however, is limited by the requirement for an external potentiostat to power and interpret the electrochemical measurement. New developments in paper-based electronics may help loosen this requirement. This review discusses the current capabilities and limitations of E mu PADs and paper-based electronics, and sketches the ways in which these technologies can be combined to provide new devices for diagnostic testing.
Supercapacitors are nowadays considered to be one of the most important electrochemical storage devices. These devices display high power and extraordinary cycle life, and they are currently used in an increasing number of applications. However, in order to further increase the applications of supercapacitors, an increase in their energy capacity appears to be necessary. Moreover, the development of safe and environmentally friendly supercapacitors is also required. In this article, we illustrate the contributions ionic liquids (ILs) might play in the development of high energy and safe supercapacitors. First, the use of ILs as electrolytes in supercapacitors is considered, and the advantages as well as challenges related to the use of this kind of electrolyte are analyzed. Next, the interaction between ILs and electrode materials is taken into account, with particular attention paid to inactive components of supercapacitor electrodes. The introduction of natural cellulose as a binder is used as an example of the contribution ILs might provide to the development of environmentally friendly supercapacitors.
The mainstream commercialization of colloidal quantum dots (QDs) for light-emitting applications has begun: Sony televisions emitting QD-enhanced colors are now on sale. The bright and uniquely size-tunable colors of solution-processable semiconducting QDs highlight the potential of electroluminescent QD light-emitting devices (QLEDs) for use in energy-efficient, high-color-quality thin-film display and solid-state lighting applications. Indeed, this year's report of record-efficiency electrically driven QLEDs rivaling the most efficient molecular organic LEDs, together with the emergence of full-color QLED displays, foreshadow QD technologies that will transcend the optically excited QD-enhanced products already available. In this article, we discuss the key advantages of using QDs as luminophores in LEDs and outline the 19-year evolution of four types of QLEDs that have seen efficiencies rise from less than 0.01% to 18%. With an emphasis on the latest advances, we identify the key scientific and technological challenges facing the commercialization of QLEDs. A quantitative analysis, based on published small-scale synthetic procedures, allows us to estimate the material costs of QDs typical in light-emitting applications when produced in large quantities and to assess their commercial viability.
Organic optoelectronics is an emerging field that exploits the unique properties of conjugated organic materials to develop new applications that require a combination of performance, low cost, light weight, and processability. For instance, disposable or wearable electronics, light-emitting diodes, smart tags, sensors, and solar cells all fall into this active area of research. Single crystals of conjugated organic molecules are, undoubtedly, the materials with the highest degree of order and purity among the variety of different forms of organic semiconductors. Electronic devices comprising these materials, such as single-crystal transistors and photoconductors developed during the last decade, are by far the best performers in terms of the fundamental parameters such as charge-carrier mobility, exciton diffusivity, concentration of defects, and operational stability. Extremely low density of defects and the resultant remarkable electrical characteristics of some of the organic single-crystal devices allow experimental access to the intrinsic charge transport properties not dominated by charge scattering and trapping. This enables basic studies of the physics of organic semiconductors, including examining the intrinsic structure-property relationship, thus providing a test bed for charge and energy transport theories. The goal of this issue of MRS Bulletin is to provide a broad overview of the state of the art of the field of organic semiconductor single-crystal materials, devices, and theory.
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.
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.
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.
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.
The ability to control the shape of metal nanocrystals is central to advances in many areas of modern science and technology, including catalysis, plasmonics, electronics, and biomedicine. This article provides a brief overview of our recent efforts toward the development of solution-phase methods for shape-controlled synthesis of metal nanocrystals. While the synthetic methods only involve simple redox reactions, we have been working diligently to understand the complex nucleation and growth mechanisms leading to the formation of metal nanocrystals with desired shapes and related properties. We hope this review will inspire new ideas and concepts in the general area of nanomaterial synthesis, expand our ability to engineer the properties of metals for various applications, and contribute to the realization of sustainable use for some of the scarcest materials.
Various life forms in nature display a high level of adaptability to their environments through the use of sophisticated material interfaces. This is exemplified by numerous biological systems, such as the self-cleaning of lotus leaves, the water-walking abilities of water striders and spiders, the ultra-slipperiness of pitcher plants, the directional liquid adhesion of butterfly wings, and the water collection capabilities of beetles, spider webs, and cacti. The versatile interactions of these natural surfaces with fluids, or special wettability, are enabled by their unique micro/nanoscale surface structures and intrinsic material properties. Many of these biological designs and principles have inspired new classes of functional interfacial materials, which have remarkable potential to solve some of the engineering challenges for industrial and biomedical applications. In this article, we provide a snapshot of the state of the art of biologically inspired materials with special wettability, and discuss some promising future directions for the field.
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.
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.
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.
Fifteen years ago, the field of cell and organ printing began with a few research groups looking to take newly developed direct-write tools and apply them to living cells. Initial experiments demonstrated cell viability and functionality post-deposition. Recently, research has begun in earnest to create three-dimensional cellular constructs that mimic both the heterogeneous structure and function of natural tissue. Several companies are now marketing cell printers, expanding access to a wider group of scientists and accelerating the pace of development. This article describes the past decade and a half of research by showing examples of some of the most sophisticated work, comparing the approaches and tools used in the field, and predicting the products that will arrive in the not too distant future.
Lighting consumes almost one-fifth of all electricity generated today. In principle, with more efficient light sources replacing incandescent lamps, this demand can be reduced at least twofold. A dramatic improvement in lighting efficiency is possible by replacing traditional incandescent bulbs with light-emitting diodes (LEDs) in which current is directly converted into photons via the process of electroluminescence. The focus of this article is on the emerging technology of LEDs that use solution-processed semiconductor quantum dots (QDs) as light emitters. QDs are nano-sized semiconductor particles whose emission color can be tuned by simply changing their dimensions. They feature near-unity emission quantum yields and narrow emission bands, which result in excellent color purity. Here, we review spectroscopic studies of QDs that address the problem of nonradiative carrier losses in QD-LEDs and approaches for its mitigation via the appropriate design of QD emitters. An important conclusion of our studies is that the realization of high-performance LEDs might require a new generation of QDs that in addition to being efficient single-exciton emitters would also show high emission efficiency in the multicarrier regime.