Showing papers in "Mrs Bulletin in 2013"

Diamond NV centers for quantum computing and quantum networks

Abstract: 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.

Condensation Heat Transfer on Superhydrophobic Surfaces

Abstract: 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.

High-performance SERS substrates: Advances and challenges

Abstract: 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.

Materials in superconducting quantum bits

Abstract: Superconducting qubits are electronic circuits comprising lithographically defined Josephson tunnel junctions, inductors, capacitors, and interconnects. When cooled to dilution refrigerator temperatures, these circuits behave as quantum mechanical “artificial atoms,” exhibiting quantized states of electronic charge, magnetic flux, or junction phase depending on the design parameters of the constituent circuit elements. Their potential for lithographic scalability, compatibility with microwave control, and operability at nanosecond time scales place superconducting qubits among the leading modalities being considered for quantum information science and technology applications. Over the past decade, the quantum coherence of superconducting qubits has increased more than five orders of magnitude, due primarily to improvements in their design, fabrication, and, importantly, their constituent materials and interfaces. In this article, we review superconducting qubits, articulate the important role of materials research in their development, and provide a prospectus for the future as these devices transition from scientific curiosity to the threshold of technical reality.

Ionic liquids in supercapacitors

Abstract: 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.

Quantifying SERS enhancements

Abstract: 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.

Nanostructured paper for flexible energy and electronic devices

Abstract: 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.

QLEDs for displays and solid-state lighting

Abstract: 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 single crystals: Addressing the fundamentals of organic electronics

Abstract: 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.

Nanoscale magnetometry with NV centers in diamond

Abstract: 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.

Paper-based electroanalytical devices for accessible diagnostic testing

Abstract: Microfluidic paper-based analytical devices (μPADs) 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 μPAD yields a device capable of performing electrochemical assays (an EμPAD). 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 EμPADs, 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 EμPADs and paper-based electronics, and sketches the ways in which these technologies can be combined to provide new devices for diagnostic testing.

Protic ionic liquids: Fuel cell applications

Abstract: 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°C succeeded. The fuel cell performance can be further improved by the optimization of the catalyst layer and with further research on PILs.

Shape-controlled synthesis of metal nanocrystals

Abstract: 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.

Superomniphobic surfaces: Design and durability

Abstract: Surfaces that display liquid contact angles greater than 150° 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.

Nitrogen-vacancy centers: Physics and applications

Abstract: Much of the motivation for exploring nitrogen-vacancy (NV) centers in diamond in the past decade has been for their potential as a solid-state alternative to trapped ions for quantum computing. In this area, the NV center has exceeded expectations and even shown an unprecedented capability to perform certain quantum processing and storage operations at room temperature. The ability to operate in ambient conditions, combined with the atom-like magnetic Zeeman sensitivity, has also led to intensive investigation of NV centers as nanoscale magnetometers. Thus, aside from room-temperature solid-state quantum computers, the NV could also be used to image individual spins in biological systems, eventually leading to a new level of understanding of biomolecular interactions in living cells.

Interfacial materials with special wettability

Abstract: 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.

Quantum dot light-emitting devices

Abstract: 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.

Ionic liquids as safe electrolyte components for Li-metal and Li-ion batteries

Abstract: This article reports the search for nonflammable, stable electrolytes based on ionic liquid (IL) compounds, able to effectively improve the needed safety and reliability of lithium batteries. The most significant results are reviewed with the aim of elucidating critical aspects governing the properties of IL electrolytes, including (1) transport properties affecting ionic conductivity and the cycling rate of battery systems, (2) electrochemical/chemical stability toward most conventional electrode materials, and (3) thermal properties determining the range of applicability. Both liquid and polymer electrolytes, adopting ILs as the main component or as an additive to standard electrolyte solutions, are considered. Very promising results, in terms of battery prototype performances in scaled-up configurations, demonstrate the validity of the use of ILs for practical applications. Even though further improvements are necessary, particularly at high current density operations in both lithium-metal and lithium-ion systems, the realization of safer, high-performance batteries based on IL electrolytes is certainly possible. It can be concluded that ILs represent a viable solution to disappointing compromises between energy density and acceptable safety features in lithium batteries.

Pure colors from core–shell quantum dots

Abstract: 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.

Single-crystal growth of organic semiconductors

Abstract: Organic single crystals are an established part of the emerging field of organic optoelectronics, because they provide an ideal platform for the studies of the intrinsic physical properties of organic semiconductors. As organic crystals have low melting temperatures and high vapor pressures and are soluble in numerous organic solvents, both solution and gas-phase methods can be used for crystal growth. The nature of the individual molecules and the interactions between molecules determine which growth method is preferred for particular materials. Organic semiconductors with very low decomposition or melting temperatures can be grown from solutions, whereas semiconductors with high vapor pressures can be grown using physical vapor transport methods. High-quality crystals can be obtained using both methods. Crystal growth and crystal engineering of multicomponent organic compounds are emerging fields that can provide a variety of new materials with different physical properties. The growth of large crystals from the melt by zone melting, the Bridgman, or the Czochralski methods has been used to produce stable materials used in wafer manufacturing or large scintillator detectors. In this article, single-crystal growth methods for organic semiconductors are discussed with the aim of preparing high-quality specimens for determination of the basic properties of organic semiconductors.

Nanoconfined light metal hydrides for reversible hydrogen storage

Abstract: 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.

Spectroscopic insights into the performance of quantum dot light-emitting diodes

Abstract: 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.

Bright and stable quantum dots and their applications in full-color displays

Abstract: Quantum dots (QDs) have inspired researchers to develop innovative optoelectronics applications, and especially the current advances in light-emitting diode (LED) displays have attained production level technology. The most challenging issues in developing practical QD displays are the design of highly efficient and stable nanostructures and control of the interfaces between the nanostructures and device components. This article highlights applications of both color-converting and current-driven QD-LEDs, with emphasis on the synthesis of materials specifically tailored for display applications and fabrication techniques that improve device performance, such as cross-linking and transfer-printing of nanocrystal thin films.

Cell and organ printing turns 15: Diverse research to commercial transitions

Abstract: 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.

Quantum computing with defects

Abstract: The successful development of quantum computers is dependent on identifying quantum systems to function as qubits. Paramagnetic states of point defects in semiconductors or insulators have been shown to provide an effective implementation, with the nitrogen-vacancy center in diamond being a prominent example. The spin-1 ground state of this center can be initialized, manipulated, and read out at room temperature. Identifying defects with similar properties in other materials would add flexibility in device design and possibly lead to superior performance or greater functionality. A systematic search for defect-based qubits has been initiated, starting from a list of physical criteria that such centers and their hosts should satisfy. First-principles calculations of atomic and electronic structure are essential in supporting this quest: They provide a deeper understanding of defects that are already being exploited and allow efficient exploration of new materials systems and “defects by design.”

Surface science for improved ion traps

Abstract: Trapped ions are sensitive to electric-field noise from trap-electrode surfaces. This noise has been an obstacle to progress in trapped-ion quantum information processing (QIP) experiments for more than a decade. It causes motional heating of the ions, and thus quantum-state decoherence. This heating is anomalous because it is not easily explained by typical technical-noise sources. Experimental evidence of its dependence on ion-electrode distance, frequency, and electrode temperature points to the surface, rather than the bulk, of the trap electrodes as the origin. In this article, we review experimental efforts and models to help identify and reduce or eliminate the source of the anomalous heating. Recent progress to reduce the heating with in situ cleaning indicates that it may not be a fundamental limit to trapped-ion QIP. Moreover, the extreme sensitivity of trapped ions to electric-field noise may potentially be used as a new tool in surface science.

Quantum photonic networks in diamond

Abstract: Advances in nanotechnology have enabled the opportunity to fabricate nanoscale optical devices and chip-scale systems in diamond that can generate, manipulate, and store optical signals at the single-photon level. In particular, nanophotonics has emerged as a powerful interface between optical elements such as optical fibers and lenses, and solid-state quantum objects such as luminescent color centers in diamond that can be used effectively to manipulate quantum information. While quantum science and technology has been the main driving force behind recent interest in diamond nanophotonics, such a platform would have many applications that go well beyond the quantum realm. For example, diamond’s transparency over a wide wavelength range, large third-order nonlinearity, and excellent thermal properties are of great interest for the implementation of frequency combs and integrated Raman lasers. Diamond is also an inert material that makes it well suited for biological applications and for devices that must operate in harsh environments.

Ionic liquids for energy applications

Abstract: There is an urgent need for new energy storage and conversion systems in order to tackle the environmental problems we face today and to make the transition to a fossil fuel-free society. New batteries, supercapacitors, and fuel cells have the potential to be key devices for large-scale energy storage systems for load leveling and electric vehicles. In many cases, the concepts are known, but the right materials solutions are lacking. Ionic liquids (ILs) have been highlighted as suitable materials to be included in new devices, most commonly as electrolytes. Attractive features of ILs such as high ionic conductivity, low vapor pressure, high thermal and electrochemical stability, large temperature range for the liquid phase, and flexibility in molecular design have drawn the attention of researchers from many different fields. In addition, there is the possibility of designing new materials and morphologies using electrochemical synthesis with ILs. In this article, we provide an introduction to ILs and their properties, serving as a base for the topical articles in this issue.

Challenges and solutions for high-efficiency quantum dot-based LEDs

Abstract: Colloidal quantum dots (QDs) hold great promise as electrically excited emitters in light-emitting diodes (LEDs) for solid-state lighting and display applications, as highlighted recently by the demonstration of a red-emitting QD-LED with efficiency on par with that of commercialized organic LED technologies. In the past five years, important advances have been made in the synthesis of QD materials, the understanding of QD physics, and the integration of QDs into solid-state devices. Insights from this progress can be leveraged to develop a set of guidelines to direct QD-LED innovation. This article reviews the fundamental causes of inefficiency in QD-LEDs understood to date and proposes potential solutions. In particular, we emphasize the challenge in developing QD emitters that exhibit high luminescent quantum yields in the combined presence of charge carriers and electric fields that appear during traditional LED operation. To address this challenge, we suggest possible QD chemistries and active layer designs as well as novel device architectures and modes of QD-LED operation. These recommendations serve as examples of the type of innovations needed to drive development and commercialization of high-performance QD-LEDs.

Paper as a novel material platform for devices

Abstract: Paper, broadly defined as thin, porous sheets, is currently being used to create novel devices for diagnostics, microfluidics, and electronics that ideally combine low cost and high performance. A “device,” in this context, can be defined as an object that serves to provide information or function to a user in response to input. This issue will highlight some of these novel devices and provide examples of potential applications. We begin with an overview of paper’s unique properties and how these properties lead to a potential for changing the integrated microfluidic and flexible electronics landscape. We then discuss methods for patterning paper as well as specific fluidic operations that are possible on paper. Finally, we conclude with an overview of electronic devices on paper and a brief outlook on the future of this emerging field.