Excitonic solar cells (XSCs) function by a mechanism that is different than that of conventional solar cells.They have different limitations on their open circuit photovoltages, and their behavior cannot be interpreted as if they were conventional p–n heterojunctions. Exciton dissociation at the heterojunction produces electrons on one side of the interface already separated from the holes produced on the other side of the interface. This creates a powerful photoinduced interfacial chemical potential energy gradient that drives the photovoltaic effect, even in the absence of a built-in electrical potential. The maximum thermodynamic efficiency achievable in an XSC is shown to be identical to that of a conventional solar cell, with the substitution of the optical bandgap in the XSC for the electronic bandgap in the conventional cell. This article briefly reviews the photovoltaic mechanism of XSCs, the limitations on their photovoltage, and their maximum achievable efficiency.

The development of dynamic substrates that can modulate the behavior of adherent cells is important for fundamental studies in cell biology, applications in biomaterials, and engineering microsystems that combine cellular and material components. This review outlines several strategies based on physical transduction schemes (including electrical, photochemical, thermal, and mechanical forces) for designing interfaces that are active and can signal changes in the behavior of attached cells.

Simple diagrams are used to show the transformation of a thin sample of intrinsic, direct-gap semiconductor from an ideal “flat-band” photopumped recombination-radiation light source into a current-driven, p–n junction “ultimate lamp,” a light-emitting diode.

This article highlights the major role Arthur von Hippel and the Laboratory for Insulation Research at the Massachusetts Institute of Technology played in the early development of the field of ferroelectricity in mixed oxides with the perovskite structure and, in particular, in the identification of ferroelectricity in barium titanate following its discovery in industrial laboratories in the United States during World War II.Very early optical and x-ray studies highlighted the characteristics of the ferroelectric domain structures in both ceramic and single-crystal BaTiO3, the elimination of domains at the Curie temperature TC, and the salient characteristics of the two low-temperature phase transitions. Perhaps the culmination of this work was the detailed studies of lamella 90° domains by Peter Forsbergh and the gorgeous patterns these could generate. This article also traces the manner in which the early studies contributed to whole industries based on perovskite ferroelectrics. The ceramic capacitor industry is now fabricating sophisticated, cofired multilayer capacitors (MLCs) with up to a thousand 1-µm-thick dielectric layers interleaved with base metal electrodes, addressing a market for some 1013 capacitors per year.

Manufacturers of large piezoelectric transducers depend almost exclusively on perovskite-structure oxide ceramics. Navy sonar systems are major customers, but spinoff has occurred into a wide range of commercial and medical ultrasound systems. The capability of current materials has improved more than tenfold over the original BaTiO3 ceramics as a result of the effective application of molecular engineering, a strong testament to the insight of the founder of this area of study.

Silicon carbide is a promising semiconductor for advanced power devices that can outperform Si devices in extreme environments (high power, high temperature, and high frequency). In this article, we discuss recent progress in the development of passivation techniques for the SiO2/4H-SiC interface critical to the development of SiC metal oxide semiconductor field-effect transistor (MOSFET) technology. Significant reductions in the interface trap density have been achieved, with corresponding increases in the effective carrier (electron) mobility for inversion-mode 4H-SiC MOSFETs. Advances in interface passivation have revived interest in SiC MOSFETs for a potentially lucrative commercial market for devices that operate at 5 kV and below.

This article provides a brief account of solution-phase methods that generate silver and gold nanostructures with well-controlled shapes. It is organized into five sections: The first section discusses the nucleation and formation of seeds from which nanostructures grow. The next two sections explain how seeds with fairly isotropic shapes can grow anisotropically into distinct morphologies. Polyol synthesis is selected as an example to illustrate this concept. Specifically, we discuss the growth of silver nanocubes (with and without truncated corners), nanowires, and triangular nanoplates. In the fourth section, we show that silver nanostructures can be transformed into hollow gold nanostructures through a galvanic replacement reaction. Examples include nanoboxes, nanocages, nanotubes (both single- and multi-walled), and nanorattles. The fifth section briefly outlines a potential medical application for gold nanocages.We conclude with some perspectives on areas for future work.

The Human Genome Project continues to reveal the genetic basis for numerous acquired and inherited diseases ranging from cancer, HIV, and heart disease to muscular dystrophy and hemophilia. With this wealth of information, the ability to design patient-specific drugs that alter the cellular machinery at the genetic level in a way that controls or treats a specific disease will increasingly become a reality. Designing nucleic acid drugs as well as engineering novel delivery vehicles that encapsulate and effectively transport genetic materials into cells provides an opportunity to enhance the understanding of disease mechanisms and may help treat or cure these diseases. This issue of MRS Bulletin on “Designer Materials for Nucleic Acid Delivery” explores the diverse materials—polymers, lipids, nanoparticles, biocompatible scaffolds, and engineered peptides—that are being evaluated for the intracellular delivery of nucleic acids. These synthetic delivery systems are actively being investigated for many research purposes that range from gene-based therapy, genetic vaccine, and RNA interference to gene function and cellular signaling studies. This area is currently being pursued by a broad group of academic, clinical, and industrial researchers at both the fundamental and applied level, motivated by the widespread implications for human health. In this introductory article, we provide a general tutorial to gene-based therapies and a brief overview of the many areas of materials research that are currently making a tremendous impact on this interdisciplinary field.We conclude with a discussion of the future challenges that materials researchers face in developing viable nucleic acid delivery vehicles.

Recent progress in various biotechnology fields, such as microfluidics, tissue engineering, and cellular biology, has created a great demand for substrates that can undergo defined remodeling with time. As a result, the latest research on materials with dynamically controllable surface properties has led to a variety of novel smart surface designs.

The following article is based on the plenary address by Luiz Davidovich (Federal University of Rio de Janeiro), presented on April 14, 2004, at the 2004 MRS Spring Meeting in San Francisco. The field of quantum information is a discipline that aims to investigate methods for characterizing, transmitting, storing, compressing, and computationally utilizing the information carried by quantum states. It owes its rapid development over the last few years to several factors: the ability, developed in several laboratories, to control and measure simple microscopic systems; the discovery of fast quantum algorithms; and the recognition that Moore's law will soon lead to the single-atom limit of elementary computing gates.Cryptography and quantum computing are among the main applications in the field.They rely on the subtle and fundamental properties of the quantum world: the unavoidable disturbance associated with measurement, the superposition principle, and the nonlocal properties of entangled states. Progress in this area is intimately connected to a deep understanding of quantum physics: recent achievements include the experimental demonstration of teleportation and detailed investigations of the role of the environment in the quantum–classical transition. This article reviews basic concepts and recent developments in the field of quantum information, emphasizing the close ties between fundamental research and possible applications.

The dye-sensitized solar cell (DSSC) provides a technically and economically viable alternative concept to present-day p–n junction photovoltaic devices. In contrast to conventional silicon systems, where the semiconductor assumes both the task of light absorption and charge carrier transport, these two functions are separated in DSSCs. The use of sensitizers having a broad absorption band in conjunction with wide-bandgap semiconductor films of mesoporous or nanocrystalline morphology permits harvesting a large fraction of sunlight. There are good prospects that these devices can attain the conversion efficiency of liquid-electrolyte-based dye-sensitized solar cells, which currently stands at 11%. In this article, we present the current state of the field, the realm of our review being restricted to the discussion of organic molecular hole conductors, which have demonstrated the best performance to date.

Three solid-oxide fuel cell (SOFC) electrolytes, yttria-stabilized zirconia (YSZ), rare-earth–doped ceria (REDC), and lanthanum strontium gallium magnesium oxide (LSGM), are reviewed on their electrical properties, materials compatibility, and mass transport properties in relation to their use in SOFCs. For the fluorite-type oxides (zirconia and ceria), electrical properties and thermodynamic stability are discussed in relation to their valence stability and the size of the host and dopant ions. Materials compatibility with electrodes is examined in terms of physicochemical features and their relationship to the electrochemical reactions. The application of secondary ion mass spectrometry (SIMS) to detect interface reactivity is demonstrated. The usefulness of doped ceria is discussed as an interlayer to prevent chemical reactions at the electrode–electrolyte interfaces and also as an oxide component in Ni–cermet anodes to avoid carbon deposition on nickel surfaces. Finally, the importance of cation diffusivity in LSGM is discussed, with an emphasis on the grain-boundary effects.

Nanoimprint lithography is a potentially low-cost, high-resolution patterning technique, but most of the surrounding development work has been directed toward tool designs and processing techniques. There remains a tremendous opportunity and need to develop new materials for specific nanoimprint applications. This article provides an overview of relevant materials-related development work for nanoimprint lithographic applications. Material requirements for nanoimprint patterning for the sub-45-nm integrated-circuit regime are discussed, along with proposed nanoimprint applications such as imprintable dielectrics, conducting polymers, biocompatible materials, and materials for microfluidic devices. Polymers available for thermal nanoimprint processing and photocurable precursors for ultraviolet-assisted nanoimprint lithography are discussed.

Ordered nanoscale pore systems such as those represented by zeolites offer many opportunities for the design of complex functional systems via self-assembly.With their large internal surface areas and tunable, well-defined crystalline pore structures that allow molecular sieving and ion exchange, zeolites can be adapted for numerous applications. The nanoscale reactors present in zeolite pore systems have been explored as structural templates for the spatial organization of numerous guests. Examples from various fields are discussed, such as the stabilization of organic dyes for the construction of energy transfer and storage systems, the construction of host–guest hybrid catalyst systems, and the encapsulation of conducting or semiconducting nanoscale wires and clusters. More complex, hierarchical forms of nanostructured matter become accessible when zeolite crystals are used as building blocks for the selfassembly of thin films or three-dimensional objects. A combination of weaker and stronger interactions ranging from dispersive forces, hydrogen bonding, and electrostatic interactions to covalent bonding can be used to build functional hierarchical constructs. Several examples and novel applications of such systems will be discussed, including oriented channel systems, chemical sensors, and hierarchical pore systems for catalytic reactions.

High-temperature superconductivity in the cuprates emerges when the localized electrons of a Mott insulator become mobile due to carrier doping. Understanding both the electronic ground state and the excited states of these systems are key challenges in physics today. Angle-resolved photoemission spectroscopy (ARPES) and inelastic neutron-scattering (INS) studies have been remarkably successful in mapping the momentum-space characteristics of the cuprate electronic structure. However, since cuprate superconductivity develops from atomically localized electrons and exhibits nanoscale disorder, a pure momentum-space description is unlikely to be sufficient. Instead, simultaneous information on electronic structure at the nanoscale in real space, and throughout momentum space, is required. Here, we describe a combination of novel spectroscopic imaging scanning tunneling microscopy (SI-STM) techniques that we have developed to achieve these apparently contradictory aims, along with the outcome of a series of SI-STM studies of the electronic structure of Bi2Sr2CaCu2O8+x.

The following article is based on the plenary presentation given by Mildred S. Dresselhaus of the Massachusetts Institute of Technology on November 29, 2004, at the Materials Research Society Fall Meeting in Boston. Advanced materials offer new promise for addressing some of the grand societal challenges of our future, including that of global energy. This article will review opportunities that have opened up at the nanoscale, with materials of reduced dimensionality and enhanced surface-to-volume ratio. Some examples of research accomplishments and opportunities at the nanoscale will be described, with special attention given to the potential for advanced materials and nanoscience to have an impact on the grand challenges related to a sustainable energy supply for the 21st century and beyond.

The possibility of recreating various tissues and organs for the purpose of regenerative medicine has received much interest. However, the field of tissue engineering has been restricted by the limitations of conventional approaches. A method to circumvent the need for traditional scaffold-based technologies is cell-sheet engineering, which uses temperature-responsive culture dishes. These surfaces, which are created by grafting the temperature-responsive polymer poly(N-isopropylacrylamide) onto ordinary culture dishes, enable the non-invasive harvesting of cells as intact sheets by simple temperature reduction. This article reviews current research on the applications of cell-sheet engineering for the reconstruction of various tissues, as well as the intelligent surfaces used by this novel technology.

We consider the fundamental limits to organic solar cell efficiency, and the schemes that have been used to overcome many of these limitations. In particular, the use of double and bulk heterojunctions, as well as tandem cells employing materials with high exciton diffusion lengths, is discussed.We show that in the last few years, a combination of strategies has led to a power conversion efficiency of ηp = 5.7% (under AM 1.5 G simulated solar radiation at 1 sun intensity) for tandem cells based on small-molecularweight materials, suggesting that even higher efficiencies are possible.We conclude by considering the ultimate power conversion efficiency that is expected from organic thinfilm solar cells.

Self-assembled block copolymer systems with an appropriate molecular weight to produce a length scale that will interact with visible light are an alternative platform material for the fabrication of large-area, well-ordered photonic-bandgap structures at visible and near-IR frequencies.Over the past years, one-, two-, and three-dimensional photonic crystals have been demonstrated with various microdomain structures created through microphase separation of block copolymers. The size and shape of periodic microstructures of block copolymers can be readily tuned by molecular weight, relative composition of the copolymer, and blending with homopolymers or plasticizers.The versatility of photonic crystals based on block copolymers is further increased by incorporating inorganic nanoparticles or liquid-crystalline guest molecules (or using a liquid-crystalline block), or by selective etching of one of the microdomains and backfilling with high-refractive-index materials. This article presents an overview of photonic-bandgap materials enabled by self-assembled block copolymers and discusses the morphology and photonic properties of block-copolymer-based photonic crystals containing nanocomposite additives.We also provide a view of the direction of future research, especially toward novel photonic devices.

We discuss three examples of emergent behavior in correlated electron matter, in which the discovery of scaling of several measured quantities with a single energy scale provides a key clue to a common magnetic origin: the emergence of itinerant (mobile) electronic behavior from the Kondo lattice of localized f electrons found in cerium- and uranium-based heavy fermion materials; the emergence of unconventional superconductivity in the cuprate and “high-temperature” heavy electron Ce- and Pu-based 1-1-5 superconductors; and the emergence of pseudogap behavior in the underdoped and optimally doped cuprate superconductors.We conclude with some speculation on ways in which these novel perspectives might be used to design new materials.