In single-walled carbon nanotubes (SWNTs), their electronic and vibrational structure as well as their charge-carrier dynamics are crucial for potential ultrasmall optical device applications. SWNT properties have now been obtained from optical absorption and time-resolved photoemission and, at the single-nanotube level, by resonance Raman scattering and photoluminescence studies. This article presents an overview of SWNT photophysics, discussing important findings for the characterization of carbon nanotube properties and directions for future research and potential applications. The unique optical properties observed in SWNTs are due to the one-dimensional confinement of electronic states, resulting in van Hove singularities in the nanotube density of states. Optical measurements of phonons, charge-carrier dynamics, and the electronic transition energy van Hove singularities are discussed.

By controlling the morphology of organic and inorganic semiconductors on a molecular scale, nanoscale p–n junctions can be generated in a bulk composite. Such a composite is typically called a bulk heterojunction composite, which can be considered as one virtual semiconductor combining the electrical and optical properties of the individual components. Solar cells are one attractive application for bulk heterojunction composites. Conjugated polymers or oligomers are the favorite p-type semiconducting class for these composites, while for the n-type semiconductor, inorganic nanoparticles as well as organic molecules have been investigated. Due to the solubility of the individual components, printing techniques are used to fabricate them.

Scanning tunneling microscopy and noncontact atomic force microscopy have been used to observe germanium growth on Si(001) and Si(111). The atomically resolved images provide invaluable information on heteroepitaxial film growth from the viewpoints of both industrial application and basic science. We briefly review the history of characterizing heteroepitaxial elemental semiconductor systems by means of scanning probe microscopy (SPM), where the Stranski–Krastanov growth mode can be observed on the atomic scale:the detailed phase transition from layer-by-layer growth to three-dimensional cluster growth was elucidated by the use of SPM. In addition, we comment on the potential of SPM for examining the spectroscopic aspects of heteroepitaxial film growth, through the use of SPM tips with well-defined facets.

A wide range of potential applications was suggested shortly after carbon nanotubes were discovered, including new super-strong materials, field-emission devices, hydrogen storage systems, novel electronic devices, and more. In this article, the actual advances in the technology of nanotubes over the last decade are examined. Particular attention is focused on current commercially viable applications and those with imminent commercial promise. The relatively large number of nanotube-related patents and nanotube-based startup companies stand in contrast to the relatively low output in commercial applications. The drive toward nanotube applications, in contrast to nanotube science, is investigated.

Many of the key challenges and opportunities in the study of matter involve complex and collective phenomena in which many parts exhibit organization without a central instruction set or clock. At the core of this enterprise in the study of matter is the search for an understanding of “emergent behavior”—that is, phenomena whose ultimate cause involves interactions between many simple units but which cannot be easily predicted from knowledge of the component parts alone. Examples of emergence are ordered convection cells in fluids, metabolic processes in cells, cognition, and stripes in superconductors. Our shorthand designation for soft, hard, and living matter exhibiting emergent behavior is “complex adaptive matter.”

The key challenges for memories that operate at the nanoscale and that are compatible with mainstream semiconductor processing are in achieving the performance characteristics that make the integration of hundreds of millions or more of such devices feasible. The concept of harnessing a single electron or just a few electrons for memory storage is very appealing, but among the issues to address are the increased variance, smaller signal, and numerous other consequences of the reduced statistics resulting from reduced number of carriers employed. Coupling a few electrons to a transistor channel provides a structure with the power gain and the necessary current for fast memory-state detection as well as compatibility with mainstream processing. Several recent ideas, such as the use of nanocrystals and defects, and decoupling storage from the read process, provide paths to addressing the issues of power, speed, technology compatibility, and variability in these structures.We discuss several approaches to nanoscale nonvolatile silicon memory, their attributes, and the underlying materials issues, with a focus on finding the design compromises necessary to enable their manufacturability.