Overview

Mark Reed

Materials form the basic underlying building block of nearly every advanced technology, and nanotechnology is no exception. Most technologies have various levels of differentiation, which can be broken down roughly into (1) materials, (2) design and fabrication, and (3) integration. In the case of some conceptual nanotechnologies, the path to achieve the last two levels is not at all clear. However, the science and technology of nanomaterials are both vibrant fields, with breathtaking advances on a daily basis. In many ways contemporary nanoscience is essentially the study of nanomaterials.

What differentiates nanomaterials from classical materials science, chemistry, and the like is the degree of control. Whereas many scientific disciplines have for more than a century dealt with phenomena and understanding on the atomic scale, the emerging field of nanomaterials strives to achieve control on that level beyond stochastic processes. One might view this field as a variant of engineering; instead of studying what is, we try to create what never wasand now on an unprecedented level.

One of the original areas of nanosciencethe production of nanometer-scale particles of various materialsis nearly as old as human civilization. Not until the past few decades have researchers begun to appreciate the unique properties of nanoparticles. With the understanding of structure and synthesis, researchers worldwide have gained nearly atomistic control over size and properties, creating such intriguing structures as "core-shell" quantum dots. Nanoparticles are finding applications in a number of diagnostic and biomedical uses and will become widespread when toxicity and functionalization issues are solved.

The unarguable poster child of nanoscience is the carbon nanotube (CNT), which are graphene sheets rolled into a perfect cylinder with mind-bending aspect ratios, often being only a nanometer in diameter but many micrometers in length. More interestingly, their electronic structure is dramatically dependent on dimension and twistan exciting advantage if these parameters can be controlled, but generally viewed as a limitation until these problems can be solved. Nonetheless, fascinating electrical, thermal, and structural properties of single-walled nanotubes (SWNTs) have been measured. The scaling of this understanding to large-scale synthesis and the use of these properties on a macroscopic scale remain challenges.

More recently, inorganic semiconducting nanowires have begun to attract attention as an alternative to nanotubes because their electronic properties are easier to control. First explored in the early 1990s, these single-crystal nanowhiskers, with dimensions of only tens of nanometers, have seen a resurgence in the past few years because of the development of a wide variety of synthesis methods. In addition, the range of materials that can be fabricated into nanowires is impressive, giving a large design space for property control. Although the exploration of these systems is still in its infancy, this field represents a rapidly expanding frontier of functional nanomaterials.

Finally, the interface between nanomaterials and numerous applications, such as biomedical, requires nonstandard materials. Whereas soft and polymeric materials are not nanomaterials, these materials produce critical fabrication and interface implementations, and an understanding of their properties is essential to modern nanomaterials applications. An extension of this field is to integrate polymeric and single-molecule structures into other nanostructures and complex hybrid materials. These represent the most challenging nanostructures investigated to date.

The field of nanomaterials has a long way to go to reach an understanding and control on the truly atomic scale. A combination of innovative synthesis approaches and new characterization techniques, and an understanding of fluctuations and control at this new length scale, is needed, and it will lead to revolutionary advances in nanomaterials and eventually in nanoscience.