In undertaking this study, the committee decided not to put hard size limitations on micro- and nano- objects and technologies. It understands these concepts as relating roughly to scale but also as differing significantly in the importance of various underlying physical and chemical mechanisms. There is no hard line between micro and nano, but there are some clear differences in the way the science and technology communities approach these regimes.
The concept of microtechnology has become somewhat familiar. One hundred million transistor computer chips are in our homes, and the public has a vague concept of the manufacturing processes, having seen many pictures of clean rooms and workers in “bunny suits.” Now, microtechnology is migrating from the electronics domain into a much broader range of technologies with the introduction of microelectromechanical systems (MEMS) and biological applications such as micro-reaction arrays for drug discovery.
A defining feature of the nanoscale is that the behavior of a material differs in fundamental ways from that observed at the macro- and the microscales. New physics and chemistry come into play. Dimensions, as well as composition and structure, impact material properties in nanoscale materials. At least two factors dominate this transition. The first is that nanometer dimensions approach characteristic (quantum) wave function scales of excitations in the material—electrons and holes, photons, spinwaves, and magnons, among others. The second is the very large surface-to-volume ratio of these structures, which means that no atom is very far from an interface and that interatomic forces and chemical bonds dominate. The large surface areas and unique interface and molecule–solid interactions at nanostructure surfaces are the basis of much of the enthusiasm driving research at the boundary between nano- and biotechnologies. The information stored in the genome and the exquisite selectivity of biochemical interactions based on chemical recognition and matching are examples of nanoscale properties where interfacial forces play a determining role.
Nanotechnology is likely to require an approach to fabrication fundamentally different from that of microtechnology. Whereas microscale structures are typically formed by top-down techniques such as patterning, deposition, and etching, the practical formation of structures at nanoscale dimensions will probably involve an additional component, bottom-up assembly. Self-assembly, a process whereby structures are built up from atomic or molecular-scale units into larger and increasingly complex structures, is widely used by biological systems. As our capabilities expand, some combination of top-down (lithographic) and bottom-up (including self-assembly) techniques probably will be employed for the efficient manufacturing of nanoscale systems.
One last word on definition: Not all things “nano” adhere to the usual nanometer dimensional scale, nanosatellites being a notable example. Nanosatellites have overall dimensions of many centimeters—and the name evolved out of the