by Nicholas V. Coppa, Nanomaterials Company
The many thoughtful definitions of nanotechnology offered in the media and literature in recent years do well in conveying a general idea. However, the most salient point, the essence of nanotechnology, is ignored or possibly unnoticed by most. At the heart of nanotechnology is the exploitation of size as an independent, fourth degree of freedom. This rarely acknowledged fact is discussed and examples presented.
March 8, 2010 - Nanotechnology is the science and engineering of manipulating size independent of temperature, pressure and composition to yield materials that express size-dependent effects or properties. Independent is the key word -- it conveys the enormous scientific, technical and commercial opportunity, while nano merely describes the length scale at which such size manipulation results in measurable and exploitable effects.
Size is the fourth independent degree of freedom because ordinarily, only three independent terms such as temperature T, pressure P and composition n, are required to define the state of a material. (Other system variables can be chosen, but the point is that generally, only three are required.) For ordinary or "macro" materials, size and variations at the macro through micro scales is unimportant since their subdivision to finer and finer powders occurs without changing the materials' intrinsic properties. For example, the familiar lustrous gray color and other optical and electrical properties of silicon remain constant as its size is reduced.
Changing material properties
The materials designer, by virtue of independent control of size, is afforded discovery and exploitation of new materials properties. When size is manipulated at the nanometer-length scale, however, the intrinsic properties of many materials are affected. When the sizes of silicon crystals are produced on the order of tens of nanometers, silicon appears golden yellow. Compositional and crystal structural analyses obtained under identical conditions of temperature and pressure reveals the yellow silicon to be identical to that of the macro (gray) silicon; the only variation is size of the crystal. The well-understood origin of such an effect is founded in the fact that there are a relatively small number of atoms within each crystal leading to an "under-development" of the electronic band structure of macro silicon. Many other optical property size-dependencies in materials exist. As particles become smaller, their surface features become more prominent. At the nanoscale, smooth surfaces or macro materials degrade to coarse surfaces prominent with lattice steps, corners, etc. The number and type of these surface features, which give rise to catalytic activity and other properties, can be selected independent of composition P, and T through the control of particle size.
Though man has been intentionally manipulating T, P and n since antiquity, the independent control of size at the nanoscale to express new materials properties has largely been unintentional until recently. Indeed, the first scientific publication devoted entirely to the reporting of control of size appeared in 1992, Nanostructured Materials (published by Elsevier Ltd.). From this perspective, it is easy to understand the infancy of the science and the enumerable technical and commercial opportunities presently before us.
The future of nanomaterials
Perhaps all known materials will be shown to express new properties through the exploitation of size as an independent degree of freedom. Such developments will do more for the solution to many of the technical and social problems facing us presently. Among the many challenges facing our society that would benefit from nanotechnology are: efficient utilization of conventional sources of energy and the rapid development of new sources, medical diagnostics and therapeutics, transportation and exploration, efficient use of and conservation of natural resources, and environmental protection.
Expression of size effects requires precise control of size. Consider the utility of light. Incoherent polychromatic light such as that radiating from a lamp or fire has limited utility, illumination. However, coherent monochromatic radiation has utility of much greater sophistication; for many readers, this text is conveyed using such radiation. The exploitation of size effects in nanomaterials will require similar sophistication. Most chemical suppliers and most companies devoted to the production of nanomaterials provide powders with a log normal distribution with a full width at half maximum of several tens of nanometers. Such materials may exhibit gross size effects, but the powder polydispersity masks the unique properties of any one size.
Analogous to the white light example above, such powders have limited or rather unsophisticated utility. Furthermore, a modest change in the average particle size, for example, 10-20nm, would not translate into a material that behaves significantly different. This is because the population of particles in one distribution does not differ significantly from the other. In analogy with the useful coherent radiation, monodisperse nanopowders have great utility. Thus, expression of size dependent, nano-enabled, useful properties of such powders are uniquely expressed as the size distribution of nanoparticles is narrowed.
So what is nanotechnology, or more importantly, what is it that nanotechnology uniquely provides to the community of nanomaterials developers? The answer is size as an independent degree of freedom.
Nicholas V. Coppa received his BA and MS degrees from Syracuse U. and PhD from Temple U. He is the managing member of Nanomaterials Company, 15 North Bacton Hill Rd., Malvern, PA 19355 USA; e-mail email@example.com.
Nanotechnology: exploiting the fourth independent degree of freedom
by Nicholas V. Coppa, Nanomaterials Company