James D. Plummer, professor of electrical engineering at Stanford University, replies:
"It is difficult to give a precise answer to this question because the term 'nanotechnology' is not well defined. It means different things to different people. Perhaps the broadest definition of a 'nanostructure' is something which has a physical dimension smaller than 0.1 micron, or 100 nanometers (billionths of a meter). Using this definition, integrated circuits will qualify as nanostructures shortly after the turn of the century. Current integrated circuits have minimum dimensions on the order of 0.35 microns. Based on current rates of development, people have projected that around the year 2005 companies will be manufacturing in high volume integrated circuits that have dimensions around 0.1 micron. These numbers refer to the lateral dimensions that are used to define the patterns in integrated circuits.
"Furthermore, the vertical dimensions in current integrated circuits are already as small as five to 10 nanometers. (That is the rough dimension of the gate insulator thickness in current MOS transistors, the most common kind of transistor.) So if size alone defines nanotechnology, then everyone of us is seeing practical benefits of it today.
"When many people think of nanotechnology, however, they think of other, more exotic kinds of devices: nanomachines or medical applications in which tiny machines circulate in the bloodstream cleaning out fat deposits from our arteries, for example. Such technologies are much further off, probably 25 years at least. In those kinds of applications, the problem often lies as much in defining a useful, appropriate application for nanotechnology as it does in actually building the nanostructure."
Nadrian C. Seeman in the department of chemistry at New York University offers another viewpoint:
"This is a very broad question, containing at least two ambiguities. The first lies in the meaning of 'nanotechnology.' To most people, nanotechnology implies an intelligently directed association of chemical components used to make objects and devices on the chemical scale, much as we make them on the macroscopic scale. How does this procedure differ in practice from ordinary chemistry? To my mind, not at all. Chemists try to assemble molecules using a combination of theoretical principles and practical experience to mold molecules that have desired structural and chemical properties. This is also what nanotechnologists do.
"The element of structure is key here. The structural goals of nanotechnology are frequently more ambitious than making a single molecule. Often nanotechnologists wish to make arrays of identical or complexed molecules, sometimes on a scale that will transcend the boundaries of the microscopic and approach the macroscopic. There are two different approaches to this end, 'top-down' and 'bottom-up.' The top-down approach is exemplified by scientists who build objects and molecular arrays using the techniques of scanning probe microscopy. The bottom-up approach is exemplified by investigators who design two- and three-dimensional chemical systems that cohere according to the rules of chemical interactions. The advantage of the top-down approach is its exquisite precision, but its disadvantage is its lack of extensive parallelism--it requires manipulating atoms and molecules practically one by one. In contrast, the bottom-up approach is massively parallel.
"There is already one highly successful nanotechnological system: we call it life. All the goals of nanotechnology are already fulfilled in living systems, and most of our attempts at nanotechnological applications can be called biomimetic, either applying the structural principles of living systems to different compounds or using the compounds of living systems for different purposes. For example, in our laboratory at N.Y.U., we have used analogs of branched DNA molecules (found in the process of genetic recombination) to form stick polyhedra, a cube and a truncated octahedron, whose edges consist of DNA double helices. The polyhedra are assembled from branched molecules that hold together because of the same sticky-ended hydrogen bonding that is used by the cell to direct replication of the genetic code. As in our work, living systems tend to make their structural components on the nanometer scale, rather than on the angstrom scale where most chemists work.
"The second ambiguity involves the term 'practical.' If there is any use, to anyone, of any scientific advance, I regard it as practical, although others might feel differently. A key near-term goal of our laboratory is the assembly of stick polyhedra into periodic arrays, in order to attack the key problem of macromolecular crystallography, the crystallization problem. We hope to assemble our polyhedra by means of sticky-ended cohesion so that they will diffract x-rays and so that crystallographers can determine their structures and the structures of suitably oriented molecular guests contained within the polyhedra; we expect to be able to form these arrays within five years. It should then be possible to make substantial advances in determining the structures of biological molecules, an advance that will have potential biomedical impacts. To me, this is a practical benefit."
Clifford P. Kubiak and Jason I. Henderson in the department of chemistry at Purdue University in West Lafayette, Ind., add this joint response:
"How far we are from realizing practical benefits from nanotechnology really depends on which aspect of nanotechnology is being considered. Some nanotechnologists envision gears, camshafts and motors engineered on the nanometer (billionth of a meter) scale. Others think of integrated circuits whose smallest features are on the scale of tens of nanometers. Still others see chemical 'self-assembly' as a means of building up larger functional structures or devices from molecular building blocks 0.5 to five nanometers in size. There is a common goal shared by all these researchers, however: to make devices that are smaller than anything now available.
"In general, there are two distinct approaches to constructing very small things: (1) to etch, chisel or sculpt small features into an existing structure or (2) to build up tiny structures from even smaller ones. Research in nanotechnology can be classified according to which approach is employed. People in the first group are using techniques such as scanning tunneling microscopy, atomic-force microscopy, electron beam lithography and other forms of lithography to define very small features, down to the atomic scale (0.1 nanometer). Miniature gears, tiny sensors and smaller integrated circuits are but a few of the objects being fabricated via this approach. The other group is building up larger structures by manipulation of molecular components, a process called chemical self-assembly. Single-electron transistors, highly ordered arrays of nanoscale metal or semiconductor clusters and microcontact stamps that can transfer submicron-scale patterns are a few of the recent breakthroughs resulting from the second approach.
"The question of how far in the future the real, practical benefits lie has been considered in the most detail by the Semiconductor Industry Association. The semiconductor industry has the largest business interest in the tremendous improvements in computer processor speeds and information storage densities that will result from nanoscale devices. The smallest feature of an Intel Pentium processor is currently on the order of 350 nanometers (0.35 micron). The industry's 'National Technology Roadmap for Semiconductors' sets a 70-nanometer minimum feature size as a goal to be realized by the year 2010. The image of a roadmap provides a nice conceptual model for nanotechnology research and development. Proved, existing technologies are the high-volume superhighways. But the next-generation technologies might be found on what are in 1996 only unimproved roads, footpaths or unblazed trails! It is widely believed that the 70-nanometer goal will not be achieved by incremental improvements of present-day lithographic processes.
"Paradigm shifts may be necessary that totally revolutionize the most fundamental architectures of logic and memory devices. A recent article in Scientific American ('Blue-Laser CD Technology,' by Robert L. Gunshor and Arto V. Nurmikko, July 1996) described blue-laser CD technology. That technology would replace the existing 820-nanometer-wavelength laser technology with 460-nanometer-wavelength blue lasers, greatly increasing the amount of data that could fit on a compact disk. All nine of Beethoven's symphonies could be played from a single audio CD, instead of just one symphony. Clearly, this technology is analogous to a paved road, and it may soon see widespread application. A radical paradigm shift to, say, single-electron transistors that take advantage of nonlinear 'staircase' current-voltage responses (which are only possible in nanostructured materials) may take many years to move from being a vague footpath to something resembling a road.
"We would like to thank professors Ronald Andres (Chemical Engineering), Supriyo Datta (Electrical Engineering), Robert Gunshor (Electrical Engineering), David Janes (Electrical Engineering) and Ronald Reifenberger (Physics) for their many helpful comments and suggestions in preparing these remarks. Interested readers may want to explore these references."
Fabrication of Submicrometer Features on Curved Substrates by Microcontact Printing. R. J. Jackman, J. L. Wilbur and G. M. Whitesides in Science, Vol. 269, pages 664-666; August 4, 1995.
"Coulomb Staircase" at Room Temperature in a Self-Assembled Molecular Nanostructure. R. P. Andres, T. Bein, M. Dorogi, S. Feng, J. I. Henderson, C. P. Kubiak, W. Mahoney, R. G. Osifchin and R. Reifenberger in Science, Vol. 272, pages 1323-1325; May 31, 1996.
Room Temperature Operation of a Single Electron Transistor Made by the Scanning Tunneling Microscope Nanooxidation Process for the TiOx/Ti System. K. Matsumoto, M. Ishii, K. Segawa, Y. Oka, B. J. Vartanian and J. S. Harris in Applied Physics Letters, Vol. 68, No. 1, pages 34-36; January 1, 1996