Page 25 - Nanotechnology an introduction

example; and human identity is essentially our life history, in which our actual chemical constitution—anyway constantly changing—plays only a
secondary role. Yet even life makes use of exquisitely identical macromolecules—nucleic acids (RNA and DNA) and proteins, and in that sense is
also alien to the spirit of classical physics.
The ultimate aim of nanotechnology—bottom-to-bottom assembly in a eutactic environment, or programmable assembly at the atomic scale—is far
closer to the true quantum world than the classical world. In order to facilitate bridging the gap between nanoscale artifacts and those suitable for
human use, “nanoblocks” have been proposed as an intermediate level of object. These nanoblocks would be produced with atomic precision, but
their own assembly into larger structures would be easier than working with atoms—self-assembly is likely to be the most convenient general route.
Every nanoblock (of a given type) would be identical to every other one, regardless of where it had been produced. Yet each nanoblock would
probably contain thousands or tens of thousands of atoms—comparable in size to the proteins of a living cell—and hence would not rank as
absolutely small according to Dirac, yet would nevertheless possess quality, specificity and individuality. In this sense nanotechnology represents a
kind of compromise between the classical and quantum realms, an attempt to possess the advantages of both. The advantages can be seen
particularly clearly in comparison with chemistry, which attempts to create entities (chemical compounds) possessing quality, specificity and
individuality, but in the absence of a eutactic environment the yield of reactions that should lead to a unique product are usually significantly below
unity. The difference between chemistry and nanotechnology is analogous to the difference between analog and digital ways of representing
information: in the latter the basic entities (e.g., zero and one) have a specific, immutable individuality (even though the voltage representing “one”
in a digital computer may actually have a value between, say, 0.6 and 1.5 V), whereas in an analog device the information is directly represented by
the actual voltage, which may be subject to some systematic bias as well as to unavoidable fluctuations.
Returning to the issue of measurement, Heisenberg has remarked that the mathematical description of a quantum object does not represent its
behavior but rather our knowledge of its behavior. This brings clarity to another classical paradox—in not a single instance is it possible to predict a
physical event exactly, since as measurements become more and more accurate, the results fluctuate. The indeterminist school deals with this by
asserting that every physical law is of a statistical nature; the opposing school asserts that the laws apply exactly to an idealized world picture, to
which an actual measurement can only approximate [133]. Clearly the latter viewpoint is appropriate to the quantum world, in which we can predict
the probability of an event. Nanotechnology in effect creates a simplified version of the world, in which only a finite set of discrete states are
available, whose occurrence (e.g., as a result of a nanofacturing process) can be predicted exactly.
Hence we can say that an ideal nano-object should be small enough to possess quality, specificity and individuality, like a quantum object, but large
enough for its state not to be destroyed by measuring one of its attributes, such as its position. Here we seem to be approaching a fundamental
definition of a nano-object, rather than merely a phenomenological one (albeit abstracted to essential characteristics), or an ostensive one.
The quantum limit corresponds to an irreducible lower limit of smallness but, depending on the phenomenon under consideration, it might be way
beyond the size scale of a single atom; for example, the ultimate lower length scale is given by the Planck length (defined solely using fundamental
constants) m.
2.9. Summary
Consideration of what happens to things when we reduce their size reveals two kinds of behavior. In one group of phenomena there is a
discontinuous change of properties at a certain size. This change can be very reasonably taken to demarcate the nanoscale. Note that qualitative
changes in behavior with size only seem to occur “near the bottom”. Hence there is no need to separate changes occurring at the nanoscale from
those occurring at the micrometer, millimeter or even meter scale because there are none of the kind we are talking about. In the other group the
properties change gradually without any discontinuous (qualitative) change occurring.
Evidently if nanotechnology is merely a continuation of the trend of ever better machining accuracy, as it was viewed by Taniguchi [159], we do not
need to worry about qualitative differences, at least not at the hardware level. In this case the upper boundary of the nanoscale must be somewhat
arbitrary, but one hundred nanometers seems entirely reasonable, from which it follows that it is really immaterial whether this boundary is taken to
be approximate or exact. This definition would fit current usage in top–down nanomanufacture (see Chapter 8)—ultraprecision machining and
semiconductor processing. In this usage, nano-objects and devices and the processes used to make them are qualitatively the same as at bigger
scales. The inclusion of phrases such as “where properties related to size can emerge” in definitions of nanotechnology (Section 1.1.1) is in this
case superfluous, other than as a warning that something unexpected might happen.
In contrast, such unexpected phenomena constitute the essential part of the definition of nanotechnology in the first group, in which the nanoscale
reveals itself as property-dependent. Furthermore, the scale may also depend on external variables such as temperature and pressure: what
appears indubitably as a nanoscale phenomenon at one temperature might cease to be thus distinguished when hotter or colder. In order to permit
comparisons between different sets of phenomena in this group, the characteristic size parameter can be normalized by the critical nanoscale-
defining length of that phenomenon. Thus, for example, when comparing nanoparticles of different materials with respect to their optical properties,
their dimensionless size would be given by r/r , where r is a characteristic length of the object under consideration, and the nanorealm concerns
B
ratios r/r < 1. Table 2.3 and summarizes some of these nanoscale-defining lengths (cf. Section 1.1.1).
B
Table 2.3 Summary of phenomenologically-based nanoscale-defining lengths
Domain Defining length Formula Typical value/nm
Surfaces (Geometry) 5
Nucleation Critical nucleus size (2.6) 5
Optics and electronics Bohr radius (2.12) 10
Magnetism Single domain size §2.6 50
Mechanics Griffith length (2.23) 50
Where there is no discontinuity, we can take the Hegelian concept of quantitative change becoming qualitative if great enough to justify the
application of the term “nanotechnology”. Usually this demands consideration of function (utility). Thus, even though the circuit elements in the
current generation of very large-scale integrated circuits with features a few tens of nanometers in length work in exactly the same way as their
macroscopic counterparts, new function (e.g., practicable personal cellular telephony) emerges upon miniaturization.
2.10 Further Reading