Page 17 - Nanotechnology an introduction

Chapter Contents

2.1 The Size of Atoms 17
2.2 Molecules and Surfaces 18
2.3 Nucleation 20
2.4 Chemical Reactivity 21
2.5 Electronic and Optical Properties 24
2.6 Magnetic and Ferroelectric Properties 27
2.7 Mechanical Properties 28
2.8 Quantum Smallness 30
2.9 Summary 33
2.10 Further Reading 34
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. The nanoscale therefore reveals itself as property-dependent. In the other group of phenomena the properties change gradually without any discontinuous (qualitative) change occurring. In this case the upper
boundary of the nanoscale is somewhat arbitrary, but one hundred nanometers seems reasonable, and it is immaterial whether this boundary is taken to be approximate or exact. The Hegelian concept of quantitative change becoming
qualitative if great enough can be used to justify the application of the term “nanotechnology”.
Keywords: molecules, surfaces, nucleation, creativity, electromagnetic properties, mechanical properties, quantum behavior, function
Before the advent of nanotechnology, and before the Système International (S.I.) was instituted, the bulk realm was typically referred to as macro
and the atomic realm as micro. Scientific consideration of the microworld really began with the invention of the microscope, very appropriately
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named as it turned out, in view of the subsequent formalization of “micro” as the name of the multiplier 10 . The Abbe limit of resolution of the
optical microscope (equation 5.2) is indeed of the order of one micrometer (the wavelength of visible light). Now that electron microscopes, and
even more recently scanning probe microscopes, can resolve features a few nanometers in size, the name “microscope” has become
anachronistic—it would be better to call these instruments ultramicroscopes or nanoscopes. The range intermediate between macro and micro
was typically referred to as meso (“mesoscale” simply means “of an intermediate scale”). This had no precise definition (except in meteorology),
but typically referred to a realm which, while being indubitably invisible (to the naked eye) and hence “micro”, and not describable using
macroscopic continuum models, could be adequately modeled without explicitly considering what were by then believed to be the ultimate
constituents of matter—electrons, protons, neutrons and so forth—but by taking into account an appropriately coarse-grained discreteness. For
example, according to this approach a protein might be modeled by considering each amino acid to be a blob with a characteristic chemical
functionality according to its unique side chain (“residue”). One might go further and model an amino acid in a protein as a structureless blob with
two integers characterizing, respectively, hydrogen bond-donating and hydrogen bond-accepting capacities; nucleotides (“bases”) in DNA (see
Section 11.1) are commonly modeled as one of A, C, G and T, with the propensities to pair with, respectively, T, G, C and A. Given that this coarse
graining neglects structure below the size of a few nanometers, it might be tempting to simply identify the nanoscale with the mesoscale. Working at
the nanoscale would then appear as mainly a matter of computational convenience, since coarse-grained models are much more tractable than
more detailed ones (consider Brownian dynamics versus molecular dynamics) and often require fewer assumptions. It may well be that a structure
characteristic of this scale, such as a lipid bilayer, could be correctly determined from the quantum electrodynamical Hamiltonian, explicitly taking
each electron and atomic nucleus into account, but this would be an extraordinarily tedious calculation (and one would need to mobilize the entire
computing resources of the world even to reach an approximate solution for any practically useful problem). However, this usage of the term would
not imply that there is any particular novelty in the nanoscale, whereas as one makes the transition from typically bulk properties to typically atomic
and subatomic properties, one passes through a state that is characteristic neither of individual atoms nor of the bulk, and it would then appear to
be reasonable to take the length range over which this transition occurs as the nanoscale.

The current consensual definition of the nanoscale is from 1 to 100 nm, although it is still disputed whether these limits should be considered exact
or approximate. Finding a consensus is analogous to a multiobjective optimization procedure in which it is not possible to perfectly satisfy every
objective considered independently because some of them conflict. A compromise is therefore sought that is as close as possible to all objectives.
In many cases there is a continuum of possible states of each attribute (each attribute corresponding to an objective) and the Pareto front on which
the compromise solutions lie is indeed a meaningful solution to the optimization problem. If, on the other hand, it is considered that there is only one
correct solution for a given objective, and anything else is wrong, the consensus might be Pareto optimal but wrong in every regard—perhaps a
little like not a single person in the population having the attributes of Quetelet's “average man”. Nevertheless such a consensual definition might still
be useful—at least it has the virtue of simplicity and probably does facilitate discussion of nanotechnology, albeit at a rather basic level, among a
diverse range of people.
This chapter goes beyond the current consensus and examines the definition of the nanoscale more deeply. According to one of the definitions of
nanotechnology discussed in Chapter 1, “nanotechnology pertains to the processing of materials in which structure of a dimension of less than 100
nm is essential to obtain the required functional performance”[36]. This functional performance is implicitly unattainable at larger sizes, hence the
definition essentially paraphrases “the deliberate and controlled manipulation, precision placement, measurement, modeling and production of
matter in the nanoscale in order to create materials, devices, and systems with fundamentally new properties and functions”[1]. The emergence of a
qualitatively new feature as one reduces the size is a key concept associated with nanotechnology. In order to define the nanoscale, therefore, we
need to search for such qualitative changes.
Given that in principle the prefix “nano” can apply to any base unit, not just the meter, one needs to justify the primacy of length in a definition of the
nanoscale. Among the different categories of attributes—length, mass, voltage, and so on—length does enjoy a certain primacy, perhaps because
man first measured things by pacing out their spacial dimension. And given that the meter roughly corresponds to one human step, we can prima
facie accept that the nanoscale is based on the nanometer. As nanotechnologists seemingly never tire of relating, the prefix “nano” is derived from
the Greek “νανoσ”, meaning dwarf, and the meaning of “very small” has been formalized by the Système International (S.I.) of scientific units
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adopting nano as the name of the multiplier 10 . Thus one nanometer (1 nm) means precisely 10 m, in other words a millionth of a millimeter or
a thousandth of a micrometer, which is somewhat larger than the size of one atom (Table 2.1).
Table 2.1 Covalent radii of some atoms