Page 95 - Nanotechnology an introduction

Baghdad, Damascus, Egypt, the Maghreb and Muslim Spain; it appeared in England around 1490. Vegetable fibers (typically based on cellulose)
are macerated until each individual filament is a separate unit, mixed with water, and lifted from it in the form of a thin layer by the use of a sieve-like
screen, the water draining through its small openings to leave a sheet of felted (matted) fiber upon the screen's surface. The first papermaking
machine was invented by Robert in France at around the end of the 18th century, and was later perfected by two Englishmen, the Fourdrinier
brothers. The machine poured the fibers out in a stream of water onto a long wire screen looped over rollers; as the screen moved slowly over the
rollers, the water drained off and delivered an endless sheet of wet paper.

With the advent of various kinds of nanofibers, it is now possible to make paper-like materials at the nanoscale. This has been attempted, most
notably using carbon nanotubes, when it is sometimes called buckypaper. The process is different from the shaking and/or stirring used to prepare
a mixed powder. Randomly oriented fibers (highly elongated objects) are rapidly placed on top of each other, with their long axes parallel to a
substratum (which is, in the case of cellulose-based writing paper, removed later on) to form a random fiber network (RFN) (Figure 8.2). The
network coheres because of numerous fiber–fiber contacts, but the structure is different from that formed by the entanglement of very long randomly
coiled polymers. Mixtures of fibers would be especially interesting for creating nanotexture.

Figure 8.2 Sketch of a two-dimensional random fiber network (RFN) used to model a felted assembly.
Simplified models, such as the random fiber network, into which the only input is the actual distribution of fiber lengths and their surface chemical
properties, are useful for calculating basic properties of felted materials, for example mass distribution, number of crossings per fiber, fractional
contact area, free-fiber length distribution and void structure.
An example of a naturally felted structure is the basement membrane assembled from extracellular matrix proteins such as laminin, used in
multicellular organisms to support cells and tissues. The deposition of fibrous proteins such as laminin in the presence of divalent cations such as
calcium allows sheets of arbitrary thickness to be assembled. Somewhat different is the giant glycoprotein mucin constituting the mucous films
lining the epithelial surfaces of multicellular organisms: the molecules are large and flexible enough to be entangled with one another.

8.1.5. Ultraprecision Engineering

Current ultrahigh-precision (“ultraprecision”) engineering is able to achieve surface finishes with a roughness of a few nanometers. According to
McKeown et al. it is achievable using 11 principles and techniques [113], which include:
• dynamic stiffness; thermal stability; seismic isolation
• rigid-body kinematic design; three-point support
• measuring system isolated from force paths and machine distortion
• high accuracy bearings
• quasistatic and dynamic error compensation.

Although some of the attributes implied by these principles and techniques would appear to call for large, massive machines, the need for a further,
twelfth, principle has become apparent, namely that of miniaturization—ultraprecision machines should be as small as is feasible taking account of
the 11 principles. This is because smaller load paths and metrology and thermal loops directly improve dynamic performance and accuracy. There
should be, therefore, ultimately convergence between assemblers and ultraprecision machine tools (UPMT).
8.2. Bottom–Up Methods
The increasing difficulty of continuing the miniaturization of classical photolithography and its derivatives in semiconductor processing (Section
8.1.1), and the extreme laboriousness of current mechanosynthesis (Section 8.3), oupled with the observation of self-assembly in nature, generated
interest in alternative fabrication technologies. The primitive idea of self-assembly (“shake and bake”) is to gather precursors in random positions
and orientations and supply energy (“shaking”) to allow them to sample configuration space. The hugeness of this space suggests that a
convergent pathway is inherent in the process in order to allow it to be completed in a reasonable time, as in protein “folding” (Section 8.2.11).
Once the precursors are in position, “baking” is applied to strengthen the bonds connecting them and fix the final object permanently.

8.2.1. Self-Assembly

Assembly means gathering things together and arranging them (fitting them together) to produce an organized structure. A child assembling a
mechanism from “Meccano” captures the meaning, as does an assembly line whose workers and machines progressively build a complicated
product such as a motor car from simpler components; an editor assembles a newspaper from copy provided by journalists (some of which might
be rejected). In all these examples the component parts are subject to constraints—otherwise they would not fit together—and their entropy S must
necessarily decrease. Since the free energy ΔG = ΔH − TΔS must then increase, in general the process will not, by itself, happen spontaneously.
On the contrary, a segregated arrangement will tend to become homogeneous. Hence in order for self-assembly to become a reality, something
else must be included. Typically enthalpy H is lost through the formation of connexions (bonds) between the parts, and provided |ΔH| exceeds |TΔS|
we have at least the possibility that the process can occur spontaneously, which is presumably what is meant by self-assembly.

The final result is generally considered to be in some sort of equilibrium. Since entropy S is always multiplied by the temperature T in order to