Page 93 - Nanotechnology an introduction

Chapter Contents

8.1 Top–Down Methods 162
8.1.1 Semiconductor Processing 162
8.1.2 Mismatched Epitaxy 163
8.1.3 Electrostatic Spray Deposition (ESD) 164
8.1.4 Felting 164
8.1.5 Ultraprecision Engineering 165
8.2 Bottom–Up Methods 166
8.2.1 Self-Assembly 166
8.2.2 Thermodynamics of Self-Organization 169
8.2.3 The “Goodness” of the Organization 171

8.2.4 Particle Mixtures 1711
8.2.5 Mixed Polymers 172
8.2.6 Block Copolymers 173
8.2.7 The Addition of Particles to the Solid/Liquid Interface 174
8.2.8 Programmable Self-Assembly 178
8.2.9 Superspheres 179
8.2.10 Biological Self-Assembly 180
8.2.11 Biopolymer Folding 181
8.2.12 Biological Growth 182

8.2.13 Self-Assembly as a Manufacturing Process 183
8.3 Bottom-to-Bottom Methods 184
8.3.1 Tip-Based Nanofabrication 185
8.3.2 Nanoblocks 186
8.3.3 Dip Pen Nanolithography 187
8.4 Summary 187
8.5 Further Reading 188
Top-down methods (exemplified by ultraprecision engineering and semiconductor processing) constitute the bulk of current industrial nanotechnology. Due to the enormous expense of the capital equipment required it is, however, impractical
for use other than for very high-volume products (such as computer or cellular phone chips) or very unique products for which a high price is affordable (giant astronomical telescopes; spacecraft). The “real” vision of nanotechnology
(especially associated with Feynman and Drexler) is based on mechanosynthesis (chemistry with positional control), possibly facilitated by using pre-constructed nanoblocks as the elementary units of fabrication. A productive nanosystem
is based on assemblers, devices that are themselves in the nanoscale, hence the method is also known as bottom-to-bottom. Because of their minute size, the only practical way to fabricate large or large quantities of entities is for the
assemblers to first assemble copies of themselves, which then all work in parallel. The practical realization of this vision is focused on tip-based methods inspired by the scanning probe instruments used in nanometrology; at present single
objects comprising of the order of ten atoms can be made in this way. Originally inspired by biology, a third approach is based on creating objects (which could be nanoblocks) capable of spontaneously assembling into useful structures. This
method is known as bottom-up or self-assembly. It has been quite successful for creating regular structures, but the creation of arbitrary geometries requires programmable self-assembly. In other words, bottom-up is presently good at
creating materials but not for creating devices. Biological nano-objects have this ability, but it is difficult to reverse-engineer them and use the knowledge to create synthetic analogs.
Keywords: top-down, semiconductor processing, ultraprecision engineering, bottom-up, self-assembly, programmable self-assembly, biological self-assembly, superspheres, bottom-to-bottom, nanoblocks
Whereas Chapter 6 considered the mainly current methods of producing nanomaterials with a regular structure (and often with a merely statistical
order), in this chapter we look at more sophisticated technologies, both present and supposed future, capable in principle of fabricating functional
artifacts of arbitrary complexity. The entire field is encompassed within the three divisions of top–down, bottom–bottom, and bottom–up; Figure 8.1
gives a summary. Top–down requires great ingenuity (and expense) in the fabrication machinery; bottom–up requires great ingenuity in the
conception of the building blocks. Bottom–bottom, the least developed of the three, requires above all ingenuity in conception.

Figure 8.1 Different modes of nanomanufacture (nanofacture).
8.1. Top–Down Methods
These share the general feature of requiring large (and also expensive, see Section 1.3.2, requiring considerable concentrations of capital)