Introduction to Nanoscience and Nanotechnology. Chris Binns

Chapter 9.

Nanobots have generated a good deal of controversy, especially ones that can play atomic lego and build anything out of atoms lying around. If this was possible, then one could, in principle, build a nanobot that moved around exploring the surface it occupied. If it was equipped with an assembler that could assemble atoms and molecules, it could make a copy of itself by rooting around and finding the atoms it needed to reproduce. Since each nanobot could make multiple copies of itself, the population could increase exponentially and would quickly produce a sufficiently vast army to build macroscopic objects. Drexler himself pointed out the doomsday scenario where the nanobots multiply out of control like a virus and eventually exist in such vast numbers that they could rearrange the atoms of the planet to produce a kind of “grey goo.” Unfortunately, this scenario has tended to hijack discussions on radical nanotechnology, and since the two branches of radical nanotechnology have been melded together in the public debate there is a general feeling that all radical nanotechnology is dangerous. The reality is that exponentially self‐replicating machines are not required for molecular manufacturing [9], and nanobots do not need to be built with assemblers to self‐replicate in order to perform useful functions, as shown in Chapter 9, Section 9.3 for the case of medical nanobots.

      There is a scientific debate about whether this technology is feasible, even in the long term, or indeed desirable, but the discussion has moved on from generalities to a consideration of the detailed processes required for molecular manufacturing (see Chapter 9 and the references therein). A frequently proposed argument in favor of radical nanotechnology is that it already exists in all living things. Biological cells are filled with what may be regarded as nano‐machines and molecular assemblers. Biology, however, is very different to the nanoscale process‐engineering path envisaged by radical nanotechnologists, as explained in Soft Machines [2]. It is fair to say that both the feasibility and timescale of radical nanotechnology divides the community. The point is that, while incremental nanotechnology exists and evolutionary nanotechnology is just coming into the frame, radical nanotechnology, if feasible, is probably decades away. Whatever the twists and turns of the debate, once we get away from the argument over nanobots, there is no doubt that the ability to produce nano‐machines and achieve safe non‐exponential molecular manufacturing will reap enormous benefits.

      It is possible that the solution to some of the more difficult technological problems involved with radical nanotechnology may arise from a better fundamental understanding of the true nature of empty space. Quantum theory predicts new types of force at very short distance scales (nanometers) arising directly out of the quantum properties of vacuum. Although we can only detect these forces with very sensitive instruments (the tools of nanotechnology in fact – see Chapter 10), to a nanoscale machine whose components are within nanometers of each other, these forces will be as natural as a part of their environment as gravity is to us. Research on these forces and how to utilise them in nanotechnology is already being undertaken by several research groups worldwide. This may be one of the missing links between biology and radical nanotechnology, that is, natural systems, whose inner workings happen on the same scale as nanomachines have evolved over billions of years and must have utilised all available forces including the exotic ones.

I.4 Bottom–Up/Top–Down Nanotechnology

Finally, in this introduction it is worth mentioning another way of categorizing nanotechnology, that is, bottom–up and top–down approaches. Everything discussed so far has been part of a bottom–up approach in which the building block (nanoparticle, molecular machine component, etc.) is identified and produced naturally, and then assembled to produce the material or device required. The top–down approach starts with a block of material and machines a device or structure out of it. This is akin to conventional engineering using lathes and millers to machine a shape out of a solid block. The modern tools of nanotechnology, however, are able to machine structures with sizes of a few nanometers, so the size of components made with a top–down approach is not much different to the building blocks of the bottom–up approach. The flexibility of top–down tools, in particular, focused‐ion beam systems (FIB's), is further enhanced by their ability to deposit material to produce nanoscale features as well as to remove it. This is beautifully illustrated in Figure I.6, which shows an example of a “wine glass” with a cup diameter 20 times smaller than the width of a human hair produced by depositing carbon. Although this is a rather big structure on the scale of nanometers, the smallest feature size that can be produced by a modern FIB is less than 20 nm.

Figure I.6 The smallest wineglass in the World (authorised by Guinness World Records). Wine glass whose cup diameter is 20 times smaller than the width of a human hair produced by deposition of carbon using a FIB machine. The structure arose from a Joint development by SII NanoTechnology, NEC, and the University of Hyogo, Japan. Although this is a rather big structure on the scale of nanometers, the smallest feature size that can be produced by a modern FIB is less than 20 nm (see Chapter 5, Section 5.2.2).

Figure I.7 Top–down nanotechnology used to attach electrodes to a nanoparticle. (a) Electron microscope image of electrodes produced by electron beam lithography to attach to a single gold nanoparticle that acts as a single‐electron transistor. (b) Artistic rendering of the set‐up showing the equivalent circuit.

      Source: Images reproduced from [10] under creative commons license CC BY‐SA 4.0.

      The two approaches (top–down and bottom–up) are complementary, and some of the most exciting research arises out of combining them. For example, if one wants to measure the electrical or magnetic properties of an individual nanoparticle, the fantastic precision of a modern top–down tool enables the production of electrodes that can attach to it. An example is demonstrated in Figure I.7, which shows source, drain, and gate electrodes, produced by electron‐beam lithography (see Chapter 5, Section 5.2.1) applied to a single gold nanoparticle acting as a SET, as described earlier.

The above is an attempt at a lightning tour of nanotechnology with generic descriptions and without addressing details. The rest of the book looks in detail at these and other aspects of nanotechnology. Chapter 1 aims to instil a feeling of how small the nanometer length scale is in comparison to macroscopic objects and why it is special. It discusses the basic conception of the discrete nature of matter starting from the original philosophical ideas of Leucippus and Demokritos of ancient Greece to the modern view of atomic structure. It also describes why the properties of pieces of matter with a size in the nanometer range (nanoparticles) deviate significantly from the bulk material and how these special properties may be used to produce high‐performance materials and devices. In Chapter 2, the discussion is broadened to include naturally occurring nanoparticles, both in the Earth's atmosphere and in space. Chapter 3 is dedicated to nanoparticles composed of carbon, and the justification for devoting a chapter to a single element is the rich variety of nanostructures produced by carbon and their importance in the rest of nanotechnology. Chapter