Introduction to Nanoscience and Nanotechnology. Chris Binns
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into macroscopic chunks produces materials with novel behavior. Finally, the lower edge of the nanoworld is defined by the size of single atoms, whose diameters vary from 0.1 (hydrogen atom) to about 0.4 nm (uranium atom). We cannot build materials or devices with building blocks smaller than atoms and so these represent the smallest structures that can be used in nanotechnology.
There are so many aspects to nanotechnology that one of the difficulties in writing about it is finding ways to organize the description into a coherent structure. This book will largely follow a classification scheme introduced by Richard Jones in his book Soft Machines [2] that helps to categorize nanotechnology into a logical framework. He defines three categories in order of increasing sophistication, that is, Incremental, Evolutionary and Radical nanotechnology. These are described below.
Figure I.1 The nanoworld. The size range of interest in nanotechnology and some representative objects.
Source: Reproduced under Creative Commons license CC BY‐SA 3.0.
I.1 Incremental Nanotechnology
All substances, even solid chunks of metal, have a grain structure and controlling this grain structure allows one to produce higher performance materials. This could mean stronger metals, magnetic films with a very high magnetization, suspensions of nanoparticles with tailored properties, etc. There are aspects of incremental nanotechnology that date back to the ancients, for example, the invention of Indian ink, probably in China around 2700 BCE, which arose from the production of carbon nanoparticles in water. Also, medieval potters in Europe knew how to produce a luster on pots by coating them with copper and silver nanoparticles [3], a process that can be traced back to ninth century AD Mesopotamia. Figure I.2 shows an electron microscope image of the glaze of a sixteenth century Italian pot, whose luster derives from the coating by 10 nm diameter copper particles.
Most modern nanotechnologists would be proud of the size control of the particles in this picture. Whereas these days a process that involved nanoparticles such as this would be proudly claimed to be nanotechnology, and thus, open the door to research funding, spin‐off companies, etc. The ancients were developing processes that did something invisible to the materials, but nevertheless allowed them to achieve visible changes. In this sense, a lot of incremental nanotechnology can sometimes be considered to be a re‐branding of other more traditional lines of research such as materials science. The nanotechnology title is still useful, however, since nanotechnology is, by its nature, multi‐disciplinary and it encourages cross‐disciplinary communication between researchers.
The aspect of incremental nanotechnology that has really changed in the modern world is the development of instruments (see Chapter 5) that can probe at the nanoscale and image the particles within materials or devices. Researchers can actually observe what is happening to the particles or grains in response to changes in processing. This not only makes development of new processes more efficient, but also leads to the discovery of completely new structures that were not known to exist and hence new applications. Nature is full of surprises when one studies sufficiently, small pieces of matter, as will be come clear throughout this book.
Figure I.2 Ancient incremental nanotechnology. Copper nanocrystals of about 10 nm in diameter on a tenth century pot, which produce a surface luster. The inset shows an increased magnification image of a single 7 nm diameter particle with atomic planes visible revealing its crystallinity.
Source: Reproduced with permission from [3].
I.2 Evolutionary Nanotechnology
Whereas Incremental Nanotechnology is the business of assembling vast numbers of very tiny particles to produce novel substances, Evolutionary Nanotechnology attempts to produce nanoparticles that individually perform some kind of useful function. They may need to be assembled in vast numbers to form a macroscopic array in order to build a device, but a functionality is built into each one. Such nanoparticles are necessarily more complex than those used in incremental nanotechnology, and will often consist of more than one material and have a surface coating of organic molecules.
An example of a single nanoparticle device is a single‐electron transistor (SET), that uses a phenomenon known as “Coulomb blockade” to effect transistor action on individual electrons (see Chapter 6). The device works in a similar manner to a field effect transistor (FET), that is, it is conducting or not conducting through the main terminals depending upon the value of a voltage applied to a third “gate” electrode. The difference is that the “on” and “off” states control single electrons hopping through the device, and the action is carried out by a single nanoparticle a few nanometers across. The smallest device that has exhibited transistor action this way is a 66‐atom aluminium nanoparticle with a diameter of about 1.5 nm [4].
The research has so far only provided proof‐of‐principle of transistor action either by finding a clever scheme to address a single nanoparticle with relatively large electrodes as illustrated in Figure I.7, or by investigating a large number of identical particles at once. The problem of how to interconnect the individual nanoparticles among themselves to form a circuit and communicate with them at the nanoscale has yet to be solved. A possibility is to use molecular wires, and it has been shown that hydrocarbon chain molecules known as thiols, shown in Figure I.3a can act as conducting wires. These form particularly strong bonds to gold nanoparticles via the sulphur atom at one end and their resistance varies according to the length of the hydrocarbon chain and the nature of any end groups attached to the molecule. Thus, there is the possibility of molecular interconnections between nanoparticles.
To illustrate the state‐of‐play consider Figure I.3b, which shows a gold nanoparticle of about 3 nm diameter with thiols attached, which can be considered as an SET. The circuit diagram for an AND gate, a fundamental component of a computer logic circuit, implemented by three FETs is shown in Figure I.3c. The circuit with the FETs implemented by three Au nanoparticles SETs is shown in Figure I.3d with the dashed lines showing the connections that we are currently unable to make. Also, included are the power electrodes drawn to scale for the current state‐of‐the‐art minimum feature size (the “process size”) of 5 nm used in the semiconductor industry. This makes the whole gate about 32 nm across as compared to the equivalent using conventional “top–down” processing (see below) of about 260 nm (Figure I.3e).
Thus, in terms of a real density, there is a factor of 50 to go in the miniaturization of circuits by exploiting nanoparticles. For this technology to come of age it is likely that some sort of self‐assembly of the thiol‐coated nanoparticles will be required, and so it is possible that devices like this could be grown chemically as illustrated in Figure I.4 rather than “cut into” silicon. There are other schemes for individual nanoscale transistors based on carbon nanotubes and graphene