Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 17

The aim of this chapter is to instill an intuitive feel for the smallness of the structures that are used in nanotechnology and what is special about the size range involved. As we will see, the importance of the nanoscale is wrapped up with fundamental questions about the nature of matter and space that were first pondered by the ancient Greeks. Three thousand years ago, they led the philosophers Leucippus and Democritus to propose the concept of the atom. These ideas will come round full circle at the end of the book when we will see that modern answers (or at least partial answers – the issue is still hot) are very much wrapped up in nanotechnology.

First of all, let us remind ourselves how small nanostructures really are. This is a useful exercise even for professionals working in the field. The standard unit of length in the metric system is the meter, originally calibrated from a platinum–iridium alloy bar kept in Paris but since 1983 it has been defined as the distance light travels in 1/299 792 458 seconds. For convenience, so that we are not constantly writing very small or very large numbers, the metric system introduces a new prefix every time we multiply or divide the standard units by 1000. Thus, a thousandth of a meter is a milli‐meter or mm (from the Roman word mille meaning 1000); a thousandth of a millimeter (or a millionth of a meter) is a micro‐meter or μm (from the Greek word mikros meaning small). Similarly, a thousandth of a micrometer (or a billionth of a meter) is a nano‐meter or nm (from the Greek word nanos meaning dwarf). As shown in Figure I.1 in the introduction (“The Nanoworld”), we will be dealing with building blocks that vary from 100 nm across down to atoms, which are 0.1–0.4 nm across.

These days instruments can directly image nanostructures, for example, Figure 1.1 shows a scanning tunneling microscope (STM – see Chapter 5, Section 5.4.1) image of a few manganese nanoparticles with a diameter of about 3 nm deposited onto a bed of carbon‐60 molecules (“bucky balls” – see Chapter 3) with a diameter of 0.7 nm on silicon. It thus displays two different nanoparticles of interest in the same image. This could just as easily be a picture of snowballs on a bed of marbles, and it is easy to lose sight of the difference in scale between our world and that of the nanoparticles. To get some idea of the scale, take a sharp pencil and gently tap it onto a piece of paper with just enough force to get a mark that is barely visible. This will typically be about 100 μm or 100 000 nm across. If the frame in Figure 1.1 were this large, it would contain about 100 000 000 of the Mn nanoparticles and 20 000 000 000 of the carbon‐60 nanoparticles, which is about three times the population of the planet.

It may seem odd to base an entire branch of science and technology around a measure of distance, especially as length scales used in science go to much smaller than nanometers and much larger than meters. This is illustrated in Figure 1.2, which shows the entire distance range encompassing present‐day scientific knowledge, plotted on a logarithmic scale, from the observable universe (10²⁷ m) to the Planck scale (10⁻³⁵ m, see Chapter 10). It is seen that nanoscience occupies a tiny sliver somewhere near the middle and doesn't immediately strike one as being of interest, but to base a field of science around it suggests that there is something special about the nanometer scale – so what is it?

Figure 1.1 Manganese nanoparticles on bucky balls. STM image (see Chapter 5, Section 5.4.1) of a few manganese nanoparticles with a diameter of about 3 nm (i.e. each one contains a few hundred manganese atoms) deposited onto a bed of carbon‐60 nanoparticles with a diameter of 0.7 nm on a silicon surface.

Source: Reproduced with the permission of the American Institute of Physics from M. D. Upward et al. [1].

Figure 1.2 Distance range encompassing all current scientific knowledge. Distance scale from the observable Universe (10²⁷ m) to the Planck scale (10⁻³⁵ m) with nanoscience based on a narrow range near the middle.

Source: The Universe image is from UCL Mathematical and Physical sciences and reproduced under creative commons 2.0 license. The solar system and atomic nucleus images are reproduced under creative commons 3.0 license.

The answer to this is best expounded with reference to philosophical questions that were originally posed regarding the fundamental nature of matter and space by Democritus and his contemporaries that still resonate today. Democritus (Figure 1.3) was born in Abdera, Northern Greece, in about 460 BC to a wealthy noble family, and he spent his considerable inheritance (millions of dollars in today's currency) traveling to every corner of the globe learning everything he could. Known as the laughing philosopher, he lived to over 100 years old so it appears to have been a good life. He wrote more than 75 books about topics ranging from magnets to spiders and their webs. Only fragments of his work survived with most of his books being destroyed in the third and fifth centuries. His lasting impact on science was to propose, with his teacher Leucippus, the concept of the atom.

Our direct experience in the macroscopic world suggests that matter is continuous and thus with nothing but our eyes for sensors, the original suggestion that matter is made from continuous basic elements such as earth, fire, air, and water seems reasonable. In fact, there are subtle indications of underlying invisible particles of matter, for example, in a dusty room traversed by shafts of sunlight, the dust particles dance around due to the motion of something invisible. Mostly, this is microscopic air currents but the glittering of the smallest particles is due to random collisions from large numbers of individual air molecules and is known as Brownian motion (see Chapter 7, Section 7.2).

Figure 1.3 Democritus. Bronze bust thought to be of Democritus at the Naples National Archaeology Museum.

Source: Odysses, https://commons.wikimedia.org/wiki/File:Bust_of_an_unknown_Greek_‐_Museo_archeologico_nazionale_di_Napoli.jpg.

For the most part, matter does appear to be a continuum but this leads to a paradox since it could then be cut into smaller and smaller pieces without end. If one were able to keep cutting a piece of matter in two, each of those pieces into two and so on, ad infinitum, one could, at least in principle, cut it out of existence into pieces of nothing that could not be reassembled. This led Leucippus and Democritus to propose that there must be a smallest indivisible piece of substance, the a‐tomon, or uncuttable from which the modern word atom is derived. They suggested the different substances in the world were composed of atoms of different shapes and sizes, which is not an unrecognizable description of modern chemistry. Once you propose atoms, however, you automatically require a “void” in which they move and the void is a concept that also produces dilemmas, which were the subject of much debate 3000 years ago. For example, is the void a “something” or a “nothing” and is it a continuum, or does it also have a smallest uncuttable piece. Whereas atoms are a familiar part of the scientific world, the true nature of the void between them is something that is still not fully understood, and many scientists believe that the void question lies at the heart of all the “big” questions about the Universe and the nature of reality. It is a subject that nanotechnology can address, and we will return to this discussion in Chapter 10.

Meanwhile, focusing on atoms, one could argue that the basic philosophy outlined above, which led to their being proposed, means that you should really attribute the a‐tomon to more fundamental constituents of atoms, such as electrons and quarks. There is a good reason to stick with atoms, however, since we are talking about constituents of materials and if we pick on a particular material, say copper, the smallest indivisible unit of “copperness” is the copper atom. If we divide a copper atom in two we get two atoms of different materials.

So what has all this got to do with nanotechnology? These days we can carry out, in practice, the Democritus mind experiment and study pieces of matter of smaller and smaller size right down to the atom. The important result is that the properties of the pieces start to change at sizes much bigger than a single atom. When the size of the material crosses into the “nanoworld” (Figure I.1), its fundamental properties start to change and become dependent on the size of the piece. This is, in itself, a strange thing as we take it for granted that, for example, copper will behave like copper whether the piece is a meter across or a centimeter across. This is not the case in the nanoworld and the onset of this strange behavior first shows up at the large end of the nanoworld scale with the magnetic properties of metals such as iron (Fe). It is worth spending a little time on this because it is a clear illustration of how the fundamental behavior of a piece of material can become dependent on its size.