Introduction to Nanoscience and Nanotechnology. Chris Binns

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Figure 1.9 Measured magnetic moments per atom in magnetic nanoparticles. Experimental measurements of the magnetic moment per atom in Fe, Co, nickel (Ni), and rhodium (Rh – a nonmagnetic metal in the bulk) nanoparticles as a function of the number of atoms in the particle. For Fe, Co, and Ni, there is a significant increase in the magnetic moment per atom over the bulk value for particles containing less than about 600 atoms. Rh becomes magnetic in particles containing less than about 100 atoms. The insets show the sharp variations in magnetic moments at very small particle sizes. Note the very dramatic change in the magnetic moment of Fe particles in going from a 12‐atom particle to a 13‐atom particle (icosahedron). A similar dip in going from 12 to 13 atoms, though not so pronounced, is also observed in Ni nanoparticles.

      Source: Adapted from I. M. L. Billas et al. [4]; A. J. Cox et al. [5]; S. Apset et al. [6]; M. B. Knickelbein [7]; F. W. Paine et al. [8].

      In the Fe curve are also shown measurements (green circles) for Fe nanoparticles supported on a graphite surface and coated with Co [9] (see text).

      Throughout the whole size range in Figure 1.9, the fundamental magnetic behavior of the particles is size‐dependent. Do not lose sight of how strange a property this is and how it runs counter to our experience in the macroscopic world. It is as strange as a piece of metal changing color if we cut it in half. If Democritus was able to do his chopping experiment down to the nanoscale on Fe, when he reached a piece 100 nm across, which would be invisible in even the most powerful optical microscope, he would say that he had not yet reached the a‐tomon as up to then there would have been no observable change in properties. When he cut in half again, he would suddenly find his piece changing from magnetically dead to the full magnetic power of Fe with every atomic magnet aligned as the piece formed a single‐domain particle. He would exclaim “I have reached the a‐tomon, lets just try and cut again.” Imagine his surprise, when he finds that the properties continue to change and on reaching 3 nm finds that when he cuts again the strength of the magnetism, in proportion to the size of the piece increases. Some of these changes can be dramatic, for example, if he was holding a 13‐atom cluster and shaved off a tiny piece to produce a 12‐atom cluster, the magnetic moment per atom would jump from 2.5μ_B to a staggering 5.5μ_B – very close to the single‐atom limit of 6μ_B. The 13‐atom piece is in a particularly stable configuration known as an icosahedron (20‐sided solid), illustrated in the figure, and for reasons beyond the scope of this chapter, the high symmetry in this atomic structure reduces the magnetism. The same effect is seen in Ni, though less pronounced, in passing through the 13‐atom size. The magnetism in very small Rh nanoparticles is particularly spiky showing peaks and troughs every two or three atoms.

      So we can now see the reason why the nanoscale is special as a size, at least as far as materials are concerned. It represents the border region between the macroscopic world and the microscopic atomic world in which the properties of pieces of matter depend on size, and they display novel behavior only found in that size scale. This highlights one of the most exciting aspects of nanoparticle research. If one considers a nanoparticle as a building block and can assemble large numbers of them to make a material, then it is possible to tailor the fundamental properties of the building block just by changing its size. It is almost as if we could add a third dimension to the periodic table, so for each element, we can choose the size of the nanoparticle building blocks, which would modify the properties of the material produced. In Chapter 5, we will look at more sophisticated ways of changing the nanoparticle building blocks.

The ability to change the fundamental properties of the building blocks will surely enable us to produce new high‐performance materials. As an example, if we deposit Fe onto a surface to make a thin film, there is a difference between depositing individual atoms, as with a conventional evaporator, and depositing whole nanoparticles containing, say, 200 atoms. This is clear from Figure 1.10, which shows a thin film of Fe produced by depositing preformed nanoparticles onto a silicon substrate in vacuum. It is clearly a random stack of the deposited particles showing they have not coalesced to form a smooth film. On this scale, a film of the same thickness produced by depositing atoms would appear smooth and featureless and it would behave differently from the nanoparticle stack.

As an instructive example of creating a high‐performance material using nanotechnology, let's go through the steps necessary to produce a magnetic material with a higher magnetization than the best conventional alloy. The most magnetic practical metal available to the designers of machines and devices is Fe–Co alloy in an approximately equal mixture. This metal has an average atomic magnetic moment of 2.45μ_B – about 10% higher than bulk Fe and has been known since 1912⁴. This magnetization known as the Slater–Pauling limit acts as a fundamental bound to the performance of a swathe of technologies ranging from electric motors to magnetic recording and despite a century of searching, no higher performance material has been found.