Introduction to Nanoscience and Nanotechnology. Chris Binns

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.

Figure 1.10 Morphology of nanoparticle film. STM image (see Chapter 5, Section 5.4.1) with an area of 100 nm × 100 nm of a thin film produced by depositing 3 nm diameter Fe nanoparticles onto a silicon substrate in vacuum. It is clearly a random stack of the deposited particles and the film properties will be different to those of a smooth film that would be formed by depositing Fe atoms.

      Source: Reproduced with the permission of the American Institute of Physics from M. D. Upward et al. [10].

      Yet, if we look at the data for Fe nanoparticles in Figure 1.9, the magnetization is higher than the Slater–Pauling limit for all sizes below about 300 atoms. This, however, is for isolated nanoparticles in a vacuum and it is not immediately clear how to make a material out of them while preserving the high value of the magnetic moment. About 20 years ago, the journey from isolated nanoparticles to materials started with experiments on size‐selected Fe particles deposited in ultra high vacuum (UHV) onto graphite surfaces [9]. This work, showed that, for particles smaller than about 3 nm, the enhanced magnetic moments were retained when the particles were on a support and also revealed the source of the additional magnetic moment. Magnetism in atoms arises from two contributions, that is, the magnetic moment due to the orbital motion of the electrons, which can be considered as a tiny current loop, and from the electron “spin,” which can only be understood from a quantum mechanical perspective. In a transition metal such as Fe, virtually all the magnetism is due to the spin with the orbital moment “quenched” almost to zero. In a nanoparticle, which has a very high proportion of surface atoms with a reduced co‐ordination relative to the bulk, some of the orbital moment reappears and in addition, the spin moment is enhanced. The experiments mentioned above [9] showed that about half the enhancement of the magnetic moment in small particles comes from the spin moment and the other half from the orbital moment.

Although these measurements moved from free beam nanoparticles to those supported on a surface, they were still for isolated nanoparticles and did not show directly how to make a high moment material. Depositing an entire film of nanoparticles reduced the effect of the surface as the particles came into contact and both the orbital and spin moments converged with the bulk values. To make matters worse, films made by depositing preformed nanoparticles on a surface such as the one shown in Figure 1.10 are highly porous with an atomic density around 60% that of the bulk so the magnetic field produced by such a film would be less than that generated by bulk Fe. The same set of experiments, however, also showed that coating the Fe nanoparticles with Co while they were on the surface did not remove the enhanced orbital moment, and the Fe spin moment increased even further so that a total moment of 2.6μ_B/atom was recorded for 200‐atom nanoparticles, compared to 2.2μ_B/atom in the bulk. The data from the Co‐coated nanoparticles supported on a surface are plotted on the Fe nanoparticle curve in Figure 1.9 and it is observed that they show magnetic moments as high as the free nanoparticles but in a film that could be removed from its UHV environment without converting the Fe to oxide.

Figure 1.11 Producing nanostructured films by cluster beam deposition. Nanostructured films can be produced by depositing preformed nanoparticles from a UHV cluster source onto a substrate along with an atomic beam from a conventional hot oven evaporation source. (a) Fe nanoparticles in a Co matrix. (b) Co nanoparticles in an Fe matrix.

These experiments suggested a method to produce a thin film with a magnetization exceeding the Slater–Pauling limit, which is illustrated in Figure 1.11a. If Fe nanoparticles from a UHV nanoparticle source (see Chapter 5, Section 5.1.2) are deposited onto a surface in conjunction with an atomic beam from a conventional hot oven evaporator source, then an atomic film (matrix) containing embedded nanoparticles is created. The atoms fill all the gaps between the particles producing a film with the bulk density and each particle will retain its enhanced orbital and spin moments found in the aforementioned study. In addition, since the matrix has a very high proportion of interface atoms, it should display enhanced magnetic moments as well. Figure 1.11a shows Fe nanoparticles embedded in a Co