Introduction to Nanoscience and Nanotechnology. Chris Binns

as a method of high‐quality nanoparticle synthesis (see Chapter 5, Section 5.1.9).

      Interestingly, magnetic nanoparticles with a similar atomic structure have been found on a piece of meteorite known to have come from Mars [3] and this was taken as evidence that there was once life on Mars, though this analysis is controversial. Mars no longer has a significant planetary magnetic field, which disappeared some four billion years ago indicating that the nanofossils, if that is what they are, must be truly ancient. There are, however, localized magnetic fields around magnetic minerals on the surface that could have been used by magnetic bacteria more recently, though still in the distant past.

Figure 1.6 Magnetic bacterium using single‐domain particles. The Magnetic Bacterium (Magnetospirillum gryphiswaldense) from river sediments in Northern Germany. The lines of (permanently magnetized) single‐domain magnetic nanoparticles, appearing as dark dots, align the body of the bacterium along the local direction of the Earth's magnetic field, which in Germany is inclined at 55° from horizontal. This means that the bacterium will always swim downward toward the sediments where it feeds.

      Source: Reproduced with the permission of the Int. J. Microbiol. from D. Schüler [2].

Figure 1.7 Size‐dependent behavior in nanoparticles. For particles smaller than 10 nm, quantum effects start to become apparent. In this size range, the proportion of atoms that constitute the surface layer starts to become significant reaching 50% in 2 nm diameter particles. Below about 3 nm, the strength of magnetism per atom starts to increase as shown in the inset for measurements on Co nanoparticles (see text).

Formation of single‐domain particles is only the onset of size effects in the nanoworld. If we continue the Democritus experiment and continue to cut the particles into smaller pieces, other size effects start to become apparent (Figure 1.7). In atoms, the electrons occupy discrete energy levels, whereas in a bulk metal, the outermost electrons occupy energy bands, in which the energy, for all normal considerations, is a continuum. For nanoparticles smaller than 10 nm, containing about 50 000 atoms, the energy levels of the outermost electrons in the atoms start to display their discrete energies. In other words, the quantum nature of the particles starts to become apparent. In this size range, a lot of the novel and size‐dependent behavior can be understood simply in terms of the enhanced proportion of the atoms at the surface of the particles. In a macroscopic piece of metal, for example, a sphere 2 cm across, only a tiny proportion of the atoms, less than 1 in 10 million, are on the surface atomic layer. A 10 nm diameter particle, however, has 10% of its constituent atoms making up the surface layer and this proportion increases to 50% for a 2 nm particle. Surface atoms are in a different chemical environment to the interior and either exposed to vacuum or interacting with atoms of a matrix in which the nanoparticle is embedded. Novel behavior of atoms at the surfaces of metals has been known for decades thus, for example, the atomic structure at the surface is often different from a layer in the interior of a bulk crystal. When such a high proportion of atoms comprise the surface, their novel behavior can distort the properties of the whole nanoparticle.

      Returning to magnetism, a well‐known effect in sufficiently small particles is that, not only are they single domains but also the strength of their magnetism per atom is enhanced. The inset in Figure 1.7 shows the measured strength of magnetism (or the magnetic moment per atom) for Co nanoparticles as a function of size. The data are described in more detail below.

A method for measuring the strength of magnetism (or the magnetic moment) in small free particles is to form a beam of them (see Chapter 5, Section 5.1.2) and pass them through a nonuniform magnetic field as shown in Figure 1.8. The amount the beam is deflected from its original path is a measure of the nanoparticle magnetic moment, and if the number of atoms in the particles is known, then one obtains the magnetic moment per atom.

Magnetic moments of atoms are measured in units called Bohr magnetons³ or μ_B (after the Nobel laureate Neils, Bohr) and the number of Bohr magnetons specifies the strength of the magnetism of a particular type of atom. For example, the magnetic moments of Fe, Co, Ni, and Rh atoms within their bulk materials are 2.2μ_B, 1.7μ_B, 0.6μ_B,and 0μ_B (Rh is a nonmagnetic metal), respectively. Figure 1.9 shows measurements of the magnetic moment per atom in nanoparticles of the above four metals as a function of the number of atoms in the particle. In the case of Fe, Co, and Ni, a significant increase in the magnetic moment per atom over the bulk value is observed for particles containing less than about 600 atoms. Perhaps most surprisingly, sufficiently small particles (containing less than about 100 atoms) of the nonmagnetic metal Rh become magnetic.

Figure 1.8 Measuring the magnetic moment in free nanoparticles. The magnetic moment in free nanoparticles can be measured by passing a beam of them through a nonuniform magnetic field and measuring the deflection in their path.

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,