Introduction to Nanoscience and Nanotechnology. Chris Binns
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Название: Introduction to Nanoscience and Nanotechnology
Автор: Chris Binns
Издательство: John Wiley & Sons Limited
Жанр: Отраслевые издания
isbn: 9781119172253
isbn:
Figure 1.5 Single‐domain particles. Domain formation in Fe to minimize energy. Below a critical size (approx. 100 nm), the energy balance favors just a single domain and the piece of Fe stays permanently and fully magnetized. Arrows show the direction of magnetization.
Nature makes good use of this magnetic size effect. Bacteria such as the one shown in Figure 1.6 have evolved, which use strings of magnetic nanoparticles to orient their body along the local magnetic field lines of the Earth. The strain shown in the figure, which is found in Northern Germany, lives in water and feeds off sediments at the bottom. For a tiny floating life‐form such as this knowing up and down is not trivial. If the local field lines have a large angle to the horizontal, as they do in Northern Europe, then the string of magnetic nanoparticles makes the body point downwards and all the bacterium has to do is to swim, knowing that it will eventually find the bottom.
The intelligence of evolution is highlighted here. If the particles are single‐domain particles, then they will stay magnetized forever, so forming a string of these ensures that the navigation system will naturally work. If the bacterium formed a single piece of the material the same size as the chain of particles, a domain structure would form and it would become magnetically dead. The nanoparticles are composed of magnetite (Fe3O4) rather than pure Fe but the argument is the same. There is currently research devoted to persuading the bacteria to modify the composition of the nanoparticles by feeding them with cobalt (Co)‐containing minerals 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 μB (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 2.2μB, 1.7μB, 0.6μB,and 0μB (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.