Physical Properties 51 Introduction

The microworld is governed by atoms and molecules, while the macroworld consists of materials which contain a mind-boggling (or infinite) number of atoms or molecules. For

Figure 19. Zero-field-cooled (ZFC) and field-cooled (FC) (in a field of 100 mT) magnetization curves of the Fe(B) nanocapsules. After [122], Z. D. Zhang et al., Phys. Rev. B 64, 024404 (2001). © 2001, American Physical Society.

atoms and molecules, we face directly a world that can be described only by quantuam mechanics, in which chemical bonds, reactivity of a molecule, electronic orbitals and spins, the energy band of the electrons, etc. are most important factors. In the macroworld, we see distinct phases of materials—liquid, solid, and gas—and talk of physical properties like magnetization, electrical resistance, thermal conductivity, internel energy, specific heat, stiffness, and so on. The bulk consists of atoms and molecules, so the properties of the atoms and molecules should decide the properties of the bulk materials. The physical properties of bulk materials can be described well either by quantuam mechanics or by theories based on classical mechanics. According to the energy band theory, the energy levels of a bulk metal are in general continuum (or so-called quasi-continuum) because the number of electrons in the bulk is infinite. The cyclic condition and the translation symmetry in three dimensions of bulk materials lead to the plane wave character of electrons. The physical properties of bulk are the thermody-namical averages of the properties of an infinite number of electrons. Classical mechanics can be treated as an approximation of quantuam mechanics in a continuous media or in a continuum limit for the macroworld.

One would like to see how things change as one goes from atoms and molecules to small assemblies of them, to large assemblies, to larger assemblies, and finally to the bulk. One would like to see how various atoms, which do not appear very different individually, work together to form bulk materials with abundance properties, like metals, insulators, and semiconductors. One would like to see whether the properties of the assemblies of atoms and molecules differ with those of bulk and how the assemblies of atoms and molecules behave. The world between atoms, molecules, and the bulk is so-called mesoworld. This is a challenging problem for scientists and only a little progress has been made in the area of the mesoscopic world.

It is clear that the physical properties of the mesoworld are quite different than those of the atoms, the molecules, and the bulk. The mesoworld is a bridge connecting with two ends: one is the microworld with the discreteness of the energy levels; another is the macroworld with the continuum of the energy band. As a limited system, the discreteness of the energy levels is still held in the mesoworld because of the limited number of electrons. The small is different in an essential way when one (or more) of the physical dimensions of the material approaches a length-scale characteristic to a physical phenomenon (with different phenomena being characterized by different length scales), and similarly in the time domain. If the energy gap were larger than the thermal energy, the magnetostatic energy, the electrostatic energy, the photon energy, or the condensation energy of the superconductivity, the physical properties of the system would differ with those of bulk due to the quantum size effect. The cyclic condition and the translation symmetry would be broken down in the mesoworld, altering the physical properties of the systems. The quantum interference of electrons in the confined states could become more pronounced in the mesoscopic systems. The properties of the mesoscopic systems could be affected also by the magic number effect, the breakdown of the symmetry at the surfaces/interfaces, the formation of core/shell structure, the presence of metastable phases, etc. The classical mechanism can be used for description of the properties of the mesoscopic systems.

The properties of the mesoscopic systems depend sensitively on size, shape, structure, surface, etc. The sensitivity of sufficiently small materials aggregating to various shapes could change the properties of the materials. Most importantly, spontaneous shape selection occurs in the mesoscopic systems, originating from the ability of finite systems to adjust their shape and the structure in order to minimize the free energy of the systems. Small is beautiful not only since the limited systems could show the fascinating phenomena that do not exist in either the macroworld or the microworld, but also since the mesoscopic systems could be designed and constructed artificially, which could be so different from those formed naturally. For instance, the superlattices or multilayers could be grown artificially which show exchange coupling and the giant magnetoresistance effect that have been successfully applied in the reading head of the hard disk of a computer. The quantum dots could be designed to be applicable for high-density magnetic recording. Nano-tubes and nanowires could be used as elementary parts of an apparatus like a quantum computer. Basic research of the structure and properties of the nanostructured materials and related issues underlies future technologies, from nano-scale machines, nanotribological systems, cellular injections, and nanocatalysis, to miniaturization of electronic circuitry and novel information storage and retrieval systems.

There is no doubt that nanocapsules are classified to the mesoscopic systems because nanocapsules are in nanoscales, having nanostructures. Indeed, nanocapsules are matter in intermediate states between bulk and atomic materials, which offer an opportunity to investigate dimensionally confined systems in basic research areas. Indeed, understanding the differences of the mesoworld to those of the bulk is extremely important for studying the properties of nanocapsules. The optical absorption, electric resistance, magnetic, and dynamic properties of nanocapsules are very different from the bulk counterparts, which would be applicable in various fields. For instance, nanocapsules could show strong band-edge photoluminescence shifting. Magnetic nanocapsules could show superparamagnetism or the exchange bias. Graphite encapsulated nanocapsules have numerous possible applications due to their novel properties and their ability to survive rugged environments. In Sections 5.2-5.5 we shall represent the optical, electronic, magnetic, and dynamic properties of the nanocapsules (and also the nanoparticles for some materials), respectively. The biomedical properties of the nanocapsules will be introduced in the next section.

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