Nanoparticles: Synthesis in Polymer Substrates
Read full article onlineFull Article
Advanced materials composed of inorganic nanoparticles are currently one of the most dynamic areas of scientific research. These particles represent significant fundamental and commercial interest with a wide range of applications including the next generation of optics, electronics, catalysts, and sensors. In the past 20 years, with the increasing developments in nanoengineering, nanoelectronics, and nanobioelectronics, functional building blocks with which to construct microstructures and devices are under intensive investigation. Increasing attention has been paid to metal or semiconductor nanoparticles, which have also been called nanocrystals, nanoclusters, quantum dots, and Q particles.
Nanoparticles are in the size range of 1–100 nm, which lies between single atoms or molecules and bulk materials, and their chemical and physical properties differ markedly from those of the bulk solids. The reasons for these properties can be attributed to high surface area and quantum size effect, which is caused by the reduced size in three dimensions. One typical example is that the melting temperature of nanoparticles strongly depends on the particle size and is substantially lower than the bulk melting temperature. The melting point of 2.5-nm Au nanoparticles is ∼ 40% lower than that of bulk gold. Similar behavior has been observed for CdS nanoparticles. At the same time, with the decrease in the number of atoms in the particles, their electronic properties also start to change. This is the so-called quantum size effect, which can be observed as a blue shift in the optical band gap or exciton energy. When the size of particles is reduced to nanometer scale, the number of atoms at the surfaces or grain boundaries of the crystalline regions is comparable to the number of those that are in the crystalline lattice itself. Physical and chemical properties, which are usually determined by the molecular structure of the bulk lattice, become increasingly dominated by the defect structure of the surface. During the preparation of nanoparticles, engineering of the size and surface structure is of both theoretical and practical importance, because it leads to mechanical, chemical, electrical, optical, magnetic, electro-optical, and magneto-optical properties that are substantially different from those observed for the corresponding bulk materials. Chemists, biologists, and materials scientists have studied and reviewed the preparation and characterization of metal and semiconductor nanoparticles in detail, focusing on their physical and chemical properties.
Nanoparticles can be synthesized from a variety of materials with controllable sizes, shapes, and structures, but because of their high specific surface area and low surface energy, nanoparticles are not stable in ambient circumstances, and they must be incorporated in certain kinds of materials. Materials such as reverse micelles and surfactant vesicles, glass, zeolites, and polymers have been used as the substrate to prepare composite materials containing nanoparticles. During the preparation of such materials, the substrate not only can prevent the nanoparticles from aggregation, but also can accurately control their size and surface structure. In addition, in applications for optical, electrical, and magnetic devices, nanoparticles are mostly used in the form of thin films. Currently, such films can be made by spin coating, spraying, chemical deposition, electrical deposition, Langmuir–Blodgett (LB) technique, and layer-by-layer self-assembly (LBL) method.
In this article, we will focus on the preparation of nanoparticles in polymer matrices and the assembly of nanoparticles at interfaces.