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All conventional materials like metals, semiconductors, glass, ceramic or polymers can in principle be obtained with a nanoscale dimension. The spectrum of nanomaterials ranges from inorganic or organic, crystalline or amorphous particles, which can be found as single particles, aggregates, powders or dispersed in a matrix, over colloids, suspensions and emulsions, nanolayers and films, up to the class of fullerenes and their derivates. Also supramolecular structures such as dendrimers, micelles or liposomes belong to the field of nanomaterials. Generally there are different approaches for a classification of nanomaterials, some of which are summarised in table 1. The main classes of nanoscale structures are summarised below table 1.
Table 1. Classification of nanomaterials with regard to different parameters.
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Dimension
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3 dimensions < 100nm
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Particles, quantum dots, hollow spheres, etc.
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2 dimensions < 100nm
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Tubes, fibers, wires, platelets, etc.
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1 dimension < 100nm
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Films, coatings, multilayer, etc.
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Phase composition
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Single-phase solids
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Crystalline, amorphous particles and layers, etc.
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Multi-phase solids
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Matrix composites, coated particles, etc.
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Multi-phase systems
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Colloids, aerogels, ferrofluids, etc.
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Manufacturing process
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Gas phase reaction
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Flame synthesis, condensation, CVD, etc.
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Liquid phase reaction
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Sol-gel, precipitation, hydrothermal processing, etc.
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Mechanical procedures
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Ball milling, plastic deformation, etc.
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Introduction to Nanoparticles
Nanoparticles are constituted of several tens or hundreds of atoms or molecules and can have a variety of sizes and morphologies (amorphous, crystalline, spherical, needles, etc.). Some kinds of nanoparticles are already available commercially in the form of dry powders or liquid dispersions. The latter is obtained by combining nanoparticles with an aqueous or organic liquid to form a suspension or paste. It may be necessary to use chemical additives (surfactants, dispersants) to obtain a uniform and stable dispersion of particles. With further processing steps, nanostructured powders and dispersions can be used to fabricate coatings, components or devices that may or may not retain the nanostructure of the particulate raw materials. Industrial scale production of some types of nanoparticulate materials like carbon black, polymer dispersions or micronised drugs have been established for a long time.
Metal Oxide Nanopowders, Compound Semiconductors and Alloys
Another commercially important class of nanoparticulate materials are metal oxide nanopowders, such as silica (SiO2), titania (TiO2), alumina (Al2O3) or iron oxide (Fe3O4, Fe3O3). But also other nanoparticulate substances like compound semiconductors (e.g. cadmium telluride, CdTe, or gallium arsenide, GaAs) metals (especially precious metals such as Ag, Au) and alloys are finding increasing product application.
Fullerenes or Dendrimers
Beside that, the range of macromolecular chemistry with molecule sizes in the range of up to a few tens of nanometers is often referred to as nanotechnology. Molecules of special interest that fall within the range of nanotechnology are fullerenes or dendrimers (tree-like molecules with defined cavities), which may find application for example as drug carriers in medicine.
Nanowires and Carbon Nanotubes (Nanorods)
Linear nanostructures such as nanowires, nanotubes or nanorods can be generated from different material classes e.g. metals, semiconductors or carbon by means of several production techniques. As one of the most promising linear nanostructures, carbon nanotubes can be mentioned, which can occur in a variety of modifications (e.g. single- or multi-walled, filled or surface modified). Carbon nanotubes are expected to find a broad field of application in nanoelectronics (logics, data storage or wiring, as well as cold electron sources for flat panel displays and microwave amplifiers), and also as fillers for nanocomposites for materials with special properties. At present, carbon nanotubes can be produced by CVD methods on a several tons per year scale and the gram quantities are already available commercially.
Nanolayers
Nanolayers are one of the most important topics within the range of nanotechnology. Through nanoscale engineering of surfaces and layers, a vast range of functionalities and new physical effects (e.g. magnetoelectronic or optical) can be achieved. Furthermore, a nanoscale design of surfaces and layers is often necessary to optimise the interfaces between different material classes (e.g. compound semiconductors on silicon wafers), and to obtain the desired special properties. Some application ranges of nanolayers and coatings are summarised in table 2.
Table 2. Tunable properties by nanoscale surface design and their application potentials.
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Mechanical properties (e.g. tribology, hardness, scratch-resistance).
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Wear protection of machinery and equipment, mechanical protection of soft materials (polymers, wood, textiles, etc.).
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Wetting properties (e.g. anti-adhesive, hydrophobic, hydrophilic).
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Anti-graffiti, anti-fouling, Lotus-effect, self-cleaning surface for textiles and ceramics, etc.
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Thermal and chemical properties (e.g. heat resistance and insulation, corrosion resistance).
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Corrosion protection for machinery and equipment, heat resistance for turbines and engines, thermal insulation equipment and building materials, etc.
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Biological properties (biocompatibility, anti-infective).
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Biocompatible implants, abacterial medical tools and wound dressings, etc.
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Electronical and magnetic properties (e.g. magnetoresistance, dielectric).
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Ultra-thin dielectrics for field-effect transistors, magneto-resistive sensors and data memory, etc.
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Optical properties (e. anti-reflection, photo- and electro-chromatic).
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Photo- and electro-chromic windows, anti-reflective screens and solar cells, etc.
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Nanopores
Materials with defined pore-sizes in the nanometer range are of special interest for a broad range of industrial applications because of their outstanding properties with regard to thermal insulation, controllable material separation and release, and their applicability as templates or fillers for chemistry and catalysis. One example of nanoporous material is aerogel, which is produced by sol-gel chemistry. A broad range of potential applications of these materials include catalysis, thermal insulation, electrode materials, environmental filters and membranes as well as controlled release drug carriers.
Note: A complete list of references can be found by referring to the original text.
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