Elemental inorganic nanotubes

A simple way of building nanotubes is the use of elements that are able to adopt a planar, layered structure. In this context, carbon is special due to its different possible hybridizations. Both two- and three-dimensional networks, such as graphene and diamond, can be built by the same element. Therefore, it has been a challenge for theoreticians and experimentalists to search for other nanotubular structures built up from just one element. Silicon has been a natural candidate since it belongs to the same main group as carbon. The stability of silicon nanotubes has been predicted theoreti-cally. However, an experimental observation or synthesis has not been achieved to date. 

In comparison to carbon, the absence of an sp hybridisation in silicon complicates the stabilization of a silicon monolayer. As a result, silicon layers have a buckled structure, which additionally have free valences. These have to be saturated, e.g. by hydrogen, in order to stabilize the systems.  

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This chapter is structured as follows we will discuss the structure of carbon and inorganic nanotubes in general in Section 2, followed by synopses of studies of structural properties of elemental inorganic nanotubes and intrinsically twisted inorgnic nanotubes in Sections 3 and 4, respectively. Section 5 discusses the encapsulation of materials in and the filling process of inorganic nanotubes, whereas Section 6 features inorganic fullerene-like structures. We conclude in Section 7. 

If organic chemistry is considered to be the chemistry of carbon , then inorganic chemistry is the chemistry of all elements except carbon. In its broadest sense, this is true, but of course there are overlaps between branches of chemistry. A topical example is the chemistry of the fuller-enes (see Section 13.4) including Ceo (see Figure 13.5) and C70 this was the subject of the award of the 1996 Nobel Prize in Chemistry to Professors Sir Harry Kroto, Richard Smalley and Robert Curl. An understanding of such molecules and related species called nanotubes involves studies by organic, inorganic and physical chemists as well as by physicists and materials scientists. 

A dispersion is instead a mixture in which the less abundant compound is dispersed, but not molecularly dissolved, in the other component. Examples are a dispersion of a solid phase (powder, nanoparticles, nanocrystals, nanotubes, etc) in a solvent, which is called a suspension, or a dispersion of an immiscible liquid phase in a second liquid, which is called an emulsion. Milk and mayonnaise are familiar examples of emulsions. Aerosols are dispersions of tiny liquid droplets or solid particles in a continuous gaseous phase. The science of aerosols is particularly relevant in order to design filter elements able to remove droplets and particles from air, which can be performed with very high efficiency by polymer nanofibers (Section 4.3.1). Finally, another way to indicate homogeneously mixed dispersions, emulsions or aerosols of nano- or microparticles is colloids. Colloidal dispersions of inorganic nanocrystals or organic nanofibers are familiar examples for nanotechnologists. 

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