Metal—organic semiconductor, electronic structure

As in the MD method, PES for KMC can be derived from first-principles methods or using empirical energy functionals described above. However, the KMC method requires the accurate evaluation of the PES not only near the local minima, but also for transition regions between them. The corresponding empirical potentials are called reactive, since they can be used to calculate parameters of chemical reactions. The development of reactive potentials is quite a difficult problem, since chemical reactions usually include the breaking or formation of new bonds and a reconfiguration of the electronic structure. At present, a few types of reactive empirical potentials can semi-quantitatively reproduce the results of first-principles calculations these are EAM and MEAM potentials for metals and bond-order potentials (Tersoff and Brenner) for covalent semiconductors and organics. 

The alternatives to metal-based semiconductors are organic semiconductors, but they struggle to reach efficiencies of 5%, although there are hints in the scientific literature of polymers that are comparable to silicon in their semiconducting properties. Even so, polymers rely on double bonds along the backbone of the polymer to provide the structure along which electrons and holes can move, but double bonds are always susceptible to oxidation by the oxygen of the atmosphere so they have to be protected. 

The electronic properties of organic conductors are discussed by physicists in terms of band structure and Fermi surface. The shape of the band structure is defined by the dispersion energy and characterizes the electronic properties of the material (semiconductor, semimetals, metals, etc.) the Fermi surface is the limit between empty and occupied electronic states, and its shape (open, closed, nested, etc.) characterizes the dimensionality of the electron gas. From band dispersion and filling one can easily deduce whether the studied material is a metal, a semiconductor, or an insulator (occurrence of a gap at the Fermi energy). The intra- and interchain band-widths can be estimated, for example, from normal-incidence polarized reflectance, and the densities of state at the Fermi level can be used in the modeling of physical observations. The Fermi surface topology is of importance to predict or explain the existence of instabilities of the electronic gas (nesting vector concept see Chapter 2 of this book). Fermi surfaces calculated from structural data can be compared to those observed by means of the Shubnikov-de Hass method in the case of two- or three-dimensional metals [152]. 

Trans-polyacetylene, tra 5-(CH) was the first highly conducting organic polymer [1,2]. The simple chemical structure, -CH- units repeated (see Fig. IVB-la), would imply that each carbon contributes a single p electron to the tr-band. As a result, the rr-band would be half-filled. Thus, based upon this stmcture, an individual chain of neutral polyacetylene would be a metal since the electrons in this idealized metal could move only along the chain, polyacetylene would be a one-dimensional (Id) metal. However, experimental studies show clearly that neutral polyacetylene is a semiconductor with an energy gap greater than 1.5 eV. Rudolf Peierls [86] showed many years ago that Id metals are... 

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