Semiconductors bond length

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

The carbides with the NaCl structure may be considered to consist of alternating layers of metal atoms and layers of semiconductor atoms where the planes are octahedral ones of the cubic symmetry system. (Figure 10.1). In TiC, for example, the carbon atoms lie 3.06A apart which is about twice the covalent bond length of 1.54 A, so the carbon atoms are not covalently bonded, but they may transfer some charge to the metal layers, and they do increase the valence electron density. 

Tab. 14.2 Bulk Moduli for standard Croup IV, lll-V, and ll-VI semiconductors. The theoretical values are from a semi-empirical formula requiring the measured bond length as input.

For Aat = 1, corresponding to a half-filled band, kj = njla and for Aat = 2, corresponding to a filled band, = r/a (see Fig. 1.28). In fact for CTSs Mt = Q-Fet us consider briefly the rather simple polyacetylene molecule, -(CH) , in which each carbon is o bonded to only two neighbouring carbons and one hydrogen atom with one n electron on each carbon (the orbital pointing perpendicularly to the chain direction). If the carbon bond lengths were equal, with one n electron per formula unit, it would imply a metallic state E < 0) as discussed above. However, neutral polyacetylene is a semiconductor with an energy gap of approximately 1.5 eV. The reason for this discrepancy is discussed next. ... 

The dependence of Tc on pressure is studied for a variety of reasons. In a chemical sense, bond lengths are shortened, and orbital interactions are increased. The volume decrease leads in principle to a rise in carrier density. In reality, however, not only do vibrational frequencies change, but crystal structure and symmetry are often affected by high pressure. Numerous materials undergo semiconductor to metal phase transitions as a function of pressure. Increasing pressure can often be considered analogous to a decrease in temperature. 

In the experiment discussed above, no directional dependence of the pair interaction is attempted. Pair interactions are simply assumed to be isotropic on the W (110) surface. The pair interaction, in general, should depend both on the direction of the adatom-adatom pair bond and on the bond length. Thus pair energies should therefore be measured for each possible pair bond. A preliminary study in this direction has been reported by the same authors for Si-Si interaction on the W (110) surface.94 Si-Si interaction is of particular interest since (1) Si atoms interact with one another in solid state by forming covalent bonds rather than metallic bonds it would be interesting to see how the interaction of Si adatom pairs on a metal surface is different from that of metal adatom pairs (2) semiconductor-metal interfaces are technologically important... 

Dietl et al. 2001c). There exists another mechanism by which strain may affect 7c. It is presently well known that the upper limit of the achievable carrier concentration is controlled by pinning of the Fermi level by impurity or defect states in virtually all compound semiconductors. Since the energies of such states in respect to bands vary strongly with the bond length, the hole concentration and thus 7c will depend on strain. 

This salt (NPrQn, /V-propylquinolinium) consists of tetramerized TCNQ chains. It undergoes a phase transition at 220 K which is considered to be a second-order metal-to-semiconductor transition. Optical reflectivity measurements on crystals, with the light polarized in the chain direction, indicate that the charge distribution on the TCNQ sites in the tetrads is less uniform at 100 K than at 300 K. This is as in TEA(TCNQ)2 (see Section III.A.3). However, estimation of this charge distribution from the bond lengths at 300 K [38] gives ambiguous results for this material [63]. 

Alternation of the CC bond lengths along the chain and the existence of a large energy gap are well-established facts in PA (see Chapter 12, Section II.C.2). However, since each carbon atom contributes one tt electron, there is at first sight no obvious reason why CC bonds should not be equivalent. If they were, and taking into account the electron spin, the tt electrons should generate a half-filled band such a material is a metal. If there is bond alternation, the one-dimensional unit cell is doubled and a gap opens at the Brillouin zone boundary the material is a semiconductor. 

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