One-dimensional semiconductor

One dimensional conjugated carbon polymers can occur in many configurations as depicted in Figure 2 where also we included some chains with nitrogen and sulfur for later reference. Also included there are inorganic one dimensional semiconductors, like SbSI and SbSBr for later comparison. Besides the depicted one-dimensional system others like TCNQ- and KCP-salts could be included here as well but rough measurements of their nonlinear coefficients gave deceptively small values which combined with their ill-characterisation make them poor candidates for nonlinear optical devices. 

Agrawal, G. P. Cojan, C. Flytzanis, C. "Nonlinear Optical Properties of One-Dimensional Semiconductors and Conjugated Polymers," Phys. Rev. 1978, B17, 776. 

One-dimensional semiconductor nanorods, because of their unique electronic, optical, and mechanical properties, are attractive dopants for liquid crystals. Hence, composites of these two distinctive materials undoubtedly have great potential for applications in electronic and optical devices. 

Fig. 10.2. One-dimensional semiconductor model of interacting 2/>z-electrons in polymer chain from C-C- bonds of alternating length (it-electron system of polyacetylene) [9] (a)-the configurations of chain with repeat union 2a (b)-energy band scheme for 2/>z-electrons G the gap between valent and conduction bands.

TEA(TCNQ)2 [triethy lammonium-di(7,7,8,8-tetracyano-p-quinodi-methane)], a one-dimensional semiconductor... 

In this salt (MDT = l-methyl-l,4-dithianium), there is a brick wall stacking arrangement of TCNQ dimers. It is a quasi-one-dimensional semiconductor up to 300 K with an energy gap Ec = 0.22 eV. The magnetic susceptibility follows quite well a Bonner-Fisher law with / = 76 K. At room temperature x = 9.5 x 10-4 emu/mol and there is a maximum = 14.5 x 10 4 emu/mol at Tm = 100 K. There is also a probable spin-Peierls transition at 5.5 K [64]. 

The initial application of quantum mechanics to the electronic states of a perfect linear conjugated chain, as in the Htickel model discussed in Section 4.2.5 and above, led to a model of a one-dimensional semiconductor with well-defined valence and conduction bands. This labelling of the electronic states is widespread in the literature. On the other hand, when electron correlation is included, the electronic states are more localised and an exciton description is more appropriate. The disorder present in all but a few exceptional cases inevitably leads to the conclusion that the electronic states must be localised by chain defects. The extent to which the electronic states of conjugated polymers are localised, i.e. deviate from the band model, has been a matter of debate. There is a growing body of experimental and theoretical evidence, discussed in Sections 9.4.2 and 9.4.3 below, that suggests that the exciton description is closer to the truth. 

Although interesting within the framework of polymer physics and material science this would not be sufficient to attract so many workers from areas outside of conventional polymer research. Additional interest arouse because of the unusual structure of the polymers obtained via solid-state polymerization of diacetylenes and because of the mechanistic features related to its formation. Polydiacetylenes exhibit a fully conjugated and planar backbone in the crystalline state and are thus considered the prototype study object as far as the nature and physical behavior of polyconjugated macromolecules are concerned Theoretical discussions of the electronic structure of these polymers (2) lead to a description in terms of a wide band one-dimensional semiconductor... 

In addition to systems with one-dimensional metallic properties, there are a considerably larger number of planar metal complexes where stacking interactions in the solid state give rise to unusual properties including highly anisotropic conductivity behavior but where electronic or structural factors lead to thermally activated conductivities 10, 11), Such one-dimensional semiconductors constitute an important area of study within the general topic of solids with one-dimensional interactions, and their study has provided much useful information regarding structure-property relationships. 

If Eq. (5) is not satisfied, it is hypothesised that the tube may exhibit semiconducting properties [59-61]. Therefore, one third of these tubules may be onedimensional metals and the other two thirds one-dimensional semiconductors. 

D Avanzo, P. C. Vanzi, M. Dutton, R. W. "One-Dimensional Semiconductor Device Analysis (SEDAN)" Stanford Electronics Laboratory, Technical Report No. G-201-5, Stanford, California 1979. 

This restricts the Geld to discrete molecular systems. Thus, some very interesting work carried out with more extended wires, such as carbon nanotubes (Refs. [7,8]) and one-dimensional semiconductors (Ref. [9]), will not be dealt with in this entry. 

High absorption cross section for photon harvesting. Ideally, the electronic structure of conjugated polymers is that of a one-dimensional semiconductor in which the absorption coefficient increases steeply above the band gap absorption. The absorption coefficient of the frequently used poly(p-phenylene vinylene) polymer MDMO-PPV (Figure 10.2.) reaches 10 cm just... 

Barth S, Hernandez-Ramirez F, Holmes JD, Romano-Rodriguez A (2010) Synthesis and applications of one-dimensional semiconductors. Prog Mater Sd 55 563-627... 

This is noteworthy since a one-electron tight-binding treatment of PDAs as one-dimensional semiconductors in the... 

Abstract The linear and nonlinear optical properties of polysilanes, and in particular poly(di-n-hexylsilane), have been investigate by vacuum UV spectroscopy and by two-photon induced processes. The electronic structure of the polymer inferred from these measurements is consistent with models which view the polymer as a one-dimensional semiconductor quantum wire. The exciton related optical nonlinearities are also large enough to allow polysUanes to be considered for various nonlinear switching applications. 

Some recent studies of the electronic properties of poly silanes have suggested that the first UV transition is not a band-to-band transition, but is instead excitonic in nature [13,16,17,18].These predict that the onset of band-to-band transitions must therefore be at higher energies, with an exciton binding energy of an eV or more, as is common for Frenkel excitons in molecular systems. Since the vacuum ultraviolet (VUV) specmim has not been measured until now, the question of whetha- a one-dimensional semiconductor band gap model or a molecular orbital model is appropriate for the polysilanes has been unresolved. 

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