Charged solitons, formation

Fig. 4.2 A short segment of t rans -polyacetylene is shown with an abrupt (idealized) reversal of the bond alternation pattern (see text). Top- a neutral soliton with an unpaired spin and an energy state near the middle of the electron energy gap. Middle the addition of an electron results in the formation of a spinless negatively charged soliton. Bottom the extraction of an electron from the top results in the formation of a spinless positive soliton. The optical transitions associated with the charged solitons are indicated as arrows on the right.

Fig. 2.3. (a) and (b) Structure of the soliton S and antisoliton S (the filled circles represent the electrons still bound to the polymer chain), (c) normalized displacement in the C-H positions due to formation of the soliton, and (d) energy levels of the neutral and the charged solitons (the arrows show the electrons with spins up or spins down). The figure is adapted from different figures given in Ref. [14]. 

SoKtons produced in polyacetylene are delocalized over approximately 12 CH units, with the maximum charge density to the dopant counterion. Soliton formation results in the creation of a new localized electronic state which is in the middle of the energy gap. At a high level of doping the charged sohtons produce soliton bands that can merge to behave hke a metalhc conductor. 

We find a value for a at the peak of the mid-gap absorption band of 1.2 x 10 5 cm, somewhat lower than the value found for the solitons due to the extrinsic doping in the Schottky diodes, and we consider that the differences are due to the high level of disord in the surface layer of polyacetylene at the interface with the silicon dioxide in this MIS device. It is important to note that the value of a is constant throughout the region of accumulation, and that it reaches this value at the onset of accumulation. It provides, therefore, very clear evidence that almost all the charge that is injected into the structure, which is measured by the diffoential capacitance, is stored in states which give rise to the new optical behaviour which is characteristic of the formation of a charged soliton-like excitation on the polymw chain. We have no evidence for the presence of trap states, which if present would have reduced the value of a, particularly at the onset of accumulation. 

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