Semiconducting state

The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

The most unusual and interesting feature of these polymers is their capacity to switch between insulating and conducting (or semiconducting) states. All other materials, with the only additional exception of some intercalation compounds, are normally found only as conductors or semiconductors or insulators, without the facility to switch between these states. 

The most striking implication of the electron lattice coupling in ID chains is the appearance of the semiconducting state the equal bond ID lattice (metallic state) is unstable (33) with respect to a lattice distorsion and this so called static Peierls instability is the origin of the opening of the intrinsic band gap at the edge of the B.Z. with an infinite density of states there and the presence of band alternation. 

Katayama S, Kobayashi A, Suzumura Y (2006) Pressure-induced zero-gap semiconducting state in organic conductor a-(BEDT-TTF)2l3 salt. J Phys Soc Jpn 75 054705/1-6... 

Figure 6.11 shows the resistivities of some alloys (V - xCrJjC as functions of temperature. The addition of Cr, as already stated, increases the temperature at which the transition to the metallic state occurs. At higher temperatures a transition back to the semiconducting state is predicted by the phase diagram of Fig. 6.3. Particularly remarkable, however, are the very low conductivities...

Optical study indicates that at low temperatures the low-energy electronic properties of some organic metal-like conductors (e.g., TTF-TCNQ) are dominated by charge density wave (CDW) effects. Frequency-dependent conductivity of TTF-TCNQ, obtained from the IR reflectance, at 25 K displays a double-peak structure with a low-frequency band near 35 cm-1 and a very intense band near 300 cm-1 [45]. The intense band may be ascribed to single-particle transitions across the gap in a 2kF (Peierls) semiconducting state, while the 35-cm-1 band is assigned to the Frohlich (i.e., CDW) pinned mode. Low-temperature results based on the bolometric technique [72,73] (Fig. 15) confirm the IR reflectance data. Such a con-... 

It is instructive to compare the T-dependences of s and A in this elementary case. The s function is represented schematically together with the A function in Fig. 1. One has simply, for T < Tx, s = A = A0, and for T > T2, s = A = A At T = T0, [dPNdT1] = 0, [dsldT = 0, and then s is maximum at the transition [15]. In consequence, when a material presents a well-defined semiconducting state with a constant energy gap 2A at low temperature, any subsequent increase in the slope function s upon heating is in fact the indication of a decrease in this gap. These opposite behaviors of s and A have sometimes been the source of misleading interpretations in the past. 

Undoped polymers, which remain in the semiconducting state. In this case charge injection is operated by photoexcitation (simultaneous creation of an electron in the conduction band and a hole in the valence band), or by action of an electric field at a junction. 

Conjugated polymers in their undoped, semiconducting state are electron donors upon photoexcitation (electrons promoted to the antibonding it band). The idea of using this property in conjunction with a molecular electron acceptor... 

Table 2 summarizes [391 the electrical conductivity of several charge-transfer crystals, most of which have a definite Peierls transition to a semiconducting state below a temperature T. In some cases, however, there is instead a very broad maximum in the conductivity vs. temperature plot, and the compound retains its high conductivity to the lowest temperatures measured, without ever going superconducting. One such salt, Cu(DMDCNQI)2, even reaches ct = 5x 10 S cm at 0.5 K. 

In relation to the primary topic of this section, nanotubes are interesting structures to contemplate. In the semiconducting state, the gaps can range from 0.7 to 1.4 eV and hence could provide sufficient overlap with the solar spectrum for good collection efficiency. In the metallic state they could also serve as 1-D quantum wires forming a barrierless conduction path between donor and acceptor states and facilitate charge... 

The structure of this equation and its curves with N(T) given either by Equation 4 or 5 do not depend generally on dimensionality, d the values for the parameters for the three dimensionalities are of course different, but the evolution of the curves from the semiconductive state to the semimetallic state is the same for all three dimensions. Hence, Equation 5 can be used in Equation 7 to illustrate the predictions of the model. Such is shown in Figure 5 wherein a(T) / cr(200) is plotted for E = 10 eV, n = 1, and Ef is varied from 0 to 2 x 10 eV. In this8sequence, the curves vary from that for a semiconductor at ji to that for a semimetal (a degenerate semiconductor) at d. If curve a were extended to higher temperatures, then it would display a broad maximum, or minimum in the resistivity. The maximum is attributable to the competition... 

It must be emphasized, of course, that the model presented above is intended to describe only part of the evolution from the semiconductive state to the superconductive state and beyond into the semimetallic state. It is intended to describe only the normal state at the onset of the evolution of the superconductive state dictated by the density of carriers. To complete the description it is necessary to recognize the generation of the paired electron state and the resulting strong diamagnetic susceptibility which approaches - —. A thermochemical model describing the equilibria... 

The second important low-temperature feature of Figure 10 is the puzzling similarity among the apparent activation energies in the semiconducting states... 

Coulomb coupling leads to a semiconducting state 8. It is highly unlikely therefore that materials with still much higher conductivity will be found in these types of charge transfer salts. 

The results of X-ray scattering thus on the one hand show that around 180 K, the Peierls transition to the CDW ground state occurs, and on the other, that above the Peierls transition over a large temperature range, fluctuations between the metallic and the semiconducting states are present. 

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