A photoinduced electrical conductivity switching is observed in the molecular conductor of ␣-͓bis͑ethylenedithio͒tetrathiafulvalene͔ 2 I 3 at different temperatures with different irradiation light intensities. The threshold voltage for the differential-negative-resistance effect appearing in the current-voltage characteristic curve decreases and increases, respectively, as the light intensity is increased and as the temperature decreases below the insulator-metal phase transition. The potential application of molecular conductor in bidirectional light-activated thyristor devices is demonstrated.

We have studied the thermoelectric power of an α-(BEDT-TTF) 2 I 3 system paying particular attention to the new type of zero-gap state (ZGS). α-(BEDT-TTF) 2 I 3 exhibits a charge ordering at ambient pressures and transitions from its charge order to ZGS when hydrostatic pressure is applied. We observed a sharp peak at the charge order transition temperature due to the anisotropy change in the conductivity. Above 1.3 GPa, thermoelectric power characteristics of the ZGS could be detected.

In this chapter, the wavefunction analysis is demonstrated, applied to the organic charge transfer salts composed of electron donor and electron acceptor molecules. Scanning tunneling microscopy (STM) images of the surface donor layers in the three charge transfer salts, α-(BEDT-TTF) 2 I 3 , β-(BEDT-TTF) 2 I 3 , and (EDO-TTF) 2 PF 6 , are analyzed with the atomic π electron orbitals of sulfur, oxygen, and carbon atoms. We have deduced three different kinds of surface molecular reconstructions as follows: (1) charge redistribution in α-(BEDT-TTF) 2 I 3 , (2) translational reconstruction up to 0.1 nm in β-(BEDT-TTF) 2 I 3 , and (3) rotational reconstruction transforming the 1D axis from the a axis to the b axis in (EDO-TTF) 2 PF 6 . Finally, it is concluded that the surface reconstruction is ascribed to the additional gain of the cohesive energy of the π electron system, provoked by the reduced steric hindrance with the anions of the missing outside double layer. The investigations of the surface states provide not only interesting behaviors of the surface cation layer, but also important insights into the electronic states of a lot of similar charge transfer crystals, as demonstrated in α-(BEDT-TTF) 2 I 3 .