Intercalation compounds of large organic molecular cations in the layered dichalcogenide 2H-TaS2

We report on the first successful intercalation of organic cations in large crystals of 2H-TaS2 by electrochemical methods. The following species have been intercalated: Tetramethylammonium, methylviologen and methylene blue. Whereas the first species can be intercalated in chemical equilibrium like the hydrated metal ions, the intercalation of the other two more complex cations is accompanied by side reactions. The most complex system is methylene blue/TaS2 where we observe three different phases depending on the preparation conditions. The electrical resistivity as a function of temperature has been measured by a 4-point ac method on single crystals immediately after intercalation. All samples are superconductors, some of them having a transition temperature to the superconducting state T-c of 4.9 K, which is quite high for organic molecule intercalation compounds of 2H-TaS2. The anomaly of the resistivity at 77 K in the empty host lattice is suppressed in all of the intercalated samples even at the very low charge transfer of n = 0.06.