Tetrathiofulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) form a well-known charge-transfer complex, in which an electron from the highest occupied molecular orbital of TTF is transferred to TCNQ. In TTF–TCNQ crystals, this charge transfer leads to metallic conduction, which is unusual for organic materials. Only recently has it been shown that such charge transfer also occurs when two single crystals of the molecules are laminated onto each other, and that this results in the formation of a conducting interface. The details of such interfaces are yet to be explored, and Bertram Batlogg and co-workers now elucidate the microscopic processes that contribute to their formation. The researchers demonstrate that, when two crystals of TTF and TCNQ are placed next to each other at room temperature, sublimation and redeposition of TTF molecules can induce a conducting surface layer on TCNQ, albeit very slowly. They suggest that local sublimation facilitates the formation of conductive interfaces in laminated crystals, and show that cooling slows down the process.
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Martin, C. Up and away. Nature Mater 11, 745 (2012). https://doi.org/10.1038/nmat3422