Modulation of the Structure and Electronic Density of Molecular Chains on Organic Conductor Surfaces

Abstract

We used scanning tunneling microscopy (STM) to observe the bc-surface of the triethylammonium-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)2)charge-transfer complex crystal, which is terminated with the arrays of TCNQ chains. Three kinds of distinct structures of the quasi-one-dimensional (1D) TCNQ chains were found on different surface terraces, that is, the TCNQ molecules within a chain exhibit the monomerization, tetramerization, and octamerization, respectively. Each structure corresponds to a respective electron density distribution along the chain direction. The latter two reflect the different characteristics of the bulk and surface energy-band filling, respectively. The monomer structure indicates that the single molecular orbital was probed by STM due to the weakening of the overlapping between TCNQ molecules induced by the enlarging of the intermolecular distance. The two-dimensionality of the surface exerts an apparent influence on the quasi-1D TCNQ chains, with the specific manifestation of enhancing the interchain coupling even at the cost of the weakening of the intrachain interaction.