Scanning tunneling microscopy (STM) images of the (001) face (ab face) of the organic conductors (TMTSF)2X (TMTSF = tetramethyltetraseienafulvalene; X = ClO4-, ReO4-) reflect the surface molecular corrugation and anisotropic stacking of TMTSF molecules along the  direction. The lattice constants of the (001) face determined from STM for (TMTSF)2ClO4 are a = 7.3 ± 0.2 Å, b = 7.8 ± 0.2 Å, and γ = 69.5 ± 2°, and for (TMTSF)2ReO4 they are a = 7.5 ± 0.2 Å, b = 8.1 ± 0.2 Å, and γ = 70 ± 2°, in excellent agreement with the known crystal structure. Height profile analysis of the tunneling current assigned to localized TMTSF electronic states is in agreement with the angle between the (001) face and the direction of the short axis of the TMTSF molecules. The STM images also are consistent with the high degree of electronic anisotropy of (TMTSF)2ClO4, with a broad electronic state density associated with the pseudo-one-dimensional TMTSF stacks and a highly localized state density associated with Se-Se contacts between stacks along the  direction. The results are corroborated by STM of isomorphous (TMTSF)2ReO4 which exhibits negligible tunneling current associated with the Se-Se states in agreement with the slightly larger Se-Se interstack distances in this compound. The lack of tunneling current in the interstack region of (TMTSF)2ReO4 signifies reduced electronic interaction transverse to the stacking direction, consistent with previously reported electronic properties of (TMTSF)2ReO4. The results indicate that STM of a crystal plane can detect subtle differences in electronic structure that are responsible for bulk electronic properties in these materials.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry