Comparison of the Symmetry Breaking in the Surface Molecular Structures of One- and Two-Dimensional Bis(ethylenedithio)tetrathiafulvalene Compounds

Abstract

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were performed on the cationic molecular surfaces of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)-based organic conductors with one-dimensional (β-(BEDT-TTF)2AsF6) and two-dimensional (κ-(BEDT-TTF)2Cu(NCS)2) conductivity. For the β-(BEDT-TTF)2AsF6 surface, highly similar STM/AFM images to those of β-(BEDT-TTF)2PF6 were observed. Two types of alternately aligned molecular rows existed in the direction of the crystal a-axis; one is characterized by bright and distinct protrusions, and the other is a little darker with less distinct rows forming a zigzag structure. On the other hand, in the case of κ-(BEDT-TTF)2Cu(NCS)2, similar one-dimensional modulation was confirmed by AFM for the first time in addition to the STM observation. However, the observed difference in the molecular images was only in their brightness, and their shapes were quite similar. Since the surface structures were confirmed by both STM and AFM, the obtained results indicate that the observed modulations are associated with the structural changes in the surface molecular arrangements. The modulations were discussed in relation to the crystal structures.