Scanning Tunneling Microscopy of Layered Molecular Conductors

Scanning tunneling microscopy (STM) of single crystals of semiconducting organic crystalline charge-transfer salts [R₁(R₂)NCH₂CH₂OCH₂CH₂]⁺[TCNQ]₂^- (1a: R₁ = C₂H₅, R₂ = C₂H₅; 1b: R₁ = CH₃, R₂ = C₂H₅; 1c: R₁ = CH₃, R₂ = CH₃, monoclinic; 1d: R₁ = CH₃, R₂ = CH₃, triclinic) and (Ph(H)NCH=CHCH=N(H)Ph)⁺(TCNQ)₂^- (TCNQ = tetracyanoquinodimethane) is described. The STM data reveal tunneling current features that compare favorably with the electronic properties and crystal structure of the salts. The local density of states (LDOS) exhibit orientation and corrugation consistent with the topography and electronic structure of dimerized TCNQ stacks contained in the molecular layers that define the exposed low-energy crystal planes. In the case of la, crystallographically inequivalent TCNQ layers that alternate through the crystal are revealed in STM data by sequential etching of the layers. The two dimorphs lc and Id reveal dramatically different tunneling current contrast due to the different molecular motifs of their respective crystal faces, as surmised from the crystal structure. While the lattice constants determined from the tunneling current contrast correspond favorably with the values expected based on single-crystal X-ray structures, in some cases there appears to be a slight compression of the surface layer. This behavior is consistent with weak interaction between the molecular layers of the charge-transfer salt, and the tendency of the molecular constituents in the surface layer to achieve a higher packing density in order to lower the crystal surface free energy. These studies reveal that STM can provide substantial insight into the structure of organic crystals, and can probe molecular-level structural and electronic features.