Snapshots of Crystal Growth: Nanoclusters of Organic Conductors on Au(111) Surfaces

Mono- and multilayer crystalline nanoclusters of tetrathiafulvalene-tetracyanoquinodimethane ((TTF)-(TCNQ)), a low-dimensional organic conductor in the bulk form, can be formed readily on Au(111) surfaces by vapor phase sublimation under ambient conditions. Scanning tunneling microscopy of monolayer (TTF)(TCNQ) films reveals a two-dimensional density of states (DOS) that is consistent with the arrangement of TTF and TCNQ molecules in the ac face of bulk (TTF)(TCNQ), in which the molecular planes are nearly parallel to the Au(111) substrate. In contrast, clusters with thicknesses corresponding to two or three molecular layers exhibit a transformation to a highly anisotropic DOS that can be attributed to interlayer molecular overlap in segregated TTF and TCNQ molecular chains along the c axis, which can be described as "molecular wires". The orientation of the crystalline (TTF)(TCNQ) clusters is preserved throughout the crystal growth sequence, leading to meso- and macroscopic (TTF) (TCNQ) needles that are oriented perpendicular to the Au(111) substrate. These studies provide visualization of crystal growth from the initial stages of nucleation to macroscopic crystals and a revealing example of the changes in electronic structure that occur during the evolution of molecular (TTF)(TCNQ) nuclei into a bulk crystalline phase.