SNAPSHOTS OF CRYSTAL-GROWTH - NANOCLUSTERS OF ORGANIC CONDUCTORS ON AU(111) SURFACES

Mono- and multilayer crystalline nanoclusters of tetrathiafulvalene-te tracyanoquinodimethane ((TTF)-(TCNQ)), a low-dimensional organic condu ctor in the bulk form, can be formed readily on Au(111) surfaces by va por phase sublimation under ambient conditions. Scanning tunneling mic roscopy of monolayer (TTF)(TCNQ) films reveals a two-dimensional densi ty 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 molecu lar planes are nearly parallel to the Au(111) substrate. In contrast, clusters with thicknesses corresponding to two or three molecular laye rs exhibit a transformation to a highly anisotropic DOS that can be at tributed to interlayer molecular overlap in segregated TTF and TCNQ mo lecular chains along the c axis, which can be described as ''molecular wires''. The orientation of the crystalline (TTF)(TCNQ) clusters is p reserved 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 gro wth 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 cryst alline phase.