SELF-ASSEMBLY OF LOW-DIMENSIONAL MOLECULAR NANOCLUSTERS ON AU(111) SURFACES

Two-dimensional nanoclusters of (TTF)(TCNQ) (TTF = tetrathiafulvalene, TCNQ = tetracyanoquinodimethane)and Li(+)TCNQ(-), formed on Au(111) s urfaces by vapor phase sublimation under ambient conditions prior to g rowth of bulk crystals of these low-dimensional organic conductors, ha ve been observed with scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The molecular planes of the constituents i n individual nanoclusters are oriented perpendicular to the Au(111) su bstrate, while the clusters exhibit azimuthal orientations conforming to the 3-fold Au (110) directions. The nanocluster morphology and stru cture suggest that self-assembly of the nanoclusters is governed by sp ecific interactions between the molecular species and the substrate an d molecular diffusion along [110] troughs on the Au(111) substrate sur face. In the case of the (TTF)(TCNQ) nanoclusters, TTF and TCNQ molecu les assemble into molecular rows normal to the stacking direction, wit h intermolecular distances along the stacking direction which are near ly identical to those observed in bulk (TTF)(TCNQ). In contrast, the i ntermolecular spacings between TCNQ molecules along the molecular stac king axis in Li(+)TCNQ(-) nanoclusters are substantially larger than t hat observed in bulk M(+)TCNQ(-) salts. The large intermolecular spaci ng in Li(+)TCNQ(-) nanoclusters is consistent with Coulomb repulsion b etween fully reduced rho = 1 TCNQ(-) anion sites (rho = formal charge) . The smaller spacings observed for (TTF)(TCNQ) nanoclusters are consi stent with reduced Coulomb repulsion, in agreement with the fractional charge known to exist in the bulk material (rho = 0.59(+) and 0.59(-) for TTF and TCNQ sites, respectively). The preferred direction of gro wth of the nanoclusters is transverse to the molecular stacking axes i n both compounds, whereas the macroscopic morphologies reflect preferr ed growth parallel to the stacking direction. These observations indic ate that morphology and molecular packing of crystal nuclei at the nan oscale are not necessarily identical to the corresponding characterist ics observed at the macroscopic scale.