Tip-induced structural rearrangements of alkanethiolate self-assembled monolayers on gold

A method for preparing self-assembled monolayers of decanethiolate on Au(111) with large (ca. 50-100 nm) regions of hexagonal (root 3 x root 3 R30) lattice is reported. Such samples could be transformed from this structure to a c(4 x 2) superlattice under the influence of a scanning tunneling microscope (STM) tip. These images were comparable to those obtained by thermally annealing the sample. Using an atomic force microscope tip in friction force mode (FFM), the c(4 x 2) superlattice could be observed on thermally annealed samples, but pressing upon the sample with a moderate tip load (as high as 80 nN) did not induce this structure. Comparative analysis of the results obtained by these two techniques suggests that this transformation occurred as the result of the tip pressing upon the surface. In addition to this final structure, intermediate structures were observed during these tip-induced transformations. The STM-induced transformation revealed an intermediate structure of p2 symmetry. The FFM-induced transformation revealed two intermediate structures: one also of p2 symmetry and the other of p1 symmetry that was a formal p(3 x 1) superlattice of the original, hexagonal SAM structure. The orientation of the FFM tip-induced intermediates could be affected by the scan direction, suggesting that these intermediate structures involved reorientation and/or conformational changes in the alkanethiolate tails. It was noted that the size of the atomically flat Au(111) terrace under study had an effect on the structure found and on the speed of the reconstruction; small terraces were found to have a c(4 x 2) superlattice structure without scanning upon or thermal annealing of the sample. STM even at low tunneling currents (as low as 6 pA) induced this reconstruction, suggesting that, at these currents, the influence of the tip upon the surface was considerable. Specifically, the transformations induced by the STM tip at this set-point tunneling current indicate that the degree of influence of the tip upon the sample is at least as great as that of an FFM pushing on the surface with a 30 nN force.