J. Phys. Chem. B 1997, 101, 5263-5276
5263
Tip-Induced Structural Rearrangements of Alkanethiolate Self-Assembled Monolayers on Gold Igor Touzov and Christopher B. Gorman* Department of Chemistry, North Carolina State UniVersity, Box 8204, Raleigh, North Carolina 27695 ReceiVed: NoVember 25, 1996; In Final Form: May 5, 1997X
A method for preparing self-assembled monolayers of decanethiolate on Au(111) with large (ca. 50-100 nm) regions of hexagonal (x3×x3 R30) lattice is reported. Such samples could be transformed from this structure to a c(4×2) 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(4×2) 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(3×1) 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(4×2) 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.
I. Introduction Self-assembled monolayers (SAMs), particularly those composed of organothiolates on gold, are of increasing interest for use in the construction of well-defined organic surfaces.1,2 One way to exploit SAMs is as ordered surfaces upon which to perform nanoscale synthetic manipulations. SAMs have already seen utility in sub-micrometer lithographic patterning schemes3-6 and may be useful in molecular electronics demonstrations.7,8 Thus, it is of interest to establish conditions for the formation of monolayers with a high degree of perfection over nanometer to micrometer length scales and to understand the range and relative stability of packing arrangements of the organothiolate molecules on the surface. These questions have been explored at some length using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Both thinner (4-8 carbons in the tail group)9-13 and thicker (10 and greater carbons in the tail group)7,14-28 monolayers have been studied. In the former case, tail group interactions (e.g. van der Waals attractions, a significant force in promoting a given packing arrangement) can be small, giving rise to many phases, some of which appear to have a lower packing density than is found in monolayers composed of longer chains. The study of thicker SAMs is more significant to us as these have denser alkanethiolate packing arrangements and greater stability.15,16,23,29 Such SAMs are likely to provide the most stable foundation and/or matrix for nanoscale chemistry. In investigations of 10 carbon and longer monolayers, it is common to find two alkanethiolate arrangements for chains X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
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containing tails of greater than or equal to 10 carbons.7,14-28 Often both can be found in a ca. 20 × 20 nm image. The first alkanethiolate arrangement is hexagonal and is assigned to an overlayer structure that is x3×x3 R30 with respect to the underlying Au(111) lattice. The second is of lower symmetry and is found to be a c(4×2) superlattice based on the first overlayer. The simplest model for this superlattice is one in which all of the sulfurs in the SAM are found at 3-fold hollows in the Au(111) surface, but two sets of alkanethiolates exist that differ by the twist angle of the plane defined by their alltrans hydrocarbon backbones.30,31 More recently, models in which the sulfurs move from these 3-fold hollows have been proposed, and the likely relaxation of the positions of the top layers of gold atoms in response to this change has been acknowledged.23,32,33 To date, several features of this reconstruction are unclear. For example, it is uncertain which of these structures is energetically most stable, whether these structures can be interconverted, what barriers for activation of this conversion exist, and whether any intermediate structures can exist during a conversion process. In this paper, it is shown that the hexagonal lattice is not the minimum energy structure but that, as the result of the interaction of a scanning probe with the SAM or upon thermal annealing, irreversible changes to lower energy, lower symmetry structures could be induced. It is assumed that this first result is caused by a mechanical interaction between the probe and the SAM as the process can be caused and followed by both STM and AFM probes. Although the force between the tip and sample cannot be measured or controlled in an STM experiment, AFM experiments could be performed in which there was some control over © 1997 American Chemical Society
5264 J. Phys. Chem. B, Vol. 101, No. 27, 1997 the force. By controlling the force between the tip and the sample, a set of metastable, intermediate structures could be induced and observed by an AFM tip during the conversion from hexagonal to c(4×2), giving some insight as to the process by which this conversion might occur. Although it is not definite that the STM or AFM tip induces the reconstruction by the same mechanism as would be found during a thermal annealing cycle, these techniques offer the ability to observe this reconstruction as it happens, a feature that is difficult or impossible to implement while thermally annealing the sample. These results indicate why both the hexagonal and c(4×2) structures have been observed in the past and augment these observations by indicating the relative stability of these alkanethiolate packing arrangements on Au(111). Further, they indicate that in AFM and, less obviously, in low-current STM of alkanethiolate SAMs on gold, the probe does not image the surface innocently. In fact, even at set-point currents considered to be small (
