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The self-assembly mechanism of alkanethiol monolayers on the (111) surface of gold was discovered with the use of an ultrahigh-vacuum scanning tunneling microscope. Monolayer formation follows a two-step process that begins with condensation of low-density crystalline islands, characterized by surface-aligned molecular axes, from a lower density lattice-gas phase. At saturation coverage of this phase, the monolayer undergoes a phase transition to a denser phase by realignment of the molecular axes with the surface normal. These studies reveal the important role of molecule-substrate and molecule-molecule interactions in the self-assembly of these technologically important material systems.

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Using an ultrahigh-vacuum scanning tunneling microscope we have discovered evidence for a novel mechanism by which Au vacancy islands form during assembly of alkanethiol monolayers on Au(111). Our results suggest a model whereby excess Au atoms are forced out of the surface layer by relaxation of the compressed herringbone reconstruction. This creates adatoms on, and vacancies in, the surface layer. On large terraces the vacancies nucleate into islands while the adatoms migrate and adsorb at ascending step edges. At saturation coverage of alkanethiols the surface exhibits ≈6% of a monolayer of vacancy islands. These results show that complex interactions between the assembling thiols and the herringbone reconstruction influence the mesoscopic aspects of the final monolayer surface.

Using ultrahigh vacuum scanning tunneling microscopy, we have characterized the structural phases of decanethiol on Au(111) at coverages below saturation. As coverage increases, the monolayer sequentially adopts five discrete structural phases. At low surface coverage, decanethiol exists as a lattice gas. Above a critical surface coverage, the molecules condense into islands of a commensurate crystalline lattice. These islands grow in equilibrium with the lattice gas until saturation. As coverage increases, the surface layer sequentially undergoes two first-order phase transitions, first to a metastable phase then to a stable phase. The first three condensed phases are characterized by alignment of the molecular axes with the surface plane but with discretely increasing degrees of out-of-plane interdigitation. Above saturation coverage of the densest surface-aligned phase, the monolayer undergoes an edge-mediated melting transition. The evidence suggests that the resulting fluid is a supercooled, two-dimensional liquid. The highest-density phase, characterized by alignment of the molecular axes close to the surface normal, grows by homogeneous nucleation from this supercooled liquid. These data provide a fundamental understanding of the mechanistic pathway of molecular monolayer self-assembly.