Coverage-dependent phases and phase stability of decanethiol on Au(111)

Using ultrahigh vacuum scanning tunneling microscopy, we have characterized the structural phases of decanethiol on Au( 111) at coverages below satura tion. As coverage increases, the monolayer sequentially adopts five discret e structural phases. At low surface coverage, decanethiol exists as a latti ce gas. Above a critical surface coverage, the molecules condense into isla nds of a commensurate crystalline lattice. These islands grow in equilibriu m 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. Ab ove 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-d ensity phase, characterized by alignment of the molecular axes close to the surface normal, grows by homogeneous nucleation from this supercooled liqu id. These data provide a fundamental understanding of the mechanistic pathw ay of molecular monolayer self-assembly.