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 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.