Low-Temperature Scanning Tunneling Microscopy Investigation of Bis(phthalocyaninato)yttrium Growth on Au(111): From Individual Molecules to Two-Dimensional Domains

We show a 4.8 K STM observation of a double-decker bis(phthalocyaninato)yttrium (YPc2; Pc = phthalocyanine) molecule adsorption on Au(111) substrate. An eight-lobed structure was imaged as the submolecule STM contrast of a single molecule both in an isolated state and in a molecule film. This feature arises from the top Pc group, where both sides of the four phenyl rings are highlighted. As an isolated molecule, the adsorption orientation is determined by the lower Pc, the diagonal axis of which aligns parallel to the close-packed direction of Au(111). In a 2D film, a near-square molecule lattice appears with a unit of similar to 1.47 x 1.38 nm(2), and one of the lattice vectors is rotated by similar to 15 degrees from the close-packed direction. A tentative model is provided to illustrate the molecule array where neighboring molecules are rotated by similar to 30 degrees from each other. In this way, the lower Pcs should align along the [10 (1) over bar] and [(2) over bar 11] directions (or their equivalent directions) alternately. All these facts illustrate the molecule-substrate and the molecule-molecule interactions in the initial adsorption and in the film accumulation.