Scanning Tunneling Microscopy Study of Vanadyl Phthalocyanine on Ag(100)

Magnetic molecules such as vanadyl phthalocyanine (VOPc) have garnered attention for their numerous applications in quantum information, photovoltaics, and medical imaging. Recent progress has been made in controlling the spin degree of freedom in molecules adsorbed on conducting surfaces via chemical synthesis. We used scanning tunneling microscopy (STM) to characterize two different coverage levels of VOPc sublimated onto a Ag(100) surface. At sub-monolayer coverage, we observe two distinct adsorption configurations: one in which the V-O group protrudes up into vacuum, and one where it points down into the substrate. Density functional theory calculations support these assignments, and predict the adsorption sites relative to the Ag(100) lattice. At higher coverages, the molecules self-assemble with only one orientation into a square lattice with a periodicity of 1.36nm, in addition to a moiré pattern of 4.36nm related to the interlayer stacking. Topographic images show a preference for stacking of an even number of molecular layers stabilized by out-of-plane dipolar interactions. Scanning tunneling spectroscopy reveals distinct electronic structure of the even-layer and odd-layer coverages.