A Scanning Tunneling Microscopy and Spectroscopy Study of Vanadyl Phthalocyanine on Au(111):  the Effect of Oxygen Binding and Orbital Mediated Tunneling on the Apparent Corrugation

Abstract

Submolecular resolution STM images of vanadyl phthalocyanine (VOPc) are reported. Unlike previously studied metal phthalocyanines, the image has neither a simple dark or bright spot in the center. Rather, it appears to have nodal planes running through the center of the molecule. The dark central region is attributed to the oxygen atom reducing the number of available states near the Fermi energy. The apparent height of the Pc ring is very bias dependent having a value of ∼0.1 nm between -1.5 and 0.2 V (sample) bias. The apparent height increases rapidly to a plateau of about 0.55 nm at +1.0 V sample bias. I -V and dI/dV data taken over VOPc islands are very different from the same measurements made on exposed Au(111) substrate adjacent to the islands. Strong peaks in dI/dV are seen at +1.38 and -1.8 V sample bias. The +1.38 V feature is associated with orbital mediated tunneling via the π* LUMO of the Pc ring. Occasionally, the tip-surface interaction generates anomalous dark areas in the VOPc islands.

Metal-free phthalocyanine (H2Pc) molecule adsorbed on the Au(111) surface: formation of a wide domain along a single lattice direction

“…It was reported that H 2 Pc molecules form a quasi-square lattice on highly oriented pyrolytic graphite surface [25], and similar ordering was observed for FePc [11,12], CoPc [2,[13][14][15], NiPc [11], CuPc [2,15,16] and SnPc [17].…”

“…The ridge of the herringbone structure is perpendicular to one of these quasi-close-packed directions. For an MPc film, quasi-square lattices were commonly formed when the coverage was close to a monolayer, as clearly demonstrated for the molecules of CoPc [2,[13][14][15] and FePc [11,12].…”

“…In studying the adsorption of the double-layer molecules, it might be necessary to compare the results with those obtained on metal-free Pc, as the bottom Pc layer does not contain a metal atom. Contrary to numerous reports of MPc on the Au(111) surface [2,[11][12][13][14][15][16][17], the studies of metal-free Pc film growth in the low-coverage regime are scarce.…”

“…The Pc molecule can contain various metal atoms at its center, and such metal-Pc (MPc) complexes are expected to play an important role in controlling the molecular device via their spins. Research in this direction has been accelerated by a recent success in the synthesis of lanthanide phthalocyanine molecules with single-molecule magnet properties [2][3][4][5]. Different from normal MPc, these molecules have a metal atom sandwiched by two Pc ligands.…”

A photoelectron diffraction investigation of vanadyl phthalocyanine on Au(111)

“…This led to an estimated coverage of 0.015 ML. For comparison, we note that the published STM images of the ordered layer of VOPc formed on Au(111) [10] indicate that the area occupied by each VOPc molecule is approximately 200 Å 2 , implying a molecular coverage for this single complete molecular layer of ~0.035 ML. Our coverage is thus ~50% of the saturation coverage of this first molecular layer.…”