Scanning Tunneling Microscopy of Metal Phthalocyanines:  d⁷ and d⁹ Cases

Abstract

Scanning tunneling microscopy (STM) images of cobalt(II) phthalocyanine (CoPc), copper(II) phthalocyanine (CuPc), and mixtures of the two adsorbed on the Au(111) face are reported. Based upon the stability and ease of obtaining molecular images, CoPc appears to adsorb more strongly than CuPc on Au(111), but both species provide images showing submolecular structure. The mixed CoPc and CuPc films also provide high-quality images showing details of the internal structure of the metal phthalocyanine. A particularly exciting aspect of this work is the strong influence of the metal ion valence configuration on the observed tunneling images. Unlike CuPc, wherein the central metal appears as a hole in the molecular image, the cobalt atom in CoPc is the highest point (about 0.3 nm) in the molecular image. These data are interpreted as indicating that the Co(II) d 7 system has significant d-orbital character near the Fermi energy while the Cu(II) d 9 system does not. This interpretation is consistent with theoretical calculations that predict a large contribution of cobalt d-orbitals near the Fermi energy, and with inelastic electron tunneling spectra that show d-orbital-related bands within 1 eV of the Fermi energy. An intriguing aspect of this work is that it may be possible to chemically identify the different metal phthalocyanines simply by their appearance. This can be used to advantage in the study of surface diffusion, 2-d sublimation, and the surface thermodynamics and kinetics of adlayer formation. 7198

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.…”

Different approaches to adjusting band offsets at intermolecular interfaces

“…Both polyaniline [12,[18][19][20][21] and copper phthalocyanine (CuPc) thin films [8,13,[22][23][24][25][26][27] (schematically illustrated in Figure 5) can be produced by vapor deposition compatible with ultrahigh vacuum photoemission. These films can be grown very thin (100 Å or less) and are free of solvent contamination.…”

“…Indeed, with adsorption on metallic substrates copper phthalocyanine exhibits an electric dipole as the molecular center copper atom is not in the same plane as the surrounding ligand, and adopts a position that is typically closer to the interface [27]. The changes in the copper phthalocyanine orientation and dipole make this metalorganic semiconductor a useful adsorbate for the study of effects of dipole interactions at a heterojunction interface [13].…”