Site-dependent donation/backdonation charge transfer at the CoPc/Ag(111) interface

Interface interaction of transition metal phthalocyanines with strontium titanate (100)

We study interface properties of CoPcF x and FePcFx (x = 0 or 16) on niobium-doped SrTiO3(100) surfaces using mainly X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. For all studied molecules, a rather complex, bidirectional charge transfer with the oxide substrate was observed, involving both the macrocycle and the central metal atom. For molecules of the first monolayer, an electron transfer to the central metal atom is concluded from transition metal 2p core level photoemission spectra. The number of interacting molecules in the first monolayer on the oxide surface depends on the central metal atom of the phthalocyanine, whereas the substrate preparation has minor influence on the interaction between CoPc and SrTiO3(100). Differences of the interaction mechanism to related TiO2 surfaces are discussed.

Phthalocyanine arrangements on Ag(100): From pure overlayers of CoPc and F16CuPc to bimolecular heterostructure

We have utilized scanning tunneling microscopy (STM) and low energy electron diffraction to determine the structural properties of two types of metal-phthalocyanines (MPcs), i.e., cobalt-phthalocyanine (CoPc) and hexadecafluorinated copper-phthalocyanine (F₁₆CuPc) on the Ag(100) surface. For coverage close to one monolayer, both systems form long-range ordered structures with square unit cells. The size and rotation of the unit cell with respect to the silver lattice depend on the chemical composition of MPc. Both types of molecules prefer adsorption with around a 30° angle between the molecular axis and the [011] silver direction. The CoPcs mainly arrange in a (5 × 5)R0 phase; however, two additional local arrangements, a 26 × 26 R 1 1 ○ and a (7 × 7)R0, were detected by STM. The F₁₆CuPcs form a 29 × 29 R 2 2 ○ structure. The co-adsorption of CoPc and F₁₆CuPc on the Ag(100) surface in a 1:1 ratio leads to the formation of a compositionally ordered chessboard-like 5 2 × 5 2 R 4 5 ○ structure. During filled states imaging, the different appearance of the central part of each MPc allows us to distinguish CoPcs from F₁₆CuPcs. Regardless of the applied voltage polarity, the ligands of F₁₆CuPcs appear brighter than the ligands of CoPcs.

Mixing of MnPc electronic states at the MnPc/Au(110) interface

Manganese-phthalocyanines form assembled chains with a variety of ordered super-structures, flat lying along the Au(110) reconstructed channels. The chains first give rise to a ×5 symmetry reconstruction, while further deposition of MnPc leads to a ×7 periodicity at the completion of the first single layer. A net polarization with the formation of an interface dipole is mainly due to the molecular π-states located on the macrocycles pyrrole rings, while the central metal ion induces a reduction in the polarization, whose amount is related to the Mn-Au interaction. The adsorption-induced interface polarization is compared to other 3d-metal phthalocyanines, to unravel the role of the central metal atom configuration in the interaction process of the d-states. The MnPc adsorption on Au(110) induces the re-hybridization of the electronic states localized on the central metal atom, promoting a charge redistribution of the molecular orbitals of the MnPc molecules. The molecule-substrate interaction is controlled by a symmetry-determined mixing between the electronic states, involving also the molecular empty orbitals with d character hybridized with the nitrogen atoms of the pyrrole ring, as deduced by photoemission and X-ray absorption spectroscopy exploiting light polarization. The symmetry-determined mixing between the electronic states of the Mn metal center and of the Au substrate induces a density of states close to the Fermi level for the ×5 phase.

On-surface manipulation of atom substitution between cobalt phthalocyanine and the Cu(111) substrate

An on-surface atom substation has been manipulated by thermal annealing which transmetalizes CoPc to CuPc for various applications.

A Ru–Ru pair housed in ruthenium phthalocyanine: the role of a “cage” architecture in the molecule coupling with the Ag(111) surface

A combined experimental and theoretical study on the Ru–Pc dimer on Ag(111) proves surface-to-molecule charge transfer and predicts magnetic moment quenching.

Iron phthalocyanine on Cu(111): Coverage-dependent assembly and symmetry breaking, temperature-induced homocoupling, and modification of the adsorbate-surface interaction by annealing

We have examined the geometric and electronic structures of iron phthalocyanine assemblies on a Cu(111) surface at different sub- to mono-layer coverages and the changes induced by thermal annealing at temperatures between 250 and 320 °C by scanning tunneling microscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy. The symmetry breaking observed in scanning tunneling microscopy images is found to be coverage dependent and to persist upon annealing. Further, we find that annealing to temperatures between 300 and 320 °C leads to both desorption of iron phthalocyanine molecules from the surface and their agglomeration. We see clear evidence of temperature-induced homocoupling reactions of the iron phthalocyanine molecules following dehydrogenation of their isoindole rings, similar to what has been observed for related tetrapyrroles on transition metal surfaces. Finally, spectroscopy indicates a modified substrate-adsorbate interaction upon annealing with a shortened bond distance. This finding could potentially explain a changed reactivity of Cu-supported iron phthalocyanine in comparison to that of the pristine compound.

Sticking with the Pointy End? Molecular Configuration of Chloro Boron-Subphthalocyanine on Cu(111)

In this combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study, we investigate self-assembly of the dipolar nonplanar organic semiconductor chloro boron-subphthalocyanine (ClB-SubPc) on Cu(111). We observe multiple distinct adsorption configurations and demonstrate that these can only be understood by taking surface-catalyzed dechlorination into account. A detailed investigation of possible adsorption configurations and the comparison of experimental and computational STM images demonstrates that the configurations correspond to "Cl-up" molecules with the B-Cl moiety pointing toward the vacuum side of the interface, and dechlorinated molecules. In contrast to the standard interpretation of adsorption of nonplanar molecules in the phthalocyanine family, we find no evidence for "Cl-down" molecules where the B-Cl moiety would be pointing toward the Cu surface. We show computationally that such a configuration is unstable and thus is highly unlikely to occur for ClB-SubPc on Cu(111). Using these assignments, we discuss the different self-assembly motifs in the submonolayer coverage regime. The combination of DFT and STM is essential to gain a full atomistic understanding of the surface-molecule interactions, and our findings imply that phthalocyanines may undergo surface-catalyzed reactions hitherto not considered. Our results also indicate that care has to be taken when analyzing possible adsorption configurations of polar members of the phthalocyanine family, especially when they are adsorbed on comparably reactive surfaces like Cu(111).

Ordered growth of vanadyl phthalocyanine (VOPc) on an iron phthalocyanine (FePc) monolayer

The growth and characterisation of a non-planar phthalocyanine (vanadyl phthalocyanine, VOPc) on a complete monolayer (ML) of a planar phthalocyanine (Iron(II) phthalocyanine, FePc) on an Au(111) surface, has been investigated using ultra-high vacuum (UHV) scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). The surface mesh of the initial FePc monolayer has been determined and shown to correspond to an incommensurate overlayer, not commensurate as previously reported. Ordered islands of VOPc, with (1 × 1) epitaxy, grow on the FePc layer at submonolayer coverages. The individual VOPc molecules occupy sites directly atop the underlying FePc molecules, indicating that significant intermolecular bonding must occur. It is proposed that this interaction implies that the V[double bond, length as m-dash]O points down into the surface, allowing a Fe-O bond to form. The detailed appearance of the STM images of the VOPc molecules is consistent with previous studies in other VOPc growth studies in which this molecular orientation has been proposed.

Electronic structure at transition metal phthalocyanine-transition metal oxide interfaces: Cobalt phthalocyanine on epitaxial MnO films

The electronic structure of the interface between cobalt phthalocyanine (CoPc) and epitaxially grown manganese oxide (MnO) thin films is studied by means of photoemission (PES) and X-ray absorption spectroscopy (XAS). Our results reveal a flat-lying adsorption geometry of the molecules on the oxide surface which allows a maximal interaction between the π-system and the substrate. A charge transfer from MnO, in particular, to the central metal atom of CoPc is observed by both PES and XAS. The change of the shape of N-K XAS spectra at the interface points, however, to the involvement of the Pc macrocycle in the charge transfer process. As a consequence of the charge transfer, energetic shifts of MnO related core levels were observed, which are discussed in terms of a Fermi level shift in the semiconducting MnO films due to interface charge redistribution.

The influence of polar (0001) zinc oxide (ZnO) on the structure and morphology of vanadyl phthalocyanine (VOPc)

Metal oxide thin films are increasingly utilized in small molecular organic photovoltaic devices to facilitate electron transport and injection.

Organic semiconductor density of states controls the energy level alignment at electrode interfaces

Minimizing charge carrier injection barriers and extraction losses at interfaces between organic semiconductors and metallic electrodes is critical for optimizing the performance of organic (opto-) electronic devices. Here, we implement a detailed electrostatic model, capable of reproducing the alignment between the electrode Fermi energy and the transport states in the organic semiconductor both qualitatively and quantitatively. Covering the full phenomenological range of interfacial energy level alignment regimes within a single, consistent framework and continuously connecting the limiting cases described by previously proposed models allows us to resolve conflicting views in the literature. Our results highlight the density of states in the organic semiconductor as a key factor. Its shape and, in particular, the energy distribution of electronic states tailing into the fundamental gap is found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt interface dipoles to extended band-bending regions.

Understanding and being able to predict alignment between the electrode Fermi energy and the transport states in the organic semiconductor is important. Here, the authors report an electrostatic model, capable of reproducing the full range of interfacial energy level alignment regimes.

CoPc and CoPcF16 on gold: Site-specific charge-transfer processes

Interface properties of cobalt(II) phthalocyanine (CoPc) and cobalt(II) hexadecafluoro-phthalocyanine (CoPcF16) to gold are investigated by photo-excited electron spectroscopies (X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and X-ray excited Auger electron spectroscopy (XAES)). It is shown that a bidirectional charge transfer determines the interface energetics for CoPc and CoPcF16 on Au. Combined XPS and XAES measurements allow for the separation of chemical shifts based on different local charges at the considered atom caused by polarization effects. This facilitates a detailed discussion of energetic shifts of core level spectra. The data allow the discussion of site-specific charge-transfer processes.