Alexander Gerlach

Charged and metallic molecular monolayers through surface-induced aromatic stabilization

Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

Adsorption-induced intramolecular dipole: correlating molecular conformation and interface electronic structure.

The interfaces formed between pentacene (PEN) and perfluoropentacene (PFP) molecules and Cu(111) were studied using photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunneling microscopy measurements, in conjunction with theoretical modeling. The average carbon bonding distances for PEN and PFP differ strongly, that is, 2.34 A for PEN versus 2.98 A for PFP. An adsorption-induced nonplanar conformation of PFP is suggested by XSW (F atoms 0.1 A above the carbon plane), which causes an intramolecular dipole of approximately 0.5 D. These observations explain why the hole injection barriers at both molecule/metal interfaces are comparable (1.10 eV for PEN and 1.35 eV for PFP) whereas the molecular ionization energies differ significantly (5.00 eV for PEN and 5.85 eV for PFP). Our results show that the hypothesis of charge injection barrier tuning at organic/metal interfaces by adjusting the ionization energy of molecules is not always readily applicable.

Optical properties of pentacene and perfluoropentacene thin films.

The optical properties of pentacene (PEN) and perfluoropentacene (PFP) thin films on various SiO(2) substrates were studied using variable angle spectroscopic ellipsometry. Structural characterization was performed using x-ray reflectivity and atomic force microscopy. A uniaxial model with the optic axis normal to the sample surface was used to analyze the ellipsometry data. A strong optical anisotropy was observed, and enabled the direction of the transition dipole of the absorption bands to be determined. Furthermore, comparison of the optical constants of PEN and PFP thin films with the absorption spectra of the monomers in solution shows significant changes due to the crystalline environment. Relative to the monomer spectrum, the highest occupied molecular orbital to lowest unoccupied molecular orbital transition observed in PEN (PFP) thin film is reduced by 210 meV (280 meV). A second absorption band in the PFP thin film shows a slight blueshift (40 meV) compared to the spectrum of the monomer with its transition dipole perpendicular to that of the first absorption band.

Organic molecular beam deposition: fundamentals, growth dynamics, and in situ studies

We review recent progress on thin film growth by organic molecular beam deposition (OMBD). We give a brief overview of growth physics with emphasis on the specific characteristics of organic materials, such as weak van der Waals binding forces and conformational and orientational degrees of freedom of the molecular building blocks. Two recent developments in experimental studies of OMBD will be discussed in more detail: (1) we will give examples for real-time and in situ growth studies during deposition of the organic semiconductors pentacene, diindenoperylene, and PTCDA and (2) we will give an overview of high precision in situ investigations of the first molecular monolayer, in particular using the x-ray standing wave technique.

Adsorption-induced distortion of F16CuPc on Cu(111) and Ag(111): an X-ray standing wave

The adsorption geometry of perfluorinated copper-phthalocyanine molecules

Substrate-dependent bonding distances of PTCDA: A comparative x-ray standing-wave study on Cu(111) and Ag(111)

({\mathrm{F}}_{16}\mathrm{CuPc})

Real-time observation of oxidation and photo-oxidation of rubrene thin films by spectroscopic ellipsometry

on Cu(111) and Ag(111) is studied using x-ray standing waves. A detailed, element-specific analysis taking into account nondipolar corrections to the photoelectron yield shows that on both surfaces the molecules adsorb in a lying down, but significantly distorted configuration. While on copper (silver) the central carbon rings reside 2.61 \AA{} (3.25 \AA{}) above the substrate, the outer fluorine atoms are located 0.27 \AA{} (0.20 \AA{}) further away from the surface. This nonplanar adsorption structure is discussed in terms of the outer carbon atoms in

Photoluminescence spectroscopy of pure pentacene, perfluoropentacene, and mixed thin films

{\mathrm{F}}_{16}\mathrm{CuPc}

Optical spectra obtained from amorphous films of rubrene: Evidence for predominance of twisted isomer.

undergoing a partial rehybridization

Structure and morphology of coevaporated pentacene-perfluoropentacene thin films.

(s{p}^{2}\ensuremath{\rightarrow}s{p}^{3}).