B.-E. Schuster

Locally resolved core-hole screening, molecular orientation and morphology in thin films of diindenoperylene deposited on Au(111) single crystals

Organic electronics has reached the market in a very short period of time, since the first organic light emitting device has been demonstrated.[1] Growth of organic thin films has been the focus of intensive investigations that in the recent years have gained knowledge about the involved mechanisms: substrate-molecule interaction versus molecule-molecule interaction, preparation parameters, substrate morphology (i.e., roughness, local defects, steps), and post growth treatment.[2–6] Here we investigate thin films of diindenoperylene deposited on Au(111) single crystals by using photoelectron emission microscopy (PEEM). We also discuss the effect of the growth on the film structure using the molecular orientation (i.e., the angle between the molecular axis and the substrate).

Molecular orientation in diindenoperylene thin films deposited on polycrystalline gold

We present x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine structure spectroscopy measurements taken on diindenoperylene (DIP) thin films of different thicknesses deposited on polycrystalline gold. Our investigations show that DIP films present flat lying molecules in the first layer, while the angle between the molecular axis and the surface increases with thickness. By using XPS, we find evidence for Stranski–Krastanov growth mode.

Resonant Raman spectra of diindenoperylene thin films

Resonant and preresonant Raman spectra obtained on diindenoperylene (DIP) thin films are interpreted with calculations of the deformation of a relaxed excited molecule with density functional theory (DFT). The comparison of excited state geometries based on time-dependent DFT or on a constrained DFT scheme with observed absorption spectra of dissolved DIP reveals that the deformation pattern deduced from constrained DFT is more reliable. Most observed Raman peaks can be assigned to calculated A(g)-symmetric breathing modes of DIP or their combinations. As the position of one of the laser lines used falls into a highly structured absorption band, we have carefully analyzed the Raman excitation profile arising from the frequency dependence of the dielectric tensor. This procedure gives Raman cross sections in good agreement with the observed relative intensities, both in the fully resonant and in the preresonant case.

Electric field assisted effects on molecular orientation and surface morphology of thin titanyl(IV)phthalocyanine films.

Herein we present electric field assisted effects on the molecular orientation, the polymorphism, and the surface morphology of thin titanyl(IV)phthalocyanine (TiOPc) films. The ability of electric fields to affect the thin film structure of polar molecules is demonstrated using titanyl(IV)phthalocyanine as a model compound exhibiting both a permanent and an induced electric dipole moment. Thin films of TiOPc prepared by organic molecular beam deposition (OMBD) in the absence and in the presence of an electric field during the thin film growth are characterized using polarization dependent Raman spectroscopy and atomic force microscopy (AFM). The distinctive pattern of vibrational modes of the phthalocyanine skeleton indicates different molecular orientations in these thin films: Raman spectra of regions where an electric field is present during thin film growth reveal a preferential molecular orientation with an inclination angle of the molecular plane (pseudoplanar macrocycle) with respect to the substrate plane of nearly 90 degrees. Contrary to that, in regions where the electric field was absent, the molecules adopt predominantly a configuration with a smaller tilt angle (approximately 60 degrees). In addition, an electric field assisted change is apparent in AFM images: A large amount of well-formed steplike crystallites lying parallel to the substrate is observed when no electric field was present, whereas in the case when an electric field was applied during thin film growth the crystallites exhibit a tilt with respect to the substrate plane.

Direct observation of step-edge barrier effects and general aspects of growth processes: morphology and structure in diindenoperylene thin films deposited on Au(100) single crystals

We investigate thin films of diindenoperylene deposited on Au(100) single crystals by using real time in situlow energy electron microscopy (LEEM) in synergy with X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. We find evidence for Stranski–Krastanov growth, with island nucleation prevalently at step bunches, and a lower probability of island nucleation at terraces. We also directly observe the effect of the Ehrlich–Schwoebel barrier at the substrate and in the film morphology, in particular inside the islands that show a non-homogeneous thickness due to a rough front growth, because of the additional energy required for the diffusing molecules to surmount a downward step.

Unusual energy shifts in resonant photoemission spectra of organic model molecules

We study the electronic structure of zinc phthalocyanine (ZnPc) and 1,4-octa-decyl substituted zinc phthalocyanine [(Dec)(8)PcZn] thin films (approximately 6-15 nm) using resonant photoemission spectroscopy and X-ray absorption spectroscopy (XAS) at room temperature and at liquid He temperature. From XAS we conclude that the probability amplitude of the lowest unoccupied molecular orbital is located predominantly at the inner C and N atoms of the molecules. Nonlinear energy shifts in resonant photoemission were observed; large shifts are explained by reduced electrical conductivity of inhomogeneously oriented molecules.

Characterization of the morphology and composition of commercial negative resists used for lithographic processes

We present a spectroscopic and microscopic characterization of the chemical composition, structure, and morphology of two commercial negative resists using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). For this purpose, films of a novolak-based resist (ma-N 2400) and hydrogen silsesquioxane (HSQ) are treated under different conditions (temperature, deep ultraviolet (DUV) exposure, CHF3 plasma). Topographic AFM images show that both heating and DUV exposure strongly affect the surface morphology of as-prepared ma-N 2400 resist films. These different treatment conditions also lead to decreasing roughnesses, which indicates structural reorganization. Furthermore, the decrease of the photoactive compound (bisazide) in the ma-N 2400 resist films, observed in FTIR spectra, suggests cross-linking of the resist after CHF3 plasma treatment, heating, or DUV exposure. XPS measurements on different CHF3 plasma-treated surfaces reveal that a structurally homogeneous fluorine-containing polymer is generated that is responsible for an enhanced etch resistance. FTIR measurements of HSQ films show a correlation between the degree of HSQ cross-linking and baking time.