I. Biswas

Buried interfacial layer of highly oriented molecules in copper phthalocyanine thin films on polycrystalline gold.

The growth of copper phthalocyanine thin films evaporated on polycrystalline gold is examined in detail using near edge x-ray absorption fine structure spectroscopy and surface sensitive x-ray photoemission spectroscopy. The combination of both methods allows distinguishing between the uppermost layers and buried interface layers in films up to approximately 3 nm thickness. An interfacial layer of approximately 3 ML of molecules with an orientation parallel to the substrate surface was found, whereas the subsequent molecules are perpendicular to the metal surface. It was shown that even if the preferred molecular orientation in thin films is perpendicular, the buried interfacial layer can be oriented differently.

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).

Tetra-t-butyl magnesium phthalocyanine on gold: Electronic structure and molecular orientation

In this work we have investigated the electronic structure and the molecular orientation of (t-Bu)(4)PcMg (tetra-t-butyl magnesium phthalocyanine) on polycrystalline and single crystalline gold substrates using photoemission spectroscopy and x-ray absorption spectroscopy, and we compare the results to the unsubstituted PcCu (copper phthalocyanine). The C 1s photoemission spectrum is described similar to unsubstituted relatives with an additional component for the aliphatic substituents. The variation of the excitation energy causes distinct differences in the shape of the C 1s spectrum, which is very useful for the analysis of the molecular orientation in the uppermost layer. It is shown that despite of the sterically demanding substituents, ordered sublimed films of (t-Bu)(4)PcMg are accessible, the orientation of the molecules, however, is different from the orientation of the unsubstituted relatives.

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.

Nanoscale Assembly of Paramagnetic Organic Radicals on Au(111) Single Crystals

The successful thin-film deposition of a pyrene-substituted nitronyl nitroxide radical under controlled conditions has been demonstrated. The electronic properties, chemical environment at the interface, and morphology of the thin films have been investigated by a multitechnique approach. Spectroscopic and morphological analyses indicate a Stranski-Krastanov growth mode and weak physisorption of molecules onto the metallic surface. Electron spin resonance (ESR) spectroscopy shows that evaporation processes and deposition do not affect the paramagnetic character of the molecules. Useful concepts for the engineering of new, purely organic-based magnets, which may open the way to fruitful exploitation of organic molecular-beam deposition for assembly on solid surfaces in view of future technological applications, are presented.

Electronic Structure and Interface Properties of a Model Molecule for Organic Solar Cells

We study the electronic structure of 4,7-bis(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (MTBT) and its interface properties with gold using X-ray photoemission spectroscopy (XPS), valence-band ultraviolet photoemission spectroscopy (UPS), X-ray absorption spectroscopy (XAS), as well as resonant photoemission (ResPES). MTBT can be regarded as a model molecule for PCPDTBT, a promising candidate for efficient bulk heterojunction solar cells. Almost no contribution of sulfur and only a weak contribution of nitrogen to the HOMO level is found. At the interface with gold, a strong chemical interaction between the sulfur of the benzothiadiazole and gold occurs, which may have consequences for interface properties in devices.

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.

Orientation of Differently Substituted Phthalocyanines: First Layers and Thin Films

The molecular orientation of the first layers and thin films of differently substituted phthalocyanines on gold substrates is compared. Due to a relatively strong molecule-substrate interaction the orientation is parallel to the sample surface for all materials in the first layers. At higher film thickness the orientation is determined by the substituents. Materials with short substituents prefer standing orientation, implying the substituents do not inhibit the intermolecular interactions. Molecules with long substituents are not able to build up the same aggregates, they grow disordered in the upper layers.

From interfaces to surfaces: soft x-ray spectromicroscopy investigations of diindenoperylene thin films on gold.

We present the results of photoemission electron microscopy investigations on diindenoperylene (DIP) thin films deposited on polycrystalline gold, prepared in order to have a roughness much larger than the molecular size. Our investigations revealed the ability of the DIP molecule to form well-organized films, exhibiting a different molecular orientation with respect to the already known λxa0and σ phases. In locally thicker film regions, the energy of the films is minimized by a molecular arrangement that has an asymptotic tendency to the σ phase.