Florian von Wrochem

The molecular organization and electrostatic properties of dithiocarbamate-anchored self-assembled monolayers on Au(111) are studied by spectroscopic experiments and theoretical simulations.

Many molecules used to fabricate organic semiconductor devices carry an intrinsic dipole moment. Anisotropic orientation of such molecules in amorphous organic thin films during the deposition process can lead to the spontaneous buildup of an electrostatic potential perpendicular to the film. This so-called giant surface potential (GSP) effect can be exploited in organic electronics applications, and was extensively studied in experiment. However, presently an understanding of its molecular origin is lacking. Here we model the physical vapor deposition process of seven small organic molecules employed in OLED applications with atomistic simulations. We are able to reproduce experimental results for a wide range of strength of the GSP effect. We find that the electrostatic interaction between the dipole moments of the molecules limits the GSP strength and identify short range van der Waals interactions between the molecule and the surface during deposition as the driving force behind the anisotropic orientation. We furthermore show how the GSP effect influences the energy levels responsible for charge transport, which is important for the design of organic semiconductors and devices.

Nanotribological studies reveal the high mechanical stability of thin films based on Fe ^II -terpyridine oligomers, which according to theory is attributed to the high cohesive energy resulting from Coulomb interactions among the ionic constituents.

Electron transfer across proteins plays an important role in many biological processes, including those relevant for the conversion of solar photons to chemical energy. Previous studies demonstrated the generation of photocurrents upon light irradiation in a number of photoactive proteins, such as photosystem I or bacteriorhodopsin. Here, it is shown that Sn‐cytochrome c layers act as reversible and efficient photoelectrochemical switches upon integration into large‐area solid‐state junctions. Photocurrents are observed both in the Soret band (λ = 405 nm) and in the Q band (λ = 535 nm), with current on/off ratios reaching values of up to 25. The underlying modulation in charge‐transfer rate is attributed to a hole‐transport channel created by the photoexcitation of the Sn‐porphyrin.

Ultrathin molecular layers of Fe^II-terpyridine oligomers allow the fabrication of large-area crossbar junctions by conventional electrode vapor deposition. The junctions are electrically stable for over 2.5 years and operate over a wide range of temperatures (150–360 K) and voltages (±3 V) due to the high cohesive energy and packing density of the oligomer layer. Electrical measurements reveal ideal Richardson–Shottky emission in surprising agreement with electrochemical, optical, and photoemission data.

Treatment of a gold surface with a solution of C₁₈H₃₇HgOTs under ambient conditions results in the formation of a covalently adsorbed monolayer containing alkyl chains attached directly to gold, Hg(0) atoms, and no tosyl groups. It is stable against a variety of chemical agents. When the initial deposition is performed at a positive applied potential and is followed by oxidative electrochemical stripping, the mercury can be completely removed, leaving a gold surface covered only with alkyl chains. The details of the attachment structure are not known. The conclusions are based on infrared spectroscopy, X-ray and UV photoelectron spectroscopy, ellipsometry, contact angle goniometry, differential pulse polarography, and measurements of electrode blocking and electrochemical admittance.