Electronic structure of dipeptides in the gas-phase and as an adsorbed monolayer.

Peptide-based molecular electronic devices are promising due to the large diversity and unique electronic properties of biomolecules. These electronic properties can change considerably with peptide structure, allowing diverse design possibilities. In this work, we explore the effect of the side-chain of the peptide on its electronic properties, by using both experimental and computational tools to detect the electronic energy levels of two model peptides. The peptides include 2Ala and 2Trp as well as their 3-mercaptopropionic acid linker which is used to form monolayers on an Au surface. Specifically, we compare experimental ultraviolet photoemission spectroscopy measurements with density functional theory based computational results. By analyzing differences in frontier energy levels and molecular orbitals between peptides in gas-phase and in a monolayer on gold, we find that the electronic properties of the peptide side-chain are maintained during binding of the peptide to the gold substrate. This indicates that the energy barrier for the peptide electron transport can be tuned by the amino acid compositions, which suggests a route for structural design of peptide-based electronic devices.

High-resolution core-level photoemission measurements on the pentacene single crystal surface assisted by photoconduction.

Upon charge carrier transport behaviors of high-mobility organic field effect transistors of pentacene single crystal, effects of ambient gases and resultant probable 'impurities' at the crystal surface have been controversial. Definite knowledge on the surface stoichiometry and chemical composites is indispensable to solve this question. In the present study, high-resolution x-ray photoelectron spectroscopy (XPS) measurements on the pentacene single crystal samples successfully demonstrated a presence of a few atomic-percent of (photo-)oxidized species at the first molecular layer of the crystal surface through accurate analyses of the excitation energy (i.e. probing depth) dependence of the C1s peak profiles. Particular methodologies to conduct XPS on organic single crystal samples, without any charging nor damage of the sample in spite of its electric insulating character and fragility against x-ray irradiation, is also described in detail.