Keiichirou Yonezawa

Quantitatively identical orientation-dependent ionization energy and electron affinity of diindenoperylene

Molecular orientation dependences of the ionization energy (IE) and the electron affinity (EA) of diindenoperylene (DIP) films were studied by using ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The molecular orientation was controlled by preparing the DIP films on graphite and SiO2 substrates. The threshold IE and EA of DIP thin films were determined to be 5.81 and 3.53 eV for the film of flat-lying DIP orientation, respectively, and 5.38 and 3.13 eV for the film of standing DIP orientation, respectively. The result indicates that the IE and EA for the flat-lying film are larger by 0.4 eV and the frontier orbital states shift away from the vacuum level compared to the standing film. This rigid energy shift is ascribed to a surface-electrostatic potential produced by the intramolecular polar bond (>C−-H+) for standing orientation and π-electron tailing to vacuum for flat-lying orientation.

Post-growth surface smoothing of thin films of diindenoperylene

We applied in situ x-ray reflectivity and ultraviolet photoelectron spectroscopy to study the impact of annealing on low temperature (200 K) deposited organic thin films of diindenoperylene (DIP) on SiO2 and indium tin oxide (ITO). At 200 K, DIP is crystalline on SiO2 and amorphous on ITO. Upon heating to room temperature, the roughness of DIP is reduced on both substrates, from 1.5 nm to 0.75 nm (SiO2) and from 0.90 nm to 0.45 nm (ITO). The smoothing is accompanied by crystallization of the surface molecules, whereas the bulk structure of the films does not strongly reorganize.

Hole-phonon coupling effect on the band dispersion of organic molecular semiconductors

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (> 1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.The charge transport properties in organic semiconductors are affected by the impact of molecular vibrations, yet it has been challenging to quantify them to date. Here, Bussolotti et al. provide direct experimental evidence on the band dispersion modified by molecular vibrations in a rubrene single crystal.

Charge transfer states appear in the π-conjugated pure hydrocarbon molecule on Cu(111)

We report on the results of experimental and theoretical studies on the electronic structure of gas-phase diindenoperylene (DIP) and DIP-monolayer (ML) on Cu(111). Vapor-phase ultraviolet photoelectron spectroscopy (UPS) was realized for 11.3 mg of DIP, giving reference orbital energies of isolated DIP, and UPS and inverse photoemission spectroscopy of DIP-ML/graphite were performed to obtain DIP-ML electronic states at a weak interfacial interaction. Furthermore, first-principles calculation clearly demonstrates the interfacial rearrangement. These results provide evidence that the rearrangement of orbital energies, which is realized in HOMO–LUMO and HOMO–HOMO−1 gaps, brings partially occupied LUMO through the surface-induced aromatic stabilization of DIP, a pure hydrocarbon molecule, on Cu(111).

Structure Matters: Combining X-Ray Scattering and Ultraviolet Photoelectron Spectroscopy for Studying Organic Thin Films

We discuss the relationship between organic film structure and ultraviolet photoelectron spectroscopy (UPS) data. As a useful method for obtaining detailed structural data we first introduce shortly the advantages of X-ray scattering. By combining such structural data and electronic information from UPS new insights in the fundamental principles of organic electronics can be obtained. On the basis of single layer and heterostructures we discuss the dependence of the electronic level alignment and the spectral shape of the HOMO band on the structural properties of organic thin films. Interestingly the intrinsic molecular shape of a compound has a large impact on its electronic response to changes in crystal quality.