Orientation of pentacene molecules on SiO2:From a monolayer to the bulk

摘要

Near edge x-ray absorption fine structure(NEXAFS)spectroscopy is used to study the orientation of pentacene molecules within thin films on SiO2 for thicknesses ranging from monolayers to the bulk(150 nm).The spectra exhibit a strong polarization dependence of the pi* orbitals for all films,which indicates that the pentacene molecules are highly oriented.At all film thicknesses the orientation varies with the rate at which pentacene molecules are deposited,with faster rates favoring a thin film phase with different tilt angles and slower rates leading to a more bulklike orientation.Our NEXAFS results extend previous structural observations to the monolayer regime and to lower deposition rates.The NEXAFS results match crystallographic data if a finite distribution of the molecular orientations is included.Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent pi* orbitals,indicating a breakup of the pentacene molecule.