The electronic properties of Pentacene thin films: A synchrotron radiation and angle-resolved photoemission study

Angle resolved photoemission spectroscopy (ARPES) was used to study the Pentacene/Bi(111) interface and electronic properties of the resulting crystalline Pentacene (Pn) films. Thickness dependent studies indicate that Pn grows epitaxially on the Bi substrate. Pentacene highest occupied molecular orbital (HOMO) features are identical for sub-monolayer and thick films which suggest a thickness independent film morphology. A weak Pn/Bi interaction was revealed, but the Bi substrate polarizes the Pn molecules which results in a lowering of the HOMO binding energy by 180 meV and 80 meV for the first and second monolayers respectively. The interface dipole (ID) is fully developed over the first ∼18 A of Pn coverage and has a value of ID = 310 meV. The hole injection barrier (HIB) across the interface is phih = 1.03 eV. ARPES reveals the band dispersions of the two HOMO-derived bands. A comparison to first-principles calculations shows a significantly smaller band width indicating that the molecular interactions are weaker than predicted by theory. From thermal broadening the electron-phonon (e-p) coupling parameter of lambda = 0.36 and an effective Einstein energy of oE = 11 meV are determined. The value of oE indicates that dominant contributions to the e-p effects come from intermolecular vibrations. The e-p coupling narrows the HOMO bandwidth by 15% between 75 K and 300 K. The effective hole mass m* at the valence band maximum M¯ is anisotropic and when compared to field effect mobility measurements shows that the band structure plays a noticeable role in the carrier mobility at 300 K.;The evolution of the vacuum level, HIBs, IDs and ionization energies were studied at the interfaces in metal/Pn/metal heterostructures which contain semimetals Bi and Sb. The vacuum levels of the metal overlayers are very nearly equal to those of the metal substrates. The Pn/metal interfaces are abrupt (lambdaeff = 5 A), whereas the metal/Pn interfaces extend over ∼100 A.