ARTICLE pubs.acs.org/JPCC
Electronic Band and Orbital Properties of Cs-Doped Pentacene Thin Films E. Annese,*,†,‡ J. Fujii,‡ I. Vobornik,‡ and G. Rossi†,‡ † ‡
Dipartimento di Fisica, Universita di Modena e Reggio Emilia, via Campi 213/A, I-41100 Modena, Italy TASC Laboratory, IOM-CNR, SS 14, km 163.5, I-34149 Trieste, Italy ABSTRACT: We studied the electronic band structure of Cs-doped pentacene films grown on Cu(119). The Cs doping of pentacene films grown on Cu(119) determines relevant changes in the band structure. Angle-resolved photoemission spectroscopy reveals a formation of a new electronic state at ∼0.6 0.7 eV below the Fermi energy. This state results from the filling of the lowest unoccupied molecular orbital as also shown by X-ray absorption at C 1s edge. At doping level of ∼1.8 Cs atom per molecule it disperses, forming an electronic band of ∼130 meV width along the direction perpendicular to the molecular plane, while no dispersion is observed in plane. Our results indicate that the anisotropic charge delocalization mediated by Cs atoms along the molecular stacking direction takes place in doped pentacene films.
1. INTRODUCTION Aromatic organic molecules deposited on metallic surfaces are model systems for understanding organic electronics. Among organic materials, pentacene has been widely studied as a prototypical molecular semiconductor with highly anisotropic charge transport and high carrier mobility.1 Pentacene can be grown in well-ordered structures with either flat lying densely packed molecules (up to a critical thickness) or perpendicular dense packing similar to the stable bulk “herringbone” structure. The choice of substrate and the growth kinetics determines the sequence of the pentacene structures at the interface. The ability of organic materials to transport charge can be modified by synthesizing organic inorganic interfaces in which the introduction of electron donor or acceptor dopants directly modifies the molecule molecule and molecule substrates interactions. Molecular electronic bands occur in pentacene films due to the overlap of the π-orbitals of adjacent molecules. These bands were directly observed by angle-resolved photoelectron spectroscopy, for example, in pentacene on Bi(001),2,3 in pentacene on graphite along the direction perpendicular to the molecular plane,4 and in pentacene on Cu(119).5 Theory also predicts band formation, albeit with systematically larger energy dispersion then observed in experiments.6 Doping in organic devices plays a similar role as in inorganic semiconductors. It is usually obtained by incorporating atoms or molecules in the film, which easily release or capture electrons (n- and p-type doping), modifying the electronic structure of the host material. For instance, the position of the molecular orbitals of polyaniline film shifts with respect to the Fermi level (EF) after the doping with both charge donor and acceptor dopant.7 Na doping shifts polyaniline molecular states toward the greater r 2012 American Chemical Society
binding energy of both the occupied and unoccupied molecular orbitals with increasing of electron population in the molecule and a decreasing in the density of states close EF. In the case of CuPc film (grown on Ag, Al substrates) Cs doping induces shift of the EF toward the broaden lowest unoccupied molecular orbital (LUMO), where it aligns.8 Alkali metals act as electron donors and fill the LUMO9 and may induce a superconducting phase in the molecular film similarly to alkali-doped fullerene.10 Recently, superconductivity was observed in K-doped picene, isomer of pentacene. In K-doped picene, the modification of HOMO (highest occupied molecular orbital) and LUMO bands upon K doping was predicted without a rigid-band shift.11 Temperature-dependent conductivity measurements showed a metallic behavior of K-intercalated pentacene film with a K concentration below one atom per molecule and an insulating behavior for higher doping concentration.9 An enhancement of pentacene conductivity (transport measurements) was found by both n- and p-type doping,9,12 14 a fact that is also understood by theoretical analysis.15 The change of electrical conductivity corresponds to a structural change of the molecular film.12 14 Iodine-doped pentacene showed different structural and electronic properties depending on the doping concentration.16 The highest conductivity in iodine-doped pentacene was observed in correspondence with highly oriented film structure, where I atoms form chains between the pentacene layers along the growth direction.9 K-intercalated pentacene crystal was predicted to induce Received: April 17, 2011 Revised: December 18, 2011 Published: January 13, 2012 2382
dx.doi.org/10.1021/jp203572z | J. Phys. Chem. C 2012, 116, 2382–2389
The Journal of Physical Chemistry C a geometrical structural transition as a function of K concentration:15 a layer-by-layer π π stacking arrangement of neighboring pentacene molecules replaces the herringbone structure characteristic of pristine pentacene. The geometrical rearrangement favors the π π overlap along the direction of the long molecular axis and results in larger HOMO and LUMO-derived electronic band dispersion (specifically for LUMO: ∼ 700 meV in doped film vs 500 meV in pristine pentacene). The investigation of the degree of electron delocalization as a function of doping may provide hints on how the material performances depend upon doping. Electron delocalization should manifest itself with observable energy dispersion of the electronic bands. So far limited experimental data on the dispersion of the molecular electronic states as a function of doping are available. We present experimental results on the electronic band structure of Cs-doped pentacene film grown on Cu(119) by means of high energy and momentum resolution ARPES (angleresolved photoemission spectroscopy) with polarized synchrotron radiation. The Cs doping generates a dispersive electron state involving the populated LUMO (hereafter we indicate as former LUMO, fLUMO). The fLUMO dispersion is confined in the molecular stacking direction, while no dispersion is observed for the HOMO derived electron states (fHOMO). Doping appears therefore to greatly affect the molecular states of the adsorbed pentacene.
2. EXPERIMENTAL DETAILS The pentacene film was grown on Cu(119), a vicinal surface of Cu(001) with 11.45 Å wide terraces separated by monatomic steps. The substrate preparation and the film growth procedure are described in ref 5. During the preparation of a single monolayer (ML) film, the substrate was kept at 370 K, while for higher coverages it was kept at room temperature (RT). Cs was sublimated through a SAES-Getters dispenser at pressure
