5703
2008, 112, 5703-5706 Published on Web 03/22/2008
Charge Transfer and Polarization Screening at Organic/Metal Interfaces: Distinguishing between the First Layer and Thin Films H. Peisert,*,† A. Petershans,†,‡ and T. Chasse´ † Institute of Physical and Theoretical Chemistry, UniVersity of Tu¨bingen, Auf der Morgenstelle 8, 72076 Tu¨bingen, Germany, and Institute of Physical and Theoretical Chemistry, UniVersity of Tu¨bingen, Auf der Morgenstelle 8, 72076 Tu¨bingen, Germany, and Institute for Technical Chemistry, Water- and Geotechnology DiVision, Forschungszentrum Karlsruhe GmbH, D-76021 Karlsruhe, Germany ReceiVed: January 23, 2008; In Final Form: February 25, 2008
We studied the core hole screening at organic/metal interfaces using combined photoemission spectroscopy (PES) and X-ray excited Auger electron spectroscopy (XAES) as a function of the organic layer thickness. As a model system for organic semiconductor/metal interfaces, magnesium phthalocyanine was evaporated onto gold foil. It was found that the screening of double hole final states is remarkably increased for molecules directly at the interface, whereas further layers are effected very weakly. The screening mechanism is discussed in terms of both charge transfer and electronic polarization.
In organic semiconductors, the electronic polarization of the dielectric medium by charge carriers is a fundamental property of their electronic structure since the energy scale is usually greater than transfer integrals or thermal energy.1 As a result, the charged electronic states are highly localized, and charge transport occurs largely by site-to-site hopping at room temperature.2 The transport gap Et, that is, the energy difference between the transport levels for holes and electrons, has a substantial polarization energy contribution.3 The polarization affects therefore the charge carrier transport in organic materials significantly. In addition, at surfaces and interfaces, the electronic polarization is different compared to that for the bulk organic material,1,4,5 which may affect the charge carrier injection.6,7 However different data for both the size and the spatial extent of polarization effects were reported so far. In order to study electronic polarization effects at organic/ metal interfaces, we focus in this work on the screening mechanisms of the core hole at these interfaces. As a model system, we choose magnesium phthalocyanine (MgPc) on gold, a system where no chemical interaction occurs. The interfaces were studied using core-level X-ray photoemission spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and valence band ultraviolet photoemission spectroscopy (UPS). The measurements were performed using a multichamber UHV system (base pressure of 2 × 10-10 mbar), equipped with a Phoibos 100 cylindrical hemispherical analyzer (SPECS), a monochromatic Al KR source, and a He discharge lamp. Organic films were evaporated in a stepwise manner onto a sputtercleaned gold foil. The pressure during the evaporation of the PCs was less than 5 × 10-8 mbar, and the evaporation rate was about 0.01 nm/s. The film thickness was controlled by a quartz microbalance calibrated using the attenuation of XPS * To whom correspondence should be addressed. † University of Tu ¨ bingen. ‡ Forschungszentrum Karlsruhe GmbH.
10.1021/jp800674z CCC: $40.75
intensities during the initial deposition steps. Due to careful comparison of single scans, possible radiation damages could be excluded; several series of spectra with different radiation exposure times were taken. In photoemission spectroscopy, the removal of an electron from orbital i above the vacuum level affects the whole electron system; the remaining electrons of the environment screen the photohole. This can be discussed within the initial state - final state framework. The screening is described by the dynamical or one-hole relaxation energy RD, and the corresponding chemical shift of the binding energy EB can be expressed according to ∆EB(XPS) ) ∆V - ∆RD, where V reflects the potential of the initial state charge contribution. Due to the different final states in XPS (one hole) and XAES (two holes), such final state (FS) effects affect, in particular, shifts in Auger spectra, and thus, the comparison of energetic shifts in XPS and XAES enables the estimation of the electronic relaxation energy (screening ability). Within the Auger parameter approach, the XAES shift may be expressed by (∆EB(XAES) = ∆V 3∆RD) on the EB scale,8 and thus, ∆RD may be semiquantitatively derived via the modified Auger parameter ∆R′ ) ∆EB(XPS) - ∆EB(XAES). The Auger parameter shift contains contributions from intra-atomic screening, relaxation by charge transfer, and environment screening.9 It can be shown that ∆R′ correlates well with the change of the electronic polarization energy for the core hole (∆R′ = 2∆RD).10-12 Presuming similar intramolecular screening and excluding extramolecular charge transfer within the time scale of the photoemission, ∆RD can be correlated with the change of the polarization energy induced by the redistribution of environmental charges. Although RD is larger for core holes compared to valence holes, changes of these terms are comparable in a good approximation and correlate with frequently discussed changes of the electronic (valence) polarization energy (∆EP or ∆P+). We stress that in the model applied, we assume that the on-site Coulomb interaction U is © 2008 American Chemical Society
5704 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Figure 1. MgPc/Au: Comparison of Mg 1s core-level photoemission spectra (a) to Mg KLL Auger spectra (b) of incrementally deposited MgPc on polycrystalline Au. All spectra were normalized to the same peak height. The appearance of a well-resolved additional feature at the earliest stages of deposition only in the Auger spectra points to a high screening at the interface.
of the same magnitude for several film thicknesses, although changes of U were observed in some cases13 (see below). It was shown that final state effects (denoted in the following as screening) do not only affect the measured absolute core level EB in solids but especially the measured EB at interfaces observed, for example, in Xe multilayer systems or at metal/ organic interfaces.14-16 The extra-atomic relaxation energy RDea is determined in macroscopic dielectric models by the polarization charge (1 - 1/)e, where is the optical dielectric constant of the environment. For ultrathin films, the substrate is responsible for an additional image charge screening essentially determined by the optical dielectric constants. The screening is indirectly proportional to the distance from the substrate image plane.10,15,16 The formation of organic/metal interfaces is often accompanied by the formation of interface dipoles and strong energetic shifts within the first nanometers of the organic film (typically about 2 nm).16-25 Beside polarization effects, further contributions have been proposed in the literature to explain the size and nature of dipoles between metals and nonpolar organic molecules such as a reduction of the metal work function at the interface (pillow effect);17-19 a partial charge transfer may occur at the interface. In this context, the low density of states in bulk organic materials, which do not allow a short-range band bending as known from inorganic semiconductors, is discussed intensively.16,19,20 However, a band-bending-like mechanism can be proposed considering the fact that the energy levels in organic molecular semiconductors are exponentially or Gaussian-like distributed.21 Furthermore, the existence of additional states, induced by the metal into the organic semiconductor (induced density of interface states (IDIS) model), describe well the level alignment at a metal/organic interfaces. A unified model combines even the IDIS model and the pillow effect.22 Furthermore, a change in the molecular orientation might be an important origin of band-bending-like shifts in molecular electronic states.25 In order to study polarization effects at interfaces, we analyze the contribution of screening to the measured BE as a function of the coverage. In Figure 1, we compare Mg 1s photoemission core level spectra and Mg KLL(1D) Auger spectra of incrementally deposited films of MgPc on polycrystalline gold.8 In both cases, a strong shift of spectral features to higher EB with increasing film thickness can be observed, as expected for
Letters
Figure 2. Modified Auger parameter ∆R′ ) ∆EB(XPS) - ∆EB(XAES) for the interface and the bulk signal. The average difference ∆R′ for the two components of about 1.8 eV could be understood only partly by polarization screening; significant contributions by chargetransfer screening have to be considered.
ultrathin films
