Coverage dependent organic–metal interaction studied by high-resolution core level spectroscopy: SnPc (sub)monolayers on Ag(1 1 1)

https://doi.org/10.1016/j.elspec.2009.03.023Get rights and content

Abstract

We study the electronic structure of tin-phthalocyanine (SnPc) molecules adsorbed on a Ag(1 1 1) surface by high-resolution photoelectron spectroscopy. We particularly address the effect of different SnPc coverages on the interaction and charge transfer at the interface. The results give evidence for a covalent molecule–substrate interaction, which is temperature and coverage dependent. The valence and core level spectra as well as the work function measurements allow us monitoring subtle differences in the strength of the interface interaction, thus demonstrating the sensitivity of the methods. The results consistently show the effect of charge exchange between substrate and molecules which obviously leads to a net charge transfer into the SnPc molecules, and which is increased with decreasing coverage. Surprisingly, the Sn3d core levels are neither effected by variations of charge transfer and interaction strength, nor by a possible “Sn-up” or “Sn-down” orientation, which have been observed for sub-monolayers.

Introduction

The interaction at organic–metal interfaces is crucial for electronic devices, since it influences the electronic structure [1], [2], [3] and the morphology in the vicinity of the interface [4], [5], [6], and consequently also the charge transport through metal–organic contacts. High-resolution photoelectron spectroscopy (PES) can provide valuable information about the electronic structure at the interface. In general, the interface interaction and, in particular, the interplay between the geometric and the electronic structure [7], [8] is very complex. However, model systems, which allow manipulating the characteristics of the interface, can provide access to systematic data and thus allow developing a comprehensive understanding of these aspects. In this context, ultra-thin films of tin-phthalocyanine (SnPc) molecules adsorbed on Ag(1 1 1) are a suitable model system. Recent studies of the geometric structure of SnPc/Ag(1 1 1) employing spot profile analysis low energy electron diffraction (SPALEED) and the x-ray standing wave (XSW) method demonstrated the existence of various adsorption phases depending on temperature and coverage. Moreover, in some phases continuous variations of the adsorbate superstructure can be prepared [9], [10].

In the following we study the electronic structure of SnPc (sub-)monolayers on Ag(1 1 1) as a function of coverage and temperature. The valence photoelectron data, which provide information about the delocalized frontier orbitals are complemented by core level photoemission spectroscopy, which allows observing the influence of the interaction induced charge redistribution within the molecule. Moreover, work function measurements give additional insight into the charge distribution on the surface. Consequently, this extensive study allows a general statement about the electronic structure and the interaction at the SnPc/Ag(1 1 1) interface.

Section snippets

Experimental

High-resolution core and valence level PES data were recorded at BESSY II at the UE52-PGM undulator beamline (EE > 14,000 at 400 eV photon energy, with cff = 10 and 20 μm exit slit). For the PES experiments the setup was adjusted to normal emission geometry with 60° angle of incidence with respect to the surface normal and p-polarized light. The SCIENTA R4000 electron analyzer was operated with a constant pass energy of 50 eV and 300 μm entrance slit, resulting in an energy resolution of ΔE = 35 meV.

Effect of adsorption on the valence states

Fig. 1 presents the PES spectra of the highest occupied valence levels of a 10 ML and a 1.00 ML sample of SnPc/Ag(1 1 1) recorded at room temperature. The 10 ML spectrum contains a double peak located at EB = 1.40 eV and 1.71 eV, respectively. With respect to previous studies of various phthalocyanines the double peak signature corresponds to the highest occupied molecular orbital (HOMO), a π (ring) orbital, which is distributed over the porphyrine-like ring [12], [13]. The double peak shape can be

C1s spectra

Due to their localized nature core levels can provide site specific information on the electronic structure and on the local influence of the molecule–substrate interaction. Fig. 2 compares the C1s spectra recorded for a 10 ML film of SnPc/Ag(1 1 1) (a), to the data of the (sub-)ML samples at room temperature (b) and at 130 K (c). In order to allow a better comparison of the line shape, all spectra were normalized to the maximum of the main peak after background subtraction. The 10 ML spectrum (a)

Workfunction

The surface potential barrier or the work function Φ, respectively, is very sensitive to the distribution of charge at the surface. In a simple picture, the work function results from an electronic charge cloud which is protruding out of the surface. The work function is thus sensitive to the crystallographic orientation and the morphology of the surface. A rough morphology, e.g. reduces the work function (Smoluchowski-effect [21]). Besides this morphological effect, adsorbates can also

Summary

We studied systematically the electronic structure and the interaction at the SnPc/Ag(1 1 1) interface as a function of coverage and temperature. High-resolution PES gives access to the valence band, core levels and the work function. From the experimental data a consistent picture of the electronic structure and the interaction at the interface can be developed.

For the SnPc monolayer we observe a metallic state at the Fermi level. It can be associated to the former LUMO, which is shifted below

Acknowledgements

We thank the BESSY staff and D. Batchelor for support during the beamtime as well as I. Kröger, C. Stadler, C. Kumpf, S. Kera and N. Ueno for stimulating discussions. Financial support by the EU and by the DFG GRK 1221 is acknowledged. One of us (E.U.) thanks the Fond der Chemischen Industrie for funding.

References (21)

  • K. Seki et al.

    Thin Solid Films

    (2001)
  • M. Fahlman et al.

    Synthetic Metals

    (1996)
  • A. Schöll et al.

    Journal of Electron Spectroscopy and Related Phenomena

    (2003)
  • S. Kera et al.

    Thin Solid Films

    (1998)
  • S. Kera et al.

    Journal of Electron Spectroscopy and Related Phenomena

    (2007)
  • Y. Zou et al.

    Surface Science

    (2006)
  • A. Bendounan et al.

    Surface Science

    (2007)
  • G. Dufour et al.

    Surface Science

    (1994)
  • J.C. Fuggle et al.

    Solid State Communications

    (1978)
  • Y. Hirose et al.

    Physical Review B

    (1996)
There are more references available in the full text version of this article.

Cited by (45)

  • Thin Films of Organic Molecules: Interfaces and Epitaxial Growth

    2018, Molecular Beam Epitaxy: from Research to Mass Production
  • Heteromolecular metal-organic interfaces: Electronic and structural fingerprints of chemical bonding

    2015, Journal of Electron Spectroscopy and Related Phenomena
    Citation Excerpt :

    The model for the line-shape analysis we have used is shown in red and blue color coding for PTCDA and CuPc carbon species, respectively. It is based on own high-resolution and high-statistics XPS measurements and in good agreement with models from literature [3,64–67]. The best fits are shown as green lines.2

  • Surface chemistry of porphyrins and phthalocyanines

    2015, Surface Science Reports
    Citation Excerpt :

    Sn complexes. The considerable experimental [196–199,201,361,362,520] and theoretical work [199,200,362,608,616] devoted to the SnPc/Ag(111) system has revealed a rather complex bonding situation with coverage- and temperature dependent adsorbate–substrate and intermolecular interactions. Related changes in the ligand-to-substrate interaction were revealed by measurement of the adsorption height using NIXSW (cf. Section 4.1.4) [197]: When the coverage was increased from 0.8 to 1 ML, the ligand-Ag(111) distance for SnPc↓ increased by 0.17 Å, while the Sn–Ag(111) distance remained nearly constant.

  • Optical and electronic interaction at metal-organic and organic-organic interfaces of ultra-thin layers of PTCDA and SnPc on noble metal surfaces

    2013, Organic Electronics
    Citation Excerpt :

    In fact, the considered molecules are already known for a strong electronic interaction with the Ag(1 1 1) surface according to a metal–organic hybridization of wavefunctions. This effect is often denoted as chemisorption in the literature and has been widely studied by photoelectron spectroscopy (PES) and electron energy loss spectroscopy (EELS) [26–31]. Accordingly, the electronic and consequently the optical fingerprints of PTCDA or SnPc in contact with Ag(1 1 1) are strongly altered compared to the respective free molecules due to the partial filling of their former LUMO states.

View all citing articles on Scopus
View full text