In order to determine the energy level alignment of the interface between cobalt and

Transcription

1 SUPPLEMENTARY INFORMATION Energy level alignment of the CuPc/Co interface In order to determine the energy level alignment of the interface between cobalt and CuPc, we have performed one-photon photoemission (1PPE) experiments. The excitation source was the s-polarized 4th harmonic (photon energy 5.9 ev) of a 100fs Ti:Sapphire oscillator (Spectra Physics Tsunami). The laser light angle of incidence is set by geometry to 45, while the spectra are taken in normal electron emission. Photoemission spectra are recorded by a commercial cylindrical sector analyzer (Focus CSA 300) equipped with an additional spin detector based on spinpolarized low-energy electron diffraction (Focus SPLEED). For this study spinresolution was not requested and spin-integrated spectra were calculated by averaging the countrate of the four SPLEED channeltrons (giving respectively the number of majority and minority spin electrons along two perpendicular quantization axes). The achieved energy resolution is 150 mev, the acceptance angle of the analyzer is ±13. Some of the recorded 1PPE spectra are shown exemplarily in Supplementary Figure 1 as a function of the binding energy E B below the Fermi energy E F (negative binding energies have been chosen for occupied states below E F ). The spectra have been recorded for different CuPc coverage ranging from 0 ML to 16 ML. The spectrum of the bare cobalt substrate (0 ML coverage) shows the know behaviour described in [1], with a maximum at E = 0. 4 ev originating from photoemission from a spin up 3d bulk B band with 5 symmetry. By increasing CuPc coverage, two main changes in the spectrum occur: Page 1 of 6

2 1. The cobalt spectral feature at E = 0. 4 ev is rapidly attenuated by CuPc B deposition, while the progressive formation of a broader peak originating from photoemission from the highest occupied molecular orbital (HOMO) of CuPc is observed. The position of the HOMO onset is at 0.7 ev below the common Fermi level (E F ) of the CuPc/Co interface. By assuming a band gap of 1.6 ev, as reported in [2,3,4], the position of the lowest unoccupied molecular orbital (LUMO) of CuPc can be set at 0.9 ev above E F. 2. The low energy cut-off of the spectra shifts to lower binding energies, giving rise to wider spectra. From the width W of the 1PPE spectra, we can calculate the work-function φ according to: φ = W hν = ( W 5.9) ev. The dependency of φ on CuPc coverage is shown in Supplementary Figure 2. The cobalt bare substrates has a work-function of φ = (5.1±0.1) ev. The work-function decreases drastically in the initial stage of the thin-film formation up to CuPc coverage of 1 ML, for which the valueφ = (4.2±0.1) ev is reached. The lowering of the work-function for sub-monolayer CuPc coverage can be understood as a reduction of the surface dipole formed at the clean cobalt surface. This reduction is proportional to the density of the adsorbed molecules at the metal surface [5]. After the cobalt surface is completely covered with CuPc, the work-function should remain approximately constant and approach the bulk value for increasing CuPc thickness. Indeed, for deposition of more than 1 ML CuPc, the work-function has a constant value of φ =(4.1±0.1) ev. The two distinct slopes fitted in the behaviour φ in Supplementary Figure 2 confirm this behaviour. The clear break in the slope occurring near 1 ML can be taken a strong indication for Page 2 of 6

3 the growth of a homogeneous first monolayer of CuPc, covering completely the cobalt substrate. The high quality film growth in the sub-monolayer regime has been confirmed by in-situ scanning tunnelling microscopy (STM) measurements performed on the same system [6]. References [1] Andreyev, O. et al. Spin-resolved two-photon photoemission study of the surface resonance state on Co/Cu(001). Phys. Rev. B 74, (2006). [2] Yoshimura, D., et al. The electronic structure of porphyrin/metal interfaces studied by UV photoemission spectroscopy. Synthetic Metals 86, (1997); [3] Yan, L., Watkins, N.J., Zorba, S., Gao, Y. & Tang C. W. Direct observation of Fermi-level pinning in Cs-doped CuPc film. Appl. Phys. Lett. 79, (2001); [4] Yan, L., Watkins, N.J., Zorba, S., Gao Y. & Tang, C.W. Thermodynamic equilibrium and metal-organic interface dipole. Appl. Phys. Lett. 81, (2002). [5] Ellis, T.,Park, K.T., Hulbert, S.L., Ulrich, M.D. & Rowe, J.E. Influence of substrate temperature on epitaxial copper phthalocyanines studied by photoemission spectroscopy. J. Appl. Phys. 95, (2004). [6] Heimer, K., et al. Morphology and electronic properties of copper phthalocyanine (CuPc) on epitaxial grown Co/Cu(100) substrates studied by photoemission techniques, NEXAFS, Auger and STM. In preparation. Page 3 of 6

4 Supplementary Figure Legends Supplementary Figure 1. Photoemission experiments performed to determine the energy level alignment of the CuPc/Co interface. The spectra are shown as a function of the binding energy and for different CuPc coverage values (from 0 ML to 16 ML). The position of the LUMO onset of CuPc at -0.7 ev is marked in the spectra. Supplementary Figure 2. Dependence of the work-function on CuPc coverage. Page 4 of 6

5 Supplementary Figure 1 Page 5 of 6

6 Supplementary Figure 2 Page 6 of 6