Contrasting Pentacene on Cu(110) and Ag(110): interactions revealed by valence band tomography

Transcription

1 Contrasting Pentacene on Cu(110) and Ag(110): interactions revealed by valence band tomography Ules Thomas Surface Science Group, KFUni Graz

2 Outline Angle Resolved UPS; Orbital Tomography Influence of the substrate on the orbital structure? - Pentacene on Ag(110) vs. on Cu(110) similar but different

3 Method Kinetic Energy Binding Energy UV Photoemission Spectroscopy (UPS) q = 0 q = 30 UPS - Spectrum E vac Secondary electrons hn. q = 40 h n E F F N(E) specular geometry Density of States only parallel momentum k II conserved Growth of ordered molecular layers is essential for the determination of the band structure.

4 Method ARUPS; Tomography The Toroidal Analyzer collects Photoelectrons with takeoff angles of ±80 with one shot! Toroidal Electron Spectrometer for Angle Resolved Photoelectron Spectroscopy with Synchrotron Radiation at BESSY II

5 Method Angle Resolved UV Photoemission Spectroscopy (ARUPS) 4dimensional dataset

6 Method Angle Resolved UV Photoemission Spectroscopy (ARUPS) Spectrum: ϴ = 0 = NE 4dimensional dataset

7 Method Angle Resolved UV Photoemission Spectroscopy (ARUPS) Spectrum: q = 0 ϴ = 0 = NE 4dimensional dataset Bandmap:

8 Method Angle Resolved UV Photoemission Spectroscopy (ARUPS) Spectrum: ϴ = 0 = NE 4dimensional dataset Bandmap: Momentum map:

9 The system: Substrates Cu(110) Ag(110) 3.6 Å Å [001] [110] 2.55 A [001] [110] A Γ Ag X Ag

10 The system: Molecule Pentacene (5A) 2.44 A 6,36 Å apex band Å linking band Calculated Orbitals

11 Pentacene (5A) + Photoemission LUMO HOMO HOMO-1 Fermis Golden rule formula Assumption: Final state is a plane wave Fourier Transform of initial state Berkebile S. Doctoral thesis, 2009 DFT of molecular orbitals Fourier Transform of DFT orbitals

12 From momentum space to real space momentum map add phase info 1 D F.T. in z direction Extrapolation of 2dim. in z Exp. back F.T. HOMO DFT orbital Plane wave approximation for the final state is good!

13 5A on the two substrates 5A monolayer on Ag(110) 5A monolayer on Cu(110) 0,0 0,0 M [1-10] M [1-10] [001] [110] [001] [110]

14 HOMO-2 Binding Energy w.r.t. E F [ev] HOMO-1 HOMO Binding Energy w.r.t. E F [ev] LUMO 5A on Ag(110) EDC [001] Γ Experiment E B = 0.1 ev Theory E B = 1.2 ev EDC 45 to [001] Γ E B = 2.4eV [001] k [Å -1 ] E B = 3.1eV

15 LUMO HOMO Momentum maps of 5A on Ag(110) Theory Experiment Isolated molecule Monolayer structure + substrate 5A monolayer Substrate E B = 0.1eV E B = 0.1eV E B = 1.2eV E B = 1.2eV

16 HOMO-1 HOMO-2 Momentum maps of 5A on Ag(110) Theory Experiment Isolated molecule Monolayer structure + substrate 5A monolayer E B = 2.4eV E B = 2.4eV Substrate E B = 3.1eV E B = 3.1eV

17 5A on Ag(110) Momentum maps compare well to the theoretical maps of the isolated molecule

18 Binding Energy w.r.t. E F [ev] Binding Energy w.r.t. E F [ev] 5A on Cu(110) LUMO HOMO Experiment Theory EDC [1-10] E B = 1eV EDC 45 to [1-10] E B = 1.5eV

19 Closer look at the LUMO LUMO, ev LUMO, - 1 ev Compare to DFT Theory Experiment Isolated molecule Monolayer structure + substrate 5A monolayer Substrate

20 What is the origin of the structured LUMO? Intermolecular Dispersion? X 5A EDC [1-10] Γ 5A Γ 5A Γ X 5A Γ 5A Dispersion of the LUMO calulated for the 5A Cu(110) unit cell without the substrate sp band k=0, Γ point, no node => high binding E k = π/a, X 5A point, node => low binding E Theory: Dispersion without the substrate only ~10meV! Experiment: ~200meV! a

21 Summary Simple FT of orbitals explains ARUPS very well Weakly bound: 5A on Ag(110) orbitals are ~ as isolated molecule Strongly bound: 5A on Cu(110) - modification in energy and momentum - intermolecular dispersion mediated through substrate interaction

22 Acknowledgements Eva Reinisch Michael Ramsey Georg Koller Margareta Wagner Stephen Berkebile Daniel Lüftner Peter Puschnigg BESSY II Markus Ostler Thomas Seyller Thank you for your attention!!