Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 96: Ultrafast Electron Dynamics at Surfaces and Interfaces

O 96.8: Vortrag

Freitag, 20. März 2015, 12:30–12:45, MA 004

Dynamics of exciton formation and decay in SP6 films on ZnO(10-10) — •Lea Bogner, Laura Foglia, Jan-Christoph Deinert, Clemens Richter, Martin Wolf, and Julia Stähler — Fritz-Haber-Institut, Faradayweg 4-6, 14195 Berlin

Zinc oxide (ZnO) is a promising transparent electrode material for various optoelectronic applications. On the ZnO surface, the spirofluorene derivative SP6 forms smooth amorphous films from which, after resonant excitation, charge transfer to the ZnO conduction band has been observed [1]. We investigate the electronic properties and the excited states dynamics of 20 nm thick SP6 films adsorbed on ZnO(10-10) using time-resolved two-photon photoemission (2PPE). In this system, optical spectroscopy showed that resonant excitation of the SP6 molecules leads to the formation of an excitonic state which relaxes vibrationally (2-6 ps) and decays on a timescale of 200-300 ps [2]. Complementarily, time-resolved 2PPE provides access to absolute binding energies and fast electron dynamics on fs timescales, which we attribute to exciton formation. Photon energy and repetition rate-dependent measurements indicate a strong intermolecular coupling of the excited states leading to electron emission from a long-lived dark state (1 eV above E_F) due to energy transfer rather than photoemission by the probe pulse. Possible energy transfer mechanisms such as intermolecular Coulombic decay or Förster resonance energy transfer will be discussed.

[1] S. Blumstengel et al., Phys. Rev. B 77, 085323, 2008

[2] L. Foglia et al., to be published