Berlin 2008 – scientific programme
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O: Fachverband Oberflächenphysik
O 93: Surface Chemical Reactions
O 93.8: Talk
Friday, February 29, 2008, 12:00–12:15, MA 043
Chemicurrent studies on bimetallic surfaces — •Beate Schindler, Eckart Hasselbrink, and Detlef Diesing — Fachbereich Chemie and Centre for Nanointegration (CeNIDE), Universität Duisburg-Essen, D-45117 Essen, Germany
Chemical reactions on metal surfaces like adsorption and desorption reactions may dissipate their excess energy to substrate and adsorbate vibrations or to electronic degrees of freedom. This means excitation of e-h pairs directly at the surface. If the energy of the excited electrons is larger than the metal work function, the excess energy could be measured by light emission. Electronic surface excitations with smaller energies can be detected as internal currents in thin film tunnel or Schottky devices. We use tantalum–tantalum oxide–top metal tunnel junctions. As top metal 5 nm thick Pt films or 12 nm thick Au films were used. Both enable a ballistic transport of excited electrons (holes) with excess energies up to 1 eV from the surface to the oxide interface. The excited carriers are then detected in the tantalum backelectrode as a tunnel current. We compare the tunnel current traces induced by bunches of hydrogen atoms (5·1015 atoms in 20 sec) on the Au and Pt surfaces. For weak chemisorption systems constant chemicurrents are typical, which scale linearly with the atom flux jH. For strong chemisorption systems like Pt-H exponentially decaying chemicurrent traces (e−jH· t) are typical. Evaporating Au on top of Pt one can monitor the change of the chemicurrent trace from an exponentially decreasing type to a constant current type. This clearly shows that the adsorbate-surface chemistry determines the e-h pair excitation.