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Regensburg 2000 – scientific programme

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O: Oberflächenphysik

O 11: Postersession (Eröffnung)

O 11.40: Poster

Monday, March 27, 2000, 19:00–22:00, Bereich C

Photoelectron Spectroscopy of Conjugated Organic Materials: Surface Charging and Its Removal — •D. Pop1, R. Weber1, N. Böwering1, B. Winter1, N. Koch2, I. V. Hertel1, and G. Leising21Max-Born-Institut, Max-Born-Str. 2A, D-12489 Berlin — 2Institut für Festkörperphysik, TU Graz, Petersgasse 16, A-8010 Graz, Austria

Ultraviolet photoelectron spectroscopy (UPS) on conjugated

organic semiconductors is a key experimental approach to obtain

insight into basic physical properties of such materials, as well

as interfaces of these with other organic and inorganic materials.

In such UPS studies the binding energy of electrons (EB) relativ

to the Fermi-energy EF is very sensitive to impurities,

radiation-induced sample degradation or surface charging.

Especially the latter two mechanisms are expected to become

important when very intense light sources are used, as in our

case the U125 undulator at BESSY II. We have prepared thin films

of para-sexiphenyl (6P) in situ and have studied the dependence

of the 6P spectral features (lineshape and EB) on the

concentration of the radiation-induced defect centers in the

organic material. One striking observation was an increasing

surface charging for increasing damage of the 6P film. By

shining a laser of appropriate energy (3.1 eV, corresponding to

the energy-gap of 6P) onto the sample surface, it was possible

to reduce the surface charging, as judged from the shift of EB

relative to EF.

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