Doping and defect inhomogeneities of polypyrrole tosylate films as revealed by μ-NEXAFS

Abstract

The photoemission electron microscope (PEEM) based techniques micro-near-edge X-ray absorption fine structure (μ-NEXAFS) and micro-photoelectron spectroscopy (μ-PES) have been recently installed at the BESSY-I HE-TGM beam lines as novel techniques for material analyses. In the PEEM microscope, photoemitted electrons are imaged by electrostatic lenses having a lateral resolution of 40 nm. Images with a field of view between 800 and 20 μm are obtained. NEXAFS enables the simultaneous recording of the near-edge fine structures of the absorption coefficient while varying the incident photon energy. For μ-PES, at a fixed photon energy, core level and valence band spectra can be recorded by an electrostatic energy analyser to fit in the optical axis of the PEEM microscope. Spectra are recorded by analysing all the electrons of the PEEM image or by selecting segments of the image with a continuous iris aperture. The minimum size of the analysed spot is in the order of a few micrometers. Here, we use these techniques to determine the electronic structure of polypyrrole films within a well defined surface area. We found surface structures of such polypyrrole films which are corrugated in the micron range as well as areas that appear almost smooth. In addition, the PEEM microscope gives patterns due to a non-homogeneous surface potential distribution. In such patterns bright irregular stripes appear between dark patches. The stripes have a typical width in the order of several micrometers.