Ion beam analysis of electropolymerized porphyrin layers

Abstract

Homogeneous porphyrin layers (5–200 nm) obtained by electropolymerization of zinc porphyrins on conducting glass, In 2O 3:Sn(ITO), were studied with Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD). These organic layers are sensitive to ion beam induced damage. Therefore, in order to optimize the analysis conditions, the influence of MeV ion bombardment on the composition and thickness of the layers was systematically investigated over a wide range of projectile and energy combinations. With RBS, the porphyrin coverage is determined from the yield of the heavy metal constituent of the porphyrin and from the substrate peak shifts. The metal and carbon constituents are not significantly subject to depletion under 2.0 MeV He bombardment. However, when ERD is employed to analyse the light element composition of the layers, degradation of the films is observed. The ion beam alters the layers into a thinner, mainly carbon and zinc containing, film. A double exponential decay of hydrogen with ion dose is found, while carbon degradation can be described with a single exponential. We report values for the degradation cross sections for several types of ion beams. The degradation cross sections of H and C depend almost quadratically on the electronic stopping power of the primary projectile ( σ ∼ ( dE dx ) 1.8 ) and seem not to be influenced by thermal or current density effects. Although the use of heavier particles results in an increased damage of the organic layers, ERD analysis employed with heavy particles in combination with low energies is more accurate, due to the increased recoil cross section of the particles. The results of the analyses indicate that the composition of the organic layers is equal to the composition of the zinc porphyrin molecule, with a slight excess of hydrogen.