Determination of the inelastic mean freepath for SiO2 and the BBL polymer : A study with XPS and HAXPES

Abstract: This study targets methodology development using Photoelectron Spectroscopy (PES) as a tool for investigating material surfaces. When analysing samples with X-ray Photoelectron Spectroscopy (XPS) and Hard X-ray Photoelectron Spectroscopy (HAXPES), it is important to know the probing depth of the samples. The probing depth can show where possible chemical changes may occur in the material. This information together with chemical shifts can be used to help understand why a material acts in a certain way and how to chemically change it to strengthen it, reduce aging, hinder certain interactions and more. By calculating the inelastic mean free path (IMFP) the probing depth is approximated. One hard and two soft materials were analysed during the thesis. The hard material comprises the investigation of the technological relevant oxidation layer on silicon, a layer that is rather well defined and is used here as a model compound to validate the method. The investigation of the soft material has bearing on studies of organic solar cells and consider conductive polymer films poly(benzimidazobenzophenanthroline) (BBL) and its pyridine analogue PyBBL. The hard material silica (SiO2) was analysed with photoelectron spectroscopy using Ga Kα (9258 eV) x-rays, while the soft solar cell polymers were analysed with both Al Kα (1486 eV) and Ga Kα (9258 eV). For each of the materials a few different samples with varied film thicknesses were studied to obtain a conclusive mean value. Atomic force microscopy (AFM) was done on all the samples after the XPS and HAXPES measurements to study the topography and surface roughness to validate the IMFP results. The results show that SiO2 had an IMFP of 17.97 nm, which is within the calculated values from other sources. The silica samples also had a low surface roughness which further confirms the accuracy of the results. The polymer samples show a high disparity of the IMFP from the Al Kα measurements as well as a high surface roughness from the AFM. The values from the Ga Kα measurements were within the expected range but show signs of contamination. Thus, the results for the polymer samples were not reliable