Modelling of angle-resolved X-ray photoelectron spectroscopy (ARXPS) intensity ratios for nanocharacterisation of closely packed shell-core nanofibres

Shell-core nanofibres are structured nanoparticles that are increasingly of technological importance. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) is potentially an excellent technique to characterise surfaces formed by this type of nanoparticles. We present both analytical and Monte Carlo models predicting the ARXPS intensity ratios of a monolayer of shell-core nanofibres on a flat substrate as a function of the photoelectron emission angle, the core size and the shell thickness. In the analytical model, the XPS intensities are calculated by integrating over one whole nanofibre following the photoelectron trajectories towards the detector using a generalized XPS measurement expression. The effects of nanoparticle structure, the influence from neighboring nanoparticles and the dependence of attenuation length on material composition are all accounted for. The results are distributions of XPS intensity from shell and core at various emission angles from which the ARXPS intensity ratios are obtained. In parallel we develop a Monte Carlo simulation code to cross validate it in tractable special cases and to extend its potential application to a wider range of geometry. A few artificial shell-core structured nanofibres of different geometrical and material parameters are used to test the two models. Agreement between them is excellent. Their potential applications are illustrated and discussed using scenarios corresponding to measuring oxidized, passivated, coated or contaminated nanoparticles and to monitoring a process of oxidation or passivation.