Sebastian Thiess

In-operando and non-destructive analysis of the resistive switching in the Ti/HfO2/TiN-based system by hard x-ray photoelectron spectroscopy

Resistive switching in Ti/HfO2/TiN was investigated in-operando by hard x-ray photoelectron spectroscopy. In comparison with the virgin-state, ON- and OFF-states show enhanced Ti/TiOx/HfO2 interface oxidation, resulting from an oxygen-gettering activity of Ti. The formed TiOx layer acts in the resistive switching process as an oxygen reservoir in exchange with the non-stoichiometric HfO2−δ. A Ti1+/Ti3+ valence change redox reaction occurs between OFF- and ON-states. The peak shifts are attributed to space charge potentials created by the varying oxygen vacancy concentration at the interface. A push-pull model of oxygen vacancies as a function of voltage polarity is proposed to describe the mechanism.

Study of the Interface between Rhodium and Carbon Nanotubes

X-ray photoelectron spectroscopy at 3.5 keV photon energy, in combination with high-resolution transmission electron microscopy, is used to follow the formation of the interface between rhodium and carbon nanotubes. Rh nucleates at defect sites, whether initially present or induced by oxygen-plasma treatment. More uniform Rh cluster dispersion is observed on plasma-treated CNTs. Experimental results are compared to DFT calculations of small Rh clusters on pristine and defective graphene. While Rh interacts as strongly with the carbon as Ti, it is less sensitive to the presence of oxygen, suggesting it as a good candidate for nanotube contacts.

Photoelectron Emission Excited by a Hard X-ray Standing Wave

The following chapter describes the basics and some applications for the case that photoelectrons are emitted from a hard X-ray interference field instead of by a travelling wave. The dipole approximation holds astonishingly well even for hard X-rays as far as the magnitude of the transition matrix element is concerned. However, the forward-backward asymmetry caused by higher order multipole terms needs to be considered when the photoelectron is emitted by the coherent action of two X-ray waves travelling in different directions. This has implications on the chosen experimental set-up which will be briefly discussed together with other experimental aspects. Finally, some examples of X-ray standing wave analysis using hard X-ray photoelectron spectroscopy will be presented yielding the geometric and electronic structure of (crystalline) materials with pm resolution.