Public defence: Core-hole Clock Spectroscopy Using Hard X-rays: Exciting States in Condensed Matter

  • Date:
  • Location: Ångströmlaboratoriet, Lägerhyddsvägen 1 Häggsalen
  • Doctoral student: Fredrik Johansson
  • Contact person: Fredrik Johansson
  • Disputation

This thesis is about how electrons move from one place to another, that is charge transfer dynamics. Charge transfer dynamics is an important property governing chemical and physical changes that form the base for many applications such as electronics, optoelectronics and catalysis. The fundamental aspect is how charge transfer manifests in the constituent materials and their interfaces building up these devices. The basic method used is synchrotron radiation based electron spectroscopies.

Using core-hole clock spectroscopy it is possible to study dynamic processes in the femtosecond and attosecond regimes - here we study the if the core-excited electron decays back into the core hole (local decays), or if the core excited electron have been tunneled away from the atomic site before the core-hole decays. Spectroscopically we can discern the two situations since one of the processes is photon energy dependent and one is not. Knowledge of the life-time of the core hole, and measuring the probability of the core-excited system decaying one way or the other makes it possible to calculate a charge transfer time. Using hard X-rays to create excited state with deep core-holes allow us to study high kinetic energy Auger electrons, also deep core-holes tend to be short lived, which gives access to short time-scales.

Bulk crystals of 2D materials have been used as model systems here owing to their well-known properties. Using those it has been demonstrated that the regime of observable times using the mentioned method can be extended with an order of magnitude compared to previous studies. Our results present themselves on time-scales on par with the atomic unit of time. The highly selective nature of resonant X-ray excitations allows the anisotropic unoccupied electronic structure of bulk 2D crystals to be mapped out, here the example of SnS2 is presented. This shows that this is a direct probe of the unoccupied band structure.

With core-hole clock spectroscopy the charge transfer time dependence on relative concentrations of blends between the low band-gap polymer PCPDTBT, with PCBM (functionalized fullerenes). This is a common prototypical system for organic photovoltaics. The charge transfer time decreases with increasing intermixing, up to a point where is starts getting slower, the same trend as the efficiency of solar cell devices made with the same mixing. The method employed here is chemically specific and probes the local surrounding energy landscape at the site of excitation – this is different from other techniques that utilize optical excitations which are non-local in character.

The synthetization of bulk heterostructures and thin films, and the disentanglement of core-ionized states are also investigated using spectroscopic and scattering techniques.