Dynamics of excited electronic states in functional materials

Abstract: Non-equilibrium processes involving excited electron states are very common in nature. This work summarizes some of the theoretical developments available to study them in finite and ex-tended systems. The focus lays in the class of Mixed Quantum-Classical methods that describe electrons as quantum-mechanical particles but approximate ionic motion to behave classically. In particular, Non-Adiabatic Molecular Dynamics and Real Time Density Functional Theory are described and applied to answer questions regarding non-equilibrium dynamics in diverse functional materials. First, the effect of phase boundaries and defects in monolayer MoS2 sam-ples is studied. This material has been suggested as a good candidate to substitute silicon in many applications, such as flexible electronics and solar cells. It is known that defects and dif-ferent polymorphs are present in experimental samples, and therefore it is extremely important to understand how realistic samples perform. We present how the electron-hole recombination times are accelerated in presence of defects, as well as how the structural changes in sam-ples that mix several phases of MoS2 affect their electronic structure. After that, rectangular graphene nanoflakes are explored. As an application to finite systems, we show in rectangu-lar graphene nanoflakes how different magnetic configurations have distinct optical absorption spectra and how this can be used for opto-electronic applications. Furthermore, the high har-monic generation for different magnetic couplings is studied, showing how some harmonics can be suppressed or enhanced depending on the underlying electronic structure. Finally, dif-fuse scattering in SnSe is investigated in an experimental collaboration in order to understand how phonon-phonon interactions affect the scattering dynamics, which may lead to profound insight into its thermoelectric properties.