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The interaction of radiation with matter at the nanoscale has an inexhaustible range of applicationsin electronics, biotechnology and medicine. At the nanoscale, the length scale wherethe classical and quantum worlds meet, quantum effects dominate the light¿matter interactionand unique phenomena arise. This work addresses fundamental questions on the overlap ofquantum theory, non-equilibrium thermodynamics and material science.As the exact description of these quantum phenomena is not feasible, we discuss how the openquantum system approach can be used to study thermal relaxation and thermo-electric transportat the nanoscale. The basic concepts of thermal relaxation are studied from first principles. Asthe conditions for relaxation are connected with the non-Markovian nature of the equation ofmotion, we discuss a time-local stochastic Schr¿odinger equation. Remarkably, this equationcan describe thermal relaxation and transport dynamics correctly. Furthermore, this thesis introducesa thermal transport theory where the temperature field is established by radiation ofclassical blackbodies. The combination of this theory with the techniques of time-dependentcurrent density functional theory provides an ab initio tool to study thermal transport in manybodysystems. This approach is general and can be adapted to describe both electron andphonon dynamics. In this way, combined with the time-dependent current DFT, it provides aunified way to investigate ab initio electrical and thermal transport beyond linear response. Theobservation of thermo-electric transport in macroscopic bodies does not disturb the system orchange the flow of energy. However, when moving towards the nanoscale, measurements mayinfluence the system and has to be considered. We demonstrate that the choice of location ofthese local measurements provides control of the direction of the energy flow and of the particlecurrents separately. These results seem to violate the second law of thermodynamics. Bytreating decoherence as a thermodynamic bath we resolve this contradiction. In order to furtheradvance the applications of light¿matter interactions for realisable materials, the electronic andoptical properties of 2D layered semiconductors are studied. 2D materials have establishedtheir place as candidates for the next generation of opto-electronic devices. Specifically, theelectronic and optical properties of TiS3 and In2Se3 are theoretically investigated within DFTand many-body perturbation theory. This work constitutes a first step towards exploiting thetrichalcogenide family in 2D opto-electronical applications, such as chemical sensors, passiveoptical polarisers, fast photodetectors, and battery technologies.
