TIME RESOLVED OPTICAL MEASUREMENTS ON DIFFERENT CARBON NANOTUBES ARCHITECTURES

Multi Wall Carbon Nanotubes (MWCNT) and Single Wall Carbon Nanotubes (SWCNT) can be grown in several architectures. A crucial aim is the theoretical and experimental study of the fundamental dynamics of photo-excited charge carriers into these ensembles. In fact the understanding of the charge transfer dynamics and of the exciton interaction is of great importance to improve the efficiency of the Carbon Nanotubes (CNT) based applications, as sensor, bio-medical, energy storage and photovoltaic technologies. In this thesis, performing time resolved optical measurements with several experimental techniques, we analyze the fundamental dynamics in different CNT architectures. Charge transfer mechanisms from semiconductor to metallic are evidenced in unaligned SWCNT, whereas this process is not present in the vertically aligned. The excitonic behavior, revealed in all ensembles, are an interesting novelty in aligned MWCNT. The study of this behavior allows to addressed a long debated question about the graphite-like or SWCNT-like behavior of MWCNT, unveiling that the MWCNT electronic structure under 3 eV presents structured peaks like the Van Hove Singularities in SWCNT. Non-linear excitonic effects are analyzed and, controlling the light polarization direction with respect to the CNT long-axis, we are able to select and unveil, in MWCNT architectures, different optical responses, evidencing linear and non-linear effects. In particular exciton-exciton annihilation and Multiple Exciton Generation (MEG) non-linear processes are discussed. The initial studies on a possible presence of MEG in MWCNT can pave the way to disentangle the complex processes occurring in the photo-excited MWCNT in the violet light region. The possibility to control the effects as a multiple generation of excitons represent crucial challenges in order to improve the photovoltaic performances of MWCNT based devices. This thesis represents the initial step of a wide project whose ultimate goal is to improve the efficiency of photovoltaic devices based on SWCNT or MWCNT. In order to address this aim, heterogeneous systems are considered, in which different CNT architectures are combined with nanoparticles or organic groups. Starting from the basic systems and adding in subsequent stages the different components up to the complete devices, for each step the optical response will be studied.