Magnetic resonance studies of spin systems in semiconductor nanocrystals

This thesis presents an experimental study of systems of spins provided by electronic dopants and by charge trapping defects in semiconductor NCs, by means of magnetic resonance spectroscopy techniques. Here, we have investigated issues that have performance-limiting effects on the properties of semiconductor NCs for their use in technological applications. Namely, we have studied the electronic doping of semiconductor NCs. Doping is critical to control the behavior of semiconductors, which would otherwise be electrically insulating. We have further investigated charge trapping defects in semiconductor NCs, which can have a negative impact on the conductivity of semiconductor NCs by capturing charge carriers from delocalized electronic states of the NCs. Moreover, we addressed the origin of magnetic anisotropy in NCs of diamagnetic materials. In this thesis, we report investigations using quantitative electron paramagnetic resonance (EPR) measurements concerning the efficiency of electronic doping of Si NCs with P atoms and its dependence on the environment surrounding the NCs. From quantitative EPR measurements, we estimate doping efficiencies in the NCs that are consistent with the incorporation of most P dopants as substitutional donors in the NCs. We further observe that the doping efficiency of the NCs varies by several orders of magnitude depending on the NCs surrounding environment due to a strong compensation of donors by molecules adsorbed to the NCs surface. Using temperature-dependent EPR spectroscopy, we further show that the ionization energy of P dopants in Si NCs increases with respect to their bulk counterpart due to confinement. We use temperature-dependent EPR spectroscopy to study the interaction between multiple P dopants incorporated in a single Si NC and its impact on the electronic structure of these NCs. We experimentally probe the exchange interaction in pairs of P donors (donor dimers) in Si NCs via a deviation of their triplet-state magnetic resonance from Curie paramagnetism. We showed that the exchange coupling of closely spaced donors can be well described by effective mass theory, enabling the calculation of many dopant configurations and allowing the consideration of statistical effects crucial in NC ensembles. We find that donor dimers induce discrete states in a NC, and that their energy splitting differs by up to three orders of magnitude for randomly placed dimers in a NC ensemble due to an enormous dependence of the exchange energy on the dimer configuration. We also investigate systems of spins induced by charge trapping defects and how these defects can affect the doping of NCs. We identify the presence of two charge states of a defect in CdSe NCs using EPR spectroscopy, combined with electronic tuning of NCs via chemically induced Ag doping. From light-induced EPR, we show that these defects have a central role on Fermi level pinning of NC ensembles. By analyzing the dependence of the EPR signal of the defects on the concentration of Ag dopants, we further demonstrate that the defects act as effective electron traps in the NCs. From temperaturedependent EPR, we estimate a lower limit for the ionization energy of the studied defects. Based on the characteristics of the EPR spectrum of the observed defect, we propose that it is associated to Se vacancies with the paramagnetic state being the positively charged state of the defect. Moreover, we show that magnetic interactions between spins associated to defects in NCs can induce magnetic anisotropy effects in NCs ensembles that are not expected to occur in their bulk counterpart. Using angulardependent ferromagnetic resonance (FMR) spectroscopy, we measure the magnetic anisotropy in different random ensembles of CdSe NCs by recording magnetic resonance spectra for various orientations of the external magnetic field. The observed angular dependencies of resonant field are different for apparently similar CdSe NC ensembles. We show that the shape and amplitude of the FMR angular variation can be well described by a simple model that considers magnetic dipole-dipole interactions between dipoles located at the NCs surface. The surface dipoles may originate from dangling bonds on surface sites that are not passivated by ligands. From our calculations, we find that for different random ensembles of NCs the strength of the magnetic anisotropy induced by dipole-dipole interactions may take values spanning four orders of magnitude, depending on the specific arrangement of the NCs in the ensemble and the specific distribution of the surface dipoles in each NC. This huge variability may justify the disparity of magnetic anisotropies observed in our experiments.