Spectroscopy of excitons in CdSe/CdS colloidal nanocrystals

Abstract

Since their discovery in the 1980s, colloidal semiconductor nanocrystals have been intensely studied theoretically and experimentally because of their great potential for a wide range of applications in, for example, optoelectronics, spintronics or biolabeling. One of the main obstacles with respect to such applications is the improvement of the radiative properties, i.e. the quantum yield, of nanocrystals, which exhibit strong non-radiative channels, such as Auger scattering. In CdSe nanocrystals, the radiative recombination of excitons is strongly affected by the construction of the exciton fine structure, which consists of states with dipole allowed and forbidden transitions. Notably, in perfectly spherical nanocrystals the exciton ground state is optically forbidden in the electric dipole approximation, implying a long radiative lifetime, which favors non-radiative recombination and thereby degrades the optical properties of such nanocrystals. This thesis elucidates how the exciton fine structure and exciton dynamics can be affected by tailoring the nanocrystal size and shape. It is shown, that the introduction of a shell of different bandgap material from the core greatly extends the possibilities in this regard, since it gives the ability to control the electron-hole wavefunction overlap, which determines radiative recombination and the construction of the exciton fine structure. Furthermore, exciton spin dynamics in external magnetic fields and the polarization properties of the photoluminescence were studied by time- and polarization-resolved photoluminescence spectroscopy. It is shown, that the spin relaxation rate is strongly dependent on the core size in core/shell nanocrystals, whereas it is weakly dependent on the size and shape of the shell. A theoretical model for describing the circular polarization of nanocrystals with anisotropic shell shapes in external magnetic fields is elaborated. Finally, the mechanisms, which enable radiative recombination of excitons with forbidden optical transitions, are studied by fluorescence line narrowing. The experimental results support a recent theory, which predicts the activation of radiative recombination of those excitons by surface dangling bonds.