Charge Transfer Processes in the Excited Dynamics of II-VI Semiconductor Nanocrystals

In large molecular systems such as DNA, supramolecular complexes and dendrimers, functional groups located at different parts of the molecular structure can act as charge donors or acceptors, and photoinduced intramolecular charge transfer can occur. An analogous scenario can be found in colloidal semiconductor nanocrystals, most evident in type-II heterostructures, where the relative band-alignment of the constituent materials are in a stagger configuration. Such a configuration, provides an energetically favourable situation for an photo-generated electron to be transferred from one material to the other, confining the electron and the hole in different domains of the nanostructure. A less obvious scenario in nanocrystals is when the core is thought of as the donor group, and the surface as the acceptor group. In such a scenario, the localization of electron or hole at surface defect sites, a process that occurs in every nanocrystal, can be thought of as an ``intramolecular" charge transfer.\n\nThe studies presented in this dissertation are an attempt to further understand charge transfer processes in semiconductor nanostructures, in particular, those occurring within the same nanocrystals. This is carried out by a combination of spectroscopic techniques and modelling. First, time-resolved fluorescence measurements are used to investigated surface trapping/de-trapping dynamics in CdSe and CdSe/CdS/ZnS core/shell/shell quantum dots. A kinetic model, in which trapping/de-trapping is described with Marcus' classical electron transfer theory, is used to analyzed our results, yielding excellent agreement between model and experiment. Second, the influence of temperature and solvent environment in the optical spectra of CdSe/CdTe nanorods are examined. Solvatochromic shifts in these heterostructures are found to be larger than those observed in core-only quantum dots. Finally, ultrafast dynamics and biexciton states in CdSe/CdTe quantum dots are probed using two-dimensional optical spectroscopy. The fine structure of the lowest exciton and biexciton states are calculated for a model system with type-II band-alignment and simulations of 2D spectra are performed.