| Optical excitation in a semiconductor creates two charge carriers, an electron and a hole. In the semiconductor, the atoms of the lattice move to solvate these two charge carriers. The extent of this motion may be monitored using resonance Raman spectroscopy. Since the amount of lattice motion depends upon the spatial extent of the optically excited wavefunction, the resonance Raman spectrum contains information about the electronic states of the semiconductor. While the electronic states of an infinite semiconductor are well understood, less is known about how the electronic states vary with size of the crystal. Using chemical synthesis, nanocrystals of semiconductor materials which have radii between 10A and 60A may be readily prepared. These nanocrystals have the bulk bonding geometry on the interior but are terminated at their surfaces with an organic layer which prevents further aggregation and growth.; This thesis presents the results of experiments which use resonance Raman spectroscopy to probe optically excited wavefunctions in two polar semiconductors, CdS, and CdSe, as the size of the nanocrystallite is varied. The dependence of the Raman spectrum on temperature, laser wavelength, laser polarization, and method of synthesis was also investigated. Since the interpretation of the resonance Raman data requires detailed modeling, this thesis also presents the results of some theoretical investigations of how the optically excited wavefunction and the response of the lattice to this wavefunction change with size. The model presented here is an extension of the polaron transform methods previously used to describe the lattice response in infinite solids. |
