Assembly and optical studies of cadmium selenide quantum dot nanostructures

Semiconductor quantum dots (QD's) have unique optical properties different from the bulk material due to the quantum confinement effects on their band structure. In particular, the enhanced optical absorption and luminescence from CdSe QD's is dependent upon the QD diameter; an effect similar to a particle-in-a-box model where the transition energy of the particle is increased as the length is decreased. To explore the collective properties of QD's and to make the connection to macroscale dimensions important for device components, we have fabricated ordered QD assemblies and have developed a model describing their unique luminescence properties.; Surface-capped CdSe QD's are assembled into 2-D arrays at the air-water interface using a Langmuir film balance. The resulting monolayers (and multilayers) are close-packed glasses, as observed using TEM. The luminescence from these CdSe QD solids is strongly influenced by the adsorption of water molecules onto the surface of the QD's. Light-induced alterations in the surface states following adsorption of water (photo-activation) results in quasi-reversible luminescence changes in the QD layers of over an order-of-magnitude. We have thoroughly investigated the dependence of this photo-activation on the size of the quantum dot, the surface capping ligand of the quantum dot, and the environment of the quantum dot. Our data shows that the rate of the photo-activation is strongly influenced by the size, surface functionality, and the surroundings of the QD's. We have developed a water adsorption model based on an “effective diameter” for the QD, which explains the photo-activation phenomenon. This model explains the complex dynamics of both the luminescence quantum yield as well as the exciton lineshape and suggests the possibility of using QD assemblies as chemical sensors.; Finally, we have used the photo-activation phenomenon in conjunction with near-field excitation as a unified direct probe to measure energy transport in QD assemblies and to estimate the excited state diffusion length. Furthermore, we have shown that our scanning techniques can photo-activate selected regions of QD solids, hence offering an application of nanoscale photolithography to QD arrays.