UC Santa Cruz

Semiconductor nanostructures, also known as quantum dots (QDs), have shown great promise as optical materials in solar cells, light emitting devices, and as fluorescent probes. When combined with other materials, the properties of QDs can be modified to suit a particular application. QD heterostructures were synthesized and investigated using time resolved spectroscopy in order to understand how to control the fate of excitons. A one pot approach to the synthesis of CdSe/ZnSe/ZnS core/shell/shell QDs was developed, and the shell growth was shown to be an effective means of extending the exciton lifetime and increasing fluorescence quantum yield. Both the extended lifetime and increased fluorescence were attributed to passivation of surface states by the double shell. Furthermore, this approach was adapted to alloyed CdxZn(1-x)Se QDs to produce CdxZn(1-x)Se/ZnSe/ZnS alloyed core/shell/shell QDs with high fluorescence quantum yield. Transient absorption spectroscopy showed that the shell coating slowed exciton recombination on the ps timescale by passivating surface states. Time resolved fluorescence revealed identical ns dynamics before and after shell growth, this was attributed to very fast surface state recombination that can is complete within a nanosecond. In addition, homogenous CdSe/TiO2 composite nanorods and pure CdSe nanorods were grown using physical vapor deposition. SEM images showed that the CdSe and CdSe/TiO2 nanorod arrays are vertically aligned, with very similar morphology. Ultrafast transient absorption spectroscopy showed that the composite nanorods exhibit very fast electron injection from CdSe into TiO2. The fast electron injection is due to the large interfacial area between the two materials in the composite nanorods.