Several groups have shown that non-toxic semiconductor systems (mostly zinc selenide) can be doped with guest ions, such as copper and manganese, to make phosphorescent dots with significantly reduced toxicity. This was the subject of our first paper, where we demonstrated a highly reproducible method to synthesize non-toxic manganese doped zinc selenide QDs which has the form of a ZnSe core, a ZnS shell doped with manganese, and a final ZnS cap (Nano Lett, 2007, 7, 3429). We have shown that energy transfer from the core to the shell is highly efficient; we further demonstrated that the phosphor may transfer the core excitation to an organic dye bound to the surface of the water solubilized ZnSe/ZnMnS/ZnS QD. Shown here are several samples of this material under UV light excitation. At the time, our brightness (measured by “quantum yield”) was a record, although several groups have now reported doped systems with better efficiency at this point. Regardless, we moved away from synthesizing doped QDs using traditional methods to address an issue that at the time was a major problem- the fact that the number of dopants added to a QD could not be controlled.
Quantum Dots for renewable energy
We are presently developing catalytic materials that may impact world energy consumption. We will likely run out of petroleum products for energy production in the year 2050, and without a clean alternative fuel (something other than coal) we will be living out the worst parts of a Mad Max movie. Photocatalysis is a promising source of energy however photocatalytic materials are polycrystalline or amorphous which obfuscates analytical analysis of the catalytic centers. We are presently applying the techniques used to synthesize highly fluorescent NCs for creating highly efficient and crystalline semiconductor photocatalysts for energy production. Specifically, we are synthesizing tantalum nitride and oxynitride NCs as they can collect a lot of solar energy (which tantalum oxide can’t) yet still break water into hydrogen as shown by this energy level diagram.
Thusfar, we have synthesized NCs of tantalum nitride and oxynitride (see J. Phys. Chem C, 2011, 115, 647-652) and are examining their catalytic efficiencies. The nitride materials can be highly crystalline while the oxynitrides are amorphous; however, the oxynitride is very easy to synthesize in large quantities. Shown here is one of our best nanocrystals of tantalum nitride NCs. We have also recently demonstrated that we can synthesize tantalum oxynitride via a very simple sol-gel method in very high yield; this work has recently been submitted for publication.