Doped CdSe Quantum Dots: How a Pebble Affects the Mountain

Quantum Dots (QDs) are a fascinating material that bridges the gap between atomistic interactions and bulk properties. Comprised of several hundred to perhaps a few thousand atoms for the very largest of particles, they are the perfect basis for studying quantum effects, such as confinement. Different combinations of elements yield widely varying changes in Bohr radii and thus effective changes to the bandgap. The research into QDs has far advanced beyond the single crystal and binary systems. Exotic combinations of cores and shells and trinary systems now tend to dominate the most cutting edge research. Practically, however, it is the most well understood materials that can yield the most interesting results. We study how the addition of a dopant atom into the host crystal structure affects the overall electronic structure of the QD. Since CdSe is very well known, the effects should be noticeable and attributable to the new dopant. Using synchrotron techniques, notably x-ray absorption near edge spectroscopy (XANES), we can directly probe the electronic states. We find an interesting result when adding even tiny amounts (~4 atoms/dot) of dopants can affect the overall electronic structure of the system. While this observation is important, it is far more useful to understand the cause of these changes. To that end, a correction to the current theoretical model is proposed and tested. The correction term yields results in excellent agreement with experiment, i.e. within experimental error, and is determined generally so as to apply predictively to other systems that are well understood.