www.acsprf.org

This project funded by ACS aims to better understand the underlying difficulties of doping of semiconductor quantum dots. We address questions how the dopant impacts the nanoparticles during growth in colloidal solutions. As a model system, we have been investigating CdSe nanoparticles grown from single precursors in the heterogeneous growth regime when multiple distinct sized nanoparticles are present.[i] In the past we have explored the effect of indium dopant on the growth kinetics and we started expanding our studies to gallium and tin doped CdSe nanoparticles. Chemical analyses of the particles have shown that all of these dopants are found within the CdSe particles.[ii] The indium dopant accelerated the growth of CdSe units, but the gallium dopant exhibit a saturation effect in the acceleration of particle growth.[iii] In contrast to the indium and undoped CdSe nanoparticles, we have found that the tined doped samples produce twinning planes in the CdSe nanoparticles. Terahertz spectroscopy shows that the tin doped CdSe nanoparticles exhibit an upward shift in their Froehlich band at 208 cm^-1. In addition, we have started collaboration with Prof. Prashant Kamat at the University of Notre Dame to investigate the transient absorption spectra of our doped quantum dots. We have found that there is an interesting 70 meV blue shift of the transient 1S bleach.[iv]

We have also found that the addition of ZnS shell greatly impacts the temperature dependent photoluminescence of the particles. The strong temperature dependent component of the doped nanoparticles disappears when the ZnS shell thickness is increased. At this point, we speculate that the dopant could migrate into the shell as a result of the better size match of with the ZnS crystal lattice.

In addition to the above studies, we realized that CdSe material will not be sufficient to address solar energy needs on the global scale. We started exploring the synthesis of sustainable nanoparticles that could be used as a platform for our doping studies. We are the first to develop the colloidal synthesis of crystalline b-FeSi₂ nanoparticles.[v] These particles will be used in our future doping studies. In addition, based on our previous observation of heterogeneous growth of the CdSe nanoparticles, we have developed an approach called the quantized Ostwald Ripening[vi] that could be a potential method to produce size focusing of nanoparticles in a colloidal solution.

We hypothesize that the doped CdSe quantum dots will be more electronegative than the neutral quantum dots. We have performed in situ photoluminescence experiments to observe the dissolution and etching of quantum dots. We found increased etching rates in doped quantum dots. The rate of etching increases with the lower ionization energy of the dopant.[vii]

We have successfully secured a larger collaborative NSF funding to address “Climate Change and Energy: Basic Science, Impacts, and Mitigation Science Initiative”. Within this proposal we just started investigating indium doped CdSe nanorod/P3HT composites with single molecule spectroscopy. The results on indium doped CdSe quantum dots from the ACS funding were important to secure our NSF funding.

Several students have worked on the project over the period of years. Summer REU student Fadzai Fungura helped us obtaining terahertz data on doped cDSe quantum dots. Dr. Pinar Dagtepe, Dr. Naween Dahal and Dr. Christopher Tuinenega finished their PhD. Dr. Dagtepe and Dr. Dahal have secured postdoc positions and Dr. Tuinenga is currently in the process of finding postdoc position. Christopher Lewis also secured a job very recently at Halliburton. The ACS grant provided opportunity to all of these students to obtain the necessary experience to secure their employment. In 2009, Chris Tuinenga obtained funds from German Academic Exchange Service and spent a semester in Germany at BASF to learn how prepare solar cells. In 2010, Pinar Dagtepe received a travel fellowship to present her research on conference (Nanomaterials and Nanocatalysis for Energy, Petrochemicals and Environmental Applications) in Cairo sponsored by the US-Egypt Advanced Studies Institute.