Spectroscopy of Multiexcitonic process in 'giant' semiconductor nanocrystals
Semiconductor nanocrystal quantum dots (NQDs) have long been known to possess attractive physicochemical properties, such as facile chemical preparation methods, tunability of emission wavelength, and high photoluminescence (PL) quantum yields (QYs) of the excitonic (X) emission. However, due to the strong confinement of the charge carriers in the nm-scale volumes, non-radiative processes such as Auger recombination severely affect the carrier dynamics of multiexcitonic processes in nanocrystals. Auger process is commonly believed to be the root cause of the undesired phenomenon of florescence intermittency or “blinking” observed in nanocrystals.
This project, in collaboration with groups of Dr. Han Htoon and Dr. Jennifer Hollingsworth at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory, studiesfluorescence intermittency and multiexciton dynamics in individual core/shell CdSe/CdSnanocrystals. Very recently, a new breed of multishellCdSe/(n)CdS, (2<n<20) “giant” nanocrystals (g-NQDs) had been developed that exhibit drastically different photophysical properties, including nearly complete blinking suppression and much reduced Auger recombination rates, allowing spectroscopic visualization of multiexcitonic emission lines.
We are performing experiments that aim to study exciton-exciton interactions in individual g-NQDs by the means of microscope-based (μPL),time-tagged, time-resolved single photon counting (TCSPC)and single photon correlation (photon antibunching) methods. Recently, it has been shown that large shell g-NQDs have biexcitonicquantum yields that can reach unity in some g-NQDs. Later, we have shown that shell thickness can be employed as a tuning parameter to suppress single exciton blinking and application of thick CdS shells leads to the emergence of a single PL emitting state in such g-NQDs. In addition to shell thickness dependence, our recent experiments point to a strong dependence of the biexcitonic emission on the size of the CdSe core.  This is relevant information for the continuous development of the non-blinking NQDs and is important to understand physics of multiexcitonic interactions which could be important in the development of quantum information sources and lasing applications.
 Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.; Bussian, D. A.; Klimov, V. I., and Hollingsworth, J.A., “Giant Multishell CdSe Nanocrystal Quantum Dots with Suppressed Blinking”, J. Am. Chem. Soc. 2008,130, 5026-5027
 H. Htoon, A. V. Malko, D. A. Bussian, J. Vela, Y. Chen, J. A. Hollingsworth, and V. I. Klimov, “Highly emissive multiexcitons in steady-state photoluminescence of individual “Giant” CdSe/CdS core/shell nanocrystals”, Nano Lett., 10(7), 2401-2407(2010)
 Y-S. Park, A. V. Malko, J. Vela, Y. Chen, Y. Ghosh, F. Garcia-Santamaria, J. A. Hollingsworth, V. I. Klimov, and H. Htoon “Near-unity quantum yield of biexciton emission from individual CdSe/CdSnanocrystals measured by two photon photoluminescence spectroscopy”,Phys. Rev. Lett., 106, 187401 (2011)
 Malko, A. V.; Park, Y.-S.; Sampat, S.; Galland, C.; Vela, J.; Chen, Y.; Hollingsworth, J. A.; Klimov, V. I.; and Htoon, H., “Pump-Intensity- and Shell-Thickness-Dependent Evolution of Photoluminescence Blinking in Individual Core/Shell CdSe/CdSNanocrystals”, Nano Lett. 11, 5213 (2011)
 B. D. Mangum, S. Sampat, Y. Ghosh, J. A. Hollingsworth, H. Htoon,andA. V. Malko, “Influence of the Core Size on Biexciton Quantum Yield of Giant CdSe/CdSNanocrystals”, submitted to ACS Nano