University of Texas at Dallas
Monday, September 20, 2010
Excitonic Pathways in Semiconductor Nanocrystals Uncovered by Time-resolved Single Dot Microscopy: From Non-blinking Giant Quantum Dots to Organic/inorganic Energy Transfer
In this talk I report our recent advances in ultrafast spectroscopy of colloidal nanocrystals (NCs). Following our initial observation of ultrafast Auger recombination of multiexcitons (MX) in regular CdSe NCs and subsequent observation of stimulated emission in CdSe films, a strong research effort has been applied to understand and engineer dynamics of MX states in such NCs, specifically to address carrier multiplication and fluorescence intermittency (blinking) issues. Recently, we developed a new class of “giant” CdSe/CdS multishell nanocrystals of CdSe cores overcoated with multiple layers of inorganic shells (CdS) and observed complete blinking suppression at all observed time scales, from milliseconds to minutes for dots with n>12 monolayers of CdS. In single dot photoluminescence (PL) measurements of such g-NCs we observed emission from multiexcitons in steady-state PL at low (10 K) temperatures. By analyzing steady-state and dynamical signatures of MX states we found that rates of Auger non-radiative recombination are comparable or even less than radiative recombination rates of MX in such gNCs, allowing significant fraction of multiexcitons to recombine radiatively. In following experiments, we studied hybrid film structures consisting of a thin layer of NCs “anchored” to a monolayer of J-aggregates (JA) of a cyanine dye. Time-resolved and steady-state PL measurements indicated strong Forster energy transfer from NCs to JA layer. Such energy transfer might be of importance for hybrid photovoltaic and LED technologies.