Measurement of the separation dependence of resonant energy transfer between CdSe/ZnS core/shell nanocrystallite quantum dots

Abstract

The separation dependence of the interaction between two resonant groups of CdSe$/$ZnS nanocrystallite quantum dots is studied at room temperature. A near-field scanning optical microscope is used to bring a group of monodisperse $\ensuremath{\sim}$6.5-nm-diam nanocrystallite quantum dots, which are attached to the microscope probe, into close proximity of an $\ensuremath{\sim}$8.5-nm-diam group of nanocrystallite quantum dots which are deposited on a solid immersion lens. Information extracted from photoluminescence, photoluminescence excitation, and absorption curves as well as numerical calculations of the energy levels show that the third excited excitonic energy level of the large quantum dots nearly matches the ground excitonic energy level for the small quantum dots. Quenching of the small quantum dots' photoluminescence signal is observed as they approach the large quantum dots. On average, the separation between microscope probe and solid immersion lens changed in the 15--50-nm range. The transition probability between these two groups of quantum dots is calculated to be $(2.60\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}47}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{6})/{R}^{6}$, within the experimentally obtained range of transition probabilities $(0.70\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}47}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{6})/{R}^{6}$--$(11.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}47}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{6})/{R}^{6}$ . The F\"orster radius, as a signature of energy transfer efficiency, is experimentally found to be in the 14--22-nm range.