Issue
EPL
Volume 86, Number 3, May 2009
Article Number 37003
Number of page(s) 6
Section Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties
DOI http://dx.doi.org/10.1209/0295-5075/86/37003
Published online 13 May 2009
EPL, 86 (2009) 37003
DOI: 10.1209/0295-5075/86/37003

Radiative and nonradiative recombination of photoexcited excitons in multi-shell–coated CdSe/CdS/ZnS quantum dots

Y. Fu1, H. Ågren1, J. M. Kowalewski2, 3, H. Brismar2, 3, J. Wu4, Y. Yue4, N. Dai4 and L. Thylén5, 6

1   Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology - S-106 91 Stockholm, Sweden, EU
2   Department of Applied Physics, Royal Institute of Technology - Stockholm, Sweden, EU
3   Department of Woman and Child Health, Karolinska Institutet - Stockholm, Sweden, EU
4   National Lab for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences 500 Yutian Road, 200083 Shanghai, China
5   Department of Microelectronics and Applied Physics, Royal Institute of Technology - Kista, Sweden, EU
6   Kista Photonics Research Center - S-160 40 Stockholm, Sweden, EU

fyg@theochem.kth.se

received 3 March 2009; accepted in final form 16 April 2009; published May 2009
published online 13 May 2009

Abstract
Colloidal quantum dots (QDs) have been widely studied for nanophotonics and bioimaging applications for which the lifetime of their fluorescence is of critical importance. We report experimental and theoretical characterizations of dynamic optical properties of multi-shell–coated CdSe/CdS/ZnS QDs. Quantum-mechanical studies of fundamental optical excitations and Monte Carlo simulations of energy relaxation mechanisms indicate that the excitonic states are densely compacted in the QDs and are easily photoexcited by the laser pulse in the presence of nonradiative electron-phonon interactions. For spherical QDs, the decay time of spontaneous radiative emission of individual photoexcited excitonic states with zero angular momenta is found to be only tens of picoseconds. In our multi-shell QDs, high-energy excitonic states of nonzero angular momenta have to go through a number of nonradiative electron-phonon interaction steps in order to relax to zero–angular-momentum excitonic states for radiative emission, resulting in an effective fluorescence peak at about 2 ns in the photoncount-time relationship. This explains the measured long average fluorescence lifetime of 3.6 ns. Such a long lifetime facilitates the applications of colloidal QDs in areas such as QD-based solar cells, bioimaging and metamaterials.

PACS
73.21.La - Quantum dots.
73.22.-f - Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals.

© EPLA 2009