Nanoscale physics
Kinetics of Photoluminescence of Exciton Complexes in CdSe/CdS/CdZnS Quantum Dots

Pavlenko V.I.1, Dobynde I.I.1, Beloussov I.V.1, Ozol D.I.2
1 Institute of Applied Physics, 5, Academy str., Chisinau, MD-2028, Republic of Moldova. 
E-mail: pavlenko@phys.asm.md 
2 Moscow Institute of Physics and Technology, 9 Institutskiy per., Dolgoprudny, Moscow Region, 141700, Russian Federation.

We used highly luminescent CdSe/CdS/CdZnS colloidal quantum dots (QDs) prepared by a standard chemical synthesis method. A sample was studied by transmission electron microscopy, optical absorption spectroscopy, and luminescence spectroscopy. It was determined that the diameter of the CdSe nucleus of QD amounts to 5.4 nm with the size dispersion of 8%. The thickness of the CdS/CdZnS shell was 2.6 nm. The time-resolved photoluminescense in QDs was excited by a single 30 ps pulse of the second harmonic (539 nm) of a mode-locked YAlO3: Nd3+ laser at room temperature. The spectral decomposition of the PL signal was performed using a polychromator. The PL kinetics was recorded by a streak camera. The spectral resolution was about 5 meV and 9 meV for the time-integrated mode and the time-resolved mode, respectively, and an overall time resolution of about 50 ps was obtained.
The laser pulse excites radiation, mainly from the p-states of electrons and holes, that occurs in a wide energy range and reaches a maximum value at 200 ps on the time scale of the experiment. Simultaneously, radiation appears from the single excitons in s-states. It has an almost unchanged spectral range and reaches a maximum at 250 ps. The emission of biexcitons appears with even greater delay and much slower. Its growth ceases at 350 ps when two previous PL bands are already significantly reduced. The spectral range where this radiation occurs also does not almost change. Starting from 350 ps the decay of triexcitons is accompanied by the decay of biexcitons within ∼140 ps. The decrease of radiation from single excitons within this time interval slows down. After 700 ps only a very slowly decreasing emission of single excitons occurs that can be described by a simple exponential curve. Using the exponential function for fitting the measurements only at times greater than 700 ps we obtain a value of ∼10 ns for the decay time of a single exciton.