Thermal Activation of Luminescence Processes in PbSe Colloidal Quantum Dots
Written by Ariel Kigel
Ariel Kigel, Georgy Maikov, Aldona Sashchiuk and Efrat Lifshitz
Schulich Department of Chemistry and Solid State Institute, Technion – Israel Institute of Technology Technion City, Haifa 32000, Israel
This work demonstrates the significant affect of thermally activated processes on the ground-state exciton emission of PbSe core colloidal quantum dots (CQDs). The variation of the photoluminescence properties with temperature showed two thermal activation thresholds: the first in the temperature range 1.4–7K connected with activation of acoustic phonon assisted dark exciton decay, and the second in the temperature range 100–200K, connected with activation of bright excitons. The thermal activation energy of the bright exciton showed a size dependent, varying from 16.8 meV to 13 meV for CQDs with diameters of 3.1 nm to 4.1 nm, respectively. This study also shows that the temperature coefficient of the energy gap and the optical phonon coupling were reduced with the decrease of the diameter, while the acoustic phonon coupling grew with the decrease of the diameter. The dark exciton lifetime was found to be ~10 mics, while the bright exciton lifetime varied between 450 ns to 500 ns, and it increased with the decrease of the CQDs’ diameter. The emission quantum yield of the PbSe CQDs was measured in various solvents (hexane, chloroform, water, glass solution and polymer) and exhibited an optimal value of 60% in hexane solution. In general, the results supplied useful information for the utilization of the PbSe CQDs in various opto-electronic devices, and shed light on the controversial theoretical models regarding the magnitude of the dark-bright states splitting and the involvement of other nonradiative relaxations.