Low temperature studies of the photoluminescence from colloidal CdSe nanocrystals prepared by the hot injection method in liquid paraffin

This article describes the results from low temperature studies of the photoluminescence (PL) from colloidal CdSe nanocrystals prepared by the hot injection method in liquid paraffin. The synthesized nanocrystals were taken from the reaction mixture at different time intervals, their sizes were all within the quantum confinement range, and showed the typical size-dependent UV-vis absorption, PL and Raman spectral characteristics. The PL spectra and their temperature dependences were measured from room temperature (295 K) down to temperatures as low as 15 K, demonstrating that the exciton PL transitions followed the Varshni equation. The thermal quenching activation energy below 100 K was relatively low, while the luminescence was strongly quenched as the temperature was increased above 100 K, probably pointing at change in the degree of carrier localization. The smallest (2.6 nm in size) CdSe nanocrystals isolated in the beginning of the nanocrystal growth had a relatively symmetric size distribution and showed a single exciton emission band, while the samples isolated at later stages of the growth process showed negatively skewed asymmetric size distribution, indicating that the growth mechanism of nanocrystals could be explained by the model of Ostwald ripening. The obtained results are expected to be helpful for better understanding of the PL properties of quantum dots and their formation and growth in non-polar organic solvents. (C) 2014 Elsevier B.V. All rights reserved.