Synthesis, characterization, and its PL dynamics of colloidal type II CdTe/CdSe nanocrystals

We describe our improved synthesis and optical properties of high quality type II CdTe/CdSe nanocrystals (NCs). Specifically, clear shell-thickness dependences have been observed in the absorption and photoluminescence (PL) spectra and PL decay profiles as well. The magnitude of the lowest absorption band decreases drastically with large redshift as the shell thickness increases. The origin will be discussed on the bases of the model where the spatial configuration of the lowest electron-hole pair in the NCs changes from that of type I to type II as the shell thickness increases. As for the PL lifetime of the lowest electron-hole excitations, substantial increase is observed with increasing shell thickness. This can also be understood by considering the spatial configuration; spatial overlap between electron and hole wavefunctions decreases with increasing shell thickness, thus the lifetime increases. As for the NCs with extremely thin shell (∼1 ML; 1 ML = 0.35 nm), the PL lifetime seems much longer than expected. This suggests that the thin shells seem imperfect and work rather a kind of trap sites than layers. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)