Photoluminescence donor-acceptor band splitting in phase transition of CdSe nanoparticles

Abstract

Undoped cadmium selenide nanoparticles (CdSe-NP's) were grown on glass substrates by chemical bath synthesis (CBS). The particle size was controlled by means of the bath temperature (Tb), which was chosen within the interval 0–80 °C. X ray diffraction patterns indicate that CdSe-NP's grow in the hexagonal wurtzite (WZ) crystalline phase for low Tb values and in the cubic zinc-blende (ZB) structure for higher ones. The WZ → ZB transitions occurs at the critical temperature Tbc ≅ 40 °C. Considering the CdSe-NP's like spheres as an approximation, the average size, calculated by using the Scherrer formula, is in the range 7.0–12.2 nm. The Arrhenius plot of the natural logarithm of the diameter versus the inverse of the absolute temperature (Tb + 273.6 K) reveals two slopes which were identified as activation energies related with two thermally activated processes of the growing NP's phases. The photoluminescence spectra show, in general, three type of emissions, which were associated with the more probable electronic transitions: a) near band edge emission, b) donor or acceptor level-band, c) donor-acceptor (DAP). The spectrum when Tb = 40 °C shows more PL bands, that is because the DAP band splits in two bands. This splitting has been associated to the presence of tetrahedral and octahedral Cd-interstitial donor levels in ZB and WZ CdS, respectively, when mixed phases are present in the material at the critical temperature of phase transition.