Synthesis of highly luminescent CdTe/CdS core-shell nanocrystals by optimization of the core and shell growth parameters

CdTe NCs were grown in aqueous solution through a hot injection method. The size of NCs was controlled by changing the refluxing time in a wide range of 1-20 h. The corresponding photoluminescence quantum yield was quite different for different sizes of nanocrystals and was saturated in long refluxing times. Different sizes of CdTe NCs within 2.5-4 nm and PL quantum yields between 28 and 50% were applied for the synthesis of core/shell CdTe/CdS nanostructures. The CdS shell was thermally over grown on the surface of different CdTe NCs for the luminescence improvement. The shell formation time was also changed in a wide range of 10-120 min to get the highest photoluminescence efficiencies. The range of changes and the maximum value of the PL quantum yield of the CdTe/CdS NCs were quite different for various sizes of core CdTe nanoparticles. A maximum PL quantum yield of 64% was achieved for the core-shell nanocrystals with cores prepared in 6 h of the refluxing time and CdS shell formed in 60 min of heating. Besides, there was a similar oscillating behaviour for the absorbance and PL quantum yields of the core-shell NCs during the shell formation process. This was qualitatively investigated and attributed to the successive formation of the perfect CdS shells on the NCs surface. The XRD, TEM and XPS analyses were performed to confirm the shell formation.