STRONGLY PHOTOLUMINESCENT CDTE NANOCRYSTALS BY PROPER SURFACE MODIFICATION

CdTe nanoclusters were prepared in aqueous solution by the reaction be tween Cd2+ and NaHTe in the presence of thioglycolic acid. Under reflu x, the clusters start to crystallize and show a narrow band emission. The photoluminescence efficiency of CdTe nanocrystals strongly depends on the pH value of the colloidal solution. The maximum quantum yield at room temperature is approximately 18% when the pH value of the CdTe solution is brought to 4.5 by using thioglycolic acid. The optical sp ectroscopy studies imply that the pH-dependent behavior of the CdTe na nocrystals' fluorescence is caused by structural changes on the surfac e rather than the size of the nanocrystals. Systematic absorption and fluorescence studies on dialyzed samples suggest that in the acidic ra nge a shell of cadmium thiol complexes is formed around the CdTe core. Thus, the fluorescence quantum yield is enhanced dramatically when th e solution is made acidic. In contrast, such a shell can also be produ ced in the alkaline range, but only after the CdTe nanocrystal crude s olution is purified by dialysis.