Transition from type-I to type-II CdTe/CdS core/shell quantum dots synthesized in water at low temperature

Ung Thi Dieu Thuy and Nguyen Quang Liem

Abstract

This paper reports the results on the syntheses of high-quality (with luminescence quantum yield reaching ~70%) CdTe and CdTe/CdS core/shell quantum dots (QDs) in water at temperatures below 100 °C. The same CdTe seed QDs (of 2.5 nm in size to emit luminescence at 494 nm) were used as a stock to develop CdTe/CdS core/shell QDs at 90 °C with different CdS shell thicknesses. By progressively increasing the CdS thickness we could make CdTe/CdS core/shell QDs changing from type-I into type-II quantum structure to emit strong luminescence peaking in the spectral range of 550–800 nm. By exciting directly the CdTe core QDs in a type-II CdTe/CdS core/shell structure, we could determine the luminescence decay-time to be as long as 190 ns which is very much longer than that from the type-I ones