Sequential Synthesis of Type II Colloidal CdTe/CdSe Core–Shell Nanocrystals

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Abstract
Colloidal type II CdTe/CdSe nanocrystals were synthesized by sequential addition of a tri‐n‐octylphosphine telluride (TOPTe)/TOP solution and several shell‐precursor solutions to a CdO/TOP solution; the shell‐precursor solutions consisted of CdO and TOPSe in TOP. For the growth of the CdTe core, the TOPTe/TOP solution was swiftly added to the CdO/TOP solution at a higher temperature (300 °C) than the growth temperature (250 °C). For the growth of the CdSe shell, in contrast, the CdO/TOPSe/TOP solution was slowly added to the CdTe/TOP solution at a lower temperature than the growth temperature (200–240 °C). The temporal evolution of the optical properties of the growing core–shell nanocrystals was monitored in detail. During the growth of the CdSe shell, the core–shell nanocrystals exhibited interesting changes in photoluminescence (PL) properties. The highest PL efficiency (≈38 %) was detected from core–shell nanocrystals with a CdSe shell thickness of 0.4–0.5 nm (indicated by TEM); the formation of the first monolayer is proposed. Our synthetic approach is well suited to a practical realization of engineering materials with bandgaps in the near‐IR and IR spectral ranges.