Syntheses and photophysical properties of type-II CdSe/ZnTe/ZnS (core/shell/shell) quantum dots

Abstract

Syntheses of CdSe/ ZnTe/ ZnS (core/shell/shell) type-II quantum dots (QDs) are reported. Structural characterization was made via TEM, EDX, XPS and XRD. Photophysical properties were investigated via the interband emission (CdSe -> ZnTe) and its associated quantum efficiency as well as relaxation dynamics. In comparison to the weak emission (Phi(f) similar to 4 x 10(-3) in toluene) of CdSe/ ZnTe (3.9/ 0.5 nm), capping ZnS (0.4 nm in thickness) enhances the CdSe -> ZnTe interband emission by similar to 30 fold (Phi(f) similar to 0.12), whereas the peak wavelength shifts only slightly to the red. By encapsulating dihydrolipoic acid ( DHLA), water-soluble CdSe/ ZnTe/ ZnS QDs were also prepared, and they exhibited an interband emission at y930 nm with an emission yield of similar to 0.1. Femtosecond pulse excitation (lambda(ex) similar to 1200 nm) measurement estimated a two- photon absorption cross section, sigma, of similar to 70 x 10(-50) cm(4) s photon(-1) for DHLA-capped CdSe/ ZnTe/ ZnS in water, supporting its suitability for the use as near-infrared (NIR) dyes in biomedical imaging.