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 (Φ_f ∼ 4 × 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 ∼30 fold (Φ_f ∼ 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 ∼930 nm with an emission yield of ∼0.1. Femtosecond pulse excitation (λ_ex ∼ 1200 nm) measurement estimated a two-photon absorption cross section, σ, of ∼70 × 10⁻⁵⁰ cm⁴ 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.