Structural and optical properties of ZnSe:Eu/ZnS quantum dots depending on interfacial residual europium

Highlights: • ZnSe:Eu/ZnS core/shell quantum dots (QDs) prepared in situ for the first time. • Luminescence intensity of core/shell QDs three times that of uncoated core QDs. • The core/shell system showed decreased Eu²⁺ emission but only the Eu³⁺ sharp peaks in the red at 579, 592, 615, 651, and 700 nm. • The core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44 nm. • The core/shell structure may contain Eu₂O₃ bonding the over-coated ZnS surface on the Eu³⁺-doped ZnSe core. A multimodal emitter comprising of ZnSe:Eu/ZnS (core/shell) quantum dots (QDs) by adding a ZnS precursor in situ during synthesis. ZnSe/Eu²⁺/Eu³⁺/ZnS actives both core and core/shell. QDs prepared with the ZnS precursor displayed a luminescence intensity three times that of ZnSe QDs due to the passivation effect of the Shell. While the core QDs display the 450–550 nm emission of Eu²⁺ (⁴F₆⁵D₁ → ⁴F₇), the core/shell system showed no Eu²⁺ emission but only the sharp peaks in the red at 579, 592, 615, 651, and 700 nm due to the electronic transitions of ⁵D₀ → ⁷F_n (n = 0–4) depending on leisurely decreased with increased reaction time. These results are in agreement with Eu 3d spectra of XPS analysis results. Microscopic analyses show that the core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44 nm. The lattice constant in the central portion of the core/shell QDs are around d₁₁₁ = 3.13 Å, which is between the outside and inside ring patterns (d₁₁₁ = 3.27 and 3.07 Å, respectively). This shows the effective over-capping of shell onto the core QDs. The core/shell structure may contain Eu₂O₃ bonding the over-coated ZnS surface on the Eu³⁺-doped ZnSe core.