Influence of Eu doping on the structural and optical properties of Zn_1-xEu_xSe quantum dots

We have successfully fabricated Zn_1-xEu_xSe (x = 0–0.1) quantum dots (QDs) with an average size of ~5 nm. X-ray diffraction analyses indicated that the zincblende-wurtzite structural transition occurred around x = 0.05. This leads to a small change in value of the band-gap energy E_g from 3.13 eV for x = 0–3.03 eV for x = 0.1. By studying their photoluminescence (PL) spectra, we observed two emission regions. The first one around 400–415 nm is a broad peak associated with the band-edge emission, while the other consists of sharp peaks due to the electronic transitions (⁵D₀ - ⁷F_j, j = 0–4) of Eu³⁺ ions in the ZnSe host lattice. Similar to the variation tendency of E_g, the excitonic emission peak also shifts towards low energies when x increases. Among the ⁵D₀ - ⁷F_j transitions, the radiative transition with j = 2 peaked at 618 nm has the strongest intensity, particularly for x = 008. Time-resolved PL spectra of both emission regions can be described by a multi-exponential decay function. The average time constants of the Eu³⁺-related emission at 618 nm vary in the range of 2.4–4.8 ms, which is much longer than those recorded for the band-edge emission. We also analyzed the emission features of QDs through the CIE chromaticity coordinates and found the QDs with x = 0.08, showing neutral white light.