Enhanced Optical Properties of Green-Emitting InP Quantum Dots with Novel Mn-Doped Shells

Introduction:<br/>Quantum dots (QDs) have made advances in display technology with tunable emission wavelengths, sharp spectra, and high quantum yield (QY). InP QDs are attracting attention as an environmentally friendly alternative to cadmium-based QDs due to the need to comply with environmental regulations. However, InP QDs currently underperform them in terms of QY and full-width at half-maximum (FWHM).[1] This performance gap is primarily attributed to two factors. First, the broad size distributions of InP QDs lead to wider emission spectra.[2, 3] Second, the relatively narrow bandgap of the ZnSe intermediate shell of InP/ZnSe/ZnS QDs, which is the current common structure for InP QDs, results in insufficient electron confinement, especially in the green emission range.[4]<br/>Therefore, this study introduces InP/Zn(Mn)Se/Zn(Mn)S as novel shell materials that aim to enhance electron confinement by widening the bandgap and reducing the core-shell interfacial defects by improving lattice match.<br/><br/>Experimental Methods:<br/>InP cores were synthesized by a reaction between indium and phosphorus precursors at 300 °C under an argon-purged environment. Then, epitaxial growth of InP/Zn(Mn)Se shells on the InP cores was conducted, and Zn(Mn)S shells were grown on that sequentially. We prepared samples with different adding ratios of Mn to Zn, <b><i>X</i></b> and compared their optical properties, which are ultraviolet-visible absorption (UV-Vis), photoluminescence (PL), and time-resolved PL. In addition, material analysis data was collected by X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX).<br/><br/>Results and Discussion:<br/>TEM analysis confirmed spherical InP/Zn(Mn)Se/Zn(Mn)S QDs with an average diameter of 6.6 nm. PL measurements revealed that QYs of the InP/Zn(Mn)Se/Zn(Mn)S QDs were significantly improved compared to the Mn non-doped ones. When <b><i>X</i></b> = 0.10, the QY reaches around 85% in the green-emission region, with the longest fluorescence lifetime in this study. These PL enhancements originate from the lattice matching and the bandgap engineering between InP-core and Zn(Mn)Se-shell. The data of EDX and XRD indicate Mn-doping into the ZnSe and ZnS lattices and that lattice constants of shells have become closer to that of InP-core with an increase of the <b><i>X</i></b>. Furthermore, an increase of Mn proportion expected to widen the bandgap of Zn(Mn)Se shell to a value between 2.7 eV (for ZnSe) and 3.4 eV (for sphalerite-type MnSe). These effects result in the reduction of interfacial defect, enhancement of electron confinement, and improvement of radiative recombination rate. However, above <b><i>X</i></b> = 0.10, QYs decreased slightly (70% in <b><i>X</i></b> = 0.15 and 60% in <b><i>X</i></b> = 0.20). These results are likely due to Mn clustering or changes in the crystal structure of MnSe/MnS.<br/><br/>Conclusion and Outlook:<br/>Zn(Mn) Se and Zn(Mn) S shells significantly improve the optical properties of green-emitting InP QDs. Future work includes extending to red and blue emission, studying long-term stability, and scaling up synthesis. This work bridges the gap between eco-friendly and high-performance QDs, paving the way for sustainable, advanced applications.<br/><br/>References:<br/>[1] S. M. Click and S. J. Rosenthal,<i> Chem. Mater.</i> 35, 822–8368 (2023).<br/>[2] P. Ramasamy, N. Kim, Y. S. Kang, O. Ramirez, J. S. Lee, <i>Chem. Mater.</i> 29, 6893–6899 (2017).<br/>[3] A. Okamoto, S. Toda, M. Hirakawa, H. Bai, M. Tanaka, S. Seino, T. Nakagawa, H. Murakami, <i>ChemistrySelect</i> 7, e202104215 (2022).<br/>[4] A. Okamoto, H. Bai, S. Toda, M. Huang, H. Kajii, K. Kawai, H. Murakami, <i>ChemNanoMat</i> 9, e202200534 (2023).