Developing Codoped Inorganic Nanoparticle Emitters by Selective Chromophore Excitation for Color Generation Applications

Codoped inorganic nanoparticles (NPs) offer a robust platform to access unique luminophores and generate multiplex assays. This work systematically investigates the emission properties in the trivalent lanthanide cation codoped calcium fluoride [Ca(TbLn)F2] [Ln = Sm, Yb, Tm] NPs, and rationalizes them in the broader perspective with due emphasis to our previous findings on the Ca(TbEu)F2 and Ca(TbDy)F2 NPs. A charge trapping mediated Tb3+→Ln3+ electronic interaction is found to be guided by the relative positioning of the Ln2+ ground energy level with respect to the initially populated energy level at the Tb3+ electronic structure. Further corroboration on this is examined with Sm–Eu codoped CaF2 NPs as a control, where involvement of Ln2+ energy level is not possible. A spectral overlap mediated mechanism is proposed in this case, resulting in weaker Sm–Eu forward and back energy transfer. Codopant induced symmetry breaking effects are found to be remarkable in the Ca(TbSm)F2 NPs, which emerge as an alternate avenue to brighten Ln3+. This is in sharp contrast to the Ca(TbEu)F2 NPs, where Tb–Eu electronic energy transfer is more dominant than the symmetry breaking effects. Interestingly, in both Ca(TbSm)F2 and Ca(TbEu)F2 NPs, Tb3+ brightens Sm3+/Eu3+ emission by almost a comparable magnitude. The codoped NPs are found to be generating more distinct emission, compared to the singly doped NPs, as a function of excitation wavelength. This excitation wavelength dependence adds a dimension to the codopant-mediated multiplexing for luminophore development. Thus, a judicious selection of (a) codopants and (b) excitation wavelengths can generate a range of colors, in an as-desired way, and are suitable for intended applications. Finally, the Ca(TbSm)F2 NPs fabricated over a blue light emitting diode (LED) are demonstrated for codopant guided white light generation.