Yb3+/Er3+ co-doped Dion–Jacobson niobium layered perovskites as NIR-to-green upconversion materials

Abstract

Here, the upconversion behaviour of Yb³⁺/Er³⁺-codoped layered Dion–Jacobson calcium-niobium perovskites was investigated. This lanthanide pair is majorly responsible for the efficient green and red anti-Stokes-like emissions in rare-earth fluorides. Layered perovskites of the composition KCa₂Nb₃O₁₀ were synthesized by the ceramic method by replacing 1% per mol of Ca²⁺ by lanthanide ions in such a way that the Yb³⁺:Er³⁺ molar proportion varied from 2 : 1 to 20 : 1. Non-doped and four doped materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and emission spectroscopy. All materials showed similar XRD profiles, which indicated the isolation of well crystalline single phases. On the other hand, the morphologies accessed by SEM were different: microparticles with well-defined edges were replaced by particles with poorly defined edges, rounded features, and intergrowth aspect when the niobate was doped. The Raman spectra showed modifications in the bands below 200 cm⁻¹ attributed to low-phonon-energy transitions. The doped layered niobates presented almost pure-green emission (excitation at 980 nm), which could be related to the low energy values of the vibrational levels, precluding energy loss. Additionally, the results indicated that the defects generated by the lanthanide ion isomorphic-aliovalent substitution in Ca²⁺ sites were mainly in the interlayer region. This coupled behaviour reduced the non-radiative decays of the upconversion centres, leading to the formation of an efficient emitting material. The spectroscopic emission data showed a two-photon major mechanism to excite the emitting levels via an energy transfer upconversion (ETU) population process. The lifetime results indicated that increasing the Yb³⁺ concentration did not lead to non-radiative decay paths. The high efficiency and the versatility of the layered niobate hosts can provide new insights into the design of efficient morphology-controlled materials.