Exploring the upconversion tunability of unilamellar [SrTa2O7 nanosheets derived from Aurivillius Bi2SrTa2O9 ]2-

This study presents the first report on high-yield and large-sized unilamellar [SrTa2O7]2- nanosheets with upconversion (UC) properties derived from the Aurivillius phase Bi2SrTa2O9. By employing three-step exfoliation process using exfoliating agents with only moderate size differences-large enough to enable exfoliation while preventing excessive strain-we successfully obtained nanosheets with lateral dimensions reaching up to the micron scale. This controlled exfoliation approach minimizes nanosheet fragmentation, allowing the retention of large lateral sizes. One of the key advantages of Aurivillius phase perovskite oxides for UC applications is their relatively low lattice phonon energy, which is comparable to other well-known efficient UC host materials. The nanosheets were synthesized through a solid-state reaction with Er(III), Ho(III), Tm(III), and Yb(III) lanthanide doping, followed by selective acid washing and exfoliation. Structural analysis confirmed the successful incorporation of lanthanides into the perovskite layers, preserving UC behavior in the exfoliated nanosheets. The effect of sensitizer-to-activator ratio on UC emission was systematically investigated, revealing tunable multicolor emission. The nanosheets exhibit comparable emission characteristics to their bulk counterparts, demonstrating their potential for integration into optical applications. These findings highlight the viability of Aurivillius-based nanosheets as functional UC materials with tunable emission properties.