Dimensionality and Surface‐State Effect on Upconversion and Optical Thermometry in Yb ³⁺ /Ho ³⁺ ‐Doped KCa ₂ Nb ₃ O ₁₀ and Its Exfoliated Two‐Dimensional Nanosheets

Lanthanide‐doped layered perovskites offer structurally robust, low‐phonon oxide environments that are attractive for upconversion‐based optical thermometry. Here, Yb ³⁺ /Ho ³⁺ ‐codoped KCa ₂ Nb ₃ O ₁₀ layered perovskites and their exfoliated Ca ₂ Nb ₃ O ₁₀ ⁻ nanosheets were prepared to elucidate how dimensionality reduction and surface‐state modification influence upconversion emission and luminescence intensity ratio (LIR) thermometry. Under 980 nm excitation, the samples exhibited strong upconversion emissions at 546 and 661 nm, corresponding to the ⁵ F ₄ / ⁵ S ₂ → ⁵ I ₈ and ⁵ F ₅ → ⁵ I ₈ transitions of Ho ³⁺ , following a two‐photon absorption mechanism. Temperature‐dependent upconversion spectra revealed a maximum relative sensitivity of 0.55% K ⁻¹ at 573 K with repeatability over multiple heating–cooling cycles. The exfoliated Ca ₂ Nb ₃ O ₁₀ ⁻ : Yb ³⁺ /Ho ³⁺ nanosheets maintained the upconversion activity with a stable temperature response up to 473 K. In addition, controlled post‐treatments were employed to tailor surface states in Langmuir–Blodgett‐assembled nanofilms composed of Ca ₂ Nb ₃ O ₁₀ ⁻ : Yb ³⁺ /Ho ³⁺ nanosheets; atomic force microscopy measurements reveal a pronounced reduction in nanoscale adhesion energy after thermal annealing followed by laser irradiation, indicating weakened tip–surface interactions consistent with surface decontamination and photon‐assisted surface modification. Overall, this work highlights Yb ³⁺ /Ho ³⁺ ‐doped layered perovskites and their two‐dimensional nanosheets as a versatile oxide platform for upconversion emission, thermometry, and surface‐engineered photonic interfaces.