Interfacial Stress‐Modulated Mechanosensitive Upconversion Luminescence of NaErF₄ Based Heteroepitaxial Core–Shell Nanoparticles

Abstract The resulting interfacial stress in heteroepitaxial nanocomplex usually has negligible influence on its performance, but at high pressures (HP) the effect can be significantly amplified. In order to unravel the underlying mechanism and search for high quality pressure sensors, a simple object is employed, NaErF₄@NaLnF₄ (Ln = Y, Lu, Gd) and its Tm³⁺‐doped derivatives, to comparatively study the effect of interfacial stress on upconversion (UC) luminescence. Specifically, the doping enhances greatly mechanosensitivity because of the anisotropic local geometry of the dopant ions of which the structural distortion is amplified by HP. The generated compressive strain does extend into the core as witnessed by the effect of tensile strain in the shell on the core and the fact that Lu‐shelled nanoparticles are the most sensitive to external force among Tm³⁺‐doped system. This compressive strain in concert with the external HP intensifies the coupling between Er³⁺ and Tm³⁺, making the attenuation of green UC emission more pronounced. The red to green UC emission ratio of this material, robust both in structural and optical properties, increases linearly with pressure in the range from ambient to 9 GPa. The study opens a new horizon for understanding the effect of interfacial stress on UC luminescence.