Unveiling the mechanism behind shell thickness-dependent X-ray excited optical and persistent luminescence in lanthanide-doped core/shell nanoparticles†

Abstract

Lanthanide-doped fluoride nanoparticles (NPs) exhibit tunable X-ray excited optical luminescence (XEOL) and X-ray excited persistent luminescence (XEPL) properties, demonstrating promising applications in X-ray imaging. However, the mechanisms underlying shell thickness-dependent variations in XEOL and XEPL intensities remain unclear. In this work, we utilize homogeneous NaYF₄:Tb@NaYF₄ and heterogeneous NaYF₄:Tb@NaLuF₄ core/shell NPs to investigate the role of shell thickness. Our results reveal an optimal shell thickness of approximately 3 nm for both XEOL and XEPL, which contrasts with behaviors observed in upconversion systems. The shell layer effectively passivates NP surface defects, reducing energy migration from activators to these defects. However, it also absorbs input X-ray photons, which can diminish X-ray absorption in the core layer. Our findings contribute to the design of lanthanide-doped core/shell NPs with enhanced XEOL and XEPL performances.