Quantification of the Activator and Sensitizer Ion Distributions in NaYF₄:Yb³⁺, Er³⁺ Upconverting Nanoparticles Via Depth‐Profiling with Tender X‐Ray Photoemission

Abstract The spatial distribution and concentration of lanthanide activator and sensitizer dopant ions are of key importance for the luminescence color and efficiency of upconverting nanoparticles (UCNPs). Quantifying dopant ion distributions and intermixing, and correlating them with synthesis methods require suitable analytical techniques. Here, X‐ray photoelectron spectroscopy depth‐profiling with tender X‐rays (2000–6000 eV), providing probe depths ideally matched to UCNP sizes, is used to measure the depth‐dependent concentration ratios of Er³⁺ to Yb³⁺, [Er³⁺]/[Yb³⁺], in three types of UCNPs prepared using different reagents and synthesis methods. This is combined with data simulations and inductively coupled plasma‐optical emission spectroscopy (ICP‐OES) measurements of the lanthanide ion concentrations to construct models of the UCNPs’ dopant ion distributions. The UCNP sizes and architectures are chosen to demonstrate the potential of this approach. Core‐only UCNPs synthesized with XCl₃·6H₂O precursors (β‐phase) exhibit a homogeneous distribution of lanthanide ions, but a slightly surface‐enhanced [Er³⁺]/[Yb³⁺] is observed for UCNPs prepared with trifluroacetate precursors (α‐phase). Examination of Yb‐core@Er‐shell UCNPs reveals a co‐doped, intermixed region between the single‐doped core and shell. The impact of these different dopant ion distributions on the UCNP's optical properties is discussed to highlight their importance for UCNP functionality and the design of efficient UCNPs.