Probing the Structure of NaYF₄ Nanocrystals using Synchrotron-Based Energy-Dependent X-ray Photoelectron Spectroscopy

Abstract

Understanding the structure and chemical speciation of the synthesized lanthanide-doped NaYF₄ nanocrystals is of paramount importance to improve and optimize their physical and chemical properties. Hence in this work we employ synchrotron-based high-resolution X-ray photoelectron spectroscopy (XPS) measurements to analyze lanthanide-doped and undoped NaYF₄ nanocrystals. These measurements revealed that there are two doublets for the yttrium ions in the nanocrystal instead of the single doublet in case all Y³⁺ ions have the same chemical environment, generally observed around a binding energy of 160 eV (3d photoelectrons of Y³⁺). This second doublet (binding energy ∼ 157.5 eV) was convoluted with the first doublet (binding energy ∼ 160 eV), and the intensity of this doublet increased with a decrease in excitation X-ray energy. The second doublet was confirmed to belong to the yttrium ions as doped and undoped NaYF₄ nanocrystals exhibit this second peak. The peaks were deconvoluted showing that the second peak is also a doublet with the ratio of the peaks being 2:3. This is exactly the same as what we have observed for the first doublet of the 3d photoelectrons of yttrium ions. In addition, we observe an increase in intensity of the second doublet in comparison to the original 3d doublet of the yttrium ions as the excitation energy is decreased. This suggests that the second doublet is from surface yttrium ions in the NaYF₄ nanocrystal. To confirm our hypothesis, a shell of NaYbF₄ or NaTmF₄ was grown over the NaYF₄ nanocrystal and the second doublet for the yttrium ions was not observed. This is an additional confirmation that the second doublet is indeed from the surface yttrium ions. This implies that the yttriums on the surface of the nanocrystals have a (slightly) different chemical speciation than their counterparts inside the nanocrystals. We attribute the new chemical speciation of surface yttrium ions to the different chemical environment they encounter than their counterparts inside the nanocrystal.