Lanthanide distribution in NaLuF4:Gd,Yb,Er upconversion nanocrystals by EFTEM and EELS

Abstract

Lanthanide‐doped nanoparticles (NPs) have gained interest within the last decade due to their photon upconversion properties. Upconversion is a multi‐photon process in which two or more lower energy photons are converted to a higher energy photon by step‐wise energy transfer between an absorber ion and an emitter ion (1). These upconversion nanoparticles (UCNPs) are a promising alternative to traditional organic fluorphores and quantum dots in the area of bioimaging because their excitation wavelength (980 nm) lies within an optical window where there is the least absorption and scattering by biomolecules (lower background signals), and they do not exhibit photobleaching or photoblinking. One of the main challenges limiting their application is a trade‐off between luminescence intensity and size of the NP. Smaller NPs exhibit less toxicity as they can be excreted through the urinary system, and they are better for intracellular imaging as the smaller size prevents interference with molecular trafficking within the cell, pharmacokinetics, and protein function; however, upconversion luminescence intensity decreases with size due to, in part, surface quenching from the presence of surface defects (2). Few reports exist on the synthesis of sub‐10 nm UCNPs, and even fewer exist on the synthesis of sub‐5 nm UCNPs that show visible upconversion emission (3‐6). We have recently synthesized two sizes of NaLuF 4 :Gd, Yb, Er UCNPs (ca. 4 nm and ca. 12 nm), through a facile one‐pot method, that both show bright upconversion luminescence upon excitation by a 980 nm laser. In order to better understand their luminescent properties, we have used energy filtered transmission electron microscopy (EFTEM) and electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) to investigate the distribution of the different lanthanides within the nanocrystal. Measurements were performed on an HF‐3300 instrument operating at 300 kV. The results suggest that Lu is enriched in the outer (shell) region of the nanocrystal, while Gd is enriched in the inner (core) region of the nanocrystal (Figure 1). Based on these results, we propose that the Lu shell protects the upconverting core from surface quenching, thereby allowing even 4 nm NPs to show upconversion luminescence. We also propose that the formation of the core‐shell structure is mediated by the way the individual lanthanides nucleate in solution – Gd nucleates first to form the core, while Lu nucleates later to form the shell. This proposal is also in accordance with previous reports on the nucleation of lanthanide NPs (7). EFTEM and EELS in STEM are shown to be valuable tools to characterize the structure of lanthanide‐doped UCNPs and to determine structure‐property relationships, and can aid in the further development of these materials for various applications.