Direct Evidence of Significant Cation Intermixing in Upconverting Core@Shell Nanocrystals: Toward a New Crystallochemical Model

• Published in: Chemistry of materials 2017-11, Vol.29 (21), p.9238-9246
• Main Authors: Hudry, Damien, Busko, Dmitry, Popescu, Radian, Gerthsen, Dagmar, Abeykoon, A. M. Milinda, Kübel, Christian, Bergfeldt, Thomas, Richards, Bryce Sydney
• Format: Article
• Language: English
• Subjects: NANOSCIENCE AND NANOTECHNOLOGY
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.7b03118

Core@shell design represents an important class of architectures because of its capability not only to dramatically increase the absolute upconversion quantum yield (UCQY) of upconverting nanocrystals (UCNCs) but also to tune energy migration pathways. A relatively new trend toward the use of very thick optically inert shells affording significantly higher absolute UCQYs raises the question of the crystallographic and chemical characteristics of such nanocrystals (NCs). In this article, local chemical analyses performed by scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDXS) and X-ray total scattering experiments together with pair distribution function (PDF) analyses were used to probe the local chemical and structural characteristics of hexagonal β-NaGd0.78Yb0.2Er0.02F4@NaYF4 core@shell UCNCs. The investigations lead to a new crystallochemical model to describe core@shell UCNCs that considerably digresses from the commonly accepted epitaxial growth concept with sharp interfaces. The results obtained on ultrasmall (4.8 ± 0.5 nm) optically active cores (β-NaGd0.78Yb0.2Er0.02F4) surrounded by an optically inert shell (NaYF4) of tunable thickness (roughly 0, 1, 2, and 3.5 nm) clearly indicate the massive dissolution of the starting seeds and the interdiffusion of the shell element (such as Y) into the Gd/Yb/Er-containing core giving rise to the formation of a nonhomogeneous solid solution characterized by concentration gradients and the lack of sharp interfaces. Independently of the inert shell thickness, core/interface/shell architectures were observed for all synthesized UCNCs. The presented results constitute a significant step toward the comprehensive understanding of the “structure–property” relationship of upconverting core@shell architectures, which is of prime interest not only in the development of more efficient structures but also to provide new physical insights at the nanoscale to better explain upconversion (UC) properties alterations.