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

Abstract

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 c...