Mono- to few-layer non-van der Waals 2D lanthanide-doped NaYF4 nanosheets with upconversion luminescence

NaYF4 is an efficient host material for lanthanide-based upconversion luminescence and has attracted immense interest for potential applications in photovoltaics, lasers and bioimaging. However, being a non-van der Waals (non-vdW) material, there have been thus far no reports on exfoliation of bulk NaYF4 to nanosheets and their upconversion luminescence properties. Here, we demonstrate for the first time the fabrication of lanthanide-containing NaYF4 2D nanosheets using a soft liquid-phase exfoliation method and report on their optical, electronic and chemical characteristics. The nanosheets exfoliated from NaYF4:Yb,Er microcrystals consisting mainly of β-NaYF4 become enriched in α-NaYF4 post exfoliation and have a large micron-sized planar area with a preferential (100) surface orientation. X-ray absorption spectroscopy confirms that both Yb and Er doping ions are retained in the exfoliated nanosheets. Through centrifugation, NaYF4 2D nanosheets are successfully obtained with thicknesses ranging from a monolayer to tens of layers. Optical analysis of individual nanosheets shows that they exhibit both optical down-conversion and upconversion properties, albeit with reduced emission intensities compared with the parent microparticles. Further exploration of their electronic structure by density functional theory (DFT) calculations and photoelectron spectroscopy reveals the formation of surface F atom defects and a shrinkage of the electronic bandgap in ultrathin nanosheets. Our findings will trigger further interest in non-vdW 2D upconversion nanomaterials.