Controllable synthesis of ultrasmall core-shell hexagonal upconversion nanoparticles towards full-color output

Lanthanide-doped upconversion nanomaterials in particular the ultrasmall (<10 nm) nanoparticles have recently gained substantial attention owing to their bright potential in diverse frontier applications such as multicolor displays and biomedical applications. However, it is still lack of mechanistic researches on crystal growth and a fine-tuning of luminescent properties for the ultrasmall nanoparticles. Herein, we report the controllable synthesis of ultrasmall hexagonal core-shell nanoparticles through controlling the crystal growth kinetics processes. By precise manipulation of the reaction temperature and lanthanide ions doping, the size of utrasmall nanoparticles can be reduced to 2.51 nm still with the hexagonal phase, which is critically important for upconversion. Besides, an epitaxial growth of a NaGdF 4 inert-shell layer outside the core nanoparticles leads to a 23.5 times enhancement of the upconversion emission still with the ultrasmall size (6.21 nm). Moreover, the RGB primary color emissions are achieved through properly lanthanide ions doping in these ultrasmall nanoparticles and a set of multicolor output covering the visible spectral range are further obtained by precisely adjusting the mix ratio of the RGB ultrasmall nanoparticles, which shows a great potential in full-color displays and anti-counterfeiting applications.