Sensitization of sub 10 nm Yb$^{3+}$-doped NaYF$_{4}$ nanoparticles with visible light through 1,2,3,4,5,6,7-heptafluro-8-hydroxyanthracene-9,10-dione chromophore

Uniform sub 10 nm Yb$^{3+}$-doped NaYF$_{4}$ nanoparticles were prepared using a conventional hydrothermal method. Yb$^{3+}$ ions doped inside the NaYF$_{4}$ nanoparticles can be sensitized with 1,2,3,4,5,6,7-heptafluro-8-hydroxyanthracene-9,10-dione (HL), which is bounded onto the surface of the nanoparticle, through the so called ``antenna effect.'' Strong sensitization is achieved with the broad visible light excitations. The overall near infrared (NIR) emission from Yb$^{3+}$ ions is increased by a factor of 5 as a result of the broad and strong absorption of HL chromophore compared with the ytterbium's intrinsic absorption during the 980 nm regime. Interestingly, an energy migration process from Yb$^{3+}$ ions on the surface to inner-side Yb$^{3+}$ ions of doped nanoparticle is demonstrated by the time-resolved spectroscopy method. It gives a direct evidence that the local environment between the surface and center of the nanoparticle is different. We believe our material will contribute to the NIR emitters with a strong visible absorption for the bio-materials.