Brightness attenuation mechanisms of Er3+ self-sensitized upconversion nanocrystals under 1.5 μm pumping

Abstract The Er3+ self-sensitized upconversion nanoparticles (UCNPs) under 1.5 μm pumping presents great promises due to their high luminescence efficiency and safe excitation wavelength. However, its practical application ability are limited due to unsatisfactory brightness induced by organic compounds. We have even observed completely quench when it dispersed in polymer. Herein, the effects of high-frequency vibration modes from the organics on luminescence properties of β-NaYF4:Er3+ excited by 1.5 μm were studied in detail. The results showed that decreased emission of β-NaYF4:Er3+ in organic compounds containing C H bonds is mainly attributed to the excitation energy attenuation induced by molecules. The C H bonds in aliphatic molecules possessing strong absorption in near infrared region will dissipate the 1.5 μm photons energy. Equally importantly, an ignored quenching pathway was considered that the Er3+ 4I13/2 state is directly quenched by the near infrared vibrations of C H at ~7000 cm−1. The de-excitation of the Er3+ first-excited state eventually affects the population of upper electronic levels. After considering to avoid the above quenching pathways, the ligand-free NaYF4:Er@NaYF4 UCNPs exhibited satisfactory brightness upon 1.5 μm excitation and could be used for anti-counterfeiting labels. Our findings will promote the developments of many advanced applications based on Er3+ doped UCNPs.