Lanthanide doped upconversion nanocrystals, showing bright future in diverse fields, are typically excited by ≈700–1000 nm light when Nd3+ and Yb3+ are used as sensitizers. Thus far, extending the excitation range of upconversion nanocrystals is still a formidable challenge. Herein, a new type of upconversion nanocrystals is reported, using Er3+ ions as sensitizers, which can be excited by 1532 nm light located in the second near‐infrared biological window. Through Er3+ sensitization, upconversion emission from a series of activators, including Nd3+, Ho3+, Eu3+, and Tm3+, is obtained and can be modulated by Yb3+ codoping. In addition, Er3+ sensitized photon upconversion of Ho3+ and Tm3+ can be further enhanced by shell coating. It is found that Er3+ sensitized upconversion processes are mainly dependent on the energy transfer between Er3+ ions and activators. Considering the demonstration of anticounterfeiting by using this newly designed nanocrystal, it is anticipated that these results can bring more opportunities to upconversion nanomaterials in other aspects, ranging from lasing to super resolution imaging.

The perfect energy level overlap of 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 in Er 3+ ions with those of 5 F 3 , 5 F 4 / 5 S 2 , and 5 F 5 in adjacently codoped Ho 3+ ions allows efficient interenergy transfer. Therefore, in addition to routine activators, Er 3+ or Ho 3+ can further act as sensitizers to transfer the upconverted energy to nearby Ho 3+ or Er 3+ , resulting in enhanced upconversion luminescence due to the emission overlap. Proper codoping of Er 3+ /Ho 3+ or Ho 3+ /Er 3+ obviously elevates the maximum doping concentration (thus producing additional upconverted photons) to a level higher than that causing luminescence quenching and significantly enhances upconversion emissions compared with those of singly Er 3+ or Ho 3+ -doped host materials. Indeed, the so-far strongest red upconversion emission under 1532 nm excitation was obtained in LiYF 4 :Er/Ho@LiYF 4 nanoparticles and Ho 3+ -sensitized Er 3+ upconversion emissions excited by 1150 nm laser was simultaneously discovered. With great enhancement compared with that of singly Ho 3+ doped counterparts, this work demonstrates the generality and rationality of our design strategy.

This paper presents an investigation on the fluorescent properties of semiconductor CdTe quantum dots (QDs) self-assembled on the surface of PVP (polyvinylpyrrolidone)-capped silver nanoparticles (NPs) by the ligand field effect. A significant 2.5-fold enhancement in the integrated fluorescence intensities, red shift of fluorescence peak, and obvious decrease of lifetime were observed in the CdTe QDs assembled on the Ag NPs in comparison with the pure CdTe QDs. The fluorescence enhancement factor and red shift were found to depend on the Ag NP concentration. The fluorescence enhancement was attributed to a highly localized electromagnetic field on the Ag NPs generated by the surface plasma and the change in the surface trap state of the CdTe QDs originating from plasma oscillations in the Ag NPs. It is first proposed that the surface passivation of CdTe QDs is also an important factor for metal-enhanced fluorescence. The surface defects of CdTe QDs can be modified by the Cd-O coordination interaction between the CdTe QDs and PVP molecules, which will cause the trap state density and luminescence lifetime to decrease. The surface passivation of CdTe QDs can also improve fluorescence quantum yield and lead to the red shift of the fluorescence peak. Compared with previous reports, the occurrence of the self-assembly of CdTe QDs on the surface of PVP-capped Ag NPs is fairly simple and easy. From a practical point of view, the combination of CdTe QDs with Ag NPs may lead to the fluorescence enhancement, which could be utilized in a variety of chemical and biological detection applications.