The past few years witnessed extensive emergence of short-wavelength upconversion（UC） emission stimulated photoactivation studies. However, low efficiency of multi-photon process greatly limits further applications. Here, ultraviolet（UV） upconversion emissions originated from multi-photon process of Tm^3＋ were studied with Nd^3＋-sensitized NaGdF4：Yb,Tm@NaYF4：Nd,Yb core/shell nanoparticles. Crucial factors, including the contents of sensitizers Nd^3＋, Yb^3＋ and activator Tm^3＋, as well as the excitation power density were investigated based on the UV emission. Spectral results showed that high contents of Nd^3＋ in shell region up to 50%（molar fraction hereafter） and Yb^3＋ of 10% were essential to mediate the energy transfer via the core/shell interface and facilitate multi-photon UV emissions. Compared with segregated activator and sensitizer, a core/shell strategy with isolated Nd^3＋ in the shell was important for higher UV emission. Although the upconverting process was initiated with Nd^3＋→Yb^3＋, the short-wavelength emissions were intrinsically coming from four- and five-photon process. The optimized nanoparticles were found to be able to manipulate the configuration transition of azobenzene molecules, and it could be promising for near infrared（NIR） triggered optical switches applications.
To break through the bottle-neck of quantum yield in upconversion （UC） core-shell system, we elucidated that the energy transfer efficiency in core-shell system had an evident contribution from the charge transfer of interface with related to two factors： （i） band offsets and （2） binding energy area density. These two variables were determined by material intrinsic properties and core-shell thickness ratio. We further unraveled the mechanism of non-radiative energy transfer by charge transfer induced dipole at the inter- face, based on a quasi-classical derivation from F6rster type resonant energy transfer （FRET） model. With stable bonding across the interface, the contributions on energy transfer in both radiative and non-radiative energy transfer should also be accounted together in Auzel＇s energy transfer （ETU） model in core-shell system. Based on the discussion about interface bonding, band offsets, and forma- tion energies, we figured out the significance of interface bonding induced gap states （IBIGS） that played a significant role for influ- encing the charge transfer and radiative type energy transfer. The interface band offsets were a key factor in dominating the non-radiative energy transfer, which was also correlated to core-shell thickness ratio. We found that the energy area density with re- lated to core/shell thickness ratio followed the trend of Boltzman sigmoidal growth function. By the physical trend, this work contrib- uted a reference how the multi-layered core-shell structure was formed starting from the very beginning within minimum size. A route was paved towards a systematic study of the interface to unveil the energy transfer mechanism in core-shell systems.
