Energy Transfer Upconversion (ETU) with rare earth (RE) doped nanocrystals has drawn a lot of attention in recent years. Although Yb³⁺ sensitized ETU is relatively efficient, many applications, such as biodetection and bioimaging, still suffer from low absorption cross section (σ_ABS) of RE ions and low quantum yield of ETU phenomenon. Due to around 5-fold larger σ_ABS of Nd³⁺ ions and low σ_ABS of water at ~800 nm (⁴I_9/2→⁴F_5/2 :Nd³⁺) in respect to ~980 nm (²F_7/2→²F_5/2 : Yb³⁺) of conventional Yb³⁺ sensitizer, Nd³⁺ ions are considered to be a good alternative for improving pumping efficiency [1, 2]. Interestingly, the energy transfer between the Nd^{3+ 4}F_3/2 and Yb^{3+ 2}F_5/2 states reached nearly 90% efficiency. It was demonstrated that the Yb³⁺ ions can act as efficient energy migrators, facilitating energy transfer from Nd³⁺ ions to actual activator ions [1]. Additionally, the use of Nd³⁺ ions as sensitizers significantly minimizes the overheating, which is caused by conventional 980 nm excitation through water absorption. On the other hand, the introduction of Nd³⁺ ions as sensitizers may directly quench the up-conversion emission, due to the energy transfer between Nd³⁺ and activators. For this reason, separating the Nd³⁺ and activator spatially is necessary. A promising method to avoid parasitic ET processes could be designing a core-shell structure where the activator and Nd³⁺ ions will be separated in the core and shell, respectively. In our work the impact of the concentration of the neodymium ions on the efficiency of upconversion energy (800 nm and 975 nm excitation) was investigated. A relatively high concentration of primer sensitizing Nd³⁺ ions doped in the shell host allows for effective absorption of the excitation NIR energy (around 800 nm) and subsequent energy transfer to secondary sensitizer Yb³⁺ ions and followed by excitation and emission from activator Tb³⁺ ions.

Acknowledgement

The research was supported by Wrocław Research Centre EIT+ and under the project “The Application of Nanotechnology in Advanced Materials” NanoMat (POIG.01.01.02-02-002/08) financed by the European Regional Development Fund (Innovative Economy Operational Programme, 1.1.2) 

Reference

[1] X. Xie, N. Gao, R. Deng, Q. Sun, Q.-H. Xu, and X. Liu, JACS, 135 (2013) 12608-12611

[2] Y.F. Wang, G.Y. Liu, L.D. Sun, J.W. Xiao, J.C. Zhou, and C.H. Yan, Acs Nano, 7, (2013),  7200-7206