Unraveling the Electronic Structures of Neodymium in LiLuF4 Nanocrystals for Ratiometric Temperature Sensing.

Nd3+-doped near-infrared (NIR) luminescent nanocrystals (NCs) have shown great promise in various bioapplications. A fundamental understanding of the electronic structures of Nd3+ in NCs is of vital importance for discovering novel Nd3+-activated luminescent nanoprobes and exploring their new applications. Herein, the electronic structures of Nd3+ in LiLuF4 NCs are unraveled by means of low-temperature and high-resolution optical spectroscopy. The photoactive site symmetry of Nd3+ in LiLuF4 NCs and its crystal-field (CF) transition lines in the NIR region of interest are identified. By taking advantage of the well-resolved and sharp CF transition lines of Nd3+, the application of LiLuF4:Nd3+ NCs as sensitive NIR-to-NIR luminescent nanoprobes for ratiometric detection of cryogenic temperature with a linear range of 77-275 K is demonstrated. These findings reveal the great potential of LiLuF4:Nd3+ NCs in temperature sensing and also lay a foundation for future design of efficient Nd3+-based luminescent nanoprobes.