Unraveling the Electronic Structures of Neodymium in LiLuF(4) Nanocrystals for Ratiometric Temperature Sensing

Nd(3+)‐doped near‐infrared (NIR) luminescent nanocrystals (NCs) have shown great promise in various bioapplications. A fundamental understanding of the electronic structures of Nd(3+) in NCs is of vital importance for discovering novel Nd(3+)‐activated luminescent nanoprobes and exploring their new applications. Herein, the electronic structures of Nd(3+) in LiLuF(4) NCs are unraveled by means of low‐temperature and high‐resolution optical spectroscopy. The photoactive site symmetry of Nd(3+) in LiLuF(4) 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 Nd(3+), the application of LiLuF(4):Nd(3+) 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 LiLuF(4):Nd(3+) NCs in temperature sensing and also lay a foundation for future design of efficient Nd(3+)‐based luminescent nanoprobes.