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.