Trivalent neodymium (Nd³⁺) ion doped near-infrared (NIR) luminescent nanocrystals (NCs) have recently attracted considerable attention owing to their superior features as excellent NIR-to-NIR luminescent nanoprobes for deep-tissue bioimaging and temperature sensing.

However, it remains notoriously difficult to unravel the electronic structures of Nd³⁺ in NCs and assign its CF transition lines in the NIR of interest because of the line broadening and multiple sites of Nd³⁺ in NCs. 

In a recent study published in Adv. Sci., the research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences (CAS) reported for the first time the electronic structures of Nd³⁺ in LiLuF₄ NCs.  

Researchers used Eu³⁺ ion as the structural probe to unveil the local site symmetry of Ln³⁺ dopants in LiLuF₄ NCs by means of high-resolution photoluminescence (PL) spectroscopy, time-resolved PL (TRPL) spectroscopy, and site-selective PL spectroscopy at 10 K.  

A single spectroscopic site of S₄ symmetry for Ln³⁺ dopants was identified in LiLuF₄ NCs, which is consistent with the crystallographic site symmetry of Lu³⁺ in LiLuF₄ lattice.  

By virtue of temperature-dependent PL spectroscopy, researchers found that a total number of 36 CF transition lines of Nd³⁺ in LiLuF₄ NCs in the NIR region were unequivocally assigned.  

Furthermore, by taking advantage of the well-resolved CF transition lines from the thermally coupled Stark sublevels of ⁴F_3/2 of Nd³⁺, researchers showed the application of LiLuF₄:Nd³⁺ NCs as NIR-to-NIR luminescent nanoprobes for ratiometric detection of cryogenic temperature with a wide linear range of 77–275 K and the highest relative temperature sensitivity (S_r) of 0.62% K^-1, which is comparable to the best S_r value for Nd³⁺-activated luminescent nanothermometers previously reported.  

The unambiguous revelation of photoactive site symmetry and electronic structures of Nd³⁺ in inorganic NCs is of vital importance for future design and development of Nd³⁺-based NIR luminescent nanoprobes toward versatile applications such as cryogenic temperature sensing for space and energy exploration. 

Schematic representation of the ratiometric detection of cryogenic temperature based on LiLuF₄:Nd³⁺ nanoprobes. (Image by Prof. CHEN’s Group) 

Contact: 

Prof. CHEN Xueyuan 

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

Email: xchen@fjirsm.ac.cn