Researchers Unravel the Electronic Structures of Neodymium in LiLuF4 Nanocrystals

Recently, trivalent neodymium (Nd³⁺) ion-doped near-infrared (NIR) luminescent nanocrystals (NCs) have gained significant attention due to their advanced features such as outstanding NIR-to-NIR luminescent nanoprobes for deep-tissue bio-imaging and temperature sensing.

However, resolving the electronic structures of Nd³⁺ in NCs and assigning its CF transition lines in the preferred NIR is still challenging due to the line broadening and multiple sites of Nd³⁺ in NCs.

In a recent research reported in Advances Science, a team of researchers headed by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter (FJIRSM) of Chinese Academy of Sciences described the electronic structures of Nd³⁺ in LiLuF₄ NCs for the first time.

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

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

Using temperature-dependent PL spectroscopy, they identified that a total number of 36 CF transition lines of Nd³⁺ in LiLuF₄NCs in the NIR region were explicitly assigned.

In addition, by leveraging the well-resolved CF transition lines from the thermally coupled Stark sublevels of ⁴F_3/2 of Nd³⁺, scientists demonstrated the application of LiLuF₄:Nd³⁺ NCs as NIR-to-NIR luminescent nanoprobes for ratiometric detection of cryogenic temperature with an extensive linear range of 77–275 K and the highest relative temperature sensitivity (S_r) of 0.62% K⁻¹, which is analogous to the best S_r value for Nd³⁺-activated luminescent nanothermometers which were described earlier.

The unequivocal discovery of photoactive site symmetry and electronic structures of Nd³⁺ in inorganic NCs is significantly important for future design and the development of Nd³⁺-based NIR luminescent nanoprobes toward flexible applications, for example, cryogenic temperature sensing for energy and space exploration.