Spectroscopic Properties of Nd³⁺-Doped Sr₂LaF₇ Nanoparticles

Lanthanide-doped nanocrystals are widely known systems thanks to their emission characteristics, such as their exceptional thermomechanical properties, chemical stability, large effective Stokes shifts, narrow emission spectra, and long lifetimes. Alkali-earth lanthanide nanophosphors (M₂LnF₇, where M represents Ca, Sr, Ba, etc., and Ln³⁺ stands for Y, La, Gd, Lu, etc.) have been proposed as good host materials. They possess elevated upconversion (UC) luminescence properties and can be grown in dimensions suitable for biomedical imaging applications.

Neodymium (Nd) ions possess intense emissions across various infrared regions, at wavelengths around 900 nm, 1064 nm, and 1300 nm. These emissions originate from transitions between the ⁴F_3/2 state and the lower-lying ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 energy states, respectively.

In this work, we present a thorough investigation of the spectroscopic properties of Sr₂LaF₇ phosphors with different Nd³⁺ ion concentrations (x=1, 2, 3, and 5 mol%). The samples were synthesized using the hydrothermal method and structurally and morphologically characterized. Powder X-ray diffraction analysis confirmed that the materials crystallize in a cubic crystal structure while transmission electron microscopy shows nanoparticles with an average particle size of ~ 40 nm.

All the Nd³⁺ ion concentrations that we studied present a slightly sublinear trend in the emission spectra as a function of the pump power. This indicates the presence of some loss in the energy transfer processes. Moreover, we studied the emission trend as a function of the concentration at a fixed pump power. This study suggests that the optimum concentration for the maximum emission intensity is 3% Nd. The trend shows a decrease in intensity at higher concentrations.

We will present both the emission trends and the lifetime of the ⁴F_3/2 level as a function of the sample temperature, which will range from -190 °C to 600 °C, with insights into the possible energy transfer processes.