Lanthanide-Dependent Clustering in Yb$^{3+}$/Ln$^{3+}$ Co-Doped CaF$_2$ Nanocrystals: Correlating Spectroscopic Signatures with DFT Insights

TL;DR

This work elucidates heterogeneous lanthanide clustering in CaF2 nanocrystals co-doped with Yb3+ and Ln3+ by combining hydrothermal synthesis, PXRD/DLS/TEM, and high-resolution FTIR with first-principles DFT calculations. FTIR reveals cluster-related bands whose intensities depend on the co-doped Ln3+ ion, while DFT identifies two dominant cluster motifs (neutral C4v Ln-containing aggregates and negatively charged hexamer derivatives) whose concentrations trend with the Ln series. The calculated formation energies and cluster populations reproduce the experimental observations, linking spectral features to specific defect structures. These insights advance understanding of nanoscale clustering mechanisms and inform strategies to optimize luminescence in CaF2-based nanomaterials for applications in bioimaging, thermometry, and lighting.

Abstract

The formation of heterogeneous lanthanide-ion clusters in CaF$_2$ was investigated experimentally and computationally. CaF$_2$ nanoparticles co-doped with 20~mol\% Yb$^{3+}$ and 2~mol\% Ln$^{3+}$ (Ln$^{3+}$ = Ce$^{3+}$, Pr$^{3+}$, Nd$^{3+}$, Sm$^{3+}$, Eu$^{3+}$, Gd$^{3+}$, Ho$^{3+}$, Er$^{3+}$, and Tm$^{3+}$) were synthesized via a hydrothermal method. The structural and morphological properties were characterized using powder X-ray diffraction, dynamic light scattering, and transmission electron microscopy techniques. High-resolution Fourier transform infra-red spectroscopy revealed the presence of Yb$^{3+}$ isolated cubic centers and various cluster sites. The relative concentration of the clusters varied with the choice of the co-doping ion. Calculations based on density functional theory were used to estimate the formation energies and local coordination structures of different clusters. The calculations indicate that the neutral $C_{4v}$ aggregations containing Ln$^{3+}$ tend to decrease across the lanthanide series, while the negatively charged derivatives of hexameric clusters are relatively constant. This variation matches the experimental results. This study advances understanding of the clustering mechanisms in lanthanide-doped CaF$_2$ nanoparticles and has implications for luminescence optimization in advanced nanomaterials.

Figures (4)

• Figure 1: (a) Powder X-ray diffraction patterns of CaF$_2$ nanoparticles doped with 20 mol% Yb$^{3+}$ and 2 mol% Ln$^{3+}$ nanoparticles. The diffraction patterns of cubic CaF$_2$ (ICDD Card No 04-002-4443) are also depicted. (b) Hydrodynamic sizes measured in dynamic light scattering of the nanoparticles. (c) TEM images of CaF$_2$: 20 mol% Yb$^{3+}$/ 2 mol% Ho$^{3+}$ nanoparticles.
• Figure 2: FTIR absorption spectra Yb$^{3+}$${}^2\mathrm{F}_{7/2}{\rightarrow}{}^2\mathrm{F}_{5/2}$ inter-multiplet measured at 10 K for singly-doped 20 mol% Yb$^{3+}$ and co-doped 20 mol% Yb$^{3+}$/2 mol% Ln$^{3+}$ CaF$_2$ nanoparticles.
• Figure 3: Coordination structure of (a) $O_h$$\mathrm{Ln}_\mathrm{Ca}^\bullet$, (b) $C_{3v}$$\bigl[\mathrm{Ln}_\mathrm{Ca}{:}\mathrm{F}_i\bigr]^\times$, (c) $C_{4v}$$\bigl[\mathrm{Ln}_\mathrm{Ca}{:}\mathrm{F}_i\bigr]^\times$, (d, e) $\bigl[3\mathrm{Ln}_\mathrm{Ca}{:}3\mathrm{F}_i\bigr]^\times$ and (f, g) $\bigl[3\mathrm{Ln}_\mathrm{Ca}{:}4\mathrm{F}_i\bigr]^\prime$ clusters in alkaline earth fluoride system. The legend is as follows: blue: Ca$^{2+}$ ion, green: F$^-$ ion, red: Ln$_\mathrm{Ca}^\bullet$ substitutional, yellow: F$_i^\prime$ interstitial, white: $V_\mathrm{F}^\bullet$ vacancy.
• Figure 4: Behaviour of heterogeneous clusters across the lanthanide series in Ln$^{3+}$/Yb$^{3+}$ doped CaF$_2$. (a) Ratio of the peak areas obtained from experimental FTIR absorption spectra. (b) Concentration of dominant clusters obtained from the DFT calculations. Other clusters contain up to three Ln and Yb ions combined have concentrations far too low to be shown on this scale.