Enhanced Optical Temperature Response of Er³⁺‑Doped Hexagonal NaGdF₄ Glass Ceramics via Tm³⁺ Co‑Doping

Background

Infrared‑to‑visible up‑conversion (UC) in trivalent lanthanide‑doped materials has attracted attention for infrared detection, solar cells, and optical thermometry [1–10]. Er³⁺ is especially attractive for temperature sensing because its two pairs of thermally coupled levels (²H_11/2,⁴S_3/2) and (²D_7/2,⁴G_9/2) exhibit temperature‑dependent fluorescence intensity ratios (FIR) [13]. Reported maximum sensitivities of Er³⁺‑based UC materials range from 0.0052 K⁻¹ to 0.0086 K⁻¹, yet most studies neglect the role of excitation power, which can markedly affect the population of thermally coupled levels [18–20]. NaGdF₄ is a low‑phonon, chemically stable host that has shown promising UC behavior and temperature sensitivity for Er³⁺ [23]. However, the impact of pump power and co‑doping with Tm³⁺—which can mediate energy transfer and modify emission—remains unexplored. This study investigates Er³⁺ single‑doped and Er³⁺/Tm³⁺ co‑doped NaGdF₄ glass ceramics, focusing on how Tm³⁺ concentration and excitation power alter their optical temperature response.

Methods

Glass‑ceramic samples with the composition 70.1SiO₂–4.3Al₂O₃–1.8AlF₃–2.3Na₂CO₃–18.5NaF–(2.4–x)Gd₂O₃–0.6Er₂O₃–xTm₂O₃ (x = 0, 0.05, 0.10, 0.15, 0.20) were prepared by melt‑quenching. Each 20 g batch was ground, melted at 1600 °C for 45 min in a corundum crucible, cast into brass molds, and annealed at 700 °C for 20 h to induce crystallization. The resulting transparent ceramics were labeled NGF1–NGF5. Phase purity was confirmed by XRD (Cu Kα, 1.54056 nm), while morphology and grain size were examined by TEM and HRTEM. Luminescence was recorded with an Acton SpectraPro SP‑2300 spectrometer under 980‑nm excitation, and temperature‑dependent spectra were acquired using an INSTEC HCS302 system.

Results and Discussion

Structural analysis (Fig. 1) confirms the formation of hexagonal NaGdF₄ nanocrystals (~30–55 nm) with clear (111) lattice fringes (0.23 nm) and XRD peaks matching JCPDS 27‑0667. Absorption spectra (Fig. 2) reveal characteristic Er³⁺ transitions (378, 405, 488, 520, 652, 972, 1532 nm) and Tm³⁺ peaks at 450 and 1206 nm. Co‑doping alters the 800‑nm absorption, indicating combined Er³⁺–Tm³⁺ energy transfer.

Under 980‑nm excitation, NGF1–NGF5 display distinct up‑conversion emissions (Fig. 3a). With increasing Tm³⁺ content, the 509‑nm green line diminishes, the 542‑nm green intensity decreases, while the 660‑nm red emission initially rises then falls. The red‑to‑green intensity ratio (Fig. 3b) peaks and then stabilizes, evidencing a tunable color output driven by Tm³⁺‑mediated energy transfer.

The energy‑transfer pathways (Fig. 4) involve ground‑state absorption (GSA) and excited‑state absorption (ESA) of Er³⁺, followed by non‑radiative relaxation to the 4S_3/2 and 4F_9/2 levels. Energy transfer from Er³⁺ 4S_3/2 to Tm³⁺ 3H₆ (→Tm³⁺ 3F₄) quenches green emission, while transfer from Er³⁺ 4I_11/2 to Tm³⁺ 3F₄ populates the red‑emitting 4F_9/2 level. Additional ET pathways involving Tm³⁺ 3H₅ further enhance the red emission.

Temperature‑dependent UC spectra (Fig. 5) show that the 4S_3/2 green band weakens with increasing temperature, while the 2H_11/2 band (509 nm) rises due to thermal population. The Boltzmann‑type relationship between the 2H_11/2 and 4S_3/2 intensities (Eq. 3) is well fitted (Fig. 8), confirming the thermally coupled nature of these levels.

Thermal quenching ratios R_Q (Fig. 6) vary with excitation power: at low power (66.8 mW cm⁻²), R_Q for 660 nm is large, indicating significant temperature‑induced quenching, whereas at high power (322.4 mW cm⁻²) the quenching behavior differs, underscoring the power dependence of the emission dynamics.

Log‑log plots of emission intensity versus pump power (Fig. 7) yield slopes n < 2 but > 1 for both green (542 nm) and red (660 nm) bands, confirming a two‑photon UC mechanism that persists across the temperature range.

Relative sensitivity S_R (Eq. 4) reaches 0.001 K⁻¹ at 334 K for Er³⁺‑only samples and 0.00081 K⁻¹ at 292 K for Er³⁺/Tm³⁺ co‑doped samples (Fig. 9). The sensitivity peak shifts to lower temperatures upon Tm³⁺ incorporation, extending the useful range into the near‑room‑temperature domain. Sensitivity also depends on pump power: higher power generally reduces S_R, but in co‑doped samples a maximum appears near 322.4 mW cm⁻².

Compared with other Er³⁺‑based UC thermometers, the Er³⁺/Tm³⁺/NaGdF₄ system exhibits competitive or superior sensitivities, attributable to its large energy separation (ΔE) between thermally coupled levels and the robust host lattice.

Conclusions

Er³⁺‑doped and Er³⁺/Tm³⁺ co‑doped NaGdF₄ glass ceramics fabricated by melt‑quenching display tunable visible emission from green to red under 980‑nm excitation. The optical temperature response—characterized by emission spectra, thermal quenching, FIR, and sensitivity—depends strongly on both Tm³⁺ concentration and pump power. The maximum relative sensitivity of 0.001 K⁻¹ at 334 K (Er³⁺ only) and 0.00081 K⁻¹ at 292 K (co‑doped) demonstrates the viability of these materials for precise optical thermometry across 298–573 K. The findings highlight the importance of co‑doping and excitation conditions in tailoring UC‑based temperature sensors.