Hydrothermal Synthesis of 19 nm Sc₂O₃:Er³⁺,Yb³⁺ Nanoparticles with 4× Superior Red Upconversion Luminescence

Abstract

We report a simple oleic‑acid mediated hydrothermal route to synthesize ~19 nm Sc₂O₃:Er³⁺,Yb³⁺ nanoparticles (NPs). X‑ray diffraction, TEM, and upconversion luminescence (UCL) measurements confirm a cubic bixbyite host with excellent crystallinity. Under 980 nm excitation, the hydrothermal (HT) samples exhibit a four‑fold increase in red UCL compared to solvothermal (ST) counterparts at identical dopant levels. The enhancement is attributed to reduced surface –OH groups and extended lifetimes, leading to higher quantum efficiency. Power‑dependent studies reveal slopes of 2.5 (red) and 2.1 (green) in log I vs. log P plots, indicating the participation of three‑photon processes at low pump densities. The HT Sc₂O₃:Er³⁺,Yb³⁺ NPs are therefore promising candidates for low‑power NIR‑to‑visible upconversion applications in biomedicine and imaging.

Introduction

Infrared‑to‑visible upconversion is central to next‑generation lasers, solar concentrators, and bio‑imaging. Co‑doping Er³⁺ with high‑concentration Yb³⁺ yields the most efficient energy‑transfer (ET) upconversion system, where 980 nm photons absorbed by Yb³⁺ are relayed to Er³⁺, producing green (≈550 nm) and red (≈660 nm) emissions. Selecting an oxide host with high thermal, mechanical, and chemical stability—such as cubic Sc₂O₃—can further boost performance due to its short Sc–Sc bond length, which accelerates Yb³⁺→Er³⁺ ET.

Experimental

Sample Preparation

Sc₂O₃, Er₂O₃, and Yb₂O₃ (≥99.9 %) were dissolved in dilute HNO₃, mixed with absolute ethanol (20 mL) at stoichiometric ratios, and stirred to form a clear solution. A 2 mL NaOH solution was added dropwise, followed by 1 mL oleic acid and vigorous stirring for 1–2 h. The suspension was sealed in a 50 mL Teflon‑lined autoclave and heated at 180 °C for 24 h. After cooling, the precipitate was washed, dried at 80 °C, and annealed at 700 °C for 2 h. For comparison, identical stoichiometry samples were prepared by a solvothermal (ST) route under the same annealing conditions.

Characterization

Phase purity was verified by Cu‑Kα XRD (Rigaku D/Max IIA). TEM (JEM‑2000EX, 200 kV) revealed monodisperse spheres with a mean diameter of 19 ± 2 nm. UCL spectra were recorded with a Hitachi F‑7000 spectrometer, excited by a 980 nm diode laser (≤3 mW mm⁻²). FT‑IR spectra (Nicolet IS50) confirmed the removal of surface hydroxyls. Lifetime measurements employed an OPO at 980 nm and a Tektronix TDS 3052 oscilloscope.

Results and Discussion

Structural and Morphological Analysis

XRD patterns matched the cubic bixbyite structure (JCPDS 84‑1884). Increasing Yb³⁺ content shifted peaks to lower angles, reflecting lattice expansion due to the larger ionic radius of Yb³⁺. TEM images (Fig. 2) confirmed uniform 19 nm spheres, a size optimal for cellular uptake.

Upconversion Luminescence

HT samples display markedly stronger UCL than ST counterparts. Red emission at 660 nm is enhanced by a factor of 4 (Fig. 3). This is surprising because smaller particles usually exhibit weaker UCL, indicating that surface chemistry and longer excited‑state lifetimes dominate the enhancement. FT‑IR spectra (Fig. 4) show reduced –OH absorption in HT samples, reducing multi‑phonon relaxation (MPR) losses.

Energy‑Transfer Mechanism

Power‑dependent studies yield slopes n = 2.5 (red) and n = 2.1 (green) in log I vs. log P plots, confirming the involvement of three‑photon processes in addition to the typical two‑photon Yb³⁺→Er³⁺ ET. The red emission originates from the sequence: Yb³⁺(²F₅/₂)→Er³⁺(⁴I₁₁/₂) (ET ①), followed by MPR to ⁴I₁₃/₂ and a cross‑relaxation back‑transfer (CRB) to populate ⁴F₉/₂ (ET ②). The green emission arises from sequential ET ③–ET ⁵, with additional cross‑relaxation between Er³⁺ ions.

Lifetime Analysis

Decay curves for both red and green transitions are longer in HT samples (Fig. 10), reflecting reduced surface quenching and higher quantum yields. The similarity of lifetimes for ⁴F₉/₂ and ⁴S₃/₂ transitions supports the CRB‑driven population of ⁴F₉/₂.

Comparison with Other Sesquioxides

Sc₂O₃ hosts yield stronger UCL than Y₂O₃ and Lu₂O₃ under identical conditions (Fig. 11). The shorter Sc–Sc distance (3.27 Å) and stronger crystal field lead to a red‑shift (~8 nm) of the ⁴F₉/₂ emission relative to Y₂O₃, indicative of larger Stark splitting.

Conclusions

We have demonstrated that oleic‑acid mediated hydrothermal synthesis produces ~19 nm Sc₂O₃:Er³⁺,Yb³⁺ nanoparticles with four‑fold stronger red upconversion luminescence than solvothermal equivalents. The enhanced performance stems from minimized surface hydroxyls, extended lifetimes, and efficient Yb³⁺→Er³⁺ energy transfer, including three‑photon pathways. These NPs exhibit superior quantum efficiency, making them ideal for low‑power NIR‑to‑visible applications in bio‑imaging and optical sensing.