Analysis of the NaYF₄:Yb³⁺, Er³⁺ nanocrystals: up-conversion luminescence, crystal structure and morphology influenced by the dopant concentration and annealing temperature

Abstract

NaYF₄:Yb³⁺,Er³⁺ nanocrystals were prepared using the combustion method. The samples were then annealed at 600 and 700°C for 2 h, respectively, to evaluate the effects of annealing temperature and the variation of the dopant concentration (Yb³⁺-Er³⁺). X-ray diffraction (XRD) analysis revealed the hexagonal phase (β-NaYF₄) for the as-prepared and the nanocrystals annealed at 600°C, as well as the cubic phase for the 700°C annealed nanocrystals. Scanning electron microscopy (SEM) images revealed a variety of morphologies, including spherical, cubic, hexagonal and porous particles, as dopant concentrations (10–30 mol% of Yb³⁺, 1–3 mol% of Er³⁺) and annealing temperatures were varied. Green and red up-conversion emission peaks centred at ~483, 542 and 665 nm attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of erbium, respectively, were observed under 980 nm laser excitation for all Yb³⁺-Er³⁺ co-doped NaYF₄ nanocrystals. UV–Vis analysis revealed that the nanocrystals exhibit six characteristic peaks centred at ~487, 520, 654, 802,972 and 1526 nm ascribed to ⁴I_15/2 → ⁴F_7/2 (~487 nm), ⁴I_15/2 → ⁴H_11/2 (~520 nm), ⁴I_15/2 → ²F_9/2 (~654 nm), ⁴I_15/2 → ⁴I_9/2 (~802 nm), ⁴I_15/2 → ⁴I_11/2 (~972) and ⁴I_15/2 → ⁴I_13/2 (~1500 nm) transitions of Er³⁺, respectively. The absorption peak at ~972 nm ascribed to the ²F_7/2 transition of Yb³⁺ overlaps with the ⁴I_11/2 absorption transition of Er³⁺. Yb³⁺-Er³⁺ doped NaYF₄ nanocrystals’ up-conversion luminescence is studied for possible application in optical devices and solar cells.