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

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 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 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 4I15/2 → 4F7/2 (~487 nm), 4I15/2 → 4H11/2 (~520 nm), 4I15/2 → 2F9/2 (~654 nm), 4I15/2 → 4I9/2 (~802 nm), 4I15/2 → 4I11/2 (~972) and 4I15/2 → 4I13/2 (~1500 nm) transitions of Er3+, respectively. The absorption peak at ~972 nm ascribed to the 2F7/2 transition of Yb3+ overlaps with the 4I11/2 absorption transition of Er3+. Yb3+-Er3+ doped NaYF4 nanocrystals’ up-conversion luminescence is studied for possible application in optical devices and solar cells.