Tuning of structural, laser power-dependent and temperature dependent luminescence properties of NaYF₄:Yb, Er (Y: 88%, Yb: 10 and Er: 2%) submicron crystals using Cr³⁺ ion doping

In this work, we show the influence of Cr³⁺ ion concentration (1–7 mol%) on the crystalline phase and shape, laser power dependent luminescence response, and temperature response in physiological temperature range for NaYF₄:Yb, Er (Y: 88%, Yb: 10% and Er: 2%) submicron crystals prepared via hydrothermal method. The atomic absorption spectral studies indicate the successful incorporation of the Cr³⁺ ions into NaYF₄:Yb, Er (Y: 88%, Yb: 10% and Er: 2%). The detailed investigation on the crystal structure using XRD and FESEM studies elucidate that even the 1 mol% addition of Cr³⁺ ions lead to the phase of the crystal from mixed to hexagonal while retaining the shape of the crystals as hexagonal microprism. The FTIR and diffuse reflectance studies further confirms the presence of Cr³⁺ ions in the host lattice. The luminescence studies carried out confirm that the green as well as red emission of the samples is affected greatly with the Cr³⁺ ion concentration with an optimum concentration of 5 mol % and further addition leads to substantial quenching in the luminescence. The laser power dependent studies unambiguously show that the transitions ²H_11/2 → ⁴I_15/2 (G1), ⁴S_3/2 → ⁴I_15/2 (G2) and ⁴F_9/2 → ⁴I_15/2 (R) transitions are multi-photon process. The temperature dependent studies in the physiologically relevant region (30–60 °C) of the thermodynamically coupled states of G1 and G2 follows a linear behavior and can be explored as optical thermometer. The results provided here are relevant for the potential practical applications in bioimaging, thermometry and luminescent devices.