Study of synthesis temperature effect onß-NaGdF4: Yb3+, Er3+ upconversion luminescence efficiency and decay time using maximum entropy method.

Upconversion materials have several advantages for many applications due to their great potential in converting infrared light to visible. For practical use, it is necessary to achieve high intensity of UC luminescence, so the studies of the optimal synthesis parameters for upconversion nanoparticles are still going on. In the present work, we analyzed the synthesis temperature effect on the efficiency and luminescence decay ofß-NaGd0.78Yb0.20Er0.02F4(15-25 nm) upconversion nanoparticles with hexagonal crystal structure synthesized by anhydrous solvothermal technique. The synthesis temperature was varied in the 290 °C-320 °C range. The synthesis temperature was shown to have a significant influence on the upconversion luminescence efficiency and decay time. The coherent scattering domain linearly depended on the synthesis temperature and was in the range 13.1-22.3 nm, while the efficiency of the upconversion luminescence increases exponentially from 0.02 to 0.10% under 1 W cm-2excitation. For a fundamental analysis of the reasons for the upconversion luminescence intensity dependence on the synthesis temperature, it was proposed to use the maximum entropy method for luminescence decay kinetics processing. This method does not require a preliminary setting of the number of exponents and, due to this, makes it possible to estimate additional components in the luminescence decay kinetics, which are attributed to different populations of rare-earth ions in different conditions. Two components in the green luminescence and one component in the red luminescence decay kinetics were revealed for nanoparticles prepared at 290 °C-300 °C. An intense short and a weak long component in green luminescence decay kinetics could be associated with two different populations of ions in the surface quenching layer and the crystal core volume. With an increase in the synthesis temperature, the second component disappears, and the decay time increases due to an increase in the number of ions in the crystal core volume and a more uniform distribution of dopants.

Synthesis of Iron Oxide Nanoclusters by Thermal Decomposition.

Herein, we report a novel one-step solvothermal synthesis of magnetite nanoclusters (MNCs). In this report, we discuss the synthesis, structure, and properties of MNCs and contrast enhancement in T2-weighted MR images using magnetite nanoclusters. The effect of different organic acids, used as surfactants, on the size and shape of MNCs was investigated. The structure and properties of samples were determined by magnetic measurements, TGA, TEM, HRTEM, XRD, FTIR, and MRI. Magnetic measurements show that obtained MNCs have relatively high saturation magnetization values (65.1-81.5 emu/g) and dependence of the coercive force on the average size of MNCs was established. MNCs were transferred into an aqueous medium by Pluronic F-127, and T2-relaxivity values were determined. T2-Weighted MR phantom images clearly demonstrated that such magnetite nanoclusters can be used as contrast agents for MRI.