NaGdF4:Dy3+ nanocrystals: new insights into optical properties and energy transfer processes.

NaGdF4:Dy3+ nanocrystals (NCs) have been synthesized using a precipitation technique. The structural characteristics and morphology of the materials were analyzed using X-ray diffraction patterns and scanning electron microscopy images, respectively. The photoluminescence excitation spectra, emission spectra and decay curves of all samples were recorded at room temperature. The color feature of Dy3+ luminescence was estimated using CIE chromaticity coordinates and the correlated color temperature. The radiative properties of the Dy3+:4F9/2 level in the material were analyzed within the framework of JO theory. In NaGdF4:Dy3+ NCs, the energy transfer from Gd3+ to Dy3+ causes an enhancement in the luminescence of the Dy3+ ions. The rate of the processes taking part in the depopulation of Gd3+ ions was estimated. The energy transfer between Dy3+ ions leads to the luminescence quenching of NaGdF4:Dy3+. In this process, the dipole-dipole interaction, which is found by using the Inokuti-Hirayama model, is the dominant mechanism. The characteristic parameters of the energy transfer processes between Dy3+ ions have also been calculated in detail.

Reduction Mechanisms of Cu2+-Doped Na2O-Al2O3-SiO2 Glasses during Heating in H2 Gas.

Controlling valence state of metal ions that are doped in materials has been widely applied for turning optical properties. Even though hydrogen has been proven effective to reduce metal ions because of its strong reducing capability, few comprehensive studies focus on practical applications because of the low diffusion rate of hydrogen in solids and the limited reaction near sample surfaces. Here, we investigated the reactions of hydrogen with Cu2+-doped Na2O-Al2O3-SiO2 glass and found that a completely different reduction from results reported so far occurs, which is dominated by the Al/Na concentration ratio. For Al/Na < 1, Cu2+ ions were reduced via hydrogen to metallic Cu, distributing in glass body. For Al/Na > 1, on the other hand, the reduction of Cu2+ ions occurred simultaneously with the formation of OH bonds, whereas the reduced Cu metal moved outward and formed a metallic film on glass surface. The NMR and Fourier transform infrared results indicated that the Cu2+ ions were surrounded by Al3+ ions that formed AlO4, distorted AlO4, and AlO5 units. The diffused H2 gas reacted with the Al-O-···Cu+ units, forming Al-OH and metallic Cu, the latter of which moved freely toward glass surface and in return enhanced H2 diffusion.