Relation between Excitation Power Density and Er3+ Doping Yielding the Highest Absolute Upconversion Quantum Yield

The upconversion quantum yield (UCQY) is one of the most significant parameters for upconverter materials. A high UCQY is essential for a succesful integration of upconversion in many applications, such as harvesting of the solar radiation. However, little is known about which doping level of the rare-earth ions yields the highest UCQY in the different host lattices and what are the underlying causes. Here, we investigate which Er3+ doping yields the highest UCQY in the host lattices β-NaYF4 and Gd2O2S under 4I15/2 → 4I13/2 excitation. We show for both host lattices that the optimum Er3+ doping is not fixed and it actually decreases as the irradiance of the excitation increases. To find the optimum Er3+ doping for a given irradiance, we determined the peak position of the internal UCQY as a function of the average Er−Er distance. For this purpose, we used a fit on experimental data, where the average Er−Er distance was calculated from the Er3+ doping of the upconverter samples and the lattice parameters of the host materials. We observe optimum average Er−Er distances for the host lattices β-NaYF4 and Gd2O2S with differences <14% at the same irradiance levels, whereas the optimum Er3+ doping are around 2× higher for β-NaYF4 than for Gd2O2S. Estimations by extrapolation to higher irradiances indicate that the optimum average Er−Er distance converges to values around 0.88 and 0.83 nm for β-NaYF4 and Gd2O2S, respectively. Our findings point to a fundamental relationship and focusing on the average distance between the active rare-earth ions might be a very efficient way to optimize the doping of rare-earth ions with regard to the highest achievable UCQY.

Nitrogen-Doping in ZnO via Combustion Synthesis?

We report the synthesis and characterization of colored ZnO-based powders via solution combustion reaction of urea and zinc nitrate hexahydrate in varying molar ratios between 1:1 and 10:1. Among other techniques, we employ X-ray diffraction, nuclear magnetic resonance, and Raman spectroscopy to characterize the products. Within a narrow range of reactant ratios, we reproducibly find an unidentified, crystalline precursor phase related to isocyanuric acid next to ZnO. Finally, we complement our investigations by performing Prompt Gamma Activation Analysis (PGAA) on selected products in order to directly determine elemental bulk compositions and compare these with X-ray photoelectron spectroscopy (XPS) measurements. Our data show traces of nitrogen mainly on the surface of the particles, and thus we question the solution combustion method as a reliable synthesis toward N-doped ZnO. Furthermore, we exclude nitrogen as being responsible for the appearance of the four controversially discussed Raman bands superimposed onto the spectrum of pure ZnO (at 275, 510, 582, and 643 cm–1) and show that the combination of PGAA and XPS is an excellent and complementary method to obtain information about the distribution of the elements in question.