Nanostructure and luminescence properties of amorphous and crystalline ytterbium–yttrium oxide thin films obtained with pulsed reactive crossed-beam deposition
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The nanostructure of ytterbium-doped yttrium oxide thin films, produced using pulsed laser ablation of a Yb–Y alloy target together with a pulsed flow of oxygen, is examined using X-ray and electron diffraction as well as Scanning Transmission Electron Microscopy (STEM). As-produced coatings are amorphous and become nanocrystalline cubic yttria after annealing. STEM images taken in the Bright-Field (BF) and in the High-Angle Annular Dark-Field (HAADF) modes reveal different aspects of the nanostructure of yttria. Simulations of the bixbyite structure of yttria indicate that dark spots arranged in a honeycomb structure seen in the STEM-BF mode arise from the absence of oxygen ions at regular crystallographic locations, while those seen on the HAADF images arise from cationic distortions. These results spectacularly exemplify the complementarity of the BF and HAADF imaging modes. Luminescence properties of amorphous and crystalline samples are also studied. Excitation of Yb3+ ions with an infrared (IR) laser diode produce both IR luminescence from excited Yb3+ and visible luminescence from holmium impurities present in the starting materials. Yb3+ emission bands become increasingly narrower as crystallization takes place, testifying for the transition from inhomogeneous to homogeneous crystal field. Increased lifetime and more intense luminescence observed after annealing imply reduced nonradiative relaxation and higher quantum efficiency.
KeywordsY2O3 Scan Transmission Electron Microscopy Dark Spot Upconversion Luminescence Scan Transmission Electron Microscopy Image
We thank David Troadec, IEMN-CNRS, France for the sample preparation, Pierre St-Onge, Université de Moncton, for technical assistance, and Ralf Bruening, Mount Allison University, for X-ray diffraction data and useful discussions. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).
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