Structural analysis and correlative cathodoluminescence investigations of Pr (doped) niobates

Changizi, Rasa; Scheu, Christina (Thesis advisor); Schneider, Jochen M. (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

In the last 40 years, great attention has been given to lanthanide (doped) compounds. These are optical materials that can be used in various applications to make lasers, LEDs as well as biological imaging systems. The reason for the good luminescent behaviour of lanthanide ions lies behind the inner shell 4f-4f electronic transitions which give rise to sharp emission lines. To develop lanthanide (doped) compounds with brighter emission lines it is essential to understand their optical properties and the mechanism of energy transfer between lanthanide ions and surrounding ions within the crystal. This PhD work is about the luminescent properties of Pr (doped) niobates and focuses on the reasons behind the great luminescent properties of PrNbO4. Polycrystalline samples were studied which have been prepared with a solid-state route. Two phases, namely Pr3+:Ca2Nb2O7 (with a cubic crystal structure) and PrNbO4 (crystalized in a monoclinic structure) were produced as μm-sized particles. In addition, Pr3+ doped Ca2Nb3O10 nanosheets were investigated. In the first part of the thesis, Pr3+:Ca2Nb2O7 and PrNbO4 were compared regarding their crystal structure, chemical composition and emission spectra for individual particles. Scanning electron microscopy in addition to energy dispersive X-ray spectroscopy were performed to investigate the morphology and chemical composition of the particles. By using a cathodoluminescence spectrometer attached to the scanning electron microscope, emission spectra for each phase were acquired. The crystal structure of individual particles was revealed using transmission electron microscope. The correlation between the optical properties and the crystal structure was obtained. Brighter emissions lines were observed for PrNbO4. Higher Pr content was the main reason for this finding. In this phase, both K and Ca from the initial KCa2Nb3O10 host were substituted by Pr. As a result, Pr3+ ions occupy the sites with C2 symmetry which perturbs the parity forbidden rule and yields sharp f-f transitions.PrNbO4 and the underlying reasons for the good luminescent behaviour of this material were further investigated and are explained in the second part of this thesis. Presence of defects (regions with high and low density of twins) was identified within the particles. The effect of such defects on the luminescent properties were examined via a correlative study. Backscattered electron imaging and emission spectra were acquired at the same location. Focused ion beam lift out from the same region was done to study the twin’s structure. On each particle two areas including twin free and twinned regions were observed. The results indicated that the regions with higher density of twins show more intense emission lines compared to the areas with less density of twins. The twins were formed during the synthesis and belong to phase transformation twinning type. Coherent structure for the twins was recognized. Third part of the thesis is dedicated to the study of lanthanide 2D nanosheets synthesized with different Pr content. Elemental analysis for single nanosheets was performed using transmission electron microscopy revealing an average Pr concentration of 0.9 at% - 1.8 at% within the [Ca2Nb3O10]- sheets, respectively. Luminescent properties of the 2D nanosheets was compared to the bulk material. Additional transitions in the visible region were observed. Structural characterization of the nanosheets was carried out using high resolution transmission electron microscopy. Thin nanosheets (3 nm) correspond to one triple Ca2Nb3O10 layer surrounded by charge compensating TBA+ molecules. Thicker nanosheets (12 nm) with the same chemical composition were observed indicating that a set of 4 nanosheets were stacked on top of each other. This thesis confirms that electron microscopy is a great tool to get insight about the chemical composition and the crystal structure of lanthanide (doped) compounds. Moreover, it explains the necessity of obtaining emission spectra for each individual particle by using a cathodoluminescence detector within the electron microscope. This enables a precise comparison between the luminescent behaviour of different phases. And it describes why PrNbO4 with higher density of twins are the better candidates for being used in optical applications. 

Institutions

• Division of Materials Science and Engineering [520000]
• Materials Analysis Teaching and Research Area [521220]