Cathodoluminescence, SEM and EDX analysis of CaF2 and Tm2O3 pellets for radiation dosimetry applications

https://doi.org/10.1016/j.radphyschem.2021.109621Get rights and content

Highlights

  • A simple dry thermal process was performed to activate CaF2 with Tm2O3 (3 mol%).

  • EDX spectra and composition maps showed the average homogeneity of the specimens.

  • SEM/CL-RGB images displayed a new LC in the blue wavelength (450–475 nm) due to the material fabrication process.

Abstract

Preparation and synthesis of luminescent materials has a special interest in medicine, industry and research due to the wide variety of applications offered by phosphors. In this study, pellets of a mixture of CaF2 and Tm2O3 which were pressed and thermally treated up to 600 °C were prepared in air. Combined SEM (scanning electron microscopy), EDX (energy dispersive X-ray analysis) and CL-RGB (cathodoluminescence red-green-blue) microanalysis and, complementarily spectral CL analysis of this phosphor were performed. EDX analysis showed that the activator was homogeneously distributed in the prepared specimens. The additional CL emission can be attributed to the properties of some luminescent centers that were apparently created during the annealing. In addition, it seems that other trapping centers were created by new structural defects during the preparation process in the CaF2 lattice. The combined techniques SEM/EDX/CL-RGB have confirmed their capabilities in the characterization of the prepared materials. This contribution provides a process and characterization methods used in the fabrication and characterization of a phosphor that can also be used to optimize the luminescent emission of other similar synthetic materials with rare earth oxides as activators.

Introduction

Rare-earth elements (REE)-activated materials are of great interest due to their luminescent properties. Synthesis of REE-based phosphors is usually employed in the fabrication of LEDs, lasers and luminescent detectors, with many applications in medicine, industry, and research where REE oxides are the main raw material used in the synthesis of these kinds of phosphors (Cui and Hope, 2015). In particular, trivalent rare-earth ions such as Tm3+ can be introduced in some matrix (e.g. CaF2 and CaSO4), enabling luminescent REE-activated materials to be fabricated (Vasconcelos et al., 2014; Madhusoodanan et al., 1999).

The creation of trapping and recombination centers and the corresponding energy levels in the bandgap provide a new electronic configuration that will be reflected in the luminescence stimulation mechanism by thermoluminescence (TL) (Madhusoodanan et al., 1999), optically stimulated luminescence (OSL) (Asfora et al., 2016), up-conversion (UC) (Cao et al., 2008) or near-ultraviolet excitation (NUV-e) (Cai et al., 2014). Electron-hole energy is transferred to the Tm3+ ion (Upadeo et al., 1994) when returning to the ground state and blue light is produced by the three transitions: 1D23F4, 3P03F4 and 1G43H6. These luminescent emissions are expected when thulium oxide has been diffused in the CaF2 matrix. However, this phosphor has previously been synthesized via wet chemical synthesis (Vasconcelos et al., 2014) and combustion synthesis (Asfora et al., 2016; Vasconcelos et al., 2016), which are protracted processes and complex infrastructures (Cao et al., 2008; Cai et al., 2014).

In this study, powders of Tm2O3 and CaF2 were homogeneously mixed and pressed. This was then followed by a simple dry heating process and was preliminarily studied as a new TL material suitable for radiation dosimetry applications (Rodríguez et al., 2020). Going beyond this previous work, the structure and composition of the resulting material was characterized by scanning electron microscopy (SEM) at room temperature to identify non-uniformities or local changes in chemical composition in micrometric scale. While the EDX technique estimates the qualitative elemental composition in addition to the changes of atomic fractions in the prepared samples (Goldstein et al., 2018), the CL-RGB signal provides a filtered Red-Green-Blue (RGB) image of the luminescent center (LC) emissions with high spatial resolution (typically 1 μm) (Boggs and Krinsley, 2006). The RGB contrasted images in the visible spectrum has been compared to CL spectra for providing the luminescent characterization of the fabricated material.

Section snippets

Preparation of specimens

The main feature of this preparation procedure was its simplicity. For the raw materials, Tm2O3 powder (99.9% of purity) was supplied by ALFA laboratory, while the CaF2 SUPRAPUR was supplied by MERCK laboratory. The CaF2 is a high-grade Standard Reference Material certified by the reference NIST180 Fluorspar NIST® SRM® 180, with the following metal trace basis (<150 ppm), obtained by qualitative spectrochemical analysis: Fe (0.1–1.0%); Al, Ba, Mg, Pb, Si and Sr (0.01–0.1%); Cu, K, Mn, Na, Ti

SEM/EDX analysis

Fig. 1, Fig. 2, Fig. 3 display SEM/EDX composition maps of the thermally prepared specimens (3-600-1h, 3-600-3h, and 3-600-7h) in false colors. Maps of Ca, Tm, F and O in yellow, green, blue and red respectively, are displayed by insets (a), (b), (c) and (d). Circles indicating the presence of Ca and Tm are highlighted in the insets (a) and (b). The absence of each element is shown in black. Thus, insets (a) display the presence of Ca within white circles, while insets (b) also highlight them

Discussion

In an attempt, to find a simple annealing procedure in air, nearly pure thulium oxide and calcium fluoride were used to make pellets of a novel luminescent material. The choice of Tm2O3, which already has its lattice saturated with oxygen, could prevent external oxygen contamination at temperatures well below the melting point, as opposed to TmF3 (the usual form of dopant) which oxidizes rapidly and requires an oxygen-free atmosphere. Despite the possibility of oxygen contamination with the use

Conclusions

In this work, the fabrication and microanalysis characterization of an easy-to-obtain and low-cost luminescent material have presented here. A combined method of pressure (60 MPa) and thermal treatment giving rise to pellets of a phosphor consisting of a mixture of Tm2O3 and CaF2 was carried out. SEM/EDX microanalysis permits to conclude that all specimens were homogeneously prepared and no elemental composition changes can be highlighted because of preparation process. On the other hand, the

Author contribution statement

Rafael Rodriguez: Conceptualization, Methodology, Investigation. Virgilio Correcher: Resources, Supervision, Investigation. Jose Maria Gomez-Ros: Funding acquisition, Supervision. Jose Luis Plaza: Resources, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work has been partially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under Grant FIS2015-64793-C02.

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