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Luminescence on HfO2 crystals: synthesis, heat treatment effect, related defects, and their application in ionising radiation dosimetry.

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Author(s):
Alvaro de Farias Soares
Total Authors: 1
Document type: Doctoral Thesis
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
Defense date:
Examining board members:
Sonia Hatsue Tatumi; Angela Mitie Otta Kinoshita; Susana de Souza Lalic; Nilo Francisco Cano Mamani; Sebastião Gomes dos Santos Filho
Advisor: Sonia Hatsue Tatumi
Abstract

As a consequence of using radiation, measurements of control and protection were established to avoid human beings and the environment from its harmful effects. In this instance, different kinds of materials are used to measure radiation doses, as is the case of dosimeters. Luminescent dosimeters based on thermoluminescence (TL) and optically stimulated luminescence (OSL) have been investigated since the middle of the 19th century, and nowadays, commercial detectors are available. Those techniques effectively allow for studying materials properties, shedding light on the emission features and underlying mechanisms, and in the characterisation of them. In this regard, hafnium oxide (HfO2), which has recently been widely studied in electronics, has poorly been studied regarding its luminescence, with few reports on its TL. Therefore, this study aimed to do a more in-depth investigation of the luminescence features of HfO2, the influence of heat treatment on them, as well as the potential application of it as an ionising radiation dosimeter. For that, samples were synthesised by using the precipitation method followed by different heat treatments at 1000, 1200, 1400 and 1600 °C for 2 hours. Crystalline structure was evaluated by X-ray diffraction (XRD), whilst luminescent was approached by analysing the fluorescence, TL, and OSL emissions. Moreover, electron spin resonance (ESR) was carried out to understand the defects that play a role in those phenomena. Results demonstrated that the HfO2 nanocrystals were successfully produced in a monoclinic arrangement, with crystallite sizes up to 41 nm which increased with the heat treatment temperature. The TL glow curves are composed of several peaks, indicating the presence of different levels of electron traps, with the most prominent peak at about 130 °C. TM-Tstop and deconvolution analyses demonstrated that the peaks have activation energies of up to 1.6 eV, and that heat treatment at higher temperatures led to the improvement of the TL intensity and benefited the creation of deep traps. Similarly, the OSL signal was intensified by the heat treatment, showing the presence of two components in the continuous-wave (CW) mode and two peaks in the linear-modulated (LM) one, when using blue stimulation. Spectral measurements indicated that the material has a bright emission band in the blue region that can vary from 2.44 to 2.71 eV, depending on the technique. This characteristic blueish emission is mainly associated with oxygen vacancies of V+ type, which was enhanced by the heat treatment and attested by the ESR signal (estimated g values of 1.898, 1.957 and 1.985). The mechanism in which the OSL takes place might be related to a non-linear optics process of two or three-photon absorption. The sample calcined at 1600 °C had the best luminescent response and the evaluation of its infrared-stimulated luminescence (IRSL) dosimetric properties confirmed the potential use as a radiation detector due to the good reproducibility (variation lower than 3 %), intense signal, and linear doseresponse. Furthermore, to the best of the author\'s knowledge, this is the first report of IRSL for the material, as well as of the correlation of its luminescence to the ESR centres. (AU)

FAPESP's process: 19/20121-3 - Study of morphological and luminescent properties of HfO2 nanocrystals synthesized by precipitation method
Grantee:Alvaro de Farias Soares
Support Opportunities: Scholarships in Brazil - Doctorate (Direct)