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Title: Ferroelectric relaxor ceramics with tunable photoluminescence properties
Authors: Sun, Hailing
Degree: Ph.D.
Issue Date: 2018
Abstract: Photoluminescence (PL) activity in ferroelectrics has received much attention recently. After doping with trace amount of luminescent ions, such as lanthanide (Ln) ions, ferroelectrics acquire unique photoluminescence while retain the inherent ferroelectric properties. As one type of multifunctional materials, Rare-earth (RE) elements doped ferroelectric oxides with both luminescence and ferroelectric properties have great potential applications in future optoelectronic devices. Especially, the rapidly spectroscopic modulation and multicolor tuning of photoluminescence activities are highly essential for not only exploring the underlying mechanism processes but also the practical applications, such as cell and tissue imaging, information processing and display media, medical diagnosis, as well as anti-counterfeit technique. One proposed approach of directly controlling the PL emissions is via introducing an external electric field in a real-time way. The work aims to address an interesting and technologically important area of materials development in electric field-modulated photoluminescence. As known, ferroelectrics are certain materials that have a spontaneous polarization that can be reversed by the application of an external electric field (E). For ferroelectrics, it is cogent that the crystalline symmetry changes induced by phase transition, polarization, and strain bridge a modification relationship between the way of an external electric field and the corresponding photoluminescence behavior. This fact resulted from a coupling effect between ferroelectric materials and external E field stimulus has inspired the study of the real-time modulation of photoluminescence properties in an in-situ approach. Since the ferroelectric materials can be divided into several different types, such as, normal ferroelectrics and relaxor ferroelectrics with individual features, so the novel use of different ferroelectric material to achieve the field-modulated photoluminescence would provide individually interesting photoluminescence changes and underlying mechanism. It has been known that Ba₀.₈₅Ca₀.₁₅Ti₀.₉₀Zr₀.₁₀O₃ (abbreviated as BCTZ) lead-free ferroelectric ceramics possess excellent piezoelectricity around the morphotropic phase boundary (MPB) deriving from a tri-critical point according to the phase diagram. Inspired by its remarkable phase transition around room temperature, the BCTZ ceramic is chosen as the classically normal ferroelectric host to investigate the structurally symmetric transformation and thus the photoluminescence modulation performance under electric field. It shows that the 0.2mol%Pr-doped Ba₀.₈₅Ca₀.₁₅Ti₀.₉₀Zr₀.₁₀O₃ perovskite bulk ceramic could response to the electric field change resulted from its remarkable polarization behavior and phase revolution. Along with the in-situ X-ray diffraction, it is demonstrated that the manipulation of photoluminescence is realized via modulating the local crystalline symmetries around Pr³⁺ induced by external electric field. Our results reveal that an applied electric field induces not only typical polarization switching and minor crystal deformation, but also a major tetragonal-to-rhombohedral phase transformation of the ceramic. The electric field-induced phase transformation is irreversible and engenders the dominant effect on photoluminescence emissions as a result of an increase in structural symmetry. After it is completed in a few cycles of electric field, the photoluminescence emissions become governed mainly by the polarization switching, and thus slightly vary reversibly with the modulating electric field.
In contrast, 0.94BNT-6BaTiO₃ (BNT-6BT) was studied in detail as a typically relaxor ferroelectric host. Our results reveal that the 1 mol% Eu can lower the ferroelectric-relaxor transition temperature (TF-R) of BNT-6BT and the compositional disorders could disrupt the long-range ferroelectric macro-domain into random nano-domains, such that the ceramic could become an ergodic relaxor (ER) with psedocubic symmetry at room temperature, giving a constricted polarization hysteresis loop and sprout strain curve with negligible remanent strain at zero electric field. Due to the presence of random polar nanoregions (PNRs) with different symmetries of P4bm and R3c which are weakly correlated and free to rotate, after electric field excitation the system quickly comes back to the original state, so that the resulting usable strain is reversible and large (~ 0.43% at 7 kV/mm). The Eu-doped BNT-6BT ceramic also exhibits strong down-conversion PL emissions that can be facilely modulated by an external electric field. The real-time PL modulation is almost reversible and of high degree (~ 40%) attributed to the nucleation and tunable transformation of PNRs in cycles of E-field evidenced by in-situ Raman analysis and a comprehensive comparison to pure BNT-6BT. In-situ X-ray diffraction analysis further confirms this ER nature and only sees a shift of XRD patterns to lower angles induced by lattice distortion, which is consistent with the internal strain reversal under electric field. Therefore, it is suggested that the real-time electric field-modulation of photoluminescence excitation/emission is attributed to the reversible distortion changes in structural symmetry (interactions of the PNRs lead to giant strain) and then the local crystal field of rare-earth ions in the ergodic relaxor. Moreover, not limited to the unitary emission channel modification of single element doping, we expanded the experiments to dual channels performance by two rare-earth elements co-doping in relaxor ferroelectric host for the ease and flexibility of modulating the multicolor photoluminescence. So, we carried out a comprehensive investigation on the evolution of ergodicity degree, structural symmetry changes and corresponding PL modulation in the Tb and Eu co-doped BNT6BT relaxor system using the facile strategy of in-situ electric field. By virtue of the introduction of rare-earth element, the induced chemical and charge disorders could benefit the formation of randomly dynamic and weakly correlated polar nanoregions, which facilitates a reversible transition between polar nanoregions and unstable ferroelectric state under an electric field, engendering a large strain and symmetry changes. These should be helpful for the realization of regulating the physical couplings (tunable photoluminescence-relaxor ferroelectrics) in multifunctional inorganic ferroelectrics with a high ergodic state by reversibly tuning the structural symmetry. Also, the mechanism of the color modulation was elucidated based on the competitive or cooperative action of resonance energy among Tb and Eu dopants. Within the scope of current contribution in relaxor ferroelectrics, we realize the reversible color tuning (~100%) in two rare-earth elements co-doped ergodic relaxors, this could also be expanded to other soft ferroelectrics in ergodic state, for high repeatability and flexible use of multicolor photoluminescent activities tuning at ambient temperature.
Subjects: Hong Kong Polytechnic University -- Dissertations
Rare earths -- Optical properties
Optoelectronics -- Materials
Pages: xxiv, 153 pages : color illustrations
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