Author: Wu, Xiao
Title: Rare-earth-doped KNN-based ceramics for photoluminescent and electro-optic applications
Advisors: Kwok, K. W. (AP)
Degree: Ph.D.
Year: 2015
Subject: Rare earths -- Optical properties.
Optoelectronics -- Materials.
Semiconductor doping -- Materials.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Physics
Pages: xxvi, 185 leaves : illustrations ; 30 cm
Language: English
Abstract: The photoluminescence (PL) properties in various rare-earth (RE) doped materials have been extensively investigated. Visible (Vis) up-conversion emission and near-infrared (NIR)/middle-infrared (MIR) emissions have attracted considerable attention due to their applications in biological/medical imaging and data storage, as well as optical communications, medical and military areas. Er³⁺ is the most popular activator among the RE ions used for up-conversion PL. When the PL properties are combined with other properties such as ferroelectricit into a single entity multifunctional performance can be realized. The corresponding multifunctional material has potential application in the optoelectronic fields. This work aims to develop new lead-free RE-doped (K0.5Na0.5)NbO3 (KNN)-based multifunctional ceramics with good PL, ferroelectric, piezoelectric and electro-optic (EO) properties. The mechanisms and influencing factors of PL have been systematically studied. Our results have shown that the PL performances of the Er-doped KNN-based ceramics can be effectively adjusted by regulating the concentration of activator (i.e. Er³⁺), changing the host structural properties (e.g. lattice distortion by Li+-doping, local asymmetry induced by phase transition or polarization switching), controlling the homogeneity and grain size of ceramics, and introducing sensitizers (e.g. Pr³⁺, Yb³⁺). Er-doped KNN ceramics (abbreviated as KNN:Er-x, where xis mol % of Er³⁺) have first been prepared by the solid state reaction (SSR) method. Raman results reveal that the ceramics have low phonon energy (~860 cm-1) and thus slow multi-phonon relaxation rate. The PL properties, including the up-conversion Vis emission, down-conversion NIR and MIR emissions, emission colors and lifetimes as well as the energy transfer processes have been systemically investigated. The relationship between the intensity of Vis up-conversion PL and laser power indicates an efficient two-photon process when x ≤ 2. The Er-doping also induces a phase transition from orthorhombic to cubic-like at x = 2, and inhibits the grain growth due to the donor-type nature. Both the change of crystallographic structures and the concentration quenching effect induced by Er3+ions have great influences on PL properties. The resulting effects on the dielectric, ferroelectric and piezoelectric properties have also been investigated. At large x, the ceramic transforms normal ferroelectric to relaxor-like. At x = 1 and x = 2, the ceramics manifest relatively good piezoelectric properties (d₃₃), high remanent polarizations (Pr) dielectric constants (εr), low dielectric losses (tan δ), as well as outstanding PL performances. The effects of Li-doping on dielectric, ferroelectric and PL properties of the Er-doped KNN ceramics have then been investigated. The samples are 2 mol% Er-doped (K₀.₅Na₀.₅)1-xLixNbO₃ (Er-KNLN-x) ceramics and fabricated by the SSR method. At x < 0.06, the ceramics possess a single-phase perovskite structure. At larger x, the crystal structure transforms from orthorhombic to tetragonal. Under an excitation of 980-nm laser, the ceramics exhibit intense up-conversion emissions as well as strong down-conversion emissions in NIR and MIR regions. The optimum doping level of Li for up-conversion PL is 0.08. Probably due to the induced structure distortion and reduced local symmetry, the PL intensities of the green, red as well as MIR emissions are enhanced by the doping of Li+. The Li-doping is effective in establishing a dynamic circulatory energy process to further enhance the PL intensity of the MIR emission at the expense of the NIR emission. At the optimum doping level of Li+ (x = 0.06), the full bandwidth at half maximum of the MIR emission reaches a very large value of ~250 nm, demonstrating that the ceramic is a promising candidate for high-power 2.7-μm ceramic lasers. The Er-KNLN-x ceramics also exhibit good ferroelectric properties, and thus they should have great potential for multifunctional optoelectronic applications.
A sol-gel method has been applied to prepare the KNN:Er-x and Er-KNLN-0.08 ceramics for enhancing the PL emissions. As compared to the SSR method, the sol-gel process can reduce the sintering temperature, producing ceramics with better compositional homogeneity and uniform grains. All these are beneficial for improving both the up-conversion and luminescent efficiencies of the ceramics, and thus leading to very strong green emissions even at a low quenching concentration of 2 mol%. The sol-gel-derived Er-KNLN-0.08 ceramics have better up-conversion PL properties (higher emission intensity) than the KNN:Er-2 (Li-free) and SSR-derived Er-KNLN-0.08 ceramics. KNN ceramics codoped with Er³⁺or Er³⁺/Yb³⁺have also been studied. The SSR method has been used to prepare both the KNN codoped with 2 mol% Er and y mol% Pr(Er-KNN-Pr-y) and KNN codoped with 1 mol% Er and z mol% Yb (Er-KNN-Yb-z) ceramics. The relationships between Vis up-conversion emissions, NIR and MIR emissions of Er³⁺ have been studied by 980-nm excitation. The effects of Pr3+ and the energy transfer processes between Er³⁺ and Pr³⁺ have been investigated. By selecting appropriate excitation wavelength, simultaneous Vis down-conversion emissions of both Er³⁺ and Pr³⁺ can be obtained. Moreover, the PL intensity of the ceramics can be enhanced by the remanent polarization resulting from by the poling process. Our results also show that Yb³⁺ is a superior sensitizer to significantly enhance the Vis up-conversion emissions (one order of magnitude higher) and partly improve the MIR MIR emissions (at low Yb content). The transparent ceramics (K₀.₅Na₀.₅)1-xLixNb1-xBixO₃ (KNN-LB-x) and 1 mol% Er-doped (K₀.₅Na₀.₅)1-xLixNb1-xBixO₃ (Er-KNN-LB-x) have been successfully manufactured by pressureless sintering through the same SSR method. Due to the effective suppression of grain growth by Bi-doping, the ceramics possess fine and cubic crystalline grains with dense structure. The ceramics are optical transparent, exhibiting high optical transmittance (~70% for KNN-LB-x and ~50% for Er-KNN-LB-x) in the NIR region (~900 nm). The good optical transparency may also be derived from the cubic-like crystal structure and relaxor-like characteristics. Both the transparent ceramics exhibit strong EO response, giving a large effective linear EO coefficient, i.e. 120-200 pm/V. The PL and ferroelectric properties of the Er-KNN-LB-x transparent ceramics have also been investigated for exploring the multifunctional photonic applications, such as optical-electro integrated materials and devices.
Rights: All rights reserved
Access: open access

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