Study on rare earth doped double perovskite tungstate for solid-state lighting and optical thermometry

Abstract

Because of the excellent chemical durability, intriguing luminescence behavior and large rare earth ion admittances, the rare earth doped double perovskite tungstates have attained great achievement and progress in various fields including solid-state lighting, flat panel displays, optical temperature sensors, and optical biomarkers. The purpose of this dissertation is to develop the new single-phased phosphors for solid-state lighting and optical thermometry. For the double perovskite structure, three kinds of phosphors are studied. And various theoretical methods were comprehensively applied in (Li, Na, K) LaMgWO6: Eu3+ phosphors to achieve the rational design of phosphors. The detailed structural characteristics were analyzed by using geometric optimization and Rietveld analysis for the first time. The plane-wave density functional theory (DFT) was used for all electronic structure, electronic distribution, and phonon dispersion calculations. The evolution of the octahedral tilting in the (Li, Na, K) LaMgWO6 crystal structures was studied in detail. The relationship between the crystal structure, environmental factors, Judd-Ofelt intensity parameters and fluorescence properties were analyzed. Combined with experimental results and theoretical calculations, the detailed luminescence process is deduced starting from the different ionic radius of Li+, Na+, and K+. Notably, the redshift of charge transfer band (CTB) can be ascribed to the value variation of environmental factors, which can represent the ionic radius, coordination number, covalency, and polyhedral tilting to qualify comparison and even quantify analysis. To further study the photoluminescence properties of KLaMgWO6: Eu3+ phosphors, a variety of Eu3+-activated KLaMgWO6 phosphors were synthesized by a traditional high-temperature solid-state reaction method in an air atmosphere. By using geometric optimization and Rietveld analysis, the detailed structural properties were derived for the first time. The band structure, electronic distribution, and phonon dispersion were calculated using the plane-wave density functional theory. Meanwhile, the optical band gap of KLaMgWO6 host was measured with an ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS). The CTB of KLaMgWO6: Eu3+ phosphors are situated at the ultraviolet and near-ultraviolet region from 250-410 nm. Combined with theoretical calculations and experimental results, the detailed luminescence process is deduced. The phosphors show intense absorption in the near ultraviolet-blue region and exhibit strong red emissions with CIE coordinates of (x=0.6474, y=0.3360) under 344 and 394 nm excitation. Excellent luminescence properties make it have potential application in the fabrication of white light-emitting diodes (w-LEDs). Simultaneously realizing the efficient bifunctional application of thermochromic phosphors on light emitting diodes (LEDs) and ratiometric thermometer is significant. Herein, the doped Er3+ ions are introduced as an activator into double perovskite LiLaMgWO6 host lattice. The developed phosphors can be efficiently excited by near-ultraviolet light emitting diodes (NUV-LEDs) chip and show bright green emission mainly at 527 and 543 nm, as well as very low thermal quenching. In addition, its chemical stability is studied, demonstrating excellent application potentials. Furthermore, the temperature sensing properties of LiLaMgWO6: 0.01Er3+ in a wide range of 303-483 K were analyzed, which shows a good exponential relationship between ratiometric intensity and temperature (R2 > 0.999) as well as high sensitivity (2.24% K-1). Such a developed system not only optimizing the performance for the solid light emitting but also provides an excellent platform for designing high-sensitive optical thermometry. Furthermore, Er3+-activated NaLaMgWO6 phosphors were prepared by a traditional high-temperature solid method. Based on the double perovskite structure of NaLaMgWO6 host, we observe the desirable PL property of Er3+. When excited at about 378 nm, the as-obtained materials can emit strong green light. When applied to the temperature sensor based on the fluorescence intensity ratio (FIR) principle, the prepared phosphors show excellent sensitivity ranged from 303-483 K. With elevated operation temperature, the sensitivity is about 2.23% K-1 at 483 K which resulting from the sensitive thermally coupled levels (2H11/2 and 4S3/2) of Er3+ ions in the double perovskite structure. The calculated relative sensitivity of the temperature sensor was 1.04%K−1 at 303 K. Especially, besides high sensitivity, the behavior of superior water resistance is an equally thrilling discovery. Therefore, it is demonstrated that the as-prepared Er3+-activated NaLaMgWO6 phosphors have a promising potential application in both near-UV solid-state lighting and non-contact optical thermometry. Finally, the Er3+/Yb3+-codoped NaLaMgWO6 phosphors were synthesized via a traditional high-temperature solid-state reaction method. The temperature sensing performance was thoroughly investigated by studying the temperature-dependent up-conversion emission intensity ratio in the range of 293-533 K. A remarkable enhancement of green UC emission, as well as enhanced temperature sensitivity, were observed by increasing the Yb3+ concentration. Furthermore, the temperature sensing properties of NaLaMgWO6: 0.01Er3+,0.06Yb3+ show a good exponential relationship between ratiometric intensity and temperature (R2 > 0.999). The maximum absolute sensor sensitivity was 2.29% K-1 at 533 K. Furthermore, when the pump power of the 980 nm laser increased from 200 to 1000 mW, a slightly elevated temperature from 293-307 K was achieved in the compounds. Eventually, using the prepared phosphors and a 940 nm NIR chip, a green-emitting LED device was developed to confirm the applicability of our prepared phosphors for solid-state lighting. As a temperature probe, the prepared phosphor detected that the temperature increased from 286 K to 315 K when the drive current was increased from 90 mA to 300 mA. These results suggest that the Er3+/Yb3+-codoped NaLaMgWO6 phosphors have a potential application in solid-state lighting and optical thermometry.