## 5 Luminescence and Imaging  
Y\({}_{2}\)O\({}_{3}\) is a white solid thermal material that is stable and is used for conferring orange to red in the television picture tube. It offers purity of color and stability. Y\({}_{2}\)O\({}_{3}\) is used for emission display vacuum fluorescent display, and its physical properties were exploited in the plasma display panel and the electroluminescence, cathodoluminescence, and thermoluminescence fields [28]. Y\({}_{2}\)O\({}_{3}\) use in solar energy conversion devices, UV radiation emitters, and telecommunication devices has been reported, due to its high bandgap [116; 117]. This showed an effective permittivity and clean transparency to IFR radiation [118; 119; 120; 121], with a melting point of 2439 \({}^{\circ}\)C [119]. Due to the ionic radius of Y\({}^{3+}\) being similar to rare-earth doping Y\({}_{2}\)O\({}_{3}\) nanoparticles, they have been used as rare-earth-doped yttrium materials [122]. Several studies demonstrated that Y\({}_{2}\)O\({}_{3}\) has diverse applicationsin material sciences. Y\({}_{2}\)O\({}_{3}\) potential in thermal stability, superior infrared transmission range ((1-8) micron wavelength), and erosion resistance ability enables them to be used as a unique material in protection domes for the infrared sensor, temperature sensor, and solar cell sensors. Moreover, they have been used in ultrafast sensors in X-rays. The research reports indicated that the nanoparticles can be synthesized with different sizes, shapes, and optical properties by various methods. The appealing property of emission of red light was used for the manufacture of fluorescent lamps. The current study showed that Y\({}_{2}\)O\({}_{3}\) doped with rare-earth ions had been used for solid-state lighting purposes [123]. The Y\({}_{2}\)O\({}_{3}\) doped Eu\({}^{3+}\) ions (Y\({}_{2}\)O\({}_{3}\): Eu\({}^{3+}\)) act as a main oxide-based red-emitting phosphor, which is used widely in the lighting industry and solid-state laser-based devices [124]. Luminescence investigation confirmed that the majority of the Eu\({}^{3+}\) ions occupy the Y\({}_{2}\)O\({}_{3}\) site, and luminescence intensity increases with particle size/crystallinity. While using PL, its images (Figure 6) are visible to the naked eye, and from this, the photoluminescence (612 nm) that is related to Eu\({}^{3+}\) is high for 5 mol.% Eu\({}^{3+}\) doping and more intense in the 900 \({}^{\circ}\)C annealed Y\({}_{2}\)O\({}_{3}\): Eu\({}^{3+}\)[125]. At the same time, there is a uniform emission of Eu\({}^{3+}\) throughout the sample annealed at 600 \({}^{\circ}\)C, while the comparatively less intense Eu\({}^{3+}\) emission could be due to OH bonds associated with the matrix at such relatively low annealing temperatures [126]. The PL lifetime decreases with the dopant concentration from ((2.26 to 1.77) and from (2.35 to 1.81)) ms in the case of (600 and 900) \({}^{\circ}\)C annealing, respectively. The luminescence observed was strong in Y\({}_{2}\)O\({}_{3}\): Eu\({}^{3+}\) with slow decay (maximum decay time of 2.35 ms), and it is a good candidate for interesting applications, such as biosensors and red components for white light LEDs. It could be a promising approach for fabricating optoelectronic thin films with high optical quality [125]. Y\({}_{2}\)O\({}_{3}\): Eu\({}^{3+}\) phosphor powders were prepared by the Sol-combustion synthesis method using sulfur-containing organic fuel in ethanol-aqueous solution, and investigated how the structure, morphology, and luminescence intensities of these phosphors are affected by varying fuel-to-host ratios (S/Y) [127]. Upon excitation, Eu\({}^{3+}\) doped systems indicate many Stark energy levels in the visible region. Luminescence transitions corresponding to the 5D0-7 FJ manifold in the orange-red region are of practical significance [128]. The emission spectrum of the Eu\({}^{3+}\) ion-doped in a Y\({}_{2}\)O\({}_{3}\) matrix with different S/Y molar ratios was obtained.