Phase, Morphology, And Photoluminescence Characteristics Of Europium Doped Zinc Oxide Nanopowders For Luminescent Applications

Abstract

Among various inorganic oxides, zinc oxide (ZnO) demonstrates a versatile material due to its high electron mobility, better thermal and chemical stability as well as it acts as an excellent host matrix to accommodate various rare-earth ions, which resulting in different luminescent applications. Trivalent rare-earth activator such as Eu3+-doped ZnO with controllable particle size and shape has gained more interest because of its potential applications in numerous fields, including luminescence. In the past few years, Eu3+-ions in ZnO matrix have been focused for the development of a highly red-emitting Eu3+- doped ZnO phosphor with distinct and well-defined morphology. A well-defined morphology is highly required to obtain the richness of red colour and improve the photoluminescence characteristics of Eu3+-doped ZnO nanophosphor. The prime objective of this research work is to develop deep red-emitting as well as unique and well-established morphology of Eu3+-doped ZnO nanophosphors by considering three different zinc-based precursors such as zincnitrate, zinc-acetate, and zinc-chloride through precipitation route with the help of sodium borohydride. In addition, influence of precursors, calcination temperatures, secondary phases, and dopant concentrations on the photoluminescence characteristics of Eu3+-doped ZnO have been explored. Different characterization such as XRD, FTIR, Raman, DRS, Photoluminescence characteristics (asymmetric ratio, colour coordinates, colour purity, and CCT), luminescence lifetime, and quantum yield have been performed on different samples. Motivation for selecting three different zinc-based precursors has been discussed by comparing the phase and powder morphology of undoped and Eu3+-doped ZnO samples. Further, the influence of calcination temperature on the phase, morphology, and photoluminescence characteristics of 3 mol% Eu3+-doped ZnO samples has been explored. Pure phase ZnO has been observed up to 800 °C. The secondary phases, such as EuBO3 and Eu(BO2)3, were developed due to the presence of borate (BO3 and BO4) and hydroxyl (O-H/B-OH) groups. A broad transition of 5D0―› 7F2 was observed at 613 nm for all samples calcined up to 800 °C. However, it was found splitting in nature and broadened due to non-homogenous Eu3+-ions and secondary phases. Deep red emission was observed for calcined (400 °C) nitrate-based sample as well as calcined (800 °C) acetate and chloride-based samples with better colour purity. The involvement of crystalline nature of secondary phases at higher temperature does not hinder, rather moderately improve the photoluminescence characteristics. To find out the nucleation of these secondary phases and its effect on photoluminescence properties, all precursor-based samples with 5 mol % Eu3+ dopant have been analysed at 800 °C. The nucleation of secondary phases was not properly crystallized for acetate and chloride-based samples; however, it was significantly evolved for nitrate-based samples. Flower-like morphology with larger particle size may induce fewer defects and decrease the nonradiative rate, resulting in an increase in the emission intensity of acetate-based samples. It was also found that the induced secondary phases do not hamper but considerably achieved a better R/O ratio, colour coordinates, and colour purity. Furthermore, influence of dopant concentration on the phase, morphology, and photoluminescence behaviour of Eu3+-doped ZnO samples has also been explored in this research work. Distinct morphologies were formed in Eu3+-doped ZnO samples by varying the dopant concentration at different temperatures. The electric dipole transition 5D0 7F2 (613 nm) was more intense than the intensity of magnetic dipole transition 5D0 7F1 for all the samples. The emission intensity has been dependent on the dopant concentration and morphology as well as particle size. Additionally, the 3 mol% based Eu3+-doped ZnO samples calcined (at 400 °C, 600 °C, and 800 °C) exhibited the colour coordinates in red/deep red region with better colour purity as compared to the other composition (Eu3+-dopant) at different temperatures. Finally, this research work demonstrated the prototype applications by utilizing the appropriate precursor-based red-emitting Eu3+-doped ZnO sample by considering the valuable photoluminescence characteristics in the latent fingerprint detection and anti-counterfeiting. Latent fingerprint has been explored on multiple surfaces and compared with scanned fingerprints. Besides, QR-code was printed by screen printing technique and applied for anti-counterfeiting.