Structural, Spectroscopic and Photophysical Features of Rare earth Activated Phosphate Phosphors

Abstract

The solid state lighting technology and its progressive fundamental scientific research supplements one of goals in the Sustainable Development Goal (SDG), to ensure the access to affordable, reliable, sustainable and modern energy for all; in terms of energy efficient light that enhances quality of human life and productivity of workforce. In this endeavor, this thesis work aspires to study a new set of phosphate based phosphors, and evaluation of their structural, spectroscopic as well as the photophysical properties. The phosphors are prepared by solid state-reaction methods and are designed by the sensitizer-activator approaches by incorporating rare-earth ions as light emitting centers. The material science description of the synthesis techniques used to synthesize different series of inorganic phosphate based phosphors are outlined here. The experimental exploration and characterization of the phosphors along with the results based on the structural, morphological, vibrational and optical band gaps are discussed in a systematic manner. The x-ray photoelectron spectroscopy characterization and photophysical properties of synthesized phosphors are examined in this work and are systematically documented. ABPO4 (A and B are mono- and divalent cations, respectively) have excellent thermal stabilities for potential efficient host materials as well as advantages of diverse structural frame works depending on the relative size of A and B cations. In this class, a series of maricite structured NaCd1-xPO4: xEu3+ are synthesized and various structural as well as optical properties are studied extensively. With Eu3+, increased broadening of band gap is explained by Burstein–Moss (BM) effect. This phosphor exhibits emissions rich in red at 617 nm due to hypersensitive transition line 5D0 → 7F2 due to the induced electric dipole (ED) under suitable excitations at 393 nm and the colour co-ordinate of the emission spectrum is concentrated to a very confined spot in the colour space indicates a high colour purity. In this phosphors, a wide bandgap and rigid structure along with sharp red emission are found suitable for design considering of LED phosphors which may find applications in pcLEDs. In an attempt to synthesize and characterize in details of a broad spectrum emitting phosphors that covers the span of visible light, another series of phosphors NaCd1-xPO4: xDy3+ (x = 0.02 – 0.1) are prepared by adopting conventional solid-state reaction routes. The photoluminescence characteristics are recorded at room temperature, in which the excitation maxima are observed at 347 nm, indicating a near-ultraviolet (UV) excitation which can be iuseful for light-emitting diodes with a UV chip. The emission spectra show three major emission lines at 480 nm (4F9/2 → 6H15/2) a blue emission, a yellow emission around 570 nm (4F9/2 → 6H13/2) and a red emission around 662 nm (4F9/2 → 6H11/2). Therefore, the resulting spectrum is nearly white under UV excitation, which enables them to be tuned suitably with co-activator ions or by means of sensitizations and can be useful as solid-state lighting sources. A wide bandgap of 5.72 eV and a small variation is observed with increasing concentrations while studying the optical reflectance properties of a series of Sm3+ doped phosphors constituted with the CsMgPO4 host. A systematic study of photoluminescence properties of Sm3+ doped CsMgPO4 phosphors is carried out and along with investigations of the structural as well as morphological, vibrational and photoelectron emission characteristics. The room temperature excitation and emission spectra, reveals that the phosphor emits the orange rich red light under the suitable excitation of 402 nm in the UV region. The emission peaks confirm the characteristic Sm3+ 4f-4f transitions. The temperature-dependent photoluminescence (TD-PL) is also explored for the selective composition with x = 0.02 (optimum doping), which has been recorded from 30 °C to 210 °C and is showing good thermal stability even at 150 °C. The thermal quenching mechanisms are discussed based on the configuration coordinate model of excitation and emission. The prepared phosphors are found to exhibit near thermal stability compared to the commercially available red phosphors. = The PLQY for this optimum composition is estimated to be 42.1 %. The color coordinates are found to lie in the orangish-red region of the color space. Thus the prepared phosphors CSMP: x Sm3+ can be useful as a red component in designing UV excitable chip-based phosphorconverted white LED applications. In a similar context, a series of phosphate phosphors KZnPO4: xEu3+, yBi3+ (x = 0 – 0.06, y = 0.005-0.05) are prepared and a systematic study of photoluminescence properties along with structural as well as morphological and vibrational characteristics are carried out. The concentration quenching of Eu3+ occurs at x = 0.04 and considering this as the optimum level of Eu3+ doping concentration, Bi3+ is added to act as the sensitizer for further enhancing the PL emission and mediate the efficient energy absorption processes. This chapter is aimed to explore the co–doping approach of activators and sensitizers in phosphors