DEVELOPMENT OF EFFICIENT COLOR TUNABLE RARE EARTH DOPED TUNGSTATE PHOSPHOR FOR SOLID STATE LIGHTING AND ANTI-COUNTERFEITING APPLICATIONS

Abstract

The substantial growth of industrial production and economic sectors has exacerbated the global energy crisis and environmental pollution, attracting significant attention from researchers worldwide. The rising global electricity demand, growing energy consumption, and the urgent need for energy conservation have emerged as critical concerns. One promising solution to address these challenges lies in the adoption of the solid state lighting (SSL) technology. The SSL technology has demonstrated widespread applicability across various domains, particularly through the implementation of phosphor converted light emitting diodes (pc-LEDs), which represent a viable alternative to conventional incandescent and fluorescent lighting systems. This is attributed due to their remarkable luminous efficiency, compact design, reduced energy consumption, high brightness, less emission of harmful gases, extended operational lifetimes, and rapid switching capabilities. In pc-LEDs, phosphor materials are integrated with LED chips and emit light in specific spectral regions upon appropriate excitation. Given these advantageous properties, pc-LEDs are regarded as the next generation of SSL technology, poised to revolutionize the lighting industry. Notably, white pc-LEDs have emerged as the preferred solution for general illumination applications due to their superior energy efficiency, longevity, environmental safety, and overall performance. The realization of phosphor based w-LEDs can be accomplished through two primary approaches: (i) employing a suitable combination of red, green, and blue (RGB) phosphors excited by a UV LED chip, or (ii) utilizing a single yellow emitting phosphor in conjunction with a blue LED chip. However, the first approach may suffer from issues such as the xii reabsorption of blue light by red and green phosphors, while the second approach often lacks a red spectral component. These limitations can lead to deteriorate color rendering index (CRI), compromised color saturation, and instability in color temperature. To overcome these challenges, it is imperative to develop single phase phosphor materials doped with an optimized combination of rare earth (RE) ions capable of generating white light emission via efficient energy transfer mechanism. Alternatively, the design and synthesis of novel red emitting phosphors with enhanced luminescent performance are crucial to improve key optical parameters, including luminous efficacy, CRI, and CCT stability in w-LED applications. In addition, the phosphor enables a wide range of applications, including solar cells, biosensing, and other photonic devices, owing to its adaptable chemical, physical, and luminescent features. Further, phosphor based material can be used, especially in anti-counterfeiting applications. The counterfeiting of currency and consumer goods poses significant challenges, leading to economic losses and health risks that adversely impact both fundamental research and industrial applications. Consequently, advanced anti-counterfeiting technologies are imperative to safeguard the economy and ensure the authenticity of consumer products in the face of a promptly expanding counterfeit market. Effective anti-counterfeiting techniques must be inherently resistant to duplication and straightforward to authenticate. In recent years, photo responsive materials, including rare earth (RE) activated fluorescent materials and quantum dots, have emerged as innovative solutions for anti-counterfeiting applications. Among these, RE-activated fluorescent nanomaterials have garnered particular attention for their critical and strong emission bands, exceptional stability, and environmentally friendly properties, making them highly promising candidates for next generation anti-counterfeiting technologies. Phosphor materials composed of a suitable host matrix and an activator element have emerged as key components in the advancement of technologies such as w-LEDs and anti-counterfeiting applications. Over recent decades, inorganic phosphors activated by RE ions have dominated xiii the fabrication of pc-LEDs, primarily due to the unique luminescent properties intrinsic to RE ions. Among various inorganic oxide hosts, tungstate-based materials have gained considerable interest for use in luminescent devices, attributed to their broad excitation spectra, wide range of emission colors, and superior thermal and chemical stability. Additionally, tungstates offer cost effectiveness and energy efficient synthesis processes. The current research work deeply focused on the development of environmental friendly new perovskite tungstate (BiYWO6 ) phosphor doped with appropriate rare earth ions, for applications in pc-w-LEDs and anti-counterfeiting applications. In the present thesis work, efforts have been made to synthesize various rare earth doped BiYWO 6 phosphors, to investigate their structural, morphological, optical, luminescent properties, and also to evaluate their potential utility for solid state lighting and anti-counterfeiting applications. The present thesis comprises eight chapters, and a brief summary of each chapter is presented as follows: Chapter 1 starts with a brief introduction outlining the origin of the problem, the motivation of the research work, and an overview of the current literature on the development of efficient solid state lighting and anti-counterfeiting applications. It includes the fundamental concepts of photoluminescence, energy transfer processes, and the utility of rare earth doped inorganic crystalline materials for lighting and display devices. The shortfalls and limitations of the existing lighting technology and how to overcome these problems have been discussed. This chapter also highlights the importance of the selected host material (BiYWO6 ) and outlines the objectives of the thesis work at the end of the chapter. Chapter 2 included the experimental methods used to synthesize and characterize rare earth doped BiYWO6 phosphors. It describes in detail the various methods employed to synthesize the desired phosphors, followed by the different analytical techniques used to assess their suitability for solid state lighting and anti-counterfeiting applications. The thermal, structural, morphological, compositional, optical, and photoluminescent properties of the xiv synthesized phosphors were investigated using thermogravimetric analysis (TGA)-differential scanning calorimetry (DSC), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), along with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), and spectrofluorophotometer, respectively. A brief introduction to each of these characterization techniques has been discussed in this chapter. Chapter 3 describes the synthesis of the crystalline monoclinic phase of Eu3+ doped bismuth yttrium tungstate (BYW: Eu3+ ) phosphors via the sol-gel combustion (SGC) procedure. X-ray diffraction (XRD) outcomes indicate the pure monoclinic crystalline phase formation of the BYW: Eu 3+ phosphors. The irregular shape and agglomerated dense packaging of the particles of BYW: Eu3+ phosphors have been revealed with the help of field emission scanning electron microscopy (FE-SEM) analysis. Defuse reflectance spectra (DRS) revealed the numerous absorption peaks in the UV/n-UV and visible regions. When excited with blue light, the BYW: Eu3+ phosphors radiate several emission peaks, and an intense emission peak has been observed at 613 nm (red region). CIE coordinates of the BYW: Eu3+ phosphors are positioned in the red section of the chromaticity diagram. The aforementioned results of the BYW: Eu3+ phosphors indicate their promising properties for usage in the field of white light emitting diodes (wLEDs) and other photonic device applications. [Part of this work has been communicated to ChemistrySelect (Under review)] (IF: 2.0) Chapter 4 presents the optimization of synthesis method as well calcination temperature of the europium activated bismuth yttrium tungstate (BYW: Eu3+ ) phosphors, synthesized using four different techniques, namely solid state reaction (SSR), sol-gel combustion (SGC), co-precipitation (CP), and hydrothermal (HT) methods. Relative investigations such as thermal, structural, morphological, and luminescence characterizations, have been carried out to optimize the synthesis process of BYW: Eu 3+ phosphor. The xv luminescent spectral profiles indicate the strong absorption in the blue region ( ex =465 nm) and intense emission in the red region ( em =613 nm) ascribed to the 5 D0 →7 F2 transition. The comparative photoluminescence (PL) results signify that the phosphor synthesized by the CP method at a calcination temperature 900 °C exhibits the strongest emission compared to the phosphor synthesized via other methods (SGC, SSR, and HT) and shows an emission intensity nearly twice that of the phosphor synthesized via the SSR method. Further, the PL intensity was enhanced with the activator concentration of Eu3+ ions up to 20 mol%. The calculated CIE chromaticity coordinates (0.654, 0.345) of 20.0 mol% Eu3+ doped BYW sample were located in the red region, which is comparable with the commercially available red emitting phosphors Y2 O3 : Eu3+ (0.645, 0.347) and Y2 O2 S: Eu3+ (0.647, 0.343). The average PL decay time of the synthesized phosphor was found to be in the microseconds range. The obtained results suggest that the BYW: Eu3+ phosphor synthesized by the CP method exhibits distinctive PL characteristics with good morphology, which can be employed as an intense red emitting component in photonics devices. [Part of this work has been published in the Journal of Materials Science: Materials in Electronics 35 (2024) 2106] (IF: 2.8) Chapter 5 explains the structural and spectroscopic features of single phase Dy3+ activated BiYWO6 phosphors synthesized using co-precipitation method. X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) measurement techniques were employed to examine the structural, morphological, compositional, elemental composition as well as their oxidation states, and photoluminescent characteristics of the Dy3+ : BiYWO6 phosphors, respectively. Photoluminescence studies of the Dy3+ activated BiYWO6 phosphor reveal that the CIE color coordinates phosphor falls in near white region under 290 nm wavelength. The average decay time ( ) for the 4 F9/2 energy state of Dy3+ ions has been found to be in microseconds. The PL spectra measured from room temperature to 448 K xvi ensure good thermal stability of the phosphor of the as-synthesized phosphor. Therefore, the synthesized phosphor might be a considered as a promising candidate for UV excited phosphor- converted white LEDs (pc-wLEDs). [Part of this work has been published in the Applied Physics A 130 (2024) 748] (IF: 2.5) Chapter 6 describes the studies of energy transfer and color tunable luminescent properties of Dy3+ /Eu3+ co-activated BiYWO6 phosphors synthesized by the co-precipitation route. The Dy3+ /Eu 3+ co-activated BYW phosphor demonstrates intense emission peaks in the blue, yellow, and red regions of the PL spectrum. Application of Dexter’s energy transfer formulation with Reisfeld’s approximation revealed that the energy transfer mechanism encompasses a non-radiative dipole-dipole (d-d) interaction between Dy3+ and Eu3+ ions. The CIE chromaticity coordinates (x, y) and the correlated color temperature (CCT) values of the synthesized BYW: Dy3+ /Eu 3+ phosphors were estimated. Based on these values, it was observed that the emission color of the synthesized phosphors can be tuned from the warm white light to the red region by precisely adjusting the concentration of Eu3+ ions and excitation wavelengths, respectively. These findings confirm that the synthesized Dy3+ /Eu3+ co-activated BiYWO6 phosphors can be a promising candidate for white light and anti-counterfeiting applications. [Part of this work will be communicated to Journal of Luminescence] (I.F.: 3.6) Chapter 7 focused on the development of wavelength dependent multicolour emitting BiYWO6 : Sm3+ phosphors using the co-precipitation technique. The single phase formation with a monoclinic structure of the as-synthesized BiYWO6 : Sm 3+ (BYW: Sm3+ ) phosphors was confirmed by the X-ray diffraction (XRD) patterns. The FE-SEM micrograph of the BYW: xSm 3+ , x = 3.0 mol% phosphor, illustrates the irregular and uniform distribution of closely spaced particles. The EDX result fully discloses the compositional behaviour of the synthesized BYW: xSm 3+ , x = 3.0 mol% phosphor. The Sm 3+ activated BYW phosphor exhibits a strong deep red emission centered at 646 nm (4 G5/2 → 6 H9/2 ), when excited with 290 nm, whereas an xvii intense red emission around 600 nm (4 G5/2 → 6 H7/2 ) is observed under blue excitations at 406 and 465 nm. The CIE chromaticity characteristics for the synthesized BYW: Sm3+ phosphor have been computed under selected excitations. The estimated average decay time was found in the microseconds range for BYW: Sm 3+ phosphor under UVn-UV/blue excitations. All the findings mentioned above validate the potential of the synthesized BYW: Sm 3+ phosphor for solid-state lighting (SSL), and anti-counterfeiting applications. [Part of this work has been communicated to the Journal of Molecular Structure (Minor revision)] (I.F.: 4.7) Chapter 8 summarizes the relevant conclusions based on the results obtained in the previous chapters, outlines the future scope of the work and highlights the social impact of this research work.