반도체 광원의 응용에 있어 열적 안정성이 우수하고 희토류 이온이 첨가된 NaCaPO4 형광체의 합성 및 발광특성 : Synthesis and Luminescent Properties of Rare Earth Activated Thermally Stable NaCaPO4 Phosphors for Solid State Lighting Applications

다국어 초록 (Multilingual Abstract)

Nowadays, solid state lighting (SSL) is a pivotal engineering technology that promises to fundamentally alter and improve lighting systems in future. Recently, one of the rapidly developing fields of research for new and efficient phosphors is the SSL technology especially white light emitting diodes (w-LEDs), which can compete with traditional incandescent and fluorescent lamps. This technology has the potential to attract significant scientific attention for the development of fourth generation light sources in recent years due to their excellent characteristics in comparison to conventional light sources including long lifetime, high efficiency, energy saving, low power consumption, low cost, reliability, small size and the environmental friendliness. The widespread use of solid state based lighting is of great importance to significantly reduce the global electricity consumption and the use of fossil fuels. Moreover, the features of long lifetime and mercury-free would contribute in solving environmental problems. Nowadays, the most popular approach for fabricating w-LED is by coating a yellow emitting phosphor on a blue emitting InGaN LED. However, the current generation white LEDs have many disadvantages.
To overcome the deficiencies of the commercially available phosphor converted white LEDs (pc-white LEDs) as mentioned above, thermally stable orthophosphate host is selected for the current study to produce white light emission with the strategy of combining near ultraviolet (near-UV) chip with blue and yellow color (or) tricolor [red, green and blue (or) blue, cyan and orange] emitting phosphor. In addition, the near-UV based pc-LED is expected to have great potential in the field of solid-state lighting, when comparing with blue LED based pc-LED. Therefore, optimization of the red/green/blue/orange emission with the high absorption of n-UV light is needed for rare earth doped phosphors to develop highly efficient white light emitting diodes.
Motivated by the above facts, white light, green, orange, orange-red color emitting trivalent rare earth activated NaCaPO4 phosphors were synthesized by using a conventional solid state reaction (SSR) method. Moreover, Eu3+ doped NaCaPO4 has also been synthesized by two other synthesis techniques such as molten salt synthesis (MSS) and sol-gel combustion (SGC) method. The synthesis procedure, sintering temperature have been optimized based on the results of TGA-DTA and emission properties. The phase and the structure of the as-prepared powders were characterized by using X-ray diffraction data and revealed the formation of pure NaCaPO4 with orthorhombic structure using Reitveld refinement analysis. In addition, thermal, morphological and structural properties have been investigated by employing TGA-DTA, FE-SEM, FT-IR and Raman techniques and the obtained results are presented in this thesis. Research has been further extended to study the luminescent and colorimetric properties of rare earth doped phosphors.
The excitation and emission spectra were measured to characterize the luminescent properties of all rare earth doped NaCaPO4 (NCP) phosphors and the results are as follows:
For NaCaPO4:Dy3+ phosphors, sharp emission peaks were observed at 482 nm (blue) and 575 nm (yellow) upon 367 nm excitation, which are attributed to the characteristic 4F9/2 →6HJ (J =15/2 and 13/2) transitions of trivalent Dy3+ ions, respectively. The suitable control of the blue and yellow intensity ratio is expected to realize a white luminescent system. The lifetime of the 4F9/2 level was measured by exciting Dy3+ ions at 355 nm excitation. The Commission Internationale de l’Eclarage (CIE) color coordinates fall in the white light region (x = 0.32, y = 0.37) under different UV and NUV excitations. These results indicate that NaCaPO4:Dy3+ phosphor could be a potential candidate for near-UV based white light emitting diodes (w-LEDs) (Section: 3.1)
For NaCaPO4:Tb3+ phosphors, the excitation spectrum exhibited one broad band located in the UV -region at 274 nm and is assigned to 4f8→ 4f75d1 transition and some other excitation bands in the longer wavelength region are attributed to f-f transitions within the Tb3+ (4f8) configuration. Among all the excitation peaks, the strong 4f-4f transition at around 370 nm has a higher intensity. The emission spectra were measured upon 370 nm excitation and the most intense peak is observed in the green region at 547 nm, corresponding to the 5D4 → 7F5 transition. Analysis of the emission spectra with different Tb3+ concentrations revealed that the optimum dopant concentration for these NCP phosphors is about 5 mol% of Tb3+. The emitting color of Tb3+ doped NaCaPO4 phosphor was discussed based on the chromaticity coordinates and are indicated in CIE diagram. The excellent luminescent properties of Tb3+ doped NaCaPO4 green phosphor makes it as a potential candidate to use in near-UV based w-LEDs (Section: 3.2).
For NaCaPO4:Sm3+ phosphors, the excitation spectra indicate that this phosphor can be effectively excited by NUV and blue light. The emission spectra indicated that the strong emission peak at wavelength of 599 nm originated from the transition of 4G5/2 →6H7/2. The optimum concentration of Sm3+ is determined as 1.0 mol% based on the concentration dependent emission spectra. These results suggest that the NaCaPO4:Sm3+ phosphor is a promising orange emitting phosphor under 404 nm excitation with CIE coordinates of x= 0.545, y= 0.410, which might be used in the development of materials for LEDs and other optical devices in the visible region (Section: 3.3).
For NaCaPO4:Eu3+ phosphors, excitation spectra indicates the strong absorption in n-UV and blue region due to intraconfigrational f-f transitions of Eu3+ ions. The emission spectra exhibit strong orange-red emission at 595 nm corresponds to 5D0 → 7F1 transition under n-UV (λex=392 nm) excitation. The SGC route synthesized phosphor indicates intense emission than that of the SSR and MSS method. Therefore, series of Eu3+ ion doped NaCaPO4 phosphor has been prepared by SGC method. The effect of concentration on the emission intensity and concentration quenching mechanism has been discussed in detail. The Commission International de I’Eclairage (CIE) chromaticity coordinates (0.621, 0.377), (0.620, 0.378) and (0.622, 0.376) are nearly equal for the phosphors synthesized by SSR, MSS and SGC method, respectively (Section: 3.4).
Thus, this thesis is devoted to synthesize and development of efficient rare earth doped NaCaPO4 phosphors with enhanced luminescent properties to generate whtie light and multicolor emission and further to assess the feasiblity to use these phosphors in near-UV based white LEDs.

목차 (Table of Contents)

• CONTENTS
• Contents ………………………………………………………………………. i
• List of Figures ……………………………………………………………….... iv
• List of Tables ………………………………………………………………….. viii
• Abstract ……………………………………………………………………….. ix
• Chapter 1. Introduction and Motivation 1
• 1.1. General Introduction and Motivation: An overview 1
• 1.1.1. Introduction and Importance of the present host as a phosphor 3
• 1.2. Fundamentals of Luminescence and Phosphor 6
• 1.2.1. Photoluminescence 7
• 1.2.2. Phosphors 10
• 1.3. Fundamentals of Rare Earth Ions 14
• 1.3.1. An introduction to rare earth elements 14
• 1.3.2. Optical transitions in rare earth ions 16
• 1.3.3. Significance of trivalent rare earth ions as activators 20
• 1.3.4. Applications of phosphors 21
• 1.4. White LEDs and Solid State Lighting Technology 24
• 1.4.1. Approaches to generate white light 27
• 1.4.2. Basics of colorimetric quantities 29
• 1.5. Objectives of the Present Work 32
• Chapter 2. Synthesis and Characterization Methods 34
• 2.1. Synthesis of Rare Earth doped NaCaPO4 Phosphor 34
• 2.1.1. Solid state reaction method 35
• 2.1.2. Molten salt synthesis 38
• 2.1.3. Sol-gel combustion method 39
• 2.2. Characterization Methods 41
• 2.2.1. Thermogravimetric & Differential thermal analysis (TGA-DTA) 41
• 2.2.2. Powder X-ray diffraction (XRD) 42
• 2.2.3. Field emission scanning electron microscope (FE-SEM) 44
• 2.2.4. Fourier transform infrared and Raman spectroscopy 45
• 2.2.5. Photoluminescence (PL) spectroscopy 48
• Chapter 3. Results and Discussion 52
• 3.1. White Light Emission from Thermally Stable NaCaPO4:Dy3+ Phosphor
• for near UV based w-LEDs 52
• 3.1.1. Thermal analysis 52
• 3.1.2. X-Ray diffraction (XRD) analysis 53
• 3.1.3. FE-SEM analysis 54
• 3.1.4. Luminescent properties 55
• 3.2. Synthesis and Luminescent Features of NaCaPO4:Tb3+ Green Phosphor
• for near UV based LEDs 63
• 3.2.1. Crystalline structure and morphology 63
• 3.2.2. FT-IR spectrum 65
• 3.2.3. Luminescent properties 66
• 3.3. Luminescent Properties of Orange Emissive Sm3+ Activated NaCaPO4
• Phosphor for Optical Devices 75
• 3.3.1. X-ray diffraction and FE-SEM 75
• 3.3.2. FT-IR and Raman spectra 77
• 3.3.3. Luminescent spectroscopy 79
• 3.3.3.1. Excitation 79
• 3.3.3.2. Emission 80
• 3.3.3.3. CIE chromaticity coordinates 84
• 3.4. Comparative Investigations on Structural, Morphological and
• Luminescent Properties of Eu3+- doped NaCaPO4 Phosphor Synthesized
• by SSR, MSS and SGC Methods 86
• 3.4.1. Crystal structure analysis 86
• 3.4.2. Morphological analysis 90
• 3.4.3. Luminescent properties 92
• 3.4.4. CIE chromaticity coordinates 98
• Chapter 4. Conclusions and Scope for Future Work 100
• 4.1. Conclusions 100
• 4.2. Scope for Future Work 105
• List of Publications and Research Work Presented at Conferences 106
• Acknowledgements 110
• References 112