Airton Germano Bispo Junior Light-emitting diodes based on Eu3+, Eu2+ or Tb3+-doped silicates for lighting and Circadian rhythm regulation São José do Rio Preto 2019 Câmpus de São José do Rio Preto Airton Germano Bispo Junior Light-emitting diodes based on Eu3+, Eu2+ or Tb3+-doped silicates for lighting and Circadian rhythm regulation Tese apresentada como parte dos requisitos para obtenção do título de Doutor em química, junto ao Programa de Pós-Graduação em Química, do Instituto de Biociências, Letras e Ciências Exatas da Universidade Estadual Paulista “Júlio de Mesquita Filho”, Câmpus de São José do Rio Preto. Financiadora: FAPESP – Proc.. 2016/20421-9 CAPES Orientadora: Profª. Drª. Ana Maria Pires Coorientador: Prof. Dr. Sergio Antonio Marques de Lima São José do Rio Preto 2019 Sistema de geração automática de fichas catalográficas da Unesp. Biblioteca do Instituto de Biociências Letras e Ciências Exatas, São José do Rio Preto. Dados fornecidos pelo autor(a). Essa ficha não pode ser modificada. 1. LEDs. 2. Iluminação de estado sólido. 3. Terras raras. 4. Sol-gel. 5. Luminescência. I. Título. Tese (doutorado) - Universidade Estadual Paulista (Unesp), Instituto de Biociências Letras e Ciências Exatas, São José do Rio Preto Orientadora: Ana Maria Pires Coorientador: Sergio Antonio Marques Lima Bispo Junior, Airton Germano Light-emitting diodes based on Eu3+, Eu2+ or Tb3+-doped silicates for lighting and circadian rhythm regulation / Airton Germano Bispo Junior. -- São José do Rio Preto, 2019 178 f. : il., tabs. B622l Airton Germano Bispo Junior Light-emitting diodes based on Eu3+, Eu2+ or Tb3+-doped silicates for lighting and Circadian rhythm regulation Tese apresentada como parte dos requisitos para obtenção do título de Doutor em química, junto ao Programa de Pós-Graduação em Química, do Instituto de Biociências, Letras e Ciências Exatas da Universidade Estadual Paulista “Júlio de Mesquita Filho”, Câmpus de São José do Rio Preto. Financiadora: FAPESP – Proc.. 2016/20421-9 CAPES Comissão Examinadora Profª. Drª. Ana Maria Pires UNESP – Campus de Presidente Prudente Orientadora Profª. Drª. Marian Rosaly Davolos UNESP – Campus de Araraquara Prof. Dr. Fernando Aparecido Sigoli UNICAMP Prof. Dr. Carlos José Leopoldo Constantino UNESP - Campus de Presidente Prudente Prof. Dr. Paulo Cesar de Sousa Filho UNICAMP Presidente Prudente 30 de agosto de 2019 To my parents Maria José Santander Bispo and Airton Germano Bispo ACKNOWLEDGEMENTS My parents Maria José Santander Bispo and Airton Germano Bispo for sure are the real responsible for this study because they always encouraged me to study and highlighted me how education can change someone’s life. Fortunately, I got the opportunity to be born into a family with the opportunity of enabling my education and I am very proud of that. For giving me the inspiration to be a good person. Everything is for you! I thank my sister Michele Santander Bispo and my niece Julia Maria Bispo Reis for the companionship. It is worth pointing out the importance of my aunts Maria Antonia Santander dos Anjos, Ivone Santander Tardim, Nair Santander, my uncle Roberto Santander and my cousins Carla Santander dos Anjos, Carlos Romão Santander dos Anjos Souza and Andrea Santander dos Anjos in my life and in my education. My grandfather Romão Santander (in memoriam) and grandmother Alice Santander Centeio (in memoriam) for being with me all the time, taking care of me. Professor Ana M. Pires opened the doors of her lab and introduced me to the luminescence subject, something that now I am fascinated with. She is also an inspiration of the kind of person and professional I want to be: smart, hardworking, kind, polite, and ethical. Professor Sergio A. M. de Lima, co-head of LLuMes group, is acknowledged for my scientific education during the graduation and Ph.D., for the help in writing papers, data interpretation, and discussion. I also thank both for their friendship. All my university classmates: Fernanda, Monica, Jéssica, Ariane and André for sharing with me hard study moments and friendship. A memorable acknowledge to my buddy André for all the friendship and help, my housemate Edy for the friendship, and Nagyla and Rebeca for sharing happy moments with me. All my friends of LLuMes for the companionship and the scientific discussions: André, Alessandra, Alessandro, Bianca, Bruno, Camila, Edy, Felipe, Filipe, João, Leonardo, Luis, Nagyla, Rebeca, Renan, Rodolpho and Vytor. Professors Luis D. Carlos and Rute A. S. Ferreira for opening the doors of their lab in Aveiro and giving me the opportunity to finish my study. A special thanks to Professor Rute Ferreira for all the patience in teaching me some fundamentals on rare- earth spectroscopy and helping me in writing papers, data interpretation, and discussion. All my fellows from Aveiro: Caixeta, Fernanda, Fernando, Gosia, Joana Costa, João Ramalho, Justyna, Rita Bastos, Rita Frias, Luis, Rodolfo, Sofia, and Talita for making easier my stay in Portugal. All the collaborators of this study: Professor Celso X. Cardoso for the synthesis with PVDF and BO. LAB-MEV and MSc Glenda G. Souza for MEV measurements. Sol-gel research group from UNIFRAN coordinated by Professors Eduardo J. Nassar and Professor Lucas A. Rocha for the photoluminescence measurements. Laboratório Multiusuário de análises químicas of IQ-UNESP and Alberto C. Alécio and Naira C. Pesquero for the diffuse reflectance measurements. Laboratório de difração de raios X coordinated by Professor Silvio R. Teixeira and MSc Wagner D. Macedo Júnior for the XRD measurements. Núcleo de inovação tecnológica em borracha natural coordinated by Professor Aldo E. Job for the thermal analysis measurements. Laboratório de compósitos e cerâmica funcional coordinated by Professor Silvania Lanfredi Nobre and MSc Fabiano R. Prachedes, Leonardo P. de M. Simões, MSc Gisele S. Silveira and MSc Eliane A. Namikuchi for the XRD measurements. Laboratório de filmes nanoestruturados e espectroscopia coordinated by Professor Carlos J. L. Constantino, and Dr. Sabrina A. Camacho, MSc José D. Fernandes, MSc Rafael J. G. Rubira and Dr. Cibely da S. Martin for Raman measurements. University of Aveiro, Professors Luís D. Carlos and Rute A. S. Ferreira for receiving me in phantom-g. Dr. Carlos D. S. Brites from phantom-g for the quantum yield measurements. Dr. Alexandre M. P. Botas and Dr. Sandra F. H. Correia from phantom-g for helping me with the photoluminescence measurements. MSc Marita A. Cardoso for the help with the fabrication of PMMA films. Dr. Andrei Kovalevsky from CICECO for the help in the heat treatments. Dr. Rosário T. Soares from Chemistry department of University of Aveiro for XRD measurements and Rietveld Refinement. I thank IBILCE and the post-graduation in chemistry. This work was carried out with the support of the Coordination of Superior Level Staff Improvement - Brazil (CAPES) - Financing Code 001, to which I thank. I thank FAPESP for granting the research grant under process no. 2016/20421-9 São Paulo Research Foundation - FAPESP (FAPESP). “[…] Hey you, don't tell me there's no hope at all, Together we stand, divided we fall” Gilmour, D.; Manson, N.; Waters, R.; Wright, R. The wall, Harvest Records, 4 vinyl, track 1 (Vinyl 2). RESUMO Diodos emissores de luz branca (WLEDs) são as principais fontes de luz branca para iluminação e fundo de tela de displays devido ao alto brilho (800 lm) e eficácia luminosa (150 lm·W-1), alto tempo de vida útil (50.000 horas), baixo consumo energético (8,5 W), baixo preço (60 dólares em 20 anos de uso) e baixa toxicidade comparados a lâmpadas incandescentes e fluorescentes. Além dos WLEDs, LEDs monocromáticos têm ganhado atenção nos últimos anos devido ao comum consenso e entendimento da influência da luz na regulação do ritmo circadiano Humano e de plantas. Entretanto, os principais desafios são obter WLEDs com parâmetros ópticos de temperatura de cor correlata (4.500 K) e índice de renderização de cor (> 90) desejáveis e contornar o “green gap” na fabricação de LEDs emissores de luz verde. Desta forma, o objetivo desta tese de doutorado foi fabricar LEDs emissores de luz branca ou monocromática para iluminação, luz de tráfico e controle do ritmo circadiano. Para esta proposta, luminóforos emissores de luz verde (Ba2SiO4:Eu2+ e Ba2SiO4:Tb3+), amarela (Sr2SiO4:Eu2+) e vermelha (Ba2SiO4:Eu3+) foram sintetizados pela metodologia sol-gel, caracterizados e dispersos em filmes poliméricos (PVDF ou PMMA), aos quais foram utilizados para recobrir LEDs emissores na região espectral do UV. As condições de síntese dos luminóforos foram variadas a fim de otimizar a composição das fases e a emissão dos dopantes. A dispersão dos luminóforos em PMMA potencializa a emissão dos ativadores luminescentes, sendo que os luminóforos baseados em Ba2SiO4:Eu3+ e Ba2SiO4:Tb3+ mostram potencial para serem combinados a LEDs UV (250 nm), fazendo um dispositivo multifuncional que emite luz e também radiação UV, importante para o controle do ritmo circadiano de plantas e desinfecção em agricultura (cultivo indoor). Já os LEDs emissores de luz verde construídos combinando LEDs UV (365 nm) e filmes de Ba2SiO4:Eu2+/PMMA apresentaram eficácia luminosa e estabilidade radiante entre as melhores reportadas, sendo uma alternativa para suprir a ausência de LEDs emissores de luz verde com alta eficácia luminosa. Finalmente, WLEDs construídos combinando LEDs UV (395 nm) e filmes de Sr2SiO4:Eu2+/BAM:Eu2+/PMMA apresentaram valores de eficácia luminosa e estabilidade radiante entre os melhores reportados e temperatura de cor dependente da proporção dos luminóforos, podendo ser ajustada para aplicações em iluminação diurna (CCT = 6.000 K) e noturna (CCT = 4.500 K), além de usos em fototerapia. Palavras-chaves: LEDs. Iluminação de estado sólido. Terras raras. Sol-gel. Luminescência. ABSTRACT White-light-emitting diodes (WLEDs) are the main light sources for indoor and outdoor lightings as well as for backlighting of displays due to their high brightness (800 lm) and luminous efficacy (150 lm·W-1), long lifespan (50,000 hours), low power consumption (8.5 W), low cost (60 dollars over 20 years of use) and environmentally friendly properties compared to the traditional incandescent and fluorescent bulbs. Beyond WLEDs, attention has currently been paid to monochromatic LEDs due to the common consensus on the light impact on human and plant circadian rhythm regulation. Nonetheless, the main challenges are to come up with WLEDs featuring desirable correlated color temperature (4,500 K) and color rendering index (> 90) and work around the “green gap” drawback in the fabrication of green-emitting LEDs. Therefore, the goal of this Ph.D. thesis is to report on the fabrication of white or monochromatic-emitting LEDs, and for this propose, UV-to-green (Ba2SiO4:Eu2+ e Ba2SiO4:Tb3+), yellow (Sr2SiO4:Eu2+) and red (Ba2SiO4:Eu3+) downshifting converter phosphors were synthesized by the sol-gel route, fully characterized and dispersed as polymeric films (PVDF or PMMA), to which were used to coat commercial UV LEDs. The synthesis conditions of the phosphors were changed aiming to optimize phase composition and emission intensity of the dopants. PMMA plays the role of enhancing the luminescent activator emission, and both Ba2SiO4:Eu3+ and Ba2SiO4:Tb3+ phosphors feature the required characteristics to be used as coatings of UV LEDs (250 nm), making multifunctional prototypes emitting UV and red light for simultaneous application in indoor farms by regulating the plant circadian rhythm and as a disinfection agent. On the other hand, the green-emitting LEDs built by coating UV LEDs (365 nm) with Ba2SiO4:Eu2+/PMMA films match high luminous efficacy and radiant stability, among the best values reported so far, being a practicable alternative to supply the absence of commercially-available high-efficient green-emitting LEDs. Finally, WLED prototypes built by combining UV LEDs (395 nm) and Sr2SiO4:Eu2+/BAM:Eu2+/PMMA films display luminous efficacy and radiant stability among the best reported, and correlated color temperature depending on the phosphor mixture proportion, that may be tuned for daylight (6,000 K) and night light (3,500 K) applications, as well as phototherapy and backlighting of displays. Keywords: LEDs. Solid-state lighting. Rare earth. Sol-gel. Luminescence. LIST OF FIGURES Figure 1.1 Temporal development of the luminous efficacy of different kinds of lamps…….. 22 Figure 1.2 (a) Global commercial lighting revenue forecast, (b) Forecast of shipments of commercial lamps and luminaires. (c) Lighting inventory, electricity consumption, and lumen production……………………………………………... 22 Figure 1.3 External quantum efficiency of conventional monochromatic LEDs emitting in the visible spectral region……………………….……………………………………. 26 Figure 1.4 Goals of the thesis. ………………………………………………………………. 29 Figure 2.1 Photoluminescence mechanism in a crystalline matrix doped with an activator ion (A) excited (a) indirectly by the matrix and (b) directly…………………………. 33 Figure 2.2 Scheme of energy transfer mechanisms by (a) dipole-dipole and (b) exchange interactions. (c) Diagrams for electropole radiators……………………………… 36 Figure 2.3 (a) Schematic of a p-n junction in LEDs. (b) Architecture of CM-LEDs, PC-LEDs and hybrid-LEDs…………………………………………………………………. 38 Figure 2.4 Different architectures of WLEDs……………………………………………….. 39 Figure 2.5 Schematic diagram of LED packaging…………………………………………… 40 Figure 2.6 (a) Sunlight CCT dependence over the day. (b) CCT of white-emitting bulbs compared to the sunlight CCT. (c) CCT dependence on the (x,y) 1,931 CIE color coordinates………………………………………………………………………… 41 Figure 2.7 (a) Representation of the image projection by the human eye of an object illuminated by lighting sources with different CRI values. (b) Correlation between the Ri value and different colors set up by the Munsell code……………………… 42 Figure 2.8 Response of the human eye sensibility to light as a function of the wavelength… 43 Figure 2.9 Human circadian clock……………………………………………………………. 45 Figure 2.10 Partial energy diagram of Eu3+ 4f6 configuration…………………………………. 51 Figure 2.11 (a) Schematic energy level diagram of Eu2+ into a crystalline solid. (b) Representation of crystalline field effect acting on the 4f65d energy level of Eu2+. 53 Figure 2.12 Partial energy diagram of Tb3+……………………………………………………. 54 Figure 2.13 (a) Ba2SiO4 unit cell representation. (b) Representation of the Ba9, Ba10 and SiO4 polyhedra………………………………………………………………………….. 55 Figure 2.14 (a) α and (b) β-Sr2SiO4 unit cell representations and SrO9, SrO10 and SiO4 polyhedra………………………………………………………………………….. 56 Figure 2.15 Structure of (a) PVDF and (b) PMMA polymers…………………………………. 57 Figure 3.1 Scheme of the SiO2:Ba2+,Tb3+ xerogel calcination by using charcoal as an in situ source for CO……………………………………………………………………… 63 Figure 3.2 (a) Powder X-ray diffractograms and (b) FTIR spectra of BSXTb samples……… 66 Figure 3.3 (a) Rietveld plot of the BS1Tb sample. (b) Unit cell obtained for Ba2SiO4……….. 67 Figure 3.4 Raman spectra (300 K) of the BSXTb…………………………………………….. 71 Figure 3.5 SEM images of the BSTb phosphors……………………………………………… 72 Figure 3.6 (a) UV-Vis diffuse reflectance spectra of BSTb. (b) Magnification of the region between 270-520 nm………………………………………………………………. 72 Figure 3.7 Bandgap calculation considering direct transition for BSTb……………………… 73 Figure 3.8 (a) Excitation (300 K, 542 nm) and (b) emission (300 K, 250 nm) spectra of BSTb. 74 Figure 3.9 Magnification in the 300 – 500 nm range of the excitation spectra of BSTb……… 74 Figure 3.10 Area under the transitions coming from the 5D3 and 5D4 states as a function of the Terbium concentration. (b) Ratio of the integrated areas of the transitions coming from the 5D3 and 5D4 states as a function of the Terbium content. (c) 1931 CIE chromaticity diagram of the phosphors (λexc = 250 nm)…………………………… 75 Figure 3.11 (a) Plot of log(x) versus log(I/x). (b) Cross-relaxation rate (WCR) as a function of the Tb amount. (c) 5D3 lifetime as a function of the Terbium amount. (d) Dependence of the RG/B/R0 on WCR……………………………………………….. 77 Figure 3.12 Luminescence decay curves fixing excitation wavelength at 250 nm and emission wavelength at (a) 414 nm and (b) 542 nm…………………………………………. 78 Figure 4.1 (a) Powder X-ray diffractograms and (b) FTIR spectra of BSXEu samples………. 86 Figure 4.2 Raman spectra (300 K) of the BSEu samples. Laser 514 nm. ……………………. 88 Figure 4.3 a) SEM images of BS5Eu. (b) EDS spectrum of the BS5Eu sample. (c) Chemical mapping by EDS of BS5Eu. (d) Superposition of the Ba, Eu, and Si distribution on the BS5Eu sample surface……………………………………………………… 88 Figure 4.4 (a) UV-Vis diffuse reflectance spectra of BSEu. (b) Magnification of the region between 310-500 nm………………………………………………………………. 89 Figure 4.5 Bandgap calculation considering direct transition of the BSXEu samples……….. 90 Figure 4.6 (a) Excitation spectra (300K, 612 nm), (b) Emission spectra (300 K, 393 nm) and (c) 1,931 Commission Internationale d´Eclairage (CIE) chromaticity diagram of the phosphors……………………………………………………………………… 91 Figure 4.7 (a) Powder XRD of the Eu3+-based phosphors calcined for 2 or 10 hours. (b) Ba2SiO4 unit cell representation…………………………………………………… 92 Figure 4.8 Excitation spectra (300 K) monitored at 610.14 nm of Eu3+-based phosphors…… 94 Figure 4.9 (a) Photo of the Eu-10 h sample under UV radiation exposition (255 nm). (b) Absolute emission quantum yield (q), (c) Emission spectra (300 K, 255 nm). (d) 1,931 CIE chromaticity diagram of Eu-2h and Eu-10h……………………………. 95 Figure 4.10 (a) High-resolution emission spectra (14 K) monitoring different excitation wavelength in the 5D0→7F0 transition region. (b) Representation of Eu3+ local sites (Eu1-3, EuD1-3 and EuA1,2)………………………………………………………….. 96 Figure 4.11 High-resolution emission spectra (14 K) excited at 255 nm and 393 nm in the (a) 5D0→7F1 and (b) 5D0→7F2 transition region………………………………………. 97 Figure 4.12 Excitation spectra (14 K) of the (a) Eu-2h and (b) Eu-10h samples monitored at distinct wavelengths around the 5D0→7F0 transitions…………………………….. 98 Figure 4.13 High-resolution emission spectra (14 K) monitoring different excitation wavelengths……………………………………………………………………….. 99 Figure 4.14 (a) Diffuse reflectance spectra of Eu-2h- and Gd-2h samples. (b) Arithmetic difference between the Eu-2h- and Gd-2h-related reflectance spectra within the range of 200-360 nm………………………………………………………………. 100 Figure 4.15 Emission decay curves (14 K) excited at 393 nm and monitored at distinct wavelengths around the 5D0→7F0 transitions for the Eu-2h sample………………. 101 Figure 4.16 Emission decay curves (300 K) excited at 393 nm and monitored at 578.2 nm for the Eu-2h sample………………………………………………………………….. 101 Figure 4.17 Emission decay curves (14 K) excited at 393 nm and monitored at distinct wavelengths around the 5D0→7F0 transitions for the Eu-10h sample……………… 102 Figure 4.18 Temperature-dependent emission spectra monitored at 393 nm of the Eu-2h phosphor in the (a) 5D0→7F0, (b) 5D0→7F1 and (c) 5D0→7F2 transitions region….. 104 Figure 5.1 Representation of the goals of chapter 5………………………………………….. 107 Figure 5.2 (a,c) Excitation spectra (300 K, 612 nm) of BSXEuYTb samples. (b,d) Magnification of the range between 325–500 nm………………………………… 109 Figure 5.3 (a,b) Emission spectra (300 K, 250 nm) of BSXEuYTb. (c) Excitation spectrum (300 K, 441 nm) and emission spectrum (300 K, 340 nm) of the undoped Ba2SiO4 matrix……………………………………………………………………………… 110 Figure 5.4 (a) Excitation spectra (300 K, 435 nm) of BSXEu1Tb series. (b) Energy transfer mechanisms between Tb3+ and Eu3+………………………………………………. 111 Figure 5.5 Linear fitting of log(x) versus log(I/x) for BSXEy1Tb series considering the emission intensity at (a) 612 nm and (b) 545 nm…………………………………. 112 Figure 5.6 CIE diagram of BSXEuYTb samples excited at 250 nm…………………………. 113 Figure 5.7 Emission decay curves excited at 250 nm and monitored at (a) 370 nm, (b) 545 nm and (c) 612 nm………………………………………………………………… 133 Figure 6.1 Scheme of the UV and red-emitting multifunctional LED architecture for a potential application in indoor farming……………………………………………. 118 Figure 6.2 Images of the PVDF membranes: (a) PVDF:2BSEu, (b) PVDF:5BSEu, (c) PVDF:10BSEu, (d) PVDF:20BSEu………………………………………………. 121 Figure 6.3 X-ray diffractograms of PVDF:BSEu films………………………………………. 122 Figure 6.4 SEM images of PVDF:BSEu films. The histogram was done by counting 200 PVDF particles……………………………………………………………………. 123 Figure 6.5 Chemical mapping of the PVDF:10BSEu film…………………………………… 124 Figure 6.6 (a) TG thermograms and (b) DTG of PVDF:BSEu films………………………… 124 Figure 6.7 (a) DSC scans of the PVDF:BSEu films. (b) Magnification of the DSC scans in the region between -67.5 ºC and -15 ºC…………………………………………… 125 Figure 6.8 (a) Excitation spectra (300 K, 610 nm) and (b) emission spectra (300 K, 250 nm) of PVDF:10BSEu compared to BS4Eu…………………………………………… 127 Figure 6.9 (a) UV–Vis absorption spectra of the PMMA:MEu-2h films. (b) Absorbance values at 600 nm for the PMMA:MEu-2h films………………………………….. 128 Figure 6.10 Excitation spectra (300 K) monitored at 610.14 nm for the PMMA films……….. 129 Figure 6.11 (a) Photo of the PMMA:2Eu-10h film under UV radiation (255 nm) exposition. (b) Emission spectra (300 K, 255 nm) of the films. (c) CIE chromaticity diagram of the films (255 nm). (d) Absolute quantum yield (q) for the PMMA:2Eu-10h film compared to its excitation spectrum…………………………………………. 130 Figure 6.12 (a) Excitation spectrum (300 K, 450 nm) and (b) emission spectrum (300 K, 255 nm) of the undoped PMMA film…………………………………………………. 130 Figure 6.13 (a) Excitation spectra (14 K) of the PMMA:2Eu-10 h film monitored at distinct wavelengths around the 5D0→7F0 transitions. (b) Emission spectra (14 K) of the PMMA:2Eu-10h film monitoring different excitation wavelength………………. 131 Figure 6.14 High-resolution emission spectra (14 K) monitored at 393 nm of the PMMA:2Eu- 10h film in the 5D0→7F0 transition region compared to the Eu-2h and Eu-10h samples……………………………………………………………………………. 132 Figure 6.15 Emission decay curves (14 K) excited at 393 nm and monitored at distinct wavelengths around the 5D0→7F0 transitions for the PMMA:2Eu-10h film……… 132 Figure 7.1 Architecture of the near-UV-emitting LED coated with green-emitting phosphors and applications as traffic signals, displays and regulation of human circadian rhythm…………………………………………………………………………….. 137 Figure 7.2 (a) 365-B LED prototype under white light exposition. (b) Emission spectra of the 365-A LED, 365-B LED and 365-C LED prototypes operating at 3.2 V………… 138 Figure 7.3 Emission spectra dependence on the operating voltage for 5 different prototypes of the 365-B LED. ………………………………………………………………… 139 Figure 7.4 Powder XRD of B2S……………………………………………………………… 140 Figure 7.5 (a) Excitation and (b) emission spectra monitored at 505 nm and excited at 366 nm (300 K), respectively. In (a) the emission quantum yield is also plotted. Pictures of (c) B2S and (d) film B under UV radiation (350 nm) exposition. (e) CIE color coordinate diagram. ……………………………………………………. 141 Figure 7.6 Emission decay curves (14 K or 300 K) excited at 355 nm and monitored at 508 nm for the B2S (left) and film-B (right)…………………………………………… 142 Figure 7.7 Time-resolved emission spectra (14 K) monitored at 366 nm at different starting delay values (0SD0.04 10−3 s) for B2S……………………………………….. 143 Figure 7.8 Temperature-dependent (a) absolute and (b) normalized emission spectra excited at 366 nm for B2S…………………………………………………………………. 145 Figure 7.9 Configurational coordinate diagram for B2S……………………………………… 145 Figure 7.10 Temperature-dependent emission spectra (300 – 450 K) excited at 366 nm for B2S sample……………………………………………………………………………... 146 Figure 7.11 (a) Temperature-dependent emission intensity of the Eu9 site excited at 366 nm for B2S. (b) Plot of ln(I0-IT/IT) vs. 1/T………………………………………………… 147 Figure 7.12 (a) Photo and (b) emission spectra dependence on the operating voltage of the 365- B LED. (c) (x,y) 1,931 CIE coordinate-dependence on the operating voltage……. 148 Figure 7.13 Normalized emission spectra dependence on the operating voltage for the 365-B LED prototype…………………………………………………………………….. 150 Figure 7.14 State-of-the-art for radiant flux stability of downshifting phosphor-converted green-light emitting LEDs………………………………………………………… 151 Figure 8.1 Architecture of WLEDs built by coating near-UV-emitting LEDs with PMMA/BAM/S2S films and applications in indoor and outdoor lighting and circadian rhythm control………………………………………………………….. 156 Figure 8.2 Pictures of the S2S/BAM powder and film under UV radiation (350 nm) exposition…………………………………………………………………………. 157 Figure 8.3 Emission spectra of five different (a) S2S(60)BAM(40)-LED and (b) S2S(40)BAM(60)-LED prototypes operating at 3.1 V……………………………. 158 Figure 8.4 Powder XRD of the S2S sample. …………………………………………………. 159 Figure 8.5 (a) Excitation spectra (300 K, 550 nm) compared to the absolute quantum yield (q), (b) emission spectra (300 K, 365 nm), (c) CIE color coordinate diagram (300 K) of S2S and film…………………………………………………………………. 160 Figure 8.6 Selective (a) excitation (300 K), (b) emission (300 K), (c) excitation (14 K) and (d) emission (14 K) spectra monitored at different wavelengths for the S2S sample. 162 Figure 8.7 High resolution excitation spectrum (14 K) monitored at 450 nm of the S2S sample……………………………………………………………………………... 163 Figure 8.8 Emission decay curves (14 K or 300 K) of S2S and S2S(100)/BAM(0) excited at 355 nm and monitored at different emission wavelength…………………………. 164 Figure 8.9 Time-resolved emission spectra (14 K) monitored at 366 nm by changing the starting-delay (SD) for the S2S sample……………………………………………. 165 Figure 8.10 Temperature-dependent (a) absolute and (b) normalized emission spectra monitored at 366 nm of the S2S sample………………………………………….. 166 Figure 8.11 (a) Photos, (b) emission spectra and (c) CIE color coordinate diagram of the LED prototypes operating at 3.1 V……………………………………………………… 167 Figure 8.12 CRI of the fabricated WLED prototypes compared to the YAG:Ce3+-based commercial LED…………………………………………………………………... 167 Figure 8.13 Operating voltage-dependent emission spectra of S2S(40)BAM(60)-LED and S2S(60)BAM(40)-LED prototypes……………………………………………….. 168 Figure 8.14 Excitation and emission spectra of the (a) S2S/BAM powder mix and (b) S2S/BAM immobilized in the PMMA……………………………………………. 170 Figure 8.15 State-of-the-art of PC-WLEDs comparing the radiant flux stability (%) over 100 hours of operation…………………………………………………………………. 171 LIST OF TABLES Table 1.1 Comparison of typical market prices for various lighting sources………………….. 21 Table 1.2 State-of-the-art of WLEDs comparing luminous efficacy (LE), correlated color temperature (CCT) and color rendering index (CRI) as figures of merit………….. 24 Table 1.3 Target LE for different kind of WLED approach………………………………….. 25 Table 1.4 Monochromatic LEDs for human phototherapy and plant and food technology….. 25 Table 1.5 Figure of merit of the emission quantum yield (q) of M2SiO4:RE (M = Sr or Ba and RE = Eu2+, Eu3+ or Tb3+) phosphors……………………………………………….. 26 Table 2.1 Selection rules for ET processes of RE ions……………………………………….. 37 Table 2.2 State-of-the-art of phosphors applied in PC-WLED……………………………….. 44 Table 2.3 Human circadian rhythm dependence on the white-LED CCT values and application in phototherapy and lighting. ………………………………………….. 47 Table 2.4 LED emission features for human and plant circadian rhythm control…………….. 48 Table 2.5 Selection rules for f–f transitions between two spectroscopic levels………………. 49 Table 2.6 Main Ba2SiO4 and Sr2SiO4-based phosphor synthesis……………………………… 56 Table 3.1 Doping proportions and amounts of reagents added, assuming 1.0000 g of the product. ……………………………………………………………………………. 63 Table 3.2 Refinement conditions……………………………………………………………... 64 Table 3.3 Average bond length (Ba-O), polyhedral distortion index (D), lattice parameters (a, b and c) and cell volume (V) determined for the phosphors. ……………………… 68 Table 3.4 Ba-O bond length for Ba1 (CN 10) and Ba2 (CN 9) sites of BSTb. ……………….. 68 Table 3.5 Ionic Radii, bond distance and difference of ionic radii (Dr) between Tb3+ and Ba2+. 69 Table 3.6 Crystallite size (ε, nm) for the four most intense plans of the samples. …………… 69 Table 3.7 Tb-Tb critical distance (Rc), 5D3 and 5D4 lifetime values (τ), cross-relaxation probability (WCR) and cross relaxation efficiency (ηCR) of BSTb…………….……. 77 Table 3.8 Radiative decay probability (Arad), non-radiative decay probability (Anrad), total decay probability (Arad+nrad) and 5D3 state quantum efficiency (η) of BSYTb.. ……. 80 Table 4.1 Doping proportions and amounts of reagents added, assuming 2.0000 g of the product……………………………………………………………………………… 84 Table 4.2 Refinement conditions……………………………………………………………… 84 Table 4.3 Crystallite size (ε, nm) for the three most intense planes of BSEu…………………... 87 Table 4.4 Refinement parameters obtained from the XRD measurement……………………. 93 Table 4.5 Polyhedral information of the potential sites for Eu3+ substitution………………… 94 Table 4.6 Emission quantum yield (q) for the Eu3+-based phosphors………………………… 96 Table 4.7 Energy of the components for the 5D0→7F0 transitions and 5D0 lifetime (τ) values of each component excited at 393 nm (14 K) for the Eu-2h and Eu-10h. …………. 97 Table 5.1 Doping proportions…………………………………………………………………. 108 Table 5.2 Tb/Eu emission rate, and critical distance (Rc) between Eu3+ and Tb3+……………. 110 Table 5.3 Tb→Eu ET efficiency, Tb3+ 5D3 (τTb 5D3) and 5D4 (τTb 5D4) lifetime values and Eu3+ 5D0 lifetime values (τEu). …………………………………………………………… 114 Table 6.1 2θ Bragg angle for the α-PVDF phase planes and thickness of the membranes……. 122 Table 6.2 Loss weight in different temperature ranges obtained from TG and maximum decomposition temperature (max.) of PVDF films………………………………… 125 Table 6.3 Glass transition temperature (Tg), melting temperature (Tm), beginning of the melting process (Tonset), heat of fusion (ΔHm), entropy variation in the melting process (ΔSm) and crystallinity degree (χ)…………………………………………. 126 Table 6.4 Film thickness (Thick.) and luminous flux (Φ) compared to the powder phosphors. 129 Table 6.5 Energy (cm-1) and 5D0 lifetime (τ) values of the components for the 5D0→7F0 transitions of the PMMA:Eu-10h sample………………………………………….. 133 Table 7.1 1 Thickness (µm) of the films and luminous flux (ϕV, lm), luminous efficacy (LE, lm.W−1), and input electric power (Pel, W) of the 365-A, 365-B and 365-C LED prototypes operating at 3.2 V………………………………………………………. 138 Table 7,2 Figure of merit of the quantum yield (q, %) for the B2S phosphor……………….. 142 Table 7.3 Eu2+ 4f65d state lifetime (×10−6 s) values measured at 14 and 300 K………………. 143 Table 7.4 Comparison of the thermal stability between the B2S sample and other Ln-based phosphors…………………………………………………………………………… 147 Table 7.5 Figure of merit of luminous efficacy (LE, lm.W-1) for the state-of-the-art of green- emitting PC-LEDs………………………………………………………………….. 149 Table 7.6 Assignments of the curves represented in Figure 7.14…………………………….. 151 Table 8.1 Thickness of the S2S/BAM films measured by optical microscopy……………….. 157 Table 8.2 Refinement parameters obtained from the XRD measurement. The refinement factors converge to Rp = 2.10 %, Rwp = 2.74 %, and χ2 =1.55……………………… 160 Table 8.3 Figure of merit of absolute emission quantum yield for the S2S phosphor comparing excitation wavelength (λexc) and calcination temperature (Tcalc)…………………… 161 Table 8.4 Positions of ZPLs observed in the 14 K high-resolution excitation spectrum and energy difference between the 4f6(7F0)5d1→8S7/2 and 4f6(7FJ)5d1→8S7/2 transition of the S2S sample compared to the Eu3+ 7FJ (J = 0- 6) energy level energies………… 163 Table 8.5 Eu2+ 4f65d state lifetime values (μs) for the Eu9 (14 and 300 K) and Eu10 (14 K) local sites of the S2S and S2S(100)/BAM(0) samples. ……………………………. 164 Table 8.6 Luminous efficacy (LE, lm.W-1), correlated color temperature (CCT, K) and color rendering index (CRI) dependence on the operating voltage (V) for the S2S(40)BAM(60)-LED and S2S(60)BAM(60)-LED prototypes………………….. 168 Table 8.7 Phosphor quantum yield (ηyield), ratio between the energy of the excitation and emission photons (ηstokes), self-absorption efficiency (ηSA), extraction efficiency (ηext), phosphor efficiency (ηp), experimental wall-plug efficiency (WPE) and theoretical luminous efficacy (LE) of all the fabricated LED prototypes………….. 169 Table 8.8 Assignments of the curves represented Figure 8.15……………………………….. 171 LIST OF ABBREVIATIONS 365-B LED Green-emitting LED prototype B2S Ba2SiO4:Eu2+ Ba9 Barium site in Ba2SiO4 with CN = 9 Ba10 Barium site in Ba2SiO4 with CN = 10 BO Buriti oil BSXEu Eu-doped Ba2SiO4 (X %) calcinated for 2 hours BSXEuYTb Eu and Tb-doped Ba2SiO4 BSXTb Tb-doped Ba2SiO4 (X %) CD Carbon dot CCT Correlated color temperature CFL Compact fluorescent lamp CIE 1,931 Commission internationale de l'éclairage CRI Color rendering index CM-LED Color-mixed LED CN Coordination number CR Cross-relaxation CTB1 Charge transfer band from Eu3+ in Ba local sites CTB2 Charge transfer band from Eu3+ -O2- associates D Donator specie D* Excited donator specie D-D Dipole-dipole DED Electric dipole strength DMD Magnetic dipole strength DSC Differential scanning calorimetry D-Q Dipole-quadrupole DR Diffuse reflectance spectroscopy DTG Differential thermal analysis DRX X-ray diffraction ED Electric dipole oscillator EDS Energy-dispersive X-ray spectroscopy EHT Electron high tension EM Magnetic dipole oscillator Ephoton Energy of a photon EQ Electric quadrupole oscillator EQE External quantum efficiency ET Energy transfer Eu1-3 Eu local sites in Ba2SiO4 or BaSiO3 lattice EuA1,2 Eu local sites related to Eu3+-O2- associates EuD1-3 Defect-related Eu local sites FTIR Fourier transform infrared spectroscopy FWHM Full-width-at-half-maximum HID High-intensity discharge lamp ICDD International Centre for Diffraction Data ipRGCs Intrinsically photosensitive retinal ganglion cells IR Infrared radiation LE Luminous efficacy LED Light-emitting diodes Ln-X Ba2SiO4: Ln (Ln = Eu or Gd) and X = 2 h and 10 h (calcination time) MW Molar weight Near-UV LED Near-UV-emitting LED chip OLED Organic light-emitting diode PC-LED Phosphor-converted LED PL Photoluminescence PLE Excitation spectrum PLED Polymer light-emitting diode PMMA Poly(methyl methacrylate) PMMA:MEu-X PMMA films with different amount of BSEu PVDF Polyvinylidene fluoride PVDF:XBO PVDF films with X mL of BO PVDF:yBSEu PVDF films with 400 mL of BO and different amount of BS4Eu Peletric Electric power Poptical Optical power QD Quantum dots Q-Q Quadrupole-quadrupole RE Rare Earth RGB Red, blue and green S2S Sr2SiO4:Eu2+ S2S(X)BAM(Y) PMMA films doped with X % of S2S and Y % of BAM SAD Seasonal Affective Disorder SD Starting delay SEM Scattering electron microscope SI International System of Units SMD Surface mounted LED Sr9 Strontium site in Sr2SiO4 with CN = 9 Sr10 Strontium site in Sr2SiO4 with CN = 10 SSL Solid-state lighting Tcalc. Calcination temperature Td Fluorescent bulb with 15.9 mm of diameter TEM Transmission electron microscopy TEOS Tetraethyl orthosilicate TG Thermogravimetric analysis TWh Terawatt-hour UV UV radiation WPE Wall-plug efficiency WLED White-emitting LED XRD X ray diffraction YAG Yttrium Aluminium Garnet ZPL Zero-phonon line LIST OF SYMBOLS 10Dq Strength of the ligand field 2S+1LJ Russell-Saunders term (x,y) 1,931 CIE color coordinates ⟨𝒇𝑵𝝍𝑱|𝑼(𝝀)|𝒇𝑵𝝍′𝑱′⟩ 𝟐 Reduced matrix elements °C Degree Celsius η Quantum efficiency of an emitting state ηCR Cross-relaxation efficiency ηext Extraction efficiency nPC-LED LED efficiency ηSA Self-absorption efficiency ηstokes Ratio between the energy of the excitation and emission photons ηyield Phosphor quantum yield λ Wavelength λem Emission wavelength λex Excitation wavelength τrad Radiative lifetime χ Lorentzian field correction χ2 “Chi squared” ϕV Luminous flux μm Micrometer ΔE Thermal activation energy ΔE1 Energy barrier for the energy transfer from the Eu10 to the Eu9 site in the Ba2SiO4 ΔE2 Energy barrier for the energy transfer from the Eu9 to the Eu10 site in the Ba2SiO4 ΔHm Heat of fusion ΔSm Entropy variation in the melting process Ω2 e Ω4 Judd-Ofelt intensity parameters A01 Einstein’s coefficient for spontaneous emission Å ångström A amper A Acceptor specie A* Excited acceptor specie Anrad Non-radiative decay rate Atp Crystal field parameters Arad Radiative decay rate cm Centimeter e Electron charge Ea Electron affinity of the ligand atoms eV Electronvolt g Gram g∙cm-3 Gram per centimeter g∙mol-1 Gram per mol h Plank’s constant Hz Hertz I Electric current J Total angular quantum number J∙g-1 Joule per gram J g−1K-1 Joule per gram Kelvin k Boltzmann constant K Kelvin L Total orbital angular momentum quantum number Lm Lumen Ln3+ Lanthanide(III) lm∙W-1 Lumen per watts me Mass of the electron mJ∙pulse MiliJaule per pulse mL Milliliter ms Millisecond Mol Mol n Refractive index nm Nanometer P Oscillator strength pm Picometer q Emission quantum yield R0 Förster distance Rexp Expected R factor Ri Munsell code Rc Critical distance RWP weighted profile R-factor s Second S Total spin quantum number T1/2 Temperature at which the emission intensity is half of that at 14 K IT/I300K Ratio between the intensities at a given temperature T and at 300 K T8 Fluorescent bulb with 25.5 mm of diameter T12 Fluorescent bulb with 38.1 mm of diameter Tg Glass transition temperature Tm Melting temperature TWh Terawatt-hour V Volts W Watts WCR Cross-relaxation rate TABLE OF CONTENTS CHAPTER 1 – INTRODUCTION .............................................................................. 21 1.1 State-of-the-art ........................................................................................................ 21 1.2 Motivation, challenges and justifications ............................................................. 27 1.3 Goals ........................................................................................................................ 28 1.4 Thesis organization ................................................................................................. 29 1.5 References................................................................................................................ 30 CHAPTER 2- BACKGROUND .................................................................................. 33 2.1 Phenomenon of luminescence ................................................................................ 33 2.1.1 Fundaments of luminescence ................................................................................ 33 2.1.2 Characteristics of photoluminescence ................................................................... 34 2.2 Solid-state lighting .................................................................................................. 37 2.2.1 Light-emitting diodes ............................................................................................ 37 2.2.2 Photometric quantities ........................................................................................... 40 2.2.3 Requirement of ideal phosphors for PC-WLEDs .................................................. 44 2.2.4 Light and Circadian rhythm control ...................................................................... 45 2.3 Rare-earth ions ....................................................................................................... 48 2.3.1 Spectroscopic properties ........................................................................................ 48 2.3.2 Eu3+ ion .................................................................................................................. 50 2.3.3 Eu2+ ion .................................................................................................................. 52 2.3.4 Tb3+ ion .................................................................................................................. 54 2.4 Silicate-based phosphors ........................................................................................ 55 2.5 On the polymeric matrices applied in this study. ................................................ 57 2.6 References................................................................................................................ 58 CHAPTER 3 – TUNABLE BLUE-GREEN EMISSION AND ENERGY TRANSFER PROPERTIES IN Ba2SiO4:Tb3+ ........................................................... 62 3.1 Introduction ............................................................................................................ 62 3.2 Experimental procedure ........................................................................................ 62 3.3 Results ...................................................................................................................... 66 3.3.1 Structural characterization ..................................................................................... 66 3.3.2 Morphology ........................................................................................................... 71 3.3.3 Band gap evaluation .............................................................................................. 72 3.3.4 Photoluminescence ................................................................................................ 74 3.4 Conclusions ............................................................................................................. 80 3.5 References................................................................................................................ 80 CHAPTER 4 – SOL-GEL SYNTHESIS OF Eu3+-DOPED Ba2SiO4 ....................... 83 4.1 Introduction ............................................................................................................ 83 4.2 Experimental procedure ........................................................................................ 83 4.3 Optimization of Eu3+ concentration in Ba2SiO4:Eu3+ .......................................... 86 4.3.1 Structural characterization ..................................................................................... 86 4.3.2 Morphology ........................................................................................................... 88 4.3.3 Band gap evaluation .............................................................................................. 89 4.3.4 Photoluminescence ................................................................................................ 91 4.4 Optimization of the calcination time ..................................................................... 92 4.5 High-resolution photoluminescence ...................................................................... 96 4.6 Conclusions ........................................................................................................... 104 4.7 References.............................................................................................................. 105 CHAPTER 5 – ENERGY TRANSFER BETWEEN Tb3+ AND Eu3+ IN BARIUM ORTHOSILICATE PHOSPHORS .......................................................................... 107 5.1 Introduction .......................................................................................................... 107 5.2 Experimental procedure ...................................................................................... 108 5.3 Results .................................................................................................................... 109 5.4 Conclusions ........................................................................................................... 115 5.5 References.............................................................................................................. 115 CHAPTER 6 – RED-LIGHT-EMITTING COATINGS FOR LEDs APPLIED TO PLANT CIRCADIAN RHYTHM CONTROL ........................................................ 117 6.1 Introduction .......................................................................................................... 117 6.2 Experimental procedure ...................................................................................... 118 6.3 Characterization of PVDF-based films .............................................................. 121 6.3.1 Structural characterization ................................................................................... 121 6.3.2 Morphology ......................................................................................................... 123 6.3.3 Thermal behavior of PVDF-films ....................................................................... 124 6.3.4 Photoluminescence .............................................................................................. 126 6.4 Characterization of the PMMA-based films ...................................................... 127 6.5 Conclusions ........................................................................................................... 133 CHAPTER 7 - GREEN-EMITTING LEDs BASED ON Ba2SiO4:Eu2+ AND NEAR- UV-EMITTING LEDs ............................................................................................... 136 7.1 Introduction .......................................................................................................... 136 7.2 Experimental procedure ...................................................................................... 137 7.3 Characterization of the B2S and B2S/PMMA phosphors ................................ 140 7.3.1. Structural characterization .................................................................................. 140 7.3.2 Steady-state photoluminescence .......................................................................... 141 7.3.3 Time-resolved photoluminescence ...................................................................... 142 7.3.4 Temperature-dependent emission spectra ........................................................... 144 7.4 Green-emitting LED prototype characterization .............................................. 148 7.5 Conclusions ........................................................................................................... 152 7.6 References.............................................................................................................. 152 CHAPTER 8 - WHITE-EMITTING LEDs BASED ON Eu2+-DOPED SILICATE ...................................................................................................................................... 155 8.1 Introduction .......................................................................................................... 155 8.2 Experimental procedure ...................................................................................... 156 8.3 S2S and S2S(100)/BAM(0) characterization ...................................................... 159 8.3.1. Structure and phase composition of S2S ............................................................ 159 8.3.2 Steady-state photoluminescence .......................................................................... 160 8.3.3 Selective excitation and emission spectra ........................................................... 161 8.3.4 Time-resolved spectroscopy ................................................................................ 163 8.3.5 Temperature-dependent emission spectra ........................................................... 165 8.4 Characterization of the WLED prototypes ........................................................ 166 8.5 Conclusions ........................................................................................................... 171 8.6 References.............................................................................................................. 172 CHAPTER 9 – FINAL REMARKS .......................................................................... 174 9.1 Conclusions ........................................................................................................... 174 9.2 Perspectives for futures investigations ............................................................... 175 9.3 Papers published by the authors during the Ph.D. ............................................ 176 21 CHAPTER 1 – INTRODUCTION 1.1 State-of-the-art Have you ever thought how would be human life without lighting? Or better, how would society evolve in the dark? Thanks to Thomas Edison that manufactured the first electric bulb based on incandescence at the end of the XIX century, we do not ask ourselves those questions.1 The lighting evolution has not stopped with Edison’s invention since, in 1926, Edmund Germer patented the modern fluorescent lamp, opening up new opportunities of lighting by using bulbs with better efficiency and color qualities compared to incandescent sources.2 However, the most exciting lighting technology came up in 1996 with Shuji Nakamura at Nichia labs, who invented the first efficient white-emitting LED (WLED) based on a blue-emitting LED coated by a yellow-emitting phosphor, starting the LED boom.2 Over the past 23 years, the luminous efficacy (LE) of WLEDs have improved from 25 lm/W to almost 200 lm/W, and this huge increase is correlated with the fabrication of high-efficient blue-light-emitting diodes by Hiroshi Amano, Shuji Nakamura, and Isamu Akasaki, who were laureates with the Nobel prize of Physics in 2014.2 Nowadays, WLEDs are estimated to achieve at about 60 % of penetration in 2020 in various market segments as indoor (offices, homes, shops) and outdoor (streets, traffic signals) lighting, displays (backlighting for displays, digital cameras, security equipment, mobile phone, etc.), automotive lighting and medical applications.3,4 This huge and outstanding expansion of WLED commercialization compared to the traditional incandescent and fluorescent lamps is due to their relative low cost-benefit (60 dollars over 20 years of use), high brightness (800 lm), long lifespan (50.000 hours), compact size and shape, environmentally-friendly properties, low power consumption (8.5 W) and high LE (150 lm.W-1), as represented in Figure 1.1 and Table 1.1.5 Table 1.1 Comparison of typical market prices for various lighting sources. CFL = Compact fluorescent lamp. Lighting Source Price ($/klm) Halogen Lamp (A19 43W; 750 lumens) $2.5 CFL (13W; 800 lumens) $2 Fluorescent Lamp and Ballast System (F32T8) $4 LED Lamp (A19 12W; 800 lumens, dimmable) $16 LED 6” Downlight (11.5W; 625 lumens) $43 OLED Luminaire $1,400 Source: Adapted from Bardsley et al. 6 22 Figure 1.1 Temporal development of the luminous efficacy of different kinds of lamps. Source: Adapted from Mitch.4 A recent report from the US Department of energy6 points out that the traditional bulb replacement by WLEDs is expected to reduce the lighting sector consumption by 15 % in 2020 and by 40 % in 2030, saving 261 TWh (equivalent to the total energy consumption of at about 24,000,000 homes in the US). A recent example from the commercial lighting sector is also shown in Figure 1.2, highlighting the energy saves due to the replacement of traditional bulbs by LED lamps in the US. Figure 1.2 (a) Global commercial lighting revenue forecast, (b) Forecast of shipments of commercial lamps and luminaires. (c) Lighting inventory, electricity consumption, and lumen production. Abbreviations: HID (High-intensity discharge lamp), CFL (Compact fluorescent lamp), T12 (fluorescent bulb with 38.1 mm of diameter), T8 (fluorescent bulb with 25.5 mm of diameter), T5 (fluorescent bulb with 15.9 mm of diameter). Source: Adapted from Bardsley et al.6 1 2 10 20 100 70 200 300 1879 1911 1937 1985 2009 2020 Year L u m in o u s ef fi ca cy (l m /W ) Edison’s bulb Incandescent bulb Fluorescent lamp Compact fluorescente lamp LED bulb 2013 2014 2015 2016 2017 2018 2019 2020 2021 0 5 10 15 20 25 30 35 40 45 50 55 60 65 (a) U S b il li o n d o la r Year LED HID CFL T12 T8+T5 Halogen Incandescent 2013 2014 2015 2016 2017 2018 2019 2020 2021 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5(b) B il li o n s o f u n it s Year LED HID CFL T12 T8+T5 Halogen Incandescent Number of bulbs Energy use Lumen production Residential Comercial Industrial Outdoor 71 % 25 % 2 % 2 % 25 % 50 % 8 % 17 % 8 % 60 % 11 % 21 % (c) 23 However, although most of the governments are providing financial support for solid-state lighting (SSL), many countries have still experienced less than 10 % of LED bulbs in the lighting market.6 Therefore, the basic research on SSL has an enormous impact on energy savings in the world and a vast field of opportunities taking into account the improvement of WLED properties, namely the luminous efficacy and the color qualities as correlated color temperature (CCT) and color rendering index (CRI).4 The main commercially-available WLED is based on the combination of the GaN blue-emitting LED chip and the blue-to-yellow downshifting converter Y3Al5O12: Ce3+ (YAG:Ce3+ - Yttrium Aluminium Garnet) phosphor.7 The combination of the yellow with the remaining blue light generates a bluish-white light sensation to the human eye. Yet, although this approach displays high LE, it also features high CCT dependent on the operating voltage and time of use, and poor CRI due to the absence of red-emitting components.8 It is important to point out that much attention has currently been paid to the color qualities of WLEDs since, in the last few years, many studies have shown that light has a huge impact on the human circadian rhythm.9 Those disadvantages, combined to the higher price compared to the traditional lighting bulbs, create a barrier between the customer and the WLED market, even that its lifespan is immensely higher.5 In order to overcome those issues, the SSL engineering is now focused on three different research fields: (1) the fabrication of high-efficient and stable blue-to-red downshifting-converter phosphors to improve the CRI and decrease the CCT of the YAG- based WLED, (2) the fabrication of high-efficient near-UV-to-visible downshifting converter phosphors to be used as coatings of near-UV-emitting LEDs because the human eye does not see UV radiation, solving the CCT stability problem or (3) the fabrication of high-efficient and stable green or red-emitting semiconductor LED chips to be combined to the blue-emitting chip.6 However, for the best of our knowledge, all the compositions with red-emitting phosphors for the issue (1) decrease the luminous efficacy of the YAG-based LED, as shown in the figure of merit of LE, CCT and CRI, Table 1.2. Furthermore, the human eye does not have good sensitivity in the red spectral region, requiring high amounts of red- emitting phosphor in the LED composition to be detected by the retina.10 For issue (2), there is a lack of high-efficient and stable near-UV-to-visible downshifting converter phosphors. Considering issue (3), there is a lack of green-emitting semiconductor materials that is known as “green-gap”, minutely discussed as follow.11 https://en.wikipedia.org/wiki/Yttrium https://en.wikipedia.org/wiki/Aluminium https://en.wikipedia.org/wiki/Oxygen 24 Table 1.2 State-of-the-art of WLEDs comparing luminous efficacy (LE), correlated color temperature (CCT) and color rendering index (CRI) as figures of merit. The values reported for incandescent and fluorescent lamps are also reported. Composition LED chip / nm LE / lm.W-1 CCT / K CRI [ref] Incandescent [1] - 26 2,812 43 12 Fluorescent [2] - 60 3,753 100 12 YAG 460 164 5,468 78 12 YAG and K2GeF6:Mn4+ 460 125 3,882 90.4 13 YAG and K2SiF6:Mn4+ 460 116 3,900 89.9 14 YAG and K2(Si,Ge)F6:Mn4+ 460 145.33 6,110 70.5 15 YAG and CsNaGeF6:Mn4+ 460 176.3 3,783 92.5 16 YAG and Rb3AlF6:Mn4+ 460 167.11 4,053 88.6 17 YAG and CdS:Cu/ZnS 460 37 3,357 89 18 YAG and CdSe/CdS/ZnS 460 32 3,865 88 19 YAG and K2TiF6:Mn4+ 455 116 3,556 81 20 YAG and Li3Mg2SbO6:Mn4+ 454 87 3,254 81 21 YAG and Ba0.8Sr0.2Mg3SiN4:Eu2+ 450 120 4,000 96 22 YAG and Cs2GeF6:Mn4+ 450 141.5 3,673 84.9 23 Lu3(Al/Ga)5O12:Ce3+ and Ca1−xLixAl1−xSi1+xN3:Eu2+ 450 101 3,036 95 24 Lu3(Al/Ga)5O12:Ce3+ YAG and Sr[Li2Al2O2N2]:Eu2+ 449 - 2,700 91 25 RbNa2K(Li3SiO4)4:Eu2+, (Sr,Ba)2SiO4:Eu2+ and CaAlSiN3:Eu2+ 395 10.37 3,707 70.9 26 CsNa2K(Li3SiO4)4:Eu2+, (Sr,Ba)2SiO4:Eu2+ and CaAlSiN3:Eu2+ 395 5.19 3,331 71.5 26 CdZnSeS/ZnS 390 222.7 6,029 95.1 27 Phosphor-in-glass [3] 385 27.19 2,984 84.2 28 Multi-color phosphor-in-glass 385 27.8 4,245 92.6 29 (Ba,Sr,Ca)BP2O8:Eu2+ and CaAlSiN3:Eu2+ 380 - 5,995 91 30 Cs(1x)RbxVO3 365 94.8 5,178 82 31 BAM, YAG 310 - 4,437 93.8 32 BAM and S2S 390 120 4,390 72 This study [1] Tungsten filament, [2] Mercury-vapor and phosphors (λexc = 250 nm), [3] CaAlSiN3:Eu2+, Ba2MgSi2O7:Eu2+, and (Sr,Ba)3MgSi2O8:Eu2+. Source: Own authorship. The US Department of energy has brought up a multi-year program plan to overcome all the barriers that are currently found in SSL, Table 1.3.6 In all those projections, the CRI value is expected to be greater than 85 and the CCT, lower than 4,500 K. It is important to decrease the CCT value of the commercial WLED because the human circadian system is exquisitely sensitive to blue-rich light, especially at night, leading to many diseases such as diabetes, insomnia, and depression.33 Yet, although light may have undesirable consequences to human health depending on the wavelength and CCT, it may be useful in medical applications and indoor farms, in the last case, by increasing the biomass production rate of plants, opening up new and exciting frontiers in SSL engineering to fabricate high-brightness monochromatic LEDs for several applications, Table 1.4.4 25 Table 1.3 Target LE for different kind of WLED approach. λem represents the emission wavelength, FWHM is the full-width at half maximum of the emission band, RGB is the red, green and blue emission, RGBA is the red, green, blue and Amber emission, R9 is the Munsell code for the red emission quantification, LE is the luminous efficacy, CM-LEDs are the color-mixed LEDs, and PC-LEDs are the phosphor-converted LEDs. RGB CM-LED with CCT of 3,000 K and CRI of 85 (R9 > 0) Emissions Blue LED Green LED Red LED Representation λem (nm) 464 546 612 FWHM (nm) 20 20 20 LE (lm/W) Current Target 133 191 PC-LED with CCT of 3,000 K and CRI of 85 (R9 > 0) Emissions Blue LED Green phosphor Red phosphor Representation λem (nm) 464 536 612 FWHM (nm) 20 100 100 LE (lm/W) Current Target 123 189 Hybrid-LED with CCT of 3,000 K and CRI of 85 (R9 > 0) Emissions Blue LED Green phosphor Red LED Representation λem (nm) 459 539 612 FWHM (nm) 20 100 20 LE (lm/W) Current Target 165 231 RGBA CM-LED with CCT of 3,000K and CRI of 85 (R9>0) Emissions Blue LED Green LED Amber LED Red LED Representation λem (nm) 460 539 590 615 FWHM (nm) 20 20 20 20 LE (lm/W) Current Target 85 153 Source: Own authorship. Table 1.4 Monochromatic LEDs for human phototherapy and plant and food technology. LED emission spectral range Application for human healthy Application for plant and food technology Red (630 nm) Rhinitis treatment, wound healing and anti- inflammatory34 Algae growth, microalgae cultivation, plant tissue culture34 Green (550 nm) Correct hyperpigmentation, eliminate skin spots35 Algae growth, bacteria and microalgae cultivation34 Blue (480 nm) Seasonal Affective Disorder (SAD), non- seasonal depression and bipolar disorder therapies9 Algae growth, astaxanthin production34 UV (250 nm) Disinfection, water treatment34 Disinfection, water treatment 34 Source: Own authorship. The main commercially-available monochromatic LEDs are based on AlInGaN or AlGaInP (Indium aluminum gallium nitrite or Aluminum gallium indium phosphide) semiconductor and their bandgap may be tuned from 0.7 eV to 3.4 eV by changing the composition, within the 365 – 1,900 nm spectral range.36 Nonetheless, the main challenge on the fabrication of monochromatic LEDs lies on the previously mentioned “green gap”, i.e., the absence of high-efficient semiconductors in the green spectral region. This issue 26 arises from the abrupt decrease of the external quantum efficiency (EQE) of the InGaN- based LEDs in the green-yellowish spectral range. On the other hand, the EQE of the AlGaInP-based LEDs increases only from 600 nm, as represented in Figure 1.3.36 Figure 1.3 External quantum efficiency of conventional monochromatic LEDs emitting in the visible spectral region. Source: Adapted from Seong et al.37 To address the “green gap” drawback, a feasible alternative is combining a near- UV-emitting LED chip (near-UV LED) with near-UV-to-green downshifting converter phosphors thanks to the huge improvement of the WPE of near-UV LEDs achieved in the last few years.38 Therefore, the main challenge on the fabrication of high-efficient white or monochromatic LEDs lies on the improvement of the phosphor properties as the emission quantum yield and the thermal and structural stabilities. In this study, we have chosen silicate-based phosphors, well-known as coatings of PC-LEDs. The state-of-the-art of Ba2SiO4 and Sr2SiO4 based-phosphors comparing the emission quantum yield and the annealing temperature as figures of merits is shown in Table 1.5. Table 1.5 Figure of merit of the emission quantum yield (q) of M2SiO4:RE (M = Sr or Ba and RE = Eu2+, Eu3+ or Tb3+) phosphors. The excitation wavelength (λexc), emission wavelength (λem) and the calcination temperature (T. calc.) to get the phase are also provided. Sample T. calc. / ºC λexc/ nm λem/ nm q / % [ref] Ba2SiO4:Eu2+ 1,200 450 508 0.53 39 Sr2SiO4:Eu2+ 1,200 400 550 0.60 39 Ba2SiO4:Eu3+ 1,300 250 612 - 43 Ba2SiO4:Tb3+ 1,450 250 545 - 40 Ba2SiO4:Eu3+, Tb3+ - - - - - Source: Own authorship. 350 400 450 500 550 600 650 0 20 40 60 80 100 AlInGaN AlGaInPE x te rn al q u an tu m e ff ic ie n cy / % Wavelength "Green-gap" 27 In this study, attention was placed on the improvement of the synthesis of silicate- based phosphors, since, although there are several methods for the phosphor obtention,41,42,43 some points as high calcination temperature (1,200 ºC-1,500 ºC) and low emission quantum yield (50 %) still need to be addressed. To contextualize, the Ba2SiO4 matrix is a current subject of research in the LLuMeS research group. The study of this host for rare earth ions has started at about in 1995 44 with Professor Ana Maria Pires under supervision of Professor Marian Rosaly Davolos. In this study, it was investigated the barium silicate matrix synthesis by the solid-state route and its use as host for Eu2+ and/or Eu3+ as precursor for BaZnSiO4:Eu3+,Mn2+ phosphor for application in fluorescent lamp.45 In 2010, Master Diego Ariça Ceccato started, in LLuMeS laboratory, the investigation of the Ba2SiO4 synthesis by the sol-gel route, and its use as an electrochemical sensor.46 Finally, taking advantage of the sol-gel synthesis, we started in 2013 a scientific initiation scholarship to optimize the Eu3+ concentration in the Ba2SiO4 host. Then, in 2015, during the Ph.D., we have started to develop all the study showed in this thesis. 1.2 Motivation, challenges and justifications We got involved in this study motived by three main points: (i) to apply deep-UV- to-visible downshifting converter phosphors as coatings of deep-UV-emitting LEDs, fabricating multifunctional UV and visible-emitting LED prototypes to be used in indoor farms, (ii) to work around the “green gap” issue by coating near-UV-emitting LEDs with Ba2SiO4:Eu2+ green-emitting phosphors, making green-emitting LED prototypes and (iii) to improve the color qualities of WLEDs by combining near-UV-to-visible downshifting converter phosphors and commercial near-UV-emitting LED chips. The reason behind point (i) lies on the potential application of a multifunctional UV and visible-emitting LED in indoor farms because of the UV radiation may be applied as an antibacterial agent, and the visible light is helpful to enhance the photosynthesis rate, by controlling the plant Circadian rhythm, as previously highlighted in Table 1.4. Already about point (ii), the use of near-UV-emitting LEDs coated by green- emitting phosphors seems to be an alluring approach to overcome the “green gap” drawback. 28 Considering point (iii), there are many challenges to be overcome concerning the fabrication of high-efficient and stable WLEDs with desirable CCT and CRI values such as (a) the fabrication of thermally-stable phosphors with high emission quantum yield, (b) to elect an ideal phosphor mix to fill all the visible spectrum in order to get desirable color qualities, (c) to get photostable LED prototypes with no changes in the color emission over the time of use and (d) how to process the phosphor particles as coatings. To cope with point (i), we chose to synthesize red-emitting phosphors based on Ba2SiO4:Eu3+ and blue or green-emitting phosphors based on Ba2SiO4:Tb3+, all processed as polymeric films. Already to fill points (ii) and (iii), we chose the Ba2SiO4:Eu2+ and Sr2SiO4:Eu2+ phosphors, respectively, dispersed in polymeric films. The justifications on their selection lie on the thermal stability (1,000 ºC), relatively low phonon frequency of the matrices (800 cm-1), transparency to UV radiation, desirable emission quantum yield, and the possibility of hosting divalent and trivalent dopant cations. The role of the phosphor processing as films lies on the decrease of the light scattering, and the improvement of the heat dissipation, to which are drawbacks currently found in the traditional protocol for the phosphor processing as coatings of LED chips (dispersion in silicone, epoxy resin or polyurethane). 1.3 Goals The goal of this study is to fabricate light-emitting diodes by coating UV-emitting LED chips with UV-to-visible downshifting converter phosphors dispersed in polymeric matrices, in order to come up with prototypes featuring potential to be applied in lighting, devices, agriculture or phototherapy through the control of the Circadian rhythm of plants and humans, Figure 1.4. The specific goals are: 1. To synthesize deep-UV-to-visible downshifting converter phosphors based on Eu3+ and/or Tb3+ doped Ba2SiO4 phosphor; 2. To use Eu3+ as spectroscopic probe aiming to understand the impacts of the doping on the Ba2SiO4 network; 3. To understand the energy transfer process between Tb3+ – Tb3+ and Eu3+ – Tb3+ in the Ba2SiO4 matrix; 4. To elect a suitable polymeric matrix to disperse the phosphors in order to make films with controlled thickness; 29 5. To use the Ba2SiO4:Eu3+,Tb3+ phosphor as coatings of deep-UV-emitting LEDs; 6. To synthesize a green-emitting phosphor based on Ba2SiO4:Eu2+ and use it as coating of near-UV emitting chips in order to fabricate green-emitting LED prototypes; 7. To synthesize a yellow-emitting phosphor based on Sr2SiO4:Eu2+ and use it as coating of near-UV emitting chips combined to a blue-emitting phosphor (BAM:Eu2+), aiming to fabricate a WLEDs featuring tunable CCT. Figure 1.4. Goals of the thesis. Source: Own authorship. 1.4 Thesis organization Chapter 1 – Introduction: Discussion on the state-of-the-art of LEDs and silicate- based phosphors. Chapter 2 - Background: Presentation of the main concepts used to discuss the results. Chapter 3 – Tunable blue-green emission and energy transfer properties in Ba2SiO4:Tb3+: Discussion on the synthesis of green/blue-emitting phosphors, impacts of the Tb3+-doping on the Ba2SiO4 network, energy transfer properties and quantum efficiency of Tb3+. This chapter was written based on a paper published in the Journal of Luminescence.47 Ba/Sr Eu3+, Tb3+, Eu2+ Si O PVDF or PMMA Phosphor Film LED prototype fabrication Ba2SiO4:Eu2+ Sr2SiO4:Eu2+ + BAM:Eu2+ Ba2SiO4:Eu3+ Ba2SiO4:Tb3+ Phosphor UV and light emission 250 nm LED Color mix 395 nm LED Film 365 nm LED Film Film Green emission 30 Chapter 4 – Sol-gel synthesis of Eu3+-doped Ba2SiO4: Synthesis of red-emitting phosphors, optimization of the synthesis and impacts of the doping on the defect-related structure of the Ba2SiO4 lattice. Part of this chapter was written based on a paper published in the RSC Advances.48 Chapter 5 – Energy transfer between Tb3+ and Eu3+ in barium orthosilicate phosphors. Discussion on the tunable red-blue light emission in Ba2SiO4:Eu3+,Tb3+, energy transfer properties and use of Tb3+ as a sensitizer to Eu3+. This chapter was written based on a paper published in the Journal of Luminescence.49 Chapter 6 – Red-light-emitting coatings for LEDs applied to plant circadian rhythm control: Fabrication of polymeric films containing Ba2SiO4:Eu3+ and evaluation of these films as coatings of deep-UV-emitting LEDs. Part of this chapter was written based on a paper published in the Materials Chemistry and Physics.50 Chapter 7 - Green-emitting LEDs based on Ba2SiO4:Eu2+ and near-UV-emitting LEDs: Synthesis of the Ba2SiO4:Eu2+ phosphor in the powder form or dispersed in PMMA and fabrication of green-emitting LEDs. Chapter 8 - White-emitting LEDs based on Eu2+-doped silicate: Synthesis of the Sr2SiO4:Eu2+ phosphor in the powder form or dispersed in PMMA and fabrication of white-emitting LEDs. Chapter 9 – Final remarks: Presentation of the final remarks and some proposals for future studies in the research group. 1.5 References 1 FRIEDEL, R. D.; ISRAEL, P.; FINN, B. S. Edison's electric light, Bibliography, 1987, Rutgers University Press. 2 CHO, J. et al. White light-emitting diodes: History, progress, and future, Laser & Photonics Reviews, 2017, v. 11, n. 2, p. 1600147-17. 3 JACOBY, M. Tuning phosphors for better white light Advances in the inorganic powders boost the efficiency and appeal of LED bulbs, Chemical and Engineering News, 2018, v. 96, n. 46, p. 28-33. 4 PATTISON, P. M; HANSEN, M.; TSAO, J. Y. LED lighting efficacy: Status and directions, Comptes Rendus Physique, 2018, v. 19, n. 3, p. 134-145. 5 MITCH JACOBY, Tuning phosphors for better white light, C&EN, 2018, v. 96, n. 46, p. 28-33. 6 BARDSLEY, N. et al. Solid-State Lighting Research and Development Multi-YearProgram Plan, Building Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, 2014 (DOE/EE-1089). 7 CHEN, L. et al. Light Converting Inorganic Phosphors for White Light-Emitting Diodes, Materials, 2010, v. 3, n. 3, p. 2172-2195. 8 BAI, X. et al. Efficient and tuneable photoluminescent boehmite hybrid nanoplates lacking metal activator centres for single-phase white LEDs, Nature Communications, 2014, v. 5, n. 0, p. 5702-8. 31 9 LEGATES, T. A.; FERNANDEZ, D. C.; HATTAR, S. Light as a central modulator of circadian rhythms, sleep and affect, Nature Reviews, 2014, v. 15, n. 7, p. 443–454. 10 OH, J. H.; YANG, S.J.; DO, Y. R. Healthy, natural, efficient and tunable lighting:four-package white LEDs for optimizing the circadian effect, color quality and vision performance, Light: Science & Applications, 2014, v. 3, n. 0, p. 141-9. 11 ZHAO, M. et al. Next-Generation Narrow-Band Green-Emitting RbLi(Li3SiO4)2:Eu2+ Phosphor for Backlight Display Application, Advanced Materials, 2018, v. 30, n. 38, p. 1802489-7. 12 PATTISON, P. M. et al. LEDs for photons, physiology and food, Nature, 2018, v. 543, n. 0, p. 493 – 500. 13 HONG, F. et al. Room-temperature synthesis, optimized photoluminescence and warm-white LED application of a highly efficient non-rare-earth red Phosphor, Journal of Alloys and Compounds, 2019, v. 775, n. 0, p. 1365-1375. 14 LV, L. et al. The formation mechanism, improved photoluminescence and LED applications of red phosphor K2SiF6:Mn4+, Journal of Materials Chemistry C, 2014, v. 2, n. 20, p. 3879–3884. 15 ZHENG, F. et.al, Reliability of fluoride phosphor K2XF6:Mn4+ (K2SiF6:Mn4+, K2(Si,Ge)F6:Mn4+, K2TiF6:Mn4+) for LED application, Journal of Materials Science: Materials in Electronics, 2018, v. 29, n. 24, p. 21061–21071. 16 JIANG, C, et al. Mn4+-Doped Heterodialkaline Fluorogermanate Red Phosphor with High Quantum Yield and Spectral Luminous Efficacy for Warm-White-Light-Emitting Device Application, Inorganic Chemistry 2018, v. 57, n. 23, p. 14705−14714. 17 DENG, T. et al. Implementation of high color quality, high luminous warm WLEDusing efficient and thermally stable Rb3AlF6:Mn4+ as red color converter, Journal of Alloys and Compounds, 2019, v. 795, n. 0, p. 453-461. 18 WANG, X. et al. Doped Quantum Dots for White-Light-Emitting Diodes Without Reabsorption of Multiphase Phosphors, Advanced Materials, 2012, v. 24, n.20, p. 2742–2747. 19 WANG, X. LI, W. SUN, K. STABLE efficient CdSe/CdS/ZnS core/multi-shell nanophosphors fabricated through a phosphine-free route for white light-emitting-diodes with high color rendering properties, Journal of Materials Chemistry, 2011, v. 21, n. 24, p. 8558-8565. 20 ZHU, H. et al. Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes, Nature communications, 2014, v. 5, n. 0, p. 4312-10. 21 WANG, S. et al. Mn4+-activated Li3Mg2SbO6 as an ultrabright fluoride-free red-emitting phosphor for warm white light-emitting diodes, RSC Advances, 2019, v. 9, n. 6, p. 3429- 3435. 22 OSBORNE, R. A. et al. Ba(1-x)SrxMg3SiN4:Eu narrow band red phosphor, Optical Materials, 2018, v. 84, n. 0, p. 130–136. 23 WANG, Z. et al. Highly efficient red phosphor Cs2GeF6:Mn4+ for warm white light-emitting diodes, RSC Advances, 2015, v. 5, n. 100, p. 82409-82414. 24 WANG, L. et al. Ca1−xLixAl1−xSi1+xN3:Eu2+ solid solutions as broadband, color-tunable and thermally robust red phosphors for superior color rendition white light-emitting diodes, Light: Science & Applications, 2016, v. 5, n. 0, p. 16155. 25 HOERDER, G. J. et al. Sr[Li2Al2O2N2]:Eu2+—A high performance red phosphor to brighten the future, Nature Communications, 2019, v. 10, n. 0, p. 1824-9. 26 ZHAO, M. et al. Discovery of New Narrow-Band Phosphors with the UCr4C4-Related Type Structure by Alkali Cation Effect, Advanced Optical Materials, 2018, v. 1801631, n. 6, p. 1-9. 27 LE, T. et al. Highly Luminescent Quantum Dots in Remote-Type Liquid-Phase Color Converters for White Light-Emitting Diodes, Advanced Materials Technology, 2018, v. 3, n. 3, p. 1800235-9. 28 PENG, Y. et al. Luminous efficacy enhancement of ultraviolet-excited white light-emitting diodes through multilayered phosphor-in-glass, Applied Optics, 2016, v. 55, n. 18, p. 4933- 4938. 29 JIANG, P. Thermally stable multi-color phosphor-in-glass bonded on flip-chip UV-LEDs for chromaticity tunable WLEDs, Applied Optics, 2017, v. 56, n. 28, p. 7921- 7926. 32 30 SU, S. et al. Near UV-pumped bluish-white emitting K(Ba,Sr,Ca)BP2O8:Eu2+ phosphors, Journal of Alloys and Compounds Volume. 2013, v. 575, n. 0, p. 309-313. 31 PAVITRA, E. et al. Evolution of highly efficient rare-earth free Cs(1x)RbxVO3 phosphors as a single emitting component for NUV-based white LEDs, Journal of Materials Chemistry C, 2018, v. 6, n. 46, p. 12746- 12757. 32 Li, H. et al. Synthesis and Luminescence Properties of Bi3+-Activated K2MgGeO4: A Promising High- Brightness Orange-Emitting Phosphor for WLEDs Conversion, Inorganic Chemistry, 2018, v. 57, n. 19, p. 12303−12311 33 FIGUEIRO, M. G. An Overview of the Effects of Light on Human Circadian Rhythms: Implications for New Light Sources and Lighting Systems Design, Journal of Light & Visual Environment, 2013, v. 37, n. 2, p. 51-61. 34 YEH, N. et al. Applications of light-emitting diodes in researches conducted in aquatic environment, Renewable and Sustainable Energy Reviews, 2014, v. 32, n.0, p. 611–618. 35 KLEIN, R. M. Effects of green light on biological systems, Biological reviews of the Cambridge Philosophical Society, 1992, v. 67, n. 2, p.199-284. 36 JEONG, H. et al. Indium gallium nitride-based ultraviolet, blue, and green light emitting diodes functionalized with shallow periodic hole patterns, Scientific Reports, 2017, v. 7, n. 0, p. 45726-9. 37 SEONG, T. et al. III-Nitride Based Light Emitting Diodes and Applications, Topics in Applied Physics, 2013, springer. 38 MATAFONOVA, G. BATOEV, V. Recent advances in application of UV light-emitting diodes for degrading organic pollutants in water through advanced oxidation processes: A review, Water Research, 2018, v. 132, n. 1, p. 177-189. 39 SATO, Y. et al. Large redshifts in emission and excitation from Eu2+ activated Sr2SiO4 and Ba2SiO4 phosphors induced by controlling Eu2+ occupancy on the basis on crystal-site engineering, Optics and Photonics Journal, 2015, v. 5, n. 11, p. 326-333. 40 DA-WEI, H. et al. VUV Luminescent Properties of M2SiO4:Re (M =Mg, Ca, Ba) (Re= Ce3+, Tb3+ ), Chinese Journal of Luminescence, 2007. v. 28, n.1, p. 53-57. 41 AWATE, V. et al. Synthesis, characterization and luminescence studies of rare earth activated Sr2SiO4 phosphor: a review, Journal of Materials Science: Materials in Electronics, 2018, v. 29, n.6, p. 4391–4401. 42 SZCZODROWSKI, K. et al. The role of compensation defects in Eu3+ stabilization under reductive atmosphere in Sr2SiO4 matrix, Journal of Alloys and Compounds, 2018, v. 748, n.5, p. 44-50. 43 WANG, Z. et al. Luminescent properties of Ba2SiO4:Eu3+ for white light emitting diodes, Physica B, 2013, v. 411, n.15, p. 110–113. 44 PIRES, A. M.; DAVOLOS, M. R.; MALTA, O.L. Eu3+-O2-associates luminescence in Ba2SiO4, Journal of Luminescence, 1997, v. 72-74, n. 0, p. 244-246. 45 PIRES, A. M.; DAVOLOS, M. R. Luminescence of Europium(III) and Manganese(II) in Barium and Zinc Orthosilicate, Chemistry of Materials, 2001, v. 13, n. 1, p. 21-27. 46 RAYMUNDO-PEREIRA, et al. Study on the structural and electrocatalytic properties of Ba2+- and Eu3+- doped silica xerogels as sensory platforms. RSC Advances, 2016, v. 6, n. 106, p. 104529-104536. 47 BISPO-JR, A.G. et al. Tunable blue-green emission and energy transfer properties in Ba2SiO4:Tb3+ obtained from sol-gel method, Journal of luminescence, 2019, v. 214, n. 0, p. 116604-8. 48 BISPO-JR, A. G. et al. Red phosphor based on Eu3+-isoelectronically doped Ba2SiO4 obtained via sol– gel route for solid state lightning, RSC Advances, 2017, v. 7, n. 85, p. 53752–53762. 49 BISPO-JR, A. G. et al. Energy transfer between terbium and europium ions in barium orthosilicate phosphors obtained from sol-gel route, Journal of Luminescence, 2018, v. 199, n. 0, p. 372–378. 50 BISPO-JR, A. G. et al. Red-light-emitting polymer composite based on PVDF membranes and Europium phosphor using Buriti Oil as plasticizer, Materials Chemistry and Physics, 2018, V 217, n. 0, p. 160-167. 33 CHAPTER 2- BACKGROUND 2.1 Phenomenon of luminescence 2.1.1 Fundaments of luminescence The phenomenon of luminescence is a process in which non-thermal radiation is produced as the return of a portion of energy absorbed from an independent source.1 The phenomenon of luminescence may occur in all kind of materials, condensed or not, organic or inorganic, crystalline or not, and it may be classified according to the excitation source.1 In this study, emphasis will be placed on photoluminescence and electroluminescence. Electroluminescence is a process in which a material (usually a semiconductor) emits light as response to an electric field or an electric current, resulting in radiative recombination of holes and electrons.2 Already in the photoluminescence, a material is excited by photons in the UV, visible or IR spectral range.1 Specifically, in the case of crystalline systems, the photoluminescence may be classified according to the nature of the emission: (i) intrinsic luminescence, a process correlated with network emission and (ii) emission coming from impurities, named as dopant or luminescent activator.3 In the emission arising from the luminescent activator, the excitation process occurs simultaneously by two different pathways, (i) the activator is indirectly excited by an intramolecular energy transfer coming from the matrix to the dopant and/or (ii) the activator is directly excited through transitions associated to its ground and excited states, as represented in Figure 2.1.4 Figure 2.1 Photoluminescence mechanism in a crystalline matrix doped with an activator ion (A) excited (a) indirectly by the matrix and (b) directly. Source: Own authorship. Ba2+ Ba2+ A Ba2+ Ba2+ Ba2+ 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− UV Light Heat Activator emissionActivator excitation Ba2+ Ba2+ A Ba2+ Ba2+ Ba2+ UV Light Heat 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− 𝑆𝑖𝑂4 4− Matrix excitation Energy transfer Activator emission (a) (b) 34 In both mechanisms represented in Figure 2.1, non-radiative processes as heat dissipation, vibrational relaxation and/or phonon release are competitively associated with the emission.4 Among the main hosts for luminescent activators are oxides, sulfides, silicates, and oxysulfides, and as activators, transition metal and rare earth ions (RE3+ or RE2+).5 Finally, the photoluminescence phenomenon may be also classified according to the energy of the excitation photons as Stokes process (or downshifting) and anti-Stokes process.1 In the Stokes emission, the emitted photons feature lower energy than the excitation photons and in the anti-Stokes process, the opposite occurs. For the sake of clarity, some figures of merit used for photoluminescence quantification and qualitative aspects of energy transfer are pointed out as follow. 2.1.2 Characteristics of photoluminescence Lifetime of an emitting state (τ) A greatness extremely important to the photophysics of photoluminescent processes is the lifetime of the emitting state. The lifetime is the needed time to the initial population of a state to decrease to 1/e, and it may also be expressed as the inverse of the velocity constant (K) related to an electronic relaxation.6 The lifetime is also dependent on the radiative (Arad) and non-radiative (Anrad) transition probabilities, as represented in Equation 2.1.6 𝜏 = 1 𝐴𝑟𝑎𝑑 + 𝐴𝑛𝑟𝑎𝑑 (2.1) Quantum efficiency of an emitting state (η) The quantum efficiency (η) of an emitting state is the ratio of the radiative (Arad) and total (Atotal) transition probabilities involved in the deactivation of the emitting state, as represented by Equation 2.2.7 𝜂 = 𝐴𝑟𝑎𝑑 𝐴𝑡𝑜𝑡𝑎𝑙 = 𝐴𝑟𝑎𝑑 𝐴𝑟𝑎𝑑 + 𝐴𝑛𝑟𝑎𝑑 (2.2) Theoretically, for downshifting converter phosphors, the quantum efficiency is expected to be 100 % in a process without any non-radiative pathways. Yet, there are several non-radiative processes that may deactivate an excited state including multiphoton relaxation, cross-relaxation processes, heat release, electronic defect levels among others.8 35 Absolute emission quantum yield (q) The absolute emission quantum yield (q) of a radiation-induced process is defined by the number of emitted photons by a sample divided by the number of the absorbed ones.8 Qualitative aspects of energy transfer processes In this study, energy transfer (ET) processes among RE3+ was investigated. The ET between two luminescent centers takes place by two different pathways: radiatively or non-radiatively.9 For the radiative process, the transfer occurs by two steps: First, the donator emits radiation and then, the acceptor absorbs this radiation, as highlighted in Equation 2.3 and 2.4, where D represents the donator and A, the acceptor species. In this case, the energy transfer efficiency depends on the superposition of the emission spectra: higher efficiencies are achieved as the Stokes shift decreases.* D* → D + hv (2.3), A + hv → A* (2.4) The non-radiative energy transfer occurs by just one step and does not involve radiation absorption or emission, as represented in Equation 2.5. In this case, the energy transfer is not directional and occurs directly by the interaction of both donator and acceptor. The two main models for non-radiative processes were first introduced by Förster and Dexter.9 D* + A → D + A* (2.5) The Förster model is associated with dipole-dipole interactions among donator and acceptor.10 For this, it is necessary that the oscillations coming from the electric field of the excited states of donator and the ground state of acceptor are resonant, plus overlap between the emission of donator and the excitation of acceptor and correct spatial orientation between the electric dipole of both states involved in the process. In the dipole-dipole mechanism, Figure 2.2 (a), the interactions among donator and acceptor occur by the overlap between their dipolar electric field, operating by an electric filed oscillator arising from the donator and do not require a Van Der Waals contact, or orbital overlap.11 * Stokes shift is the energy difference of the absorption and emission bands that arise from the same electronic transition. 36 To examine the electric field oscillator of the donator, it is necessary to consider the electron as a harmonic oscillator that may experience oscillations (like electronic vibrations) into the direction of some cartesian axis in the crystalline network. In the classic mechanical model, an electron in the ground state of harmonic oscillation does not oscillate, but the donator has an excited electron, to which, according to this classic theory, correspond to an excited state of harmonic oscillator. This electron experience periodic harmonic oscillations with a natural frequency (ν0) and some of these oscillators randomly dispersed in the network create an oscillating electric dipole close to the electric dipole oscillator of the electromagnetic radiation. Thus, the donator (but not the acceptor) is idealized as having an electric dipole oscillator that creates an oscillating electric field around the donator.11 Figure 2.2 Scheme of energy transfer mechanisms by (a) dipole-dipole and (b) exchange interactions. (c) Diagrams for electropole radiators. Source: Adapted from Ye et al.11 Förster reported the energy transfer by interactions of electric dipoles from both donator and acceptor, but some phenomena have not been foreseen, and Dexter expanded the Förster model considering the electron exchange or multipolar interactions between donator and acceptor. Thus, the Dexter theory is observed almost exclusively in quite short distances or in cases that the Förster mechanism is forbidden. This mechanism is also known as electron exchange or overlap mechanism, and it is limited by distances of 4 Angstroms, since it requires an orbital overlap, Figure 2.2 (b).12 A Rc D* e e Excited state Ground state 1 22 2 D* A Dipole-dipole dipole-dipole interactions 2 2 A* 1 2 dipole-dipole interactions D (a) A Rc L D* Excited state Ground state 1 22 2 Electron transfer Hole transfer D* 2 22 1 A D A* Electron exchange (b) (c) Dipole Quadrupole - + + + - - FF 37 In the Dexter model, exchange interactions and multipole dipole-quadrupole and quadrupole-quadrupole interactions are introduced, and the energy transfer probabilities depend on R-8 and R-10, respectively, being R the distance between donator and acceptor. This theory may be correlated with the electron transfer in oxy-reduction reactions in transition metal complexes, mostly those ones that involve the inner-sphere mechanism.9 I. G. Van Uitert (1967)13 came up with a model that it is possible to determine the mechanism of energy transfer between donator and acceptor by measuring the emission intensity (or lifetime) of the acceptor in different concentrations by applying Equation 2.6, where x represents the acceptor concentration, I the emission intensity (or lifetime), β is a constant, and θ = 6, 8 or 10, and represents electric dipole–dipole, dipole– quadrupole or quadrupole–quadrupole interactions, respectively. This model is applied for ET processes between RE3+ as Tb3+→Tb3+, Eu3+→Eu3+ or Eu3+→Tb3+ and the selection rules of those processes are highlighted in Table 2.1. 𝐼 𝑥 = [1 + 𝛽(𝑥)𝜃/3] −1 ( 2.6) Table 2.1 Selection rules for ET processes of RE ions (RE = RE2+, RE3+). EQ is the electric quadropole- quadrupode ET, 2λ represents multipolar interactions. Mechanism Selection rule Exemple (i) RE–RE ET ED ΔJ = 0, 1 Ce3+ and Eu2+: 5d - 4f Forced ED 6 ≥ ΔJ ≥ 0 Eu3+: 5D0 - 7F2,4,6 EQ ΔJ ≤ 2 Yb3+: 2F5/2–2F7/2 Exchange | J – J’| = 0, 1 Eu3+: 5D1 - 5D0 (ii) Ligand–Ln ET Dipole-2λ pole and D-D | J – J’| < λ ≤ J + J’ Eu3+ transfer from 7F0 to 5D2, 5L6, 5G6, 5D4. 7F0 to 5D0,1 forbidden. Exchange | J – J’| = 0, 1 ET involving Eu3+ 7F0–5D0 forbidden, but relaxed by J-mixing; Eu3+ 7F0–5D1 Source: Reproduced from Tanner at al. 14 2.2 Solid-state lighting 2.2.1 Light-emitting diodes The solid-state lighting (SSL) is a field of lighting engineering that studies all kinds of lightings that use semiconductor light-emitting diodes (LEDs), organic light- emitting diodes (OLED), or polymer light-emitting diodes (PLED). In this study, we will focus on LEDs, and the spontaneous emission of them occurs by radiative recombination of electron-holes due to an electric field (electroluminescence phenomenon). Theoretically, this process may occur infinitely, increasing the device lifespan and cost- benefit compared to the traditional lightings.15 38 The fundamental on the LED architecture is a p-n junction, Figure 2.3 (a), and in this approach, the n layer is composed by electrons as majority load conductors and the p layer uses holes for the same purpose. The cathode is connected to the negative terminal and the anode to the positive one, making that the electrons from the n terminal be repelled to the depletion region of the “p-n” junction and tunnel to the p terminal. The same occurs for the holes in the p terminal and this charge movement generates the emission by the electron-hole recombination.16 Figure 2.3 (a) Schematic of a p-n junction in LEDs. (b) Architecture of CM-LEDs, PC-LEDs and hybrid- LEDs. CB = conduction band, VB = valence band. Source: Adapted from Held.16 There are several methods to get white light from WLEDs, and the main ones are highlighted in Figure 2.3 (b). All those approaches are based on color-mixed LEDs (CM- LEDs), phosphor-converted LEDs (PC-LEDs), or hybrid-LEDs. In CM-LEDs, the color mix is achieved from the intrinsic electroluminescence of LED chips (semiconductor material emission). Already the PC-LED architecture is predicated on the use of near- UV-emitting LED chips coated by a phosphor mixture. In this case, the LED chip emits radiation by electroluminescence within the near-UV spectral window and this radiation is used to excite the phosphor mixture on the LED chip through photoluminescence. Finally, in Hybrid-LEDs, the LED chip emits light by electroluminescence, a portion of this energy is used to excite a mixture of phosphors, and the combination of both LED chip and phosphor emissions generates the white light sensation.17 Among these three approaches to fabricate WLEDs, there are different color combinations to get white light, as highlighted in Figure 2.4, pointing out LE, CCT and CRI of the devices. p type n type Hole Electron CB VB Fermi level Recombination Color mix Blue LED Green LED Red LED Color mix Phosphor mix Blue LED CM-LED Hybrid-LED Color mix Phosphor mix UV LED PC-LED (b)(a) 39 Figure 2.4 Different architectures of WLEDs. The terms R (red), G (green), B (blue) and A (Amber) corresponds to the different combinations of color emissions to get white light. Source: Own authorship. In this study, particular attention is placed on PC-LEDs due to the relatively-high wall-plug efficiency (WPE) and external quantum efficiency (EQE) achieved in the last years for near-UV emitting LEDs, and the non-dependence of the white light emitted by the WLED on the excitation source (near-UV LED chip), to which guarantee the stability of the correlated color temperature (CCT) over the time of use.18 Moreover, other advantages of near-UV-emitting LEDs compared, for instance, to blue-emitting LEDs are less current drooping and significantly less binning, producing greater photon density at higher currents.18 The total efficiency (nPC-LED) of PC-LEDs depends on several factors represented in Equation 2.7, being ηyield the phosphor quantum efficiency, ηstokes the ratio between the energy of the excitation and emission photons, ηSA the self-absorption efficiency and ηext the extraction efficiency and it is expected to be 1 due to the high refractive index of both phosphor and the near-UV-emitting LED chip. All those parameters must be optimized to get the highest efficiency in the WLED. 𝜂𝑃𝐶−𝐿𝐸𝐷 = 𝜂𝑦𝑖𝑒𝑙𝑑𝑥𝜂𝑒𝑥𝑥𝜂𝑠𝑡𝑜𝑘𝑒𝑥𝜂𝑆𝐴 (2.7) It is worth pointing out the effect of the phosphor composition and packaging structure on the efficiency of the PC-LED. Traditionally, the LED packaging uses a Blue-emitting chip Color mix Phosphor High LE (150 lm/W) High CCT (> 6,000 K) Poor CRI (< 75) Hybrid WLED Blue-emitting chip Color mix Phosphor Low LE (< 120 lm/W) Desirable CCT ( 4,000 K) High CRI (> 90) Hybrid WLED RGB CM-LED Color mix Low LE (< 120 lm/W) Desirable CCT ( 4,000 K) High CRI (> 90) LED chip RGBA CM-LED Color mix Low LE (< 100 lm/W) Desirable CCT ( 4,000 K) High CRI (> 90) LED chip Blue-emitting chip Color mix Low LE (120 lm/W) Desirable CCT (> 4,000 K) High CRI (> 90) Hybrid WLED Phosphor UV-emitting chip Color mix RGB PC-WLED Phosphor Low LE (120 lm/W) Desirable CCT (> 4,000 K) High CRI (> 90) UV-emitting chip Color mix PC-WLED Phosphor Low LE (120 lm/W) High CCT (> 4,000 K) Poor CRI (< 75) Blue and red chip Color mix Low LE (120 lm/W) Desirable CCT (> 4,000 K) High CRI (> 90) Hybrid WLED Phosphor 40 dispersing method, based on the phosphor particles blended in silicone or organic resins (polyurethane, epoxy) that are directly dropped onto the LED chip surface, Figure 2.5 (a).19 Although those methods are the mainstream for commercially-available PC-LEDs, they feature several disadvantages such as: (i) at about 60 % of the LED light is backscattered by the phosphor and reabsorbed by the LED chip since the phosphor blend is tightly close to the LED chip, Figure 2.5 (a); (ii) poor heat dissipation due to the silicone or organic resin, resulting in thermal quenching of the phosphor luminescence and (iii) the silicone resin may crack over the time of use.20 Figure 2.5 Schematic diagram of LED packaging. (a) Phosphors dispersed in silicone or organic resins, (b) phosphors processed as films. Source: Own authorship. To cope with all those limitations, our strategy in this study is to process the phosphors as films, Figure 2.5 (b), by dispersing the particles of the phosphors in polymeric matrices as Polyvinylidene fluoride (PVDF) or Poly(methyl methacrylate) (PMMA), making films with tunable thickness, decreasing the light scattering.21,22,23 2.2.2 Photometric quantities 1,931 Commission Internationale de L'éclairage (CIE) color coordinates The (x,y) color coordinates were defined by the Commission Internationale de L'éclairage (CIE) in 1931 adopting a standard colorimeter that represents the color attributes in a tridimensional diagram. The cartesian coordinates are represented as following in Equations 2.8, being X, Y and Z, representations of the integrals in all visible spectral range, Equations 2.9.24 The �̅�(𝜆), �̅�(𝜆) 𝑎𝑛𝑑 �̅�(𝜆) spectral stimulus represent the human eye sensibility to the electromagnetic radiation corresponding to the red, green and blue primary colors, respectively.24 𝑥 = 𝑋 𝑋 + 𝑌 + 𝑍 , 𝑦 = 𝑌 𝑋 + 𝑌 + 𝑍 , 𝑧 = 𝑍 𝑋 + 𝑌 + 𝑍 (2.8) FilmPhosphor blend Near-UV-emitting LED Near-UV-emitting LED Phosphor mix Phosphor film (a) (b) 41 𝑋 = ∫ 𝐼(𝜆)�̅�(𝜆)𝑑𝜆, 𝑌 = ∫ 𝐼(𝜆)�̅�(𝜆)𝑑𝜆, ∞ 0 ∞ 0 𝑍 = ∫ 𝐼(𝜆)�̅�(𝜆)𝑑𝜆 ∞ 0 (2.9) Correlated color temperature (CCT) The CCT of a bulb is the temperature in Kelvin of an ideal heated black-body radiator that radiates the same color of the light source. Strictly speaking, this is a greatness that measures the